Geochemical approaches to sedimentation, provenance, and tectonics
S. M. McLennan, S. Hemming, D. K. McDaniel, G. N. Hanson, 1993. "Geochemical approaches to sedimentation, provenance, and tectonics", Processes Controlling the Composition of Clastic Sediments, Mark J. Johnsson, Abhijit Basu
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Geochemical and isotopic approaches to constraining provenance of sedimentary rocks complement the information inferred from petrography. Geochemical approaches have several advantages, including applicability to both matrix-rich sandstones and shales and ability to constrain provenance age and geochemical history. Five provenance components, or terrane types, have been defined on the basis of whole-rock chemical and Nd-isotopic composition, including Old Upper Continental Crust, Recycled Sedimentary Rocks, Young Undifferentiated Arc, Young Differentiated Arc, and various Exotic Components, such as ophiolites. Among the most important geochemical characteristics that define these provenance types are Nd isotopic composition (reflecting average provenance age), europium anomalies (reflecting intracrustal igneous differentiation processes), large-ion lithophile element enrichments (provenance composition), alkali and alkaline earth depletions (weathering and alteration), Zr and Hf enrichments (heavy mineral enrichments), and high Cr abundances (ultramafic sources).
Pb isotopic compositions of whole rocks and framework (quartz, feldspar) and accessory (e.g., zircon, monazite) grains constrain the age and crustal history of sediment sources. The U-Pb system may be used to date the time of crystallization of small populations and, for favorable circumstances, individual sand-sized grains of detrital quartz. The Early Proterozoic Pokegama Quartzite contains detrital quartz populations that give a Pb-Pb age of 2647 ± 16 Ma, consistent with detrital zircon ages and with the age of the Archean Superior Province, from which this formation is mainly derived. The initial Pb isotopic composition may be approximated by the Pb isotopic composition of leached feldspars, due to their low U/Pb and Th/Pb ratios. Pb isotopic compositions of detrital feldspars may also provide information about sedimentary provenance. K-feldspars from the Pokegama Quartzite and Early Proterozoic Chelmsford Formation fall within the field of Archean Superior Province igneous K-feldspars. They are distinct from K-feldspars found in other potential provenance terranes, including the Penokean Orogen or, in the case of the Pokegama, the Minnesota River Valley gneisses.