Thermotectonic history of SE China since the Late Mesozoic: insights from detailed thermochronological studies of Hong Kong

The late Mesozoic Yanshanian volcanic arc affected an extensive region of SE China, but the conclusion of magmatism and later evolution are not fully understood. Widespread Yanshanian ignimbrites and their contemporaneous granites exposed in Hong Kong represent a microcosm of this magmatic arc. To constrain the post-magmatic thermal history of the region, we present zircon and apatite fission-track analyses from these rocks. Double dating using laser ablation inductively coupled plasma mass spectrometry U–Pb and fission-track techniques on detrital zircons from post-volcanic Cretaceous sediments is used to further constrain the tectonothermal evolution. The resulting dataset and thermal modelling suggest that the igneous rocks and Cretaceous sediments together experienced post-emplacement or post-depositional heating to &gt;250 °C, subsequently cooling through 120–60 °C after <em>c</em>. 80 Ma. The heating reflects the combined effects of an enhanced geothermal gradient and burial. We interpret the enhanced gradient to represent continuing Yanshanian magmatic activity until <em>c</em>. 100–80 Ma, much later than previously considered. Our data also indicate a long-term, slow cooling (<em>c</em>. 1 °C myr^–1) since the early Cenozoic, linked to <em>c</em>. 2–3 km of erosion-driven exhumation. The thermotectonic history of Hong Kong reflects the mid-Cretaceous transition of SE China from an active to a passive margin bordered by marginal basins that formed in the early Cenozoic.

晚中生代燕山期（Yanshanian）火山弧曾影响中国东南部广袤区域，但有关其岩浆作用及后续演化的结论尚未完全阐明。香港广泛出露的燕山期熔结凝灰岩及其同期花岗岩，堪称该岩浆弧的微观缩影。为约束该区域的岩浆后热历史，我们对这些岩石开展了锆石（zircon）与磷灰石（apatite）裂变径迹分析。针对火山作用后形成的白垩纪沉积物中的碎屑锆石，我们采用激光剥蚀电感耦合等离子体质谱法（laser ablation inductively coupled plasma mass spectrometry）铀-铅（U-Pb）与裂变径迹双定年技术，以进一步约束其构造热演化过程。本数据集与热模拟结果显示，火成岩与白垩纪沉积物共同经历了侵位后或沉积后的加热过程，温度超过250℃，随后在约80 Ma之后冷却至120~60℃。该加热事件反映了地温梯度升高与埋藏作用的共同影响。我们认为，地温梯度升高代表燕山期岩浆活动持续至约100~80 Ma，这一时间远晚于此前的认知。本研究数据还表明，自早新生代以来，区域经历了长期缓慢的冷却过程，速率约为1℃/百万年，这与约2~3 km的侵蚀驱动剥露作用密切相关。香港的热构造历史反映了中国东南部在白垩纪中期从活动大陆边缘向被动大陆边缘的转变，而该被动大陆边缘边界伴随早新生代形成的边缘盆地。