苏鲁超高压变质带东北端多种成因类型变基性岩:来自岩石学、同位素年代学及地球化学属性的制约

发布时间：2018-06-05 14:51

本文选题：变基性岩 + 锆石U-Pb年代学　；参考：《岩石学报》2017年09期

【摘要】：苏鲁超高压变质带是扬子板块与华北板块在三叠纪俯冲-碰撞的产物。变基性岩是苏鲁超高压变质带内出露最广泛的岩石类型之一,研究其岩石学、年代学、地球化学属性及成因机制,对于揭示扬子板块与华北板块之间的俯冲-碰撞-折返的动力学过程具有重要的科学意义。以(退变)榴辉岩为代表的超高压变质岩石广泛出露在威海-荣成一带,少量出露在乳山地区。锆石LA-ICP-MS U-Pb定年结果显示,(退变)榴辉岩的原岩时代为792~760Ma,峰期榴辉岩相变质时代为243~226Ma,后期角闪岩相退变质时代为221~207Ma。非榴辉岩相变质的基性岩(麻粒岩和斜长角闪岩)主要出露在乳山地区,其原岩形成时代应不晚于古元古代(1939Ma),峰期麻粒岩相变质时代为1895~1870Ma,后期角闪岩相退变质时代为1848~1806Ma,与胶北地体变基性岩的原岩时代和变质时代十分相似。全岩地球化学研究结果表明,(退变)榴辉岩的原岩显示高Fe拉斑玄武岩的特点,根据其稀土和微量元素特征,可将(退变)榴辉岩进一步划分为A、B和C三组。在球粒陨石标准化稀土配分模式和原始地幔均一化蛛网图解上,A、B和C三组样品分别具有轻稀土弱亏损、轻稀土弱富集和轻稀土富集的特点。轻稀土富集或弱富集型(退变)榴辉岩的原岩地球化学性质与岛弧或大陆玄武岩相似,它们的源区可能与深部富集地幔或受流体交代的地幔楔存在密切的成因关系;而轻稀土亏损型(退变)榴辉岩的原岩可能来自于亏损地幔的部分熔融。由此可见,(退变)榴辉岩的原岩具有成因多样性的特点。乳山地区的基性麻粒岩和斜长角闪岩的原岩也具有高Fe拉斑玄武岩的地球化学属性,Al2O3与Mg O呈正相关变化,TiO_2、P_2O_5与MgO表现出一定程度的负相关性。绝大多数非榴辉岩相变质基性岩的球粒陨石标准化稀土配分模式和原始地幔均一化蛛网配分曲线具有微右倾或明显右倾的特点。上述特征表明,研究区绝大多数非榴辉岩相变质的基性岩原岩来自于富集地幔,少数来自于原始地幔或亏损地幔,并经历了斜长石和辉石的分离结晶以及不同程度的部分熔融过程。由此可见,乳山地区出露的非超高压变质基性岩的原岩具有与胶北地体(高压)基性麻粒岩相近的成因特点。岩石学、同位素年代学和地球化学特征的综合对比研究结果表明,在苏鲁超高压变质带东北端的威海-荣成-乳山地区,既存在与华北板块古老变质基底相关的变基性岩,也存在与华南板块北缘新元古代变质基底相关的超高压榴辉岩,表明三叠纪时期华北板块东南缘胶北地体的部分古老变质基底曾卷入到扬子板块与华北板块之间的俯冲-碰撞造山过程,随后与超高压岩石一起抬升折返,形成当今的构造混杂岩带。
[Abstract]:The Sulu UHP metamorphic belt is the product of the Triassic subduction-collision between the Yangtze plate and the North China plate. Metamorphic rocks are one of the most widely exposed rock types in the Sulu UHP metamorphic belt. The petrology, geochronology, geochemical properties and genetic mechanism of these rocks are studied. It is of great scientific significance to reveal the dynamic process of subduction, collision and reentry between the Yangtze plate and the North China plate. The ultrahigh pressure metamorphic rocks represented by eclogite are widely exposed in Weihai-Rongcheng area and a few in Rushan area. The results of zircon LA-ICP-MS U-Pb dating show that the protolith age of the metamorphic eclogite is 792 ~ 760 Ma. the metamorphic age of the peak-eclogite facies is 243 ~ 226Ma. the retrograde metamorphic age of the amphibolite facies is 221 ~ 207Ma. Non-eclogite metamorphic basic rocks (granulite and amphibolite) are mainly exposed in the Rushan area. The age of its original rock formation should not be later than that of Gu Yuan, the peak granulite facies metamorphism is 1895 ~ 1870 Maand the late amphibolite facies retrograde metamorphism is 1848 ~ 1806 Ma, which is very similar to the original rock age and metamorphic age of the basic rocks of the Jiaobei terrane. The geochemical results of the whole rock show that the protolith of the metamorphic eclogite shows the characteristics of high Fe tholeiite. According to its rare earth and trace element characteristics, the eclogite can be further divided into three groups: AHB and C. On the basis of the standardized REE partition model of chondrites and the primitive mantle homogenization cobweb diagram, the three groups of samples, namely, Aneb and C, have the characteristics of light rare earth weak deficiency, light rare earth weak enrichment and light rare earth enrichment, respectively. The geochemical properties of LREE enriched or weakly enriched eclogite are similar to those of island arc or continental basalt, and their source regions may be closely related to the deep enriched mantle or the mantle wedge which is metasomatic by fluid. The source rocks of LREE depleted eclogite may come from partial melting of depleted mantle. It can be seen that the original rocks of the eclogite have the characteristics of genetic diversity. The basic granulites and amphibolites of Rushan area also have geochemical properties of high Fe tholeiite. There is a positive correlation between Al _ 2O _ 3 and MgO. The correlation between TiO2P _ 2O _ 5 and MgO is negative to some extent. The normalized REE distribution model of chondrites and the homogenized cobweb distribution curves of the primitive mantle in most non-eclogite metamorphic basic rocks have the characteristics of slightly right inclination or obvious right dip. The above characteristics indicate that the majority of the noneclogite metamorphic primary rocks in the study area come from the enriched mantle and a few from the primitive mantle or depleted mantle. It also underwent the separation and crystallization of plagioclase and pyroxene and the partial melting of different degrees. It can be seen that the original rocks of the exposed non-ultrahigh pressure metamorphic basic rocks in the Rushan area are similar to those of the Jiaobei terrane (high pressure) basic granulites. A comprehensive comparative study of petrology, isotopic chronology and geochemistry shows that in the Weihai-Rongcheng Rushan area at the northeast end of the Sulu ultrahigh pressure metamorphic belt, there are both altered basic rocks associated with the ancient metamorphic basement of the North China plate. There are also ultrahigh pressure eclogites associated with the Neoproterozoic metamorphic basement in the northern margin of the South China plate. The results indicate that part of the ancient metamorphic basement of the Qiaobei terrane in the southeastern margin of the North China plate during the Triassic period was involved in the subduction-collision orogenic process between the Yangtze plate and the North China plate, and then uplifted and reversed along with the ultrahigh pressure rocks. Forming the present-day tectonic melange zone.
【作者单位】：中国地质科学院地质研究所;
【基金】：国家自然科学基金重点项目(41430210) 中国地质调查局地质大调查项目(DD20160121) 中国地质科学院基本科研业务费项目(YYWF201703)联合资助
【分类号】：P588.3;P59

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