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 b oundins within quartzofeldspathic gneisses or pelitic gneisses characterize d by amphibolite facies parageneses. The petrographic features and reaction textures testify to three main metamorphic stages: (1) the peak eclogite f acies 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 tra ce 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 f or Sm-Nd and U-Pb isotopic dating. The Sm-Nd isotopic data yield a whole ro ck-garnet-omphacite isochron of 500 +/- 10 Ma age. The U-Pb isotopic measur ement of zircons from the same eclogite shows that the four grain populatio ns 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 ecl ogites, and reflects the existence of a mountain root related to continenta l subduction during Early Paleozoic. Moreover, based on the comparison with similar rocks in the northern margin of the Qaidam basin, a displacement o f 350-400 km for the Altyn Tagh fault have been estimated. (C) 2001 Elsevie r Science B.V. All rights reserved.