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

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...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 of 1890 +/- 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/Na2O 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 epsilon(Nd)(t) = -4. 7 similar to -4. 1, I-Sr = 0. 7050 similar to 0. 7110 and epsilon(Hf)(t) = -3. 5 similar to 2. 2, which is consistent with the isotope composition of the similar to 2. 17Ga gneissic granites, fine-grained gneisses at 1890Ma. 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 SiO2 (<71%), high content of FeOT + MgO + TiO2 (expect for the sample SC-1), low Sr/Y, (La/Yb)(N), and 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% similar to 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/TiO2 ratios, 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.国家973项目SCI(E)中文核心期刊要目总览(PKU)中国科技核心期刊(ISTIC)中国科学引文数据库(CSCD)yangmc@pku.edu.cn; binchen@pku.edu.cn061573-15883