Regional differences in PB isotopic compositions of feldspars in plutonic rocks of the northern Appalachian Mountains, U.S.A., and Canada: A geochemical method of terrane correlation

Abstract

Feldspar Pb isotopic compositions from Paleozoic tonalitic to leucogranitic Caledonian plutons from the northern Appalachian Mountains of the U.S.A. and Canada range from nonradiogenic to radiogenic relative to the evolution curve of Stacey and Kramers (1975). Pb isotopic values in the feldspars are generally independent of modal and major and trace element contents of the plutons. Plutons with the least radiogenic Pb isotopic signatures (Northern group) have intruded allochthonous rocks closest to the North American craton, and plutons with the most radiogenic Pb (Southern group) have intruded Avalonian basement. Feldspars from plutons in the Central group have intermediate Pb isotopic compositions between the Northern and Southern groups and do not define an isotopically unique group. The predominant source of the felsic magmas was the continental lithosphere as indicated by the general Pb isotopic similarity of the plutonic feldspars with either Grenville or Chain Lakes basement in the case of the Northern group and with Avalonian basement rocks in the case of the Southern group. The source region for plutons in the Central group may have contained a mixture of Grenvillian and Avalonian components. Plutons in the northern Appalachians are as a group more evolved in their Pb isotopic compositions relative to Caledonian plutons in the British Isles.

Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England

“…6, data from twelve plutons in our study area correspond quite closely with the field of results determined for plutonic rocks in the Central Maine province of Ayuso (1986). Ayuso (1986) and Ayuso and Bevier (1991) suggested that the isotopic ratios in the Central Maine province are the result of mixing of lead from Grenville-type (%1.0-1.2 Ga) basement (their Northern Maine group) and Avalonian-type (%0.60-0.65 Ga) basement (their Coastal Maine group). We suggest, however, that their data for the Central Maine province results from mixing of lead in sedimentary aprons that spread oceanward from peri-Laurentian (including Grenville) and peri-Avalonian terranes, rather than from the actual basements.…”

“…This approach works well in regions, such as northern New Brunswick, where VMS and plutonic feldspars both yield strongly radiogenic leads that are characteristic of the underlying Gander terrane and of Avalon and associated terranes (Slack et al, 1991;Ayuso and Bevier, 1991). In northern New Hampshire, however, VMS and plutonic feldspar Pb isotope compositions do not agree.…”

“…Plutonic feldspar-lead isotope data have provided a broad regional framework for basement terrane identification (Ayuso and Bevier, 1991), and they are specifically that the latest Ordovician to Early Silurian Lost Nation pluton on the west side of the Bronson Hill axis was partly derived from Grenville-like basement, whereas the Early Devonian Fairlee-Bethlehem-Kinsman suites were derived from Avalon-like basement east of the BHA axis. Accordingly, it is reasonable to place the RIL either near the Silurian tectonic hinge along the western margin of the Central Maine-Matapedia trough, or within the trough (Fig.…”

“…Because the feldspar-Pb isotopic composition of the granite occurs in the intermediate field for central Maine plutons ( Fig. 6; Table 2) of Ayuso and Bevier (1991), a source in Grenville-like or Avalon-like basement is unlikely. Alternatively, the source might have been sea-floor flysch (Dead River Formation), but the granite is considerably more evolved than the ''flyschmelt'' granodiorites of the eastern Gulf of Alaska (Barker et al, 1992).…”

U-Pb Zircon Ages and Pb Isotope Geochemistry of Gold Deposits in the Carolina Slate Belt of South Carolina

“…Also, shown are isotopic fields for the Grenville (Vermont, Ontario, and Cape Breton Island). Sources of data include: Ayuso and Bevier (1991), R.A. Ayuso and N. Ratcliffe (unpub. data), DeWolffe and Mezger (1994), Ayuso et al (1996), and Whalen et al (1997, and references therein).…”