Volume 38: January - December 1997

Issue 3: March 1997

Abstract

Igneous charnockites and related rocks from the Bjerkreim-Sokndal layered intrusion (Southwest Norway): a jotunite (hypersthene monzodiorite)-derived A-type granitoid suite

  • Igneous charnockites and related rocks from the Bjerkreim-Sokndal layered intrusion (Southwest Norway): a jotunite (hypersthene monzodiorite)-derived A-type granitoid suite

    • J-C. Duchesne and E. Wilmart2 1Laboratoires Associes de Geologie, Petrologie et Geochimie, Universite de Liege, B-400, Sart Tilman, Belgium, 2Groupe des Sciences de la Terre, Laboratoire Pierre Sue, Cen Saclay, F-91191, Gif-sur-Yvette, France and Corresponding author

    ABSTRACT

    The upper part of the Bjerkreim-Sokndal layered intrusion (BKSK) comprises a suite of charnockitic granitoids (mangerite, quartz mangerite and charnockite) with some jotunites. The petrology and geochemistry (major and trace elements) of eight main rock types (88 samples) are considered. The rocks form a suite of K20-rich alkali-calcic granitoids of A-type, deriving from jotunitic magma. Two trends of inferred liquids are distinguished: the main liquid line of descent which passes from jotunites through two-pyroxene quartz mangerites to amphibole charnockites, the second one which encompasses olivine-bearing quartz mangerites and charnockites and is rooted into mangeritic liquids. Comparison with the Tellnes rock series reveals that contamination by leucogranitic material has taken place simultaneously with fractional crystallization in both BKSK trends. Zr, Rb, Cs and Th are particularly sensitive to assimilation and the higher water content in BKSK. Zircon and apatite have saturated at the liquidus of quartz mangerite around 900oC, close to the fayalite-magnetite-quartz (FMQ) buffer. The two-pyroxene quartz mangerites cannot have been equilibrated with the mangerite cumulates on which they rest. A major influx of magma took place when mangeritic cumulates were being deposited. It mingled and partly mixed with the resident contaminated magma.

    Keywords: A-type granite; charnockite; fractional crystallization; assimilation; layered intrusion

    Pages: 337 - 369

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