Dissecting Complex Magmatic Processes: an in-depth ...

JOURNAL OF PETROLOGY

VOLUME 53

NUMBER 9

PAGES 1887^1911

2012

doi:10.1093/petrology/egs037

Dissecting Complex Magmatic Processes: an in-depth U^Pb Study of the Pavia Pluton, Ossa^Morena Zone, Portugal S. M. LIMA1*, F. CORFU2, A. M. R. NEIVA1 AND J. M. F. RAMOS3 DE POMBAL, 3000-272 COIMBRA, PORTUGAL 2

DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF OSLO, POSTBOX 1047, BLINDERN, N-0316 OSLO, NORWAY

3

LNEG, RUA DA AMIEIRA, APARTADO 1089, 4466-901, S. MAMEDE DE INFESTA, PORTUGAL

RECEIVED JUNE 14, 2011; ACCEPTED MAY 7, 2012 ADVANCE ACCESS PUBLICATION JUNE 22, 2012

The build-up of large magmatic complexes can proceed piecemeal over periods of several million years through sequences of complex processes of magma production, differentiation, assimilation, final crystallization and subsequent metasomatic modification. All these stages can produce or modify minerals used as geochronometers, such as zircon, monazite and titanite. The present study exemplifies such complex relationships, also demonstrating how a systematic approach with comprehensive sampling and careful high-resolution U^Pb analyses can yield a coherent picture of the entire magmatic process. The study was conducted on the Pavia pluton, an elongated Variscan intrusion in the Ossa^Morena Zone of Portugal. The geochronological data show that the Pavia pluton was emplaced by the amalgamation of multiple magma pulses into the crust, over a period of c. 11 Myr. An early event at 340 Ma, revealed by xenocrystic zircon, preceded the magmatic activity at the exposed level of the pluton, but is recognized as the main magmatic event elsewhere in the Ossa^Morena Zone. A second event at 328 Ma formed tonalite, trondhjemite and granodiorite, and subordinate differentiates in the central domains of the pluton (units I and II). A third event at c. 324 Ma emplaced granodiorite in the flanking domains III^V and the contemporaneous and widespread two-mica granite in domain VI, together with late rhyodacite porphyries, microgranodiorites, aplite^pegmatite and pegmatite dikes. A fourth event at 319^317 Ma was characterized by the emplacement of some microgranites and pegmatite dikes. These two last magmatic events also had an effect on the previously emplaced rocks, causing local overgrowths and isotopic resetting of minerals. The occurrence of a fifth

*Corresponding author. Telephone: (þ351) 239860521. Fax: (þ351) 239860501. E-mail: [email protected]

magmatic event at depth at 313 Ma is the inferred cause of the hydrothermal activity responsible for local zircon, monazite and titanite resorption and/or recrystallization and for some of the textures exhibited by the main rock-forming minerals.The magmatic episodes were interspersed with periods of quiescence; this cyclicity presumably reflects an external control by the transtensional tectonic regime of the Ossa^Morena Zone.

ID-TIMS U^Pb geochronology; magmatic evolution; multi-stage zircon growth; Ossa^Morena Zone; Pavia pluton

KEY WORDS:

I N T RO D U C T I O N Detailed mapping and petrological studies combined with increasingly precise U^Pb geochronological data (Coleman et al., 2004; Matzel et al., 2006; Saint-Blanquat et al., 2006; Lipman, 2007; Michel et al., 2008; Schaltegger et al., 2009; du Bray et al., 2011; Menand et al., 2011) suggest that many plutons grow incrementally by the accretion of successive and relatively small batches of magma. The initial influx of magma heats the crustal environment, allowing later intrusions to remain partially molten for longer periods of time (e.g. Glazner et al., 2004; Michaut & Jaupart, 2011). This process can take from a few hundred to several million years, depending on the geodynamic setting and magma source fertility. Piecemeal emplacement

ß The Author 2012. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@ oup.com

Downloaded from http://petrology.oxfordjournals.org/ at 00500 Universidade de Coimbra on August 28, 2015

GEOSCIENCES CENTER AND DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF COIMBRA, LARGO DO MARQUE“S

1


VOLUME 53

GEOLOGIC A L S ET T I NG The Ossa^Morena Zone The OMZ is one of the tectonostratigraphic subdivisions of the Iberian Massif (Fig. 1a), which corresponds to the western extension of the European Variscan Belt. The OMZ

SEPTEMBER 2012

was part of a Cadomian magmatic arc accreted during the Late ProterozoicÊarly Cambrian to the outer continental margin of Gondwana near the West African Craton and then affected by rifting in the Cambrian to Early Ordovician (Sa¤nchez-Garc|¤ a et al., 2008). Its northern boundary, the Tomar^Badajoz^Co¤rdoba Shear Zone, separates tectonic domains with NE and SW vergence, respectively (Franke et al., 2005). To the south the OMZ is delimited by the BejaÂcebuches oceanic amphibolites, located at the southern end of the Beja Igneous Complex in Fig. 1 (e.g. Franke et al., 2005; Azor et al., 2008). Although controversy still exists (e.g. Azor et al., 2009; Pin & Rodr|¤ guez, 2009; Ribeiro et al., 2010), geochronological data published by Azor et al. (2008) suggest that these amphibolites do not represent parts of a Variscan ophiolite sequence (as the Rheic Ocean seems to have been consumed before the beginning of the Carboniferous) but may instead have formed during a widespread extensional magmatic event recorded all along SW Iberia in the Early Carboniferous. In the Early^Middle Mississippian (360^325 Ma) the OMZ underwent extension involving crustal deformation, metamorphism of the deep crust, generation of metamorphic complexes and the emplacement of voluminous magmas (Pereira et al., 2009). The Variscan metamorphism (epizonal to catazonal) is of the high-temperature^low-pressure (HT^LP) type (Franke et al., 2005) and based on Rb/Sr and K/Ar geochronological studies of the OMZ [compilation by Castro et al. (2002)] the magmatic activity lasted until the beginning of the Permian. Three major metamorphicîgneous domains are recognized in the western part of the OMZ (Fig. 1b): the Northeast Alentejano Massif, the E¤vora Massif and the Beja Igneous Complex, from north to south. The Pavia pluton is part of the E¤vora Massif (sensu Carvalhosa, 1983). This massif is mainly composed of Ediacaran, Cambrian and Ordovician country rocks affected by medium- and high-grade metamorphism coeval with the emplacement of several mafic to felsic intrusive bodies (Moita et al., 2009). Calc-alkaline volcanism also occurred. Based on deformation heterogeneity, three tectonic units separated by important transcurrent zones were recognized in the E¤vora Massif: the Montemor-o-Novo Shear Zone and the E¤vora high-grade and medium-grade metamorphic terrains (Fig. 1b). The main pervasive foliation strikes N508W and is usually associated with strong mylonitization and is affected by folding (Pereira et al., 2007).

The Pavia pluton The Pavia pluton (Figs 1b and 2) is an intrusive complex, elongated east^west, and, although mainly granitic (sensu stricto; s.s.) in composition, comprises rocks ranging from tonalite to two-mica granite. They are metaluminous (tonalite) to slightly peraluminous, calcic to calc-alkalic. All the granites (sensu lato; s.l.) are magnesian, show moderate to strong enrichment in Ca over alkalis, and are

1888


of magma batches is accompanied by subsidiary magmatic processes such as fractionation and assimilation that modify the magmas and involve important quantities of fluid, which then further modifies the already solidified plutons and country rocks. The integrated result of all these processes can be difficult to unravel based solely on the mineralogical and textural features of the rocks; however, a useful contribution can be provided by geochronology. Zircon is the most widely used and reliable mineral in U^Pb dating because (1) it is a common accessory mineral in a wide variety of rocks, (2) it contains U concentrations well above those of its host-rock, and (3) it discriminates strongly against the daughter element Pb during crystallization (e.g. Davis et al., 2003). Furthermore, zircon is able to survive multiple magmatic, metamorphic and erosional processes that destroy most of the other common minerals (Corfu et al., 2003). Four processes can affect U^Pb ages, complicating dating but also contributing extra information: Pb loss owing to radiation damage, U^Pb resetting by recrystallization, inheritance and assimilation. Whereas older inherited or assimilated zircon components are generally easily recognized during dating, it is more difficult to identify and properly interpret zircon grown in the earliest stages of the evolution of the magma (termed ‘antecrysts’; Miller et al., 2007, and references therein), and to distinguish them from the ‘main magmatic’, syn-emplacement generation. Further challenges can arise when late or post-magmatic hydrothermal zircon growth is present. In addition to zircon, magmatic rocks generally contain other useful geochronometers such as monazite or titanite, which provide useful complementary information on magmatic and post-magmatic processes. This study presents a detailed examination, using U^Pb isotope dilution thermal ionization mass spectrometry (ID-TIMS), of a multiphase plutonic complex from the western part of the Variscan Ossa^Morena Zone (OMZ; Fig. 1). The Pavia pluton, CiborroÂldeia da Serra area, covers c. 30 km 9 km and consists of tonalite, granodiorite and two-mica granite, with rare enclaves of older phases and many crosscutting dikes of porphyry, microgranite and pegmatite. The resulting data reveal a considerable degree of complexity, much of which can be explained by one or a combination of the processes mentioned above. They provide some exceptional insights into the evolution of complex magmatic systems and also important clues concerning the interpretation of U^Pb data.

NUMBER 9

LIMA et al.

PAVIA PLUTON, OSSA^MORENA ZONE

enriched in large ion lithophile elements (LILE) with respect to high field strength elements (HFSE). Their chemical composition shows a strong affinity to that of normal continental arc granites (Lima et al., 2011). The pluton intruded both the Neoproterozoic Gneiss^Migmatite Complex, metamorphosed in the Early Carboniferous, and Middle or Upper Cambrian^Lower Ordovician(?) schists, metabasites and metapelites (Moura and Carvalhal Formations; Pereira et al., 2007). Because of the low relief of the study area and the occurrence of small and isolated outcrops (locally intensely altered), the majority of the contacts between the main granitic phases cannot be observed. However, the boundaries between the main granitic intrusions and the minor intrusions are almost always found. The basic field relations are

summarized in Table 1 and selected field photographs are shown in Fig. 3. The S. Geraldo tonalite (unit I) is a coarse-grained biotite4amphibole tonalite. To the south, unit I is in fault contact with a medium-grained biotite trondhjemite closely associated with a biotite4muscovite granodiorite (unit II). The contact is marked by enclaves of metabasite and metapelite belonging to the Carvalhal Formation. Further south, there is a predominance of mediumgrained, two-mica, slightly porphyritic granodiorite (unit III), which shows a general eastward decrease in muscovite with increasing biotite content. Unit IV, to the north of the S. Geraldo tonalite, is a medium-grained, porphyritic biotite granodiorite with titanite or muscovite as accessory phases. This borders another medium-grained, slightly

1889


Fig. 1. (a) Location of the Ossa^Morena Zone within the Iberian Massif. The rectangle corresponds to the area represented in (b). (b) Main structures of the western Ossa^Morena Zone (modified after Silva & Pereira, 2004).


