Crustal anatexis and granite genesis in the Socorro-Guaxupe thrust nappe, southeastern Brazil: some constraints from elemental geochemistry

Abstract

Three suites of granites were produced at ca. 620-630 Ma by partial melting of a crustal section exposed as a high-grade allocthonous terrane (the Socorro-Guaxupe Thrust Nappe, SGTN) south of the Sao Francisco Craton, SE Brazil. The striking chemical and petrographical contrasts observed reflect their genesis from different protoliths under varying P, T, a(H2O). The Sao Jose do Rio Pardo (SJRP) suite is made up of folded tabular bodies with a predominant consanguineous association of foliated mangeritic to granitic rocks which resulted mostly from in situ crystal fractionation of low mg#, Ca and Sr quartz mangeritic magmas. The very low contents of radiogenic (Rb, Cs, Th, U) trace-elements and H2O inferred for their parent melts imply a depleted granulite source which, as deduced from Zr saturation thermometry, was melted under abnormally high T (up to 1,000oC). High K/Rb and Ba/Sr ratios and positive Eu anomalies seem to result from disequilibrium melting involving K-feldspar as a reactant phase, probably as a reflection of the rather dry nature of the magmas and of fast rates of melt extraction. The Pinhal-type granites occur as countless small bodies of equigranular, in part nebulitic, pink biotite granite closely associated to migmatitic gneisses. These granites are interpreted as products of biotite dehydration-melting at ca. 850°C from tonalitic-granodiorite protoliths, as indicated by their fractionated REE patterns (suggestive of residual garnet), moderately negative Eu anomalies and higher Rb/Sr as compared to the other suites (both suggestive of a plagioclaserich, biotite-poor residuum, possibly a granulite). The garnet-biotite Nazare Paulista-type granites, also closely associated to migmatitic gneisses, correspond to products of lower-temperature (700-800oC, as inferred from zircon and monazite saturation thermometry) melting of protoliths with an important metasedimentary component. Comparatively high Ca and Sr and slightly positive to negligible Eu anomalies, besides low Rb and Ba contents point to a residuum with abundant biotite and either plagioclase-poor or bearing a Ca-rich plagioclase. The widespread anatexis in this section of Neoproterozoic crust appears to reflect the influence of a huge thermal anomaly of a magnitude unusual in collisional orogens, that was responsible for the generation, in a short time interval, of granite types normally found in different tectonic environments (e.g., the SJRP mangerite-charnockite-granite suite and the peraluminous leucogranites associated to the Nazare Paulista suite).