| 苏鲁造山带威海古元古代泥质麻粒岩锆石U-Pb年龄和Hf同位素特征及其构造属性 |
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| 引用本文： | 熊志武,续海金,王攀,章军锋,刘强.苏鲁造山带威海古元古代泥质麻粒岩锆石U-Pb年龄和Hf同位素特征及其构造属性[J].地球科学,2021,46(2):504-526. |
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| 作者姓名： | 熊志武 续海金 王攀 章军锋 刘强 |
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| 作者单位： | 中国地质大学地球科学学院, 湖北武汉 430074 |
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| 摘 要： | 威海地区出露古元古代泥质麻粒岩，其构造属性仍存在争议.泥质麻粒岩以透镜体的形式出露在花岗质片麻岩中，透镜体从核部到边部的岩性逐渐变化:未变形的粗粒泥质麻粒岩、面理化的细粒泥质麻粒岩、石榴黑云片麻岩和混合岩化麻粒岩.粗粒泥质麻粒岩，粗粒斑状变晶结构，块状构造；细粒泥质麻粒岩，细粒斑状变晶结构，面理发育；石榴黑云片麻岩，斑状变晶结构，片麻状构造，上述3种岩石的主要矿物组合均为石榴子石+黑云母+斜长石（反条纹长石）+石英+矽线石；混合岩，条带状构造，暗色残余体主要矿物组合为石榴子石+斜长石+黑云母+石英+矽线石，浅色体矿物组合为石英+斜长石+钾长石.所有样品均有金红石、锆石和独居石等副矿物.粗粒泥质麻粒岩中的锆石颗粒均为浑圆近等粒，具有典型的麻粒岩相变质锆石的特征:锆石CL图像为均一的云雾状或补丁状结构，低的Th/U比值（0.01~0.30），强烈的Ce正异常和Eu负异常，HREE的亏损及高的Hf/Y比值（19~537）.利用锆石Ti温度计获得的变质温度为788~892℃（加权平均值为837±24℃）.锆石U-Pb定年获得上交点年龄为1 863±18 Ma，206Pb/238U加权平均年龄为1 832±23 Ma.εHf（t）值为-3.4~-4.9（加权平均值为-4.23±0.35），相应的两阶段模式年龄（TDM2）为2 716±107 Ma~2 807±93 Ma（加权平均值为2 767±44 Ma）.细粒泥质麻粒岩中的锆石也具有麻粒岩相锆石的CL和微量元素特征，Ti含量温度计获得的变质温度为804~909℃（加权平均值为845±23℃），锆石U-Pb上交点年龄为1 823±14 Ma，谐和206Pb/238U年龄加权平均值为1 812±13 Ma，εHf（t）为-3.7~-5.7（加权平均值为-4.67±0.37），TDM2为2 705±133 Ma~2 826±116 Ma（加权平均值为2 766±46 Ma）.石榴黑云片麻岩中的锆石也具有麻粒岩相变质锆石的CL和微量元素特征，锆石Ti含量计算变质温度为785~923℃（加权平均值为820±32℃），锆石U-Pb上交点年龄为1 807±22 Ma，εHf（t）为-4.5~-9.0（加权平均值为-6.07±0.48），TDM2为2 742±90 Ma~3 020±92 Ma（加权平均值为2 839±41 Ma）.混合岩中的锆石大部分具有核-边结构.根据Ti含量温度计获得的混合岩中麻粒岩相变质锆石核的变质温度为754~875℃（加权平均值为818±30℃），U-Pb上交点年龄为1 822±19 Ma，εHf（t）值为-4.3~-6.3（加权平均值为-5.47±0.35），TDM2为2 742±82 Ma~2 864±91 Ma（加权平均值为2 814±43 Ma）.可见，麻粒岩、石榴黑云片麻岩和混合岩经历了相同的麻粒岩相峰期变质作用（~1.8 Ga），具有相同的原岩属性，即晚太古代（2.7~2.8 Ga）的地壳物质.麻粒岩透镜体从核部到边部岩性的变化，可能受到晚三叠纪碰撞造山作用的不同程度改造.因此，威海超高压地体中出露的泥质麻粒岩透镜体，在构造亲属性上可能属于华北克拉通地壳物质，成因上可能与哥伦比亚超大陆的演化有关，在三叠纪大陆俯冲碰撞过程中卷入造山带.
