TIMING AND ORIGIN OF MIDCONTINENT RIFT ALKALINE MAGMATISM, NORTH-AMERICA - EVIDENCE FROM THE COLDWELL COMPLEX

The Coldwell Complex represents the largest alkaline intrusion associated with the Midcontinent Rift System in North America. This complex contains a plethora of rock types that have previously been subdivided into three intrusive centers. A detailed U-Pb zircon/baddeleyite age study of five samples indicates that the majority of the complex was emplaced into "cold" Archean crust at 1108 +/- 1 Ma and likely experienced a rapid cooling history. These data, combined with published U-Pb zircon/baddeleyite results for other rift related igneous activity, document the contemporaneous production and emplacement of tholeiitic and alkaline magmas at the onset of rifting. The Sr-Nd-Pb isotopic compositions of selected minerals from different phases of the complex display considerable scatter that is best explained by the presence of magmas with different initial isotopic compositions. The initial Sr and Nd isotopic compositions for clinopyroxene and plagioclase from one of the earliest gabbro phases (epsilon(Nd) = +0.5 to +1.6; epsilon(Sr) = +2.4 to +3.1) are identical to published data for primitive olivine tholeiites from the rift and indicate that the majority of magmas, both tholeiitic and alkaline, have a uniform, nearly chondritic isotopic composition. This very reproducible isotopic composition for rift magmastism can be explained by the dominance of a well-mixed mantle plume signature in magma genesis. The shift in isotopic compositions observed for the more evolved granite and syenite samples (epsilon(Nd) = -4.6 to -6.4; epsilon(Sr) = +10.2 to +13.8) combined with less radiogenic Pb isotopic signature is consistent with derivation of these magmas from or interaction with an older granulite facies lower crust. The chondritic isotopic signature typical of most MRS volcanic and plutonic rocks is quite distinct from published results on associated carbonatites (epsilon(Nd) = +2.1 to +4.5; epsilon(Sr) = -8.0 to -11.5) indicating the presence of at least two distinct subcontinental mantle isotopic reservoirs in this region.