Transformation Of Gabbro To Coesite-bearing Eclogite During Deep Subduction In Yangkou Area, North Sulu UHP Terrane

A gradational sequence of main metamorphic rocks transformation crop out at Yangkou Beach completely, which are metagabbro, transitional eclogite and coesite-bearing eclogite from the core to the rim. Metagabbro preserves primary gabbroic texture and primary plagioclase broke down to pesudomophy of Ab + Ms + Zo + Grt + Qtz and so on. Aboundent of fine-grained Grt and Omp coronas mainly develop along the boundaries between plagioclase and clinopyroxene. Transitional eclogite still preserve primary gabbroic texture, fine-grained Grt and Omp coronas are Coarser and microstructure characteristics are very similar with eclogite, the typical mineral assemblage are Grt + Omp + Phe + Qtz. Coesite-bearing eclogite contains Grt + Omp + Phe + Rt + Coe and without gabbroic texture, whereas a few fine-grained symplectite of hornblende and plagioclase develop along the boundary or the rim of garnet and omphcite. Above shows the protolith of coesite-bearing eclogite is gabbro. Gabbro transformed to transitional eclogite during the collision of Sino-krean plate and Yangtz plate, with the deeper subduction to mantle depth, the transitional eclogite transformed to coesite-bearing eclogite. After that, all these UHP rock exhumed to low crust depth.Based on the mineral genetic analysis, the composition of garnet, clinopyroxene and phengite in the metagabbro or transitional eclogite are different with the coesite-bearing eclogite obviously. (1) Garnet: The garnet in metagabbro is high in and low in MgO, and Alm = 45-64%, Spes = 1-4%, Pyr = 4-14%, Gro = 26-44%; While coesite-bearing eclogite is low in and high in MgO, and Alm = 44-49%, Pyr = 17-30%, Gro = 23-35%, Spes = 1%; The garnets in gnesis show the diffusion zoning and are characteristic with low MgO and high MnO, Alm = 40-49%, Pyr = 1-2%, Gro = 29-43%, Spes = 13-25%, from core to rim, Gro decreases and Spes increases, which indicates a exhumation with P and T both down. (2) Clinopyroxene: The relic augites develop growth zoning in metagabbro, the content of Na₂O and Al₂O₃ increase from core to rim obviously, the Jd content increases from 0.2% to 10.5% accordingly. The coronas omphacite has higher content of Na₂O and Al₂O₃, and Jd content varies 14-23%; While the omphacite in coesite-bearing eclogite has the highest content of Na₂O and Al₂O₃, and Jd varies 42-65%. (3) Phengite: The silicon content of phengile increases from metagabbro to coesite-bearing eclogite, while the phengite in the gneiss shows well diffusion zoning, from core to rim, Si and Mg content decrease while Al and Fe increase, respectively. Mg and Fe show negative linear relation. (4) Amphibole and plagioclase in retrogressive phase are Ca-Homblende and albite, respectively.The metamorphic evolution of UHP rocks in Sulu Yangkou area can be divided into 3 phases: Quartz eclogite-facies progressive metamorphism, UHP coesite-bearing eclogite-facies metamorphism and amphibolite-facies retrogressive metamorphism. The mineral assemblage of Quartz eclogite facies progressive metamorphism are Grt + Omp + Phe + Qtz + Rt, yielded T = 540℃- 630℃at P = 1.8 - 2.3 Gpa; The mineral assemblage of UHP coesite-bearing eclogite-facies metamorphism are Grt + Omp + Phe + Coe + Rt, yielded T = 660℃- 800℃at P = 3.12 - 4.09 Gpa; The mineral assemblage of amphibolite facies retrogressive metamorphism are Hb1 + P1 + Qtz, yielded T = 528℃- 666℃at P = 0.8 - 1.1 Gpa;The Zircon U-Pb age dating from metagabbro, eclogite and ultramafic rocks yielded that the protolith were formed in paleoproterozoic - Neoproterozoic. The protolith was subducted to mantle depth about 65 km to Quartz eclogite-facies progressive metamorphism at early Triassic (246 - 244 Ma), and then subducted to deeper mantle depth about 100-120 km to UHP coesite-bearing eclogite-facies metamorphism at Late Triassic (227 - 225 Ma). About 16 Ma later, these rocks were exhumed to mid-low crust depth about 20-30 km and overprinted by an amphibolite-facies retrogressive metamorphism. The exhumation rate deduced from U-Pb age and metamorphic P-T conditions is about 0.5 - 0.6 cm/y, which indicates a rapid exhumation.