RESIDUAL LIQUID ORIGINS OF FE-TI-ZR MINERALIZATION IN THE RAFTSUND INTRUSION, ÅRSTEINEN, LOFOTEN-VESTERÅLEN ARCHIPELAGO, NORTHERN NORWAY - INSIGHT FROM MINERAL TRACE ELEMENT COMPOSITIONS AND MACRO TO MICROSCALE TEXTURES

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 suites. The origin of these mineralized rocks is 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 been invoked to explain occurrences of these rocks in AMCG suites.

At Årsteinen in the Lofoten-Vesterålen archipelago Fe-Ti-Zr-enriched zones occur as ≤ 3 cm veins in a mingling zone between equigranular fayalite-clinopyroxene-monzonite and a porphyritic orthopyroxene-clinopyroxene monzonite. The mineralized zones have planar contacts with the porphyritic monzonite, but protrude between mesoperthitic feldspar grains in the equigranular monzonite suggesting the mineralization is related to the equigranular monzonite. The Mg# and Fo% in clinopyroxene and olivine are highest in the mineralization (35.0 and 6.17) compared to the equigranular monzonite (22.2 and 2.50). Higher modal proportions of Fe-Ti-oxides in the mineralized zones, coupled with subsolidus exchange between ferromagnesian phases and Fe-Ti-oxides at high temperatures may promote diffusional exchange, resulting in higher than expected Mg/Fe ratios in these minerals. Lower Sr contents, more negative Eu anomalies, and higher ΣREEs in apatite of the mineralization suggest this phase crystallized in a more evolved magma, and after apatite in the monzonites. Additionally, significant feldspar crystallization could drive the residual liquid to become enriched in HFSEs, which may explain the notably higher modal proportions of zircon in the mineralization. Integrating field relationships with trace element mineral compositions suggest that the Årsteinen mineralization was the result of crystallization of a residual Fe-rich liquid at the margins of the equigranular monzonite.