南海新生代碱性玄武岩中斜长石矿物的化学成分及意义

CHEMICAL COMPOSITION OF PLAGIOCLASE IN CENOZOIC ALKALI BASALT FROM THE SOUTH CHINA SEA

南海新生代碱性玄武岩中存在两种不同粒径的斜长石矿物。其一为斜长石斑晶,常见熔蚀麻点,是岩浆上升、压力降低时发生熔蚀作用并在骤冷条件下形成的;其二斜长石微晶,半定向或杂乱分布于火山玻璃中,其中空骸晶结构表明斜长石微晶是在淬冷条件下迅速结晶形成的。斜长石斑晶具弱成分环带,斑晶边部的An值稍高或接近于斜长石微晶。微晶斜长石An值与岩浆喷出后的水深以及喷发位置距离岩浆主通道的远近存在一定联系。本区的斜长石斑晶形成温度明显低于冲绳海槽地区,而类似于东海陆架地区;斜长石微晶的结晶温度类似于冲绳海槽,表明两地区在岩浆喷出海底后淬火结晶的物理化学条件相似。结合同样品中橄榄石斑晶研究结果以及已有的地球物理学和岩石学方面的资料,可能反映了地幔柱快速上涌使早期部分熔融及结晶分异作用较弱,岩浆本身温度高提供了早期结晶形成的斑晶与寄主岩浆进一步充分反应的热量。计算的斜长石斑晶温度不能反映源区温度特征,后者应高于本文所计算的斜长石斑晶的结晶温度。

There exist two types of plagioclases of different sizes in Cenozoic alkali basalts which had been dredged from the South China Sea, one of which is plagioclase phenocryst with corrosion phenomena and the other is plagio- clase microlite which is distributed in volcanic glass semi-directionally or randomly. Its hollow skeletal texture indi-cates that it was formed by rapid crystallization under quenching conditions. Plagioclase phenocryst has weak zoning in composition, the An value in the rim of which is slightly higher than or similar to that of plagioclase microlite. There exist certain relationships between the An value of plagioclase microlite and water depth at which magma erupted and the distance between the erupting location and the main magma passage. The formation temperature of plagio-clase phenocryst obviously is lower than that of the Okinawa trench and similar to that of continental shelf in the East China Sea. The formation temperature of plagioclase microlite is similar to that of the Okinawa trench, implying that there exited similar physical and chemical conditions during eruption and quenching crystallization. In combination with olivine chemistry from the same host rocks, and published geophysics or petrology data, it may be reflected that rapid upwelling of mantle plume in the South China Sea resulted in weak earlier partial melting or fractional crystalli- zation, and high magma temperature itself provided the heat source for further interaction between the earlier pheno- crysts and the host magma. The calculated temperature of plagioclase phenocryst cannot represent that of the magma origin, the latter is higher than the former.