High-potassium intrusives from southeastern Papua

Abstract

This paper presents petrographic, major element, and trace element data from high-potassium ‘shoshonitic’ rocks of Miocene age which intrude Eocene submarine basalts in southeastern Papua. The intrusives fall into two distinct but overlapping groups, a ‘near-saturated’ group ranging from gabbro to syenite with regular petrographic and chemical variations, which is either slightly nepheline normative or quartz normative, and a nepheline normative ‘undersaturated’ group which shows wide variations in texture, modal mineralogy, and chemistry. Biotite-bearing pyroxenites are associated with the intrusives but their genetic relationship to the intrusives is unknown. The intrusion of shoshonitic rocks at the beginning of a period of major tectonic activity in southeastern Papua shows that high-potassium magmas can be generated in areas of active tectonism and may form part of the island arc ‘magmatic’ association.