唐古拉坳陷中-新生代高钾钙碱性火山岩及其成岩构造环境

GEOCHEMISTRY AND TECTONIC ENVIRONMENT OF MESOZOIC-CENOZOIC HIGH-K CALC-ALKALINE VOLCANIC ROCKS IN THE TANGGULA DEPRESSION, NORTHERN XIZANG (TIBET), CHINA

藏北唐古拉坳陷中央隆起带与北部凹陷带海相体罗－白垩纪地层中，夹杂一套以高钾安山岩、高铝玄武岩为主体的高钾钙碱性火山岩系列。主要岩石类型有玄武岩、高铝玄武岩、高钾安山岩、英安岩、流纹岩及流纹质角砾凝灰岩。根据火山岩产出特征和全岩同位素年龄测定，将本区火山活动划分为中生代晚期和老第三纪两个旋回。这套火山岩系的REE、相容元素和不相容元素分布特点与造山带火山岩类似；在构造判别图解上，其投影点均落在造山带大山岩区域内。对火山岩特征分析表明，它们的形成与俯冲作用有关，显示出在俯冲造山带的构造背景下演变而来。

Mesozoic-Cenozoic high-K calc-alkaline volcanic rocks are widely distributed in the Tanggula Depression. The volcanic rocks mainly include basalt, high-Al basalt, andesite, dacite, rhyolite and/or rhyolitic pyroclastic rocks. Rb-Sr and K-Ar isotope studies indicate that the volcanic rocks and intrusive rocks similar in diagenetic age, which involved two cycles: one is from Late Jurassic to Early Cretaceous, i. e., 148. 8 ~ 106. 5 Ma; and the other is Eocene, i. e., 49. 1 ~40. 8 Ma. Trace elements in the volcanic nds are characterized by low Ti and Ta and high K and Rb, as is encountered in orogenic volcanic rocks. The REE patterns in the rocks are characterized by LREE enrichment. The volcanic rocks in this area axe related to plate subduction. In a word, the Mesozoic-Cenozoic high-K calcalkaline volcanic rocks along the central rise sone and northern hollow ane of the Tanggula Depression and their tectonic settings are closely related to the subduction of the Lhasa plate, i. e., the rocks were developed in a subduction orogenic belt.