华北克拉通吉南夹皮沟新太古代石榴黑云斜长片麻岩的变质P-T-t演化及构造意义

Metamorphic P-T-t evolution and tectonic significance of Neoarchean garnet-biotite-plagioclase gneisses in Jiapigou,southern Jilin,North China Craton

华北克拉通东部陆块新太古代末期的构造体制是近年来备受学界关注的热点议题。本文对地处华北克拉通东部陆块东北部的吉南夹皮沟石榴黑云斜长片麻岩进行了详细的岩相学、矿物化学、变质相平衡模拟、传统地质温压计和锆石U-Pb年代学研究,以明确其变质P-T-t演化,探讨构造过程和动力学机制。石榴黑云斜长片麻岩主要由石榴石、黑云母、斜长石、角闪石、石英和少量钛铁矿组成,记录了逆时针P-T轨迹,可以划分为三个变质阶段。岩石第一阶段的矿物组合为黑云母+斜长石+角闪石+石英+钛铁矿(+熔体)(Bt 1+Pl 1+Amp 1+Qz+Ilm(+L)),记录的温压条件为790~854℃和0.55~0.88GPa,之后通过升压略降温到达压力峰期阶段,在这个过程中角闪石分解、石榴石生长、基质型斜长石的X_(An)值从核到边降低。第二阶段的矿物组合为石榴石+黑云母+斜长石+角闪石+石英+钛铁矿(+熔体)(Grt+Bt_(2)+Pl_(2)(核部)+Amp_(2)+Qz+Ilm(+L)),相平衡模拟获得该阶段的温压条件为670~745℃和1.10~1.24GPa,传统地质温压计的结果为707~771℃和0.99~1.18GPa,两者可相互佐证,可能代表了岩石的压力峰期阶段。第三阶段的矿物组合为黑云母+斜长石+石英+钛铁矿(Bt_(3)+Pl_(2)(边部)+Qz+Ilm),形成于压力峰期后的降温降压过程,石榴石边部分解形成斜长石,该类斜长石的X_(An)值从核到边逐渐升高。变质锆石U-Pb定年获得2483±8Ma(MSWD=3.2)的年龄,结合稀土配分模式和锆石Ti温度计计算结果,将该年龄解释为岩石压力峰期阶段前的变质时代。考虑到吉南变质地体新太古代末期岩浆活动与变质作用的时间重叠性、区域上的“穹窿-龙骨”构造样式、前人的变质作用资料以及石榴黑云斜长片麻岩记录的升压轨迹,本文认为吉南新太古代末期变质作用与重力凹沉构造有关。

The Late Neoarchean tectonic regime in the Eastern Block of the North China Craton has been hotly debated in recent years.In this contribution,detailed petrography,mineral chemistry,metamorphic phase equilibria modeling,traditional geothermobarometers and zircon U-Pb geochronology were carried out for garnet-biotite-plagioclase gneisses from Jiapigou,southern Jilin in the northeastern part of the Eastern Block,to clarify its metamorphic P-T-t evolution and explore the tectonic process and dynamic mechanism.Garnet-biotite-plagioclase gneisses mainly consist of garnet,biotite,plagioclase,amphibole and quartz,with minor ilmenite.The rocks record a counterclockwise P-T path and can be divided into three metamorphic stages.The mineral assemblage in the first stage of the rocks is biotite+plagioclase+amphibole+quartz+ilmenite(+melt)(Bt 1+Pl 1+Amp 1+Qz+Ilm(+L)),which P-T conditions are 790~854℃and 0.55~0.88GPa.Following a slight cooling compression process to the pressure peak stage,this process is characterized by the breakdown of amphibole,the growth of garnet,and a decrease in X_(An) from core to rim in the matrix-type plagioclase.The mineral assemblage in the second stage consists of garnet+biotite+plagioclase+amphibole+quartz+ilmenite(+melt)(Grt+Bt_(2)+Pl_(2)(core)+Amp_(2)+Qz+Ilm(+L)).The P-T conditions of this stage from phase equilibria modeling are 670~745℃and 1.10~1.24GPa,which are supported by the results(707~771℃and 0.99~1.18GPa)using traditional geothermobarometers,and this stage may represent the pressure peak stage of the rocks.The mineral assemblage in the third stage consists of biotite+plagioclase+quartz+ilmenite(Bt_(3)+Pl_(2)(rim)+Qz+Ilm).This assemblage formed during the cooling and decompression process after the pressure peak stage,causing the breakdown of garnet to form plagioclase,and the X_(An) of this type of plagioclase gradually increases from core to rim.Metamorphic zircon U-Pb dating yields an age of_(2)483±8Ma(MSWD=3.2),interpreted as the metamorphic age before the pressure peak stage based on the REE patterns and Ti-in-zircon thermometer calculations.Considering the temporal overlap between magmatic activity and metamorphism in the Late Neoarchean,the regional“dome and keel”structure,previous metamorphic data,and the compression process recorded by the garnet-biotite-plagioclase gneisses from southern Jilin,it is suggested that the metamorphism in southern Jilin during the Late Neoarchean may be related to sagduction.