山东省麻粒岩与麻粒岩相变质作用--研究现状及对前寒武纪大地构造演化的启示

Granulite and Granulite Facies Metamorphism in Shandong Province——Research Status and Implications to Precambrian Geotectonic Evolution

本文系统收集整理了近年来与山东省境内麻粒岩有关的研究资料,将山东省麻粒岩划分为沂水麻粒岩带、平度—栖霞麻粒岩带、海阳所—威海麻粒岩带三个麻粒岩相带,总结了各带的地质概况、变质温压条件及P—T—t轨迹、变质时代及演化过程,探讨了麻粒岩的成因。认为山东省各麻粒岩带均是多期次麻粒岩相变质作用的产物,其中2.7Ga、2.5Ga两期麻粒岩相变质作用均为中低压条件下的产物,发生时间与区内TTG的形成时间较为一致,P—T—t轨迹为逆时针型,反映当时形成的大地构造背景为地幔底辟引发的拉张环境;而1.9Ga、210Ma两期麻粒岩相变质作用则高压、中低压麻粒岩都有,反映其非均质变质环境的特点,P—T—t轨迹为顺时针型,指示其与俯冲—碰撞造山过程有关。其中1.9Ga期与古元古代胶辽吉活动带发生俯冲—碰撞造山过程有关,而210Ma期则是华北—扬子两大板块三叠纪碰撞产生的榴辉岩的退变质产物。结合省内基础地质研究的最新进展,对山东省前寒武纪大地构造演化作了部分探讨,并建立了年代学格架。认为平度—栖霞麻粒岩带古元古代高压麻粒岩相变质和苏鲁超高压变质带三叠纪的榴辉岩相变质事件可能是胶东地区太古宙绿岩带金元素活化迁移并在中生代成矿的决定性因素。山东省太古宙大地构造演化具有一定的旋回性,每个旋回由绿岩带—基性—超基性侵入体—TTG花岗岩类—热变质事件组成,很好地符合TTG的玄武质岩浆底侵成因模式。自1.9Ga才有高压麻粒岩的形成,暗示了具有一定规模的板块之间的俯冲活动始于1.9Ga之前的古元古代。鲁西陆块与胶辽陆块在古元古代可能并不是两个完全独立的块体,其巨大差异可能是由于两个陆块的大地构造位置差异和后期抬升剥蚀程度不同而引起。

Objectives： Many research on granulites in Shandong Province have been done recent years, but there are still argues on protolith, age, process, genesis and tectonic setting. Basic geology study in Shandong Province have made great progress due to some precise age gained recently especially on Precambrian. It is very necessary to restudy and summarize granulites in Shandong Province systematically, discuss forming process of granulites and its implications for Precambrian geotectonic evolution of Shandong Province. Methods： Results of previous studies on granulites in Shandong Province are systematically collected. Geological background, metamorphic temperature and pressure, P--T--t path, metamorphic age, evolution process of granulites are summarized, while genesis is discussed. Furthermore, Precambrian geotectonic evolution of Shandong Province is initially discussed combined with latest advances in local basic geological research. Results： Granulites in Shandong Province lay in three belts： Yishui granulite belt, Pingdu--Qixia granulite belt, and Haiyangsuo--Weihai granulite belt. Granulite facies metamorphism in Shandong Province at 2.7Ga and 2.5Ga are product of low or middle pressure condition, simultaneous with Archean TTG and having an anticlockwise P--T--t path, suggesting that their tectonic background is an extension environment caused by mantle diapirism. While granulite facies metamorphism in Shandong Province at 1.9Ga and 210Ma are product of both high pressure and low--middle pressure condition, with a clockwise P--T--t path, suggesting that their metamorphic environment is anisotropic and having a relationship with subduction or collision process. Thereinto, 1.9Ga granulite is related to subduction or collision process of Jiao-Liao-Ji belt [ Jiaodong （Eastern Shandong）-- Liaoning--Jilin ] belt） in Paleoproterozoic, while 210Ma granulite is a retrograde metamorphism product of eclogite made by collision of North China Craton and South China Craton in Triassic. Conclusions ： High pressure granulite facies metamorphism of Pingdu--Qixia granulite belt in Paleoproterozoic and eclogite facies metamorphism of the Sulu ultra-high pressure metamorphic belt in Triassic might be a determinative factor of activation, transportation, and mineralization in Mesozoic of Archean greenstone belt gold in Jiaodong Peninsula. Archean geotectonic evolution of Shandong Province has a certain character of cycling. Each cycle is made up of greenstone belt, mafic--uhramafic intrusions, TTG granites, and heat metamorphic event. It can be well explained by the TTG genesis model of basaltic magma underplating. There is no high pressure granulite until 1.9Ga, implies that large scale subduction between plates began in Paleoproterozoic before 1.9Ga. Luxi block and Jiaoliao block might not be completely separated in Paleoproterozoic. Great differences between these two blocks might be caused by their differences in tectonic positions and degree of uplift--erosion in later period.