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)

The Bjerkreim-Sokndal layered intrusion is part of the Rogaland anorth osite Province of southern Norway and is made of cumulates of the anor thosite-mangerite-charnockite suite. This study presents experimental phase equilibrium data for one of the fine-grained jotunite (Tjorn loc ality) occurring along its northwestern lobe. These experimental data show that a jotunitic liquid similar in composition to the Tjorn jotun ite, but slightly more magnesian and with a higher plagioclase compone nt is the likely parent of macrocyclic units (MCU) III and IV of the i ntrusion. The limit of the olivine stability field in the experimental ly 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. Ex perimentally determined Fe-Ti oxide equilibria compared to the order o f cumulus arrival in the intrusion show that the oxygen fugacity was c lose to FMQ (fayalite-magnetite-quartz) during the early crystallizati on. It subsequently decreased relative to this buffer when magnetite d isappeared from the cumulus assemblage and then increased until the re entry of this mineral. Calculated densities of experimental liquids sh ow a density increase with fractionation at 7, 10 and 13 kbar due to t he predominance of plagioclase in the crystallizing assemblage. At 5 k bar and 1 atm (FMQ-1), where plagioclase is the liquidus phase, densit y first increases and then drops when olivine (5 kbar) or olivine + il menite (1 atm: FMQ-1) precipitate. At 1 atm and NNO (nickel-nickel oxi de), the presence of both magnetite and ilmenite as near liquidus phas es induces a density decrease. In the Bjerkreim magma chamber, oxides are early cumulus phases and liquid density is then supposed to have d ecreased during fractionation. This density path implies that new infl uxes 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.