Geology topics

Michael J. Dorais

Publications and source records attributed to Michael J. Dorais.

3 recordsLinked to original sources

USGS2013

P-T-t conditions, Nd and Pb isotopic compositions and detrital zircon geochronology of the Massabesic Gneiss Complex, New Hampshire: isotopic and metamorphic evidence for the identification of Gander basement, central New England

We present new evidence for the assignment of the Neoproterozoic Massabesic Gneiss Complex of New Hampshire to the Gander terrane rather than the Avalon terrane. The majority of Avalonian (sensu stricto) igneous and meta-igneous rocks as defined in Maritime Canada have positive whole-rock ɛ Nd compared to more negative values for Gander rocks, although there is a region of overlap in ɛNd between the two terranes. Our samples from areas in Connecticut previously thought to be Avalonian and samples from the Willimantic dome have the same isotopic signatures as Maritime Canada Avalon. In contrast, samples from the Clinton dome of southern Connecticut plots exclusively in the Gander field. The majority of the orthogneiss samples from Lyme dome (coastal Connecticut), Pelham dome (central Massachusetts) and Massabesic Gneiss Complex also plot in the Gander field, with a few samples plotting in the overlap zone between Gander and Avalon. U-Pb age distributions of detrital zircon populations from quartzites from the Massabesic Gneiss Complex more closely approximate the data from the Lyme Dome rather than Avalon. Additionally, the similarity of the P-T-t path for the rocks of the Massabesic Gneiss Complex (established by thermobarometry and 40 Ar/ 39 Ar dating of amphibole, muscovite, biotite and K-feldspar) with that established in the Ganderian Lyme dome of southern Connecticut strengthens the assignment of these rocks to a single Gander block that docked to Laurentia during the Salinic Orogeny. The identification of Ganderian isotopic signatures for these rocks all of which show evidence for Alleghanian metamorphism, supports the hypothesis that Neoproterozoic Gander lower crustal rocks underlie southern New Hampshire, Massachusetts, and Connecticut, and that all rocks of the overlying Central Maine trough that largely escaped high-grade Alleghanian metamorphism are allochthonous. We suggest that during the Alleghanian, the docking of Gondwana caused Avalon to wedge into Gander, metamorphosing and partially melting the Massabesic Gneiss Complex to the observed P-T-t conditions, with the complex forming an uplifted sheet that was back-thrusted over the Avalonian wedge.

New Hampshire↗

USGS2011

Bedrock geologic map of the Grafton quadrangle, Worcester County, Massachusetts

The bedrock geology of the 7.5-minute Grafton, Massachusetts, quadrangle consists of deformed Neoproterozoic to early Paleozoic crystalline metamorphic and intrusive igneous rocks. Neoproterozoic intrusive, metasedimentary, and metavolcanic rocks crop out in the Avalon zone, and Cambrian to Silurian intrusive, metasedimentary, and metavolcanic rocks crop out in the Nashoba zone. Rocks of the Avalon and Nashoba zones, or terranes, are separated by the Bloody Bluff fault. The bedrock geology was mapped to study the tectonic history of the area and to provide a framework for ongoing hydrogeologic characterization of the fractured bedrock of Massachusetts. This report presents mapping by G.J. Walsh, geochronology by J.N. Aleinikoff, geochemistry by M.J. Dorais, and consists of a map, text pamphlet, and GIS database. The map and text pamphlet are available in paper format or as downloadable files (see frame at right). The GIS database is available for download. The database includes contacts of bedrock geologic units, faults, outcrops, structural geologic information, and photographs.

Massachusetts↗

USGS2010

Contrasting P‐T‐t paths: Thermochronologic evidence for a Late Paleozoic final assembly of the Avalon Composite Terrane in the New England Appalachians

Strongly contrasting pressure‐temperature‐time paths for the Avalon composite terrane and the structurally overlying Putnam‐Nashoba zone in eastern New England obtained from thermochronologic and thermobarometric data are best explained by a late Paleozoic underthrusting of cover rocks by the Avalon composite terrane. We present new Ar and U‐Pb thermochronologic data that show that in the southern Hope Valley zone, Permian (280 Ma) anatectic metamorphic conditions of 700°C and 6 kbar were quenched by relatively rapid cooling (12°C/m.y.) and exhumation (0.5 km/m.y.) for ∼40 m.y. In contrast, peak metamorphic conditions in the Putnam‐Nashoba zone predate Silurian intrusions, and slower cooling (3.5°C/m.y.) began at about 400 Ma. One‐dimensional thermal modeling suggests that these two belts were not in thermal equilibrium during the Permian metamorphism of the Avalon composite terrane. Because of the absence of high‐grade Alleghanian metamorphism in rocks overlying the Avalon terrane, we conclude that high‐grade Alleghanian metamorphism in the Avalon terrane occurred east of rocks now overlying it and that significant motion between Avalon and this cover occurred after peak Alleghanian metamorphism. Similarly contrasting metamorphic histories between Avalon inliers (Willimantic window, Massabesic complex gneiss, Pelham dome) and their cover rocks reveals the regional significance of this boundary. The core rocks all show Permian cooling, but the cover rocks show post‐Acadian cooling ages decreasing from east to west to the Pelham area, where hornblende ages in Avalon and cover differ by only 35 rather than 80 m.y. Model calculations show that thermal equilibrium between instantaneously thrusted blocks of rocks is generally obtained in tens of millions of years. Consequently, underthrusting of Avalon is constrained to be middle Mississippian or younger. Because the leading edge of the underthrusting block would have been heated the longest and would have most closely approached thermal equilibrium with its cover, core rocks of the Pelham dome must have been relatively close to this leading edge. Thus Carboniferous to Permian underplating from a generally eastward direction best explains these thermochronologic relationships.

Connecticut, Maine, Massachusetts, New Hampshire, ↗