Coesite in metasediments from the Muzhaerte valley, southwestern Tianshan

• 来源期刊: 科学通报（英文版）
• 年，卷（期）: 2019,（2）
• 页码范围: 78-80
• 页数: 3页
• 语种: 英文

摘要

The sediments,including calcareous,siliciclastic and volcanoclastic,usually occupy ＞90％ in volume of an accretionary complex and thus,if involved in subduction zones,their maximum return depths (i.e.,peak pressures) are fundamental to unravel the geodynamic processes during the convergence of plates [1].As a unique example of accreted complex returned from sub-arc depths (＞90 km) worldwide,the metamorphic belt of southwestern Tianshan,NW China has been focused on and hotly discussed in the last two decades [2-5].Coesite is considered to be diagnostic of the maximum return depths for silica-saturated rocks,the lack of which in some samples often aroused disputation regarding the return depths among the researchers [6,7].Up to now,there have been abundant reports of coesite-bearing metasediments in the Akeyazi area (Fig.1) and their occurrences in adjacent regions are still unclear,although ultrahigh-pressure (UHP) conditions have been claimed based on phase relations of carbonate minerals for a pelitic schist from the Muzhaerte valley,ca.30 km west of the Akeyazi coesite localities [9].In this study,we carefully examined two metasedimentary samples (albite schist and quartz-rich eclogitic rock) from the Muzhaerte metaophiolitic complex and identified very small coesite grains in porphyroblastic garnet.