Journal of Petrology Advance Access originally published online on November 4, 2004
Journal of Petrology 2004 45(12):2481-2505; doi:10.1093/petrology/egh074
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Journal of Petrology 45(12) © Oxford University Press 2004; all rights reserved
Magmatic Evolution of the Skye Igneous Centre, Western Scotland: Modelling of Assimilation, Recharge and Fractional Crystallization
1 DEPARTMENT OF GEOLOGICAL SCIENCES, CENTRAL WASHINGTON UNIVERSITY, ELLENSBURG, WA 98926, USA
2 DEPARTMENT OF GEOLOGICAL SCIENCES AND INSTITUTE FOR CRUSTAL STUDIES, UNIVERSITY OF CALIFORNIA, SANTA BARBARA, SANTA BARBARA, CA 93106, USA
RECEIVED SEPTEMBER 1, 2003; ACCEPTED AUGUST 30, 2004
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONTHE SKYE IGNEOUS CENTRE,...EC-RAFC GEOCHEMICAL TRAJECTORIESDISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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The Skye igneous centre, forming part of the British Tertiary magmatic province, developed over a
7 Myr period (61–54 Ma) and is characterized by a complex suite of lavas, hypabyssal and intrusive rocks of picritic to granitic composition. The intrusion of magma from mantle to crust at 2x10–3km3/yr (6 Mt/yr) advected magmatic heat of roughly 0·2 GW averaged over the period of magmatism supporting an ‘excess’ heat flux of about 130 mW/m2, or about twice the present-day average continental heat flow. The volume of new crust generated at Skye (15000 km3) spread over the present-day area of Skye corresponds to 9 km of new crust. The geochemical evolution of the Skye magmatic system is constrained using the Energy-Constrained Recharge, Assimilation, and Fractional Crystallization (EC-RAFC) model to understand variations in the Sr- and Pb-isotopic and Sr trace-element composition of the exposed magmatic rocks with time. The character (composition and specific enthalpy) of both assimilant and recharge magma appears to change systematically up-section, suggesting that the magma reservoirs migrated to progressively shallower levels as the system matured. The model of the magma transport system that emerges is one in which magma batches are stored initially at lower-crustal levels, where they undergo RAFC evolution. Residual magma from this stage then migrates to shallower levels, where mid-crustal wall rock is assimilated; the recharge magma at this level is characterized by an increasingly crustal signature. For some of the stratigraphically youngest rocks, the data suggest that the magma reservoirs ascended into, and interacted with, upper-crustal Torridonian metasediments. KEY WORDS: assimilation; EC-RAFC model; geochemical modelling; magma recharge; Skye magmatism
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONTHE SKYE IGNEOUS CENTRE,...EC-RAFC GEOCHEMICAL TRAJECTORIESDISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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The compositional diversity of igneous rocks on Earth reflects the complex interplay between physical and chemical processes during magma generation, segregation, ascent, storage, mixing, crystallization and eruption. Bowen (1928)
noted the essential connection between fractional crystallization and crustal assimilation. He recognized that the release of latent heat as a result of the cooling and crystallization of magma could, in favorable circumstances, raise wall-rock temperatures above the local solidus, Ts(ai, p), where ai represents the activity of the independent chemical components and p is the pressure. Physical mobilization and subsequent addition of anatectic melt and country-rock residue to a magma body leads to chemical contamination, which can influence the course of crystallization. Significant heat transfer and magma compositional evolution occur even when no country-rock partial melting takes place, as magmatic heat removal is a requirement for crystallization and fractional crystallization changes the composition of derivative liquids as governed by the principles of phase equilibria. In addition to fractional crystallization and assimilation, the addition of new magma batches (recharge), which often are compositionally and thermally distinct, also influences magma evolution via the process of magma mixing and, if the specific enthalpy of the recharge magma is higher than that of the magma it intrudes, by providing additional energy for country-rock heating, anatexis and possible additional contamination. A high magma recharge rate both enhances anatexis and increases the probability that wall-rock partial melts will gain access to the evolving magma. To quantify magma contamination, it is convenient to define an effective mass of country rock, 
, that acts as a sink for magma-derived heat on the time-scale of magma compositional and thermal evolution (102–106yr). The mass ratio of assimilant melt added to magma, 
, to the mass of country rock acting as a sink, , is a convenient measure of the importance of chemical contamination. Here, 
represents the fraction of anatectic melt generated in country rock that is added to (or contaminates) the evolving magma. The parameter 
serves as a measure to gauge the importance of magma contamination by assimilation. Similarly, Mr and Mc serve as metrics to gauge the importance of recharge and fractional crystallization, respectively. It is desirable to relate 
, Mr and Mc to the chemical evolution of magma via a relationship that couples magma energetics to compositional evolution, to account for the complexities of simultaneous recharge and fractional crystallization. Trace-element concentrations and isotope ratios are especially useful petrogenetic markers. One of the outstanding challenges in igneous petrology and geochemistry is to quantify RAFC phenomena and discriminate shallow-level signatures from those imparted to the primary magma by variable degrees of melting of heterogeneous mantle sources at specific environmental conditions of ai, pressure and temperature.
In this study, we use the Energy-Constrained Recharge, Assimilation, and Fractional Crystallization (EC-RAFC) geochemical modelling tool (Bohrson & Spera, 2001, 2003; Spera & Bohrson, 2001, 2002) to examine the complex petrologic history of a small portion of the British Tertiary Igneous Province (BTIP). Our goal is to assess quantitatively the relative importance of assimilation, recharge and fractional crystallization in the petrologic evolution of the compositionally diverse Skye Tertiary igneous centre in western Scotland. Previous workers have attributed the geochemical diversity of the Skye magmatic rocks to multiple heterogeneous mantle sources (e.g. asthenosphere, lithosphere, depleted lithosphere, etc.) along with a variety of crustal-level processes. The Skye igneous centre was chosen because it has a long history of investigation (e.g. Harker, 1904; Anderson & Dunham, 1966; Moorbath & Welke, 1969; Thompson et al., 1972; Dickin, 1981; Thompson, 1982; Mussett et al., 1988; Dagley et al., 1990; Gibson, 1990; Emeleus & Gyopari, 1992; Scarrow, 1992; Hamilton et al., 1998) and because it exhibits a complex evolution that provides a stringent test of the EC-RAFC model. Available geochemical data show unequivocal evidence for the involvement of open-system processes in the petrogenesis of intrusive and extrusive centres throughout 7 Myr of Skye Tertiary magmatism; the EC-RAFC model was specifically derived to accommodate open-system behaviour. Because of the complexity and protracted history of magmatism, the EC-RAFC simulator was applied sequentially to each of three distinct petrologic lineages defined on the basis of spatial relatedness, geochronology and petrological characteristics, as gleaned from the published literature. The purpose is to develop an improved quantitative understanding of the roles that shallow and deep-level processes played at Skye and to forge a link between regional-scale material and energy balances and the record preserved in exposed igneous rocks.
