今天是2019年07月23日

河北围场朝阳地中泥盆世高Sr/Y花岗岩成因及对兴蒙造山带演化的约束

DOI:10.3799/dqkx.2018.350

作者:王树庆,胡晓佳,赵华雷,刘文刚,杨泽黎

联系地址:中国地质调查局天津地质调查中心,天津 300170

摘要:兴蒙造山带位于华北板块和西伯利亚板块之间,具有复杂的构造演化历史,其构造格局和拼合机制一直存在较大争议.本次在兴蒙造山带南部的朝阳地识别出一套形成于伸展背景的中泥盆世高Sr/Y花岗岩,可对早古生代末期弧陆拼贴过程进行约束.朝阳地花岗岩锆石U-Pb年龄为386.8±2.9 Ma,属于中泥盆世.岩石地球化学分析显示高SiO2、Al2O3、Na2O含量,中等CaO、K2O含量,较低FeOt、MgO含量的特征,Na2O/K2O比值均大于1,属于中钾钙碱性I型花岗岩;A/CNK范围为 1.02~ 1.08,属于弱过铝质系列;微量元素具有富集Rb、Ba、K等大离子亲石元素及Sr,弱亏损Nb、Ta、P、Ti等高场强元素的特征,Sr/Y比值高(103~146);具有轻稀土富集的配分模式((La/Yb)N=9.9~14.6),重稀土分馏较弱,(Gd/Yb)N范围为 1.45~ 1.97,具有弱Eu正异常( δEu值1.31~1.80),地球化学分析表明,高Sr/Y比值继承自岩浆源区而非加厚地壳的熔融.朝阳地侵入岩均具有富集的Sr-Nd-Hf同位素组成,εNd(t)和εHf(t)均为负值(范围分别为-18.6~-17.9,-38.30~-23.59),锆石Hf模式年龄范围为 2 825~ 3 745 Ma,集中于2 800~3 000 Ma,Nd模式年龄较集中(2 584~2 642 Ma).综合岩石学、元素及同位素地球化学分析,朝阳地高Sr/Y侵入岩是中泥盆世在区域伸展背景下,由具有高Sr/Y比值的古老基性下地壳物质受减压和上涌软流圈加热共同作用发生部分熔融形成,代表泥盆纪白乃庙岛弧与华北陆块碰撞后区域伸展背景.

关键字:中泥盆世;高Sr/Y花岗岩;弧陆碰撞;下地壳源区;华北陆块北部;兴蒙造山带;地球化学.

Petrogenesis of Middle Devonian Chaoyangdi High Sr/Y Granite and Constraints on the Evolution of Xing-Meng Orogeny

Author:Wang Shuqing , Hu Xiaojia , Zhao Hualei , Liu Wengang , Yang Zeli

Author Address:Tianjin Center of China Geological Survey, Tianjin 300170, China

Abstract:Xing-Meng orogeny is the accretionary orogeny between Siberia and North China plates with complex multi-stage subduction and accretion, and its tectonic evolution. has remained controversial for a long time. To reveal the arc-continent collision in the end of Early Paleozoic, this paper presents a study on Chaoyangdi Middle Devonian high Sr/Y granite in southern part of Xing-Meng orogeny zircon U-Pb dating results yield a formation age of 386.8±2.9 Ma, with the corresponding age of Middle Devonian (D2). Geochemical data shows that these rocks have high SiO2, Al2O3 and Na2O, moderate CaO and K2O, but low FeOt and MgO contents with the Na2O/K2O>1 and calcalkaline series signature. The A/CNK ratios range from 1.02 to 1.08, indicating these samples are weak peraluminous series. Trace element analysis results display enrichment of LILEs, such as Rb, Ba, K and Sr, but depletion of HFSE, i.e., Nb, Ta, P and Ti, with high Sr/Y ratios (103-146), similar with adakitic-affinity. These rocks have strong fractionation between LREE and HREE ((La/Yb)N=9.9-14.6), no obvious HREE fractionation ((Gd/Yb)N=1.45-1.97) and weak positive Eu anomalies (δEu=1.31-1.80). All these geochemical characteristics suggest that high Sr/Y signature is inherited from magma source. Sr-Nd-Hf analyses show enriched isotopic signatures-both εHf(t) and εNd(t) are negative(-38.30 to -23.59 and -18.6 to -17.9) with old two-stage model age (2 825 to 3 745 Ma for Hf, 2 584 to 2 642 Ma for Nd), indicating an old enriched crustal source. Based on petrology, geochemistry and Hf-Sr-Nd isotopic characteristics, we conclude that the Chaoyangdi Middle Devonian intrusive rocks were formed by partial melting of ancient basic lower crust with high Sr/Y ratios and formed by postcollisional extension and heating of asthenosphere upwelling, indicating the regional extension after collision between the Bainaimiao arc and the North China Block.

Keyword:Middle Devonian; high Sr/Y; arc-continent collision; lower continental crust source; northern part of North China Craton; Xing-Meng orogeny; geochemistry

 

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作者简介:王树庆(1983-),男,高级工程师,硕士,从事岩石学、地球化学研究及地质调查工作,主要从事造山带构造-岩浆作用研究.ORCID: 0000-0002-4973-7535. E-mail: 89617984@qq.com

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