摘要
琼北新近纪碱性橄榄玄武岩中广泛出露大型地幔岩捕虏体 (最大尺度 2 0cm) ,但前人尚未在第四纪玄武岩中发现过此类捕虏体。经过对采自第四纪玄武岩中 14 0余个样品的微观结构观测 ,发现天然氧化作用揭示出样品中部分粗粒橄榄石晶体发育位错构造 ;同时由橄榄石和辉石晶粒组成的某些集合体型斑晶的结构 ,与新近纪玄武岩中大型地幔岩捕虏体的结构相同 ,从而确定第四纪玄武岩中存在橄榄石捕虏晶和微型地幔岩捕虏体 (最大尺度为 4mm)。捕虏晶和微型捕虏体的存在表明第四纪玄武岩岩浆与新近纪一样都来源于上地幔 ;同时 ,地幔岩捕虏体由大型演变为微型反映了岩浆上升速度的逐步降低 。
Mantle-derived xenoliths of various sizes are abundant in Tertiary basalts of Northern Hainan Island, but no mantle xenolith has been found in Quaternary basalts of this region. Microstructural observation on 140 thin sections cut from Quaternary basalt samples reveals that the naturally oxidized large olivine grains exhibit well-developed dislocation sub-structures. In addition, the polycrystalline micro-fragments consisting of olivine and pyroxene grains are characterized by textures typical of mantle xenoliths from Tertiary basalts. All these may indicate that mantle-derived xenocrysts and micro-xenoliths (the largest may reach 4mm in size) do exist in Quaternary basalts of Northern Hainan Island. The occurrence of mantle-derived xenocrysts and micro-xenoliths in Quaternary basalts implies that the Quaternary basaltic magma came from the upper mantle in similar to the Tertiary basaltic magma. Moreover, the decrease in size of mantle xenoliths from mesoscopic scale to microscopic scale indicates the decrease of ascending velocity of the magmas and the weakening of volcanic activity.
出处
《地震地质》
EI
CSCD
北大核心
2003年第B12期33-42,共10页
Seismology and Geology
基金
科技部社会公益项目研究专项资金 (2 0 0 0DIA10 2 0 0 0 3)资助
关键词
第四纪
玄武岩
地幔
岩捕虏体琼北
Quaternary basalt, mantle-derived xenolith, northern Hainan Island
