全吉地块金泉山—化石沟一带古生代 花岗质岩体地球化学及其构造意义

柴达木盆地北缘之全吉地块花岗质岩体大量发育,具多期次多阶段特征。通过对全吉地块金泉山—化石沟一带古生代花岗质岩体岩石学、岩石化学特征及单颗粒锆石U-Pb同位素定年,发现该区花岗岩有4次侵入,侵入时代分别为早奥陶世(471~476Ma)、中奥陶世(459±5Ma)、早志留世(423±4Ma)和中泥盆世(366±2Ma)。岩石地球化学研究显示,该4期花岗岩均具典型的钙碱性特征,轻稀土富集、重稀土轻度亏损、Eu轻微负异常—正异常,大离子亲石元素K2O、Rb、Ba、Th等相对强烈富集,高场强元素Nb、Ta、Hf、Zr及Yb明显亏损,除第4期岩体即具I型,又具S花岗岩特征外,其它各期次均属I型花岗岩,总体显示岩体具壳源特征,为板块碰撞前消减地区花岗岩,研究推测,金泉山—化石沟一带古生代花岗质岩体第1、2组年龄为全吉地块与柴达木陆块碰撞的时代,第3组年龄反映了深俯冲地下的板块由于拆沉而折返的时代,第4组年龄为碰撞隆起后造山带伸展、滑塌的时代。

GEOCHEMISTRY OF PALEOZOIC JINQUANSHAN-HUASHIGOU GRANITOIDS IN QUANJI MASSIF AND ITS TECTONIC SIGNIFICANCE

There are large quantities of Caledonian granitoids with multi stages Quanji massif in the north margin of Qaidam Block. Four phases of Caledonian -Variscan period granitoid intrusions were determined in Jinquanshan-Huashigou area by the study of LA-ICP/MS ages of zircons, lithological and geochemical characteristics of the granitoids. Each of them formed during Early Ordovician （471±476Ma）, Middle Ordovieian （459±5Ma）, Early Silurian （423±4Ma）, Middle Devonian （366±2Ma）. Intermediate acid intrusive rocks are occurred in the first phase of intrusion. The second phase of intrusion mainly consist of intermediate rocks. Both the third and the fourth phase of intrusions are mainly composed of acid rocks. Geochemical characteristics of the granitoids reveal that they were Z-type granites and magma production of igneous rock melt from orogenic belt. The granites were characterized as talc-alkaline island-arc granites, LREE enrichment, a deficit of HREE, enrichment of large-ion lithophile elements and depletion of high field strength elements （HFSE）. Geochemical characteristics suggest the granitoids were complex of combined action among partial melting of the lower crust, fractional crystallization of magma and assimilation and contamination of the upper crust, for the upper mantle under plating caused by the ocean crust subduction.