TECTONOMAGMATIC ORIGIN OF ORDOVICIAN K-BENTONITES IN CENTRAL PA BASED ON ZIRCON PETROCHRONOLOGY

Pervasive K-bentonite deposits throughout eastern and central North America reveal a history of explosive volcanism related to the Taconic Orogeny during the Ordovician. The Deicke (B12) and the Millbrig (B14) bentonite deposits are regionally correlated across the southern and central Appalachians and the midwestern US. These ash beds are generally thought to be sourced from a volcanic arc during subduction between Laurentia and off-shore microcontinents, but many uncertainties remain regarding the nature of the tectono-magmatic setting(s), including pre-eruptive storage conditions, composition and physical properties of the magmas, the relatedness of deposits to each other, and the number of sources (volcanic centers). To better understand the tectonic and magmatic conditions before, after, during, and between the deposition of the Deicke and Millbrig ashes, we used zircon phenocrysts separated from five horizons (B11, B12, B13, B14, and B15) collected from a carbonate transgressive sequence near State College, PA. We employed CL imaging of zircon, laser-ablation split-stream ICPMS of U-Pb isotopes and trace elements, and SIMS analysis of O isotopes in our investigation. Clustered median U-Pb ages (median ages: B11=457 Ma, B12=481 Ma, B13=453 Ma, and B14=455 Ma) suggest that there were potentially multiple eruptive centers active simultaneously. Trace element plots (e.g. U vs Yb, Ti vs Hf), and model melt REE (Nd-Gd ~250-35), are consistent with cooling, evolving magmas that erupted from a granitoid source material. Analysis of Eu/Eu* suggests that there is variability in the crustal storage depth for magmas represented in the State College, PA sequence (B13 ~35 km; B14 ~45 km median) (Tang et al., 2021). There is general agreement that the Millbrig (B14) erupted off-shore of what is now the southern Appalachians (Kolata et al., 1996, Zhang and Huff, 1994), but the eruptive location(s) of the other samples is still uncertain. Zircons with inherited cores of Grenville age are present, though rare in these bentonites: B11: 968-1044 Ma (n=2); B12: 806-970 Ma (n=2); B13: 914-1513 Ma (n=2); and B14: 806-1856 Ma (n=9). Subsequent work with more inherited cores may clarify if B11-B15 magmas erupted from the same volcanic system(s) in the same Grenville terrane(s).