A New Caledonian Khondalite Series in West Kunlun, China: Age Constraints and Tectonic Significance

Abstract

The Kangxiwar ductile strike-slip shear zone marks the southern boundary of the West Kunlun terrane, a large, nearly E-W trending metamorphic terrane in the western Qinghai Tibet Plateau region. This ductile shear zone is ∼7 km wide, and consists of mylonitized khondalites. Protoliths of the khondalites were alumina-rich pelitic sedimentary and subordinate volcanic rocks. The pelitic khondalites have pronounced positive Th anomalies and subdued positive Ce and Zr anomalies, whereas the metavolcanic rocks have positive Nb and Zr anomalies. Both types of khondalite are LREE enriched, and show weak HREE depletions and moderate negative Eu anomalies. P-T conditions for the formation of the khondalites are estimated to be 6.8 kbar and 700°C. The khondalites formed in the Caledonian orogeny (428-445 Ma) and underwent strong shear deformation during the Indosinian (250-210 Ma). SHRIMP dating of detrital zircons in the khondalites suggests that they were derived from an older metamorphic basement, probably older than 644 Ma. The Kangxiwar khondalites are similar in their protoliths, trace- element and rare-earth element geochemistry, P-T conditions, and age of formation to those of the South Altyn Tagh khondalite series. This lateral correlation suggests that the West Kunlun and Altyn Tagh terranes were once contiguous, and provides evidence for the existence of a Caledonian orogenic belt in this region.