中沙群岛近海表层沉积物中的火山灰及其对构造环境的响应

南海中沙近海表层沉积物的轻组分(粒径为63~125 μm)中主要包括有褐色火山玻璃、无色火山玻璃以及火山渣等3种火山灰类型。在硅碱图(TAS)及Al2O3、FeO*、MgO、CaO、Na2O、K2O对SiO2的变化趋势图中可以判断,本区可能存在双峰式火山活动,褐色火山玻璃代表中-基性端元,而无色火山玻璃代表酸性端元。无色火山玻璃与褐色火山玻璃具有共同的岩浆源区,结晶分异可能是源区岩浆演化的主要方式。这种双峰式火山活动暗示火山活动源区处于大陆拉张减薄环境,支持西沙海槽是一个正在活化的伸展大地构造现象这一观点,同时肯定基于地球物理学和岩石学研究资料所提出的海南地幔柱的存在。

CHARACTERISTICS OF VOLCANIC ASH IN SURFACE SEDIMENTS AROUND ZHONGSHA ISLANDS: RESPONSE TO TECTONIC SETTING IN THE NORTH MARGIN OF THE SOUTH CHINA SEA

Volcanic ash in surface sediments around Zhongsha Islands of the South China Sea mainly include three kinds of volcanogenic detritus:that is,brown volcanic glass,colorless volcanic glass and volcanic scoria.TAS diagram for these three kinds of volcanogenic detritus,combined with variation diagram for Al2O3,FeO*,MgO,CaO,Na2O,K2O vs SiO2,shows that there may exist bimodal volcanic activity in the north margin of the South China Sea,brown volcanic glass and colorless volcanic glass represent basic end member and acidic end member respectively,and crystallization differentiation is the main process for magma evolution.The bimodal volcanic activity implies that the source of volcanic activity lay in continental extensional setting,supports that Xisha trench is a phenomenon for ongoing or activating extensional geodynamic setting,and affirms that there exists Hainan plume based on geophysical and petrologic data.