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 .

Abstract The Ślęża Ophiolite is one of several thrust-bounded crustal slices dominated by metabasites in the western Sudetes. The apparent field association of serpentinites, gabbros and amphibolitic components led previous workers to consider that this lithological assemblage represented an ophiolite sequence. Fieldwork suggests that the ophiolite is now highly inclined, partly overturned, so that an ophiolitic pseudostratigraphy can be deduced, grading from serpentinites and gabbros in the south to metabasite lavas in the north. The recent discovery of pillow lava structures (at Gozdnica Hill, to the west of Sobótka town) confirms that the volcanic top of the ophiolite lies in the northern section, as might be expected from the ophiolite model. The gabbros have undergone greenschist facies metamorphism with the random development of low-grade amphibole. The volcanic portion of the sequence comprise metamorphosed dolerites and basalts partly within the contact aureole of the Variscan Strzegom-Sobótka granite. Previous work dated plagiogranites associated with the gabbros at about 400–420 Ma (U-Pb zircon ages). Geochemical data suggest that the gabbros are distinct and apparently not comagmatic with the volcanic section of sheeted dykes and lavas. The gabbros, in particular, although very depleted in incompatible elements are dissimilar to supra-subduction zone ophiolites, exhibiting instead N-MORB-like light REE depleted patterns. Depletion is both a feature of the cumulate character of many of the gabbros, as well as a source effect (especially the uniformly low Nb content). The metabasalts and metadolerites, on the other hand, are a well-evolved single comagmatic suite with high incompatible element contents, Zr/Y approximately 3–4, and generally flat to light REE-depleted patterns. The geochemical dichotomy of the plutonic and volcanic segments calls into question a simple interpretation of the body as a single-stage coherent stratiform ophiolite. Chemical comparison with Sudetic metabasites from within the nearby Rudawy-Janowickie and Kacazawa Complexes shows that the Ślęża metabasites have a number of features in common, including the presence of both low-Ti (gabbros) and high-Ti (dykes and lavas) chemical groups. The correlation of the gabbros, dykes and lavas with the low-Ti and high-Ti (Main Series) metatholeiites respectively, seen throughout the Bohemian Massif, as well as the Sudetes, places them within the regional collage of Palaeozoic crustal blocks separated by the Saxothuringian Seaway. Comparison with Bohemian Massif metabasites also indicates that sediment contamination of the Ślęża Ophiolite sources was not an important process and that an enriched plume source played no part in the generation of the ophiolitic melts. The two Ślęża chemical groups were derived from variably depleted asthenospheric mantle sources. Simple modelling suggests that the volcanic segment could have been derived by 10–15% partial melting of a depleted N-MORB source, whereas the plutonic segment represents around 30% partial melting of a more depleted source. To develop varying degrees of depletion in an oceanic environment, the two sources could be related via incremental partial melting of a shallow MORB-type source.