Isocon analysis of migmatization in the Front Range, Colorado, USA

Isocon analysis has been applied to five sets of leucosome, mafic selvages and immediately adjacent mesosome in the migmatites from a 15-m outcrop in the Colorado Front Range. The results show: (i) mafic selvages formed from the adjacent mesosome by loss of felsic components and therefore the mesosomes are indeed palaeosomes or protoliths; (ii) the leucosomes did not form in a closed system from the palaeosome (in which case the material lost from the palaeosome during selvage formation would become the leucosome). The observed volumes and compositions of leucosomes require that the present leucosome must contain some material in addition to the felsic components lost from the selvages. The materials that must be added are leucotonalitic to granitic in composition, varying greatly in K/(Na + Ca) ratio. The trend in leucosome composition can be reproduced by assuming that a metasomatic exchange, KNa + Ca, modified originally leucotonalitic leucosomes to more K-rich compositions. These leucosomes most likely formed by injection of silicate melts accompanied, or followed, by metasomatism. The trend of leucosome compositions in this study reflects the general trend in the leucosome compositions which have been published from other areas, indicating that the proposed mechanism can be applicable to other regional migmatites.