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THE SKYE IGNEOUS CENTRE, WESTERN SCOTLAND |
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TOPABSTRACTINTRODUCTIONTHE SKYE IGNEOUS CENTRE,...EC-RAFC GEOCHEMICAL TRAJECTORIESDISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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The Skye igneous centre (Fig. 1) is part of the BTIP, the site of voluminous mafic to silicic magmatism in mainland western Scotland and the offshore Hebridean islands, associated with the opening of the North Atlantic Ocean (Saunders et al., 1997
). On Skye, the bulk of igneous activity is hypothesized to have occurred in less than 3 Myr (Chambers & Pringle, 2001), although the total duration of magmatism was 7 Myr. The first event was eruption of a 1·2–1·7 km thick succession of flood basalts (the Skye lavas) exposed in north and west–central Skye over an area of 1550 km2 (Thompson, 1982; Emeleus, 1991; England, 1994; Williamson & Bell, 1994). The Skye Lavas dominantly consist of transitional to alkalic basalt compositions (including both hy and ne normative basalts) of the Skye Main Lava Series (SMLS). The relative depletion of heavy rare earth elements is consistent with residual garnet in the SMLS source region and hence suggests melt segregation of parental SMLS liquids from peridotitic mantle beneath relatively thick (80 km) lithosphere within the stability field of garnet (Ellam, 1992; White & McKenzie, 1995). In west–central Skye, a later tholeiitic magma type, the Preshal More basalt (PMB), lies unconformably upon the volumetrically dominant SMLS and exhibits strong chemical similarity to mid-ocean ridge basalt (MORB) (Thompson, 1982). The SMLS to PMB compositional change has been interpreted as representing progressive upward movement of the region of partial melting because of lithospheric thinning during the early stages of opening of the North Atlantic basin in the Paleocene. Presumably, the transport of mantle heat by magma advection was a contributing factor to lithospheric thinning. A maximum age for the Skye lavas has been estimated at 60·53 ± 0·08 Ma by Hamilton et al. (1998), based on the U–Pb zircon age of the Rum central complex. Clasts of the Rum central complex are present in conglomerates intercalated with Skye lavas in the lower portion of the lava field. Based on zircon-bearing pegmatites in gabbros of the Cuillin centre, which intrude the Skye Lavas, Hamilton et al. (1998) estimated an upper age limit for the Skye lavas of 58·91 ± 0·07 Ma and hence a maximum duration of magmatism of 1·6 ± 0·3 Myr. Jurassic sediments that underlie Skye lavas host an extensive intrusive suite, the Little Minch Sill Complex (Gibson & Jones, 1991). Intrusive relationships indicate that the sill complex is marginally younger than the Skye lavas (Gibson, 1990).
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Towards the end of the period of flood basalt volcanism, a central intrusive complex developed, in which four magmatic centres, each comprising multiple intrusions, formed in succession as the focus of igneous activity moved progressively eastward. From west to east, these are the Cuillin centre, which is dominantly mafic, and the Srath na Creitheach, Western Redhills, and Eastern Redhills centres, which are dominantly granitic (e.g. Emeleus, 1982
; Emeleus & Gyopari, 1992). Based on gravity data, the Cuillin centre is believed to be an approximately cylindrical mass extending to at least 16 km beneath the surface (e.g. Bott & Tucson, 1973). Although the earliest Cuillin magmas have SMLS compositions, the bulk of the centre displays a strong compositional affinity with PMB lavas. Numerous dykes of the main Skye swarm cut the SMLS and chemically are similar to PMB tholeiites, probably injected laterally from the Cuillin centre (Walker, 1993). Dickin & Exley (1981) reported an age of 59·30 ± 0·7 Ma for the youngest Cuillin centre intrusion, the Coire Uaigneich Granophyre. The Srath na Creitheach centre is small and poorly documented, and is not included in this investigation. The Western and Eastern Redhills granites are approximately 2 km thick (Goulty et al., 1992). The presence of hybrid bodies in the Western and Eastern Redhills centres suggests that basaltic magma (evidence of recharge) was available during the formation of the granitic centres. Further evidence of this is intrusion of mafic to intermediate cone sheets and dykes throughout the history of the central complex (Emeleus & Gyopari, 1992). Age determinations have been reported for the Loch Ainort granite of the Western Redhills centre (Table 1) (58·58 ± 0·3 Ma; Chambers & Pringle, 2001) and the Beinn an Dubhaich granite of the Eastern Redhills centre (Table 1; Outer Granite) (53·5 ± 0·8 Ma; Dickin, 1981). These relations are summarized in Table 1.
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A purpose of this paper is to develop a semi-quantitative account of Skye geochemical evolution based on the fundamental processes of assimilation, magma mixing and fractional crystallization, without calling on exotic processes or special mantle sources. We use Pb- and Sr-isotope and Sr-concentration data, along with inferred crustal assimilant compositions and the melting behaviour of possible crustal contaminants, to model the relative importance of assimilation, recharge and fractional crystallization, develop first-order quantitative estimates of material budgets (i.e. relative masses of assimilant, recharge magmas and cumulates) and address the implications for the growth and differentiation of crust in the Skye province. Our goal is to connect regional-scale ‘geophysical’ estimates for crustal growth rates and heat transport summarized in the next section to the petrological evolution recorded in the composition of exposed intrusive, hypabyssal and extrusive igneous rocks at Skye.
