Petrogenesis And Tectonic Implications Of The Late Paleozoic Granites From The Xingxingxia Area Of The Central Tianshan

The Central Tianshan Microcontinent?CTM?in the southern Central Asian Orogenic Belt is characterized by Precambrian crustal basements and Paleozoic to Mesozoic?500²40 Ma?magmatic rocks.These magmatic rocks are characterized by magmatic“flare-up”during Late Carboniferous and Early Permian.However,both the crustal evolution of the CTM during the Paleozoic and the tectonic setting of Late Paleozoic magmatic rocks have not been well constrained.In this paper,we report whole-rock geochemistry,zircon U–Pb ages,Hf isotopes and trace elements for the Late Carboniferous granites and the Early Permian granitic rocks from the Xingxingxia area of the CTM.The Late Carboniferous granites,including the granodiorite and monzogranite,show calc-alkaline and metaluminous to weak peraluminous affinities.They further show enrichment in large iron lithospheric elements?Cs,Rb,Ba and K?,depletion in high field strength elements?Nb,Ta,P and Ti?and significant Eu anomalies with fractionated rare earth element patterns,consistent with the geochemical characteristics of subduction-related magmas.Furthermore,they are characterized by arc metaluminous magmas evolution.It is suggested that amphibole,zircon,apatite and plagioclase are mainly fractional crystallization minerals during magma evolution.Zircon trace elements also suggest a subducted-related continental arc setting with relatively high U/Yb ratios and low Hf contents.Highly variable zircon?_Hf?t?values?-0.1 to+12.9?of the Late Carboniferous granites suggest that their magmas were formed by magma mixing of mantle-and crustal-derived magmas.We suggest that they were generated in a continental arc setting related to the subduction of the North Tianshan Oceanic Plate along the north margin of the CTM.Early Permian?ca.280 Ma?granitic intrusions,including quartz diorite and dioritic enclave,amphibole granite,biotite granite,dioritic porphyry dyke and a granitic dyke,were revealed from the Xingxingxia area of the CTM by LA-ICP-MS zircon U–Pb dating.These granitic rocks show magnesian and calc-alkaline affinities and are characterized by Nb,Ta,P,Ti and MREE depletion[?Dy/Yb?_N=1.02¹.51]with no significant Eu anomalies,consistent with typical active continental margin magmatism.Zircon trace element geochemistry of these Early Permian granitic rocks also indicates a continental arc setting with high U/Yb and Gd/Yb ratios.Among these granitic intrusions,the dioritic porphyry dyke has negative zircon?_Hf?t?values?-8.0 to-5.7?,suggesting that the magma was produced by partial melting of the ancient crustal basements of the CTM.The rest intrusions show positive but largely varied zircon?_Hf?t?values of+4.1 to+10.9?5 samples?,suggesting that the magmas were probably derived from the mantle wedge with variable contributions from ancient crustal component during the magma evolution.These granitic intrusions were coeval with high-temperature metamorphic rocks,mafic-ultramafic rocks and Cu-Ni sulfide deposits in the CTM.Therefore,we propose a geodynamic model of oceanic ridge subduction of the North Tianshan Ocean during Early Permian involving upwelling of asthenospheric mantle through slab window and partial melting of the mantle wedge.The multiple-derived magmas,including those from mantle wedge and/or ancient crustal basement,and their blendings formed the studied Early Permian granitic associations in the CTM.In conclusion,we believe that the North Tianshan Ocean was in a continuous oceanic subduction process during the Late Paleozoic,and the final closure of the North Tianshan Ocean may have occurred in the Late Permian or Early Triassic.In addition,the zircon?_Hf?t?values of the Late carboniferous and Early Permian granitic rocks in the CTM plot above the crustal basement evolution area of the CTM.Therefore,the crustal evolution of the CTM during the Late Paleozoic was dominated by crustal growth with some reworking of ancient crust.