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

Abstract

Whole rock elemental and Sr-Nd isotope geochemistry and in situ zircon Hf isotope geochemistry were used to identify the sources of the Neoproterozoic granites from the Socorro batholith, Socorro-Guaxupe Nappe (SGN), South Brasilia Orogen, Brazil. Zircon trace elements and Hf isotope geochemistry provided information about sources and crystallization (T, $f_{O_2}$) conditions. Three main types of granites built the bulk of the batholiths, beginning with probably pre-collisional ∼640–630 Ma charnockites, and ending with ∼610 Ma voluminous post-collisional high-K calc-alkaline (HKCA) I-type granites (Braganca Paulista-type). Several types of leucogranites were generated from 625 to 610 Ma, spanning the interval from collisional to post-collisional tectonics. Two charnockite bodies occur in the study area: the ∼640 Ma Socorro charnockite has remarkable chemical similarities with Braganca Paulista-type granites, but higher ε _Nd(t)= −6.1 and average zircon ε _Hf(t)= −9.1 and lower ⁸⁶Sr/⁸⁷Srt (0.709 3) values, indicative of more juvenile and water-poor source. The ∼633 Ma Atibaia charnockite has distinct geochemical signature (lower Mg^# and Sr content; higher Zr), more negative ε _Nd(t)= −14.1, similar average zircon ε _Hf(t)= −8.9, and much higher ⁸⁶Sr/⁸⁷Srt=0.719 7, probably reflecting a larger component from old crust. The predominant ∼610 Ma Braganca Paulista-type granites were emplaced in a post-collisional setting, and correspond to porphyritic biotite-hornblende monzogranites of high-K calc-alkaline character with 61 wt.%–67 wt.% SiO₂, high Mg^# (39–42), Sr/Y (19–40), La/Yb (12–69), highly negative ε _Nd(t) (−12.3 to −12.9) and zircon ε _Hf(t) (−12 to −17) and ⁸⁷Sr/⁸⁶Srt=0.711 9–0.713 1. These features are interpreted as indicative of magma generation in a thickened crust, where melts from enriched mantle sources emplaced in the lowermost crust, heated host old continental crust rocks (gneisses and granulites) and partially mixed with their melting products. Leucogranite plutons (SiO₂>72 wt.%) occurring in the southern portion of the batholith have a range of geochemical and isotope signatures, reflecting melting of crustal sources in space and time between ∼625 Ma (Bocaina Pluton) and ∼610 Ma (Bairro da Pedreira Pluton). Highly negative ε _Nd(t) (−16.2) and average zircon ε _Hf(t)= −16, and high ⁸⁷Sr/⁸⁶Srt (0.715 6–0.717 1) are consistent with relatively old ortho- and paragneiss sources similar to those which generated regionally abundant migmatites and anatectic granites in the collisional to post-collisional setting.

Keywords

U-Pb dating / Socorro-Guaxupé Nappe / high-K calc-alkaline granites

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