Mineral Lamination Development in Layered Gabbros of the British Palaeogene Igneous Province: A Combined Anisotropy of Magnetic Susceptibility, Quantitative Textural and Mineral Chemistry Study

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Abstract

Quantitative textural analyses including anisotropy of magnetic susceptibility (AMS) and crystal size distribution measurements were combined with detailed field structural measurements and major element profiling of cumulus plagioclase crystals to study the development of igneous textures in the ‘Fluxion Gabbros’ of the Ardnamurchan Centre 3 intrusion and the Druim Hain Layered Gabbro, Isle of Skye, both in NW Scotland. Both lithologies studied exhibit mineral lamination (sometimes intensely developed), historically attributed to magma flow at a melt–crystal mush interface. Unlike the Ardnamurchan laminated gabbros, the Druim Hain gabbros lack evidence of sedimentary structures or visible linear alignments of mineral grains on planes of lamination. Syn-magmatic deformation is common in the Ardnamurchan rocks, suggesting movement of unconsolidated cumulate. Plagioclase and clinopyroxene are cumulus in both cases, and magnetite is also cumulus in the Ardnamurchan laminated gabbros. Magnetite occurs as cumulus and intercumulus crystals in the Druim Hain intrusion, in some instances in quantities up to 30 vol. %. Petrographic observation suggests that the Ardnamurchan rocks have undergone a degree of sub-solidus readjustment at grain boundaries. The Druim Hain rocks exhibit an inherited-unmodified impingement texture (with good examples in some cases of melt-present textural equilibrium) although overgrowth on crystals has also occurred in some magnetite-rich samples. Bulk magnetic susceptibilities at both localities are generally (10–230) × 10–3 (in SI system), consistent with a magnetite-dominated susceptibility. The magnetic foliation in each locality is parallel to the visible mineral lamination in almost all samples. AMS measurements on the Ardnamurchan gabbros reveal that magnetic lineations are oriented confocally around the intrusion; that is, consistently plunging downdip on lamination planes. These AMS data are supported by plagioclase and magnetite lineation orientations measured by image analysis on thin-sections cut parallel to the mineral lamination. On the other hand, a lack of visible lineations in the Druim Hain rocks is further emphasized by randomly oriented lineations returned by the AMS and image analysis techniques. Crystal size distribution (CSD) and mineral chemical data suggest the presence of a single population of plagioclase crystals at both localities, with evidence of a simpler crystallisation history at Druim Hain. Our combined datasets indicate that the Ardnamurchan fabrics formed in the crystal mush, and have subsequently been affected (mildly degraded) by sub-solidus recrystallization. In contrast, the Druim Hain fabrics may have formed through primary in situ crystallization. Compaction has affected some of the Druim Hain samples, giving rise to solid-state deformation features and a coarser grain size. No correlation is observed between the strength of the AMS fabric and the volume of magnetite present in the Druim Hain gabbros, suggesting that clinopyroxene and plagioclase control the shape of magnetite crystals, and hence the AMS fabrics.

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