VOLUME 53

NUMBER 9

SEPTEMBER 2012

Mineralogical evidence for subsolidus reactions and hydrothermal activity

Fig. 2. Geological map of the CiborroÂldeia da Serra area (SW border of Pavia pluton) showing the sample locations. Modified from Zbyszewski et al. (1979, 1980), Carvalhosa & Zbyszewski (1994) and Carvalhosa (1999).

porphyritic biotite granodiorite with accessory muscovite (unit V). The most evolved rock is a coarse-grained, porphyritic two-mica granite (unit VI), which covers about one-half of the surface of the study area and extends

The U^Pb data presented below suggest that several of the samples investigated underwent important post-magmatic modifications that caused recrystallization or new growth of zircon, titanite and monazite. The petrographic evidence is not always easily seen, owing to the small size and scarcity of these accessories in thin section. However, several samples provide clues that are relevant for the interpretation. Reworking phenomena are also observed in the rock-forming minerals. Some examples are discussed below. In the biotite granodiorite 1 of domain IV (sample C73), there is an unusual coexistence of monazite and titanite. The latter is younger and presumably a secondary phase, probably formed from biotite (Ferry, 1979; Enami et al., 1993). However, in polished thin section the titanite is subhedral and fractured, with the fractures being filled by chlorite. It has Al þ Fe3þ and F contents ranging from 0·078 to 0·147 and 0·028 to 0·075 a.p.f.u., respectively (Table 2), falling within the accepted ranges for titanite of magmatic origin (Enami et al., 1993). The analyzed titanite grains are also U-rich (401^452 ppm; Supplementary Data Appendix, available for downloading at http:///

1890


further NE beyond the area of Fig. 2. A fault oriented NW^SE marks the contact between this granite and unit V. The plutonic rocks contain rare surmicaceous and microgranular enclaves and granitic and amphibolitic xenoliths. There are also many crosscutting rhyodacite porphyries, microgranites (s.l.) and pegmatite dikes, predominantly oriented NE^SW and NW^SE (Fig. 2). The pegmatite dikes are mainly constituted by quartz þ albite þ orthoclase þ microcline þ muscovite biotite garnet. Quartz veins occur mainly within units V and VI. The Pavia pluton is anisotropic, exhibiting in two dimensions an elliptical shape (Figs 1b and 2). The approximately east^west orientation of the long axis of the ellipse, the almost parallel and east^west-trending contacts between the intrusive units (following the same orientation as the pluton borders and important faults), allied to the occurrence of two main dike systems oriented perpendicularly to each other and with high length/width ratios, indicate that the emplacement of the pluton occurred under stress. Although in some locations the rock foliation appears to be magmatic (e.g. orientation of enclaves concordant with the foliation of the host-rock in units I, III and VI; preferential orientation of phenocrysts in unit VI), most of these fabrics are very probably younger deformational structures formed during D3 [at 305^295 Ma, according to Ribeiro et al. (2010)], as shown by the internal deformation of minerals (e.g. microkinking of feldspar and mica, bending of plagioclase twins and internal recrystallization of quartz).

LIMA et al.


Table 1: Basic field relations of each minor intrusion dated Rock

Ref.

Host

Orientation

Contacts

Foliation

Host foliation

Enclave1

C69B

Unit I (C51)

N458E

Sharp

N458E

N458E

Porphyry2

C16

Unit I (C51)

N508E

Sharp

–

alt.

Porphyry2

C20P

Unit I (C51)

East–west

Sharp

–

N708W

Microgranodiorite

C24A

Unit I (C51)

N408E

Sharp

N608W

N708E

Aplite–pegmatite

CP4

Unit I (C51)

N258W

Sharp

–

N708W

Pegmatite3

CP11

Unit II (C66)

N55–708W

Sharp (L)

–

–

Enclave4

C55E

Unit III (C55)

4

Sharp

4

4

CP20

Unit III (C55)

East–west

Sharp

–

alt.

Aplite–pegmatite

CP27

Unit IV (C73)

N508W

Sharp

N508W

N708W

Enclave5

C2E

Unit V (C1)

not obs.

not obs.

N658W

not obs.

Microgranite6

C79

Unit V (C1)

N708E

not obs.

N608W

not obs.

Aplite–pegmatite

CP2

Unit V (C1)

N458W

Sharp

N458W

N458W

Aplite–pegmatite

CP7

Unit VI (C49)

N608W

Sharp

–

–

Aplite–pegmatite

CP8

Unit VI (C49)

N45–508E

Sharp (L)

–

–

alt., the host was altered; —, not foliated; (L), lobated limits; not obs., not observed. 1 Both the enclave and the host-rock are cut by 1 mm thick feldspathic veins oriented east–west. 2 The two rhyodacitic porphyries are petrographically, chemically and isotopically distinct. 3 The rock is cut by 1 mm quartz veins with increasing thickness towards the core of the pegmatite oriented N408E, N508E and N808E. Some also cut the host-rock. 4 The granitic block is not in situ but the orientation of the enclaves is concordant with the foliation of the host, which near this location is oriented N108W. 5 The enclave is cut concordantly by an aplite–pegmatite dike. Although the host (C1) was not observed in this location, nearby its foliation is oriented N508W. 6 The contact between this dike and the host was not observed so its orientation is according to the published geological maps.

www.petrology/oxfordjournals.org), supporting the primary origin of the titanite in C73. In this granodiorite and in the biotite trondhjemite from unit II (sample C11), titanite also occurs as reaction rims on ilmenite in close association with biotite (Fig. 4a). A similar texture was reported by Harlov et al. (2006) for titanite from metamorphic rocks and interpreted as a result of hydration and oxidation reactions during metamorphism. This titanite also has Al þ Fe3þ and F contents suggesting a magmatic origin (0·073^0·124 and 0·018^0·051 a.p.f.u., respectively; Table 2). The titanite was likely formed by a fluid-mediated, subsolidus reaction, probably involving ilmenite. Corrosion features are also found elsewhere in the intrusive suite. For example, corrosion of plagioclase borders is observed in the biotite granodiorite 2 (unit V) and its enclaves (Fig. 4b). Epidote is intensely corroded in the S. Geraldo tonalite (unit I), where it also occurs intimately associated with allanite (Fig. 4c), and in the microgranular enclave hosted in this tonalite. Another interesting aspect is the corrosion and subsequent growth of new plagioclase with a different twinning orientation, observed in microgranular enclaves hosted in the two-mica granodiorite (unit

III) and in the two-mica granite (unit VI; Fig. 4d). In the latter case the new plagioclase grains are optically zoned. Dynamic recrystallization of quartz is observed in all rocks but with variable degrees of intensity (Fig. 4e and f).

M E T H O D S A N D A N A LY T I C A L TECH NIQUES A total of 21 samples covering the entire range of the outcropping igneous rocks were dated by U^Pb ID-TIMS at the University of Oslo on zircon, titanite and monazite. The samples were crushed, at the Universities of Aveiro and Coimbra (Portugal), by progressive grain-size reduction using a jaw crusher. After sieving, the material with a grain size below 0·42 mm was washed to remove dust and light minerals and dried using a water-bath. Magnetic minerals were removed by free-fall with a Franz magnetic separator. The remaining rock-forming minerals were separated by a combination of heavy-liquid flotation and magnetic separation. The grains to be analyzed were selected by hand-picking under a binocular microscope. Zircon grains from samples C2, C20P, C49, C51, C66, C73 and C79 were selected for both mechanical and chemical

1891


Pegmatite


VOLUME 53

NUMBER 9

SEPTEMBER 2012

abrasion. In the other samples zircon was only chemically abraded. The majority of the titanite and monazite fractions were mechanically abraded. Mechanical abrasion (MA) followed the method developed by Krogh (1982) and the

chemical abrasion (CA) followed the three main steps of Mattinson’s (2005) ‘CA-TIMS’ technique: (1) annealing at 9008C for 3 days; (2) partial dissolution with HF þ HNO3 (12:1); (3) cleaning using 6N HCl. Subsequently the selected

1892


Fig. 3. Field relations between the main granitic phases and the subsidiary phases (enclaves and dikes). (a) Diorite microgranular enclave (C69B) hosted in unit I tonalite (C51) with an interpretative scheme (a1). (b) Rhyodacite porphyries cutting unit I tonalite (C51). Detail of a slightly discordant crosscutting vein (b1) and of the contact between the porphyry C20P and the host (b2). (c) Microgranodiorite dike (C24A) cutting unit I tonalite (C51) with an interpretative scheme (c1). At the contact the host-rock is slightly discolored, corresponding to a metasomatic effect caused by the intrusion of C24A. (d) Quartz-diorite microgranular enclaves (C55E) hosted in unit III granodiorite (C5). (e) Aplite^pegmatite dike (CP27) cutting unit IV granodiorite (C73). Detail of the contact between the two granitic phases (e1). (f) Aplite^pegmatite dike (CP8) cutting unit VI granite (C49). Detail of the contact between the two granitic phases (f 1).

LIMA et al.


Table 2: Representative microprobe analyses of titanite in the biotite granodiorite (C73, unit IV) and in the biotite trondhjemite (C11, unit II) Sample C73 1

Sample C11 2

3

4

5

6

7

8

9

29·915

29·574

29·612

30·370

30·680

30·550

30·830

30·655

30·376

36·131

37·262

38·065

35·977

37·398

36·697

37·932

37·729

37·847

Al2O3

2·719

1·687

1·451

2·158

1·396

1·912

1·742

1·552

1·226

Fe2O3

1·602

1·097

0·798

1·643

0·793

1·067

1·188

1·161

1·106

Cr2O3

0·056

0·030

0·000

0·018

0·000

0·000

0·029

0·022

0·056

P2O5

0·194

0·203

0·190

0·169

0·363

0·340

0·295

0·243

0·318

MnO

0·028

0·095

0·168

0·127

0·107

0·143

0·147

0·131

0·107

MgO

0·385

0·009

0·004

0·873

0·024

0·001

0·015

0·012

0·016

CaO

28·093

28·553

28·620

26·992

28·187

28·386

27·900

27·725

28·121

Na2O

0·000

0·000

0·007

0·024

0·042

0·020

0·029

0·036

0·017

K2O

0·017

0·000

0·000

0·026

0·000

0·027

0·004

0·009

0·001

F

0·712

0·266

0·377

0·234

0·174

0·496

0·285

0·301

0·226

H2O*

0·650

0·601

0·340

0·902

0·497

0·439

0·598

0·515

Total

100·50

99·38

99·63

99·51

99·66

100·08

100·99

100·09

0·463 99·88

O¼F

0·30

0·11

0·16

0·10

0·07

0·21

0·12

0·13

0·10

Total

100·20

99·27

99·47

99·41

99·59

99·87

100·87

99·96

99·78

Si

1·000

1·000

1·000

1·000

1·000

1·000

1·000

1·000

Ti

0·908

0·948

0·967

0·891

0·917

0·903

0·925

0·926

1·000 0·937

Al

0·107

0·067

0·058

0·084

0·054

0·074

0·067

0·060

0·048

Fe3þ

0·040

0·028

0·020

0·041

0·019

0·026

0·029

0·028

0·027

Cr

0·001

0·001

0·000

0·000

0·000

0·000

0·001

0·001

0·001

P

0·005

0·006

0·005

0·005

0·010

0·009

0·008

0·007

0·009

Mn

0·001

0·003

0·005

0·004

0·003

0·004

0·004

0·004

0·003 0·001

Mg

0·019

0·000

0·000

0·043

0·001

0·000

0·001

0·001

Ca

1·006

1·034

1·036

0·952

0·984

0·996

0·970

0·969

0·992

Na

0·000

0·000

0·000

0·002

0·003

0·001

0·002

0·002

0·001

K

0·001

0·000

0·000

0·001

0·000

0·001

0·000

0·000

0·000

F

0·075

0·028

0·040

0·024

0·018

0·051

0·029

0·031

0·024

OH

0·072

0·067

0·038

0·100

0·055

0·049

0·066

0·057

0·051

(cations)

3·090

3·087

3·091

3·022

2·991

3·015

3·006

2·997

3·019

Al þ Fe3þ

0·147

0·095

0·078

0·124

0·073

0·100

0·096

0·088

0·075

Formulae calculated on the basis of 1Si; all Fe2þ from the microprobe analyses was recalculated as Fe3þ. *H2O calculated based on OH ¼ Al þ Fe3þ – F.

single- and multi-grain fractions were washed, weighed on a microbalance and spiked with a mixed 202Pb^205Pb^235U tracer. Dissolution and chemical separation were carried out according to the method of Krogh (1973) modified by Corfu (2004). The samples were loaded on an outgassed Re filament with Si-gel and H3PO4 and analyzed using a MAT 262 mass spectrometer. The standard used was NBS 982Pb þ U500. The data corrections and reduction follow the procedures of Corfu (2004). Data plotting and age calculations were performed using Isoplot 4.1, a Berkeley Visual

Basic add-in for Microsoft’s Excel for data analysis and graphical presentation of geochronology, earth science and other radiogenic isotopic data only (Ludwig, 2009). Decay constants are those of Jaffey et al. (1971), with the exception of l235U for which we use the value proposed by Mattinson (2010). Analytical precision is given by the uncertainty propagated for each analysis, with a confidence degree of 2s. Zircon grains similar to those analyzed were separated for catholuminescence (CL) imaging using a JEOL JSM 6460LV electron microscope.

