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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.

The generation of granitic rocks by melting of flyschoid sediments in an accretionary prism is addressed in this study of 50 Ma silicic igneous rocks in the Gulf of Alaska, near Cordova, Alaska. Plutons of relatively homogeneous biotite and biotite-hornblende granodiorite and minor tonalite and granite are scattered through the Paleocene and Eocene Orca Group. Two masses of cointrusive gabbro and minor dacite dikes also were intruded here. The Orca Group consists of flysch, (quartzofeldspathic graywacke and argillite of turbidite or deep-sea fan origin) and of minor basaltic rocks and pelagic sedimentary rocks. The Orca represents the youngest and structurally lowest part of a late Cretaceous to Eocene composite accretionary prism that extends for 2100 km along the Gulf of Alaska. The plutons 2 are 5-150 km in plan and represent less than perhaps 5% of the total volume of this part of the prism. These granitic rocks are unusual in that they were emplaced in a forearc environment during the last stages of deformation of the prism. The three intrusive bodies chosen for study (the McKinley Peak, Rude River, and Sheep Bay plutons) show a range of chemical and initial isotopic compositions (SiO2 = 66.3-71.3%, Na20 = 2.8-3.6%, K20 = 1.8-3.0%, eNa = +2.1 to --3.3, 87 86 206 204 207 204 Sr/ Sr = 0.7051-0.7067, Pb/ Pb = 19.04-19.20, Pb/ Pb = 15.60-15.66, and 2øspb/204pb = 38.59-38.85). Relative to the other two plutons, the McKinley Peak pluton generally shows slightly lower K20, higher A120 3, higher eNa, and lower 87Sr/86Sr ratios. All three plutons, however, have similar, well-defined minor and trace element abundances characterized by relative enrichment in light rare earth elements and depletion in high field strength elements. The granodiorites and flysch of the Orca Group show overlapping elemental and isotopic compositions. The only clearly defined chemical differences between the flysch and the granodiorites are weak negative Eu anomalies in the granodiorites and slightly lower Ca and higher Na contents in the flysch. The Nd and Sr isotopic compositions of the Rude River and Sheep Bay plutons completely overlap those of the flysch. The McKinley Peak pluton, however, shows discretely higher eNa and slightly lower 87 Sr/86Sr values than those of the flysch. The Pb isotopic compositions of the flysch and the Rude River pluton also overlap, but Pb of the other two plutons is slightly less radiogenic. Our chemical data, modeling, and comparison with Conrad et al.'s (1988) melting experiments of graywacke indicate that the granodiorite orginated by large fractions (65-90%) of melting of the Orca Group graywacke and argillite. Plagioclase, pyroxene(s), and biotite(?) were residual to melting at about 850ø-950øC and at low H20 activities. The distinct chemical and isotopic compositions of the McKinley Peak pluton probably result from variations in the character of the flysch at depth in the prism, rather than from mixing between melts of the flysch and mafic magmas injected into the prism itself. However, basaltic magmas...

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