SHRIMP U-Pb Geochronology of the Socorro Batholith and implications for the Neoproterozoic evolution in SE Brazil

ABSTRACT: The Socorro Batholith is one of the most expressive granite manifestations associated with the Neoproterozoic evolution in SE Brazil, occupying large areas (~1,200 km 2 ) in the southern portion of the Socorro-Guaxupé Nappe. A U-Pb zircon SHRIMP dating program was developed to determine the ages of the main components of this batholith, identified in previous detailed mapping projects. High-K calc-alkaline (HKCA) porphyritic biotite-hornblende granites with relatively low (60-67 wt%) SiO 2 are the most voluminous component of this and other large “syn-tectonic” batholiths in the SGN (Água Limpa and Pinhal-Ipuiúna) and neighboring domains located south of it in the Apiaí and São Roque Domains of the Ribeira Fold Belt. Two samples collected in widely separated localities at the northern and southern part of the Socorro batholith yield similar ages of magmatic crystallization, respectively 610.1 ± 7.0 and 608.3 ± 6.6 Ma. A more fractioned (> 72 wt% SiO 2 ) granite reported in the literature as related to a younger event (“Socorro II magmatism”, as opposed to the previous “Socorro I” HKCA granites) yield a precise age that is clearly older (624.4 ± 3.6 Ma), and contemporary to anatectic granites and migmatites that were produced during a prolonged period of high-grade metamorphism (635-605 Ma) that affected the SGN. Our data thus indicates that at least part of the HKCA magmatism that constitutes the Socorro batholith post-dates the high-P metamorphism associated to continental collision, and may have been a source of heat and volatiles to the high-T metamorphism responsible for partial melting of the upper portions of the crustal section represented by the SGN. Two charnockitic rocks that show transitional contacts with granites of the Socorro batholith were also dated. The Socorro Charnockite is aged 641.6 ± 4.1 Ma, which overlaps those of regional orthogneisses (in part also of charnockitic character) considered as associated with a pre-collisional tectonics (subduction-related?). However, it is reported to transition to granites that are very similar to the HKCA granites of the Socorro batholith, which are yet undated. The Atibaia Charnockite has distinct geochemical affinity (lower mg# and Sr content; higher Zr), a younger age (633.3 ± 6.2 Ma), and may signal a different tectonic setting at the end of the period of plate consumption as yet poorly characterized