1893


SiO2 TiO2


VOLUME 53

NUMBER 9

SEPTEMBER 2012

Titanite microprobe analyses were obtained using a CAMECA SX-100 at Laboratorio de Geolog|¤ a y Geocronolog|¤ a dos Servicios Comunes de Investigacio¤n (University of Oviedo, Spain). The analyses were conducted with an acceleration potential of 15 keV and an emission current of 15 nA.

R E S U LT S A total of 140 analyses (94 of zircon, 20 of titanite and 26 of monazite) were obtained. Based on the field relations, the 21 samples were divided into six units, each unit labeled according to the dominant igneous rock, but encompassing the associated enclaves and dikes. The U^Pb data for all

1894


Fig. 4. Mineralogical evidence for the occurrence of subsolidus reactions in response to late magmatic events and related hydrothermal activity. (a) Titanite rimming ilmenite (sample C11, unit II). Backscattered electron image (a1). (b) Corrosion of plagioclase rims (sample C2E, xenolith in unit V). (c) Corrosion of epidote rimming allanite (sample C27, unit I). (d) Corrosion and growth of new plagioclase with a different twinning orientation (sample C60, unit VI). (e, f) Dynamic recrystallization of quartz (samples C42, unit IV and C55, unit III, respectively). Photomicrographs (a) and (c) are in plane-polarized light and (b), (d), (e) and (f) are in cross-polarized light. Mineral abbreviations follow Whitney & Evans (2010): Aln, allanite; Ap, apatite; Bt, biotite; Ep, epidote; Ilm, ilmenite; Ms, muscovite; Pl, plagioclase; Ttn, titanite; Qz, quartz.

LIMA et al.


Concordia diagrams interpretation Unit I: S. Geraldo biotite4amphibole tonalite This group includes one biotite4amphibole tonalite (C51), one dioritic microgranular enclave (C69B), two rhyodacite

porphyries (C16 and C20P), one microgranodiorite (C24A) and one aplite^pegmatite (CP4) dike. The zircon grains from sample C51 selected for MA and CA were prismatic, euhedral, with oscillatory to complex growth zoning, locally affected by resorption (Fig. 5, grains A and B, respectively), and transparent with melt and opaque mineral inclusions, and fractures oriented parallel to the crystal width. In every batch of grains it was possible to identify some overgrowths, both as cup-shaped tips (Fig. 5, grain A) and as homogeneous mantles surrounding the entire grain (Fig. 5, grain C). Seven of the nine analyses define a main age cluster at 328^325 Ma with two other analyses giving older and younger ages of 331·2 1·1 Ma and 319·4 1·2 Ma, respectively (Table 3; Fig. 6a). Because the data for enclave C69B, discussed below, give an age of 328·53 0·80 Ma and enclaves cannot be younger than their host-rock, the oldest analysis in C51 must be interpreted as an early crystallization phase (‘antecryst’), predating the enclave’s incorporation into the magma. The main cluster of seven zircon data points for C51 shows some scatter, probably caused by mixing effects with the younger generation; four overlapping analyses yield an age of 328·24 0·55 Ma and the oldest analysis (no. 2) alone yields 328·60 0·70 Ma, which is considered to best reflect the age of this tonalite. Titanite is also a constituent mineral. It is orange and opaque with frequent inclusions of opaque minerals. The analyzed titanite grain is slightly reversely discordant and yields an age of 323·7 1·4 Ma. The youngest age of 319·4 1·2 Ma was obtained from a zircon overgrowth cut off from the tip of a prismatic zircon grain. A potential interpretation of this youngest generation as being magmatic, thus implying that the tonalite was constructed over 11 Myr, is contradicted by the lack of a continuous decrease in the U^Pb ages, the presence of crosscutting dikes with sharp contacts to the host (samples C16, C20P, C24A and CP4) with ages of 328^324 Ma (see discussion below) and the presence of the 323·7 Ma titanite in this sample and 326·07 Ma titanite in the enclave (below). It is thus likely that the 319 Ma zircon overgrowths formed by post-magmatic overprints related to the youngest magmatic event recorded in the complex (see Unit IV, sample CP27; Unit V, samples C79 and CP2). Several zircon grains from the microgranular enclave C69B were selected for CA. Two distinct morphologies occur, both showing patchy zoning (Fig. 5, grains D and E): (1) prismatic and elongated, colorless to light orange, translucent with frequent melt inclusions; the majority of the grains are fractured, with the fractures oriented perpendicularly to obliquely to the crystal length; (2) colorless to pale yellow, anhedral to subhedral, flat and bent translucent grains. Patchy zoning may reflect strain experienced by the zircon during final magmatic emplacement (Corfu et al., 2003), which can explain the predominance of ‘bent’

1895


the samples are provided in the Supplementary Data Appendix. The main characteristics and the ages of each analyzed fraction are presented in Table 3 and CL images of the majority of the analyzed zircon populations are presented in Fig. 5. The complexity of the concordia diagrams obliges us to always take into consideration the data obtained for the entire unit and, on a broader scale, for the entire pluton. In a separate study, an aplite^pegmatite dike (CP6), cutting discordantly the amphibolite-facies schists (Carvalhosa, 1999) of the Moura Schists Formation,was dated by U^Pb ID-TIMS on U-rich inclusions in garnet. Based on five analyses the emplacement of this dike was constrained at 318·36 0·32 Ma, with an older fraction giving an age of 322·30 0·70 Ma (Lima et al., 2012). In some samples of this study zircon grains were selected for both mechanical and chemical abrasion. The use of these two types of abrasion showed that in most cases mechanical abrasion can be as efficient as chemical abrasion. However, in samples C2E (no. 99) and C49 (no. 123) mechanically abraded fractions yielded younger ages, reflecting the limitations of this type of abrasion. In this study we base our zircon and titanite ages on 206 Pb/238U (2s errors). In some cases there is some dispersion in 207Pb/235U (not reflected by 206Pb/238U) that may be due to excess 207Pb. This effect is caused by the initial incorporation of 231Pa, an intermediate product of the 235 U decay chain (e.g. Anczkiewicz et al., 2001; Parrish & Noble, 2003). Monazite is typically reversely discordant, which is due to its ability to incorporate much more initial 230Th than required to reach a state of secular equilibrium, finally containing more 206Pb than it should have based on its age and amount of 238U that has decayed (e.g. Parrish, 1990). This process does not affect the 235U decay chain; thus the ages of monazite are based on 207Pb/235U (2s errors). Overall, the zircon ages are more self-consistent and reliable than those of monazite and titanite, and are generally taken as the preferred indicator of the age of the rock. However, in samples CP20 (unit III) and CP2 (unit V) the age yielded by monazite is favored; in sample CP20 because all the zircon grains are considered to be inherited or xenocrystic and in sample CP2 because zircon is intensely metamict and reset. Although monazite is hundreds of times more radioactive than zircon it is always crystalline, owing to its ability to restore its structure at low T (60^1508C) and the fact that the energy barrier to recrystallization in phosphates is much lower than in silicates (Seydoux-Guillaume et al., 2007).


VOLUME 53

Table 3: Main characteristics and ages for zircon, titanite and monazite fractions selected for U^Pb ID-TIMS analysis An.

Characteristics

Sample C51, tonalite Zrc M_CA (pr., fract.)

331·2 1·1

2

Zrc M_CA (pr., fract.)

328·6 0·7

3

Zrc M_MA (pr., with melt incl.)

327·9 1·4

Zrc 1_CA (ovgr. fract.)

327·7 1·5

5

Zrc M_CA (pr., fract.)

327·0 2·0

6

Zrc 4_CA (ovgr. fract.)

326·2 1·0

7

Zrc 1_CA (pr.þovgr., fract.)

325·7 1·4

8

Zrc M_MA (pr., with melt incl.)

325·3 1·2

9

Zrc 1_CA (only the ovgr.)

319·4 1·2

10

Ttn 2_MA (dark and op. frag.)

Zrc 1_CA (bent grain, transl.)

328·7 1·4

Zrc 6_CA (pr. grains and frag., transl.)

328·4 1·1

Zrc 7_CA (bent grains and frag., transl., fract.)

326·7 1·1

14

Ttn 1_MA (lt brown frag., slightly transl., fract.)

326·4 0·9

15

Ttn 1_MA (reddish brown frag., op.)

325·8 0·8

18

Zrc 1_CA (clear pr. tip, fract.) Zrc 1_CA (pr. tip, light brown fract.)

330·0 1·4 328·7 1·0 323·0 1·0

19

Zrc 1_CA (clear tip with a ovgr.)

322·0 1·0

20

Mnz 1_MA (frag., pale yellow, transl.)

332·7 4·9

21

Mnz 1_MA (frag., pale yellow, transl.)

Zrc 1_CA (clear tip)

323·5 1·6

493·2 2·5

Zrc 1_MA (tip, with op. incl.)

327·4 1·2

24

Zrc 1_MA (frag., fract.)

325·1 1·5

25

Zrc 1_MA (clear frag.)

324·8 1·5

26

Ttn 4_MA (op., honey to reddish brown frag.)

323·5 1·0

27

Ttn 1_MA (light brown, slightly transl., op. incl.)

321·2 1·3

28

Ttn 3_NA (op., honey to reddish brown frag.)

320·2 3·1

Ttn 1_MA (brown, slightly transl., frag.)

Zrc 2_CA (clear pr. frag.)

312·8 1·7

330·1 1·2

31

Zrc 1_CA (tip, light yellow, fract.)

327·5 1·2

32

Zrc 2_CA (clear pr.)

323·8 1·3

33

Zrc 5_CA (pr. frag. þ 1 tip, transl., fract.)

323·5 0·7

34

Ttn 1_MA (anh., reddish brown, op., mag.)

327·5 2·1

35 36

Ttn 5_MA (anh., brown, op., mag.)

329·3 3·7 328·9 1·1

39

Zrc 1_CA (pr. with melt incl., fract.)

328·5 1·2

40

Zrc 1_CA (by., fract.)

328·2 0·8

41

Zrc 1_CA (pr. core with melt incl.)

319·3 0·7

42

Mnz 1_MA (anh., pale yellow, fract., sup. alt.)

322·6 2·8

43

Mnz 1_MA (anh., pale yellow, fract.)

321·5 3·4

44


329·5 1·4

45

Zrc 5_CA (clear pr. and 1 frag., fract.)

328·1 0·7

46

Zrc 1_MA (pr. tip,)

327·4 0·7

47

Mnz 1_MA (pale yellow, transl.)

326·5 4·1

48

Zrc 5_CA (pr., light brown, fract.)

49

Zrc 8_CA (clear pr., transl., rare melt incl., fract.) 327·6 0·6

329·0 0·7

50

Zrc 1_CA (clear pr. tip, transl.)

327·4 1·3

51

Ttn_9_MA (anh., honey, slightly transl., fract.)

327·3 0·8

52

Ttn_6_MA (anh., honey brown, slightly transl.)

327·2 0·8

53

Zrc 1_CA (long pr., transl., fract.)

337·6 0·7

54

Zrc 1_CA (pr.)

328·3 0·6

55

Zrc 1_CA (pr., broken)

328·3 0·6

56

Zrc 1_CA (pr. tip)

327·9 0·7

57

Mnz 1_MA (pale yellow, subh., sup. alt.)

329·6 1·4

58

Zrc 4_CA (pr. frag., fract., turbid)

340·7 1·3

59

Zrc 1_CA (euh. flat grain, fract., very turbid)

329·0 1·5

60

Zrc 1_CA (pr. tip)

325·1 1·2

61

Zrc 2_CA (pr. tips, fract.)

324·5 1·2

62

Zrc 1_CA (pr. tip, fract.)

