腾冲地体坡仑山岩体锆石U-Pb年龄和地球化学组成

滇西特提斯构造域广泛发育中-新生代岩浆作用，是长期复杂的造山作用的结果。出露于腾冲地块的坡仑山花岗岩 体锆石U-Pb年龄和地球化学特征显示，坡仑山岩体主要岩石类型为斑状花岗闪长岩，以全碱含量高为特征，属于高钾钙碱 性系列，富集大离子亲石元素、亏损高场强元素（Nb、Ta、Ti和P）。锆石U-Pb定年结果指示岩体的侵位时代为66~63 Ma， 是新特提斯洋汇聚过程的产物。部分锆石颗粒或晶区具有高的U和Th含量特征，其U-Pb年龄集中在35~30 Ma，明显年轻 于岩体侵位时代，而与高黎贡山变质带变质-变形时代吻合，可能反映高U-Th锆石同位素体系遭受热事件重置。全岩 Sr-Nd同位素组成显示富集特征，初始87Sr/86Sr比值介于0.7158和0.7164之间，初始εNd值为-14.5至-15.3，对应Nd模式年龄 值为2.04~2.10 Ga，推测岩浆源区可能以古元古代地壳物质为主。

Zircon U-Pb Ages and Geochemical Composition of the Polunshan Pluton in the Tenchong Block,Western Yunnan

Mesozoic to Cenozoic magmatic activities, widely distributed in the Tethyan belt of western Yunnan, are induced by the long-time complicated orogenic processes. Zircon U-Pb age data and geochemical characteristics of the Polunshan granite, exposed in the Tenchong block, are reported in this study. The Polunshan granite pluton is composed mainly of porphyritic coarse-grained granodiorites. Geochemically, this granite is characterized by high alkali contents and belongs to the high-potassic calc-alkaline rock type. It enriches in large-ion lithosphile elements and depletes in high field-strength elements (Nb, Ta, Ti and P). Results of the zircon U-Pb dating suggest emplacement ages of 66~63 Ma for the magma(s) of the Polunshan granite, pointing to a magmatic product related to the closure of the Neo-Tethys Ocean. Many analysed zircon grains or crystal domains have high contents of U and Th. They yield U-Pb ages of about 35~30 Ma, significantly younger than emplacement time of the granite, but similar to the age of deformation or metamorphism of the Gaoligongshan metamorphic belt. This phanomenon implies that the U-Pb isotopic system of those zircons of high Th-U-contents was reset during the late thermal event. The Polunshan granite exhibits entrenchment feature in Sr-Nd isotopic composition, with initial 87Sr/86Srratios of 0.7158 to 0.7164 and initial εNd values of -14.5 to -15.3, corresponding to Nd modal ages of 2.04~2.10 Ga, suggesting that Paleoproterozoic crustal material as a major source of the magma(s).