| 冀西北早白垩世岩浆岩的地球化学特征及其地球动力学背景 |
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| 引用本文： | 李创举,包志伟.冀西北早白垩世岩浆岩的地球化学特征及其地球动力学背景[J].地球化学,2012(4):343-358. |
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| 作者姓名： | 李创举 包志伟 |
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| 作者单位： | 1. 中国科学院广州地球化学研究所矿物学与成矿学重点实验室,广东广州510640
2. 中国科学院研究生院,北京100049 |
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| 基金项目： | 国家重点基础研究发展计划（2012CB416602）;国家自然科学基金（40773018） |
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| 摘 要： | 冀西北张家口地区晚中生代发生了大规模的中酸性岩浆活动,通过精确的锆石 U-Pb 年代学研究,获得东坪金矿东侧北栅子碱性花岗岩的侵位年龄为(130.5±1.5) Ma,其周围出露的张家口组粗面质火山岩的喷发年龄为(127.8±3.9) Ma,为早白垩世同期岩浆作用的产物.这些岩浆岩总体属于高钾钙碱性-钾玄质系列,富碱,低 Mg#(30~43);微量元素组成上具有大离子亲石元素(LILE)、轻稀土元素(LREE)相对富集(ΣLREE/ΣHREE =14.1~23.0,(La/Yb)N =20.8~42.2), Eu 弱负异常(δEu =0.62~0.97),高场强元素(Nb、Ta、Ti和 P)不同程度亏损的地球化学特征,显示出同源岩浆演化的趋势.火山岩和花岗岩(87Sr/86Sr)i 平均值分别为0.7075和0.7078,全岩εNd(t)和锆石εHf(t)值差别明显,火山岩εNd(t)=–15.9~–13.6,εHf(t)=–18.7~–13.5,而花岗岩εNd(t)及εHf(t)则显著低于火山岩,分别为–16.8~–15.9和–24.7~–18.4.地球化学以及同位素特征表明它们是经历过强烈改造的前寒武纪下地壳与中生代底侵形成的玄武质下地壳部分熔融的产物,花岗岩岩浆主要源于经历过强烈改造的前寒武纪下地壳部分熔融,而张家口组粗面质火山岩的源区可能含有相对较多的年轻地幔物质,两者可能存在源区混合作用.北栅子碱性长石花岗岩和大面积张家口组火山岩的喷发,反映了该地区在早白垩世构造体制从挤压-伸展的转折,这种构造体制转变可能与早白垩世燕山构造带开始垮塌以及岩石圈强烈伸展减薄有关
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| 关 键 词： | 早白垩世 花岗岩 中生代 岩石圈减薄 北栅子岩体 河北省 |
Geochemical characteristics and geodynamic implications of the Early Cretaceous magmatisms in Zhangjiakou region, northwest Hebei Province, China |
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| LI Chuang-ju,and BAO Zhi-wei.Geochemical characteristics and geodynamic implications of the Early Cretaceous magmatisms in Zhangjiakou region, northwest Hebei Province, China[J].Geochimica,2012(4):343-358. |
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| Authors: | LI Chuang-ju and BAO Zhi-wei |
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| Institution: | 1＊1. CAS Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou510640, China;2. Graduate University of Chinese Academy of Sciences, Beijing 100049, China |
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| Abstract: | Late Mesozoic intermediate-acidic magmatisms are widespread in Zhangjiakou region, northwest Hebei Province. Precise zircon U-Pb chronolgical study show that both of the widespread trachytic volcanic rocks of Zhangjiakou Formation and the Beizhazi alkali granite located to the east of the Dongping gold deposit were proximately contemporary magmatisms, which were formed in （127.8±3.9） Ma and （130.5±1.5） Ma, respectively. The rocks are of high K calc-alkaline to shoshonitic, high in alkali, and low in Mg# （30~43）, and are characterized by relatively enrichments of LILE and LREE （（La/Yb）N = 20.8~42.2）, weak negative Eu anomalies （δEu = 0.62~0.97）, and depletions of HFSEs, e.g., Nb, Ta, Ta and P. The major and trace element geochemistry of volcanic rocks and granites shows crystallization fractionation trends. Even though the volcanic rocks and graniteshave similar Sr isotope compositions, with average （87Sr/86Sr）i values of 0.7075 and 0.7078, respectively, Nd and zircon Hf isotope compositions are significantly different, i.e., εNd（t） = -15.9~ -13.6, εHf（t） = -18.7~ -13.5 for the volcanic rocks, while εNd（t） = -16.8~ -15.9, εHf（t） = -24.7~ -18.4 for the granites. Element and isotope geochemical characteristics of the rocks demonstrate that they were derived from partial melting of the Precambrian lower crust which was strongly overprinted by Mesozoic underplating and metamorphism. The sources of trachytic volcanic magmas of the Zhangjiakou Formation is likely to contain more juvenile mantle material of Mesozoic underplated basaltic lower crust, and magam mixing in deep source might be involved in the generation of the volcanic-magmatic series. The occurrence of the widespread Zhangjiakou Formation and Beizhazi alkali granite suggests a tectonic regime of transition from compression to extension in the Early Cretaceous, which probably related to the collapse of the Yanshanian orogen and intensive lithospheric thinning in eastern China. |
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| Keywords: | Early Cretaceous granite Mesozoic lithospheric thinning Beizhazi rock Hebei Province |
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