Magma and energy budget
Given a surface area of 1550 km2 for the Skye lavas and Little Minch Sill Complex (see Fig. 1 and Emeleus & Gyopari, 1992) and a mean lava pile thickness of 1·5 km, the total volume of lavas and sills is approximately 2300 km3. Based on an eruption interval of 1·6 Myr, a mean eruptive rate of the Skye lavas and Little Minch Sill Complex is 1·5 x 10–3 km3/yr. This figure is consistent with a wide range of other crustal-level open magmatic systems (e.g. Crisp, 1984; White et al., 2003). The volumes of the Cuillin and Redhills centres are approximately 104 and 250 km3, respectively (based on data in Fig. 1 and Emeleus & Gyopari, 1992). Thus, the integrated mean ratio of intrusive to extrusive magma production was 5 in the Skye province. The mean rate of magma production (as opposed to rate of eruption) during the 7 Myr period of magmatism at Skye was 1·7 x 10–3 km3/yr. The similarity of this figure to the mean eruptive rate estimated for Skye lavas and Little Minch Sill Complex suggests that the mean rate of magma generation remained approximately constant during Skye igneous activity, although the ratio of intrusive to extrusive mass was not constant, but instead markedly increased as time progressed in the interval 61–54 Myr. Hamilton et al. (1998) estimated a magma production rate of 2·2 x 10–3 km3/yr for the bulk of the igneous activity at Skye based on an estimate of 1400 km3 for the combined volume of the lava succession, the mafic and ultramafic rocks of the Cuillin complex, and several high-precision Pb–U zircon dates. The agreement between our volumetric rate estimate and that of Hamilton et al. (1998) gives some confidence in the quality of these estimates, at least to ±20%. Adopting a figure of 2 x 10–3 km3/yr implies formation of 1·4 x 104km3 of new crust over the 7 Myr interval of Skye magmatism. Averaged over the area of Skye, this corresponds to a crustal thickness increment of 9 km. The volumetric magma generation rate (
) of 2 x 10–3km3/yr (6 Mt/yr) in the mantle beneath Skye can be used to estimate the magma heat power and ‘excess’ heat flux transferred from mantle to crust or atmosphere. The average magma heat power, 
, transferred from mantle to crust during the 7 Myr period of magmatism at Skye computed from
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is magma density (2800 kg/m3), Cp is the isobaric specific heat capacity (1200 J/kg K), Tm is the magma temperature (1500 K), 
is the mean crustal temperature (800 K) and 
Hcrystallization is the specific latent heat of crystallization for basalt (380 kJ/kg). Values for the parameters are taken from Spera (2000). Distributed uniformly over the area of the Skye igneous province (1650 km2), this magma heat power corresponds to an ‘excess’ heat flux of 130 mW/m2. For comparison, the global mean heat flux through continental crust is currently about 57 mW/m2. Hence, magmatism at Skye played an important, indeed dominant, role in regional geothermics and crustal growth in the Paleocene.
Previous models
It is impossible to provide a comprehensive review of the truly voluminous literature pertaining to the geology and petrology of Skye. Here, we briefly review a few recent studies most germane to the present study. Table 1 presents an overview of the chronology, petrology and petrography of igneous rocks from the Skye Tertiary igneous centre used in this modelling study; it shows compositional ranges and textural features indicative of open-system processes and is arranged stratigraphically, from oldest at bottom, to youngest at top. The lithologic divisions of the map units shown on Fig. 1 are based on compilations from Bell (1976), Mussett et al. (1988), Dagley et al. (1990) and Hamilton et al. (1998). Many workers have emphasized the complex evolution of Skye magmas inferred from the lack of correlation and monotonicity between differentiation indices (such as SiO2 and MgO), incompatible trace-element concentrations and Sr–Nd–Pb-isotope systematics. Some early studies suggested that parental magmas inherited their compositional traits from a heterogeneous Archaean sub-continental lithospheric mantle source and did not interact with the continental crust en route to the surface (e.g. Beckinsale et al., 1978; Pankhurst & Beckinsale, 1979). More widely accepted hypotheses, however, involve variable degrees of mantle-source melting at a range of depths (Scarrow & Cox, 1995) or polybaric fractional crystallization in combination with selective crustal contamination, either by migration of elements in a fluid phase (e.g. Moorbath & Thompson, 1980; Thompson et al., 1980; Dickin, 1981) or by a silicate melt composed of fusible components of Lewisian gneiss (e.g. Thompson et al., 1982, 1986; Dickin et al., 1987; Geldmacher et al., 2002). In short, a menagerie of petrologic processes and source materials has been called upon to explain the petrogenesis of Skye Tertiary rocks.
Nature of the crust beneath Skye
Geophysical studies indicate that the crust beneath Skye is approximately 25–28 km thick and is dominated by Archaean (Lewisian) orthogneisses (Bott & Tucson, 1973; Bamford et al., 1977; Bott & Tantrigoda, 1987; Chadwick & Pharoah, 1998). Granulite-facies Lewisian gneisses (LG) form the lower crust and are overlain at mid-crustal levels by amphibolite-facies Lewisian gneisses (LA) (Weaver & Tarney, 1980, 1981). Both crustal types crop out on the Scottish mainland. The granulite complex is compositionally bimodal (i.e. tonalite and basic granulite), and exposed portions contain a greater proportion of mafic and ultramafic material than the amphibolite complex (Wood, 1980). The uppermost crust is made up of late Precambrian [Torridonian (T)] metasediments with ages in the range 1·1–0·75 Ga, based on a few radiometric and palaeomagnetic dates (Gass & Thorpe, 1976; Dickin & Exley, 1981). T rocks include fluviatile arkosic sandstones and siltstones (Dickin & Exley, 1981) with intercalated shale beds and thin calcareous lenses present in the lower part of the succession. In the upper part, arkoses predominate. On Skye, T metasediments are overlain by a thin (900 m) veneer of Mesozoic sediments (Gass & Thorpe, 1976).
LG, LA and T sedimentary rocks have different mean isotopic compositions and ranges (Table 2). On average, amphibolite-facies Lewisian rocks are more radiogenic (Sr and Pb) than the granulite-facies rocks (Dickin et al., 1984). In comparison with Lewisian rocks, T metasediments generally have higher 87Sr/86Sr and 206Pb/204Pb. The range of 208Pb/204Pb for T sediments lies within the range of Lewisian amphibolite (LA) gneiss (Dickin & Exley, 1981; Dickin et al., 1984). These relations are summarized in Table 2.
