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

In addition to the Triassic Hong'an low-T-high-P eclogite and the Xinxian coesite-bearing kyanite-glaueophane 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 degreesC and 14-19 kbar, and retrograde metamorphism of 530-685 degreesC 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 deltaD values for eclogites at Xiongdian and at Hujiawan range from 8.8 parts per thousand to 11.2 parts per thousand, whereas delta(18)O value of the Yangchong eclogite is as low as 4.3 parts per thousand. Whole-rock D values range from 55 parts per thousand to - 75 parts per thousand 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 Sujiahc 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. (C) 2002 Elsevier Science B.V. All rights reserved.