南苏鲁超高压变质带东海ZK703钻孔榴辉岩的变质作用  

Metamorphism of the Eclogites from the ZK703 drillhole in Donghai. South Sulu (Jiangsu-Shandong) Ultrahigh-pressure Metamorphic Belt, Eastern China

作　　者：张泽明[1] 许志琴[1] 徐惠芬[1] 

机构地区：[1]中国地质科学院地质研究所,北京100037

出　　处：《地质学报》1999年第4期321-333,共13页Acta Geologica Sinica

基　　金：国家自然科学基金(编号49772142);国土资源部大陆动力学开放研究实验室资助

摘　　要：东海ZK703钻孔位于苏鲁超高压变质带南部,钻孔深558m,穿过的岩性主要为榴辉岩、黑云斜长片麻岩和石榴橄榄岩,及少量多硅白云母石英片岩、石榴石多硅白云母石英片岩、蓝晶石石英岩和硬玉石英岩。钻孔揭示榴辉岩呈层状产出,具中等倾角,稳定延伸到钻孔深度以下。超高压矿物或其后成合晶假象在这些岩石中的保存,以及岩石之间的接触关系表明,这是一个整体经历了超高压变质作用的表壳岩和基性-超基性岩组合。据原生次要矿物,超高压榴辉岩可分成5种类型,其中以富金红石榴辉岩为主,多硅白云母榴辉岩和石英榴辉岩次之,夹少量蓝晶石多硅白云母榴辉岩,含极少量次要矿物的普通榴辉岩与石榴橄榄岩伴生。岩相学研究揭示超高压榴辉岩至少经历了3期变质作用,即峰期超高压榴辉岩相,以柯石英、高硅多硅白云母和金刚石的出现为特征;角闪岩相退变质作用,以角闪石、斜长石和黑云母等后成合晶矿物的形成为特征;绿帘角闪岩相,角闪石+钠长石+绿帘石共生。超高压榴辉岩峰期变质作用的pt条件是700～860C,>2.8～3.0GPa,退变质作用pt轨迹表现为早期的近等温降压过程和晚期的降温、降压过程。显微结构和矿物微区化学成分变化表明,超高压矿物减压退变质形成的后成合晶和后成合晶冠状体矿物是域平衡反应或不平衡反应的产物。合晶矿物的?The ZK703 drillhole with a depth of 558m is located in the Donghai area of the southern margin of the Sulu (Jiangsu-Shandong) ultrahigh-pressure metamorphic belt, eastern China. A large number of eclogites, minor gneisses, jadeite quartzite and garnet peridotites, and rare phen-gite quartz schist and kyanite quartzite occurs as layers in the drillhole, in which the eclogite layers extend further downward below the level of drillhole bottom. The preservation of ultrahigh-pressure metamorphic minerals and their symplectites, and contact relationship and protolith types of various rocks indicate that the drillhole consists of metamorphic supracrustal rocks and basic-ultrabasic rock assemblages that have been subjected to a coherent ultrahigh-pressure meta-morphism. According to their minor minerals, the eclogites can be divided into five types, i. e. the rutile-rich eclogite, phengite eclogite, quartz eclogite, kyanite phengite eclogite and common eclogite. Two stages of retrograded assemblages are observed in the eclogites. The first stage is represented by the formation of albite + amphibole symplectite or symplectitic corona after om-phacite, plagioclase + amphibole after garnet, plagioclase + biotite after garnet or phengite, and plagioclase after kyanite; the second stage involved total replacement of omphacites and garnets by amphibole + albite + epidote. The reaction texture and micro-region composition of symplectitic minerals show that the retrograde reaction of ultrahigh-pressure minerals is a domainal equilibration or disequilibration reaction. The symplectitic mineral compositions are mainly controlled by the replaced minerals and the diffusion rates of elements involved in the reactions. Peak metamorphic pt conditions of the eclogites are >2. 8-3. 0GPa and 700-860℃. The retrograde pt path of the eclogites shows a feature of near isothermal decompression in the early stage and decreasing temperature and pressure in the late stage.

关 键 词：钻孔 榴辉岩 岩相学 矿物化学 超高压变质带 

分 类 号：P588.3[天文地球—岩石学]