The timing of tectonic transition from compression to extension in Dabieshan: Evidence from Mesozoic granites

Researches over the last 20 years show that the orogenic belt remains rather active after plate colli-sion1,2]. A complete orogenic cycle in the last period of the Wilson cycle can be defined by three stages of development3]: (1) horizontal contraction and crustal thickening due to collision, as well as formation of topography and the crustal and lithospheric root; (2) eclogite facies metamorphism of the crustal root; and (3) delamination of the crustal root or lithospheric mantle, extension…

The timing of tectonic transition from compression to extension in Dabieshan:Evidence from Mesozoic granites

With the methods of single grain zircon U-Pb and hornblende-biotite Ar-Ar dating, a U-Pb zircon age (135.4(2.7) Ma of Liujiawa high Sr/Y granite and 40Ar-39Ar plateau ages (139.0(1.0) Ma and (125.3(0.2) Ma of hornblende and biotite respectively of Fenliupu high Sr/Y granite in the Dabieshan high- and ultrahigh-pressure metamorphic belt are yielded. Following the emplacement of the high Sr/Y granites, extension-related mafic intrusions and late granitoids with negative Sr and Eu anomalies and isotopic ages of 105-130 Ma were emplaced. Geochemical comparison between the two types of granitoids shows that, from early to late, the alkalinities, the K2O contents and the Sr/Y ratios as well as FeO/(FeO MgO) ratios of the rocks increase, while the abundance of Sr reduces, suggesting that as time goes on the depth of crustal melting decreases. Rheological stratification of the crust due to early Cretaceous partial melting of crustal rocks resulted in crustal extension and deep-buried rock exhumation. Therefore, the high Sr/Y granite that appeared at ~135 Ma is the product of the tectonic transition from early shortening to late overall extension.