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）从古特提斯动力学域（palaeo-Tethyan dynamic domains）向古太平洋动力学域（palaeo-Pacific dynamic domains）的构造转换问题至今仍存在争议。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契机。本文记录了南岭东段（eastern Nanling Range）翁岗花岗质岩体（granitic pluton）的一套岩石学、年代学及地球化学数据。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(t)值介于+2.1至+7.7之间。样品具有高SiO₂、Zr+Nb+Ce+Y、FeOt/MgO、Ga/Al及Y/Nb比值，且亏损Nb–Ta、Zr–Hf、Ba、Sr、Ti及Eu元素，地球化学特征与A₂型花岗岩（A₂-type granite）相似。其初始⁸⁷Sr/⁸⁶Sr比值介于0.70885至0.70983之间，ε_Nd(t)值介于-2.9至-1.1之间，与华南地块东南部早古生代基性岩（mafic rocks）的同位素特征相近。翁岗A₂型花岗岩起源于早古生代时期底侵至下地壳的基性岩的部分熔融。华南地块东南部的中生代A型花岗岩可划分为229–215 Ma（晚三叠世）、197–152 Ma（侏罗纪）及135–92 Ma（白垩纪）三个阶段，三者在地球化学特征与空间展布上均存在差异。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背景，而白垩纪A型花岗岩则受古太平洋动力学域控制。晚三叠世-早侏罗世及晚侏罗世-早白垩世期间几乎无A型花岗岩出露。侏罗纪A型花岗岩形成于板内伸展背景，是从古特提斯动力学域向古太平洋动力学域构造转换的响应。因此，翁岗A₂型花岗岩的产出表明该构造转换可能始于早侏罗世早期。