Abstract
Three types of eclogites were identified in the Xiaoxinzhuang area in the northern Sulu ultrahigh pressure (UHP) terrene based on their petrographic, compositional characteristics and locations. They are composed of garnet, omphacite, amphibole, epidote, phengite, quartz/coesite, rutile, apatite, ilmenite and kyanite. Garnet in eclogite exhibits weak compositional zoning, which shows an increase in Xgr and a decrease in Xpy from core to mantle, and a decrease in Xgr and a slight increase in Xpy from mantle to rim. Phengite inclusions in garnet show higher Si, up to 3.424 p.f.u., than those in the matrix. Pseudosections calculated using THERMOCALC in the NCKFMASHTO system for three representative samples record three stages of metamorphism: (I) prograde stage, (II) post- Pmax decompression and heating to the Tmax stage and (III) retrograde stage. Stage-I was recorded in garnet cores with mineral assemblage of garnet + omphacite ± amphibole ± lawsonite + phengite + quartz + rutile, and the P-T condition is constrained at 23.5–26.4 kbar and 623–655 °C. The Pmax, 41.5 kbar at 801 °C, is revealed from garnet enclosed by coarse-grained garnet with the mineral assemblage of garnet + omphacite + phengite + coesite + rutile. Stage-II produced garnet rim with mineral assemblage of garnet + omphacite + amphibole + quartz + rutile + metabasite melt, which constrained the P-T conditions of 21.4–23.0 kbar and 869–924 °C. Stage-III, recorded by unzoned garnet grain with the mineral assemblage of garnet + omphacite + amphibole + ilmenite + rutile + metabasite melt, constrained P-T conditions of 13.5–16.4 kbar and 813–852 °C. The data suggest that the rocks in the Xiaoxinzhuang area were subducted to a depth of over 135 km and underwent an UHP metamorphism. The P-T-t path revealed by the Xiaoxinzhuang eclogites is different from those in other areas of the Sulu UHP terrane, suggesting that they represent different rock slices during the subduction and exhumations.
Highlights
The Sulu-Dabie ultrahigh pressure (UHP) metamorphic belt in eastern China is the longest UHP terrane in the world, which was formed by the subduction of the Yangtze craton beneath the North China craton (NCC) during the middle-late Triassic period [1,2,3,4]
The study area is close to the Wulian—Qingdao—Yantai Fault (WQYF) that is considered as the northwestern boundary of the Sulu HP-UHP metamorphic belt
This paper presents the petrography and mineral chemistry of eclogites from the Xiaoxinzhuang area and evaluates its P-T metamorphic evolution using THERMOCALC 3.45 by Powell et al [14] and as used in Wei et al [15]
Summary
The Sulu-Dabie UHP metamorphic belt in eastern China is the longest UHP terrane in the world, which was formed by the subduction of the Yangtze craton beneath the North China craton (NCC) during the middle-late Triassic period [1,2,3,4]. Ye et al [8] considered that the subducted Yangtze craton reached > 200 km deep with peak P-T conditions up to 70 kbar and 1000 ◦C based on the exsolution of clinopyroxene from garnet. Previous studies on Sulu eclogites were carried out in Rongcheng [3], Yangkou Bay [4,8,11] and Donghai [3,12,13], located in the northern and southern parts of the Sulu UHP terrane, respectively. The eclogites in the Xiaoxinzhuang area in the southern part of the Sulu UHP terrane have not been examined. The study area is close to the Wulian—Qingdao—Yantai Fault (WQYF) that is considered as the northwestern boundary of the Sulu HP-UHP metamorphic belt. This paper presents the petrography and mineral chemistry of eclogites from the Xiaoxinzhuang area and evaluates its P-T metamorphic evolution using THERMOCALC 3.45 by Powell et al [14] and as used in Wei et al [15]
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