Preliminary results from detailed geological mapping of the contact zone between metavolcanic and metasedimentary rocks in the Pontiac Subprovince, Lac Bellecombe area, Quebec Published Tuesday, 19 December 2017

This report summarizes preliminary results from the first field season of a two year M.Sc. research project conducted by the first author. The research is part of Metal Earth, a large seven-year project headed by the Mineral Exploration Research Centre at the Harquail School of Earth Sciences in Sudbury, Ontario. The study area is located ~17 km south of Rouyn-Noranda, near Lac Bellecombe, Quebec, where mafic–ultramafic metavolcanic rocks occur within metasedimentary rocks of the Pontiac Subprovince (Figure 1).

Mafic and ultramafic metavolcanic rocks occur as a narrow (50–1000 m) but laterally extensive (10–25 km) east-northeast-trending belt within metasedimentary rocks of the Pontiac Subprovince, south of Rouyn-Noranda. The objective of the research is to determine whether emplacement of the metavolcanic rocks into the Pontiac metasedimentary rocks occurred during deformation or during deposition of the metasedimentary succession. A secondary goal is to characterize the metavolcanic succession, its stratigraphy and deformation history. Previous work elsewhere in the Pontiac Subprovince interpreted the contact between the mafic–ultramafic metavolcanic and metasedimentary rocks as structural (Camiré et al., 1993; Camiré and Burg, 1993). This interpretation was based on sheared contacts between metasedimentary rocks and mafic–ultramafic metavolcanic rocks (Camiré and Burg, 1993), and the uncontaminated rare-earth element pattern of the metavolcanic rocks (Camiré et al., 1993).

Fieldwork involved 1:2000 scale mapping of the metavolcanic succession and surrounding metasedimentary rocks, and 1:100 grid mapping of outcrops located at or near the contact between the metasedimentary and metavolcanic rocks. Samples of the different rock types were collected for petrographic and lithogeochemical analysis. Samples collected for geochronology were obtained from a tonalitic dyke that crosscuts bedding to help constrain the age of deformation events and of undisturbed Pontiac metasedimentary rocks. The metavolcanic and metasedimentary rocks have been regionally metamorphosed from upper-greenschist to upper-amphibolite facies; however, the ‘meta’- prefix has been omitted to improve the readability of the text.

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