藏北羌塘地区新生代火山岩岩石特征及其成因探讨

羌塘地区新生代火山岩在岩性上以粗安岩、粗面岩占优势,富碱（尤其是K2O）和LREE,属高原钾玄岩－高钾钙碱性系列,其岩性和地球化学特征与青藏高原北部可可西里及中昆仑的新生代火山岩完全可以对比。根据K－Ar同位素定年数据可将该火山活动划分为四个喷发阶段,时代分属始新世—渐新世。羌塘地区火山岩的形成与新生代以来高原深部岩石圈的演化关系密切,可能与青藏高原隆升早期阶段的陆内俯冲作用有关。

PETROLOGIC FEATURES AND GENESIS OF CENOZOIC VOLCANIC ROCKS, QIANGTANG AREA, NORTHERN TIBETAN PLATEAU

Cenozoic volcanic rocks in the Qiangtang area are located in northern Tibetan plateau, with a thicknessranging from 200 to 1000m, and are characterized by potassic lavas which consist of latidacite and latite.The volcanic rocks in the Qiangtang area belong to shoshonite and high-K calc-alkaline series. They arehighly enriched in alkaline, large-ion lithophile elements (Rb, Sr, Ba, and Th), LREE and depleted inhigh- field strength elements (Ta, Er, Hf and Ti). Chondrite-normalized REE pattems in the volcanicrocks show no significant Eu anomalies, and indicate that the source was plagioclase-free and probably located in the lower crust or upper mantle. The Cenozoic volcanic rocks which have similar petrologic and geochemical characteristics have been reported in northern Hoh Xil and Kunlun area. The volcanic rocks in theQiangtang area are mid-Eocene to Oligotene in age according to their K-Ar isotopic data. The intraplatesubduction southward Qiangtang played an important role in Cenozoic volcanism, early uplift of the Tibetanplateau and regional extension.