Metamorphism in the Agnew Lake Area, Sudbury District, Ontario, Canada
Metamorphism occurred in the Agnew Lake area about 1600 m. y. ago during the Penokean orogeny. Mafic igneous, pelitic meta-sedimentary, and quartzofeldspathic rocks were metamorphosed under conditions ranging from the low greenschist (chlorite-grade) to low almandine amphibolite (staurolite-grade) facies. The metamorphic zones have a nodal distribution. The sequence of mineralogic and assemblage changes with advancing metamorphic grade can be portrayed diagrammatically and by equations. There is regular partitioning of some elements, notably Mn, among coexisting biotite, staurolite, and garnet. Study of the interrelationships of metamorphic porphyroblasts and fabric elements provides evidence for a threefold division of events; an early “plastic” phase of deformation accompanied by growth of “low-grade” minerals such as muscovite and chlorite; the major period of metamorphism and the growth of “high-grade” minerals such as staurolite in a static environment; a late “brittle” deformation phase with minor “retrograde” metamorphism. The composition of pyrrhotites which have supposedly equilibrated with pyrite during metamorphism indicate metamorphic temperatures of 400°–570° C in the upper greenschist and almandine amphibolite facies. Calculations based on the pyrrhotite data indicate thermal gradients of 75°–150° C per km.