西藏扎雪地区始新世钾玄岩特征及其构造环境  

Characteristics and tectonic setting of Eocene shoshonite in Zaxue area of Tibet

作　　者：单久库 

机构地区：[1]黑龙江省地球物理勘察院,哈尔滨150036

出　　处：《世界地质》2009年第2期171-178,共8页World Geology

基　　金：国土资源部地质调查项目(200213000013)

摘　　要：西藏扎雪地区钾玄岩包括浅成侵入的石英二长斑岩和帕那组的火山岩，分布于冈底斯岩浆弧带中段。石英二长斑岩侵入于前奥陶纪松多岩群和石炭-二叠系来姑组中，并被帕那组火山岩呈火山沉积不整合覆盖。钾玄岩中的石英二长斑岩K—Ar年龄为54．42Ma，帕那组火山岩中的粗安岩K—Ar年龄为41．46～45．14Ma，时代为始新世。岩石学和岩石化学特征显示典型的钾玄岩特征。在稀土配分曲线上，显示轻稀土富集，重稀土亏损，存在弱至中等的负铕异常。微量元素在原始地幔标准化图解上，出现Rb、Th、Hf明显的峰和Ba、Sr明显的谷，类似于“岛孤”环境的岩石特征；而大离子亲石元素Ba、K和高场强元素Hf、Zr的相对富集则反映了板内钾玄岩的特点。扎雪地区钾玄岩具有岛弧和板内火成岩的双重地球化学特点，与青藏高原上广泛发育的后碰撞火成岩极其相似。微量元素和稀土元素特征显示该套钾玄岩形成于陆壳加厚、部分熔融环境，是印度板块和欧亚板块后碰撞阶段的产物。The shoshonites in Zaxue area of Tibet, includifig the hypabyssal intrusive quartz-monzonite porphyry and the volcanic rocks of Pana Formation, distribute in the central part of Gangdese magmatic belt. The quartz-monzonite porphyry intrusived in the Pre-Ordovician Songduo Group and Carboniferous-Permian Laigu Formation, unconformably covered by the volcanic rocks of Pana Formation. The K-Ar age of quartz-monzonite porphyry in shoshonites is 54. 42 Ma, the K-Ar age of trachyandesite in Pana Formation volcanic rocks is 41.46-54.42 Ma, which indicates the Eocene in age. Their petrological and lithogeochemical characteristics show of typical shoshonites. The REE pattern displays LREE is rich, HREE is depletion and there is weak-medium Eu negative anomaly. The curve of trace element distribution patterns normalized by the primitive mantle demonstrates Rb, Th, Hf as a wave peak and Ba, Sr as wave trough , indicating the shoshonites were formed in the island-arc setting （IAT）. The richment of LILE （ Ba, K） and HFSE （ Hf, Zr） reflects the feature of plate shoshonites. Zaxue shoshonites have both geochemical characteristics of IAT and plate igneous rocks, which is consistent with the postcollision igneous rocks in Tibet. The characteristics of REE and trace element show the shoshonites formed in continental crust thickening and partial melting setting, is a product of the post-collision between Indo and Eurasia Plates.

关 键 词：钾玄岩 构造环境 始新世 扎雪地区 西藏 

分 类 号：P588.145[天文地球—岩石学] P595[天文地球—地质学]