Magnetic fabric and emplacement of dykes of lamprophyres and related rocks of the Central Bohemian Dyke Swarm (Central European Variscides)

Abstract

Numerous dykes of lamprophyres and various types of granitoid, syenitoid, dioritoid, and gabbroid porphyries of Variscan age crop out in the area of the Central Bohemian Plutonic Complex and adjacent high-to low-grade metamorphic units of the Moldanubian Zone and Teplá-Barrandian Unit. Magnetic fabric in fourteen dykes of lamprophyres and related rocks was investigated. Mostly, the magnetic foliation is roughly parallel to the dyke plane and the magnetic lineation is horizontal with the relics of originally steep fabrics. This type of magnetic fabric originated through magma flow in which the larger surfaces of the magnetic minerals were oriented parallel to the dyke plane and their longer dimensions were parallel to the magma flow. In two localities, the so-called inverse fabrics were found in which the maximum and minimum susceptibility directions were swapped. The dykes of lamprophyres and related rocks were emplaced into already juxtaposed and cooled Teplá-Barrandian Zone, Central Bohemian Plutonic Complex and western Moldanubian Zone not later than 339 Ma. Parallel orientation of dykes giving a steep intrusive contacts mainly in W(NW)-E(SE) trend was caused by the regional stress field of ~WNW-ESE convergence (arc-parallel stretching) during the Variscan Orogeny.

Permo-Carboniferous hypabyssal magmatism in northern Portugal: the case of the Lamas de Olo microgranite and lamprophyre dykes

“…When compared to other Variscan lamprophyres (Štemprok et al, 2014), the Lamas de Olo specimen is depleted in Na 2 O and CaO, which is presumably due to the fluid-promoted feldspar alteration (Förster et al, 2007). Previous studies (Oliveira et al, 2019) have shown that the magnetic fabric of this lamprophyre is near-type III (Hrouda et al, 2016) because while the average magnetic lineation is subperpendicular to the dyke trend (40°/N138°E), the average magnetic foliation is oblique (N105°E-57°SW). Such magnetic fabrics have been typically attributed to postmagmatic processes (Raposo & Ernesto, 1995).…”

Simultaneous Free Flow and Forcefully Driven Movement of Magma in Lamprophyre Dykes as Indicated by Magnetic Anisotropy: Case Study from the Central Bohemian Dyke Swarm, Czech Republic

“…The younger MPC is composed of peraluminous (S-type) anatectic granitoids (c. 330-300 Ma), and it was formed by extensive migmatitization and isothermal decompression of the surrounding pelitic rocks. The third group is represented by potassic-ultrapotassic rocks (346-337 Ma) that form several isolated plutons ( Figure 1a) and dyke swarms [8,[36][37][38]. The last group of Variscan igneous rocks have mafic and ultramafic composition (gabbro, norite, troctolite, pyroxenite) and an age of around 340 Ma [39][40][41].…”

“…The structural aspects of these processes have been investigated both from the theoretical and practical points of views (see, for example, [2,3] and references therein). These investigations used various indicators of magma flow, which are unfortunately scarce, and the anisotropy of magnetic susceptibility (AMS), which can be easily measured on almost all types of dyke rocks (e.g., [4][5][6][7][8][9][10][11][12][13][14]).…”

“…Much less information is available about magnetic fabric in potassic-lamprophyre dykes (e.g., [19,20]) with source magma generated in the mantle depths (e.g., [21][22][23][24]). Among the potassic-ultrapotassic dykes of the Central Bohemian Dyke Swarm, Hrouda et al [8] revealed the first three of the above four magnetic fabric types. Interestingly, there is one dyke, the composite lamprophyre dyke near the town of Dobříš, where all three magnetic fabrics can be found: normal in the central part, and intermediate and even inverse magnetic fabrics in the other parts of the dyke.…”

Geophysical characterization and modeling of a Neoproterozoic intrusive volcaniclastic lamprophyre in southernmost Brazil

“…Studies concerning the physical properties of lamprophyric rocks (sensu Le Maitre 2002) in the literature are rare (e.g., Rao 2008, Nishad et al 2013, Hrouda et al 2016, while studies applying geophysical techniques are almost non-existent (e.g., Machek et al 2014). Considering the relationship between lamprophyres and some types of ore deposits and the frequent use of gamma spectrometric, magnetometric, and gravimetric surveys in the early stages of prospection, this work provides results that can be useful in exploration.…”