The Precambrian Loon Lake pluton, Ontario, consists of two main zones—a core of monzonite with a rim of younger quartz monzonite. Several isolated bodies of older diorite and syenodiorite occur within the pluton. The variations in the chemical and mineralogical composition of diorites and syenodiorites are due to both magmatic differentiation and hybridization. The trends of the variations of major elements, Rb, Tl, Sr, Ba, and rare earth elements in monzonite are consistent with fractional crystallization mainly of feldspars; this fractionation probably involved flowage differentiation. Fractional crystallization and contamination of monzonitic magma by anatectic granitic melt probably played a dominant role in the genesis of quartz monzonite. Monzonite and quartz monzonite are believed to have formed from a magma of lower crustal or upper mantle origin. While part of this magma intruded as monzonite, another part which evolved further generated quartz monzonite.