Abstract

To test the suitability of using shale geochemistry as a provenance indicator of a source with an ultramafic component, we used I.C.P. Mass Spectrometry to analyze 130 samples from seven localities from Newfoundland to New York. These samples represent mud from the foreland basin produced during the collision of the Taconic are with North America in the Ordovician. The sandstones in the flysch contain detrital chromite and other detritus that indicate that the source contained ultramafic rocks, presumably from ophiolites emplaced during collision. This setting is an ideal test case for examining the spatial and temporal variation in Cr and Ni geochemistry of shale derived from ophiolite-bearing highlands. Shale samples with high concentrations of Cr and Ni have a Cr/Ni ratio of nearly equal 1.4, which is approximately the Cr/Ni ratio for ultramafic rocks ( nearly equal 1.6), suggesting only minor geochemical partitioning, but sandstones have a Cr/Ni ratio of > 3.0, suggesting significant sedimentary fractionation. Analyses of samples taken upsection in a single stratigraphic section suggest that proximity to the source influences Cr and Ni concentrations. A decrease in Cr and Ni concentrations through time suggests that uplift and erosion of the thrust complex (non-ophiolitic) diluted the ultramafic signal. We attribute significant along-strike variation in Cr and Ni concentrations to the relative proportion of ultramafic rocks in the source region. This case study shows that Cr and Ni geochemistry of shale from basin strata can be used to determine the lateral and temporal variability of ultramafic rocks in an active orogenic setting.

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