ABSTRACT West Avalonia is a composite terrane that rifted from the supercontinent Gondwana in the Ordovician and accreted to Laurentia during the latest Silurian to Devonian Acadian orogeny. The nature and extent of West Avalonia are well constrained in Nova Scotia, New Brunswick, and Newfoundland, Canada, by U-Pb detrital zircon data and/or isotope geochemistry of (meta)sedimentary and igneous rocks. The southeastern New England Avalon terrane in eastern Massachusetts, Connecticut, and Rhode Island has generally been interpreted as an along-strike continuance of West Avalonia in Canada, but the ages and origins of metasedimentary units along the western boundary of the Avalon terrane in Massachusetts and Connecticut remain poorly constrained. In this study, new detrital zircon U-Pb and Lu-Hf laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) data from three samples of metasedimentary units along the western boundary of the southeastern New England Avalon terrane in Connecticut and Massachusetts were compared with existing data to test whether these metasedimentary units can be correlated along strike. The data were also compared with existing detrital zircon U-Pb and εHf data in New England and Canada in order to constrain the extent and provenance of West Avalonia. The maximum depositional age of two of the three detrital zircon samples analyzed in this study, based on the youngest single grain in each sample (600 ± 28 Ma, n = 1; 617 ± 28 Ma, n = 1) and consistency with existing analyses elsewhere in the southeastern New England Avalon terrane, is Ediacaran, while that of the third sample is Tonian (959 ± 40 Ma, n = 4). Detrital zircon analyses of all three samples from this study showed similar substantial Mesoproterozoic and lesser Paleoproterozoic and Archean populations. Other existing detrital zircon U-Pb data from quartzites in the southeastern New England Avalon terrane show similar Tonian populations with or without Ediacaran grains or populations. Most published detrital zircon U-Pb data from (meta)sedimentary rocks in West Avalonia in Canada yielded Ediacaran youngest detrital zircon age populations, except for a quartzite unit within the Gamble Brook Formation in the Cobequid Highlands of Nova Scotia, which showed a Tonian maximum depositional age, and otherwise a nearly identical detrital zircon signature with rocks from the southeastern New England Avalon terrane. All samples compiled from the southeastern New England Avalon terrane and West Avalonia in Canada show main age populations between ca. 2.0 Ga and ca. 1.0 Ga, with major peaks at ca. 1.95, ca. 1.50, ca. 1.20, and ca. 1.00 Ga, and minor ca. 3.1–3.0 Ga and ca. 2.8–2.6 Ga populations. The εHf ( t ) values from the three samples yielded similar results to those from West Avalonia in Canada, suggesting that both regions were derived from the same cratonic sources. The εHf ( t ) values of all West Avalonian samples overlap with both Amazonia and Baltica, suggesting that there is a mixed signature between cratonic sources, possibly as a result of previous collision and transfer of basement fragments between these cratons during the formation of supercontinent Rodinia, or during subsequent arc collisions.

ABSTRACT 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 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 ca. 647–643 and ca. 745–733 Ma age populations and minor zircon up to ca. 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, Massachusetts, as far northwest as (but not including) the North Attleboro Formation quartzite and associated rocks in North Attleboro, Massachusetts, and as far west as Warwick, Rhode Island, 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 it may 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.

ABSTRACT Southeastern New England is largely composed of Ediacaran granitoid and related volcanic rocks formed during the main phase of arc-related magmatism recorded in West Avalonian lithotectonic assemblages extending through Atlantic Canada to eastern Newfoundland. In situ Lu-Hf analyses presented here for zircons from the Dedham, Milford, and Esmond Granites and from the Lynn-Mattapan volcanic complex show a restricted range of εHf values (+2 to +5) and associated Hf- T DM model ages of 1.3–0.9 Ga, assuming felsic crustal sources. The most evolved granites within this suite lie in a belt north and west of the Boston Basin, whereas upfaulted granites on the south, as well as the slightly younger volcanic units, show more juvenile Hf isotopic compositions. Similar inferences have been drawn from previously published Sm-Nd isotopic signatures for several of the same plutons. Collectively, the isotopic compositions and high-precision U-Pb geochronological constraints now available for southeastern New England differ in important respects from patterns in the Mira terrane of Cape Breton Island or the Newfoundland Avalon zone, but they closely resemble those documented in the Cobequid and Antigonish Highlands of mainland Nova Scotia and New Brunswick’s Caledonia terrane. Particularly significant features are similarities between the younger than 912 Ma Westboro Formation in New England and the younger than 945 Ma Gamble Brook Formation in the Cobequid Highlands, both of which yield detrital zircon age spectra consistent with sources on the Timanide margin of Baltica. This relationship provides the starting point for a recent model in which episodic West Avalonian arc magmatism began along the Tonian margin of Baltica and terminated during diachronous late Ediacaran arc-arc collision with the Ganderian margin of Gondwana.

