青海治多地区晚三叠世石英闪长岩地球化学特征及成岩动力学背景

对羌塘地块东北缘甘孜-理塘缝合带南侧日啊日曲石英闪长岩进行了系统的锆石U-Pb年龄、主量-微量元素及Sr-Nd同位素分析, 以探讨其岩石成因及地球动力学意义.LA-ICP-MS锆石U-Pb定年结果表明, 石英闪长岩侵位年龄为218±1 Ma, 为晚三叠世岩浆活动的产物.岩石SiO₂含量介于53.02%~62.06%之间, 富Al₂O₃(15.84%~17.00%)和CaO(6.71%~8.94%), 贫TiO₂(0.49%~1.01%)和P₂O₅(0.04%~0.12%), 具有较高的MgO(3.31%~5.66%)和Mg#(50~62), 属准铝质钙碱性系列; 岩体稀土元素总量较低(38.05×10-6~61.58×10-6), 轻重稀土分馏不明显, LILE富集, HFSE亏损, 具有含量较高的Cr(33.45×10-6~176.64×10-6)和Ni(13.34×10-6~43.62×10-6).全岩(87Sr/86Sr)_i比值较为一致(0.706 8~0.707 9), ε_Nd(t)变化于-5.5~-1.6.主微量元素及同位素结果表明日啊日曲石英闪长岩具有高镁闪长岩的特征, 与赞岐岩地球化学特征类似, 为俯冲沉积物来源的熔体交代岩石圈地幔形成的尖晶石相金云母二辉橄榄岩低度(10%~15%)部分熔融的产物, 岩浆演化过程中经历了辉石、角闪石等矿物的分离结晶.微量元素构造判别图解表明岩石为俯冲环境下的弧岩浆岩, 结合区内蛇绿岩、研究区南部陆缘弧火山岩和义敦岛弧埃达克岩的发现, 认为古特提斯洋在晚三叠世时期可能仍处于消减状态, 日啊日曲高镁石英闪长岩形成于甘孜-理塘洋南西向俯冲过程中. 

Petrogenesis and Geodynamic Setting of Late Triassic Quartz Diorites in Zhiduo Area,Qinghai Province

Zircon U-Pb age, major and trace elements and Sr, Nd isotope compositions of the Riariqu quartz diorites from the south of the Ganzi-Litang suture zone, northeastern Qiangtang terrane are studied to decipher its petrogenesis and geodynamic significance. LA-ICP-MS zircon U-Pb dating yields an emplacement age of 218±1 Ma for the intrusion. Bulk-rock analyses show that these rocks belong to the calc-alkaline series, with SiO₂ contents ranging from 53.02% to 62.06%. They are enriched in Al₂O₃ (15.84%-17.00%) and CaO (6.71%-8.94%), depleted in TiO₂ (0.49%-1.01%) and P₂O₅ (0.04%-0.12%), defining their metaluminous characteristics. All samples have high concentrations of MgO (3.31%-5.66%), Cr (33.45×10-6-176.64×10-6) and Ni (13.34×10-6-43.62×10-6), and resultant high Mg# (50-62). These rocks are enriched in LREE and LILE, and depleted in HFSE with low REE contents (38.05×10-6-61.58×10-6). The (87Sr/86Sr)_i ratios range from 0.706 8 to 0.707 9, and ε_Nd(t) values vary from -5.5 to -1.6. The geochemical and Sr-Nd isotopic compositions of the Riariqu quartz diorites are similar to those of high-Mg dioritoids/sanukitoids. We contend that the Riariqu quartz diorites were derived from low-degree partial melting (10%-15%) of a phlogopite-bearing spinel lherzolite mantle, which was metasomatized by sediment-derived melts. Fractional crystallization of pyroxene and amphibole might also occur during the magma evolution. The geochemical characteristics indicate that the Riariqu quartz diorites are arc-related magmatic rocks, and were generated in a subduction-related tectonic setting. Combined with the ophiolites and continental marginal arc lavas in the study area, and the regional adakites in the Yidun terrane, it is suggested that the Riariqu quartz diorites might be generated during the southwestward subduction of Ganzi-Litang Paleo-Tethys Ocean in the Late Triassic.