桂东北燕山早期岩浆作用与伸展动力学

Early Yanshanian Magmatism and Extensional Geodynamics in Northeastern Guangxi, South China

摘要: 桂东北位于南岭构造带东段、扬子与华夏地块拼合边界，是华南早中生代构造体制由特提斯向古太平洋俯冲体系转换的关键构造单元。本文系统整合锆石U-Pb年代学、Hf-Sr-Nd同位素及区域构造–沉积资料，厘定桂东北燕山早期(早侏罗世–中侏罗世，175~150 Ma)岩浆序列、岩石成因与伸展构造响应。结果表明，区内燕山早期岩浆岩以A型花岗岩、双峰式侵入岩和基性脉岩组合为特征，锆石U-Pb年龄集中于175~150 Ma，锆石εHf(t)随成岩时代逐渐升高，指示幔源物质持续加入。区域广泛发育拆离断层、走滑–伸展断裂、张性节理及断陷红盆，构成典型的陆内伸展构造系统。动力学分析表明，印支期陆陆碰撞增厚地壳后发生的岩石圈拆沉与软流圈上涌，叠加早侏罗世古太平洋板块初始俯冲产生的远程拉张应力，共同驱动了桂东北大规模壳幔相互作用与伸展型岩浆活动。本研究为理解华南早中生代构造体制转换及陆内伸展动力学提供了关键约束。

Abstract: Northeastern Guangxi is located in the eastern segment of the Nanling tectonic belt, at the suture boundary between the Yangtze and Cathaysia blocks, and serves as a key tectonic unit recording the Early Mesozoic transition of South China from the Tethyan to the Paleo-Pacific subduction regime. This study systematically integrates zircon U-Pb geochronology, Hf-Sr-Nd isotopic data, and regional structural-sedimentary records to constrain the magmatic sequence, petrogenesis, and extensional tectonic response of Early Yanshanian (Early to Middle Jurassic, 175~150 Ma) magmatism in northeastern Guangxi. The results show that the Early Yanshanian igneous rocks in the area are characterized by A-type granites, bimodal intrusive rocks, and mafic dike swarms. Zircon U-Pb ages cluster at 175~150 Ma, and zircon εHf(t) values increase progressively with decreasing emplacement age, indicating continuous input of mantle-derived materials. Regionally, detachment faults, strike-slip extensional faults, extensional joints, and fault-controlled red basins are widely developed, constituting a typical intracontinental extensional tectonic system. Dynamic analysis suggests that lithospheric delamination and asthenospheric upwelling following the Indosinian continent-continent collision and crustal thickening, superimposed by far-field extensional stress from the initial Early Jurassic subduction of the Paleo-Pacific Plate, jointly drove large-scale crust-mantle interaction and extensional magmatism in northeastern Guangxi. This study provides key constraints for understanding the Early Mesozoic tectonic regime transition and intracontinental extensional dynamics in South China.