Paleoproterozoic granulite-facies metamorphism and anatexis in the Oulongbuluke Block, NW China: Respond to assembly of the Columbia supercontinent

Abstract

The Oulongbuluke Block, which is located in the northeastern margin of the Tibet Plateau, has traditionally been considered to be a fragment of the Tarim Craton. Here we present a systematic petrologic, geochemical, and zircon U-Pb and Hf isotopic investigation on mafic granulite and migmatite in the Oulongbuluke Block. The mafic granulite is mainly composed of clinopyroxene, orthopyroxene, plagioclase, amphibole and quartz, with peak metamorphic P-T contions of 6.5–8.8kbar, 745–770°C. Macroscopic and microscopic observations provide strong evidence for in situ partial melting of the felsic gneiss involving breakdown of biotite within the Oulongbuluke Block. The Pl-rich leucosomes with positive Eu anomalies and higher Sr contents were generated as the early-formed feldspar cumulate, and the Kfs-rich pegmatite with negative Eu anomalies and lower Sr contents may represent percolating fractionated melt that was trapped during cooling. Zircon U–Pb dating and Hf isotopic analyses on the mafic granulite and migmatite of the Oulongbuluke Block reveal two distinct age populations: the early Paleoproterozoic (∼2.37Ga) and late Paleoproterozoic (1.93–1.92Ga). The ∼2.37Ga magmatic zircon cores of the migmatite have εHf(t) values between −4.3 and 0.4, with two-stage Hf model ages (TDMC) mainly between 2.82Ga and 3.05Ga. The age of 1.93–1.92Ga obtained from the mafic granulite and migmatite is interpreted as the age of Late Paleoproterozoic metamorphism and anatexis. Most of the 1.93–1.92Ga metamorphic and anatectic zircons have significantly lower 176Lu/177Hf ratios but higher 176Hf/177Hf (t) values than the inherited magmatic zircon cores, which demonstrates that both the zircon U–Pb and Lu–Hf isotope compositions were significantly reset during metamorphism and anatexis. The late Paleoproterozoic metamorphic and anatectic event coincided with global orogenic events that are recorded in many continental fragments, which suggests their link to the Columbia supercontinent.