冈底斯中段林子宗火山岩岩石地球化学特征

Petrological and Geochemical Characteristics of the Linzizong Volcanic Rocks in the Central Gangdise Area

广泛发育在冈底斯岩浆岩带中的林子宗火山岩及其与下伏地层间的区域不整合提供了印度-亚洲大陆碰撞的重要证据。谢通门地区的林子宗火山岩早期以中基性-中性岩为主,夹少量流纹质凝灰岩,晚期以流纹质火山岩为主。岩石学和地球化学研究表明,这套火山岩早期以钙碱性为主,带有较多陆缘火山岩特征,中期开始出现标志陆内活动的钾玄岩,晚期更多地显示了加厚陆壳条件下火山岩的特点,记录了由新特提斯俯冲消减末期过渡到印度-亚洲大陆碰撞的信息。中基性岩浆来源于俯冲带的地幔源区,长英质岩浆形成于加厚地壳的部分熔融。结合区域同位素年龄资料,可以认为林子宗火山岩中高钾流纹质火山岩是印度-亚洲大陆碰撞阶段陆壳缩短加压升温引起部分熔融的产物。

The widely developed Linzizong volcanic rocks in the Gangdise magmatic belt and the regional unconformity between the volcanic rocks and the underlain strata are important evidences for the collision of Indian-Asian continents. The Linzizong volcanic rocks distributed in the Xietongmeng area are mainly early stage intermediate-basic and intermediate volcancic rocks with minor rhyolite-tuff and late stage rhyolitic volcanic rocks. The petrological and geochemical studies show that the Linzizong volcanic rocks recorded the information of the transitional processes from the later Neo-Tethyan oceanic plate subduction to the Indian -Asian continental collision, as the early stage Linzizong volcanic rocks are mainly calc-alkali rocks with obvious characteristics of continental margin-arc volcanic rocks,the middle stage Linzizong volcanic rocks contain shoshonite reflecting intra-continental volcanism and the late stage Linzizong volcanic rocks show features of volcanic rocks formed under crust thickening. It is believed that the intermediate-basic magma was derived from a mantle source under subduction zone, while the felsic magma was generated by partial melting of thickened crust. In combination with regional K-Ar isotopic dating data,it is suggested that the high K rhyolitic volcanic rocks was formed by partial melting of the thickened continental crust due to the increase of pressure and temperature caused by the collision of Indian-Asian continents.