INTERPRETING DETRITAL ZIRCON PROVENANCE IN THE CONTEXT OF REGIONAL STRATIGRAPHIC AND TECTONIC MODELS: AN EXAMPLE FROM LATE ORDOVICIAN–EARLY DEVONIAN COVER ROCKS ON GANDERIA, EASTERN MAINE

SHRIMP U-Pb ages of zircon grains in Late Ordovician—Early Devonian Ganderian cover rocks in eastern Maine yield interesting and, in some instances, startling insights into the provenance and tectonic association of the host rocks and their sources.

1. Not all of the detrital zircons have been recycled from older source rocks. ∼ 422 – 430 Ma zircons are distributed over a broad geographic area in several Silurian units. These grains are not much older than their host rocks suggesting that there would not have been enough time for them to have crystallized, been incorporated in a plutonic or volcanic rock, exhumed and eroded, and incorporated in formations thousands of meters thick. They are most likely airborne, erupted during early stages of activity in the Eastport-Mascarene arc or other local sources.

2. The immediate source of sediment for these cover strata was internal (i.e. Ganderian), with little if any external input from Laurentia or Avalonia. Significant populations of zircon grains with familiar ∼1,000 Ma and ∼600 Ma ages in these rocks are cited by several workers, most recently Dokken et al. (2018), as evidence for external provenance. However, currently accepted tectonic models postulate what would have been major barriers to zircon transport from either of these adjacent source regions. It is highly unlikely that zircons could have been transported from Laurentia across the deep-water (originally 600-800 Km-wide) Central Maine basin, and from Avalonia across the Eastport-Mascarene island arc/trench complex at the trailing edge of Ganderia

3. Ganderia most probably separated from Amazonia after the breakup of Rodinia. Comparison of age spectra of zircons from our study with timing of zircon-producing events on peri-Gondwanan Laurentian, West African), and Amazonian plates supports the conclusion by van Staal et al. (2012) that Ganderia separated from Amazonia rather than from either potential source