PALEOPROTEROZOIC METAMORPHIC EVOLUTION OF TONALITIC GNEISSES AND ASSOCIATED MIGMATITES FROM THE CERRO EL CRISTO AREA OF THE TANDILIA BELT, RÍO DE LA PLATA CRATON

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

Heidelberg

Congreso; The 23rd Latin American Colloquium on Earth Sciences; 2014

Universität Heidelberg

The Tandilia belt in central-eastern Argentina represents the southernmost portion of the Paleoproterozoic Río de la Plata craton. As P-T paths were rarely estimated for metamorphic rocks of this craton, we selected the area of Cerro El Cristo for studying its P-T evolution. In this area, which is located ~65 km southeast of the town of Tandil, three main lithologies can be recognized: (1) amphibole(Am)-biotite(Bt) gneisses, (2) garnet(Grt)-Bt-bearing migmatites and (3) metamorphosed pegmatites and granites. In order to determine the P-T evolution of such rocks we applied the PERPLE_X software package for calculating P-T pseudosections for a gneiss and a migmatite in the 11-component system Si-Ti-Al-Fe-Mn-Mg-Ca-Na-K-O-H. Information on the experienced P-T conditions was obtained by electron microprobe analysis of the minerals and the contouring of the pseudosections by isopleths of chemical parameters of these minerals. The selected mesosome of a stromatic metatexite (0224) contains about 30 vol.% quartz (Qtz), 20 vol.% each of K-feldspar and plagioclase (Pl), 15 vol.% Bt and almost 10 vol.% Grt, which formed porphyroblasts by a peritectic melt reaction. Grt core compositions of pyr4.5 (gro+andr)10 spes6 alm79.5 crystallized at about 5.4 kbar, 670 °C, and a melt proportion of less than 40%. The outermost Grt rim of pyr3.5 (gro+andr)17 spes6 alm73.5 points to about 6.8 kbar and 645 °C, where the proportion of melt was less than 30%. As we assume that only Grt rims were in equilibrium with the analyzed bulk-rock, we subtracted the Grt cores from the bulk rock, but obtained similar P-T conditions for the final garnet crystallization (6.9 kbar and 642 ºC). The modal composition (vol.%) of the modeled gneiss (0324) is: Qtz(28)+Pl(32)+Bt(24)+Grt(5)+Am(7)+Accessories(4). Zoned Grt porphyroblast also allowed us to derive two metamorphic stages. Core compositions of pyr6.5(gro+andr)26spes12alm55.5 yielded P-T conditions of 8.9 kbar and 640 °C at solidus state. Grt rim compositions of pyr4.5 (gro+andr)33 spes12 alm50.5 resulted in 9.6 kbar and 580 °C. The derived P-T path is characterized by P increase from 5.5 to 9.5 kbar at cooling by nearly 100 °C. We interpret this unusual anti-clockwise P-T path by tectonic transport from mid-crustal to deeper levels along a major suture zone, eventually the contact zone between the Rio de la Plata craton and a colliding crustal plate in the south. The cooling of the studied rocks, being part of the Rio de la Plata craton, was caused by relatively cold, fastly buried material of the downgoing plate.