Ultrahigh-Pressure Metamorphism and P-T-t Paths for the Eclogites from the Central Areas of Sulu Orogen, Eastern China

Abstract

Eclogites from the Guanshan and Yangkou areas of the Sulu orogen consist of garnet, omphacite, phengite, amphibole, quartz/coesite, rutile, and ilmenite. Garnet exhibits weak compositional zoning where Xgr decreases from the core to the mantle and then increases towards the rim, coupled with an increase in Xpy from the core to the mantle and then decrease towards the rim. Phase equilibria modelling with pseudosections calculated using THERMOCALC in the NCKFMASHTO system for the Guanshan and Yangkou eclogites records two stages of metamorphism: (I) prograde associated with quick subduction (Stage-I) and (II) retrograde associated with quick exhumation (Stage-II). Stage-I is recorded in the core-mantle zoning of garnet and Si content in phengite in the Guanshan and Yangkou eclogites with a mineral assemblage of Grt-Omp-Amp-Phg-Qtz-Rt ± Lws, and the P-T conditions are constrained at 22–26 kbar and 600–615 °C in Guanshan, while 24–26 kbar and 595–600 °C in Yangkou. The peak P-T conditions (Pmax = 33 kbar; T = 685 °C) of Guanshan eclogites are revealed by the maximum Si content in phengite and the minimum Xgr in the garnet mantle with the mineral assemblage of Grt-Omp-Phg-Coe-Rt ± Lws. The value of Pmax suggests that the subduction depth of the Guanshan eclogites exceeds 110 km. Stage-II is recorded in the mantle-rim zoning of garnet, and its P-T conditions are estimated to be 12–15 kbar and 780–820 °C for the Guanshan eclogites reflected by the assemblage of Grt-Omp-Amp-Pl-LL-Qtz-Rt ± ilm, and 13–14 kbar and 770–790 °C for the Yangkou eclogites by the assemblage of Grt-Omp-Amp-Pl-LL-Qtz-Rt. The two stages of metamorphism in the study areas are overall consistent with the regional metamorphic events, from ultra-high-pressure eclogite facies, through high pressure eclogite facies, to amphibole eclogite facies, with the ages of 245, 227 and 195 Ma, respectively.