东昆仑祁漫塔格小圆山地区辉长辉绿岩年龄、地球化学特征及对区域构造演化的约束

Zircon U-Pb ages,geochemical features and constraints on regional tectonic evolution of gabbro-diabase in Xiaoyuanshan area,Qimantage of East Kunlun Mountains

通过东昆仑祁漫塔格小圆山辉长辉绿岩岩相学、年代学和岩石地球化学研究,探讨其岩浆作用及其源区、成岩构造环境、年龄及地质意义。结果表明,辉长辉绿岩为拉斑玄武岩系列岩石,SiO_(2)含量为48.27%~50.06%,全碱Na_(2)O+K_(2)O含量低,为3.10%~4.04%,且钠较钾富,MgO含量为7.81%~8.60%,TFe_(2)O_(3)含量为10.69%~12.00%,TiO_(2)含量为1.32%~1.76%;∑REE平均为58.13×10^(-6),δEu值为1.02~1.24,平均为1.12,呈轻微正异常;稀土元素球粒陨石标准化配分模式图上呈Gd略亏损的近平坦曲线,具有轻、重稀土元素分异不明显的轻稀土元素弱富集特征;岩石明显富集大离子亲石元素Cs、Rb、Ba、K、Sr,活泼的不相容元素U和Th,轻稀土元素及Pb,相对亏损高场强元素Nb、Ta和P。采用SIMS锆石U-Pb定年方法,获得小圆山辉长辉绿岩的年龄加权平均值为415±16 Ma(MSWD=5.0)和243±11 Ma(MSWD=4.1),前者代表辉长辉绿岩的结晶年龄,后者应代表晚期深部岩浆上升侵位到辉长辉绿岩中形成的锆石结晶年龄,是对中三叠世末期伸展活动的响应。结合岩石地球化学、构造特征和区域地质背景,认为小圆山辉长辉绿岩是拉张型岛弧背景下亏损的岩石圈地幔平衡部分熔融的产物,受到一定程度地壳物质的混染。

This paper discusses magmatism,source area,diagenetic tectonic environment,age and geological significance of Xiaoyuanshan gabbro-diabase(XGD)in Qimantage,East Kunlun,China,through the study of petrography,chronology and rock geochemistry.The test data indicated that the content ranges of SiO_(2),Na_(2)O+K_(2)O,MgO,TFe_(2)O_(3),and TiO_(2) were 48.27%~50.06%,3.10%~4.04%(Na_(2)O>K_(2)O),7.81%~8.60%,10.69%~12.00%,and 1.32%~1.76%respectively,supporting that XGD should belong to tholeiitic series rocks.The average value of∑REE,in addition,was 58.13×10^(-6),and theδEu value was 1.02~1.24 with its average of 1.12,showing slightly negative Eu anomaly.The normalized distribution pattern of REE chondrites presented a nearly flat curve with a slight loss of Gd,and had weak enrichment characteristics of LREE with no obvious differentiation between LREE and HREE.The rock was obviously enriched in large ion lithophile elements(Cs,Rb,Ba,K and Sr),active incompatible elements(U and Th),relatively depleted in high field strength elements(Nb,Ta and P),LREE,and Pb.The crystallization age of XGD and the zircon crystallization age formed by the late deep magma rising and emplacement into XGD were respectively obtained to be 415±16 Ma(MSWD=5.0)and 243±11 Ma(MSWD=4.1)by SIMS zircon U-Pb method.And the latter indicated a response to the extension activity at the end of the Middle Triassic.Conclusively,it was considered that XGD was a product from partial melting of the depleted lithospheric mantle balance in the extensional island arc background,and was contaminated by crustal materials to a certain extent.