The earliest Jurassic A-type granite in the Nanling Range of southeastern South China: petrogenesis and geological implications

The tectonic transition from the palaeo-Tethyan to palaeo-Pacific dynamic domains in the South China Block (SCB) is still a matter of debate. The A-type granites collected from the southeastern SCB offered an opportunity to illustrate this tectonic transition. This article records a set of petrographic, geochronological, and geochemical data for the Wengong granitic pluton from the eastern Nanling Range. LA-ICP-MS zircon U–Pb dating shows a crystallization age of 196.9 ± 4.4 Ma with <i>ε</i>_Hf(<i>t</i>) values ranging from +2.1 to +7.7. The samples have high SiO₂, Zr+Nb+Ce+Y, FeOt/MgO, Ga/Al, and Y/Nb and are depleted in Nb–Ta, Zr–Hf, Ba, Sr, Ti, and Eu, similar to those of the A₂-type granite. Their initial ⁸⁷Sr/⁸⁶Sr ratios range from 0.70885 to 0.70983 and the <i>ε</i>_Nd(<i>t</i>) values range from −2.9 to −1.1, close to those of the Early Palaeozoic mafic rocks in the southeastern SCB. The Wengong A₂-type granite was derived from partial melting of the mafic rocks underplated into the lower crust during the Early Palaeozoic. The Mesozoic A-type granites in the southeastern SCB can be subdivided into 229–215 Ma (Late Triassic), 197–152 Ma (Jurassic), and 135–92 Ma (Cretaceous). They differ in geochemical and spatial distribution characteristics. The Late Triassic A-type granites were formed in the post-collision extensional setting associated with the palaeo-Tethyan dynamic domain, whereas the Cretaceous A-type granites were under the control of the palaeo-Pacific dynamic domain. The A-type granites were hardly exposed during the Late Triassic–Early Jurassic and Late Jurassic–Early Cretaceous. The Jurassic A-type granites were formed in the intra-plate extensional setting, a response to the tectonic transition from the palaeo-Tethyan to palaeo-Pacific dynamic domains. Thus, the occurrence of the Wengong A₂-type granite indicates that this tectonic transition possibly initiated at the earliest Early Jurassic.

华南地块（South China Block, SCB）内从古特提斯（palaeo-Tethyan）动力学域向古太平洋（palaeo-Pacific）动力学域的构造转换问题至今仍存在较大争议。采自华南地块东南部的A型花岗岩（A-type granites）为阐明该构造转换过程提供了宝贵契机。本文报道了南岭东段翁岗花岗岩体的一套岩相学、年代学与地球化学数据。激光剥蚀电感耦合等离子体质谱（LA-ICP-MS）锆石U-Pb定年结果显示其结晶年龄为196.9±4.4 Ma，ε_Hf(<i>t</i>)值介于+2.1至+7.7之间。该岩体样品具有高SiO₂、Zr+Nb+Ce+Y、FeOt/MgO、Ga/Al及Y/Nb比值，且显著亏损Nb-Ta、Zr-Hf、Ba、Sr、Ti及Eu元素，其地球化学特征与A₂型花岗岩一致。样品的初始⁸⁷Sr/⁸⁶Sr比值介于0.70885至0.70983之间，ε_Nd(<i>t</i>)值介于-2.9至-1.1之间，与华南地块东南部早古生代镁铁质岩石的同位素组成相近。研究表明，翁岗A₂型花岗岩起源于早古生代时期底侵至下地壳的镁铁质岩石的部分熔融作用。华南地块东南部的中生代A型花岗岩可划分为三个时代阶段：229–215 Ma（晚三叠世）、197–152 Ma（侏罗纪）以及135–92 Ma（白垩纪），三者在地球化学特征与空间分布上均存在显著差异。晚三叠世A型花岗岩形成于与古特提斯动力学域相关的碰撞后伸展环境，而白垩纪A型花岗岩则受控于古太平洋动力学域。在晚三叠世-早侏罗世及晚侏罗世-早白垩世两个时期，A型花岗岩几乎未见出露。侏罗纪A型花岗岩形成于板内伸展环境，是构造体系从古特提斯动力学域向古太平洋动力学域转换的响应。因此，翁岗A₂型花岗岩的产出指示该构造转换可能始于早侏罗世初期。