magmatism in the Variscan suture zone, southern I - Springer Link

Int J Earth Sci (Geol Rundsch) (2014) 103:1433–1451 DOI 10.1007/s00531-014-1034-5

Original Paper

U–Pb geochronology and petrology of the late Paleozoic Gil Marquez pluton: magmatism in the Variscan suture zone, southern Iberia, during continental collision and the amalgamation of Pangea Evan R. Gladney · James A. Braid · J. Brendan Murphy · Cecilio Quesada · Christopher R. M. McFarlane Received: 1 October 2013 / Accepted: 13 May 2014 / Published online: 31 May 2014 © Springer-Verlag Berlin Heidelberg 2014

Abstract The origin of plutonic complexes that stitch suture zones developed during collision is not well understood. In southern Iberia, the Pulo du Lobo suture zone (PDLZ) is intruded by the syn- to postcollisional Gil Marquez pluton (GMP), thought to be part of the Sierra Norte Batholith. U–Pb (LA-ICPMS, zircon) data on various phases of the GMP yield from oldest to youngest: (1) a 354.4 ± 7.6 Ma unfoliated gabbro; (2) a 345.6 ± 2.5 Ma foliated intermediate phase; (3) a 346.5 ± 5.4 Ma unfoliated porphyritic granite; (4) a 335.1 ± 2.8 Ma unfoliated biotite granite. This sequence is consistent with cross-cutting relationships observed in the field. The range in ages is consistent with interpretations that the GMP is part of the composite (ca. 350–308 Ma) SNB. Inherited ages preserved in the GMP intermediate and felsic phases indicate that its magmas traversed through South Portuguese Zone and PDLZ crust during emplacement. The ca. 345 Ma emplacement of the late kinematic foliated intermediate phase constrains the age of late-stage strike slip deformation within the PDLZ, and the lack of a foliation in the

Electronic supplementary material The online version of this article (doi:10.1007/s00531-014-1034-5) contains supplementary material, which is available to authorized users. E. R. Gladney · J. A. Braid · J. B. Murphy (*) Department of Earth Sciences, Saint Francis Xavier University, Antigonish, NS B2G 2W5, Canada e-mail: [email protected] C. Quesada Instituto Geológico y Minero de España, c/Rios Rosas 23, 28003 Madrid, Spain C. R. M. McFarlane Department of Earth Sciences, University of New Brunswick, PO Box 4400, Fredericton, NB E3B 5A3, Canada

older gabbro indicates that is was not proximal to a shear zone neither at the time of emplacement, nor during its subsequent history. The unfoliated porphyritic granite and unfoliated biotite granite cut the foliation of the intermediate phase indicating emplacement during the waning stages of collision, while the ca. 335 Ma biotite granite intrudes the Santa Ira Flysch, thereby providing a tight constraint for the latest stage of deformation in the PDLZ. Keywords Gil Marquez pluton · Sierra Norte Batholith · Southern Iberia · U–Pb geochronology · Variscan suture zone · Collisional magmatism

Introduction Although magmatism in collisional settings is well documented (e.g., Dilek 2006; Gutiérrez-Alonso et al. 2011), the origin of plutonic complexes that stitch suture zones which developed during collision is not well understood. In ancient collisional belts, this task is particularly difficult because these zones are commonly destroyed or obscured by younger geologic events. However, in southern Iberia, the suture zone, which records the final amalgamation stages of Pangea in the Late Paleozoic, is exposed and crosscut by syn- to postcollisional intrusive igneous rocks (Eden 1991; Braid et al. 2010, 2012; Quesada 1991; Pereira et al. 2012). This suture zone, known as the Pulo do Lobo Zone (PDLZ), is the southernmost expression of the Variscan orogen in western Europe and formed during the closure of the Rheic Ocean and the resulting terminal collision between Laurussia and Gondwana (Matte 2001; Stampfli and Borel 2002; Gutiérrez-Alonso et al. 2008, 2011; Braid et al. 2011). The PDLZ is widely interpreted to be

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Int J Earth Sci (Geol Rundsch) (2014) 103:1433–1451

Fig. 1 (Inset) Setting of the Rheic suture with respect to Variscan and Cadomian Massifs in southeastern Europe (adapted from Nance et al. 2010). An overview of the geology of the South Portuguese and Pulo do Lobo zones (adapted from Braid et al. 2012)

