オフィオライトかんらん岩の成因 (オフィオライト<特集>)
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概要
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Origin of spinel peridotites of upper mantle derivation, especially that of ophiolitic peridotites, is discussed mainly on the basis of compositional relationship between olivine (Fo content) and chromian spinel (Cr/(Cr+Al) atomic ratio=Cr<SUP>*</SUP> ratio). Spinel peridotites are distributed in a relatively narrow band, the olivine-spinel mantle array, on the Fo-Cr<SUP>*</SUP> plane. The olivine-spinel mantle array (=OSMA) may by a trend for residual peridotites and have a fertile tip at Fo87, Cr<SUP>*</SUP>=0.08 and a refractory tip at Fo93, Cr<SUP>*</SUP>=0.95.Lherzolite is distributed in a fertile part of the OSMA (Cr<SUP>*</SUP><0.6) and harzburgite, in a refractory part (Cr<SUP>*</SUP>>0.4). In a subsolidus stage, the Fo-Cr<SUP>*</SUP> relation in spinel peridotites is notaltered by temperature variation but is severely altered by a metasomatic process.<BR>Mantle peridotites from know tectonic settings are summarized as follows; lherzolite with Cr<SUP>*</SUP> of 0.6-0.1 (mostly 0.4-0.2) from the ocean floor, lherzolite with Cr<SUP>*</SUP><0.4 (mostly around 0.1) from the oceanic hot spot, harzburgite-lherzolite with Cr<SUP>*</SUP> of 0.6-0.1 from the island arc or the marginal basin (Japan island arcs), and harzburgite-lherzolite with Cr<SUP>*</SUP> of 0.8-0.4 from the fore-arc area. Subcontinental upper mantle is mostly made up by lherzolite with Cr<SUP>*</SUP> less than 0.2.<BR>Olivine and chromian spinel are also early precipitating phases from primary or nearly primary magmas. Variation of the Cr<SUP>*</SUP> ratio of chromian spinel monitored by the Fo content of coexisting olivine makes a "fractionation line" on the Fo-Cr<SUP>*</SUP> plane. The cumulative peridotite, which always contains chromian spinel, is directly correlated with its parental magma on the Fo-Cr<SUP>*</SUP> plane. The residual peridotite for each magma suite could be estimated by extrapolating a fractionation line back to the OSMA as follows; lherzo lite with Cr<SUP>*</SUP><0.6 (mostly 0.6 to 0.4) for MORB, lherzolite with Cr<SUP>*</SUP><0.5 (mostly 0.5 to 0.2) for alkali basalts, harzburgite with Cr<SUP>*</SUP>>0.9 for boninites, harzburgite with Cr<SUP>*</SUP> of 0.9-0.7 for high-magnesia andesites or high-magnesia, high-silica arc tholeiites, harzburgitelherzolite with Cr<SUP>*</SUP><0.7 for arc subalkalic basalts, harzburgite with Cr<SUP>*</SUP> of ca. 0.7 for intraplate tholeiites. The genetical consanguinity between residual peridotites and cumulate or volcanic rocks within an ophiolite complex could be examined in terms of the Fo-Cr<SUP>*</SUP> relationship.
- 社団法人 東京地学協会の論文
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