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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 understandingof the changes in mineralogy between the grey and red granitoids to evaluate the red- dening phenomenon. However, detailed optical micros- copy as well as scanning and transmission electron microscopy provided evidence that the grey granitoids have already undergone intensive alteration with respect to feldspar replacement reactions prior to the fracturing causing the reddening.We illustrate the affiliations and rel- ative timings between the multiple feldspar replacement reactions and demonstrate that their complex association with fluids implies that the replacements may be due to sequential fluid infiltration events. In addition, these obser- vations suggest that such granitic rocks are secondary, sub- solidus products created by fluid-induced re-equilibration of their original magmatic mineralogy. This should be taken into closer consideration when studying granitic rocks. GEOLOGICAL SETT ING AND SAMPLES The bedrock in the Simpevarp and Laxemar areas 30 km north of Oskarshamn, SE Sweden is dominated by the Sm�land granitoids and dioritoids of the Trans- Scandinavian Igneous Belt (TIB) (Fig. 1). Formation of TIB granitoids can be subdivided into three major pulses of magmatism between 1�85 and 1�66 Ga; TIB 1 (1�81^1�77 Ga), TIB 2 (1�72^1�69 Ga) and TIB 3 (1�69^1�66 Ga) (Larson & Berglund, 1992). U^Pb dating of the Sm�land granitoids from the Simpevarp and Laxemar areas reveals that they are between 1�79 and 1�80 Ga in age, and there- fore belong to the TIB 1 generation (Kornfa« lt et al., 1997; Wahlgren et al., 2004). The TIB is about 1600 km long, up to 150 km wide, nearly north^south in orientation and situ- ated between older Svecofennian (c. 1�9 Ga) crust in the ENE and younger rocks from the Swedish Gneiss Region in the SW (—ha« ll & Larson, 2000). Further A-type granitic intrusions are found to the north and south of the investi- gated areas: the Go« temar granite (1�45 Ga) and the Uthammar granite (1�44 Ga), respectively (Kresten & Chyssler, 1976). The Simpevarp area is dominated by three major rock types: (1) fine-grained dioritoid in the southern part of the peninsula and in the central part of the A« vro« granite; (2) medium-grained quartz monzodiorite in the eastern part JOURNAL OF PETROLOGY VOLUME 50 NUMBER 5 MAY 2009 968 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 of the peninsula; (3) porphyritic granite to quartz monzo- diorite in the north and at H�lo« and A« vro« (Wahlgren et al., 2004). The dominant rock types have overlapping compositional variations and are mainly distinguished by texture and grain size. Minor intrusions and enclaves of intermediate to mafic composition (diorite to gabbro) are common. Contact associations and mixing as well as min- gling relationships between the different rock types in the Simpevarp area imply that they were formed almost syn- chronously (Kornfa« lt et al., 1997; Wahlgren et al., 2004). This is confirmed by U^Pb zircon and titanite geochronol- ogy that gives ages of 1802�4 Ma (zircon) for the quartz monzodiorite and 1793�4 Ma (titanite) and 1800� 4 Ma (zircon and titanite) for the A« vro« granite (Wahlgren et al., 2004). Based on field relationships the following chronos- tratigraphy for the dominant and subordinate rock types is suggested (beginning with the youngest): (1) fine- to medium-grained granite and pegmatite; (2) fine-grained mafic rock; (3) medium- to coarse-grained granite; (4) A« vro« granite; (5) quartz monzodiorite; (6) diorite to gabbro; (7) fine-grained dioritoid as the oldest rock type (Wahlgren et al., 2004). Up to 80% of the Laxemar area is dominated by the A« vro« granite, which comprises a suite of porphyritic rocks that varies in composition from quartz monzodiorite to granite and includes quartz dioritic and quartz monzonitic varieties. As in the Simpevarp area, a characteristic feature of the A« vro« granite is the occurrence of enclaves of intermediate to mafic composition. The A« vro« granite has been observed to intimately mix with the equigranular quartz monzodiorite. Gradual contact rela- tionships indicate that the two rock types formed contemporaneously. Within the study area several localized shear zones of ductile and brittle^ductile nature occur. Red staining appears to have affected larger rock volumes in the Simpevarp area than in the Laxemar area (Nilsson et al., 2004) and is mainly concentrated along fractures, although Fig. 1. (a) Geological map of the Simpevarp and Laxemar area showing the locations of the investigated drillcores KSH03 and KLX04 (mod- ified from Drake et al., 2008). (b) Simplified overview of the main geological units in southern Sweden highlighting the location of the investi- gated area (black square) and theTrans-Scandinavian Igneous Belt (TIB) (modified from Kornfa« lt et al., 1997). PLU« MPER & PUTNIS HYDROTHERMAL HISTORYOF GRANITIC ROCKS 969 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 in many places it has also affected the rock between meso- scopic fractures (Wahlgren et al., 2004). Drake et al. (2008) considered that the hydrothermal alteration that caused the red staining occurred at temperatures between 250 and 4008C. This is based on analysis of low-temperature mineral parageneses in fractures and associated altered granitoids; the upper temperature is limited by the stabil- ity of prehnite and chlorite (Deer et al., 1992; Liou et al., 1983; Spear, 1993), whereas the lower temperature limit is defined by the breakdown of prehnite below �2508C (Liou, 1971; Liou et al., 1983). Furthermore, the lower tem- perature estimation is consistent with the observation that laumonite is absent in sample KSH03 (Drake et al., 2008, and references therein). In this area prehnite is commonly replaced by laumonite, which has an upper stability of around 220^2808C (Liou, 1970; Frey et al., 1991; Bucher & Frey, 2002). Drillcores were obtained during site investigation by the Swedish Nuclear Fuel andWaste Management Co. (SKB) for a nuclear waste repository in the Oskarshamn region. Samples were obtained from several drill sites.We focused on two cores (KLX04, KSH03) that contain both grey and red-stained granitoids, showing a clear association between the red staining and fractures (Fig. 2). The most prominent red staining is observed in drillcore KSH03. Several polished thin sections and thick sections were prepared incorporating both the red and grey granitoids as well as the transition zone between them. In addition, rock samples of the macroscopically least altered granitoid were studied from a shallower drillcore level, located several tens to hundreds of metres away from the fracture. The geological map in Fig. 1a shows the loca- tions from which the cores were taken; KSH03 is a grano- diorite drillcore from 102�55m depth and KLX04 a quartz monzodiorite drillcore from 322�25m below the present-day surface. Detailed descriptions of the drillcores have been given by Nilsson et al. (2004) and Carlsten et al. (2006). ANALYT ICAL AND THEORET ICAL TECHNIQUES Analytical techniques To investigate the nature of the hydrothermal alteration the two drillcores were studied with transmitted light microscopy, scanning electron microscopy (SEM), particu- larly focusing on back-scattered electron (BSE) imaging and energy-dispersive X-ray (EDX) analysis, and finally using transmission electron microscopy (TEM). Quantitative electron probe microanalysis (EPMA) and laser ablation high-resolution inductively coupled plasma mass spectrometry (LA-HR-ICP-MS) analysis