Kiosk domain, Central Gneiss Belt, Grenville Province, Ontario: A Labradorian palimpsest preserved in the ductile deep crust

摘要

The Kiosk domain is a thrust sheet within a multi-component Grenvillian thrust stack, the polycyclic (evidence for pre-Grenvillian orogeny present) Bonfield-Kiosk-Algonquin stack. The stack is overlain to east and west by thrust sheets of predominantly monocyclic gneiss and uplifted along a ramp of Archean parautochthonous rocks. Central to the thrust stack, the Kiosk domain contains a homoclinal southeast-inclined core locally preserving granulite fades assemblages. Predominantly amphibolite fades shear zones, bounding and within the stack, and southeast trending folds overprint the margins of the core fabric. Finally, the Kiosk basal shear zone is locally reworked by extension. Monzonitic orthogneiss in Kiosk domain contain zircon with cores indicating protolith ages of ca. 1640 Ma, and dark mantles yield an age of migmatization (and possibly of granulite facies) of ca. 1450 Ma. However, metadiabase dykes containing granulite assemblages cut the ca. 1450 Ma leucosome and share a southeast trending LS fabric with the country rock of the homoclinal core suggesting the possibility of an early Grenvillian, granulite event. Dating the amphibolite fades overprint is generally challenging, thin zircon rims yielding ages with large errors suggest an Ottawan (ca. 1090-1030 Ma) age for stacking, consistent with a 1078 +/- 28 Ma age for a shear zone which cuts the internal fabric of the stack. The Kiosk domain thus preserves successively younger Grenvillian events from core to rim interpreted to represent early thickening (homoclinal core LS fabric), retrogression-related softening (pegmatites), nappe flow (marginal shears and folds), and post convergence extension (localised shearing).