摘要:
南祁连北缘东段刚察大寺花岗岩的形成时代和成因一直存在争议。本文对其进行了锆石LA-ICP-MS U-Pb年代学和岩石地球化学研究,进而约束其岩石成因和形成的构造背景。结果表明,刚察大寺花岗岩主要由花岗闪长岩和黑云母二长花岗岩组成;花岗闪长岩中锆石自形程度较好,阴极发光图像显示振荡生长环带,具有较高的Th/U比值(0.27~0.91),表明其为岩浆成因;最小一组岩浆锆石的206Pb/238U加权平均年龄为(435±4) Ma,即岩体形成于早志留世;花岗闪长岩和黑云母二长花岗岩具有类似的地球化学属性,主量元素具有高硅、富碱、富铝和贫镁的特征,其SiO2=65.52%~74.23%、(K2O+Na2O)=6.95%~8.24%、Al2O3=12.33%~15.26%、MgO=0.31%~1.32%, A/CNK介于0.85~1.05之间,为准铝质-弱过铝质的亚碱性系列岩石;整体富集轻稀土元素和大离子亲石元素(Rb、Ba、K)、亏损重稀土元素和高场强元素(Nb、Ta),具有Eu的负异常(δEu=0.51~0.80)和Sr、P、Ti的明显亏损。南祁连北缘东段刚察大寺花岗岩为I型花岗岩,起源于高温低压条件下中、上地壳物质的部分熔融,结合区域构造演化暗示,刚察大寺早志留世花岗岩形成于活动大陆边缘的构造背景。
Abstract:
The formation age and petrogenesis of the Gangchadasi granites in the eastern segment of the northern South Qilian block remain controversial. New LA-ICP-MS zircon U-Pb dating and whole-rock geochemical analysis were conducted in this study to constrain the petrogenetic and tectonic processes during the formation of the granites. The Gangchadasi granites are mainly composed of granodiorite and biotite-monzogranite. Zircons from the granodiorite show euhedral shape, oscillatory CL zoning and high Th/U ratios (0.27 ~ 0.91), indicative of a magmatic origin. The weighted mean of the youngest group of concordant206Pb/238U ages yielded an emplacement age of (435±4) Ma. The granodiorite and biotite-monzogranite share similar geochemical characteristics, such as high content of SiO2 (65.52% ~ 74.23%), total alkali ((Na2O+K2O)=6.95% ~ 8.24%), and aluminum (Al2O3=12.33% ~ 15.26%), and low amounts of magnesium (MgO=0.31% ~ 1.32%). Their A/CNK ratios range from 0.85 to 1.05, indicating a transitional composition between metaluminous and peraluminous. The granites are enriched in large ion lithophile elements (e.g., Rb, Ba, and K) and light rare earth elements and depleted in high field strength elements (e.g., Nb and Ta) and heavy rare earth elements, and exhibit prominent negative Eu anomaly (δEu=0.51 ~ 0.80) and depletion of Sr, P, and Ti. It is held that the Gangchadasi granites are I-type granites derived from partial melting of a middle-upper continental crust under the condition of high temperatures and low pressures. In combination with regional tectonic evolution, the authors suggest that the Gangchadasi granites probably formed in an active continental margin during Early Silurian.
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