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The source of granites: inferences from the Lewisian complex ANTONIO CASTRO Departamento de Geología, Universidad de Huelva, Campus de El Carmen, Avda. de las Fuerzas Armadas s/n, 21071 Huelva, Spain Synopsis The origin of granitic rocks has been the subject of intense debate. It is broadly accepted that granites are generated in the continental crust. However, the composition of the source rocks and the processes controlling melting reactions remain controversial. Fluid-absent melting is a plausible mechanism for granite generation but is not supported by experimental data. Fluid-present melting is favoured by experiments but the presence of a free water phase is very unlikely in the deep crust. Isotopic ratios are also paradoxical because Nd isotopes indicate a crustal (recycled) source but the low 87Sr/86Sr ratios displayed by many granite batholiths require a substantial mantle contribution. Furthermore, mantle–crust hybridization mechanisms are difficult to apply in terms of magma mixing and assimilation according to models of the physical/rheological behaviour of silicic magmas. Here I present a new, source-based, genetic approach for granitic rocks in which all these paradoxes are satisfactorily explained. The model is supported by new constraints from laboratory experiments and observations of field relationships in the Lewisian complex of northern Scotland. The process starts with the invasion of crustal regions by water and K-rich monzodiorite (andesite) magmas derived from hybridized regions of the subduction-modified mantle wedge. These intermediate magmas supply the water and potassium necessary for crustal melting and granite batholith generation. Introduction Granites (sensu latto) are the most common magmatic rocks forming the continental masses of the Earth. They have been the subject of geological and geochemical studies addressing the implications of granite magma generation in the formation and evolution of the continental crust with time (Taylor & McLennan 1985; Windley 1995). However, the origin of granite magmas has been among the most hotly debated topics in the history of geology (e.g. Read 1948). Granites have some intriguing and paradoxical features that have puzzled geologists for decades, and still they do. The study of these paradoxes may help us understand the complex processes involved in the generation of granite batholiths. Here, I show that a combination of geological, experimental and geochemical arguments may be used in a new hypothesis for the origin of granites that accounts for most of their intriguing features. The recognition of the required source compositions and the understanding of the mechanisms involved in the generation of these source regions are integral to the model. Field observations of rock complexes from the lower continental crust exposed in the Archaean basement of northern Scotland have been crucial in the formulation of a coherent source-based model for granite generation. Laboratory experiments (Tuttle & Bowen 1958) were used to demonstrate the magmatic origin of granites and have also constrained the pressure and temperature conditions for granite melt generation (Thompson 1982; Clemens & Vielzeuf 1987; Vielzeuf & Holloway 1988). However, due to the fact that granite rocks have a composition close to the thermodynamic minimum in the system quartz–albite–orthoclase (Tuttle & Bowen 1958; Holtz et al. 2001a), a wide variety of rock compositions are theoretically suitable as source materials for the production of granite melts under different conditions. The paradox is that none of the known crustal sources wholly satisfies all the essential geochemical and isotopic features of granites. The thermodynamic minimum also has the effect that residual melts left after crystal fractionation from a wide variety of magmas will tend towards granite composition. Again, isotopic features do not match the expected ratios produced by such closed-system processes. Granites are enriched in radiogenic isotopes with respect to the composition of the parental magma from which they are supposedly derived. By contrast with basalts, for which the composition of the mantle peridotite source is recognized, granites do not have a unique solution for the composition of the source region. Thus, the problem of source lies at the heart of the granite problem, and the source problem is a matter to be dealt with using a combination of constraints imposed by field observations, laboratory experiments and isotope geochemistry. I will show in this paper that the problem of the origin of granites is essentially related to the identification of their source(s) and to the geological processes that lead to the availability of appropriate sources. In summary, it can be postulated that the granite-making processes are closely related to source-making processes. Scottish Journal of Geology 40, (1), 49–65, 2004 50 A. CASTRO F. 1. Initial Sr isotopic compositions and Nd model ages of granitic rocks ranging in age from the Early Archaean to the Late Cainozoic, plotted against their crystallization ages. Initial Sr isotopic ratios (a) are compared to evolutionary lines for Sr isotopic compositions of the continental crust, chondritic uniform reservoir (CHUR) and depleted mantle (DM). Peraluminous leucogranites (S-type, e.g. the Himalayan granites) occur as relatively small intrusions and most are granitic sensu stricto. They plot above the average crustal evolution line, suggesting that they represent pure crustal melts. In contrast, I-type granitoids that form large batholithic complexes (and their volcanic equivalents) are chiefly of granodioritic composition and plot below the crustal evolution line but generally above CHUR. They define a crustal array coincident with the lower crust array (McCulloch & Bennett 1994). The participation of crustal recycled materials in the origin of batholith-forming granodiorites is also demonstrated by the Nd model ages of these rocks that are systematically older than their crystallization ages (b). Data sources are: Sete Voltas, Brazil (Martin et al. 1997); Wyoming (Frost et al. 1998); Yilgarn (Champion & Sheraton 1997); Superior Province (Dunphy & Ludden 1998); Rodinia (Wareham et al. 1998); Antarctica (Zhao et al. 1997); Late Panafrican (Abdel Rahman 1990; Da Silva et al. 2000; Moghazi et al. 1998); Nova Scotia (Clarke et al. 1988); Iberian Massif (Moreno-Ventas et al. 1995; Villaseca et al. 1998; Castro et al. 1999); New Zealand (Pickett & Wasserburg 1989); Himalaya (Inger & Harris 1993; Le Fort et al. 1987). The problem of granite sources Chappell & White (1974) proposed the first genetic classification of granite types based on the nature and composition of their sources. They distinguished between S-type granites (sedimentary source) and I-type granites (igneous source). The S-I classification may be applied to any granite batholith with reasonable success. However, the existence of granites with transitional features (Patiño Douce 1999) and the lack of field and experimental proof of the igneous source for I-type granites (Wall et al. 1987) have caused some difficulties in the application of this simple scheme. The fact that I-type granites have hybrid isotopic signatures, intermediate between crust and mantle (Fig. 1), has led some authors to propose processes of magma mixing and assimilation, implying several crustal sources and mantle-derived magmas (Collins 1996; Castro et al. 1991). However, there is a systematic evolution of these intermediate (hybrid?) isotopic ratios of