Lewisian Complex Of Northwest Scotland Research Articles

Mixed metamorphic and fluid graphite deposition in Palaeoproterozoic supracrustal rocks of the Lewisian Complex, NW Scotland

AbstractGraphite deposits may form alternatively by metamorphism of sedimentary rocks and from fluids. Both types occur in supracrustal successions within the Lewisian Complex of Northwest Scotland, and similarly in Palaeoproterozoic supracrustal rocks across the North Atlantic region in Canada, Greenland and Scandinavia. Carbon isotope compositions show that the graphite in Scotland had a mixed origin from metamorphism of sedimentary organic matter (schists) and the decarbonation of limestones (marbles). Raman spectroscopy shows that most of the graphite in Scotland exhibits some structural disorder, unlike the complete order in graphite vein ore deposits across the region. Exceptionally, where graphite was precipitated from fluid, in albitized rock in Tiree and Scardroy, it is fully ordered. While organic matter may survive granulite facies metamorphism without being transformed to fully ordered graphite, it can yield commercially more valuable ordered graphite when mobilized in a fluid.

U-Pb isotopic evidence for the accretion of different crustal blocks to form the Lewisian Complex of northwest Scotland

Single zircon and titanite U-Pb SHRIMP data presented for tonalite-trondhjemite-granodiorite (TTG) suite gneisses and an ultramafic rock from the northern and central regions of the Lewisian Complex of northwest Scotland, show that protolith ages of tonalitic gneisses in the northern region (2800–2840 Ma) are significantly younger than those in the central region (2960–3030 Ma). Further evidence of a major (2490–2480 Ma) metamorphic event in the central region is documented by a metamorphic zircon associated with a granulite facies ultramafic body. A dioritic gneiss from the northern region has also been dated at c. 2680 Ma. The northern region therefore does not comprise reworked central region rocks and consequently the old models for the evolution of the Lewisian which were based upon this concept need replacing. It is instead proposed that two distinct crustal blocks, now the northern and central regions, were tectonically juxtaposed along a boundary corresponding to the Laxford Front. Juxtaposition would appear to have occurred in Proterozoic times, as it must have postdated the 2490–2480 Ma (?Inverian) metamorphism recorded only in the central region, and the emplacement of granite sheets restricted to the northern side of the boundary. The first recorded event common to both regions is resetting of titanite ages associated with c. 1750 Ma Laxfordian amphibolite facies metamorphism. Zircon inheritance in rocks of both regions is scarce. Within one zircon from the northern region a c. 3550 Ma core was found. This represents the oldest known material from the region.

Isotope systematics of Precambrian marbles from the Lewisian complex of northwest Scotland: implications for PbPb dating of metamorphosed carbonates

We present Pb-isotopic data from the late-Archaean Lewisian marbles of Tiree, Iona, and Glenelg, each locality representing a different tectonic history since the mid-Proterozoic. Data from Tiree (unaffected Precambrian foreland to the Caledonian) yield an age of 1656 ± 23 Ma which is within error of broadly defined Laxfordian reworking events, both locally and throughout the Lewisian. Nd-isotopic data confirm a late-Archaean protolith, although low model μ 1 values require considerably lower pre-Laxfordian U Pb ratios. An age pf 1418 ± 56 Ma is obtained from the Iona marble (? foreland, buried beneath Caledonian nappes), again with a low μ 1 and negative ϵ Nd( t). This age is too young for the Laxfordian and might indicate some disturbance of UPb systematics during Caledonian events. No meaningful age data are obtained from the Glenelg marbles (Lewisian inlier in Caledonian thrust sheet) where Grenvillian (∼ 1.1 Ga) eclogite metamorphism preceded Caledonian events and Pb-isotopic compositions appear not to have been homogenised. Leaching experiments for the Tiree and Iona marbles reveal that radiogenic Pb is hosted in silicate phases, with unradiogenic Pb in the carbonates, a reversal of the situation commonly observed in sedimentary carbonates. If a sedimentary precursor is assumed, redistribution of U and Pb probably occurred during high-grade metamorphism recorded by the age regression, accompanied by significant UPb fractionation to yield higher post-metamorphic U Pb ratios. The significance of non-zero Th U ratios (between ∼ 1 and ∼ 3, post-Laxfordian) remains unclear, but may not have any bearing upon the age or nature of the protolith. Our study provides further evidence that Pb-isotopic studies of marbles can yield reliable geochronological information on metamorphic events in the absence of later tectono-metamorphic overprinting, and is therefore a potentially useful technique for reconnaissance geochronology in metasupracrustal belts. However, caution should be exercised in applying this technique to terrains with complex histories.

