Origin of Ophiolitic Peridotites

Abstract

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^* ratio). Spinel peridotites are distributed in a relatively narrow band, the olivine-spinel mantle array, on the Fo-Cr^* plane. The olivine-spinel mantle array (=OSMA) may by a trend for residual peridotites and have a fertile tip at Fo87, Cr^*=0.08 and a refractory tip at Fo93, Cr^*=0.95.Lherzolite is distributed in a fertile part of the OSMA (Cr^*<0.6) and harzburgite, in a refractory part (Cr^*>0.4). In a subsolidus stage, the Fo-Cr^* relation in spinel peridotites is notaltered by temperature variation but is severely altered by a metasomatic process.
Mantle peridotites from know tectonic settings are summarized as follows; lherzolite with Cr^* of 0.6-0.1 (mostly 0.4-0.2) from the ocean floor, lherzolite with Cr^*<0.4 (mostly around 0.1) from the oceanic hot spot, harzburgite-lherzolite with Cr^* of 0.6-0.1 from the island arc or the marginal basin (Japan island arcs), and harzburgite-lherzolite with Cr^* of 0.8-0.4 from the fore-arc area. Subcontinental upper mantle is mostly made up by lherzolite with Cr^* less than 0.2.
Olivine and chromian spinel are also early precipitating phases from primary or nearly primary magmas. Variation of the Cr^* ratio of chromian spinel monitored by the Fo content of coexisting olivine makes a “fractionation line” on the Fo-Cr^* plane. The cumulative peridotite, which always contains chromian spinel, is directly correlated with its parental magma on the Fo-Cr^* 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^*<0.6 (mostly 0.6 to 0.4) for MORB, lherzolite with Cr^*<0.5 (mostly 0.5 to 0.2) for alkali basalts, harzburgite with Cr^*>0.9 for boninites, harzburgite with Cr^* of 0.9-0.7 for high-magnesia andesites or high-magnesia, high-silica arc tholeiites, harzburgitelherzolite with Cr^*<0.7 for arc subalkalic basalts, harzburgite with Cr^* 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^* relationship.