granitoids with time since the Late Archaean (Fig. 1a). This temporal evolution lies between the average crust and mantle and is very close to the lower crust array (e.g. McCulloch & Bennett 1994). This may imply that the Sr isotopic signatures of I-type granites are inherited from a lower crust dominantly composed of tonalites of Archaean age (cf. Weaver & Tarney 1980a, Weaver & Tarney 1980b; Weaver & Tarney 1981). This temporal evolution recorded by granodiorite (I-type) batholiths, may be defined as an Archaean crustal (AC) array for Sr isotopes (Fig. 1a). The comparison of Nd model ages of granites (based on a depleted mantle model) with the ages of generation (Fig. 1b) suggests that recycled (sedimentary?) material is involved in the composition of I-type granites. This is so even if the chondrite uniform reservoir (CHUR) model is used for model age calculations. However, the addition of this sedimentary component with a high Rb/Sr would produce Sr isotopic ratios higher than expected in I-type granites. The explanation of these isotopic ratios for Sr and Nd by processes of magma mixing and/or assimilation leads to the need to THE SOURCE OF GRANITES invoke some mechanism able to produce the observed decoupling between Sr and Nd; whatever mechanism is involved has to produce low Sm/Nd and low Rb/Sr. An alternative way is to assume that I-type granites typically with low 87Sr86/Sr initial ratios are generated from a crustal source that has this particular composition. Comparison with Archaean crust array (Fig. 1a) suggests that tonalites of the lower crust (e.g. Weaver & Tarney 1981) are good candidates to produce Sr isotopic ratios intermediate between mantle and crust with no implication of hybridization processes such as magma mixing or assimilation. This simple model of an Archaean crust source for I-type granites receives strong support from field relations and laboratory experiments that will be discussed later on in this paper. The model also accounts for the decoupled behaviour of the Nd isotopic ratios of granite batholiths. If no other source components but rocks of the Archaean array are involved in the generation of I-type granite batholiths, the calculated model ages would be homogeneous for any age of generation. Model ages are systematically older than the age of generation (Fig. 1b) but far from being homogeneous they are positively correlated with the age of generation. This correlation implies that a Nd-enriched component with low 143Nd/144Nd (crustal component) is incorporated into the source of granites at the time of generation. Although this component may have evolved in the continental crust (e.g. sediments) before granite magma generation, it may be incorporated by varied mechanisms, one of them being the addition of mafic magmas from enriched portions of the mantle by processes of subduction. This mafic magma may have no significant effect on the Sr isotopic ratios of the crustal source as most Sr is supplied by this old crust. In summary, the Sr isotopic ratios of the I-type granites are controlled by the isotopic signatures of an old (Archaean?) crustal source of dominantly tonalite composition (e.g. Weaver & Tarney 1981), and the Nd isotopic signatures are modified by the incorporation of a recycled component (sedimentary?) either by participation of sediments at the source region or by input of mafic magmas from subduction-enriched regions of the mantle. Although this recycled sedimentary component is normally enriched in Rb, with respect to Sr, due to weathering processes and is potentially rich in radiogenic Sr, the effect on the Sr isotopic ratio of the granite magma may be reduced for two main reasons: first, the low amount of Sr supplied by the sedimentary component in comparison with the richness in Sr of the plagioclase-rich tonalite source; and second, the residence time of these sedimentary reservoir may be as short as several million years if they come from subducted mélanges formed in marginal basins around the continental masses. Thus, in the light of the systematic and decoupled variations of the Sr and Nd isotopic ratios, magma mixing and assimilation are very unlikely to account for the intermediate isotopic signatures of I-type granites. Furthermore, magma mixing is limited by viscosity differences between magmas of contrasting composition 51 (Sparks & Marshall 1986) and is only effective in local situations of highly energetic flow in magma conduits (Koyaguchi 1985) or flow zones in magma chambers (Castro et al. 1990). Accordingly, magma mixing is only recognized in the geological record as a local-scale process. Assimilation also is very limited by physical factors. First, large-scale motions of magmas in large magma chambers are necessary to homogenize the resulting hybrid magmas. Second, large inputs of basaltic magma into the continental crust are necessary for the production of hybrid granite melts (Patiño Douce 1995). Furthermore, there is no evidence in the geological record for the processes of basalt–pelite reaction zones on a scale large enough for the production of major granodiorite batholiths. Moreover, the chemical equilibrium required between hybrid melts and hybrid restites produced by the basalt–pelite assimilation reaction (Patiño Douce 1995) is contrary to the observed differences in Nd and Sr isotopic ratios (Castro et al. 1999) between granites and mafic rocks supposed to be the product analogues of the assimilation reaction. Finally, large masses of basaltic intrusions coeval with batholith generation in late orogenic processes have not been identified in the geological record. All these observations severely weaken the case for assimilation processes as the driving mechanism for the generation of the large granodiorite batholiths. Partial melting of K-andesites (Roberts & Clemens 1993) has been proposed for the generation of granodiorite (I-type) batholiths. The advantage of this mechanism is that the hybrid signatures of the I-type granites are accounted for by inheritance from the hybrid andesite source. However, the variable isotopic signatures, mainly in terms of Nd isotopic ratios, observed in I-type granites require the addition of other crustal materials to the source region and the extraction of magma pulses incompletely homogenized with these sources. Furthermore, in order to extract the large volumes of granite melt necessary for the formation of batholiths, large amounts of water have to be added to the andesite source region. Fluid-absent melting of K-rich andesites generates only small melt fractions (less than 5 vol%) for temperatures of about 900(C (Roberts & Clemens 1993; Castro & Patiño Douce 2001). Andesites are not recognized as source rocks in the geological record, and this is also a major handicap for this hypothesis. Experiments have also shown that K-rich granodiorites cannot be partial melts of basaltic source materials (Patiño Douce 1999). It has been suggested that granodiorites may form by partial melting of Archaean tonalites (Rapp 1997). This is in good agreement with inferences from isotopic ratios, as mentioned before, and with the possible existence of an Archaean crustal array deduced from the Sr isotopic ratios of I-type granites. However, the melting process is not simple as inferred from Nd isotopic ratios, according to which an enriched mantle component is needed at the source region. In order to further investigate this possibility we have conducted partial melting experiments on three different tonalitic starting 52 A. CASTRO F. 2. Compositions and melt fractions of melts derived from