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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 (Allgre 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et al., 1996), and is superseded by widespread intracrustal melting, migmatization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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