Implications Based on LA-ICP-MS Zircon U-Pb Ages of Eclogite and Its Country Rock from Jianggalesayi Area, Altyn Tagh, China

On the basis of Cathodoluminescence (CL) images and LA-ICP-MS in situ trace element analyses of zircons for finding the metamorphic age of eclogite and its country rock (garnet-biotite gneiss) from the Jianggalesayi area, Altyn Tagh, China, it is found that the zircon grains in eclogite mostly show homogeneous internal texture, and that few grains have residual cores, showing the typical magmatic origin characterized by the enrichment in HREE and high Th/U ratios generally higher than 0.4. On the other hand, the zircon rims and the homogeneous grains show that the REE pattern is flat or slightly depleted in HREE, which is the typical of metamorphic zircons coexisting with garnet. In addition, the zircon grains in garnet-biotite gneiss display a core-mantle-rim zoning texture, where the cores show the features of detrital zircons, and the mantles show the metamorphic features of zircons coexisting with garnet. By the LA-ICP-MS in situ dating of zircons, the metamorphic ages of eclogite, garnet-biotite gneiss, and the protolith of eclogite are calculated as 493 ± 4.3 Ma, 499 ± 27 Ma, and 754 ± 9 Ma, respectively. The metamorphic age of eclogite is almost the same as that of its country rock paragneiss and is about 250Ma later than its protolith age, which indicates that the ultrahigh-pressure (UHP) eclogite is the product of continental crust deep subduction.