Inversión metamórfica en el orógeno famatiniano de la Sierra de San Luis, Argentina

Abstract

P-T conditions of Famatinian metamorphism (c. 465 Ma) were determined along a cross section in the central southern part of the Sierras de San Luis, Eastern Sierras Pampeanas. A continuity from low to high grade metamorphism is recorded here with an inverted disposition of metamorphic zones, i.e., higher grade rocks structurally overlying lower grade rocks. Foliation dip decreases accordingly downward in a fan-like arrangement. Shear zones are boundaries to the main metamorphic zones. The cross section is located on the eastern side of a large scale transpressional double vergence positive flower structure which involves most of the Sierra de San Luis. Calculated peak P-T values are: 790º and 800ºC and 7.2 to 8.9 kbar for the gneiss zone, 600º to 650ºC and 5.9 to 6.4 kbar for schist zone, and 550 ºC and 4.7 kbar for the transition schist – phyllite zone. Highest grade rocks show a clockwise P-T path with a nearly isothermal decompression to P values which are similar within error to those of the lower grade rocks. The latter suggests that metamorphic inversion was largely synmetamorphic and that it was driven by movement along shear zones and ductile flow within domains. Reactivation of shear zones is invoked however to explain some mistmach of P values particularly among the lower grade zones.