Petrogenesis and tectonic implications of Triassic A-type granites in southeastern China: insights from zircon U–Pb–Hf isotopic and whole-rock geochemical compositions of the Luoguyan and Guiyantou granites in northwestern Fujian Province

Triassic A-type granites in eastern South China Block (SCB) are abundant in the Wuyi–Yunkai tectonic domain and provide an important opportunity to explore the early Mesozoic evolution of continental crust of the SE part of the SCB. We carried out U–Pb zircon dating, Lu–Hf isotope analyses of zircon, and whole-rock geochemical analyses for two granitic plutons, the Guiyantou (GYT) and Luoguyan (LGY) granites, from northwestern Fujian Province. LA–ICP–MS U–Pb zircon analyses yielded ages of 232 ± 4 to 231 ± 7 Ma and 221 ± 5 Ma (Middle-Late Triassic) for the GYT and LGY granites. These two granites belong to metaluminous to weakly peraluminous high K calc-alkaline A-type granite that are enriched in K, Al, light rare earth element and Rb, Th, U, and Pb, and depleted in Nb, Ta, P, and Ti. Their rare earth element patterns are highly fractionated with (La/Yb)<i>_N</i> ratios of 2–21 and strong negative Eu anomalies (Eu/Eu* = 0.02–0.31). <i>In situ</i> Hf isotopic analysis of zircon from the GYT and LGY granites yielded <i>ε</i>_Hf(<i>t</i>) values ranging from –11.5 to –1.1, with corresponding two-stage Hf model ages from 1.98 to 1.33 Ga, from which it is inferred that the GYT and LGY magmas formed by partial melting of Proterozoic metasedimentary rock in the Cathaysia block. The two granites were emplaced at 232 and 221 Ma and together with Triassic A-type granites in coastal region of the SCB, which is in agreement with an extensional tectonic setting in the Middle-Late Triassic. We suggest that the Middle-Late Triassic A-type granites in eastern SCB were probably formed in an intracontinental, post-orogenic extensional regime that collision was between the SCB and an ‘unknown block’ or the eastern extension of Indochina block.

华南克拉通（South China Block, SCB）东部的三叠纪A型花岗岩（A-type granite）在武夷-云开构造域（Wuyi–Yunkai tectonic domain）中广泛分布，为探究华南克拉通东南部大陆地壳的早中生代演化提供了重要契机。本次研究对闽西北地区的桂岩头（GYT）和罗锅岩（LGY）两个花岗岩体开展了锆石U-Pb定年（U–Pb zircon dating）、锆石Lu-Hf同位素分析及全岩地球化学测试；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，桂岩头花岗岩的形成年龄为232±4 Ma至231±7 Ma，罗锅岩花岗岩为221±5 Ma，二者均形成于中晚三叠世。这两个岩体均属于准铝质至弱过铝质的高钾钙碱性A型花岗岩，富K、Al、轻稀土元素（light rare earth element）以及Rb、Th、U、Pb，相对亏损Nb、Ta、P、Ti；其稀土元素配分模式具强烈分馏特征，球粒陨石标准化La/Yb比值[(La/Yb)ₙ]为2~21，且存在显著的负铕异常（Eu/Eu*=0.02~0.31）。对两个岩体的锆石进行原位Hf同位素分析（in situ Hf isotopic analysis），结果显示其εHf(t)值介于-11.5至-1.1之间，对应的两阶段Hf模式年龄为1.98~1.33 Ga，据此推断桂岩头与罗锅岩岩浆源于华夏地块（Cathaysia block）内元古代变质沉积岩（Proterozoic metasedimentary rock）的部分熔融。该两个岩体分别形成于232 Ma和221 Ma，结合华南克拉通沿海地区三叠纪A型花岗岩的分布特征，指示研究区在中晚三叠世处于伸展构造环境；我们认为，华南克拉通东部的中晚三叠世A型花岗岩可能形成于陆内造山后伸展构造环境（intracontinental, post-orogenic extensional regime），该构造背景对应华南克拉通与某“未知地块”或印支地块（Indochina block）东延部分的碰撞事件。