Geochemistry and fractional crystallization history of the Falcon Lake Intrusive Complex

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Tirschmann, Patricia A.
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The Falcon Lake Intrusive Complex (FLIC) is a small (9 km2) compositionally zoned, composite intrusion located in the southwestern part of the Superior Structural Province. The complex was emplaced in Archean metavolcanic and metasedimentary rocks of the Lake of the Woods greenstone belt and represents a post-tectonic intrusion in its original intrusive orientation. The complex is comprised of six lithologic units: four outer gabbroic units (Gabbro #1-Gabbro #4), a central diorite-granodiorite unit and a quartz monzonite core. A K-AR (Wanless, 1963) date on biotite from the core rocks yielded a minimum age of 2.3 Ga. Geochemical data and field relationships indicate that the rock types comprising the FLIC represent a genetically-related magma suite derived from a LREE-enriched parental source with "alkaline-like" characteristics. The lithologies present, however do not represent a single continuous liquid line of descent. Gabbro #1 and Gabbro #2 are geochemically the most primitive rock types of the complex. Gabbro #3, Gabbro #4, as well as some of the more mafic dioritic rocks, are distinctly more evolved, but are geochemically indistinct from each other. The dioritic-granodioritic and quartz monzonitic rocks collectively display major and trace element trends consistent with a single liquid line of descent. The overall compositional zonation from oldest gabbroic rocks inward to youngest quartz monzonitic rocks can be explained by a combination of multiple intrusion and fractional crystallization processes. Formation of the mafic portions of the FLIC involved multiple intrusions of a) more primitive basic magma(s) and b) more evolved basic magmas. Evolution of the earlier, more primitive mafic units (Gabbro #1 and Gabbro #2) involved the fractionation of clinopyroxene and plagioclase while the later, more evolved, mafic units (Gabbro #3, Gabbro #4 and some of the diorites) were dominated by plagioclase crystallization and accumulation. Subsequent evolution of the intermediate to felsic portions of the complex appears to have taken place via inward fractional crystallization, the later stages of which involved upward movement and convective homogenization of mobile felsic differentiates followed by rapid cooling and crystallization forming the quartz monzonite core.