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 postdates 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.

Petrogenesis of Granites from the Ediacaran Socorro Batholith, SE Brazil: Constraints from Zircon Dating, Geochemistry and Sr-Nd-Hf Isotopes

“…The Bragança Paulista-type granites were mostly emplaced at ~610 Ma, based on two SHRIMP U-Pb zircon ages reported in our previous work (610±7 and 608±7 Ma;Toledo et al, 2018), which are equivalent to the result obtained in one sample by Ebert et al (1996) (610±10 Ma). A new age determination presented in this contribution yields a similar age (cf.…”

“…It corresponds to the typical "Salmão-type" granite referred by Campos Neto et al (1984a), who reported typical mineral proportions as 33% quartz, 34% K-feldspar, 28% plagioclase, 4% biotite and accessory apatite, zircon, opaque minerals and monazite totaling 1%. Our previous U-Pb dating of sample BRP-03 by SHRIMP indicated a crystallization age of 624±4 Ma, with inherited zircon cores of Mesoproterozoic to Paleoproterozoic ages (1 500 and 1 780 Ma; Toledo et al, 2018).…”

“…Modern geochronological and isotopic data (in situ Hf isotope determination in zircon, U-Pb in situ dating by SHRIMP) to support inferences on the sources of magmas and their tectonic significance are relatively scarce in Socorro batholith. A recent work by our group (Toledo et al, 2018) reported five U-Pb zircon ages by SHRIMP, ranging from 608±7 (HKCA Bragança Paulista-type granite) to 642±4 Ma (Socorro charnockite).…”

“…The tectonic setting of these extensive elongated HKCA batholiths is still uncertain. They have been considered as products of active continental margin magmatic arcs (Heilbron et al, 2017(Heilbron et al, , 2013(Heilbron et al, , 2004Trouw et al, 2013;Janasi and Ulbrich, 1991;Campos Neto et al, 1984a), but recent works associated some of them with the post-collisional period (Toledo et al, 2018;Janasi et al, 2016;Meira et al, 2015) and therefore not directly linked to active ocean plate consumption.…”

Geochronology and petrogenesis of the southern portion of Socorro Granitic Batholith based on U-Pb dating and isotope geochemistry

“…The new geochronological data presented in this work confirms our previous studies (Toledo et. al., 2018) that suggested a wide time interval for the build up of Socorro Batholith.…”