海南岛乐东地区志仲岩体锆石U-Pb年代学、Hf同位素研究及其构造意义  

Zircon U-Pb Ages,Hf Isotopic Composition of Zhizhong Granitic Intrusion in Ledong Area of Hainan Island and Their Tectonic Implications

作　　者：温淑女[1,2] 梁新权[1] 范蔚茗[1] 王岳军[1] 池国祥 梁细荣[1] 周云[1] 蒋英[1] 

机构地区：[1]中国科学院广州地球化学研究所,同位素地球化学国家重点实验室,广东广州510640 [2]中国科学院大学,北京100049 [3]Department of Geology,University of Regina,Regina,Saskatchewan,Canada S4S 0A2

出　　处：《大地构造与成矿学》2013年第2期294-307,共14页Geotectonica et Metallogenia

基　　金：中国科学院重大项目(编号:KZCX-YW-15-1);国家973项目(编号:2010CB808900);国家科技重大专项(2011ZX5023-004-011);国家自然科学基金项目(批准号:40872080,41072081)联合资助

摘　　要：海南岛分布大量的海西-印支期的花岗岩,关于这些花岗岩的成因及其成岩背景,特别是海西期花岗岩,一直是争论的焦点。本文采用LA-ICP-MS对海南岛乐东地区志仲花岗岩体进行了高精度的锆石U-Pb同位素年代学及锆石Hf同位素组成特征的研究。结果显示,两个花岗岩样品获得接近的272 Ma的结晶年龄,表明该花岗岩体形成于早二叠世晚期。锆石原位Hf同位素组成εHf(t)值变化于-3.7至+2.8之间,对应的两阶段模式年龄t DM2集中在1.12～1.53 Ga,表现出壳幔混合源区特征。据此,推测志仲花岗岩的形成岩浆很可能来源于海南中元古代火成岩的重熔,同时可能有新生地幔物质的加入。其两阶段模式年龄t DM2与前人研究的Nd二阶段模式年龄计算的结果(1.3～1.6 Ga)相近,表明中元古代是该区一期重要的地壳分异的时间,即海南岛在中元古代发生过重要的地壳增生事件。结合区域构造-岩浆活动-沉积记录及古地磁资料,我们趋向于认为海南岛内海西期岩浆作用应该是古特提斯演化过程中构造-岩浆作用的产物,并非古太平洋西向俯冲-碰撞的结果。Late Paleozoic to Early Mesozoic( Hercynian to Indosinian) granitic intrusions widely distribute in Hainan Island,South China. However,no consensus has been achieved in terms of the related tectonic regime of these magmatism activities,the Late Paleozoic granites,in particular. Aiming to provide insights into the time of intrusion,petrogenesis and tectonic setting of the Late Paleozoic granites,zircons from the Zhizhong granite,which is located in the Ledong area in southwestern Hainan Island,were analyzed. In-situ zircon U-Pb geochronology and Hf isotope analysis of zircons were analyzed in Northwestern University using LA-ICPMS methods. Zircons from two samples( HN0832 and HN0909) yield similar weighted mean206Pb /238U ages of 272. 4 ± 1. 5 Ma( n = 21) and271. 7 ± 3. 0 Ma( n = 16),respectively. The ages are interpreted as the intrusive age,i. e.,the Zhizhong granite intruded in late Early Permian. The εHf( t) values of the zircons range from-3. 7 to +2. 8,while the Hf two-stage modal ages vary from 1. 12 Ga to 1. 53 Ga with a peak at 1. 33 ～ 1. 41 Ga,demonstrating a mantle input for the Zhizhong granite. Thus,a mixed crust-mantle source is envisaged. Furthermore,the parent melts of the Zhizhong granite possibly derived from the melting of the Mesoproterozoic igneous rocks beneath Hainan Island with significant contribution of asthenosphere melts. Moreover,the age peaks of the two-stage Hf modal ages( 1. 3 ～ 1. 4 Ga)of the Zhizhong Granite are similar to the whole rock two-stage Nd modal ages( 1. 4 ～ 1. 6 Ga) of the coeval granites in the adjacent region,which implies that Mesoproterozoic is an important episode of crustal growth in Hainan Island. Combining the regional deformation,sedimentary records and palaeomagnetic data,it can be deduced that the late Early Permian magmatism in Hainan Island was probably geodynamically linked to the collision between the Indochina and South China Block,following the closure of the Paleo-Tethys Ocean rather than the westward subduction of the Paleo-Pacific Plate ben

关 键 词：LA—ICP—MS 锆石U—Pb定年 HF同位素 志仲岩体 海南岛 

分 类 号：P597[天文地球—地球化学]