tonalite starting material as function of temperature (data in Appendix). Squares show the compositions of melts formed by melting of Fuente del Oro Bt-tonalite (1) and its composition. Circles show the composition of melts formed by melting of Average Archaean TTG (Condie 1997) and its composition (2). Diamonds show the composition of melts formed by melting of Zarza Bt-tonalite and its composition (3). The numbers near to the squares, circles and diamonds are the corresponding melt fractions in vol%. The numbers inside squares, circles and diamonds indicate the pressure (kbar). materials that have compositions comparable to those of characteristic Archaean tonalities (see Table A1 in Appendix). Dehydration melting of these starting materials generates liquids of granitic composition sensu stricto, that are depleted in Ca relative to K-rich granodiorites (Fig. 2). Additionally, only very small amounts of these liquids are produced (<10%) and only at high temperatures (>900(C). Dehydration melting of Archaean tonalites is therefore not a viable mechanism for producing the voluminous K-rich granodiorites that are present in all cratons (Fig. 3). Melting of tonalites with added H2O circumvents some of these difficulties, because up to c. 20% of granodiorite melt is formed by this process at temperatures of 800(C or lower (Fig. 2). However, melts rapidly become tonalitic at higher degrees of melting as they inevitably approach the source composition. Thus, fluid-present melting of tonalites by the addition of water alone does not produce melts with the required granodiorite composition. I propose that a possible solution to this paradox is the supply by mantle magmas of water and K to the tonalitic source region in the lower continental crust. These fluids may trigger partial melting of tonalitic rocks that form a great part of the Archaean protocontinents and produce the large late Archaean granodiorite batholiths. This hypothesis has been inferred from field and geochemical relations of the tonalite–trondhjemite–granodiorite (TTG) complex of the Scottish Lewisian as I will show later, and also finds strong support from the Sr and Nd isotopic signatures of F. 3. Age relations (a) and relative volume proportions (b) of granodiorite batholiths from several cratons around the world. The diachronous distribution of the oldest K-granite batholith in different cratons implies that major periods of crustal recycling spanned a long period of time from 3.0 to 2.0 Ga. Ages for K-granites and associated tonalite–trondhjemite complexes are taken from precise SHRIMP and single zircon U–Pb determinations from: Quebec (Dunphy & Ludden 1998), Nigeria (Dada 1998), Namibia (Seth et al. 1998), Siberia (Jahn et al. 1998), South America (Vanderhaeghe et al. 1998), Antarctica (Harley et al. 1998), Australia, Yilgarn (Champion & Sheraton 1997), South Africa (Kroner et al. 1999), Canada (Corfu et al. 1998), W Central Africa (Feybesse et al. 1998), North America (Frost et al. 1998), Australia, Pilbara (Smith et al. 1998) and India (Mishra et al. 1999). Relative abundances of K-granites in different shield areas have been estimated from geological maps and estimations in De Witt & Ashwall (1997). These correspond to the following key letters in (b): a, Amazonian craton; b, Greenland; c,d, Baltic Shield; e, Siberia; f, Slave Province; g, Superior Province; h, Wyoming Province; i, West Africa; j, Sao Francisco; k, Zimbawe; l, Kaapvaal; m, n, Tanzania and NE Zaire; o, Indian Shield; p, Madagascar; q, Yilgarn; r, Pilbara (see Appendix for data sources). I-type granites as mentioned above. Other alternative mechanisms will be considered first. Compositional variations in granites have been interpreted according to the restite model (Chappell et al. 1987) as due to variable degrees of melt-from-restite separation in the source and during subsequent ascent and emplacement. However, as mentioned above, the observed isotopic variations among compositional facies in granite batholiths imply the existence of open-system processes in which interchange between distinct isotopic THE SOURCE OF GRANITES reservoirs is a necessary condition. It is thus difficult to account for this interchange if magma mixing and assimilation are ruled out. An attempt to solve the paradox was proposed by Shirey & Hanson (1984) for the granodiorite–sanukitoid association of the Late Archaean (2500 Ma) batholiths of the Superior Province (North America craton). They suggested that part of the crustal inheritance of the mantle-derived sanukitoid (Mg-andesites) rocks comes from an enriched mantle source and that granodiorite batholiths may be produced by fractionation within the continental crust of this mantle-derived melt (Stern et al. 1989). Recently, we have demonstrated experimentally that melting of composite experimental charges that simulate melanges composed of sediments and fragments of oceanic crust, introduced into the mantle regions by subduction, may give rise to water and K-rich melts of andesitic (= sanukitoid, = monzodiorite) composition. The process of fractionation from this parental sanukitoid, however, remains obscure. The problem is that granodiorites, supposed to be fractionated from the parental sanukitoid have more crustal isotopic signatures than the parental sanukitoid, implying large-scale processes of assimilation of crustal materials (Stern et al. 1989). However, the ability for assimilation of granitoid rocks is essentially restricted to local-scale processes. The evidence from field and structural studies (Fernández et al. 1997; Fernández & Castro 1999a) indicates that large convective movements in large magma chambers, required for large-scale homogenization, are very unlikely in granite batholiths. These field-based studies indicate that emplacement is related to pulse-in-pulse intrusions in association with crustal extension processes rather than to large diapiric bodies, in a way similar to the dyke-in-dyke emplacement of basalts at the midocean ridges. The implication is that each pulse retains distinctive geochemical and textural features inherited from the source region and not acquired during emplacement. The high rates of granite emplacement (Petford et al. 2000; Fernández & Castro 1999b) are a limiting factor to the capability of granite magma to assimilate country rocks and to homogenize the magma composition at the scale of kilometres. This is in agreement with the differences found in the content of microgranular enclaves, colour index, etc. between granitic pulses closely related between them which, at the same time, show sharp, intrusive contacts. All these relations indicate that heterogeneities in granite pulses forming the large batholiths are inherited from the source and not acquired during ascent and final emplacement. The problem is thus transferred to the processes that produce the necessary heterogeneities in the source region. Processes in the source: cooking the crust for granite generation Heterogeneities in the source are the most plausible cause to account for the heterogeneities of granite batholiths. However, the successful application world- 53 wide of the S-I classification scheme for granite batholiths surprisingly implies that heterogeneities are comparable among different continental areas and have been so since the Late Archaean, when granodiorite batholiths appeared for the first time in the geological record. Furthermore, granite types (S and I) are produced at particular stages in the orogenic process, I-type granites being generally associated with late-orogenic processes related to lithospheric