ABSTRACT Avalonia and Ganderia are composite microcontinental fragments in the northern Appalachian orogen likely derived from Gondwanan sources. Avalonia includes numerous Neoproterozoic magmatic arc sequences that represent protracted and episodic subduction-related magmatism before deposition of an Ediacaran–Ordovician cover sequence of mainly siliciclastic rocks. We characterized the nature of the basement on which these arcs were constructed using zircon grains from arc-related magmatic rocks in Atlantic Canada that were analyzed for their Lu-Hf isotope composition. The majority of zircon grains from Avalonia are characterized by initial 176 Hf/ 177 Hf values that are more radiogenic than chondritic uniform reservoir, and calculated crust formation Hf T DM (i.e., depleted mantle) model ages range from 1.2 to 0.8 Ga. These data contrast with those from Ganderia, which show typically positive initial εHf values and Hf T DM model ages that imply magmatism was derived by melting of crustal sources with diverse ages ranging from ca. 1.8 to 1.0 Ga. The positive distribution of initial εHf values along with the pattern of Hf T DM model ages provide a clear record of two distinct subduction systems. Cryogenian–Ediacaran magmatism is interpreted to have resulted from reworking of an evolved Mesoproterozoic crustal component in a long-lived, subduction-dominated accretionary margin along the margin of northern Amazonia. A change in Hf isotope trajectory during the Ediacaran implies a greater contribution of isotopically evolved material consistent with an arc-arc–style collision of Ganderia with Avalonia. The shallow-sloping Hf isotopic pattern for Paleozoic Ganderian magmatism remains continuous for ~200 m.y., consistent with tectonic models of subduction in the Iapetus and Rheic Oceans and episodic accretion of juvenile crustal terranes to Laurentia.

The eastern flank of the Appalachian orogen is composed of extensive Neoproterozoic–early Paleozoic crustal blocks that originated in a peri-Gondwanan setting. Three of these blocks record the evolution of Neoproterozoic magmatic-arc systems, including Carolinia in the southern Appalachians and Ganderia and Avalonia in the northern Appalachians. Relationships among these three crustal blocks are important for understanding both the accretionary history of the orogen and the evolution of the Iapetus and Rheic Oceans, first-order geographic features of the Paleozoic globe. Traditionally, Carolinia and Avalonia have been considered to represent a single microcontinental magmatic arc that accreted to Laurentia in the middle to late Paleozoic. The early lithotectonic history (ca. 680–570 Ma) of the two blocks is obscure; however, their latest Neoproterozoic-Paleozoic histories are distinct. This disparity is manifest in the first-order features of (1) timing and style of magmatic-arc cessation and (2) the nature of their Paleozoic lithotectonic records. Magmatic arc activity ceased in Avalonia in the late Neoproterozoic (ca. 570 Ma), succeeded by extension-related magmatism and sedimentation that was transitional into a robust latest Neoproterozoic–Silurian platformal clastic sedimentary sequence. This platform was tectonically unperturbed until the Late Silurian–Early Devonian. In contrast, Carolinia records late Neo-proterozoic tectonothermal events coeval with arc magmatism, which extended into the Cambrian; a relatively thin Middle Cambrian shallow-marine clastic sequence is preserved unconformably atop the Carolinia arc sequences. Subsequently, Carolinia experienced widespread Late Ordovician–Silurian deformation and metamorphism. However, we note striking similarities between Carolinia and Ganderia; specifically, in Ganderia, like Carolinia, late Neoproterozoic tectonism was accompanied by arc magmatism that extended into the Cambrian. Ganderian arc rocks are capped unconformably by a Middle Cambrian to Early Ordovician clastic sequence, and they were tectonized in the Late Ordovician–Silurian, similar to relations in Carolinia. Independent studies indicate that the Late Ordovician–Silurian tectonism in both blocks was related to their accretion to Laurentia. Thus, Carolinia and Ganderia show parallel development of first-order lithotectonic characteristics for two endpoints in their global strain path, i.e., their Gondwanan source region and their accretion to Laurentia. Consequently, we posit that Carolinia appears to be more closely affiliated with Ganderia than with Avalonia. The recognition of this linkage between Appalachian peri-Gondwanan realm crustal blocks in light of paleomagnetic and isotopic data leads to a unified model for the accretion of these blocks to the eastern margin of Laurentia.

