Abstract

The amphibolites of the Late Proterozoic-early Paleozoic Ashe and Alligator Back Formations in western North Carolina represent metamorphosed tholeiitic basalts, unrelated to the mafic dikes of continental tholeiitic affiliation that intrude the Grenville-age basement rocks. On the basis of their geochemistry, the amphibolites are divisible into three compositional groups (I, II, and III), the protoliths of which appear to have evolved from different parental magmas at a spreading center. Group II (intermediate-Ti) amphibolite, the predominant variety in the study area, constitutes a highly fractionated suite (Zr ≃ 40-200 ppm, TiO2 ≃ 0.7-2.3 wt.%) with geochemical characteristics similar to those of N-type MORB formed from a heterogeneous mantle source. Group III (high-Ti) amphibolite is enriched in high-field-strength incompatible elements (Zr, Nb, Ti, Y, REE's), especially Ti (TiO2 = 2.8-3.4 wt.%), and is interpreted as T-type MORB on the basis of its Zr/Nb and Y/Nb ratios. Group I (low-Ti) amphibolite, restricted to the Ashe Formation, is characterized by extremely low abundances of incompatible elements, including Ti (TiO2 < 0.45 wt.%), and U-shaped REE profiles, requiring the mixing of a depleted MORB-type mantle source with a fluid phase enriched in LREE's.

The juxtaposition of depleted, low-Ti basalt (group I amphibolite) and MORB-like basalts (group II and group III amphibolites) is suggestive of a back-arc basin setting, as has been postulated for many ophiolites, but it can also be accounted for by multi-stage melting of an upper-mantle source adjacent to a mantle plume in a mid-oceanic-ridge environment. In either case, the Ashe and Alligator Back amphibolites represent metamorphosed oceanic crust material generated at a spreading center.

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