Conflicting mineral and whole-rock isochron ages from the Late-Archaean Lewisian complex of northwestern Scotland: Implications for geochronology in polymetamorphic high-grade terrains

We report major and trace element (including rare-earth element, REE) geochemistry and whole-rock Sm Nd geochronology on two suites of mafic/ultramafic inclusions in an agmatite complex within a trondhjemitic host at Gruinard Bay, in the Late Archaean Lewisian Complex of northwest Scotland. A suite of medium-grained equigranular amphibolites (main amphibolite suite, MAS) has broadly chondritic high field strength element (HFSE) ratios (Ti/Zr ca. 115, Ti/Y ca. 318, Y/Cr ca. 0.39), flat heavy REE patterns at five to fifteen times chondrites, usually with mild light REE enrichment (La N < 50) and small Eu anomalies. This is consistent with an origin as basaltic liquids which may represent suitable mafic precursors for tonalite-trondhjemite-granodiorite (TTG) suite magmas, as argued previously by Rollinson and Fowler (1987). A Sm Nd regression line (MSWD = 3.4) obtained from the MAS yields an age of 2.943 ± 0.091 Ga with an initial ε Nd value of +2.69 ± 0.62. A suite of coarser grained hornblendites and metagabbros (hornblendite-metagabbro suite, HMS) displays significant geochemical differences from the MAS, exemplified by significantly light REE-enriched patterns in which the hornblendites have the highest total REE with La N/Nd N< 1 and large negative Eu anomalies, and the most felsic metagabbros have low total REE and large positive Eu anomalies. Samples from the HMS yield a Sm Nd isochron (MSWD = 0.52) age of 2.846 ± 0.073 Ga with an initial ε Nd value of +1.10 ± 0.72. The HMS geochemistry is consistent with an origin as amphibole and plagioclase cumulates to the local TTG magmas. Combining the new data with previously published data from the host trondhjemites at Gruinard Bay (Whitehouse, 1989a) yields a Sm Nd isochron (MSWD = 1.09) age of 2.795 ± 0.028 Ga with an initial ε Nd value of +0.66 ± 0.38. The relatively low initial ε Nd values for the HMS and combined HMS-trondhjemite isochrons compared to contemporaneous depleted mantle (ε Nd(2.8) ca. + 2.1 to + 3.3) is consistent with derivation of a parental magma from light-REE-enriched components of a mafic (basaltic) precursor which was itself derived from a depleted mantle source approximately 100–200 Ma previously. The MAS suite at Gruinard Bay may be representative of such a mafic precursor. Our new geochronological data differ significantly from the ca. 3.3 Ga mineral Sm Nd and Pb Pb regression ages presented recently for two members of the HMS by Burton et al. (1994) which, if correct, require radical revision of the early geological history of the Lewisian Complex. Comparison of the relatively old mineral ages with our new whole-rock age contradicts the conventional assumption that mineral isotope systematics are less robust to later disturbance than whole-rock systematics. However, we argue that extensive retrogression, evident in all lithologies at Gruinard Bay, makes them less than ideal samples for a mineral isotopic study and it is likely that the ca. 3.3 Ga ages may be spurious, the product of disturbance of isotope systematics during the Laxfordian. On the basis of the geochronology presented here, evolution of the Lewisian closely follows generally accepted models for late-Archaean crust formation.

