Determining early orogenic processes within the Pamir-Tibet orogen represents a critical step toward constructing a comprehensive model on the tectonic evolution of the region. Here we investigate the timing and cause of prograde metamorphism of Cenozoic metamorphic rocks from the Pamir plateau through Lu-Hf geochronology, U-Pb rutile thermochronology, and garnet thermometry. Regional prograde metamorphism and heating to 750-830 °C, as constrained by thermometry, occurred between 37 and 27 Ma. Prograde growth of garnet first occurred in the South Pamir and spread to the Central Pamir during the following 10 m.y. The early metamorphism is attributed to high mantle heat flow following the ca. 45 Ma break-off of the Indian slab south of the Pamir. Our investigation confirms a long-lived thermal history of the Pamir deep crust before the Miocene, and provides a causal link between break-off, enhanced mantle heat flow, and prograde heating of the subduction hanging wall.
INTRODUCTIONThe Pamir-Tibet orogen is Earth's largest and highest plateau and a prime natural laboratory for investigating how plate dynamics, regional tectonics, and surface uplift and erosion interact. Refining models for this orogen, and collisional orogens in general, requires knowledge about how the crust thickens, and how its thermal and mechanical structure changes during and after collision. Tectonic processes occurring at depth particularly impact regional dynamics. However, investigating these processes in the Pamir-Tibet orogen is hindered by the scarcity of exposed Cenozoic metamorphic rocks and the general difficulty in reconstructing their prograde his-
Metamorphic rutile from granulite facies metapelitic rocks of the Archean Pikwitonei Granulite Domain (PGD; Manitoba, Canada) provides constraints on the systematics of trace elements in rutile during high-temperature conditions and subsequent slow cooling. Compositional profiles and maps of the Zr concentrations in rutile grains (120-600 lm) from three metapelitic gneisses were acquired by electron probe micro-analysis, using a spatial resolution of down to 2 lm. Simultaneously, profiles were analysed for Nb, Cr and V, which have significantly different diffusion characteristics in rutile. The profiles of all elements show relatively homogeneous concentrations within most grains, but significant inter-grain differences even within a single thin section. Some rutile grains display a slight concentration decrease from a neighbouring garnet towards the matrix for all measured elements. The lack of diffusion profiles for all analysed elements shows that these are highly immobile in rutile and that distributions of these elements are primary and preserve prograde information. The Nb and Cr concentrations overlap with ranges that are ascribed to different provenances indicating that source discrimination based on these elements is not possible in all cases. High retentiveness for Zr implies that the Zr-in-rutile geothermometer is highly robust to diffusive re-equilibration, even during very slow cooling (<2°C Ma )1 ) from granulite facies conditions. Most grains have high Zr contents (3000-4600 ppm). Differences between high Zr contents suggest that during growth under vapour-absent conditions there may not be saturation of Zr in rutile, even if zircon is present. Therefore, several rutile grains need to be analysed in a sample to obtain a useful minimum peak temperature. The highest Zr concentrations correspond to 900°C. This is significantly higher than previous peak temperature estimates of 820°C based on two-feldspar thermometry. On a regional scale this implies that part of the PGD was affected by ultra-high temperature (UHT) metamorphism. It also implies that rutile is able to preserve primary compositions even to UHT conditions. This study shows that, if combined with textural information, Zr-in-rutile has the potential to be a very useful tool for estimating rutile crystallization temperatures and peak metamorphic conditions. For granulite facies rocks, Zr-in-rutile yields more reliable peak metamorphic temperatures than most other exchange geothermometers, which tend to partially re-equilibrate by diffusion during cooling.
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