Margaret D. Thompson

Late Neoproterozoic paleogeography of the Southeastern New England Avalon Zone: Insights from U-Pb geochronology and paleomagnetism

Integrated geochronologic and paleomagnetic investigation of the arc-related Lynn and Mattapan volcanic complexes around Boston, Massachusetts, establishes a Neoproterozoic paleopole for the Southeastern New England Avalon Zone. Four new U-Pb zircon dates obtained using isotope dilution and thermal ionization mass spectrometry (ID-TIMS) methods range from 597.4 ± 1.5 Ma to 595.7 ± 1.6 Ma, in agreement with published Lynn-Mattapan results of ca. 596 Ma. The mean direction calculated from the A component of magnetization at nine Lynn-Mattapan sites has a declination (D) of 319.9° and an inclination (I) of 57.0° (α95 = 7.4°); the paleopole based on this mean direction is 206°E, 60°N. These results pass both reversals and fold tests and are regionally consistent among lithologies that include basalt, andesite, rhyodacite, and rhyolite, which represent both volcanic and subvolcanic phases. Most sites also contain stable B and C magnetic components with directions that suggest Ordovician to Permian remagnetizations. A broadly comparable A component direction with D = 109.0° and I = −63.4° (α95 = 9.4°, n = 11 samples) was obtained from one site in the Squantum Member of the Roxbury Conglomerate (maximum age of 593 Ma) overlying the volcanic sequence. The Lynn-Mattapan results indicate that southeastern New England occupied a mid-latitude position at ca. 595 Ma, similar to those reported from Avalonian terranes in maritime Canada. The most likely peri-Gondwanan position for Avalonia consistent with these paleomagnetic data lies off the West African margin. Using poles from the East Sahara craton (Egypt) and the Rio de la Plata craton (Brazil) has permitted reconstruction of West Gondwana without recourse to Laurentian proxies.

Avalonian perspectives on Neoproterozoic paleogeography: Evidence from Sm-Nd isotope geochemistry and detrital zircon geochronology in SE New England, USA

Arc-related Northern Appalachian Avalonian terranes developed in the ocean surrounding Rodinia beginning around 760 Ma, so paleogeographic constraints on these terranes have implications for the margins of Rodinia itself in Neoproterozoic time. The first Sm-Nd isotopic data together with new detrital zircon ages from SE New England suggest that at least this segment of Avalonia contains rocks with contributions from crustal sources in Baltica, rather than Amazonia as commonly argued. Eight samples from 610 to 600 Ma Dedham, Dedham North, Milford, Fall River, Esmond, and Westwood granites in SE New England yield e Nd (600) values from −1.2 to +2.2 and depleted mantle model ages ranging from 2.2 Ga to 1.0 Ga. The younger model ages, reflecting largely juvenile magmatic components, are typical of other Avalonian terranes in Atlantic Canada. The oldest model ages can be produced by mixing juvenile mantle extractions with 10%-15% Baltic crust. The isotopic composition of Baltic crust in these calculations is proxied by Svecofennian metasedimentary rocks with detrital zircon suites comparable to those obtained from younger than 912 Ma quartzite of the Westboro Formation intruded by the Dedham Granite north of Boston (Massachusetts). An Avalonia-Baltica connection is also consistent with available paleomagnetic data placing both areas at moderate south paleolatitude between ca. 615 Ma and ca. 575 Ma during Ediacaran deposition of their respective Squantum-Gaskiers and Varanger glacial deposits.