Fig. 2 A summary of the geology of the GMP and host Pulo do Lobo zone, with sample locations shown

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Fig. 3 Field relationships within the GMP, and with surrounding host rocks. a Unfoliated gabbro intruded by foliated intermediate phase. b Unfoliated gabbro intruded by biotite granite. c Parallel foliation shared between granodiorite and quartz diorite, demonstrating coeval emplacement and supporting magma mingling processes. d Biotite granite intruding foliated intermediate phase, and intersecting the foliation. e Biotite granite intruding host Pulo do Lobo schist and transecting the foliation. f Porphyritic granite intruding a quartz wacke of the Ribeira de Limas Formation (high angle to the intrusive contact)

a classic accretionary complex that records the collision between these continents (Fig. 1; Eden 1991; Quesada and Dallmeyer 1994; Onézime et al. 2003; Braid et al. 2011) and separates a portion of Laurussia (South Portuguese Zone, SPZ) from Gondwana (Ossa Morena Zone, OMZ). The SPZ and PDLZ are crosscut by the Sierra Norte Batholith (Fig. 1), for which published data suggest intrusion occurred between ca. 350 and 300 Ma (Dunning et al. 2002, de la Rosa 1992; Braid et al. 2010, 2012). This study focuses on the composite Gil Marquez pluton (GMP), an important component of the Sierra Norte Batholith, which intrudes the PDLZ suture zone. The GMP is comprised primarily of gabbro, granite, and granodiorite. These rocks have the potential to elucidate the late-stage tectonic evolution of this collisional setting and may provide insights into processes responsible for the generation of magmas within suture zones, as well as temporal constraints for the emplacement of the host rocks within the suture zone. The GMP contains both foliated and nonfoliated components suggesting their syn- to postcollision emplacement

(Castro et al. 1995; de la Rosa et al. 2002; Braid et al. 2012). However, (1) the timing of pluton emplacement, (2) the timing and origin of the foliation relative to regional deformation, and (3) the age relationships between the internal phases of the pluton remain poorly understood. In this paper, we provide new U–Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of magmatic, inherited, and xenocrystic zircons from different components of the GMP. Taken together with petrographic and field observations, these data provide insights into the age(s) and emplacement mechanism(s) of the GMP, its likely crustal sources, and the relationship between fabric development in both the pluton and the suture zone host rocks.

Geologic setting The late Paleozoic collision of Gondwana and Laurussia resulted from the closure of the Rheic Ocean and was a major event in the formation of the Variscan orogenic

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Fig. 4 Enclaves, Xenoliths, and the magmatic aureole of the GMP. a Ribeira de Limas quartz wacke Xenolith hosted in foliated granodiorite. The foliation of the granodiorite wraps the xenolith and displays a pressure shadow. b Mafic dyke intruding unfoliated gabbro and foliated granodiorite. The gabbro was previously intruded by the granodiorite, with a fine-grained chill margin present. c Magmatic enclave hosted in coeval granodiorite and quartz diorite providing further evidence of magma mingling processes. d Enclave hosted in foliated granodiorite, stretched in the foliation direction. e Xenoliths of Ribeira de Limas and Gil Marquez gabbro in a biotite granite. f Cordierite growth in the Ribeira de Limas host rocks, within the northern magmatic aureole of the GMP

belt and the amalgamation of Pangea (Matte and Ribeiro 1975; Franke 1989, 2000; Matte 2001; Stampfli and Borel 2002; Onézime et al. 2003; Van Der Voo 2004; Murphy and Nance 2008; Braid et al. 2010, 2011; Weil et al. 2010; Gutiérrez-Alonso et al. 2008, 2011; Martínez Catalán 2011). In southern Iberia, the Pangean suture zone is unusually well exposed and separates the parautochthonous Ossa Morena Zone (Gondwana) from the South Portuguese Zone (SPZ) (Laurussia) (Onézime et al. 2002, 2003; Nance et al. 2010; Braid et al. 2011, 2012). Abundant 355–300 Ma calc-alkaline magmatism along the southern margin of the OMZ is interpreted to be genetically linked to subduction beneath the Gondwanan margin (Jesus et al. 2007; Castro et al. 2002) marking the oblique closure of the Rheic Ocean and juxtaposition of the SPZ to the south (Quesada 1991). During this convergence, oceanic metasedimentary rocks, sedimentary mélange, mafic mélange, and flysch deposits of the PDLZ are thought to have accreted on the OMZ upper plate and deformed during continued sinistral convergence (Fig. 1; Eden 1991; de la Rosa et al. 2002;

