Petrology and geochronology of HP–UHP rocks from the South Altyn Tagh, northwestern China

Abstract

Detailed field and petrological studies on the South Altyn Tagh in the last decade show that high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks are mainly exposed at Jianggalesayi and Qingshuiquan in the west, and at Yinggelisayi in the middle of the terrane. The South Altyn Tagh UHP rocks occur as lensoid blocks in granitic gneiss, paragneiss and marble and include eclogite, kyanite-garnet-bearing pelitic gneiss, magnesite-bearing garnet lherzolite, K-feldspar-bearing garnet clinopyroxenite and garnet-bearing granitic gneiss. Zircon U–Pb SHRIMP and LA-ICP-MS dating, combined with CL and trace element analysis yield geochronological data for different South Altyn Tagh HP/UHP rocks as follows: metamorphic ages of 493 ± 4 and 499 ± 27 Ma for the Jianggalesayi eclogite and its country rock respectively, 509 ± 12, 489 ± 8 and 487 ± 10 Ma for Yinggelisayi magnesite-bearing garnet lherzolite, K-feldspar-bearing garnet clinopyroxenite and garnet-bearing granitic gneiss, and 475 ± 4 and 486 ± 5 Ma for Qingshuiquan garnet clinopyroxenite and kyanite-bearing garnet-biotite gneiss. Magmatic protolith ages of eclogite, garnet lherzolite, garnet clinopyroxenite and garnet-bearing granitic gneiss are 754 ± 9, 844 ± 9, 821 ± 27 and 842 ± 18 Ma, respectively. Metamorphic ages of these rocks are consistent within errors, indicating that the South Altyn Tagh HP/UHP rocks were formed during 475–509 Ma; these are 20–80 Ma older than dated North Qaidam HP/UHP rocks (420–457 Ma). The occurrence of possible stishovite in pelitic gneiss and the older HP/UHP metamorphic ages suggest that the South Altyn Tagh belt may not necessarily be a western extension of the North Qaidam HP/UHP metamorphic belt, offset along the Altyn Tagh strike-slip fault.