Seismic velocities of granulite-facies xenoliths from Central Ireland: Implications for lower crustal composition and anisotropy

Tectonophysics 407 (2005) 81 – 99 www.elsevier.com/locate/tecto

Seismic velocities of granulite-facies xenoliths from Central Ireland: Implications for lower crustal composition and anisotropy Riana van den Berg a,*, J. Stephen Daly a,1, Matthew H. Salisbury b,2 b

a Department of Geology, University College Dublin, Belfield, Dublin 4, Ireland Geological Survey of Canada-Atlantic, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada

Received 17 January 2005; received in revised form 5 July 2005; accepted 11 July 2005 Available online 30 August 2005

Abstract V p and V s values have been measured experimentally and calculated for granulite-facies lower crustal xenoliths from central Ireland close to the Caledonian Iapetus suture zone. The xenoliths are predominantly foliated and lineated metapelitic (garnet– sillimanite–K-feldspar) granulites. Their metapelitic composition is unusual compared with the mostly mafic composition of lower crustal xenoliths world-wide. Based on thermobarometry, the metapelitic xenoliths were entrained from depths of c. 20– 25 F 3.5 km and rare mafic granulites from depths of 31–33 F 3.4 km. The xenoliths were emplaced during Lower Carboniferous volcanism and are considered to represent samples of the present day lower crust. V p values for the metapelitic granulites range between 6.26 and 7.99 km s
1 with a mean value of 7.09 F 0.4 km s
1. Psammite and granitic orthogneiss samples have calculated V p values of 6.51 and 6.23 km s
1, respectively. V s values for the metapelites are between 3.86 and 4.34 km s
1, with a mean value of 4.1 F 0.15 km s
1. The psammite and orthogneiss have calculated V s values of 3.95 and 3.97 km s
1, respectively. The measured seismic velocities correlate with density and with modal mineralogy, especially the high content of sillimanite and garnet. V p anisotropy is between 0.15% and 13.97%, and a clear compositional control is evident, mainly in relation to sillimanite abundance. Overall V s anisotropy ranges from 1% to 11%. Poisson’s ratio (r) lies between 0.25 and 0.35 for the metapelitic granulites, mainly reflecting a high V p value due to abundant sillimanite in the sample with the highest r. Anisotropy is probably a function of deformation associated with the closure of the Iapetus ocean in the Silurian as well as later extension in the Devonian. The orientation of the bulk strain ellipsoid in the lower crust is difficult to constrain, but lineation is likely to be NE–SW, given the strike-slip nature of the late Caledonian and subsequent Acadian deformation. When corrected for present-day lower crustal temperature, the experimentally determined V p values correspond well with velocities from the ICSSP, COOLE I and VARNET seismic refraction lines. Near the xenolith localities, the COOLE I line displays two lower crustal layers with in situ V p values of 6.85–6.9 and 6.9–8.0 km s
1, respectively. The upper (lower

* Corresponding author. Present address: Department of Geology, Private Bag XI, University of Stellenbosch 7602, South Africa. Fax: +27 21 808 3129, +353 1 283 7733. E-mail addresses: [email protected] (R. van den Berg), [email protected] (J.S. Daly), [email protected] (M.H. Salisbury). 1 Fax: +353 1 283 7733. 2 Fax: +1 902 426 6152. 0040-1951/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2005.07.003

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velocity) layer corresponds well with the metapelitic granulite xenoliths while the lower (higher velocity) layer matches that of the basic granulite xenoliths, though their metamorphic pressures suggest derivation from depths corresponding to the presentday upper mantle. D 2005 Elsevier B.V. All rights reserved. Keywords: Lower crustal xenoliths; Metapelitic granulites; Seismic velocities; Anisotropy

1. Introduction Seismic refraction studies have been used for decades to estimate the composition of the lower continental crust (e.g., Christensen and Mooney, 1995), but the results have been difficult to confirm due to the rarity of outcrops of lower crustal rocks at the Earth’s surface. Xenoliths provide a robust alternative means of testing petrological models of the crust based on seismology. The purpose of the present study is to investigate the composition and anisotropy of the lower crust beneath central Ireland by determining compressional (V p) and shear wave (V s) velocities of crustal xenoliths and comparing these values with data available from deep seismic refraction lines conducted in Ireland (Fig. 1). In particular, lines COOLE I (Jacob et al., 1985) and ICSSP (Lowe and Jacob, 1989) intersect close to xenolith localities in central Ireland while the VARNET lines (Landes et al., 2000, 2003) lie to the southwest. While similar velocity studies have been carried out by various authors, both on lower crustal and mantle xenolith samples (e.g., Padovani et al., 1982; Jackson and Arculus, 1984; Reid et al., 1989; Parsons et al., 1995; Rabbel et al., 1998) as well as exposed lower crustal rocks (e.g., Christensen and Fountain, 1975; Christensen, 1979; Burke and Fountain, 1990; Burlini and Fountain, 1993; Zappone et al., 2000), the majority of these studies were carried out on mafic granulites, eclogites and a variety of mantle rocks. The Irish xenoliths are unusual in that many consist of granulite-facies metapelites (Table 1). Although

