Contrasting styles of Taconian, Eastern Acadian and Western Acadian metamorphism, central and western New England

Abstract

The two major Early to Middle Palaeozoic tectonic/metamorphic events in the northern Appalachians were the Taconian (Middle to Late Ordovician) in central to western areas and the Acadian (Late Silurian to early Middle Devonian) in eastern to west-central areas. This paper presents a model for the Acadian orogenic event which separates the Acadian metamorphic realm into eastern and western belts based on distinctively different styles. We propose that the Acadian metamorphism in the east was the delayed consequence of Taconian back-arc lithospheric modification. East of the Taconian island arc, thick accumulations of Late Ordovician and Silurian sediments, coupled with plutons rising along a magmatic arc, produced crustal thermal conditions appropriate for anomalously high-T, low-P metamorphism accompanied by major crustal anatexis. In this zone, upward melt migration was coupled with subsequent E-W crustal shortening (possibly due to outboard collision with the Avalon terrane) to produce mechanical conditions that favoured formation of fold and thrust nappes and resultant tectonic thickening to the west (and probably to the east as well).The basis for the distinction between the Eastern and Western Acadian events lies in the contrasting styles of metamorphism accompanying each. Evidence for contrasting metamorphic styles consists of (1) estimated metamorphic field gradients (MFGs) based on thermobarometric studies, and (2) petrological evidence for contrasting P-T trajectories. West of the Acadian metamorphic front, the Taconian zone has an MFG in which peak temperatures of 400-600" C were reached at pressures of about 4-6 kbar, with both P and T increasing to the east. Near its western edge, the Western Acadian metamorphic overprint has a similar MFG to the Taconian, and is mainly discriminated by 4"Ar/"Ar dating and microtextural evidence. East of this narrow zone, the Western Acadian overprint is characterized by progressively higher temperatures (600-725" C) and pressures (6.5-10 kbar, or more) to the east, yielding an overall MFG that lies along, or slightly above, the kyanite-sillimanite boundary on a P-T diagram. There is little or no plutonism accompanying Western Acadian metamorphism.In contrast, thermobarometry in the Eastern Acadian, east of the Bronson Hill Belt, yields high-T, intermediate-P conditions for the highest grade rocks known in New England: T = 650-750" C, P = 4.5-6.5 kbar for granulite facies assemblages which apparently formed along an 'anticlockwise' P-T path. The Bronson Hill Belt lies geographically between the Eastern and Western Acadian zones and shows transitional petrological behaviour: anomalously high temperatures at intermediate pressures, but a 'clockwise' path with decompression cooling.Radiometric dating indicates peak Taconian conditions may have been achieved as early as 475 Ma in the Taconian hinterland and as late as 445Ma in the Taconian foreland (including the Taconic allochthons). Eastern Acadian magmatism may have started as early as 425 Ma, and most nappe-stage def...

Oriented multiphase needles in garnet from ultrahigh-temperature granulites, Connecticut, U.S.A.

“…The easternmost of these (Eastford fault) cuts the 379 Ma Canterbury Gneiss, thus establishing a maximum age for fault movement (U-Pb zircon dating of Wintsch et al 2007). The CMT of northeastern CT underwent high-grade metamorphism during Acadian/ Neoacadian orogenesis (Devonian; Rodgers 1981; Robinson and Tucker 1982;Schumacher et al 1989;Armstrong et al 1992;Wintsch et al 1992Wintsch et al , 2009Robinson et al 1998;Thomson 2001); however, the precise timing of events awaits future geochronological studies.…”

Diachronous supracrustal extension in an intraplate setting and the origin of the Connecticut Valley–Gaspé and Merrimack troughs, northern Appalachians

“…In the Gaspé Peninsula and New Brunswick, this Taconian suture is overlain by younger deposits or affected by faulting, and both troughs form a composite basin (Fig. On the basis of different P/T paths, metamorphic conditions and ages, Armstrong, Tracy & Hames (1992) have separated the New England Acadian orogen into Western and Eastern belts (Fig. The Connecticut Valley-Gaspé and Merrimack troughs are characterized by contrasting tectonic and metamorphic histories attributed to the Acadian orogeny.…”

“…LL -Logan's Line; SF -Squirtgun fault; SPF -Sennebec Pond fault, which is interpreted as the Medial New England-Composite Avalon boundary in coastal Maine (West & Hubbard, 1997). (b) P-T diagram showing postulated typical P-T trajectories for the Western and Eastern Acadian belts as defined by Armstrong, Tracy & Hames (1992). See text for discussion.…”

“…Other symbols as in Figure 1. From Armstrong et al (1992). (b) P-T diagram showing postulated typical P-T trajectories for the Western and Eastern Acadian belts as defined by Armstrong, Tracy & Hames (1992).…”

Crustal redistribution, crust–mantle recycling and Phanerozoic evolution of the continental crust

“…This estimate is supported by geological evidence. For example, the mountains formed by the Acadian Orogeny in eastern North America after closure of the Iapetus Ocean reached peak metamorphism around 395 Ma (Armstrong et al, 1992), but were truncated by a peneplain erosion unconformity of Late Carboniferous age (~320 Ma) in western Ireland (Graham, 1981), suggesting~75 million years was required to remove the excess crust by erosion. Surface processes are quite efficient at recycling continental crust over timescales much shorter than the age of the planet.…”