Abstract

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.

Within individual cyclic units there are cryptic variations in mg# of pyroxenes (En) and olivine (Fo). Notably, olivines in CUB rocks have lower Ni concentrations despite higher Fo contents than in CUA rocks. Also, incompatible trace element ratios (e.g., Zr/Y and Ce/Yb) are distinct between the two types of cyclic units. These mineralogical and geochemical attributes are interpreted to indicate that the CUA and CUB units were generated by fractional crystallization of two compositionally distinct parental magmas. The CUA parental magma was more siliceous, had higher concentrations of Ni and incompatible trace elements, and had a lower Ce/Yb ratio relative to the CUB parental magma. In seeking out possible parent magma analogues for the CUA and CUB within the NUC, it was found that gabbroic rocks forming the Eastern Marginal Zone and hornblende gabbro dikes crosscutting parts of the Layered Series, respectively, have comparable geochemical attributes.

The non-systematic sequencing of CUA and CUB units comprising the Layered Series is interpreted to have resulted from the random recharge of CUA and CUB parental magmas feeding the NUC. When similar magma types successively recharged the NUC chamber, or when a CUA magma recharged the chamber previously filled by a CUB, the recharging magma evidently had a higher density than the resident magma and flowed along the cumulus floor to produce sharp cyclic unit boundaries. When more siliceous CUA magma recharged the chamber previously filled by CUB magma, its lower density resulted in its pluming into the resident magma and producing a complex hybrid cyclic unit boundary.

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