Journal of Conference Abstracts

The Satellites Dykes, Zimbabwe: Part of the Parental Magma of the Great Dyke?

H. M. Stubbs (stubbsh@geol.port.ac.uk), R. P. Hall (hallp@geol.port.ac.uk) & D. J. Hughes (hughesd@geol.port.ac.uk)

Department of Geology, University of Portsmouth, Portsmouth PO1 3QL, U.K.

The Zimbabwe craton is intruded by mafic dykes and associated sills of many different ages and orientations. The largest of these intrusive events was the emplacement of the Great Dyke at the end of the Archaean (~2460Ma). The Great Dyke is a major ultramafic-mafic intrusion consisting of a wide range of cumulates and no individual part of it can be taken to represent a truly magmatic composition. Thus, in common with other major layered complexes, the nature of its parental magma remains enigmatic.

The Great Dyke has two parallel satellite dykes, the East dyke and the Umvimeela (West) Dyke. The East dyke consists mainly of gabbro whereas the Umvimeela dyke consistently contains orthopyroxene and comprises gabbro and gabbronorite. These much smaller, discontinuous, locally bifurcating, dykes were far less severely affected by crystal settling and layering processes than the Great Dyke and therefore may provide a clearer picture of parental magma compositions of the Great Dyke event. Both of them have quartz tholeiitic compositions but show early magnetite crystallisation and therefore only very weak Fe-enrichment. Consistent incompatible trace element ratios, such as Y/Zr of c.6, suggest dominantly gabbro fractionation-controlled evolution. REE data implies a magma source which was LREE-enriched and HREE-depleted.

The satellite dykes have clearly not undergone the same extreme differentiation and mixing processes associated with large layered intrusions such as the Great Dyke, Bushveld, and Stillwater Complexes and unlike these large-scale complexes, there is no evidence that the Great Dyke satellites were derived from more than one mafic magma. It is postulated that the Great Dyke event was initiated by a late-Archaean mantle plume and whereas the Great Dyke probably comprises mixtures of plume core, margin and local mantle compositions, the satellite dykes were generated by interaction between the plume margin and local mantle.

The Role of a Palaeoproterozoic Mantle Plume in the Fragmentation of the Archaean Megaprovince Kenorland, North America

M. A. Summers¹ (summersm@geol.port.ac.uk), R. P. Hall¹ (hallp@geol.port.ac.uk), D. J. Hughes¹ (hughesdj@geol.port.ac.uk) & G. L. Snyder² (0101-303-236-0214)

¹ Department of Geology, University of Portsmouth, Portsmouth PO1 3QL, U.K.

² U.S. Geological Survey, Federal Center, Denver, CO 80225, USA.

Recent high-precision geochemical and isotopic data suggest that episodes of continental fragmentation and associated magmatism may be caused by ascending mantle plumes impinging on the base of the lithosphere. The Laramie mountains of the southeast Wyoming Province comprise Archaean granite-greenstone terranes which have been penetrated by abundant northeast-trending Palaeoproterozoic (2.1 - 2.01 Ga) mafic and ultramafic intrusives which are collectively referred to as the Kennedy dyke swarm. Petrologically, these comprise tholeiitic metadolerites and noritic to picritic dykes, which despite their continental setting possess unequivocal plume affinities.

A petrogenetic model for the evolution of the Kennedy dyke swarm involves an ascending mantle plume which encroached upon the base of the Sub-Continental Lithospheric Mantle (SCLM). Noritic magmas appear to have been derived by plume-induced melting of a refractory harzburgitic SCLM. Picritic magmas, enriched in incompatible elements, were derived from the hot plume core. Tholeiitic magmas derived from cooler portions of the plume appear to be dominantly represented by enriched MORB-type dykes, but there is a continuum of compositions from incompatible element-enriched, lower mantle-derived melts which exhibit affinities with certain OIB suites (Reunion), through compositions similar to oceanic plateau basalts (Ontong Java Plateau), to depleted upper mantle MORB.

Dykes of a similar age and geochemical diversity (the Kenora-Kabetogama swarm) occur in the neighbouring Superior Province in northern Minnesota and southwestern Ontario. Palaeoproterozoic continental reconstructions suggest that the Wyoming and Superior provinces were part of an Archaean megaprovince (Kenorland), and that the Kennedy dykes may have been contiguous with the Kenora-Kabetogama swarm. These dykes may in turn have been part of a larger, radiating swarm which include the Biscotasing and Marathon swarms of the Superior Province. It is postulated that this episode of plume-related magmatism was responsible for initiating the fragmentation of Kenorland into the now separate Wyoming and Superior Provinces.