EARLY ARC-RELATED UHP METAMORPHISM PRESERVED IN THE COMPOSITE ARC BELT, GRENVILLE OROGEN, ONTARIO, CANADA

Ultra-high pressure (UHP) metamorphic rocks, including sapphirine assemblages, occur throughout the Laurentian margin of the Grenville Orogen. Herein, UHP rocks are reported from the Composite Arc Belt (CAB) from the Pakenham structure (PS) in northeastern Sharbot Lake domain. The main units of the PS are 1) layered, moderately to highly strained, quartzofeldspathic, variably migmatitic gneiss ± garnet; 2) moderately to highly strained, tonalite gneiss, with a LA-ICP-MS zircon age of 1299±4.7 Ma; and 3) layered and non-layered, variably migmatitic, mafic gneiss ± garnet. All 3 gneiss units are older than gneissic granodiorite emplaced into the PS at 1263±2 Ma. There is no evidence that the PS was subject to the ca 1170 Ma high-grade metamorphism seen in the Wolf Grove structure 10 km to the southeast.

In the PS, peraluminous sapphirine (approximate formula 7MgO.9Al₂O₃.3SiO₂₀ (7:9:3)) occurs as small needles in labradorite (An₅₃) and hercynite coronae surrounding kyanite in rusty psammitic gneiss (quartz-potassium feldspar-kyanite-rutile-pyrite) on Sugarbush Road (396566E 5012207N, NAD 83, zone 18). Texturally this symplectite resembles those in the coeval Sveconorwegian orogen interpreted to have formed during decompression from eclogite facies (P>15 kbar) to granulite and upper amphibolite facies. Present in adjacent layers are almandine garnet (Pyr₃₃Alm₅₄And₅Sps₇Grs₁), quartz-potassium feldspar (melt?), and quartz-potassium feldspar-biotite-corundum-rutile-pyrite domains. The Sugarbush occurrence differs from other global sapphirine occurrences in that orthopyroxene or cordierite are absent, there are no associated mafic to ultramafic pods or lenses of retrogressed eclogite, and the host quartzose gneiss is not magnesium-rich. The host rock of the Sugarbush occurrence was likely associated with a volcanogenic-massive sulphide hydrothermal alteration system.

Regional geochronology suggests that the event forming the sapphirine coronae occurred before 1263 Ma, i.e. before regional assembly of CAB at 1250-1240 Ma and before the ca 1160 Ma Shawinigan and ca 1070 Ma Ottawan orogens. Sapphirine-bearing rocks can form in arc settings (e.g., southwest Japan, Woodlark basin) and it is proposed that the PS initially formed in a tectonic setting akin to the D’Entrecasteaux Islands of modern-day Papua-New Guinea.