The Koldal and Løyning intrusions, Rogaland

J. Richard Wilson, Rikke Kirk Andersen, Thomas Flemming Schou
2021 Norwegian Journal of Geology  
The Koldal and Løyning intrusions are small (1.8 km 2 and 0.25 km 2 ,respectively), dominantly noritic to gabbronoritic bodies emplaced in the foliated southern margin of the Egersund-Ogna anorthosite massif. Modal layering has a different orientation in each of three lobes constituting the Koldal body. The Høgåsen and Forefjellet lobes consist dominantly of leuconorites/norites and have similar An% and Mg# opx compositions to each other. The Koldal lobe is gabbronoritic with similar An% but
more » ... e Fe-rich orthopyroxenes than the other two lobes. A minor mangeritic unit may represent a fractionation product of the Koldal gabbronorites. The Høgåsen and Forefjellet lobes crystallised in two related (and maybe connected) small chambers with sloping floors. Parental magma to the Koldal gabbronorites was emplaced from an underlying evolving chamber and also crystallised on a sloping floor. The underlying chamber eventually produced mangeritic magma which was emplaced into the preexisting lobes. The narrow, elongate Løyning intrusion consists of leuconorites to the north and norites ± olivine to the south. Modal layering dips 55-80° to the south. The leuconorites and norites have different An% and Mg# opx compositions and may not be related to each other by simple in situ fractionation. Modal layering in the noritic rocks is laterally discontinuous and contains autoliths, consistent with its formation on a steeply sloping crystallisation front. Modal grading suggests that the rocks young to the south. The fact that the relatively primitive norites overlie the more evolved leuconorites suggests that the norites are the crystallisation product of an influx of relatively primitive magma. J. Richard Wilson et. al The Koldal and Løyning intrusions, Rogaland Most of the igneous rocks in the RAP are characterised by containing orthopyroxene as the dominant mafic mineral; they belong to the charnockitic suite, for which there is a special nomenclature (Le Maitre, 2002) . Jotunites are monzonorites and mangerites are monzonites with orthopyroxene. The term anorthosite is used for plagioclase-rich rocks with less than 10% mafic minerals. With between 10% and 35% mafic minerals (mostly orthopyroxene and ilmenite) the term leuconorite is used. This paper presents new geological maps of the Løyning and Koldal intrusions and new mineral compositional data. The aim is to develop models for the crystallisation of these two small layered intrusions that are both emplaced in the marginal zone of the Egersund-Ogna anorthosite. It emerges that our models differ from those presented by Vander Auwera et al. (2006) .
doi:10.17850/njg101-2-1 fatcat:qtn2mwx4dzembnxh3atujcgsny