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

Abstract

In this thesis field observations and laboratory data are related to the regional cooling and uplift/erosion history of the Grenville Province in Ontario. The conclusions reached during this investigation provide important constraints for tectonic models of the evolution of this ancient mountain belt. Equally important, these results provide a quantitative measure of the time involved in mountain building processes, thus yielding fundamental insights into phenomena that continually act to reshape the face of our planet. Thermochronological ($\\sp{40}$Ar/$\\sp{39}$Ar) data are presented from 76 mineral separates of hornblende, muscovite, biotite, phlogopite, and K-feldspar. Samples were selected from regionally metamorphosed gneiss, amphibolite, metasediment, marble, metagabbro and pegmatite across the two major metamorphic belts of the Grenville Province, the Central Metasedimentary Belt (CMB) and the Central Gneiss Belt (CGB). When combined with published temperature estimates for closure to argon diffusion in the phases analyzed, cooling rates from $\\sim$500$\\sp\\circ$C to $\\sim$120$\\sp\\circ$C of 1-4$\\sp\\circ$C/Ma are calculated across the entire Grenville Province of Ontario. Regional uplift/erosion rates for the Grenville Orogen of Ontario have been estimated from the $\\sp{40}$Ar/$\\sp{39}$Ar data, a retrograde P-T path for rocks of the CGB, and an upper time constraint provided by flat, overlying Cambro-Ordovician sediments. These erosion rates of 0.03-0.07 km/Ma are consistent with estimated rates of other Proterozoic or Archean granulite terranes, but are an order of magnitude slower than active orogens such as the Alps and Himalayas. Different terranes within the CMB preserve hornblende $\\sp{40}$Ar/$\\sp{39}$Ar cooling ages that vary by as much as 200 Ma. The preservation of distinct ages between terranes is interpreted to result from sub 500$\\sp\\circ$C tectonism that resulted in the exposure of different structural levels by late extension. Twenty-two of the hornblendes used for thermochronology have been quantitatively analyzed for major elements by microprobe, Fe$\\sp{2+}$/Fe$\\sp{3+}$ by wet chemistry, and for H$\\sb2$O by manometric measurement. Formulae have been determined for the hornblendes on the basis of O + OH + Cl + F = 24 which indicate an oxyamphibole component of up to 24% for some samples. Normalization schemes based only on microprobe data are discussed with best results obtained assuming Si + Ti + Al + Fe + Mn + Mg = 13 cations. Water activities (aH$\\sb2$O) calculated from hornblende equilibria are typically low ($<$0.01) because of the exponential dilutions in hornblende (tremolite) activity required by present activity-composition models. An oxyamphibole component of 25% further reduces any amphibole component and the H$\\sb2$O activity by as much as 50% below that calculated with simplifying assumptions usually made regarding X$\\sb{\\rm OH}$ in the hydroxyl site (e.g., X$\\sb{\\rm OH}$ = 1 or X$\\sb{\\rm OH}$ = 1 $-$ X$\\sb{\\rm Cl} - \\rm X\\sb{\\rm F}$). These findings indicate that different amphibole normalization schemes have a marked effect on the activity calculated for a specific amphibole or H$\\sb2$O, and should be carefully evaluated.