DECOUPLING OF WHOLE-ROCK ND AND ZIRCON U-PB AND HF ISOTOPE SEDIMENT PROVENANCE DETERMINATIONS: EXAMPLES FROM NEOPROTEROZOIC-CAMBRIAN SEDIMENTARY ROCKS ALONG THE WESTERN MARGIN OF LAURENTIA

Although detrital zircon U-Pb age spectra and Hf isotopic compositions are used to define the sources of bulk siliciclastic sediments, zircons are durable accessory minerals that carry information on the sources of the zircon but not necessarily of the bulk of the sediments in which they are entrained. In contrast, light rare earth elements are contributed largely by detrital clay minerals and monazite in siliciclastic sediments, and so whole-rock Nd isotopic data may provide a better indicator of the bulk sediment source(s). To test whether detrital zircon U-Pb and Hf isotopic data and whole-rock Nd isotopic compositions provide different, yet complementary, information on sediment provenance, we compared available whole-rock and zircon isotopic data from cratonic Neoproterozoic-Cambrian Wood Canyon Formation (WCF) and underlying, off-craton, Neoproterozoic Stirling Quartzite and Johnnie Formation deposited along the western margin of Laurentia. Coarse- and fine-grained sedimentary rocks from throughout the WCF contain both high ( ~-5 to -10) and low (<-15) ε_Nd(0) whole-rock detritus. The low ε_Nd(0) sedimentary rock samples overlap the isotopic compositions expected for the products of erosion of the local Paleoproterozoic Mojave Province. The high ε_Nd(0) portions of the WCF align along a ~1.1 Ga reference isochron, suggesting that these sediments were derived from Mesoproterozoic source rocks, either locally or from more distal “Grenville” source regions in present-day Texas. No high ε_Nd(0) sedimentary rocks are found in underlying Stirling Quartzite or Johnnie Formation and yet all of these sedimentary rocks contain, in varying proportions, ~1.1 Ga detrital zircon with similar ε_Hf(0) (-15 to -20) to those found in high ε_Nd(0) parts of the WCF. We conclude that the ~1.1 Ga detrital zircons were derived from high ε_Hf(0), Mesoproterozoic source regions, irrespective of the sources of the bulk sediment as determined by their whole-rock Nd isotopic compositions. These results demonstrate that detrital zircon age distributions alone only yield a partial picture of the locus of weathering and erosion that actually produced the bulk of the sedimentary detritus in which the detrital zircon are intermixed.