Neoproterozoic granitic magmatism in a lower to middle crust section (Socorro-Guaxupé Thrust Nappe, SE Brazil): Age progression and crust zonation evidenced by U-Pb dating and Sr-Nd-Hf-Pb isotope geochemistry

The Socorro-Guaxupé Thrust Nappe (SGTN) is a metamorphic high-grade allochthonous terrane situated in southeast Brazil, interpreted as a segment of a Late Neoproterozoic magmatic arc that developed along the margin of a continental plate. The SGTN exposes a crustal section with granulites, orthogneisses, and charnockites in its lower portions (∼8–10 kbar) whereas migmatitic para- and orthogneisses are intruded by younger granites in its upper portions (∼4–5 kbar). In this work the quite variable (meta)-plutonic rocks of the western part of the SE segment of the SGTN (the Socorro Domain), were dated by U-Pb zircon SHRIMP and LA-ICP-MS. The oldest are charnockites and orthogneisses (650–630 Ma) that predominate in the deeper portions, whereas the middle portions of the crust (5–6 kbar) are intruded by a voluminous suite of less deformed porphyritic hornblende-biotite granitic rocks of intermediate SiO 2 content (60–71 wt%) and high-K calc-alkaline (HKCA) affinity dated to 620–610 Ma (Bragança-Paulista type granites). Country rocks vary from banded granulites and gneisses in the lower crust to a package of para- and orthogneisses associated with abundant anatectic granites produced by widespread migmatization between 625 and 605 Ma. Notably, the plutonic rocks show a consistent decrease of εNd(t) and εHf(t) from −4/+1 to −10 in the deeper orthogneisses and charnockites to < -10 in the shallower HKCA granitoids, which also show higher initial 207 Pb/ 206 Pb, consistent with a greater proportion of an older component in their source. This pattern suggests an upward-propagating magma column in a stratified crust, which incorporated partial melts from sources with greater crustal residence time or greater proportion of such rocks as the magmas reached the middle and upper sections of the SGTN.