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EC-RAFC GEOCHEMICAL TRAJECTORIES |
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TOPABSTRACTINTRODUCTIONTHE SKYE IGNEOUS CENTRE,...EC-RAFC GEOCHEMICAL TRAJECTORIESDISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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In the section below, we apply the EC-RAFC simulator in order to quantify petrogenetic processes relevant to the Skye Tertiary igneous rocks. We have broken the complex
7 Myr magmatic history of Skye into three main lithologic lineages (L1, L2 and L3). The lineages are defined based on geologic criteria, including spatial relatedness, petrologic affinity and radiometric and palaeomagnetic geochronology deduced from previous workers (Thompson, 1969; Thompson et al., 1972; Dickin, 1981; Dickin & Exley, 1981; Bell, 1983; Bell & Harris, 1986; Tantrigoda, 1988; Dagley et al., 1990; Gibson, 1990; Emeleus & Gyopari, 1992; Scarrow, 1992; Bell et al., 1994; Hamilton et al., 1998), as summarized in Table 1. These lineages provide the framework for the computation of EC-RAFC geochemical trajectories. Additional lineages may be present, but are not considered here because of lack of geochemical data. Table 3 provides the salient features of the defined lineages, including cross-reference to the lithologic units summarized in Table 1, the approximate range of observed silica contents, Pb- and Sr-isotope ratios, Sr abundance, likely crustal contaminants (see Table 2) and the name of each EC-RAFC trajectory. There are three EC-RAFC trajectories for lineage L3 (EC3A, EC3B and EC3C), two for lineage L2 (EC2A and EC2B) and five for lineage L1 (EC1A, EC1B, EC1C, EC1D and EC1E). Hence, 10 distinct EC trajectories were used to model the 7 Myr magmatic evolution at Skye. Sr- and Pb-isotope data and Sr-concentration data used to gauge the quality of the EC models are shown in Fig. 2a–d. In that figure and on computed EC-RAFC trajectories, the raw data are plotted (broken into L1, L2 or L3) to facilitate comparison between observation and model.
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The EC-RAFC model provides information on any trace-element and isotope ratio of interest. Unfortunately, the trace-element and isotope geochemistry database on Skye rocks relevant to L1, L2 and L3 is incomplete. Compared with other geochemical data, abundant published Sr-isotope data do exist for a wide range of Skye Tertiary igneous rocks and, consequently, we have focused upon 87Sr/86Sr, Sr abundance and (to a lesser extent) Pb-isotopes in the modelling efforts. The available Nd-isotopic dataset (e.g. Dickin et al., 1984
, 1987; Gibson, 1990; Scarrow, 1992; Ellam & Stuart, 2000) is insufficient for our purposes. However, as a wider range of geochemical measurements on single unaltered samples become available, the validity of the EC-RAFC ‘solutions’ developed in this study can be explicitly and easily tested. A great value of the EC-RAFC model is that it leads to quantitative predictions for relative masses and compositions of derivative melts, cumulate rocks, enclaves and preserved mixed magmas in a relative temporal sequence.
Definition of petrologic lineages
Criteria used to define the lineages are discussed in detail in this section. The bulk of the Skye lava field is made up of light rare earth element (LREE)-enriched transitional alkali basalts and less abundant hawaiites–trachytes of the SMLS (Thompson et al., 1972, 1982; Scarrow, 1992). SMLS MgO contents can be as low as 1 wt % and as high as 14 wt %. Most of the SMLS magnesian (>9 wt % MgO) lavas do not have geochemical features traditionally associated with primary magmas; the lavas often display unradiogenic Pb- and radiogenic Sr-isotope compositions similar to those of the underlying Archaean Lewisian gneisses (Scarrow, 1992). Thompson et al. (1972) showed that within the SMLS, there are products of two contrasting differentiation series: Fe-rich basalts–benmoreites exposed in northern and central Skye and Fe-poor intermediate compositions, which have been sampled mainly in central Skye.
In our classification scheme, lineage L1 (Table 3) consists of SMLS low-Fe intermediate-type Skye lavas that mainly occur in the upper portion of the lava field (Dickin et al., 1984). Based on major and trace-element and Pb-isotopic similarity to SMLS-type lavas (Dickin et al., 1984), the earliest intrusion of the Cuillin centre (the Outer Unlayered Gabbro; Table 1) and all Western and Eastern Redhills intrusions (Table 1) are also included in L1.
Lineage L2 (Table 3) rocks include PMB-type Skye lavas from the uppermost part of the lava pile, as well as all intrusions of the Cuillin centre except the Outer Unlayered Gabbro (Table 1). PMB flows are MORB-like, LREE-depleted, high-calcium, alkali-poor olivine tholeiites. These are intercalated with SMLS-type flows at the top of the Skye lava field in west–central Skye. SMLS and PMB lavas are distinct on element–element, element–isotope and isotope–isotope plots such as [Sr] versus 87Sr/86Sr and 206Pb/204Pb versus 208Pb/204Pb (Fig. 2a and d). Unlayered Cuillin intrusions have major element abundances similar to the PMB-type lavas and the Cuillin complex has been interpreted as coeval with the PMB lavas (Thompson et al., 1972; Thompson, 1982; Scarrow, 1992; Stuart et al., 2000). The PMB flows are believed to represent the remnants of an originally significant lava shield that developed above the Cuillin centre (Williamson & Bell, 1994).
Lineage L3 (Table 3) comprises SMLS Fe-rich lavas that are mainly ne normative at low 87Sr/86Sr. These form most of the lowermost flows and are volumetrically dominant in the Skye lava field (Table 1). On the basis of compositional and spatial affinities (Gibson, 1990), the sills of the Little Minch Sill Complex (Table 1) are categorized in lineage L3. Little Minch Sill Complex sills intrude Jurassic sediments in northern Skye and on nearby islands (Fig. 1) (Gibb & Gibson, 1989). Intrusive relationships indicate that the Little Minch Sill Complex is marginally younger than the majority of SMLS lavas (Gibson, 1988). Olivine cumulate-rich picrite, picrodolerite and alkali dolerite are the main lithologies (Gibb & Gibson, 1989). The sills can be mono- or polylithologic and the presence of internal chills at lithologic boundaries and within lithologic units suggests that multiple intrusions occurred. The picrodolerite unit, which exhibits element and isotope signatures similar to SMLS lavas, is considered in this study. The crinanite and picrite units are chemically and texturally distinct (Gibson & Jones, 1991). These units are not examined here because of a lack of relevant chemical data.
