Kyanite-garnet granulite from the Andrelândia nappe system, Brasília belt, registers two late Neoproterozoic metamorphic cycles

Abstract

The southern Brasília belt (SBB) evolved during the Neoproterozoic amalgamation of Western Gondwana. The Andrelândia nappe system (ANS) is part of the SBB and a key area to understand the tectono-metamorphic evolution of the SBB due to the ongoing controversy concerning whether peak metamorphism occurred at >650 Ma, >618 Ma or 605 Ma in the upper nappe of the ANS. A kyanite-garnet-bearing felsic granulite from the Três Pontas-Varginha nappe (TPVN) in the ANS was investigated to constrain the pressure-temperature (P-T)-time evolution. For this purpose, we applied thermodynamic modelling with P-T pseudosections contoured by garnet compositional isopleths and monazite in-situ dating by LA-ICP-MS. Early garnet (Gt I) formed in the P-T range of 11-20 kbar and 495-595 °C based on the garnet core composition. This growth was followed by resorption at retrograde conditions. Mineralogical changes during a second metamorphic cycle are discernible by new growth of garnet (Gt II) with a significantly different composition compared to Gt I. The P-T climax reached 13 ± 1.5 kbar and 800 ± 50 °C, accompanied by partial melting. The two metamorphic cycles are assigned to two successive burial-exhumation events that ended with nappe stacking and formation of the TPVN. Monazite occurring in garnet (Gt II) and in the matrix contains Y concentrations of 0.1-3.5 wt.% independent of the textural position, but positively correlated with HREE contents, supporting the formation of all monazite during corrosion of Gt I and early growth of Gt II. Dating of monazite yielded a concordia age of 617.1 ± 3.8 (2σ) Ma. We assign this age to the end of the first P-T loop and beginning of the second loop. Literature data suggest two ages, close to 650 Ma and 605 Ma, for the P-T climax in the TPVN. The age difference of ca. 45 Ma fits a multiple metamorphic evolution similar to Phanerozoic orogens, characterized by a difference in time between two metamorphic peaks in the range of tens of million years (15-60 Ma) within a single orogenic cycle. This points to an orogenic evolution in the late Neoproterozoic (Brasiliano/Pan-African orogenesis) that had the character of modern mountain-building processes.