Sao Bento do Sapucai Shear Zone: Constraining age and P-T conditions of a collisional Neoproterozoic oblique shear zone, Ribeira Orogen, Brazil

摘要

The Sao Bento do Sapucai Shear Zone is a subvertical NE-SW anastomosing shear zone of at least 100 km length and few km wide. It affects orthogneisses, paragneisses and igneous rocks of the Socorro-Guaxupe Nappe, the uppermost nappe of a nappe stack, which constitutes the Southern Brasilia Orogen, part of a Neoproterozoic orogenic system surrounding the Sao Francisco Craton. The Socorro-Guaxupe Nappe is interpreted as the root of a magmatic arc. The nappe stack is partly affected by superposed deformation related to the slightly younger Ribeira Orogen. The mylonitic foliation of the shear zone dips steeply to both SE and NW, and contains a stretching lineation with shallow plunge predominantly to NE and locally to SW. Kinematic indicators including sigma and delta objects, asymmetric mica fish and synthetic shear bands point to oblique dextral movement with a SE-up component. Evidence of quartz and feldspar recrystallization-mechanisms was applied to estimate temperature during mylonitization as between 450 degrees C and 650 degrees C. The geothermobarometry calculated from the composition of mineral rims, partially or fully recrystallized during mylonitization, yielded a temperature of 479 +/- 66 degrees C and a pressure of 2.94 +/- 1.56 kbar. Geochronological investigation by LA-ICP-MS of U-Pb in zircon and monazite was applied in order to constrain the age of rocks which were affected by, or cross-cut the shear zone. Metamorphic rims on detrital zircon grains collected from a paragneiss indicate a main metamorphic episode at ca. 613 Ma. Analysis of a gabbronorite affected by the shear zone yielded a crystallization age of ca. 610 Ma; a pegmatitic leucogranite which cross-cuts the shear zone, produced various zircon ages between 656 and 611 Ma, interpreted as inheritance. Monazite crystals of the same leucogranite yielded an age of 552 Ma, interpreted as the age of crystallization. Hence the age of the shear zone is constrained between 610 and 552 Ma. The data produced in this study fit well the proposed evolutionary history of the Socorro-Guaxupe Nappe, where a magmatic arc developed between 680 and 630 Ma, followed by continental collision and associated metamorphism ranging from 630 to 600 Ma. The younger shear zone is related to the tectonics of the superimposed Ribeira Orogen in the time span 600-560 Ma.