辽东地区古元古代变基性岩床的成因和构造意义  

Petrogenesis and Tectonic Implications of the Paleoproterozoic Metamorphic Mafic Sills in Liaodong Area

作　　者：胡国辉[1,2] 张琪琪 王伟 张拴宏[1,2] 李建锋 HU Guohui;ZHANG Qiqi;WANG Wei;ZHANG Shuanhong;LI Jianfeng(Institute of Geomechanics,Chinese Academy of Geological Sciences,Beijing 100081,China;Key Laboratory of Paleomagnetism and Tectonic Reconstruction,Ministry of Natural Resources,Beijing 100081,China)

机构地区：[1]中国地质科学院地质力学研究所,北京100081 [2]自然资源部古地磁与古构造重建重点实验室,北京100081

出　　处：《大地构造与成矿学》2021年第5期1023-1043,共21页Geotectonica et Metallogenia

基　　金：国家重点研发计划项目(2018YFC0603802)资助。

摘　　要：辽东地区胶‒辽‒吉造山带发育大量的变基性岩墙/床,这些岩墙/床对研究造山带古元古代构造演化过程具有重要意义。本文通过对辽东地区辽河群中的变基性岩床进行岩石学、岩石地球化学和锆石LA-ICP-MS U-Pb年代学研究,探讨其成因和形成的构造背景,有助于深入认识胶‒辽‒吉造山带古元古代地质演化过程。辽东地区变基性岩床顺层侵入于辽河群大石桥组大理岩中,发生角闪岩相变质作用,岩石类型以斜长角闪岩和石榴斜长角闪岩为主,主要矿物为角闪石和斜长石,其他矿物有石榴子石、石英、黑云母、磁铁矿和黄铁矿等。变基性岩床样品的锆石具特征的环带结构和较低的Th/U值(0.01~0.34),指示锆石为变质成因,4个样品的锆石LA-ICP-MS U-Pb定年结果(1854±11 Ma、1846±12 Ma、1847±9 Ma和1848±7 Ma)表明基性岩床发生变质作用的时间为~1.85 Ga,与华北克拉通最终碰撞拼合的时间一致。根据基性岩床与胶‒辽‒吉造山带其他地区~2.1 Ga的基性岩具有相似的野外产状、岩石学和地球化学特征,推断其侵位时代可能为~2.1 Ga。基性岩床样品的SiO2含量为47.07%~52.18%,K_(2)O+Na_(2)O含量为1.78%~4.70%,MgO含量为3.92%~8.59%,属于拉斑玄武岩系列。大部分样品的稀土总量较低(47.3×10^(−6)~109.5×10^(−6)),富集轻稀土元素和大离子亲石元素(如Rb、Ba、La),亏损高场强元素(如Nb、Ta),Zr、Hf没有明显的亏损且未发生分馏作用。所有样品的Al_(2)O_(3)、CaO和Fe_(2)O_(3)^(T)含量在主量元素相关性图解上分布规律且与MgO含量呈较好的相关性,Nb/La值(0.52~0.73)随SiO2和MgO含量的变化较小,表明岩浆在侵位过程中受地壳混染的程度较低,似“岛弧”型地球化学特征是富集岩石圈地幔源区遭受古俯冲组分改造的结果。结合已有的辽河群变质火山‒沉积岩、辽吉花岗岩和基性岩的研究成果,胶‒辽‒吉造山带在~2.1 Ga处于陆内�The metamorphic mafic sills that widespread in the Jiao-Liao-Ji orogenic belt in Liaodong area are of great significance to study the Paleoproterozoic tectonic evolution of the orogenic belt.To reveal their petrogenesis and tectonic setting,and thus constraint the Paleoproterozoic geological evolution of the Jiao-Liao-Ji orogenic belt,a detailed petrographic observation,zircon U-Pb dating and whole-rock geochemical analyses of the metamorphic mafic sills were carried out.The metamorphic mafic sills in the Liaodong area emplaced into the thick marble of the Dashiqiao Formation of the Liaohe Group and underwent amphibolite facies metamorphism.They consist of amphibolite and garnet amphibolite,mainly composed of hornblende and plagioclase,and minor garnet,quartz,biotite,magnetite and pyrite.The characteristic zoning structures and low Th/U ratios(0.01–0.34)of the zircon grains from the metamorphic mafic sills suggest a metamorphic origin.LA-ICP-MS dating of zircons from four mafic sills yielded U-Pb ages of 1854±11 Ma,1846±12 Ma,1847±9 Ma,and 1848±7 Ma,respectively,suggesting a metamorphic age of~1.85 Ga,which is synchronous with the final amalgamation of the North China Craton.Based on the similar field occurrence,petrographic and geochemical features with those of the ca.2.1 Ga mafic intrusions in the Jiao-Liao-Ji orogenic belt and elsewhere,their emplacement age may be also ca.2.1 Ga.Whole-rock geochemistry shows that the metamorphic mafic sills are tholeiitic in composition(SiO2:47.07%–52.18%,K_(2)O+Na_(2)O:1.78%–4.70%,MgO:3.92%–8.59%),and enriched in light rare earth elements and large ion lithophile elements(i.e.,Rb,Ba and La),but depleted in high field strength elements(i.e.,Nb and Ta).In addition,there is not significant Zr and Hf depletions and fractionation effect.All mafic samples show regular distributions in Al_(2)O_(3),CaO and Fe_(2)O_(3)^(T) contents and good correlations with MgO contents in major element covariant plots,and Nb/La ratios(0.52–0.73)do not change with SiO2 and MgO content

关 键 词：古元古代 基性岩床 锆石U-PB定年 辽东地区 华北克拉通 

分 类 号：P581[天文地球—岩石学] P595[天文地球—地质学]