Structural and Geochemical Interactions Between Magma and Sedimentary Host Rock: The Hovedøya Case, Oslo Rift, Norway

Abstract

Two end‐member conceptual models are used to describe deformation of the Earth's crust induced by magma intrusion. “Mode I” fracturing assumes tensile or opening‐mode, elastic deformation, while “Mode II” fracturing assumes plastic shear‐mode deformation around a viscous indenter. Field observations of both mechanisms exist, but it remains unclear which mechanism dominates in which conditions. We describe intrusion geometries, host rock deformation, and geochemical magma‐host rock interactions around 53 exceptionally preserved, tephrite‐basanite Permian dike segments of 0.5‐ to 30‐cm thickness. These thin dikes, that is, “dikelets,” intruded Late‐Ordovician carbonate‐rich sedimentary rocks on Hovedøya island, Oslo Rift, Norway. Dikelets emplaced in preexisting fractures dominantly created cavities ahead of their narrow, tapering tips and are associated with bent host rock, broken bridges, and stepped segmented geometries. Other tips are blunt with dense brittle fracturing around them. Also, cross‐sectional intrusion segment opening profiles deviate from parabola‐shaped profiles typical for elastic media. The observations demonstrate that dominant opening‐mode host rock deformation can coexist with shear‐mode deformation locally. Alignment of most dikelet segments along the dominant host rock fracture directions highlights the control of local structural orientations on magma emplacement. Analysis of bulk major and trace element compositions, in situ micro‐XRF sample analysis and carbon and oxygen stable isotope compositions, suggests that thermochemical interactions between magma and the carbonate‐rich host rock produced a low‐viscosity mixture of magma, pore water, and gas. We propose that such low‐viscosity hybrid fluid may assist in the intrusion of magma in sedimentary rocks by filling the cavity ahead of propagating sheet intrusion tips.

Emplacement and Segment Geometry of Large, High-Viscosity Magmatic Sheets

“…The low values of thickness-to-length aspect ratios of sheet intrusions have been interpreted as indicators of emplacement as Mode I tensile fractures [2,[17][18][19][20]. In agreement with this model, field observations of sharp intrusion tip geometries have been mostly described for low-viscosity magma [18,[21][22][23]. However, observed tips of high-viscosity magma intrusions exhibit blunt shapes [21,[24][25][26], suggesting another emplacement mechanism than Mode I tensile fracturing [21,24,27].…”

Segment tip geometry of sheet intrusions, II: Field observations of tip geometries and a model for evolving emplacement mechanisms

“…We can consider the potential for post-emplacement overprinting through detailed field-based textural and structural characterisation of the host rock and intrusion [e.g., Bons et al, 2004]. Few studies have focused on syn-emplacement variations in host rock and magma properties, which may cause a transition in the intrusion tip geometry and emplacement mechanism [Poppe et al, 2020]. Tapered through to superelliptical sill segments outcrop in the Neist Point study area, within no correlation between host rock lithology and emplacement mechanism ( Fig.…”

“…An issue that is generally omitted in intrusion growth models, is that magma and host rock properties can change during the lifetime of an intrusive event [Poppe et al, 2020].…”

“…Tensile failure is therefore inhibited, and the segment tip may inflate to accommodate the driving pressure, producing a rounded tip geometry [e.g. Cañón-Tapia and Merle].Few studies have documented the presence of tapered and rounded segment geometries in the same study area [exceptions includeBaer, 1991;Kjøll et al, 2019, Poppe et al, 2020 Baer [1991]. suggests that dykes may propagate in a poorly cemented sandstone by a cyclic process of brittle fracture and viscous fingering that depends on the availability of magma-related fluids and local resistance of the host rock to fluidization.…”

Segment tip geometry of sheet intrusions, I: Theory and numerical models for the role of tip shape in controlling propagation pathways.

“…Intrusion segments are generally treated as representing the staged growth of through-going sheets, with isolated segments inferred to represent the earliest stages of the process (Delaney and Pollard, 1981;Baer and Reches, 1987;Rickwood, 1990;Schofield et al, 2012), before linkage to create the sheet-like form of sills, dykes, or inclined sheets. Field-based observations of intrusive segment tips tend to be the lateral edges of the segments as viewed in the axis of bulk magma transport (e.g., Galland et al, 2019), rather than the terminal end tip region at the distal extremities of the segment (e.g., Poppe et al, 2020). In either case, observations of segment tips are of a system that did not grow further than the observed state, and therefore represent the arresting form of segments, rather than the growing form.…”