Allmendinger, 1986). The basement rocks of the eastem Sierras Pampeanas con- ... group from the Sierra de Quilmes and Sierra de Ancasti al shown in Table 1.
Geological Society of America Special Paper 241 1990
Granite plutonism of the Sierras Pampeanas; An inner eordilleran Paleozoie are in the southern Andes Carlos W. Rapela Centro de Investigaciones Geológicas, Universidad Nacional de La Plata, Calle 1, No. 644, 1900 La Plata, Argentina Alejandro Toselli Facultad de Ciencias Naturales, Universidad Nacional de Tucumán, Casilla de Correo 91, Miguel Lillo 205, 4000 San Miguel de Tucumán, Argentina Larry Heaman Department 01 Mineralogy and Geology, Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 206, Canada Julio Saavedra U.EL Mineralogía Geoquímica, e.S.Le., Apartado 257, Salamanca, Spain
ABSTRACf The eastem zone of the Sierras Pampeanas in central and northwestem Argentina is characterized by a Paleozoic granitoid series emplaced at dift'erent structurallevels in polymetamorphic terranes. Three main plutonic groups have been recognized on the basis of their ages and their relation to the deformational history. Gl (Late Precambrian lo Cambrian): Scattered concordant bodies of gabbros, amphibolitized norites, amphibolites, diorites, and tonalites were intruded during an early deformation and regional metamorphism (DI, MI). This group is the only one to show tholeütic affinities. G2 (Ordovician to Early Devonian): Syn-D2 (Early Ordovician) and late D2 (Middle to Late Ordovician) subgroups consist of small batholiths, plutons, and concordant bodies of tonalitic to granitic composition that were intruded during or shortly alter the D2 and M2 (amphibolite to granulite facies) episode. These granitoids, characterized by their peraluminous chemistry, contain primary muscovite, spessartite gamet, and magmatic epidote. Most initial87Sr/86Sr ratios are between 0.705 and 0.707. Some Upper Ordovician units have initial 87Sr/86Sr ratios greater than 0.710, suggesting an important upper-crustal contribution. Minimum crystallization pressures of 4 to 5 kbar are inferred from the granitoid paragenesis. The melts are inferred to be water- and volatile-rich as they produced ubiquitous tourmaline-bearing pegmatite swarms during the final stages of crystaUization. This plutonism seems to be related to the Famatinian magmatic arc that was located 400 to 800 km east of the present Peru-Chile trench. These syn-D2 and late D2 granitoids are thought to be part of an inner back-arc zone of this early Paleozoic arco Post-D2 (Late Ordovician to Early Devonian) granitoids consist of large batholiths (e.g., Achala and Velasco batholiths) and widespread smaller plutons that were emplaced during the later stages of the early Paleozoic (Famatinian) orogeny. Common features of the larger bodies include the presence of muscovite, aluminum silicates, and a dominant porphyritic phase characterized by microcline microperthite megacrysts in a groundmass of medium- to coarse-grained quartz, oligoclase, and biotite. Mioeralized pegmatites (Li, Be, Nb, Ta) and U deposits related to post-D2 granites have high levels of LIL (large ion lithophiles: K, Rb, LREE) and HFS (high field strength: Y, Nb) elements. The post-D2 group exhibits compositional features similar to alkali-calcic arc granitoids developed at a great distance from the subduction margin, in Rapela, C. W., Toselli, A., Reaman, L., and Saavedra, J., 1990, Granite plutonism of the Sierras Pampeanas; An inner cordilleran Paleozoie are in tbe southem Andes, in Kay, S. M., and Rapela, C. W., eds., Plutonism from Antarctica to Alaska: Boulder, Colorado, Geological Society of America Special Paper 241.
