广东三水盆地双峰式火山岩:空间展布、岩石学特征及其盆地动力学意义

SPATIAL DISTRIBUTION AND PETROLOGICAL CHARACTERISTICS OF THE BIMODAL VOLCANIC ROCKS FROM SANSHUI BASIN, GUANGDONG PROVINCE: IMPLICATION FOR BASIN DYNAMICS

广东三水盆地内广泛分布有晚白垩纪-始新世火山岩,在化学成分上他们表现为以玄武岩类(SiO2=44.45%～54.46%;K2O+Na2O=3%～6%)和粗面岩-流纹岩(SiO2=59.23%～73.27%;K2O+Na2O=7.5%～12%)为主的双峰式特征。这些火山岩大致由十三期火山活动所形成,其中,早期形成以粗面岩-流纹岩为主的火山岩分布在盆地的边缘,晚期以玄武岩类为主的火山活动出现在盆地的中部。火山岩的时空分布特征反映了随着盆地断陷程度的不断加深导致岩浆由来自浅部岩浆房向深部岩浆房转化的过程。盆地内岩浆岩主要受区域地幔上涌-部分熔融所控制,盆地的断陷过程诱发了不同深度岩浆房内岩浆的喷发。三水盆地火山岩的不对称分布暗示了盆地的东侧向沿海方向的拉伸更加强烈。

Late Cretaceous and Cenozoic volcanic rocks are widely distributed within the Sanshui basin, Guangdong province. These volcanic rocks consist of basalts （SiO2 =44.45% -54.46% ; K2O＋ Na2O =3% -6% ） and trachyte/rhyolites （SiO2 = 59. 23% - 73. 27% ; K2O ＋ Na2O = 7. 5% - 12% ） , and display bimodal features in chemical composition. They were formed by thirteen - stage eruption. In the earlier stages （from first to third stage） trachyte and rhyolites dominated volcanies that are located at the eastern and western margins were generated, in contrast, at late stages （from fourth to thirteen stages） volcanic basalts were formed and located at the center of the basin. These features suggest significant transition from shallow magma chamber to deep ones, along with stronger and deeper rifting. Both felsic and mafic magmas were sourced from upwelling mantle by decompressed melting and fractional crystallization processes. Active rifting had resulted in eruptions from shallow to deep, while asymmetric extension forces caused more extensive crustal thining in the east side, which may be genetically associated with the formation of Zhujiangkou basin.