New evidence for protolith ages of Lewisian granulites, northwest Scotland

Research Article| November 01, 1995 New evidence for protolith ages of Lewisian granulites, northwest Scotland C. R. L. Friend; C. R. L. Friend 1Department of Geology, Oxford Brookes University, Oxford OX3 OBP, United Kingdom Search for other works by this author on: GSW Google Scholar P. D. Kinny P. D. Kinny 1Department of Geology, Oxford Brookes University, Oxford OX3 OBP, United Kingdom Search for other works by this author on: GSW Google Scholar Geology (1995) 23 (11): 1027–1030. https://doi.org/10.1130/0091-7613(1995)023<1027:NEFPAO>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation C. R. L. Friend, P. D. Kinny; New evidence for protolith ages of Lewisian granulites, northwest Scotland. Geology 1995;; 23 (11): 1027–1030. doi: https://doi.org/10.1130/0091-7613(1995)023<1027:NEFPAO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The granulite facies Scourian rocks in the central region of the Lewisian complex of northwest Scotland play an important role in the understanding of the development of mid-Archean high-grade gneiss complexes. Whereas the later, largely Proterozoic history of the Scourian has been clarified in a detailed U-Pb study, unraveling the Archean ages of the protoliths using conventional isotope techniques has proved impossible. In order to advance our understanding of the deep crustal processes responsible for these granulites, it is essential to establish the age(s) of accretion of the protoliths and their subsequent tectonothermal history. A combined cathodoluminescence (CL) and sensitive high-mass resolution ion microprobe (SHRIMP) single-zircon study has revealed a hitherto unrecognized morphological complexity in zircons from the type localities of the granulites. Use of CL allowed identification of relict oscillatory igneous zoning, metamorphic overgrowths, and irregular areas of recrystallization. From the SHRIMP data, an age of ca. 2960 Ma is inferred for the gneiss protoliths, which were altered considerably during an important metamorphic event ca. 2490 Ma. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Archaean crustal development in the Lewisian complex of northwest Scotland

THE Lewisian complex of northwest Scotland is typical of many Archaean terrains and has a well documented history starting ∼2,700 Myr ago1–6. Here we present new isotopic data that extend this history back to 3,300 Myr, and provide some insight into how the earliest continental crust may have formed. The Lewisian is dominated by tonalite, trondhjemite and granodiorite (TTG) rocks, which require a mafic lithospheric source, rather than being direct mantle melts7–11. But the mafic and ultramafic rocks in the high-grade granulite-facies part of this terrain show little evidence of a significantly older crustal history12,13 and precursor material to the TTG lithologies has not yet been identified. Here we show that older amphibolite material has survived at a lower metamorphic grade. Coexisting amphibolite minerals yield indistinguishable 207Pb–206Pb and 147Sm–143Nd ages of 3,310 ± 27 Myr and 3,298 ± 73 Myr, respectively. These data are consistent with an origin for much of the Lewisian terrain by the re-melting of preexisting lithosphere, with an isotopic signature similar to that of the amphibolites studied here.

Effects of lithology, cataclasis and melting on the composition of fault-generated pseudotachylytes in Lewisian gneiss, Scotland

Petrological studies of fault-generated pseudotachylytes, formed during coseismic slip, need to consider cataclastic processes as well as rapid, closed-system melting and freezing in order to account for the observed similarities and the variations between the chemical compositions of pseudotachylytes and their host-rocks. Whole-rock analyses of pseudotachylytes and their gneissic host-rocks from the Lewisian Complex of northwest Scotland indicate that the composition of the pseudotachylyte is sensitive not only to the bulk host-rock composition but also to its mineralogy and fabric. In anisotropic (e.g., gneissic) rocks, selective cataclastic and melting effects may both contribute to determining the whole-rock composition of pseudotachylytes. Electron microprobe analyses of pseudotachylyte matrices indicate more rapid decomposition and fusion of hydrous ferromagnesian phases, such as biotite and amphibole, and to a lesser extent feldspar relative to quartz. The pseudotachylytes of this study appear to have formed neither by ultracataclasis alone nor by classical equilibrium partial melting or total fusion, but through the interaction of selective cataclastic and melting processes controlled by the lithological layering and mineralogical heterogeneity in their host-rocks. This interpretation may be widely applicable to pseudotachylytes generated by faulting in foliated host-rocks.

Comment on “Pb-isotopic evidence for U-Th-Pb behaviour in a prograde amphibolite to granulite facies transition from the Lewisian complex of north-west Scotland: Implication for Pb-Pb dating” by M. J. Whitehouse

The application of Pb-Pb isochron dating is subject to uncertainties not only from the degree of Pb isotope homogenization during a geological event being dated, but also from the secondary disturbance of unknown ages and the presence of trace uranium in the investigated samples. A three-dimensional Pb-Pb isochron plane method can probably throw light on the Pb isotope dating of whole-rock samples if concentrations of trace uranium and lead can be accurately determined.