unusual, some velocity studies of felsic rocks and metapelites similar in composition to the Irish xenoliths have been carried out (Table 2). These include V p and V s measurements on a garnet–sillimanite–K-feldspar-bearing granulite xenolith from the same locality as the present study (Evans, 1980) and on felsic granulite xenoliths from South Australia (Jackson and Arculus, 1984). V p and V s measurements have also been obtained from granulite-facies outcrop samples from the Lapland Granulite Terrane (Kern et al., 1993) and the Ivrea Zone (Barruol and Kern, 1996) as well as on lower grade mica-rich rocks from the same area (Burlini and Fountain, 1993). V p measurements have also been carried out on lower crustal xenoliths from New Mexico (Reid et al., 1989) that closely resemble the Irish xenolith suite, and on a lowergrade garnet–sillimanite schist xenolith from the Colorado Plateau (Padovani et al., 1982). Zappone et al. (2000) carried out V p measurements on metapelites of slightly lower metamorphic grade from the Betic chain, southern Iberian Peninsula, and also calculated seismic anisotropy for these rocks. Burlini and Fountain (1993) reported V p values of outcrop samples of granulite-facies metapelites from the Ivrea–Verbano Zone. The following section offers a brief outline of the geological setting of the Irish granulite-facies xenoliths, followed by a description of the samples used for petrophysical investigations. The experimental methods are followed by a discussion of the results, and a comparison of experimental values with deep seismic refraction V p data. A discussion on seismic anisotropy of the lower crust concludes the paper.

Fig. 1. Sketch map of Ireland and Britain showing major Caledonian deformational features including the Iapetus suture (A BROAD BAND OF DEFORMATION THAT IS WIDER THAN THE BROKEN LINE (IST) IMPLIES), the location of the central Ireland and Scottish xenolith localities discussed in the text and seismic refraction lines in Ireland (COOLE I, ICSSP and VARNET). Schematic crustal section for the COOLE I line, close to the xenolith localities, shows V p and density values from Lowe and Jacob (1989) and calculated pressures (Van den Berg, 2005). 1-D velocitydepth model for the VARNET A line from Landes et al. (2000) is shown for comparison. Pressures from thermobarometry (Van den Berg, 2005) are shown together with calculated depths for the best characterized samples of metapelite (error crosses) and mafic granulite (error crosses with square symbols).

R. van den Berg et al. / Tectonophysics 407 (2005) 81–99

83

COOLE I

6.0 - 6.2

2.75

10 6.6 - 6.69

2 6

94 269

10

2.84

20

30

Pressure (MPa) 600 800 1000

P (MPa)

Depth (km)

Density (gcm-3) 2.74

GG

Depth (km)

0

Vp (kms-1) <6

604 6.85 - 6.9

2.87

6.9 - 8.0

2.90

HB

716 800

14

DRB6-5

18 22

D14

26

GG

M

IST SU

AB5-1 CH19

30

SC25 SC10

34

100 km

SCD

DR CC

COOLE I ICSSP

CH

N

MP

IST

VARNET B VARNET A

0

Vp (kms-1) 6.40

VARNET A Depth (km)

5.90

10

6.10 6.30 6.25

GG HB SU IST MP SCD

6.55

20 6.90

30

7.20 8.10 8.20

40 8.35

M

Great Glen Fault Highland Boundary Fault Southern Uplands Fault Iapetus Suture Trace Midland Platform Southern limit of Caledonian deformation Xenolith localities: DR Dungolman River CC Clare Castle CH Croghan Hill Scottish xenolith localities (Halliday et al., 1993) Approximate location of velocity sections

M Moho

50

Geotectonic zone boundary Seismic refraction lines

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Table 1 Modal mineralogy and calculated seismic velocities for lower crustal xenoliths from central Ireland Rock type

Mp

Mp

Mp

D5

Mig

D14

Mp

D44

Mp

Sample

A44

Cores1

X 10

Y

X 20

Y 40

X 10

Z 70

X 30

Y

DRB6-2 XY

Z

Sil Grt Qtz Kfs Chl Op3 Bt Pl Spl Opx V pcalc4 V scalc4

37.0 29.8 2.2 24.4 1.4 4.4 0.8 * – – 8.10 4.56

36.0 29.2 5.4 20.4 4.0 4.4 0.6 * – – 8.38 4.76

17.6 23.0 7.6 45.2 2.8 1.2 * 2.6 – – 6.95 3.93

21.4 29.0 9.4 32.8 4.2 3.2 * * – – 7.61 4.34

22.0 34.4 15.8 25.8 1.6 * 0.2 * 0.2 – 7.31 4.24

24.8 27.6 14.4 29.8 1.6 1.8 * * * – 7.34 4.23

27.0 48.8 0.4 8.0 13.2 2.6 – – – – 9.87 5.60

45.8 25.2 12.0 12.8 4.2 * – – – – 8.26 4.77

0.6 12.4 18.8 57.2 9.8 0.6 * 0.6 – – 6.66 3.83

1.4 21.0 12.4 43.6 19.8 1.8 * * – – 7.98 4.57

Ps

G orth

MG

2

CCV

SC45

FB3-1

SC25

25.5 23.9 20.1 30.0 – 0.6 – – – – 7.02 4.09

5 21 42 30 * 2 – – – – 6.51 3.95

– 2 26 25 * 2 5 40 * – 6.23 3.97

– 28 1 – – 4 7 31 – 29 7.22 4.48

Mp = metapelite; Mig = migmatite; Ps = psammite; G orth = granitic orthogneiss; MG = mafic granulite. 1: Core orientations (X parallel to lineation, Y normal to X in plane of foliation, Z normal to foliation; numbers indicate angular deviation relative to lineation) of xenolith samples for which seismic velocities have been measured experimentally. 2: Data for CCV from Evans (1980). 3: Includes ilmenite and rutile. P 4: Calculated using V = 100 / [ (modi / v i)] (Voigt, 1928; Reuss, 1929; Hill, 1952) where modi is the modal abundance and v i is the individual mineral seismic velocity. Mineral seismic velocities are from Christensen (1982) and Sumino and Anderson (1984) selecting appropriate mineral compositions that correspond to the xenolith mineral compositions obtained from electron microprobe analyses. * = abundance less than 0.1; – = not detected. Mineral abbreviations: Bt = biotite, Chl = chlorite, Grt = garnet, Kfs = K-feldspar, Op = opaque, Opx = orthopyroxene; Pl = Plagioclase, Spl = spinel, Sil = sillimanite, Qtz = Quartz.

