The mineralogy and petrology of layered series nepheline syenite within Center II of the Coldwell Complex

Abstract

The Coldwell Complex is situated within the Archean Schreiber-White River metavolcanic and metasediment of the Superior Province. Spanning over 25km in diameter, it is the largest alkaline intrusion in North America. The 1108 ± 1 Ma age of the Coldwell Complex and its close spatial proximity supports a strong relationship to the magmatism of the Keweenawan Midcontinent Rift. Early studies define three magmatic centers of the Coldwell Complex, which in order of intrusion are Center I, Center II, and Center III. Center I consists of an oldest phase gabbro, which borders a ferroaugite syenite to the east and north. Center II includes a nepheline-bearing biotite-gabbro and several intrusions of nepheline syenite, and Center III is composed of four syenites which in order of intrusion are: magnesiohornblende syenite, contaminated ferroedenite syenite, ferroedenite syenite, and quartz syenite. This study evaluated the formation of the layered series nepheline syenite in Center II of the Coldwell Complex. Field mapping and sampling were completed along the shoreline of Neys Provincial Park, where extensive exposures of massive, hybrid, and layered syenite; together with xenolith-rich zones, biotite-gabbro, lamprophyre dykes and pegmatitic syenite of diverse composition are preserved. In the layered series, perthitic K-feldspar with secondary plagioclase forms the cumulus phase. Feldspar observed through cathodoluminescence (CL) imaging demonstrated multiple feldspar species within single crystals. K-feldspar ranged in orthoclase component from 87-100%, whereas plagioclase exsolution and alterations were characterized as albite, with anorthite components ranging from 0-3%. Amphibole (classified as ferro-pargasite) and minor apatite represent post-cumulus phases, forming interstitially to feldspar laths or in amphibole-rich laminae at the base of individual layers. Biotite, also a post-cumulus phase, was classified as the iron-rich end member annite. Fluorapatite is the most abundant accessory mineral, hosting light rare earth elements (LREE), along with less abundant britholite, wöhlerite, pyrochlore, titanite and other minor accessory minerals hosting incompatible elements. Layers commonly display modal grading from amphibole-dominant laminae, to a mixture of amphibole and feldspar, followed by a section of ‘normal syenite’ (dominantly feldspar with disseminated amphibole). Hydrodynamic processes are favoured for the origin of the layered series, specifically surge-type density currents and separation and reattachment vortex cells, are proposed to have formed the modally graded cumulate layers. In conjunction with graded layers, other magmatic “sedimentary” features including slumping, scour channels, flame structures, load casts, and various stages of hybridization and deformation of mafic xenoliths produced during hydrodynamic processes, indicate a strong convecting current operating during the formation of these rocks. The formation of the layered series was a product of fractional crystallization, varying deposition mechanisms, and reworking through erosion or deformational processes.