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Simancas et al. 2005; Braid et al. 2010; Dahn et al. 2014). However, recent detrital zircon data show that the metasedimentary rocks are not derived from either the SPZ or the OMZ, suggesting the PDLZ likely had a more complex tectonic history (Braid et al. 2011). The exposed geology of the SPZ contains mostly Devonian-Carboniferous sedimentary and bimodal volcanic sequences of the Iberian Pyrite Belt (IPB), which consists from oldest to youngest: (1) upper Devonian Phyllite Quartzite (PQ) Group (Braid et al. 2012); (2) Late Famennian to middle Visean age Volcano Siliceous Complex (VSC), hosting the volcanogenic massive sulfide (VMS) mineralization (Dunning et al. 2002; Rosa et al. 2008); and (3) Upper Visean to the Serpukhovian turbiditic flysch group (Schermerhorn 1971; Oliveira et al. 1986). The PQ rocks were deposited in a sub-tidal environment in a sand bar and fan delta system on a shallow-marine continental platform. Detrital zircon data from PQ host rocks yield age populations dominated by ca. 1.8–2.3 and ca. 0.5–0.7 Ga, with minor 2.5–2.9 Ga zircons (Braid et al. 2011). The


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Fig. 5 Petrography of gabbro (ERG66). a–d Photomicrographs displaying medium-grained unfoliated gabbro, comprised of hornblende (hb), plagioclase (pl), and augite (aug) with minor apatite, zircon (zr),

titanite, skeletal Ilmenite (op), and trace chlorite (chl) and calcite (cal); the plagioclase exhibits subophitic intergrowths with augite and hornblende. e Macroscopic field sample of gabbro

Fig. 6 Petrography of quartz diorite (ERG21). a–b Photomicrographs displaying strongly foliated, coarse-grained quartz diorite, comprised of andesine (pl), biotite (bt), and hornblende with minor quartz (~10–15 %, (qtz), orthopyroxene, apatite (ap), k-feldspar, zir-

con (zr), and opaque minerals. c–d Myrmekitic texture exhibited by an intergrowth of quartz and plagioclase. e Macroscopic field sample of quartz diorite

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Fig. 7 Petrography of biotite granite (ERG11). a–b Photomicrographs displaying coarse-grained biotite granite, comprised of quartz (qtz), albite-oligoclase (pl), microcline, biotite (bt) with minor apatite

(ap), zircon, titanite, and opaque minerals. c Perthitic texture between albite (ab) and microcline (mc). d Oscillatory zoning in plagioclase. e Macroscopic field sample of biotite granite

VSC is comprised of mafic and felsic rocks, with tuffites, siltstones, purple shales, and minor limestones, with the latter dated by conodonts and cephalopods constraining its depositional age as upper Famennian-Tournaisian to upper Visean (Braid et al. 2012). The PDLZ is comprised of four polydeformed faultbounded lithotectonic units (Fig. 1; Braid et al. 2011): (1) quartz–mica schists and local quartzite mélange of the Pulo do Lobo Formation; (2) quartz wackes and phyllites of the Ribeira de Limas Formation (RDL), which contains palynomorphs interpreted as evidence for a GivetianFrasnian depositional age (Oliveira et al. 1986; Giese et al. 1988); (3) hectometer- to meter-scale internally deformed olistostromal quartzites in a polydeformed phyllite–quartzite matrix (Alajar mélange); (4) tectonically emplaced mafic blocks in a volcaniclastic and schistose matrix of the Peramora mélange (Eden 1991; Braid et al. 2011, 2012; Dahn et al. 2014). The age of the protolith of the imbricated sedimentary sequences in the Peramora mélange is constrained by paleontological data (Eden and Andrews 1990) that yield a Givetian to Famennian age (ca. 390–360 Ma). The Santa Ira Flysch (SIF) unconformably overlies all four PDLZ lithotectonic units and is comprised of relatively simply deformed graywackes, shales, and siltstones (Fig. 1; Braid et al. 2011). A late Devonian to early Carboniferous depositional age has been inferred from the spores and

