Abstracts
Abstract
Monazite and sphene, separated from sillimanite-grade schists, gneisses and two-mica granites in the Central Maine Terrane 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 of 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 teiranes 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.
RESUME
L'application de la syst&natique U-Pb a permis la datation de la monazite et du sphene ex traits de schistes m£tamorphis6s au grade de la sillimanite, de gneiss et de granites a deux micas provenant de la Laniere de Central Maine et de la Fosse de Merrimack. Les ages ainsi obtenus imposent des contraintes premises sur l*6poque du paroxysme me'tamorphique et du plutonisme en Nouvelle-Angleterre septentrionale. Dans la Laniere de Central Maine, le me'tamorphisme est d'age acadien (£odevonien). Dans la Fosse de Merrimack, d'un cote" a Fautre de la zone de failles de Campbell Hill-Nonesuch River-Norumbega, lem6tamorphisme date de 1' A116ghanien(?) (Permien). Les ages des granites mettent en evidence un episode distinct de magmaiisme deVonien qui caracteiise la Laniere de Central Maine.
Dans cette portion de l'Orogene appalachien, la laniere presentant un haut degre" de me'tamoiphisme est composite. Elle comprend des blocs crustaux ayant subi des Episodes distincts de me'tamorphisme de haut degre' qui d^butent possiblement a rOTdovicienmoyenpours'dtalerjusqu'au Permien. Les ages du paroxysme mdtamorphiqueconfortentrhypothesevoulanique les lanieres de Central Maine et de Merrimack aienl eu3es histoires tec tono-mfitamorphiques differ entes et que leur juxtaposition a un meme grade metamorphique ne soit que coincidence.
L'hisloire de la zone de failles de Campbell Hill-Nonesuch River-Norumbega est active, complexe et s '&helonne d'environ 360 Ma jusqu'a au moins 250 Ma. Cette frontiere forme un candidal plausible pour le Front alleghanien (varisque) occidental en Nouvelle-angleterre. La juxtaposition finale des lanieres de Central Maine et de Merrimack s'effectua peut-etre au M6sozo'ique le long de failles en extension bordanl les lanieres, possiblement par reactivation des accidents compressifs acadiens et/ou des structures transprcssives allfighaniennes.
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