THE ORIGIN OF ANORTHOSITES AND RELATED ROCKS FROM THE LOFOTEN ISLANDS, NORTHERN NORWAY - I - FIELD RELATIONS AND ESTIMATION OF INTRINSIC VARIABLES

Crystallization temperatures of the 1.8 Ga Lofoten anorthosites are es timated from pyroxene thermometry, and pressure is derived from solvin g simultaneously the equilibria CaAl2SiO6 (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 + Mt(ss) Ilm(ss) + Plag (An(57-47))] crystallized under polybaric conditions at pressures between 4 and 9 kbar and at temperatures between 1140 and 1 185 degrees C. The sodic Eidsfjord complex [Cpx + Opx + Mt(ss) + Ilm(s s) + Plag [An(48-44))] crystallized at 1100-1135 degrees C at a maximu m pressure of 7.3 kbar. This technique may provide a means to estimate crystallization pressure and a(SiO2) in many types of intrusive and e xtrusive rocks. Coeval mangerites and charnockites intruded subsequent ly at similar to 4 kbar and temperatures between greater than 925 degr ees C and 800 degrees C, respectively, indicated by the succession of the mafic phase assemblages (CPx + OPx; Cpx + Opx + Ol; Cpx + Pig + Ol ; Cpx + Ol) that reflect continuous fractionation to higher Fe/Mg rati os. The evidence for polybaric crystallization of the FBC quantitative ly supports the common model that generation of Proterozoic anorthosit es involves initial crystallization at depth (crust-mantle boundary) a nd intrusion as a crystal-rich mush. Detailed estimation of intrinsic parameters (P, T, f(O2), a(SiO2), f(HCl)) indicates a systematic relat ionship between the Phase assemblages in anorthosites, ferrodiorites, mangerites and charnockites, which is compatible with fractional cryst allization of a mafic parental magma.