Microchemistry of amphiboles near the roof of a mafic magma chamber: Insights into high level melt evolution

Lithos 148 (2012) 162–175

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Microchemistry of amphiboles near the roof of a mafic magma chamber: Insights into high level melt evolution J. Brendan Murphy a,⁎, Stephanie A. Blais a, Michael Tubrett b, Daniel McNeil a, Matthew Middleton a a b

Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia, Canada B2G 2W5 MicroAnalysis Facility - Inco Innovation Centre (MAF-IIC) c/o Memorial University of Newfoundland, 230 Elizabeth Avenue, P.O. Box 4200, St. John's, NL, Canada A1C 5S7

a r t i c l e

i n f o

Article history: Received 7 February 2012 Accepted 9 June 2012 Available online 18 June 2012 Keywords: Amphibole Appinite Magma chamber Laser ablation microprobe Rare earth elements

a b s t r a c t The Late Neoproterozoic Greendale Complex, located within the Avalon terrane of Nova Scotia, is a suite of appinitic rocks ranging from ultramafic to felsic in composition that were intruded during regional ensialic arc magmatism and crystallized at shallow crustal levels under conditions of high pH2O. Amphibole is the dominant mafic mineral in ultramafic to mafic rocks and displays the extraordinary variability in texture and modal abundance that is characteristic of appinite suites. These features allow sensitivity of amphibole composition (major, trace and REE) to the evolution of water-rich magma to be investigated. All amphiboles in mafic and ultramafic rocks are calcic, with (Ca + Na)B ≥ 1.34 and SiIV between 6.1 and 7.3. They predominantly range in composition from tschermakite to magnesiohornblende and display a dominance of edenite (Na,KA + Al IV = SiIV) substitution. Although each sample exhibits remarkably uniform Mg# over a wide range in Si of up to one formula unit, the mafic rock amphiboles are characterized by lower (0.5 to 0.7) Mg#, compared to the ultramafic rocks (0.7 and 0.9). REE profiles are bow-shaped, and are characterized by depletion in LREE (La/Sm ≈ 0.61), a slight depletion in HREE (Gd/Yb ≈ 1.55) as well as a negative Eu anomaly, which is attributed to co-precipitation of plagioclase. REE and trace element profiles of ultramafic amphiboles are divided into two groupings. Group A amphiboles occur in all specimens analyzed and their REE profiles are very similar to the whole-rock analyses of the mafic rocks and to those predicted from amphibole/melt partition coefficients. In contrast, Group B amphiboles display relative enrichment in light REEs (La/Sm ≈ 2.05), have lower ΣREE, and lack a negative Eu anomaly relative to Sm and Gd. Group B amphiboles are more enriched in Th and U and show a more pronounced depletion in Nb, Ti, Y and HREE. Group B amphiboles probably grew in a reaction relationship with olivine and pyroxene, and their contrasting profiles suggest that partition coefficients traditionally used for petrogenetic modeling may not be appropriate for such amphiboles. Groups A and B are virtually indistinguishable with respect to the major elements used for amphibole classification, suggesting that REE and selected trace element data, when combined with textural observations, may provide important additional insights into phase equilibria and conditions of crystallization. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Because of their ability to accept a wide range of major and trace elements within its mineral structure, the chemical composition of amphiboles can yield important constraints about the physical conditions of magmas at the time of amphibole crystallization (e.g. Anderson and Smith, 1995; Holland and Blundy, 1994). In igneous rocks, the composition and stoichiometry of amphibole have been used as a geobarometer (e.g. Anderson, 1996, 1997; Hammarstrom and Zen, 1986; Johnson and Rutherford, 1989) and as a geothermometer (e.g. hornblendeplagioclase, Holland and Blundy, 1994).

⁎ Corresponding author. Tel.: + 1 902 867 2481. E-mail address: [email protected] (J.B. Murphy). 0024-4937/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2012.06.012

Amphibole is the dominant mineral in the “appinite” suite of igneous rocks, which are medium- to coarse-grained plutonic rocks ranging from mafic to felsic in composition in which the amphibole displays extraordinary variability in texture and modal abundance (Pitcher, 1997). These features, together with the abundance of pegmatitic gabbro and diorite with acicular and skeletal amphibole, and the presence of explosion breccias indicate that appinitic magma is characterized by high pH2O (Bowes and McArthur, 1976; Pitcher, 1997). Geochemical and isotopic studies in the Caledonide orogen of northwestern Europe, where the appinite suite was first defined (Bailey and Maufe, 1916), indicate a genetic connection with hornblende-rich calc-alkaline lamprophyre (Rock, 1991) and with shoshonite (Fowler, 1988) and imply a broad genetic connection with subduction processes. The Neoproterozoic Greendale Complex, Antigonish Highlands, Nova Scotia has many of the characteristics of appinite suites

