Geochronology, Petrogenesis And Geological Significance Of The Mangqisu Pluton In The South Tianshan

The Mangqisu granite is a large complex pluton in the central part of the Erbeng Mountain tectonically belong to the South Tianshan Orogenic Belt.This granite pluton consists mainly of three units,i.e.,biotite granodiorite unit in the east,biotite granite unit in the west and two-mica granite unit in the central.In this study,we competed an integrated investigation of the field geology,petrology,geochronology?zircon U-Pb dating?,isotopic geochemistry?Lu-Hf and Sr-Nd isotopes?,whole-rock geochemistry and mineral chemistry?EPMA and TIMA scanning?,to trace the magmatic source,petrogenesis and tectonic setting.The Mangqisu granite has SiO₂ ranging from 65.34%to 73.12%.Samples from eastern and western parts of the pluton show high Al₂O₃?14.40%-16.51%?,A/CNK?1.00-1.11,the average is 1.05?,Na₂O+K₂O?7.26%⁸.52%?,and are of high-K calc-alkaline and weakly peraluminous series.The central granite unit displays Al₂O₃?14.17%-14.92%?,?A/CNK=1.09-1.14,with an average of 1.12?,Na₂O+K₂O?8.36%⁸.80%?,suggesting that they are of calc-alkaline and strong peraluminous series.All samples of the Mangqisu granite are enriched in large ion lithophile elements?LILEs,such as K,Rb,Ba?,and depleted in high field-strength elements?HFSEs?such as Nb,Ta and Ti.Their rare earth element distribution patterns are consistently characterized by enrichment in LREE?La/Yb?_N=9.58⁴4.94)and depletion in HREE?LREE/HREE=8.00²2.97?with negative Eu anomalies??Eu=0.67⁰.81 in the eastern unit;?Eu=0.48⁰.78 in the western unit;?Eu=0.44⁰.48 in the central unit?.The eastern and western granite units show high Sr(164.70×10^-6616.80×10^-6)and low Y(4.90×10^-618.5×10^-6),with high Sr/Y?10.0⁴5.5?ratios.Such a feature is comparable to that of adakite,and implies that the South Tianshan Orogenic Belt had experienced crustal thickening before the emplacement of the Mangqisu pluton.Whole-rock Sr-Nd isotopic and in situ zircon Hf isotopic analysis yield negative?_Nd?t??-6.89^-10.07?and?_Hf?t?values?-14.43^-0.49?,corresponding to two-stage model age(t_DM2?Hf?)varying from 1.30 Ga to 2.20 Ga.The compositions of feldspar crystals from the three parts of the pluton vary greatly;they are mostly andesine withsubordinate albite and orthoclase in the eastern granite unit,oligoclase and orthoclase with a small amount of albite and anorthose in the western unit,and mainly oligoclase and albite in the central unit.Muscovite in the granite occurs mainly as a primary phase;biotite grains from the eastern and western units are of magnesian biotite,and ones from the central granite unit are of magnesian biotite and ferribiotite.Combined with regional geological background,we propose that the Mangqisu granie pluton was derived from partial melting of a lower crustal source composed mainly of Paleo-Mesoproterozoic meta-igneous rocks of South Tianshan at early stage of post-collisional settings,and that the parental magma had experienced variable degrees of fractional crystallization.