摘要
对华北克拉通新太古代和古元古代的构造模式有多种不同认识,有必要进行深入的变质作用研究。本文选取了辽北清原地区中性麻粒岩、石榴方辉石岩和变质基性岩墙,开展系统的岩相学观察、矿物化学分析、相平衡模拟和锆石-独居定年研究,以阐明其变质演化过程和大地构造意义。中性麻粒岩和石榴方辉石岩均发育两期麻粒岩相组合。中性麻粒岩第一期斜长石发育复杂成份环带,从核到幔部其钙长石含量(X_(An))降低,然后再向边部升高;石榴方辉石岩中第一期石榴石与斜方辉石互相包裹。第一期麻粒岩相变质作用P-T轨迹为逆时针型,包含峰期前升压至峰期和峰后降温降压至固相线两个变质阶段。依据两个样品中观测的峰期矿物组合在P-T视剖面图中的稳定范围,并结合斜长石幔部成份,确定峰期温压条件为1.0~1.2GPa/890~1000℃。石榴方辉石岩在峰前升压过程,斜方辉石转变为石榴石,形成石榴石包裹斜方辉石的结构;在峰后降温降压过程中,石榴石又转变为斜方辉石,导斜方辉石包裹石榴石。中性麻粒岩和石榴方辉石岩的第二期组合以形成石榴石+石英和黑云母+石英±单斜辉石±钾长石后成合晶和冠状体为特征。变质基性岩墙只发育第二期矿物组合,为高压麻粒岩组合,其P-T轨迹为顺时针型。根据变质斜长石中最小X_(An),角闪石中最大Ti含量和石榴石幔部最高镁铝榴石含量等值线,确定峰期温压条件为~1.15GPa/830℃。锆石定年表明第一期高温-超高温麻粒岩相变质作用峰后冷却时间为2.49~2.48Ga,第二期高压麻粒岩相变质时间为~1.83Ga,独居石定年获得峰后退变质年龄为~1.75Ga。结合其他区域地质特征,本文认为第一期高温-超高温麻粒岩相变质作用受太古宙特有的垂直沉落构造控制,第二期高压麻粒岩相变质作用与沿华北克拉通北缘发生的碰撞造山事件有关。
There are different understandings of the Neoarchean and Paleoproterozoic tectonic evolution of the North China Craton,so it is necessary to conduct detailed studies of its metamorphism.In this paper,intermediate granulites,garnet-orthopyroxene rocks,and metabasic dykes in the Qingyuan terrane of northern Liaoning were selected to carry out systematic studies of petrography,mineral chemistry,phase equilibria modeling,and zircon-monazite dating to document their metamorphic evolution and tectonic significance.Both intermediate granulites and garnet-orthopyroxene rocks are recognized to show two stages of granulite facies assemblages formed during the Neoarchean and Paleoproterozoic,respectively.The first phase plagioclase(Pl 1)in the intermediate granulite has complex compositional zoning with X An decreasing from core to mantle followed by increasing towards the rim,and in garnet-orthopyroxene rock the first phase garnet and orthopyroxene are enveloped between each other.The first phase granulite facies metamorphism shows anticlockwise P-T paths involving the pre-peak compression to the peak and post-peak decompressional cooling to a solidus.According to the stabilities of the observed peak assemblages in P-T pseudosections and the composition of the Pl 1 mantle,the peak P-T conditions are constrained to be 1.0~1.2GPa/890~1000℃for the first phase metamorphism.In the garnet-orthopyroxene rocks,orthopyroxene is transformed into garnet during the pre-peak compression as revealed from the texture of garnet enveloping orthopyroxene,and vice versa,orthopyroxene is formed by consuming garnet during the post-peak decompression cooling as indicated by garnet enveloped by orthopyroxene.The second phase granulite facies assemblages of the intermediate granulites and garnet-orthopyroxene rocks are characterized by the symplectites or coronae of garnet+quartz and biotite+quartz±clinopyroxene±K-feldspar.The metabasic dykes develop only the second phase assemblages of high-pressure granulite facies,and have clockwise P-T paths.According to the minimum X An in metamorphic plagioclase(Pl b),the maximum Ti content in amphibole,and the maximum X Py in garnet mantle,the peak P-T conditions are determined to be~1.15GPa/830℃.Zircon dating shows that the first phase high-ultrahigh temperature granulite facies metamorphism occurred during 2.49~2.48Ga,representing the time of the post-peak cooling process,and the second phase high-pressure granulite facies metamorphism occurred at~1.83Ga.Monazite dating gave a post-peak retrograde age at~1.75Ga for the second phase metamorphism.Combined with other regional geological characters,we propose that the first phase high-ultrahigh temperature granulite facies metamorphism is controlled by the Archean unique vertical tectonics/sagduction,and the second phase high-pressure granulite facies metamorphism is related to the collision orogenic event along the northern margin of the North China Craton.
作者
崔润泽
魏春景
段站站
CUI RunZe;WEI ChunJing;DUAN ZhanZhan(MOE Key Laboratory of Orogenic Belts and Crustal Evolution,School of Earth and Space Sciences,Peking University,Beijing 100871,China;College of Earth Sciences,Hebei GEO University,Shijiazhuang 050031,China)
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2023年第8期2257-2278,共22页
Acta Petrologica Sinica
基金
国家自然科学基金项目(41872057、418930834)资助。
