Cooling and inferred uplift/erosion history of the Grenville Orogen, Ontario: Constraints from ⁴⁰Ar/³⁹Ar thermochronology

Abstract

Stepwise 40Ar/39Ar degassing experiments of 57 mineral separates of hornblende, muscovite, biotite, and perthitic microcline have been used in conjunction with petrologic observations to place regional constraints on the postmetamorphic cooling and the inferred uplift and erosion history of the Grenville Orogen in Ontario. The 40Ar/39Ar data support an interpretation of slow, nearly uniform cooling (1 ø-4 ø C/m. y.) from temperatures of ~500øC to below ~ 150øC. In the Central Gneiss Belt (CGB) hornblendes cooled through Ar closure between 930 and 1025 Ma, whereas in the Central Metasedimentary Belt (CMB) hornblendes record the following range in 4øAr/39Ar cooling ages: 1104 Ma in the Frontenac terrane, 1007-1067 Ma in the Sharbot Lake terrane, 919-1026 Ma in the Elzevir terrane, and 972 Ma in the Central Metasedimentary Belt Boundary Zone. Regional uplift/erosion rates of 0.03-0.14 km/m.y. have been estimated for the Grenville Orogen in Ontario based on the 4øAr/39Ar data, a model retrograde P-T path for rocks of the CGB, and an upper time constraint provided by flat, overlying Cambrian and Ordovician sediments. These erosion rates are consistent with rates estimated for other Proterozoic or Archcan granulite terranes but are an order of magnitude slower than active orogens such as the Alps and Himalayas. A regular variation in hornblende 40Ar/39Ar cooling ages is observed in rocks that traverse highly strained often mylonitic shear zones that separate the four major terranes of the CMB. The pattern of 40Ar/39Ar ages is interpreted to reflect late-tectonic extension, consistent with field observations in the Central Metasedimentary Belt Boundary Zone and elsewhere in the CMB. Up to 13 km of vertical displacement is inferred for some rocks in the CMB between the time they cooled below closure to argon diffusion in hornblende (~500øC) and their exposure at the surface (~25øC).1Now at Copyfight 1991 by the American Geophysical Union. Paper number 91TC00859. 0278-7407/91/91 TC-00859510.00 U/Pb, Sm/Nd, Rb/Sr, K/Ar, or 4øAr/39Ar), for which retention of the daughter isotopes or fission tracks is temperature dependant [e.g., Cliff, 1985]. The 4øAr/39Ar technique is especially useful for cooling studies because potassium-bearing minerals are abundant in many igneous and metamorphic rocks and because these minerals begin to quantitatively retain argon over a wide range of distinct temperatures. The 4øAr/39Ar dating technique has been successfully applied in many cooling history studies of regionally metamorphosed rocks [e.g., Dallmeyer et al., 1975; Dallmeyer and Sutter, 1976; Berger and York, 1981b; Harrison and McDougall, 1981; Sutter et al., 1985; Horton et al., 1987]. The Grenville Province (or Orogen, as used throughout this paper) refers to the 1.0-1.2 Ga old, mostly high-grade metamorphic terrane, including the Adirondacks, exposed from Georgian Bay to the Labrador coast [Moore, 1986; Woussen et al., 1986]. Rocks considered to have genetic affinities with the Grenville Orogen have been reported from the Appalachian...

Tectonic Setting and Evolution of the Grenville Orogen: An Assessment of Progress Over the Last 40 Years

“…Recognition of the orogenic lid and its tectonic juxtaposition with exhumed mid crust along normal sense shear zones provided evidence for the crustal scale of orogenic collapse and permitted integration of the robust, but previously stranded 'old' 40 Ar/ 39 Ar data (e.g. Reynolds 1989;Cosca et al 1991Cosca et al , 1992Cosca et al , 1995Corrigan and van Breemen 1997;Streepey et al 2004;Selleck et al 2005) into a coherent tectonic model. The lid was subsequently named the Ottawan Orogenic Lid (OOL) by Rivers (2012).…”

“…At the same time, another geochronological technique, 40 Ar/ 39 Ar analyses of hornblende, muscovite and biotite, was providing a database of 'apparent ages' relating to the time of cooling through ~500°C, 350°C and 300°C respectively (e.g. Cosca et al 1991Cosca et al , 1992. Overall, when excess argon ages were discounted, these supported the preliminary conclusions of Harper (1967) based on the K/Ar method that post-orogenic cooling was fast in the foreland near the Grenville Front, but anomalously slow in the hinterland where isotopic closure in some areas did not occur until 950 Ma, some 80-100 Ma after the metamorphic peak.…”

Global events across the Mesoproterozoic–Neoproterozoic boundary: C and Sr isotopic evidence from Siberia

“…1300 Ma (McLelland et al, 1996), marking the beginnings of Rodinia assembly. The protracted assembly event ultimately resulted in continentcontinent collision and the uplift of a global chain of mountain belts (Dalziel, 1991;Hoffman, 1991), which reached peak thermal metamorphism asynchronously over the interval 1160-1050 Ma (Mezger et al, 1991(Mezger et al, , 1993McLelland et al, 1996;Cosca et al, 1998), and unroofing occurred nearly concurrently with peak thermal events (Cosca et al, 1991(Cosca et al, , 1992(Cosca et al, , 1998. Unroofing continued until 850-800 Ma at a uniformly low rate (Cosca et al, 1991;Mezger et al, 1991;Cosca et al, 1992;McLelland et al, 1996;Cosca et al, 1998).…”

Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France

“…. Cooling rates for lower-crustal granulite terranes that experienced erosion-controlled exhumation are c. 1-4°C Ma-' (Giletti et al, 1988;Cosca et al, 1991;Mezger et al, 1991), whereas tectonic exhumation of metamorphic core complexes can yield extreme cooling rates of up to 300°C Ma-' (Hill et al, 1992;Zeck et a/., 1992).…”