Paleozoic crustal evolution and tectonic switching in the Northeastern Tianshan: insights from zircon Hf isotopes of granitoids

Abstract

Decoding the crustal and tectonic evolution of ancient accretionary orogens is not always straightforward. Here, four episodes of Paleozoic granitoids have been identified with distinct zircon–Hf isotopic characteristics from the Northeastern Tianshan. The first stage granitoids in the Dananhu–Harlik arc system are characterized by highly positive zircon εHf(t) values and short crustal incubation times with a rising event signature, suggesting a northward trench advance for the Kangguer Ocean. During the second stage, granitoids in the Dananhu and Kangguer belts have high zircon εHf(t) values and short crustal incubation times, but with a decreasing event signature for the Dananhu granitoids, implying a reworking of the juvenile arc crust. However, the near-zero εHf(t) values and the longest crustal incubation times of the Yamansu granitoids in this stage elucidate an origin from a Precambrian basement. These variations suggest that the northern trench of the Kangguer Ocean retreated southward while the southern trench advanced southward. During the third stage, the enlarged ranges of zircon εHf(t) values and crustal residence ages as well as crustal incubation times for the Dananhu and Kangguer granitoids show an interaction of juvenile material and the pre-existing crust, whereas the highly positive zircon εHf(t) values with a sharp rising event signature of the Yamansu granitoids suggest an significant crustal growth, indicating that a northward trench advance and a southern trench retreat for the Kangguer Ocean. However, the last stage granitoids in the Northeastern Tianshan entirely exhibit decreasing zircon εHf(t) values and long crustal incubation times, demonstrating a reworking of the pre-existing juvenile crust with minor input of ancient crustal materials in a post-collisional setting.Supplementary material: Table S1 and S2 and analytical methods are available at https://doi.org/10.6084/m9.figshare.c.5197889