Petrology and geochronology of eclogites from the western segment of the Altyn Tagh, northwestern China

The eclogites from the western segment of the Altyn Tagh occur as lens or boundins within quartzofeldspathic gneisses or pelitic gneisses characterized by amphibolite facies parageneses. The petrographic features and reaction textures testify to three main metamorphic stages: (1) the peak eclogite facies stage (P > 15 kbar, T = 665-880 degreesC), (2) the granulitic facies stage (P = 11-14 kbar, T= 670-800 degreesC) and (3) the amphibolite facies stage (P = 6.3-9.5 kbar, T= 619-738 degreesC) during decompression. The trace element, rare earth element and Sm-Nd isotopic data suggest that most of the eclogites have protolith features resembling transitional type (T-type) mid-ocean ridge basalt (MORB). The well preserved eclogite was selected for Sm-Nd and U-Pb isotopic dating. The Sm-Nd isotopic data yield a whole rock-garnet-omphacite isochron of 500 +/- 10 Ma age. The U-Pb isotopic measurement of zircons from the same eclogite shows that the four grain populations are near concordant and are well plotted on concordia, giving a weighted mean age of 503.9 +/- 5.3 hla. The two different geochronological methods yield a similar age which is interpreted as the peak metamorphic age of eclogites, and reflects the existence of a mountain root related to continental subduction during Early Paleozoic. Moreover, based on the comparison with similar rocks in the northern margin of the Qaidam basin, a displacement of 350-400 km for the Altyn Tagh fault have been estimated. (C) 2001 Elsevier Science B.V. All rights reserved.