Publication Type:
Book ChapterSource:
Geological Society of America, 1998 annual meeting, Geological Society of America (GSA), Boulder, CO, United States, Volume 30, p.96 (1998)ISBN:
0016-7592Keywords:
absolute age, Canada, Chromite, chromitite, dates, eastern canada, emplacement, geochronology, gneisses, grain size, Grenville Front, Huronian, igneous rocks, magmas, melts, mesoproterozoic, metaigneous rocks, metals, Metamorphic rocks, metapyroxenite, mineral composition, nesosilicates, North America, Ontario, orthosilicates, oxides, plutonic rocks, Precambrian, precipitation, proterozoic, rare earths, silicates, Sudbury District Ontario, ultramafics, upper Precambrian, variations, zircon, zircon groupAbstract:
A suite of 25-30 metapyroxenite bodies, generally less than 500 m in size, occur within the Grenville Front tectonic zone east of Sudbury, Ontario. The metapyroxenites generally occur along the flanks of metagabbroic and meta-anorthositic rocks of the East Bull Lake intrusive suite (EBLI), at the contact with host migmatitic gneiss. The better preserved bodies have a general podiform shape, and contain a blackwall alteration zone present at the contact with the migmatitic gneiss. Some bodies occur in tectonic contact with the adjacent gneisses, but are nonetheless spatially associated with EBLI rocks. Mineralogically, these bodies consist of roughly equal amounts of orthopyroxene phenocrysts (0.5-5 cm in size) in a matrix of fine-grained Mg-cummingtonite, cummingtonite, magnesium hornblende and tremolite; locally, olivine is preserved in bodies proximal to the Grenville Front. Little variation in grain size and phenocryst content is observed between bodies. Zircons present within orthopyroxene from a body near the Grenville Front give an emplacement age of 2468+ or -5 Ma, consistent with the age of the EBLI suite (2.47-2.49 Ga), whereas zircons from the matrix give Mesoproterozoic ages. The SiO (sub 2) content of the metapyroxenite is 46-50.5 wt. %, MgO 21-27 wt. %, Ni 0.75-1. 1 wt. %, with 37-63 ppm Zr, 5-19 ppm Y and (REE = 30 ppm. The bodies contain 2.9-4.7 wt. % Cr, present as Cr-spinel, Fe-chromite and chromite inclusions in olivine and orthopyroxene. Mg#'s are 78-82 and Cr#'s are 20-38. CIPW norms suggest a primary mineralogy dominated by olivine (4-22%), hypersthene (32-62%) and anorthite (13-21%). Mesonorms suggest a metamorphic mineralogy dominated by pyroxene and amphibole, as observed. The bodies have chemical affinities to high-Al chromitites from ophiolitic complexes, most notably Cr#0.2%, CaO>1. 5%, and Al (sub 2) O (sub 3) >1.2%. High-Al chromitites are found in back-arc spreading and rifting environments, consistent with the previously determined rift-setting for the lower Huronian Sgp. and the EBLI suite. Chromite precipitation was likely triggered by mixing of evolved and primitive magmas, which is suggested by the moderate Zr, Y and REE contents of these rocks. In addition, removal of the pyroxene from the host would increase the silica content of the melts, moving melts from the olivine-chromite cotectic into the primary phase field of chromite.
Notes:
GeoRef, Copyright 2018, American Geological Institute.<br/>1999-017273<br/>East Bull Lake Intrusion
