Metamorphic evolution of high-grade metagabbros from La Huerta range, Western Pampean Ranges, San Juan, Argentina

CASTRO DE MACHUCA, BRÍGIDA; MORATA, DIEGO; ARANCIBIA, GLORIA; BELMAR, MAURICIO; PONTORIERO, SANDRA

Florencia

Congreso; 32° International Geological Congress; 2004

Union of Geological Sciences and the Mediterranean Consortium

Metaluminous igneous rocks of Famatinian age (Early-to-Mid Ordovician) are well exposed along the eastern flank of La Huerta range, Western Pampean Ranges, San Juan province, Argentina. As several authors pointed out, magmatic activity and regional metamorphism in the area may be correlated with an active continental margin due to the accretion of the Precordillera Terrane to the southwestern margin of Gondwana. Metatonalites and diorites are predominant, with lesser amounts of metagabbros and ultramafic rocks suggesting a deep level of exposure of the magmatic arc. The igneous rocks intrude a Proterozoic-Lower Paleozoic metamorphic basement made up of paragneisses which grade into migmatites farther north, amphibolites and marbles of medium-to-high grade. The magmatic belt has a NNW-SSE trending. This study discusses the changes experienced by the metagabbros and the P-T conditions during the regional metamorphic evolution. Metagabbronorites and olivine-gabbros form small bodies within the magmatic complex. They are massive, dark-colored and coarse-grained, sometimes displaying relic igneous textures difficult to differentiate from regular mosaic texture due to recrystallization. Unzoned Mg-rich olivine (compositional variations ranging from Fo83 to Fo75 in different samples) and Ca-rich plagioclase (An100-99) appear as igneous relics, with accessory ilmenite and magnetite. Abundant coronas of orthopyroxene (En82-76Fs18-23Wo0-1)+Al-rich spinel and clinopyroxene (En47-44Fs6-8Wo50-46)+Al-rich spinel rimming olivine are present. Ca-rich amphibole (Mg-hastingsite and pargasite) is present as external rim of these coronas. At Pl-Ol boundaries the complete Ol->Opx+Spl->Cpx+Spl->Am+Spl->Pl reaction sequence can be observed, indicating a continuous metamorphic evolution from low-pressure granulites (absence of garnet) to upper amphibolite facies conditions. Opx-Cpx geothermometry gives typical granulitic temperature range of 863-930 ºC. Al-in barometry in amphibole gives pressures ranging from 8-8.5 kbar. After regional metamorphism, local mylonitization along discrete NW-SE to NNW-SSE striking ductile shear zones occurs. Scarce kinematic indicators record a reverse sense of shear. Highly deformed metagabbros are fine-grained and strongly foliated. Mafic minerals porphyroclasts set in a finer recrystallized mass of Pl+Opx+Cpx+Am while Pl is dominant as matrix constituent. Finally, metagabbros are local and partially replaced by retrograde low-T hydrated minerals.