TIBETAN TECTONICS FROM 40AR/39AR ANALYSIS OF A SINGLE K-FELDSPAR SAMPLE

40Ar/39Ar data on an alkali feldspar sample from the Quxu pluton, Gangdese batholith, southern Tibet, allow a detailed assessment of unroofing and uplift history between 35 and 18 Ma. The 39Ar Arrhenius plot for this sample shows departures from a linear relationship between the effective diffusion parameter, log(D/r2), and reciprocal temperature, which we interpret to be the result of a distribution of distinct diffusion-domain sizes. We use an alternative way of plotting the Arrhenius data that exhibits domain size versus cumulative % 39Ar released during step heating. The 40Ar/39Ar age spectrum of the sample has features, such as local age plateaus, that are most easily explained in terms of the distinctive closure age of particular domains. The fact that the same distribution of diffusion-domain sizes explains both the Arrhenius data and the age spectrum is an indication that the diffusion properties operating in the laboratory are those of the sample while it was in its natural environment. Modelling of the age spectrum with a distribution of domain sizes results in the recovery of a continuous cooling-history segment rather than a single time-temperature datum. We demonstrate the robustness of the cooling-curve determination by showing the large misfits to the age spectrum that arise from relatively small changes in the cooling history. The best-fit cooling curve for the Quxu sample shows a decreasing rate of cooling in the time interval 35-18 Ma, followed by a very rapid cooling beginning at about 18 Ma. We have used a thermal model for the conductive cooling of an unroofing pluton to estimate the rate of unroofing required to explain the Quxu cooling curve, and find that in the 35-20 Ma time interval, the primary control of the thermal evolution is the conductive loss of magmatic heat with little or no unroofing (unroofing rates of approximately 0.05 mm/yr) followed by a brief period (< 5 Ma) of very rapid unroofing with rates of order 2 mm/yr.