High-grade metamorphism of the Sierras de La Huerta and Valle Fertil, San Juan province, NW Argentina

MOGESSIE, ABERRA; GALLIEN, FLORIAN; SCHNEIDER, ISABELLA; CASTRO DE MACHUCA, BRIGIDA; BJERG, ERNESTO; DELPINO, SERGIO; PREVILEY, LORENA; PONTORIERO, SANDRA; MEISSL, ESTELA

San Salvador de Jujuy

Congreso; XVII Congreso Geológico Argentino; 2008

Asociación Geológica Argentina

The Sierras de La Huerta and Valle Fertil located in the San Juan Province, NW Argentina, belong to the Western Sierras Pampeanas.They consist of a Proterozoic gneissic basement intruded by gabbroic to granodioritic rocks in Early to Middle Ordovician times (Famatinian Orogeny). The abundant gneissic rocks which are strongly deformed and metamorphosed at granulite facies conditions are composed mainly of quartz - plagioclase - biotite - garnet - sillimanite - cordierite +/- potassium feldspar +/- spinel +/- magnetite. The tonalitic - granodioritic, gabbroic - dioritic rocks are considered to provide the heat source for a regional contact metamorphism at about 6 - 7 kbar, with temperatures in excess of 860 °C. Applied geothermobarometry based on the mineral parageneses (Grt - Sil - Qtz - Pl) in the high-grade gneisses result in a pressure of 6 -7 kbar (GASP) at temperatures of 700 - 800°C(garnet-biotite-, cordierite- spinel- thermometry). For the coronitic gabbros the temperatures range between 800 - 900 °C (Cpx-Opx thermometry) at pressures around 6-7 kbar. The metagabbros show well preserved igneous textures. They are composed of Pl (An70-100) + Opx (En85-60Fs15-40Wo0-1>) + Cpx (Wo50-40En45-50Fs5-10) + Ol (Fo58-70) + Ilm + Mag. Spinel (an Al - rich hercynite) is found in the matrix, in symplectites with amphibole and as exsolution lamellae in clinopyroxene. The Cr content in spinel is variable and reaches 10 wt% Cr2O3 in the exsolutions in clinopyroxene. Amphibole is mostly pargasitic hornblende [K0.07Na0.5Ca1.9Mg3.0Fe2+0.9Fe3+0.12Al2.5Ti0.09Si6.2O22(OH,F,Cl)2. Coronitic textures, which developed at olivine-plagioclase phase boundaries are very common, composed of Ol - Opx - Cpx-Am-Spl symplectites - Pl suggesting the H2O absent reaction Ol + Pl = Opx + Cpx + Spl, and the H2O consuming reaction Ol + Pl + H2O = Opx + Am + Spl. They are interpreted to be a result of slow cooling of the basic-ultrabasic bodies under low-granulite to high-amphibolite metamorphic conditions. The presence of large amounts of symplectites (spinel-amphibole, clinopyroxene-spinel) in most of the studied coronitic gabbro samples may indicate an open system where diffusion and transport of elements has been possible. Abundant magnetite symplectites in olivine and magnetite indicate increasing oxygen fugacity quite above the QFM-buffer. Metamorphic peak coincided with corona formation and was related to the Famatinian magmatic intrusive activity. Later, metagabbros were locally cut by narrow ductile shear zones associated with mylonitization under high-to medium-grade metamorphic conditions. Evidence of ductile deformation in cordierite and chessboard extinction in quartz from paragneisses suggests temperatures above 700 °C at a pressure > 5 kbar for the mylonitization event. In the Sierra de Valle Fertil, siliceous marbles of various thicknesses occur interlayered with granulite gneisses and migmatites. Marble outcrops in contact with metagabbroic rocks contain granulite facies parageneses represented by sapphirine + spinel + corundum, surrounded by clinochlore and tremolitic amphibole. This parageneses is confined to peraluminous pockets of about 400um diameter and could be interpreted as an originally fairly closed system. Several reaction textures can be observed in the dolomite-free scapolite - wollastonite - grandite - clinopyroxene - quartz - calcite marbles. These are quartz-calcite replacement textures around wollastonite, and grandite coronas developed between scapolite-calcite and wollastonite- scapolite. These textures are consistent with isobaric cooling from ~ 850°C to 750°C at 6.5 kbar.