1.1. Geological setting The continental crust underlying most of Ireland was assembled during the terminal closure of the Iapetus Ocean in the Silurian toward the end of the Caledonian orogeny (Soper et al., 1992). The oblique collision of Laurentian and Avalonian lithosphere is marked by the Iapetus Suture Zone (ISZ), a ~50 km wide NE–SW-trending, NW-dipping structure, extending to deep crustal levels across Ireland and the UK (e.g., Beamish and Smythe, 1986; Freeman et al., 1988; Soper et al., 1992; Woodcock and Strachan, 2000; Fig. 1). Caledonian deformation in central Ireland involved a major strike-slip component (Soper et al., 1992; Dewey and Strachan, 2003), which explains the absence of extensive crustal thickening and uplift in the Irish Midlands as well as the lack of exposed deep basement rocks. Due to the presence of a thick cover of Carboniferous sediments in central Ireland, seismic studies have become an essential tool in understanding the deep crustal structure. The only direct samples of the lower crust in central Ireland

are xenoliths brought to the Earth’s surface by Lower Carboniferous volcanic pipes (Strogen, 1974; Daly et al., 1993; Van den Berg and Daly, 2001). These unique samples occur above the ISZ and compositionally appear to be derived from Avalonian lithosphere to the south (Van den Berg, 2005). The vast majority of the granulite-facies xenoliths are metapelites with the mineral assemblage garnet + quartz+ K-feldspar + rutileF sillimanite Fplagioclase F biotite F ilmenite. A large number of these samples contain granitic leucosomes, probably as a result of partial melting. Mafic granulite samples containing orthopyroxene are extremely rare and the vast majority of mafic granulites have been severely altered. The Irish xenolith suite is therefore unusual in composition when compared to the mostly mafic lower crustal xenoliths from world-wide occurrences (e.g., Griffin and O’Reilly, 1987; Rudnick, 1992; Ruiz, 1992; Rudnick and Fountain, 1995; Kempton et al., 2001; Downes et al., 2001; Embey-Isztin et al., 2003) including the well-known Scottish suite (Halliday et al., 1993; Upton et al., 1983, 2001). However, they appear

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Table 2 Bulk density, laboratory seismic velocity and anisotropy values for selected felsic and metapelitic granulite-facies rocks from outcrop and xenolith studies reported in the literature Sample, minerology

Sample no.

Avg q (g cm
3)

Evans (1980); Clare Castle xenolith; granulite-facies metapelite Grt, Sil, Kfs, Qtz CCV 3.04

Mean V p (km s
1)

Avg A (%)

Mean V s (km s
1)

Avg A (%)

7.36y

*

4.15y

*

3.83

*

Jackson and Arculus (1984); South Australia lower crustal xenoliths; felsic granulite Grt, Kfs, Qtz, Ky, Bt, Rt 824 2.94 7.16

2.23

Reid et al. (1989); New Mexico lower crustal xenoliths; pelitic paragneisses Sil, Grt, San, Qtz, P1 1975 3.04 7.26 Sil, Grt, San, Qtz, P1 1977 2.86 6.73

19.16 7.43

* *

* *

Burke and Fountain (1990); Ivrea Bt, P1, Sil, Grt, Qtz, Op, Ap Grt, Qtz, Sil, Op, Mym, Bt, Ap Qtz, Grt, Sil, Op, Alt, P1, Bt Qtz, Grt, Sil, P1, Qtz, Bt, Op

3.83 2.18 7.91 16.05

* * * *

* * * *

Kern et al. (1993); Lapland Granulite Terrane; granulite-facies metapelites Grt, Qtz, Kfs, Sil, P1, Rt 1403 3.11 6.99 Grt, Qtz, Kfs, Sil, Bt, Op 58049 3.10 6.88

11.00 2.39

3.71 4.09

*

Barruol and Kern (1996); Ivrea Zone metapelites Qtz, Kfs, Sil, Grt, Op 90VS19 3.10 Qtz, P1, Kfs, Bt, Grt, Sil 91V08 3.01 Bt, Kfs, Qtz, Sil, Grt 87VS57 2.86 Qtz, Kfs, P1, Bt, Sil, Grt 91V09 2.85

10.14 5.34 15.70 8.42

3.94 4.00 3.69 3.69

4.57 3.50 17.33 8.13

Zone metapelites IV-5 2.91 IV-7 3.11 IV-23 2.95 IV-24 3.00

6.53 7.35 6.95 7.60

6.91 6.74 6.31 6.17

2.25

y V p and V s measured at 400 Mpa; if unspecified, measurements at 600 MPa; A = anistropy; * = data not available. Mineral abbreviations (in addition to those in Table 1): Ap = apatite, Ky = kyanite, Mym = Mym = myrmekite, Rt = rutile, San = sanidine, Alt = unspecified alteration.

to be similar to the metapelitic xenoliths derived from the lower crust beneath Colorado and New Mexico (Reid et al., 1989). Thermobarometry has been carried out on a suite of metasedimentary xenoliths compositionally similar to those used for petrophysical studies and including sample D14. The best constrained samples (Fig. 1) indicate metamorphic pressures of between 550 and 700 MPa for the metapelitic samples and pressures above 850 MPa for mafic (orthopyroxene-bearing) garnet granulites (Van den Berg, 2005). Given the limited amount of tectonic deformation since the Carboniferous and the thin-skinned nature of the Variscan events in Ireland (e.g., Landes et al., 2000), it is likely that the metapelitic xenoliths were entrained at depths corresponding to these pressures (20–25 F 3.5 km, Van den Berg, 2005). U–Pb zircon dating (Van den Berg, 2005), indicates that the metamorphic assemblages and migmatitic leucosomes developed during

the Devonian in the latest stages of the Caledonian Orogeny. This together with the lack of observed metamorphic mineral zoning (Van den Berg, 2005) rules out the possibility that the xenoliths might represent high-level metamorphic basement derived from shallower crustal levels.

