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The Mars Hill terrane (MHT), a lithologically diverse belt exposed between Roan Mountain, North Carolina–Tennessee, and Asheville, North Carolina, is distinct in age, metamorphic history, and protoliths from the structurally overlying Eastern Blue Ridge and underlying Western Blue Ridge. MHT lithologies include diverse granitic gneisses, abundant mafic and sparse ultramafic bodies, and mildly to strongly aluminous paragneisses. These lithologies experienced metamorphism in the granulite facies and are intimately interspersed on cm to km scale, reflecting both intrusive and tectonic juxtaposition. Previous analyses of zircons by high-resolution ion microprobe verified the presence of Paleoproterozoic orthogneiss (1.8 Ga). New data document a major magmatic event at 1.20 Ga. Inherited and detrital zircons ranging in age from 1.3 to 1.9 Ga (plus a single 2.7 Ga core), ubiquitous Sm-Nd depleted mantle model ages ca. 2.0 Ga, and strongly negative ε Nd during Mesoproterozoic time all attest to the pre-Grenville heritage of this crust that was suggested by previous whole-rock Pb and Rb-Sr isotope studies. A single garnet amphibolite yielded a magmatic age of 0.73 Ga, equivalent to the Bakersville dike swarm, which cuts both the MHT and the adjacent Western Blue Ridge. Zircons from this sample display 0.47 Ga metamorphic rims. Zircons from all other samples have well-developed ca. 1.0 Ga metamorphic rims that date granulite-facies metamorphism. Silica contents of analyzed samples range from 45 to 76 wt%, reflecting the extreme diversity observed in the field and the highly variable protoliths. The MHT contrasts strikingly with basement of the adjacent Eastern and Western Blue Ridge, which comprise relatively homogeneous, 1.1 to 1.2 Ga granitic rocks with initial ε Nd values near 0. It appears to have more in common with distant Paleoproterozoic crustal terranes in the Great Lakes region, the southwestern United States, and South America.

A variety of lithostratigraphic units may be outlined within the pre-Cretaceous crystalline basement of the Atlantic and Gulf Coastal Plains. An expanse of undeformed granite occurs within central peninsular Florida; biotite within the pluton yields 40 Ar/ 39 Ar plateau dates of ca. 525 to 530 Ma. The high-level character of the granite and its emplacement age are similar to post-kinematic granitic plutons within northern portions of the Rokelide orogen in Guinea, West Africa. A sequence of calc-alkaline, felsic volcanic-volcaniclastic rocks, together with associated hypabyssal granite, occurs throughout the basement terrane. These are variably altered and display markedly discordant 40 Ar/ 39 Ar age spectra. Crystallization ages are likely best represented by ca. 675 to 690 Ma Rb-Sr whole-rock and 40 Ar/ 39 Ar hornblende plateau ages previously reported for correlative calc-alkaline granites exposed in southeastern Senegal. A Lower Ordovician–Devonian sedimentary sequence with non-Laurentian paleontological characteristics occurs within the Suwannee basin of the northern Florida subsurface. Detrital muscovite from Lower Ordovician sandstones in the succession records 40 Ar/ 39 Ar plateau ages of ca. 500 to 510 Ma. The sequence is similar to that within the Bové basin in Senegal and Guinea. A small area of high-grade metamorphic rocks occurs in the east-central Florida subsurface. Hornblende from this terrane records 40 Ar/ 39 Ar plateau ages of ca. 485 to 495 Ma. The character and post-metamorphic cooling history of these rocks are similar to that of units within the Rokelide orogen in Sierra Leone and Liberia, West Africa. In the subsurface Wiggins Arch of southwestern Mississippi, hornblende and biotite from interlayered gneiss and amphibolite record similar 40 Ar/ 39 Ar plateau ages of ca. 300 to 310 Ma. Phyllite from a lower-grade metasedimentary sequence penetrated in the Wiggins Arch of southwestern Alabama records a 40 Ar/ 39 Ar whole-rock plateau age of ca. 315 Ma. Characteristics of the pre-Cretaceous basement units suggest that they represent an extension of the Mauritanide, Bassaride, and Rokelide orogens of West Africa. There is no apparent record of Paleozoic tectonothermal activity in central and southern portions of the Atlantic and Gulf Coastal Plain subsurface basement terrane. This is in marked contrast to complete late Paleozoic reworking of basement units penetrated in the area of the Wiggins Arch. This suggests that a major dextral transcurrent fault system was likely active during the late Paleozoic, and that proximal basement units were directly involved in collisional aspects of Pangea assembly. The various units of the Atlantic and Gulf Coastal Plain basement are not correlative with any of the northern non-Laurentian terranes exposed in the southern Appalachian orogen (e.g., Carolina terrane of the eastern Piedmont) which had earlier accreted to exterior positions along the eastern margin of the North American craton. These were transported into their present structural positions along a basal décollement during late Paleozoic collision of Gondwana and Laurentia, and are separated from the stranded Gondwana units of the Coastal Plain basement by a suture approximately marked by the Brunswick-Altamaha magnetic anomaly.