The Origin of Anorthosites and Related Rocks from the Lofoten Islands, Northern Norway: I. Field Relations and Estimation of Intrinsic Variables

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These calculations indicate that the calcic Flakstadoy anorthosite INTRODUCTION AND GEOLOGICAL [FBC, Cpx ± Ol ± Opx + Mtss + Ilmss + Plag (An57–47)] SETTING crystallized under polybaric conditions at pressures between 4 and 9 kbar and at temperatures between 1140 and 1185°C. The sodic The commonly observed association of Proterozoic masEidsfjord complex [Cpx + Opx + Mtss + Ilmss + Plag sif-type anorthosites [see Ashwal (1993) and references (An48–44)] crystallized at 1100–1135°C at a maximum pressure therein] with jotunitic and mangeritic or charnockitic of 7·3 kbar. This technique may provide a means to estimate rocks is still not satisfactorily explained. Some models crystallization pressure and aSiO2 in many types of intrusive and consider them to be comagmatic and relate these rocks extrusive rocks. Coeval mangerites and charnockites intruded subby processes of liquid immiscibility (Philpotts, 1981) or a sequently at ~4 kbar and temperatures between greater than 925°C specific fractionation trend (Owens et al., 1993); others and 800°C, respectively, indicated by the succession of the mafic describe them as coeval, but not comagmatic (Duchesne phase assemblages (Cpx + Opx; Cpx + Opx + Ol; Cpx + et al., 1989; Duchesne, 1990). A typical Proterozoic Pig + Ol; Cpx + Ol) that reflect continuous fractionation to anorthosite–mangerite–charnockite association is exhigher Fe/Mg ratios. The evidence for polybaric crystallization of posed in the Lofoten Islands, off the NW coast of Norway, the FBC quantitatively supports the common model that generation which is dominated by mangerites and charnockites. In of Proterozoic anorthosites involves initial crystallization at depth this paper, the first of a series on these rocks, we present (crust–mantle boundary) and intrusion as a crystal-rich mush. the field observations and phase petrology, and determine the conditions under which the intrusions were emplaced. Detailed estimation of intrinsic parameters ( P, T, fO2, aSiO2, fHCl)