Metamorphic P-T Path Differences between the Two UHP Terranes of Sulu Orogen, Eastern China: Petrologic Comparison between Eclogites from Donghai and Rongcheng

Abstract

The Sulu Orogen constitutes the eastern part of the Sulu-Dabie Orogen formed by Triassic collision between the Sino-Korean and Yangtze plates. An HP Slice I and two UHP slices II and III with contrasting subduction and exhumation histories within the Sulu Orogen were postulated. This study presents the metamorphic P-T paths of eclogites from the two UHP belts constructed by petrography, mineral chemistry and Perple_X P-T pseudosection modeling in the MnC(K)NFMASHO system. Eclogites from Slice III mainly consist of omphacite, garnet and quartz, with minor rutile, ilmenite, amphibole and phengite. Eclogites from Slice II show a porphyroblastic texture with epidote porphyroblasts and garnet, omphacite, phengite, quartz and rutile in matrix. Pseudosection modeling reveals that eclogites from Slice II witness a peak metamorphism of eclogite-facies under conditions of 3.1–3.3 GPa and 660–690 ºC, and a retrograde cooling decompression process. The eclogites from Slice III record a heating decompressive P-T path with a peak-P stage of 3.2 GPa and 840 ºC and a peak-T stage of 2.4 GPa and 950 ºC, suggesting an apparent granulite-facies metamorphism overprint during exhumation. Both eclogites recorded clockwise P-T paths with peak P-T conditions suggesting a subduction beneath the Sino-Korean Plate to ~100–105 km depth. Combined with tectonic scenarios from previous studies, it is concluded that the two UHP crustal slices in the Sulu terrane have a similar geodynamic evolution, but the UHP rocks in Slice II exhumed after the eclogitic peak-pressure conditions earlier than that of Slice III. The existence of Slice II diminished the buoyancy force on Slice III, resulting in a granulite-facies overprint on Slice III. The Sulu orogenic belt is made up of different crustal slices that underwent different subduction and exhumation histories, rather than a single unit.