acritarchs (Eden 1991), although Dahn et al. (2014) suggest that these ages may be misleading. The PDLZ is intruded by the GMP, which is a component of the ca. 350 and 300 Ma Sierra Norte Batholith (Figs. 1, 2; de la Rosa 1992; Dunning et al. 2002; Braid et al. 2011). The Sierra Norte Batholith is a voluminous composite batholith, comprised of granitic, tonalitic, gabbroic and dioritic compositions, which intruded the PDLZ and SPZ and is considered to be syn- to posttectonic with respect to the Variscan deformation in the region (Fig. 1; Simancas 1986; de la Rosa et al. 1993; Soriano and Casas 2002; Braid et al. 2010). The SNB has been interpreted to represent either: (1) the plutonic counterpart of coeval VSC exposed in the SPZ (Soler 1980; Schultz et al. 1987) or (2) late-orogenic intrusive complexes, unrelated to the VSC volcanism (Simancas 1986). Based on whole rock Rb–Sr isotopic data, de la Rosa et al. (1993, 2002) interpreted the SNB granitoids to be the products of mixing/mingling of magmas derived from melting of the lithospheric mantle and the lower crust in an active continental margin setting. Published age data suggest that the components of the GMP could range in age from 359 to 325 Ma (Kramm et al. 1991; Giese et al. 1993; de la Rosa et al. 2002; Braid et al. 2012). According to de la Rosa (1992), emplacement of the GMP occurred after the main deformation phase of the Variscan orogeny. However, the intermediate phases

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Fig. 8 Concordia plots and SEM photomicrographs of gabbro (ERG66). a Concordia plot of magmatic age of gabbro. b Macroscopic field sample of gabbro. c Concordia plot of inherited zircon population. d SEM image of a subhedral stubby inherited zircon with

heterogeneous overgrowth zoning, and spot analyses location e SEM image of an oval ‘soccer ball’ inherited zircon with heterogeneous zoning, and spot analyses location

display a steep (70–90o) east–west foliation, which is parallel to the main orogenic fabric in the host rocks and is interpreted to reflect late stages of regional deformation of the PDLZ during its emplacement and crystallization (Castro et al. 1995). In this context, intrusion and emplacement of the GMP have been interpreted to be synkinematic with respect to the final (transpressional) phase of regional deformation (Castro et al. 1995).

localities throughout the pluton (Dahn et al. 2014). The gabbro is intruded by all other phases (Figs. 2, 3), and xenoliths of gabbro are common in all phases of the GMP, especially in the biotite granite (Fig. 4e), indicating that the gabbro is the oldest component of the GMP. The foliated intermediate phases intrude the gabbro, but are crosscut by the unfoliated biotite granite (Figs. 2, 3) and by the porphyritic granite, indicating that these intermediate phases are older than both granite phases. The compositionally intermediate phases display mingling textures between mafic and felsic magmas and show evidence of hybridization, ranging in composition from quartz diorite to granodiorite. The foliated granodiorite, biotite granite, and porphyritic granite also intrude the lithologies of the PDLZ to the north, east, south, west (Figs. 2, 3), and contain abundant xenoliths and enclaves of the PDLZ metasedimentary rocks (Fig. 4a, d). Dahn et al. (2014) define the contact aureole in further detail. Some, but not all, enclaves and xenoliths are elongate parallel to the foliation (Fig. 4d). The tectonic foliations are commonly continuous with the regional foliation in the PDLZ wall rock,

Field relationships Four main intrusive phases of the GMP were documented and are from oldest to youngest (Fig. 2): (1) an unfoliated gabbro that comprises a significant portion of the GMP; (2) a foliated intermediate phase comprised of quartz diorite, tonalite, and granodiorite, which is the most abundant phase in the GMP; (3) an unfoliated porphyritic granite; and (4) an unfoliated biotite granite. The unfoliated gabbro intrudes the RDL unit of the PDLZ, and the intrusive contact is exposed in numerous

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Fig. 9 Concordia plots and SEM photomicrographs of gabbro (ERG81A). a Concordia plot of magmatic age of gabbro. b SEM image of euhedral magmatic zircon with oscillatory zoning, and spot

analyses location. c Concordia plot of inherited zircon population. d SEM image of morphology’s of zircons present in the gabbro. e Macroscopic field sample of gabbro

although local occurrences of the foliation at a high angle to the wall rock also occur (Fig. 3f, e). Porphyritic granite intrudes both the gabbro and foliated granodiorite (Figs. 2, 3) and is intruded by the biotite granite phase (Figs. 2, 3), providing evidence that the biotite granite is the youngest phase in the GMP. Additionally, alkali granite (Monte Chico Pluton) intrudes the Peramora Mélange (Fig. 2) (PDLZ) to the southwest of the town of Aroche (Dahn et al. 2014). Although an intrusive contact between alkali granite pluton and the PDLZ is exposed, its contact with the main GMP pluton is not exposed and the relationship is poorly understood (Dahn et al. 2014).