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(Murphy et al., 1991), and is a local representative of regional arc magmatism (Murphy and Nance, 2002). The complex primarily consists of mafic to intermediate rocks whose mineralogy is dominated by amphibole and plagioclase. The widespread presence of pegmatitic hornblende gabbro, textures such as miarolitic cavities and abundant roof pendants indicates that the complex is exposed near the roof of the magma chamber, affording a rare opportunity to document processes in these specialized environments. As recent studies on the crystal chemical parameters in the wall-rocks have constrained its depth of emplacement between 2 and 3 kbars (Abad et al., 2011), the Greendale Complex provides the possibility of examining factors that influence the chemistry of amphiboles in water-rich magmas at these shallow crustal levels. Recent advances in instrumentation facilitate the analysis of selected trace and rare earth elements (REE) in the amphiboles. Because of its presence in all lithologies in the complex, its fresh composition and its ability to accept a wide range of trace elements and REEs into its chemical structure (preferentially the M4 site, Tiepolo et al., 2000), the sensitivity of the amphibole composition to magma evolution can be examined. 2. Geologic setting The 607 ± 2 Ma Greendale Complex (Fig. 1) is a local representative of regionally extensive 635–590 Ma subduction-related magmatism that characterizes the Avalon terrane in the Canadian Appalachians (Keppie et al., 2003; Murphy and Nance, 1989; Murphy et al., 1997a, 2004). The complex is exposed in a coastal section in the northern highlands between the Hollow and Greendale faults where it intrudes Late Neoproterozoic host rocks of the Georgeville Group, an arc-related sequence dominated by polydeformed greenschist facies volcanic and sedimentary rocks (Murphy et al., 1990; Fig. 1). The complex was emplaced during later stages of this deformation (Murphy and Hynes, 1990). U–Pb dating of Georgeville Group provides a maximum depositional age of 613 ± 5 Ma (Keppie et al., 1998), indicating a limited time interval between the deposition, deformation and metamorphism of the Georgeville Group rocks and the intrusion of the Greendale Complex. Murphy et al. (1997b) document a close correspondence in elemental and isotopic (Sm–Nd) geochemistry between the plutonic rocks of the Greendale Complex and the volcanic rocks of the Georgeville Group. Many of the features of the Greendale Complex are typical of the appinite suite of rocks (Murphy and Hynes, 1990; Murphy et al., 1991) and are indicative of crystallization of a volatile-rich magma (e.g. Bowes and McArthur, 1976; Rock, 1991). The complex is a roughly semi-circular body (Fig. 1) with a diameter of approximately 5 km and predominantly consists of steeply-dipping E–W oriented intrusive sheets, from 5 cm to 5 m in width (Fig. 2a), which range from felsic to ultramafic in composition (Murphy and Hynes, 1990). Therefore many of the samples described herein are taken to represent discrete injections of magma from a subjacent chamber. Although the complex is dominated by hornblende-rich porphyritic gabbro, it is remarkably heterogeneous from thin section to outcrop scale. There is abundant field evidence for mixing and mingling among ultramafic, mafic, intermediate, and felsic components (see Fig. 3 in Murphy et al., 1997b). Contacts between intrusive sheets vary from sharp to gradational, and from planar to irregular and scalloped. Mingling along contact zones produces rocks of intermediate composition, consisting of trains of autoliths and autocrysts. Autoliths of mafic rock in a felsic host typically display various stages of assimilation and disintegration, producing a hybrid rock of intermediate composition. Felsic rocks account for about 10% of the complex and are interpreted as late stage; they commonly occur as dykes ranging from 2 cm to 2 m in width that crosscut mafic and ultramafic rocks. The thinner dykes typically terminate in pegmatitic lenses. The felsic dykes are predominantly restricted to the complex, with isolated

