GEOCHEMICAL COMPARISON OF MID CONTINENT RIFT RELATED GABBROIC ROCKS OF THE COLDWELL COMPLEX AND MAFIC SILLS AND DIKES LOCATED ALONG THE NORTH SHORE OF LAKE SUPERIOR, CANADA

Geochemical data for seven gabbroic rock suites in the Coldwell Complex including five that are host to significant copper and palladium mineralization are compared to mafic sills and dikes located along the North Shore of Lake Superior. Data are displayed in a series of molar element ratio and trace element variation diagrams that were selected to best exhibit process related trends that highlight the differences between the various gabbroic units. Gabbroic bodies examined from the Coldwell include the Two Duck Lake and Geordie Lake gabbros; the Eastern Layered Gabbro Series; and an earlier and prevalent fine grained gabbro. North Shore gabbroic units include the Logan Sills, Nipigon Sills, Riverdale Sill, Pigeon River Dikes, Cloud River Dikes and the Mt Mollie Dike (after Hollings, et al. 2010).

In Pearce element and molar element ratio diagrams the rocks are related to the minerals involved and agree in general with field observations. Rocks from the North Shore mafic sills and dikes plot in a relatively tight group as is expected from the rather limited range of rock types, whereas the diverse mineral assemblages observed in Coldwell Complex gabbros result in greater separation of the individual gabbro suites and a relatively larger spread of the data along process related trend lines.

Apatite fractionation apparently plays an important role in many rock units as revealed by strong positive correlation between P₂O₅ and rare earth elements, uranium and thorium. Plots of La/Gd vs La or Gd/Yb vs Gd conveniently separate mafic sills and dikes and Coldwell Complex gabbros into nearly separate elongate fields that are, in general, oriented with a positive slope. It appears that the individual trends, being a result of a unique combination of initial REE abundance and bulk partition coefficient in a dynamic system, may be useful in identifying MCR magmatic suites.

An important conclusion is that all Coldwell Complex gabbros except the fine grained gabbro, are enriched in incompatible elements with higher Ta/Zr, Nb/Zr and La/Yb than north shore sills and dikes and could have originated by a smaller degree of partial melting in the mantle. Interestingly, the fine grained gabbro appears to be analogous to the sills and dikes and represent an earlier more primitive phase of magma evolution.