青海玉树地区扎喜科岩体形成时代、地球化学特征及构造意义研究

Emplacement Age,Geochemical Characteristics and Tectonic Significance of Zhaxike Quartz Diorite in Yushu Area,Qinghai Province

青海玉树地区的扎喜科石英闪长岩体出露于西金乌兰-金沙江缝合带西段。地球化学特征表明,岩石的轻稀土富集,(La/Yb)N为2.56～4.37,Eu负异常,富集大离子亲石元素,亏损高场强元素,K*在4.18～4.89范围内,RbN/YbN比值范围在9.77～12.09间,这些特点均显示了岛弧岩浆岩的特征;运用Maniar主量元素判别法判定其为IAG型花岗岩(造山型花岗岩类),形成于板块俯冲阶段,在Pearce的构造判别图上也得到了很好的验证。通过对其进行角闪石40Ar/39Ar测年得到(222.3±1.7)Ma的年龄数据,代表其侵位时代,是印支期岩浆活动的产物。结合岩体的岩石学、岩石地球化学特征以及年代学研究结果可以初步确定,至晚三叠世,即约222Ma,西金乌兰-金沙江缝合带在沿扎喜科岩体处仍处于岛弧环境,板块俯冲作用仍在继续。这一结果对古特提斯末期的演化历史给予了岩石学和年代学的限制。

Zhaxike quartz diorite is located in Yushu area of Qinghai province, and is ex Jinshajiang suture zone. Petrologic and geochemical studies suggest that the samples posed along the western have the characteristics of negative Eu anomalies （5Eu=0.6-0.76） and LREE enrichment （（La/Yb）N=2.56-4.37）. All rock samples are enriched in large ion lithophile elements （LILE） and depleted in high field strength elements （HFSE）, K＊ values range from 4.18 to 4.89, and RbN/YbN ratios range from 9.77 to 12.09. These data show that Zhaxike quartz diorite is magmatic rocks of island arc. The authors used both major elements and trace elements to determine its tectonic setting. The tectonic discrimination diagrams show that Zhaxike quartz diorite is island arc granitoids （IAG）related to plate subduction. The 40Ar/39Ar plateau age of hornblende shows that the intrusion of Zhaxike quartz diorite took place at about （222±1.7） Ma in Indosinian period. Based on the study of petrology, geochemistry and chronology, the a setting in the late the petrologic and uthors preliminarily hold that Xijinwulan-Jinshajiang suture zone remained in the island arc Triassic period, and the subduction of the plate still continued. The results of the study provide chronologic constraints for the last evolution of the Paleo-Tethys.