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?  

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?

作　　者：毕君辉[1] 邢德和[2] 葛文春[1] 杨浩[1] 董玉[1] BI Junhui;XING Dehe;GE Wenchun;YANG Hao;DONG Yu(College of Earth Sciences, Jilin University, Changchun 130061, China;Shenyang Center of Geological Survey, China Geological Survey, Shenyang 110034, China)

机构地区：[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

出　　处：《地学前缘》2018年第3期295-308,共14页Earth Science Frontiers

基　　金：中国地质调查局地质调查项目(12120114085601)

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LAICP-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SiO_2(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、Fe_2O_3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ε_(Hf)(t)值介于-12.00^+6.88,二阶段Hf模式年龄(T_(DM2))介于3 467~2 310Ma。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。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 0.34-0.92 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（0.24-0.42）indicate the rocks largely belong to the subalkalic volcanic rocks,and their high CaO,MgO and Fe2 O3 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 7.57-11.13,7.60-11.53 and 0.14-0.22,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 ε（Hf）（t）values for meta-acid volcanic rocks ranged between-12.00 and＋6.88,with two-stage Hf model ages（T（DM2））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

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 

分 类 号：P588.14[天文地球—岩石学] P59[天文地球—地质学]