We examined the Rb-Sr and Sm-Nd systematics of three different, high-level to subvolcanic granitoid plutons and their microgranular enclaves. The host granitoids are adamellites and granodiorites, while the enclaves are mostly microtonalites with igneous textures. The enclaves are rounded in shape and vary from 1 m across. Contacts with the enclosing granitoids are sharp and, mostly, nonlobate. The microtonalitic enclaves are similar in mineralogy to their hosts. Initial 87S/86Sr are all ≤0.7070 and initial ϵNd os -3.0 to -6.6 for both the enclaves and their hots. This precludes a major input of an unmodified, mantel-derived component, at the time of magma genesis. Nd isotopic contrasts between enclaves and hosts are 1 &egr;-Nd-unit at most. These data indicate broadly cogenetic origins for enclave-host systems, though some enclaves have slightly less ''crustal'' signatures than the enclosing granitoids.

There are several possible interpretations. (1) There may have been nearly complete isotopic homogenization between enclaves and host magmas derived from different starting parents. (2) The enclaves and granitoids could all have purely crustal origins. Enclaves might represent magmas derived from slightly less evolved protoliths or parental magma, or they could be concentrations of early formed crystals, with interstitial melt. (3) Basaltic magma, from the depleted mantle, could have thoroughly mixed with a large volume of crustal melt, in the lower crust. (4) There could have been a significant input of melt derived from an enriched mantle source. Several of these mechanisms may have played roles in the genesis of the granitoids and their enclaves. Nd isotopes do not always allow unequivocal recognition of mantle components in granitoid magmatism. We argue that enclaves of this type are probably not products of major mixing processes relevant to granitoid protoliths and large-scale crustal evolution. They commonly result from second-order mingling of broadly cogenetic magma fractions that may have a variety of different origins. ¿1990 American Geophysical Union