西秦岭东段印支期花岗质岩浆作用及构造演化:地球化学、锆石U-Pb年代学及Hf同位素约束  

Granitoid magmatism and tectonic evolution in the eastern part of the West Qinling:Constraints from geochemistry,zircon U-Pb chronology and Hf isotopic

作　　者：李雪峰[1] 李永胜[2] 董国臣[1] 吕鑫[2] 夏清 LI XueFeng;LI YongSheng;DONG GuoChen;Lü Xin;XIA Qing(School of Earth Science and Resources,China University of Geosciences(Beijing),Beijing 100083,China;Development Research Center,China Geological Survey,Beijing 100037,China)

机构地区：[1]中国地质大学(北京)地球科学与资源学院,北京100083 [2]中国地质调查局发展研究中心,北京100037

出　　处：《岩石学报》2021年第6期1691-1712,共22页Acta Petrologica Sinica

基　　金：国家重点研发计划“深地资源勘查开采”重点专项(2018YFC0603704);中国地质调查局二级项目“整装勘查区矿产地质调查与找矿预测”(DD20190159)联合资助。

摘　　要：西秦岭造山带东段发育大量印支期花岗岩,由北向南贯通整个西秦岭造山带。本文选取西秦岭东段的柴家庄、太白、周家山和迷坝4个花岗质岩体进行岩石学、地球化学、锆石U-Pb定年和Lu-Hf同位素研究。这些花岗质岩体的岩性主体为石英闪长岩-花岗闪长岩-二长花岗岩,属高钾钙碱性系列,形成时代在237~219Ma之间,并显示出两期岩浆活动的特点,早期柴家庄岩体形成于236.6±2.9Ma,晚期的太白、周家山和迷坝岩体形成于220.12~218.9Ma。早期柴家庄二长花岗岩显示出类似埃达克岩的一些地球化学特点,如高含量的Sr(578×10-6~661×10-6)和高的(La/Yb)N比值(37.44~41.73),低含量的Y(3.48×10-6-5.50×10-6)和Yb(0.3×10-6~0.5×10-6),以及弱的负Eu异常(δEu=0.85~1);在晚期花岗岩中,迷坝和周家山岩体同样表现高Sr含量(Sr>400×10-6)、低Yb含量(Yb<2×10-6)的特点,但太白花岗岩则类似于喜马拉雅型花岗岩(Sr<400×10-6,Yb<2×10-6)。锆石Hf同位素组成显示,柴家庄、太白、周家山和迷坝岩体的εHf(t)值分别为-5.1~2.2、-4.8~-0.9、-6.0~-2.7和-6.3~-3.9,对应的二阶段模式年龄(tDM2)依次为2153~1495Ma、2120~1767Ma、2227~1932Ma、2249~2033Ma,表明西秦岭东段花岗质岩体的源区主要为古元古代-中元古代壳源物质,柴家庄岩体可能有部分新生下地壳物质加入,并且整体表现出结晶基底组成由北向南逐渐变老的趋势。显示埃达克岩特点的花岗岩可能形成于加厚下地壳的部分熔融,而其喜马拉雅型的花岗岩则是正常下地壳部分熔融的产物。三叠纪时期发生的陆陆碰撞导致地壳加厚,加厚下地壳在不同深度发生的部分熔融,形成本区具有埃达克或者喜马拉雅型地球化学特点的花岗质岩体。In the eastern part of the West Qinling Orogenic Belt,a large number of Indosinian granites are developed,which run through the entire West Qinling from north to south.In this paper,a detailed study of the petrology,geochemistry,zircon U-Pb chronology and Lu-Hf isotopic composition were made for the Chaijiazhuang,Taibai,Zhoujiashan,and Miba plutons in the eastern part of the West Qinling.The lithology of these plutons is quartz diorite,granodiorite and monzogranite.Geochemical data suggest that these granitic rocks are belonging to high-K calc-alkalic series.Zircon LA-ICP-MS data indicate that these plutons formed from 237 Ma to219 Ma,and shows two stages of magmatic activity.The Chaijiazhuang monzogranite has the geochemical characteristics of adakite,such as high Sr(578×10-6~661×10-6),(La/Yb)N ratios(37.44~41.73),and low Y(3.48×10-6~5.50×10-6)and Yb(0.3×10-6~0.5×10-6),and weakly negative Eu anomalies(δEu=0.85~1.00).The Miba and Zhoujiashan plutons also show the characteristics of high Sr(Sr>400×10~6)and low Yb(Yb<2~10-6),but Taibai monzogranite is similar to Himalaya-type granite(Sr<400×10-6,Yb<2×10-6).The zircon in situ Lu-Hf isotopic analysis shows that theεHf(t)values of the Chaijiazhuang,Taibai,Zhoujiashan,and Miba plutons are-5.1~2.2,-4.8~-0.9,-6.0~-2.7,and-6.3~-3.9,respectively,and the corresponding two-stage model ages(tDM2)are 2153~1495 Ma,2120~1767 Ma,2227~1932 Ma,and 2249~2033 Ma.The Hf isotopic compositions of zircons suggest that the granitoids in the eastern part of the West Qinling were mainly derived from Paleoproterozoic to Mesoproterozoic crustal materials,and a little of juvenile crust material might be added in the source area of Chaijiazhuang pluton.At the same,the composition of crystalline basement tends to age gradually from north to south.The granitoids that show the characteristics of adakite may be formed from partial melting of the thickened lower crust,while the Himalaya-type granitoids are the products of partial melting of the normal lower crust.The continental collision

关 键 词：早中生代 花岗岩类 锆石U-Pb定年 LU-HF同位素 西秦岭 

分 类 号：P588.121[天文地球—岩石学] P597.3[天文地球—地质学]