The assumption that SMLS- and PMB-type magmas have distinct parental liquids is supported by gravity data showing that both the Red Hills and Cuillin centres of the Skye central complex are associated with distinct, strong, positive Bouguer gravity anomalies. These are attributed to the presence of a large mass of mafic to ultramafic rock, extending to depths of at least 15 km beneath the central complex (Bott & Tucson, 1973). In addition, results from palaeomagnetic investigations show that the L2 rocks are reversely magnetized, whereas the L1 rocks have normal polarities. Therefore, there must have been at least two periods when mafic magmas were introduced into the Skye centre; those for L2 were intruded when the Earth's magnetic field polarity was reversed, and those of L1 when the field was normal. No exposed record of intrusions associated with the L3 lavas is preserved.
Alteration
Petrographic, isotopic and mineralogical investigations (e.g. Bailey et al., 1924; Tilley & Muir, 1962; Taylor & Forrester, 1971; Forrester & Taylor, 1977; Ferry, 1985) showed that BTIP rocks have interacted widely with heated groundwater. This led to the suggestion that Pb- and Sr-isotope compositions in BTIP rocks may have undergone significant post-solidification alteration. All subsequent geochemical work has had to take this problem into account.
Several studies have concluded that alteration is minimal in carefully selected samples (Thompson et al., 1972; Hawkesworth & Morrison, 1978; Dickin et al., 1980; Moorbath & Thompson, 1980; Scarrow & Cox, 1995). For example, Dickin et al. (1980) showed that significant isotopic disturbance in the Coire Uaigneich Granophyre is found only within 1·5 cm of major fractures. Skye lavas typically have amygdaloidal upper and lower layers, and massive central portions (Scarrow & Cox, 1995). Hawkesworth & Morrison (1978) measured vertical Sr-isotope variation in a BTIP lava flow from a greenschist-facies zone of hydrothermal alteration on Mull and found that the whole-rock 87Sr/86Sr isotope composition of the flow centre was the same as that of separated fresh pyroxene. They concluded that the flow centre was essentially unaffected by hydrothermal alteration. Subsequent workers have minimized the effects of alteration on the Skye geochemical dataset by collecting samples from the interior parts of intrusions and lava flows. Data used to assess the quality of the EC-RAFC trajectories in this study are restricted to those interpreted as being minimally altered by previous workers.
EC-RAFC model: overview
EC-RAFC tracks the trace-element and isotopic composition of melt, cumulates, country-rock partial melts and enclaves during simultaneous recharge, assimilation and fractional crystallization. EC-RAFC is formulated as an initial value problem based on a set of 3 + t + i + s coupled differential equations, where the number of trace elements, radiogenic and stable-isotope ratios simultaneously and self-consistently modelled are t, i and s, respectively. Solution of the EC-RAFC equations provides values for the average wall-rock temperature (Ta), mass of melt within the magma body (Mm), mass of cumulates (Mc) and enclaves (Men), mass of wall rock involved in the thermal interaction (), mass of anatectic melt assimilated (
), and concentration of t trace elements and i + s isotopic ratios in melt (Cm), cumulates (Cc), enclaves (Cen), and anatectic melt (Ca) as a function of magma temperature (Tm). Input parameters include the equilibration temperature (Teq), the initial temperature and composition of pristine melt (
, 
, 
), recharge melt (
, 
, 
) and wall rock (, 
, 
), temperature-dependent trace-element distribution coefficients (Dm, Dr, Da), heats of transition for wall rock (ha), pristine melt (hm) and recharge melt (hr), and the isobaric specific heat capacity of assimilant (Cp,a), pristine melt (Cp,m) and recharge melt (Cp,r). The magma recharge mass function, Mr(Tm) is specified a priori and defines how recharge magma is added to standing magma. In addition to detailed geochemical information generated in a complete EC-RAFC trajectory, a few specific measures are especially useful. These include 
, an indication of the extent of partial melting in the country rock surrounding the magma body, , a measure of the extent of magma contamination, 
Mr, the sum of all increments of recharge, and Mc, the mass of cumulates formed by fractional crystallization. Details of the EC models have been described by Spera & Bohrson (2001, 2002, 2004) and Bohrson & Spera (2001, 2003). In this work, the parameter is set to unity.
The models were run iteratively to maximize agreement between model and observed data, while maintaining geologically reasonable values for input parameters. Geochemical data for assimilants (T, LA and LG) and for liquid (melt) compositions are taken from the literature sources cited above and are summarized in Tables 1–3. Laboratory studies (see, e.g. Bergantz & Dawes, 1994; Petford & Gallagher, 2001; Spera and Bohrson, 2002, for a summary) constrain melt productivity functions [fa(Ta), fm(Tm) and fr(Tr)] for the end-member compositions used in EC-RAFC models. Melt-productivity curves exhibit non-linear sigmoid forms in melt fraction versus temperature coordinates at fixed pressure and volatile species fugacity (e.g. fO2, fH2O, fCO2). Multicomponent equilibrium thermodynamic calculations (e.g. MELTS, Ghiorso, 1995) similarly exhibit sigmoidal melt-productivity curves. The melt-production curves used in this study are based on both experiments and MELTS calculations for appropriate compositions based on protoliths in Table 2 and magma bulk compositions. Phase transition enthalpies and specific heats are taken from compilations by Spera (2000) and Spera & Bohrson (2001). The temperature dependence of trace-element partition coefficients yields a range of distribution coefficients for a particular temperature range and are taken from the literature or estimated.
Choice of Sr–Pb-isotope composition of parental magma
Each lineage corresponds to an evolutionary trend starting from a well-defined and distinct parental liquid. Here, we describe the rationale for selection of Pb- and Sr-isotope initial values. The unradiogenic 206Pb/204Pb and 208Pb/204Pb signatures of many of the earliest-erupted Skye Tertiary rocks (SMLS) have long been proposed as evidence of interaction with Lewisian granulite (LG)-facies crust (e.g. Moorbath & Welke, 1969; Dickin, 1981; Dickin et al., 1984). The PMB lavas form a separate trend that extends towards Pb-isotopic values similar to LA gneiss (Fig. 2d). Those Skye lavas with the most radiogenic Pb are of similar composition to the North Atlantic End Member (NAEM)—a hypothetical uncontaminated mantle-derived melt composition proposed by Ellam & Stuart (2000), which is defined by the convergence of Pb-isotope trends of rocks from numerous localities within the BTIP and elsewhere in the North Atlantic Igneous Province (Fig. 2d). According to Ellam & Stuart (2000), Pb-isotope variations in rocks from different locations within the BTIP indicate local variation in the nature of crustal contaminants. We identified as parental compositions NAEM-like Skye lavas. These are present within each lineage and generally have the lowest 87Sr/86Sr values (0·70300–0·70308). An assessment of the changes in Sr-isotopes up-section, coupled with covariations in MgO- and Pb-isotopes, suggest that recharge and assimilation add radiogenic Sr.
