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盐场北山二长花岗岩位于柴北缘西端青海冷湖地区.岩体高硅(74.98%~76.92%),富钾(4.44%~5.93%),贫镁(0.04%~0.07%)和钙(0.43%~0.69%).里特曼指数介于2.43~2.79属于高钾钙碱性岩石系列,铝饱和指数平均为1.01,属弱过铝质花岗岩.岩体富集大离子亲石元素(K、Rb、Ba、Pb),亏损高场强元素(Nb、Ta),具岛弧花岗岩地球化学特征.LAICP-MS锆石U-Pb测年结果表明:岩体形成于252±3Ma;锆石Hf同位素初始比值176 Hf/177 Hf分布于0.282 958~0.283 032之间,并具有极高的正εHf(t)值(8.75~14.50),其平均两阶段模式年龄TDM2(Hf)为385Ma,反映岩体源区可能为玄武质下地壳.另外,εHf(t)基本位于亏损地幔演化线附近,指示该玄武质下地壳来源于亏损地幔.结合区域地质背景和岩石地球化学特征,认为盐场北山二长花岗岩与泥盆纪镁铁-超镁铁岩有关,由于宗务隆裂谷小洋盆的向南俯冲,引发其上覆玄武质新生地壳发生熔融形成,进而揭示晚二叠世末柴北缘处于火山弧或活动陆缘的构造环境.
Salt Mountain Beishan monzogranite is located in the Lenghu area of Qinghai Lake in the western end of northern Qaidam Basin. The contents of high silicon (74.98% -76.92%), potassium (4.44% -5.93%), magnesium (0.04% -0.07%) and calcium ~ 0.69%). The Rittman index ranged from 2.43 to 2.79, belonging to the high-K calc-alkaline rock series with an average aluminum saturation index of 1.01, belonging to weak peraluminous granites. The rock mass is enriched in large ion lithophile elements (K, Rb , Ba, Pb), depleted high field strength elements (Nb, Ta) and island arc-arc granite.The zircon U-Pb dating of LAICP-MS shows that the formation of the rock mass is 252 ± 3Ma, the zircon Hf isotope The initial ratio 176 Hf / 177 Hf is distributed between 0.282 958 and 0.283 032 and has a very high positive εHf (t) value (8.75-14.50). The average two-phase model age TDM2 (Hf) is 385Ma, In addition, εHf (t) is located near the depleted mantle evolution line, indicating that the lower crust of the basalt is derived from the depleted mantle. Combined with regional geological background and lithogeochemical characteristics, it is considered that the salt field Beishan monzonitic granite In connection with the Devonian mafic-ultramafic rocks, due to the subduction of the small basin of the Zongliu rift to the south, the basaltic upper crust of the upper Cretaceous is triggered to melt and form Revealing the tectonic setting of the northern margin of Late Permian in the volcanic arc or active margin.