Highly retentive core domains in K-feldspar and their implications for ⁴⁰Ar/³⁹Ar thermochronology illustrated by determining the cooling curve for the Capoas Granite, Palawan, The Philippines

<div><p>K-feldspar from the late Miocene Capoas Granite on Palawan in The Philippines appears to contain highly retentive diffusion domains that are closed to argon diffusion at near-solidus temperatures during cooling of this ∼7 km-diameter pluton. This is an important result, for K-feldspar is commonly considered not retentive in terms of its ability to retain argon. Closure temperatures for argon diffusion in K-feldspars are routinely claimed to be in the range ∼150–400°C but the release of ³⁹Ar from irradiated K-feldspar during furnace step-heating experiments <i>in vacuo</i> yields Arrhenius data that imply the existence of highly retentive core domains, with inferred closure temperatures that can exceed ∼500–700°C. These high closure temperatures from the Capoas Granite K-feldspar are consistent with the coincidence of ⁴⁰Ar/³⁹Ar ages with U–Pb zircon ages at <i>ca</i> 13.5 ± 0.2 Ma. The cooling rate then accelerated, but the rate of change had considerably slowed by <i>ca</i> 12 Ma. Low-temperature (U–Th)/He thermochronology shows that the cooling rate once again accelerated at <i>ca</i> 11 Ma, perhaps owing to renewed tectonic activity.</p>