324·4 0·9

Sample C55E, quartz-diorite microgranular enclave

Sample C24A, microgranodiorite dike 30

Zrc 1_CA (py. tip, transl.)

Sample C5, two-mica granodiorite

23

29

Zrc 1_CA (pr., fract., transl.)

38

UNIT III

Sample C20P, rhyodacite to dacite porphyry 22

37

Sample CP11, aplite–pegmatite dike

Sample C16, rhyodacite porphyry

17

Age (Ma)

Sample C11, trondhjemite

13


Characteristics

Sample C66, Bt4Ms granodiorite

323·7 1·4

12

16

An.

UNIT II

Sample C69B, diorite microgranular enclave 11

Table 3: Continued

63

Zrc 1_CA (clear concave frag.)

64

Zrc 1_CA (clear py. tip, melt incl.)

343·4 2·0 335·2 1·1

65

Zrc 11_CA (frag., very turbid)

334·8 1·1

66

Zrc 5_CA (frag., fract.)

334·2 0·7

67

Ttn 4_MA (subh., brownish yellow, transl.)

331·5 1·2

68

Ttn 8_MA (subh., brownish yellow, transl.)

330·4 1·5

Sample CP20, pegmatite dike

324·1 1·2

Ttn 4_MA (anh., honey brown, transl., non-mag.) 323·4 0·8

(continued)

69


338·0 1·1

70


334·2 1·3

71

Zrc 1_CA (clear pr., fract.)

329·1 1·3

72

Mnz 1_MA (anh., melt incl.)

323·9 2·0

73

Mnz 1_MA (anh., pale yellow)

321·9 1·8

74

Mnz 1_MA (anh., pale yellow)

319·5 3·2

(continued)

1896


4

SEPTEMBER 2012


Age (Ma)

UNIT I

1

NUMBER 9

LIMA et al.


Table 3: Continued An.

Characteristics

Table 3: Continued Age (Ma)

An.

Characteristics

Age (Ma)

UNIT IV

113

Mnz 1_MA (pale yellow grain, sup. alt.)

316·3 2·7

Sample C73, Bt granodiorite 1

114

Mnz 1_MA (pale yellow grain, transl.)

316·2 1·5

Zrc 1_MA (frag.)

326·7 1·6

UNIT VI

76

Zrc M_CA (clear pr. and frag.)

325·3 1·3

Sample C49, two-mica granite

77

Zrc 1_CA (clear pr., fract.)

324·3 1·5

115

Zrc 1_CA (anh. grain, turbid)

341·9 0·8

78

Zrc 1_MA (clear pr., fract.)

321·8 1·4

116

Zrc 1_CA (clear frag.)

323·0 1·4

79

Ttn 1_MA (honey brown, transl.)

319·9 0·8

117

Zrc 1_CA (core, transl.)

320·3 0·7

80

Ttn 1_MA (dark brown, op., fract., sup. alt.)

318·8 1·3

118

Zrc 1_CA (subh. pr., fract.)

319·1 1·4

81

Mnz 1_ MA (anh., yellow, transl., fract.)

323·2 2·3

119

Zrc 1_CA (py tip, fract.)

317·9 1·0

82

Mnz 2_NA (anh., yellow, transl., op. incl.)

314·3 13·2

120

Zrc 1_CA (subh. pr., fract.)

316·7 1·3

121

Zrc 1_CA (pr., fract.)

314·1 1·9

83

Zrc 1_CA (pr., op. incl., very turbid)

327·8 1·0

122

Zrc 1_MA (clear frag., fract.)

313·0 1·9

84


321·1 1·5

123

Zrc 2_MA (clear frag.)

307·8 0·7

85

Zrc 1_CA (subh. tip, transl.)

318·9 1·1

124

Mnz 1_NA (pr., yellow, transl.)

320·5 2·8

86

Zrc 5_CA (pr., transl., fract.)

318·6 2·3

125

Mnz 2_NA (subh., yell., transl. with op incl.)

317·1 1·8

87

Mnz 1_NA (euh., lemon yell., minor sup. alt.)

317·1 12·7

126

Mnz 1_NA (subh., yellow, transl.)

315·4 1·8

88

Mnz 1_NA (euh., pale green)

290·1 43·6



UNIT V

127


339·0 1·3

Sample C1, Bt granodiorite 2

128

Zrc 1_CA (piece of a flat grain, transl.)

338·6 0·8

89


323·7 1·1

129

Zrc 1_CA (pr.)

338·0 0·8

90

Zrc 1_CA (bp., light brown, turbid)

323·5 0·6

130

Zrc 1_CA (flat slab, transl.)

336·6 0·8

91

Zrc 1_CA (pr., transl.)

321·9 1·1

131

Zrc 1_CA (long pr. tip)

336·6 0·8

92

Zrc 2_CA (clear pr., transl.)

320·1 1·0

132

Zrc 1_CA (pr.)

334·4 1·7

93

Mnz 1_MA (subh., pale yellow)

327·4 9·2

133

Zrc 1_CA (pr. tip)

339·7 6·5

94

Mnz 1_MA (subh., pale yellow)

324·1 5·7

134

Zrc_1_CA (tip)

336·9 1·2

135

Zrc 1_CA (pr. tip, light orange, very turbid)

324·5 0·8

Sample C2E, Bt4Amph granodiorite enclave 95

Zrc M_CA (clear frag., fract.)

328·1 1·2

136

Zrc 1_CA (pr., light orange)

324·3 0·7

96


327·6 0·8

137

Zrc 1_CA (clear pr. grain, central piece)

311·1 2·6

97

Zrc 3_CA (pr., fract., melt incl.)

327·4 0·7

138

Mnz 1_NA (subh., colorless, planar fract.)

334·5 9·8

98

Zrc 8_CA (pr., turbid)

327·0 0·8

139

Mnz 1_MA (subh., lemon yellow, planar fract.)

327·7 29·8

99

Zrc 7_MA (pr. frag., rare incl. and rare fract.)

325·4 0·7

140

Mnz 1_MA (subh., light yellow, planar fract.)

317·7 9·7

100

Ttn 2_MA (brown, transl.)

323·8 1·2

101

Ttn 3_MA (brown, fract.)

318·1 1·3

The 206U/238Pb ages are given for zircon and titanite and the 207Pb/235U ages (italic) are given for monazite. Both ages were corrected for fractionation, spike, blank and initial common Pb (error calculated by propagating the main source of uncertainty). An., analysis. Mnz, monazite; Ttn, titanite; Zrc, zircon; CA, chemically abraded; MA, mechanically abraded; anh., anhedral; bp, bipyramidal; euh., euhedral; fract., fractured; frag., fragment; incl., inclusion; lt, light; M, multi-grain fraction; mag., magnetic fraction; met., metamictic; pr., prism; py, pyramidal; op., opaque; ovgr., overgrowth; sup. alt., superficial alteration; transl., translucent; yell., yellow. The number corresponds to the number of grains that compose each fraction.

Sample C79, microgranite dike 102

Zrc 1_CA (pr. tip)

318·0 1·7

103

Zrc 1_CA (bp., fract., pale yellow)

317·9 1·3

104

Zrc 1_MA (pr., fract., pale yellow)

317·2 0·8

105

Zrc 1_MA (bp., clear incl., pale yellow)

316·5 1·2

106

Ttn 2_MA (op., dark with op. incl.)

319·6 1·8

107

Ttn 1_MA (slightly transl.)

316·0 0·7

Sample CP2, aplite–pegmatite dike 108

Zrc 1_MA (met. tip)

289·8 1·1

109

Zrc 1_MA (met. tip)

227·5 1·6

110

Mnz 1_MA (pale yellow grain)

320·4 0·8

111

Mnz 1_MA (pale yellow grain, transl.)

318·1 1·1

112

Mnz 1_MA (pale yellow grain)

317·9 2·0

(continued)

crystals in this sample. In population 2 melt inclusions are rarer and smaller than in population 1 and fracturing is also less important, with the existing fractures being parallel to the crystal length. The age of this enclave is defined by the two older analyzed zircon fractions (nos 11 and 12)

1897


75


VOLUME 53

NUMBER 9

SEPTEMBER 2012


Fig. 5. Cathodoluminescence images of representative zircon grains from the majority of the dated samples.

1898

LIMA et al.



Fig. 6. Concordia diagrams showing the U^Pb data for zircon, titanite and monazite of the intrusive rocks from Unit I: C51, biotite4amphibole tonalite; C69B, microgranular enclave; C16 and C20P, rhyodacite porphyries; C24A, microgranodiorite dike; CP4, aplite^pegmatite dike. Error ellipses are drawn at 2s. CA, chemically abraded; MA, mechanically abraded.

1899


VOLUME 53

SEPTEMBER 2012

yellow, translucent and inclusion-free, with some of the grains displaying oblique fracturing. Two analyses of zircon coincide with two of brown titanite, defining a common age of 323·58 0·43 Ma, which is considered the magmatic age (Fig. 6e). Another titanite and two zircon fractions have slightly older ages, no. 30 with 330·1 1·2 Ma and the discordant no. 31 projecting to a Precambrian upper intercept age (2854 890 Ma). These older analyses are interpreted to represent assimilated or inherited components. Zircon grains selected for abrasion from the aplite^pegmatite CP4 are euhedral and colorless with fractures oriented mainly perpendicularly to the crystal length. Some grains are inclusion-free and others have abundant melt inclusions. Zircon exhibits oscillatory zoning with minor evidence of resorption (Fig. 5, grains L and M). Grain M is surrounded by a homogeneous mantle that corresponds to secondary growth. Four zircon analyses give 328·44 0·54 Ma (Fig. 6f). A fifth zircon analysis (no. 41) is normal discordant, reflecting Pb loss. Monazite is anhedral, pale yellow and inclusion-free. The two monazite fractions yield a younger age of 322·1 2·2 Ma, which is probably a resetting age.

Unit II: biotite4muscovite granodiorite This group consists of one biotite4muscovite granodiorite (C66) that crops out intimately associated with a biotite trondhjemite (C11) and one crosscutting pegmatite (CP11). The zircon grains extracted from sample C66 were generally euhedral and prismatic (Fig. 5, grains A and B), locally showing evidence of complex growth masked by resorption (grain B). The grains selected for abrasion are clear, transparent and inclusion-free, whereas the fraction selected for CA also included some ‘rusty’ grains with evident fracturing and melt inclusions. The three analyzed zircon fractions from sample C66 gave an age of 328·23 0·59 Ma (Fig. 7a). The analyzed monazite is anhedral, pale yellow and slightly translucent, and yields an age of 326·5 4·1 Ma, which is, within error, the same as the zircon age. The zircon grains from the biotite trondhjemite (C11) selected for CA are euhedral, mainly prismatic, show oscillatory zoning (Fig. 5, grains C and D), and are colorless and translucent, with some melt and opaque mineral inclusions. The fracturing is more important in the largest grains, where fractures are oriented preferentially perpendicularly to the crystal length. Coexisting titanite is anhedral, honey brown in color, and, although some opaque mineral inclusions occur, it is mainly translucent. The three zircon fractions give a concordia age of 328·42 0·47 Ma with an MSWD ¼ 3·7, but one of the analyses (no. 48) gives an older age of 329·0 0·7 Ma, and probably represents an early crystallization phase (Fig. 7b). The more precise of the younger zircon data points (no. 49) has an age of 327·6 0·6 Ma, which is the

