Geochemistry and provenance of the Middle Ordovician Austin Glen Member (Normanskill Formation) and the Taconian Orogeny in New England |
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| Authors: | BARBARA BOCK S M MCLENNAN & G N HANSON |
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| Institution: | Department of Geosciences, ESS Building, State University of New York at Stony Brook, Stony Brook, NY 11794-2100, USA (E-mail: Scott.McLennan@sunysb.edu;Gil@pbisotopes.ess.sunysb.edu) |
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| Abstract: | The Austin Glen Member of the upper Middle Ordovician Normanskill Formation is a sandstone-shale flysch succession deposited in the foreland of the Taconian Orogen. Petrographic, major and trace element, and Nd–Pb isotopic data provide substantial constraints on its provenance. Lack of K-feldspar and paucity of plagioclase, in addition to the dominance of sedimentary rock fragments, indicate that the source was dominated by recycled, sedimentary components. Major and trace element data support this conclusion and indicate that the provenance of both shales and sandstones was the same. No evidence of an ophiolitic or volcanic component was observed. Interpretation of Nd isotopic characteristics are complicated by a partial resetting of the Nd isotope system at about the time of sedimentation but indicate that the provenance of the Austin Glen Member had a long-term history of light rare earth element (LREE) enrichment (average TDM = 1·8 Ga). Furthermore, Nd isotopic compositions are extremely homogeneous (?Nd = –8·1 ± 0·6; 1 s.d.; n = 23) at 450 Ma, the approximate depositional age, indicating either a single source or very well-mixed sources. 207Pb/204Pb ratios are variable but within the range of Pb isotopic compositions typically described as Grenvillian. The range of 207Pb/204Pb is greater than expected for the range of 206Pb/204Pb and suggests an additional component of Pb, possibly introduced during diagenesis. The immediate source of the Austin Glen Member may have been the accretionary prism that developed as older sediments of the Laurentian margin were scraped off the basin floor, incorporated within the accretionary prism and shed into the basin. No evidence indicating the arrival of an undifferentiated island arc or continental fragment during the Taconian Orogeny has been found. The data acquired during this study can be explained almost exclusively by Grenville Province source components but with possible additional contributions from older Laurentian terranes and Late Proterozoic rift volcanics that are not readily quantified but likely to have been minor. Accordingly, we conclude that the Taconian Orogeny in New England involved either: (1) a continental arc that involved exclusively Laurentia; (2) collision of a continental block with identical geochemical characteristics as Laurentia; or (3) essentially no detritus from any exotic colliding block (island arc or continental fragment) reached the foreland basin at the time of Austin Glen deposition. |
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