GEOTHERMOBAROMETRY OF A SILLIMANITE-GARNET-BEARING GRANITE OF THE CERRO SAN VERÁN, SOUTHEASTERN TANDILIA BELT

MARTINEZ JUAN CRUZ; MASSONNE, HANS -JOACHIM; DRISTAS A. JORGE; GRAFF, AILÍN AYELÉN

Córdoba

Congreso; XIX Congreso Geológico Argentino; 2014

Asociación Geológica Argentina

The igneous-metamorphic basement of the Tandilia belt in central-eastern Argentina is predominantly composed of high-grade metamorphic rocks (gneisses, migmatites and granulites) as well as magmatic-arc derived plutonic rocks which originated in a relatively short-lived Paleoproterozoic orogeny between 2.25 and 2.08 Ga (Hartmann et al., 2002; Cingolani et al., 2002; Pankhurst et al., 2003, Massonne et al., 2012). Pegmatites and leucogranites, spatially related to a migmatitic complex, are partly mylonitized syn- to post-tectonic phases of the Paleoproterozoic orogeny (Dalla Salda et al., 1988). A small leucogranite pluton (1.5 x 1.5 km), emplaced in garnet biotite-bearing migmatites, make up the San Verán hill located ~65 km southeast of the town of Tandil. This leucogranite is composed of K-feldspar + quartz + plagioclase ± muscovite ± biotite ± garnet ± sillimanite + zircon and opaque minerals. The granite is medium to coarse grained, often with pegmatitic appearance. Under the microscope, a fine-grained dynamic recrystallization matrix (up to 40 vol.%) is recognized, both types of feldspar porphyroclasts show core and mantle texture and besides ondulatory extinction, quartz porphyroclasts may display chess-board texture. Perthitic K-feldspar shows crosshatched crystal twinning whereas plagioclase is commonly saussuritized. The peraluminous character of this rock is confirmed by the presence of prismatic sillimanite (rarely up to 7 vol.%) and garnet (occasionally up to 15 vol.%) crystals in some igneous facies. The sillimanite-garnet-bearing facies was studied in detail in order to constrain its conditions of formation. For doing this, the selected bulk-rock (0624) of this peraluminous granite was evaluated in the 11-component system Si-Ti-Al-Fe-Mn-Mg-Ca-Na-K-O-H with the PERPLE_X software package through modeling of a P-T pseudosection. The pseudosection was calculated for the range of 0.1 -1.2 GPa and 450 - 800 ºC and contoured by isopleths of parameters such as molar fractions of end members in garnet and Si per formula unit in muscovite. Xenomorphic garnet crystals are almandine rich and chemically different from garnet in the country rock, migmatites and gneisses. Element mapping with the electron microprobe shows that garnet crystals are almost homogeneous, slightly richer in pyrope at their cores with composition of: pyr6.7 (gro+andr)2 spes5 alm86.3 and pyr5.2 (gro+andr)2 spes5 alm87.8 for core and rim, respectively. Muscovite is characterized by a low Si content around 3.10 - 3.07 a.p.f.u. According to the contouring in the P-T pseudosection, the earliest crystallization of garnet was registered in a field where sillimanite and muscovite coexist at 0.55 GPa and 680 ºC with a melt proportion of less than 12 vol.%. A short cooling path at decreasing pressure was determined through garnet rims, still at a temperature above the solidus curve: 0.52 GPa and 665 ºC. Final crystallization of melt occurred at temperatures between 640 and 630 ºC at upper crustal levels constrained by isopleths of the slightly variable Si p.f.u. in muscovite. The history obtained by field and petrographic observations and the geothermobarometric thermodynamic evaluation suggests that the studied sillimanite-garnet-bearing leucogranite was emplaced at middle to lower crustal conditions as a result of syn- to postcollisional anatexis of surrounding rocks.