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dykes extending no more than 5 m into the host rock. Some terminations are interpreted as small-scale pull-apart structures related to movement along the Hollow Fault (Fig. 2b). Offsetting of felsic veins as well as growth of crystals inward from the wall rock is also evidence of dextral shear and extension during intrusion (Murphy and Hynes, 1990). Hornblende pegmatitic gabbros typically display a “stacked-log” orientation of hornblende crystals (Fig. 2c), defining a linear fabric. Similar appinitic features have been described in coeval plutons in the neighboring Cobequid Highlands (Pe-Piper et al., 2010) which was juxtaposed with the Antigonish Highlands prior to the late Paleozoic opening of a sedimentary basin between them (Murphy et al., 2011; Waldron, 2004). Until recently, only a very broad constraint on the maximum depth of emplacement of the complex, given by the lower greenschist metamorphic grade of the host rock, was available. However a study of crystal–chemical parameters (illite crystallinity, b-cell dimension) of white micas in the Georgeville Group shales indicates that deformation of the host rock occurred at ca. 3–5 kbar under lowermost greenschist to anchizone conditions (Abad et al., 2011). As the Greendale Complex is interpreted to have intruded during the waning stages of this deformation (Murphy and Hynes, 1990), these data constrain the maximum depth of emplacement of the complex. On the other hand, data from the contact aureole of the adjacent, but posttectonic, 580 Ma Georgeville pluton indicate 1–2 kbar for its depth of emplacement (Abad et al., 2011), and provide a minimum estimate for the depth of emplacement of the Greendale Complex. Taken together, these data imply that the Greendale Complex was probably emplaced at a depth equivalent to 2–3 kbar. The presence of pegmatitic hornblende gabbro (Fig. 2d), textures such as miarolitic cavities and abundant roof pendants indicates that the complex is exposed near the roof of the pluton. The predominant E–W oriented sheets are thought to reflect emplacement along fractures due to repeated extensional failure in the host rock at the roof of the magma chamber (Murphy and Hynes, 1990). 3. The Greendale Complex lithologies and mineralogy Detailed descriptions of the lithologies of the Greendale Complex are given in Murphy et al. (1991). Mafic to intermediate lithologies dominate the Greendale Complex and range from amphibole gabbro, diorite, to quartz diorite in composition. Amphibole gabbro is characterized by roughly equal modal abundance of amphibole and labradorite, with very minor augite (generally b5%). Diorite and quartz diorite are also composed primarily of amphibole and plagioclase, although quartz is more abundant. All mafic to intermediate rocks vary considerably in grain size from a fine grained “salt and pepper” texture to a pegmatite, which contains euhedral amphibole crystals up to 15 cm in length. The amphibole in these pegmatites is acicular and skeletal and also encloses cores of plagioclase. Such textures are attributed to rapid growth from a water-rich magma. The contacts between fine grained and pegmatitic lithologies vary from sharp and planar to gradational and irregular. All lithologies, irrespective of grain size, exhibit both non-foliated and foliated textures. In many instances, hornblende pegmatite occurs in dykes, from 0.5 to 10 m in width, with coarse amphibole defining a linear fabric that is perpendicular to the bounding walls of the dyke. All fabrics are interpreted to have formed during the emplacement and cooling of the complex (Murphy and Hynes, 1990). Although generally fresh in all lithologies, locally amphibole is pseudomorphed by chlorite. Plagioclase shows polysynthetic twinning in stubby lath-shaped crystals, and is typically extensively saussuritized. Most fresh plagioclase is labradorite in composition (An65–An60), although normal to oscillatory zoning ranging from An65–An40 also occurs. Ultramafic rocks consist of pyroxene hornblendites and olivine– pyroxene hornblendites, account for about 5–10% of the lithologies

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Fig. 1. (a) Tectonic map of the northern Appalachian orogen (after Hibbard et al., 2007). CBI, Cape Breton Island; NB, New Brunswick; NE, New England; NL, Newfoundland; NS, Nova Scotia; QUE, Quebec. The inset shows the location of the northern Antigonish Highlands, within the Avalon terrane (Avalonia) of Nova Scotia. (b) Geological map of the northern Antigonish Highlands, showing the Greendale Complex, located between the Hollow and Greendale faults, intruding the Georgeville Group (after Murphy et al., 1997b). Geochronological data: underlined = 40Ar–39Ar data (h, hornblende; m, muscovite); boxed, U–Pb data (see text for details).

exposed, and occur as discontinuous sheets and pods which are interpreted as boudins developed late in the cooling history of the complex (Murphy and Hynes, 1990). These rocks are dominated (45 to 90 modal %) by large crystals of dark red-brown amphibole, up to 4 cm in length, with good cleavages, and simple twinning. These amphiboles poikilitically enclose medium grained, rounded to subhedral olivine and clinopyroxene (or more rarely, hypersthene) chadacrysts.

Olivine contains inclusions of chromite. It is typically fresh, incipient alteration to talc or serpentine occurs. Amphibole also occurs as an interstitial phase together with plagioclase, phlogopite and opaques. Taken together, these observations suggest that amphibole in the ultramafic rocks occurred in two distinct environments: (a) synchronous growth with plagioclase in the interstitial liquid, and (b) in a classic reaction relationship with olivine and/or pyroxene.

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(a)

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(b)

(d)

(c)

Fig. 2. Representative field photographs showing the textural complexities of the Greendale Complex. (a) Example of vertical layering defined by alternating layers of mafic and ultramafic compositions. (b) Thick (ca, 5 m wide) ultramafic layer intruded by a conjugate set of felsic veins. (c) Mafic porphyry displaying a “stacked log” linear fabric defined by amphibole oriented perpendicular to vein walls. Felsic veins are also visible in both photos cross cutting mafic and ultramafic rocks. (d) Mafic porphyry showing evidence of co-precipitation of amphibole and plagioclase. Skeletal amphibole enclosing plagioclase is visible, as are local “rosettes” of amphibole.

Microprobe analyses indicate that the olivine ranges from Fo86 to Fo75. Clinopyroxenes are endiopside, augite and sub-calcic augite in composition, with Mg# ranging from 79 to 62. Clinopyroxene also contains trace amounts of Ti, Al, Na, and K. Orthopyroxene is hypersthene in composition with Mg# between 0.86 and 0.85 and very low Ca (b0.05). Plagioclase occurs as a minor (b5%) interstitial phase. Although rare grains with An75 occur, most samples contain plagioclase ranging from An60–50, as well as albite, which is interpreted as a secondary phase. Minor accessory phases include phlogopite, which occurs as an anhedral intergranular mineral ranging up to 3 mm in length and has a Mg # of 91 to 89. Anhedral spinel up to 1 mm in diameter occurs in fractures within olivine grains. The spinel is Fe and Cr rich (Mg# = 0.32; Cr# = 0.95). The ultramafic rocks are cut by late stage veins containing magnetite, hematite and apatite. Felsic rocks typically range from granodiorite to tonalite in composition. They occur in veins, including conjugate sets or intricate networks that typically terminate in pegmatitic lenses. They are composed primarily of quartz and altered plagioclase which are typically albite in composition. Amphiboles occur as rare lath-shaped or

