A Petrogenetic Model for LREE-Enriched Basalt and Gabbros in the Coldwell Alkaline Complex, Midcontinent Rift, Ontario

David John Good, University of Western Ontario, Earth Sciences, London, ON, Canada, Pete N Hollings, Lakehead University, Thunder Bay, ON, Canada, Cundari M Robert, Ministry of Northern Development and Mines, Thunder Bay, Canada and Doreen E. Ames, Geological Survey of Canada, Ottawa, Canada

Contact First Author: David John Good; dgood3@uwo.ca

Abstract ID#: 34139

 

English Abstract:
The Coldwell Alkaline Complex (CAC) is associated with the Midcontinent rift event. The eastern and northern margin of the CAC consists of numerous cross-cutting gabbroic to ultramafic intrusions that are associated with magma conduit style Cu-Pd deposits. A package of basalt flows that overlie syenite near the centre of the CAC is linked to the gabbroic bodies by common geochemical characteristics. This study presents a petrogenetic model for the mafic rocks based on a comprehensive geochemical and isotopic study of the basalt and gabbroic bodies.

Observations are centred on the highly unusual LREE abundances in the basalt, and on trend lines for La vs. Zr or La vs. Th variation diagrams that resemble effects due to contamination, but evidence suggests contamination did not occur by assimilation of crustal material, as follows: (a) there is a negligible increase in Th, Zr and SiO2 with increasing La; (b) absolute abundances of HFSE in basalt are comparable to E-MORB; and (c) Nd and Sr isotopic signatures are near CHUR. The evidence suggests LREE enrichment occurred at the time of partial melting from a LREE enriched mantle source, and not by crustal contamination.

Excluding LREE abundances, the composition of the initial Coubran basalt magma is interpreted to be comparable to Mamainse Point volcanic group 1. This interpretation is supported by similar Th/Nb and Tb/Yb values, and similar abundances of LILE, HREE, HFSE, and major elements. However the extruded Coubran basalt magma must have undergone fractional crystallization at depth in order to explain the present low Ni and Mg Numbers.

A proposed geochemical model for fractional crystallization of various basalt magma compositions that lie along the La-Zr or La-Th trajectories can explain the trace element compositions of five prominent gabbroic bodies in the CAC (Two Duck Lake gabbro, layered troctolite sill, Geordie Lake gabbro, gabbroic anorthosite and layered olivine gabbro).

Back to: Chemical Scene Investigation of Large Igneous Provinces (CSI-LIPs) II