EC-RAFC modelling: results and petrogenetic implications
Each of the three petrologic lineages is further divided into sublineages for the purpose of comparing observations (geochemical data) with EC-RAFC modelling trajectories. It is important to note that within a given lineage, the sublineages form an evolutionary sequence such that the letter ‘A’ denotes the first sublineage and subsequent sublineages are denoted alphabetically. Within a lineage, successive model input parameters follow from the previous model. For example, the input for model EC1B is the final state of model EC1A. We distinguish data from EC-RAFC models by using, for example, the label L1A to refer to a sublineage of rocks and the symbol EC1A to refer to a computed EC-RAFC geochemical trajectory.
Lineage 1: sublineages, model results and petrogenetic implications
L1: sublineages. For the purpose of modelling, L1 data are subdivided into five sublineages based on spatial and compositional characteristics. Each sublineage is modelled as a separate EC-RAFC trajectory, labelled EC1A, EC1B, EC1C, EC1D, and ECE on the diagrams that follow. L1A consists of Skye lavas of SMLS low-Fe intermediate type. The lavas are hawaiites, mugearites and benmoreites that contain plagioclase and magnetite phenocrysts as well as sparse-to-absent olivine phenocrysts (Thompson et al., 1972). The L1A data form a negative 206Pb/204Pb–87Sr/86Sr trend. The most evolved L1A samples have 87Sr/86Sr similar to the least radiogenic L1B samples (Fig. 3).
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At the lowest 87Sr/86Sr values, L1B includes samples from the upper part of the Skye lava field (Table 1) and the Outer Unlayered Gabbro (OUG) of the Cuillin centre (Table 1); other samples are from intrusions in the northern part of the Western Redhills centre (Table 1) and the Outer Granite (Table 1) of the Eastern Redhills centre (Fig. 3). Samples in L1C come from intrusions exposed in the southern part of the Western Redhills centre (Table 1), from the basaltic andesite facies of the Rubha'an Eireannaich mafic–silicic composite sill (Table 1), and the Outer Granite (Table 1) of the Eastern Redhills centre (Fig. 3). There is field evidence that the Western Redhills intrusions may have distinct petrogenetic histories. Intrusions in the southern part of the Western Redhills centre are separated from a group of intrusions in the northern part of the centre by a mass of vent agglomerates, crushed gabbro and basaltic lavas (Emeleus & Gyopari, 1992
). Table 1 summarizes the igneous chronology at the northern and southern parts of the centre, respectively. Sr-isotopic values within L1B and L1C increase progressively with increasing stratigraphic height, according to the sequence outlined in Table 1. The L1B and L1C samples display a broad positive 206Pb/204Pb–87Sr/86Sr correlation.
The Western Redhills centre is renowned for preserving hybrid rocks that display a wide range of magma-mixing phenomena (Wager et al., 1965; Thompson, 1980; Vogel et al., 1984). The presence of mafic inclusions in some granitic intrusions (e.g. the Glamaig Granite and Northern Porphyritic Granite; Table 1), as well as the hybrid nature of the Marscoite and Marscoite–Glamaigite mixed suites (Table 1), indicate that recharge was important in the magmatic evolution of L1B and L1C magmas. Remnants of Tertiary basalt on top of some granites show that the granitic magmas intruded into high levels in the crust (Gass & Thorpe, 1976).
All samples in L1D and L1E are from either the Outer Granite (Table 1) or the rhyolite–intermediate facies of the Rubha'an Eireannaich sill (Table 1) of the Eastern Redhills centre. The Rubha'an Eireannaich sill is one of a suite of mafic–silicic composite sills (Harker, 1904; Bell & Pankhurst, 1993) in which boundaries between marginal basaltic andesite and a central rhyolite portion are gradation in terms of mineralogy and texture (Bell & Pankhurst, 1993). There are no internal contacts. Bell & Pankhurst (1993) performed serial sampling across the sill and observed that compositional profiles for major and trace elements and Sr-isotopes exhibit continuous gradients. The Outer Granite is a group of intrusions that is peripheral to a second group of intrusions, the Inner Granite (Bell & Harris, 1986). Because of a scarcity of Sr-isotope, major- and trace-element data, the Inner Granite is not considered in this study.
L1: model results. Input parameters for all EC1 simulations are shown in Table 4. In the EC1A model, the parental sample has an initial Sr-isotope value of 0·70315 and 4·26 wt % MgO. The modelled country-rock Sr-isotope ratio and Sr-concentration parameters are similar to published values for leucocratic to mesocratic LG-facies gneiss (Weaver & Tarney, 1980; Dickin, 1981). Invoking LG as the contaminant is consistent with the negative correlation between 206Pb/204Pb and 87Sr/86Sr and also with thermal parameters that suggest the magma chamber resided within the lower crust. For consistency with the presence of plagioclase (e.g. Thompson et al., 1972), Sr is modelled as compatible in the host magma and incompatible in the country rock in EC1A and all other EC-RAFC simulations for lineage L1 data. MELTS (Ghiorso, 1997) liquidus temperature simulations based on the major-element chemistry of the parental sample yield a magma liquidus temperature close to 1302°C at 1 GPa (a pressure equivalent to lower-crustal depths) (Thompson et al., 1972).
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The 87Sr/86Sr value of the recharge magma is higher than that of the uncontaminated host magma in EC1A and all other EC1 models. In EC1A, five episodes of magma recharge are modelled to coincide with sharp observed increases in Sr concentrations or deviations in 206Pb/204Pb towards higher values. Sr is incompatible in the recharge magma. Table 4 shows the temperatures and normalized masses at which recharge magma mixes with host magma.