extension (e.g. the Late Archaean batholiths (Ridley et al. 1997), the newer granites of the Scottish Caledonides (Stephens & Halliday 1984) or the late granodiorites of the Variscan belt (Capdevila et al. 1973; Corretgé et al. 1977)). Consequently, the heterogeneity of the source region, far from being accidental, must be the result of some systematic, orogenic-related process. This observation is key to understanding the processes involved in granite generation. The invasion of continental crust (mainly the lower crust) by mantle-derived (water-rich) basic magmas is a plausible explanation for this heterogeneous yet systematic source. These mafic magmas (e.g. K-rich andesites) may crystallize at depth at the base of the continental crust and they may give rise to biotiterich monzodiorite rocks plus exsolved fluids that infiltrate the tonalitic crust reducing its melting point and triggering granite magma generation. Granites produced from this heterogeneous source will mimic all these heterogeneities for major elements, colour index, enclave content, isotopic ratios etc. An important observation is that this source-based genetic model accounts for the complex geochemical relationships of granite batholiths that were outlined in an earlier section. However, the process is not a simple one based exclusively on the generation of granodiorite magmas by partial melting of a composite source. Field relations indicate that it is more complex and may imply processes of mass transfer from the intruding basic magma to the surrounding crust and reaction between mafic magmas and anatectic melts. The presence of intrusive layers of diorite composition is a common feature of these complex source regions of granite generation. These have been described as inclusions within granite batholiths in many parts of the Caledonian and Variscan orogenic belts of Western Europe. Mafic and intermediate (monzodiorite) rocks form part of the so-called appinite suite in the British Isles (Pitcher 1997) or the vaugnerites of the Massif Central in France (Sabatier 1991). In the Late Archaean batholithic complexes, the same intermediate rocks have been referred to as members of a sanukitoid– granodiorite series (Shirey & Hanson 1984) due to the compositional similarities that these rocks share with the sanukite (Mg-rich andesites) volcanic rocks of the Setouchi belt in Japan (Tatsumi 1982). In deep-seated regions of the continental crust, where these intermediate magmas are emplaced, they may contribute to the generation of granodiorite batholiths by means of two different, but closely related, mechanisms. First, if the intermediate magmas, which may carry important amounts of water in dissolution (e.g. Grove et al. 2002), 54 A. CASTRO are emplaced at the upper continental crust into a metasedimentary complex (e.g. pelites and greywackes), they supply heat and water, exceeding the structural OH groups coordinated in mica and amphibole, to the surrounding pelitic rocks, enhancing the formation of anatectic melts. These granitic melts will react with the mafic materials, rock and magma, and may become richer in Ca and consequently granodioritic in composition. Recently developed experiments on microgranular enclave dissolution in hydrous granitic magmas (Castro et al. 2002) demonstrate that this mechanism is favoured by decompression during magma ascent and/or crustal extension due to the low solubility of water in the granitic magma at low pressure. In summary, this mechanism will produce a granodiorite melt by enclave dissolution and Ca addition to an originally Ca-poor anatectic melt. This mechanism may act from the moment of invasion of the anatectic zone by intermediate magmas and may continue during ascent and emplacement. The second mechanism of granodiorite generation, and possibly the most important, is related to the invasion of tonalite crust (Archaean) by waterrich mantle magmas. This is favoured at deep crustal levels where tonalite crust is a dominant rock, especially in the Late Archaean and Lower Proterozoic terranes (e.g. Weaver & Tarney 1980a,b). In contrast with the former mechanism, in this situation granodiorites are generated by addition of water and potassium from a mantle-derived magma to the tonalite source. This mechanism has been inferred from field relations in the Lewisian complex of northern Scotland and has been tested by means of laboratory experiments (unpublished data). The most outstanding field relations will be summarized here. Inferences from the Lewisian complex The isotopic and experimental constraints discussed in the previous section clearly favour a genetic model based on the presence of an Archaean source region at the base of the continental crust. Consequently, observations of field relations between granodiorite, tonalite and associated mafic rocks in these deep-seated Archaean regions are crucial to test the suggested hypothesis. The Lewisian complex of northern Scotland constitutes one of the best preserved portions of an Archaean craton (e.g. Park & Tarney 1987). The complex is considered part of the North Atlantic craton (see recent terrane reconstruction by Friend & Kinny 2001) also cropping out in Greenland and eastern North America. In general terms, the Lewisian complex has all the characteristic features of any Archaean craton, dominated by siliceous igneous rocks of tonalite–trondhjemite–granodiorite (TTG) composition (Rollinson 1996). Although complex in detail, all Archaean cratons share the same basic structure which has been called the TTG association (Condie 1997). The earliest silica-rich igneous rocks are Na-rich tonalites and trondhjemites (TT). These rocks are intruded by younger K-rich granodiorites (G). Two other observations are important. In the first place, the two compositionally distinct magmatic episodes (TT vs. G) are separated in time by several tens to several hundreds of millions of years (Ridley et al. 1997). Secondly, the exposed volumes of K-rich granodiorites are at least comparable to, and in many cases greater than, those of Na-rich tonalites and trondhjemites. The late character of granodiorite batholiths with respect to the main tectonic phases is a common feature in all the Proterozoic and Phanerozoic orogens. They are normally related to large extensional phases during the late stages of orogenesis. The geological observations about the volume ratios and time sequence must be taken into account by any genetic model for granite batholiths. Recycling of granodiorites into new granodiorites is only possible if water is available in the source region. This mechanism, however, finds little or no support from field relations and is considered very unlikely due to the difficulties in having a free-water phase at the required depths of generation in the continental crust (Yardley & Valley 1997). Partial melting of a source composition different from granodiorite is the most plausible process for the generation of new granodiorite batholiths. As granodiorite batholiths appear in the geological record at the end of the Archaean and Early Proterozoic, the study of genetic processes of these ‘very early’ granodiorite magmas is of great value because there is no previous crustal rock with this composition. The most abundant crustal rocks available as source material for granodiorite magmatism in Archaean complexes (e.g. Lewisian) are the tonalites and trondhjemites of Late Archaean age (Corfu et al. 1994; Friend & Kinny 1995; Kinny & Friend 1997) that were generated by partial melting of oceanic basaltic material (Rollinson 1996). This is