Petrology and petrogenesis of cumulate peridotites and gabbros from the marum ophiolite complex, Northern Papua New Guinea

The Marum ophiolite complex in northern Papua New Guinea includes a thick (3-4 km) sequence of ultramafic and mafic cumulates, which are layered on a gross scale from dunite at the base upwards through wehrlite, lherzolite, plagioclase lherzolite, pyroxenite, olivine norite-gabbro and norite-gabbro to anorthositic gabbro and ferrogabbro at the top. Igneous layering and structures, and cumulus textures indicate an origin by magmatic crystallization in a large magma chamber(s) from magma(s) of evolving composition. Most rocks however show textural and mineralogical evidence of subsolidus re-equilibration.The cumulate sequence is olivine and chrome spinel followed by clinopyroxene, orthopyroxene and plagioclase, and the layered sequence is similar to that of the Troodos and Papuan ophiolites. These sequences differ from ophiolites such as Vourinos by the presence of cumulus magnesian orthopyroxene, and are not consistent with accumulation of low pressure liquidus phases of mid-ocean ridge-type olivine tholeiite basalts.The cumulus phases show cryptic variation from Mg- and Ca-rich early cumulates to lower temperature end-members, e.g. olivine Mg_93-78, plagioclase An_94-63. Co-existing pyroxenes define a high temperature solidus with a narrower miscibility gap than that of pyroxenes from stratiform intrusions. Re-equilibrated pyroxene pairs define a low-temperature, subsolidus solvus. Various geothermometers and geobarometers, together with thermodynamic calculations involving silica buffers, suggest the pyroxene-bearing cumulates crystallized at ∼1200 °C and 1-2 kb pressure under low f_O2. The underlying dunites and chromitites crystallized at higher temperature, ∼1300-1350 °C. The bulk of the cumulates have re-equilibrated under subsolidus conditions: co-existing pyroxenes record equilibration temperatures of 850-900 °C whereas olivine-spinel and magnetite-ilmenite pairs indicate final equilibration at very low temperatures (≤600 °C).Magmas parental to the cumulate sequence are considered to have been of magnesian olivine-poor tholeiite composition (>50 per cent SiO₂, ∼15 per cent MgO, 100 Mg/(Mg + Fe²⁺) ∼78) rich in Ni and Cr, and poor in TiO₂ and alkalies. Fractionated examples of this magma type occur at a number of other ophiolites with similar cumulate sequences. Experimental studies show that such lavas may result from ial melting of depleted mantle lherzolite at shallow depth. The tectonic environment in which the complex formed might have been either a mid-ocean ridge or a back-are basin.