北祁连造山带东端早古生代岩浆作用过程——来自甘肃天水长沟河闪长质片麻岩锆石年龄、微量元素及Hf同位素的证据  

Paleozoic magma evolution at the eastern end of Northern Qilian orogenic belt:Evidence from the zircon U-Pb ages, trace elements and Hf isotopic composition of Changgouhe dioritic gneiss

作　　者：张佳瑶 何艳红 陈亮[1] 徐可心[1] ZHANG Jiayao;HE Yanhong;CHEN Liang;XU Kexin(State Key Laboratory of Continental Dynamics,Northwest University/Department of Geology,Northwest University,Xi’an 710069,Shaanxi,China)

机构地区：[1]西北大学大陆动力学国家重点实验室/西北大学地质学系

出　　处：《地质通报》2019年第10期1626-1636,共11页Geological Bulletin of China

基　　金：国家自然科学基金项目《早古生代胶-辽-吉带岩浆作用:地球化学和年代学研究》(批准号:41421002);国家自然科学创新群体项目《大陆构造与动力学》(批准号:41102121)

摘　　要：甘肃天水地区长沟河闪长岩位于北祁连造山带东端,发育片麻状构造面理,长期以来被认为是形成于前寒武纪的侵入岩体。用LA-ICP-MS技术测得2组岩浆锆石的206Pb/238U年龄为463.3±2.3Ma(MSWD=0.52,n=11)和443.8±2.6Ma(MSWD=0.44,n=9)。锆石微量元素分析表明,2组年龄具有一致的微量元素组成,说明二者晶出于同源封闭的岩浆体系。t-Eu/Eu*投图显示,约460Ma的岩浆锆石无Eu异常,与幔源岩浆锆石稀土元素特征一致。而约440Ma的岩浆锆石具有负Eu异常,指示母岩开始发生斜长石的结晶分离,与深熔作用过程一致。因此,约460Ma为长沟河闪长质片麻岩原岩的形成年龄,约440Ma为长沟河闪长质片麻岩深熔岩浆的结晶年龄。Hf同位素分析测试结果显示,约460Ma和约440Ma两组锆石的εHf(t)值分别为8.23~11.57和6.36~8.03,指示460Ma发育幔源新生地壳岩浆作用,而440Ma壳源再造活动增强。Outcropped at the eastern end of the Northern Qilian orogenic belt in Tianshui area of Gansu Province, Changgouhe dioritic intrusion has traditionally been considered as Precambrian intrusive rocks due to their gneiss structure. The results of LAICP-MS zircon U-Pb dating in this study show two groups of magmatic zircon crystallization ages at 463.3±2.3 Ma(MSWD=0.52,n=11) and 443.8 ± 2.6 Ma(MSWD=0.44, n=9). Zircon trace element analyses show that these two groups share the similar trace element compositions, indicating that they crystallized in a closed magmatic system. t-Eu/Eu~*diagram shows that the magmatic zircons of about 460 Ma have no Eu anomalies, similar to the REE patterns of mantle-derived zircons. The negative Eu anomaly of about 440 Ma magmatic zircons indicates that the parent magma began to crystallize plagioclase, consistent with the process of the anatexis. Therefore, about 460 Ma is the formation age of Changgouhe dioritic gneiss, and about 440 Ma is the crystallization age of anatexis magma of Changgouhe dioritic gneiss. Hf isotope analysis shows that εHf(t) values of about 460 Ma and about 440 Ma zircons are 8.23~11.57 and 6.36~8.03 respectively, indicating that mantle-derived juvenile crustal magma was developed at about 460 Ma and that more involvement of crustal remelting took place at about 440 Ma.

关 键 词：北祁连造山带东端 长沟河闪长质片麻岩 LA-ICP-MS锆石U-Pb年龄 微量元素 HF同位素 

分 类 号：P597.3[天文地球—地球化学] P534.4[天文地球—地质学]