AGE OF ECLOGITE-FACIES METAMORPHISM AND EXHUMATION IN NORTHWESTERN BHUTAN

Across northern Bhutan, cryptic south-directed thrusts place granulite-facies rocks of the Greater Himalayan Sequence (GHS) over amphibolite-facies rocks belonging to the same unit. Within this northern high-T region, rare garnet-bearing mafic rocks are interpreted as retrogressed eclogite, based on textural information that includes the presence of symplectite after omphacite. Previous studies have suggested that eclogite-facies metamorphism peaked at ~16–14 Ma, with subsequent rapid exhumation at initially high temperatures.

We report new zircon U/Th–Pb analyses from three granulitized eclogites collected in the Jomolhari region of northwest Bhutan, about 40 km SW of the previous locality and separated from it by a klippen of structurally higher Tethyan Sedimentary Sequence rocks. All contain grt (partially replaced by hbl + plag ± opx symplectites) and cpx–plag symplectite after former omphacite. Zircon cores and rims were analyzed by split stream LA–ICPMS at the UC Santa Barbara geochronology facility. Two samples yield a relatively tight cluster of lower intercept and concordant dates from 18–17 Ma in one sample, and ~20–16 Ma in the other. A few rim analyses in the second sample yield dates as young as 15–13 Ma. The third sample yields older concordant to slightly discordant zircon cores ranging from 28–22 Ma, with no younger ages. All these data are consistent with previously reported 36–18 Ma U/Th–Pb monazite ages from pelites in this region.

These new zircon dates indicate that the GHS in the Jomolhari Massif resided at high temperatures in the mid- to lower orogenic crust for a prolonged period prior to c. 18 Ma. The Jomolhari domain was then rapidly exhumed between 18–15 Ma, potentially 2–3 Ma earlier than similar rocks in northern Bhutan (~40 km northeast from Jomolhari). Whether this difference represents either a coherent tectonic unit (during prograde–peak conditions) across north and northwest Bhutan that exhumed diachronously, or separate evolution within units following different kinematic paths, is currently being investigated.