广西南部三叠纪强过铝质火成岩岩石化学特征的动力学意义

THE GEODYNAMIC SIGNIFICANCE OF PETROCHEMICAL FEATURES OF TRIASSIC STRONGLY PERALUMINOUS IGNEOUS POCKS IN THE SOUTHERN PART OF GUANGXI PROVINCE

为深化对广西南部地区印支期造山运动深部动力学机制的理解,采用该地区三叠纪强过铝质火成岩的岩石化学数据提取其源区熔融温度以及成分特征方面的信息。结果显示:广西南部三叠纪强过铝质火成岩属于高钾钙碱性系列,形成于印支期后碰撞构造背景中。它们的Al2O3/TiO2小于100,CaO/Na2O多数大于0 3、少数小于0 3。同时,这些强过铝质火成岩的Al2O3/TiO2与其SiO2含量有正相关趋势,而CaO/Na2O与SiO2含量呈明显的负相关关系。这些岩石化学特征表明,强过铝质火成岩部分源于泥质岩的部分熔融,部分源于砂岩或正变质岩的部分熔融,但不排除后者可能是泥质岩部分熔融产物和玄武质岩浆相混合的结果;同时这些强过铝质火成岩形成源区的温度高(>=875℃)。因此,广西南部三叠纪时期的深部能量来自以底侵玄武岩浆加热作用所体现出来的软流圈地幔的平流热传递过程。

To understand the dynamic regime of Indosinian orogeny in the southern part of Guangxi Province, the petrochemical data were used to get information on melt temperature and source composition of Triassic strongly peraluminous igneous rocks (SPIRs). The results show that the Triassic SPIRs in the area studied belong to high potassic calc-alkaline series, which were formed in the post-collision setting during Indosinian orogeny. Nearly all these rocks have low Al2O3/TiO2 ratios (< 100), and most of them have high CaO/Na2O ratios (> 0.3), but the others have lower CaO/Na2O ratios (<0.3). Meanwhile, there is a positive correlation trend between SiO2 content and Al2O3/TiO2 ratio, and significant negative correlations between SiO2 content and CaO/Na2O ratio. These petrochemical features indicate that a part of these SPIRs were formed by partial melting of pelitic rocks, and the others by partial melting of sandstones or meta-igneous rocks. However, the latter also may be derived from the mixture of pelitic rock's melt and basaltic magma underplated beneath the crust. Meanwhile, the melting temperature of strongly peraluminous magma source is high (>= 875 ). Therefore, the deep thermal mechanism of Indosinian orogeny in the southern part of Guangxi Province is the advective heat transfer from asthenospheric mantle, through heating by underplating basaltic magma.