Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development

Magmatism that accompanied the 1.1 Ga Midcontinent rift system (MRS) is attributed to the upwelling and decompression melting of a mantle plume beneath North America. Five distinctive flood-basalt compositions are recognized in the rift-related basalt succession along the south shore of western Lake Superior, based on stratigraphically correlated major element, trace element, and Nd isotopic analyses. These distinctive compositions can be correlated with equivalent basalt types in comparable stratigraphic positions in other MRS localities around western Lake Superior. Four of these compositions are also recognized at Mamainse Point more than 200 km away in eastern Lake Superior. These regionally correlative basalt compositions provide the basis for determining the sequential contribution of various mantle sources to flood-basalt magmatism during rift development, extending a model originally developed for eastern Lake Superior. In this refined model, the earliest basalts were derived from small degrees of partial melting at great depth of an enriched, ocean-island-type plume mantle source ( E , , ( , , , , value of about 0), followed by magmas representing melts from this plume source and interaction with another mantle source, most likely continental lithospheric mantle ( E , , ( , , , , < 0). The relative contribution of this second mantle source diminished with time as larger degree partial melts of the plume became the dominant source for the voluminous younger basalts (E, , ( , , , , value of about 0). Towards the end of magmatism, mixtures of melts from the plume and a depleted asthenospheric mantle source became dominant ( E , , ( , , , , = 0 to +3). RCsumC : Le magmatisme contemporain de la mise en place du systkme du rift Midcontinental, il y a 1,l Ga, est attribuk au soulkvement et a une fusion par baisse de pression d'un panache mantellique localis6 sous I'AmCrique du Nord. Le long de la rive sud dans la partie occidentale du lac SupCrieur, cinq compositions distinctes ont kt6 reconnues parmi les coulees de basalte de la succession basaltique associke au rift, sur le fondement de correlations stratigraphiques Ctablies pour les elements majeurs et en traces, et les analyses isotopiques du Nd. Ces compositions distinctes peuvent etre mises en correlation avec les types de basalte equivalents qui occupent des niveaux stratigraphiques comparables dans les autres localitks du systkme du rift Midcontinental voisines du secteur occidentale du lac Superieur. On a identifie Cgalement quatre de ces compositions a Mamainse Point, c'est-a-dire a plus de 200 km de distance, dans la region orientale du lac SupCrieur. Ces correlations regionales fondkes sur les compositions des basaltes fournissent les elements de base pour determiner la contribution sequentielle des diffkrentes sources mantelliques au magmatisme des coulees de basalte durant le dkveloppement du rift, perfectionnant le modkle developpk initialement pour la region orientale du lac SupCrieur. Dans ce modkle raffine, les premiers basaltes formes derivaient de faibles degres de fusion partielle a grande profondeur d'un panache mantellique enrichi de type oceanique-insulaire (E, , ( , , , , proche de 0), suivis de magmas composes des liquides de fusion partielle du panache et d'une interaction avec une autre source mantellique, plus probablement issue du manteau lithospherique continental (E, , ( , , , , < 0). La contribution relative de cette deuxikme source mantellique diminuait avec le temps, au fur et a mesure qu'augmentait la proportion de fusion partielle du panache qui finalement est devenue la source dominante alimentant les volumineuses coulees de basalte plus jeunes , , , proche de 0). Les derniers stades du magmatisme ont kt6 domines par les melanges de magmas derives du panache et d'une source appauvrie du manteau asthCnosphCrique (E, , ( , , , , = 0 a + 3). [Traduit par la redaction] Introduction emplaced within about 22 Ma (Davis and Green 1997), and provides an example of a structurally intact continental rift The Ga Midcontinent rift system (MRS) at least that nearly formed an oceanic basin. The last decade has been lo6 km3 of rocks (Cannon 1992) which were a period of intense data gathering about all aspects of the rift. This effort coincided with the publication of rapidly changing Received April 19, 1996. Accepted November 28, 1996. concepts regarding the general development of continental S.W. Nicholson' and K. J. Schulz. United States Geological rifts, flood-basalt provinces, and mantle plumes (Cox 1980; Survey, National Center, MS 954, Reston, VA 20192, R.S. White et a1. 1987; McKenzie and Bickle 1988; Richards U.S.A. et al. 1989; R.S. White and McKenzie 1989, 1995; CampS.B. Shirey. Department of Terrestrial Magnetism, Carnegie bell and Griffiths 1990; Arndt and Christensen 1992; R.S. Institution of Washington, 5241 Broad Branch Road NW, White 1992; Turner and Hawkesworth 1995). Models develWashington, DC 20015, U.S.A. oped from these concepts suggest that progressive melting of J.C. Green. Department of Geology, University of a mantle plume head can produce voluminous flood basalts Minnesota, Duluth, Duluth, MN 558 12, U.S.A. distributed over a region up to 1 x l o6 km2 or more (e.g., I Corres~onding author (e-mail: s w n i c h @ u s ~ s . ~ o v ~ . R.S. White and McKenzie 1989: R.S. White 1997). The w u u z I Can. J . Earth Sci. 34: 504-520 (1997) 1997 NRC Canada