Paleoproterozoic Metamorphism Of Supracrustal Rocks In The Korla Area, North Tarim Craton

The Tarim Craton is one of the three old cratons in china. Precambrian basement are well outcropped in the Kuruktag uplift. We carried out zircon U-Pb and Lu-Hf isotope and mineral composition microprobe measurements on garnet-mica schists and amphibolites, as well as geochemical analysis on amphibolites, of the Xingditage Group in order to obtain the maximum depositional age, the metamorphic age and P-T condition of the Xingditage Group, reveal the crustal growth history of the studied area and infer the tectonic setting of the protoliths of amphibolites.Zircon U-Pb age results of four garnet-mica schists indicate that they all record the1.85Ga high grade metamorphic event. The detrital zircons in samples09T-7,8,9either were dissolved to form the new metamorphic zircons or experienced the solid state recrystallization. These zircons have the same upper intercept age of1.85Ga and the nearly identical176Hf/177Hf. This metamorphic event of the Tarim Craton was response to the amalgamation of Columbia supercontinent. Sample T1preserved the source information of the detrital zircons with four small age peaks. The youngest age peak of detrital zircon in sample T1is2.0Ga, indicating that the Xingditage Group was deposited between2.0-1.85Ga and is firstly constrained as the Late Paleoproterzoic strata.The protoliths of amphibolites can be subdivided to three types according to their geochemical character:alkaline OIB, E-MORB and basic rocks contaminated by the crust materials. Element geochemical modelling indicates that both alkaline OIB and E-MORB samples were generated from the enriched mantle source. Some of basic rocks contaminated by the crust materials could be formed by the AFC process, but the magma source are N-MORB compostion, rather than alkaline OIB or E-MORB compostions. Two amphibolite samples(09T-2,09T-10) were carried on zircon U-Pb and Lu-Hf isotope study. The results show that sample09T-2was crystallied at2.7Ga, and then experienced metamorphism at2.35Ga and1.85Ga, respectively. The Hf isotope data indicate that the Tarim Craton has juvenile crust input at2.7Ga, which was mixed by the≥3.5Ga old crust materials. The other amphibolite sample09T-10records the2.21Ga metamorphic event, the average crust residence time of the source of this sample is3.6-3.1Ga.The metamorphic P-T conditions of garnet-mica schists and amphibolites are:1) the metamorphic temperatures of garnet-mica schists are590-800℃and the metamorphic pressures are10-17Kbar (maybe higher than the truth), indicating that the Xingditage Group have experienced high-grade metamorphism and caused the reset of U-Pb and Lu-Hf isotope of detrital zircons;2)The metamorphic temperatures of amphibolites are580-790℃, and metamorphic pressures are5.8-8.4Kbar, except for two samples having lower pressures of2.7Kbar and2.9Kbar. The P-T data indicate the samples have experienced amphibolite facies metamorphism, which is in agreement with the typical amphibolite facies mineral assamblages of amphibole+plagioclase. Did the metamorphism of garnet-mica schists and amphibolites of the Xingditage Group occur at the same episode, which metamorphic face series did the Xingditage Group reach and from which tectonic setting was the metamorphism result (collisional orogen or extentional setting that the magma underplate results in metamorphism) are several major problems needed to be further investigated.