Garnet-bearing lherzolites and discrete nodule suites from the Malaita alnoite, Solomon Islands, and their bearing on the nature and origin of the Ontong Java Plateau

Abstract

A brecciated alnoite volcanic centre in central-north Malaita is of deep-seated origin and contains xenolithic suites that normally characterise kimberlites in continental cratonic settings. There is a wide range of xenolith types. The unique occurrence of garnet lherzolite (unmodified mantle material) xenoliths is especially exceptional in a supposed island arc situation or oceanic submarine plateau. A pyroxene geotherm calculated from the garnet-bearing nodules indicates a steeper gradient than those constructed for continental kimberlites. A high-temperature inflection, defined by discrete bronzite and subcalcic diopside nodules, is suggestive of a lithosphere thickness of 110 km. These considerations, the age of the Malaita pipe, the crustal nature and thickness of the Ontong Java Plateau (of which Malaita is an edge), its suggested low heat flow values and its geological history support the notion that the Plateau is continental, possibly even a 'proto-continent' as suggested by Kroenke (1974).