The εNd values for six large-volume (100-3000 km3) ash-flow tuffs and associated lavas from the multicyclic central caldera cluster of the San Juan volcanic field in south-central Colorado are between those of Proterozoic crust in the region and mantle-derived basaltic magmas, and the values generally become progressively higher in progressively younger tuffs and lavas. The increase in the ϵNd values of the tuffs, from -8.0 to -6.0 with decreasing age, can be modeled by assimilation and crystal fractionation of a mantle-derived magma, accompanied by an increase of ∼4 units in εNd values of the assimilated crust. The postulated increase in &egr/Nd values of the crust is envisioned to have occurred by hybridization of the crust through continued injection of mantle-derived magmas during the life of the magmatic system. Decreasing 206Pb/204Pb ratios observed in progressively younger tuffs following the initiation of caldera-related volcanism cannot, however, be solely explained by addition of mantle-derived magmas to the crust, but are more likely to reflect the transfer of lower-crustal Pb into the upper crust as the magmatic system evolved. Input of large volumes (>300 000 km3 in the San Juan volcanic field) of mantle-derived magma resulted in extensive hybridization of preexisting crust, suggesting that large-scale silicic volcanism involves generation of large quantities of new crust.