东昆仑南缘布青山构造混杂带亿可哈拉尔花岗闪长岩年代学、地球化学特征及构造意义研究

摘要

Located in Buqingshan tectonic mélange belt on the southern margin of Eastern Kunlun Orogeny, Yikehalaer granitoid comprises mainly of grayish white coarse grained granodiorite and fine grained granodiorite with gneissic structure and porphyaceous texture. The zircons of the rock assume typical magmatic zoning structure, suggesting the characteristics of magmatic zircon. The LA-ICP-MS zircon U-Pb age is (436.9±5.7) Ma, suggesting the formation of granitoid in Early Silurian. The authors studied in detail the geological and geochemical characteristics of Yikehalaer granitoid, and the results show that SiO2 content ranges from 66.08%to 72.22%, Na2O content ranges from 4.61%to 5.01%, and K2O content ranges from 0.95%to 3.59%, similar to the characteristics of high potassium calc-alkali granite. The trace elements and rare earth elements (RRE) data show that the Yikehalaer granitoid is also classified as a adakitic granite, with high content of Sr ranging from 280×10-6 to 493×10-6, low content of Y ranging from 3.76×10-6 to 11.7×10-6, low content of Yb ranging from 0.28×10-6 to 0.86×10-6, and high content of Sr/Y ranging from 23.93 to 125.0. Moreover, the chondrite-normalized REE patterns show rightly-dipping style, suggesting enrichment of LREE, with no obvious Eu anomaly. Petrogenetic research shows that the granitoid was most probably derived from the partial melting of the 10%~20% garnet amphibolite connecting the subducting oceanic crust. Regional tectonic research shows that the East Kunlun paleo-ocean basin in proto-Tethys domain experienced subduction from Cambrian until Early Silurian, which led to an important oceanic crust melting event at the late stage of oceanic crust subduction in Early Silurian.%亿可哈拉尔花岗闪长岩体呈构造混杂块体出露于东昆仑南缘布青山构造混杂带,主要岩性为灰白色片麻状、似斑状粗粒花岗闪长岩和细粒花岗闪长岩。该岩体 LA-ICP-MS 锆石 U-Pb 测年结果为(436.9±5.7) Ma,形成于早志留世。岩石地球化学特征显示,岩体具有高硅(66.08%~72.22%)、富钠(4.61%~5.01%)、弱过铝(A/CNK 介于1.04~1.11)钙碱性花岗岩特征,微量元素显示出典型的埃达克质花岗岩特征,即高 Sr(280×10-6~493×10-6)、低 Y(3.76×10-6~11.7×10-6)、Yb(0.28×10-6~0.86×10-6)、高Sr/Y(23.93~125.0)、轻重稀土元素分馏强烈、稀土元素配分曲线为向右倾斜型及铕异常不明显特征。岩石成因研究表明,亿可哈拉尔花岗闪长岩与俯冲洋壳板片熔融密切相关,为由俯冲洋壳变质形成的约含10%~20%的石榴石角闪岩部分熔融形成,源区熔融残留物主要为石榴石与角闪石。区域构造研究表明原特提斯构造域东昆仑古洋盆于寒武纪开始发生俯冲,俯冲作用可能持续至早志留世(437 Ma),并发生洋壳熔融事件。