OCGC Seminar: Evolution of a Neoproterozoic Continental Flood Basalt Province - The Franklin Sills and Natkusiak Lavas of Victoria Island

 

Dr. Jean Bédard 
Howard Street Robinson Lecturer (2014-15),
Geological Survey of Canada, Quebec

The Franklin Sills and Natkusiak lavas of Victoria Island

Wednesday, March 18th, 2015
2:30 p.m.  

3120 Herzberg Laboratories
Carleton University 

 

Abstract

The Neoproterozoic (ca.716 Ma) Franklin Large Igneous Province formed during the breakup of Rodinia. The Natkusiak continental flood basalts (≤1 km thick, preserved as 2 lobes in a syncline) are the extrusive phase of the Franklin event, and erupted onto a shallow-water continental platform. An underlying fluvial sandstone, the Kuujjua Fm., pinches out towards the NE, suggesting pre-eruptive thermal doming, possibly associated with arrival of a mantle plume. The lowermost extrusive unit (ca 50-100 m thick) is a primitive basalt (7-11 wt% MgO), and is tentatively interpreted as agglutinate (welded fire fountain deposits) erupted from multiple vents. The unit is characterized by LREE-LILE-enrichment, high L/HREE, high87/86Sri  (up to 0.70791), intermediate εNd (4.0-8.1) and εHf (0.03-6.7), high 208/204Pb, 207/204Pb and 206/204Pb ratios; indicating either an enriched source, extensive crustal contamination, or influx of enriched fluids from the footwall. A hiatus in eruptive activity is marked by deposition of red-weathering volcaniclastic rocks that contain matrix-supported conglomerates (lahars or damburst deposits?) that fill palaeovalleys. Two differentiation cycles of laterally extensive basaltic (10-6% MgO) sheet flows were then deposited above the basal lavas and volcaniclastics. Both cycles show upward shifts in phenocryst populations, decreases in Mg#, Cr and Ni, and increases in incompatible element concentrations, consistent with up-section fractional crystallization. Only cycle 1 sheet flow basalts are exposed in the SW. These have higher εNd (7.7-9.6), lower 87/86Sri (0.70251-0.70605), higher εHf (4.1-9.7), and lower 208/204Pb, 206/204Pb and 207/204Pb than basal basalts. The cycle 1 sheet flow basalts in the NE have trace element and isotopic trends that differ from those in the SW (c.150km separation), indicating regional-scale isotopic heterogeneity in the source and/or contaminants of these lavas. Alternatively, isotopic heterogeneities may have been enhanced during ascent through the crust in a compartmentalized feeder system. On the scale of the entire magmatic system, the secular decrease in incompatible trace element concentrations, L/HREE ratios, and radiogenic isotope signatures could be interpreted as a decrease in the degree of host contamination with time. Changing magma composition could also reflect a shift from a fertile mantle source to a less enriched source, possibly associated with upwelling of asthenospheric mantle during the separation of Siberia from Laurentia.

The plumbing system that fed the lavas is dominated by sills, with localized fault-guided dykes. Two magma populations have been identified: Younger cotectic diabasic sills with trace element signatures matching the sheet flow lavas, and older sills (based on cross-cutting relationships), commonly with olivine-enriched bases, that match the basal basalts. Field, geochemical and isotopic evidence imply that steep, dyke-like feeders were sites of preferential wall rock assimilation and allowed melt to ascend between sills. In-conduit assimilation locally triggered sulfide immiscibility.  Mineral chemical data  and isotopic data imply that many sills were replenished by ol-charged slurries that propagated for 10s of km along strike, gradually mixing with the diabasic host.