1900


at 328·53 0·80 Ma, but a slight decrease in the zircon 206 Pb/238U ages is observed (Fig. 6b). Coexisting titanite is light to reddish brown, slightly translucent to opaque and inclusion-free, and yields a slightly younger age of 326·07 0·60 Ma. Selected zircon grains from the rhyodacite porphyry C16 are euhedral, mainly prismatic, colorless and with rare melt inclusions. CL images show the presence of oscillatory zoning (Fig. 5, grains F and G) and the occurrence of overgrowths (grain F). Monazite is lemon yellow, translucent and inclusion-free. It is possible to distinguish two discrete groups, each constrained by two zircon and one monazite analyses (Fig. 6c). The older age is 329·18 0·84 and 332·7 4·9 Ma and the younger one is 322·47 0·74 and 323·5 1·6 Ma, for zircon and monazite, respectively. The younger monazite is reversely discordant. The older age of 329·18 0·84 Ma is equal to the age of the host (sample C51), which is 328·6 0·7 Ma. Thus, it is possible that this represents an assimilated rather than a magmatic component. However, one monazite grain also has the same age and this cannot be an assimilated phase from the host as it does not have monazite as a constituent mineral. It is therefore suggested that the older age represents the porphyry emplacement age and the younger one results from a later post-magmatic overprint that caused resetting and/ or the crystallization of a new generation. The other dated rhyodacite porphyry, sample C20P, provided euhedral to subhedral, transparent to light brown zircon grains with frequent melt and opaque mineral inclusions. The grains exhibit growth zoning disrupted by minor resorbed domains (Fig. 5, grains H and I). Fractures develop obliquely to parallel to the crystal width. The three zircon fractions 23^25 overlap, yielding an age of 325·97 0·87 Ma, but the MSWD value is 4·7, indicating some scatter (Fig. 6d). An inherited component is present in the discordant grain (no. 22), which projects to an upper intercept age of 585 24 Ma (95% confidence level). Several titanite fragments with colors varying from orange to dark brown, opaque and with abundant opaque mineral inclusions, were also selected for analysis. Three titanite fractions (nos 26^28) are concordant and yield an age of 322·50 0·77 Ma, probably reflecting post-magmatically reset ages. Another titanite grain, very rich in U (1440 ppm), gives a slightly reversely discordant age of 312·8 1·7 Ma. Because titanite resetting occurred, zircon CL images show minor resorption (Fig. 5, grains H and I). As the zircon grains are fairly rich in U, it appears more likely that the spread in ages is caused by Pb loss, thus the age of zircon analysis no. 23 at 327·4 1·2 Ma is considered the most likely age for this porphyry. Zircon grains from microgranodiorite dike sample C24A are mainly prismatic, euhedral to subhedral, with faint to oscillatory growth zoning disrupted by resorption (Fig. 5, grains J and K, respectively), colorless to pale

NUMBER 9

LIMA et al.


Unit III: two-mica granodiorite

Fig. 7. Concordia diagrams showing the U^Pb data for zircon, titanite and monazite of the intrusive rocks from Unit II: C66, biotite4muscovite granodiorite; C11, biotite trondhjemite; CP11, aplite^pegmatite dike. Error ellipses are drawn at 2s. CA, chemically abraded; MA, mechanically abraded.

This group includes one biotite4muscovite granodiorite (C5), one quartz-dioritic microgranular enclave (C55E) and one pegmatite (CP20) dike. Zircon tips and crystals selected for CA from sample C5 are mainly long prismatic, exhibiting growth zoning disrupted by resorption (Fig. 5, grains A, B and C); some bipyramidal crystals and one flat euhedral grain were also chosen for analysis. The tips are colorless, clear and translucent with only rare melt inclusions. Melt inclusions are, however, abundant in complete grains. The inclusions are locally surrounded by an orange halo that gives the same color to the host grain. The three younger zircon fractions indicate an age of 324·63 0·60 Ma (Fig. 8a). Two older concordant ages of 340·7 1·3 Ma (fraction 58) and 329·0 1·5 Ma (fraction 59) also occur. The age of fraction 58 is easily excluded as the magmatic age of this rock, as one microgranular enclave hosted in C5 (sample C55E) yields a younger age (334·55 0·50 Ma; see discussion below). Given the abundance and relative homogeneity of the zircon in this sample and the concentration of zircon data around 324 Ma (fractions 60^62), the age given by fraction 59 is more easily explained by assimilation of material; for example, from rocks of units I and II formed at 328 Ma. The microgranular enclave (C55E) is characterized by the occurrence of colorless to light brown prismatic zircon grains (Fig. 5, grains D and E), in part with textures suggesting internal resorption (grain D). Although not abundant, some anhedral, flat and curved grains also occur. Zircon grains with this morphology are usually more clear and translucent than the euhedral, prismatic and bipyramidal morphologies. The majority of the grains selected for CA contain melt inclusions and are intensely fractured, especially perpendicular to the crystal length. The age of this enclave is constrained by analyses 64^66, which give an age of 334·55 0·50 Ma (Fig. 8b). An older age of 343·4 2·0 Ma is given by a concave zircon fragment characterized by an

1901


suggested age for this rock. This is corroborated by the two titanite fractions, which yield an average age of 327·23 0·55 Ma. The zircon fraction from the pegmatite dike, sample CP11, selected for CA, is composed of prismatic grains and tips, euhedral to subhedral, colorless to orange, with abundant melt and opaque mineral inclusions. Fracturing is important, being oriented perpendicularly to obliquely to the crystal length. Three zircon analyses give 328·20 0·38 Ma (Fig. 7c). A fourth zircon analysis (no. 53) yields 337·6 0·7 Ma. Monazite is anhedral and pale yellow, and gives an age of 329·6 1·4 Ma, which is, within error, the same as the age given by the zircon and the age of its host-rock (biotite4muscovite granodiorite, sample C66).


VOLUME 53

NUMBER 9

SEPTEMBER 2012

Unit IV: biotite granodiorite 1

Fig. 8. Concordia diagrams showing the U^Pb data for zircon, titanite and monazite of the intrusive rocks from Unit III: C5, two-mica granodiorite; C55E, microgranular enclave; CP20, pegmatite dike. Error ellipses are drawn at 2s. CA, chemically abraded.

This group consists of one biotite granodiorite (C73) and one aplite^pegmatite (CP27) dike. The selected zircon grains from sample C73 are euhedral to subhedral and mainly prismatic, showing oscillatory zoning (Fig. 5, grains A, B and C). Secondary growths occur as homogeneous mantles surrounding the entire grain, and as cup-shaped growths over the tips (Fig. 5, grains A and C, respectively). Generally they are clear and transparent but ‘rusty’ zones (mainly on the rims) are common. Opaque mineral and melt inclusions occur and fractures are predominantly oblique and parallel to the crystal length. The three oldest zircon analyses indicate an age of 325·39 0·83 Ma, with another grain giving a slightly younger age of 321·8 1·4 Ma (Fig. 9a). The younger zircon age may reflect Pb loss or it may result from a mixture with a younger zircon generation. Zircon overgrowths have been identified in some grains (Fig. 5, grains A and C); moreover, the occurrence of a later event at around 319 Ma is recorded in titanite as discussed below. Sample C73 contains both monazite and titanite as accessory minerals. Monazite is anhedral, yellow and translucent with small and rare opaque mineral inclusions. The two analyzed fractions are slightly reversely discordant and have a mean age of 322·9 2·2 Ma (Fig. 9a). Titanite is orange to dark brown, opaque to slightly translucent, with abundant opaque mineral inclusions and is the youngest phase, yielding an age of 319·56 0·66 Ma. Several zircon grains from aplite^pegmatite dike CP27 were selected for CA. They are prismatic, euhedral, clear, colorless to pale yellow with rare melt inclusions; they exhibit oscillatory zoning (Fig. 5, grains D and E) and are highly fractured. The dike age is defined by three overlapping zircon data points, including the most precise analysis (no. 85), at 319·53 0·80 Ma (Fig. 9b). An older zircon age, 327·8 1·0 Ma, was also recognized. Monazite is euhedral with color ranging from lemon yellow (no. 87) to

1902


unusually high Th/U ratio (34·95). Coexisting titanite is brownish-yellow, very clear, translucent and with rare melt inclusions. The analyzed titanite grains are younger than the zircon at 331·12 0·93 Ma, which is probably a resetting age. The zircon grains from the pegmatite vein (CP20) selected for CA are euhedral, colorless to light orange and inclusion-free. Three zircon grains from this sample yield concordant but progressively younger ages of 338·0 1·1, 334·2 1·3 and 329·1 1·3 Ma which are older than the age suggested for the host-rock (C5). Thus, the age given by the three analyzed monazite grains (slightly reverse discordant), 322·3 1·3 Ma, is preferred (Fig. 8c). The monazite grains are anhedral, pale- to lemon-yellow, inclusion-free and non-fractured.

LIMA et al.


pale green (no. 88, with very high Th/U) and yields progressively younger ages, probably indicating later resetting.

Unit V: biotite granodiorite 2 This group consists of one biotite granodiorite (C1), one biotite4amphibole granodiorite enclave (C2E), one microgranite (C79) and one aplite^pegmatite dike (CP2). The zircon population from sample C1 selected for CA is composed of colorless, translucent, mainly prismatic and some bipyramidal crystals with rare melt inclusions. They exhibit growth zoning (Fig. 5, grains A and B). Three overlapping zircon fractions (nos 89^91) gave an age of 323·20 0·49 Ma, mainly constrained by the most precise fraction (no. 90; Fig. 10a). A younger zircon fraction (no. 92) with an age of 320·1 1·0 Ma is viewed as a potentially younger component as, in this sample, some overgrowths were identified (Fig. 5, grain A). Coexistent monazite is pale to lemon yellow, exhibiting rare inclusions and microfractures. The monazite mean age of 325·0 4·9 Ma is, within error, the same as the zircon age. The zircon grains extracted from the biotite4amphibole granodiorite enclave, sample C2E, are euhedral to subhedral and prismatic, with oscillatory zoning (Fig. 5, grains C and D), colorless to pale brown, with abundant opaque minerals and melt inclusions. Four analyses (nos 95^98) are concordant to slightly discordant with a slight dispersion across the concordia curve, possibly owing to an inherited component (Fig. 10b). However, their ages overlap, yielding a mean of 327·41 0·40 Ma. A fifth fraction has been partially reset. Coexisting titanite is chocolate brown, clear and translucent. Inclusions are rare but are mainly of opaque minerals. The two titanite fractions

analyzed yielded progressively younger ages, 323·8 1·2 and 318·1 1·3 Ma, both representing resetting. The zircon grains from microgranite vein C79 selected for abrasion are prismatic, euhedral, clear and slightly colored (pale yellow), with some melt inclusions. They show complex zoning (Fig. 5, grains E and F). The four analyzed zircon fractions give an age of 317·24 0·57 Ma (Fig. 10c). Coexisting titanite is orange brown, varying from opaque to slightly translucent. Some fragments have opaque mineral inclusions. Although concordant and similar in age (319·6 1·8 and 316 0·7 Ma), the two fractions plot slightly above and below the cluster of zircon data, suggesting some perturbations of their U^Pb ratios. The few zircons found in the aplite^pegmatite dike (CP2) cutting C1 are strongly metamict and rich in U (Supplementary Data Appendix). Two tips of these metamict grains were mechanically abraded but they remained normally discordant and yielded the expected younger ages (289·8 1·1 and 227·5 1·6 Ma; Fig. 10d). Monazite is anhedral to subhedral and pale yellow. The majority of the selected grains are inclusion-free, except for rare opaque mineral inclusions. They are slightly reversely discordant but form a cluster at 317·43 0·76 Ma. One monazite fraction (no. 110) is slightly older with an age of 320·4 0·8 Ma, possibly representing an assimilated component from the host (C1).

Unit VI: two-mica granite This group consists of one two-mica granite (C49) and two aplite^pegmatite dikes (CP7 and CP8). The U^Pb dataset for sample C49 is rather complex, showing a great variation in ages, mainly in zircon but also in monazite. Zircon grains selected for abrasion are

1903


Fig. 9. Concordia diagrams showing the U^Pb data for zircon, titanite and monazite of the intrusive rocks from Unit IV: C73, biotite granodiorite 1; CP27, aplite^pegmatite dike. Error ellipses are drawn at 2s. CA, chemically abraded; MA, mechanically abraded.