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| 关 键 词： | 泥质麻粒岩 古元古代 锆石 构造属性 苏鲁造山带 地球化学 地质年代学 |
| 收稿时间： | 2019-12-19 |
Zircon U-Pb Age and Hf Isotope of Paleoproterozoic Pelitic Granulites at Weihai,Sulu Orogen:Implications for Tectonic Affinity |
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| Xiong Zhiwu,Xu Haijin,Wang Pan,Zhang Junfeng,Liu Qiang.Zircon U-Pb Age and Hf Isotope of Paleoproterozoic Pelitic Granulites at Weihai,Sulu Orogen:Implications for Tectonic Affinity[J].Earth Science-Journal of China University of Geosciences,2021,46(2):504-526. |
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| Authors: | Xiong Zhiwu Xu Haijin Wang Pan Zhang Junfeng Liu Qiang |
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| Affiliation: | (School of Earth Sciences,China University of Geosciences,Wuhan 430074,China) |
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| Abstract: | Paleoproterozoic pelitic granulites have been reported in Weihai region,the tectonic affinity of which is still under controversy.The pelitic granulite occur as a lens in the granitic gneisses and the lithologies of the lens gradually changes from the core to the margin,from undeformed coarse-grained granulite,foliated fine-grained granulite,garnet-biotite-gneiss to migmatized granulite.The coarse-grained pelitic granulite exhibits coarse-grained porphyroblastic texture.The fine-grained pelitic granulite shows a weakly foliated layer with a fine-grained porphyroblastic texture.The garnet-biotite-gneiss displays porphyroblastic texture.The three samples described above mainly comprise of garnet+biotite+plagioclase(antiperthite)+quartz+sillimanite.The migmatite is made up of melanosomes and leucosomes,in which the former mainly consists of garnet+plagioclase+biotite+quartz+sillimanite and the latter has mineral assemblage of quartz+plagioclase+K-feldpsar.All of the four samples contain the accessory mineral assemblage of rutile+zircon+monazite.Zircons from the coarse-grained pelitic granulite are round or elliptical in shape and show homogenous cloudy zoning or patchy zoning in internal structure,and they have low Th/U ratios(0.01-0.30,mostly<0.1),flat HREE patterns with positive Ce and negative Eu anomalies and high Hf/Y ratios(19-537,mostly>100).These characteristics are consistent with the granulite-facies zircon growth.Ti-in-zircon thermometers give temperature of 788-892℃(mean=837±24℃).These metamorphic zircons yield an upper intercept age of 1863±18 Ma and a concordant 206Pb/238U age of 1832±23 Ma,εHf(t)values ranging from-3.4 to-4.9(mean=-4.23±0.35)and corresponding TDM2 are between 2716±107 Ma and 2807±93 Ma(mean=2767±44 Ma).The morphology and trace element characteristics of the zircon grains from fine-grained pelitic granulite also suggest a typical granulite-facies metamorphic zircon.The calculated metamorphic temperature is 804-909℃(mean=845±23℃).Metamorphic zircons define an upper intercept age of1823±14 Ma with a tight cluster concordant206Pb/238U age of 1812±13 Ma.TheεHf(t)values of them range from-3.7 to-5.7(mean=-4.67±0.37).Calculated TDM2 are from 2705±133 Ma to 2826±116 Ma(mean=2766±46 Ma).Zircon grains from the garnet-biotite-gneiss also grew under granulite-facies condition according to the CL and trace element features.The metamorphic temperature calculated based on the Ti contents in zircon is from 785℃to 923℃(mean=820±32℃).U-Pb dating of zircons gives a discordant upper intercept age of 1807±22 Ma.TheεHf(t)values range from-4.5 to-9.0(mean=-6.07±0.48).TDM2 range from 2742±90 Ma to 3020±92 Ma(mean=2839±41 Ma).Most of the zircons from the migmatite exhibit a core-rim texture comprising of granulite-facies metamorphic core and a narrow anatectic rim.The metamorphic cores give temperature ranging from 754℃to 875℃(mean=818±30℃)and yield an upper intercept age of 1822±19 Ma.TheεHf(t)values are of-4.3 to-6.3(mean=-5.47±0.35)and corresponding TDM2 are between 2742±82 Ma and 2864±91 Ma(mean=2814±43 Ma).In conclusion,the pelitic granulite,garnet-biotite-gneiss and migmatite have experienced the same granulite-facies metamorphism at about~1.8 Ga and they share a common protolith which was derived from Neoarchean(2.7-2.8 Ga)crust.The changed lithologies from core to the margin of the pelitic granulite lens may be caused by the collisional orogeny during the late Triassic.Thus,the pelitic granulites in Weihai region have the tectonic affinity to NCC and formed when involved in the evolution of supercontinent Columbia,then they incorporated into the Sulu ultrahigh-pressure metamorphic belt during the collision between the Yangtze and the North China cratons and subsequently exhumed in Triassic. |
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| Keywords: | pelitic granulite Paleoproterozoic zircon tectonic affinity Sulu orogen geochemistry geochronology |
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