Rapakivi granite, anorthosite and charnockitic plutonism

Abstract

THE close association of massive anorthosite and charnockitic granitic rocks is well documented1. Rare earth element (REE) investigations2–4 have indicated that the massive charnockite (mangerite) associated with anorthosite is not comagmatic with it but represents a distinct magma fraction. Bridgwater et al.5 describe a charnockitic mantle around a plutonic suite of norites, monzonites and rapakivi granites from the Kap Farvel district of south Greenland which they consider to represent a parallel situation to the anorthosite–mangerite association. They believe contact metamorphic processes accompanied by anatexis produced the charnockitic phases. The charnockites are, therefore, dependent on the associated igneous suites but are not comagmatic with them. Duchesne et al.7 have investigated the REE content of monzonoritic rocks considered consanguineous with the anorthosites of the South Rogaland district of southern Norway. The absence of any Eu anomaly in the monzonoritic rocks, as compared with the marked positive Eu anomaly found for the anorthosites, places severe constraints on any postulated common parent magma for this rock association. A magma produced by partial fusion of upper mantle Kaersutite has been suggested7. A link between anorthosite and rapakivi granite6 has been proposed, rapakivi granite being a high level crystallisation related to and contemporaneous with anorthosite emplacement at greater depth. From their study of the Pikes Peak Batholith, Colorado, Barker et al.8 proposed a composite model for the origin of the rock spectrum gabbro–anorthosite–syenite–granite (including rapakivi granite). Convecting, mantle-derived, alkaline olivine basalt magma is believed to produce a quartz–syenitic magma from K2O-poor lower crustal rocks which in turn reacts with the granodioritic to granitic rocks of higher crustal levels to produce biotite and biotite–hornblende granites. Anorthosite is a precipitate phase from intermediate liquids. Our results from an REE investigation of plutonic charnockitic rocks and associated granites from south-west Sweden suggest that rapakivi granite may be associated with the charnockites satellitic to anorthosite rather than with anorthosite itself.