Field and microstructural evidence demonstrating the interplay between seismicity and the emplacement of lamprophyres in the dry mid to lower crust

Lamprophyre dykes are enigmatic volatile-rich, mantle-derived igneous melts that are often contemporary with lithospheric extension. Despite a rich literature on the petrology and geodynamic implications of lamprophyre intrusions into the continental crust, the emplacement mechanisms of these dykes (i.e. structural relationship with the host rock and structures, mode of intrusion, speed of magma ascent, etc) into the dry mid–lower crust is poorly constrained.

In the Jotun Nappe of south-central Norway, Proterozoic gabbro gneisses are overprinted locally by mutually crosscutting lamprophyre dykes, pseudotachylytes (coseismic-derived quenched frictional melts), and mylonitized pseudotachylytes. Mylonitized pseudotachylytes form networks of small-scale (cm- to dm-scale) ductile shear zones, orientated in roughly three sets of orientations, that separate relatively undeformed gabbro gneiss blocks, while pristine pseudotachylytes dissect these blocks and are bounded by the ductile shear zones – akin to observations from Lofoten, Norway (e.g. Jostling Block; Campbell et al., 2020). Pseudotachylytes and mylonitized pseudotachylytes have similar mineral assemblages containing plagioclase, K-feldspar, clinopyroxene, amphibole, Fe-Ti-oxides, with the mylonitized versions also containing garnet porphyroblasts and biotite in addition. Lamprophyre dykes (<1.5m wide), strike dominantly NW-SE, are either undeformed or are incorporated into viscous shear zones that are comprised primarily of mylonitized pseudotachylytes. Many of the undeformed lamprophyres show some amount of viscous shearing localized to <5 cm at the contact with the host rock, otherwise pristine undeformed dykes display primary igneous fabrics and textures. Injection veins of the dyke into the host rock are common, while dyke tips form sharp <45° indentations into the gabbro gneiss. The host rock around jogs is bleached and exhibits numerous small shear fractures filled with dyke material that can easily be misidentified for pseudotachylytes. Lamprophyres have a matrix composed of biotite, plagioclase, dolomite, orthopyroxene, amphibole, Fe-Ti-oxides, and apatite with xenocrysts of orthopyroxene surrounded by a corona of clinopyroxene, amphibole, biotite. Pristine pseudotachylytes crosscut the dykes, offsetting them by up to an apparent ~50 cm and dragging dyke material along the length of the pseudotachylyte surface.

Structural relationships between mylonitized pseudotachylytes and pseudotachylytes suggest that viscous creep along the shear zone network concentrated stresses towards the interior of the gabbro gneiss blocks, which resulted in failure of the blocks and the formation of pristine pseudotachylytes (Zertani et al., 2025). Because the dominant orientation of the lamprophyre dykes is orthogonal to the most dominant orientation of the ductile shear zones, we suggest the lamprophyres exploited transient crustal weaknesses caused by the stress drops during rupturing of the blocks, which created permeably fracture networks for the dykes to ascend through the gneiss. This study demonstrates through field and microstructural observations that lamprophyre intrusions are fundamentally linked to seismicity in the dry mid–lower crust.

Campbell et al., (2020). Nature Communications,  https://doi.org/10.1038/s41467-020-15150-x

Zertani et al., (2025). Geophysical Research Letters,  https://doi.org/10.1029/2024GL114350