华北克拉通丰镇碳酸岩中榴辉岩捕虏体岩石学研究:现今板块体制古元古代开始启动证据

关于现今板块构造体制何时启动是目前地球科学研究的焦点问题。本文在原报道的古元古代丰镇火成碳酸岩中发现的榴辉岩捕虏体基础上,开展了详细的岩石学研究。该榴辉岩捕虏体分为两种类型:即相对富石榴石的Fz-2和贫石榴石的Fz-16,它们产于同一地点,且具有相同的矿物成分和结构构造特征。Xu et al.(2018)的研究表明该捕虏体具有1839±26Ma和1766±7Ma的独居石U-Th-Pb年龄且具有大洋辉长岩原岩的全岩成分特征。本文通过进一步的岩相学研究发现该榴辉岩至少经历了两期变质阶段:M1,角闪石/绿帘石-榴辉岩阶段;M2,硬柱石-榴辉岩阶段。具有放射状裂纹包裹特征的柯英石假象在石榴石变斑晶和基质绿辉石中以包体形式出现。以蓝晶石与黝帘石共存为特征的柱状硬柱石假象,也偶尔以包体形式存在于石榴石中。变斑晶石榴石分为富含包体的核部和比较干净的边部。石榴石从中心到边部具有明显的镁铝榴石含量增加和钙铝榴石含量降低的环带特征,通过相平衡模拟和等值线投图得到其温压范围为2.6~3.7GPa和655~670℃,记录了从M1到M2的近等温增压的进变质过程。通过石榴石边部-绿辉石-蓝晶石-石英的地质温压计计算得到温压条件为3.0GPa、734℃。金红石中的锆含量温度计也给出了相似的温度条件,即在2.6~3.7GPa压力时为601~685℃。石榴石边部的柯石英假象和硬柱石假象支持了M2硬柱石-榴辉岩阶段的存在,这表明丰镇古元古代榴辉岩可能是目前发现的世界上最古老的低温超高压变质岩。同时,我们得到该榴辉岩代表的进变质过程中的地温梯度为216±35℃/GPa,证明至少在~1.8Ga以来代表现今板块构造体制的板块冷俯冲作用就开始启动了。

Petrologic study of the eclogite xenoliths in fengzhen carbonatite, North China Craton: Evidence for the start of modern-style plate tectonic regime in Paleoproterozoic

The onset of modern plate tectonic regime on Earth became increasingly controversial in recent years because of the lack of continuously newly found records of subduction and collision.Here,we present the study of the ultra-high pressure eclogite xenoliths in the Paleoproterozoic Fengzhen carbonatite complex,including the garnet-rich(Fz-2)and the omphacite-rich(Fz-16)samples.All samples are collected from the same site and exhibit identical mineral chemistry and textures.The xenoliths display whole-rock compositions indicative of the protoliths of oceanic gabbro.At least two metamorphic stages are recognized in these eclogites:epidote/amphibole-eclogite stage M1 and lawsonite-eclogite stage M2.Coesite pseudomorph within radial cracks occurs as inclusions in garnet and omphacite.Columnar lawsonite pseudomorph as aggregates of coexisted kyanite and zoisite occasionally present as inclusion in garnet.The porphyroblastic garnet exhibits an inclusion-rich core and an inclusion-poor rim.The garnet displays compositional zoning,with the increasing pyrope and decreasing almandine content from core to rim.The phase equilibria modeling records the prograde path from amphibole-eclogite phase to lawsonite eclogite phase,at pressure and temperature of 2.6~3.7 GPa and 655~670℃,recorded from garnet zonation.The garnet-omphacite-phengite-kyanite-quartz thermobarometry yields a pressure and temperature of 3.0 GPa and734℃.The Zr-in-rutile thermometer also gives a similar temperature range of 601~685℃at 2.6~3.7 GPa.The coesite and lawsonite pseudomorphs within the garnet and omphacite support the existence of the lawsonite-eclogite stage,representing the oldest record of low temperature ultra-high pressure metamorphic rock in the world.With a thermal gradient of 216±35℃/GPa,the eclogite rocks supports the onset of modern-style cold subduction prior to~1.8 Ga.