吉南地区古元古代双岔巨斑状花岗岩成因及其构造意义：岩石学、年代学、地球化学和Sr-Nd-Hf同位素证据  

Petrological,geochronological,geochemical and Sr-Nd-Hf isotopic constraints on the petrogenesis of the Shuangcha Paleoproterozoic megaporphyritic granite in the southern Jilin Province: Tectonic implications

作　　者：杨明春[1] 陈斌[2] 闫聪[1] 

机构地区：[1]北京大学地球与空间科学学院、造山带与地壳演化教育部重点实验室,北京100871 [2]合肥工业大学资源与环境工程学院,合肥230009

出　　处：《岩石学报》2015年第6期1573-1588,共16页Acta Petrologica Sinica

基　　金：国家973项目(2012CB416603)资助

摘　　要：双岔岩体位于华北克拉通东部陆块辽吉活动带内,岩性主要为巨斑状石榴石黑云母二长花岗岩。LA-ICP-MS锆石U-Pb年代学研究表明该岩体形成于1890±21Ma。岩石A/CNK>1.1属于强过铝质岩石,CaO/Na2O>0.3,微量元素显示Nb、Ta、Sr、Ti、P的亏损,指示与弧岩浆的亲缘性。全岩同位素特征是:εNd(t)=-4.7^-4.1,ISr值介于0.7050~0.7110,εHf(t)值变化范围为-3.5~2.2,与~2.17Ga条痕状花岗岩和变粒岩-浅粒岩在1890Ma时的同位素组成大致吻合,指示其源岩应主要为成熟度较低的变质砂岩,主要源区为条痕状花岗岩和变粒岩-浅粒岩。双岔花岗岩具有较低的SiO 2含量(<71%),较高的FeO T+MgO+TiO 2含量(除SC-1外都大于4%),较低的Sr/Y和(La/Yb)N比值及较高的Rb/Sr比值以及明显的Eu负异常,说明源岩可能有部分变质玄武岩卷入。Nd同位素模拟结果显示双岔巨斑状花岗岩的源岩中可有20%~30%的变质玄武岩。部分样品的Rb/Sr、Rb/Ba比值特征与泥质原岩相似,显示源区也有少量的泥质岩。岩石低的Al2O3/TiO 2比值及变质熔融残余的石榴石和夕线石,说明岩浆形成于高温中压环境,上涌的软流圈加热地壳使源岩发生部分熔融,是辽吉活动带由挤压向伸展转换的产物,标志着辽吉带古元古代造山作用的结束。The Shuangcha megaporphyritic granitic pluton is located in the Liao-Ji mobile belt,Eastern Block of the North China Craton. It mainly consists of megaporphyritic garnet biotite monzogranite. LA-ICP-MS U-Pb zircon dating gives the emplacement age of1890 ± 21 Ma for it. The granites belong to strongly peraluminous rocks for their A / CNK ratios greater than 1. 1. Geochemical data show that their CaO / Na2 O ratios are greater than 0. 3. They are depleted in Nb, Ta, Sr, Ti, P, showing arc magma affinity.Simultaneously,they have very enriched isotope characteristics with εNd（ t） =- 4. 7 -- 4. 1,ISr= 0. 7050 - 0. 7110 and εHf（ t） =- 3. 5 - 2. 2,which is consistent with the isotope composition of the - 2. 17 Ga gneissic granites,fine-grained gneisses at 1890 Ma. All these characteristics show that the source of the granites are of low maturity,and the source rocks may be mainly the metamorphosed psammite which was formed by the sedimentary diagenesis of the gneissic granites and fine-grained gneisses＇ weathering products. The relatively low content of SiO 2（ 71%）,high content of FeO T＋ MgO ＋ TiO 2（ expect for the sample SC-1）,low Sr/Y,（ La/Yb）Nand high Rb / Sr ratios,together with the pronounced negative Eu anomaly show that some metabasalts may contribute the source rocks. Nd isotope modeling suggests that 20% - 30% of the source rocks are the metabasalts. Some of the samples have similar Rb / Sr,Rb / Ba ratios with the pelitic source rocks,revealing that there are also minior pelitic rocks in the source rocks. The granites have low Al2O3/TiO 2ratios,metamorphogenetic garnet and sillimanite,indicating that the melt was produced at high temperature and medium pressure;the ascending asthenosphere heated the crust and triggered the partial melting of the source rocks. The granites were the product of stretching stress switching from the extrusion stress,marking the end of the Paleoproterozoic orogeny in the Liao-Ji Belt.

关 键 词：辽吉活动带 后碰撞 巨斑状花岗岩 华北克拉通 

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