Prograde, peak, and retrograde P–T paths from aluminium in orthopyroxene: High‐temperature contact metamorphism in the aureole of the Makhavinekh Lake Pluton, Nain Plutonic Suite, Labrador

Abstract

AbstractStatic heating during intrusion of the 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 neighbouring orthopyroxene, preserves a temperature gradient from 700 to 900 °C at distances between 5750 and 20 m from the intrusion, reaffirming the robustness of this thermometry technique. Intracrystalline and intergranular variations of Al zoning in orthopyroxene are well‐preserved, suggesting that little diffusional modification of Al growth zoning occurred. Maximum Al2O3 in orthopyroxene ranges from c. 2.0 wt% at 5750 m from the intrusion to a maximum of 4.3 wt% at the contact. Individual orthopyroxene grains show decreasing Al from core to rim in samples < 500 m from the intrusion, while those at greater distances show an increase from core to rim. These features are interpreted with the aid of numerical models for conductive heat flow in the aureole. Coronas in samples close to the intrusion grew at high temperatures and along T‐t paths dominated by cooling, so maximum Al content in orthopyroxene in these samples occurs in the cores of grains that grew during the earliest stages of garnet consumption. In contrast, the corona‐forming reactions in rocks further from the contact proceeded along prograde heating paths, so maximum Al content in orthopyroxene occurs in the rims of grains that grew during the final stages of garnet consumption. These results document the ability of Al‐in‐orthopyroxene thermometry to preserve a detailed record of thermal histories in contact‐metamorphic granulites; they suggest that similar intracrystalline and intergranular variations of Al zoning in orthopyroxene in regional granulites may also preserve portions of both the prograde and peak‐T evolution.