Timing and conditions of peak metamorphism and cooling across the Zimithang Thrust, Arunachal Pradesh, India

The Zimithang Thrust juxtaposes two lithotectonic units of the Greater Himalayan Sequence in Arunachal Pradesh, NE India. Monazite U–Pb, muscovite 40Ar/39Ar and thermobarometric data from rocks in the hanging and footwall constrain the timing and conditions of their juxtaposition across the structure, and their subsequent cooling. Monazite grains in biotite–sillimanite gneiss in the hanging wall yield LA-ICP-MS U–Pb ages of 16 ± 0.2 to 12.7 ± 0.4 Ma. A schistose gneiss within the high strain zone yields overlapping-to-younger monazite ages of 14.9 ± 0.3 to 11.5 ± 0.3 Ma. Garnet–staurolite–mica schists in the immediate footwall yield older monazite ages of 27.3 ± 0.6 to 17.1 ± 0.2 Ma. Temperature estimates from Ti-in-biotite and garnet–biotite thermometry suggest similar peak temperatures were achieved in the hanging and footwalls (~ 525–650 °C). Elevated temperatures of ~ 700 °C appear to have been reached in the high strain zone itself and in the footwall further from the thrust. Single grain fusion 40Ar/39Ar muscovite data from samples either side of the thrust yield ages of ~ 7 Ma, suggesting that movement along the thrust juxtaposed the two units by the time the closure temperature of Ar diffusion in muscovite had been reached. These data confirm previous suggestions that major orogen-parallel out-of-sequence structures disrupt the Greater Himalayan Sequence at different times during Himalayan evolution, and highlight an eastwards-younging trend in 40Ar/39Ar muscovite cooling ages at equivalent structural levels along Himalayan strike

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

U–Pb 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 LA–MC–ICP–MS U–Pb 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 temperature–time and Pb diffusion data suggest that these ages are best explained by rapid cooling from peak temperature conditions of ∼800 °C at 14 Ma in the granulite-bearing unit and ∼650 °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 U–Pb lower intercept cooling age of 14.6 ± 1.2 Ma. Diffusion modelling suggests that this age is too old to be consistent with the temperature–time paths inferred for the rutile-bearing samples. Instead, the titanite age suggests cooling from ∼650 °C at an earlier time of 17–15 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 °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

Detrital zircon U-Pb geochronology, trace-element and Hf isotope geochemistry of the metasedimentary rocks in the Eastern Himalayan syntaxis: Tectonic and paleogeographic implications

The origin of the Greater Himalayan Sequence in the Himalaya and the paleogeographic position of the Lhasa terrane within Gondwanaland remain controversial. In the Eastern Himalayan syntaxis, the basement complexes of the northeastern Indian plate (Namche Barwa Complex) and the South Lhasa terrane (Nyingchi Complex) can be studied to explore these issues. Detrital zircons from the metasedimentary rocks in the Namche Barwa Complex and Nyingchi Complex yield similar U–Pb age spectra, with major age populations of 1.00–1.20 Ga, 1.30–1.45 Ga, 1.50–1.65 Ga and 1.70–1.80 Ga. The maximum depositional ages for their sedimentary protoliths are ~ 1.0 Ga based on the mean ages of the youngest three detrital zircons. Their minimum depositional ages are ~ 477 Ma for the Namche Barwa Complex and ~ 499 Ma for the Nyingchi Complex. Detrital zircons from the Namche Barwa Complex and Nyingchi Complex also display similar trace-element signatures and Hf isotopic composition, indicating that they were derived from common provenance. The trace-element signatures of 1.30–1.45 Ga detrital zircons indicate that the 1.3–1.5 Ga alkalic and mafic rocks belt in the southeastern India is a potential provenance. Most 1.50–1.65 Ga zircons have positive εHf(t) values (+ 1.2 to + 9.0), and most 1.70–1.80 Ga zircons have negative εHf(t) values (− 7.1 to − 1.9), which are compatible with those of the Paleo- to Mesoproterozoic orthogneisses in the Namche Barwa Complex. Provenance analysis indicates that the southern Indian Shield, South Lhasa terrane and probably Eastern Antarctica were the potential detrital sources. Combined with previous studies, our results suggest that: (1) the Namche Barwa Complex is the northeastern extension of the Greater Himalaya Sequence; (2) the metasedimentary rocks in the Namche Barwa Complex represent distal deposits of the northern Indian margin relative to the Lesser Himalaya; (3) the South Lhasa terrane was tectonically linked to northern India before the Cambrian

Probing the depths of the India-Asia collision : U-Th-Pb monazite chronology of granulites from NW Bhutan

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 orthopyroxene-bearing felsic gneiss yields LA-MC-ICP-MS U-Th-Pb 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