苏鲁造山带池庄超高压榴辉岩中变质脉：大陆俯冲带超临界流体活动的证据  

Veins in ultrahigh-pressure eclogites from Chizhuang in the Sulu orogen,eastern China: Evidence for supercritical fluids in continental subduction zones

作　　者：田野[1] 黄建[1] 回迎军 肖益林[1] 

机构地区：[1]中国科学院壳幔物质与环境重点实验室、中国科学技术大学地球和空间科学学院,合肥230026

出　　处：《岩石学报》2015年第7期1818-1840,共23页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41303015;41090372;41273037)资助

摘　　要：高压-超高压变质岩中的变质脉能够反映俯冲带变质流体的组成和演化。为了探究大陆俯冲带超临界流体活动及伴随的元素迁移,本文系统地研究了苏鲁造山带南部江苏东海池庄地区的超高压榴辉岩及变质脉。变质脉主要是由石英、石榴石、绿辉石、多硅白云母、蓝晶石、黝帘石、金红石和锆石等矿物组成,与寄主榴辉岩的矿物组成类似。相比于榴辉岩,脉体中的石榴石更加富集重稀土元素(HREE);黝帘石强烈富集轻稀土元素(LREE)。变质脉和榴辉岩中各主要矿物的氧同位素组成在误差范围内一致(石英的δ18O分别为2.42‰和2.79‰;石榴石为-0.30‰和0.010‰;绿辉石为0.25‰和0.071‰),说明变质脉的形成与榴辉岩释放的内部流体有关。综合已有的研究,发现大别-苏鲁造山带不同地区的变质脉和榴辉岩具有极不均一的氧同位素组成,说明在陆壳深俯冲和折返过程中,流体活动有限。利用矿物温压计得到变质脉的峰期变质温压条件为692±65℃和3.6±0.3GPa,脉体中锆石U-Pb定年结果表明锆石的形成时代为218±2.4Ma,指示变质脉形成于深俯冲陆壳折返初期的超高压变质阶段。变质脉中矿物组合和矿物的主微量元素特征说明成脉流体富集Si、Al、Ca、K、LILE、REE和HFSE等元素,表明成脉流体可能是溶解能力极强的超临界流体。Metamorphic veins in high-pressure/ultrahigh-pressure metamorphic rocks provide a unique opportunity to trace the composition and evolution of metamorphic fluids in subduction zones. To explore the activity of supercritical fluids and their associated element mobility during deep subduction and exhumation of the continental crust, we have systematically investigated the ultrahigh pressure （UHP） eclogites and associated metamorphic veins at Chizhuang in the Sulu orogen. Petrographic observations indicate that the veins are mainly composed of quartz, garnet, omphacite, phengite, kyanite, zoisite, rutile and zircon, similar to the host eclogites. Compared to the host eclogites, the garnets in the veins contain relatively high concentrations of heavy rare-earth elements （HREE） and the zoisites in the veins contain strongly high concentrations of light rare-earth elements （LREE）. Minerals from both vein and eclogite have similar 0 isotopic compositions within errors （ quartz δ18 O values are 2.42‰ and 2.79‰, garnet - 0.30‰ and 0. 010‰; omphacite 0.25‰ and 0. 071‰, respectively）. This indicates that the vein-forming fluids are internally derived. Compilation of published data for veins and their host eclogites from different localities in the Dabie-Sulu orogenic belt indicates highly heterogeneous O isotopic compositions, suggesting that fluid activity in deeply subducted continental crust is limited. Mineral geothermobarometry yields 692 -＋ 65~C and 3.6 ＋ O. 3GPa for the peak metamorphic conditions of vein formation in Chizhuang, while zircons from the vein yield U-Pb ages of 218 ＋ 2.4Ma. This indicates that the vein formed during the initial exhumation of deeply subducted continental crust under UHP conditions. Mineral assemblages in the veins combined major and trace element concentrations of these minerals indicate that the vein-forming fluids are enriched in Si, A1, Ca, K, LILE, REE, and HFSE. This suggests that such fluids are probably supercritical fluids which have a great ca

关 键 词：榴辉岩 变质脉 元素迁移 超临界流体 超高压变质 苏鲁造山带 

分 类 号：P588.348[天文地球—岩石学] P597.3[天文地球—地质学]