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

Static 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 Al 2 O 3 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 contactmetamorphic 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.

Oxygen isotopic studies of the interaction between xenoliths and mafic magma, Voisey’s Bay Intrusion, Labrador, Canada

“…Low temperatures obtained from mineral pairs using biotite may be indicative of the low closing temperature of biotite or isotopic exchange during retrogression and low-temperature alteration. The majority of quartz-plagioclase mineral pairs indicate temperatures between 660 and 815°C, which is consistent with the results of previous studies based on mineral equilibria (e.g., Berg and Docka, 1983;Lee, 1987;McFarlane et al, 2003). However, the wide range of temperatures recorded by the entire system indicate oxygen isotopic disequilibrium, which is consistent with expected isotopic trends in rocks that contain both high (garnet, pyroxene, quartz) and low (biotite, plagioclase, K-feldspar) closing temperature minerals (e.g., Eiler et al, 1993).…”

“…1). Details of the geology of the Nain area have been given by several authors (e.g., Berg and Docka, 1983;Wardle, 1983;Lee, 1987;Wardle et al, 1990a,b;Ryan et al, 1995;Ryan, 2000;McFarlane et al, 2003). A brief description of regional and local geology is presented here.…”

Arrested development – a comparative analysis of multilayer corona textures in high-grade metamorphic rocks

“…Typically, the latter scenario arises in contact aureoles characterized by rapid heating and cooling (Johnson et al, 2004 - Fig. 8g; Mcfarlane et al, 2003;Ings and Owen, 2002;Barboza and Bergantz, 2000;Wheeler et al, 2004;Daczko et al, 2002;Dasgupta et al, 1997;Joesten and Fisher, 1988), but it can also occur in shock-heated rocks within large impact structures (Gibson, 2002;Ogilvie, 2010).…”

“…Nonprogressive coronas develop in a separate P -T event to those that generate the peak assemblage (e.g. Johnson and Carlson, 1990;Gibson, 2002;McFarlane et al, 2003).…”

Thermodynamic equilibrium at heterogeneous pressure

“…11dinVrijmoed et al 2013). Diffusion of Al in Opx is usually assumed slow, mostly based on Al diffusion experiments in Cpx(Smith and Barron 1991;McFarlane et al 2003)…”