exemplified by the relations found in basic agmatites and granites in the Gruinard Bay region of the Lewisian complex (Rollinson 1987). Furthermore, Late Archaean granodiorite batholiths normally appear intruded into the upper crust, uprooted from their source region, in most cratonic areas (e.g. Ridley et al. 1997). Only a few exceptions exist in which deep crustal regions are observable, the Lewisian complex being one of these deep-crustal areas where observations from the lower and middle continental crust can be made. Most studies of the mainland Lewisian agree in a subdivision into three main regions (Sutton & Watson 1951; Park & Tarney 1987) (Fig. 4): (1) the Northern Region, north of Loch Laxford – this is named the “Rhiconich terrane” according to the modern terrane subdivision (Friend & Kinny 2001); (2) the Central Region (Scourian), between Loch Laxford and Gruinard Bay – this is the “Assynt terrane” according to Friend & Kinny (2001); and (3) the Southern Region, between Gruinard Bay and Loch Torridon. The deepest continental crust is represented by the tonalite– trondhjemite granulite facies gneisses of the Assynt terrane. These are the Scourie gneisses in the classical terminology, which are dated at 3030–2960 Ma (Friend & Kinny 1995; Kinny & Friend 1997). The Rhiconich terrane, north of the Laxford front (Fig. 4), is characterized by the presence of numerous granodiorite THE SOURCE OF GRANITES 55 gneisses (tonalite–trondhjemite complexes) separated in age by a time span of more than 200 Ma in most cratons, and representing a volume proportion of about half of the whole silicic crust (Fig. 3). Ferrodiorites of the Rhiconich terrane (Northern Region) F. 4. Geological sketch of the Lewisian complex of Northern Scotland (Mainland). Terrane division according to Friend & Kinny (2001). sheet intrusions cross-cutting previous structures of the host tonalite–trondhjemite banded grey gneisses. Hornblendites and mafic ferrodiorites are also intercalated between the host grey gneisses and the granodiorite sheets. Radiometric dates by SHRIMP U–Pb determinations on zircon grains from these granite sheets have been used to constrain the timing of amalgamation of the Rhiconich and Assynt terrains at about 1740 Ma (Friend & Kinny 2001), in agreement with age estimations of shearing along the boundary terrane represented by the Laxford front (Corfu et al. 1994). The age of the tonalite–trondhjemite banded gneisses in the Northern Region (the Rhiconich terrane) is about 2840– 2680 Ma (Kinny & Friend 1997), slightly younger than similar gneissic complexes of the Central Region (Assynt terrane) dated at 3030–2960 Ma (Kinny & Friend 1997). The most significant feature of the Rhiconich terrane is the production of large volumes (c. 30 vol% of the whole area) of granodiorite magma dated at 1854 13 Ma according to SHRIMP zircon determinations (Friend & Kinny 2001) from a typical granite sheet at Loch na Fiacail, 1 km north of Loch Laxford. Significantly, these granodiorites contain 2500–2650 Ma relict zircon grains (Friend & Kinny 2001) coincident with the age of generation of the host tonalite–trondhjemite gneisses. These observations favour the derivation of granite sheets from the host tonalite gneisses. However, as pointed out by Friend & Kinny (2001), it is unlikely that the K-poor tonalite gneisses can produce the large volumes of K-rich granites of the Rhiconich terrane. This observation is crucial in addressing the problem of granite generation from Archaean TTG complexes. In other cratons the proportion of granodiorite batholiths to tonalite–trondhjemite complexes is similar to that found in Rhiconich, but at a larger scale. In this sense, the Rhiconich terrane of the Lewisian complex may be considered as a microcosm of cratonic architecture: late granodiorite batholiths emplaced into Archaean grey Apart from granodiorite sheets, the Rhiconich terrane is characterized by the presence of hornblende-rich bands of ferrodiorite composition. These may be the Fe-rich equivalents of the sanukitoid suits (Mg-rich) that appear associated with Late Archaean granodiorite batholiths in other cratonic areas such as the Superior Province in the North America craton (Stern et al. 1989; Evans & Hanson 1997) and the Darwar craton in India (Moyen et al. 2001). The main difference with typical sanukitoids is the lower Mg# (Mg# = mol Mg/Mg+Fe) of the Lewisian ferrodiorites and the lower light rare earth element (LREE) contents as shown later. They may represent the products of a water-rich intermediate (andesitic) magmatism that invaded the Archaean tonalite–trondhjemite continental crust at the time of granodiorite generation, in a way similar to sanukitoids in other cratonic areas. The inference is that they may have supplied water and potassium to the tonalitic crust and may have triggered partial melting with the consequent production of granodiorite magmas. In the absence of radiometric dating of these mafic rocks, field relations may be used to constrain age relations. Figure 5a shows the characteristic field relations observed between one of these ferrodiorite bodies, a granodiorite sheet and the hosting tonalite–trondhjemite banded gneisses. The granodiorite sheet clearly cross-cuts the foliation of the tonalite host and may appear locally as concordant with the foliation. Some layers of the tonalite are trapped in the granodiorite magma but they retain their original orientation. The upper contact of the granite sheet with the ferrodiorite layer is more complicated. The mafic layer is locally foliated but the contact with the granite sheet shows lobate shape with complex relations that resemble the contact between two magmas. Thus, these relations indicate that the mafic bodies may have a magmatic origin and that this mafic magmatism was coeval with the generation of granodiorites from melting of the tonalite host. Also relevant is the presence of fine K-feldspar bands in the tonalite host parallel and oblique to the main foliation in the vicinity of the granite sheet. The relations shown in Figure 5b are indicative of the dyke-like geometry of the mafic rocks. In this case the host is a granodiorite magma. From field observations at the contact between both, it is impossible to elucidate which is intruding into which. Possibly they were coeval magmas, the granodiorite generated in situ by melting of the tonalite host, and the ferrodiorite magma generated from a mantle source and intruded into a primitive tonalitic crust of the type forming the Assynt terrane (Scourie gneisses). Other relevant relations between granodiorites 56 A. CASTRO F. 5. Field relations between granodiorite sheets, tonalite gneisses and mafic rocks in the Rhiconich terrane of the Lewisian complex north of Laxford front. (a) Relations at the “Rock Stop” in the road from Scourie to Cape Wrath [NC 233488]. (b, c) Agmatitic relations between diorites and granodiorite at Kinlochbervie [NC 222562]. and mafic bodies may be observed in the area of Kinlochbervie in the Rhiconich terrane. Some representative exposures are shown in Figure 5b, c. Here, the mafic rocks appear to form agmatite complexes in which mutual relations between mafic bands and granites clearly indicate partial melting of the amphibole-rich diorites (Fig. 5b). In places, it is possible to recognize partially digested blocks of mafic rocks surrounded by a THE SOURCE OF GRANITES heterogeneous granodiorite. In detail, the contacts between these rock fragments and the enclosing granodiorite are transitional, in such a way that mineral aggregates and single crystals are partially or