PROTEROZOIC MANTLE XENOLITHS IN ULTRAMAFIC DYKES NEAR WAWA, ONTARIO: IMPLICATIONS FOR THE LITHOSPHERIC MANTLE UNDERNEATH THE CENTRAL NORTH AMERICAN CRATON

The Wawa area of northern Ontario contains mantle xenolith suites in both Proterozoic and Archean dykes providing a unique window into the lithospheric mantle beneath the central North American Craton. The present study focuses on the Proterozoic xenoliths and the ultramafic dykes (~1.1 Ga) which host them, and includes new analyses of the Alona Bay picritic basalts of the Keweenawan flood basalt. The best known dyke, Nicholson dyke, has the same age (Sage, 1997) as the nearby Alona Bay basalts. The Proterozoic host dykes have been considered in the past to be kimberlites (R.Sage, 1997; Kamensky, 2002), but they have high Fe contents and are more similar in composition to aillikites (Francis, 2003). One exceptional locality, Fletch, has somewhat lower Fe content and appears to be transitional between kimberlite and aillikite.

There are a variety of xenoliths in these dykes, including: lherzolites, harzburgites, eclogites and dunites. The Cr/Al and Ni/Al arrays of the eclogites from East C are identical to those of the group 1 Mamainse Point picrites (Shirey, 1994), but the eclogites from Nicholson dyke have higher Al and lower Cr and Ni contents, more similar to Alona Bay basalts (equivalent of group 5 Mamainse Point; Shirey 1994). The close correspondence between the eclogite xenoliths and picritic lavas suggest that the eclogites may be the result of crystallization of Keweenawan magmas at elevated pressures.

The mantle xenolith suite is, however, dominated by depleted harzburgites, with the low Fe-contents typical of cratonic lithospheric mantle (Boyd, 1989), despite the general high Fe contents of their Proterozoic host dykes. The Wawa harzburgites do not extend to the extreme orthopyroxene enrichments seen in xenoliths suites in kimberlites from the Kapvaal and Siberian cratons. They have lower Fe and higher Si contents than the Proterozoic harzburgite xenoliths of the North American Cordiliera, indicating that the lithospheric mantle beneath the central North American Craton is more refractory and colder then the surrounding lithospheric mantle (Francis, 2003).