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