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

Abstract

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 facies assemblages. Predominantly amphibolite facies 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. 1640Ma, and dark mantles yield an age of migmatization (and possibly of granulite facies) of ca. 1450Ma. However, metadiabase dykes containing granulite assemblages cut the ca. 1450Ma 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 facies overprint is generally challenging, thin zircon rims yielding ages with large errors suggest an Ottawan (ca. 1090–1030Ma) age for stacking, consistent with a 1078±28Ma 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).The record of Paleo- early Mesoproterozoic igneous-metamorphic history preserved in the Kiosk core is complemented by detrital zircon data from a quartzite member of a metasedimentary assemblage. The detrital zircon display an age concentration spanning ca. 1500–2000Ma with a Penokean (ca. 1850Ma) maximum. Pre-Labradorian components of the age spectrum are similar to published spectra from the Penokean Upper Animikie sediments from the Lake Superior region and may represent reworking of these during early Pinwarian deposition on the Labradorian basement. We speculate that the Kiosk metasediments, metamorphosed at high grade with their hosts at ca. 1459Ma represent a pre-or early-Pinwarian back-arc (age of youngest detrital grain, ca. 1500Ma) in Labradorian crust inverted in the Pinwarian Orogeny and consistent with A-type plutonism of this age present in the Central Gneiss Belt and interpreted to have been emplaced in extended back-arc crust. The Kiosk domain is thus established as a Labradorian member of the peri-Laurentian Great Paleoproterozoic Accretionary Orogen (Condie, 2013), having received detritus from other members of the accretionary orogen as well as Laurentia.