Early Cretaceous extension in South China: constraints from east–west-trending A-type granite belts and growth strata in terrigenous basins

The late Mesozoic tectonic domain of Southeast China transitioned from compression to extension because of the Paleo-Pacific plate rollback. However, the precise timing of the initiation of extensional tectonics remains unclear. This work focuses on the E–W-trending granite belt and the growth strata of Early Cretaceous red beds in Southeast China to establish the extensional tectonic model in Southeast China. The petrogenesis of four granitic plutons from the E–W-trending granite belt in NE Jiangxi is traced on the basis of zircon U–Pb ages and whole-rock major and trace element abundances to elucidate lithosphere evolution in the study area. U–Pb zircon dating reveals the granite emplacement at 130–135 Ma. Geochemical results demonstrate that all granite intrusions can be identified as A-type granites derived from crustal and a few mantle sources respectively through lithospheric thinning and magma upwelling under the extension background. Field observation shows the enlargement of the sedimentary area of the early Cretaceous Huizhou Formation (Fm.) in the Huangshan Basin closely after (or simultaneously) 139 Ma Shiling volcanic activities. The dip angles of the lower part of Huizhou Fm. vary from 49° to 25° towards the basin centre, and the thickness exhibited a slightly increasing trend towards the lower part within the same bed. These phenomena are consistent with the shape of the growth strata. A model, in which the NE–SW-oriented structure lines transfer to the E–W-trending tectono-magmatic belts by meeting and inheriting the previous E–W-trending tectonic axis, is established in the present work. Lithospheric thinning, magma upwelling, and crustal remelting occur within an oblique extension stress field since ca. 140 Ma. Consequently, these phenomena are influenced by the Paleo-Pacific plate rollback and the inheritance of a structural framework from the regional basement series.受古太平洋板块回卷（Paleo-Pacific plate rollback）作用影响，中国东南部晚中生代构造域由挤压环境转向伸展环境。然而，伸展构造作用启动的精确时限仍不明确。本研究聚焦中国东南部东西向花岗岩带与早白垩世红层生长地层（growth strata），旨在构建中国东南部的伸展构造模型。以赣东北地区东西向花岗岩带中的4个花岗岩体为研究对象，基于锆石U-Pb年代学（zircon U–Pb ages）、全岩主量与微量元素丰度（whole-rock major and trace element abundances）数据，追溯其岩石成因，以阐明研究区的岩石圈演化过程。锆石U-Pb定年结果显示，花岗岩侵位时代为130~135 Ma。地球化学分析表明，所有花岗岩侵入体均属于A型花岗岩（A-type granites），其源区分别为地壳与少量地幔，形成于伸展背景下的岩石圈减薄（lithospheric thinning）与岩浆上涌（magma upwelling）过程。野外实地观测表明，黄山盆地早白垩世惠州组（Huizhou Formation）的沉积范围在139 Ma石岭火山活动（Shiling volcanic activities）后（或同期）迅速扩大。惠州组下部地层的倾角向盆地中心由49°逐渐变为25°，且同一层位内下部厚度呈现轻微增大的趋势，上述现象均符合生长地层的形态特征。本研究建立了如下构造模型：北东向构造线通过交汇并继承早期东西向构造轴，最终转变为东西向构造-岩浆带。约140 Ma以来，斜向伸展应力场下发生岩石圈减薄、岩浆上涌与地壳重熔作用。综上，上述构造与岩浆作用均受古太平洋板块回卷，以及区域基底构造格架继承性的共同影响。