77
78
c. W. Rape/a and Others
the later stages of are evolution. G3 (Late Devonian to CarboniJe,ous): Homblendebiotite alkalic-like plutons culminate granitoid activity in the Sierras Pampeanas. These granitoids have high levels of HFS and LIL elements, and some have high initial 87Sr/86Sr ratios (0.710). The G3 granitoids are interpreted to be within-plate plutons indirectly related to the beginning of late Paleozoic subduction on the Gondwana margino INTRODUcnON
\ \
The Sierras Pampeanas in central and northwestem Argentina (Fig. 1) are a seriesof north-south-trending mountain blocks separated by tectonic valleys.The presentlandformsare the result of uplift on reverse faults during the upper Tertiary part of the Andean orogeny. The limits of the structural domain of the Sierras Pampeanas province are coincident with a nearly horizontal segment of the Nazca plate at the same latitude (e.g.,Jordan and Allmendinger, 1986). The basement rocks of the eastem Sierras Pampeanas consist primarily of phyllites, banded schists,gtleisses,and less commonly, marblesand amphibolites.The metamorphicgrade ranges from greenschist to amphibolite and, in a few cases, granulite facies.This metamorphic complex is intruded by numerous granitoids and less abundant mafic and ultramafic rock bodies. As recognized by Caminos (1979), the easOOmzone of the Sierras Pampeanas is distinct from the westem Sierras Pampeanas (Fig. 1). The westem zone is characterized by an absence of major batholiths; abundant ultramafic, mafic, and carbonate-bearing rocks; and metamorphic assemblages that generally indicate higher-pressuremetamorphism in the west than in the east. Although the Famatina sysOOm, located between the eastem and westem zones (Fig. 1), has not been considered part of the Sierras Pampeanas (Aceñolaza and others, 1972; Caminos, 1979; de Alba, 1979), the characteristicsof its early and middle Paleozoie magmatism can be correlated with Pampean igneous evolution. A remarkable feature of the Famatina sysOOmnot found in the Sierras Pampeanas is the voluminous cale-alkaline volcanics interlayered with marine fossiliferous sediments of Early Ordovieian Arenigian age (Aceñolaza and Toselli, 1984). Discontinuous outerops of Ordovician volcanics appear to the north ofthe Famatina Range, reaching the Argentine Puna (Aceñolaza and Toselli, 1984; Coira and others, 1982; see Fig. 1). Detailed studies in Sierra de Maz (Kilmurray and Dalla Salda, 1971a); Cerro Valdivia (Kilmurray and Dalla Salda, 1971b); Sierra de Quilmes (Rapela, 1976a); Sierra de San Luis (Kilmurray and Dalla Salda, 1977); Sierra de Ancasti (Willner and Miller, 1982);Sierra de Pie de Palo (Dalla Salda and Varela, 1982) and Sierra de Córdoba (Dalla Salda, 1984; 1987) show clear struetural evidence of several major metamorphie-tectonie episodes.An attempt to correlate the tectonicand igneoushistory of the central (Sierra de Ancasti) and northem (Sierra de Quilmes) regions of the eastem zone is presented in Figure 2. During the second metamorphie-tectonie event in the Early Ordovician (M2 and D2, see Fig. 2), the regional metamorphie
grade reaehed a maximum in some sectors (granulite facies partíal melting of metagraywackes and metapelites). Three main granitoid groups are recognized in the eas Sierras Pampeanas usingchronological eriteria (Fig. 2): G1, I Precambrian to Cambrian granitoids; G2, Ordovician to E Devonian (Famatinian) granitoids; and G3, Late Devoniar Carboniferous granitoids. A further subdivision of the G2 grc based on temporal relations with the second and m~ metamorphic-tectonie event (e.g.,syn-D2, laoo-D2,etc.) has bt used to describe the field occurrences of these granitoids (G. zález Donorino, 1950; Rapela and others, 1982). Gl (Late Precamb,ian lo Camb,ian granitoids) Small bodies of gabbro, homblendite, diorite, and tonal intruded the metamorphie complex before and during an eal deformational phase and regional metamorphism (D 1 and M Fig. 2) in the Sierra de Ancasti (e.g., Sehalamuek and othe: 1983; Lottner, 1986;Lottner and Miller, 1986). Relictsof this( event are aIso widespread in the northem sector of the Sierr; Pampeanas (e.g.,amphibolitizednorites of the Sierra de Quilme Toselli and others, 1978). Selected ehernical analyses of the G group from the Sierra de Quilmes and Sierra de Ancasti al shown in Table 1. G2 (Ordov;cian
