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Hydrothermal alteration of Tertiary igneous rocks from the Isle of Skye, northwest Scotland

II. Granites


Hydrothermal alteration of Tertiary granites from Skye involed the reaction of igneous amphibole, biotite, ferroaugite, fayalite, alkali feldspar, plagioclase feldspar, magnetite, and ilmenite with aqueous fluid primarily to combinations of secondary alkali feldspar, chlorite and montmorillonite. Lesser amounts of muscovite, calcite, epidote, prehnite, zeolite, and pyrite also developed. During mineralogical alteration of the granites there was a net addition to rock of H and a net loss of Ca, Fe, and probably Na. There is no positive evidence for a significant change in their Al, Si, Mg, or K contents. Hydrothermal alteration of Skye granite involved not only large-scale migration of 18O, 16O, D, and H but also of Ca, Fe, and probably Na. Mineral thermometry combined with textural observations suggests that chemical, isotopic, and most mineralogical alteration were synchronous and occurred at 350°–450° C. Textural observations further suggest that the mechanism of isotopic exchange between granite and hydrothermal fluid primarily was solution/reprecipitation.

Mineral thermometry and oxygen fugacity barometry indicate that some minerals in the granites — clear alkali and plagioclase feldspars, Fe-Ti oxides, biotite, fayalite, and probably amphibole and pyroxene — have compositions that record physical conditions during crystallization or at high temperatures slightly below the crystallization temperature (650°–750° C). These minerals have chemistries that were later unaffected by the hydrothermal event. Mineral thermometry also indicates that other minerals — turbid feldspar and all secondary minerals — have compositions that record physical conditions of hydrothermal alteration (350°–450° C). The isotope chemistries of the high-temperature minerals are inferred to be those in unaltered “normal” granite while the isotope chemistries of the low-temperature minerals are inferred to be those of minerals equilibrated with the hydrothermal fluid at low temperature. The close correspondence between the isotopic and major element composition of minerals in altered granites leads to an integrated mineralogic/isotopic model that considers the granites as composed of two families of minerals: an“igneous” family and a “hydrothermal” family.

Comparison of data for altered Skye granites with data from hydrothermally altered synmetamorphic granites from Maine shows the important control that depth exerts on the isotopic composition and mineral chemistry of altered granites. Because the hydrothermal fluids that altered epizonal Skye granites were meteoric in origin, altered rocks are isotopically lighter than “normal” granite. In part because the hydrothermal fluids that altered the mesozonal granites from Maine were metamorphic in origin, altered rocks are isotopically heavier then “normal” granite. The hydrothermal event on Skye was shallow and hence short-lived; mineral-fluid reactions did not go to completion. Minerals in altered Skye granites therefore show large departures from both chemical and isotopic exchange equilibrium. The hydrothermal event in Maine was deeper and hence long-lived; reactions more closely approached equilibrium. Minerals in altered Maine granites therefore show a much closer approach to both chemical and isotope exchange equilibrium.

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Ferry, J.M. Hydrothermal alteration of Tertiary igneous rocks from the Isle of Skye, northwest Scotland. Contr. Mineral. and Petrol. 91, 283–304 (1985).

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  • Hydrothermal Fluid
  • Hydrothermal Alteration
  • Fayalite
  • Exchange Equilibrium
  • Altered Rock