Ophiolite-contaminated andesites, trachybasalts, and cognate inclusions of Mount Lamington, Papua New Guinea: anhydrite-amphibole-bearing lavas and the 1951 cumulodome

Abstract

The catastrophic explosive eruptions in January 1951 from Mount Lamington were followed by a year-long episode of activity which was thoroughly described by Taylor (1958). However, only preliminary major-element analyses and petrographic observations have previously been reported from this volcano. The present study was prompted by the description of anhydrite and numerous ultramafic inclusions in the lavas of Mount Lamington, together with reports of similar inclusions and anomalously high Ni and Cr contents in other volcanics of southeastern Papua. Mount Lamington, together with three other major andesitic centers — Mount Victory, Mount Trafalgar and Hydrographers Range — rest on a northeast-dipping ophiolite sheet, the Papuan Ultramafic Belt (PUB); there is no evidence for a Benioff-Wadati zone beneath this part of Papua. The lavas of Mount Lamington are porphyritic trachybasalt and high-Si, medium-to-high-K andesite. Phenocrysts are plagioclase, amphibole, and biotite, and less common minerals are magnetite, pyroxene, apatite, cristobalite, zircon and sulphide. Olivine and orthopyroxene grains and clusters are found as foreign fragments, and anhydrite is a minor constituent, particularly of pristine samples. Numerous inclusions of xenolithic harzburgite and amphibole-biotite-plagioclase-dominated, coarse-grained igneous inclusions are also present. The lava suite is characterized by enrichments in several incompatible elements relative to intermediate lavas from Bougainville Island and New Britain. Products of the 1951 eruption have Ni and Cr concentrations in the ranges 23–45 ppm and 54–111 ppm, respectively. There are dramatic compositional zonations in the constituent phases of both lavas and inclusions. For example, amphibole zones range from compositions compatible with crystallization from pristine intermediate magmas, to Mg and Si-rich and alkali-poor components. Spinels range from refractory, Al-poor chromite to magnetite in composition. Contamination of rising andesitic magmas by the PUB is demonstrated conclusively for Mount Lamington lavas, and overall interaction with crustal rocks in other andesitic provinces may be more generally prevalent than has previously been recognized. The high Ni and Cr contents of the lavas have resulted from partial digestion of PUB material, and are not the result of direct partial melting of an upper-mantle source; neither do the high values exclude derivation of the intermediate magmas by fractional crystallization of basalt below the ophiolite. Isotopic analysis indicates the source of the sulphur in anhydrite was of primary magmatic rather than sedimentary origin. Two possible sources for the sulphur are either remobilized sulphides in the PUB, or, more speculatively, low-pressure breakdown of sulphate meionite (scapolite), possibly present in the lower crust of southeastern Papua.