Title: Sedimentologic and Geochemical Evidence for Middle Ordovician Near-Trench Volcanism in the Central Appalachian Orogen
Abstract: The Jonestown volcanic rocks are a minor but important component of the Greenwich slice of the Hamburg klippe of eastern Pennsylvania. Sedimentary rocks of the Greenwich slice are arranged in faultbounded, coarsening-upward sequences of pelagic deposits (Early to Middle Ordovician) overlain by hemipelagic and turbidite deposits (Middle Ordovician). Petrographic and geochemical characteristics of the sedimentary rocks indicate derivation from continental rather than volcanic sources. The coarsening-upward sequences of the Greenwich slice are inferred to record advance of a site on oceanic lithosphere toward, and ultimately into a trench by Middle Ordovician time. Rocks of the Jonestown volcanic suite were extruded onto and intruded into the inferred Middle Ordovician near-trench and trench axis mudstone and sandstone. Immobile element concentrations of the Jonestown suite are typical of mid-oceanic ridge basalts and, along with the lack of associated shallow-water and/or volcaniclastic sedimentary rocks, argue against magmatism related to an ocean island-trench encounter. Normal fault-related magmatism on the subducting plate seaward of the inner trench slope may explain some of the stratigraphic relations between the igneous and sedimentary rocks but raises more questions than it answers (e.g., mechanism of magma generation). The association of the MORB-like Jonestown volcanic rocks and near-trench sedimentary rocks of the Greenwich slice is best explained by either extension of a leaky transform fault into the trench or migration of a mid-oceanic ridge toward the trench. Geochemical and geological differentiation of these scenarios is not possible at the present time.
Publication Year: 1986
Publication Date: 1986-01-01
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 11
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