The timing of peak high-grade metamorphism in central-eastern New England

Abstract

Monazite and sphene, separated from sillimanite-grade schists, gneisses and two-mica granites in the Central Maine Terr ane and Merrimack Trough, have been dated using the U-Pb system. The ages constrain the timing of peak high-grade metamorphism and plutonism in northern New England. In the Central Maine Terrane metamorphic ages are Acadian (early Devonian); in the Merrimack Trough, across the Campbell Hill-Nonesuch River-Norumbega fault zone, the age o f metamorphism is Alleghe nian!?) (Permian). The granite ages outline a distinct pulse of Devonian magmatism, characteristic of the Central Maine Terrane. The high-grade terrane of this part of the Appalachian Orogen is composite. It is made up of crustal blocks that experienced discrete pulses of high-grade metamorphism beginning perhaps as long ago as the pre-Middle Ordovician and extending into the Permian. Ages of peak metamorphism support the hypothesis that the Central Maine and Merrimack terranes had different tectono-metamorphic histories and are coincidentally juxtaposed at the same metamorphic grade. The Campbell Hill-Nonesuch River-Norumbega fault zone has had an active and complex history, beginning approximately 360 Ma and lasting at least to 250 Ma. This boundary is a likely candidate for the western Alleghenian (Variscan) Front in New England. The final juxtaposition of the Central Maine and Merrimack terranes may have occurred during the Mesozoic along extensional, terrane-bounding faults, possibly the reactivated Acadian compressional and/or Alleghenian transpressional structures.

Paleozoic orogens in New England, USA

“…This view was not entirely supported by radiometric ages on plutons but was confirmed by a series of ages around 390-400 Ma on metamorphic monazite in high grade mica schists within or near the large sillimanite-grade metamorphic high of central New Hampshire which locally reaches conditions for the formation of sillimaniteorthoclase-garnet-cordierite assemblages (Eusden & Barreiro 1988). It was naturally assumed that the nearby granulite-facies metamorphic high of central Massachusetts, also with sillimanite-orthoclase-garnet-cordierite assemblages, would be the same age, although there are substantial differences in metamorphic character between the two zones.…”

“…285 to 275 Ma and from ca. 282 to 261 Ma (Eusden & Barreiro 1988). Permian Ar-Ar hornblende release spectra also show that a Late Paleozoic thermal event reached amphibolite grade (West 1993).…”

Dextral transpression at the Casco Bay restraining bend, Norumbega fault zone, coastal Maine

“…2A) as the Casco Bay shear-zone system (Swanson, 1995). Individual faults in the Casco Bay area include the South Portland, Cape Elizabeth, Broad Cove, and Flying Point fault zones and they serve as a linking structure to the Nonesuch River-Campbell Hill fault to the southwest (Eusden and Barreiro, 1988;Hussey and Newberg, 1978). Continuation of these faults to the southwest, however, is problematic, hampered by poor exposure.…”

Insights into the Acadian orogeny, New England Appalachians: a provenance study of the Carrabassett and Kittery formations, Maine

“…1) is a challenge to this interpretation Aleinikoff et al 2007). Instead, the geochronology of Avalonia and inboard peri-Gondwanan terranes document a Pennsylvanian-Late Permian orogenic event (Eusden and Barreiro 1988;Spear and Harrison 1989;Tucker and Robinson 1990;Dallmeyer and Takasu 1992;Wintsch et al 1992;Getty and Gromet 1992;West 1993;Lux and West 1993;Walsh et al 2007). …”