The Conlara Metamorphic Complex: A Pampean metamorphic event in the Sierra de San Luis, Argentina

LÓPEZ DE LUCHI, M.G.; CERREDO, M.E.; STEENKEN, A.; SIEGESMUND, S.; WEMMER, K.; MARTINO, R. D.

San Salvador de Jujuy

Congreso; XVII Congreso Geológico Argentino; 2008

Asociación Geológica Argentina

The Sierra de San Luis consists of NNE trending metamorphic complexes named Nogolí, Pringles and Conlara (Sims et al. 1997). These units are separated by the two narrow phyllite belts of the San Luis Formation (SLF) (Prozzi and Ramos 1988). The Conlara Metamophic Complex (CMC) is the easternmost of these complexes. The western margin of the CMC is affected by the mylonitic Río Guzmán shear zone whereas its eastern margin to the Sierra de Comechingones (Sims et al. 1997) is controlled by the Las Lajas and Guacha Corral shear zones. López de Luchi (1986) and López de Luchi and Cerredo (2001) proposed a polyphase evolution of the metamorphic series that encompasses at least three ductile deformation phases which produced foliations, banding, folding and localized high temperature shear zones. The penetrative, dominant foliation, S2, locally associated with banding in some metaclastic schists is related to the second deformation (D2). An earlier, relic S1 is preserved in microlithons as biotite polygonal arcs or as thin folded leucocratic veins with S2 axial plane cleavage. The penetrative S2 foliation is in turn affected by upright, generally isoclinal, D3 folds with N-NE/S-SE axial planes. Bt-Qz mylonitic shear zones are related with D3. The metamorphic series of CMC comprises both metasedimentary and metaigneous rocks. The former are dominated by metagreywackes and scarce metapelites with lesser amounts of tourmaline schists and turmalinites. Likely controlled by a D3 related major structure (fold?) distinct metasedimentary assemblages are recognized on cartographic plan: in the central northern areas of CMC fine-grained schists, banded medium-grained schists with subordinated interlayers of tourmaline schists dominate, whereas along the eastern and southeastern areas mediumgrained schists (either banded or not) with rare amphibolite interlayers crop out. The central fine to medium-grained schist association is characterized by S2 oriented biotite-quartz-plagioclase±muscovite ± garnet assemblages, whereas the eastern medium-grained schists bear the non-diagnostic biotite (S2 oriented)-plagioclase assemblage, rarely garnet occurs as a prograde phase displaying post S2 plagioclase replacement. Muscovite blasts overprinting S2 planes are widely distributed on M domains of all schist types. The assemblages in the metaclastic rocks of the CMC are characterised by the paucity of diagnostic paragenesis, in fact biotite (and rare relic garnet) is the only AFM phase present. The determination of metamorphic grade is largely based on the use of the garnet in the banded schist together with calculations on the Ti thermometer in biotite. The absence of typical diagnostic minerals is attributed to the composition of protolith rocks because semipelitic rocks have unsuitable composition for aluminosilicate minerals to form. The mineral assemblage observed within the semipelitic banded schists is typical with biotite +quartz + plagioclase+/- garnet. The rocks are subaluminous (i.e. they mostly plot below the chlorite projection in the AFM diagram), Mg-rich (bulk XFe between 0.3 and 0.47) which precludes the formation of both aluminosilicate phases and Fe-rich minerals (except for some rare samples with XFe > 0,5 (i.e 59 - 0.51- and 103 -0.518). Mineral composition data indicate plagioclase of dominant oligoclase composition (An 25-32), slightly Fe-rich biotites (XFe 0.49 to 0.56); rare garnet in banded schists is almandine-rich (0.67-0.72) with spessartine contents between 0.101 and 0.118. The metaigneous components of CMC encompass basic terms (likely emplaced as sills within the sedimentary pile) and granitic rocks. These in turn, belong to two distinct groups: an earlier one which underwent all the prograde metamorphic evolution of the CMC being emplaced between D1-D2, and a later one which were emplaced later synkinematically with the late D3 deformation and displays a down-T path of deformation microstructures (from the submagmatic stage to low-T greenschist facies conditions). The earlier granitic stage is represented by orthogneisses, and locally migmatites, very widespread within the CMC. They mainly outcrop to the south of Renca Batholith and in an extended northeastern belt, mostly hosted in the medium-grained schists. Rocks are mainly tonalitic to granodioritic and show an intimate interpenetration with host schists that appear as lenses. Discrete orthogneissic bodies rarely reach some tens- meters, more often they form decimeter sized layers interbedded with the host schist displaying migmatic structures. Rocks are typically banded, generally medium (to coarse ??) grained with irregular distribution of S2 oriented biotite (XFe 0.52-0.55) which locally forms mm to cm large clots, plagioclase is slightly more sodic than in host schists (An 23-26); rarely garnet occurs surrounded by retrograde post- S2 plagioclase coronas. Minor amounts of S2 aluminum silicate (generally sillimanite) locally occur in association with albite (An 2-8) indicating that conditions of first sillimanite isograde (upper amphibolite facies) have been attained. The later granitic stage comprises Early Ordovician monzogranite to tonalitic plutons (i.e. La Tapera, El Peñón, El Salado(?)) and a group of large granitic pegmatites which share comparable deformation microstructures allowing to infer a synchronic emplacement. These bodies commonly display a folded cartographic pattern (i.e La Tapera pluton) with D3 NE- trending axial planes (López de Luchi and Cerredo 2001). Microstructures within La Tapera (López de Luchi and Cerredo 2001) and El Peñón (Steenken et al. 2005) granitoids argue for the continuity from magmatic to high temperature solid-state deformation probably indicating the syn-kinematic emplacement of the batholith with respect to D3. These igneous bodies enclose xenoliths of host schists displaying D3 folds; long axes of xenoliths have a NE trend parallel to the dominant foliation in the host suggesting a magmatic component of flow. Deformation/metamorphism relationships are consistent for the different components of the CMC with peak metamorphic conditions being nearly synchronous with the second deformation (D2), followed by a decompression path (as indicated by plagioclase coronas around garnet both in banded schists and orthogneisses) related to D3. Late deformation events are represented by non penetrative, discontinuous low-T shear zones within the schists and metagranitoids, especially conspicuous in the Sierra de San Felipe and characterized by sericite-quartz ± chlorite finegrained discontinuous shear bands. Geochronological constraints suggest that D1/2 history took place during the Pampean Orogeny (cf. Steenken et al. 2005). Steenken et al. (2006, 2007) indicate that the sedimentary precursors of the Conlara Metamorphic Complex were deposited at ~590 Ma. Folded xenoliths of the D2 banded schists are observed in pervasively solid-state deformed granitoids like the Early Ordovician (497 ±8 Ma) the El Peñón batholith. This evidence argues against the assumption that the entire evolution of the Sierra de San Luis could be addressed to post-Pampean events (Whitmeyer and Simpson 2004). The restricted low greenschist facies overprint is comparable (both in strike and composition) to the mylonitic paragenesis characteristic of Río Guzmán Shear Zone. This age of the Devonian event which closely matches the regional biotite cooling ages within de CMC would have been responsible for final unroofing of the complex.