藏南特提斯喜马拉雅带始新世隆子-恰嘎次火山岩区:雅拉香波二云母花岗岩的高位岩浆体系

已有的数据表明,大约在中始新世(44～40Ma),西藏特提斯喜马拉雅带(雅拉香波穹窿地区)经历了一次特别的地壳深熔作用,产生了大量高Sr/Y比值的二云母花岗岩.在雅拉香波穹窿南部的隆子-恰嘎地区,发育一套流纹质次火山岩,以小岩体或岩脉形式侵位于侏罗纪日当组的砂岩和页岩之中.岩相学观察、锆石U-Pb年代学、全岩元素和同住素(Sr、Nd)地球化学数据表明:(1)该次火山岩形成于约43～41Ma,与北部二云母花岗岩体的形成年龄相似；(2)该次火山岩经历了强烈的岩浆演化后期的岩浆-热液作用和钙长石分离结晶作用,导致该套岩浆岩强烈的Eu负异常、明显降低的Sr含量和锆石岩浆增生边的普通Pb和U浓度明显升高；和(3)该套次火山岩和二云母花岗岩属于同一岩浆过程的不同的构造层次,在时间、空间和成岩物质来源上具有一致性,同属于中始新世青藏高原主碰撞时中下地壳部分熔融的产物.

The Mid-Eocene subvolcanic field in the Lhunze-Qiaga area, Tethyan Himalaya, southern Tibet: A high-level magmatic suite related to the Yardio two-mica granite.

Recent studies in the Yardoi gneiss dome (YGD) of the Tethyan Himalaya, have demonstrated that the Tethyan Himalayan belt experienced a major episode of crustal melting at Mid-Eocene (44~40Ma) time, represented by the formation of a suite of high Sr/Y two-mica granites. Further south of YGD, a suite of subvolcanic rocks of rhyolitic composition occurs in the Lhunze-Qiaga area and intruded into black sandstones and yellow-greenish shales of the Tethyan sedimentary sequence. Petrographic observations, zircon U-Pb age and geochemical data show that: (1) the Lhunze-Qiaga rhyolitic subvolcanic rocks formed at ca. 43~41Ma and may represent the shallow counterpart of the two-mica granites crystallized at a much deeper level; (2) in contrast with the two-mica granites, the subvolcanic rocks had experienced intensive hydrothermal-magmatic interactions as well as An-rich plagioclase fractional crystallization as evidenced by pronounced negative Eu anomalies, much lower Sr concentrations, and enhanced common Pb and U concentrations in the magmatic zircon; and (3) these two suites of rocks formed a Mid-Eocene igneous edifice under crustal thickened conditions.