completely incorporated to the granodiorite magma (Fig. 5c). Finally, granite pegmatites form discordant dykes that appear to intrude mafic bodies and tonalite banded gneisses. These dykes are late with respect to the process of granodiorite generation and may represent a late stage of fluid activity in relation to the final cooling and crystallization of granodiorites. In order to characterize these mafic rocks and to compare with similar rocks associated with granite batholiths, representative samples collected from fresh exposures and road cuts in the area from Laxford Bridge to Kinlochbervie have been analysed for major and trace elements including the rare earth elements (REE). Some representative samples were also analysed for oxygen isotopes (Table 1). Figure 6 shows the main geochemical features of these mafic rocks and their comparison with other rocks of similar composition in the region. They have silica contents in the range of typical basalts and basaltic andesites and plot in the calc-alkaline and high-K calc-alkaline series fields (Fig. 6b). However, the high FeO content means that most of them plot in the tholeiite field in the AFM diagram (Fig. 6c), resembling the composition of the mafic rocks of the Central Region and Gruinard Bay (Rollinson 1987; Weaver & Tarney 1980b). The calc-alkaline affinity seems clear for these Rhiconich (Northern Region) mafic rocks. However, other geochemical features are not similar to typical calc-alkaline andesites and basalts. For instance, the chondrite-normalized REE patterns (Fig. 6d) are flat for most samples with slight LREE enrichment for a few. These LREE-enriched amphibolites resemble the REE patterns of diorites from Gruinard Bay (Rollinson 1996). However, major element geochemistry is quite different from these diorites, as mentioned above. A strong similarity is found with the REE patterns of the basic gneisses of the Assynt terrane (Scourie) according to data from Rollinson (1996) and Weaver & Tarney (1980b) (Fig. 6d). The mafic bodies from the Rhiconich terrane show a congruent variability in element–element plots and in the patterns of REE. These are indicative of a magmatic origin. However, the ultimate origin of these basic magmas still remains unclear. There is no correlation between the silica content and LREE enrichment, suggesting that part of the observed geochemical variations may be due to metasomatic changes produced in the course of metamorphism (cf. Rollinson 1996). There are important similarities between these mafic rocks of the Rhiconich terrane and the “basic gneisses” of Assynt terrane (Central Region) studied by Weaver & Tarney (1980b). Accordingly, these mafic rocks may be considered as derived from crystallization of a mantle-derived basaltic to basaltic andesite magma (Weaver & Tarney 1980b). The mantle origin is also supported by the constant and low oxygen isotopic ratios (Fig. 6e) with values of 18O around +5.7‰. The ferrodiorite with the highest 57 18O value (+7‰) differs from the other two analysed samples in the presence of K feldspar. However, this sample shows no enrichment in LREE and has a high Cr content (193 ppm). It is interesting to note that one of the samples with a mantle-like oxygen isotopic ratio is a biotite-rich amphibolite with K2O = 3.44 wt% and SiO2 = 44.11 wt%. It seems that the mantle source for this K-rich ferrodiorite was enriched in K and Rb, but not in LREE before melt extraction. The implication is that these ferrodiorites may represent mantle-derived magmas that carried important amounts of water and K from an enriched mantle region, possibly subductionrelated (cf. Weaver & Tarney 1980b). They intruded into a tonalite–trondhjemite continental crust and crystallized at depth within the crust. On crystallization, these water-rich magmas released the water exceeding that coordinated in hydrous phases (mainly hornblende). This water carried other soluble elements, mainly K, and metasomatized the crustal region surrounding mafic intrusions. The net effect of these K-rich fluids is the formation of granodiorite magmas by reaction with the tonalite host with the consequent production of granodiorite batholiths. Figure 7 shows schematically the process that may account for the field and geochemical relations observed in the Rhiconich terrane of the Lewisian complex. The process may be described as follows. In the initial stage all the water is retained by the intrusive ferrodioritic magma coming from melting of a subduction-related (enriched) hybridized mantle. Emplacement and crystallization of this magma in the lower continental crust may release water and K to the surrounding crustal rocks promoting partial melting (congruent melting). The relations observed in the migmatized ferrodiorites of the Lewisian complex indicate that even the monzodiorite rocks are incorporated into the melt by this mechanism. Laboratory experiments using two-layer capsules (water-rich andesite and tonalite) have been performed in order to simulate this interaction process between a tonalite source and fluids exsolved from a crystallizing andesite magma. These experiments have shown definite evidence for the transfer of water and K from the mafic magma layer to the tonalite at the conditions of 1000(C and 6 kbar. The whole tonalite layer becomes granodioritic and, at the same time, produces a large proportion of granodiorite melt. The only source for water and K in the capsule is a K-rich fluid phase that has been released from the mafic layer on crystallization. The fraction of granodiorite melt is near 50 vol%. The K2O content of this melt is about three times the original content of the tonalite. The K2O content of the whole layer is doubled with respect to the original composition of the tonalite. In summary, the tonalite layer is transformed into a granodiorite magma, with a melt fraction of about 50 vol%, by input of K released from the adjacent andesite. The result is a melt that will have components in part derived from the crust that hosted the ferrodiorite intrusions, and in part derived from the crystallization and melting of the monzodiorite rocks. 270 72 46 26 163 34 62 4 185 9 20 3 12 4 1 5 1 6 1 4 1 4 1 16 2 1 Hb Amphibolite enclave in Gd Loch na Fiacail 233488 50.74 1.22 12.84 16.08 4.61 0.23 7.52 2.71 1.37 0.15 1.8 99.27 A80116 274 493 450 141 169 16 97 4 323 11 26 4 18 4 2 4 1 4 1 2 0 1 0 10 1 1 5.3 Bt rich amphibolite Loch na Fiacail 233488 44.11 1.94 12.01 16.86 10.71 0.21 6.85 1.77 3.44 0.14 1.22 99.26 A80111 162 230 139 20 200 13 25 3 178 10 21 3 11 2 1 3 0 2 1 1 0 1 0 7 2 0 5.2 Hb Amphibolite band Loch Laxford 227476 53.58 0.54 15.27 9.56 7.43 0.14 7.72 3.75 0.76 0.12 0.81 99.68 A80120 208 364 168 47 196 19 85 4 185 9 17 3 10 3 1 3 1 3 1 2 0 2 0 13 1 1 Mafic layer Laxford Bridge 235468 50.08 0.62 14.12 12.14 8.31 0.21 8.58 3.27 1.65 0.01 1.35 100.34 A80131 259 194 101 21 239 29 91 4 276 9 21 3 13 4 1 5 1 5 1 3 1 3 1 9 0 0 7.0 Amphibolite band Loch Inchard 237559 50.22 1.26 14.52 12.99 5.53 0.19 8.34 3.27 1.55 0.05 1.18 99.1 A80136 260 83 43 31 163 25 68 7 227 13 28 4 16 4 1 5 1 5 1 3 0 3 0 18 2 1 Amphibolite migmatite Kinlochbervie 222562 49.75 1.29 13.18 15.45 5.55 0.22 7.76 2.84 1.67 0.15 0.97 98.83 A80137 265 104 48 21 135 25 45 7 158 13 29 4 17 4 1 5 1 5 1 3 0 3 0 9 3 1 Amphibolite migmatite Skerricha jct 247510 49.37 1.51 13.11 15.96 5.47 0.22 9.05 2.82 1.21 0.13 0.8 99.65 A80144 Major elements analysed by XRF at the University of Oviedo. Trace elements determined by ICP-MS at the University of Huelva. Oxygen isotopes determined at the University of Salamanca (analytical details in the Appendix) Location Coordinates (NC) SiO2 TiO2 Al2O3 Fe2O3 MgO MnO CaO Na2O K2O P2O5 LOI Total Trace elements (ppm) V Cr Ni Rb Sr Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Pb Th U 18O (‰) Rock type Sample TABLE 1 Representative analyses of dioritic rocks of the Rhiconich area (Lewisian complex) north of Laxford front. 