was also performed to examine the elemental gains and losses that occurred during the feldspar replacement reactions, in particular the albitization of oligoclase. For phase identification, especially of the alkali feld- spars, Raman spectra were collected with a high-resolution JobinYvon HR800 Raman spectrometer using the 532 nm line of a 14 mW Nd^YAG laser. Thescattered Raman light was analysed with a 100� objective lens in a 1808 backscattering geometry and a charged-coupled device (CCD) detector. After the light passed a 100 mm entrance slit it was dispersed by a grating of 1800 grooves per mm. A confocal hole of 1000 mmwas used for all measurements. Quantitative EPMA was performed using a JEOL JXA 8600M Superprobe equipped with wavelength-dispersive spectrometers to obtain mineral compositions.The acceler- ating voltage was 15 kV and the counting time 5^10 s on peak. When possible, minerals were analysed with a defo- cused beam to prevent loss of alkalis and to gain an aver- age quantitative analysis of the altered plagioclase grains. Standardization was carried out using a selection of natu- ral mineral standards, including hypersthene for Si, kya- nite for Al, fayalite for Fe, diopside for Ca, jadeite for Na, sanidine for K, rhodonite for Mn and rutile for Ti. The ZAF correction procedure was applied to correct for matrix effects. Fig. 2. Photograph of the polished drillcore KSH03, which is representative of the grey (left side) and red-stained (lighter colour, right side) granitoids with direct association with a sealed fracture (arrow). Prehnite is the main fracture filling.Traces of red staining can be found further from the fracture than is macroscopically visible. JOURNAL OF PETROLOGY VOLUME 50 NUMBER 5 MAY 2009 970 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 Trace element concentrations were analysed by LA-ICP- MS utilizing a New Wave UP103HE 193 nm ArF high- energy excimer laser coupled to a single-collector magnetic sector ICP mass spectrometer (Finnigan Thermo Element2). The spot size was varied from 2 to 20 mm to obtain optimal spatial resolution and to avoid mixed sig- nals. An average analysis of albite and sericite was pro- duced by selecting a spot size between 10 and 20 mm. Laser frequency and laser energy were set to 5Hz and 9 J/cm, respectively. An NIST silicate glass SRM 612 was used for standardization. Data reduction was performed using the GLITTER� software (Access Macquarie Ltd.). Selected samples from each drillcore were prepared for optical, SEM and TEM analyses. Thin sections (30 mm in thickness) were made for polarized transmitted light microscopy. Before SEM analyses the thin sections were coated with a carbon layer of 15^20 nm thickness and then examined with a SEM JEOL 840 and a field emission (FE) SEM JEOL JSM 6300F, both equipped with a BSE detector and EDX spectrometer (OX2000 from Oxford Instruments, INCA system). Total porosity distribution was calculated from two-dimensional images created using the SEM. For TEM analysis, several 3mm discs were removed from the thin sections and glued to copper grids using dif- ferent sizes of gratings and slots. Final thinning was exe- cuted with a Gatan Duo-mill ion polishing system. TEM analyses were performed in a JEOL 3010 EX TEM operat- ing at 300 kV, equipped with an EDX spectrometer (link ISIS) and a post-column elemental mapping device (Gatan Image Filter, GIF). Crystallographic orientation and identification of submicron inclusions in the alkali feldspars were determined from diffraction patterns. All analytical measurements were performed at the Institut fu« r Mineralogie, University of Mu« nster in conjunc- tion with the Interdisciplinary Centre of Electron Microscopy and Microanalysis (ICEM), with the excep- tion of Raman spectroscopy, which was conducted at the Institut fu« r Anorganische und Analytische Chemie, University of Mu« nster. Theoretical approach to feldspar equilibria To gain a better understanding of equilibria between feld- spars and fluids an appropriate graphical model needed to be established. Examining the change in molarity of Naþ and Kþ in a fluid and the concurrent effects on the coexist- ing minerals is one possible method. Real fluid^solid phase systems (e.g. the hydrothermal feldspar replacement reaction we have studied) are in constant flux and thus never reach true thermodynamic equilibrium. However, we can apply thermodynamic models to systems close to equilibrium and therefore to areas of local equilibrium, such as at reaction interfaces, and to residual fluid in pores. As any disequilibrium condition relaxes instantaneously to an equilibrium state, we can assume that the system remains in local equilibrium (Knapp,1989). We have generated equilibrium constants for aqueous species dissociation reactions at appropriate temperatures and pressures for the hydrothermal alteration processes observed utilizing SUPCRT92 (Johnson et al., 1992) with the slop98 database (GEOPIG, 1998). Using these equilib- rium constants activity diagrams were constructed between albite, sericite and K-feldspar. The pressure effect on feldspar equilibrium was also investigated. Thermodynamic equilibrium modeling using SUPCRT92 has been used previously to good effect when modelling various fluid^feldspar interactions (e.g. Wibberley & McCaig, 2000). PETROLOGY AND MINERAL CHEMISTRY Grey granitoids Microcline islands in single oligoclase grains The dominant feldspar phase in the investigated granitoids is oligoclase. These grains are equigranular, fine- to medium-grained (up to �5mm) crystals with embayed grain boundaries (Fig. 3a) with compositions in the range of An24Ab76 to An30Ab70 (average An26Ab74) for KLX04 and An27Ab72 to An30Ab68 (average An28Ab72) for KSH03 (Table 1). In back-scattered SEM images the oligo- clase phase has a high porosity distribution of up to 3% (Fig. 4). During analysis we noticed that the samples also contain irregular-shaped, porosity-free, cross-hatched twinned K-feldspar islands, varying from 100 mm up to 1cm (Fig. 3), which are consistently found in almost every oligoclase grain in the granitoids. The bulk compositions of these islands are in the range of Ab10Or90 to Ab3Or97 (Table 1). The islands are usually distributed within the central section of a single oligoclase grain in both unal- tered and altered areas (Fig. 3), but can also be found in the outer parts of oligoclase grains. Under cross-polarized light all the K-feldspar islands in a single oligoclase grain are optically continuous and extinguish in the same direc- tion (Fig. 3b and c). Additionally, most of the islands’ cross-hatched twinning is parallel to the polysynthetic twinning of the surrounding oligoclase. Back-scattered electron imaging of the K-feldspar islands showed no porosity or intragranular fractures and only minor perthi- tic exsolution (Fig. 3d). Raman spectroscopy of the K-feld- spar islands identified them as microcline; these were clearly distinguishable from other K-feldspar structural types, such as sanidine and orthoclase, because of the dif- ference in their Al^Si ordering (Mernagh, 1991; Freeman et al., 2008). Progressive replacement of oligoclase by albite and sericite The transformation of the observed oligoclase to albitic feldspar (average An9Ab91) occurs at sharp interfaces PLU« MPER & PUTNIS HYDROTHERMAL HISTORYOF GRANITIC ROCKS 971 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 along polysynthetic twinning (Fig. 4a) and, to a smaller extent, along intragranular micro-scale, albite-filled frac- tures that cut inconsistently through the oligoclase grains (Fig. 4d). Albitization along the polysynthetic twinning can be subdivided into different stages. The first stage is a crystallographically defined replacement of oligoclase by albite along several polysynthetic twins (Fig. 4a). In the second stage progressive replacement along the oligoclase twinning combines to form albitic patches (Fig. 4b and c). The well-defined oligoclase^albite replacement interface in the BSE images is also detectable as a sharp composi- tional change from An26Ab72 to An9Ab91 (Fig. 5). The albite is noticeably porous and associated with numerous crystals of fine-grained mica (sericite) thatare up to �40 mm in length (Fig. 6a). Epidote is also observed and is restricted to the albitized areas (Fig. 4a). Subsequent TEM investigations of the replaced areas show that the albite also contains scattered sub-micron inclusions of sericite (Fig. 6b). Thus, it is likely that the K content of albite determined by electron microprobe has been overesti- mated and therefore the Na content is underestimated (Fig. 5d). To estimate elemental gains and losses during the albiti- zation of oligoclase, sericite was also included in the min- eral analyses because it is intricately associated with the albite. To evaluate fluctuations in composition the Gresens’ equation for metasomatic changes was used (Gresens,1967; Grant,1986).This model evaluates composi- tional variations assuming constant volume, mass or the constant concentration of one component. Average repre- sentative electron microprobe analyses of oligoclase (olg) and albite (ab) (Table 2) were used to balance the reaction 100golgþ0�8gSiO2þ0�04gFeO þ0�5gNa2Oþ2�4gK2O ¼93gab ðincludingmsÞþ0�33gAl2O3þ4�1gCaO ð1Þ assuming constant volume and with sericite (ms) included in the albite. Transmission electron microscopy: focusing on sericite As illustrated by the back-scattered electron imaging (Fig. 6a), sericite grains are commonly associated with the porosity generated during albitization. Figure 6b shows Fig. 3. Occurrence of scattered microcline relicts (Or93) in highly porous oligoclase. (a) Optical photomicrograph under crossed polars giving an overview of the distribution of microcline relicts in a single oligoclase grain. (b, c) Magnified view of (a), to illustrate the crystallographic continuity of the microcline relicts by rotating the microscope stage (crossed polars). (d) Back-scattered electron image showing the contrast between the oligoclase grain and the irregularly shaped microcline relict in (a). Mc, microcline; Pl, plagioclase; Ab, albite; Ms, muscovite (ser- icite) (after Kretz, 1983). JOURNAL OF PETROLOGY VOLUME 50 NUMBER 5 MAY 2009 972 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 one example of sericite intimately related to an albite pore. However, subsequent detailed TEM investigations of the albite patches revealed a more complex association between albite and sericite that is dominated by a complex diffraction contrast. This contrast is extensive in the albi- tized patches as seen in Fig. 6c and d. The sericite grains appear blurred in the TEM images (Fig. 6c) as a result of the intricacy of the high contrast, but can be clearly identi- fied with EDX analysis. This makes it difficult to ascertain the nature of the association between the sericite and the albite pore. The origin of this contrast in the albite patches is unclear, as it could be dependent on lattice strain, which could be due to the presence of tiny sericite particles, from a high defect density or from elastic strain associated with the replacement process. Red granitoids K-feldspathization and associated Fe-oxide inclusions The most obvious alteration in the Simpevarp and Laxemar areas is the red staining located in the vicinity of fractures. To investigate this, several thin sections of the interface between the grey and red granitoids were prepared, as well as sections from the red granitoid itself for comparison. Subsequent transmitted light microscopy and BSE imaging of the red-stained oligoclase reveals that they have undergone intensive K-feldspathization (Fig. 7). Within a single oligoclase grain the formation of secondary K-feldspar is restricted to areas that have previously under- gone albitization and sericitization, leaving the remaining oligoclase unaffected (Fig. 7a and b). K-feldspathization appears to have pseudomorphically replaced the oligoclase where the albitization of oligoclase occurs almost through- out an entire original grain. Further high-magnification back-scattered (BSE) imaging of these areas revealed that the K-feldspar has replaced the sericite in a pseudomorphic manner (Fig. 7c). A complex texture has developed where the relationship between albite, sericite and K-feldspar is not clearly visible and the determination of parent and product phases is insufficient to fully interpret the replace- ment texture. The secondary K-feldspars contain numerous inclusions, resulting in a turbid appearance in thin section, which cannot be resolved by optical microscopy. At high magnifi- cation in back-scattered electron images these inclusions Table 1: Representative electron microprobe analyses of oligoclase, albite and microcline relicts in KLX04 and KSH03 Sample: KLX04 KSH03 Analysis no.: 10 73 71 72 45 50 65 67 66 81 10 84 Mineral: Olg Olg Ab Ab Mc relict Mc relict Olg Olg Ab Ab Mc relict Mc relict wt% SiO2 62�13 61�94 67�49 66�38 65�87 65�69 62�95 63�01 69�74 67�93 66�24 65�95 Al2O3 23�92 24�18 21�08 20�85 18�05 18�13 23�70 23�59 19�72 20�43 17�87 18�04 FeO 0�03 0�06 0�00 0�00 0�00 0�08 0�10 0�09 0�01 0�00 0�00 0�03 CaO 5�17 5�28 1�74 1�68 0�01 0�01 5�92 5�54 0�21 1�77 0�02 0�01 Na2O 8�02 7�97 9�84 10�26 0�35 0�77 8�27 8�31 11�88 10�55 0�70 0�61 K2O 0�16 0�08 0�10 0�09 16�04 15�19 0�17 0�19 0�18 0�11 15�68 15�38 Total 99�43 99�51 100�25 99�26 100�32 99�87 101�11 100�73 101�74 100�79 100�51 100�02 Structural formula based on 5 cations (a.p.f.u.) Si 2�76 2�75 2�94 2�93 3�02 3�02 2�76 2�77 2�99 2�95 3�03 3�03 Al 1�25 1�27 1�08 1�08 0�98 0�98 1�22 1�22 1�00 1�05 0�96 0�98 Fe 0�00 0�00 0�00 0�00 0�00 0�00 0�00 0�00 0�00 0�00 0�00 0�00 Na 0�69 0�69 0�83 0�88 0�03 0�07 0�70 0�71 0�99 0�89 0�06 0�05 Ca 0�25 0�25 0�08 0�08 0�00 0�00 0�28 0�26 0�01 0�08 0�00 0�00 K 0�01 0�00 0�00 0�00 0�94 0�89 0�01 0�01 0�01 0�00 0�92 0�90 mol% An 26�0 26�6 8�8 8�3 0�0 0�0 28�1 26�6 1�0 8�4 0�1 0�0 Ab 73�0 72�8 90�6 91�2 3�2 7�2 71�0 72�3 98�1 90�9 6�3 5�7 Kfs 1�0 0�5 0�6 0�5 96�8 92�8 0�9 1�1 1�0 0�6 93�6 94�3 Olg, oligoclase; Ab, albite; Mc, microcline. PLU« MPER & PUTNIS HYDROTHERMAL HISTORYOF GRANITIC ROCKS 973 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 directly associated with the replacement interface, seen as the brighter back-scattering spots (Fig. 7c), and were revealed as Fe oxides by EDX analysis. Raman spectros- copy can distinguish between different feldspars and thus was conducted on several secondary K-feldspars from both drillcores, identifying them as orthoclase (Fig. 8). The Raman spectra obtained show typical orthoclase modes along with a second order band at �1320 cm^1, which is characteristic of a second-order scattering by hematite. To further investigate the relationship between the orthoclase interface and the iron oxide inclusions, as well as to determine any microstructural relationships between the inclusions and the feldspar, transmission elec- tron microscopy samples were prepared from the optical thin sections. TEM imaging revealed open pores in the secondary orthoclase matrix of several hundred nano- metres in cross-section that contain numerous polycrystal- line inclusions of plate-like rosettes and needles (Fig. 9a). Electron diffraction patterns verify that the inclusions are hematite (Fig. 9b and c; see also Golla-Schindler et al., 2006; Putnis et al., 2007a), supporting the Raman spectra observations (Fig. 8). Interconnection and density estima- tions of the orthoclase pores were not possible for these samples, even though pores that are observable in a typical TEM field of view (several mm2) can be found in every thinned section. DISCUSS ION Although our initial aim was to gain a better understand- ing of the reddening phenomenon by investigating the changes in feldspar mineralogy between the red and grey granitoids, using the latter as a reference, we unexpectedly discovered that the grey granitoids themselves had already undergone multiple feldspar replacement reactions. Consequently, further work was undertakento obtain a better insight into the multiple metasomatic feldspar replacement reactions observed and the relative timings between them. In the following section, we discuss the feld- spar reactions in the sequence in which they occur, as indi- cated by the microstructural observations. Fig. 4. Back-scattered electron images of the microstructural development in oligoclase grains as albitization progresses along polysynthetic twinning and intragranular fractures. (a) The progressive albitization along several polysynthetic twins. The close association between albite, fine-grained mica (sericite) and epidote should be noted. (b) The beginning of albitic patch formation by expanding replacement along twin- ning, resulting in patches such as (c). (d) Intragranular fracturing can initiate further fluid infiltration, also resulting in albitization. Pl, plagio- clase; Ab, albite; Ms, muscovite (sericite); Ep, epidote (after Kretz, 1983). JOURNAL OF PETROLOGY VOLUME 50 NUMBER 5 MAY 2009 974 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 Feldspar replacement reactions Searching for the protolith: relict microcline in porous oligoclase To understand the origin of the crystallographically con- tinuous microcline relicts found in the highly porous oligo- clase three possible scenarios need to be taken into consideration: (1) the microcline relicts represent mag- matic mineral inclusions; (2) the islands are post-magmatic exsolution products from oligoclase grains; or (3) the growth of oligoclase is at the expense of microcline, leav- ing intragranular relicts of pristine microcline behind. First, crystallographically continuous inclusions scat- tered throughout a single grain are not likely to form from melts because communication between nuclei during crystallization in intragranular pores is limited and would not produce the texture observed. It is expected that microcline crystallized from a melt would preferentially remain at three-grain junctions with melt^solid dihedral angles of less than 608 (Holness, 2006). This has recently been observed for microcline in migmatites, which crystal- lizes from melt films at plagioclase grain boundaries and not within single grains (Holness & Sawyer, 2008). Fig. 5. (a) Back-scattered electron image of an oligoclase^albite intracrystalline replacement interface chosen for element distribution mapping via electron probe micro-analysis and for elemental traverse analysis. Element distribution maps of (b) Na and (c) Ca of the reaction interface. It should be noted that the transition from the parent oligoclase to the product albite is sharp on the micrometre scale. Bright spots in (c) are epidote grains. (d) Chemical profile along a traverse, dashed white line in (a), showing the abrupt change in Ca and Na content at the oligo- clase^albite interface, acquired by spot analyses with a focused beam. All elemental data are displayed as mole percent. The slight increase in K content beginning at the intracrystalline oligoclase^albite interface proceeding into albite should be noted. It is obvious that the K content determined by electron microprobe has been overestimated, and therefore the Na content (albite) underestimated, as a result of scattered sub- micron inclusions of sericite. Pl, plagioclase; Ab, albite; Ms, muscovite (sericite) (after Kretz, 1983). PLU« MPER & PUTNIS HYDROTHERMAL HISTORYOF GRANITIC ROCKS 975 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 In contrast, in the rocks from this study, the inclusions are intragranular with random shapes and sizes. In addition, a slow crystallizing melt that forms microcline would have reacted with the surrounding mineral producing reaction rims; however, BSE imaging has shown no evi- dence of reaction with the oligoclase grains (Fig. 3d). An origin by solid-state post-magmatic exsolution would create a better defined textural relationship between the distribution of the microcline islands and the host oligo- clase, controlled by diffusion distances and strain distribu- tion [see, for example, exsolution textures in alkali feldspars (Brown & Parsons, 1988)]. Instead, the distribu- tion of microcline inclusions within the oligoclase is essen- tially random. The high porosity of the oligoclase grains compared with the porosity-free microcline also argues against a con- temporaneous origin. Previously reported porosity distri- butions in plagioclase grains, ranging from 1 to 2% by volume, are thought to represent former fluid inclusions Fig. 6. (a) Back-scattered electron image showing the contrast between albite and sericite as well as the close association of the latter with albite porosity.The black spots in (a) are pores. (b^d) Bright-fieldTEM images: (b) the detailed microstructure of two sericite (ms) grains in different orientations in direct association with an open albite pore; (c) the complex image contrast at the interface between sericite and albite; (d) moire¤ fringes showing the position of sericite grains. [Note the lack of any sharp interface between sericite and albite in (b)^(d).] Ab, albite; Ms, muscovite (sericite) (after Kretz, 1983). Table 2: Representative Gresens analysis of the albitization of oligoclase for sample KLX04 assuming constant volume Olg Ab þ Ms G/L average average (n¼ 7) (n¼ 5) wt% SiO2 61�90 62�70 0�83 Al2O3 23�95 23�62 �0�33 FeO 0�08 0�04 �0�04 CaO 5�52 1�57 �4�08 Na2O 7�98 8�48 0�51 K2O 0�15 2�48 2�40 Density r 2�65 2�73 G/L, gain/loss; Olg, oligoclase; Ab þ Ms, albite including sericite. JOURNAL OF PETROLOGY VOLUME 50 NUMBER 5 MAY 2009 976 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 or cleavages (Montgomery & Brace,1975). However, Smith & Brown (1988) suggested that most plagioclase pores are of secondary origin and are generated by deuteric or hydrothermal alteration (Parsons, 1978). The characteristi- cally high porosity of up to 3% in the oligoclase grains and the crystallographic continuity of relict microcline suggest that the oligoclase crystallized at the expense of the microcline. Experiments recently conducted by Niedermeier et al. (2009) examined the production of orthoclase via the re-equilibration of albite in a KCl- bearing hydrothermal fluid and have shown that the transformation is consistent with an interface-coupled dissolution^reprecipitation mechanism (Putnis & Putnis, 2007b). Investigations on the natural albitization of plagio- clase from the Bamble Sector in South Norway also support the re-equilibration of feldspars in the presence of a fluid phase via this process (Engvik et al., 2008). These observations suggest that the replacement of microcline by oligoclase could also occur by this mechan- ism, especially as the generation of porosity is an impor- tant consequence of this process (Putnis, 2002; Putnis & Putnis, 2007b). Porosity is generated whenever the vol- ume of the parent mineral dissolved is less than that of the volume reprecipitated. This volume change is depen- dent not only on differences in molar volume between the solid phases, but also on their relative solubilities. Thus it is possible to generate porosity even when the molar volume of the product solid is higher than that of the parent (Putnis et al., 2005). An increase in porosity would be expected upon the dissolution of microcline and repreci- pitation of oligoclase as the molar volume decreases by �8%. Fig. 7. Back-scattered electron images of an oligoclase grain in the red-stained granitoid showing the complex relationship that develops during the replacement of sericite by orthoclase. (a) Overview image illustrating the restriction of K-feldspathization to areas previously albitized and sericitized. The two relict microclines (both crystallographically continuous) in the lower left part of the oligoclase grain shouldbe noted. (b) Close-up of the area affected by K-feldspathization in (a). (c) A high-magnification image [outlined area in (b)] reveals the pseudomorphic replacement of sericite by orthoclase.The two Fe oxide inclusions (brighter back-scattering spots in the white ellipse), which form at the reaction interface between sericite and orthoclase and become incorporated into the growing orthoclase, should be noted. Further Fe oxide inclusions are found in the lower left and upper right corners. The black spots are pores. Pl, plagioclase; Mc, microcline; Ab, albite; Ms, muscovite (seri- cite); Ep, epidote; Or, orthoclase (after Kretz, 1983). PLU« MPER & PUTNIS HYDROTHERMAL HISTORYOF GRANITIC ROCKS 977 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 Fig. 9. (a) A typical TEM image of polycrystalline hematite needles in an open pore within the red-stained granitoid orthoclase (Or). (b) The electron diffraction pattern can be clearly indexed as hematite in (c), using the EMS software package (Stadelmann,1987). Fig. 8. (a) Raman spectra of hematite-bearing orthoclase with hematite second-order scattering mode at �1320 cm^1. The phases can be iden- tified through comparison with the reference spectra (b) and (c), from the RRUFF Project (Downs, 2006). (d) Raman spectra of a microcline relict shown in Fig. 3, which can be distinguished unambiguously from orthoclase because of the characteristic band positions and the evolution of the peak triplet between 450 and 520 cm^1 with increasing Al^Si order (Freeman et al., 2008). JOURNAL OF PETROLOGY VOLUME 50 NUMBER 5 MAY 2009 978 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 The microcline^oligoclase transformation observed in trondhjemite from Alabama, USA (Drummond et al., 1986) and Sicily, Italy (Fiannacca et al., 2005, 2008) points to a scenario where metasomatism and fluid flow cause the almost entire replacement of microcline by oligoclase. In such a situation the replacement may result in an alkali ion exchange process, as suggested by Orville (1963), where Kþmoves to low-temperature and Naþ to high-tem- perature regions. Therefore, fluids moving in the initial stages of a cooling granitic complex will tend to enrich feldspars in Naþ and transfer their Kþ to the fluid. From these arguments we propose that microcline was replaced by oligoclase in the early high-temperature stages of alteration. Oligoclase^albite transformation and the role of sericite Albitization is a well-known feldspar replacement phe- nomenon that has been described in a range of rock types from various geological settings. Numerous experimental investigations on the replacement of one feldspar by another in the presence of a fluid phase illustrate that these processes are rapid even on a laboratory timescale (e.g. O’Neil & Taylor, 1967; Labotka et al., 2004; Niedermeier et al., 2009; Ho« velmann et al., in preparation). Therefore, whenever feldspars in rocks come into contact with hydrothermal fluids it is virtually inevitable that such re-equilibration reactions will take place. However, the contemporaneous appearance of sericite with albite has been almost completely neglected even though its formation is one of the most common types of hydrothermal alteration found in granitic rocks (Que & Allen, 1996). Further progress in understanding the rela- tionship between the formation of albite and associated sericite requires the identification of textural relationships, the pathways taken by the fluid and its chemical evolution with regard to input and output fluxes, as well as the influ- ence of the fluid on the origin of sericite. The first step towards these aims is to identify the microstructural evolu- tion of the albitization and associated formation of sericite, as it provides important information about the mechanism of these processes. The most obvious textural relationship is the close asso- ciation of sericite with the albite porosity. To understand the process by which this texture arises, the transformation of oligoclase to albite needs to be examined in greater detail. The replacement of oligoclase by albite illustrates the most important characteristics of fluid-induced min- eral replacement reactions: (1) a sharp replacement interface, even on the micro- metre scale, as illustrated by BSE images (Figs 4 and 5a) and element distribution maps (Fig. 5b and c), shows no evidence for significant solid-state diffusion between parent and product phases; (2) porosity development in the reaction product; (3) preservation of the original external volume occupied by the oligoclase crystals and their crystallographic orientation; (4) additional phases, in this case sericite and epidote, co-precipitate during the albitization reaction. The observation of the features described above, in con- junction with the results of recent investigations into the albitization of metagranitic rocks (Sandstro« m et al., 2009) and tonalites (Engvik et al., 2008), indicates that this pro- cess can be ascribed to an interface-coupled dissolution^ reprecipitation mechanism (Putnis & Putnis, 2007b). Generating porosity is a key feature of this mechanism, as it allows the fluid to infiltrate the product phase and access the reaction interface. The newly generated pores also provide sites for the nucleation and growth of co-preci- pitates. For this to arise the pores must be interconnected during the time of reaction. SEM and TEM observations were not able to provide further information about pore connectivity in the present samples, but observations in analogue experiments involving salt systems reveal that porosity produced during mineral replacement is coar- sened and eventually eliminated by textural equilibration, which follows the chemical equilibration in the mineral (Putnis et al., 2005). Further detailed TEM investigations of the relationship between sericite and albite show that it is more complex (Fig. 6c and d) than was observed with previous analytical techniques. The distinct association of sericite with albite pores in the BSE images (Fig. 6a) is no longer clear inTEM images because of the unusual diffrac- tion contrast in the albite, which is probably due to disloca- tions, a high point defect population and elastic strain in this area. This unusual strain contrast is also evident in natural albitized oligoclase from tonalites (Engvik et al., 2008) and has been recently observed to arise in experi- mental investigations on the albitization of plagioclase feldspars (Ho« velmann et al., in preparation). However, areas where sericite can be clearly attributed to open albite nanopores were identified (Fig. 6b and c). To gain additional insight into the interlinked evolution of albite and sericite an investigation into element mobili- ties during replacement reactions and changes in fluid activity was undertaken, which included the consequences for mineral stability. One method of determining quantita- tive element mobility is by evaluating mass transfer on the basis of gains and losses (Gresens, 1967; Grant, 1986). This can be performed in an isocon diagram (Grant, 1986), which represents a graphical solution to the original com- position^volume relationships in metasomatism as out- lined by Gresens (1967). In a Gresens diagram, concentrations of components in the altered mineral or rock are displayed versus those in the original mineral or rock. The actual concentrations of these components in the altered mineral are then compared with those calcu- lated from the assumptions of constant volume, constant PLU« MPER & PUTNIS HYDROTHERMAL HISTORYOF GRANITIC ROCKS 979 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 mass or constant concentration of one component (here we use Al). From the displacement of the data points relative gains and lossescan be obtained. In this study all three of the above-mentioned possibilities are shown in Fig. 10. The presence of sericite inclusions in close association with albite indicates that Al3þ may be released from the oligoclase, as suggested by Leichmann et al. (2003). Al3þ mobility during metasomatism has also been documented by Nijland & Touret (2001). As albitization is the dominant alteration process it is reasonable to assess element mass transfer by comparing the formation of albite both accounting for and disregarding sericitization (Fig. 10). Under the assumption of constant volume Al3þ appears to be immobile when sericite is accounted for, but is revealed as mobile when it is neglected. This implies that the Al3þ released from the oligoclase during albitization was incor- porated into the associated sericite. Furthermore, it sup- ports a cogenetic evolution of albite and its inclusions, as it is consistent with the textural relationship observed in BSE andTEM images (Fig. 6). As the albitization of oligo- clase (An20) involves a fluid phase, which introduces Na þ and Si4þ and releases mobile Al3þ and Ca2þ, the following reaction is an appropriate description of this alteration: Na0�8Ca0�2Al1�2Si2�8O8 ðolgÞ þ 0�2Na þ ðaqÞ þ0�2H4SiO4 ðaqÞ ¼ NaAlSi3O8 ðabÞ þ 0�2Ca 2þ ðaqÞ þ0�2Al3 þ ðaqÞ þ 0�8OH�ðaqÞ: ð2Þ Reaction (2) assumes that silica was supplied by the pore fluid in the form of Si(OH)4 (Merino, 1975), rather than from quartz. This is further supported by the observation that quartz grains in all samples have not been heavily dis- solved or deformed during the hydrothermal alteration. Assuming constant volume, Gresens analysis of albitized parts (including sericite) and oligoclase indicates that per 100 g of oligoclase, 4�1g of CaO is released. This and other components that are released during the associated break- down of biotite and hornblende are incorporated into the epidote that coexists with the albitized sections of oligoclase. Even if all of the Al3þ released from oligoclase was incorporated into ideal sericite KAl2AlSi3O10(OH)2 there would be insufficient K to produce the amount found in these samples. This discrepancy is also evident in the K content of the albite^sericite intergrowths by comparing the chemical analyses given inTables 1 and 2, as well as in the net gains and losses of the oligoclase vs albite^sericite intergrowths (Fig. 10). A more feasible source for the K concentration required to initiate sericitization in the oli- goclase has been rarely discussed. Engvik et al. (2008) sug- gested that the externally derived fluids containing the Naþ needed for albitization could also bring in Kþ. The activities of Kþ (aKþ) and Na þ (aNaþ) in this fluid have important effects on the process of albitization and the associated sericitization, as well as concurrent repercussions on the albite and sericite mineral stabilities. Temperature and pressure, which are key parameters con- trolling these alterations (Hemley & Jones, 1964), also influence activities. Using SUPCRT92 (Johnson et al., 1992) we calculated equilibrium constants to construct an activity diagram that relates fluid composition to albite, sericite and K-feldspar in equilibrium at 200MPa and var- ious temperatures (Fig. 11a). Using this diagram we can follow the changes in fluid composition during the feldspar replacement reactions. The removal of Naþ from the fluid during the albitization of oligoclase would be expected to locally decrease the aNaþ. Furthermore, the incomplete replacement reaction to pure albite (An0), approximately An9 in all analysed albite patches, points to a limited Na reservoir. As the pore volumes in albite are small, we assume that local equilibrium is reached within the pores. Therefore, fluid remaining in the albite pores shortly after the reaction would be low enough in aNaþ to enter the seri- cite stability field (Fig. 11a; arrow 1). Sericite would then precipitate because the aKþ in the externally derived fluid is also expected to be in the stability field of sericite. This is also supported by the observation that with decreasing temperature the stability field of albite shrinks, favouring sericitization. Textural and chemical observations, as well as theoreti- cal considerations based on thermodynamic equilibrium modeling, suggest that albitization and sericitization are directly linked to each other and develop as cogenetic alteration products. K-feldspathization and reddening around fractures The phenomenon of reddening in the vicinity of fractures in the granitoid is directly related to the K-feldspathization of sericite (Fig. 7c). The formation of secondary orthoclase is favoured by high aKþ=aHþ in the fluid (Fig. 11a; arrow 2), which implies that conditions of high aKþ or pH are required. Metasomatic replacement of sericite was partly accomplished by the following reaction: KAl2AlSi3O10ðOHÞ2 ðmsÞþ6SiO2 ðqtzÞþ2KClðaqÞ ¼ 3KAlSi3O8 ðorÞþ2HClðaqÞ: ð3Þ Fracturing would produce a pressure change from litho- static to hydrostatic pressure, which also favours the feld- spar side of the equilibrium and the production of HCl. This effect is evident from the calculated equilibrium con- stant increase for the sericite^orthoclase reaction at 3008C by approximately one and a half orders of magnitude when a drop in pressure from 200 to 70MPa is considered (Fig. 11b). The production of the red staining itself is caused by hematite precipitation during the K-feldspathization (Figs 7c and 9a). The observed microtexture of hematite needle JOURNAL OF PETROLOGY VOLUME 50 NUMBER 5 MAY 2009 980 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 formation in orthoclase pores could not be produced by the solid-state exsolution of hematite from a feldspar matrix, nor is it consistent with the argument of co-crystal- lization from a magma.The presence of hematite in associ- ation with the replacement interface between sericite and orthoclase, as well as in pores throughout the orthoclase grains, indicates that there is a direct relationship between the replacement reaction and the formation of hematite. Putnis et al. (2007a) recently showed that hematite rosettes and needles formed in pores at the reaction interface between K-feldspar and plagioclase, with no evidence of exsolution or co-crystallization processes. The formation of hematite within pores in alkali feldspars is consistent with a subsolidus replacement origin, where hematite pre- cipitation is a coproduct dependent on the Fe content of the fluid. The question arises about the location of the Fe Fig. 10. Isocon diagram after Grant (1986), showing net gains and losses of oligoclase vs albite, (a) disregarding and (b) accounting for sericite. It should be noted that Al is mobile in (a) when only albite is considered under the conditions of constant volume or mass, but appears to remain constant in (b) when sericite is accounted for with albite. PLU« MPER & PUTNIS HYDROTHERMAL HISTORYOF GRANITIC ROCKS 981 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 source for hematite formation. Three different origins can be examined: (1) an Fe source exists within the rock (e.g. biotite and hornblende breakdown); (2) the infiltrating hydrothermal fluid responsible for secondary feldspar for- mation also contains significant amounts of Fe; (3) re-equi- libration of a feldspar containing Fe in solid solution (Fe3þ substituted for Al3þ in the tetrahedral sites) releases Fe into solution to precipitate as hematite in pores linked with the feldspar replacement process (Nakano et al., 2005). The last option is not likely to be applicable in the investigated rocks as the albite and sericite do not contain enough Fe for the extensive hematite precipitation observed (Table 1). Drake et al. (2008) conducted Mo« ssbauer spectroscopy to determine changes in the Fe3þ/ Fetot ratio betweenthe red-stained and grey granitoids, and showed that the total amount of Fe remains very simi- lar, indicating an in situ source. Therefore, the Fe source probably originates from the chloritization of biotite and the breakdown of hornblende, which are observed in con- junction with the feldspar replacement reactions. Hydrothermal fluids Fluid system and origin An evaluation of the fluid system is crucial in any discus- sion of major element metasomatism caused by fluid flow because the extent of metasomatism is directly related to compositional changes in the fluid along the flow pathway. Fluid composition may vary along the flow path, even in a homogeneous rock, because the composition of a fluid in equilibrium with a fixed mineral assemblage varies with temperature and pressure (Dipple & Ferry, 1992). Despite the extensive research into fluid composition based on fluid inclusion analyses, drill-hole sampling (Touret, 2001; Yardley & Graham, 2002; Yardley et al., 2003) and fluid^rock interaction models (Reed, 1982; Dolejs & Wagner, 2008), which are often limited to a restricted range of compositions, our understanding of the fluid chemistry that drives fluid^mineral reactions in the Earth’s crust is incomplete. This is also the case for the granitic rocks studied here. Hence, we have made a quali- tative prediction of the fluid composition by extrapolating information about the fluid from comparisons of mineral reaction products and their parent phases using composi- tional as well as textural changes. Overall, the observed complex metasomatism does not require the infiltration of a chemically exotic fluid, but also cannot be easily explained by a single fluid infiltration event. A sequential infiltration of multicomponent aqueous fluids consti- tutes a more likely explanation for the several feldspar replacement reactions found in these plutonic rocks. Fig. 11. (a) Activity^activity diagram showing the stability of albite, sericite and orthoclase at various temperatures and 200MPa. The dia- gram was constructed using equilibrium constants calculated by the SUPCRT92 program (Johnson et al., 1992). Inset (b) illustrates the effect of pressure on the replacement reaction of sericite by orthoclase where the equilibrium constant K of reaction (3) gradually increases with decreas- ing pressure within the range of a lithostatic to hydrostatic pressure drop. JOURNAL OF PETROLOGY VOLUME 50 NUMBER 5 MAY 2009 982 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 The introduction of a single fluid containing sufficient Ca, Na and K to drive the replacement of microcline by oligo- clase and subsequently transform the oligoclase to albite and sericite is questionable. Furthermore, the fracturing and K-feldspathization observed, especially the presence of the red hematite precipitate, point to the possibility of sequential infiltration events. Variability in the degree of sericitization throughout the granitoids suggests that fluid/ rock ratios may have varied geographically within the plu- tonic complex, or even on the mesoscopic and microscopic scale. This would be dependent on the degree of fluid access. Combining mineral chemical analyses and textural observations for different mechanisms of fluid infiltration, discussed below, we suggest a scenario of sequential fluid infiltration events. However, the origin of the fluids caus- ing the hydrothermal alteration is at present unknown. Drake et al. (2008) suggested that, on the basis of wall- rock and fracture dating as well as similarities in element concentration, the majority of the red staining in the area is related to fluid circulation during the emplacement of the nearby Go« temar and Uthammar granites. It is uncer- tain whether the two intrusions are initiators of the entire sequence of feldspar replacement reactions observed or if they play a key role in only the reddening event. To unravel the history of the fluid causing the feldspar replacement reactions, further detailed studies, especially of fluid inclusions, are needed. For example, Nijland & Touret (2001) successfully used fluid inclusions associated with the replacement of graphic pegmatite by a graphic albite^actinolite^clinopyroxene intergrowth from Mj�vatn, southern Norway to gain insight into the fluid composition and temperature. Although during the pres- ent study no fluid inclusions were identified, the high porosity and microfracturing observed does not preclude their presence. Furthermore, fluid inclusions in quartz can provide valuable information about feldspar alteration when they can be correlated with adjacent feldspars (Fig. 3a, large quartz grain at the lower left side). Combined with recent achievements in oxygen isotope analysis by in situ ion-microprobe techniques (Cole et al., 2004; Bowman et al., 2009), systematically obtained fluid inclusion analysis could be used to gain detailed informa- tion about the fluid regime and temperature path of hydro- thermal alteration when applied to granitic feldspar replacement reactions. It will therefore be of great interest to conduct fluid inclusion and oxygen isotope analyses in future studies. Mechanism of fluid infiltration The intensive reddening in the vicinity of fractures indi- cates that such fractures can act as channels for penetrat- ing hydrothermal fluids on the outcrop scale. This staining is not only restricted to these areas, but is also found between the mesoscopic fractures (Wahlgren et al., 2004). Fracturing is a well-established mechanism for fluid infiltration in low-permeability rocks both in deep crustal (Austrheim, 1987; Jamtveit et al., 1990) and shallow crustal settings (Bons, 2001; Engvik et al., 2005). Furthermore, fluid infiltration aided by fracturing on the micrometre scale has been demonstrated by several workers (Fitz Gerald & Stunitz, 1993; Oliver, 1996; Engvik et al., 2001; Jamtveit et al., 2009). Microtextures observed in the grani- toids from the Laxemar and Simpevarp areas emphasize the importance of fluid infiltration along microcracks as one way of providing fluid for mineral replacement reac- tions. This is illustrated by the transformation of oligoclase to albite along intragranular microcracks (Fig. 4d) and the observation that K-feldspathization and pale reddening are located within the grey granitoid away from the origi- nal macroscopic fractures and their intense staining. However, the introduction of fluids via microcracks alone cannot be the only mechanism that results in an almost completely replaced oligoclase grain. More pervasive fluid infiltration is provided by the instantaneously gener- ated porosity that is a crucial consequence of reactions governed by an interface-coupled dissolution^reprecipita- tion mechanism. Pre-existing high porosity in oligoclase from the replacement of microcline also ensures that con- tact is established between the reactive fluid and the parent phase. Selective albitization along single polysyn- thetic twins in the oligoclase crystals (Fig. 4a and b) sug- gests that the crystallographic orientation of twinning controls the mineral reaction and acts as another pathway for fluid infiltration into the grains. Continuous fluid infil- tration along the twinning is likely to form isolated albite patches, as in Fig. 4c. When considered in three dimen- sions, these patches may interconnect to others several tens of micrometres away, leading to the formation of larger albitized zones. Engvik et al. (2008) have also observed this selective replacement along polysynthetic twinning in TEM investigations of oligoclase grains and showed that it is linked to complex diffraction contrast caused by dislocation formation. Dislocation structures induced by deformation increase the rate of mineral reac- tions (White, 1975), producing the selective albitization observed. Although separately these processes would not pro- vide enough fluid for the degree of replacement observed,together microcracking, porosity generation and crystallographically controlled infiltration along twin- ning produce a pervasive mechanism for the introduction of fluids at the micrometre scale and ensure that the con- tact between the reactive fluid and parent mineral is main- tained. Furthermore, macroscopic fracturing, such as that related to the reddening, ensures further fluid infiltration via fracture flow. The combination of pervasive and frac- ture flow acted as the overall mechanism for fluid infiltration. PLU« MPER & PUTNIS HYDROTHERMAL HISTORYOF GRANITIC ROCKS 983 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 SUMMARY AND CONCLUSIONS Based on the observed replacement textures and their con- secutive relationships, we can summarize the observed hydrothermal alteration using the following proposed evo- lutionary sequence (Fig. 12). (1) Based on the microtextural observations made in this study we propose the following scenario for the for- mation of relict microcline in oligoclase. The granitic intrusion, which crystallized microcline as the domi- nant alkali feldspar, released fluids during a late stage of magmatic crystallization, which initiated the replacement of pristine microcline by oligoclase. The original protolith is observed in the studied samples as crystallographically continuous microcline relicts within highly porous oligoclase grains. However, fur- ther work is needed to fully unravel the origin of the relict microcline. (2) An externally derived Na-enriched hydrothermal fluid established the progressive albitization of oligo- clase. The contemporaneous association of sericite within albite pores indicates a requirement for further K introduction and is linked to both albite stability and aNaþ in the fluid. This intricate relationship also implies that albite and sericite are of cogenetic origin. (3) Fracturing and the infiltration of a K-bearing fluid caused the K-feldspathization of sericite and hematite precipitation in orthoclase pores, producing the red coloration. Observed K-feldspathization is restricted to previously albitized and sericitized areas and hence occurred after the albitization process. In addi- tion, it is evident that although the red coloration observed here is clearly associated with fractures, it is not necessarily restricted to them. It is likely that the reddening is a more pervasive phenomenon of feld- spar replacement reactions in the presence of a fluid throughout a granitic pluton. A direct relationship between red feldspars in granitic rocks and hydrother- mal activity has been suggested before, based on field observations (Boone, 1969). This is also consistent with low d18O values observed in many Precambrian brick red granitic rocks, which are indicative of subso- lidus re-equilibration with meteoric^hydrothermal fluids (Taylor, 1977; Hoefs & Emmermann, 1983; Fiebig & Hoefs, 2002). Consistently throughout this evolutionary sequence porosity generation and the mechanism of dissolution and reprecipitation, as described by Putnis et al. (2002), play crucial roles in the re-equilibration of granitic feldspars in the presence of a fluid phase. The present study has provided new microtextural and chemical insights into the subsolidus re-equilibration of granitic rocks with hydrothermal fluids, with particular emphasis on feldspar replacement reactions and their sequential occurrence along the hydrothermal alteration path of granitic rocks. This hydrothermal history is cer- tainly complex and in many aspects a puzzling process. Further work is required to gain a full understanding of this history, with particular emphasis on the absolute timing of the overall replacement and each of the feldspar replacement stages (i.e. albitization, K-feldspathization) as well as their corresponding fluid chemistry. Although our understanding of the mineralogical and geochemical changes during the re-equilibration of grani- tic rocks is restricted to the investigated area, it is a logical Fig. 12. Evolutionary sequence of multiple feldspar replacement reactions summarizing the observed hydrothermal alteration of the Simpevarp and Laxemar granitoids. The primordial high-temperature stage of the alteration caused the replacement of microcline by oligoclase, leaving intragranular, crystallographically continuous relict microcline behind. Hydrothermally derived fluid established the subsequent replacement of oligoclase by albite and contemporaneously associated sericite precipitation in albite pores. Later fracturing and fluid infiltration caused the K-feldspathization of sericite, and hematite precipitation in orthoclase pores resulted in reddening. JOURNAL OF PETROLOGY VOLUME 50 NUMBER 5 MAY 2009 984 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 progression that hydrothermal fluids could affect larger volumes of the Earth’s crust than is currently acknowl- edged. Furthermore, fluid inclusions within feldspar minerals in the upper crust have been calculated to con- tain the same amount of water as is incorporated in all hydrous minerals (Johnson & Rossman, 2004). Consequently, the interactions of gigantic hydrothermal systems with crustal rocks should be taken into closer con- sideration in future studies of granitic rocks and their pet- rological history. ACKNOWLEDGEMENTS We are grateful to H. Drake (Go« teborg) for providing the drillcore samples, and H. Austrheim for discussion. A. Janssen and M. MacKenzie are thanked for help with the transmission electron microscope, J. Berndt for assis- tance in microprobe and mass spectrometry analyses, and M. Menneken for operating the Raman spectrometer. Helpful comments from H. King improved and clarified the manuscript. Furthermore, the manuscript benefited from valuable comments and suggestions given by the reviewers D. E. Harlov, C. C. Lundstrom and J. L. R. Touret. 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