Lead-isotopic evidence for distinct source of granite and for distinct basement in the northern Appalachians, Maine

Lead-isotopic compositions of feldspars in plutons of the coastal lithotectonic block (Avalonian basement), central Maine (Merrimack) synclinorium, and Connecticut Valley-Gaspé synclinorium in northern Maine (Granville basement) indicate the presence of three fundamentally different sources of Devonian granitic rocks in Maine. Plutons in the coastal lithotectonic block have the most radiogenic values (²⁰⁶Pb/²⁰⁴Pb: 18.25–19.25; ²⁰⁷Pb/²⁰⁴Pb: 15.59–15.67; ²⁰⁸Pb/²⁰⁴Pb: 38.00–38.60); plutons in northern Maine are the least radiogenic (²⁰⁶Pb/²⁰⁴Pb: 18.00–18.50; ²⁰⁷Pb/²⁰⁴Pb: 15.51–15.55; and ²⁰⁸Pb/²⁰⁴Pb: 37.80–38.38). Intermediate lead-isotope values characterize the plutons in central Maine (²⁰⁶Pb/²⁰⁴Pb: 18.20–18.40; ²⁰⁷Pb/²⁰⁴Pb: 15.56–15.60; and ²⁰⁸Pb/²⁰⁴Pb: 38.00–38.30). All plutons show relatively radiogenic lead values for their ages and suggest the imprint of continental crustal sources especially in the coastal block and in central Maine. Plutons from the Connecticut Valley–Gaspé synclinorium allow for a more significant isotopic input from the subcontinental mantle. The three distinct lead-isotope groups probably identify plutons formed in different crustal fragments in a continental collisional environment that were juxtaposed after emplacement of the granites.