VOLUME 53

NUMBER 9

SEPTEMBER 2012

euhedral to anhedral, mainly prismatic with oscillatory zoning (Fig. 5, grains A and B), but some pyramidal tips were also selected. Evidence of resorption are also observed (Fig. 5, grain C). Generally, the grains selected for MA are clearer and less fractured than the ones selected for CA. Of the nine analyses, eight are essentially concordant but spread along the concordia curve (Fig. 11a). One grain (no. 115) is distinctly older than the others at 341·9 0·8 Ma and is probably affected by inheritance or corresponds to a xenocrystic phase. Seven analyses are spread between 323 and 313 Ma, whereas the youngest (no. 123) is somewhat more discordant and probably affected by Pb loss. Monazite is subhedral, lemon yellow, translucent and has opaque mineral inclusions. None of the fractions were abraded. Fraction number 124 gives an age of 320·5 2·8 Ma and the other two fractions have a mean age of 316·7 1·3 Ma.

The interpretation of this dataset is not straightforward, but it appears reasonable to conclude that the main zircon population comprises two age groups, one at about 323 Ma and the other at about 315^313 Ma. The progressively younger ages can be indicative of incremental growth for about 10 Myr. However, because multiple magmatic events are known to have occurred in the area, it is proposed that the progressively younger ages represent mixtures, in different proportions, with a late zircon population (as rare thin hollow-cup-shaped overgrowths on zircon were observed). A probable age for this later event is 313·6 1·3 Ma, given by fractions 121 and 122. These two analyses correspond to single-grain fractions that were chemically and mechanically abraded, respectively. Zircon grains in aplite^pegmatite sample CP7 comprise colorless, long, prismatic grains and tips with abundant melt inclusions, some fractured, with the fractures oriented

1904


Fig. 10. Concordia diagrams showing the U^Pb data for zircon, titanite and monazite of the intrusive rocks from Unit V: C1, biotite granodiorite 2; C2E, biotite4amphibole granodiorite (enclave); C79, microgranite dike; CP2, aplite^pegmatite dike. Error ellipses are drawn at 2s. CA, chemically abraded; MA, mechanically abraded.

LIMA et al.


DISCUSSION Temporal evolution of Pavia pluton

Fig. 11. Concordia diagrams showing the U^Pb data for zircon and monazite of the intrusive rocks from Unit VI: C49, two-mica granite; CP7 and CP8, aplite^pegmatite dikes. Error ellipses are drawn at 2s. CA, chemically abraded; MA, mechanically abraded.

The combined data define a range of ages from 343 to 313 Ma, but almost all samples record multi-stage zircon growth accompanied by one or more generations of monazite and titanite. These relationships reveal both the complexity of the magmatic evolution in the studied area and the relative resistance of the various minerals (zircon, titanite and monazite) to partial and/or complete resetting in response to superimposed magmatic^hydrothermal events.

1905


perpendicularly to obliquely to the crystal length, and colorless, subhedral to euhedral, flat grains, with rare melt inclusions. The six analyzed zircon grains yield apparent ages between 339 and 334 Ma, the three oldest ones defining an age of 338·37 0·51 Ma. The three younger zircons define an age of 336·37 0·52 Ma (Fig. 11b). Zircon grains from aplite^pegmatite sample CP8 are euhedral to subhedral, mainly prismatic, colorless to orange, with frequent melt inclusions. Internal regular growth zoning is locally transected by recrystallized domains (Fig. 5, grains D and E). The five zircon analyses yield a range of apparent ages, two at about 340^337 Ma, two others at 324 Ma and one more discordant, the latter possibly affected by 231Pa excess (Fig. 11c). The age given by this analysis is 311·1 2·6 Ma, possibly recording the inferred 313 Ma event recognized in the host granite (C49). Coexisting monazite is anhedral to subhedral, lemon yellow, inclusion-free, with planar fracturing. Three monazite fractions gave ages of 326·1 6·7 Ma, which is the same age as that of the two identical zircon analyses (nos 135 and 136) giving an age of 324·39 0·51 Ma, interpreted as the time of formation of the aplite^pegmatite. The two oldest analyses probably represent an inherited or xenocrystic component. The age of 338·37 0·51 Ma of aplite^pegmatite sample CP7 presents a paradox, being older than the host granite (323 Ma). Given that the latter is supported by the age of 324·39 0·51 Ma for the second crosscutting dike (CP8), which exhibits sharp but lobate boundaries with the host-rock (Fig. 3f), suggesting that this was partially molten at the time of the emplacement of the dike, the only reasonable explanation for the age data for CP7 is that all the analyzed zircons are inherited or xenocrystic. This is remarkable because the crystals have very well-preserved crystal shapes, many without much evidence for resorption, and they appear to be derived from a unique source. Detailed examination of the mineral separates from this sample revealed just one grain with a thin overgrowth and another turbid grain with the appearance of extremely metamict zircon (such as those in CP2). These may then be the only magmatic zircons found in the dike.


VOLUME 53

SEPTEMBER 2012

host. With this exception, the data resolve the timing of the main magmatic episodes fairly well. The simplest relationships are seen in units I and II where identical ages of 329^328 Ma were obtained, not only for the main phases, tonalite in I and trondhjemite to granodiorite in II, but also by enclaves and crosscutting rhyodacite porphyries and pegmatites (Fig. 12). The only exception in these domains of the pluton is a microgranodiorite dike (C24A) whose age of 324 Ma corresponds to that of the main activity in the flanking domains. At the direct contact with this dike the host-rock (C51) is slightly discolored (Fig. 3c), probably from a metasomatic effect caused by the intrusion of the dike, thus supporting the younger age of this rock. The remaining units III^VI all reflect the principal period of magmatism at about 324 Ma. Only the quartz-dioritic microgranular enclave in unit III represents older 335 Ma material.

Fig. 12. Summary of U^Pb ages for the outcropping granitic rocks of the Pavia pluton, including an aplite^pegmatite dike cutting schists (sample CP6) dated by U^Pb ID-TIMS using U-rich inclusions in garnet (Lima et al., 2012). The vertical bars correspond to the ages of single analyses or to the weighted mean of several analyses (see main text for further discussion). The horizontal grey bars and dashed lines represent the three main magmatic events directly responsible for the formation of the Pavia pluton: 1st ME, first magmatic event at 328 Ma, mainly responsible for the emplacement of the central units (I and II) but present as xenocrystic phases in units III and IV; 2nd ME, second magmatic event at 325^323 Ma mainly responsible for the emplacement of the flanking units (III^VI) but present in unit I as resetting ages; 3rd ME, third magmatic event at 319^317 Ma, responsible for the emplacement of subsidiary phases in units IV and V but also present as the age of secondary overgrowths in unit I.

1906


The most important information concerns the timing of the main magmatic events. In most cases this information can be extracted readily from the data from a consideration of the dominant physical features of the zircon populations, combined with the coherence of the results obtained from multiple analyses. These interpretations are further consolidated when the same ages are also given by coexisting monazite or titanite. The main exceptions are two samples from unit VI, namely aplite^pegmatite CP7 and, to some degree, two-mica granite C49. In the latter case the interpretation of the primary magmatic age at around 323 Ma, as indicated by the oldest fraction, is consistent with the coeval age of crosscutting aplite^pegmatite CP8. By contrast, the zircon appearance and isotopic data for sample CP7 alone would support an age of 339^334 Ma for its emplacement, except that such an interpretation is unacceptable considering the much younger age of the

NUMBER 9

LIMA et al.


then granite, whereby magmas with granodioritic composition were injected and emplaced sequentially for about 5 Myr. According to Annen (2011) and Michaut & Jaupart (2011), as the first emplaced magmas are in direct contact with cold country rock they crystallize rapidly, even before the next magma batch is emplaced and, with time, successive intrusions thermally equilibrate with their surroundings at progressively higher temperatures. This thermal evolution can explain why time intervals of magma accretion are so well recorded in the oldest rocks in comparison with the younger ones that were emplaced into a hot crust, allowing them to remain partially molten for longer periods (which probably permitted magmatic differentiation towards more felsic compositions). Other features, such as partial resetting of zircon (a possible mechanism in samples C69B and C20P), crystallization of new zircon (samples C49 and C51) and both zircon and monazite (a possible mechanism in sample C16), and total resetting of titanite (samples C51, C69B, C20P, C73 and C2E), also suggest prolonged magmatic activity with sequential injection of multiple magma batches and associated hydrothermal activity. Partial resetting of zircon is proposed because samples C69B and C20P show a continuous decrease in the 206Pb/238U ages of the analyzed zircon grains, a decrease that cannot be correlated with other features. Moreover, the older units (I and II) are flanked by younger ones (III and IV), whose emplacement followed the contact between the wall-rocks and the older intrusions (Fig. 2). This suggests that this pluton may have grown by sheeting rather than by ‘nested’ growth, which implies emplacement by successive small-volume batches of magma separated by periods of nearly complete solidification (Barker, 2007).

Significance of pre-magmatic zircon components

Fig. 13. Rose diagrams showing the orientation of the dated dikes.

A common feature in felsic intrusive rocks is the presence of older inherited zircon, either as cores in new magmatic crystals, or as separate, but generally corroded grains. In this study, however, much older zircon components were recorded in only two cases (C20P and C24A). This is due not necessarily to the absence of inheritance in the rocks of the Pavia pluton, but first of all to the fact that all grains with visible cores or with morphological features suggestive of the presence of inherited cores were avoided during grain selection. In contrast to this nearly total absence of inherited Precambrian zircon ages, our data register a higher abundance of zircon grains of Carboniferous age, but predating the inferred time of magmatism. Typically these grains are morphologically not greatly different from the newly formed magmatic zircons, and hence were not discriminated during selection. Concordant older outliers plot in

1907


The age of subordinate cross-cutting phases varies from being coeval to the main activity to post-dating it by up to 6 Myr. At least three generations of pegmatite dikes are recognized. The coeval generation yields 328 Ma in units I and II (CP4 and CP11) and 324 Ma in unit VI (CP8). The generations that are younger than their host can be subdivided according to the magmatic episode to which they are related. Sample CP20 of unit III yields 323 Ma and represents the final stages of the second magmatic event. Samples CP2 (unit V), CP27 (unit IV) and CP6 (cutting schists; Lima et al., 2012) with ages of 319^317 Ma are the products of a later magmatic episode. Whole-rock geochemistry suggests that all the granites (s.l.) and cross-cutting phases correspond to discrete magmatic pulses and that the analyzed enclaves could not generate the composition of the host by differentiation (Lima et al., in preparation). Coeval pegmatite dikes may represent the differentiation products of biotite4muscovite granodiorite (unit II) and two-mica granite (unit VI). The pegmatite dike (CP20, unit III) and the younger pegmatite dikes (CP2 and CP27) do not seem related to any of the main outcropping phases. Also, there is no evident relation between emplacement age and orientation of the dikes (Fig. 13). The data show that the Pavia pluton was constructed by the amalgamation of two major and some subsidiary magmatic pulses over a period of c. 11 Myr. The sequence of magmatism shows a trend towards more felsic compositions through time, from tonalite to granodiorite and


VOLUME 53

Magmatic evolution As discussed in the previous section, the range of ages between 343 and 337 Ma of concordant older outliers in some of the dated plutonic rocks are considered to result from a previous magmatic or metamorphic event of regional extent (Ribeiro et al., 2007; Azor et al., 2008; Pereira et al., 2009; Antunes et al., 2011). The oldest rock dated in the Pavia pluton is a quartz-dioritic microgranular enclave (C55E) with an age of 335 Ma. There is a significant time gap (10 Myr) between the crystallization age of this enclave and the crystallization age of its host (C5, unit III). For this reason, it is suggested that the enclaves were incorporated by the felsic magma at depth during ascent and predate the beginning of the magmatic activity in the area. This implies that the Pavia pluton is underlain by plutonic rocks formed during an earlier event at around 335 Ma, corresponding to the widespread magmatic activity recorded elsewhere in the region. Magmatism at this level of the crust began at 328 Ma with the emplacement of the tonalite, trondhjemite and granodiorite of units I and II and contemporaneous porphyries (C16 and C20P) and pegmatite dikes (CP4 and CP11). The next major event at 325^323 Ma emplaced the bulk of the remaining units III^VI. Thus during a period of 5 Myr several magma pulses were injected and emplaced sequentially, affecting intrusions that were already in place. A magmatic pause of 4 Myr between 323 and 319 Ma probably represents a period of magma storage at depth. The subsequent magmatic event at 319^317 Ma is characterized by the emplacement of microgranite (C79) and pegmatite dikes (CP27, CP2 and CP6) and is also recorded in other lithologies as zircon

