Experimental-Study of a Jotunite (Hypersthene Monzodiorite) - Constraints on the Parent Magma Composition and Crystallization Conditions (P, T, F(02)) of the Bjerkreim-Sokndal Layered Intrusion (Norway)

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Contributions to Mineralogy and Petrology
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The Bjerkreim-Sokndal layered intrusion is part of the Rogaland anorthosite Province of southern Norway and is made of cumulates of the anorthosite-mangerite-charnockite suite. This study presents experimental phase equilibrium data for one of the fine-grained jotunite (Tjorn locality) occurring along its northwestern lobe. These experimental data show that a jotunitic liquid similar in composition to the Tjorn jotunite, but slightly more magnesian and with a higher plagioclase component is the likely parent of macrocyclic units (MCU) III and IV of the intrusion. The limit of the olivine stability field in the experimentally determined phase diagram as well as comparison of the Al2O3 content of low-Ca pyroxenes from experiments and cumulates (almost-equal-to 1.5%) yields a pressure of emplacement less-than-or-equal-to 5 kbar. Experimentally determined Fe-Ti oxide equilibria compared to the order of cumulus arrival in the intrusion show that the oxygen fugacity was close to FMQ (fayalite-magnetite-quartz) during the early crystallization. It subsequently decreased relative to this buffer when magnetite disappeared from the cumulus assemblage and then increased until the reentry of this mineral. Calculated densities of experimental liquids show a density increase with fractionation at 7, 10 and 13 kbar due to the predominance of plagioclase in the crystallizing assemblage. At 5 kbar and 1 atm (FMQ-1), where plagioclase is the liquidus phase, density first increases and then drops when olivine (5 kbar) or olivine + ilmenite (1 atm: FMQ-1) precipitate. At 1 atm and NNO (nickel-nickel oxide), the presence of both magnetite and ilmenite as near liquidus phases induces a density decrease. In the Bjerkreim magma chamber, oxides are early cumulus phases and liquid density is then supposed to have decreased during fractionation. This density path implies that new influxes of magma emplaced in the chamber were both hotter and denser than the resident magma. The density contrast inferred between plagioclase and the parent magma shows that this mineral was not able to sink in the magma, suggesting an in situ crystallization process.


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