Reply to Comment by Y.-F. Zheng on “Pb-isotopic evidence for U-Th-Pb behaviour in a prograde amphibolite to granulite facies transition from the Lewisian complex of north-west Scotland: Implications for Pb-Pb dating”

In a recent paper ( Whitehouse 1989a), Pb-isotopic arrays obtained from a prograde amphibolite to granulite facies transition in the Lewisian Complex were interpreted in terms of different degrees of Pb-isotopic homogenisation and different styles of U Pb fractionation during the 2660 Ma Badcallian metamorphism. Zheng (1990) comments that post-2660 Ma U-Pb disturbance might have contributed significantly to the observed Pb-isotopic arrays, thus requiring a re-evaluation of the proposed model for U-Th-Pb behaviour at 2660 Ma. This comment raises an important point with regard to the general interpretation of Pb- (and other) isotopic data from ancient poly-phase metamorphic terranes, namely the extent to which particular events may be evaluated as significant in multi-stage isotopic evolution or dismissed as insignificant, thus reducing the number of stages and permitting more definitive modelling of earlier events. In this reply, it is argued that, for the samples studied by Whitehouse (1989a), post-Badcallian events are not represented or are unable to generate the observed Pb-isotopic arrays. The modelling of Zheng (1989) for these samples is considered to be inappropriate and misleading. The Pb-isotopic data of Moorbath et al. (1969), for which measured U Pb ratios are available, appear to indicate an U-Pb disturbance at ~900 Ma when reduced using a three-dimensional isochron plane model (ZHENG, 1990). Similar results are obtained using the approach of Gale and Mussett (1973) for reducing three-stage data to two-stage. However, ages obtained from both these methods are highly dependent upon the assumption of initial ratio homogenisation during Badcallian metamorphism. Geochemical considerations discussed by Whitehouse (1989a) and reiterated here question the validity of this assumption and any modelling based upon it.

Sm-Nd evidence for diachronous crustal accretion in the Lewisian complex of northwest Scotland

Sm-Nd whole rock analyses of several cogenetic rock suites from the Late Archaean Lewisian complex of northwest Scotland yield chronological data for a crustal accretion-differentiation superevent (CADS) in this terrane. Three layered mafic/ultramafic bodies predate Badcallian high-grade metamorphism (~ 2660 Ma), but their temporal relationship to the igneous precursors of the host quartzofeldspathic gneiss complex remains unclear. Regression of data from these samples yield the following: Achiltibuie, 2850 ± 95 Ma with ϵ Nd ( t) = +1.93 ± 0.52; Drumbeg, 2910 ± 55 Ma with ϵ Nd ( t) = +1.70 ± 0.32; Scouriemore, 2670 ± 110 Ma with ϵ Nd ( t) = +0.98 ± 0.73. These data suggest that contemporaneous sub-Lewisian mantle had a long term light rare-earth element depleted history relative to a chondritic reservoir composition. Three suites of quartzofeldspathic gneisses, the pyroxene-granulites of Scourie, the hornblende-granulites of Gruinard Bay and the amphibolites of the northern region (north of Loch Laxford), have small ranges of Sm/Nd ratios and do not yield precise regression results. If these gneisses were derived from a relatively homogeneous source region, perhaps represented by the mafic/ultramafic bodies, then Sm-Nd model ages suggest a diachronous sequence of crustal generation decreasing in age from Scourie (terrane average t dm ~ 2930 Ma) through Gruinard Bay (~ 2860 Ma) to the northern region (~ 2780 Ma). A diachronous sequence such as this suggests that for a CADS in which separate accretion and differentiation episodes may be resolved, final high-grade metamorphism may not be related to magmatic crustal generation processes.

Pb-isotopic evidence for U-Th-Pb behaviour in a prograde amphibolite to granulite fades transition from the Lewisian complex of north-west Scotland: Implications for Pb-Pb dating