2. Samples Five granulite-facies metapelites (Fig. 2) were selected from a large collection of high grade xenoliths, most of which are unsuitable for petrophysical studies due to hydrothermal alteration, reaction with the host volcanics, weathering, fracturing or small sample size. One of the metapelites (DRB6-2) is from the Dungolman river (Irish National Grid Square N1851) while the other samples are from Clare Castle (N2447). Modal mineral abundances and calculated

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R. van den Berg et al. / Tectonophysics 407 (2005) 81–99

Fig. 2. Photographs of investigated samples showing core orientations. Numbers in superscript indicate angular deviation of cores relative to the mineral lineation (Table 1).

velocities for these and several other xenoliths are given in Table 1. The samples generally exhibit a strong foliation defined by alternating sillimanite–garnet-bearing and quartzo-feldspathic bands up to 3 mm thick. Abun-

dant, coarse-grained sillimanite (up to ~5mm in length) defines a strong lineation in most cases (with the exception of DRB6-2, which contains very minor sillimanite). Quartzo-feldspathic leucosomes (predominantly quartz and perthitic K-feldspar) which occa-

R. van den Berg et al. / Tectonophysics 407 (2005) 81–99

sionally cross-cut the foliation are present in many of the metapelites, with cores cut from D44 exhibiting the highest proportion of leucosomes (Fig. 2).

3. Experimental methods Ten cores approximately 2.5 or 2.0 cm in diameter (depending on sample size) and between 4.0 and 6.5 cm in length were cut from five samples and machinelapped to produce smooth, parallel orthogonal terminations. Ideally three orthogonal cores should be used to characterize the velocity of a sample with a strong fabric (Fig. 3). Two cores from each of the five metapelitic samples were classified in terms of its angular relationship to the foliation plane and lineation in the sample (Table 1, Fig. 2). Fig. 2 illustrates the individual samples and shows the orientations of the cores relative to the foliation plane and lineation direction. Cores cut parallel or subparallel to the foliation plane and lineation direction are denoted X, while those cut parallel to foliation and perpendicular to lineation are denoted Y cores. Z cores are those perpendicular or sub-perpendicular to the foliation plane and lineation direction. In the case of metapelitic sample DRB6-2 only a very weak lineation is present, so the core cut parallel to the plane of the foliation is denoted XY (Fig. 2). Deviations of the cores from ideal X, Y, Z orientations (Fig. 3) were sometimes unavoidable due to limitations of sample size or to avoid alteration and are indicated in Table 1 and Fig. 2. Bulk densities of the xenoliths were determined by weighing the cores in air and water and using the Z

X

87

Archimedes principle and the mass/volume relationship to calculate their density. Errors in density measurements are F 0.2%. V p and V s were measured in the Dalhousie University GSC High Pressure Laboratory in Halifax, Nova Scotia, on dry cores at room temperature and hydrostatic confining pressures to a maximum of 600 MPa using the pulse-transmission method (Birch, 1960, 1961). The samples were placed between two transducers (lead zirconate for V p, lead zirconate titanate for V s) with a resonant frequency of 1 MHz and brass electrodes were placed on the transducers at each end of the core. The sample assemblies were embedded in FlexaneR (a urethane polymer resin) to prevent saturation of the cores by the pressure medium. This is a standard technique that has no effect on the experimental data. Hydrostatic confining pressures up to 600 MPa were generated using an air-driven fluid pump in conjunction with a multi-stage fluid intensifier system using low viscosity oil (Esso Monoplex) as the pressure medium. The pressure was monitored using a strain gauge on the high pressure side of the intensifier and recorded digitally. After the sending transducer was pulsed with a high-voltage spike, the received signal was displayed on a Nicolet digital oscilloscope and stacked over at least four pulse repetitions to increase the signal to noise ratio. The first break of each received wave form was manually picked at 10, 20, 40, 60, 80, 100, 150, 200, 300, 400, 500 and 600 MPa while increasing and decreasing pressure in order to determine the pulse transmission time through the sample. Velocities were calculated from the pulse transmission time and the core length. One measurement per core was carried out for V p, while two V s measurements (one for vibration parallel, and one normal to the lineation or foliation direction) were made for some cores (Table 3). Velocities at pressures above 600 MPa were extrapolated due to the increasing viscosity of the pressure medium at very high confining pressures.

4. Results Y Fig. 3. Schematic diagram showing core orientation conventions. The Z-direction is normal to the foliation and lineation, X is parallel to the lineation, and the Y-direction is normal to the lineation in the plane of the foliation.

The densities of the xenolith samples for which velocities were measured or calculated are given in Table 3. They range between 2.97 and 3.23 g/cm3, with the lowest density obtained from metapelite D14

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Table 3 Density (measured and calculated), V p (measured, calculated and temperature-corrected), V s (measured and calculated) and anisotropy data from central Ireland xenoliths Sample

A44

D5

D14

D44

DRB6-2

Rock type

Metapelite

Metapelite

Metapelite

Migmatite

Metapelite

Cores1 2

Calculated q Measured q Calculated V p3 Measured V p T-corrected V p4 Calculated V s3 Measured V sp5 Measured V sn6 DV s = V smax
V s min Averages: Calculated q Measured q Calculated V p Measured V p T-corrected V p Calculated V s Measured V s P-wave anisotropy (%)7 T-corrected anisotropy S-wave anisotropy (%)7 Poisson’s ratio8

