Late Cretaceous Transtension in the Eastern Tibetan Plateau: Evidence From Postcollisional A-Type Granite and Syenite in the Changdu Area, China

摘要

The Late Cretaceous is an important geological time interval for the Tibetan Plateau because it corresponds to the period when the tectonic regime changed from Lhasa‐Qiangtang collision to Indo‐Asian assembly. However, the nature of and controls on the change in tectonic regime are poorly constrained. In this paper, we report results of a study of two intrusions in the Changdu area of the Eastern Tibetan Plateau. Zircon U‐Pb dating shows that both intrusions formed at ca. 77.6-74.3 Ma. The Bangda intrusion has A‐type granite affinity and a peraluminous character, whereas the Ruduo intrusion is a metaluminous syenite. Both intrusions have very similar trace element compositions, slightly enriched zircon εHf(t) values (?9.3 and ?1.7), and EM‐2‐like Sr‐Nd‐Pb isotope ratios. These features of the two intrusions indicate that their magmas were derived from partial melting of an alkali‐rich basaltic lower crust and a small proportion of mantle melt. The occurrence of alkaline intrusions is consistent with Late Cretaceous extension in the Eastern Tibetan Plateau. Based on the results of this study and previous data, we propose an intraplate extensional tectonic model, in which there was NS‐NNW‐directed Late Cretaceous transtension in the Eastern Tibetan Plateau following the Lhasa‐Qiangtang collision. This extension is interpreted to have been triggered by the Bangong‐Nujiang slab break‐off at around 110 Ma and driven by the far‐field subduction of the Neo‐Tethys oceanic crust.