PETROGENESIS OF CYCLIC UNITS IN THE NORTHERN ULTRAMAFIC CENTER OF THE LAC DES ILES COMPLEX, ONTARIO, CANADA

摘要

Lithologic, mineralogical, and geochemical attributes of cyclic units are presented for the Northern Ultramafic Center (NUC) of the Lac des Iles Complex, Northwest Ontario. Detailed mapping and core logging show that the NUC is comprised of the Eastern Marginal Zone and the Layered Series. Fourteen cyclic units with two distinct paragenetic sequences of cumulus minerals are identified in the Layered Series. Cyclic Unit type A (CUA) has a cumulus paragenesis of olivine+chrome-spinel -> olivine + orthopyroxene + clinopyroxene +/- chrome-spinel -> clinopyroxene + orthopyroxene -> plagioclase + orthopyroxene +/- clinopyroxene. Cyclic Unit type B (CUB) has a simpler paragenesis of olivine+chrome-spinel -> olivine+ clinopyroxene. Boundaries are sharp between similar cyclic units and where CUA overlies CUB. In contrast, where CUB overlies CUA, the cyclic unit contact is gradational and is marked by the development of a geochemical and mineralogical hybrid unit.