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

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.

KEY WORDS:

• 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

ACKNOWLEDGEMENTS

Geoff Fraser, Fred Jourdan and an anonymous reviewer are thanked for their efforts during review. MAF acknowledges the support of an Australian Research Council Australian Research Fellowship and Discovery Projects DP0877274 and DP120103554. Mineral separation was undertaken by Shane Paxton at RSES, ANU. ⁴⁰Ar/³⁹Ar analysis was undertaken on the VG1200 in the RSES Argon Facility, ANU, where Davood Vasegh provided technical support and undertook data reduction. Data were analysed using the eArgon and MacArgon computer program written by GSL. Scanning Electron Microprobe analysis (SEM) was undertaken at the Centre for Advanced Microscopy at ANU with Dr Frank Brink. Fieldwork was supported by Professor Robert Hall at the SE Asia Research Group, Department of Earth Sciences, Royal Holloway University of London, who also provided invaluable discussion as to the tectonic significance of the two-stage cooling history as these results emerged. The University of Melbourne thermochronology laboratory received infrastructure support under the AuScope Program of NCRIS. Roland de Jesus at the Mines and Geosciences Bureau gave permission to conduct fieldwork. Edwin Mojares was our geological guide in Palawan.

SUPPLEMENTARY PAPERS

Appendix 1 ⁴⁰Ar/³⁹Ar Geochronology analytical procedures (Capoas Granite).

Appendix 2 ⁴⁰Ar/³⁹Ar geochronology data: Sample 11, PAL33a, K-feldspar.

Appendix 3 SEM data: on the Hitachi 4300 FESEM.

DISCLOSURE STATEMENT

No potential conflict of interest was reported by the authors.