SEPTEMBER 2012

secondary crystallization ages (samples C49 and C51) and titanite ages (samples C20P, C73 and C2E). At 313 Ma, another episode can be recognized as recorded in zircon grains from samples C49 and CP8, in very U-rich titanite grains in sample C20P and perhaps in a monazite from sample CP27 (whose age, however, could also be due to partial Pb loss at a later time). However, this late event was not found to date any outcropping rock and may instead represent a later hydrothermal event responsible for local processes of zircon resorption and recrystallization (Fig. 5; Group VI). Emplacement of quartz veins, mainly in units V and VI (Fig. 2), may have occurred at this time, but the available data are insufficient to confirm this. In previous geochronological studies in the OMZ the repetition of ages from 330 to 313 Ma has been observed both for granite (s.l.) emplacement and metamorphism (e.g. Castro et al., 2002, and references therein; Moita et al., 2005, 2009), showing that these events had a regional expression. The east^west elongation of all units in the Pavia pluton suggests that the space and shape of the intrusions was created and controlled by the overall stress regime that caused transtension across the OMZ (see Pereira et al., 2009). The origin and sequencing of magma input results from the complex interaction between processes of magma generation, extraction and emplacement occurring on different timescales at the source level (Annen, 2009, 2011). Pereira et al. (2007) suggested that the Variscan evolution of the crust of the E¤vora Massif, during the Late Paleozoic, was related to alternating cycles of crustal shortening and extension, owing to transcurrent movements, responsible for important perturbations in the crustal thermal regime. This can explain the periodicity of magmatism over such a long period of time (11 Myr). Jellinek & DePaolo (2003) have shown that magma storage is enhanced by regional tectonic extension, as this suppresses dike formation by dissipating magma chamber overpressure through an increase in the volume of the magma chamber. For the Pavia pluton it is thus suggested that the localization, in time and space, of the magmatic events was related to changes in the tectonic regime. The period 328^323 Ma was characterized by compression, which marked the injection of several magma pulses into the crust. At 323 Ma the tectonic regime probably changed, acquiring an extensional character for 4 Myr that resulted in decreased overpressure inside the magma chamber and a general absence of magma emplacement. Recharge of the magma chamber and magmatic differentiation may have occurred during this interval, at the end of which, at 319 Ma, the tectonic framework changed again. The magmatic chamber was compressed and the magma forced to ascend into the crust. Magma injection occurred dominantly along faults, as this event is

1908


the age interval from 343 to 328 Ma. Zircons with ages between 328 and 334 Ma occur in samples C51 (no. 1), C24A (no. 30), C11 (no. 48), C5 (no. 59), CP20 (nos 70 and 71) and CP27 (no. 83) (Table 3, Fig. 12). Those with ages close to the magmatic age of the rock can be interpreted to represent early crystallization phases (‘antecrysts’, Miller et al., 2007) indicating prolonged zircon growth in a magma chamber (samples C51 and C11). The others probably correspond to xenocrystic phases, scavenged from the earliest batches of magma crystallized during the Carboniferous and assimilated during magma ascent and/ or emplacement, or may even be inherited from the protolith(s), implying it was very young when melted (samples C24A, CP27, C5 and CP20). The ages of 337^343 Ma found in samples CP11 (no. 53), C5 (no. 58), C55E (no. 90), CP20 (no. 69), C49 (no. 115), CP7 (nos 127^132) and CP8 (nos 133 and 134) (Table 3) do not correspond to any of the dated plutonic rocks in the Pavia pluton, but they are widely recognized elsewhere in the OMZ as dating important magmatic and metamorphic events (Ribeiro et al., 2007; Azor et al., 2008; Pereira et al., 2009; Antunes et al., 2011).

NUMBER 9

LIMA et al.


characterized by dikes whose orientation follows that of the main fracture systems in the area. The time span between 317 and 313 Ma represented another period of magmatic quiescence, possibly with reactivation at 313 Ma, again as a result of tectonic controls. The fact that this age was documented in a few genetically unrelated rocks (samples C20P, CP27, C49 and CP8; Table 3 and Figs 6, 9 and 11) indicates that this last event must also have affected the entire region.

CONC LU DI NG R E M A R K S

AC K N O W L E D G E M E N T S We thank Ricardo L. Silva, Ma¤rio Monteiro and Virg|¤ lio Pereira for the help in sampling, Professor Manuel Machado Leite for giving permission to the last two to help and for the polished thin-sections, Professor Maria R. Azevedo and Maria M. Costa for the laboratory facilities at the University of Aveiro (Portugal), and Professor Andre¤s Cuesta Ferna¤ndez and M|¤ guel Ferna¤ndez Gonza¤lez for the support in the microprobe analyses at University of Oviedo (Spain). We appreciate constructive reviews by Stephen Daly and an anonymous reviewer.

FU N DI NG This work was financially supported by the Portuguese Foundation for Science and Technology (SRFH/BD/ 39948/2007 to S.M.L.), Geosciences Center and Department of Geoscience of University of Oslo.

S U P P L E M E N TA RY DATA Supplementary data for this paper are available at Journal of Petrology online.

R EF ER ENC ES Anczkiewicz, R., Oberli, F., Burg, J. P., Villa, I. M., Guenther, D. & Meir, M. (2001). Timing of normal faulting along the Indus Suture in Pakistan Himalaya and a case of major 231Pa/235U initial disequilibrium in zircon. Earth and Planetary Science Letters 191, 101^114. Annen, C. (2009). From plutons to magma chambers: Thermal constraints on the accumulation of eruptible silicic magma in the upper crust. Earth and Planetary Science Letters 284, 409^416. Annen, C. (2011). Implications of incremental emplacement of magma bodies for magma differentiation, thermal aureole dimensions and plutonism^volcanism relationships. Tectonophysics 500, 3^10. Antunes, A., Santos, J. F., Azevedo, M. R. & Corfu, F. (2011). New U^Pb zircon age constraints for the emplacement of the Reguengos de Monsaraz Massif (Ossa^Morena Zone). In: Molina, J. F., Scarrow, J. H., Bea, F. & Montero, P. (eds) VII Hutton Symposium on Granites and Related Rocks. Granada: Universidad de Granada, pp. 9^10. Azor, A., Rubatto, D., Simancas, J. F., Gonza¤lez-Lodeiro, F., Mart|¤ nez-Poyatos, D., Mart|¤ n Parra, L. M. & Matas, J. (2008).

1909


Growth of plutons by injection of multiple magma batches is not rare and has been reported from all over the world in plutons with different tectonic settings, compositions and ages. However, the processes that lead to multiple growth of granitic bodies, both in space and time, are not well understood. As a consequence, the existence of large magma chambers and the interaction between different processes such as magmatic differentiation, country-rock metamorphism and assimilation need re-evaluation. The geochronological data presented in this study, on the one hand, reflect the complexity of magmatic processes, but, on the other hand, they also draw attention to some of the problematic aspects that can affect the interpretation of U^Pb data for zircon and other accessory minerals, particularly when multiple generations are present. The identification of specific mineral growth phases and the links to U^Pb ages and magmatic or hydrothermal processes can be difficult tasks, especially when combined with Pb loss owing to radiation damage, U^Pb resetting, inheritance and assimilation. Additional difficulties arise when the contaminants are derived from sources and/or country rocks that are only 10^15 Myr older, which seems to be the situation in the Pavia pluton. In this study a sufficiently dense sampling strategy, from rocks with well-understood field relationships and relative age, and the use of titanite and monazite as complementary geochronometers to zircon have allowed us to overcome some of the uncertainties and have resulted in a more coherent data interpretation. The data show that the Pavia pluton was constructed over 11 Myr by the injection of multiple batches of magma; at least four main magmatic events are identified. The first event at around 340 Ma, identified in concordant zircon outliers, was restricted to deeper levels of the crust and preceded the magmatic activity at the now exposed levels of the pluton; however, it is recognized as the main magmatic event elsewhere in the OMZ. The second event at about 328 Ma formed tonalite, trondhjemite and granodiorite, together with minor differentiates in units I and II. The third event at about 324 Ma was responsible for the emplacement of granodiorite and two-mica granite in units III^VI, together with contemporaneous to late microgranites (s.l.) and pegmatite dikes in all units. The

fourth magmatic event at 319^317 Ma was much less important volumetrically, being characterized by the emplacement of microgranite and pegmatite dikes. The occurrence of a fifth magmatic event at 313 Ma at depth is possible, and may have induced hydrothermal activity causing metasomatism and zircon resorption and/or recrystallization. Each magmatic episode was interspersed with periods of quiescence without intrusive activity. It is suggested that the cyclic character of these phenomena (magma injection versus magma storage), and the geometry of the various intrusive phases may have been a response to the prevailing transtensional tectonic regime.


VOLUME 53

SEPTEMBER 2012

Jaffey, A. H., Flynn, K. F., Glendenin, L. E., Bentley, W. C. & Essling, A. M. (1971). Precision measurement of half-lives and specific activities of 235U and 238U. Physical Review, Section C, Nuclear Physics 4, 1889^1906. Jellinek, A. M. & DePaolo, D. J. (2003). A model for the origin of large silicic magma chambers: precursors of caldera-forming eruptions. Bulletin of Volcanology 65, 363^381. Krogh, T. E. (1973). A low-contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determinations. Geochimica et Cosmochimica Acta 37, 485^494. Krogh, T. E. (1982). Improved accuracy of U^Pb zircon age by the creation of more concordant systems using an air abrasion technique. Geochimica et Cosmochimica Acta 46, 637^649. Lima, S. M., Neiva, A. M. R. & Ramos, J. M. F. (2011). Geochemical characterization of the granitic rocks from the Pavia pluton (Ossa^Morena Zone, Portugal). In: Antunes, I. M. R. H., Almeida, J. P. F. & Albuquerque, M. T. D. (eds) Livro de Actas do VII Congresso Ibe¤ rico de Geoqu|¤ mica e XVII Semana de Geoqu|¤ mica. Castelo Branco: Instituto Polite¤cnico de Castelo Branco, pp. 253^258. Lima, S. M., Corfu, F., Neiva, A. M. R. & Ramos, J. M. F. (2012). U^Pb ID-TIMS dating applied to U-rich inclusions in garnet. American Mineralogist 97, 800^806. Lipman, P. W. (2007). Incremental assembly and prolonged consolidation of Cordilleran magma chambers: Evidence from the Southern Rocky Mountain volcanic field. Geosphere 3(1), 42^70. Ludwig, K. R. (2009). Isoplot 4.1. A geochronogical toolkit for Microsoft Excel. Berkeley Geochronology Center Special Publication 4, 76 p. Mattinson, J. M. (2005). Zircon U^Pb chemical abrasion (‘CA-TIMS’) method: Combined annealing and multi-step partial dissolution analysis for improved precision and accuracy of zircons ages. Chemical Geology 220, 47^66. Mattinson, J. M. (2010). Analysis of the relative decay constants of 235 U and 238U by multi-step CA-TIMS measurements of closed-system natural zircon samples. Chemical Geology 275, 186^198. Matzel, J. E. P., Bowring, S. A. & Miller, R. B. (2006). Time scales of pluton construction at differing crustal levels: Examples from the Mount Stuart and Tenpeak intrusions, North Cascades, Washinghton. GSA Bulletin 118, 1412^1430. Menand, T., Saint-Blanquat, M. & Annen, C. (eds) (2011). Emplacement of magma pulses and growth of magma bodies. Tectonophysics 500, 112 p. Michaut, C. & Jaupart, C. (2011). Two models for the formation of magma reservoirs by small increments. Tectonophysics 500, 33^49. Michel, J., Putlitz, B., Schaltegger, U. & Ovtcharova, M. (2008). Incremental growth of the Patagonian Torres del Paine laccolith over 90 k.y. Geology 36(6), 459^462. Miller, J. S., Matzel, J. E. P., Miller, C. F., Burgess, S. D. & Miller, R. B. (2007). Zircon growth and recycling during the assembly of large, composite arc plutons. Journal of Volcanology and Geothermal Research 167, 282^299. Moita, P., Santos, J. F. & Pereira, M. F. (2005). Dados geocronolo¤gicos de rochas intrusivas sin-tecto¤nicas do Macic o dos Hospitais (Montemor-o-Novo, Zona de Ossa^Morena). In: Fonseca, E., Silva, E., Azevedo, M., Patinha, C. & Reis, A. (eds) Proceedings VIII Congresso de Geoqu|¤ mica de Pa|¤ ses de L|¤ ngua Portuguesa, 2. Aveiro: Universidade de Aveiro, pp. 471^474. Moita, P., Santos, J. F. & Pereira, M. F. (2009). Layered granitoids: Interaction between continental crust recycling processes and mantle-derived magmatism. Examples from the E¤vora Massif (Ossa^Morena Zone, southwest Iberia, Portugal). Lithos 111, 125^141.

