造山带岩浆作用的强度和旋回性：以东昆仑古特提斯花岗岩类岩基为例

Intensity and cyclicity of orogenic magmatism: An example from a Paleo-Tethyan granitoid batholith,Eastern Kunlun,northern Qinghai-Tibetan Plateau

大陆造山带的重要特征之一是常常发育强烈的岩浆活动。如何刻画造山带古岩浆作用的强度?岩浆从深部添加到中上地壳的速率有多大?岩浆活动强弱的变化规律如何?这些都是近年来国际上有关岩浆动力学的新的研究主题。位于青藏高原北部的东昆仑造山带岩浆活动强烈,尤其发育与古特提斯构造演化相关的晚二叠世-晚三叠世岩浆活动(年龄主要介于270~200Ma之间)。发育于东昆仑东段的香日德复式花岗岩类岩基,主要由早、中、晚三叠世的石英闪长岩、二长花岗岩和花岗闪长岩等多种岩性单元构成,岩石类型多,时间跨度大(258~218Ma),是造山带岩浆活动的缩影。本文以香日德岩基为例,在深入的野外地质、岩石学和年代学研究基础上,结合各岩性单元的出露面积、U-Pb锆石年龄和假定的岩体厚度,对岩浆的体积添加速率(岩浆通量)进行了估算,发现从早到晚,该岩基的岩浆体积添加速率存在着旋回性的变化特征。其中,早三叠世石英闪长岩的岩浆体积添加速率最低,而晚三叠世花岗闪长岩的岩浆体积添加速率则最高,中三叠世介于其间。以2.5km的岩体厚度计算得到的岩浆体积添加速率分别是,早三叠世峰期的添加速率约33km^3/m.y.,中三叠世峰期添加速率约50km^3/m.y.,而晚三叠世的峰期添加速率达到了99km^3/m.y.。研究表明,从230Ma开始,东昆仑地区晚三叠世时期Hf同位素初始比值显著增大,并出现了大规模的铁多金属成矿作用。因此,晚三叠世时期该区强烈的Fe多金属成矿大爆发是在亏损地幔岩浆底侵影响下,发生大规模的花岗岩类岩浆活动和壳幔岩浆活动的结果。

Voluminous magmatism is commonly occurred in continental collision orogenic belt. The magma flux,magma addition rate and magma cyclicity are recent focuses of increasing interests on the global studies of magma dynamic in orogens. The Eastern Kunlun Orogen,located in northern Qinghai-Tibetan Plateau,provides an ideal place for such studies,since Late Permian to Early Triassic magmatism（ mainly 270 ~ 200Ma） related to Paleo-Tethys tectonic evolution is widespread. In this work,we present a case study of the Xiangride granitoid batholith which is typical for the Eastern Kunlun Orogen magmatism. The Xiangride batholith consists of variable rock types,including quartz-diorite,monzogranite and granodiorite,with an intrusive period from 258 Ma to 218 Ma. Based on detailed field investigation,petrological and geochronological studies,we conduct theoretical calculations on the magma flux and magma addition rate of this complex. Our results suggest that obvious magma cyclicity exist in terms of the magma addition rate in Xiangride complex,that is,the magma addition rate of the Early Triassic quartz diorite is the lowest,followed by the Middle Triassic monzogranite,and the Late Triassic granodiorite exhibits the highest. Given a thickness of 2. 5km for the Xiangride batholith,we obtained magma addition rates for Early,Middle and Late Triassic magmatisms varying from 33,50 to 99km3/ m. y.,respectively.Combined with the clearly high εHf（ t） values of Late Triassic magmatic rocks,we propose that the large abundance of Late Triassic skarn iron ore deposits in Eastern Kunlun Orogen are more likely genetically related to a granitoid magmatic flare-up as a result of contemporaneous depleted mantle-derived magma underplating and magma mixing in this region.