Microstructural evidence for crystallization regimes in mafic intrusions: a case study from the Little Minch Sill Complex, Scotland

Nicoli, Gautier; Holness, Marian B.; Neufeld, Jerome A.; Farr, Robert

doi:10.1007/s00410-018-1525-7

Cited by 7 publications

(18 citation statements)

References 43 publications

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“…Within the crinanite, there are a series of horizontally elongate and laterally extensive gabbro lenses up to 10 cm in thickness ( Figures 2B,C). These are similar in composition to pegmatitic gabbro lenses mentioned by other authors who have published on the area (e.g., Gibson and Jones, 1991;Nicoli et al, 2018). There are also occasional amygdale rich bands throughout the thickness of the sill (Figure 2Bii).…”

Section: Geological Settingsupporting

confidence: 81%

“…These proportions of olivine indicate that the sill lithology straddles the boundary between crinanite and picrodolerite compositions based on the classification of Gibb and Henderson (1984). However, in this paper we will refer to it as crinanite for ease of comparison with other studies in the area (e.g., Gibson and Jones, 1991;Nicoli et al, 2018).…”

Section: Composition and Petrographic Fabricsmentioning

confidence: 97%

“…This is similar to the Shiant Isles picrodolerite and crinanite sills (Gibb and Henderson, 1996;Holness et al, FIGURE 11 | Schematic diagram of initial sill emplacement as magma fingers which propagated to the SSE (A), which subsequently coalesced with diverted flow direction towards the NE (B). 2017) and the Dun Raisburgh crinanite sill of the Isle of Skye (Nicoli et al, 2018).…”

Section: Crystallization Historymentioning

confidence: 99%

“…Analcime is present as vesicle infill. Previous work in the area does not mention a crystal cargo within the crinanite at Inver Tote (e.g., Gibson and Jones, 1991), however, it is present in other sills in the region such as the Shiant Isles picrodoleritie/crinanite unit (Holness et al, 2017) and the Dun Raisburgh crinanite (Nicoli et al, 2018) which have an approximant 3% crystal cargo consisting of olivine and plagioclase.…”

Section: Geological Settingmentioning

confidence: 99%

“…There is good evidence that these relatively large volumes of magma can cool by convection (e.g., Holness et al, 2017). Evidence of convection includes asymmetrical changes in chemistry, mineralogy, crystal preferred orientation and grain size distribution across the breadth of an intrusion (e.g., Holness and Humphreys, 2003;Holness et al, 2017;Nicoli et al, 2018).…”

Section: Introductionmentioning

confidence: 99%

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The Origin and Evolution of Magnetic Fabrics in Mafic Sills

et al. 2019

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Studying extinct volcanoes where erosion has exposed dykes and sills provides direct access to the fossil remnants of magma movement, however, linking crystallized magma to emplacement dynamics is challenging. This study investigates how magma flow varies across the thickness of a thin (6 m thick) mafic sill. We use a high-resolution sampling regime to measure micro-scale variations in magnetic anisotropy, which is associated with the orientation of the magnetic particles present within the crystalline rock. Fieldwork was conducted on exposed sills of the British and Irish Palaeogene Igneous Province, Isle of Skye, Scotland. Here Jurassic sedimentary rocks have been intruded by a series of sills, of picrite to crinanite composition, from the Little Minch Sill Complex (c.60 Ma). Anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM) signals have been used to separate a crinanite sill into distinct magnetic groupings. We identified two AMS groups (the upper and lower sill margins, and the central region) and four AARM groups (the lower margin, the middle region, a region just below the upper margin, and the upper margin). Both AMS and AARM signals originate from titanomagnetite of multi-domain or vortex-state to single-domain sized grains, respectively. The AMS and AARM fabrics are aligned with each other in the margin regions preserving a history of magma flow toward the North during initial emplacement. However, in the sill interior the magnetic fabrics are oblique to each other, thus reflecting multiple origins. We suggest the AMS fabrics have recorded magma flow during sill growth, and AARM fabrics have recorded melt percolation flow as the interstitial melt migrated upward through a solidifying crystal mush. We demonstrate that when AMS and AARM are used in combination they enable a detailed understanding of magma flow and solidification dynamics to be obtained, from initial emplacement to solidification. Overall, our detailed sampling and analysis indicates that magnetic fabrics can be highly variable over small distances, supporting the suggestion of horizontal flow restriction and propagation path migration within growing sills, and that previous reports of magma flow and solidification dynamics based on under-sampled bodies may require reconsideration.

