|
Department of Geology, University of Leicester, Leicester LE1 7RH, UK
School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73019, USA
The mineral chemistry, petrology and geochemistry of representative members of the Scourie dyke swarm are described, and discussed in relation to crystallization processes in deep-seated dykes and to their petrogenesis. Four types of dykes can be defined on petrological and geochemical grounds: bronzite-picrites, norites, olivine-gabbros and quartz dolerites. The latter constitute by far the most abundant dyke-type. In the central Scourian granulite zone in particular the dykes were emplaced at depth into hot country rocks; this enables the through-flow behaviour of different magma types in dyke conduits to be compared. Whereas the dolerites and norites have chilled margins, the bronzite-picrites and olivine-gabbros have coarse grained orthopyroxene-rich and augite-rich margins respectively. This probably reflects turbulent-flow conditions in the less viscous, more ultramafic magmas which permitted rapid crystallization of pyroxenes on the dyke walls during intrusion. The picrite dykes, and to a lesser extent the olivine-gabbro dykes, display additional across-dyke modal and textural variations. These are not accompanied by significant variations in mineral composition, and can largely be attributed to crystal settling in variably inclined dyke sheets.
All dykes typically show a ‘continental’ trace element signature (enrichment in light rare-earth elements and large-ion-lithophile elements), but this must have been inherited from the sub-continental lithosphere because contamination by Lewisian gneisses would not generate the observed trace element characteristics. Both trace element and mineral chemical data indicate that the dykes were derived from at least two distinct mantle sources, that supplying the picrite and norite dykes being more refractory with respect to major elements, but showing a greater relative enrichment in light REE and LIL elements.
The emplacement of the Scourie dyke swarm represents considerable crustal extension, but also poses a thermal problem in the generation of high temperature picritic magmas. It is suggested that the dykes and their compositional characteristics reflect the processes of growth and evolution of the sub-continental lithosphere, and that the extensive retrogression of the granulites, which occurred penecontemporaneously with dyke intrusion, may also be linked with these processes.
This article has been cited by other articles:
 |
|
 |
|
  References Geological Society, London, Memoirs, 2002; 26: 73 - 76. [PDF]
|
|
|
|
|
|
R. P. Hall and D. J. Hughes Early Precambrian crustal development: changing styles of mafic magmatism Geological Society, London, Memoirs, 1995; 16: 25 - 35. [Abstract] [PDF]
|
|
