Abstracts
Abstract
Three major fault-bounded terranes comprise the eastern margin of the Appalachian Orogen In the vicinity of Boston, Massachusetts. Each has a distinctive geological history. The easternmost, the Boston-Avalon Zone, contains features common to Avalonlan terranes in the northern Appalachians. These Include voluminous Late Proterozolc (650-600 Ha) calc-alkaline granitic to dioritic plutons and related volcanic rocks associated with the "main phase" of Avalonian thermal activity. These are covered by a thin veneer of Late Proterozolc to Early Paleozoic sedimentary rocks and fluviatile Carboniferous basin deposits. This terrene can be divided into three subzones on the basis of the igneous rocks present, metamorpbic grade, and style of deformation. The metamorphism generally is no higher than the green-schist facles in the Boston area, but reaches the amphibolite faciea to the south and west. The metamorphiem ia generally assigned an Alleghanlan age although there is evidence for Late Precamhrlan deformation and metamorphism.
To the west, across the Bloody Bluff fault zone is the Hashoba Block, a terrane largely underlain by mafic volcanic rocks and volcanogenlc sedimentary rocks. During the Ordovlclan-Silurlan this terrane underwent upper amphibolite facles metamorphism and was intruded by calc-alkaline intermediate plutons and metalunlnous to peralumlnous granites.
Best of the Nashoba Block, across the Clinton-Newbury fault zone, is the Merrimack Trough, underlain by a thick sequence of flysch and calcareous flysch. Crosscuttlng Ordovician plutons establish an upper age limit for these sedimentary rocks and the ages of deformation and netamorphism.
Because of differing Late Proterozolc - Early Paleozoic histories. It Is unlikely that the Boston-Avalon Zone and Nashoba Block could have been juxtaposed prior to the Mid-Paleozoic. Whether the Nashoba Block and Merrimack Trough represent separate terranee, a composite terrane joined in the Late Precamhrlan or Early to Mid-Paleozoic, or faulted portions of a single terrane remains to he determined.
RESUME
Aux alentours de Boston, au Kassachusetts, la marge orlentale de 1'Orogene Appalachlen comprend trols lanlerea majeures bordees par des failles. Chacune a son histolre geologlque dlstincte. La plus orlentale de celles-ci, la Zone de Boston-Avalon, poasede des traits commune aux lanlerea avaloniennes du Nord-Est des Appalaches. Parmi ceux-ci, on peut mentioimer de volumlneux plutons calco-alcalina. granltlquea A dloritlques, taxdiproterozoiques (650-600 Ma) alnsi que des volcanites, apparentees a ces derniers, qui sont associees avec la "phase principale" de l'activite thermlque avalonienne. En general, le metamorphlsme ne depasse pas le facles a schlstes verts dans la region de Boston quoiqu'il attelgne le faciea a amphlbolites au sud et a l'oueat. On attribue generalement un age alleghanien au metamorphlsme bien qu'il y ait preuve d'une deformation et d'un metamorphlsme tardiprecambriens.
A l'oueat, de 1'autre cote de la Zone de failles de Bloody Bluff, se trouve le Bloc de Nashoba, une laniere formee surtout de volcanltea mafiquea et de rochea sedimentaires volcanogenea. Durant l'Ordovicien et le Silurien, cette laniere subit un metamorphisme au facles A amphlbolites superieur alnsi qu'une Intrusion de plutons neutres calco-alcallns et de granites metalumineux a peralumineux.
A l'ouest du Bloc de Nashoba, au-dela de la zone de failles de Clinton-Newbury, se trouve la Fosse de Merrimac qui renferme une pulssante assise de flyschs et de flyscha calcatrea. On etablit la 11mlte BUperleure sur l'age de ces rochea sedimentaires alnsi que lea ages de la deformation et du metamorphlsme grace aux plutons ordovlclena qui traveraent celles-ci.
La juxtaposition de la Zone de Boston-Avalon et du Bloc de Nashoba avant le Paleozoique moyen n'est guere vraisemblable en ralson de leurs hlstolres dlfferentes du Tardlproterozoique A l'Eopaleozoique. II reste encore a determiner si le Bloc de Nashoba et la Fosse de Merrimac representent des lanlerea separees. une laniere composite assemblee lors du Tardiprecambrlen ou de l.Eo- au Mediopaleozoique. ou bien des parties d'une seule et meme laniere qui sont en rapport par failles..
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