P-T-TIME EVOLUTION OF HIGH-GRADE NAPPES FROM THE NEOPROTEROZOIC SOUTHERN BRASíLIA BELT, BRAZIL

In the S sector of the Southern Brasília Belt, ENE-verging nappes record terminal collision of the Socorro–Guaxupé Arc with the subducting passive margin on the W of the São Francisco Craton. An arc-derived nappe overlies passive margin-derived nappes of the Andrelândia Nappe Complex (ANC), which lie on autochthonous basement. The ANC comprises HP granulite facies rocks at the top with metamorphic grade decreasing structurally downward to greenschist facies at the bottom. Peak metamorphic conditions in HP granulites from the uppermost Tres Pontas–Varginha (TPV) and Carmo da Cachoeira (CdC) Nappes were P ~1.5 GPa, T ≥840°C and P ~1.1–1.35 GPa, T ~830°C. SIMS U-Pb Zrn ages (Reno et al., 2009, JGS) constrain timing of: eclogite facies metamorphism of oceanic crust (²⁰⁶Pb/²³⁸U age of 678±29 Ma from a tectonic block located along the contact between the uppermost two nappes in the ANC); HP granulite facies metamorphism in the TPV nappe (²⁰⁶Pb/²³⁸U ages of 648±12/648±8 and 647±11 Ma, from Zrn that records Ti-in-Zrn crystallization T of 860–785°C during isobaric cooling, but skewed toward the lower part of the range); and, HP granulite facies metamorphism in the overlying arc-derived nappe (²⁰⁶Pb/²³⁸U age of 622±28 Ma, from Zrn that records Ti-in-Zrn crystallization T of 970–820°C). For the TPV nappe, Mnz chemical dating yields ages of 634–631 Ma and 619–614 Ma from higher Y domains, interpreted to record crystallization at the solidus, the T of which varies with composition, and 606–597 Ma from lower Y domains, interpreted to record juxtaposition with the overlying arc-derived nappe during exhumation (Reno et al., in revision, JMG); decompression is constrained by Rb–Sr multi-mineral whole rock isochron ages of ca. 590 Ma, and further isobaric cooling is constrained by ⁴⁰Ar–³⁹Ar Bt ages of 591–567 Ma (Reno et al., in press, AJS). For the CdC nappe, Mnz chemical dating yields ages of 631 Ma and 620–617 Ma from higher Y domains, interpreted to date the high-T suprasolidus growth of foliation-forming Bt during isobaric cooling following peak T, and 605–594 Ma from lower Y domains, interpreted to record growth or recrystallization of Mnz owing to crystallization of residual melt and release of aqueous fluid at the solidus (Reno et al., in revision, JMG); cooling is constrained by ⁴⁰Ar–³⁹Ar ages on Bt of 542–510 Ma (Reno et al., in press, AJS).