to Early Devonian
granito;ds)
The most extensive granitoid group of the Sierras Pampea nas was emplaced during the deformational and metamorphil events (Fig. 2) known as the Famatinian eycle of the Famatiniar orogeny (Aceñolaza and Toselli, 1976). The compositional an, field eharaeteristicsof the Famatinian granitoids depend on theil emplacement relation with the most penetrative deformation (D2). Syn-D2 g,anitoids. Th~ small concordant bodies have typically lenticular outlines (e.g., Tolombón tonalite, Rapela, 1976a; La Pampa-Unquillo granitoids, Knüver and Miller, 1982). They commonly have a foliation orienOOdparallel to the regional fabric in the country rocks. Although most are biotite and two-mica (peraluminous) tonalites, their modal composition can vary from tonalite to granite (Fig. 3 and Table 1). Late-D2 g,anitoids. These granitoids vary in size from rather homogeneous plutons (e.g., Cuehiyaco granodiorite,Rapela, 1976b; El Alto granite, Reissinger, 1983; Loma Pelada gran-
Granite plutonism
79
01 the Sie"OS Pampeanas
j i
i i i \c(
\z H= LLI~Z LLI -( CJ ::E: a: ~c(
,
ItA~AJ Faja eruptiva 01 the Puna
~vvVvlOrdovicianvolcanism Et:;)Granitoids rocks
§
;,--- Caminos 1979
,
LA RIOJA
SAN
'
JUAN
·
32°S
Metamorphic rocks Limits between zones
~
Reverse faults
~
EASTERN SIERRAStJ~ PAMPEANAS\~J ~+
\ ~
\~~ ,\:~
~
-
-
+ ++
~
1/ h
'\
~
\~
e
4
o I
50 I
100
~~ SAN LUIS Figure 1. Generalized geologica1map of Lower Paleozoic rocks in central and northwestem Argentina. 1, Achala batholith; 2, Sierra de Córdoba; 3, Alpa Corral batholith; 4, Sierras de San Luis; 5, Sierra Norte batholith; 6, Sierra de Velasco; 7, Famatina system; 8, Sierra de Ancasti; 9, Sierra de Ambato; 10, Capillitas batholith; 11, Cumbres calchaquíes; 12, Sierra de Quilmes. Principal structural features after Jordan and Allmendinger(1986).
dib,
150km.
80
c. w: Rape/a ami Others Carboniferous
Late Precambrian
01. Isoclinal foldlng. Development of planar fabrico 02:NNW-SSE folding. Schistoaity and foliation oblique to 01 structurea. 1>,).Large scale flexures related to batholltic emplacements in upper levels. D4>Folding.Chlorite growthdeveloped a 04 schiatoaity. Rotation of Ma porphyroblasts. Oso Mylonitization.
Deformation (a)
Metamorphism
Me
M1.Medium to low grade regional metamorphlsm. M2. Medium to high grade lupper amphibolite and granulite facies) regional metamorphlsm. Widespread synkinematic anatexis inthe cord-Kfap-siU Ma. Retrograde chlorite and muscovite. Mc: Local contact metamorphism related to late and post-kinematic Ipost-D2) granitic emplacement.
(b) SedimentatlonChlefly eugeosyncll Sediments-
Rb-Sr Isochron ages (granitolds roeks) O.71S~ (e)
Open symbols: Sierra de Ancaati. 1 : Sierra de Quilmea 2.6.Cumbres Calchaqules a. Achala batholith 4. Velasco batholith S.Caplllitas batholith
:! :5
0.710~
zone.
...
o.705~ ~ IIHn .0.$0... o 00 O 0eI'
00
!!.!!.EBMi
65
70
75
Si02(%) Figure 7. Si02 variation diagram for Nb, Y, Zr, La, te, Rb, and K20 from selectedgranitoid unitsofthe Sierras Pampeanas. Symbols as in Figure 5. Sources of data: Acbala batholith and Capillitassyenogranite (Rapela and Heaman, 1982); Cerro Amarillo granite, Cafayate granite, Tolombón tonalite (Rapela, unpublisbed XRF determinations); Loma Pelada granite (Saavedra, unpublisbed Y, Rb, and Zr XRF determinations).
Granite plutonism 01 the Sierras Pampeanas
87
100
Nb (ppm)
WPG VAG+syn-coIG
+ +
. 10
-
~ Syn D2
ORG
Sierra de Quilmes (Tolombon tonalite,
G2 . Late-D2 } Cafayate granite) x Post -D2 Sierra de Cordoba { ~Achalabatholith) + Cumbres Calchaquíes (Cerro Amarillo granites) G 3 e Capillitas Batholith [ (syenogranitic facies) 1.0 10
100
y (ppm) Figure 8. Nb-Y discrimination diagram for volcanie are granites (VAG), syn-collision granites (synCOLG), within-plate granites (WPG), and ocean-ridge granites (ORG) (Pearce and others, 1984). Source of data as noted in Figure 5.