broken crystal fragments, showing extensive alteration to actinolite or chlorite, and containing many inclusions. As a result of this alteration, amphibole compositional data from felsic rocks was not collected. 4. Comparative geochemistry The geochemistry of the mafic to felsic lithologies of the Greendale Complex is discussed in detail in Murphy et al. (1997b). The geochemistry of the ultramafic rocks was not included in that study because of their cumulate composition. However the compositions of all rocks were analyzed using the same procedures. Major and selected trace element (Rb, Sr, Ba, Ga, Zr, Y, Nb, Co, Cu, Pb, Zn, V, Cr and Ni) lithogeochemistry were determined by X-ray fluorescence spectrometry in the Nova Scotia Regional Geochemical Centre at Saint Mary's University, Halifax. Rare earth element analyses were determined by ICP-MS at Memorial University, Newfoundland using the procedure described by Dostal et al. (1994) for the X-ray data, and by Jenner et al. (1990) for the REE data. Major, trace and REE data for the ultramafic rocks are given in Table DR-1. The geochemistry of

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Fig. 3. (a) Chondrite-normalized (values after Sun and McDonough, 1989) REE profiles and (b) extended MORB-normalized REE diagrams for ultramafic and mafic rocks of the Greendale Complex.

other lithologies in the Greendale Complex is given in Murphy et al. (1997b). Their ultramafic composition is demonstrated by high MgO (13.4 to 23.9 wt.%), high Fe2O3t (8.6 to 12.8 wt.%), low Al2O3 (6.2 to 11.9 wt.%), and low SiO2 (44.3 to 52.1 wt.%). The most volumetrically significant minerals (amphibole and pyroxene) dominate the chemistry of the ultramafic rocks so that on most major element plots the bulk chemistry lies between their compositions. Compatible trace element abundance is highly variable, with Ni ranging from 54 ppm to 510 ppm and Cr from 602 to 2046 ppm. The ranges and abundance of these elements together with the low abundance of magmatophile elements such as Nb (b5 ppm) and Zr (32–79 ppm) are typical of

ultramafic cumulate rocks. The chondrite-normalized rare earth element (REE) distribution of the Greendale ultramafic rocks is characterized by a slight light REE (LREE) enrichment (normalized La/Sm from 1.0 to 1.6), a small negative Eu anomaly, and a minor decrease in heavy REE (HREE), with normalized Sm/Lu from 2.0 to 2.2 (Fig. 3a). Multi-element MORB-normalized plots for the ultramafic rocks (Fig. 3b) show (with the exception of Sr) significant enrichment in the large ion lithophile elements (LILE) such as K, Rb, Ba as well as in Th with only slight enrichment in Ce. These rocks show depletion in high field strength elements (HFSE) Ta, Nb, P, Ti, Zr, Hf and HREE (Sm, Y, Yb). The MORB-normalized ratios of Nb/Zr range from 1.4 to 2.1 and Nb/Yb range from 1.6 to 5.2.

Fig. 4. Major element data for amphiboles from (a) mafic and (b) ultramafic rocks of the Greendale Complex plotted on the Mg/(Mg + Fe2+) vs Si classification diagrams (according to Leake, 1978 and Leake et al., 1997, 2004). Symbols represent different thin sections.

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Fig. 5. 6-Fold Al apfu (AlV1) plotted against 4-fold Al apfu (AlIV) and the (Na + K + Ca)A vs Si diagrams for amphiboles from (a,c) mafic rocks and (b,d) ultramafic rocks. Procedure for assigning Al according to Leake (1978) and Leake et al. (2004). Solid black line in (a,b) indicates slope of 1. Symbols indicate different thin sections. Type locations for different amphibole end-member compositions are plotted according to Laird and Albee (1981).

Coeval Greendale mafic rocks (see Murphy et al., 1997b for details) are characterized by higher Al2O3, K2O, Na2O, TiO2 and P2O5, similar Ti/V (between 20 and 50), slightly lower Fe2O3t, and

substantially lower MgO (average MgO of 5.9 wt.%, compared with an 16.1 wt.% MgO in the ultramafic rocks), Ni and Cr. The REE and HFSE normalized patterns for the Greendale mafic rocks (Fig. 3a,b)

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Fig. 6. Mg/(Mg + Fe2+) vs K (a), and Ti (b) for amphibole from the mafic rocks.

Fig. 7. Mg/(Mg + Fe2+) vs K (a), and Ti (b) for amphibole from the ultramafic rocks.

shows that the mafic and ultramafic rocks have similar profiles characterized by depletion in HFSE such as Nb, Zr and Ti. However the mafic rocks are higher in LILE, HFSE, ∑REE, normalized La/Sm (1.7 to 4.6) and La/Yb (3.3 to 8.3) values.

