Protracted zircon geochronological record of UHT garnet-free granulites inthe Southern Brasília orogen (SE Brazil) : petrochronological constraints onmagmatism and metamorphism.

Abstract

The investigation of ultrahigh temperature (UHT) metamorphic rocks, and their corresponding (pressure)-temperature-time (P-T-t) history is critical to distinguish between arc- or collision-related metamorphism. This isa very challenging task if mineral assemblages are highly retrogressed and isotopic systems are disturbed.Garnet-free granulites lacking accessory minerals (chronometers) and metamorphic index minerals (thermo-barometers) located in UHT domains are examples of such complex systems. In such cases, zircon may be themain chronometer, although isotopic U-Pb data outline protracted records, making the interpretation of the datacomplex. This study focuses on the timing of magmatism and metamorphism, as well as on the thermal meta-morphic conditions of garnet-free UHT granulites of the Guaxupé nappe, southernmost Brasília orogen, locatedclose to the Paranapanema cratonic block. It presents U-Pb dating, Lu-Hf isotopes and trace element signatures ofzircon, and thermometry on metamorphic clinopyroxene and orthopyroxene from granulites. Steady176Hf/177Hf(t)in zircon cores exhibiting U-Pb dates spreading in the Concordia suggest post-crystallizationdisturbance. From those disturbed granulitic systems, minimum crystallization ages of ca. 2550 Ma, ca. 790 Ma,ca. 690 Ma and ca. 660 Ma can be retrieved. The juvenile ca. 2.55 Ga granulite is thefirst evidence of an exposedrock of the Paranapanema cratonic block, previously only inferred from geophysical data. The Guaxupé napperecords arc-related magmatic episodes in the range of 790–640 Ma, partially coeval with a long-lasting(∼80 m.y.) metamorphic event (670–590 Ma) and intrusion of basic magma (ca. 660 Ma). Thermometry onzoned clinopyroxene and orthopyroxene yields UHT conditions around 900–1000 °C. Comparing the distributionpatterns of metamorphic zircon rims and newly formed grains, we suggest two distinct metamorphic stages: i) anarc-related metamorphism (670–640 Ma), recorded by domains possibly formed by subsolidus recrystallization;and ii) a continental collision to decompression involving partial melting (630–590 Ma) associated to extensivezircon crystallization. The temporal relationship between magmatic and metamorphic ages suggests an ultra-high-temperature metamorphic event related to a magmatic arc. This arc was afterwards involved by theGuaxupé nappe stacking during the collisional stage of the southernmost Brasília orogen.