| 吉南天桥地区基性岩墙与其围岩TTG片麻岩的成因及构造意义 |
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| 引用本文： | 李鹏川,董晓杰,刘正宏,赵庆英,石强,李长海,李天瑜.吉南天桥地区基性岩墙与其围岩TTG片麻岩的成因及构造意义[J].岩石学报,2018,34(6):1581-1598. |
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| 作者姓名： | 李鹏川 董晓杰 刘正宏 赵庆英 石强 李长海 李天瑜 |
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| 作者单位： | 吉林大学地球科学学院;核工业二〇八大队 |
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| 基金项目： | 本文受国家自然科学基金（41272223）和中国地质调查局项目（12120114028001、1212011220854、121201004000150017-28、121201004000160901-7）联合资助. |
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| 摘 要： | 吉南地区太古宙基底中发育大量早前寒武纪基性岩墙群,是陆壳伸展的直接证据。对白山市东部天桥太古宙基底出露区内基性岩墙及其围岩进行了锆石U-Pb定年和地球化学分析,以确定该期伸展事件的形成机制及地质意义。天桥地区基性岩墙岩性为斜长角闪岩,侵位于TTG片麻岩中。英云闪长质片麻岩(TN1)中锆石具核-边结构,岩浆核的LA-ICP-MS测年结果为2500±6Ma,指示其形成于新太古代末期。天桥岩墙(TN3)中的锆石内部结构与TN1相同,酸性岩浆核的SHRIMP测年结果为2490±17Ma,与TN1在误差范围内一致,表明这些锆石不是基性岩墙原生锆石,而是岩墙侵位过程中在围岩中捕获的锆石,但根据岩墙仅侵位在太古宙基底中且变质程度高于周围古元古界老岭群,将其侵位年龄大致限制在新太古代末期-古元古代早期。地球化学特征显示,基性岩墙具有低SiO_2、Na_2O、K_2O含量,高CaO、MgO含量,A/CNK=0.56～0.59,属于准铝质的拉斑玄武岩系列岩石,∑REE低、配分曲线平坦,富集LILE(Rb、Ba和K),亏损HFSE(Th、U、Nb和Ta),具有与原始地幔相同的Nb/Ta、Zr/Hf比值及接近地壳的Nb/U、Ta/U比值,指示其岩浆可能来源于地幔且在上升过程中受到地壳混染,形成于板内伸展环境。TTG片麻岩具有中等的SiO_2和MgO含量,高Al_2O_3和Na_2O含量以及低CaO含量,A/CNK=1.00～1.14,属弱过铝质的钙碱性系列岩石,∑REE低、具有右倾的REE配分曲线,轻稀土富集、重稀土亏损,富集LILE(Rb、Ba、K和Sr),强烈亏损HFSE(U、Nb、Ta、Sm和Ti),其岩浆可能来源于变质玄武质岩石和极少量沉积岩的部分熔融,结合邻区TTG的研究成果,认为其形成于与俯冲相关的活动大陆边缘环境。前人研究表明,新太古代晚期板块构造体制可能已经启动,结合我们以往研究,认为新太古代晚期华北克拉通东北部可能发生了弧陆碰撞造山运动,天桥岩墙的侵位标志着新太古代末期至古元古代早期之间华北克拉通东北部进入造山后伸展环境,可能是对新太古代造山运动结束的响应。
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| 关 键 词： | 华北克拉通 吉南地区 新太古代末期-古元古代早期 基性岩墙 锆石U-Pb年龄 伸展事件 |
| 收稿时间： | 2017/12/2 0:00:00 |
| 修稿时间： | 2018/3/5 0:00:00 |
Petrogenesis and tectonic implication of the mafic dykes and its host TTG gneisses from Tianqiao area in southern Jilin Province |
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| LI PengChuan,DONG XiaoJie,LIU ZhengHong,ZHAO QingYing,SHI Qiang,LI ChangHai and LI TianYu.Petrogenesis and tectonic implication of the mafic dykes and its host TTG gneisses from Tianqiao area in southern Jilin Province[J].Acta Petrologica Sinica,2018,34(6):1581-1598. |
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| Authors: | LI PengChuan DONG XiaoJie LIU ZhengHong ZHAO QingYing SHI Qiang LI ChangHai and LI TianYu |
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| Institution: | College of Earth Sciences, Jilin University, Changchun 130061, China,College of Earth Sciences, Jilin University, Changchun 130061, China,College of Earth Sciences, Jilin University, Changchun 130061, China,College of Earth Sciences, Jilin University, Changchun 130061, China,College of Earth Sciences, Jilin University, Changchun 130061, China,College of Earth Sciences, Jilin University, Changchun 130061, China and No. 208 Geological Party of Nuclear Industry, Baotou 014010, China |
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| Abstract: | The Early Precambrian mafic dyke swarms are widely developed in the Archaean basement in southern Jilin Province, representing the extensional events of continental crust. This paper reports zircon U-Pb chronology and geochemistry of the Tianqiao mafic dykes and its host TTG gneisses in order to discuss the tectonic setting and geological implication of this extensional event. Tianqiao mafic dykes are mainly amphibolite which emplaced in the TTG gneisses. LA-ICP-MS zircon U-Pb dating shows the magmatic zircons in the tonalitic gneiss (TN1) which is the host rock of Tianqiao mafic dyke were crystallized at 2500±6Ma, suggesting that it was formed in the Late Neoarchean. The zircons in the Tianqiao mafic dyke (TN3) have the same inner structure as those in sample TN1, SHRIMP zircon U-Pb dating shows these acidic magmatic zircons in sample TN3 were crystallized at 2490±17Ma, which age is consistent with those in sample TN1, indicating these acidic magmatic zircons were captured from the host rocks during mafic dyke emplacement rather than primary zircons of Tianqiao mafic dyke. Nevertheless, the mafic dykes in Tianqiao area were only emplaced in the Archaean basement and have higher metamorphic grade than Palaeoproterozoic Laoling Group, these geological characteristics indicating that the mafic dyke was intruded during the Late Neoarchean to Early Paleoproterozoic. Geochemical characteristics show that the mafic dykes have low SiO2, Na2O, K2O contents and high CaO, MgO contents, with A/CNK ratios of 0.56~0.59, and are classified as metaluminous and tholeiite series rocks. They have lower total REE contents with a flat REE pattern, and are enriched in LILE (Rb, Ba and K), depleted in HFSE (Th, U, Nb and Ta), with Nb/Ta, Zr/Hf ratios and Nb/U, Ta/U ratios which are similar to primitive mantle and crustal rocks, respectively, indicating the magma was derived from the mantle and took place crustal contamination during magma ascent, and formed in intraplate extensional environment. The TTG gneisses in Tianqiao area have moderate SiO2, MgO, high Al2O3, Na2O and low CaO contents, with A/CNK ratios of 1.00~1.14, and are classified as weakly peraluminous and calc-alkaline rocks. They have lower total REE contents and are enriched in LREEs relative to HREEs, enriched in LILE (Rb, Ba, K and Sr), depleted in HFSE (U, Nb, Ta, Sm and Ti), indicating they were derived from partial melting of metabasaltic materials with minor sediments, and combining with the results of TTGs in adjacent regions, we suggest that it was possibly formed in a subduction related active continental margin background. Some previous studies considered that the plate tectonics has been started in the North China Craton (NCC) during the Late Neoarchean. Combining with our previous study, we suggest that an arc-continent collision orogenic process might occur during the Neoarchean in the NE of NCC. The intrusion of Tianqiao mafic dyke represents the tectonic setting was turned into post-orogenic extensional environment in the northeast of the NCC, indicating the end of Neoarchean orogenesis. |
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| Keywords: | North China Craton Southern Jilin Province Late Neoarchean-Early Paleoproterozoic Mafic dykes Zircon U-Pb age Extensional events |
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