Two end-member types of U-Pb fractionation behaviour are recognised: type I in which Pb-isotopic homogenisation occurs, and type II, in which Pb-isotopic homogenisation is incomplete or absent. Type II can be further subdivided into three categories depending upon the relationship of the post-fractionation μ 3 ( 238U 204Pb ) value to prefractionation μ 2. In type IIa, μ 3 is unrelated to μ 2; in type IIb, μ 3 = kμ 2, where k is a constant; in type IIc, μ 3 values are uniform. Each of these types of behaviour results in a predictable effect upon observed uranogenic Pb-isotope systematics which may lead to an incorrect geochronological interpretation. An example of the relationship between these specific types of U-Pb behaviour and metamorphic grade is provided from the late-Archaean Lewisian complex of N.W. Scotland. This complex was metamorphosed during the ~2660 Ma Badcallian event. Three suites of tonalitic-granodioritic grey gneisses have been analysed; pyroxene-granulite facies from Scourie; hornblende-granulite facies from Gruinard Bay; and amphibolite facies from the northern region. The present-day Pb-isotopic composition of these suites can be interpreted as representing a transition from type lib behaviour in the lowest grade rocks resulting in a rotated isochron, through type Ha in the hornblende-granulites which yield a scatter of data points, to type I in the pyroxene-granulites which yield an isochron corresponding to the Badcallian event. This transition presumably reflects increasing U-depletion with metamorphic grade and ultimately, at the highest grade, Pb mobility and/or depletion. In addition, a consideration of the thorogenic Pb-isotopic data reveals a systematic variation during prograde metamorphism. An attempt to test the model of 2660 Ma initial ratio inhomogeneity by analysis of K-feldspars from the amphibolite facies gneiss suite reveals ~1700 Ma Laxfordian resetting of their Pb-isotope systematics. This study has implications for Pb Pb geochronological data particularly in lower grade metamorphic terrains. It suggests that reliable isochrons may only be generated by type I behaviour, or by the imposition of high μ 3 values during type IIa behaviour at higher grades of metamorphism.

Granulite fades Nd-isotopic homogenization in the Lewisian complex of northwest Scotland

A published Sm–Nd whole-rock isochron of 2,920 ± 50 Myr, obtained from a wide range of lithologies in the Lewisian complex of north-west Scotland, was interpreted1 as the time of protolith formation. This date is ∼260 Myr older than estimates for the timing of high-grade metamorphism in the complex at ∼ 2,660 Myr2'3, and this period is considered to represent the duration of the Lewisian crustal accretion-differentiation superevent (CADS)4. Here we give new Sm-Nd data, obtained specifically from granulite facies tonalitic gneisses, that yield a date of 2,600 ±155 Myr. Although depleted-mantle model ages (tDM suggest >200 Myr of premetamorphic crustal residence, the regression date and its associated initial Nd-isotopic parameters demonstrate Nd-isotopic homogenization during the high-grade event, as well as the probability of general rare-earth-element (REE) mobility. Models for selective element depletion in the complex have previously assumed REE immobility since 2,920 Myr, but the data presented here suggest that a reappraisal of the depletion mechanism is required.

Petrology and deformation of metamorphosed volcanic-exhalative sediments in the Gairloch Schist Belt, N.W. Scotland

The Gairloch Schist Belt in the Archaean to Early Proterozoic Lewisian Complex of north-west Scotland is largely composed of amphibolite facies metabasites and metagreywackes. These are associated with a distinctive suite of metamorphosed volcanic-exhalative sediments including quartz-magnetite rocks, garnet-grunerite rocks and compositionally variable, siliceous calcite- and dolomite-bearing lithologies. The carbonate horizons are locally rich in sulphide and carry Cu-Zn-Au mineralization. Meta-exhalites occur within parts of metavolcanic units characterized by metamorphosed tuffs and tuffs mixed with exhalative material. Quartz-magnetite and carbonate horizons were dismembered and underwent mylonitic recrystallization during regional compression. The associated metabasic rocks in the shear zones have suffered extensive phyllonitization. This style and degree of deformation are not developed elsewhere in the immediate area which suggests that ductile shear zones in the Gairloch Schist Belt were preferentially initiated near and localized around the meta-exhalative horizons.

A geochemical study of Lewisian metasedimentary granulites and gneisses in the Scourie-Laxford area of the north-west Scotland

AbstractMajor element, trace element, and REE data for metasedimentary granulites and their retrogressed derivatives formed from Archaean parent-rocks at two localities in the Lewisian complex of north-west Scotland are presented.The metasedimentary rocks are enclosed in and intruded by metatonalites and related calc-alkaline rocks and have been highly deformed and metamorphosed along with these rocks. They are geochemically distinct from this meta-igneous suite and range from highly aluminous to highly siliceous types. Comparisons of major and trace element data with those for unmetamorphosed Archaean and post-Archaean sediments suggest that they are derived from a detrital shale-greywacke assemblage, which may have included a volcaniclastic component. High ΣREE and ΣLREE suggest that the source-rocks included fractionated felsic igneous rocks.Evidence that depletion in the large ion lithophile elements K, Rb, Th accompanied high-pressure granulite metamorphism supports the view, based on data derived from the associated meta-igneous rocks, that depletion was effected by active fluids rather than by abstraction of a partial melt and suggests that removal of K and Th began only when a considerable reduction of Rb had taken place.

