Geochemistry,Sr-Nd isotopes and tectonic implications of two generations of Late Paleozoic plutons in northern West Junggar,Northwest China

Abstract

Two generations of Late Paleozoic plutons,Early Carboniferous subduction-related plutons and Late Carboniferous-Middle Permian post-collisional A-type granites,were recently recognized in northern West Junggar,Xinjiang,and they are distinguishable by their spatial distribution and geochemistry.Early Carboniferous plutons include diorites,granodiorites,and monzonitic and K-feldspar granites,and they are exclusively restricted within the Late Paleozoic Zharma-Saur arc.The formation of these plutons may be related to the southward subduction of the Irtysh-Zaysan Ocean between Kazakhstan in the south and Altai in the north.Geochemically,they are metaluminous to weak peralunimous,with A/CNK and A/NK ratios of 0.86 ～ 1.03 and 1.24 ～ 1.86,respectively,and are characterized by relative high Al2O3,CaO,Na2O and Na2O/K2O( 1.2).They show strong fractionations between LREE and HREE((La/Lu) N =4.50 ～26.03),with negligible Eu anomalies,and enrichment in LILE(K,Rb,Ba),Pb and Sr and depletion in Nb,Ta,P and Ti.All they are typical of I-type granitoids.The Early Carboniferous plutons show a wide range of initial Sr isotopic ratios(ISr =0.70408 ～0.70912),relatively constant eNd(t) values(+6.09 ～ +7.25) and young Nd model ages(tDM1 =527 ～593 Ma).All these data suggest that Early Carboniferous plutons,except for the Abudula monzonitic granite,were probably formed by fractional crystallization of mantle-derived magma which was generated by partial melting of the metasomatized depleted mantle and underwent a magma mixing process.Additionally,the Abudula monzonitic granite was likely to be derived from partial melting of subducted slab.By contrast,Late Carboniferous-Middle Permian plutons,mainly K-feldspar granites,are widespread across all the tectonic units in northern West Junggar and adjacent areas,and they were the products of post-collisional magmatism.These plutons are metaluminous and have A/CNK and A/NK ratios of 0.92 ～ 0.99 and 1.03 ～ 1.27,respectively,low Al2O3,MgO,CaO,moderate Na2O/K2O ratios(0.9 ～1.1),high total alkali(8.6% ～9.7%) and REE concentrations(average 196 ×10-6),weak fractionations between LREE and HREE((La/Lu) N = 3.32 ～ 5.36) with striking negative Eu anomalies,significant depletion in Ba,Sr,P,Ti,Nb and Ta.All these are consistent with those of A-type granites.Their eNd(t) values(+ 5.26 ～ + 7.26) and young Nd model ages(tDM1 = 502 ～ 655Ma) indicate that Late Carboniferous-Middle Permian A-type granites might be produced by highly fractional crystallization of ascending basaltic magma,which was possibly originated from fluid-metasomatized depleted mantle and mixed with juvenile crustal melts,as a result of slab break-off in a post-collisional setting.