X 10

Y

X 20

Y 40

X 10

Z 70

X 30

Y

XY

Z

3.34 3.24 8.10 7.99 7.79 4.56 4.31 4.24 0.07

3.34 3.21 8.38 7.37 7.17 4.76 3.85 – –

3.05 3.10 6.95 7.10 6.90 3.93 4.12 4.03 0.09

3.21 3.10 7.61 7.25 7.05 4.34 4.17 4.28 0.11

3.24 2.94 7.30 7.30 7.10 4.23 4.19 4.08 0.11

3.19 3.00 7.34 7.16 6.96 4.23 4.18 – –

3.55 3.20 9.87 7.20 7.00 5.60 4.12 4.24 0.12

3.27 3.10 8.26 6.26 6.06 4.77 4.34 – –

2.79 3.10 6.66 6.65 6.45 3.83 3.87 3.82 0.05

2.97 3.03 7.98 6.64 6.44 4.57 3.86 – –

3.34 3.23 8.24 7.68 7.48 4.66 4.13 8.07 8.29 11.13 0.30

3.13 3.10 7.28 7.18 6.98 4.14 4.15 2.09 2.15 6.02 0.25

3.22 2.97 7.32 7.23 7.03 4.23 4.15 1.94 1.99 2.65 0.25

3.41 3.15 9.07 6.73 6.53 5.19 4.23 13.97 14.4 5.20 0.17

2.88 3.07 7.32 6.65 6.45 4.20 3.85 0.15 0.16 1.30 0.25

1: Core orientations of xenolith samples for which seismic velocities were measured experimentally. X parallel to lineation, Y normal to X in plane of foliation, Z normal to foliation. Numbers indicate angular deviation relative to lineation. 2: q = density (g cm
3). Calculated as q = A(modiq i / 100), where modi = modal abundance (Table 1) and q i = individual mineral density. Density data from Christensen (1982) and Sumino and Anderson (1984). 3: V p = P-wave seismic velocity (km s
1); V s = S-wave seismic velocity (km s
1). Calculated using V = 100[A(modi / Vi]
1 where modi is the modal abundance (Table 1) and Vi is the individual mineral seismic velocity, using average Voigt–Reuss values for mineral aggregates (Voigt, 1928; Reuss, 1929; Hill, 1952). Mineral seismic velocities from Christensen (1982) and Sumino and Anderson (1984), selecting appropriate mineral compositions that correspond to the xenolith mineral compositions obtained from electron microprobe analyses. 4: Temperature-corrected velocities calculated at 400 8C, i.e., for 25 km depth, using a thermal conductivity value of 2.6 W m
1 K
1 (Chapman, 1986) and a velocity temperature derivative of
5  10
4 km s
1 8C
1. 5: V sp = S-wave seismic velocity parallel to lineation direction (km s
1). 6: V sn = S-wave seismic velocity normal to lineation direction (km s
1). 7: Anisotropy calculated as 100[(V maxV min / Vaverage)]. 8: Poisson’s ratio calculated as (r = 1 / 2[(V 2p
2V2s / V 2p
V2s)].

and the highest from sillimanite-rich metapelite A44. The density values for the metapelites correlate well with values obtained for pelitic gneisses in several previous studies of lower crustal rocks, as summarized by Rudnick and Fountain (1995). The pressure dependence of V p and V s measurements (Fig. 4) follows the typical pattern of sharply increasing velocity at low pressure followed by a slower increase at higher pressure (Birch, 1960) up to 600 MPa. Only the values determined during the

descending pressure cycle were used (Fig. 4) because of hysteresis effects, attributed to the adjustment of rock porosity to changes in pressure (Christensen, 1965) in the ascending pressure runs. Velocities have not been corrected for changes in sample length at elevated confining pressure because this is significant only in the calculation of elastic constants and pressure derivatives (e.g., Brace, 1965). Core lengths are precise to 0.05 cm and travel times to 0.250 As (for V p) and 0.215 As (for V s).

Vp (kms-1)

R. van den Berg et al. / Tectonophysics 407 (2005) 81–99

8.00

8.00

7.50

7.50

7.00

7.00

6.50

6.50

6.00

6.00

89

Y and Z cores

X cores 5.50

5.50 0

100

200

300

400

500

600

0

4.40

4.40

4.20

4.20

4.00

4.00

3.80

3.80

3.60

3.60

3.40

3.40

3.20

100

200

300

500

600

3.20

X cores - normal

X cores - parallel 3.00

3.00 0

Vs (kms-1)

400

100

200

300

400

500

0

600

4.40

4.40

4.20

4.20

4.00

4.00

3.80

3.80

3.60

3.60

3.40

3.40

100

200

300

400

500

600

3.20

3.20

Z cores

Y cores 3.00

3.00 0

100

200

300

400

500

600

0

100

200

300

400

500

600

Pressure (Mpa) D14

DRB6-2 x A44

D5 + D44

D5 Yn

Fig. 4. Seismic velocity measurements versus confining pressure for experimental runs up to 600 MPa on five lower crustal xenolith samples from central Ireland.

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This results in an overall uncertainty in velocity measurement of 0.4%. Velocity values measured at 600 MPa or calculated using the average Voigt–Reuss values for mineral aggregates (Voigt, 1928; Reuss, 1929; Hill, 1952) and mineral velocities from Christensen (1982) and Sumino and Anderson (1984), are used throughout this paper although they can be extrapolated to higher pressures. Experimentally, only minor increases in velocity are observed above 600 MPa. In addition, this pressure corresponds reasonably well to the likely entrainment depths of the xenoliths (550–700 MPa, 20–25 km, see above). In comparing the xenoliths with the present-day lower crust, the measured velocities have been corrected for the present-day temperature at these depths resulting in small reductions. These calculations (Table 3) were based on a heat flow value of 70 mW m
2 (Brock, 1989) obtained from a bore-hole at Moate (near Clare Castle) by J. Wheildon (Imperial College, London) and reported by Cˇerma´k and Rybach (1979). The estimated lower crustal temperature at 25 km depth is 400 8C, which corresponds to a reduction in V p of 0.2 km s
1.

