Lindley Hanson's List: Appalachain Geology

A lost arc–back-arc terrane of the Dunnage oceanic tract recorded in clasts from the Garin Formation and McCrea mélange in the Gaspé Appalachians of Québec
Céline Dupuis1,{dagger}, Michel Malo1,*, Jean Bédard2,§, Bill Davis3 and Mike Villeneuve

Upper Ordovician Garin conglomerates (base of the Gaspé belt, Québec Appalachians) contain three igneous clast populations. (1) Calc-alkaline intermediat-felsic rocks resemble Exploits (New Brunswick, Maine) and Notre Dame (Québec, Newfoundland) subzone lavas. Clasts (monzonite-rhyodacite) give U-Pb zircon ages between 465 and 466 Ma, precluding correlation with Exploits rocks. We suggest that the suite represents peri-Laurentian continental arc magmas coeval with the Red Indian Lake Group of Newfoundland. Correlation of lavas from a Gaspé well with the Popelogan arc implies that the Red Indian Line passes through southern Gaspé. (2) Mafic tholeiitic-to-alkaline clasts resemble New Brunswick and Maine (Exploits subzone) alkaline lavas. We interpret them as magmas associated with initiation of a back-arc basin, possibly on the Laurentian margin. (3) Mafic-intermediate tholeiitic to calc-alkaline clasts have an oceanic subduction component and correlate with Notre Dame subzone lavas (Québec, Newfoundland). They are interpreted as products of spreading and off-axis magmatism of a peri-Laurentian back-arc basin. We correlate McCrea mélange lavas to the calc-alkaline intermediat-felsic and mafic-intermediate suites and interpret them as samples of a Notre Dame subzone terrane coeval with the Red Indian Lake Group of Newfoundland during initiation of a successor arc–back-arc, a terrane which is no longer preserved in New Brunswick and Maine but that provides some evidence for possible extensions of Newfoundland's geology into the Gaspé Appalachians of Québec. A mid-ocean-ridge basalt–like intrusion in the McCrea mélange (Ar-Ar age: 471.2 ± 11.2 Ma) is coeval with peri-Laurentian ophiolites in Newfoundland and may record a back-arc opening event.

Key Words: igneous clasts • cal

Contrasting styles of Taconian, Eastern Acadian and Western Acadian metamorphism, central and western New England
T. R. ARMSTRONG 1 , R. J. TRACY 1 W. E. HAMES*
1 Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0420, USA
Correspondence to * Present address: Department of Earth, Atmospheric and Planetary Sciences, M.I.T., Cambridge, MA 02139, USA.
Copyright 1992 Blackwell Publishing Ltd.
KEYWORDS
Acadian metamorphism • Appalachians • high-T/low-P • high-T/intermediate-P
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 traject

Supra–subduction zone extensional magmatism in Vermont and adjacent Quebec: Implications for early Paleozoic Appalachian tectonics
Jonathan Kim{dagger},1, Raymond Coish{ddagger},2, Matthew Evans{ddagger},2 and Gregory Dick{ddagger},2

1 Vermont Geological Survey, 103 South Main Street, Waterbury, Vermont 05671, USA
2 Geology Department, Middlebury College, Bicentennial Hall, Middlebury, Vermont 05753, USA

Metadiabasic intrusions of the Mount Norris Intrusive Suite occur in fault-bounded lithotectonic packages containing Stowe, Moretown, and Cram Hill Formation lithologies in the northern Vermont Rowe-Hawley belt, a proposed Ordovician arc-trench gap above an east-dipping subduction zone. Rocks of the Mount Norris Intrusive Suite are characteristically massive and weakly foliated, have chilled margins, contain xenoliths, and have sharp contacts that both crosscut and are parallel to early structural fabrics in the host metasedimentary rocks. Although the mineral assemblage of the Mount Norris Intrusive Suite is albite + actinolite + epidote + chlorite + calcite + quartz, intergrowths of albite + actinolite are probably pseudomorphs after plagioclase + clinopyroxene. The metadiabases are subalkaline, tholeiitic, hypabyssal basalts with preserved ophitic texture. A backarc-basin tectonic setting for the intrusive suite is suggested by its LREE (light rare earth element) enrichment, negative Nb-Ta anomalies, and Ta/Yb vs. Th/Yb trends. Although no direct isotopic age data are available, the intrusions are broadly Ordovician because their contacts are clearly folded by the earliest Acadian (Silurian–Devonian) folds. Field evidence and geochemical data suggest compelling along-strike correlations with the Coburn Hill Volcanics of northern Vermont and the Bolton Igneous Group of southern Quebec. Isotopic and stratigraphic age constraints for the Bolton Igneous Group bracket these backarc magmas to the 477–458 Ma interval. A tectonic model that begins with east-dipping subduction and progresses to outboard west-dipping

The basal Wamsutta Formation of the Narragansett Basin, Massachusetts–Rhode Island, contains Ma rhyolite that is unexpectedly 60 m.yr. older than sedimentary strata containing late Pennsylvanian floras higher in the basin fill. The transitional alkalic geochemistry of the rhyolite and associated basalt signifies Late Devonian rifting pre‐dating major basin subsidence. Surface rifting was accompanied by voluminous alkalic intrusions of the 380–370 Ma Scituate Igneous Suite into Neoproterozoic basement west of the basin and by ductile extension along shear zones confined to the pluton and neighboring units. The Maritimes Basin and adjoining areas in Atlantic Canada experienced a similar mid‐Paleozoic history.

Twelve hundred and sixty-six gravity stations were established in 15,000 square miles of New Hampshire, Maine, Massachusetts, and Vermont. Within that area three detailed surveys were made in places of particular interest. To aid in the interpretation density determinations were made on rock specimens.

