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

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.It has been proposed by other workers that CO₂ fluid in the inclusions could form from an H₂O-CO₂ fluid when H₂O is partitioned into the silicate melt. Such partitioning should result in some early H₂O-rich inclusions: H₂O must be released as the melt crystallizes. As found in migmatites from other areas, most aqueous inclusions in the Front Range rocks are obviously much younger than the early CO₂ ones. However, early H₂O-rich fluid may still be preserved, at least in three ways: (A) in rare, isolated or clustered inclusions within quartz inclusions in feldspar; (B) as inclusions in microcline porphyroblasts; (C) in hydrous alteration products of feldspar. (A) contain dilute fluids, 1 to 6 wt% NaCl equivalent. The densities of (A) as well as those of the early CO₂ inclusions found in the quartz inclusions in feldspar are appropriate for the range of P — T conditions estimated for migmatization. These early inclusions must have been preserved because of protected environment. Inclusions (B), found to contain H₂O (and possibly CO₂) by infrared analyses, must be early because they are absent from recrystallized grains. (B) and (C) are much more common in the leucosome than in the other zones suggesting that they are related to migmatization process. The concentration of early CO₂ inclusions in the leucosome is consistent with the model of migmatization in which fluid concentration in the leucosome was a cause of melting.