Cambro-Ordovician paleogeography of the Southeastern New England Avalon Zone: Implications for Gondwana breakup

Paleomagnetic measurements and U-Pb geochronology of alkalic igneous rocks intruding the Ediacaran Cambridge Argillite and the Lower Cambrian Weymouth Formation north of Boston at Nahant, Massachusetts, yield a well-constrained paleopole. The oldest intrusive phases are a syenodiorite sill and a granite dike with 206 Pb/ 238 U crystallization ages of 490.19 ± 0.90 Ma and 489.03 ± 0.82 Ma, respectively. This sequence along with undated mafic sills penetrating both the argillite and Weymouth strata are cut by 488.53 ± 0.81 Ma gabbro locally associated with 488.48 ± 0.79 Ma syenite forming a steep-sided intrusive plug. Ten paleomagnetic sampling sites within the mafic sills and the gabbro contain a stable A magnetic component with a mean direction of D = 278.5°, I = −76.7° (A 95 = 3.9°, N = 10). This is considered to be the primary magnetization direction because it passes both fold and reversals tests. The corresponding paleopole at 320LE, 34°N (A 95 = 7.2°, N = 10) places the Southeastern New England Avalon Zone at a southerly paleolatitude of 65° slightly removed from the African margin of Gondwana and suggests that bimodal Nahant magmatism is linked with latest Cambrian–Early Ordovician rifting of West Avalonia to form the Rheic Ocean. Stable B and C magnetic components in Nahant syenodiorite and syenite give rise to virtual geomagnetic poles that track Late Silurian through Carboniferous segments of the North American apparent polar wander path, indicating docking of West Avalonia by mid-Paleozoic time.

Early Rifting in the Narragansett Basin, Massachusetts–Rhode Island: Evidence from Late Devonian Bimodal Volcanic Rocks

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U-Pb zircon geochronology of Roxbury Conglomerate, Boston Basin, Massachusetts: Tectono-stratigraphic implications for Avalonia in and beyond SE New England

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Altering Avalonia: oxygen isotopes and terrane distinction in the Appalachian peri-Gondwanan realmLaboratory for Stable Isotope Science (LSIS), The University of Western Ontario, Contribution 236.

\end{document} 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.

Ash-flow stratigraphy in the Mattapan Volcanic Complex, greater Boston, Massachusetts

Ash-flow tuff in the Stony Brook Reservation contains clasts as much as 450 m long. The pyroclastic matrix of this breccia and the large size of its clasts are characteristic of “megabreccias” found in terrains of ash-flow volcanism and caldera collapse. The breccia of the Stony Brook Reservation is intruded by shallow-level (Westwood?) granite, suggesting that both caldera collapse and magmatic resurgence were important processes in the history of this part of the late Precambrian plutonic-volcanic terrain in southeastern New England.

Early to Middle Devonian granitic and volcanic rocks from the central Gulf of Maine

High-precision CA-TIMS 206Pb/238U zircon dates from sandstone and Brighton igneous rocks associated with Roxbury Conglomerate in the Boston Basin, eastern Massachusetts provide constraints on the age and tectonic significance of these deposits. Detrital zircon suites from Roxbury-related sandstones representing, in ascending order, the Franklin Park Member (proposed name), the Brookline Member and the Squantum Member establish closely comparable maximum depositional ages of 595.14 ± 0.90, 598.87 ± 0.71 and 596.39 ± 0.79 Ma, respectively. The youngest of these is the best maximum age estimate of the conglomerate. Brighton dacite near the base of the Brookline Member and amygdaloidal andesite near the top yield respective crystallization ages of 584.19 ± 0.70 Ma and 585.37 ± 0.72 Ma. These virtually identical dates support previous interpretations of these particular units as shallow intrusions and thus represent minimum ages of associated conglomerate. The Roxbury-Brighton sequence is traditionally shown as inter-fingering northward with ≤570 Ma mudstone of the Cambridge “Argillite,” but the age range of the conglomerate makes this impossible. Conglomerate lacking quartzite clasts typical of the Roxbury proper and associated with 593.19 ± 0.73 Ma rhyolite on the south side of the basin is re-assigned to the Lynn-Mattapan Volcanic Complex which rests unconformably on Dedham Granite in basement exposed west and north of the basin as well. Calc-alkaline geochemistry and ages of the 609 to 584 Ma Dedham-Lynn-Mattapan-Brighton assemblage link it with 630 to 580 Ma arc sequences in other northern Appalachian Avalonian terranes. Roxbury Conglomerate accumulated in fault-bounded intra-arc basins near the end of this magmatic cycle.