2021
The Koldal and Løyning intrusions, Rogaland
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The Koldal and Løyning intrusions are small (1.8 km2 and 0.25 km2 ,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 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.

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