Journal of Petrology, Volume 40, Issue 1, pp. 61-77

Journal of Petrology, Volume 40, Issue 1: January 1999.

The Origin of Anorthosites and Related Rocks from the Lofoten Islands, Northern Norway: II. Calculation of Parental Liquid Compositions for Anorthosites

GREGOR MARKL¹ AND B. RONALD FROST²

¹INSTITUT FÜR MINERALOGIE, PETROLOGIE UND GEOCHEMIE, ALBERT-LUDWIGS-UNIVERSITÄT, ALBERTSTRASSE 23 B, D-79104 FREIBURG, GERMANY
²DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF WYOMING, LARAMIE, WY 82071, USA

Clinopyroxene, orthopyroxene and plagioclase in the Proterozoic Lofoten anorthosites are inferred to be in equilibrium with ferrodioritic melts, on the basis of literature partition coefficients for these phases in equilibrium with basaltic melts. Ferrodiorites and high-Al gabbros occur as fine-grained dykes in spatial association with the ~1·8 Ga Lofoten anorthosites. They are linked to each other by an Fe-enrichment fractionation trend typical of tholeiites, and they are characterized by Al₂O₃ contents of 14-18·5 wt % and X_Fe [= FeO^tot/(FeO^tot + MgO)] between 0·5 and 0·8. The association of high-Al gabbros, ferrodiorites and anorthosites is common in many Proterozoic anorthosites. We infer that the high-Al gabbros to ferrodiorites represent the parental liquids from which these Proterozoic anorthosites formed. The Lofoten rocks show fractionation trends that are similar to those of high-Al gabbros and ferrodiorites from other Proterozoic anorthosite complexes. We interpret these trends in terms of incorporation of intermediate composition plagioclase into a typical tholeiitic magma with subsequent fractionation. The first process produces positive Eu anomalies and the high Al₂O₃ contents of the melts, and explains the lack of negative Eu anomalies in most of the ferrodiorites from Lofoten and Laramie.

Keywords: anorthosite;parental liquid;ferrodiorite;high-Al basalt;partition coefficients

Pages 61-77

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Please note that full-text access is only available to current subscribers [Registration & Subscription Info]
Full-text HTML This page is run by Oxford University Press, Great Clarendon Street, Oxford OX2 6DP, UK as part of the OUP Journals World Wide Web service. Comments and feedback: www-admin@oup.co.uk URL: http://www.oup.co.uk/petroj/hdb/Volume_40/Issue_01/egc005_gml.abs.html Last modification: 25 January 1999. Copyright© Oxford University Press, 1999.