(used for calibration), which was used as an internal standard to calibrate the results. Analytical methods are described in Dorais et al. (2009) and the data reduction in Longerich et al. (1996). 6. Amphibole major element data

5. Microanalytical methods Details on analytical methods used to determine the major, trace and REE compositions of amphiboles are given in Supplementary File DR-2. The data are given in Supplementary data files DR-3 (raw oxides), DR-4 (stoichiometry) and DR-5 (REE and selected trace element abundance). Major element analyses on selected minerals were determined for nine representative samples (four mafic, five ultramafic) at the Robert MacKay Electron Microprobe Laboratory in Dalhousie University, Halifax. Information on analytical procedures may be found at http://earthsciences.dal.ca/research/facility/probelab/. Electron microprobe major element data were also used to provide an internal standard for laser ablation analysis which provides the rare earth element (REE) composition of the amphiboles in varying lithologies of the Greendale Complex. The Laser Ablation Microprobe (LAM) laboratory, located in the Inco Innovation Centre of Memorial University of Newfoundland (MUN), consists of a Finnigan ELEMENT XR detector, a high resolution inductively coupled plasma mass spectrometer (HR-ICPMS), and a GEOLAS 193 mm wavelength argon-fluorine excimer laser system (http://www.mun.ca/creait/maf/LAM.php). Laser spot size for our samples was ca. 50 μm. Trace elements and rare earth elements (REEs) were analyzed, as well as major oxides including CaO

The amphibole analyses in mafic (n = 55) and ultramafic (n = 66) rocks show similar and wide ranges in major element composition. Amphiboles in mafic rocks vary from 41.53 to 49.99 wt.% in SiO2, 7.13 to 14.56 wt.% in Al2O3, 10.39 to 12.07 wt.% in CaO and 1.50 to 4.27 wt.% in TiO2. In the ultramafic rocks, amphibole compositions vary from 42.25 to 50.55 wt.% in SiO2, from 5.91 to 14.86 wt.% in Al2O3, 10.45 to 12.15 wt.% in CaO, and 0.33 to 4.28 wt.% in TiO2. Amphibole classification (after Leake et al., 2004) is based on the general chemical formula V1 1V

A0–1 B2 C5 T

8 O22 ðOH; F; ClÞ2 :

Because the water and halogen content of the amphiboles are unknown, the amphibole formula is calculated to 23(O), and the Fe 2+/ Fe 3+ ratio after the method described by Pe-Piper (1988). The amphiboles are first classified based on the number of atoms per formula unit (apfu) of Ca and Na in the B site, i.e. (Ca + Na)B. According to this classification, calcic amphiboles have (Ca + Na)B ≥ 1.00 and NaB b 0.50 apfu. All amphiboles in the mafic and ultramafic rocks of the Greendale Complex comply with this designation and are therefore plotted on the calcic amphibole classification diagrams. These

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Fig. 8. Representative analyses showing core to rim data for amphiboles in (a) a representative mafic sample (GR‐9) and (b) ultramafic sample (PG-7). Classification after Leake (1978) and Leake et al. (2004).

amphiboles typically have (Ca + Na)B ≥ 1.34 with SiIV between 6.1 and 7.3. On the Mg/(Mg + Fe 2+) vs Si classification diagram, amphiboles from mafic and ultramafic rocks both predominantly range from tschermakite to magnesiohornblende in composition (Fig. 4a,b). Each sample exhibits a remarkably uniform Mg# over a range in Si of up to one formula unit. However, mafic rock amphiboles are characterized by lower Mg# which is between 0.5 and 0.7, compared to the ultramafic rocks which have Mg# between 0.7 and 0.9. All amphiboles have Si between 6.1 and 7.3, values which are typical of igneous amphiboles (e.g. Leake, 1978). A plot of six-fold Al (Al VI, C site) vs. four-fold Al (Al IV, T site) for amphiboles from mafic and ultramafic rocks (Fig. 5) indicates that Al preferentially resides in the tetrahedral site, which, together with the negative correlation between Na + K + Ca vs Si (Fig. 5c,d), implies a dominance of edenite-type (Na,KA + Al IV = Si IV) substitution for both lithologies. Both mafic and ultramafic rock amphiboles are characterized by a gentle negative relationship between K and Mg# (Figs. 6a, 7a). One mafic amphibole (GR-15 M) and all ultramafic amphiboles show a positive relationship between Mg# and Si, and there is no obvious

relationship between Mg# and Ti in either the mafic or ultramafic amphiboles (Figs. 6b, 7b). Although local examples of oscillatory zoning occur, core-to-rim data (Fig. 8) show that the amphiboles in the mafic and ultramafic rocks are dominated by normal zoning, with higher Si (and therefore lower Al IV and Na + K) and toward the rims. 7. Amphibole trace element data 7.1. Mafic rocks REE and trace element data were gathered from amphiboles spanning the entire major element compositional range. In general, chondrite-normalized REE profiles of amphiboles from mafic rocks are remarkably consistent (Fig. 9a,b). in that they all display a gentle bow shape, displaying depletion in LREE (La/Sm ≈ 0.61), slight depletion in HREE (Gd/Yb ≈ 1.55) and a negative Eu anomaly. Trace element NMORB-normalized plots (Sun and McDonough, 1989) also show enrichment in Ba, U, K and Ti, relative to Rb, Th, Nb, Sr, and Zr, (Fig. 9c,d). In general, amphiboles with the highest K and Ba are also the most enriched in REEs. These profiles are remarkably consistent (i.e. with few crossovers) suggesting the amphiboles crystallized

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Fig. 10. Sm vs Mg/(Mg + Fe) for amphiboles from mafic rocks.

considered together (e.g. Sm, Fig. 10). However, this relationship cannot be discerned within an individual sample.

