The rare-element-enriched monzogranite-pegmatite-quartz vein systems in the Preissac-Lacorne Batholith, Quebec; I, Geology and mineralogy

The monzogranite plutons of Preissac, Moly Hill, Lamotte and Lacorne vary from biotite through two-mica to muscovite monzogranite subtypes, which display gradational, intrusive and fracture-bounded contacts. They were emplaced at P of 3.5 kbar and crystallized over the T range 750-650 degrees C. Field relations and systematic mineralogical and mineral-chemical variations within and between the monzogranite subtypes are interpreted as having been produced mainly by fractional crystallization, and subordinately by the intrusion of different batches of magma. The first process caused the early fractionation of oligoclase, biotite and Fe-Ti oxides, and the second produced albite, garnet and muscovite. The end stage of fractional crystallization was marked by the occurrence of pegmatites with beryl, spodumene and columbite-tantalite.