Sm–Nd dating and cooling history of Scourian granulites, Sutherland

The Scourian gneisses of the Lewisian complex of north-west Scotland have been the subject of many geochronological investigations. Sm–Nd whole-rock measurements suggest that the protoliths of the Lewisian complex differentiated 2.92±0.05 Gyr ago from an approximately chrondritic mantle1. Rb–Sr and Pb–Pb whole-rock ages between ∼2.8 and 2.6 Gyr reflect subsequent depletion in Rb and U (refs 2, 3) which has generally been associated with granulite facies metamorphism. Zircons which are thought to have crystallized during this metamorphism have also given U–Pb ages of 2.66 Gyr (ref. 4). This granulite facies metamorphism, involving pressures in excess of 10 kbar and peak temperatures estimated between 820±50 °C (ref. 5) and 1,250 °C (refs 6,7), affects a terrain including supracrustal rocks7 and thus implies a major tectonic event. We report here the first geochronological data measured on mineral assemblages that have also provided estimates of the conditions of metamorphism. Such measurements are possible because the major minerals of granulite facies assemblages show an unusually favourable spread in Sm/Nd ratios; garnet, especially, is strongly enriched in Sm (ref. 8). Data for three garnetiferous basic gneiss assemblages suggest that closure of the Sm–Nd system occurred significantly later than the peak of the metamorphism.

Variations in crustal level and geothermal gradient during the evolution of the Lewisian complex of northwest Scotland

The metamorphic reactions observed in the polycyclic Precambrian gneisses of parts of western Sutherland and the Outer Hebrides are discussed in the light of other evidence bearing on the tectonic and metamorphic evolution of the Lewisian complex. The evidence that early granulite facies assemblages were transformed into successively lower-pressure assemblages seems consistent with the views expressed by several authors that the complex remained continuously at deep crustal levels for nearly a thousand million years after the earliest dated metamorphic event and that it began to rise towards the earth's surface during the Laxfordian metamorphic stage. The Scourie dyke swarm appears to have been emplaced under deep-seated conditions. The Laxfordian tectonic patterns record the effects of regional uplift and the associated metamorphic assemblages suggest local steepening of the geothermal gradient.

A Discussion on global tectonics in Proterozoic times - The interrelations of fluid transport, deformation, geochemistry and heat flow in early Proterozoic shear zones in the Lewisian complex

The Lewisian complex of northwest Scotland shows a pattern of evolution typical of a number of early Proterozoic provinces. During the period 2500-1600 Ma, deformation occurred along steeply dipping shear zones, resulting in both vertical and lateral movements. The largest of these shear zones, forming the northern boundary to the Scourian granulites (Archaean), must have penetrated to considerable depth, possibly to the mantle. Modal and chemical analysis of rocks from shear zones are presented and discussed in relation to rocks sampled outside shear zones. The mineralogy and composition of all rocks deformed in the shear zones have been considerably altered by synkinematic metasomatism. In the early stages, immediately prior to and during the intrusion of the regional doleritic dyke swarm, this metasomatic activity involved addition of H 2 O and Na to the rocks. Subsequently, more significant changes in rock chemistry occurred addition of H 2 O, K, Na, loss of Fe, Ca, Mg). These changes resulted from the interaction between large volumes of water and the rocks in the shear zones along which the fluid travelled. A combination of modal and chemical data allow general chemical reactions to be written which describe the evolution of the gneisses during reworking and retrogression from pyroxene bearing granulite facies rocks to hornblende and biotite bearing amphibolite facies rocks in shear zones. The reactions are written as ionic equilibria and suggest that the fluid phase in the shear zones had a low pH. Adiabatic transport of water upwards through the crust will result in moderate warming of the fluid, and can cause large temperature increases above the preexisting geothermal gradient in rocks through which the fluid travels. It is suggested that both deformation and metamorphism in these shear zones are related to transport of fluid by hydraulic fracturing. Grain size reduction by hydraulic fracturing increases the strain rate in the shear zones. Deformation may cease in a shear zone when the fluid pressure drops and hydraulic fracturing no longer occurs. Thus fluid transport, mineral reactions, chemical changes, grain size reduction and convective heat flow will cease. A close relation should exist between the intensity of deformation, the extent of metasomatism and the thermal history in these important shear belts.