7.2. Ultramafic rocks

Fig. 9. Chondrite-normalized REE (a,b) and MORB-normalized trace element (c,d) diagrams for amphiboles from representative mafic rocks. Symbols represent different thin sections. Normalizing values from Sun and McDonough (1989).

from magma of similar composition. A simple relationship between REE content and major element chemistry is not apparent. For example, Mg# displays a slight negative relationship with REE if all data are

With the exception of the higher Mg#, (Fig. 4) the amphiboles in ultramafic rocks have broadly similar ranges in major element composition to the amphiboles in mafic rocks. However, REE data reveal two distinct groupings (here named Group A and Group B) within amphiboles from ultramafic rocks. These two groups, when plotted on REE and trace element chondrite normalized diagrams, indicate two different patterns of REE behavior (Figs. 11, 12). All samples have amphiboles with Group A compositions. In two samples (PG-6, PG-11), both Group A and Group B amphiboles occur in the same thin section. Group A amphiboles in ultramafic rocks display similar trace and rare earth element characteristics as those in the mafic rocks. The chondrite-normalized REE profiles (Sun and McDonough, 1989) for Group A amphiboles display a gentle bow shape (Fig. 11a,b) with depletion in LREE (La/Sm ≈ 0.55) and slightly depletion in HREE (Gd/ Yb ≈ 1.90), as well as a modest negative Eu anomaly. Trace element NMORB-normalized plots also show enrichment in Ba, U and Ti relative to Rb, Th, Nb, Sr, and Zr (Fig. 11c,d). There is some crossover in the profiles, but in general, those amphiboles with the highest Ba and U are also the most enriched in REEs. A chondrite-normalized REE diagram (Sun and McDonough, 1989) for Group B amphiboles (Fig. 12a) indicates relative enrichment in light REEs (La/Sm ≈ 2.05) and slight depletion in HFS and HREEs (Gd/Yb ≈ 1.50). In general, however, Group B amphiboles have lower ΣREE than Group A amphiboles. Although La and Ce abundance are similar, from Nd to Lu, REE abundance of Group B amphiboles are markedly lower than those of Group A amphiboles. In contrast to Group A, Group B amphiboles do not display a negative Eu anomaly relative to Sm and Gd. A NMORB-normalized trace element diagram (Fig. 12b) indicates that relative to Group A, Group B amphiboles are more enriched in Th and U and have lesser abundance of high field strength elements such as Nb, Ti and Y. Plots of REE and other trace elements vs. Si for Group B amphiboles give an indication of how the REEs and trace elements behave with variations in the tetrahedral site occupancy (i.e. with edenite substitution). In contrast with Th (Fig. 13a), these plots show that high field strength (HFS) and REE elements in Group B amphiboles have a positive relationship to Si, that is different from that of the Group A amphiboles, for which no obvious relationship exists (Figs. 13b, 14a,b). As HFS and REE elements reside in the M4 site

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Fig. 12. Chondrite-normalized REE (a) and MORB-normalized trace element (b) diagrams for Group B amphiboles from ultramafic rocks. Symbols represent different thin sections. Profiles for Group A amphiboles (derived from Fig. 11) are shaded in gray. Normalizing values from Sun and McDonough (1989).

Fig. 11. Chondrite-normalized REE (a,b) and MORB-normalized trace element (c,d) diagrams for Group A amphiboles from ultramafic rocks. Symbols represent different thin sections. Profiles for Group B amphiboles (derived from Fig. 12) are shaded in gray. Normalizing values from Sun and McDonough (1989).

(Tiepolo et al., 2000), these data suggest that (i) REE occupancy patterns are simpler in the earlier stages of crystallization (i.e. before plagioclase appears on the liquidus), and (ii) M4 site occupancy is

influenced by the extent of solid solution substitutions on adjacent sites. In order to gain some insight into the spatial distribution of Group A and B domains, two detailed traverses (five analyses each) were conducted across two large grains in each of two samples (PG-6 and PG-11). The locations of each analysis and the data are shown in Supplementary files DR-6 and DR-7, respectively. Each traverse yields internally consistent profiles (i.e. sub-parallel with relatively few crossovers, Fig. 15a–d), but there is no obvious regular zonation of REE content relative to the cores or rims of these grains. These profiles resemble either Group A or Group B amphiboles. Traverse PG‐6–S5 is across a hornblende grain in contact with a pyroxene pseudomorph along one edge and plagioclase along its other edge. All five analyses along this traverse show the humped pattern typical of Group A amphibole with depletion in La and Ce relative to Nd and Sm, and very gently sloping MREE to HREE. Traverse PG‐6– S6 is across a hornblende that is in contact with an olivine pseudomorph along one edge and plagioclase along its other edge. The data (five analyses) consistently display LREE enrichment and flat HREE, patterns which are typical of Group B amphiboles. The two traverses in sample PG-11, traverses S-4 and S-5, both yield quasiparallel profiles displaying a humped LREE and relatively flat HREE patterns typical of Group A amphiboles. Both PG-11 traverses provide clear evidence of a progressive development of a negative Eu anomaly with increasing REE, indicating that plagioclase was on the liquidus as the hornblende grew. These data also indicate that Group A and Group B amphiboles both occur in large grains and so there is no simple relationship between REE content and texture. Instead, the main