4.1. Compressional wave velocities (V p) The measured V p values at 600 MPa range between 6.26 and 7.99 km s
1 (Table 3) for individual cores from the metapelitic samples while the mean V p values range from 6.65 to 7.68 km s
1 (Table 3). The average of the mean values is 7.09 F 0.42 km s
1. Most of the measured values fall within the range of V p for granulite-facies metapelitic gneisses observed in previous studies, e.g., as compiled by Rudnick and Fountain (1995). However, the highest values are extreme and are more typical of mantle velocities (e.g., Burke and Fountain, 1990). Relatively high velocities have been reported previously for metasedimentary xenoliths by Jackson and Arculus (1984) and Reid et al. (1989) and are thought to be controlled by the presence of high-velocity phases such as sillimanite (9.5 km s
1; Sumino and Anderson, 1984) and garnet (8.45 km s
1; Christensen, 1982). A strong compositional control on V p is suggested by the generally close agreement between the measured and calculated values (Fig. 5) and is discussed further below. V an d en B erg et al. F ig.

11

10

D44 X

Vp (calc)

9

D44 Y A44 Y DRB6-2 Z D5 Y

8 D14 Z

D14 X A44 X

7 DRB6-2 XY D5 X CCV

6 6

8

7

9

Vp (meas) Fig. 5. V p calculated from modal mineralogy (Table 1) and mineral seismic velocities versus experimentally-determined values (Table 3). Experimental data for the central Ireland xenoliths are shown with 2 r error bars and identified by core number.

R. van den Berg et al. / Tectonophysics 407 (2005) 81–99

4.2. V p anisotropy Some of the metapelitic samples exhibit significant P-wave anisotropy. Table 3 shows that the X-cores yield the highest V p values (6.65 to 7.99 km s
1), while the Y-cores display intermediate values (6.26 to 7.37 km s
1) and the Z-cores lower values (6.64 to 7.16 km s
1). V p anisotropy ranges from 0.15% (DRB6-2) to 13.97% (D44) but, in view of the estimated 0.4% uncertainty in V p, is considered insignificant for values below about 2%. 4.3. V s and shear wave splitting Measured V s values for the X cores range from 3.87 to 4.31 km s
1 for vibration parallel to foliation (denoted V sp), and 3.82 to 4.24 km s
1 for vibration normal to foliation (V sn). Values for Y-cores range from 3.85 to 4.34 km s
1 (V sp), while a single V sn run gave a value of 4.28 km s
1. Due to the angles between the cores and the lineation direction in D5, two measurements were carried out on the Y core of this sample in addition to the two on the X core. V sp for the X core is 0.09 km s
1 higher than V sn, in keeping with the general trend for the metapelites. In the case of the Y core, however, V sn has a higher value than V sp. The reason for this may be related to the angle between the Y core orientation and the lineation direction (408, Table 3, Fig. 2). Metapelite V s values for two Z cores range from 3.86 to 4.18 km s
1. The mean V s value for the metapelites is 4.1 F 0.15 km s
1, well within the range of pelitic gneiss values determined in previous studies (e.g., Rudnick and Fountain, 1995). The available data allow calculation of shear wave splitting (DV s = V smax
V smin) for the X-cores of the metapelites and for one Y-core (Table 3). These values range from 0.05 to 0.12 km s
1, but only the higher values for D14 and D44 (Table 3) are likely to be significant. V sp values are generally higher than V sn, with the exception of the X-core values of the compositionally heterogeneous migmatitic sample D44 and the Y core values of metapelite D5, but these reversals in shear wave splitting are probably insignificant in view of the experimental uncertainty. Overall, anisotropy values (taking into account the extremes in V s of individual samples) range from 1% to 11% (Table 3).

91

4.4. Compositional control on seismic velocity and anisotropy As can be seen in Figs. 5 and 6, measured seismic velocities (Tables 2,3) for high-grade rocks are strongly influenced by bulk composition and mineralogy, as discussed by previous authors for both xenoliths and outcrop samples (Christensen and Fountain, 1975; Evans, 1980; Jackson and Arculus, 1984; Burke and Fountain, 1990; Jackson et al., 1990; Holbrook et al., 1992; Rudnick and Fountain, 1995; Barruol and Kern, 1996; Kern et al., 1999, 2002; Weiss et al., 1999). In addition, there can be an important crystallographic control on seismic anisotropy because the lineation and foliation are strongly controlled by the preferred orientation of mineral grains, especially sillimanite (Fountain, 1976; Reid et al., 1989). Most of the remaining differences between measured and calculated velocities can be attributed to differences in modal mineralogy between thin sections and cores (Fig. 5b). In agreement with older data from metapelitic xenoliths (Evans, 1980; Padovani et al., 1982; Jackson and Arculus, 1984; Reid et al., 1989) and outcrop samples (Burke and Fountain, 1990; Kern et al., 1993; Zappone et al., 2000), the central Ireland xenoliths also show positive, though weak, correlations between V p and V s and density (Fig. 6a,b). D44 and DRB6-2, which have the highest volume of migmatitic leucosomes, fall off these trends. V p and V s correlate more strongly with the modal abundances (Table 1) of sillimanite (Fig. 7a,c) and with garnet plus sillimanite (Fig. 7b,d). In the case of sample A44, which yields the highest velocity values (X-core, Table 3), the high content of sillimanite (Table 1) defines a prominent lineation in the plane of the foliation. In contrast, DRB6-2 contains very little sillimanite (Table 1) and yields the lowest velocity values (X-cores, Table 3). D44 ( Y-core) is one exception to the general pattern (Fig. 7a,b). The low velocity of this sample is due to the presence of large migmatitic leucosomes, mainly composed of quartz and K-feldspar (Fig. 3). These are probably under-represented in the available thin section for this sample. D44 also displays the highest deviation between its calculated and measured V p values (Fig. 5).

