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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. J. The Sao
Francisco Craton, pp. 557–566.
k, Zimbawe: Blenkinsop, T., Martin, A., Jelsma,
H. A. & Vinyu, M. L. The Zimbabwe Craton,
pp. 567–580.
l, Kaapvaal: Brandl, G. & de Witt, M. J. The
Kaapvaal Craton, South Africa, pp. 581–607.
m, n, Tanzania and NE Zaire: Borg, G. & Shackleton,
R. M. The Tanzania and NE-Zaire Cratons,
pp. 608–619.
o, Indian shield: Rogers, J. J. W. & Giral, R. A. The
Indian Shield, pp. 620–635.
p, Madagascar: Rambeloson, R. A. The Malagasy
Shield, pp. 636–639.
q, Yilgarn: Myers, J. S. & Swager, C. The Yilgarn
Craton, Australia, pp. 640–656.
r, Pilbara: Barley, M. E. The Pilbara Craton,
pp. 657–664.
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MS. accepted for publication 13 September 2003