Abstract Modern X-ray diffractometer, optical, and chemical techniques coupled with principles developed by recent phase-equilibria studies of the hydrous systems Ab-Or (Bowen and Tuttle, 1950), Ab-Or-Q (Tuttle and Bowen, 1958), and Ab- Or-An (Yoder, Stewart, and Smith, 1957) have been applied to the feldspars from 68 analyzed rocks belonging to a complex comagmatic series, the Laramide in trusive sequence in the Colorado Front Range. Consistent differences in composit ion, thermal state, and paragenesis of the feldspars confirm the subdivision of the rocks into two related lines of descent, based upon bulk composition and mafic mineralogy, and throw light on the pressures (H 2 O) and temperatures involved. The sodic subseries consists of abundant porphyritic-aphanitic dikes and stocks concentrated i n the fractured mineral belt proper. These rocks contain more than 59 per cent Ab/(Ab + Or) and are inferred to be the result of strong fractionation and partial devolatilization brought on by deep Laramide fracturing . The chief mafic minerals are hornblende and aegirine-augite. Most plagioclase phenocrysts are early, contain up to 16 per cent Or by weight, and are strongly zoned. They are predominantly high-temperature plagioclase in which the cores commonly have higher temperature characteristics than the sodic rims. As crystallization progressed, the high-temperature plagioclase was joined by high-temperature alkali fieldspar (hyperorthoclase cryptoperthite) containing 33–47 per cent (Ab + An) by weight. As the magma temperature continued to f all, especially under conditions of strong fractionation, all or part of the hightemperature potassic plagioclase reacted with the highly sodicliquid to yield high-temperature sodic alkali feldspar. Mantling of the plagioclase by alkali feldspar resulted in incomplete reaction and led to an increase in the Or/ (Or + Ab) ratio of the final An-poor residuum. When filter-pressed, these late liquids produced a range of An-poor rocks, including the quartz bostonites, having Or/(Or + Ab) ratios higher than that at the minimumin the hydrous system Ab-Or-Q (about 0.60 at the prevailing H 2 O pressures). These residual liquids were unusually enriched in U, Th, and other silicophile minor elements. The An-poor residual liquids yielded early phenocrysts of alkali feldspar in place of plagioclase. The high-temperature optics and composition of the phenocrysts indicate hypersolvus crystallization at H 2 O pressures close to 3000kg/cm 2 and temperatures close to 690°C. As the temperature fell and fluxes built up in the remaining liquid, two low-temperature feldspars, highly sodic plagioclase and microcline, crystallized under subsolvus conditions throughout the groundmass. The groundmass feldspars retain textures indicative of original crystallizat ion; clearly, these feldspars formed before much of the associated quartz, thus ruling out formation by subsolidus exsolution. The potassic subseries is made u p of phaneritic intrusive rocks including more mafic types, mainly earlier than the sodic subseries, plus later leucocratic types resembling normal calc-alkaline granite believed to have formed approximately at the same time as the sodic subseries. These phaneritic rocks contain less than 59 per cent Ab/(Ab + Or),are mainly restricted to the less fractured borders of the mineral belt, contain a more hydrous mafic assemblage, and are inferred to be the products of stronger reaction. The potassic subseries contains mostly lowtemperature feldspars: weakly zoned plagioclase having less than 5 per cent Or, and potassic alkali feldspar (suborthoclase microperthite) containing less than 28 per cent Ab .