粤北仁化白垩纪安山玢岩锆石年代学、地球化学和岩石成因研究

摘要

The Renhua andesitic porphyrite is exposed in the Danxia volcanic basin in the northern Guangdong, southeast of the Hunan-Guangxi Diwa series in South China. Volcanic rocks from Shibei and Miaochansi in the Renhua area have been dated to be 104.3±0.8 Ma and 105.0±0.7 Ma, respectively, using the method of LA-ICP-MS zircon U-Pb, which suggests that the Renhua andesitic porphyrites were formed in Early Cretaceous. The Shibei and Miaochansi andesitic porphyrites are of porphyritic texture, within which the phenocrysts are dominated by plagioclase and amphibole or pyroxene. They have K2O of 3.07%~3.81% and Mg# values of 49~61, with compositions similar to those of high-K calc-alkaline volcanic rocks. The rocks are enriched in large ion lithophile elements (LILEs, Rb, K, Th, U) and light rare earth elements (LREEs), and depleted in high field strength elements (HFSEs, Nb, Ta, Ti). The ratios of Zr/Y (6.56~9.10), Th/U (3.72~4.31), La/Nb (>3), Nb/La (<0.31) and (Th/Nb)N (>10) of the rocks indicate that they are probably derived from intraplate mafic magma with the contamination of crust materials. Combined with the previous studies in this area, we conclude that the andesitic porphyrites in the Renhua area may have formed by partial melting of the lithospheric mantle in an intraplate extensional tectonic setting and subsequently experienced crystallization differentiation and crust contamination during the magma ascent and emplacement. Formation of the volcanic rocks was triggered by the subduction of the Pacific plate under the Eurasian plate during the late Early Cretaceous.%仁化安山玢岩分布在华南湘桂地洼系南东侧,位于广东韶关丹霞火山盆地中。来自仁化妙禅寺和石背两地的安山玢岩 LA-ICP-MS 锆石 U-Pb 测年结果分别为105.0±0.7 Ma 和104.3±0.8 Ma,表明其形成于中生代构造活化期最强烈的早白垩世晚期。妙禅寺和石背安山玢岩均具斑状结构,块状构造。斑晶主要为斜长石和角闪石或辉石,基质组成与斑晶类似。相关图解显示安山玢岩为高钾钙碱性系列岩石,其 Mg#为49~61, K2O 含量为3.07%~3.81%。微量元素地球化学特征研究表明,这些安山玢岩明显富集轻稀土(LREE)和大离子亲石元素 Rb、K、Th 和 U,亏损高场强元素 Nb、Ta 和 Ti,无Zr、Hf 异常。Zr/Y (6.56~9.10)、Th/U (3.72~4.31)、Nb/La (＜0.31)和(Th/Nb)N (>10)等微量元素比值表明其源区为地壳物质加入板内玄武质岩浆的结果。结合前人已有的区域研究成果,作者认为仁化安山玢岩母岩浆应是在岩石圈伸展构造背景下由岩石圈地幔部分熔融形成,岩浆在上升或侵位过程中遭受了不同程度的地壳混染,经历一定程度结晶分异作用后最终形成安山质岩浆。其形成与早白垩世晚期太平洋板块向欧亚板块的俯冲和后缘拉张有关。