The physical and chemical evolution of fluids in rare-element granitic pegmatites associated with the Lacorne pluton, Quebec, Canada

摘要

Co-magmatic rare-element (Be, Li, Nb, Ta) granitic pegmatites associated with the Lacorne monzogranite pluton of the Preissac-Lacorne batholith (Quebec) range from a least evolved beryl-type in the pluton through a transitional beryl + spodumene-type to a most evolved spodumene-type in mafic country rocks. They possess internal lithologic fabrics and primary fluid inclusions that provide evidence for saturation of the magma with aqueous fluids immediately after the formation of aplite in the beryl pegmatite, and pene-contemporaneously with formation of the beryl + spodumene and spodumene pegmatites. The early fluids are represented by aqueous liquid-vapor (Type 1a) and aqueous liquid-vapor-solid (Type 2) inclusions in beryl from beryl and beryl + spodumene pegmatites and are estimated to have been trapped between 500 and 550 degrees C and at 3.5 Kb. On the basis of microthermometric measurements, analyses of the compositions of the precipitates from decrepitated inclusions and the nature of the solids (trapped), these fluids are interpreted to have been NaCl-dominated (= 16 wt% NaCl eq.) and to have contained appreciable Fe and dissolved CO2. In spodumene pegmatite, the equivalent fluid, which was trapped at 450 to 500 degrees C, contained significant Mn, Li and Cs in place of Fe, consistent with corresponding trends in the chemical evolution of the host rock.