The earliest Cambrian UHT metamorphism in the Qaidam block,western China: A record of the final assembly of Greater Gondwana?

The final assembly of Gondwana, known as the late Pan-African orogeny, is characterized by Ediacaran–early Cambrian ultrahigh temperature (UHT) metamorphism, which is widely identified within reconstructed East Gondwana. This distinctive feature likely provides a reliable criterion for identifying new Gondwanan terranes that lack paleo-geomagnetic data. Here we present zircon U–Pb geochronology and phase equilibria calculations for a variety of granulite types newly recognized from western Qaidam, China, which provide the first evidence that the Qaidam block, at least western Qaidam, experienced high-grade metamorphism in excess of 900 °C before/at 540–520 Ma. These UHT metamorphic rocks, similar to many well-known Pan-African UHT metamorphic terranes, is inferred to evolve along a clockwise P–T path that is usually related to collisional orogens. Comparison between new metamorphic zircon U–Pb ages from western Qaidam and the published age data from the UHT metamorphic terranes within East Gondwana suggests that the UHT metamorphic rocks found in western Qaidam similarly records the final assembly of Gondwana. Although the exact paleo-geographical location of the Qaidam block during the Gondwana period is unknown yet because of lacking paleo-geomagnetic data, new Pan-African UHT metamorphic record found in western Qaidam indicates, for the first time, that the Qaidam block is a Gondwanan terrane that split from this semi-supercontinent after the Pan-African orogeny.