Constraining the cooling history of the Greater Himalayan Sequence in NW Bhutan

摘要: Rutile from a mafic granulite, intermediate granulite and amphibolite within juxtaposed litho/tectonostratigraphic units in the Greater Himalayan Sequence (GHS) of NW Bhutan yield overlapping laser ablation multi-collector inductively-coupled plasma mass spectrometry U-Pb lower intercept cooling ages 10.1 ± 0.4 Ma, 10.8 0.1 Ma 10.0 respectively. Titanite that grew during an exhumation-related amphibolite-facies overprint on eclogite-facies mineral assemblage neighbouring Chomolari Massif yields age 14.6 1.2 Ma. The interpretation rate by linking thermochronometer to single closure temperature, TC (Dodson, 1973), is shown not provide as useful insight into rates matching analysed with those calculated diffusion models. Certain assumptions made calculating TC, specifically assumption high starting temperature proportional 1/time, are applicable timescales paths. Additionally, requires knowledge rate, commonly unknown parameter interest. Given increasing precision available geochronological data computationally undemanding nature simple models, there now little reason progress beyond Dodson’s 1973 formulation quest for determining rates. Numerical models constrained previously published temperature-time (Warren et al 2011) Pb-diffusion (Cherniak 2000) suggest rutile may be explained rapid (>50°C/Ma) peak P-T conditions ca. 800°C 1.0 GPa at 14 granulite-bearing unit 650 °C 12 amphibolite-bearing unit. good fit between model confirms relatively retentivity Pb suggested experimental 2000). Diffusion modelling additionally suggests titanite too old consistent paths inferred other samples. Instead, cooled 40°C/Ma 650°C earlier time 17-15 This grade rocks experienced different history rest GHS Bhutan. Together these show high-grade three apparently structural levels following metamorphism, but times. highest granulite-facies were exhumed deeper exposed, preserved, or remain unrecognized west eastern Nepal. A progressive eastwards change tectonic regime, metamorphic and/or exhumation mechanism across orogen implied thermochronologic data.