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Original Article |
1 1Department of Earth Sciences, St. F. X. University, Antigonish, NS, Canada, B2G 2W5 (e-mail: bmurphy@stfx.ca)
2 2Instituto de Geología, Universidad Nacional Autónoma de México, 04510 México D.F., Mexico
3 3Department of Geological Sciences, Ohio University, Athens, OH 45701, USA
4 4Department of Geology and Geophysics, Texas A&M, College Station, TX 77843-3115, USA
5 5Department of Geology, St. Mary's University, Halifax, NS, Canada B2H 3C3
6 6Departamento de Petrología y Geoquímica, Universidad Complutense, 28040 Madrid, Spain
7 7Natural History Museum, Cromwell Road, London SW7 5BD, UK
8 8Department of Earth Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, UK
Recent data indicating that the Piaxtla Suite (Acatlán Complex, southern Mexico) underwent eclogite-facies metamorphism and exhumation during the Devono-Carboniferous suggest an origin within the Rheic Ocean rather than the Iapetus Ocean. The Asís Lithodeme (Piaxtla Suite) consists of polydeformed metasediments and eclogitic amphibolites that are intruded by megacrystic granitoid rocks. U–Pb (zircon) data indicate that the metasediments were deposited after c. 700 Ma and before intrusion of c. 470–420 Ma quartz-augen granite. The metasedimentary rocks contain abundant Mesoproterozoic detrital zircons (c. 1050–1250 Ma) and a few zircons in the range of c. 900–992 and c. 1330–1662 Ma. Their geochemical and Sm–Nd isotopic signature is typical of rift-related, passive margin sediments derived from an ancient cratonic source, which is interpreted to be the adjacent Mesoproterozoic Oaxacan Complex. Megacrystic granites were derived by partial melting of a c. 1 Ga crustal source, similar to the Oaxacan Complex. Amphibolitic layers exhibit a continental tholeiitic geochemistry, with a c. 0.8–1.1 Ga source (TDM age), and are inferred to have originated in a rift-related environment by melting of lithospheric mantle in the Ordovician. This rifting may be related to the Early Ordovician drift of peri-Gondwanan terranes (e.g. Avalonia) from Gondwana and the origin of the Rheic Ocean.
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