地学前缘 ›› 2018, Vol. 25 ›› Issue (3): 295-308.DOI: 10.13745/j.esf.2018.03.024

• 《地学前缘》开设“学术争鸣”专栏 • 上一篇    

辽东地区北辽河群变酸性火山岩形成的时代及构造背景：古元古代陆内裂谷，还是活动大陆边缘?

毕君辉，邢德和，葛文春，杨浩，董玉   

  1. 1. 吉林大学 地球科学学院, 吉林 长春 130061
    2. 中国地质调查局 沈阳地质调查中心, 辽宁 沈阳 110034
  • 收稿日期:2018-01-18 修回日期:2018-04-16 出版日期:2018-05-15 发布日期:2018-05-15
  • 作者简介:毕君辉(1988—)，女，博士研究生，地球化学专业。E-mail：biying2746@126.com
  • 基金资助:
    中国地质调查局地质调查项目(12120114085601)

Age and tectonic setting of metaacid volcanic rocks from the North Liaohe Group in the Liaodong area: Paleoproterozoic intracontinental rift or active continental margin?

BI Junhui,XING Dehe,GE Wenchun,YANG Hao,DONG Yu   

  1. 1. College of Earth Sciences, Jilin University, Changchun 130061, China
    2. Shenyang Center of Geological Survey, China Geological Survey, Shenyang 110034, China
  • Received:2018-01-18 Revised:2018-04-16 Online:2018-05-15 Published:2018-05-15

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摘要： 辽东半岛辽河群出露于胶辽吉活动带北段，发育大量的古元古代花岗质岩石、镁铁质侵入体和火山沉积岩系。本文在河栏地区北辽河群中新识别出一套变酸性火山岩，并对其进行详细的岩石学、锆石LAICPMS UPb年代学、全岩地球化学和锆石Hf同位素的研究，以确定其形成时代、源区性质及其构造属性。变酸性火山岩中的锆石多呈自形半自形晶，发育明显的振荡环带结构，Th/U比值=0.34~0.92，指示其为岩浆成因。对岩浆锆石的定年结果表明，变酸性火山岩的原岩形成于古元古代((2 153±11) Ma)，该年龄在误差范围内与区域上广泛出露的辽吉花岗岩属于同期岩浆作用的产物。岩石地球化学特征表明变酸性火山岩具有高SiO2(64.09%~72.82%)及相对较低的Nb/Y比值(0.24~0.42)，属于亚碱性火山岩，而样品的CaO、MgO、Fe2O3等含量明显偏高，可能与后期强烈的热液活动有关；其稀土元素配分模式均呈右倾型，LREE/HREE=7.57~11.13，(La/Yb)N=7.60~11.53，δEu=0.14~0.22；在微量元素蛛网图上表现为大离子亲石元素(Rb、K)的富集和高场强元素(Nb、Ta)及Ti、P的亏损，显示出弧岩浆的特征；锆石εHf(t)值介于-12.00~+6.88，二阶段Hf模式年龄(TDM2)介于3 467~2 310 Ma。上述特征表明，研究区内变酸性火山岩的原始岩浆起源于中—新太古代增生的下地壳物质的部分熔融，在岩浆演化过程中可能伴有少量古老地壳物质的加入。结合前人有关辽吉花岗岩及区域镁铁质侵入岩、变质作用等资料，认为辽东半岛河栏地区北辽河群变酸性火山岩形成于活动大陆边缘环境，其地球动力学机制可能与龙岗地块南缘大洋板块的俯冲作用有关。

关键词: 古元古代, 变酸性火山岩, 锆石U-Pb年龄, 地球化学, 胶辽吉带

Abstract: The Liaohe Group of Liaodong Peninsula, located in the northern segment of the JiaoLiaoJi Belt, is developed mainly of deformed sedimentary and volcanic successions and felsicmafic intrusions. In this study, we report a newly recognized metaacid volcanic rocks from the North Liaohe Group in the Helan area, on which we conducted a detailed petrological study through UPb zircon dating and wholerock geochemical and LuHf isotopic analyses to determine the rocks emplacement age, magma source and tectonic nature. Zircons from metaacid volcanic rocks were euhedralsubhedral in shape and displayed obvious oscillatory zoning, which indicate a magmatic origin, in accord with the Th/U ratios of 0.340.92 in the zircons. Zircon LAICPMS UPb ages show that the volcanic rocks were emplaced in the Paleoproterozoic 2153±11 Ma and comparable to the regional Paleoproterozoic Liaoji granitoids, suggesting the two rocks are from coeval magmatism. From geochemical analysis, high SiO2 contents (64.09%72.82%) and low Nb/Y ratios (0.240.42) indicate the rocks largely belong to the subalkalic volcanic rocks, and their high CaO, MgO and Fe2O3 contents are likely related to hydrothermal activities. LuHf isotopic analysis showed that REE patterns were rightleaned, and LREE/HREE, (La/Yb)N and δEu were 7.5711.13, 7.6011.53 and 0.140.22, respectively. Indeed, these rocks were relatively enriched in LILE (e.g., Rb, K), depleted in HFSE (e.g., Nb, Ta) and Ti and P, consistent with the geochemical characteristics of island arc magmatic rocks. In addition, insitu Hf isotopic analysis revealed that zircon εHf(t) values for metaacid volcanic rocks ranged between -12.00+6.88, with twostage Hf model ages (TDM2) varying from 3467 to 2310 Ma. These features suggest that the primary magma of metaacid volcanic rocks came from partial melting of a dominantly “old” MesoNeoarchaean crustal source possibly with significant addition of juvenile materials. Based on our overall analysis, together with previously reported data on igneous rocks and metamorphism, we propose that metaacid volcanic rocks from the North Liaohe Group were formed in an active continental margin setting presumably related to subduction of the paleooceanic plate beneath the south margin of the Longgang Block.

Key words: Paleoproterozoic, metaacid volcanic rock, zircon U-Pb dating, geochemistry, Jiao-Liao-Ji Belt

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