Late Cretaceous cooling of the east-central Peninsular Ranges batholith (33°N): Relationship to La Posta pluton emplacement, Laramide shallow subduction, and forearc sedimentation

Biotite and K-feldspar Ar/Ar systems from the east-central Peninsular Ranges batholith near 33°N were affected by two distinct phases of Late Cretaceous rapid cooling. The 85-Ma biotite K-Ar isochron separates comparatively shallow rocks in the southwest that record earlier cooling (91–86 Ma) from deeper rocks in the northeast that record later cooling (78–68 Ma). Samples close to 85 Ma isochron record both episodes of rapid cooling as well as slower cooling between 86 and 78 Ma. Although the 85 Ma isochron also coincides with a steep (1–2 m.y./km) K-Ar age gradient, only localized faulting has been detected along it. We attribute 91–86 Ma cooling to denudation related to emplacement of the voluminous suite of 96 ± 3 Ma La Posta plutons. In contrast, we link cooling after 78 Ma to Laramide shallow subduction beneath the Peninsular Ranges batholith. Our numerical simulations indicate that the latter cooling phase can be explained by either pure erosional denudation or by subduction refrigeration beginning at 80 Ma. In the latter case, erosional denudation occurs during steady-state shallow subduction. While fi nal erosion depths predicted by the two models differ significantly (as much as 20 km for pure erosional denudation but only 11 km for subduction refrigeration followed by erosion), both are within the range indicated by independent thermobarometry of the eastern batholith. Based upon the similarity of independently determined denudation histories from Peninsular Ranges batholith basement rocks and forearc sediments that onlap the northern Peninsular Ranges batholith, we conclude that erosional denudation was probably the most important process between 78 and 68 Ma. We speculate that removal of lower crust and lithospheric mantle beneath the east-central Peninsular Ranges batholith during Laramide shallow subduction triggered erosional denudation and localized thrusting until the density balance between the crust and mantle was restored in latest Cretaceous–early Tertiary time.