The Precambrian Loon Lake pluton, Ontario, is a circular zoned body emplaced in metamorphosed rocks of the Grenville Province about 1075±75 m.y. ago. The intrusion of this post-tectonic pluton superimposed on country rocks, already regionally metamorphosed to amphibolite facies, a contact metamorphism of K-feldspar-cordierite hornfels facies. Various parameters indicate a maximum contact temperature range of 750-820ºC and total pressure range of 2.5-4 kb, producing anatexis of rocks in the contact aureole. Garnet-sillimanite-cordierite gneisses, abundant in this aureole, are probably a residuum after partial melting of the Apsley biotite gneiss. These residual rocks contain high Al, Fe and Mg, low alkalies and also heavy REE enrichment with negative Eu anomalies, compared to the Apsley gneiss. Leucogranites associated with garnet-sillimanite-cordierite gneisses may represent anatectic material subtracted from the Apsley gneiss. The pluton consists of two main concentric zones, a monzonitic core and outer younger quartz monzonite. Several isolated bodies of older diorite and syenodiorite occur within the pluton, mostly in the core. Intrusions of the felsic rocks probably followed in rapid succession. The pluton is also zoned with respect to the structural state of potassium feldspar, which is mostly microcline, although some central monzonites contain orthoclase: this variation was probably controlled by water distribution. Large variations in chemical and mineralogical composition of the basic rocks are due to (a) magmatic differentiation and (b) interaction with felsic magma. These rocks may not be genetically related to the pluton. The variations of major elements, Rb, Sr, Ba, Tl and REE in monzonite are consistent with fractional crystallization (mainly of feldspars) and probably involved flowage differentiation. Most chemical variations in quartz monzonite are also compatible with fractional crystallization. The behaviour of REE, however, is not readily consistent with this process. Monzonite and quartz monzonite were both formed from a single magma, which was generated either by partial melting of lower crustal/upper mantle rocks, or by fractional crystallization of basic deep-seated magma. This magma in part intruded (monzonite) and in part evolved further (quartz monzonite) and subsequently intruded. Quartz monzonite was generated by fractional crystallization of monzonitic magma and by "mixing" of monzonitic magma with anatectic granitic melt. The composition of biotite from the felsic rocks suggests oxygen fugacities slightly higher than those in equilibrium with a Ni-NiO buffer.

Doctor of Philosophy (PhD)