Abstract

The Shoo Fly Complex, the remnants of an early to middle Paleozoic subduction system, contains the Culbertson Lake allochthon. Included in the Culbertson Lake allochthon are the post-Cambrian and pre-Upper Devonian Quartz Hill, Toms Creek, and McMurray Lake cherts. Geochemical data derived from these three units plot on the Fe 2 O 3 /TiO 2 versus Al 2 O 3 /(Al 2 O 3 + Fe 2 O 3 ) and La (sub n ) /Ce (sub n ) versus Al 2 O 3 /(Al 2 O 3 + Fe 2 O 3 ) discrimination diagrams within either the continental margin-island arc field, or that part of the pelagic field overlapping the continental margin-island arc field. These relationships are consistent with the presence of argillaceous turbidites interstratified with radiolarite, and suggest a relatively distal continental-island arc setting where muddy turbidity currents episodically interrupted pelagic deposition. Specimens from the Quartz Hill and Toms Creek cherts with Al 2 O 3 /TiO 2 values > 20 make up Group I samples, whereas specimens with Al 2 O 3 /TiO 2 values < 10 are classified as Group II samples. The Al 2 O 3 /TiO 2 ratios of Group II specimens suggest derivation from mafic rock. Mean Th/Sc and Th/U values, as well as LREE-enriched patterns, are consistent with this interpretation, and indicate a source area dominated by alkalic seamount or ocean island material. The Al 2 O 3 /TiO 2 ratios of Group I samples indicate a source area with an average andesitic to rhyodacitic composition. Chondrite-normalized REE distribution patterns, as well as mean Th/Sc and Th/U values, support such an interpretation. Samples from the McMurray Lake chert have Al 2 O 3 /TiO 2 values ranging between nearly equal 19 and nearly equal 28, and REE patterns characterized by LREE enrichment, negative Eu anomalies, and slightly fractionated HREE patterns. These features, along with mean Th/Sc and Th/U values, indicate a source area dominated by old upper continental crust. The location of the source area(s) supplying material to cherts and argillites of the Culbertson Lake allochthon is unknown. However, existing tectonic models commonly portray the source of old upper continental crustal material in the Shoo Fly Complex as being located somewhere along the western North American continental margin, whereas the source of island-arc debris is portrayed as being located in a fringing arc system. Our data suggest that a relatively high-standing seamount or ocean island probably resided on the subducting oceanic plate. Thus, our work supports the idea that Al, Fe, Ti, Th, Sc, and the REEs in chert and argillite deposited in proximal continental-island arc settings can be used to assess the characteristics of sources supplying particulate matter to ancient trench systems.

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