Petrological, isotopic and fluid inclusion studies of eclogites from Sujiahe, NW Dabie Shan (China)

In addition to the Triassic Hongʹan low-T–high-P eclogite and the Xinxian coesite-bearing kyanite-glaucophane eclogite, Silurian coesite-free amphibole eclogites occur in the Sujiahe region, NW Dabie Shan of central China. A comprehensive study of petrology, Nd–Sr, O–H isotopes and fluid inclusions has been carried out for the Sujiahe eclogites. Geothermobarometry of mineral assemblages gave peak P–T conditions of 600–730 °C and 14–19 kbar, and retrograde metamorphism of 530–685 °C and 6 kbar. The eclogites have high Nd(t) values of −1.9 to 5.8, indicating that their protoliths were derived from oceanic basalts. Whole-rock δ18O values for eclogites at Xiongdian and at Hujiawan range from 8.8‰ to 11.2‰, whereas δ18O value of the Yangchong eclogite is as low as 4.3‰. Whole-rock δD values range from −55‰ to −75‰. The results exclude that the eclogite protoliths were isotopically exchanged with ancient meteoric water prior to plate subduction, thus distinguishing them from the other Dabie–Sulu eclogites. Fluid inclusions show the presence of early highly saline brine and late low-salinity aqueous fluids in the Sujiahe eclogites during the high-P metamorphism. The isotopic data suggest that the Sujiahe eclogite protolith was metamorphosed during subduction of an oceanic plate beneath the Sino-Korean Craton in the Middle Paleozoic, different from the other eclogites in the Dabie–Sulu orogen which were formed by the ultrahigh pressure metamorphism during the Triassic.