22 research outputs found
Breccia formation during intrusion of a dolerite sill: an example from Sheffield Beach, KwaZulu-Natal North Coast, South Africa.
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Structural evolution and tectonic context of the Mfongosi Group, Natal thrust front, Tugela terrane, South Africa
No full textCharacterisation of grain-size, shape and orientation of plagioclase in the Rooi Rand dyke swarm, South Africa
No full textInternational audienceMagmatic (type-A) fabric co-exists with dyke-orthogonal (type-B) fabric in both the plagioclase and opaque grain fractions in dykes of the Rooi Rand dyke swarm (RRDS). We present new data from the RRDS pertaining to the size, shape, texture and orientation of plagioclase. Texturally, the samples range from intersertal to sub-ophitic and phenocrystic (plagioclase-phyric). More than 90% of plagioclase grains are <33μm in size and the modal size is 12.3μm. The smallest, most abundant grains have modest shape ratios (1.4
When an 'inverse' fabric is not inverse: An integrated AMS-SPO study in MORB-like dykes
No full textInternational audienceMagma flow direction is recorded in the macroscopic mineral petrofabric and magnetic fabric of the MORB-like Rooi Rand dyke swarm. The bulk magnetic fabric is sub-parallel to the dyke plane, resulting from preferred orientation of grains during magma flow. This is referred to as type-A fabric. A study of mineral shape preferred orientation reveals an additional fabric, which is orthogonal to the average dyke orientation in 30% of the data. This fabric is carried by both plagioclase and opaque grains and would typically be interpreted as 'inverse'. It is referred to here as type-B fabric. However, because it is carried by the shape preferred orientation of macroscopic grains and is in most cases not coaxial to the anisotropy of magnetic susceptibility fabric, it is interpreted as having been acquired during increased grain interaction during late-stage magma flow associated with decreasing magma pressure
Significance of magnetic and petrofabric in Karoo-feeder dykes, northern Lebombo
No full textInternational audienceThe orientation of magnetic and petrofabric in 14 mafic dykes of the Northern Lebombo dyke swarm (NLDS) has been determined using the anisotropy of magnetic susceptibility (AMS) and mineral shape preferred orientation (SPO) in order to constrain the magma flow direction during dyke emplacement. The N-S striking NLDS intruded the N-S trending; 700. km long Lebombo faulted monocline between 182 and 178. Ma. The Lebombo faulted monocline, which forms the eastern-most sub-province of the Karoo large igneous province (LIP), is essentially a volcanic rifted margin with a protracted magmatic and tectonic history related to the break-up of southern Gondwana. It also forms the southern limb of the Karoo triple-rift system which has been hypothesised to have formed by a mantle plume centred on the triple junction locus. The other two arms of the rift which converge on the triple junction are the WNW trending Okavango dyke swarm (ODS) and NE trending Save-Limpopo dyke swarm (SLDS). In the NLDS it is found that the magnetic fabric, which is of magmatic origin in ~. 50% of the samples, is carried predominantly by stoichiometric magnetite. There is remarkable agreement between the orientations of the AMS and SPO fabrics and at least two dykes show coaxial AMS and mineral SPO fabric consistent with lateral magma flow from the north. This direction is supported by the imbrication of the foliations of the well-defined bulk AMS and plagioclase SPO fabric when viewed in a dyke co-ordinate system. Although steeply plunging maximum axes occur in the SPO fabric, their possible use as magma flow vectors is questioned because they do not necessarily mimic the elongation of plagioclase grains visible in orientated thin sections. The magnetic and petrofabric of the NLDS is interpreted to have developed during lateral magma flow from the locus of the Karoo triple junction
Distinct brief major events in the Karoo large igneous province clarified by new 40Ar/39Ar ages on the Lesotho basalts..
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Distinct brief major events in the Karoo large igneous province clarified by new 40Ar/39Ar ages on the Lesotho basalts..
No full textInternational audienc
