| 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 |
| 投稿时间：2025-06-26 修订日期：2025-08-03 点此下载全文 |
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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10000×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，提供了地壳熔融提供的热源，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 Block and the Yangtze Block, holds significant geological importance 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 focuses on the Cenxi granite body, adopting systematic field geological investigations, zircon U–Pb dating, whole-rock geochemistry, and zircon Hf isotopic analysis to comprehensively investigate its petrogenesis and tectonic context. The granite mainly consists of biotite monzogranite with minor quartz monzodiorite, with widespread occurrences of dark-colored monzodiorite enclaves. Geochemical characteristics include high silica content (SiO2 = 73.8%–77.0%), alkali enrichment (Na2O + K2O = 7.71%–8.82%), and a weakly peraluminous (A/CNK = 0.95–0.96) composition, as well as a high-K calc-alkaline to shoshonitic series. Key parameters, including 10000×Ga/Al, Zr+Nb+Ce+Y ratios, and zircon saturation temperatures (TZr = 826–846 ℃), collectively indicate that the granite is a typical A-type granite. LA–ICP–MS zircon U–Pb dating yields a precise crystallization age of 156.3 ± 1.1 Ma (MSWD = 1.9; n = 18), indicating formation during the Late Jurassic (Early Yanshanian). Geochemical analyses reveal significant light rare earth element (LREE) enrichment [(La/Yb)N = 6.88–8.01] and negative Eu anomalies (δEu = 0.13–0.15). Primitive mantle-normalized diagrams exhibit enrichment in large ion lithophile elements (Rb, Th, U, K, Pb) and depletion in high-field strength elements (Nb, Ta, P, 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 primarily involved partial melting of Mesoproterozoic continental crust, with contributions from mantle components. Petrographic and geochemical evidence indicate that the biotite monzogranite, quartz monzodiorite, and monzodiorite enclaves are products of different evolutionary stages of a co-magmatic system, resulting from crystallization differentiation. Regional tectonic analysis reveals that the formation of the Cenxi A-type granite is closely related to the extensional tectonic environment triggered by the subduction of the Paleo-Pacific plate. The slab rollback induced asthenospheric upwelling, providing thermal sources for crustal melting and facilitating the mixing of mantle-derived materials with crustal melts. This ultimately generated a high-temperature, low-water-activity parental magmas, which subsequently underwent significant fractional crystallization. This study provides new constraints on the dynamic mechanisms of late Mesozoic magmatism within the South China Block. |
| keywords:Granite Yanshanian period Zircon U?Pb age Geochemistry Southeastern Guangxi |
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