Cooling of a late-syn orogenic pluton: Evidence from laser K-feldspar modelling of the Carion granite, Madagascar

Since the late 1980's, it has been hypothesized that the wide range of appa rent argon ages seen within single K-feldspar samples might be due to a dis tribution of diffusion domain sizes within the mineral. To test and apply t his idea, an analytical technique that combines conventional laboratory deg assing experiments (resistance heating) with numerical inversion procedures has been developed to extract cooling history information from feldspars. A key part of the method involves careful control of temperature in the lab oratory to constrain the diffusion parameters of the feldspar samples. In o ur study, we have K-feldspar data from single crystals that mimic the types of data seen in classic resistance heater fusion experiments. Our step-hea ting data are based on using a continuous argon-ion laser with no direct co ntrol on temperature. However, with only a single added free parameter in t he model, we show that it is possible to analyze this data in the multi-dom ain style, and make some simple inferences on the nature of the cooling his tory of the Carion pluton in central Madagascar. The Carion granitic pluton in central Madagascar was intruded into warm continental crust following o rogenic events related to the final amalgamation of Gondwana. U-Pb SHRIMP d ating of the pluton yields an emplacement age of 532.1 +/- 5.2 Ma followed by relatively slow cooling as constrained by Ar-40/Ar-39 ages on hornblende , biotite and K-feldspar. Four hornblende samples yielded a mean Ar-40/Ar-3 9 age of 512.7 +/- 2.6 Ma. A biotite sample yielded an age of 478.9 +/- 1.0 Ma and modeled K-feldspar ages show cooling from 350 degrees C at 466 Ma t o 100 degrees C by 410 Ma. Collectively, the data suggest chat the pluton c ooled from 850 degrees C at 532.1 +/- 5.2 (U-Pb zircon) Ma to 500 degrees C at 512.7 +/- 2.6 Ma (40Ar/39Ar hornblende), or approximately 8 degrees C/M a slowing to similar to4 degrees C/Ma between 512 Ma and 478 Ma and finally to about 3 degrees C/Ma between 478 and 410 Ma.