GRENVILLIAN METAMORPHISM OF THE SUDBURY DIABASE DYKE-SWARM - FROM PROTOLITH TO 2-PYROXENE - GARNET CORONITE

Coronite derived from olivine- and Fe-Ti-oxide-bearing diabase or gabb ro is common in high-grade metamorphic terranes, but its progressive d evelopment from completely unmetamorphosed protolith has rarely been f ully documented. Olivine diabase of the 1.24-Ga Sudbury continental dy ke swarm, exposed across the Grenville Front southwest of Sudbury, Ont ario, provides such documentation. Progressive textural and chemical c hanges are recorded by primary igneous and secondary metamorphic miner als, in rocks whose bulk chemistry has not changed, from unmetamorphos ed diabase to coronitic metadiabase in which metamorphic conditions ap proached the granulite facies (>700 degrees C at >8.0 kbar) during Gre nvillian orogeny at similar to 1.0 Ga. Southeast-trending Sudbury dyke s are undisturbed in the Southern and Superior province foreland of th e Grenville orogen, but approaching the Grenville Front are offset by faults and locally contain greenschist-grade metamorphic assemblages. Deformed metadiabase dykes in the uplifted margin of the Grenville oro gen, known from rock chemistry and U-Pb age of primacy baddeleyite to be correlative with the Sudbury swarm, display reactions of both olivi ne and Fe-Ti oxide with plagioclase, leading to classic coronite devel opment within a few kilometers of the front. Textural relationships in dicate that peak metamorphism outlasted early deformation, compatible with a prograde P-T-t path. Of particular interest is comparison of th e reactions between Fo(70) olivine and An(60) plagioclase (xenocrysts) and Fo(35) and An(45) (matrix) in the same rocks, and increase of Ti in biotite, Na and Al in clinopyroxene toward higher grade. Comparison of metamorphism in the dykes and their host rocks indicates telescopi ng of pre-Grenvillian isograds across the front.