华北克拉通北缘白乃庙组变质火山岩锆石定年与岩石地球化学特征
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中央高校基本科研业务费专项(编号 2652013002)及国土资源部地质调查局华北地区基础地质及数据更新项目(编号 1212010510506;1212011120700)


Geochronology and Geochemistry of the Bainaimiao Metavolcanic Rocks in the Northern Margin of North China Craton
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    摘要:

    白乃庙组绿片岩是白乃庙铜金矿床赋存层位,构造上位于华北克拉通北缘古生代陆缘增生带内。岩石学、岩石地球化学特征指示该绿片岩原岩为中基性火山岩。对白乃庙组变质火山岩的锆石LA-MC-ICP-MS U-Pb年代学研究表明,该火山岩形成年龄为449±3 Ma(n=33,MSWD=4.0),为晚奥陶世岩浆活动的产物。白乃庙组变质火山岩具有低的SiO2(43.47%~54.64%)、TiO2(0.60%~1.31%)和P2O5(0.08%~0.35%),宽的MgO(3.98%~12.99%)含量,属于钙碱性系列岩石,岩石富集大离子亲石元素K、Sr等,亏损高场强元素Nb、Ta, Zr、Hf也表现为轻微的亏损。轻重稀土分馏明显,具有不同程度的Eu异常(Eu/Eu*=0.7~1.2)和轻微负的Ce异常(Ce/Ce*=0.74~1.01)。岩石地球化学特征表明白乃庙变质火山岩经历了一定程度的结晶分异,橄榄石和单斜辉石为主要的结晶相。该变质火山岩具有较低的Ti/Y(2.5~5.0)和Zr/Y(189~355)值、低的176Hf/177Hf比值和较宽的εHf(t)值,暗示源区可能为岩石圈地幔,且受到了地壳的混染。Nb和Ta强烈亏损、极低的Nb/Ta值(0.08~1.09)以及Ce的负异常表明与大洋板块的俯冲作用有关,岩浆源区可能已受到俯冲组分的影响。构造环境判别显示白乃庙变质火山岩为与消减作用有关的大陆弧岩浆岩。锆石二阶段Hf模式年龄(tDM2)变化范围为1073~1470Ma,平均值为1285Ma(n=30),暗示华北板块北缘在中-新元古代时期曾发生过一次重要的构造热事件。

    Abstract:

    Bainaimiao complex located at the collision zone of the Siberia plate and Sino Korean plate,is composed predominantly of copper bearing greenschist facies mafic to intermediate metavolcanic rocks with some quartzite, phyllite and limestone layers. The features of petrology and lithogeochmistry indicate that the protolith of these rocks are basaltic and andesitic volcanic rocks. Zircon grains extracting from metavolcanic rock in a drill hole fromhe copper mine, gave a weighted mean U-Pb age of 449±3 Ma (n=33, MSWD=4.0) ,interpreted to be of magmatic formation age. The Bainaimiao metavolcanic rocks have relatively low content of SiO2 (43.47%~54.64%), TiO2 (0.60%~1.31%) and P2O5 (0.08%~0.35%), while the MgO content vary in range of 3.98%~12.99%. They are enriched by LILE such as K and Sr,and depleted in HFSE, such as Nb、Ta、Zr、Hf. The partition of LREE and HREE is quite obvious and there are some Eu anomaly (Eu/Eu*=0.7~1.2) and slight Ce anomaly (Ce/Ce*=0.74~1.01). These rocks formed after a certain degree of fractionation of the magma. Olivine and clinopyroxene are the main crystallized phases. Low ratios of Ti/Y(2.5~5.0), Zr/Y(189~355), 176Hf/177Hf and the wide range of εHf(t)indicate the source of the magma could be from the lithospheric mantle. The depletion of Nb、Ta and the very low ration of Nb/Ta (0.08~1.09) and the slight Ce anomaly all indicate the magma contained oceanic crust component, while the relatively high Ta/Yb (0.25~0.42) and low Zr/Nb (17.1~54.0) indicate the influence of continental crust component. So, Bainaimiao complex is of continental arc origin which related to the subduction of Paleo-Asian ocean floor beneath the continental plates and the complex itself is an important component of the growth of continental crust during early Paleozoic. The tDM2 values of the zircons vary in range of 1073 Ma~1470 Ma, which suggest that the northern margin of North China Craton records the Tectonic thermal event at Middle-Late Proterozoic.

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柳长峰,刘文灿,王慧平,周志广,张华锋,唐永举.2014.华北克拉通北缘白乃庙组变质火山岩锆石定年与岩石地球化学特征[J].地质学报,88(7):1273-1287.
LIU Changfeng, LIU Wencan, WANG Huiping, ZHOU Zhiguang, ZHANG Huafeng, TANG Yongju.2014. Geochronology and Geochemistry of the Bainaimiao Metavolcanic Rocks in the Northern Margin of North China Craton[J]. Acta Geologica Sinica,88(7):1273-1287.

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  • 收稿日期:2013-11-25
  • 最后修改日期:2014-02-07
  • 录用日期:2014-02-08
  • 在线发布日期: 2014-07-16
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