Precambrian Tectonothermal Evolution History In The Oulongbuluke Block

The Oulongbuluke Block(also named Quanji Block)is located on the northeastern margin of the Qinghai-Tibet Plateau,bounded by the high-ultrahigh pressure metamorphic belt on the northern margin of the early Palaeozoic Qaidam Block to the south and the Qilian Block to the north.The Oulongbuluke Block,one of several ancient micro-continents on the southeast margin of the Tarim Craton,is regarded as a nearly extinct fragment of the Tarim Craton.Previous studies have shown that these micro-continental blocks also record evidence of the assembly and break-up of the supercontinent.Many more valuable references for the evolutionary relationship between the cratons and the global tectonic evolution pattern can be provided by the study of these micro-continental blocks.Therefore,it is of great significance to study the thermal evolution history of micro-continental blocks.In this manuscript,petrology,petrography,zircon U-Pb,Lu-Hf isotope,and whole-rock geochemical studies were carried out on the Mesoproterozoic felsic gneisses,migmatites,augen granitic gneisses and mafic granulites from Wulan County on the eastern margin of the Oulongbuluke Block.Combined with previous work,this manuscript systematically summary the genesis and tectonic background of these rocks,and discuss the Mesoproterozoic thermal evolution history,the crust evolution history of the Oulongbuluke Block,as well as the relationship with Columbia and Rodinia supercontinent evolution,and the relationship with the three cratons in China.The zircon cathodoluminescence(CL)images,U-Pb dating and Hf isotope analyses show that zircon grains from felsic gneiss and melanosomes in migmatites are composed of magmatic zircon cores and anatectic or metamorphic zircon rims.The early Mesoproterozoic magmatic age 1500-1496 Ma and anatectic or metamorphic age1454-1443 Ma are yielded from the cores and rims,respectively.The nearly identical¹⁷⁶Lu/¹⁷⁷Hf and?_Hf(t)values suggest that the Hf isotope of the zircon rims have inherited the zircon cores in a closed system.The zircon cores from the grey felsic gneiss possess?_Hf(t)values between-3.5 and+3.9,with T_DMC between 2.29 Ga and 1.99 Ga.The zircon cores from the melanosome possess?_Hf(t)values between-1.7 and+2.8,with T_DMCbetween 2.14 Ga and 2.34 Ga.The zircons from augen granitic gneisses and mafic granulites in the Oulongbuluke Block are magmatic or metamorphic origin,and yield the late Mesoproterozoic magmatic age 1105-1104 Ma and metamorphic age 1092-1081 Ma.Two samples of augen granitic gneiss possess all negative?_Hf(t)values except for one positive?_Hf(t)value,and the T_DMC ages range from 1.90 to 2.26 Ga.The early Mesoproterozoic ca.1.5 Ga felsic gneisses from the Oulongbuluke Block are composed of plagioclase,K-feldspar,biotite and quartz.The Mesoproterozoic ca.1.5Ga migmatites are mainly composed of melanosomes and leucosomes.The leucosomes generally occur in veins and in layers with melanosomes,which shows the characteristics of migmatization.The leucosomes are predominately composed of quartz,plagioclase and K-feldspar,with the characteristics of in-situ partial melting and in-source with short distance migration.The mafic granulites occur as lenses and they consist of plagioclase,hypersthene,and clinopyroxene.The late Mesoproterozoic ca.1.1 Ga augen granitic gneisses are mainly composed of K-feldspar,plagioclase,quartz and biotite.The augen K-feldspar phenocrysts are the typical marks.The whole-rock geochemical analysis results suggest that the Mesoproterozoic ca.1.5Ga felsic gneisses and melanosomes in migmatites are enriched in light REEs and relatively depleted in heavy REEs.The REEs distribution patterns show the characteristics of light REEs right dipping,relatively flat heavy REEs and negative Eu anomalies.On the primitive mantle-normalized spider diagrams,they are enriched in K and Rb,and depleted in Nb,Ta,Sr,P and Ti.Leucosomes in migmatites are characterized by lower total REEs and trace element contents,positive Eu anomalies,depletion in Th,Ti,Tb,Y and enrichment in Sr different from the felsic gneisses and melanosomes.The geochemical characteristics of the late Mesoproterozoic ca.1.1 Ga augen granitic gneisses are high-K calc-alkaline and metaluminous to weakly peraluminous,which also have the characterized by high total amount of REEs,REEs curve right dipping,negative Eu anomalies,enrichment of K,Ba,Rb and Th,and depletion in Nb,Ta,Sr,P and Ti.Combined with the previous research results,the comprehensive analysis suggest that the early Mesoproterozoic ca.1.5 Ga magmatic protoliths of felsic gneisses are I-type granite,and were produced by part melting of mafic rock in lower crust in the setting of extension under relatively low pressure conditions.The early Mesoproterozoic ca.1.5 Ga protoliths of migmatites might be produced by part melting of mafic rock in lower crust with the same nature to the protoliths of felsic gneisses.The late Mesoproterozoic ca.1.1Ga magmatic protoliths of augen granitic gneisses are I-type granite,which were produced by part melting of mafic rock in lower crust,and formed in the Andean active continental margin tectonic environment.Comparing the tectonothermal events recorded in the Oulongbuluke Block with those in other cratons,there are two stages of tectonothermal evolution events in the Mesoproterozoic,namely,ca.1.5-1.45 Ga tectonomagmatic-metamorphism/anatexis in the early Mesoproterozoic and ca.1.1 Ga tectonomagmatic-metamorphism in the late Mesoproterozoic,which are related to the breakup of Columbia supercontinent and the assembly of Rodinia supercontinent.The Mesoproterozoic crust of the Oulongbuluke Block underwent early remelting of ca.1.5-1.45 Ga and late remelting and accretion of ca.1.1Ga.The Oulongbuluke Block,Tarim Craton and North China Craton have similar evolutionary histories before the early Mesoproterozoic and good affinity,but the Oulongbuluke Block,Tarim Craton and South China Craton have good affinity during the late Mesoproterozoic-early Neoproterozoic.In addition,the South China Craton-North Qaidam Block-Oulongbuluke Block-Qilian Block-Tarim Craton united continent was formed on the margin of Rodinia supercontinent.