Mineral deformation mechanisms in granulite facies, Sierra de Valle Fértil, San Juan province: evelopment conditions constrained by the P-T metamorphic path

Abstract

In the Sierra de Valle Fértil, evidence of granulite facies metamorphism have been preserved either in the constitutive associations as in deformation mechanisms in minerals from biotite-garnet and cordierite-sillimanite gneisses, cordierite and garnet-cordierite
migmatites, metagabbros, metatonalites-metadiorites and mafic dikes. The main recognized deformation mechanisms are: 1)
quartz: a) dynamic recrystallisation of quartz-feldspar boundaries, b) combination of basal <a> and prism [c] slip; 2) K-feldspar: grain boundary migration recrystallisation; 3) plagioclase: combination of grain boundary migration recrystallisation and subgrain rotation recrystallisation; 4) cordierite: subgrain rotation recrystallisation; 5) hornblende: grain boundary migration recrystallisation.
Preliminary geothermometry on gabbroic rocks and the construction of an appropriated petrogenetic grid, allow us to establish temperatures in the range 800-850 C and pressures under 5 Kb for the metamorphic climax. Estimated metamorphic peak conditions,
preliminary geothermobarometry on specific lithologic types and textural relationships, together indicate an counter-clockwise P-T path for the metamorphic evolution of the rocks of the area. Ductile deformation of phases resulting from anatexis linked
to the metamorphic climax indicates that the higher-temperature ductile event recognized in the study area took place after the metamorphic peak. Evidence of ductile deformation of cordierite within its stability field and presence of chessboard extinction
in quartz (only possible above the Qtzα/Qtzß transformation curve), both indicate temperatures above 700 C considering pressures greater than 5 Kb. Based on the established P-T trajectory and the characteristics described above, it can be concluded
that deformation mechanisms affecting the Sierra de Valle Fértil rocks were developed entirely within the granulite facies field.