A direct comparison of the ages of detrital monazite versus detrital zircon in Appalachian foreland basin sandstones: Searching for the record of Phanerozoic orogenic events

The provenance potential of detrital monazite was investigated by in situ measurement of ²³²Th-²⁰⁸Pb dates of grains isolated from six Middle Carboniferous-Permian sandstones from the Appalachian foreland basin. Provenance assessment of these units was previously investigated by measuring U-Pb crystallization ages of detrital zircon (Thomas et al., 2004; Becker et al., 2005, 2006). Approximately 90% of the detrital zircon ages record Mesoproterozoic or older ages, with only 10% recording the three major pulses of tectonism (Taconian, Acadian and Alleghanian) that are the hallmark of the Appalachian Orogen. ²³²Th-²⁰⁸Pb ages of detrital monazite, however, strongly record the complex phases of Paleozoic orogenesis. Nearly 65% of the ages record Paleozoic events, while 35% record Neoproterozoic or older ages. In several of the analyzed sandstones, detrital monazite ages record Paleozoic orogenic events that are completely missed by detrital zircon ages, demonstrating that monazite ages more accurately reflect the character of the sediment source rocks. The inferred maximum age of sediment deposition, as determined by the youngest monazite grains, is ~550Ma younger for two of the analyzed sandstones compared to depositional constraints based on the youngest detrital zircon. The different physical properties and petrogenesis of zircon and monazite are interpreted to be factors for the dramatic differences in sediment provenance information provided by each mineral. The results from this study have important implications for determining sediment provenance, constraining maximum age of sediment deposition, and developing robust regional tectonic models.