The composition and role of the fluid in migmatites: a fluid inclusion study of the Front Range rocks

Abstract

Monophase negative-crystal shaped CO2 inclusions occurring isolated, in small clusters, or in well-healed intragranular fractures are common in the leucosome quartz of the 1700m.y.-old migmatites from the east-central Colorado Front Range. They are, however, quite rare in the mafic selvage and paleosome (host rock) quartz. The mode of occurrence suggests that these are the earliest inclusions to form. In addition to the difference in abundance of the inclusions, there is a difference in CO2-density distribution between migmatitic zones. The temperatures of homogenization for the leucosome inclusions range and ∼ +l°C from −67° C to +20° C with two maxima (at ∼ −21° C) while those for the paleosome and selvage inclusions are −37° C to +20° C with a single maximum at ∼ + 5° C. These differences between the migmatitic zones which occur on the scale of a few centimeters suggest that the formation of these inclusions was related to the migmatization process. The densities corresponding to the Th maxima are appropriate for the P-T conditions for migmatization estimated from the mineral geobarometer/geothermometer. These inclusions must contain nearly pure CO2, as their final melting temperatures (−56.5° to −57.2° C) are very close to that of the triple point of CO2. Their composition also was confirmed by Raman spectroscopic analyses.