Geochemistry and Provonance of the Middle Ordovician Austin Glen Member (Normanskill Formation) and the Taconian Orogeny in New Engalnd.

Bock, Barbara, McLennan, S. M. and Hanson, G. N. (1998) Geochemistry and Provonance of the Middle Ordovician Austin Glen Member (Normanskill Formation) and the Taconian Orogeny in New Engalnd. Sedimentology, 45 . pp. 635-655. DOI 10.1046/j.1365-3091.1998.00168.x.

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The Austin Glen Member of the upper Middle Ordovician Normanskill Formation is a
sandstone-shale ¯ysch 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
= 1á8 Ga). Furthermore, Nd isotopic
compositions are extremely homogeneous (eNd
= ±8á1 = 0á6; 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 ¯oor, 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
quanti®ed 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.

Document Type: Article
Refereed: Yes
Open Access Journal?: No
DOI etc.: 10.1046/j.1365-3091.1998.00168.x
ISSN: 0037-0746
Date Deposited: 18 Feb 2008 17:24
Last Modified: 17 Jul 2018 13:00

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