Table 4 shows , the mass of anatectic melt assimilated into the magma, Mc, the mass of cumulates in the magma body, and , the mass of country rock involved in the EC-RAFC event, for EC1A and all other EC1 models. The mass of recharge magma, 
, is specified a priori. In the EC-RAFC simulations discussed in the present study, all melt generated in the country rock is assimilated into the magma body (i.e. = 1). The ratio of the mass of anatectic melt to the mass of cumulates in EC1A (
) is higher than in other EC1 models. The input of relatively large masses of recharge magma (
) early in the EC1A RAFC episode limits the effect of fractional crystallization. As a result of the large proportion of recharge magma involved in EC1A, the proportion of anatectic melt that is incorporated (
) is greater than in other L1 EC-RAFC models.
Figure 3 shows EC1A, the computed EC-RAFC trajectory for L1A [Sr]–87Sr/86Sr data. The addition of two pulses of recharge magma (‘1’ and ‘2’, Fig. 3a) during the heating of country rock from its initial temperature to the solidus causes 87Sr/86Sr to become slightly more radiogenic. Because Sr is incompatible in the recharge magma, [Sr] of the host magma increases during these recharge episodes. When assimilation begins, 87Sr/86Sr in the magma increases significantly because of the incorporation of more radiogenic assimilant. During intervals in which the evolution of the magma body is dominated only by assimilation and fractional crystallization processes, [Sr] of the magma body decreases because of the strong compatibility of Sr in the magma and the relatively low [Sr] of incoming anatectic melt. When recharge magma is mixed into host magma, [Sr] increases.
Thermal and geochemical input parameters in the EC1B model corresponds to the most radiogenic 87Sr/86Sr samples of EC1A and EC1C input parameters correspond to the most radiogenic 87Sr/86Sr samples of EC1B (Fig. 3b). In contrast to the L1A data, the positive correlation between L1B and L1C 87Sr/86Sr and radiogenic Pb-isotopic values suggests that L1B and L1C magmas are contaminated with LA gneiss. The presence of LA xenoliths in L1B and L1C rocks (Thompson, 1969, 1981) supports this hypothesis. Country-rock Sr-isotope values in the EC1B and EC1C models are therefore similar to those of LA-facies gneiss and are consistent with thermal parameters reflecting middle- to upper-crustal conditions (e.g. Holden et al., 1987; Whitehouse, 1990). In the EC1B model, the range of the country-rock distribution coefficient reflects varying plagioclase contents in the country rock as observed in LA xenoliths (Thompson, 1980, 1981). The compatibility of Sr in the EC1B host magma is modelled to increase to accommodate observed changes in plagioclase abundance (e.g. Thompson, 1969; Vogel et al., 1984).
In the EC1B and EC1C models, the country-rock initial temperature is typical of upper-crustal conditions. Thompson (1981) performed melting experiments on LA xenoliths from L1B rocks at 1 bar pressure and observed a solidus temperature of 715°C for leucocratic and mesocratic compositions. At 952°C, the mesocratic compositions were more than 80% liquid, with several percent residual orthopyroxene and plagioclase. The modelled EC1B solidus and country-rock liquidus temperatures reflect these results. Experimentally determined solidus temperatures for L1C and L1B intrusions (Thompson, 1983; Ferry, 1985) provide approximate limits on the equilibration temperatures of L1B and L1C magmas.
The selection of Sr-concentration, Sr-isotope and distribution coefficient parameters for the recharge magma is based on the assumption that the EC1B recharge magma is similar to that which injected the EC1A magma reservoir. Plagioclase is hypothesized to join the recharge magma liquidus (i.e. the melt was multiply saturated, perhaps by cotectic crystallization) during the EC1B RAFC event, and Sr is modelled as compatible in the recharge magma in the EC1C simulation.
Of note in EC1B is the flat [Sr]–87Sr/86Sr trajectory at the lowest 87Sr/86Sr values. This indicates that this part of the differentiation history is dominated by fractional crystallization. This suggests that the country rock had a relatively low initial temperature. As a consequence, the relative mass of assimilant incorporated into the host magma body is small, with 
.
During the first addition of recharge magma in EC1B, 87Sr/86Sr increases, but although [Sr] in the recharge magma is relatively high, the first episode of recharge does not lead to an increase in [Sr] in the host magma. This is because the Sr-distribution coefficient in the host magma is high as a result of melt saturation with plagioclase-bearing cumulates. When the second pulse of magma recharge takes place, [Sr] in the host magma is lower and there is a minor increase in host magma [Sr].
The form of EC1C is broadly similar to that of EC1B (Fig. 3). However, the greater mass of recharge magma involved in EC1C (
) compared with EC1B (
), as well as the incompatibility of Sr in the recharge magma throughout EC1C, leads to greater increases in [Sr] during the addition of recharge magma. The fraction of assimilated crust in EC1C (
) and the proportion of assimilant (
) are similar to those of EC1B (
; ).
In Sr–87Sr/86Sr coordinates (Fig. 3b), data from the Outer Granite form multiple trajectories, which may reflect complex magma dynamics associated with the formation of silicic compositions or the effects of alteration (Ferry, 1985). For illustrative purposes, we have modelled only two trends, EC1D and EC1E. In the Rubha'an Eireannaich sill, the lowest initial 87Sr/86Sr values (0·70749–0·70990) are present in the ferrobasaltic andesite facies, which is chemically similar to L1C intrusions of the Western Redhills centre. The highest 87Sr/86Sr values (0·71244–0·71248) are in samples from the central rhyolite facies. These samples are chemically similar to Eastern Redhills intrusions. Samples of intermediate composition have intermediate initial 87Sr/86Sr values (0·71069–0·71092).
Initial magma model parameters for EC1D and EC1E are broadly similar to more radiogenic 87Sr/86Sr samples in E1C and country-rock geochemical parameters are similar to published values for LA (e.g. Dickin, 1981). Although the model requires a higher country-rock initial temperature for EC1E than for EC1D, in general, these initial temperatures reflect middle- to upper-crustal conditions, which, in these cases, leads to insignificant proportions of assimilant being incorporated into the EC1D and EC1E magma bodies. Despite this, the contaminated magmas have relatively high 87Sr/86Sr because the assimilant has a high Sr-isotope ratio. The EC1D model involves a larger mass of country rock (
) and a smaller ratio of mass of assimilated melt to mass of cumulates removed (
) than EC1E (
; 
).
