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Journal of Petrology | Volume 39 | Number 8 | Pages 1425-1452 | 1998
© Oxford University Press 1998

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

Gregor Markl1,*, B. Ronald Frost2 and Kurt Bucher1

1 Institut FüR Mineralogie, Petrologie Und Geochemie, Albert-Ludwigs-UniversitäT Albertstrasse 23 B, D-79104 Freiburg, Germany
2 Department of Geology and Geophysics, University of Wyoming Laramie, WY 82071, USA

Received February 10, 1997; Revised typescript accepted February 12, 1998


  Abstract

Crystallization temperatures of the 1.8 Ga Lofoten anorthosites are estimated from pyroxene thermometry, and pressure is derived from solving simultaneously the equilibria

Formula 1
and

Formula 2
These calculations indicate that the calcic Flakstadøy 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 + Mtss + 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 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, fO2, aSiO2, fHCl) 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.

KEY WORDS: anorthosite; mangerite; polybaric; QUILF; thermobarometry

* Corresponding author. Telephone: +49 761 203 6414. Fax: +49 761 203 6407.


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G. Markl and R. B. Frost
The Origin of Anorthosites and Related Rocks from the Lofoten Islands, Northern Norway: II. Calculation of Parental Liquid Compositions for Anorthosites
J. Petrology, January 1, 1999; 40(1): 61 - 77.
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