The Josephine Mountain intrusion is a Cretaceous calc-alkalictonalite-granite pluton emplaced at 22 km depth in a continental margin arc. Variable uplift of adjacent terranes in southern California since mid-Cretaceous time allows us to reconstruct the local crustal column and evaluate its role as a contaminant of mantle-derived arc magmas in this region. The parental magma of the intrusion was high-alumina basalt whose isotopic signature (87Sr/86Sr = 0.7087; δ18O = 7.5; ϵNd = −10) cannot have been generated by intracrustal assimilation of known or inferred rock types in the middle or lower crust. Such a signature could have resulted from high-pressure fractionation of primary low-alumina basalt coupled with assimilation of felsic/pelitic lower crust, partial melting of enriched subcontinental mantle followed by high-pressure fractionation, or a combination of these processes. Tonalite of the intrusion was formed by fractionation of the parent magma coupled with assimilation of local felsic wall rocks or by crustal melts similar to slightly younger granite. Assessment of the magnitude of crustal contamination is hampered by uncertainty regarding the existence and role of partial melting of previously enriched subcontinental mantle in generating the parental basaltic magma, leading to concomitant uncertainty in the fraction of new continental crust created by such arc plutonism.