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Fig. 14. Plots of (a) Sm (ppm) and (b) Yb vs Si (ppm) for amphiboles from ultramafic rocks. Group B amphiboles are circled in (a). Symbols indicate different thin sections.

Fig. 13. Plots of Th and Ti vs Si (a,b) showing differences between Group A and Group B amphiboles. Symbols indicate different thin sections.

variable appears to be whether plagioclase was on the liquidus when the amphibole grew. 8. Discussion: factors influencing amphibole geochemistry The Greendale Complex is a local representative of regionally extensive 635–590 Ma ensialic arc magmatism which characterizes the Avalon terrane. The complex predominantly consists of steeplydipping intrusive sheets of ultramafic, mafic, intermediate, and felsic compositions, with abundant evidence of mingling and mixing between these lithologies (Murphy et al., 1997b). The complex displays many mineralogical and textural similarities to appinite suites and records processes that occur near the roof of a magma chamber at a shallow crustal level.

amphibole-bearing pegmatites, in which crystals extend across the entire width of an intrusive sheet, clearly grew in situ and therefore at the same structural level as the Greendale Complex. Experimental data (e.g. Moore and Carmichael, 1998) for water-saturated mafic melts at pressures similar to those of the Greendale Complex (Fig. 16) indicate that this sequence of crystallization (i.e. plagioclase before amphibole) implies a maximum pH2O of 2.4 kbar, (an interpretation that is compatible with the data of Abad et al. (2011) from the host rock) and temperatures of crystallization on the order of 1000 °C. The applicability of this phase diagram to lower structural levels in the Greendale Complex is unclear because of uncertainties in abundance of H 2O in the magma at these levels. However, the diagram indicates that crystallization sequence of olivine–clinopyroxene–amphibole exhibited by the ultramafic rocks indicates that water-rich conditions probably did not extend to depths greater than ca. 2.7 kbar (Fig. 16).

8.1. Depth of emplacement Crystal–chemical parameters from shales in the Georgeville Group host rock suggest a depth of emplacement between 2 and 3 kbar (Abad et al., 2011). Determining the depth of emplacement from the phase relationships exhibited in the complex itself is complicated by the possibility that crystals may have been highly mobile in the water-rich (i.e. low viscosity) mafic magmas. For example, Pitcher (1997) interpreted the olivine–pyroxene–hornblende relationships in appinitic rocks to represent the disruption of olivine–pyroxene cumulates at depth, which were carried upward by water-rich mafic magma, and reacted to form amphibole as they ascended. If so, the compositions of these phases cannot be used to estimate the depth of emplacement of the Greendale Complex itself. However, the

8.2. Amphibole major element chemistry Major element data for the amphiboles from mafic and ultramafic rocks indicate that they are calcic, and are predominantly tschermakite to magnesiohornblende in composition, with Si varying from 6.1 to 7.3. The higher Mg# ratios in amphiboles from ultramafic, compared to mafic rocks (Fig. 4) are likely reflective of the more Mg-rich composition of the magma from which they crystallized. These higher ratios are consistent with phase relationships exhibited in Fig. 16 which suggest that the ultramafic cumulates are the end product of mafic magma that began crystallizing at higher temperatures than the hornblende–plagioclase dominated mafic to intermediate rocks.

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Fig. 15. Chondrite-normalized REE analyses from two detailed traverses (five analyses each) across two grains in two samples: PG-6 (a,b) and PG-11 (c,d). The locations of each analysis are shown in Supplementary File DR-6 and the data are shown in DR-7. Analytical methods are given in Supplementary File DR-2. Normalizing values from Sun and McDonough (1989).

Other than the Mg#, there is very little difference in major element chemistry between the amphiboles in ultramafic and mafic rocks. Most major element variations in the amphiboles of both

lithologies are typical of edenite-type (Na,KA + Al IV = Si IV) substitution. Taken together, these amphibole compositions are broadly similar to those in the mafic “appinitic” facies within the coeval (ca.

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Fig. 16. Simplified water-saturated pressure–temperature phase diagram (after Moore and Carmichael, 1998) showing the phase assemblage fields for basaltic andesite magma under conditions similar of those of Greendale Complex mafic magmas. Numbers beside the dots correspond to the plagioclase (An) composition in equilibrium with basaltic andesitic melts at the relevant pH2O and T. The curves show the conditions at which a phase appears on the liquidus. Note that the crystallization of plagioclase before hornblende in the mafic rocks places a maximum of pH2O of 2.4 kbars (where the hornblende and plagioclase stability curves intersect). Petrographic evidence that olivine and pyroxene crystallized before hornblende in the ultramafic rocks places a maximum pH2O of 2.7 kbars.