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Xenoliths:

8.0

Outcrop samples:

Clare Castle (Evans 1980) New Mexico (Reid et al. 1989) South Australia (Jackson and Arculus 1984) Betic Chain (Zappone et al. 2000) Colorado Plateau (Padovani et al. 1982)

Ivrea Zone (Burke and Fountain, 1990) Lapland Granulites (Kern et al. 1993) Ivrea Zone (Barruol and Kern, 1996)

A44

Vp (kms-1)

7.6

D14

7.2

D5

6.8 D44 DRB6-2

6.4

A

6.0 2.8

2.9

3.0

3.1

3.2

3.3

Density (gcm-3) 4.3 D44

4.2 D5

D14

A44

Vs (kms-1)

4.1

4.0

3.9 DRB6-2

3.8

3.7

B 3.6 2.8

2.9

3.0

3.1

3.2

3.3

-3

Density (gcm ) Fig. 6. Seismic velocity versus density for the Irish xenoliths compared with high-grade metapelitic xenoliths and outcrop samples. (a) V p versus density; (b) V s versus density. Error bars and sample labels as in Fig. 5 for the central Ireland xenoliths. Sources of other data and symbols are shown in (a).

R. van den Berg et al. / Tectonophysics 407 (2005) 81–99

93

8.0

8.0

A44 X

A44 X

7.6 D14 X

7.2 D5 X

A44 Y

Vp (kms-1)

Vp (kms-1)

7.6

D44 X

D5 Y D14 Z

6.8

D14 X

A44 Y

D5 Y

7.2 D5 X

6.8

D44 X

D14 Z

DRB6-2 Z

DRB6-2 XY DRB6-2 Z

DRB6-2 XY

6.4

6.4

D44 Y

A 6.0

0

20

10

40

30

D44 Y

6.0

50

0

20

10

Sil (%)

30

50

40

60

70

B 80

Sil + grt (%)

4.2

D44

4.2

D14

D44

D5 D14

4.1

A44

V s (kms -1)

Vs (kms-1)

4.1 4.0 3.9 DRB6 -5

D5

A44

4.0 3.9 DRB6-5

3.8

3.8 3.7 3.7

8

3.6 0

10

20

30

D

3.6

C 40

12

16

20

24

28

32

36

40

44

48

Sil + grt (%)

Sil (%) Fig. 7. Seismic velocity modal abundances of sillimanite and sillimanite plus garnet for the Irish xenoliths compared with high-grade metapelitic xenoliths and outcrop samples. (a) V p versus modal abundance of sillimanite; (b) V p versus modal abundance of sillimanite plus garnet; (c) V s versus modal abundance of sillimanite; (d) V s versus modal abundance of sillimanite plus garnet. Error bars and sample labels as in Fig. 5 for the central Ireland xenoliths. Sources of other data and symbols are shown in Fig. 6a.

Replacement of garnet by chlorite appears to have little effect on the measured velocity. Significant chloritisation is present in DRB6-2 (Z-core, V p = 6.64 km s
1) but is also present in D44 (Xcore), which has a more typical V p value of 7.2 km s
1. Their different velocities are probably better explained by contrasting sillimanite contents (1.4% and 27%, respectively) as well as the relatively high content of quartz and K-feldspar in DRB6-2 (Table 1). Poisson’s ratio (r, Table 3), determined from V p and V s data from deep seismic studies, has been used to estimate lower crustal composition (e.g., Christen-

sen and Fountain, 1975; Zandt and Ammon, 1995). Generally, felsic lower crustal rocks have r values of b0.26, while intermediate and mafic lower crustal rocks have values of 0.26–0.28 and N 0.28, respectively (Zandt and Ammon, 1995). Poisson’s ratios for the central Ireland xenoliths have been calculated using average V p and V s values (Table 3). Sample DRB6-2 has the lowest r (0.25) and A44 has the highest r (0.35). The high r of sample A44 is a direct result of its high V p value, which in turn is a consequence of abundant sillimanite. DRB6-2 on the other hand, contains very little sillimanite. D5 and D14 and

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R. van den Berg et al. / Tectonophysics 407 (2005) 81–99

20 18 16 14 D44

%Ap

12 10 8 A44

6 4 2 DRB6-5

D5

A

D14

0 10

0

20

30

40

Sil (%) 17 15 13

%As

11 A44

9 7

D5 D44

5 3

B

D14 DRB6-5

1 0

10

20

30

40

Sil (%) Fig. 8. Anisotropy versus modal abundance of sillimanite for the Irish xenoliths compared with high-grade metapelitic xenoliths and outcrop samples. (a) %A p versus modal abundance of sillimanite and (b) %A s versus modal abundance of sillimanite. Error bars and sample labels as in Fig. 5 for the central Ireland xenoliths. Sources of other data and symbols are shown in Fig. 6a.

R. van den Berg et al. / Tectonophysics 407 (2005) 81–99

migmatitic sample D44 yield lower r values ranging from 0.26 to 0.28, reflecting their high quartz and Kfeldspar content relative to sample A44 (Table 1). Various authors have attributed seismic anisotropy within the plane of foliation to the preferential orientation (lineation) of sillimanite and quartz (e.g., Fountain, 1976; Reid et al., 1989). Anisotropy values (A p) calculated from this study (Table 3) correlate with the modal abundance of sillimanite (Fig. 8a), although the dependence is less than in compositionally similar metapelite outcrop samples from the Ivrea Zone (Barruol and Kern, 1996) and in metapelitic xenoliths from New Mexico (Reid et al., 1989). In contrast to the Ivrea Zone outcrop samples, which exhibit a strong dependence of shear-wave anisotropy (A s) on modal abundance of sillimanite, the central Ireland xenoliths show only a weak positive correlation (Fig. 8b).