58 A. CASTRO THE SOURCE OF GRANITES 59 F. 6. Geochemical diagrams showing the main compositional features of ferrodiorites in the Rhiconich terrane of the Lewisian complex north of Laxford front. Samples from similar rocks in adjacent terranes are shown for comparison. Concluding remarks One of the most intriguing geochemical features of I-type granites and many peraluminous S-types is the apparently ‘decoupled’ isotopic ratios of Nd and Sr (Weaver & Tarney 1980a). These granites typically contain microgranular enclaves, have low 87Sr/86Sr, close to mantle values, and at the same time they have negative and variable Nd values, in part close to typical crustal values (McCulloch & Wassergurg 1978). The low 87Sr/ 86 Sr values of these granodiorite batholiths may be inherited from the crustal source according to the Archaean crustal array for Sr isotopes as discussed above. If granites were produced by melting alone of an Archaean tonalite source, their Nd model ages would be Archaean and independent of the age of generation. However, for Palaeozoic I-type batholiths, Nd model ages are systematically younger than those of Proterozoic batholiths (Fig. 1b). Nevertheless, these model ages are always older than ages of generation for any I-type granite around the world. Obviously, the generation of granites from Archaean tonalite sources cannot be accounted for by a simple process of partial melting in a closed system. The addition of a water-rich and K-rich fluid is required as based on experimental grounds. The inference is that this fluid may be supplied by intermediate magmas (andesitic or monzodioritic), produced from enriched mantle sources that intruded the tonalite crust at the time of granite magma generation. Not only do fluids trigger partial melting in the continental crust, but they may also modify the Nd isotopic signatures by addition of recycled and fresh 60 A. CASTRO F. 7. Idealized sketch showing the hypothetical stages in the formation of granodiorite sheets in relation to the intrusion of K-rich magmas into a tonalite crust. The crystallization of the water and K-rich magmas: (a) exsolve fluids that trigger partial melting of the tonalite host; (b) granodiorite sheets together with remnants of migmatized diorites are deformed; (c) producing the observed field relations in the Lewisian complex (northern region). components. An implication from the observed model ages of granite batholiths is that this recycled component must be younger than the Archaean crust, and also it must be younger for younger batholiths. That is, the residence times are shorter for younger batholiths in comparison with older ones. This may be accomplished by two complementary mechanisms: incorporation of fresh mantle material at the time of generation; and incorporation of recycled components (sediments) slightly older than the age of generation. Both processes are expected to be coupled in subduction environments by incorporation of subducted sediments to mantle regions where andesite magmas are generated. These andesites will carry Nd isotopic signatures intermediate between fresh mantle and recycled sediments, giving rise to a model age in the crustal granites older than the age of generation and younger than that of the Archaean source. Another major implication is the need for water to effect partial melting of the crustal source. Two main lines of argument lead to this conclusion. Firstly, the proportion of melt is in excess compared with fluidabsent melting experiments. Secondly, the expected phase assemblages resulting from a fluid-absent melting reaction, composed of typical peritectic minerals, such as orthopyroxene or garnet in metaluminous systems, is absent in these deep domains with partially melted rocks and agmatitic structures as it is the case of the Rhiconich terrain of the Lewisian complex. Only a process of congruent melting in the presence of water accounts for these features. Although the presence of a free-water phase is very unlikely in the deep continental crust (Yardley & Valley 1997), water is an essential component dissolved in silicate melts that may be coordinated in the form of OH groups to the melt structure (Burnham 1979). Water solubility is enhanced at higher pressures in such a way that a silicate magma may dissolve up to 20 wt% water at moderate pressure (Burnham 1979; Holtz et al. 2001b). Thus, a silicate magma is an important agent for water transport to the source region where granite magmas are generated. These water-bearing magmas may be the monzodiorite melts that invaded the source region before melting and granite generation. Estimations by Grove et al. (2002) indicate that andesite magmas may have a pre-eruptive water content of about 5 to 7 wt%. Only a small fraction of this water (less than 1 wt% of the whole system) will be used in the formation of hydrous minerals such as micas and amphiboles in the course of magma crystallization; most of the water will be released to the surrounding crustal rocks promoting melting and granite magma generation. The process of melting in the source region is strongly favoured if pressure is decreased by lithospheric extension and if temperature is increased by mantle upwelling. As both processes are coupled, melting of the composite source region is promoted with the consequence that even the previously intrusive monzodiorite magma may crystallize, transferring volatiles to the host and later may be affected by the melting process during decompression, as implied by the field relations observed in the Rhiconich terrane of the Lewisian complex. In the case of the Lewisian complex, the crust hosting the monzodiorite intrusions is a tonalite derived by partial melting of oceanic crust (Rollinson 1996). Consequently, all recycled components are supplied by subducted sediments that were involved in the early generation of the monzodiorites. The melts generated from this composite source will be dominantly granodiorites due to the assemblage of the hosting crustal tonalite gneiss dominated by plagioclase, quartz and amphibole. The isotopic signatures of these granodiorites will be mantle-like for Sr isotopes and crustal-like for Nd isotopes. This is because most of the Sr is supplied by the tonalite host and most of the Nd is supplied by the monzodiorite intrusive magma, which in turn is derived by melting of a subduction-enriched region of the mantle. This interpretation accounts for the paradoxical relations observed in granodiorite (I-type) batholiths in terms of isotopic signatures. Also resolved is the difficulty in finding an appropriate igneous source for granodiorites and a source for water as needed to satisfy experimental and field constraints. The igneous source for I-type granites (Chappell & White 1974) is demonstrated by these field, geochemical and experimental constraints, and the complexity of the processes that prepared the source for magma 61 THE SOURCE OF GRANITES generation are presented here in a new model. The presence of more than a single source component in the generation of granite batholiths was inferred from isotopic mixing trends of granite batholiths (Collins 1996). The nature of these source materials and the way in which they are incorporated within granite compositions