ABSTRACT The Brevard fault zone is one of the largest faults in the Appalachians, extending from Alabama to Virginia. It had a very complex history of movement and reactivation, with three movement episodes: (1) Acadian-Neoacadian (403–345 Ma) movement accompanying the thermal peak of metamorphism and deformation with dextral, southwest-directed emplacement of the Inner Piedmont; (2) ductile dextral reactivation during the early Alleghanian (~280 Ma) under lower-greenschist-facies conditions; and (3) brittle dip-slip reactivation during the late Alleghanian (260 Ma?). The Brevard is comparable to other large faults with polyphase movement in other orogens worldwide, for example, the Periadriatic line in the Alps. Two types of far-traveled, fault-bounded horses have been identified in the Brevard fault zone in the Carolinas: (1) metasedimentary and granitoid horses located along the southeastern margin of the Alleghanian retrogressive ductile dextral Brevard fault zone in North and South Carolina; and (2) limestone/dolostone horses located along the brittle, late Alleghanian Rosman thrust, the contact between Blue Ridge and Brevard fault zone rocks in North and South Carolina. Field, stratigraphic, petrographic, and Sr-isotope data suggest the carbonate horses may be derived from Valley and Ridge carbonates in the Blue Ridge–Piedmont megathrust sheet footwall. The horses of metasedimentary and granitoid rocks occur along faults that cut klippen of the southwest-directed Inner Piedmont Acadian-Neoacadian Alto (Six Mile) allochthon. New laser ablation– inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb zircon analyses from the metasedimentary mylonite component yield a detrital zircon suite dominated by 600 and 500 Ma zircons, and a second zircon population ranging from 2100 to 1300 Ma, with essentially no Grenvillian zircons, suggesting a peri-Gondwanan provenance. The granitoid component has a sensitive high-resolution ion microprobe (SHRIMP) age of 421 ± 14 Ma, similar to the ~430 Ma plutonic suite in northern Virginia and Maryland—a prominent component of the Cat Square terrane detrital zircon suite in the Carolinas. Peri-Gondwanan Neoproterozoic to Cambrian Avalon–Carolina superterrane rocks are nowhere in contact with the Brevard fault zone at present erosion level. While these far-traveled metasedimentary and granitoid horses may have originated several hundred kilometers farther northeast in the central Appalachians, they could alternatively be remnants of Avalon–Carolina superterrane rocks that once formed the tectonic lid of the southwest-directed Neoacadian–early Alleghanian (Late Devonian–early Mississippian) orogenic channel formed during north-to-south zippered accretion of Avalon–Carolina. The remnant fossil subduction zone survives as the central Piedmont suture. Avalon–Carolina terrane rocks would have once covered the Inner Piedmont (and easternmost Blue Ridge) to depths of >20 km, and have since been eroded. Data from these two suites of horses provide additional insights into the mid- to late Paleozoic history and kinematics of the Brevard fault zone, Inner Piedmont, and Avalon–Carolina superterrane. It was six men of Indostan To learning much inclined, Who went to see the Elephant (Though all of them were blind), That each by observation Might satisfy his mind. … And so these men of Indostan Disputed loud and long, Each in his own opinion Exceeding stiff and strong, Though each was partly in the right, And all were in the wrong. —John Godfrey Saxe (1816–1887) “The Blind Men and the Elephant”

The Appalachians are a Paleozoic orogen that formed in a complete Wilson cycle along the eastern Laurentian margin following the breakup of supercontinent Rodinia and the coalescence of all of the continents to form supercontinent Pangea. The Appalachian Wilson cycle began by formation of a Neoproterozoic to early Paleozoic rifted margin and platform succession on the southeastern margin of Laurentia. Three orogenies ultimately produced the mountain chain: the Ordovician Taconic orogeny, which involved arc accretion; the Acadian–Neoacadian orogeny, which involved north-to-south, transpressional, zippered, Late Devonian–early Mississippian collision of the Carolina superterrane in the southern-central Appalachians and the Avalon-Gander superterrane in the New England Appalachians, and Silurian collision in the Maritime Appalachians and Newfoundland; and the Alleghanian orogeny, which involved late Mississippian to Permian collision of all previously formed Appalachian components with Gondwana to form supercontinent Pangea. The Alleghanian also involved zippered, north-to-south, transpressional, then head-on collision. All orogenies were diachronous. Similar time-correlative orogenies affected western and central Europe (Variscan events), eastern Europe and western Siberia (Uralian events), and southern Britain and Ireland; only the Caledonide (Grampian–Finnmarkian; Caledonian–Scandian) events affected the rest of Britain and the Scandinavian Caledonides. These different events, coupled with the irregular rifted margin of Laurentia, produced an orogen that contains numerous contrasts and nonthroughgoing elements, but it also contains elements, such as the platform margin and peri-Gondwanan elements, that are recognizable throughout the orogen.