Eclogites of the D'Entrecasteaux Islands

Abstract

Eclogitic rocks of the D'Entrecasteaux Islands, Papua New Guinea, are of three types: true eclogites of omphacite-garnet-rutile; retrogressed or S-stage eclogites in which some omphacite has altered to symplectites of albite and less-jadeitic clinopyroxene (or amphibole); and rocks that are eclogites in all respects except that clinopyroxene is jadeitic diopside (Jd_<20) rather than omphacite. The rocks of the third group equilibrated in eclogite facies P-T conditions and, we conclude, are Na-poor eclogites, rather than granulites; i.e., low Na in the bulk rock is the reason for low jadeite content of clinopyroxene. Bulk rock chemical data confirm low Na and Si. Other prograde phases in the ecologitic rocks are kyanite, quartz, epidote group minerals and phengite and, in the low-Na group, orthopyroxene. Post-eclogite phases are amphibole, epidote group minerals, phengite and albite and, in the Na-poor eclogites, late phlogopite, calcic plagioclase, rare scapolite, and sulfides. The eclogitic rocks occur as lenticular boudins and small concordant tabular bodies within a 2–3 km thick sequence of migmatitic gneisses and, less commonly, in granodiorite. The gneiss sequence is bounded by detachment faults above and by younger granodiorite below, and is folded into broad antiforms. The three types of eclogite equilibrated at temperatures ranging from 530 to 840°C and pressures of 12 to 24 kbar. The metamorphic complex developed during Early Cenozoic subduction and arc-continent collison, and was elevated and exposed during Mid and Late Cenozoic crustal extension. The thermal gradient during subduction averaged 10°C/km and remained low during initial uplift, increasing to 18°C/km subsequently. Uplift averaged about 1 mMa^-1 from 60 to 5 Ma, then about 4 mMa^-1.