The Origin of Anorthosites and Related Rocks from the Lofoten Islands, Northern Norway: II. Calculation of Parental Liquid Compositions for Anorthosites

Abstract

Clinopyroxene, orthopyroxene and plagioclase in the Proterozoic INTRODUCTION Lofoten anorthosites are inferred to be in equilibrium with ferrodioritic Proterozoic massif-type anorthosites are huge intrusive melts, on the basis of literature partition coefficients for these phases bodies that contain hundreds to thousands of km of in equilibrium with basaltic melts. Ferrodiorites and high-Al gabbros nearly pure (>90%) plagioclase cumulates of intermediate occur as fine-grained dykes in spatial association with the ~1·8 composition (An50±10; Ashwal, 1993, and references Ga Lofoten anorthosites. They are linked to each other by an Fetherein). Most Proterozoic anorthosites are not associated enrichment fractionation trend typical of tholeiites, and they are with significant amounts of mafic or ultramafic cumulates. characterized by Al2O3 contents of 14–18·5 wt % and XFe [= They contain plagioclase that is relatively sodic (usually FeO/(FeO + MgO)] between 0·5 and 0·8. The association around An50) and this makes them distinct from other of high-Al gabbros, ferrodiorites and anorthosites is common in anorthositic rocks (Archean anorthosites, lunar anormany Proterozoic anorthosites. We infer that the high-Al gabbros thosites, anorthosites in layered intrusions), which contain to ferrodiorites represent the parental liquids from which these more calcic plagioclase. Proterozoic anorthosite comProterozoic anorthosites formed. The Lofoten rocks show fractionation plexes contain anorthosite sensu stricto (>90% plagioclase) trends that are similar to those of high-Al gabbros and ferrodiorites as well as variable amounts of leuco-gabbroic, -troctolitic from other Proterozoic anorthosite complexes. We interpret these trends and -noritic rocks which respectively may contain up to in terms of incorporation of intermediate composition plagioclase into 35% clinopyroxene, olivine or orthopyroxene. a typical tholeiitic magma with subsequent fractionation. The first The Proterozoic anorthosites of the Lofoten Islands in process produces positive Eu anomalies and the high Al2O3 contents northern Norway (Heier, 1960; Green & Jorde, 1971; of the melts, and explains the lack of negative Eu anomalies in Romey, 1971; Griffin et al., 1978; Wade, 1985) are most of the ferrodiorites from Lofoten and Laramie. associated with large amounts of temporally related intrusive mangerites, charnockites and gabbros (Markl et al., 1998). There are two different anorthosite intrusions present. One is a relatively calcic complex, which occurs on the island of Flakstadoy (Flakstadoy Basic Complex, FBC), and the other is a relatively sodic complex, which