Two broad features stand out on the Bouguer map, a coastal belt of positive anomalies and a gravity low over the northern half of the area. The low could be explained by assuming that the exposed granite bodies enlarge with depth and extend beneath the metamorphic rocks, forming a sheet 15,000 feet thick, or it could be ascribed to an increase of about 2 kms in the thickness of the crust.

The relationship between the local gravity features and the surface geology is described. Most of the local anomalies are due to the density contrast between the plutonic rocks and the metamorphic rocks. In general, the plutons are associated with lows, the most prominent of which is found over the White Mountain batholith. Analysis of the gravity data shows that the density contrast between the granite of this batholith and the surrounding metamorphic rocks extends downward at least 5000 feet.

The detailed survey near Lake Winnipesaukee disclosed a gravity high over the Conway Granite of the Merrymeeting stock. There is also a magnetic high over the stock. But the Conway, which belongs to the White Mountain plutonic series, has the lowest density of all the rocks exposed in the area and contains no magnetite. Beneath the surface of the stock there must be a high-density body which contains a magnetic mineral. This body is probably an older member of the White Mountain series, a dionte or gabbro, and the Conway Granite was probably emplaced along the contact of the older body, which subsided to make room for the granite.

The same detailed survey revealed a gravity low over the Pine Mountain ring-dike complex, which also belongs to the White Mountain plutonic series. Analysis of this anomaly shows that the complex forms the ape

How does one "read" a landscape? Inspired by the classic work of Hans Kurath documenting the dialect geography sub-regions of New England, Christopher J. Lenney set out to determine whether such patterns of linguistic migration were repeated in the everyday features of our man-made landscape. Through inspired conjecture and methodical fieldwork, Lenney discovered that at least six cultural and material artifacts could be mapped into similar flows and clusters: placenames, boundaries, townplans, roads, houses, and gravestones. With infectious enthusiasm and wit, Lenney guides the reader through a historical and cultural examination of how this artificial landscape came to be. Of the many possible sources of placenames, for example, there are evident patterns of Algoquian and transplanted English; there is the obvious irony of patriot and Tory honored side by side. But what do we make of the apparent hodgepodge of placename suffixes that dot our maps--the -fields, -tons, -hams, and -burys that append themselves to our life and land? And how do we explain the "Great-Big" line, a dramatic yet invisible scar across the map of Maine? The other five cultural markers similarly reveal themselves in a surprising patterning of the New England countryside--in the areas where the connected farmstead dominates, where recessed balconies or twin rearwall chimneys distinguish the scene; in the migration of gravestone cutters and their motifs, which left odd undulating waves of artistic expression throughout the region. Lenney forces the reader to reconsider the shape of the village greens, to wonder why old roads go where they go, and to question where (good neighbors and Robert Frost notwithstanding) we built stone walls. By pushing us beyond mere sightseeing to "sightseeking," Lenney dares to fundamentally alter the way we--old-time Yankee, newcomer, and tourist alike--experience and interpret the New England landscape.

Images of Wards collection

West of Boston, Mass., Castle and others (1976) recognized an up to 5km wide, possibly folded, NE-SW trending Burlington Mylonite Zone. We have extended mapping south into Natick and Framington quadrangles, and supplemented it by fixing local directions of tectonic motion, which are more variable than reported by Goldstein (1989). In Natick the mylonite zone is partly migmatized and converted into blastomylonites, forming the lithodemic Rice Gneiss and is intersected by the Dedham Granite dated ca 630 Ma. The granite also invades deformed, folded, and commonly mylonitized Westboro Quartzite. Thus mylonitization, folding, and formation of migmatitic blastomylonites are all earlier than ca 630 Ma, and can collectively be attributed to the main phase of the Avalonian orogeny that in Africa is referred to as the Pan-African I. The sense of movements in the Rice Gneiss is generally sinistral strike-slip with a NE-SW trend of foliation. Other local mylonites have more variable directions of motion.

A narrower E-W zone of mylonitization has been recognized by Grimes (M.S. thesis 1993, Boston College) and named the Nobscot Shear Zone. It affects the Milford Granite, also about 630 Ma in age, while similar but narrow shear zones affect other local granites including the Dedham. These zones, dipping steeply north and including the Nobscot, are less intensely mylonitized and are not associated with migmatites. Their age is not known, but since they affect only Precambrian rocks, they are assumed to be late Proterozoic. We attribute these zones to the second stage of the Avalonian or the Pan-African II.

The older rocks west of Boston are widely affected by numerous brittle faults. These are all of unknown age, but probably Phanerozoic. The most significant brittle fault in the Burlington area is the mid to late Paleozoic Bloody Bluff Fault. We do not associate large scale mylonitization with that fault, because the mylonites are commonly cut by undeformed or little deformed Siluro-Devonian gabbro-diorites.

Comprehesive overview of the geology of eastern New York and New Jersey

The floor of Passamaquoddy Bay, New Brunswick is densely populated with pockmarks of uncertain origin. The regional distribution has been described initially by Fader et al. (1977) based on reconnaissance sidescan. Subsequently denser multibeam coverage of nearly the entire bay floor has now confirmed their distribution and provided much more detail on their specific geometry

During the Late Paleozoic all of the continental land masses assembled into a single supercontinent, Pangaea (Figure 83). The region that would eventually become the east coast of North America was at the heart of this great landmass, and was located within the equatorial realm. During the climax of the Alleghenian Orogeny, the Appalachians were probably high, rugged mountains rivaling the modern Alps. However, as mountain building subsided at the end of the Paleozoic Era, tropical weathering and erosion eventually reduced the landscape to a low, rolling plain. Ancient river systems carried the sediment far away to basins on the continental interior of the North American Plate, and to distant shorelines around the great landmass. Pangaea held together for nearly 100 million years.