The ages of U-Pb and Sm-Nd for eclogite from the western segment of Altyn Tagh tectonic belt - Evidence for existence of Caledonian orogenic root

The eclogites of Altyn Tagh tectonic belt occur as lens within gneisses cha racterized by amphibolite-facies mineral parageneses. The well-preserved ec logite is selected for Sm-Nd and U-Pb isotopic dating. The Sm-Nd isotopic d ata yield a whole rock-garnet-omphacite isochron of (500 +/- 10) Ma. The U- Pb isotopic measurements of zircons show that the four grain populations ar e nearly concordant and well plotted on concordia curves, giving a weighted mean age (503.9 +/- 5.3) Ma. Two kinds of methods obtain a similar age whi ch reveals the peak metamorphic age of eclogites, and reflect the existence of orogenic root related to Caledonian subduction and continent-continent collision.