Mineralogy of ultramafic cumulates and tectonites from eastern Papua

Abstract

Ultramafic rocks in the Papuan Ultramafic Belt can be divided into two categories on the basis of their fabric: (1) rocks with textures which indicate origin by crystal settling from presumably basaltic magma (ultramafic cumulates); and (2) rocks with textures which suggest recrystallization in the solid state (ultramafic tectonites). It is thought that the ultramafic cumulates have formed by crystal settling from the basaltic intrusions which are now represented by the overlying gabbro zone of the Ultramafic Belt, and that the ultramafic tectonites are pre-existing upper mantle and formed a floor for the basaltic intrusions [6]. Olivine, orthopyroxene, clinopyroxene and chrome spinel from both types were analysed by electron probe. Olivine and orthopyroxene from ten ultramafic cumulates have compositions in the ranges Fo78.3–89.6 and En81.2–90.5, whereas the same minerals in thirteen samples of ultramafic tectonite have compositions in the ranges Fo91.6–93.6 and En92.1–93.4. The wide range of mineral compositions in the ultramafic cumulates is compatible with an origin by crystal settling from a fractionating magma, but the very small compositional range and uniformly highly magnesian character of minerals from the ultramafic tectonites, together with their large volume and wide geographic distribution, strongly suggest that these rocks are not recrystallized cumulates. Such characteristics would be expected from rocks which formed as a refractory residue during partial melting of primitive mantle material. However, the Ca and Al contents of many orthopyroxenes in the ultramafic tectonites are lower than would be expected for a residue in equilibrium with basaltic magma.