PETROLOGY AND MINERALOGY OF PERIDOTITES FROM THE SOUTHERN MARIANA FOREARC: IMPLICATION FOR THE METASOMATISM OF THE MANTLE WEDGE UNDER MARIANA ARC
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摘要: 描述了来自马里亚纳南部前弧的橄榄岩,并对其中的组成矿物进行了电子探针分析,结果显示来自马里亚纳南部前弧橄榄岩中普遍含角闪石,这些角闪石主要为镁角闪石-透闪石系列,其中Al2O3含量变化范围大约为0~10%。橄榄岩中尖晶石的化学组成变化规律与角闪石的出现及其成分有着较好的耦合关系:出现富Al2O3角闪石的橄榄岩中的尖晶石富Al贫Cr;贫Al2O3角闪石大量出现的橄榄岩中尖晶石相对贫Al富Fe3+。这暗示马里亚纳弧下地幔楔橄榄岩经历了两期比较明显的交代作用:①早期较高温压条件下富Al和Ca的含水流体(熔体)的交代作用,消耗斜方辉石,形成富Al2O3角闪石,并引起尖晶石的富Al贫Cr的成分演化;②晚期较低温压条件下富Ca的含水流体(熔体)的交代作用,继续交代残余的斜方辉石,形成贫Al2O3角闪石,并使尖晶石发生富Fe3+贫Al。Abstract: Peridotites from the southern Mariana forearc are described and the mineral compositions analyzed by EPMA. Amphibole which generally falls into Magnesiohornblende and Tremolite is common in these peridotites and the abundance of Al2O3 ranges from 0 wt% to 10 wt%. Composition of spinel in these peridotites is closely related to amphibole:the occurrence of amphibole rich in Al2O3 is always accompanied by enrichment of Al and depletion of Cr in spinel,and the occurrence of amphibole poor in Al2O3 by enrichment of Fe3+ and depletion of Al. These indicate there were two stages of hydrous metasomatism in mantle wedge under the Mariana arc:(1)Hydrous fluid (melt) enriched in Al and Ca reacted with mantle wedge under high pressure and temperature, resulting in the consumption of orthopyroxene and the occurrence of amphibole with high Al2O3 to lead to enrichment of Al and depletion of Cr in spinel; (2) Hydrous fluid (melt) enriched in Ca continually reacted with mantle wedge in low pressure and temperature conditions and changed the residual orthopyroxene into amphibole with low Al2O3 to lead to the enrichment of Fe3+ and depletion of Al in spinel.
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Key words:
- peridotite /
- mantle wedge /
- metasomatism /
- Mariana
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