Diffusion, closure temperatures, and accessory mineral petrogeneses in a high-temperature aureole: refining the integration of P-T and t

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Date

2003

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McFarlane, Christopher R. M.

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Static heating during intrusion of the 1322 Ma Makhavinekh Lake Pluton (MLP) caused replacement of garnet in the adjacent country rocks (Tasiuyak Gneiss) by coronal assemblages of orthopyroxene + cordierite. Thermometry based on Al solubility in orthopyroxene, applied to relict garnet and neighboring orthopyroxene, preserves a temperature gradient from 700°C to 900°C at distances between 5750 and 20 m from the intrusion, reaffirming the robustness of this thermometry technique. Intracrystalline and intergranular Al zoning in M2 orthopyroxene are interpreted with the aid of numerical models for conductive heat flow in the aureole. The results document the ability of Al-in-orthopyroxene thermometry to preserve a detailed record of thermal histories in contactmetamorphic granulites. Zircon in the Tasiuyak Gneiss formed at ~1850 Ma near the peak of the Torngat Orogeny (M1 metamorphism). These M1 zircons were reheated during contact heating at 1322 Ma (M2 metamorphism). U-Pb dating of M1 zircon, using thermal ionization mass spectrometry (TIMS) and in situ sensitive high resolution ion microprobe (SHRIMP) geochronology, tested whether temperatures in the aureole exceeded the closure temperature for Pb diffusion in zircon. TIMS analyses failed to resolve appreciable resetting towards 1322 Ma even in samples that reached ~900 ºC. In contrast, high spatial resolution SHRIMP data revealed that cores of M1 zircon in samples that reached temperatures >800 ºC are significantly younger than rims. This reversal of core and rim ages implies that Pb was preferentially lost from cores. This observation is hypothesized to be the result of Pb migration during high-temperature recovery of lattice strain induced by higher impurity concentrations (REE, P) in cores. While M1 zircon suffered intracrystalline Pb diffusion during contact heating, it did not react with the surrounding major mineral assemblage. In contrast, low-Y monazite inclusions in M1 garnet were consumed as they were exposed to the M2 coronas. New growth of high-Y monazite records both the age and temperature of metamorphism in the aureole. Dissolution and new growth of monazite occurred under anhydrous conditions, highlighting the potential use of monazite as a high-temperature thermochronometer in granulites and ultra-hightemperature rocks.

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