Fluid evolution of rare-element and muscovite granitic pegmatites from central Galicia, NW Spain

摘要

Fluid inclusions have been studied in threepegmatite fields in Galicia, NW Iberian Peninsula. Based onmicrothermometry and Raman spectroscopy, eight fluid systemshave been recognized. The first fluid may be considered to be apegmatitic fluid which is represented by daughter mineral(silicates)-rich aqueous inclusions. These inclusions are primaryand formed above 500deg C (dissolution of daughter minerals).During pegmatite crystallization, this fluid evolved to a low-density, volatile-rich aqueous fluid with low salinity (93percentH_2O; 5percent CO_2; 0.5percent CH_4; 0.2percent N_2;1.3percent NaCl) at minimum P-T conditions around 3 +- 0.5kbar and 420deg C. This fluid is related to rare-metalmineralization. The volatile enrichment may be due to mixing ofmagmatic fluids and fluids equilibrated with the host rock. Adrop in pressure from 3 +- 0.5 to 1 kbar at a temperature above420deg C, which may be due to the transition frompredominantly lithostatic to hydrostatic pressure, is recorded bytwo-phase, water-rich inclusions with a low-density vapourphase (CO_2, CH_4 and N_2). Another inclusion type isrepresented by two-phase, vapour-rich inclusions with a low-density vapour phase (CO_2, CH_4 and N_2), indicating a laststage of decreasing temperature (360deg C) and pressure(around 0.5 kbar), probably due to progressive exhumation.Finally, volatile (CO_2)-rich aqueous inclusions, aqueousinclusions (H_2O-NaCl) and mixed-salt aqueous inclusions withlow Th, are secondary in character and represent independentepisodes of hydrothermal fluid circulation below 310deg C and 0.5 kbar.