are explained by this genetic model. This model may be compatible with the two-stage process inferred for the generation of the Cordilleran batholiths (I-type) of the Andes (Atherton & Petford 1993). The new model establishes a cause–effect relationship between these two stages as the first stage, melting within the mantle, produces the water and K-rich monzodiorite melt that will supply water and hence promote melting in a second stage at the deep crustal level where the monzodiorite magma was emplaced. Part of this monzodiorite melt may reach the surface and extrude giving rise to the andesitic volcanism that characterizes continental arcs, and part may be emplaced and crystallized at depth in the crust promoting melting and the generation of granodiorite batholiths. In the case of emplacement of the water and K-rich monzodiorite magma into a pelitic or greywacke host at the medium to upper crust, the sequence of processes may be very similar to the above-described case for the lower crust. The main difference is in the composition of the host crust that will produce a granite magma richer in K in comparison with the lower crust granitoids. The incorporation of components from the monzodiorite rocks (enclaves) accounts for the apparently anomalous monzogranite and granodiorite composition of the anatectic melts. The Ca content of these anatectic granites cannot be accounted for by pure melting of the pelitic migmatites alone, as evidenced by recently published melting experiments on these protoliths (Castro et al. 2000). Together with Ca, the same process of enclave dissolution at the source region and during magma ascent will incorporate Sr with the low isotopic ratio characteristic of the mantle-derived monzodiorite. This model solves the paradox found in these S-type granites which, if derived from a pelitic or greywacke source with typical crustal values for Nd isotopes and a peraluminous composition, have surprisingly low isotope ratios for Sr, in many cases close to mantle values. The combined use of three independent lines of reasoning – (1) field and age relations in deep crust domains (Lewisian complex), (2) phase relations for granodiorite magma generation deduced from laboratory experiments, and (3) Sr and Nd isotope ratios – has led to the formulation of this new hypothesis for the generation of granodiorite batholiths. It is concluded that the generation of granite batholiths (I-type) is a consequence of complex mass and heat transfer processes that operate on a tonalite lower crust of possible Archaean age. The low Sr isotopic ratios of these granitoids are inherited from a crustal array and not acquired by hybridization at the time of generation. It is not excluded that some minor interchanges between granodiorite magma and basaltic intrusions may occur at the local scale. Acknowledgements The paper was written during a sabbatical leave in the University of St Andrews in 2001. I thank colleagues at St Andrews for the warm hospitality and the facilities. Discussions with Ed Stephens, Tony Prave and Graham Oliver were of great help. Comments and criticisms by Rob Ellam helped to improve the manuscript. Financial support for fieldwork in Scotland was supplied by grant PR2000-0372 of the Spanish Ministry of Education. Appendix Analytical techniques Major elements were analysed by X-ray fluorescence (XRF) at the University of Oviedo (Spain) using glass beads. Typical precision of XRF technique was better than 1.5% relative. Trace elements and REE were analysed with an HP-4500 ICP-MS at the University of Huelva (Spain), following digestion in a HF+HNO3 (8:3) solution, drying and second dissolution in 3 ml HNO3 and later 3 ml HCl. The average precision and accuracy for most of the elements fall in the range of 5–10% relative, and they were controlled by repeated analysis of SARM-1 (granite) and SARM-4 (norite) international rock standard. Details of analytical techniques for oxygen isotope determinations are in Recio et al. (1992). Experimental procedures and SEM-EDS analytical techniques of experimental charges are in Castro et al. (1999). Data sources In Figure 3b, relative abundances of K-granites, tonalites–trondhjemites and greenstones in Precambrian shields have been estimated from geological maps and other information published in various chapters of De Witt & Ashwall (1997) as follows (only page numbers are shown for each reference) a, Amazonian craton: Tassinari, C. C. G. The Amazonian Craton, pp. 558–566. b, Greenland: Nutman, A. The Greenland sector of the North Atlantic Craton, pp. 665–674. c, d, Baltic shield: Sorjonen Ward, P., Nironen, M. & Luukkonen, E. J. Greenstone associations in Finland, pp. 677–698. Puchtel, I.S., Shchipanski, A.A. & Zhuravlev, D. Z. The Karelian granite–greenstone terrain in Russia, pp. 699–706. e, Siberia: Dobretsov, N. N. et al. The AldanStanovick Shield, pp. 710–725. f, Slave Province: King, J. & Helmstaedt, H. The Slave Province, North-West Territories, Canada, pp. 459–479. g, Superior Province: Stott, G. M. The Superior Province, pp. 480–507. h, Wyoming Province: Wilks, M. & Harper, G. D. The Wind River Range, Wyoming Craton, USA, pp. 508–516. i, West Africa: Attoh, K. & Ekwueme, B. N. The West African shield, pp. 517–528. 62 A. CASTRO TABLE A1 Experimental data and compositions of starting materials of Figure 2. Numbers in parentheses are modal abundances of biotite (Bt), plagioclase (Pl), quartz (Qtz) and K-feldspar (Kfs) Sample Bt Tonalite gneiss (FOG) Bt (19). Pl (38). Qtz (35). Kfs (7) SiO2 63.88 TiO2 0.77 Al2O3 16.86 FeO 3.37 MnO 0.10 MgO 3.08 CaO 4.04 Na2O 3.54 K2O 2.63 LOI 1.49 Sum 99.76 Bt Tonalite (Zarza) Bt (20). Pl (40). Qtz (35). Kfs (5) 67.80 0.51 16.10 3.70 0.03 1.50 2.76 3.79 2.57 0.75 98.76 TTG Archaean average (Condie 1997) Bt (14). Pl (55). Qtz (31) 69.50 0.34 15.10 3.50 0.00 1.10 2.80 4.50 2.40 – 99.24 Melt compositions obtained in the melting experiments P (kbar) T ((C) Run no H2O (n) SiO2 TiO2 Bt Tonalite gneiss (FOG) 63.88 0.77 3 850 ME18 0 5 76.68 0.44 3 980 ME27 0 5 74.54 0.76 6 700 ME6b 5 10 75.97 0.16 6 725 ME11 5 6 76.16 0.06 6 750 ME3a 5 5 76.15 0.24 6 750 ME3b 10 6 74.45 0.19 6 775 ME33 2 6 76.17 0.26 6 850 ME16 0 3 73.40 0.29 6 900 ME19 0 4 74.49 0.36 6 980 ME25 0 5 73.37 0.69 10 675 ME28a 5 4 73.10 0.28 10 725 ME34 5 8 74.89 0.17 10 850 ME17 0 3 74.87 0.18 10 950 ME23 0 7 73.09 0.59 10 980 ME26 0 8 71.00 0.71 Al2O3 16.86 13.67 13.69 14.92 15.09 14.32 15.47 15.01 15.48 14.98 15.03 16.53 16.40 15.12 15.08 15.90 FeO 3.37 0.98 1.50 0.42 0.50 1.06 1.08 0.85 1.45 1.44 1.75 1.09 0.74 0.82 1.01 2.18 MnO 0.1 0.17 0.07 0.06 0.18 0.08 0.13 0.11 0.11 0.07 0.15 0.09 0.09 0.13 0.10 0.06 MgO 3.08 0.46 0.93 0.05 0.25 0.27 0.36 0.25 1.35 0.71 0.40 0.80 0.15 0.22 0.30 1.05 CaO 4.04 0.61 0.39 2.19 1.87 2.28 2.81 1.46 1.46 0.81 0.82 2.20 3.06 1.39 0.90 0.86 Na2O 3.54 2.44 2.75 2.49 2.63 3.22 3.36 2.58 2.53 2.58 2.61 2.31 2.40 2.74 3.20 2.97 K2O 2.63 4.54 5.37 3.74 3.26 2.39 2.16 3.32 3.93 4.55 5.18 3.62 2.10 4.52 5.74 5.28 Sum 99.76 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 Bt Tonalite (Zarza) 7 900 AC101a 7 850 AC102a 100-Sum 5.84 2.57 12.56 12.22 11.29 12.58 11.02 6.98 8.58 4.47 14.3 15.59 8.30 5.74 5.37 5 5 67.8 74.90 75.00 0.51 0.31 0.06 16.1 14.38 16.33 3.7 1.33 1.85 0.03 0.00 0.00 1.5 0.00 0.16 2.76 0.39 0.54 3.79 3.52 2.00 2.57 5.17 4.07 98.76 100.00 100.00 4.34 6.22 TTG Archaen average (Condie 1993) 7 900 AC104 0 4 69.5 74.41 0.34 0.41 15.1 16.41 3.5 1.04 0 0.00 1.1 0.41 2.8 0.96 4.5 2.85 2.4 3.52 99.24 100.00 4.23 0 0 j, Sao Francisco (Brazil): Baars, F. 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