新疆北部青河县阿斯喀尔特铍矿区花岗质岩石年代学及地球化学特征

摘要

新疆北部青河县阿斯喀尔特铍矿床的形成与岩浆活动密切相关,是中国花岗岩型铍矿床的典型代表.对矿区斑状二云母二长花岗岩进行LA-I CP-MS锆石U-Pb年龄测试,获得其加权平均年龄为(216.7±2.8) Ma(MSWD=0.48),表明该岩体形成时代为晚三叠世,据此限定阿斯喀尔特铍矿床成矿时代略晚于216Ma,为晚三叠世—早侏罗世.岩石具有高硅(ω(SiO2)=70.86％～76.34％)、富碱(ALK=5.54～9.30)、富铝(ω(Al2O3)=13.00％～14.74％,A/CNK=0.99～ 1.23)、低钛(ω(TiO2)=0.02％ ～0.18％)和镁(ω(MgO) =0.02％ ～1.21％)特征,为过铝质中钾-高钾岩石系列.稀土元素配分型式显示LREE的相对弱富集,HREE较平坦以及Eu弱-中等的负异常(δEu=0.37～0.90),呈略右倾型.微量元素Ba、Sr、Hf、Ti等具负异常,Rb、Th、K、Nb、Ta、La、Ce、Nd、Sm等具正异常,Rb/Sr比值较高(9.34～26.81),显示出S型花岗岩特征.结合区域资料,认为阿斯喀尔特铍矿矿区印支期花岗岩形成于后造山构造阶段,可能是上地壳含砂泥质岩石部分熔融的产物.%LA-ICP-MS zircon U-Pb geochronological studies were conducted for porphyritic two-mica adamellite from the Ascalt beryllium deposit in Aershat area of Qinghe County,northern Xinjiang.The results show that the zircon 206pb/238U weighted average age of the rocks is (216.7 ± 2.8) Ma(MSWD =0.48),indicating that the crystallization age of the plutons is late Triassic.The Ascalt beryllium deposit is the typical representative of the granite type beryllium deposits,closely related to the magmatic plutons.The authors hold that the beryllium deposit was formed slightly later than 216 Ma in late Triassic-early Jurassic period.The granites have high SiO2 (ω(SiO2) =70.86％ ～76.34％),ALK(ALK =5.54～9.30) and Al2O3 (ω (Al2O3) =13.00％ ～ 14.74％,A/CNK=0.99～1.23) and low TiO2 (ω (TiO2) =0.02％ ～ 0.18％) and MgO (ω (MgO) =0.02％ ～ 1.21％),suggesting middle-high K series with peraluminous features.The REE patterns are characterized by weak enrichment of LREE,with moderate-faint Eu negative anomalies (δEu =0.37 ～ 0.90).The trace elements show enrichment of Rb,Th,K,Nb,Ta,La,Ce,Nd,Sm and Rb/Sr ratios (9.34～26.81) and depletion of Ba,Sr,Hf and Ta,with the characteristics of S-type granite.Combined with the previous studies,the authors consider that the Ascalt granitic plutons were generated from the partially melting of the upper continental crust dominated by argillaceous rocks at the post-orogenic stage.