Geochemistry and geochronology of the 1.3 Ga metatonalites from the Central Metasedimentary Belt boundary thrust zone in southern Ontario, Grenville Province, Canada

Abstract

Studies of pre-Grenvillian (1.4-1.3 Ga) plutons offer insight into the dynamics of arc amalgamation and backarc rifting prior to continental collision during the Ottawan orogeny. The Central Metasedimentary Belt boundary thrust zone (CMBbtz) is a northeast-southwest-trending thrust zone consisting of metaplutonic thrust sheets enveloped in gneissic tectonites and calcitic-dolomitic marble. Tonalitic CMBbtz thrust sheets (Dysart and Redstone), located in the southern Ontario Grenville Province (Canada) are made up of upper amphibolite facies, foliation-concordant metatonalite (+ amphibole ± biotite ± accessory zircon and titanite) and amphibolite (± biotite ± clinopyroxene). These thrust sheets are thought to have formed and amalgamated onto the Laurentian margin prior to Ottawan orogeny. Major and trace element analyses show that the metatonalite rocks have calc-alkaline affi nity and amphibolite rocks have both calc-alkaline and tholeiitic affi nities, suggesting an arc environment. Laser ablation-inductively coupled plasmamass spectrometry (LA-ICP-MS) geochronology of zircon from the two thrust sheets yield igneous ages of ca. 1350-1300 Ma for diorite and granodiorite, ca. 1150-1100 Ma ages from Dysart tonalite interpreted to record metamorphic zircon growth, and a ca. 1086 Ma Ottawan metamorphic age from a Dysart amphibolite. The ca. 1150-1100 Ma metamorphic event has not been previously documented within these thrust sheets of the CMBbtz, but correlates well with thermal events in the allochthonous Parry Sound domain to the west, as do ca. 1350 Ma igneous ages of tonalite in both areas. These data support the hypothesis that the CMBbtz and Parry Sound domain may have been initially linked. Widespread ca. 1350 Ma crust along with distinct 1460-1400 Ma depleted mantle model ages (T DM ) are also consistent with a shared genesis with the Dysart-Mount Holly suite in New York and Vermont, and support the correlation between the CMBbtz thrust sheets and the Adirondack Highlands-Mount Holly belt as a rifted arc. Alternatively, the CMBbtz thrust sheets and the Adirondack Highlands-Mount Holly belt may represent contemporaneous arc development at different parts of the convergent margin; however, we support the correlation between the CMBbtz thrust sheets and the Adirondack-Highlands-Mount Holly belt.

Monazite U–Th–Pb geochronology of the Central Metasedimentary Belt Boundary Zone (CMBbz), Grenville Province, Ontario Canada

“…Two U/Pb zircon studies that were specifically conducted to assess the timing of deformation in the CMBbz (McEachern and van Breemen 1993;Timmerman et al 1998) (Rivers 2015, McLelland et al 2013. As a result, we do not find any evidence to support the interpretations of U/Pb zircon and titanite dates in this range that suggest that metamorphism may have occurred throughout the CMBbz during this interval (Mezger et al 1993, Agustsson et al 2013 follows the designation of Easton (1992) and Carr et al (2000). B) The terrane/domain subdivisions of Davidson (1984) and Carr et al (2000) are the Parry Sound (PS), the Muskoka, many other terranes in the CGB that we do not label, the Bancroft (sensu lato, we equate the Bancroft Domain with the CMBbz, an equivalence that is in first-order agreement with the diverse delineations in the literature), the Elzevir, the Mazinaw (Ma), the Sharbot Lake (SL), the are at a high angle (e.g.…”

“…A recent compilation of geochronological data from this northern region of the CMBbz (Duguet and Easton 2017) suggests that Shawinigan metamorphic and igneous dates there are widespread. Agustsson et al (2013) reported metamorphic U-Pb zircon dates of 1150-1100 Ma from the ca.…”

“…We interpret these dates as representing a second prograde event in the CMBbz, the ca. (Agustsson et al 2013). Yet the oldest U-Th-Pb monazite dates we present here from the West Guilford gneisses overlap only with the youngest ages reported by Agustsson et al (2013).…”

“…(Agustsson et al 2013). Yet the oldest U-Th-Pb monazite dates we present here from the West Guilford gneisses overlap only with the youngest ages reported by Agustsson et al (2013). This disconnect between our data and U/Pb zircon protolith ages even holds true for the Fishtail Lake sample, which is related to a suite studied in much more detail by Peck and Smith (2005).…”

Three-dimensional visualization of top-down superimposed thrust sheets in the SW Grenville Province, Ontario

“…In the Central Metasedimentary Belt boundary, the 1190 Ma crystallization ages of syntectonic granite sheets from the northern Central Metasedimentary Belt boundary indicate the beginning of ductile thrusting and crustal thickening (McEachern & van Breemen, 1993). There is also evidence for metamorphism on the Central Metasedimentary Belt boundary around 1130 Ma (Agustsson et al 2013). In the Parry Sound domain, metamorphic zircon from Parry Sound domain granulite at c. 1150 Ma dates the time of cooling from peak metamorphic conditions (Marsh et al 2012).…”