Rare granulitized eclogites exposed in the eastern Himalaya provide insight into conditions and processes deep within the orogen. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb, Ti, and rare earth element (REE) data from zircons in mafi c granulitized eclogites located in the upper structural levels of the Greater Himalayan Sequence in Bhutan show that zircon was crystallized under eclogite-facies metamorphic conditions between 15.3 ± 0.3 and 14.4 ± 0.3 Ma, within a couple million years of the later granulite-facies overprint. In conjunction with pressure estimates of the eclogite-and granulite-facies stages of metamorphism, the age data suggest that initial exhumation occurred at plate-tectonic rates (cm yr-1). These extremely rapid synconvergence exhumation rates during the later stages of the India-Asia collision require a revision of theories for the transportation and exhumation of crustal materials during continental collisions. In contrast to western Himalayan examples, the eastern Himalayan eclogites cannot be tectonically related to steep subduction of India beneath Asia. Instead, they more likely represent fragments from the base of the overthickened Tibetan crust. Based on the zircon age and trace-element data, we hypothesize that the protolith of the mafi c granulites was middle Miocene mafi c intrusions into the lower crust of southern Tibet, linked to Miocene volcanism in the Lhasa block. We suggest that a transient tectonic event-possibly the indenting of a strong Indian crustal ramp into crust under southern Tibet that had been weakened by partial melting-may have promoted exhumation of the eclogitized lower crust under Tibet. The mafi c magmatism and volcanism themselves may have been related to the convective thinning of the lithospheric mantle triggered by a reduction in the India-Eurasia convergence rate during the middle Miocene, which in turn could have facilitated the rapid extrusion of the lower crust over the earlier-exhumed middle crust.

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[1] Rocks metamorphosed to high temperatures and/or high pressures are rare across the Himalayan orogen, where peak metamorphic conditions recorded in the exposed metamorphic core, the Greater Himalayan Sequence (GHS), are generally at middle to upper amphibolite facies. However, mafic garnet-clinopyroxene assemblages exposed at the highest structural levels in Bhutan, eastern Himalaya, preserve patchy textural evidence for early eclogite-facies conditions, overprinted by granulite-facies conditions. Monazite hosted within the leucosome of neighboring granulite-facies orthopyroxenebearing felsic gneiss yields LA-MC-ICP-MS U-ThPb ages of 13.9 ± 0.3 Ma. Monazite associated with sillimanite-grade metamorphism in granulite-hosting migmatitic gneisses yields U-Th-Pb rim ages between 15.4 ± 0.8 Ma and 13.4 ± 0.5 Ma. Monazite associated with sillimanite-grade metamorphism in gneiss at structurally lower levels yields U-Pb rim ages of 21-17 Ma. These data are consistent with Miocene exhumation of GHS material from a variety of crustal depths at different times along the Himalayan orogen. We propose that these granulitized eclogites represent lower crustal material exhumed by tectonic forcing over an incoming Indian crustal ramp and that they formed in a different tectonic regime to the ultrahigh-pressure eclogites in the western Himalaya. Their formation and exhumation in the Miocene therefore do not require diachroneity in the timing of the initial India-Asia collision.

Editor: T.M. Harrison Keywords: inverted metamorphism ductile thrusting petrochronology monazite geochronologyIn the Himalayan region of Sikkim, the well-developed inverted metamorphic sequence of the Main Central Thrust (MCT) zone is folded, thus exposing several transects through the structure that reached similar metamorphic grades at different times. In-situ LA-ICP-MS U-Th-Pb monazite ages, linked to pressure-temperature conditions via trace-element reaction fingerprints, allow key aspects of the evolution of the thrust zone to be understood for the first time. The ages show that peak metamorphic conditions were reached earliest in the structurally highest part of the inverted metamorphic sequence, in the Greater Himalayan Sequence (GHS) in the hanging wall of the MCT. Monazite in this unit grew over a prolonged period between ∼37 and 16 Ma in the southerly leading-edge of the thrust zone and between ∼37 and 14.5 Ma in the northern rear-edge of the thrust zone, at peak metamorphic conditions of ∼790 • C and 10 kbar. Monazite ages in Lesser Himalayan Sequence (LHS) footwall rocks show that identical metamorphic conditions were reached ∼4-6 Ma apart along the ∼60 km separating samples along the MCT transport direction. Upper LHS footwall rocks reached peak metamorphic conditions of ∼655 • C and 9 kbar between ∼21 and 16 Ma in the more southerly-exposed transect and ∼14.5-12 Ma in the northern transect. Similarly, lower LHS footwall rocks reached peak metamorphic conditions of ∼580 • C and 8.5 kbar at ∼16 Ma in the south, and 9-10 Ma in the north. In the southern transect, the timing of partial melting in the GHS hanging wall (∼23-19.5 Ma) overlaps with the timing of prograde metamorphism (∼21 Ma) in the LHS footwall, confirming that the hanging wall may have provided the heat necessary for the metamorphism of the footwall. Overall, the data provide robust evidence for progressively downwards-penetrating deformation and accretion of original LHS footwall material to the GHS hanging wall over a period of ∼5 Ma. These processes appear to have occurred several times during the prolonged ductile evolution of the thrust. The preserved inverted metamorphic sequence therefore documents the formation of sequential 'paleothrusts' through time, cutting down from the original locus of MCT movement at the LHS-GHS protolith boundary and forming at successively lower pressure and temperature conditions. The petrochronologic methods applied here constrain a complex temporal and thermal deformation history, and demonstrate that inverted metamorphic sequences can preserve a rich record of the duration of progressive ductile thrusting.

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The South Tibetan detachment system (STDS) in the Himalayan orogen is an example of normal-sense displacement on an orogen-parallel shear zone during lithospheric contraction. Here, in situ monazite U(-Th)-Pb geochronology is combined with metamorphic pressure and temperature estimates to constrain pressure-temperature-time (P-T-t) paths for both the hangingwall and footwall rocks of a Miocene ductile component of the STDS (outer STDS) now exposed in the eastern Himalaya. The outer STDS is located south of a younger, ductile ⁄ brittle component of the STDS (inner STDS), and is characterized by structurally upward decreasing metamorphic grade corresponding to a transition from sillimanite-bearing Greater Himalayan sequence rocks in the footwall with garnet that preserves diffusive chemical zoning to staurolite-bearing Chekha Group rocks in the hangingwall, with garnet that records prograde chemical zoning. Monazite ages indicate that prograde garnet growth in the footwall occurred prior to partial melting at 22.6 ± 0.4 Ma, and that peak temperatures were reached following c. 20.5 Ma. In contrast, peak temperatures were reached in the Chekha Group hangingwall by c. 22 Ma. Normal-sense (top-to-the-north) shearing in both the hangingwall and footwall followed peak metamorphism from c. 23 Ma until at least c. 16 Ma. Retrograde P-T-t paths are compatible with modelled P-T-t paths for an outer STDS analogue that is isolated from the inner STDS by intervening extrusion of a dome of mid-crustal material.