Abstract
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.
Highlights
Differences in the pressure–temperature–time (PTt) evolution of metamorphic rocks across lithologies and tectonic structures yield information about the rates and timescales of processes that act to transform rocks in plate collision zones
Like many other in-situ LA–ICP–MS U–Pb monazite data reported from across the Himalaya, our results show that many samples yield a range of ages that spread across ~3–9 Ma
Monazite U–Pb data from rocks in the hanging and footwalls of the Zimithang Thrust at high structural levels of the Greater Himalayan Sequence in Arunachal Pradesh, NE India show that the structure juxtaposes younger (~17 to 12.7 ± 0.4 Ma) rocks in the hanging wall over older (27.3 ± 0.6 to 17.1 ± 0.2 Ma) rocks in the footwall
Summary
Differences in the pressure–temperature–time (PTt) evolution of metamorphic rocks across lithologies and tectonic structures yield information about the rates and timescales of processes that act to transform rocks in plate collision zones. The archetypal modern orogen formed by continent-continent collision, the Himalaya, provides excellent material for investigating how, when, and how fast, rocks are transported and transformed deep within collision zones and subsequently exhumed back to the surface. The high-grade rocks that comprise the core of the Himalayan orogen form what is known as the Greater Himalayan Sequence (e.g. GHS; Hodges, 2000 and references therein). This unit is separated from the overlying Tethyan Sedimentary Sequence (TSS) by the lowangle brittle–ductile normal-sense South Tibetan Detachment (STD) and from the underlying Lesser Himalayan Sequence (LHS) by the low-angle brittle–ductile Main Central Thrust (MCT). The metapelites are variably migmatized and transected by Oligocene to Miocene-aged leucogranite veins, dykes, sheets and plutons of varying dimensions, most of which were formed by partial melting of metapelites, similar in composition to those into which they intrude (e.g. Hodges, 2000 and references therein)
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