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

• Published in: Journal of Petrology
• Main Authors: Markl, Gregor, Frost, B. Ronald, Bucher, Kurt
• Format: Text
• Language: English
• Published: Oxford University Press 1998
• Subjects: Articles; Flakstadøy; Lofoten; Norway; Northern Norway
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/39/8/1425; https://doi.org/10.1093/petroj/39.8.1425

Crystallization temperatures of the 1.8 Ga Lofoten anorthosites are estimated from pyroxene thermometry, and pressure is derived from solving simultaneously the equilibria<fd id="FD1">$$\begin{array}{c}CaA{l}_{2}Si{O}_{6}\left(in\hbox{ \hspace{0.17em} }cpx\right)+Si{O}_{2}=CaA{l}_{2}S{i}_{2}{O}_{8}\left(in\hbox{ \hspace{0.17em} }plag\right)\\ NaAlS{i}_{2}{O}_{6}\left(in\hbox{ \hspace{0.17em} }cpx\right)+Si{O}_{2}=NaAlS{i}_{3}{O}_{8}\left(in\hbox{ \hspace{0.17em} }plag\right)\end{array}$$</fd>and<fd id="FD2">$$M{g}_{2}Si{O}_{4}\left(in\hbox{ \hspace{0.17em} }ol\right)+Si{O}_{2}=2MgSi{O}_{3}\left(in\hbox{ \hspace{0.17em} }opx\right)$$</fd>These calculations indicate that the calcic Flakstadøy 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 1185°C. The sodic Eidsfjord complex [Cpx + Opx + Mt ss + Ilm ss + Plag (An 48–44 )] crystallized at 1100–1135°C at a maximum pressure of 7.3 kbar. This technique may provide a means to estimate crystallization pressure and a SiO 2 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 succession 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 , f O 2 , a SiO 2 , f HCl ) indicates a systematic relationship between the phase assemblages in anorthosites, ferrodiorites, mangerites and charnockites, which is compatible with fractional crystallization of a mafic parental magma.