Petrogenesis of Early Cretaceous low-Mg adakitic rocks along the southernmost margin of the North China Craton: implications for late Mesozoic crustal evolution

This article reports systematic zircon U–Pb dating, whole-rock geochemistry, and Sr–Nd isotopic data for the Early Cretaceous Jialou granitoids along the southernmost margin of the North China Craton (NCC), adjacent to the Tongbai Orogen. These results will provide significant constrains on the crustal evolution of the southern margin of the NCC. Zircon U–Pb analyses, using laser ablation–multicollector–inductively coupled plasma–mass spectrometry, indicate that the Jialou granitoids were emplaced at ~130 Ma. The granitoids have high SiO₂, K₂O, Al₂O₃, Sr, and Ba contents, high Sr/Y and (La/Yb)N ratios, and low concentrations of MgO, Y, and heavy rare earth elements, indicating a low-Mg adakitic affinity. They have relatively high initial ⁸⁷Sr/⁸⁶Sr ratios (0.707464–0.708190) and negative εNd(t) values (–11.8 to –15.2), similar to those of the Palaeoproterozoic lower crust in the NCC. These geochemical and isotopic features indicate that the Jialou low-Mg adakitic rocks were derived by partial melting of mafic Palaeoproterozoic lower crust of the NCC at >50 km depth, leaving behind a garnet amphibolite residue. The petrogenesis of the Jialou low-Mg adakitic rocks, plus the petrogenesis of Mesozoic granitoids and lower crustal xenoliths entrained in the Late Jurassic Xinyang volcaniclastic diatreme, suggests that the continental crust along the southern margin of the NCC was thickened during the Middle Jurassic to Early Cretaceous, but thinned after 130 Ma. We propose that crustal thickening was caused by a late Middle Jurassic to Early Cretaceous intra-continental orogeny, rather than continent–continent collision between the NCC and the Yangtze Craton. We also suggest that crustal thinning and Early Cretaceous magmatism were related to subduction of the palaeo-Pacific plate, rather than post-orogenic collapse of the Qinling–Tongbai–Dabie Orogen.

本文报道了华北克拉通（North China Craton, NCC）最南缘毗邻桐柏造山带的早白垩世贾楼花岗岩类的系统锆石U-Pb定年、全岩地球化学及锶-钕同位素数据。上述研究结果将为华北克拉通南缘的地壳演化提供重要约束。采用激光剥蚀-多接收器-电感耦合等离子体质谱法开展的锆石U-Pb分析显示，贾楼花岗岩类的侵位时代约为130 Ma。该花岗岩类具有高二氧化硅（SiO₂）、氧化钾（K₂O）、三氧化二铝（Al₂O₃）、锶（Sr）及钡（Ba）含量，高锶/钇（Sr/Y）与球粒陨石标准化镧/镱[(La/Yb)N]比值，且低氧化镁（MgO）、钇（Y）及重稀土元素浓度，指示其具有低镁埃达克质亲和性。它们具备相对较高的初始⁸⁷锶/⁸⁶锶比值（0.707464~0.708190）与负的εNd(t)值（-11.8~-15.2），与华北克拉通古元古代下地壳的同位素特征相似。上述地球化学与同位素特征表明，贾楼低镁埃达克质岩石起源于华北克拉通古元古代镁铁质下地壳在深度大于50 km条件下的部分熔融，残留相为石榴角闪岩。结合晚侏罗世信阳火山碎屑角砾岩筒中所包裹的中生代花岗岩类与下地壳捕虏体的岩石成因研究，贾楼低镁埃达克质岩石的成因机制指示：华北克拉通南缘大陆地壳在中侏罗世至早白垩世期间发生了加厚，但在130 Ma之后出现减薄。我们提出，地壳加厚是由中侏罗世晚期至早白垩世的陆内造山作用所引发，而非华北克拉通与扬子克拉通之间的陆陆碰撞。同时我们认为，地壳减薄与早白垩世岩浆作用与古太平洋板块的俯冲作用相关，而非秦岭-桐柏-大别造山带的造山后垮塌。