Oxygen and strontium isotope ratios have been used to characterize source regions for granitic magmas for a transect across the northern Appalachian orogen in central and eastern Maine. The northwestern plutons (Katahdin and Seboeis) have δ18O values of 10.3-13.3 and initial 87Sr 86Sr ratios of 0.7083 and 0.7066, respectively. The central plutons (Bottle Lake and Center Pond) have lower δ18O values (8.2-9.9) and initial 87Sr 86Sr ratios (0.7043-0.7055). The southeastern plutons (Lucerne and Deblois) have δ18O values (9.0-11.0) but initial 87Sr 86Sr ratios (0.7077 and 0.7041, respectively) which are intermediate between the northwestern and central plutons. Source models derived from these results and other petrological and geochemical data reflect the juxtaposition of discrete source regions by transcurrent faulting, which may be related to oblique plate motions. This model illustrates the importance of microplate accretion in the Palaeozoic history of the northern Appalachian orogen.