北祁连石灰沟奥陶纪玄武岩流体组成及其地质意义

Chemical compositions of volatiles and their geological significance in Ordovician in alkali basalt in Shihuigou Area,North Qilian

北祁连石灰沟地区中奥陶世发育了一套海相细碧岩--碱性火山岩组合,为岛弧火山作用产物.应用分步加热法测试了该区碱性玄武岩中橄榄石斑晶和基质的流体组成.玄武岩橄榄石斑晶和基质中流体组份都具有分阶段释放的特征,可分为低温(200～400?C)、中温(400～900?C)和高温(900～1 200?C)3个温度段,流体化学组成以H2O,CO2,CO+N2为主,次要组份是H2,SO2,CH4,O2,H2S等.石灰沟碱性玄武岩发育于相对氧化的流体环境,揭示其源区具有高的氧逸度,代表古岛弧形成的流体环境.

The middle Ordovician of Shihuigou in the North Qilian has developed a suit of marine spilitealkaline volcanic rock assemblage, which is a product of the island arc volcanism. The chemical compositions of volatiles in the olivine phenocrysts and matrix in the alkali basalt from the Shihuigou of Qilian orogenic belt were determined by stepwise heating mass spectrometer to reveal mantle fluids. The results showed that volatiles in different occurrence sites are released at 200-400℃, 400-900℃, 900-1 200℃ intervals. The volatiles in basalt in Shihuigou are mainly composed of H2 O, CO2, CO＋N2, and the minor composed of H2, SO2, CH4, O2 and H2S. The alkaline volcanic rocks in Shihuigou could reflect that the basalt developed in relative oxidation fluid environment and in high fugacity of O2 environment.