foliation development, and the regional significance of its emplacement into the PDLZ suture zone. Ten samples (Fig. 2) were selected: (1) two biotite granites (ERG11, ERG57A) and one porphyritic granite (ERG65) from the unfoliated phases; (2) one quartz diorite (ERG21) and one tonalite (ERG29) from the intermediate foliated phases; (3) four unfoliated gabbro samples (ERG66, ERG81A, ERG81C, and ERG89); and (4) one unfoliated alkali granite (ERG62) from the Monte Chico intrusion to the NW of the main GMP body. Sample descriptions Gabbro

Sampling Samples were selected from the GMP for U–Pb (zircon) age dating in order to enhance the understanding of the intrusive age of different plutonic phases, the age of

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The unfoliated gabbro is medium grained and comprised of hornblende, plagioclase, and augite with minor apatite, zircon, titanite, skeletal ilmenite, and trace chlorite and calcite. The plagioclase displays sub-ophitic intergrowths with


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Fig. 10 Concordia plots and SEM photomicrographs of gabbro (ERG81C). a Concordia plot of magmatic age of gabbro. b Macroscopic field sample of gabbro. c Concordia plot of inherited zircon

population. d SEM image of subhedral fractured inherited zircon with no zoning, and spot analyses location. e SEM image of rounded subhedral inherited zircon with no zoning, and spot analyses location

augite and hornblende (Fig. 5). ERG81A, 81C, and 89 are fine to medium grained and have the same mineralogy and textures as ERG66 (with additional minor biotite) and are interpreted to belong to the same consolidated gabbroic body. The presence of minor calcite and epidote (Fig. 5) shows the original gabbro has limited secondary alteration and/or metamorphism.

oscillatory zoning) and K-feldspar megacrysts, along with interstitial biotite and hornblende.

Intermediate foliated components The quartz diorite (ERG21) is strongly foliated, coarse grained, and comprised of plagioclase (andesine), biotite, and hornblende, with minor quartz (~15 %), orthopyroxene, apatite, K-feldspar, zircon, and opaque minerals (Fig. 6). ERG 21 exhibits myrmekitic texture, pericline polysynthetic twinning, and oscillatory zoning in the plagioclase (Fig. 6b). The tonalite (ERG29) shows a moderate tectonic foliation, is coarse grained, and is comprised of plagioclase, biotite, and quartz, with minor hornblende, apatite, zircon, and opaque minerals. The foliation is defined by the preferred orientation of euhedral plagioclase (which exhibits

Porphyritic granite Porphyritic granite (ERG65) intrudes both the gabbro and intermediate foliated phases (Fig. 2). The sample has a porphyritic texture with phenocrysts of k-feldspar and consists of quartz, plagioclase, and amphibole with minor apatite, zircon, titanite, and opaque minerals. Micrographic intergrowths of quartz and k-feldspar occur in ERG65, along with albite twinning and oscillatory zoning in the plagioclase. Biotite granite The biotite granite (ERG11, 57A) is the youngest phase and is coarse grained and comprised of quartz, plagioclase (albite-oligoclase), K-feldspar (microcline), biotite, with minor apatite, zircon, titanite, and opaque minerals. Perthitic texture and oscillatory zoning are common in plagioclase (Fig. 7).

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Fig. 11 Concordia plot, relative probability plot, and SEM photomicrographs of gabbro (ERG89). a Concordia plot of magmatic age of gabbro. b SEM image of fractured euhedral magmatic zircon with heterogeneous zoning, and spot analyses location. c Zircon his-

togram/relative probability plot. d SEM image of oval ‘soccer ball’ inherited zircon with heterogeneous zoning, and spot analyses. e Macroscopic field sample of gabbro

Alkali granite

was inserted between each sample during crushing to prevent carryover and contamination of the samples. ODM screened the crushed material to 1 mm and processed the