J. Christopher Hepburn

TACONIAN OROGENY IN THE NEW ENGLAND APPALACHIANS : COLLISION BETWEEN LAURENTIA AND THE SHELBURNE FALLS ARC

Tectonic models of the Ordovician Taconian orogeny in western New England usually invoke a collision between the Laurentian margin and a magmatic arc identified as the Bronson Hill arc. However, in central Massachusetts and southern New Hampshire, rocks in the Bronson Hill arc are 454 to 442 Ma and therefore younger than much of the Taconian deformation and metamorphism in western New England and eastern New York, which began by 470 Ma. U-Pb and single-grain evaporation zircon ages combined with geochemical analyses reveal the presence of an older magmatic arc, the Shelburne Falls arc, that formed west of the Bronson Hill arc at 485 to 470 Ma. The Shelburne Falls arc formed above an east-dipping subduction zone by the Early Ordovician. The Taconian orogeny was the result of the collision between Laurentia and the Shelburne Falls arc beginning ca. 475 to 470 Ma. The younger Bronson Hill arc formed above a west-dipping subduction zone that developed along the eastern edge of the newly accreted terrane during the final stages of and subsequent to the Taconian orogeny. The Taconian orogeny ended when plate convergence between Laurentia and Iapetus was accommodated by the newly developed west-dipping subduction zone instead of by crustal shortening in the Taconian thrust belt. The tectonic history of the New England Appalachians is inconsistent with a Middle Ordovician collision between Laurentia and the proto-Andean margin of Gondwana.

Quartz Sand Surface Textural Evidence for a Glacial Origin of the Squantum “Tillite,” Boston Basin, Massachusetts

Scanning electron microscopy (SEM) of quartz sand grains separated from the Squantum “Tillite” reveals surface textures characteristic of those acquired during glacial transport. This evidence supports past theories of a middle to late Paleozoic mountain glaciation in the Boston area.

Geochemistry of Silurian-Devonian volcanic rocks in the Coastal Volcanic belt, Machias-Eastport area, Maine: Evidence for a pre-Acadian arc

The Coastal Volcanic belt in easternmost Maine consists of Silurian–Devonian volcanic rocks related to the convergence of Avalonia with the leading edge of composite Laurentia just before and during early stages of the Acadian orogeny. New major- and trace-element analyses from the Machias-Eastport area indicate that the Early to Late Silurian volcanic rocks in the Dennys, Edmunds, and Leighton Formations formed in a continental arc environment, in contrast to some earlier interpretations. These rocks have a continuous compositional range from basalt, through andesite, to rhyolite. The mafic rocks in these formations are calc-alkaline and show subduction-related geochemical signatures on various discrimination diagrams, whereas the felsic rocks have geochemical characteristics of volcanic arc granites. In contrast, the thick volcanic deposits of the younger Late Silurian Eastport Formation are bimodal and tholeiitic and have transitional to within-plate geochemical signatures. Thus, the transition over time, as the Acadian seaway closed, from subduction and arc settings to continental extension can been seen in this rock sequence, and it constrains the time of Avalonia’s docking. Regionally, volcanic rocks in the Kingston terrane of southern New Brunswick are likely correlatives of the earlier arc-volcanic rocks (Dennys, Edmunds, and Leighton Formations), while Eastport Formation analogues are found in the coastal Maine magmatic province, Cranberry Island series, and probably the Passamaquoddy Bay volcanic series, which similarly appear to have formed after the transition to an extensional environment occurred, although other extensional settings cannot be entirely ruled out. Basalts in the post-Acadian Perry Formation indicate continental extension was also active in the Late Devonian.

Geochemical evidence for a Ganderian arc/back-arc remnant in the Nashoba Terrane, SE New England, USA

New geochemical data including Sm/Nd isotopic data show evidence for an early Paleozoic arc/back-arc complex in the Nashoba terrane of southeastern New England. The Nashoba terrane lies between rocks of Ganderian affinity to the northwest and Avalonian affinity to the southeast. It consists of early Paleozoic mafic to felsic metavolcanic and metasedimentary rocks that were intruded by intermediate to felsic plutons and metamorphosed to upper amphibolite facies conditions in the mid-Paleozoic. Major and trace element geochemical data indicate that the early Paleozoic igneous rocks contain a mix of arc, MORB, and alkaline signatures, and that the terrane formed as a primitive volcanic arc/back-arc complex built on thinned continental crust. Amphibolites have +4 to +7.5εNd(500) values consistent with formation in a primitive volcanic arc with minimal crustal contamination. Intermediate and felsic gneisses have εNd(500) values between +1.2 and –0.75 indicating a mixture of juvenile arc magmas and an evolved (likely basement) source. Depleted mantle model ages of 1.2 to 1.6 Ga point to a Mesoproterozoic or older age for this source. Metasedimentary rocks yielded –6 to –8.3 εNd(500) values and 1.6 to 1.8 Ga model ages, indicating an isotopically evolved source (or sources) that included Paleoproterozoic or older material. The εNd(500) values and model ages of the intermediate and felsic and metasedimentary rocks indicate that the basement to the Nashoba terrane is Ganderian rather than Avalonian. The Nashoba terrane therefore represents a Ganderian arc/back-arc complex similar to the Cambrian Penobscot arc/back-arc seen in Maritime Canada and Newfoundland, and particularly in the Annidale and New River terranes of southern New Brunswick. This correlation has not previously been recognized in southeastern New England. The Ganderian affinity of the Nashoba terrane also extends Ganderia farther SE in New England than previously established and indicates that the Nashoba terrane did not originate as a separate oceanic arc/back-arc complex or microcontinent.

Refining temporal constraints on metamorphism in the Nashoba terrane, southeastern New England, through monazite dating

Electron-microprobe dating of monazite grains within high-grade mylonitic rocks of the Nashoba terrane in eastern Massachusetts provides new temporal constraints on metamorphism in southeastern New England. In situ dating of monazite grains from three fault zones has allowed the timing of multistage events to be discerned. Three distinct metamorphic events were detected in the Nashoba terrane. The fi rst metamorphic event (M1) occurred from 435 to 400 Ma, with an average age of 423 Ma. A second metamorphic event (M2) occurred at ca. 390 Ma and was associated with widespread migmatization. A third metamorphic event (M3) occurred during the ca. 378‐ 371 Ma time interval and was possibly associated with the Neoacadian orogeny. Intermittent monazite growth during the 360‐305 Ma interval suggests that the main phase of metamorphism in the shear zones was complete, but the highly deformed fault zones acted as a conduit for fl uid migration, which was responsible for the production of young monazite grains. By at least 345 Ma, the Nashoba terrane had cooled below the stability of sillimanite.