Deep subduction of continental crust in accretionary orogen: Evidence from U–Pb dating on diamond-bearing zircons from the Qinling orogen, central China

Abstract

Accretionary orogenic belts are considered the most important sites for the generation of continental crust. However, it is still not very clear whether in such settings the continental crust has been significantly transported back to the mantle accompanied with its lateral growth. Continental UHP rocks, the best indicators of the recycling of the continental crust, have rarely been discovered in accretionary orogenic belts, which hinder the exploration of this essential process. The possible occurrence of early Paleozoic UHP metamorphism in the Qinling orogen provides an excellent opportunity to address this question. This article reports an integrated study of U–Pb age, trace element and mineral inclusion of zircon from an amphibolite sample in the Qinling orogen. The zircon crystals show typical metamorphic growth zoning, low Th/U ratios, flat HREE patterns, and insignificant Eu anomalies. They give a weighted mean U–Pb age of 490.4±5.8Ma. Most importantly, an in situ diamond inclusion was identified from one of the zircon crystals. Therefore, the obtained 490.4±5.8Ma age was taken as registering the peak UHP metamorphism. The discovery of in situ diamond inclusion provides important evidence for the UHP metamorphism of the Qinling orogen and indicates that the North Qinling microcontinent was subducted to mantle depths of >120km when it collided with the Erlangping arc attached to the southern margin of the North China Block. Combining with previous results, we suggested that the North Qinling UHP terrane underwent fast exhumation from >120km to ca. 30km within 10Myr intervals. As a few examples of UHP metamorphism have been discovered in some accretionary orogens, more attention needs to be paid to the occurrence of continental UHP metamorphism during accretionary orogenic process. Furthermore, evident crustal growth has been documented in the North Qinling orogen during the early Paleozoic. It is inferred that the accretionary orogenic belts may play important roles in recycling of continental crust into the mantle besides generation of continental crust.