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

Abstract

A composite lamprophyre dyke from the Central Bohemian Dyke Swarm (Czech Republic) shows both indications of magma free flow (normal magnetic fabric with magnetic foliation and lineation parallel to the dyke plane) as well as those of forcefully driven magma movement (intermediate and inverse magnetic fabrics with magnetic foliation perpendicular to the dyke plane). The overall characteristics of the magnetic parameters across the dyke indicate the existence of at least two slightly differing parts that probably represent two magma pulses. The marginal part of the dyke is formed by kersantite, while toward the axial part, the composition gradually changes to spessartite, and obtains an increasing degree of amphibolization. The rocks of the dyke are inhomogeneous, both compositionally and structurally. It is likely that some portions of ascending magma were more viscous than the others, and the magnetic minerals in the more viscous magma portions may have oriented according to their longer dimensions perpendicular to the dyke, creating an inverse fabric. The lengthening perpendicular to the dyke was compensated by the vertical escape of neighboring more fluid magma, creating a normal magnetic fabric. The frequent oblique magnetic fabrics may represent transitions between the above two mechanisms.

Interpreting Inverse Magnetic Fabric in Miocene Dikes From Eastern Iceland

“…It may also happen that at the dike margins, due to the higher friction and faster cooling, the flow slows down creating the conditions for a locally laminar flow while the dike center is still under turbulent or nonlaminar conditions. Additionally, turbulence or nonlaminar flow conditions may also be created by several other local factors, such as magma propagation within irregular fractures (Gudmundsson, 1983; Kissel et al, 2010), stiffness contrasts in the country rocks (e.g., Geshi et al, 2012; Gudmundsson, 2002; Gudmundsson & Loetveit, 2005; Gudmundsson & Philipp, 2006), and compositional zonation (Eriksson et al, 2011; Hrouda et al, 2019). Even though the latter phenomena may affect the orientation of the magnetic crystals and therefore the overall AMS results, we can exclude any influence of peculiar local dike conditions thanks to the sampling strategy: in the sampling sites the dikes do not show any of above‐mentioned conditions.…”

“…Several complicating factors may be responsible for “anomalous” AMS fabrics. The occurrence of single‐domain (SD) magnetite (Ferré, 2002; Potter & Stephenson, 1988; Rochette et al, 1992), the “distribution anisotropy” of clusters of magnetic grains (Hargraves et al, 1991; Stephenson, 1994), nonflow‐parallel grain alignment by viscous fluid flow (Cañón‐tapia & Chávez‐Álvarez, 2004; Dragoni et al, 1997), a secondary overprinting of primary flow fabric due to late cooling (Almqvist et al, 2012; Ellwood, 1978; Martin et al, 2019; Mattsson et al, 2011), and the existence of two magma pulses of different composition (Hrouda et al, 2019) or tectonics (Eriksson et al, 2014; Kusbach et al, 2019; Park et al, 1988; Soriano et al, 2007) are among the main established processes that may produce such anomalous fabric. Additionally, due to the different crystallization timing of the minerals within the dike or to metasomatism processes, a composite fabric may also be found.…”

Neutron Tomography Studies of Two Lamprophyre Dike Samples: 3D Data Analysis for the Characterization of Rock Fabric

“…Its development is mostly owed to the alignment of magnetite grains at some imbrication angle, with respect to the flow direction (e.g., [ 7 ]). Although numerous works have shown the correlation of measured AMS of dike samples with magma flow direction, there is a manifold in the relation of magnetic ellipsoid axes to the flow direction and dike walls orientation: normal (symmetrical, asymmetrical), intermediate and inverse fabrics (e.g., see an example of a composite lamprophyre dike, studied in [ 8 ]). Moreover, in those situations, when the flow fabric was deformed by regional stresses (e.g., [ 9 ]) or/and the definitive evidence of flow direction in dikes is lacking, the interpretation of AMS measurements, without additional information extracted from other flow indicators, becomes difficult.…”