SEDIMENTARY RECYCLING AND LIMITS ON ACCURACY OF DETRITAL ZIRCON PROVENANCE ANALYSIS: INSIGHTS FROM DETRITAL MONAZITE GEOCHRONOLOGY AND TEXTURES

Sediment recycling poses a fundamental limitation for accurate sedimentary provenance analysis. Detrital zircon (DZ) geochronology as a provenance tool applied to clastic systems across Laurentia is particularly plagued by recycling of abundant late Mesoproterozoic zircons generated in the Grenville “zircon fertility” boom. Presence of a dominant Grenville age mode is usually cited as evidence of an eastern Laurentian/Appalachian provenance. “Double dating” of DZ grains [U/Pb geochronology combined with (U-Th-Sm)/He thermochronology] has shown that a surprisingly large fraction of DZ is recycled. There is no simple high-throughput method (e.g., CL imaging) for distinguishing recycled DZ grains. However, detrital monazite (DM) provides the means of testing for, and estimating the proportion of, recycled sedimentary components. Monazite, unlike zircon, crystallizes during diagenesis or low-grade metamorphism of clastic sediments and produces textures that clearly differentiate it from monazite grains derived mostly (at least in the Appalachian foreland) from regional metamorphic rocks in the source region. Erosion of an exposed clastic rock in which diagenetic monazite formed generates “diagenetic detrital monazite” (DDM) grains, in addition to second cycle DM grains. The DDM grains are full of low-T mineral inclusions and are easily distinguished from DM grains in reflected light or via BSE imaging. I.e., the presence of DDM grains proves recycling occurred. Counting DDM vs. DM grains provides a first order estimate of the volume of the recycled component. For example, Permo-Carboniferous fluvial quartz arenites in the Appalachian foreland contain ~35% of DDM grains in the total population of monazite grains examined, most of which yield Th-Pb ages of ca. 500-450 Ma (time of diagenetic/low grade monazite formation in late Neoproterozoic, upper Ocoee strata in the Blue Ridge). The total DDM+DM age spectrum suggests Grenville crystalline rocks were a minor source for the Permo-Carboniferous arenites, whereas the DZ age spectrum would be interpreted as Grenville rocks being the dominant source. We propose that DM (Th-Pb)- and DZ (U-Pb)-(U-Th-Sm)/He “double-double-dating” would be a means to test the utility of using DDM vs. DM to assess recycling and obtain more accurate provenance constraints.