HR: 09:35h
AN: V51B-06    [Abstracts]
TI: Describing Ancient Forearcs Through the Study of Mafic and Ultramafic Metamorphic Rocks: a Case Study from the US Appalachians
AU: * Ryan, J G
EM: ryan@shell.cas.usf.edu
AF: Department of Geology University of South Florida, 4202 East Fowler Ave. SCA 528, Tampa, FL 33620, United States
AU: Peterson, V
EM: petersvi@gvsu.edu
AF: Department of Geology, Grand Valley State University, Allendale, MI 49401, United States
AU: Yurkovich, S P
EM: yurkovich@wcu.edu
AF: Department of Earth Sciences/NRM, Western Carolina University, Cullowhee, NC 28723, United States
AB: When assessing the role of subduction in ancient orogenic rocks, metamorphism associated with collision can obscure original geochemical signatures. In the Blue Ridge province of the US Appalachians, small, discontinuous mafic and ultramafic rock exposures form a broad chain near the center of the belt, a map pattern that has led past workers to suggest they represent an ancient ophiolite (Misra and Keller 1978; Misra and McSween 1984). The focus of our 1997-2002 Blue Ridge REU Site Program was to examine the field relations, structure and geochemistry of a number of bodies in fine detail, toward understanding what these rocks represented in a convergent plate boundary setting of the late Precambrian. Our results indicate two classes of units: a) multi-lithologic bodies with contact relations and chemical signatures consistent with mafic and ultramafic cumulate rocks, and immobile trace element patterens indicating oceanic origins. These bodies are enclosed in pelitic metasediments and proximal to major faults or terrane boundaries, supporting an origin as fragments of a subducting oceanic plate. b) Podiform ultramafic and mafic bodies of varied size and lithologic complexity. The ultramafic bodies are dunites with minor pyroxenite that preserve U-shaped REE patterns and enriched isotopic signatures. The mafic rocks show major and immobile element systematics consistent with andesitic protoliths. These units occur in several Blue Ridge terranes characterized by block-in matrix rocks (Hatcher et al 2005; Raymond et al 1989). Relict eclogites point to deposition in melange sequences, suggesting that these bodies represent entrained crustal and mantle blocks from the overriding plate. While LILE patterns in such ancient rocks have been reset, their major and immobile trace element signatures are similar to those in modern subduction systems. Thus, subduction in the late Precambrian appears to have functioned much as it does today.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3621 Mantle processes (1038)
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly