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The Complex Hydrothermal History of Granitic Rocks: Multiple Feldspar Replacement Reactions under Subsolidus Conditions OLIVER PLU« MPER* AND ANDREW PUTNIS INSTITUT FU« R MINERALOGIE, UNIVERSITY OF MU« NSTER, CORRENSSTR. 24, D-48149 MU« NSTER, GERMANY RECEIVED NOVEMBER 13, 2008; ACCEPTED APRIL 10, 2009 ADVANCE ACCESS PUBLICATION MAY 18, 2009 Recurring subsolidus re-equilibration of granitic feldspars induced by fluid infiltration events provides a record of fluid^rock interactions that affect large volumes of the Earth’s continental crust.This has a direct bearing on the interpretation of the present-day granitic rock mineralogy and geochemistry. We examine Palaeoproterozoic grey and red-stained granitoids from the Simpevarp and Laxemar areas in SE Sweden, particularly focusing on consecutive feldspar replace- ment reactions, to provide an in-depth understanding of subsolidus re-equilibration of granitic rocks with hydrothermal fluids. The apparently most unaltered grey granitoids contain highly porous oli- goclase grains that enclose crystallographically continuous microcline relicts.This texture suggests that the oligoclase is already secondary and may be a replacement product of original microcline. Oligoclase is progressively replaced by albite (�An9) along polysynthetic twin- ning and intragranular fractures. The features of this replacement are characteristic of a dissolution^reprecipitation mechanism. Fine- grained mica (sericite) is closely associated with the albite porosity within micron-sized pores observable with scanning electron micros- copy as well as in nanopores imaged with transmission electron microscopy. The reddening phenomenon in the vicinity of fractures is contemporaneously related to the K-feldspathization of sericite, which is restricted to the altered oligoclase. Submicron size hematite precipitation within orthoclase pores at the replacement front results in the red coloration. The complex associations between the fluid^ feldspar reactions indicate that the replacement reactions may be due to sequential fluid infiltration events and that the granitoids have undergone extensive subsolidus re-equilibration, changing the original magmatic mineralogy. Therefore, the effects of large-scale re-equilibrations of granitic rocks through hydrothermal convection systems should be more closely considered. KEY WORDS: dissolution^reprecipitation; feldspar replacement; fluid^ rock interaction; granites; hydrothermal alteration I NTRODUCTION The hydrothermal alteration of granitic rocks indicates considerable interaction between external fluid circulation systems and crustal rocks on an enormous scale, as shown by oxygen isotope studies (Taylor, 1977; Taylor & Forester, 1979; Hoefs & Emmermann, 1983). Recent research into granitic rocks attributes their mineralogical and geochemi- cal characteristics to purely magmatic processes and typi- cally invokes multiple parental magma sources from the mantle and crust as well as magma mixing and crystal fractionation (Clemens & Vielzeuf, 1987; Millar et al., 2001; Yang et al., 2004; Kemp et al., 2007). However, these studies neglected to consider the effect of pervasive, large- scale fluid^rock interaction and the resulting subsolidus re-equilibration of granitic rocks with hydrothermal fluids, despite clear indications of the hydrothermal reset- ting of oxygen isotopes. Hydrothermal activity is likely to be a more common phenomenon in the continental crust than is currently documented, as it is a natural consequence of any thermal perturbation in fluid-rich rocks (Fyfe et al., 1978; Yardley & Shmulovich, 1995). Pervasive fluid circulation systems can extend throughout the crust and may influence several cubic kilometres of rock. Conditions of pervasive metaso- matism involving large rock volumes are well known in mid-oceanic ridge environments (e.g. Alt et al., 1986), and *Corresponding author. Present address: Physics of Geological Processes (PGP), University of Oslo, Sem Selands vei 24, N-0316 Oslo, Norway. E-mail: oliver.pluemper@fys.uio.no � The Author 2009. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@ oxfordjournals.org JOURNALOFPETROLOGY VOLUME 50 NUMBER 5 PAGES 967^987 2009 doi:10.1093/petrology/egp028 D ow nloaded from https://academ ic.oup.com /petrology/article-abstract/50/5/967/1604127 by U FO PA user on 11 June 2019 are also responsible for the generation of many of the most important ore deposits, including examples such as the Olympic Dam Mine in South Australia (Haynes et al., 1995; Barton & Johnson, 1996). The presence of crustal hydrothermal activity is usually not accounted for, unless it is associated with the formation of new minerals that are characteristic of fluid infiltration, or when succes- sive stages of replacement can be identified. A variety of studies have been performed that address subsolidus re- equilibration of granitic rocks by deuteric fluids, focusing mainly on the coarsening of cryptoperthite to patch perthite. This coarsening provides a simple petrographic marker for fluid^rock interaction at subsolvus tempera- tures (Parsons, 1978; Parsons & Brown, 1984;Worden et al., 1990; Walker et al., 1995; Lee & Parsons, 1997; Parsons & Lee, 2000; Nakano et al., 2002, 2005). Remarkably, more general investigations of subsolidus re-equilibration of granitic rocks in geochemical studies, using a broader vari- ety of petrographic markers for fluid^rock interaction, have not been undertaken. The albitization of plagioclase and alkali feldspars is a possible petrographic marker as it is one of the most frequently observed metasomatic alumi- nosilicate reactions in the upper crust of the Earth, as has recently been reviewed by Perez & Boles (2005). Albitization has received limited attention, even though this process probably marks the circulation of fluids of var- ious origins on a very large scale. More generally, the replacement of one feldspar by another is commonly asso- ciated with fluid^rock interactions and metasomatism, as Harlov et al. (1998) and Putnis et al. (2007a) have demon- strated the replacement of plagioclase by K-feldspar. Furthermore, the association of plagioclase alteration and sericitization is a form of hydrothermal alteration found extensively in granitoids (Que & Allen, 1996, and refer- ences therein). The red clouding in granitic rocks has long been attribu- ted to the presence of ferric iron oxides in alkali feldspar minerals (Boone, 1969; Taylor, 1977; Smith & Brown, 1988; Nakano et al., 2002). Putnis et al. (2007a) recently demon- strated that this red staining is due to the precipitation of hematite from a fluid within the feldspar pores. The gener- ation of porosity has been proposed as a fundamental feature of a dissolution^reprecipitation mechanism for re- equilibrating minerals in the presence of a fluid phase (Putnis, 2002; Putnis et al., 2005, 2007, 2007a; Putnis & Putnis, 2007b). Turbidity, a characteristic of replaced feld- spars, originates from porosity and is almost ubiquitously developed to varying extents within plutonic rocks (Montgomery & Brace, 1975; Parsons, 1978; Worden et al., 1990). Thus, the red clouding of feldspars can be used as another direct marker for large-scale, subsolidus, re- equilibrating crustal fluid flow. Here we present work on granitoids from the Simpevarp and Laxemar areas, SE Sweden, providing new data on the re-equilibration of granitic rocks with crustal hydro- thermal systems. Our study particularly emphasizes feld- spar replacement reactions around fractures and their textural and chemical changes. Numerous earlier studies in the area and at the nearby A« spo« Hard Rock Laboratory have illustrated that reddening in the vicinity of fractures is the most prominent style of alteration of the original grey granitoid (Drake et al., 2008, and references therein). Initially the main aim of this work was to gain an in-depth understanding
