卡洛琳地幔柱活动减弱过程中岩浆成因和源区组成演化

Magma genesis and evolution of source composition during the weakening of Caroline mantle plume activity

地幔柱活动不同阶段具有不同的岩浆作用产物,由楚克(14.8~4.3 Ma)、波纳佩(8.7~<1 Ma)、科斯雷(2~1 Ma)等洋岛构成的卡洛琳海山链是地幔柱活动不同阶段的典型例子,研究该海山链中不同洋岛地球化学特征的差异有助于加深对地幔柱晚期活动规律的认识。本研究对获取的楚克、科斯雷样品进行全岩主量、微量元素及矿物电子探针分析,并与波纳佩岛进行对比。科斯雷和楚克由霞石岩和碱性玄武岩组成,在微量元素配分模式上体现出典型碱性洋岛玄武岩的特征。这些样品的橄榄石斑晶具有与辉石岩源区夏威夷OIB橄榄石类似的高Ni、低Ca-Mn的特征,反映其地幔源区可能存在辉石岩。科斯雷霞石岩橄榄石斑晶内存在含碳酸盐的熔体包裹体,反映CO_(2)在地幔熔融和岩浆成因过程中起到了重要作用。从楚克、波纳佩到科斯雷,La/Sm比值逐渐增大,地幔熔融程度逐渐降低。Nb/Nb^(*)随着La/Sm、Sm/Yb升高和SiO_(2)降低有逐渐降低的趋势,与地幔熔融程度降低过程中CO_(2)作用的增强有关。研究认为,楚克、波纳佩、科斯雷等洋岛火山岩的地球化学变化由卡洛琳地幔柱热点活动逐渐减弱导致,随着地幔柱活动性减弱,CO_(2)在火山岩成因上起到越来越明显的作用。

The Caroline seamount chain consists of Chuuk(14.8~4.3 Ma),Pohnpei(8.7~<1 Ma),Kosrae(2~1 Ma)islands and a series of seamounts as the result of late-stage mantle plume.Geochemical variations in the seamount chain can deepen the understanding of late activity of the mantle plume.The whole rock major-and trace-elements,electron probe mineral analyses of the samples from Chuuk and Kosrae islands were conducted,and the results were compared with published data of Pohnpei.Kosrae and Chuuk islands are composed of nephelinites and alkaline basalts,reflecting typical ocean-island alkaline basalts in trace element patterns.Olivine phenocrysts in the samples are Ni-enriched but Ca-Mn–depleted,which is similar to olivines from Hawaiian OIB(ocean island basalt),suggesting the existence of pyroxenite in the mantle source.The presence of carbonate melt inclusions in the olivine phenocryst(Fo=85 mol%)of Kosrae nephelinite indicates that CO_(2) plays an important role in mantle melting and magma generation.The average La/Sm ratio of volcanic rocks gradually increases from Chuuk,Ponape,to Kosrae,which may reflect the decreasing degree of mantle melting during the weakening of the Caroline hot spot activity.In addition,the Nb/Nb^(*) ratio decreases with the increase of La/Sm,Sm/Yb ratios and the decrease of SiO_(2),indicating the enhancing effect of CO_(2)due to the decrease in mantle melting degree.Therefore,the continuous geochemical changes of volcanic rocks from Chuuk,Ponape,Kosrae islands are caused by the gradual weakening of Caroline mantle plume activity,during which CO_(2) plays an increasingly obvious role in genesis of volcanic rocks.