吉东南新太古代晚期片麻岩类的时代、成因及其对早期地壳形成演化的制约

Age,Petrogenesis and Their Constraints on Regional Crustal Evolution of Late Neoarchean Gneisses in Southeast Jilin Province

吉林省东南部通化地区广泛出露早前寒武纪花岗质片麻岩类,其岩石组成、形成时代和成因对深化认识华北克拉通东北缘早期地壳形成演化历史具有重要意义。本文系统的岩石学、锆石U-Pb年代学、元素和Lu-Hf同位素地球化学等研究结果表明,这些花岗质片麻岩类按照SiO_2质量分数总体可划分为高硅和低硅两组:前者主要由二长花岗质片麻岩、奥长花岗质片麻岩及英云闪长质片麻岩组成,其原岩形成于2 549~2 557 Ma;而后者由石英二长闪长质片麻岩及花岗闪长岩质片麻岩组成,其原岩形成于2 534~2 552 Ma;并且两组岩石都含有约2 600 Ma的捕获锆石,共同遭受了约2 500 Ma变质作用的影响。地球化学分析结果显示,低硅岩组具有较高的MgO、CaO、Na_2O质量分数,属于高钾—中钾钙碱性系列,并且富集LREE、亏损HREE和HFSE;与之相比,高硅岩组则具有较低的MgO和CaO质量分数,显示更强烈的轻、重稀土元素分馏以及Nb、Ta、P、Ti等亏损的特征;但两者均具有较弱的正或负Eu异常。结合区域最新研究成果,认为研究区低硅和高硅两组岩石应具有相同的源区,其形成可能与大洋板片俯冲、岩浆底侵引起的地壳部分熔融作用有关。此外,两组岩石具有相似的ε_(Hf)(t)值(2.72~7.95)和模式年龄(2.86~2.55Ga),暗示区域主要存在新太古代晚期地壳生长事件;结合区域内变质火山岩的研究进展,认为吉林省东南部通化地区花岗质片麻岩类可能形成于活动大陆边缘的构造背景。

The Early Precambrian granitic gneiss are widely distributed in Tonghua in southeast Jilin Province.The lithological assemblages,ages and petrogenesis of these granitic gneiss are informative to the formation and evolution of the early crust in the northeastern margin of the North China craton.Based on the systematic petrology,zircon U-Pb chronology,element and Lu-Hf isotopic geochemistry results,we divided these granitic gneisses into high silicon group and low silicon group according to their SiO2 contents.The former is mainly composed of monzogranite gneisses,trondhjemite gneisses and tonalite gneisses,and the magmatic precursors was emplaced at 2 549-2 557 Ma.While the latter is composed of quartz-monzonitic diorite gneisses and granodiorites gneisses,and their protolith may be formed at 2 534-2 552 Ma.The both groups contain zircons captured at ca.2 600 Ma,and suffered from the metamorphism for ca.2 500 Ma.The geochemical data show that the low silicon group is rich in MgO,CaO and Na2O,belonging to the high-medium K calc-alkaline series,and has the feature of higher contents of LREE,but lower contents of HREE and HFSE.In contrast,the high silicon group has the lower MgO and CaO,and exhibit a stronger fractionation between the light and heavy rare earth elements,and the elements of Nb,Ta,P,Ti are depleted.The two groups show the weak positive or negative Eu anomalies.In combination with the regional geological data in this study,we believe that the two groups were derived from the same source,and their formation may be related to the ocean plate subduction through a partial melting of the crust caused by the magma underplating.Moreover,the two groups have similarεHf（t）values（2.72 to 7.95）and model age（2.86 to 2.55 Ga）,which suggests that a crustal growth occurred in the study area in the Late Neoarchean.According to the research progress of metamorphic volcanic rocks in the region,we consider that the granitic gneisses in Tonghua may be formed at an active continental margin.