Ammonoosuc Volcanics

Ammonoosuc Volcanics
EdwardEMeyer · CC BY-SA 4.0 · source
Name	Ammonoosuc Volcanics
Type	Geological formation
Period	Ordovician–Silurian
Lithology	Basalt, andesite, rhyolite, tuff, volcanic breccia
Region	New Hampshire, Vermont, Maine
Country	United States
Namedfor	Ammonoosuc River

Ammonoosuc Volcanics is a widespread suite of volcanic and volcaniclastic rocks exposed in the northern Appalachian orogen, principally in New Hampshire, Vermont, and western Maine. The unit records submarine to subaerial eruptive episodes and synvolcanic sedimentation during the late Ordovician to early Silurian periods, and it is a key component of the Appalachian terrane collage interpreted in regional studies by institutions such as the United States Geological Survey, Harvard University, and the New Hampshire Geological Survey. The Ammonoosuc Volcanics have been central to debates involving the tectonic evolution of the northern Appalachians, including correlations with the Bronson Hill Arc, Ganderia, and the Avalonia microcontinent.

Overview

The Ammonoosuc Volcanics form part of the northern Appalachian Mountains stratigraphy and are exposed in belts that trend roughly northeast–southwest across the White Mountains, the Green Mountains, and adjacent uplands. Field mapping by groups at the University of New Hampshire, the Smithsonian Institution, and the Yale University geology departments has delineated a package of mafic to felsic lavas, volcaniclastic turbidites, and hyaloclastites that interfinger with sedimentary units recognized in the Siluro-Devonian succession. Correlations have been proposed with volcanic suites described in the Quebec Appalachians, the Gaspé Peninsula, and the Maine Appalachians.

Geology and Petrology

Petrographic and geochemical investigations by researchers from the Geological Society of America, the American Geophysical Union, and the Royal Society show that the Ammonoosuc Volcanics include tholeiitic and calc-alkaline suites composed of basalt, andesite, dacite, and rhyolite. Whole-rock geochemistry and trace-element ratios compared with data from the Iapetus Ocean realm, the Newfoundland Appalachians, and the Long Range Mountains indicate affinities ranging from island-arc basalt (IAB) to within-plate basalt (WPB). Mineral assemblages commonly include plagioclase, clinopyroxene, orthopyroxene, amphibole, and subordinate biotite; alteration products include chlorite, sericite, and epidote associated with regional metamorphism examined in studies at the University of Vermont and Colby College.

Stratigraphy and Age

Stratigraphic work by mapping projects led by the New Hampshire Geological Survey and academic teams from the University of New Hampshire established that the volcanic package unconformably overlies older Cambrian–Ordovician sedimentary rocks and is overlain by Silurian–Devonian strata in many sections. Radiometric ages from zircon U-Pb dating conducted at laboratories such as the Lamont–Doherty Earth Observatory and the Massachusetts Institute of Technology place volcanism primarily in the Late Ordovician (~450–430 Ma) with local bimodal ages extending into the Early Silurian. Biostratigraphic ties using conodonts and graptolites sampled near volcanic horizons have been correlated with chronostratigraphic frameworks developed at the Smithsonian Institution and the Natural History Museum, London.

Tectonic Setting and Origin

Interpretations of the tectonic setting are diverse: proponents from the Rutgers University and Dartmouth College favor an island-arc model related to convergence along the margin of the Iapetus Ocean and the accretion of Avalonia, while others at the University of Toronto and the Geological Survey of Canada emphasize back-arc basin or rift-related volcanism tied to continental breakup and slab rollback. Structural studies referencing the Acadian orogeny, the Taconic orogeny, and regional thrust systems documented by the New England Intercollegiate Geological Conference show that the Ammonoosuc Volcanics were translated, imbricated, and metamorphosed during successive orogenic events that assembled the northern Appalachian orogen.

Economic Resources and Mineralization

Although not a major metalliferous province, the Ammonoosuc Volcanics host localized zones of hydrothermal alteration and mineralization investigated by economic geologists at the USGS and prospecting reports archived by state geological surveys. Mineral occurrences include copper, gold, and associated sulfide minerals (chalcopyrite, pyrite) in veins and disseminations, and zeolitic alteration in tuffaceous horizons exploited in small-scale quarrying. These occurrences have been compared to mineralized volcanic margins in the Keweenaw Peninsula and the Mother Lode belt in terms of epigenetic hydrothermal processes.

History of Investigation

Early reconnaissance mapping in the 19th century by geologists associated with the Geological Survey of Canada and the nascent USGS recognized distinctive volcanic rocks in the White Mountains; systematic studies progressed through the 20th century with significant contributions from the New Hampshire Geological Survey, the University of New Hampshire, and researchers publishing in journals of the Geological Society of America. Key advances include geochronology by teams at Lamont–Doherty Earth Observatory and petrogenetic models refined by collaborative projects involving Yale University and Dartmouth College that integrated field mapping, geochemistry, and structural analysis.

Fossil Content and Paleoenvironments

Fossil preservation within the Ammonoosuc Volcanics is limited but interbedded sedimentary horizons and tuff layers contain marine fossils such as graptolites, conodonts, and rare brachiopods studied by paleontologists at the Smithsonian Institution, Yale Peabody Museum, and the Natural History Museum, London. These faunal assemblages, together with lithofacies analysis performed by teams from the University of Vermont and Colby College, indicate a range of paleoenvironments from deep-marine turbidite basins to shallower submarine-slope and volcanic-arc settings contemporaneous with the closure of the Iapetus Ocean.

Category:Geologic formations of the United States Category:Geology of New Hampshire Category:Ordovician volcanism