Timing of anatexis and melt crystallization in the Socorro–Guaxupé Nappe, SE Brazil: Insights from trace element composition of zircon, monazite and garnet coupled to U[sbnd]Pb geochronology

Abstract

The timing of partial melting and melt crystallization in granulite facies rocks of the Socorro–Guaxupé Nappe (SGN), Brazil is constrained using a combination of imaging techniques, LA-ICP-MS and EPMA dating, trace element geochemistry and thermobarometry. (Orthopyroxene)-garnet-bearing migmatite that records extensive biotite dehydration melting shows evidence for a clockwise P–T–t path. UHT peak conditions were attained at 1030±110°C, 11.7±1.4kbar, with post-peak cooling to 865±38°C, 8.9±0.8kbar. Cryogenian igneous inheritance of ca. 720–640Ma is identified in oscillatory zoned zircon cores (n=167) with steep HREE patterns. Resorbed, Y-rich monazite cores preserve a prograde growth stage at 631±4Ma prior to the partial melting event, providing an upper age limit for the granulite facies metamorphism in the SGN. REE-rich, Th-depleted monazite related to apatite records the initial stages of decompression at 628±4Ma. Multiple monazite growth episodes record melt crystallization events at 624±3Ma, 612±5Ma and 608±6Ma. Stubby, equant “soccer ball” zircon provide evidence for melt crystallization at 613±2Ma and 607±4Ma. The excess scatter in zircon and monazite age populations between 629±4 and 601±3Ma is interpreted as discontinuous and episodic growth within this age range, characterizing a prolonged metamorphic event in the SGN lasting ca. 30m.y. The development of Y+HREE-rich monazite rims at ca. 600Ma documents retrograde garnet breakdown, extensive biotite growth and the final stages of melt crystallization. Th-rich, Y+HREE-poor monazite rims at ca. 590Ma record monazite recrystallization.