Abstract Within the Appalachian–Variscan orogen of North America and southern Europe lie a collection of terranes that were distributed along the northern margin of West Gondwana in the late Neoproterozoic and early Palaeozoic. These peri-Gondwanan terranes are characterized by voluminous late Neoproterozoic ( c . 640–570 Ma) arc magmatism and cogenetic basins, and their tectonothermal histories provide fundamental constraints on the palaeogeography of this margin and on palaeocontinental reconstructions for this important period in Earth history. Field and geochemical studies indicate that arc magmatism generally terminated diachronously with the formation of a transform margin, leading by the Early–Middle Cambrian to the development of a shallow-marine platform–passive margin characterized by Gondwanan fauna. However, important differences exist between these terranes that constrain their relative palaeogeography in the late Neoproterozoic and permit changes in the geometry of the margin from the late Neoproterozoic to the Early Cambrian to be reconstructed. On the basis of basement isotopic composition, the terranes can be subdivided into: (1) Avalonian-type (e.g. West Avalonia, East Avalonia, Meguma, Carolinia, Moravia–Silesia), which developed on juvenile, c . 1.3–1.0 Ga crust originating within the Panthalassa-like Mirovoi Ocean surrounding Rodinia, and which were accreted to the northern Gondwanan margin by c . 650 Ma; (2) Cadomian-type (e.g. North Armorican Massif, Ossa–Morena, Saxo-Thuringia, Moldanubia), which formed along the West African margin by recycling ancient ( c . 2.0–2.2 Ga) West African crust; (3) Ganderian-type (e.g. Ganderia, Florida, the Maya terrane and possible the NW Iberian domain and South Armorican Massif), which formed along the Amazonian margin of Gondwana by recycling Avalonian and older Amazonian basement; and (4) cratonic terranes (e.g. Oaxaquia and the Chortis block), which represent displaced Amazonian portions of cratonic Gondwana. These contrasts imply the existence of fundamental sutures between these terranes prior to c . 650 Ma. Derivation of the Cadomian-type terranes from the West African craton is further supported by detrital zircon data from their Neoproterozoic–Ediacaran clastic rocks, which contrast with such data from the Avalonian- and Ganderian-type terranes that suggest derivation from the Amazonian craton. Differences in Neoproterozoic and Ediacaran palaeogeography are also matched in some terranes by contrasts in Cambrian faunal and sedimentary provenance data. Platformal assemblages in certain Avalonian-type terranes (e.g. West Avalonia and East Avalonia) have cool-water, high-latitude fauna and detrital zircon signatures consistent with proximity to the Amazonian craton. Conversely, platformal assemblages in certain Cadomian-type terranes (e.g. North Armorican Massif, Ossa–Morena) show a transition from tropical to temperate waters and detrital zircon signatures that suggest continuing proximity to the West African craton. Other terranes (e.g. NW Iberian domain, Meguma) show Avalonian-type basement and/or detrital zircon signatures in the Neoproterozoic, but develop Cadomian-type signatures in the Cambrian. This change suggests tectonic slivering and lateral transport of terranes along the northern margin of West Gondwana consistent with the transform termination of arc magmatism. In the early Palaeozoic, several peri-Gondwanan terranes (e.g. Avalonia, Carolinia, Ganderia, Meguma) separated from West Gondwana, either separately or together, and had accreted to Laurentia by the Silurian–Devonian. Others (e.g. Cadomian-type terranes, Florida, Maya terrane, Oaxaquia, Chortis block) remained attached to Gondwana and were transferred to Laurussia only with the closure of the Rheic Ocean in the late Palaeozoic.