The complex hydrothermal history of granites: Multiple feldspar replacement reactions under sub-solidus conditions

The hydrothermal alteration of granites indicates considerable interaction between external fluid circulation systems and crustal rocks as shown by oxygen isotope analysis. Recent studies on granitic rocks frequently explain the geochemical and isotopic signatures on the basis of magmatic processes, but neglect to consider the effects of pervasive, large-scale fluid-rock interactions.
We present work on Swedish granites from the Oskarshamn Region with particular emphasis on the multiple replacement of feldspars around shear zones, allowing a better understanding of sub-solidus reequilibration of granitic rocks with hydrothermal fluids. Detailed scanning and transmission electron microscopy studies of plagioclase show a wide variety of replacement reactions. The most unaltered granite contains plagioclase crystals which are highly porous and include crystallographically continuous K-feldspar inclusions. The textures suggest that the plagioclase is already secondary and may be a replacement product of an original K-feldspar.
The plagioclase is progressively replaced by albitic feldspar along polysynthetic twins with a compositional shift from An₂₆Ab₇₄ to An₉Ab₉₁. Epidote found along the albitised twins is likely to be due to the Ca and Al release during albitisation. The albitisation of plagioclase is contemporaneously associated with sericitisation on increasing alteration and often accompanied by sub-micron precipitates of Fe-oxides in pores. Sericitisation requires additional introduction of K into plagioclase of which the breakdown of biotite to chlorite is one possible source. Furthermore albitisation requires external introduction of an Na-bearing fluid and therefore it is reasonable that such fluid could also contain the required K. The chemical interface between plagioclase and albite is sharp on the nanometre scale, and the crystallographic orientations of plagioclase and albite are coincident within less than a degree. These characteristic features suggest an interface-coupled dissolution-reprecipitation replacement mechanism. Another commonly observed phenomenon in the vicinity of fractures in the granite is the replacement of the plagioclase by K-rich alkali feldspar.
This pseudomorphism yields an almost entire replacement of the plagioclase and the disappearance of sericite. Optical and electron microscopy imaging show that the alkali feldspars contain many inclusions which produce a cloudy pink-red coloration. It has been shown that this coloration is due to sub-micron size hematite precipitates associated with micropores in contact with the replacement front between the parent plagioclase and the product alkali feldspar. The complex associations of a variety of feldspar-fluid reactions indicate that the replacement reactions may be due to multiple fluid infiltration events and that such granites are secondary, sub-solidus products of a parent rock with a different mineralogy.