Petrology and Petrogenesis of Clinopyroxene-Rich Tholeiitic Lavas, Merelava Volcano, Vanuatu

The mineralogy, petrography and geochemistry of a suite of clinopyroxene-rich olivine tholenite lavas from Merelava island, Vanuatu are described. Located at the southern end of the Northern Trough back-arc basin, this suite displays all the characteristics of primitive island arc lavas: flat REE patterns, depleted HFSE, enrichment in K-group elements relative to LREE, highly calcic plagioclase (to An9 3 and Cr-rich spinels (cr-number≤80) Analysis of groundmass compositions demonstrates that the variation in MgO within the lava suite (from 13·7 to 4·3% MgO) represents only a small departure from a liquid line of descent. Some of the more primitive lavas contain low-Al2O3 clinopyroxene megacrysts (mg-number = 100Mg/(Mg+Fe2 + and ultramafic xenoliths, the latter ranging from fine-grained, tectonite wehrlites and chnopyroxene-bearing harzburgites, to coarse-grained cumulate wehrlites. The cumulate nodules, megacrysts and phenocrysts are shown to be co-magmatic, and an empirical compositional relationship is demonstrated for equilibrium olivine-clinopyroxene pairs, covering the observed fractionation range (mg-numberCpx=0·6375 mg-numberO1 + 35·3). On the basis that the most primitive olivine (mg-number 91 7) is close to the liquidus composition, this compositional relationship demonstrates that clinopyroxene (mg-number=94, and containing no Fe3+) was also a liquidus phase. Clinopyroxenes with mg-number>94 are the product of local oxidation during mixing of primitive, relatively reduced magmas, and more evolved, oxidized magmas. This mixing also gave rise to relatively narrow, reversely zoned, internal rims on many clinopyroxene and olivine phenocrysts, cumulus crystals, and clinopyroxene megacrysts. Fractionation modelling demonstrates that the most differentiated sample with 19 wt.% Al2O3 can be derived from the most primitive sample with 10·3% Al2O3 by removal of 48% crystals of clinopyroxene and olivine in the proportions 73:27 Plagioclase is a late crystallizing phase and has an insignificant role in the fractionation process. The parent melt composition (mg-number=77) is deduced from the most primitive olivine composition and the liquid line of descent, and is shown to contain equal amounts of MgO and CaO (137 wt.%), a high CaO/Al2O3 ratio of 1·3 and an unusually low Ni content of 137 ppm. Data from published high pressure (8–20 kb) experiments on melting of peridotite and pyrolite do not provide an explanati in for the large normative diopside component in this parent melt (38 mol.%), and a hypothesis is proposed whereby high degrees of melting of refractory Iherzolite or harzburgite+a component of lower crustal pyroxenite and/or wehrlite takes place at the base of the crust (5–55 kb). At this depth, and initially under hydrous conditions, high degrees of melting would progressively eliminate orthopyroxene and then clinopyroxene to produce a dunite residue. The liquid produced near the point of clinopyroxene elimination would be compatible with the high CaO and Sc contents, and high Sc/Ni, Cr/Ni and D1/Hy ratios of the lavas, and the refractory nature of the phenocrysts.