南苏鲁造山带的超高压变质岩及岩石化学研究

Petrology and Petrochemistry of UHP Metamorphic Rocks from the Sulu Orogenic Belt, Eastern Central China

在南苏鲁造山带核部,古老的表壳岩和花岗质侵入岩经历了三叠纪的超高压变质作用,在超高压变质岩石抬升过程中经历了强烈的角闪岩相退变质作用改造。据岩相学和岩石化学研究,可以区分出六大类典型超高压变质岩:榴辉岩、石榴石橄榄岩、石英硬玉岩、石榴石多硅白云母片岩、硬玉石英岩和石榴石绿辉石文石岩。这些岩石的角闪岩相退变质产物分别是斜长角闪岩、蛇纹岩、长英质片麻岩、长石石英云母片岩、石英岩和大理岩。地球化学研究揭示,榴辉岩的原岩很可能是形成在大陆内部构造环境的拉斑玄武岩,而石榴石橄榄岩可能是起源于亏损的残余地幔。石英硬玉岩原岩包括正变质的花岗岩和奥长花岗岩、副变质的酸性火山碎屑岩和长石石英砂岩。大面积分布的古老花岗岩很可能是形成在大陆或大陆边缘环境。长石石英云母片岩、石英岩和大理岩的原岩为沉积岩,与副变质的长英质片麻岩和基性火山岩一起构成了古老的表壳岩组合。双峰式的酸性和基性火山岩组合的存在也证明部分表壳岩是形成在大陆环境。因此,可以推测南苏鲁造山带核部的超高压变质岩原岩为形成在大陆板内环境的沉积岩-酸性和基性火山岩-花岗岩和奥长花岗岩建造。

In the southern Sulu orogenic belt, old supracrustal rocks and granitic intrusions were subjected to ultrahigh-pressure metamorphism (UHPM) in 220 to 240 Ma during the subduction and collision between the North China plate and Yangtze plate, and then amphibolite-facies retrogression during the exhumation of UHPM rocks. Based on the study of petrology and petrochemistry, these rocks can be divided into six types, i. e. eclogite, garnet peridotite, quartz jadeitite, garnet-phengite schist, jadeite quartzite and garnet and omphacite-bearing aragonite marble. They were transformed to amphibolite, serpentinite, gneiss, quartz-mica schist, quartzite and marble during the amphibolite-facies retrogression. The chemical characteristics indicate that the protolith of eclogites were possibly tholeiite with a continental affinity, whereas the garnet peridotites were derived from a depleted mantle. The protoliths of quartz jadeitite include metamorphosed granite, trondhjemite, acidic volcanic rocks and feldspar-quartz sandstone. The metamorphosed granites show an affinity of continental or continental margin. The quartz-mica schist, quartzite and marble were metamorphosed sedimentary rocks, and, together with paragneiss and mafic rock, constituted the old supracrustal series. The bimodal association of acidic and mafic volcanic rock suggests that some supracrustal rocks were formed under continental rift. Therefore, the protoliths of the UHPM rocks from the southern Sulu orogenic belt were sedimentary rocks, mafic and acidic volcanic rock, granite and trondhjemite formation with continental affinity.