丹凤花岗岩和基性捕虏体的地球化学和锆石U-Pb-Hf-O同位素:北秦岭超高压变质带两期构造抬升事件的记录  

Geochemistry and zircon U-Pb-Hf-O isotopic study of granite and mafic xenoliths in the Danfeng area:Recording two stages of tectonic uplift events in the North Qinling ultrahigh-pressure metamorphic belt

作　　者：张娟[1] 程昭 张宏福[1,2] 杨港 ZHANG Juan;CHENG Zhao;ZHANG HongFu;YANG Gang(State Key Laboratory of Continental Dynamics,Department of Geology,Northwest University,Xi’an 710069,China;School of Earth Sciences,Zhejiang University,Hangzhou 310058,China)

机构地区：[1]西北大学地质学系,大陆动力学国家重点实验室,西安710069 [2]浙江大学地球科学学院,杭州310058

出　　处：《岩石学报》2023年第1期55-73,共19页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41688103)资助.

摘　　要：北秦岭造山带中广泛出露的早古生代花岗岩及其中的基性捕虏体,对于讨论该造山带早古生代地壳及构造演化过程具有重要研究意义。本文报道了丹凤地区枣园二长花岗岩及其中的斜长辉石岩捕虏体与黄柏岔岩体中的斜长角闪岩捕虏体的全岩地球化学和锆石U-Pb-Hf-O同位素组成。斜长角闪岩和斜长辉石岩锆石U-Pb定年获得了493±8Ma原岩年龄和468±4Ma~460±5Ma的角闪岩相变质年龄。锆石U-Pb定年表明枣园花岗岩侵位于晚志留世(419±4Ma)。斜长角闪岩和斜长辉石岩SiO 2含量介于49.69%~50.73%之间,呈现镁质、准铝质和钙碱性特征;富集大离子亲石元素(LILEs)和轻稀土元素(LREE),亏损高场强元素(HFSEs);并且大部分锆石具有较高的εHf(t)值(-0.3~7.5)和锆石δ18 O值(5.61‰~8.20‰)。这些特征指示其原岩可能源于俯冲板片交代的地幔楔部分熔融。枣园花岗岩SiO 2含量为70.11%,呈现高硅碱,贫铁镁,弱过铝,富集LILEs和LREE以及亏损HFSEs的特点;与I型花岗岩的元素地球化学特征一致;花岗岩具有较高的εHf(t)值(0.1~3.9)和δ18 O值(8.72‰~9.66‰)。这些特征指示花岗岩是由蚀变的年轻地壳经黑云母脱水熔融形成。结合北秦岭高压-超高压(HP-UHP)岩石、早古生代花岗岩和基性岩研究现状,表明该区在早古生代经历两期构造抬升事件并伴随退变质和深熔作用,分别发生在中-晚奥陶世和晚志留世。The widely exposed Early Paleozoic granites and mafic xenoliths in the North Qinling Belt have important research significance for discussing the Early Paleozoic crustal and tectonic evolution in this belt.This paper reports whole-rock major and trace element and zircon U-Pb-Hf-O isotope compositions for the Zaoyuan monzogranite and its plagioclase pyroxenite xenolith,and the amphibolite xenolith in Huangbaicha pluton from the Danfeng area.Zircon U-Pb dating of the amphibolite and plagioclase pyroxenite yielded a protolith age of 493±8Ma,amphibolite-facies metamorphic ages of 468±4Ma to 460±5Ma.Zircon U-Pb dating indicates the Zaoyuan granite was emplaced at 419±4Ma.The amphibolite and plagioclase pyroxenite range in SiO 2 from 49.69%to 50.73%,exhibiting magnesian,metaluminous and calc-alkalic affinities,with enrichment in large ion lithophile elements(LILEs)and depletion in high field strength elements(HFSEs).Most of the zircon grains possess relative highεHf(t)values(-0.3~7.5)andδ18 O values from 5.61‰to 8.20‰.These features suggest that the protolith of the amphibolite and plagioclase pyroxenite might be derived from mantle wedge which has been modified by the subducting slab.The Zaoyuan granite has SiO 2 of 70.11%,with high alkalis but low iron and magnesium contents.It is weakly peraluminous.These features are consistent with those of typical I-type granite.The granite is characterized with high zirconεHf(t)(0.1~3.9)andδ18 O values(8.72‰~9.66‰).These features suggest that the granite was formed through dehydration melting of altered juvenile crust.Our data,combined with the previous work on the HP-UHP metamorphic rocks,Early Paleozoic granites and mafic rocks in the North Qinling Belt,demonstrate that the North Qinling Belt experienced two stages of tectonic uplift events accompanied with retrogression and anataxis in the Early Paleozoic,which occurred in the Middle to Late Ordovician and Late Silurian respectively.

关 键 词：早古生代 花岗岩 基性捕虏体 锆石U-Pb-Hf-O同位素 北秦岭造山带 

分 类 号：P588.12[天文地球—岩石学] P597.3[天文地球—地质学]