1910


Rheic Ocean ophiolitic remnants in southern Iberia questioned by SHRIMP U^Pb zircon ages on the BejaÂcebuches amphibolites. Tectonics 27, TC5006, 11 p. Azor, A., Rubatto, D., Marchesi, C., Simancas, J. F., Gonza¤lezLodeiro, F., Mart|¤ nez-Poyatos, D., Mart|¤ n Parra, L. M. & Matas, J. (2009). Reply to comment by C. Pin and J. Rodr|¤ guez on ‘Rheic Ocean ophiolitic remnants in southern Iberia questioned by SHRIMP U^Pb zircon ages on the BejaÂcebuches amphibolites’. Tectonics 28, TC5014, 6 p. Barker, D. (2007). Endogenous and exogenous plutons: the influence of emplacement style and contamination of granitic magma. Canadian Mineralogist 45, 63^70. Carvalhosa, A. (1983). Esquema Geolo¤gico do Macic o de E¤vora. Comunicac o‹ es dos Servic os Geolo¤ gicos de Portugal 69(2), 201^208. Carvalhosa, A. (1999). Carta Geolo¤ gica de Portugal na escala 1/50 000, Folha 36-C (Arraiolos) e respectiva not|¤ cia explicativa. Lisboa: Instituto Geolo¤gico e Mineiro. Carvalhosa, A. & Zbyszewski, G. (1994). Carta Geolo¤ gica de Portugal na escala 1/50 000, Folha 35-D (Montemor-o-Novo) e respectiva not|¤ cia explicativa. Lisboa: Instituto Geolo¤gico e Mineiro. Castro, A., Corretge¤, L. G., De la Rosa, J., Enrique, P., Mart|¤ nez, F. J., Pascual, E., Lago, M., Arranz, E., Gale¤, C., Fernande¤z, C., Donaire, T. & Lo¤pez, S. (2002). In: Gibbons, W. & Moreno, M. T. (eds) Paleozoic Magmatism, The Geology of Spain. London: Geological Society, pp. 117^153. Coleman, D. S., Gray, W. & Glazner, A. F. (2004). Rethinking the emplacement and evolution of zoned plutons: Geochronologic evidence for incremental assembly of the Tuolumne Intrusive Suite, California. Geology 32(5), 433^436. Corfu, F. (2004). U^Pb age, settings and tectonic significance of the anorthosite^mangerite^charnockite^granite suite, Lofoten^ Vesteralen, Norway. Journal of Petrology 45(9), 1799^1819. Corfu, F., Hanchar, J. M., Hoskin, P. W. O. & Kinny, P. (2003). Atlas of Zircon Textures. In: Hanchar, J. M. & Hoskin, P. W. O. (eds) Zircon. Mineralogical Society of America and Geochemical Society, Reviews in Mineralogy and Geochemistry 53, 469^500. Davis, D. W., Williams, I. S. & Krogh, T. E. (2003). Historical development of zircon geochronology. In: Hanchar, J. M. & Hoskin, P. W. O. (eds) Zircon. Mineralogical Society of America and Geochemical Society, Reviews in Mineralogy and Geochemistry 53, 145^181. du Bray, E. A., Bacon, C. R., John, D. A., Wooden, J. L. & Mazdab, F. K. (2011). Episodic intrusion, internal differentiation, and hydrothermal alteration of the Miocene Tatoosh intrusite suite south of Mountain Rainier, Washington. GSA Bulletin 123(3/4), 534^561. Enami, M., Suzuki, K., Liou, J. G. & Bird, D. K. (1993). Al^Fe3þ and FÔH substitutions in titanite and constraints on their P^T dependence. EuropeanJournal of Mineralogy 5, 219^231. Ferry, J. M. (1979). Reaction mechanisms, physical conditions, and mass transfer during hydrothermal alteration of mica and feldspar in granitic rocks from south^central Maine, USA. Mineralogy and Petrology 68, 125^139. Franke, W., Matte, P. & Tait, J. (2005). Europe/Variscan orogeny. In: Selley, R. C., Cocks, L. R. M. & Plimer, I. R. (eds) Encyclopedia of Geology. Amsterdam: Elsevier, pp. 75^86. Glazner, A. F., Bartley, J. M., Coleman, D. S., Gray, W. & Taylor, R. Z. (2004). Are plutons assembled over millions of years by amalgamation from magma chambers? GSA Today 14(4^5), 4^11. Harlov, D., Tropper, P., Seifert, W., Nijland, T. & Fo«rster, H. J. (2006). Formation of Al-rich titanite (CaTiSiO4^CaAlSiO4OH) reaction rims on ilmenite in metamorphic rocks as a function of fH2O and fO2. Lithos 88, 72^84.

NUMBER 9

LIMA et al.


Iberia): Geological characterization and geodynamic significance. Gondwana Research 17, 408^421. Saint-Blanquat, M., Habert, G., Horsman, E., Morgan, S. S., Tikoff, B., Launeau, P. & Gleizes, G. (2006). Mechanisms and duration of non-tectonically assisted magma emplacement in the upper crust: The Black Mesa pluton, Henry Mountains, Utah. Tectonophysics 428, 1^31. Sa¤nchez-Garc|¤ a, T., Quesada, C., Bellido, F., Dunning, G. R. & Gonza¤lez del Ta¤nago, J. (2008). Two-step magma flooding of the upper crust during rifting: The Early Paleozoic of the Ossa^ Morena Zone (SW Iberia). Tectonophysics 461, 72^90. Schaltegger, U., Brack, P., Ovtcharova, M., Peytcheva, I., Schoene, B., Stracke, A., Marocchi, M. & Bargossi, G. (2009). Zircon and titanite recording 1·5 million years of magma accretion, crystallization and initial cooling in a composite pluton (southern Adamello batolith, northern Italy). Earth and Planetary Science Letters 286, 208^218. Seydoux-Guillaume, A. M., Wirth, R. & Ingrin, J. (2007). Contrasting response of ThSiO2 and monazite to natural irradiation. European Journal of Mineralogy 19, 7^14. Silva, J. B. & Pereira, M. F. (2004). Transcurrent continental tectonics model for the Ossa^Morena Zone Neoproterozoic^Paleozoic evolution, SW Iberian Massif, Portugal. Geologische Rundschau 93, 886^896. Whitney, D. L. & Evans, B. W. (2010). Abbreviations for names of rock-forming minerals. American Mineralogist 95, 185^187. Zbyszewski, G., Da Veiga Ferreira, O. & Barros e Carvalhosa, A. (1979). Carta Geolo¤ gica de Portugal na escala 1/50 000, Folha 35-B (Mora) e respectiva not|¤ cia explicativa. Lisboa: Direcc a‹o Geral de Geologia e Minas^Servic os Geolo¤gicos de Portugal. Zbyszewski, G., Barros, A. & Da Veiga Ferreira, O. (1980). Carta Geolo¤ gica de Portugal na escala 1/50 000, Folha 36-A (Pavia) e respectiva not|¤ cia explicativa. Lisboa: Direcc a‹o Geral de Geologia e Minas^ Servic os Geolo¤gicos de Portugal.

1911


Parrish, R. R. (1990). U^Pb dating of monazite and its application to geological problems. Canadian Journal of Earth Sciences 27, 1431^1450. Parrish, R. R. & Noble, S. R. (2003). Zircon U^Th^Pb geochronology by isotope dilution^thermal ionization mass spectrometry (ID-TIMS). In: Hanchar, J. M. & Hoskin, P. W. O. (eds) Zircon. Mineralogical Society of America and Geochemical Society, Reviews in Mineralogy and Geochemistry 53, 182^213. Pereira, M. F., Branda‹o Silva, J., Chichorro, M., Moita, P., Santos, J. F., Apraiz, A. & Ribeiro, C. (2007). Crustal growth and deformation processes in the northern Gondwana margin: Constraints from the E¤vora Massif (Ossa^Morena Zone, southwest Iberia, Portugal). In: Linnemann, U., Nance, R. D., Kraft, P. & Zulauf, G. (eds) The Evolution of the Rheic Ocean: From Avalonian^ Cadomian Active Margin to Alleghenian^Variscan Collision. Geological Society of America, Special Papers 423, 333^358. Pereira, M. F., Chichorro, M., Williams, I. S., Silva, J. B., Fernande¤z, C., Da¤z-azp|¤ roz, M., Apraiz, A. & Castro, A. (2009). Variscan intra-orogenic extensional tectonics in the Ossa^Morena Zone (E¤voraÂracena^Lora del R|¤o metamorphic belt, SW Iberian Massif): SHRIMP U^Th^Pb geochronology. In: Murphy, J. B., Keppie, J. D. & Hynes, A. J. (eds) Ancient Orogens and Modern Analogues. Geological Society, London, Special Publications 327, 215^237. Pin, C. & Rodr|¤ guez, J. (2009). Comment on: ‘Rheic Ocean ophiolitic remnants in southern Iberia questioned by SHRIMP U^Pb zircon ages on the BejaÂcebuches amphibolites’ by A. Azor et al. Tectonics 28, TC5013, 6 p. Ribeiro, A., Munha¤, J., Dias, R., Mateus, A., Pereira, E., Ribeiro, L., Fonseca, P., Arau¤jo, A., Oliveira, T., Roma‹o, J., Chamine¤, H., Coke, C. & Jorge, P. (2007). Geodynamic evolution of the SW Europe Variscides. Tectonics 26, TC6009, 24 p. Ribeiro, A., Munha¤, J., Fonseca, P. E., Arau¤jo, A., Pedro, J.C., Mateus, A., Tassinari, C., Machado, G. & Jesus, A. (2010). Variscan ophiolite belts in the Ossa^Morena Zone (Southwest

Dissecting Complex Magmatic Processes: an in-depth ...

Dissecting Complex Magmatic Processes: an in-depth ...

Suggest Documents

Dissecting complex polyketide biosynthesis

Feedback processes between magmatic events

Time scales of magmatic processes

ZeroAccess Indepth - Symantec

Hydrovolcanic vs Magmatic Processes in Forming ...

Modulation of magmatic processes by CO2 flushing

Magmatic processes that generate chemically distinct ...

Magmatic processes associated with caldera ... - GeoScienceWorld

Magmatic processes that generated the rhyolite of

The influence of magmatic processes on the

Magmatic and Postmagmatic Processes in Tin ... - CiteSeerX

Understanding Vesuvius magmatic processes ... - Springer Link

Brainâcomputer interfaces for dissecting cognitive processes ...

Dissecting Complex Diseases in Complex Populations - ATS Journals

Dissecting Genetic Networks Underlying Complex Phenotypes: The ...

Dissecting the mammalian synaptonemal complex ... - Springer Link

Dissecting Complex Diseases in Complex Populations - ATS Journals

indepth interview - Universitas Airlangga

ZeroAccess Indepth - Symantec

INDEPTH-Uganda - PLOS

MSR InDepth

INDEPTH-Uganda - PLOS

ZeroAccess Indepth - Symantec [PDF]

Magmatic processes in developing oceanic crust revealed in a ...

Dissecting Complex Magmatic Processes: an in-depth ...