Constraining cooling histories: rutile and titanite chronology and diffusion modelling in NW Bhutan

UPb analyses of rutile and titanite commonly yield ages that constrain the timing of cooling rather than the timing of their crystallization. Rutile which grew at or close to peak temperature conditions in a mafic granulite, intermediate granulite and mafic amphibolite within juxtaposed litho/tectonostratigraphic units in the Greater Himalayan Sequence (GHS) of NW Bhutan yield LAMCICPMS UPb lower intercept cooling ages of 10.1 +/- 0.4, 10.8 +/- 0.1 and 10.0 +/- 0.3 Ma, respectively. Numerical finite-difference diffusion models constrained by previously published temperaturetime and Pb diffusion data suggest that these ages are best explained by rapid cooling from peak temperature conditions of similar to 800 degrees C at 14 Ma in the granulite-bearing unit and similar to 650 degrees C at 12 Ma in the amphibolite-bearing unit. The good fit between the model and analysed ages confirms the relatively high retention of Pb in rutile suggested by the experimental data. Titanite that grew during an exhumation-related amphibolite facies overprint on an eclogite facies mineral assemblage from the neighbouring Jomolhari Massif yields a UPb lower intercept cooling age of 14.6 +/- 1.2 Ma. Diffusion modelling suggests that this age is too old to be consistent with the temperaturetime paths inferred for the rutile-bearing samples. Instead, the titanite age suggests cooling from similar to 650 degrees C at an earlier time of 1715 Ma, implying that the high-grade rocks in the Jomolhari Massif experienced a different cooling history from the rest of the GHS in NW Bhutan. Together these data show that high-grade rocks from three apparently different structural levels of the GHS in NW Bhutan experienced rapid cooling at >40 degrees C Ma-1 at varying times. The highest grade granulite facies rocks were exhumed from deeper structural levels that are not exposed, not preserved, or not yet recognized west of eastern Nepal. A progressive along-strike change in tectonic regime, metamorphic history and/or exhumation mechanism across the orogen is implied by these thermochronologic data.