Investigation of the Fe-Ti-HFSE enrichment in the Raftsund intrusion, Årsteinen, Lofoten-Vesterålen archipelago, Northern Norway
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Iron+Ti±P±HFSE enriched rocks are found in a variety of localities worldwide, including some that are temporally and spatially related to Proterozoic AMCG (Anorthosite-Mangerite-Charnokite-Granite) suites. The origin of these enriched rocks has been widely debated and petrogenetic models such as liquid-liquid immiscibility, fractional crystallization and mineral accumulation, hydrothermal fluid enrichment, and residual liquids concentrated by filter pressing have all been proposed to explain occurrences of these rocks associated with AMCG suites. This locality is particularly distinct for two reasons, 1) abundant and large (>2.5mm) zircon, which contrasts with many Fe+Ti±P±HFSE enriched rocks where apatite is the main REE carrier 2) the mineralized zones are located as thin veins (up to 3 cm wide) located at the contact between two rock-types – an equigranular olivine-clinopyroxene-monzonite and a porphyritic orthopyroxene-clinopyroxene monzonite. Field relationships down to the microscale suggest the mineralization is related to the equigranular monzonite - the mineralized zones show planar contacts with the porphyritic monzonite, while it protrudes in between mesoperthitic feldspar grains of the equigranular monzonite. The mineralized zones likely developed from the equigranular monzonite, while the porphyritic monzonite served as a boundary where the mineralization was concentrated. Mg# and Fo% in clinopyroxene and olivine, respectively, are highest in phases of the mineralization (35.0 and 6.17) compared to the equigranular monzonite (22.2 and 2.50) and are can be suggested to be early-forming. Magnetite in the mineralization also has relatively elevated V and Al, which are known to behave as fractionation indices, and indicates earlier forming magnetite compared to magnetite in the equigranular monzonite. Strontium content and Eu/Eu* of apatite in the mineralization indicate a contradicting relationship – having more evolved signature compared to apatite of both the equigranular monzonite indicating crystallization from a melt which underwent greater feldspar fractionation. Textures show that apatite in the mineralization are anhedral, and has its morphology controlled when bordering ferromagnesian phases. Compositions and textures align with apatite being an intercumulus phase, which crystallized from a liquid which underwent greater feldspar fractionation. However, field relationships, such as steeply dipping mineralization zones do not fit with gravity-driven accumulation. Alternatively, these mineralized zones can be the product of an iron-rich residual liquid. Subsolidus exchange between ferromagnesian phases and Fe-Ti-oxides can drive the compositions of ferromagnesian silicates to higher Mg/Fe ratios. Higher modal proportions of Fe-Ti-oxides in the mineralization zones could result in apparently higher Mg/Fe ratios in clinopyroxene and olivine. Apatite compositions can be used as a fractionation indicator, and the most evolved compositions in the mineralization zones, as well as field relationships, can be used to fit the origins of the mineralization zones with crystallization of a iron-rich residual liquid at the margins of the equigranular monzonite.