石榴石、独居石和金红石微量元素和同位素的精细研究及其对胶东古元古代变质事件时代的制约

Paleoproterozoic is an important period of early-Precambrian tectonic regime transition from mantle plume-controlled vertical.growth to plate-controlled lateral accretion for North China Block (NCB). The previous studies have shown that the Paleoproterozoic granulite-facies metamorphism in NCB occurred at 1.95-1.8 Ga, which mainly focused at 1.95-1.92 Ga, 1.85-1.83 Ga and ~1.8 Ga and some interpretations have been proposed. However, it needs more work to affirm the geological meaning of these ages. Recent developments of dating on metamorphic minerals such as monazite and rutile supply more approaches to confirm meaning of the metamorphic ages. This project will carry on the elaborate microstrcture-occurrences and/or compositonal zonations of garnet and accessory minerals such as monazite and rutile for high-pressure mafic granulite, high-temperature granulite and meta-ultramafic rocks which underwent.Paleoproterozoic metamorphism in Jiaodong terrane in the eastern part of NCB. Combined with in-situ dating, it could constrain Jiaodong Paleoproterozic metamorphism periods and metamorphic stages. It is obvious that Paleoproterozoic metamorphism in NCB differed from modern plate-controlled tectonic evolutions. This project will integrate the traditional thermobarometer, trace element thermometer, isotope geochronology and closure temperature, providing constraints on the early-Precambrian thermal regime for Jiaodong.terrane.

古元古代是早前寒武纪构造体制转化的关键时期。研究表明华北克拉通古元古代麻粒岩相变质时间持续了长达150 Ma以上,大致集中在三段：1.95-1.92 Ga，1.85-1.83 Ga和~1.8 Ga。对此虽然已有研究积累，但其精准年代和代表的意义（同一变质事件的不同阶段还是不同的变质事件）仍存在争议。近年来的分析技术，已经使得对变质的副矿物如独居石、金红石的精准原位微区分析成为可能。本项申请拟以华北东部胶东地体古元古代的高压基性麻粒岩、泥质麻粒岩和变超镁铁岩为研究对象，在岩石学研究的基础上，详细地分析其造岩矿物（主要是石榴石）和副矿物（独居石，金红石等）的成分分带特征，将其产状、显微结构与原位精细定年（薄片尺度）结合起来，为该地区古元古代变质期次和变质阶段的认识提供依据。此外，基于古元古代变质体系的温压梯度明显高于现今造山带的特点对胶东地体古元古代的热流体制进行限定。

古元古代是早前寒武纪构造体制转化的关键时期。麻粒岩中的锆石年龄研究表明华北克拉通古元古代麻粒岩相变质时间持续了长达150 Ma以上,大致集中在三段：1.95-1.92 Ga，1.85-1.83 Ga和~1.8 Ga。本项目通过对胶北超镁铁质麻粒岩、基性麻粒岩和泥质麻粒岩变质矿物组合、造岩矿物主微量元素分析，揭示出胶东麻粒岩经历了峰期高压麻粒岩相变质、中压麻粒岩相变质以及角闪岩相退变质三个阶段。通过对锆石、独居石、金红石原位定年和包裹体成分分析，限定其三个变质阶段与三组年龄可以很好的对应上。在此研究的基础上，本项目对华北克拉通古元古代麻粒岩地体的变质温压条件作了统计分析，并计算了当时的地热梯度与麻粒岩地体的抬升速率，发现其明显不同于太古宙和显生宙，暗示了一种新的构造体制，可能对应于初始的板块构造。