Geochronology and geochemistry of Yanchangbeishan biotite Adamellite of Lenghu area in Qinghai and their geological significanceEI北大核心CSCD

The Yanchangbeishan biotite adamellite in the Lenghu area of Qinghai is located in the west segment of the north margin of Qaidam. The rocks have high SiO2 (73.92%-75.98%) and K2O contents (3.71%-4.78%), but low MgO and CaO contents. The alumina saturation index (A/CNK) varies from 1.03 to 1.11, indicating the biotite adamellite is peraluminous. The chondrite-normalized REE patterns of the rocks are right dipping and display weak Eu anomalies. The rock contains the Al-rich mineral like garnet and possesses normative corundum contents of 1%. It has low Th and Y contents which are negatively correlated with Rb, suggesting the source rock was sedimentary rock. Thus, the biotite adamellite belongs to S-type granite. The granite was produced by partial melting of the crust material due to the underplating of mantle-derived magma. LA-ICP-MS zircon U-Pb dating shows that the formation age of the rock is 254±4 Ma. Based on the regional geological background and the geochemical characteristics of the rocks, we consider that the formation of the Yanchangbeishan biotite adamellite was related to subduction, which is illustrated by the characteristics similar to those of the island-arc igneous rocks. Therefore, the results reveal that the north margin of Qaidam was in active continental margin tectonic environment during Late Permian epoch. ©, 2015, Science Press. All right reserved.     

青海冷湖盐场北山黑云母二长花岗岩年代学、地球化学及其地质意义

柴北缘西端青海冷湖盐场北山地区发育黑云母二长花岗岩,岩体高硅(Si O2=73.92%~75.98%)、富钾(K2O=3.71%~4.78%)、贫镁钙,铝饱和指数A/CNK介于1.03~1.11,属强过铝质花岗岩。稀土元素球粒陨石标准化曲线呈右倾型,Eu异常不明显;岩体含有富铝矿物石榴子石,刚玉标准分子约为1%,Th、Y含量低,且与Rb呈负相关性,源岩为沉积岩,具S型花岗岩特征,是幔源岩浆底侵作用引发地壳物质部分熔融的产物。LA-ICP-MS锆石U-Pb测年结果表明岩体形成于254±4 Ma。结合区域地质背景及岩石地球化学特征认为盐场北山黑云母二长花岗岩形成与俯冲作用有关,类似于岛弧环境产物,从而揭示柴北缘晚二叠世具有活动大陆边缘性质。     

柴北缘西端盐场北山铜镍硫化物矿床镁铁-超镁铁质岩体岩浆演化过程及含矿性分析

盐场北山岩体位于柴北缘造山带西段,是区内近年来新发现的具有良好铜镍矿化的镁铁-超镁铁质岩体,由橄榄岩相、辉石岩相和辉长岩相岩石组成。锆石U-Pb年代学研究表明,岩体形成时代为258.5±1.6Ma,为晚二叠世岩浆作用的产物。岩石中橄榄石Fo分子为82.8~86.7,均为贵橄榄石,辉石由斜方辉石和单斜辉石组成,斜方辉石种属为古铜辉石,单斜辉石为透辉石、普通辉石和顽透辉石。岩石地球化学特征显示,岩石属于亚碱性岩系列,主要氧化物SiO_2、Al_2O_3、CaO、Ti O_2、Na_2O+K_2O等与MgO呈规律性变化趋势,显示不同岩相岩石为同源岩浆演化的产物;岩石相对富集大离子亲石元素(Rb、Ba、Sr),而强烈亏损高场强元素(Nb、Ta、Zr、Hf、Ti),具有岛弧岩浆岩特征;锆石Hf同位素显示εHf(t)均为正值,变化范围为0.1～6.7之间,具有幔源岩浆成因特征;岩石原生岩浆为MgO含量略大于13.21%的高镁拉斑玄武质岩浆,岩浆源区为受俯冲洋壳析出流体交代地幔楔而成的富集地幔。岩石结晶过程中发生了强烈的结晶分异作用和同化混染作用,结晶分异作用在岩浆演化的初期使岩浆中硫发生初步富集,地壳混染作用是岩浆中硫化物达到饱和并发生熔离的主要因素。