湖南桂阳大义山南体(太坪山单元)花岗岩形成时代及物质来源探讨

摘要

Based on geological investigation,this paper studies the whole-rock element geochemistry,zircon U-Pb geochronology,Sm-Nd isotopic compositions and in situ zircon Hf isotope of the southern Dayishan biotite monzogranitic batholith in Guiyang City,Hunan Province.With reference to previous research,we discuss petrochemical property,forming age,material sources,as well as petrogenesis of the southern Dayishan granitic batholith.LA-ICP-MS zircon U-Pb isotopic analyses show that the southern Dayishan granite was emplaced during the Early Yanshanian,with a weighted mean n(206pb)/n(238U) age of 153.4 ± 1.1Ma (MSWD =0.30,2σ,N =21).Two inherited zircon cores yield n(207pb)/n(206pb) ages of 2449 Ma and 2498 Ma,respectively.The southern Dayishan granite is characterized by high silicon,high potassium,metaluminous to weak peraluminous as well as rich in Light Rare Earth Elements (LREEs) and some Large Ion Lithophile elements (LILEs,Rb,K),while depleted in Heavy Rare Earth Elements (HREEs) and some High Field-Strength elements (HFSE,Nb,Ta,Ti) and Ba,Sr and P.It exhibits characteristics of Zr + Nb + Ce + Y ＞ 350 * 10-6 and 10000 * Ga/A1 ＞ 2.6,being similar to A-type granites.Sm-Nd isotopic study shows εNd(t) values ranging from-6.94 to-5.19 and two-stage model ages of Nd isotope (TDM2) within the range of 1.37 to 1.51 Ga.In situ zircon Hf isotope yields εHf(t) ranging from-9.35 to-1.16 and TDm from 1.28 to 1.79 Ga.Two Paleoproterozoic inherited zircon cores yield εHf(t) of-6.82 and 1.48,respectively.The petrochemistry and isotopic characteristics indicate that the southern Dayishan granite was mainly derived from melting of Paleoproterozoic crustal metagreywackes,and maybe contaminated by a small amount of mantle material.During Early Yanshanian,the roll-back of paleo-Pacific Plate resulted in tectonic extension and thinning of the continental lithosphere of the study area.The underplating of mantle-derived magma in response to upwelling of the asthenosphere induced partial melting of the Palaeoproterozoic lower crust and produced granitic magma.The magma then invaded along deep faults and formed the southern Dayishan granitic batholith (Taipingshan Body).%在详细的野外地质调查基础上,本文通过对湖南省桂阳县大义山南体(太坪山单元)黑云母二长花岗岩的岩石化学、锆石U-Pb定年和全岩Sm-Nd同位素以及锆石原位Hf同位素的综合研究,参照前人研究成果,探讨其岩石化学性质、形成时代、物质来源以及形成机制等.LA-ICP-MS锆石U-Pb定年结果显示其n(206pb)/n(238U)加权平均年龄为153.4±1.1Ma,两颗继承锆石核n(207pb)/n(206pb)年龄分别为2449Ma和2498 Ma.岩石高硅、富碱、高钾钙碱性、准铝质到弱过铝质,富集轻稀土元素、部分大离子亲石元素(Rb、K等)和Th、U、Pb等元素,亏损部分高场强元素(Nb、Ta、Ti)和Ba、Sr、P等元素.该岩体Zr+ Nb+ Ce+Y＞350 × 10-6,10000*Ga/Al＞2.6,显示出A型花岗岩的特征.全岩Sm-Nd同位素显示εNd(t)=-6.94 ～-5.19,二阶段模式年龄TDM2=1.37 ～ 1.51 Ga.锆石原位Hf同位素显示,锆石εHf(t)=-9.35 ～-1.16,TDM2=1.28 ～ 1.79 Ga.两颗继承锆石核εHf(t)分别为-6.82和1.48.岩石化学和同位素特征表明岩石主要来源于元古代地壳变质杂岩的部分熔融,并可能有少量地幔物质的加入.燕山早期,由于俯冲的古太平洋板块后撤,研究区处于伸展的构造环境,岩石圈减薄,软流圈上涌,导致玄武质岩浆底侵,诱发上覆的古元古代下地壳部分熔融,形成花岗质岩浆,并沿着深大断裂带运移侵位,形成大义山南体(太坪山单元)黑云母二长花岗岩.