PRESSURE-TEMPERATURE PATH OF ECLOGITES FROM THE GRENVILLE PROVINCE, SOUTHERN ONTARIO

Two studied eclogites from the Grenville Province of Ontario, Canada experienced partial melting on their exhumation path. Peak mineral assemblage of the studied samples from Mattawa area included garnet, omphacite, amphibole, biotite, phengite, rutile and zircon. Garnet forms porphyroblasts which are up to 3 mm in size, subhedral in shape and contain monomineralic and polymineralic inclusions. The chemical zoning is characterized by Alm₆₂Pyp₁₅Grs₁₉Spss₅, Alm₅₇Pyp₂₃Grs₂₀Spss₁ and Alm₆₁Pyp₁₉Grs₁₉Spss₂ in the core, inner rim, and outermost rim, respectively. Numerous retrogressed textures developed on the exhumation path. Kelyphite of pargasite and oligoclase formed around garnet. Symplectite of oligoclase, diopsidic clinopyroxene and pargasite replaced omphacite. Polymineralic inclusions of oligoclase + ilmenite ± biotite ± K-feldspar ± apatite ± iron sulfides are present in garnet, though some are open to the matrix. Oligoclase in the setting displays sharp cusps into garnet. The texture suggests that the eclogites were partially melted. Pseudosection modeling of a representative eclogite along with garnet isopleths yield a prograde path from 1.75 GPa, 650 °C at subsolidus to ~1.9 GPa at 730 °C at suprasolidus conditions, along which a small proportion of partial melt formed through breakdown of amphibole and phengite with involvement of omphacite. Amphibole-plagioclase thermometry on symplectite and kelyphite yielded a T range of 720–750 °C at P ~ 0.7 GPa pointing to near isothermal exhumation. Partial melts were present at near-peak pressure conditions, which suggests that exhumation of the eclogites may have been triggered by partial melting during the collapse of the Grenville orogeny.