L1: petrogenetic implications of EC-RAFC results. EC-RAFC modelling results indicate that L1 magmas initially resided in a lower-crustal reservoir. We speculate that the repeated input of hot magma enabled magma bodies to rise to progressively shallower depths within the crust. This led to the development of mid- to upper-crustal magma reservoirs and subsequently, the Western and Eastern Redhills centres of the Skye central complex (EC1B, EC1C, EC1D and EC1E).
L1A lavas have previously been recognized as chemically and petrographically distinct from L3 and L2 lavas. All L1A lavas have been classified as Fe-poor intermediates by Thompson et al. (1972). Distinguishing petrographic features of Fe-poor lavas include the presence of plagioclase and magnetite phenocrysts, with rare-to-absent olivine. The least contaminated L1A samples come from the upper portion of the Beinn Edra stratigraphic group (Anderson & Dunham, 1966) and interdigitate with L3 lavas (Thompson et al., 1972). This implies that the L1A reservoir existed contemporaneously with the L3 reservoir. More contaminated L1A samples all come from the upper portion of the Skye lava pile (Table 1), and, therefore, are also spatially distinct from L3 lavas.
The results are in agreement with previous conclusions that L1A magmas were contaminated in the lower crust (e.g. Dickin, 1981) and that AFC may have played a role in the production of some L1 samples (Thirlwall & Jones, 1983). Thompson et al. (1972) and Thompson (1974) proposed that the L1A magmas represent the daughter products of fractional crystallization of L3 magmas. Our results suggest that the L1A and L3 magmas are derived from distinct parental magmas.
The parental magmas of L1B and L1C samples are probably the end-products of EC1A, the RAFC event associated with L1A rocks. Model results suggest that L1B and L1C magmas evolved in separate but closely linked magma reservoirs. The presence of Eastern Redhills samples at the more evolved ends of the EC1B and EC1C indicates that periodic magmatic migrations from the magma reservoirs led first to the formation of the Western Redhills centre, and then to the formation of the Eastern Redhills centre. Samples from the basaltic andesite facies of the Rubha'an Eireannaich composite sill (Table 1) plot on EC1C. However, samples from the rhyolite and intermediate portions of the sill plot away from the trend at 87Sr/86Sr values similar to those of other Eastern Redhills intrusions. Additional EC-RAFC models are required to explain the genesis of these rocks. This phenomenon, in combination with compositional profiles for major and trace elements and Sr-isotopes that show continuous gradients between the ferrobasaltic andesite and rhyolite portions of the sill (Harker, 1904; Buist, 1959; Bell, 1983), provide evidence that the evolutionary histories of the Western and Eastern Redhills centres are linked. In addition, fine-grained acid inclusions within the Glamaig granite (Table 1) of L1B are petrographically and geochemically similar to inclusions within the Beinn an Dubhaich granite of the Outer Granite (Table 1) of the Eastern Redhills centre. Further evidence of a connection between the Western and Eastern Redhills centres is provided by Pb-isotopes, with the Eastern Redhills data extending from the range of Western Redhills values towards higher 206Pb/204Pb and 208Pb/204Pb (Fig. 2d).
EC-RAFC results EC1D and EC1E support the concept that L1D and L1E magmas originated from L1B and L1C parental magmas. The L1D and L1E reservoirs, which are probably shallower than the L1B and L1C reservoirs, were contaminated by LA-facies crust.
Lineage 2: sublineages, model results and petrogenetic implications
L2: sublineages. Lineage 2 rocks include the unlayered intrusions of the Cuillin centre (Table 1) and the PMB-type Skye lavas (Table 1). As noted above, the PMB lavas are compositionally similar to many Cuillin centre intrusions (Emeleus & Gyopari, 1992). The Cuillin centre has been proposed as a possible feeder for the PMB (Walker, 1993). The Cuillin centre is a composite layered intrusion of tholeiitic affinity (Table 1) consisting of a succession of layered and unlayered gabbroic and ultramafic intrusions, an acid intrusion and minor gabbroic intrusions. These intrude the Tertiary lava flows, T sandstones, and Jurassic sediments (Gass & Thorpe, 1976; Emeleus & Gyopari, 1992). An unnamed cone sheet suite (Table 1) is closely related in space and time to the Cuillin centre (Bell et al., 1994). Some cone sheets within this suite cut the youngest intrusion of the Cuillin centre, but are older than the Western Redhills centre intrusions (Bell & Harris, 1986). In general, the cone sheets exhibit a typical tholeiitic Fe-enrichment trend and the least evolved members of the suite are similar in composition to the PMB lavas (Bell et al., 1994). The focus of the present study is on the unlayered intrusions of the Cuillin centre because these rocks are interpreted as liquid compositions (e.g. Wager & Brown, 1968; Hutchison & Bevan, 1977; Dickin et al., 1984).
The Coire Uaigneich Granophyre (CUG) is closely associated with the Cuillin centre in space and time and is chilled against Jurassic sediments. The same suite of cone sheets that cuts the layered rocks of the Cuillin centre also cuts the CUG. This provides strong evidence that the CUG is structurally related to the Cuillin centre (Bell & Harris, 1986; Emeleus & Gyopari, 1992), and hence it is included with the Cuillin centre in Table 1. According to Wager et al. (1953), the CUG crystallized under low-pressure conditions.
On the basis of radiogenic Pb-isotope variations, the data are divided into two sublineages for EC-RAFC analysis: L2A and L2B. On the 206Pb/204Pb versus 208Pb/204Pb diagram (Fig. 2d), the PMB-type Skye lavas, the Border Group (Table 1) and a dyke (Table 1) extend from values similar to the North Atlantic End Member (Ellam & Stuart, 2000) towards low 206Pb/204Pb, but flat 208Pb/204Pb. Compared with the PMB and Border Group rocks, the CUG (Table 1) data show high 206Pb/204Pb.
L2: model results. EC-RAFC parameters and results for [Sr]–87Sr/86Sr L2 data are displayed in Table 5 and Fig. 4. The presence of plagioclase phenocrysts requires that Sr is modelled as compatible in the magma in both EC2 simulations. The EC2A model requires an assimilant Sr-isotope ratio that is similar to that of LA-facies gneiss. L2B samples have high 206Pb/204Pb in comparison with other L2A samples. High 206Pb/204Pb values are a characteristic feature of T rocks (Fig. 2d; Dickin & Exley, 1981), and therefore assimilant compositional parameters are consistent with published values for T metasediments.
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