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

Abstract

The Central Metasedimentary Belt boundary zone (CMBbz) is a crustal-scale shear zone that juxtaposes the Central Gneiss Belt and the Central Metasedimentary Belt of the Grenville Province. Geochronological work on the timing of deformation and metamorphism in the CMBbz is ambiguous, and the questions that motivate our study are: how many episodes of shear zone activity did the CMBbz experience, and what is the tectonic significance of each episode? We present electron microprobe data from monazite (the U–Th–Pb chemical method) to directly date deformation and metamorphism recorded in five garnet–biotite gneiss samples collected from three localities of the CMBbz of Ontario (West Guilford, Fishtail Lake, and Killaloe). All three localities yield youngest monazite dates ca. 1045 Ma; most of the monazite domains that yield these dates are high-Y rims. In comparison with this common late Ottawan history, the earlier history of the three CMBbz localities is less clearly shared. The West Guilford samples have monazite grain cores that show older high-Y domains and younger low-Y domains; these cores yield a prograde early Ottawan (1100–1075 Ma) history. The Killaloe samples yield a well-defined prograde, pre- to early Shawinigan history (i.e., 1220–1160 Ma) in addition to some evidence for a second early Ottawan event. In other words, the answers to our research questions are: three events; a Shawinigan event possibly associated with crustal thickening, an Ottawan event possibly associated with another round of crustal thickening, and a late Ottawan event that resists simple interpretation in terms of metamorphic history but that coincides chronologically with crustal thinning at the base of an orogenic lid.

The late Mesoproterozoic to early Neoproterozoic Grenvillian orogeny and the assembly of Rodinia: Turning point in the tectonic evolution of Laurentia

“…Despite metamorphic chronometers commonly being interpreted to yield peak metamorphic conditions, some of the oldest dates associated with the Grenvillian orogeny come from the high-yttrium cores of monazite grains (red pentagons in Grenville Province panel in Fig. 2), that are interpreted to have crystallized during prograde metamorphism prior to the crystallization of garnet, which is also a sink for Y (Markley et al, 2018). An estimate of the onset of Grenvillian orogenesis using our resampling approach based on both monazite and metamorphic zircon dates gives an age of 1089 Ma, while an estimate of the onset based on the zircon dates alone would be younger at 1070 Ma.…”

“…1050-1020 Ma), giving rise to the sub-horizontal, high-strain layering in the metamorphic cores. The best constrained estimates for the time of collapse are based on U-Pb dating of monazite integrated with trace element modelling of REE+Y (rare earth element + yttrium) compositions of coexisting monazite and garnet (e.g., Markley et al, 2018;Regan et al, 2019;Williams et al, 2019). These ages imply that collapse was initiated some 40 Myr after the onset of Ottawan high-grade metamorphism in the middle crust, significantly predating thrusting and crustal shortening in the parautochthonous belt during the Rigolet phase.…”

“…1090 Ma (e.g. Corrigan et al, 2000;Markley et al, 2018), and more ca. 1070 Ma monazite and zircon rim dates (Fig.…”

“…These durations are: Shawinigan orogeny, Grenville Province 1198 to 1157 Ma; Grenvillian orogeny, Grenville Province 1089 to 981 Ma; pre-Grenvillian orogeny, Blue Ridge 1162 to 1153 Ma; Grenvillian orogeny, Blue Ridge 1070 to 994 Ma; AMCG plutonism 1174 to 1149 Ma; pre-Midcontinent Rift magmatism 1157 to 1143 Ma; Midcontinent Rift magmatism 1108 to 1086 Ma. The data come from the Canadian Geochronology Knowledgebase compilation (2013) as well as additional data sets compiled in this work from the following publications: Aleinikoff et al (2013, 2021), Bleeker et al (2020), Chiarenzelli et al (2011),Connelly (2006),Davis and Green (1997a),Dunning and Indares (2010),Fairchild et al (2017),Hamilton et al (2004), Jannin et al (2018a, 2018b,Johnson et al (2018Johnson et al ( , 2020,Jordan et al (2006),Krogh et al (1987),Labat et al (2020),Lasalle et al (2014),Markley et al (2018), McCormick et al (2017,Moecher et al (2020),Paces and Miller (1993),Peck et al (2018),Queen et al (1996),Regan et al, (2019),Shinevar et al (2021),,Southworth et al (2010), Swanson-Hysell et al (2019, 2021b,Tollo et al (2010Tollo et al ( , 2017,Williams et al, (2019),Wu et al (2017). For some data sets, new weighted means were calculated for populations of dates.…”

Iterative thermodynamic modelling—Part 1: A theoretical scoring technique and a computer program (Bingo‐Antidote)

“…Calcite-graphite carbon isotope thermometry on surrounding marbles revealed a maximal temperature between 661 and 705°C (Dunn et al, 2019). Markley, Dunn, Jercinovic, Peck, and Williams (2018) reported U-Pb ages for monazite from the same outcrop (see sample West Guilford HA13). The first age population of 1,095-1,080 Ma was interpreted as reflecting monazite growth during prograde metamorphism, whereas the second age population of 1,050-1,045 Ma was interpreted as reflecting re-equilibration during a late retrogression stage after cooling and/or decompression.…”