SHRIMP U-PB GEOCHRONOLOGY OF PRE-ACADIAN SILURIAN BASINS OF CENTRAL AND EAST-CENTRAL MAINE AND THE POCOMOONSHINE PLUTON: DIVERSE SOURCES, RAPID SEDIMENTATION, TECTONISM, AND INTRUSION

Geologic mapping in central and east-central Maine clarifies relationships among post-Taconian, pre-Acadian basin sediments and adjacent pre-Silurian Ganderian terranes. Detrital SHRIMP U-Pb zircon ages from six samples constrain Late Ordovician to Early Silurian basin formation prior to initiation of the Late Silurian Acadian foredeep proposed by Bradley et al. (2000). The new ages were obtained from sandstones from three formations in different post-Taconian, pre-Acadian belts and from one formation (Madrid) in the presumed Acadian foreland.

Lithotectonic belt	Central Maine	Waterville-Vassalboro	Aroostook-Matapedia	Fredericton
Formation (stratigraphic age) Youngest zircon population in Ma (# grains), N= Neoproterozoic G= Grenville, O=Early Ord.	Madrid (Late Sil.) ~433(2), G, O	Mayflower Hill, (Late Sil.) 429±7(3), G, O Hutchins Corner (Early Sil.) ~431(1), N,G,O	Mayflower Hill (Late Sil.) 426±5(3),G,O; ~425(1),N,G,O	Flume Ridge (Early to mid-Sil.) 431±3(7),G,O

Grenvillian (1.35-0.95 Ga) and Ordovician (490-440 Ma) zircons are present in all samples and two units also contain grains of Neoproterozoic (700-550 Ma) age, suggesting multiple or heterogeneous source areas. The youngest zircons in all samples span a narrow mid-Silurian age range. Data from the Waterville area suggest that the Hutchins Corner and Mayflower Hill formations were deposited in rapid succession. Recent discovery of felsic tuff and lava (as yet undated) in the Flume Ridge offers an opportunity for directly determining depositional age of that part of the formation.

A new SHRIMP U-Pb zircon age of 423±3 Ma for crystallization of the Pocomoonshine pluton is identical to a previous ⁴⁰Ar/³⁹Ar hornblende cooling age (West et al., 1992). Because this pluton intrudes deformed Flume Ridge strata, deposition, lithification, and folding of the Flume Ridge occurred between 431±3 Ma (age of the youngest detrital zircon population, n=7) and 423±3 Ma (age of the cross-cutting Pocomoonshine pluton).