Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

The Bushveld and Stillwater Complexes contain most of the world’s platinum-group element resources and it has long been recognized that their cumulate stratigraphies show similarites, which suggests that the magmas they formed from were similar with respect to major element compositions. At both localities a magma rich in SiO2 and MgO is needed to produced the orthopyroxene rich lower parts of the intrusions. Chill margins with the composition of Mg-rich basaltic andesite are present at both localities (the B-1 and Gp-3 chills). To produce the gabbroic middle zones of the intrusions, melts close to tholetiitic basalt composition are required. Chills with these compositions are found at both intrusions (B-2 and Gp-1 chills). Recent studies on Bushveld have provided information on the trace element content of the Bushveld magmas and cumulates indicating that the magmas were strongly enriched in LILE and LREE (Fig. 1). We have determined the trace element content of the equivalent magmas from Stillwater and they are poorer in LILE and LREE. The Mg-rich andesite (Gp-3) are closer to boninite compositions than the Busheld B-1 rocks. The tholeiitic basalt (Gp-1) show some similarities to arc tholeiites.

Fig. 1 Mantle normalized incompatible element plot of the marginal rocks of the Bushveld and Stillwater compared with modern magmas.