The origin of anorthosites and related rocks from the Lofoten Islands, northern Norway: I. Field relations and estimation of intrinsic variables

Abstract

Crystallization temperatures of the 1.8 Ga Lofoten anorthosites are estimated from pyroxene thermometry, and pressure is derived from solving simultaneously the equilibria CaAl2SiO2(in cpx) + SiO2 = CaAl2Si2O8 (in plag) NaAlSi2O6(in cpx) + SiO2 = NaAlSi3O8(in plag) and Mg2SiO4(in ol) + SiO2 = 2 MgSiO3 (in opx). These calculations indicate that the calcic Flakstadoy anorthosite [FBC, Cpx ± Ol ± Opx + Mtss + Ilmss + Plag (An57-47)] crystallized under polybaric conditions at pressures between 4 and 9 kbar and at temperatures between 1140 and 1185°C. The sodic Eidsfjord complex [Cpx + Opx + Mts + Ilmss + Plag (An48-44)] crystallized at 1100-1135°C at a maximum pressure of 7.3 kbar. This technique may provide a means to estimate crystallization pressure and aSiO2 in many types of intrusive and extrusive rocks. Coeval mangerites and charnockites intruded subsequently at ~4 kbar and temperatures between greater than 925°C and 800°C, respectively, indicated by the succesesion of the mafic phase assemblages (Cpx + Opx; Cpx + Opx + Ol; Cpx + Pig + Ol; Cpx + Ol) that reflect continuous fractionation to higher Fe/Mg ratios. The evidence for polybaric crystallization of the FBC quantitatively supports the common model that generation of Proterozoic anorthosites involves initial crystallization at depth (crust-mantle boundary) and intrusion as a crystal-rich mush. Detailed estimation of intrinsic parameters (P,T,fo2, aso2, fHcl) indicates a systematic relationship between the phase assemblages in anorthosites, farnodiorites, mangerites and charnockites, which is compatible with fractional crystallization of a mafic parental magma.