subduction-related batholiths. The remnants of the Ordovician volcanicscould define the approximate location of the axis ofthe Famatinian arc (Fig. 1). Some plutons of the post-D2 G2 group show geochemical affinitieswith anorogenic granitoids (e.g., Martin and Piwinskii, 1972, 1974) in noncompressional tectonic settings.Nevertheless, these granitoids are not true alkalic granites (Rapela, 1982). Instead, they are high-K granitoids that are enriched in LIL and HFS elements.On the Nb versosY and Rb versosNb+y discriminationdiagrarns(Figs.8 and 9) of Pearce and others (1984), they plot in the within-plate and collision fields.Their characteristics are like those of alkali-calcic granitoids that develop at great distances from the subduction margin in the later states of arc evolution(Brown and others, 1984). Late Devonian to Carboniferoos plutons (e.g., Capillitas syenogranite,Cerro Amarillo granite, G3 Group) primarily plot in or near the within-plate fieldon the discrimination diagrarnsin Figures 8 and 9. These granites are not related to the Famatinian compressivephases and are probably geneticallydistinct. During the late Paleozoic,a magmatic arc was located west of the Sierras Pampeanas in the high Andean Cordillera and the Coast Range (cf.,Parada, this volume). Therefore, the G3 group may represent intracratonic plutonic activity of a new subduction regime that was established on the Gondwana margin during the late Paleozoic.
Models for the tectonic evolution of the Paleozoic terranes in northwestem and central Argentina all suggest an eastwarddipping subduction regime,but the proposed modelsare different in many important aspects. Willner and others (1985) consider that the Cambrian Pacific margin of Gondwanaland changed from a passive to an active margin, with a flat subduction zone that became somewhat steeper north of present-day 26°S. The position of the trench was not far from that of the Andean (Mesozoic-Cenozoic) regime. A major drawback of this model is that magmatism in modem flat-slabanalogs in the Andes between 28 and 33° (Kay and others, 1987) is very different from Pampean plutonismo
The model developed by Ramos and others (1984, 1986) suggeststhat an early Paleozoictrench was located on the westem side of the modem Precordillera in Argentina on the eastem side of the present Andean chain. The 300-km-wide belt of rocks presentlybetween the Precordilleraand the Chilean trench would have been added (Chilenia terrane) to the South American conti. nentby the LateDevonian(Ramosand others,1984,1986).
The lack of precise isotopic and geochemical data, in general, but mainly in the westem Sierras Pampeanas prohibits further elaboration of these proposed tectonic models. The petrological and geochemical characteristics of the OrdovicianSilurian granitoids in the Sierras Pampeanas suggest that they represent a back-arc counterpart of the Famatinian early Paleo-
88
C. Jv. Rapela and Others
1000
syn-COLG
Rb (ppm)
WPG x
.
VAG
ORG
10
Sierra de Quilmes (Tolombón tonalite, G2 . Late -D2 } Cafayate granite) x Post-D2 Sierra de Cordoba (Achala batholith) {
~ Syn-D2
G
+ Cumbres Calchaqules (Cerro Amarillo granite) 3 . Capillitas Batholith { (syenogranitlc facies) 100
10
1000
y + Nb(ppm) Figure 9. Rb-Y+Nb discrimination diagram for volcanie are granites (VAG), syn-coIlisiongranites (syn-COLG), within-plate granites(WPG), and ocean-ridgegranites(ORG) (Pearce and others, 1984). Source of data as noted in Figure 5.
zoic arc. This pre-Andean subduction regime developed a wide north-northwest/south-southeast-trending magmatic arc that is located some 400 to 800 km east of the modero Pero-Chile trench.
REFERENCES CITED Aceñolaza, F. G., and Toselli, A. J., 1976, Consideraciones estratigráfi~ tectónicas sobre el Paleozoico inferior del Noroeste argentino: 11Congrl -
ACKNOWLEDGMENTS
Latinoamericano de Geología, Tomo 2, p. 755-763. , 1984, Lower Ordovician volcanism in northwest Argentina, in Brut, D. L., OO., Aspects of the University
Aceñolaza,
This paper benefited from helpful reviews by Calvin Miller, Travis Hudson, Denis Shaw, and Victor Ramos. At various stages this work has been supported by the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina (CONICET), and the National Research Council of Canada (Grant A0155 to D. M. Shaw and Grant 2659 to R. H. McNutt). This work is a contribution to the IGCP Project 249.
of the Ordovician
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