Basaltic to andesitic volcaniclastic rocks in the Blake River Group, Abitibi Greenstone Belt: 1. Mode of emplacement in three areas¹This article is a companion paper to Ross et al. 2011. Basaltic to andesitic volcaniclastic rocks in the Blake River Group, Abitibi Greenstone Belt: 2. Origin, geochemistry, and geochronology. Canadian Journal of Earth Sciences, 48: this issue.²MRNF Contribution BEGQ 8439-2010/2011-1. Natural Resources Canada, Earth Science Sector Contribution 20100253.

Abstract

In the Archean Blake River Group, mafic to intermediate fragmental units have controversially been proposed to have formed during the collapse of a giant submarine caldera. This paper describes and interprets these rocks, summarizing their physical characteristics, inferred origins, age relationships, and geochemical signatures. The widespread Stadacona member, south of Rouyn-Noranda, consists of several hundred meters of bedded volcaniclastic rocks interpreted to have been mostly deposited from aqueous density currents fed directly by explosive eruptions. The magmas involved in these eruptions were plagioclase-phyric tholeiitic to transitional basalts. The similarly widespread D'Alembert tuff, in the northern part of the Blake River Group, shares many physical characteristics with the Stadacona member and is thought to have a similar origin. However, the D'Alembert tuff is approximately six million years younger than the Stadacona member. It is composed mostly of transitional to calc-alkaline andesites and basaltic andesites with very distinct trace element profiles. Volcaniclastic rocks from other areas, such as Tannahill Township in Ontario and the Monsabrais area in Québec, are interpreted to represent mostly in situ to remobilized hyaloclastite, with no explosive eruptions involved in their genesis. Our observations and interpretations are not compatible with models in which the volcaniclastic units are emplaced during a catastrophic event in relation with the collapse of a giant caldera. Instead, the fragmental rocks were produced by various mechanisms at many distinct times during the evolution of the Blake River Group.

In situ trace metal analysis of Neoarchaean – Ordovician shallow‐marine microbial‐carbonate‐hosted pyrites

“…The Archaean rock record from greenstone belts shows abundant evidence for prolific volcanic resurfacing and for deposition of easily weathered volcanic deposits (e.g. Ross et al ., ). The post‐Palaeoproterozoic drop in As concentration reported here could, therefore, relate to a change in the composition of the post‐Palaeoproterozoic land surface source material – namely, a transition from a volcanic surface (e.g.…”

Igneous Rock Associations 14. The Volcanic Setting of VMS and SMS Deposits: A Review

“…Pearson and Daigneault 2009;Mueller et al 2012) is difficult (see also Ross et al 2011a, b for more discussion on the Blake River Group). Recall that in subaerial settings, calderas produced by magma withdrawal or lava effusions are summit calderas many times smaller than the shield volcanoes or stratovolcanoes on which they occur.…”