ORDOVICIAN ARC DEVELOPMENT AND EVOLUTION IN MUNSUNGUN INLIER, MAINE: NEW EVIDENCES FROM FIELD, GEOCHEMICAL, AND GEOCHRONOLOGICAL STUDIES

The Munsungun inlier is part of the Ordovician Northern Appalachian Popelogan arc. Very little was known about the Munsungun inlier compared to the development and evolution of its equivalents, notably the Victoria, Popelogan, and Bronson Hill-Ammonoosuc belts. Field mapping and new geochemical and ICP-MS zircon U-Pb data present a scenario of arc development and evolution of the inlier. The inlier contains two belts of volcanic rocks separated by a faulted sliver of black shale and conglomerate. In the SE, the Bald Mtn-Jack Mtn-Norway Bluff belt consists of tholeiitic E-MORB-like (rifted arc) basalt/diabase with enriched LREE & LILE and positive εNd, as well as arc calc-alkaline basalt, andesite, dacite, and diabase/gabbro that are depleted in Ta, Nb, and Ti and enriched in LILE with negative Eu anomaly and negative εNd values (-16.3 to -18). The belt also contains voluminous basaltic and andesitic pyroclastics. Two tuff samples from the belt yielded U-Pb concordia ages of 468.0±2.0 Ma and 471.2±4.2 Ma. The Spider Lake-Mule Brook Mtn belt in the NW consists of tholeiitic arc basalt and pyroclastics and minor arc calc-alkaline rhyolite with similar REE pattern and εNd values of 3.5 to -4.8. Two tuff samples gave U-Pb concordia ages of 454.4±1.8 Ma (Mule Brook Mtn) and 456.6±1.8 Ma (Spider Lake Fm). The ages suggest that the Munsungun arc commenced at 471-468 Ma with basaltic, andesitic, and dacitic volcanic and subvolcanic rocks. These rocks overlie mélange of the Chase Brook Fm which is part of the leading edge of ensialic Ganderia. These data suggest a northwestward trench-arc migration (current coordinates), where volcanism was reactivated at 454-456 Ma, forming Spider Lake and Mule Brook Mtn volcanic and subvolcanic rocks. Detrital zircon U-Pb data obtained from three sandstone samples from Rowe Lake Fm, Blind Brook Fm, and an unnamed formation (all of them consist of graptolitic black shale, graywacke, and/or conglomerate) show youngest zircon grains at 447.0±3.2 Ma, 445.2±1.8 Ma, and 442.8±3.2 Ma, respectively, indicating the earliest deposition ages of the formations. Their U-Pb age spectra point to a Laurentian provenance, suggesting that near the end of Ordovician, the Munsungun arc was already accreted to the Laurentia margin and the restricted successor basins began receiving sediments from the Laurentia.