Murphy, J. B., Pisarevsky, S. A., Nance, R. D. and Keppie, J. D. 2001. Animated history of Avalonia in Neoproterozoic - Early Proterozoic. In: Jessell, M. J. 2001. General Contributions: 2001. Journal of the Virtual Explorer, 3, 45-58.
Animated history of Avalonia in Neoproterozoic - Early Proterozoic

 

Cadomia-Iberia

The Tregor-La Hague terrane of Cadomia (Strachan et al., 1996) contains the only undisputed basement exposed in any of the peri-Gondwanan arc terranes. The age (Auvray et al, 1980; Piton, 1985; Samson and D'Lemos, 1998) and isotopic signature (D'Lemos and Brown, 1993; Samson and D'Lemos, 1998) of this basement (the c. 2.2-1.8 Ga Icart Gneiss) resembles that of the 2.1 Ga Eburnian basement of the West African craton (Allgre and Ben Othman, 1980). Early arc-related magmatism is recorded by the c. 746 Ma orthogneiss of the Pentevrian Complex (Egal et al., 1996), and by deformed granodioritic conglomerate cobbles with protolith ages of 670-650 Ma (Guerrot and Peucat, 1990) in the Armorican massif of northwestern France. Deformation and metamorphism of the early Cadomian arc occurred in the interval (c. 650-615 Ma) separating the early and main phases of arc magmatism (Egal et al., 1996; Strachan et al., 1996).

Cadomian magmatism between 616-570 Ma is widespread in the Armorican massif and Iberia (e.g., Quesada, 1990; Strachan et al., 1996; Miller et al., 1999) and produced voluminous late Neoproterozoic magmatic arc-related volcanic and cogenetic plutonic rocks. Coeval sedimentary successions are dominated by volcanogenic turbidites that are thought to have been deposited in arc-related basins (e.g., Dennis and Dabard, 1988; Chantraine et al., 1994; Egal et al., 1996; Strachan et al., 1996).

The main phase of arc magmatism continued to c. 570 Ma but is progressively replaced by sinistral strike-slip tectonics in the interval c. 570-540 Ma (e.g., Strachan et al., 1996), and is superseded by widespread intracrustal melting, migmatization and bimodal magmatism including post-tectonic granitoid emplacement at c. 550-540 Ma (e.g., Rabu et al., 1990; Quesada, 1990; Chantraine et al., 1994; Egal et al., 1996).

The basement isotopic Sm-Nd signatures of Cadomia together with U-Pb detrital zircon data from within its late Neoproterozoic (Brioverian) sedimentary succession (Samson et al., 1999) suggest a position near the West African craton. Thus, in contrast to Avalonia, Cadomia and Iberia appear to have originated above Paleoproterozoic crust along the continental margin of West Africa, rather than within the peri-Rodinian ocean. As a result, Avalonia and Cadomia-Iberia did not form a coherent orogenic belt until the collision of Avalonia with northern Gondwana at ca. 650 Ma.

Voluminous bimodal rift volcanism of predominantly Middle Cambrian age (Quesada, 1990, Giese and Buehn, 1994) records an important extensional event in Iberia. Widespread Arenig subsidence, recorded in the broad distribution of the Armorican Quartzite across Cadomia and Iberia (e.g., Noblet and Lefort, 1990) suggests that rifting extended into the Early Ordovician. Potential correlatives in Britain (the Stiperstones Quartzite) suggest that rifting occurred at about the same time in East Avalonia. Faunal data (eg. Cocks and Fortey, 1990; Cocks, 2000) indicate that by the Early Ordovician, Avalonia had separated from Gondwana, resulting in the birth of the Rheic Ocean, whereas Cadomia and Iberia remained along the West African portion of this margin.

Carolina/Goochland/Piedmont terranes

The oldest rocks in the Carolina terrane are ca. 670 Ma granitoid bodies of the Roanoke Rapids terrane (Hibbard et al., in press). They are interpreted as evidence of early arc magmatism broadly coeval with that in Avalonia. The basement to the Carolina terrane is not exposed. However, initial eNd values of +0.5 to +5.9 and TDM model ages of 0.7-1.1 Ga from c. 635-610 Ma volcanic rocks of the Virgilina sequence (Samson et al., 1995; Wortman et al., 2000) suggest that the Carolina terrane, like Avalonia, was located outboard from the northern Gondwanan margin until at least 700 Ma.

The Carolina terrane is dominated by a ca. 633-607 Ma juvenile arc assemblage, overlain unconformably by a 580-540 Ma mature arc sequence, followed by middle Cambrian platformal sedimentary strata that contain cool-water trilobites similar to those of Cadomia and Baltica (Samson et al., 1990; Hibbard and Samson, 1995; Wortman et al., 2000). Possible episodes of arc rifting have been documented at c. 590-570 Ma and c. 560-535 Ma (e.g., Dennis and Shervais, 1991, 1996; Shervais et al., 1996). The earlier event is probably related to a transition from arc to strike-slip tectonics and may be responsible for the unconformity between the older and younger volcanic successions. The later event may have been coeval with widespread deformation and metamorphism (Dennis and Wright, 1997; Barker et al., 1998).

The neighboring Goochland terrane has a ca. 1.0 Ga granulite facies basement that has been interpreted as either part of the Laurentian Grenville Belt, or as an exotic terrane (Glover, 1989; Rankin et al., 1989) that collided with the Carolina terrane at about 590 Ma. Piedmont terrane assemblages are dominated by a Cambro-Ordovician complex of arc, fore-arc and accretionary complexes (Hibbard and Samson, 1995) that may be a continuation of the Pampean orogeny of western South America (Keppie and Ramos, 1999).

Middle American terranes

Although their paleogeography is perhaps the least understood, the distribution of Early Paleozoic Gondwanan fauna, indicates that several terranes in Middle America have peri-Gondwanan affinities. However, they do not preserve evidence of Neoproterozoic arc activity, suggesting they were located inboard of the magmatic arc. These terranes expose basement of Pan African (Yucatan block) and Grenville (Oaxaquia and Chortis block) age (Keppie and Ortega-GutiŽrrez, 1999). The Yucatan block is thought to have been contiguous with the Florida basement until the opening of the Gulf of Mexico in the Mesozoic (e.g. Pindell et al., 1990; Dickinson and Lawton, 2001). The Grenville basement of Oaxaquia and the Chortis block is isotopically transitional between that of the Grenville Belt and the basement massifs of Grenvillian age in Columbia (Ruiz et al., 1999). Following Keppie and Ramos (1999), we position these along the Columbian margin in accordance with the paleomagnetic data of Ballard et al., (1989).

 

 

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