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The Precambrian migmatites in the central Front Range were formed during high-grade regional metamorphism, prior to the emplacement of the major granitic bodies. The leucosomes of the migmatites consist essentially of varying proportions of microcline and plagioclase together with 30 to 50 percent quartz. The concentrations of biotite along the margins of the leucosomes suggest that the migmatite formed by local segregation. This interpretation is supported by data indicating equilibrium in the concentrations of alkali and alkaline earth elements between the leucosomes and the melanosomes. Strontium isotopic compositions are also in equilibrium between adjacent leucosomes and melanosomes. All the parameters considered in this study are consistent with the migmatites having formed by segregation of metamorphic rocks; the process apparently did not involve partial melting.

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