南岭-云开交接带三叠纪构造-热改造作用的激光拉曼光谱定量化研究

The South China Plate has experienced a long-lived and polyphased tectono-metamorphic history since the amalgamation of the Yangtze and Cathaysia blocks during the Neoproterozoic. The Cathaysia block was affected by widespread magmatism from Silurian to Cretaceous times, suggesting repetitive abnormally hot thermal conditions within the lower crust and/or the mantle and implying heat transfer by the advection of molten rocks up to the upper crust. In contrast to the Jurassic and Cretaceous plutonism with narrow contact aureoles, the Silurian and Triassic magmatic events are known as being coeval with local to regional high-temperature (HT) metamorphism. The Triassic tectono-metamorphic overprints upon the pre-Mesozoic structures varies at the scale of the Cathaysia block, partly preserving Silurian structures in the Nanling belt but strongly highlighted in the Yunkai massif. This Triassic thermo-tectonic event is characterized by strong ductile deformation and anatexis within the Yunkai massif, but the northward extent of the Triassic metamorphism in the Nanling belt remains poorly constrained, and the transition between the hot ductile Yunkai massif and the weakly metamorphic and slightly deformed Nanling belt is unclear. These uncertainties raise fundamental questions concerning the thermal evolution of the southwestern domain of the Cathaysia block, including i) the intensity and distribution of the Triassic metamorphism in the basement of the Cathaysia block, i.e., what were the geothermal gradients during the Triassic? and ii) how does paleo-thermal condition evolve laterally, i.e., does the transitional zone between the Yunkai massif and the Nanling belt reflect a continuous attenuation of the Triassic thermal anomaly or is it a tectonic contact induced by more significant uplift of the Yunkai massif? To answer these scientific questions, our project aims to investigate the thermal evolution of SW Cathaysia using Raman spectroscopy on carbonaceous materials (RSCM) along several north-south cross-sections, located at the transition zone between the high-grade Yunkai massif and low-grade Nanling belt. To decipher the Mesozoic regional metamorphism from the Silurian one, our cross-sections include Late Paleozoic Beiliu, Luoding and Yangchun basins covering both the Yunkai and Nanling belts. The Upper Paleozoic series are known as lying unconformably over the basement. These basins are mostly supposed as non-metamorphic and were not studied for their thermal histories. The estimate of thermal peak metamorphism recorded by collected samples will be systematically coupled to structural and kinematic analyses, allowing us to quantify the offset of the main tectonic contacts and highlight the possible waviness of isotherms surfaces along cross-sections. Furthermore, the drawing of high-resolution paleo-isotherms along our cross-sections will be used to estimate local apparent paleo-geothermal gradient during the Triassic deformation. A 3D model highlighting the geological architecture of the Yunkai-Nanling transitional zone will be built based on these new structural and metamorphic constraints.

华夏地块自新元古代以来经历了复杂的构造演化，发育志留-白垩纪岩浆作用，深部岩浆的反复侵位使其长期处于高热流状态。相比侏罗-白垩纪侵入岩体，志留和三叠纪岩浆侵入伴随高温变质和强韧性变形。在华夏内部，三叠纪变质热事件对前中生代构造的叠加改造差异明显：云开地区极其强烈，南岭地区则保留了志留纪构造。云开地区三叠纪强韧性变形和高温变质如何向南岭延伸过渡或突变为弱变形-低级变质作用，目前尚不清楚，但涉及华夏地块热演化的基本问题：华夏基底中三叠纪变质程度的空间分布及古地温梯度如何？云开与南岭地区古地温差异是连续递减还是差异性隆升的结果？为了回答这些问题，本项目拟运用激光拉曼光谱(RSCM)分析方法，对云开-南岭交接带北流、罗定和阳春地区5个代表性上古生界沉积剖面开展古地温测量，获取变质峰期温度，结合构造运动学分析，绘制高精度古地温等值线图，厘定研究区三叠纪构造热演化历史，构建区域地壳热结构三维模型。

针对华夏地块三叠纪变质-热事件对前中生代构造差异性叠加改造的机理、中生代面状分布的陆内构造和岩浆作用的成因等问题，本项目运用野外和室内构造分析、碳质物激光拉曼光谱(RSCM)分析、变质岩温-压计算和多矿物年代学等方法，选择云开-南岭交接带、南岭-武夷山交接带的代表性变质沉积岩剖面，开展了系统的显微构造和古地温测量综合研究，获得了研究区三叠纪的变质峰期温度及其平面分布规律，绘制了高精度古地温等值线图，发现了华夏地块中生代自南向北古地温递减、内部各地块古地温梯度差异显著、块体边界古地温梯度显著升高等重要规律，这些研究结果为进一步构建区域地壳热结构三维模型奠定了重要基础，为揭示华夏地块中生代地壳流变学性质和陆内变形-岩浆作用动力学过程提供了重要依据。

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