Magmatism and rifting at the South Atlantic margin: Magma transport and emplacement mechanisms of mafic dykes from magnetic studies

The Henties Bay Outjo Dyke swarm (HOD) in NW Namibia is part of the early Cretaceous Paraná-Etendeka Large Igneous Province, which originated during the early Cretaceous breakup of western Gondwana into Africa and South America. The basaltic dykes consist dominantly of dolerites, which are compositionally equivalent to the erupted lava series and thus the HOD provides a look at the feeder systems of a flood basalt province. The dyke orientations are subvertical and most have NE-SW and minor NW-SE oriented strike directions parallel or perpendicular to the Neoproterozoic Damara Belt in which they intruded.
A petrofabric study is presented for 41 HOD dykes using the anisotropy of low-field magnetic susceptibility (AMS) with the aim to derive magma flow directions and thus better constrain emplacement mechanisms within the dyke swarm. Additionally, the anisotropy of the anhysteretic remanent magnetization (AARM) is applied to a subset of theses samples in order to detect disturbances by grain size effects on the magnetic fabric due to single-domain titanomagnetite. For individual samples also the shape-preferred orientation of plagioclase was determined for comparison with the magnetic fabric. ... mehr

At the contact with the country rock, the HOD dykes have fine-grained, vitrophyric to intersertal or glassy textures. A more or less strong preferred orientation of the phenocrysts can be recognized microscopically and is interpreted as a magmatic flow fabric. The microfabrics of the dykes are characterized by intergranular to ophitic or subophitic intergrowths of plagioclase and clinopyroxene, locally with minor contents of olivine and/or hornblende. The magnetic susceptibility (к) in the dyke rocks is predominantly controlled by late-crystallized titanomagnetite (tmt; used for solid solutions between the end members ulvospinel and magnetite), which is associated and locally intergrown with ilmeno-haematite. Tmt is the main carrier mineral of the AMS, but also of the AARM in most of the HOD rocks. Strong variations of the magnetic susceptibility are attributed to subsolidus processes during magma cooling such as high- and low-temperature oxidation of tmt. High-temperature (deuteric oxidation) is indicated by lamellae of ilmenite whereas low-temperature oxidation (maghemitization) is ob-vious from irreversible к-T curves. Both types of alteration are typically accompanied by grain size reduction of tmt. Secondary minerals such as carbonate, sericite or pseudomorphic replacement of tmt and ilmenite by leucoxene are frequent and regarded as the product of hydrothermal alteration. From hysteresis properties, alternating field demagnetization and from the comparison of AMS and AARM fabric ellipsoid orientation it is concluded that tmt occurs in various domain states within the samples from single-domain over pseudo-single domain to multi-domain states.
Magnetic susceptibility and its anisotropy in the dykes is mainly controlled by distribution anisotropy of titanomagnetite grains at boundaries and interstices of the flow-oriented silicate phenocrysts (plagioclase and pyroxene). The anisotropy is mainly due to magnetostatic interaction between very closely- spaced tmt grains. The degree of anisotropy is low in most samples with less than 10% (P’ < 1.1), supporting a primary magmatic origin. This is in accordance with the lack of evidence for metamorphic or brittle tectonic overprint.
In 66 of 110 investigated samples the AMS fabric is “normal”, with the long and intermediate fabric axes (= plane of magnetic foliation) within the dyke plane. These rock fabrics are interpreted as flow fabrics where early crystallized minerals are oriented by magma flow. Therefore, the magma flow direction is inferred from the orientation of the long magnetic fabric axis (кmax) in samples with fabric type I taken from the dyke margin. The flow directions are mostly vertical to subvertical regardless of location near or distant from the former rifted margin. This implies generation of mantle melts beneath the entire HOD region (c. 300 x 100 km). Horizontal кmax orientation is less frequent and is interpreted as local variation of the magma flow direction (horizontal) within the dyke segments or at the segment tips.
The “anomalous” magnetic fabrics of the remaining 38 samples, where the magnetic foliation is non-parallel with the dyke plane, are attributed to two mechanisms. One is the pure single-domain effect of very fine-grained titanomagnetite, which was found in 6 cases by reorientations of anomalous AMS fabrics to normal-type AARM fabrics.
The more common cases of anomalous fabric are probably related with rotation of the AMS axes by tectonic shear at the dyke walls during magma emplacement. Dyke-internal shear, generated by the velocity gradient of the magma flow between dyke walls and centre, produces symmetric imbrication of long AMS fabric axes кmax on opposite dyke walls (observed in 3 dykes). The dyke-external tectonic shear is transferred to the magma, deforming the symmetric imbrication and producing an asymmetric one. Field support for syn-emplacement tectonic shear is given by dyke segmentation geometries including locally curved segments, en-echelon arrangements, left/right-stepping displacements and lineation at the dyke walls. In the regional context, syn-emplacement shear is consistent with the observed reactivation during Gondwana breakup of Neoproterozoic transcrustal shear zones in the Damara mobile belt.
Provided that both the NE- and the NW-striking dykes intruded during early Cretaceous times, which is suggested by radiometric dating and the age relations of mutually cross-cutting NE- and NW-dykes, crustal extension must have occurred at least in two perpendicular directions to emplace them: NW-SE and NE-SW. Magma overpressure in the intruding dykes is suggested to have caused additional stresses in the country rock that influenced the orientation of adjacent dyke fissures. The dyke orientations and timing are consistent with a three-armed rift system (triple junction geometry), where the Damara Belt as the NE-striking inland arm becomes inactive while dyking and rifting continued along the coast-parallel Gariep and Kaoko Belts, finally opening the South Atlantic Ocean. Pre-existing NE-striking Panafrican structures of the Damara-Orogen likely facilitated magma intrusion in the early Cretaceous which may explain the predominance of the NE dykes.
The dominance of vertical and subvertical magma flow orientation suggests magma sources existed over a wide area beneath the dyke swarm. Published pressure estimates from clinopyroxene-melt thermobarometry of HOD dykes suggest reservoir depths of 11 to 17 km. From these mid-crustal reservoirs magma ascended steeply in propagating main fissures up to the present outcrop level (and beyond), which was then in a brittle environment at 4 to 5 km of depth. While propagating, individual dyke segments inflated due to continued magma injection. The less common horizontal кmax orientation near the segment tips is interpreted as lateral propagation of one dyke segment to the adjacent one leading to their final merging. Field observation of overlapping segment tips or broken country rock “bridges”, which previously separated two dyke segments supports this mode of dyke propagation.