Pyroxenes from the early and middle stages of fractionation of the Skaergaard intrusion, East Greenland

Abstract

SummaryCoexisting calcium-rich and calcium-poor pyroxenes have been separated from the gabbros and ferrogabbros of the Skaergaard layered basic intrusion and chemically analysed. Nine new analyses of augites, three of inverted pigeonites, and one of orthopyroxene are presented, together with optical properties. The trend of crystallization of the augites is different from that drawn by previous investigators. The stages at which the orthopyroxene and pigeonite begin and cease to erystallize are defined, and the sub-solidus exsolution and inversion textures of the pigeonites are examined in detail. The characters of the exsolution textures and the degree of pigeonite inversion (monoclinic to orthorhombic pyroxene) are shown to change gradually with height in the intrusion, and are related to a gradual change in the interval between the temperatures of crystallization and inversion.