The Scourie dyke suite: Petrogenesis and geochemical nature of the Proterozoic sub-continental mantle |
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| Authors: | B L Weaver J Tarney |
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| Institution: | (1) Department of Geology, University of Leicester, LE1 7RH Leicester, Great Britain |
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| Abstract: | The petrogenesis of bronzite-picrite, olivine-gabbro, norite and quartz-tholeiite dykes, which make up the 2.39 b.y. Scourie dyke swarm cutting the Archaean Lewisian gneisses of N.W. Scotland, is interpreted on the basis of their major and trace element geochemistry. Most of the dykes bear primary amphibole and/or phlogopite and, with one exception, are all hypersthene- or quartz-normative. Apart from one tholeiite dyke which shows relative light rare-earth element depletion, all the dykes show enrichment in light rare-earths and large-ion lithophile elements. They do not however show an equivalent enrichment in other incompatible high field strength ions such as Nb and Ta, and in this respect resemble island arc and calc-alkaline basalts. The different dyke types have distinctive rare-earth patterns and other trace element ratios which are maintained over a range of major element compositions.Petrogenetic modelling of the major and trace element compositions of the various dykes demonstrates that very few can be related by fractional crystallisation. Indeed, even with partial melting mechanisms at least two different mantle sources, with different major and trace element compositons, are required to explain the compositional differences between the major dyke types. The high degrees of mantle melting implied for the generation of the magnesium-rich bronzite-picrites suggests that their rare earth and other trace element patterns closely reflect those of their mantle source. Similar arguments, though less well constrained, can be advanced for the other dyke types. The results suggest that the sub-continental mantle source feeding the dykes was heterogeneous with respect to both major and trace elements, and that their mantle sources must have been enriched in lithophile elements. Enrichment at the time the Lewisian gneisses were generated (i.e. 2.92 b.y. ago) would be compatible with the initial 87Sr/86Sr ratios of the dykes and the inferred Rb/Sr ratios of their mantle sources. The sub-continental mantle sources have thus retained the geochemical signature of the crustgenerating processes some 500 m.y. earlier. |
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