粤西高州泥质麻粒岩变质P-T-t演化及其对华南早古生代构造背景的指示

Abstract

<p>华南陆块早古生代构造热事件的形成与演化一直是学界关注的热点议题. 本文对地处华夏地块西南部的高 州泥质麻粒岩开展了详细的岩相学、矿物化学、相平衡模拟和锆石年代学研究, 综合华夏地块已有的早古生代变 质作用资料, 探讨了华南早古生代时期的构造背景. 高州泥质麻粒岩包含三阶段矿物组合: 包裹体组合由石榴子石 核部及所包裹的钾长石、黑云母、矽线石、石英、金红石和钛铁矿组成, 相平衡模拟获得温压条件为~0.95 GPa/ 800°C(压力峰期阶段); 基质矿物组合为石榴子石变斑晶和基质中的堇青石、钾长石、黑云母、石英、矽线石和 钛铁矿, 结合矿物组合在相图中的位置、基质黑云母的Ti含量和基质钾长石的A b值, 限定温压条件为 0.57~0.69 GPa/815~865°C(温度峰期阶段); 退变质组合以石榴子石周围或后成合晶中的堇青石、钾长石、黑云 母、石英和钛铁矿以及晚期斜长石和矽线石为代表, 指示降温降压的退变质过程. 高州泥质麻粒岩的锆石U-Pb定 年获得441±1.4 Ma的变质年龄. 变质锆石的Ti温度计计算结果为693~759°C, 与岩石的退变质冷却温度一致. 华南 早古生代变质作用整体上为中压相系条件, 变质P-T轨迹均为顺时针样式, 暂无高压相系蓝片岩或榴辉岩的发现. 上述变质作用特征在大陆碰撞造山带和陆内造山带都可形成, 但碰撞造山带的另一个特点是常伴生高压-超高压 蓝片岩和榴辉岩. 从目前掌握的变质作用数据和其他地质资料分析, 华南陆块内部发育早古生代变质-岩浆-变形 作用的构造域很可能并不是发生俯冲-碰撞的“第一现场”, 引起该期构造热事件的应力来源于彼时华南的陆缘<br></p>

The formation and evolution of the early Paleozoic tectono-thermal event in South China remains a hotly debated topic within the academic community. Studies of metamorphism can provide pivotal insight into this issue. In this contribution, detailed studies of petrography, mineral chemistry, phase equilibria modelling and zircon U-Pb geochronology have been carried out on pelitic granulite from Gaozhou, southwestern part of the Cathaysia Block, South China. This paper will compare the early Paleozoic metamorphic data in the Cathaysia block to the new data provided here in order to discuss tectonic setting in South China. The pelitic granulite contains three generations of mineral assemblage, namely the inclusion, matrix and retrograde mineral assemblages. The inclusion mineral assemblage consists of garnet core and the wrapped K-feldspar, biotite, sillimanite, quartz, rutile and ilmenite within garnet. P-T conditions of the inclusion mineral assemblage are ~0.95 GPa and 800°C (Pmax stage), obtained by phase