THE GEOLOGICAL CASE FOR AVALON OFF NORTH AFRICA AT 595 MA

Avalon terranes have commonly been pictured as originating along the Amazonian margin of Gondwana based on Nd isotopic signatures of crystalline rocks and 1.0-1.3 Ga detrital zircons that could have derived from South American source belts. We suggest that the geological evidence can also be reconciled with the peri-Gondwanan African position implied by our paleopole from ca. 595 Ma Lynn-Mattapan volcanic rocks in the Southeastern New England Avalon Zone.

Source rocks that could have generated positive initial Ε_Nd values observed in Avalonian igneous rocks of maritime Canada were first pointed out in the Tocantins Province of central Brazil. Comparable results are also published, however, for Pan African belts closer to our paleomagnetically preferred site. These include the Anti-Atlas orogen that was active along the northern margin of the west African craton during Gondwana assembly, as well as the Trans-Saharan suture between the West African craton and the Saharan paleocontinent on the east.

Mesoproterozoic-age detrital zircons for which there are no source belts in the West African craton might be longitudinally derived from other terranes along the Gondwanan margin, or even from a terrane separating Avalonia from West Gondwana. Possible candidates are Middle American terranes like Ouaxaquia (eastern Mexico) where igneous and metamorphic basement rocks have yielded U-Pb zircon dates between ca. 1.1 Ga and 0.9 Ga. A second aspect of detrital zircon suites deposited in proximity to West Africa is the expected presence of grains in the ca. 2.27-2.05 Ga interval marking events loosely known as the Eburnean orogeny. Zircons of this age are rare in Avalonian-cycle sandstones, but they have been identified in quartzite of the Blackstone Group which predates Avalonian arc magmatism in southeastern New England. High concentrations of detrital zircon ages around 2.0 Ga are also reported from quartzite clasts in conglomerates from the Caledonia and Mira terranes in New Brunswick and Nova Scotia, respectively. It appears from these examples that older Avalonian sedimentary sequences could have been supplied in part by source areas of Eburnean age, but that transport patterns changed with the onset of Avalonian tectonism so that ca. 2.0 Ga Paleoproterozoic zircons are present only locally as recycled grains.