辽东半岛东南部南辽河群变质火山岩的时代、成因及其对区域构造演化的制约

辽东半岛东南部地区出露大面积南辽河群变质表壳岩系,其下部尤以广泛发育变质火山岩为特征。本文通过对黑云斜长片麻岩、斜长角闪岩以及黑云母变粒岩等岩石系统的岩石学、全岩地球化学研究,并结合锆石U-Pb年代学以及Lu-Hf的研究结果来制约其物质组成、原岩形成和变质时代、岩石成因及形成环境,进而探讨胶—辽—吉活动带的大地构造属性。锆石显微结构、微区成分及LA-ICP-MS U-Pb年代学分析结果表明,研究区南辽河群变质火山岩原岩可能形成于2.19Ga左右,并记录了1.90Ga左右的变质事件。元素和同位素地球化学分析结果表明,其原岩以玄武安山岩-安山岩和英安岩及少量的流纹岩为主,主体属于亚碱性系列,具有钙碱性演化趋势。其中,基性端元以相对较低的SiO_2质量分数,富集MgO、TFe_2O_3和轻稀土元素(LREE)及Cr、Co和Ni为特征,强烈亏损重稀土元素(HREE)和高场强元素(HFSE,如Nb、Ta和Ti),岩浆应起源于受俯冲流体或熔体交代的亏损岩石圈地幔楔;而中—酸性端元则具有高的SiO_2质量分数和较低的MgO、TFe_2O_3质量分数,富碱,富集Ba、Th、U和K,以及亏损Nb、Ta、P和Ti等特征,应来自于新生地壳物质的部分熔融。结合区域地质资料表明,南辽河群变质火山岩应形成于一个典型的活动大陆边缘的构造环境,暗示胶—辽—吉活动带北段古元古代中期以前的形成演化机制应与弧-陆碰撞作用有关。

Geochronology,Petrogenesis and Constraints on Regional Tectonic Evolution of the Meta-Volcanic Rocks in Southeastern Liaodong Peninsula

The Paleoproterozoic meta-supracrustal rocks,known as the South Liaohe Group,is widely exposed in the southeast of Liaodong Peninsula.Its lower part is characterized by the extensive metavolcanic rocks.In this paper,we study the petrology and whole-rock geochemical data of the biotite plagioclase gneiss,amphibolite,and biotite leptynites,as well as the U-Pb dating and Lu-Hf isotopedata of the zircons in these rocks.The aim is to restrict the petrogenesis of Jiao-Liao-Ji belt by researching their composition,the ages of formation and metamorphism of the protolith,and the petrogenesis.The structure,composition,and U-Pb ages of zircon indicate that the protoliths of the metavolcanic rocks in South Liaohe Group were formed at ca.2.19 Ga,and they were affected by metamorphic events at 1.90 Ga.The element and isotope geochemical analysis show that the protoliths of these rocks were basaltic andesite-andesite,dacite,and minor rhyolite;most of them belong to subalkaline affinities with calc-alkaline evolution.The basic rocks have relatively low SiO2 contents,enriched in MgO,TFe2O3,and light rare-earth elements(LREE),Cr,Co,and Ni,while depleted of heavy rare-earth elements(HREE) and high-field strength elements(HFSE,such as Nb,Ta and Ti),indicating a derivation from a partial melting of a depleted lithospheric mantle that had been altered by the fluids or melts derived from a subducted slab.However,the intermediate acid rocks have high contents of SiO2 and low contents of MgO and TFe2O3,enriched in alkalis,Ba,Th,U and K,while depleted of Nb,Ta,P and Ti,indicating that the acidic rocks could have been derived from a partial melting of juvenile crust.Taking into account the regional geology,we consider that the metavolcanic rocks of the South Liaohe Group formed under an active continental margin,and the evolution of the northern segment of the Jiao-Liao-Ji belt was related to an arc-continent collision before the middle Paleoproterozoic.