Magma Genesis and Mantle Heterogeneity in the Manus Back-Arc Basin, Papua New Guinea

Abstract

Geochemical data from back-arc volcanic zones in the Manus Basin INTRODUCTION are used to define five magma types. Closest to the New Britain arc It has long been known that the diversity of rock types are medium-K lavas of the island arc association and back-arc and chemical lineages in extensional back-arc basins basin basalts (BABB). Mid-ocean ridge basalts (MORB), BABB greatly exceeds that of mid-ocean ridges. Although lavas and mildly enriched T-MORB (transitional MORB) occur along generally similar to mid-ocean ridge basalts (MORB) are the Manus Spreading Center (MSC) and Extensional Transform known from several back-arc basins, a broad range of Zone (ETZ). The MSC also erupted extreme back-arc basin other rock types also have been found. Several workers basalts (XBABB), enriched in light rare earth elements, P, and Zr. have emphasized enrichments in certain volatile and Compared with normal MORB, Manus MORB are even more lithophile elements compared with MORB (e.g. Tarney depleted in high field strength elements and slightly enriched in fluidet al., 1977; Garcia et al., 1979; Fryer et al., 1981; Sinton mobile elements, indicating slight, prior enrichment of their source & Fryer, 1987). Fryer et al. (1981) argued that the chemical with subduction-related components. Chemical variations and modsignature of lavas erupted in back-arcs is unique to that eling suggest systematic, coupled relationships between extent of setting and proposed the term back-arc basin basalts mantle melting, prior depletion of the mantle source, and enrichment (BABB) to denote this magma series. However, the exact in subduction-related components. Closest to the arc, the greatest nature and cause of the mantle source enrichment has addition of subduction-related components has occurred in the mantle led to various competing models for the petrogenesis of with the greatest amount of prior depletion, which has melted the lavas in back-arc basins. Most of these models involve most. Variations in K2O/H2O indicate that the subduction-related source component mixing, with one component generally component is best described as a phlogopite and/or K-amphibolebearing hybridized peridotite. Magmas from the East Manus Rifts accepted to be depleted asthenosphere similar to, or even are enriched in Na and Zr with radiogenic Sr/Sr, possibly more depleted than, the source for typical MORB. The indicating crustal interaction in a zone of incipient rifting. The nature of the other components varies with author and/ source for XBABB and lavas from the Witu Islands requires a or basin. Several workers have argued that BABB have mantle component similar to carbonatite melt. concentrations of incompatible and volatile elements that are intermediate between MORB and those in island arcs lavas, and hence, have called on the subducted slab for the source of a component contributing to the