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 |
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| Authors: | M A Forster R Armstrong B Kohn G S Lister M A Cottam S Suggate |
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| Institution: | 1. Research School of Earth Sciences, The Australian National University, Canberra, ACT 2601, Australiamarnie.forster@anu.edu.au;3. Research School of Earth Sciences, The Australian National University, Canberra, ACT 2601, Australia;4. School of Earth Sciences, The University of Melbourne, Parkville, Vic 3010, Australia;5. SE Asia Research Group, Department of Earth Sciences, Royal Holloway University of London, Egham TW20 0EX, UK |
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| Abstract: | 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 in vacuo 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 ca 13.5 ± 0.2 Ma. The cooling rate then accelerated, but the rate of change had considerably slowed by ca 12 Ma. Low-temperature (U–Th)/He thermochronology shows that the cooling rate once again accelerated at ca 11 Ma, perhaps owing to renewed tectonic activity. |
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| Keywords: | dating emplacement 40Ar/39Ar geochronology U–Pb SHRIMP geochronology (U–Th)/He thermochronology K-feldspar geochronology closure temperature South China Sea hydrocarbon maturity age of unconformity |
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