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The southeastern New England Avalon Terrane (AT) accreted to the southeastern margin of the Nashoba Terrane (NT) at the onset of the Acadian orogeny (latest Silurian to Devonian). The NT represents the trailing edge of Ganderia. Rocks of the NT have previously been interpreted as having been extruded to the southeast over the AT as part of a channel flow zone (CFZ). Based on fold symmetries, it was inferred that only the top and center of this zone are located in the NT. Bedrock and structural mapping were carried out in the AT adjacent to the NT to test whether the bottom of the CFZ may be located in the AT. Data were collected from migmatitic biotite gneiss, mylonite, foliated quartzite, and gneiss. Structural data were divided into NE and SW domains. In the NE domain, foliations dip predominantly NW, and lineations plunge NE and SW. Migmatitic and gneissic rocks are absent in the SW domain, and orientations of mylonite zones and foliations in quartzite vary. Compared to the NE domain, rocks in the SW domain are strongly faulted and intruded by Ediacaran and late Silurian/Devonian granitic and gabbroic plutons. The presence of migmatite and consistency in structural orientations in the NE domain, and the general resemblance of structures to those in the NT make the NE domain a likely candidate to represent the bottom of the CFZ. U-Pb zircon data of the migmatitic biotite gneiss yielded a detrital zircon signature typical for Avalonia, with predominantly Mesoproterozoic and minor Paleoproterozoic and Tonian populations. Furthermore, zircon overgrowths are ~585 Ma, which suggests that high-grade metamorphism and partial melting occurred in the Ediacaran, i.e., not during the Acadian orogeny. Hence, the migmatitic biotite gneiss in the AT terrane does not represent the bottom of the CFZ. We believe that the Bloody Bluff Fault along the Nashoba-Avalon terrane boundary may have cut off the bottom of the CFZ. Our analysis is complemented by and provides context for high-resolution seismic imaging of the crust enabled by the ongoing GENESIS deployment of broadband seismometers across the NT. Preliminary results from GENESIS suggest a transition in crustal structure across the boundary between NT and AT, consistent with geological observations.
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U-Pb detrital zircon analysis of sedimentary rocks of the southeastern New England Avalon terrane in the US Appalachians: Evidence for a separate crustal block
The Avalon terrane of southeastern New England is a composite terrane, in which various crustal blocks may have different origins and/or tectonic histories. The northern part (west and north of Boston, Massachusetts) correlates well with Avalonian terranes in Newfoundland, Nova Scotia and New Brunswick, Canada, based on rock types and ages, U–Pb detrital zircon signatures of metasedimentary rocks, and Sm–Nd isotope geochemistry data. In the south, fewer data exist, in part because of poorer rock exposure, and the origins and histories of the rocks are less well constrained. We conducted U–Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analysis on zircon from seven metasedimentary rock samples from multiple previously interpreted subterranes, in order to constrain their origins. Two samples of Neoproterozoic Plainfield Formation quartzite from the previously interpreted Hope Valley subterrane in the southwestern part of the southeastern New England Avalon terrane and two from the Neoproterozoic Blackstone Group quartzite from the adjacent Esmond-Dedham subterrane to the east have Tonian youngest detrital zircon age populations. One sample of Cambrian North Attleboro Formation quartzite of the Esmond-Dedham subterrane yielded an Ediacaran youngest detrital zircon age population. Detrital zircon populations of all five samples include abundant Mesoproterozoic zircon and smaller Paleoproterozoic and Archean populations, and are similar to those of the northern part of the southeastern New England Avalon terrane and the Avalonian terranes in Canada. These are interpreted as having a Baltican/Amazonian affinity based primarily on published U-Pb and Lu-Hf detrital zircon data. Based on U-Pb detrital zircon data, there is no significant difference between the Hope Valley and Esmond-Dedham subterranes. Detrital zircon of two samples of the Price Neck and Newport Neck formations of the Neoproterozoic Newport Group in southern Rhode Island is characterized by large ~647–643 and ~745–733 Ma age populations and minor zircon up to ~3.1 Ga. This signature is most consistent with a northwest African affinity. The Newport Group may thus represent a subterrane, terrane or other crustal block with a different origin and history than the southeastern New England Avalon terrane to the northwest. The boundary of this Newport Block may be restricted to the boundaries of the Newport Group, or it may extend as far north as Weymouth, MA, as far northwest as (but not including) the North Attleboro Formation quartzite and associated rocks in North Attleboro, MA, and as far west as Warwick, RI, where eastern exposures of the Blackstone Group quartzite exist. The Newport Block may have amalgamated with the Amazonian/Baltican part of the Avalon terrane prior to mid-Paleozoic amalgamation with Laurentia, or have arrived as a separate terrane after accretion of the Avalon terrane. Alternatively, it may have arrived during the formation of Pangea and been stranded after the breakup of Pangea, as has been proposed previously for rocks of the Georges Bank in offshore Massachusetts. If the latter is correct, then the boundary between the Newport Block and the southeastern New England Avalon terrane is the Pangean suture zone.
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- Award ID(s):
- 1827429
- PAR ID:
- 10270638
- Editor(s):
- Kuiper, Yvette D; Murphy, J Brendan; Nance, R Damian; Strachan, Rob A; Thompson, Margaret D
- Date Published:
- Journal Name:
- Geological Society of America data repository
- ISSN:
- 1049-5886
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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The Acadian orogeny resulted from the accretion of the southeastern New England Avalon Terrane (AT) to the Nashoba Terrane (NT) - the trailing edge of Ganderia - to its northwest, in eastern Massachusetts. Ganderia and the AT are mostly Gondwana-derived. Previously, rocks of the NT were interpreted to have been extruded to the southeast over the AT as part of a channel flow zone. Only the top and center of this zone are exposed in the NT. Bedrock and structural mapping were carried out in the AT adjacent to the NT to locate the bottom of the channel flow zone. The main rock types are migmatitic biotite gneiss and mafic rock, quartzite, and igneous rocks, exposed in 10s of m to km scale blocks and lenses. Some of these rocks have been sheared and show evidence of mylonitization. Furthermore, they occur near, and in two areas are crosscut by, igneous plutons of unknown age. The foliations of migmatitic rocks, quartzites, and mylonites predominately dip NW, but the orientations of the mylonites vary, especially away from the terrane boundary. Lineations plunge NE and SW in migmatites, NE in quartzites, and NW in mylonites. Migmatitic rocks show abundant isoclinal folds. Predominantly NW to SW dipping normal faults with various slickenline orientations were observed in all rock types. The migmatitic biotite gneiss and its structures resemble those of the NT. However, U-Pb zircon data yielded a detrital zircon signature typical for Avalonia, with predominantly Mesoproterozoic and minor Paleoproterozoic and Tonian populations. Furthermore, zircon overgrowths are ~585 Ma, which suggests that the high-grade metamorphism and partial melting were Ediacaran and did not result from the Acadian orogeny and channel flow at that time. Based on the (1) blocky/lensoid outcrop pattern of rock types, (2) varied orientations of structures, and (3) abundance of faults, the area may represent a brittle fault zone that cut off the interpreted channel flow zone of the Nashoba terrane. Our structural analysis is complemented by and provides context for high-resolution seismic imaging of the crust enabled by the ongoing GENESIS deployment of broadband seismometers across the NT. Preliminary results from GENESIS suggest a transition in crustal structure across the boundary between the NT and AT, consistent with geological observations.more » « less
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