浙中印支晚期伸展事件:NNE向带状分布A型碱性花岗岩的证据

Late Indosinian extensional event in Central Zhejiang Province:Evidence from the NNE-trending A-type alkaline granite belt

出露于浙江省江绍对接带内印支晚期花岗岩呈NNE向产出,锆石U-Pb年代学研究表明其形成于230~220Ma。地球化学分析显示该系列岩石富K_2O和Al_2O_3,低Na_2O和MgO,为准铝质-过铝质的钾质-钾玄质花岗岩。岩体氧化物与SiO_2呈明显的负相关,反映浙江省印支期侵入岩的原始岩浆可能的同源性。岩体富集Th、La、Ce等大离子亲石元素及轻稀土元素,亏损Ba、Sr、P、Ti等高场强元素,轻重稀土分馏明显,(La/Yb)_N=12.87~140.02,δEu=0.1~0.7,具高的10~4×Ga/Al值及Zr、Nb、Ce、Y含量,为板内A型花岗岩的特征。岩石有较高的I_(Sr)值(0.7069~0.7203)和较低的εNd(t)值(-14.9^-4.1),其二阶段Nd同位素模式年龄为1.33~2.21Ga。地球化学特征指示浙江印支期花岗岩是伸展环境下,具有壳幔作用和地壳物质的部分熔融而形成。结合该区发现的同期基性岩墙群,说明浙江境内在230Ma构造环境转换已经完成,板内沿古碰撞带拉张伸展和陆内裂谷作用,超基性-基性-碱性岩浆的底侵形成壳幔作用及部分熔融地壳物质。

The NNE-trending granites distributed along the Jiangshan-Shaoxing fault zone,Zhejiang province,was formed in230~220 Ma of late Indosinian according to zircon U-Pb dating.Geochemical data indicates that the igneous rocks are high in Al_2O_3and K_2O and low in Na_2O and MgO,and belong to a type of metaluminous-peraluminous high-K calcalkaline shoshonitic granites.The oxides and their negative correlation with SiO_2demonstrate that the late Indosinian granites in Zhejiang Province should be from a homologous magma.They are concentrated in LILE（Th、La、Ce）and LREE,but depleted in high field-strength elements,such as Ba,Sr,P,Ti.Whole-rock LREE/HREE ratios are（（La/Yb）_N=12.87~140.02）withδEu=0.1~0.7.And the 10^（4 ）Ga/Al ratios and（Zr＋Nb＋Ce＋Y）contents are also high.In addition,the characteristics of SrNd isotopic show high value of I_（Sr）（0.7069~0.7203）and low value ofεNd（t）（-14.9^-4.1）with two-stage depleted mantle Nd model ages of 1.33~2.21 Ga.The geochemical characteristics indicate that the Indosinian granites are intraplate A-type granites formed in an extensional environment under combined crust-mantle effect and partial melting of crust material.In consideration of the contemporaneous mafic dyke swarms in some places,it is supposed that the tectonic transformation was completed 230 Ma in Zhejiang Province.With the intracontinental rifting along the ancient collision zone,the underplating invasion of ultrabasic-basic-alkaline magma caused the crust-mantle interaction and partial melting of the crust.