Plume magmatism and crustal growth at 2.9 to 3.0 Ga in the Steep Rock and Lumby Lake area, Western Superior Province

The greenstone belts of the western Superior Province are predominantly 2.7 8 to 2.69 Ga and provide evidence of oceanic and are volcanism during the a ccretionary phase of development of the Superior Province. There is also sc attered evidence of Meso-Archean crust (predominantly 2.9 to 3.0 Ga) within the western Superior Province. The Meso-Archean greenstone belts commonly contain platformal sediments and unconformably overlie granitoid basement. The platformal sediments occur associated with komatiitic and tholeiitic vo lcanic rocks that suggest a history of magmatism associated with rifting du ring the Meso-Archean. The central Wabigoon Subprovince is a key area of Me so-Archean crust and in its southern portion comprises the Steep Rock, Finl ayson and Lumby Lake greenstone belts. The Steep Rock greenstone belt uncon formably overlies 3 Ga continental basement and contains platformal sedimen ts succeeded by komatiitic and tholeiitic volcanic rocks. The Lumby Lake gr eenstone belt contains thick sequences of mafic volcanics, a number of koma tiite horizons, and thin platformal sedimentary units. The two belts are jo ined by the predominantly mafic volcanic Finlayson greenstone brit. The vol canics throughout these three greenstone belts may be correlated to some ex tent and a range of basaltic and komatiite types is present. Al-undepleted komatiites present in the Lumby Lake greenstone belt have an Al2O3/TiO2 rat io ranging from 14 to 27 and (Gd/Yb)(N) from 0.7 to 1.3. These are divided into basaltic komatiites with generally unfractionated mantle-normalised mu lti-element profiles, and spinifex-textured high-Mg basalts with slightly l ight REE enriched multi-element profiles and small negative Nb and Ta anoma lies. The unfractionated basaltic komatiites represent high degree partial melts of the upper mantle whereas the spinifex-textured high-Mg basalts rep resent evolutionary products of the komatiite liquids following olivine and chromite fractionation and crustal contamination. Al-depleted komatiites a re present in both the Lumby Lake and Steep Rock belts and have Al2O3/TiO2 ratio ranges from 2.5 to 5. These display strong enrichment in the light RE E and Nb and strong depletion in the heavy REE and Y ((Gd/Yb)(N) = 2-4). Th ey represent a deep mantle plume source generated from a high degree of par tial melting in the majorite garnet stability field. The basaltic flows in all three greenstone belts are predominantly slightly light REE depleted an d represent a slightly depleted upper mantle source. Basalts spatially asso ciated with the unfractionated basaltic komatiites and the slightly Light R EE enriched spinifex-textured high-Mg basalts are also slightly enriched in light REE and have negative Nb and Ta anomalies. These basalts represent e volved products of the primitive basaltic komatiites and enriched spinifex- textured high-Mg basalts after further crustal contamination and olivine an d clinopyroxene fractionation. The geochemical stratigraphy in the Lumby La ke belt is consistent with an ascending mantle plume model. The light REE d epleted basalts were derived from upper mantle melted by an ascending mantl e plume. These are overlain by the unfractionated basaltic komatiites and t heir evolutionary products which represent hotter plume head material deriv ed from a mixture of plume mantle and entrained depleted upper mantle. In t urn, these are overlain by strongly light REE and HFSE enriched komatiites that represent a deep plume source that has not been mixed with depleted ma ntle and are, therefore, likely to have been derived from a plume core or t ail. Volcanism was protracted in these three greenstone belts lasting ca. 70 Ma and combined stratigraphic evidence from the Lumby Lake and Steep Rock belt s suggests that more than one plume may have ascended and tapped the same m antle sources, over time, within the area. Plume magmatism and rifting of c ontinental platforms thus appears to have been an important feature of crus tal development in the Meso-Archean. (C) 1999 Elsevier Science B.V. All rig hts reserved.