Robert Cundari's Honours thesis abstract

Thesis Title: 
Geology and Geochemistry of the Devon Volcanics, South of Thunder Bay, Ontario

A unit of mafic rock inDevonTownship, south ofThunder Bay, was mapped by Tanton (1931) and was termed Rove Formation Basalts.  Geul (1970) mapped this unit as aLogandiabase sill.  The primary objective of this study was to characterize the unit and test whether it is of intrusive or volcanic origin.  This was achieved by mapping the unit at a scale of 1:20 000, and through petrographic and geochemical analysis.

The unit is exposed on a plateau 7 km long and 0.8 to 1.0 km wide.  The unit is 4 to 6 m thick and is in apparent conformable contact with the underlying shales of the Paleoproterozoic Rove Formation, where a pronounced chilled margin consists of variolitic material up to 20 cm thick.  The flow-top also exhibits a variolitic texture ~15 cm thick.  The presence of ropy flow top and amygdules as well as quench textures, support a volcanic origin. 

Major element chemistry reveals a tholeiitic, intermediate composition with samples plotting in the basaltic andesite to andesite fields as well as in the basaltic trachy-andesite to trachy-andesite fields on a TAS diagram.  The unit typically has an intergranular texture consisting of randomly oriented plagioclase laths with interstitial chlorite, an alteration product of primary augite.  Most samples contain minor serpentine (after olivine), opaque minerals, secondary quartz, oxides, pyrite and calcite.  Amygdules are present in most samples and are infilled with some combination of calcite, quartz, chlorite and pyrite. Lower flow contacts and flow-tops are typically glassy with abundant spherulites that sometimes coalesce into bands.

Rare-earth element geochemistry shows the unit to be relatively enriched in both HREEs and LREEs, similar to the ultramafic sills of the Nipigon Embayment as well as the Riverdale Sill (Hollings et al., 2007, 2009).  A primitive mantle-normalized REE plot shows that the volcanic unit  is characteristic of an Ocean-Island Basalt, but with a negative Nb anomaly, most likely the result of lower crustal contamination.  This evidence is further supported by an ∊Nd(t=1100Ma) of -3.48, which also suggests contamination of the unit by a lower crustal source.  The trace element characteristics of the volcanic unit suggest an origin in Keweenawan time as they are geochemically similar to volcanic units of the MCR (Hollings et al., 2007) rather than Paleoproterozoic volcanic units of the Gunflint Formation.

Field, petrographical and geochemical analysis suggests that the unit is of volcanic origin with evidence towards emplacement during Keweenawan Midcontinent rifting.  Thus, we are proposing that the unit be renamed the Devon Volcanics.

Rob is currently working on his MSc thesis at Lakehead University

For more information about this thesis contact Dr Pete Hollings