610–605 Ma, Keppie et al., 1990) Jeffers Brook Plutonic Complex in the adjacent Cobequid Highlands, which is thought to have crystallized at pressures less than 4 kbar (Pe-Piper, 1988).

8.3. Amphibole trace and rare earth chemistry The high abundance of compatible trace elements (e.g. Ni, Cr) and the low abundance of magmatophile elements (e.g. Nb, Zr) are typical of ultramafic cumulate rocks. All REE profiles of amphiboles from mafic rocks display a gentle bow shape, with depletion in LREE, a slight depletion in HREE, and a negative Eu anomaly. These profiles are similar to those expected from amphibole/melt partition coefficients for REE (see compilation by Rollinson, 1993, p. 113). Normalized trace element plots also show enrichment in K, Ba, and Ti, and depletion in Th, Nb, Ta, Sr, Zr, and Hf. The profiles are sub-parallel, with few crossovers. Although it is unclear to what extent the high field strength element content of amphiboles may passively record the calc-alkaline chemistry of the magma, the negative Eu anomaly suggests that composition of the amphibole was influenced by coprecipitation with plagioclase, an interpretation that is consistent with petrographic observations. Petrographic observations from the ultramafic rocks indicate that amphibole grows in two distinct environments within the same sample; (a) late-stage synchronous growth with plagioclase in the interstitial liquid, and (b) earlier growth in reaction relationship with olivine and/or pyroxene. These distinct environments may be reflected in the contrasting REE and normalized trace element profiles which show that the amphiboles from ultramafic rocks can be divided into two groupings. Group A amphiboles display very similar trace and rare earth element characteristics to the amphiboles in the mafic rocks, and may have a similar origin, implying that the amphiboles crystallized at the same time as plagioclase. Their profiles are similar to those expected from generally accepted amphibole/melt partition coefficients. As the ultramafic rocks are cumulates, this interpretation implies

co-precipitation of Group A amphiboles with plagioclase growing from the interstitial liquid. The different REE profiles, as well as the crossovers on the normalized trace element plot suggest that Group B amphiboles have a different and more complex origin than Group A amphiboles. The absence of a Eu anomaly indicates that these amphiboles grew before plagioclase was on the liquidus, implying pH2O > 2.7 kbar. If so, these amphiboles probably grew in a reaction relationship with olivine and pyroxene. Given that these minerals are typically LREE depleted (e.g. Rollinson, 1993), it is unlikely that the Group B amphiboles inherited their signature from these minerals. It is possible, however, that their composition was influenced by relatively LREE-enriched interstitial liquid that would have been trapped between these minerals. If so, Group B amphiboles may have started to grow at depth, before being carried upward by water-rich magma in the manner described by Pitcher (1997). Moreover, their profiles suggest that in these situations, the partition coefficients traditionally used for petrogenetic modeling may not be appropriate. Significant variations in the REE profiles of amphiboles may be more common than realized hitherto. For example, in an ion microprobe study of glass-rich ultramafic xenoliths, Downes et al. (2004) documented amphiboles similar in composition to Group A and Group B in the core and rim (respectively) of an amphibole in xenolith MER-1. More generally, our data illustrate that trace and REE element analyses of amphiboles offer additional insights into their sensitivity to the crystallization history of the magma. Group A and B amphiboles are virtually indistinguishable on the basis of the major element chemistry that is used to classify them. However, it can be distinguished on the basis of their HFS and REE abundance, and, when combined with textural observations, these differences are connected to the phase relationships at the time of crystallization. Our data also suggest that caution should be exercised when using amphibole/ melt partition coefficients for petrogenetic modeling, as these may vary depending in the mode of origin of the amphibole. The positive relationship between HFS and REE elements with the extent of edenite substitution in Group B amphiboles suggests that M4 site occupancy (Tiepolo et al., 2000) is influenced by solid solution substitutions affecting adjacent sites (see Blundy and Wood, 2003). This potential co-dependency between adjacent sites is typically not considered when partition coefficients of trace elements are employed in petrogenetic modeling. Our study shows that such dependencies should be considered for amphiboles because of the variable environments in amphibole forms in mafic magmas. Recent advances in analytical capabilities should allow these dependencies to be further investigated. Although each ultramafic dyke represents a separate magma injection, the overall similarity in the major element composition of olivine and pyroxene, together with the dominance of normal zoning in hornblende and the parallel REE patterns suggests a rather simple history involving fractionation and limited exchange with externally-derived melts (e.g. Borghini and Ramponi, 2007; Claeson and Meurer, 2004). Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.lithos.2012.06.012.

Acknowledgments Supported by the Natural Sciences and Engineering Research Council of Canada Discovery and Research Capacity grants to JBM. We are grateful to Mike Fowler, two anonymous reviewers and journal editor Nelson Eby for the very detailed and constructive suggestions which led to significant improvements in the content and the presentation, and to Randy Corney and Jiggs Diegor for cheerful technical assistance.

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