Peraluminous to metaluminous (S-type) granitoids occur in the central and eastern parts of the Peninsular Ranges batholith, southern California, as highly deformed plutons and anatectic migmatites within metapelitic prebatholithic wallrocks of upper amphibolite fades. The granitoids overlap in age with, and have undergone a structural history similar to that of, the Early Cretaceous I-type granitoids that occur mainly to the west. Initial 87Sr/86Sr ratios greater than 0.7079 and δ18O greater than 11.8 indicate a significant metasedimentary component in the source rocks of the S-type granitoids. An I-S line that defines the westernmost extent of these rocks lies close to gravity, age, isotopic, and prebatholithic wallrock contrasts within the batholith, and together these features probably mark a fundamental change in the Mesozoic crust. If the western part of the batholith represents an Upper Jurassic and Lower(?) Cretaceous accreted tectonostratigraphic terrane, then the zone that contains these features may represent a terrane boundary.
We propose that the S-type granitoids and associated migmatites developed at midcrustal levels as a result of collision and crustal thickening, and rose as diapirs, some dragging an envelope of migmatite and wallrock with them. The zone of intense metamorphism and deformation that now forms the axis of the batholith has been uplifted relative to the western part of the batholith, possibly as a consequence of unloading.