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Section: Geological Settingsupporting

confidence: 81%

Section: Composition and Petrographic Fabricsmentioning

confidence: 97%

Section: Crystallization Historymentioning

confidence: 99%

Section: Geological Settingmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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The Origin and Evolution of Magnetic Fabrics in Mafic Sills

et al. 2019

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The Hebridean Igneous Province plumbing system: A phase equilibria perspective

Nicoli

Matthews

2019

Lithos

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Pyroxenite Xenoliths Record Complex Melt Impregnation in the Deep Lithosphere of the Northwestern North China Craton

Dai

Zheng

Griffin

et al. 2020

Journal of Petrology

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Transformation of refractory cratonic mantle into more fertile lithologies is the key to the fate of cratonic lithosphere. This process has been extensively studied in the eastern North China Craton (NCC) while that of its western part is still poorly constrained. A comprehensive study of newly-found pyroxenite xenoliths from the Langshan area, in the northwestern part of this craton is integrated with a regional synthesis of pyroxenite and peridotite xenoliths to constrain the petrogenesis of the pyroxenites and provide an overview of the processes involved in the modification of the deep lithosphere. The Langshan pyroxenites are of two types, high-Mg# [Mg2+/(Mg2++Fe2+)*100 = ∼ 90, atomic ratios] olivine-bearing websterites with high equilibration temperatures (880 ∼ 970 oC), and low-Mg# (70 ∼ 80) plagioclase-bearing websterites with low equilibration temperatures (550 ∼ 835 oC). The high-Mg# pyroxenites show trade-off abundances of olivine and orthopyroxene, highly depleted bulk Sr-Nd (ƐNd = +11.41, 87Sr/86Sr = ∼0.7034) and low clinopyroxene Sr isotopic ratios (mean 87Sr/86Sr = ∼0.703). They are considered to reflect the reaction of mantle peridotites with silica-rich silicate melts derived from the convective mantle. Their depletion in fusible components (e.g., FeO, TiO2 and Na2O) and progressive exhaustion of incompatible elements suggest melt extraction after their formation. The low-Mg# pyroxenites display layered structures, convex-upward rare earth element patterns, moderately enriched bulk Sr-Nd isotopic ratios (ƐNd = -14.20 ∼ -16.74, 87Sr/86Sr = 0.7070 ∼ 0.7078) and variable clinopyroxene Sr-isotope ratios (87Sr/86Sr = 0.706-0.711). They are interpreted to be crustal cumulates from hypersthene-normative melts generated by interaction between the asthenosphere and heterogeneous lithospheric mantle. Combined with studies on regional peridotite xenoliths, it is shown that the thinning and refertilization of the lithospheric mantle was accompanied by crustal rejuvenation and that such processes occurred ubiquitously in the northwestern part of the NCC. A geodynamic model is proposed for the evolution of the deep lithosphere, which includes long-term mass transfer through a mantle wedge into the deep crust from the Paleozoic to the Cenozoic, triggered by subduction of the Paleo-Asian ocean and the Late Mesozoic lithospheric extension of eastern Asia.

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Microstructural evidence for crystallization regimes in mafic intrusions: a case study from the Little Minch Sill Complex, Scotland

Cited by 7 publications

References 43 publications

The Origin and Evolution of Magnetic Fabrics in Mafic Sills

The Origin and Evolution of Magnetic Fabrics in Mafic Sills

The Hebridean Igneous Province plumbing system: A phase equilibria perspective

Pyroxenite Xenoliths Record Complex Melt Impregnation in the Deep Lithosphere of the Northwestern North China Craton

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