Michael A. Hamilton

Monazite geochronology of UHP and HP metamorphism, deformation, and exhumation, Nordøyane, Western Gneiss Region, Norway

Abstract U-Th-Pb monazite geochronology is combined with previous structural analysis and quantitative estimates of metamorphic conditions to date the thermotectonic evolution of UHP and HP plates (820 °C, 39 kbar; 780 °C, 18 kbar) metamorphosed during the Late Silurian-Early Devonian collision between Baltica and Laurentia. The Upper Plate contains a microdiamond-bearing, kyanitegarnet- graphite gneiss and associated kyanite eclogites, independently indicating probable diamond-forming conditions. In situ dating of monazite in the microdiamond sample, using the SHRIMP II at the Geological Survey of Canada, yielded ages of 415 ± 6.8 Ma for those included in garnet and 398 ± 6 Ma for those in the matrix. These ages compare to 408.0 ± 5.6 and 397.5 ± 4.4 Ma determined using the electron microprobe at the University of Massachusetts. Both methods also identified complexly zoned detrital cores up to 150 micrometers in diameter with ages of 1100-950 Ma and scattered grains with ages of 900-500 Ma, but no ages of 1680-1650 Ma equivalent to the local Baltica basement were found. Agreement between the two techniques allowed evaluation of monazite age domains (198 analyses) from the microdiamond rock and a kyanite-garnet-sillimanite mylonite produced from it, using a combination of high-resolution element imaging and trace-element analysis of U, Th, Pb, and Y. This comparison yielded three mean ages of 407.0 ± 2.1 Ma, 394.8 ± 2.3 Ma, and 374.6 ± 2.7 Ma. Combining this geochronology with previous P-T estimates, we propose that the UHP unit reached its maximum depth of 125 km, at a maximum age of 407 Ma when monazite was included in garnet, and experienced 65 km of exhumation at an average rate of 10.9 mm/year during top-southeast thrusting that brought it into contact with the HP unit. Following these events, both units were exhumed together at an average rate of 3.8 mm/year until reaching a depth of 37 km at 395 Ma, where these rocks experienced extensive re-equilibration, and top-west and left-lateral shearing. After 395 Ma, these units continued to be exhumed at an average rate of 0.8 to 1.4 mm/year until 375 Ma, the time of last equilibration of asymmetric monazite porphyroclasts in the mylonite. The exhumation histories of these units record a change in mechanism from syncollisional exhumation through late- to post-orogenic collapse that was a consequence of plate reorganization

Detrital zircon geochronology and provenance of the Torridonian, NW Scotland

Between 30 and 50 single detrital zircons from each of four specimens of the Stoer Group and two specimens from the unconformably overlying Torridon Group were analysed on the GSC SHRIMP II ion probe. 207Pb/206Pb ages of zircons from the Stoer Group range between 3.00 and 1.74 Ga with 95% concentrated between 2.93 and 2.48 Ga. The Bay of Stoer Formation has a small mode c. 2.55 Ga, matching overgrowth ages on some older grains. A few grains between 1.92 and 1.74 Ga occur in each of the Stoer Group specimens. These data are consistent with conventional provenance information and sedimentology, which indicate that the bulk of the Stoer Group probably was derived from local basement of the Lewisian Gneiss Complex and that deposition occurred adjacent to basin-margin growth faults. The two Torridon Group samples have similar detrital zircon age profiles with distinctive modes at 1.80 Ga, 1.66 Ga and 1.10 Ga, interpreted to represent sources of Ketilidian, Labradorian and Grenvillian affinity, respectively. A less well defined cluster c. 2.85–2.55 Ga reflects reworking of the underlying Stoer Group and possibly direct contribution from the Lewisian Gneiss Complex. The youngest concordant detrital zircon yields a maximum age of 1060±18 Ma for the Torridon Group (Applecross Formation). Our data, together with palaeocurrents from the Applecross Formation, suggest that the Torridon Group could have been deposited by a late to post-Grenvillian intermontane or foreland trunk river system flowing northeasterly, parallel to the Grenvillian orogenic belt.

Integrated Paleomagnetism and U‐Pb Geochronology of Mafic Dikes of the Eastern Anabar Shield Region, Siberia: Implications for Mesoproterozoic Paleolatitude of Siberia and Comparison with Laurentia

This article reports the first joint paleomagnetic and U‐Pb geochronologic study of Precambrian diabase dikes in the Anabar Shield and adjacent Riphean cover of Siberia. It was undertaken to allow comparison with similar published studies in Laurentia and to test Proterozoic reconstructions of Siberia and Laurentia. An east‐trending Kuonamka dike yielded a provisional U‐Pb baddeleyite emplacement age of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Neoproterozoic palaeogeography of the Cadomia and Avalon terranes: constraints from detrital zircon U–Pb ages

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Zircon U-Pb geochronology of the Ottawan Orogeny, Adirondack Highlands, New York: regional and tectonic implications

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Comparative isotopic and chemical geochronometry of monazite, with implications for U-Th-Pb dating by electron microprobe: An example from metamorphic rocks of the eastern Wyoming Craton (U.S.A.)

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