ARCHEAN HIGH-MG QUARTZ-MONZODIORITE SUITE - A REEVALUATION OF THE PARENTAL MAGMA AND DIFFERENTIATION

Quartz-monzodiorite is generally considered to represent the parent ma gma of high-Mg quartz-monzodiorite plutonic suites; thus such suites a re thought to be melts from a metasomatized mantle wedge. It is propos ed here that hornblende tonalite, which is present in the feeder dikes of high-Mg quartz-monzodiorite plutonic suites, better represents the parental magma. Hornblende tonalite has a higher compatible element c ontent with a relatively lower incompatible element content and a lowe r (La/Yb)(n) ratio than quartz-monzodiorite, suggesting that the tonal ite is the more primitive magma of the suite. The hornblendite unit of the high-Mg quartz-monzodiorite plutonic suite is interpreted to repr esent the cumulate, and the quartz-monzodiorite the differentiated liq uid. To test this hypothesis and explain the differentiation of the un its of a high-Mg quartz-monzodiorite plutonic suite, the Houghton plut on in the northern Abitibi greenstone belt, Quebec, was studied. Resul ts show that crystal fractionation and equilibrium crystallization can not sufficiently enrich incompatible elements in the quartz-monzodiori te while keeping the compatible element contents high. Feeder dike pet rography suggests that the magma chamber was periodically replenished. The model that best explains the formation of the different units of this suite is a periodically replenished magma chamber with a crystall ization sequence of hornblende followed by hornblende and plagioclase. Such a magma chamber undergoing crystal fractionation will produce th e high incompatible and compatible element contents typical of quartz- monzodiorite magmas. Not all quartz-monzodiorite magmas of the site ca n be explained by a replenishment of the hornblende tonalite. As ident ified in the feeder system, the replenishing magma changed at some tim e to one represented by leucotonalite-to-quartz-diorite. By changing t he replenishing magma from the hornblende tonalite to the leucotonalit e-to-quartz-diorite unit, after the differentiated liquid reached 64% SiO2, it is possible to model all the units in the plutonic suite. The hornblendite unit can be easily modeled for the major oxides, but the inclusion or exclusion of accessory phases rich in trace elements (al lanite) in the samples renders trace element behavior erratic, and the hornblendite cannot be successfully modeled.