Timing of tectonometamorphism across the Green Mountain anticlinorium, northern Vermont Appalachians: 40Ar/39Ar data and correlations with southern Quebec

Abstract

In the pre-Silurian lithotectonic units of the northern Vermont Appalachians, the timing of orogenesis and tectonometamorphism has traditionally been ascribed to the combined effects of the Middle Ordovician Taconian orogeny and Middle to Late Devonian Acadian orogeny. However, numerous geo chronological studies throughout the Northern Appalachians, including neighboring southern Quebec, have obtained Silurian and Early Devonian age data that document more or less continuous tectonometamorphic activity throughout the Ordovician-Devonian. The structural and metamorphic evolution of northern Vermont can be separated into three regional phases, which are characterized by distinct structures, fabrics, and metamorphic parageneses. The fi rst phase (D 1 ), associated with westward emplacement of various thrust slices leading to crustal thickening and regional metamorphism, and the second phase (D 2 ), characterized by bivergent structures and metamorphic overprint, have both been considered to be Taconian. The third phase, the structure and fabric of which are also observed in the Silurian-Devonian rocks to the east, is considered to be Acadian. We present new step-heating and spot fusion 40 Ar/ 39 Ar geochronological data on amphibole and fabric-forming muscovite from samples taken across the Green Mountain anticlinorium, which, coupled with published data, provide improved age constraints on tectonometamorphism of D 1 (latest Cambrian to Middle Ordovician), D 2 (Silurian-Early Devonian), and D 3 (Middle Devonian) events. By comparing structural and metamorphic characteristics, and now timing, these phases are interpreted to be correlative to the tripartite tectonometamorphic evolution documented in southern Quebec, and they further exemplify the along-strike diachronism of tectonism induced by the inherited irregular geometry of the Laurentian margin. Ar-Ar age Castonguay et al. (2007) Ar-Ar ageCambrian and Ordovician ophiolitic and suprasubduction zone metamorphosed magmatic and associated metasedimentary rocks from a peri-Laurentian arc system.Ordovician ophiolitic and suprasubduction zone metamorphosed magmatic and associated metasedimentary rocks from a peri-Gondwanan arc system. Proterozoic Grenville basement of Laurentia.Silurian to Devonian syn-Acadian metasedimentary rocks and associated metaigneous rocks.Neoproterozoic to Cambrian rift-related clastic metasedimentary rocks and associated metavolcanic rocks.Middle Devonian granitic rocks. Mesozoic intrusive rocks.Cambrian and Ordovician clastic and carbonate platform rocks overlain by Middle to Upper Ordovician Taconic clastic wedge rocks.Neoproterozoic-Ordovician Iapetan drift facies continental slope and rise metasedimentary and associated metaigneous rocks.

Evidence of Late Ediacaran Hyperextension of the Laurentian Iapetan Margin in the Birchy Complex, Baie Verte Peninsula, Northwest Newfoundland: Implications for the Opening of Iapetus, Formation of Peri-Laurentian Microcontinents and Taconic – Grampian Orogenesis

“…8), and graphitic pelite in a sequence that underwent TaconicGrampian blueschist-facies metamorphism . Blueschist facies metamorphism is also preserved in correlative OCT rocks in northern Vermont (Doolan et al 1982;Castonguay et al 2012), emphasizing the link between high-pressure metamorphism and OCT assemblages established in the Alps ) and in the Taconic -Grampian orogen of the Appalachian -Caledonian mountain belt. A discontinuous horizon of serpentinite bodies has also been documented in the Easdale Subgroup of central and northeastern Scotland (Garson and Plant 1973).…”

Taconic Metamorphism Preserved in the Baie Verte Peninsula, Newfoundland Appalachians: Geochronological Evidence for Ophiolite Obduction and Subduction and Exhumation of the Leading Edge of the Laurentian (Humber) Margin During Closure of the Taconic Seaway

“…Advocate ophiolite complex). The exceptional preservation of the early D 1 structures suggested that this area may have escaped the intense overprinting by Salinic orogenesis, similar to parts of southern Quebec and northern Vermont Appalachians (Castonguay et al 2001(Castonguay et al , 2012. In this contribution we present new 40 Ar/ 39 Ar and in situ U-Pb zircon ages from polymetamorphic units of the Baie Verte Peninsula.…”

“…Syn-to post-collision exhumation in the East Pond Metamorphic Suite was significantly more protracted and resulted in growth of retrograde amphibolite-facies mineral assemblages, which occurred prior to the 440 Ma hornblende cooling age. Laird et al 1984;Castonguay et al 2012). 40 Ar/ 39 Ar data from the Quebec Appalachians (496-461 Ma; Whitehead et al 1995Whitehead et al , 1996Castonguay et al 2001Castonguay et al , 2007Tremblay et al 2011;De Souza et al 2012) obtained from BVOT ophiolite (e.g.…”

Taconian orogenesis, sedimentation and magmatism in the southern Quebec-northern Vermont Appalachians: Stratigraphic and detrital mineral record of Iapetan suturing

“…Mines ophiolite; UT-Underhill Thrust. Asterisks indicate the location of sampling sites for the U-Pb and 40 Ar/ 39 Ar data presented in this study: A-samples 07BUNKER and 10BUNKER01; B-09SV01; C-09SV02; D-09MILAN01; E-09MILAN02; F-08M44. informally referred to as the Connecticut Valley sequence ( fig.…”

“…In this contribution we present detailed field observations and detrital zircon U-Pb and muscovite 40 Ar/ 39 Ar geochronological data for volcanic and sedimentary rocks of the Dunnage zone and the Connecticut Valley-Gaspé trough of southern Quebec. It will be shown that these syn-to post-orogenic units form an almost continuous lithostratigraphic record of Ordovician to Devonian basin formation and ongoing tectonics.…”