GEOCHEMISTRY AND PROVENANCE OF THE MIDDLE ORDOVICIAN AUSTIN GLEN MEMBER (NORMANSKILL FORMATION) AND THE TACONIAN OROGENY IN NEW-ENGLAND

The Austin Glen Member of the upper Middle Ordovician Normanskill Form ation 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. L ack of K-feldspar and paucity of plagioclase, in addition to the domin ance of sedimentary rock fragments, indicate that the source was domin ated by recycled, sedimentary components. Major and trace element data support this conclusion and indicate that the provenance of both shal es and sandstones was the same. No evidence of an ophiolitic or volcan ic component was observed. Interpretation of Nd isotopic characteristi cs are complicated by a partial resetting of the Nd isotope system at about the time of sedimentation but indicate that the provenance of th e Austin Glen Member had a long-term history of light rare earth eleme nt (LREE) enrichment (average T-DM = 1.8 Ga). Furthermore, Nd isotopic compositions are extremely homogeneous (epsilon(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. Pb-207/Pb-204 rati os are variable but within the range of Pb isotopic compositions typic ally described as Grenvillian. The range of (207)pb/Pb-204 is greater than expected for the range of Pb-206/Pb-204 and suggests an additiona l component of Pb, possibly introduced during diagenesis. The immediat e source of the Austin Glen Member may have been the accretionary pris m that developed as older sediments of the Laurentian margin were scra ped off the basin floor, incorporated within the accretionary prism an d shed into the basin. No evidence indicating the arrival of an undiff erentiated island are or continental fragment during the Taconian Orog eny has been found. The data acquired during this study can be explain ed 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 li kely to have been minor. Accordingly, we conclude that the Taconian Or ogeny in New England involved either: (1) a continental are that invol ved exclusively Laurentia; (2) collision of a continental block with i dentical geochemical characteristics as Laurentia; or (3) essentially no detritus om any exotic colliding block (island are or continental f ragment) reached the foreland basin at the time of Austin Glen deposit ion.