藏南昂仁县桑桑地区林子宗群火山岩的形成时代和地球化学特征

桑桑地区林子宗群火山岩Sr、Nd同位素地球化学特征和SHRIMP锆石U-Pb地质年代学数据表明：①林子宗火山岩以高钾流纹岩为主,属于高钾钙碱性系列岩石,形成于49.8Ma±0.92 Ma,属于帕那组火山岩地层;②林子宗火山岩稀土元素配分模式较为一致,相对于HREE,强烈富集LREE;③林子宗火山岩具有相对低的初始Sr同位素值（87Sr/86Sr（i）=0.70488～0.70569）和较高的初始Nd值（εNd（i）=-1.38～-1.58）;④总体上富集大离子亲石元素,亏损高场强元素,与岛弧型火山岩的地球化学特征类似。桑桑地区林子宗火山岩在形成过程中明显受到角闪石和斜长石分离结晶作用的影响。可能是俯冲的新特提斯洋板片断离或变陡,进而导致经历过俯冲交代作用的富集岩石圈地幔甚至局部下地壳发生部分熔融,形成的初始岩浆发生混合作用,并在近封闭条件下发生高度分离结晶作用的产物。结合已有的结果,认为冈底斯带南带普遍经历了50Ma左右的岩浆作用。

Timing and geochemistry of the Linzizong Group volcanic rocks in Sangsang area,Ngamring County,southern Tibet

The Linzizong Group volcanic rocks in Sangsang area consist mainly of high-K rhyolite formed at 49.85±0.92Ma（SHRIMP zircon U-Pb age）,belonging to Pana Formation.They are characterized by（1） enrichment of LREE and depletion of HREE;（2） pronounced negative Eu anomalies;（3） relatively low initial Sr（87Sr/86Sr（i）=0.70488～0.70569） and high εNd values（εNd（i）=-1.38～-1.58）;and（4） relative enrichment of LILEs and depletion of HFSEs,similar to things of arc-type volcanic rocks.These data indicate that fractional crystallization of amphibole as well as plagioclase played an important role in the formation of the Sangsang volcanic rocks.The Sangsang volcanic rocks might have been derived from melting of mature continental materials and a metasomatic lithospheric mantle source induced either by the roll-back or by the break-off of the subducted Neo-Tethyan slab.The data obtained by the authors,in combination with those from the literature,further demonstrate that the southern Gangdise belt must have experienced a large-scale magmatic event at approximately 50 Ma.