华北克拉通辽北清原地体新太古代基性麻粒岩变质作用演化

Evolution of metamorphic processes in the Neoarchean mafic granulites of the Qingyuan Terrane in northern Liaoning,North China Craton

对华北克拉通新太古代的构造演化模式有多种不同的认识,需要进行更加深入的变质作用研究。通过对辽北清原地体基性麻粒岩进行系统的岩相学观察、矿物化学分析、相平衡模拟和锆石定年研究,以阐明其变质演化过程和大地构造意义。研究选择的基性麻粒岩样品分为含石榴石域(19DJ07-GD)和不含石榴石域(19DJ07-NGD)两类,含石榴石的区域呈条带状且分布不均匀。两种区域都发育两期麻粒岩相组合。在含石榴石域,第一期变质矿物组合为石榴石+单斜辉石+斜方辉石+角闪石+黑云母+斜长石+石英。其中,第一期斜长石(Pl_(1))发育复杂成分环带,钙长石摩尔分数(x_(An))从核部到幔部升高,然后再向边部降低;第一期角闪石(Amp_(1))的Ti成分环带同样为从核部到幔部升高再向边部降低。通过矿物组合和相应的成分环带推测第一期麻粒岩相变质作用具有逆时针型P-T轨迹,包含峰期前升温升压阶段以及峰后降温降压阶段。通过相平衡模拟约束峰期温压条件为0.8~0.9 GPa/900~950℃,达到高温—超高温(high-ultrahigh temperature)变质条件。锆石定年结果表明变质作用峰后冷却时间为2498±6.9 Ma(MSWD=0.39)。综合区域上的“穹隆-龙骨”构造、逆时针的变质轨迹以及和TTG岩浆活动晚期脉冲几乎一致的表壳岩变质时间,表壳岩超高温麻粒岩相变质作用被认为受太古宙特有的垂向构造/沉落(sagduction)构造体制控制。第二期变质组合以局部生长的石榴石+石英±单斜辉石的后成合晶/冠状体为特征,代表一期与古元古代造山事件有关的高压麻粒岩相变质作用。

Multiple interpretations exist regarding the tectonic evolution model of the Neoarchean North China Craton,requiring a more in-depth study of metamorphic processes.Systematic petrographic observations,mineral chemical analysis,phase equilibrium modeling,and zircon dating were conducted on the basic granulites from Qingyuan in northern Liaoning to elucidate their metamorphic evolution processes and geological significance.The selected samples of mafic granulites were divided into the garnet-bearing domain(19DJ07-GD)and garnet-free domain(19DJ07-NGD),with the garnet-bearing region displaying a banded and inhomogeneous distribution.Both domains exhibit two generations of granulite facies assemblages.In the garnet-bearing domain,the first-generation metamorphic mineral assemblage includes garnet+clinopyroxene+orthopyroxene+hornblende+biotite+plagioclase+quartz.Notably,the first-generation plagioclase(Pl_(1))exhibits a complex compositional zoning,with anorthite content(x_(An))increasing from the core to the mantle and then decreasing towards the rim.Similarly,the titanium component zoning in the first-generation amphibole(Amp_(1))follows a pattern of increasing from the core to the mantle and then decreasing towards the rim.Based on mineral assemblages and corresponding component zoning,it is inferred that the first-generation granulite facies metamorphic process followed a counterclockwise P-T path,involving a pre-peak P-T rise stage and a post-peak P-T drop stage.Phase equilibrium modeling constrains the peak conditions at 0.8~0.9 GPa/900~950℃,indicative of high-ultrahigh-temperature(HT-UHT)metamorphism conditions.Zircon dating results yielded a post-peak cooling age of 2498±6.9 Ma(MSWD=0.39).Considering the regional“dome-and-keel”tectonics,the counterclockwise P-T path,and the metamorphic timing of supracrustal rock nearly synchronous with late-stage TTG magmatic pulses,the UHT metamorphism of the supracrustal rocks is believed to be controlled by the unique Archean vertical tectonics/sagduction system.The second-generation metamorphic assemblage is characterized by locally formed coronas or symplectites of garnet+quartz±clinopyroxene,representing high-pressure(HP)granulite facies metamorphism associated with a Paleoproterozoic orogenic event.