中祁连西段花岗岩类的地球化学特征及构造意义

中祁连西段石板墩地区北部及南部发育有辉长岩、闪长岩及花岗岩。北带岩体的LA-ICPMS锆石U-Pb年龄为(469.3±2.8)Ma,(461.2±3.3)Ma和(470.0±2.5)Ma;岩石Si O2质量百分含量为53.2%~66.11%,高Al,Ti,Mg,Fe,Ca,K,Na,A/CNK1.1;富集大离子亲石元素Rb,K和Pb,亏损高场强元素Nb,Ta,Ce,Sr,Hf,Ti;REE总量较高,LREE富集、HREE近平坦型分布,中等的负Eu异常,δEu=0.6;此外,在构造判别图中,样品落于岛弧环境。南带岩体LA-ICP-MS锆石U-Pb年龄为(470.9±2.8)Ma和(472.3±4.2)Ma,岩石Si O2质量百分含量较高(67.13%~70.73%),高Al,Mg,Fe,Ca,富Na贫K,A/CNK1.1;岩石表现为明显的高Sr低Yb,Y的特点,富集大离子亲石元素R,K,Sr等,亏损高场强元素Nb,Ta,P,Ti等;REE总量较低,LREE富集、HREE亏损,轻重稀土分异明显,轻微的负Eu异常至正异常,δEu=0.74~1.18。研究表明,北带岩体为地幔楔部分熔融产生的,形成于岛弧环境;南带岩体为消减的大洋岩石圈板块部分熔融产生的埃达克岩,是北祁连向南俯冲导致的岩浆作用的产物,"中祁连西段"是早古生代时期在"残留的微陆块"基础上形成的一个岛弧增生杂岩地体。

Geochemical Characteristics and Tectonic Significance of the Granotoids in the Western Section of the Mid-Qilian

Granotoids, mainly gabbro, diorite and granite, are greatly rich in the northern and southern parts of Shibandun area, western section of the Mid-Qilian. LA-ICP-MS zircon and U-Pb dating methods indicate that the age of the rock from the northern belt were（469.3±2.8）Ma,（461.2±3.3）Ma and (470.0±2.5) Ma, respectively. The SiO₂ content of the northern rock mass ranges from 53.2% to 66.1% with high content of the Al, Ti, Mg, Fe, Ca, K, Na in the rock mass and A/CNK<1.1. The rock analysis shows enrichment of large ion lithophile elements (LILE) (e.g., Rb, K) and Pb, and depletion of high field strength elements (HFSE) (e.g., Nb, Ta, Ce, Sr, Hf and Ti). The results also indicate LREE enriched pattern with high ∑REE abundance, flat HREE and moderate negative Eu anomaly (δEu=0.6). Tectonic discrimination diagrams showed that the samples are located in the arc environment. The age of the rock from the southern belt are respectively （470.9±2.8） Ma and （472.3±4.2） Ma based on LA-ICP-MS zircon U-Pb dating methods. The southern rock mass contains high amount of the Al, Mg, Fe, Ca and is rich in Na and poor in K (A/CNK<1.1). Rock analysis results show the enrichment of large ion lithophile elements (LILE) (e.g., Rb, K and Sr) and depletion of high field strength elements (HFSE) (e.g., Nb, Ta, Ti and P), and also indicate a LREE enriched pattern with rather low ∑REE, depleted HREE and obvious differentiation in Eu, which is from slight negative to positive anomaly (δEu=0.74~1.18). Our study shows that the northern belt was formed by partial melting of mantle wedge in an island-arc environment, while the southern belt is adakites generated by partial melting of Adakites, and is also the product from the magmatic process by the North Qilian southward subduction. The western part of the Mid-Qilian is an island arc hyperplasia complex created on a remained micro continental in early Paleozoic.