| 桂东南岑溪燕山期A型花岗岩年代学、地球化学特征及其构造意义 |
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| 引用本文：宋兴旺,谢志鹏,王建荣,张源,窦磊,王维.2026.桂东南岑溪燕山期A型花岗岩年代学、地球化学特征及其构造意义[J].地球学报,47(4):750-769. |
| DOI：10.3975/cagsb.2025.091711 |
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| 基金项目:本文由广东省区域地质调查成果更新与综合编图项目(编号: 2024-02)和云南省新一轮找矿暨地勘基金项目(编号: Y202401; Y202504)联合资助。 |
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| 中文摘要:桂东南地区作为华夏地块与扬子地块的关键构造结合带, 其燕山期A型花岗岩的成因机制对揭示华南板块中生代构造与岩浆演化过程具有重要的地质意义。本文以岑溪花岗岩体为研究对象, 通过系统的野外地质调查、锆石U-Pb年代学、全岩地球化学及锆石Hf同位素分析, 综合探讨了该岩体的岩石成因及其构造背景。该岩体主要由黑云母二长花岗岩及少量石英二长岩组成, 且普遍存在暗色二长岩包体。地球化学特征表现为高硅(SiO2=73.8%~77.0%)、富碱(Na2O+K2O=7.71%~8.82%), 以及准铝质(A/CNK=0.95~0.96)和高钾钙碱性-钾玄质的岩石系列, 特别是特征参数10 000×Ga/Al、Zr+Nb+Ce+Y等和锆石饱和温度(TZr=826~846 ℃), 均指示该岩体为典型的A型花岗岩。LA-ICP-MS锆石U-Pb定年结果表明, 黑云母二长花岗岩的精确成岩年龄为 (156.4±1.1) Ma(MSWD=1.9; n=18), 指示其形成于晚侏罗世早燕山期。岩石的地球化学分析揭示显著的轻稀土元素(LREE)富集((La/Yb)N=6.88~8.01)和负铕异常(δEu=0.13~0.15), 原始地幔标准化图解呈现大离子亲石元素(Rb、Th、U、K、Pb)富集和高场强元素(Nb、Ta、P、Ti)亏损的特征。锆石原位Hf同位素组成(εHf(t)= ?2.84 ~ +2.84, 均值+0.035)及相应的二阶段模式年龄(T2DM=1.39~1.03 Ga, 均值1.20 Ga)表明, 岩浆源区以中元古代陆壳物质的部分熔融为主, 伴有幔源组分参与。岩相学和地球化学证据表明, 黑云母二长花岗岩、石英二长岩及二长岩包体为同源岩浆经结晶分异作用形成的不同演化阶段产物。通过综合区域构造背景分析, 岑溪A型花岗岩的形成与古太平洋板块俯冲引发的伸展构造环境密切相关。俯冲板片的后撤导致软流圈上涌, 提供了地壳熔融的热源, 并促使幔源物质与壳源熔体的混合, 最终形成了高温、低水活度的母岩浆, 并经历显著的分异演化过程。该研究为华南晚中生代岩浆岩的动力学机制提供了新的约束。 |
| 中文关键词:花岗岩 燕山期 锆石U-Pb年龄 地球化学 桂东南 |
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| Geochronology, Geochemical Characteristics and Tectonic Implications of the Yanshanian A-type Granites in Cenxi, Southeastern Guangxi |
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| Abstract:The southeastern Guangxi region, a crucial tectonic junction between the Cathaysia and Yangtze Blocks, is essential for understanding the Mesozoic tectonic and magmatic evolution of the South China Block. The petrogenesis of the Yanshanian A-type granites in this region is key to revealing these processes. This study focused on the Cenxi granite body and applied systematic field geological investigations, zircon U-Pb dating, whole-rock geochemistry, and zircon Hf isotopic analyses to investigate its petrogenesis and tectonic context. The granite mainly comprises biotite monzogranite with minor quartz monzodiorite and widespread dark-colored monzodiorite enclaves. The geochemical characteristics include high silica content (SiO2 = 73.8%–77.0%), alkali enrichment (Na2O + K2O = 7.71%–8.82%), a metaluminous composition (A/CNK = 0.95–0.96), and a high-K calc-alkaline to shoshonitic series. Key parameters, including 10 000×Ga/Al, Zr+Nb+Ce+Y ratios, and zircon saturation temperatures (TZr = 826–846 ℃), indicate that the granite is a typical A-type granite. LA-ICP-MS zircon U-Pb dating yields a crystallization age of (156.4±1.1) Ma (MSWD = 1.9; n = 18), indicating formation during the Late Jurassic (Early Yanshanian). Geochemical analyses reveal light rare earth element (LREEs) enrichment [(La/Yb)N = 6.88–8.01] and negative Eu anomalies (δEu = 0.13–0.15). Primitive mantle-normalized diagrams show enrichment in large-ion lithophile elements (Rb, Th, U, K, and Pb) and depletion in high-field-strength elements (Nb, Ta, P, and Ti). Zircon in-situ Hf isotopic compositions (εHf(t) = ?2.84 to +2.84, average +0.035) and corresponding two-stage model ages (T2DM = 1.39–1.03 Ga; average 1.20 Ga) suggest that the magma source mainly involved partial melting of Mesoproterozoic continental crust, with contributions from mantle components. Petrographic and geochemical evidence indicates that the biotite monzogranite, quartz monzodiorite, and monzodiorite enclaves represent different evolutionary stages of a co-magmatic system formed through crystallization differentiation. Regional tectonic analysis indicates that the formation of the Cenxi A-type granite is closely linked to the extensional tectonic environment triggered by subduction of the Paleo-Pacific Plate. Slab rollback induced asthenospheric upwelling, providing heat for crustal melting and facilitating the mixing of mantle-derived materials with crustal melts. This process ultimately generated high-temperature, low-water-activity parental magmas that underwent significant fractional crystallization. This study provides new constraints on the dynamic mechanisms of Late Mesozoic magmatism in the South China Block. |
| keywords:granite Yanshanian Period zircon U-Pb age geochemistry southeastern Guangxi |
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