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JOSLAND LAKE SILLS: U-PB AGE AND TECTONOSTRATIGRAPHIC IMPLICATIONS (PARTS OF NTS 63K AND 63N)

The age, geochemistry and field relationships of the mafic, layered Josland Lake sills require a re-evaluation of the tectonic history of the Flin Flon Belt and adjacent parts of the Kisseynew Domain. A sample of quartz diorite from the upper, Fe-rich portion of a differentiated mafic sill northwest of Reed Lake has yielded a U-Pb zircon age of 1886 ± 3 Ma (95% confidence level, MSWD = 0.68) that is interpreted to be the age of crystallization of the sill. The sill, 24 km long and up to 2 km thick, belongs to the suite of Josland Lake sills. These sills have a unique geochemical fingerprint throughout their 100 km regional extent from Snow Lake and Reed Lake, in the Flin Flon Belt, to Kississing Lake, on the south flank of the Kisseynew Domain. They are interpreted to represent a major igneous province, which includes the 1881 +3/-2 Ma Mikanagan Lake sills in the Flin Flon area that formed at the close of early juvenile-arc volcanism, before the pre-1876 Ma tectonic amalgamation into the Amisk collage. The trace-element geochemistry of the sills and their common occurrence in early volcaniclastic basin fill suggest that they are a product of arc rifting. Elevated platinum-group elements (PGEs) and gold deposits in the sills make them a mineralexploration target. Because the sills intrude the Amisk collage, the Reed Lake ocean-floor assemblage and the Snow Lake arc assemblage, these various host rocks may have been part of a composite arc and back-arc terrane that separated into segments during the 1881 to 1886 Ma rifting. Structural slices of ca. 1.85 Ga metaturbidite, extending from Reed Lake into the Kisseynew Domain along volcanic arcand back-arc–assemblage boundaries, suggest that the composite terrane was reassembled during the continental collision tectonics that involved closing of the Kisseynew basin and deformation of the 1.85 Ga turbidite.