5. Xenoliths as lower crustal samples — comparison with deep seismic refraction data Three seismic refraction experiments have been shot in the vicinity of the central Ireland xenolith localities (ICSSP, Jacob et al., 1985; COOLE I, Lowe and Jacob, 1989 and VARNET, Landes et al., 2000, 2003, Fig. 1). The VARNET data were derived from lines in southwest Ireland, while the ICSSP line ran NE–SW, roughly parallel to the ISZ. The COOLE I refraction line extended approximately north— south, intersecting the main Caledonian strike and cross-cutting the ICSSP line in the vicinity of the xenolith localities in central Ireland (Fig. 1). Interpretation of the V p data from the seismic lines indicates that the lower crust is approximately 10 km thick with an average V p of 6.9 km s
1. The Moho is located at approximately 30 km depth (deduced from wide-angle reflections) and was modeled as a transitional zone beneath central and eastern Ireland and shallows slightly towards the coast in agreement with gravity observations (Murphy, 1981). V s data are not available from any of these studies. Fig. 1 shows simplified crustal sections constructed using seismic velocity (V p) data from Landes et al. (2000), Jacob et al. (1985) and Lowe and Jacob (1989) and a density model from Lowe and Jacob (1989). It shows calculated pressures for depths to 29

95

km below central Ireland, in the vicinity of the xenolith localities. Seismic data from the COOLE I line indicate that the lower crust beneath central Ireland can be divided into two layers. An upper layer between 22 and 26 km depth has V p values between 6.85 and 6.9 km s
1, while the lower layer comprises a Moho transition zone with velocities increasing from 6.9 to 8.0 km s
1. The Moho lies at a depth of 29 km. The average density of the upper layer is 2.87 g/cm3 while the lower layer between 26 and 29 km has an average density of at least 2.90 g/cm3. Pressures calculated using these densities range from 604 to 716 MPa at depths between 22 and 26 km (the upper deep crustal layer) and reach 800 MPa at 29 km depth (the lower layer). Thermobarometric calculations from the metapelitic xenoliths (see above) yield metamorphic pressures corresponding to the upper of these two layers suggesting that the metapelites are derived from this layer. Lower crustal velocities determined from the seismic refraction studies compare well with our experimental data within the given error bars (mean 7.09 F 0.42 km s
1, Table 3). The presence of such high velocity material in the lower crust agrees with observations from other authors (e.g., Jackson and Arculus, 1984; Reid et al., 1989), who suggested that lower crustal velocities greater than 7 km s
1 may be related to metasedimentary granulites, which are an important lower crustal constituent in certain regions. Most of the differences between experimental xenolith velocities and the seismic data (Fig. 1) can be explained when the experimentally-determined velocity values are corrected for present-day lower crustal temperatures (Table 3). This reduces the velocities by c. 0.2 km s
1, resulting in a mean V p value of 6.88 km s
1. In addition, the metapelites are accompanied in the lower crust by granulite-facies rocks of lower seismic velocity including psammite and granitoid orthogneiss (Van den Berg, 2005), which may be under-represented in the xenolith population. This could arise as a result of farther transport of these xenoliths in the entraining volcanic magma due to their generally lower densities. Relatively low velocities are to be expected for such siliceous lithologies, even those metamorphosed under granulitefacies conditions. Evans (1980) reported experimentally-determined V p values for siliceous high-pressure

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granulites from the NE Ox Mountains (Sanders et al., 1987) ranging from 5.75 to 6.09 km s
1. As can be seen in Table 1, a granulite-facies psammite and a granitic orthogneiss from Clare Castle have calculated V p values of 6.51 and 6.23 km s
1, respectively. Mixing small amounts (say up to 10% of each) with the metapelites with the highest temperature-corrected V p values would maintain the good agreement with the seismic refraction results. A general problem in lower crustal xenolith studies is the extent to which the xenolith population is representative of the crustal composition (e.g., Rudnick, 1992). As already argued, it is likely that the lower crustal layer between 22 and 26 km is not made up exclusively of metapelites. Over-representation of the garnet–sillimanite–K-feldspar gneiss bulk compositions in the xenolith population could result in part from their refractory nature. The fact that they had already experienced partial melting and probable melt extraction (Van den Berg, 2005) makes them less susceptible to assimilation by the entraining magma. By contrast, more fertile bulk compositions such as the granitic orthogneiss and psammite may be significantly reduced in the population by melting and assimilation. The V p value of 7.22 km s
1 calculated for the mafic granulite (SC25, Table 1) agrees well with the Lowe and Jacob (1989) data. Thermobarometry yields pressures of 850 and 910 MPa for mafic granulites SC25 and SC10, respectively (Fig. 1), which indicate derivation from depths greater than 30 km, i.e., apparently within the mantle. However, it is clear from their basic composition that these samples cannot represent the bulk mantle. This apparent discrepancy, together with the presence of such high velocity material as the metapelites in the lower crust suggests that identifying the base of the crust on seismic velocity grounds alone could be subject to systematic error leading to a mismatch between the bpetrologicalQ and seismic Moho. Alternatively, the Moho could have shallowed by tectonic rebound or the mafic xenoliths could represent veins of frozen basic melt trapped in the mantle section. Velocity measurements and geochronology to test the origin of the mafic xenoliths are not currently possible due to their rarity, small size and extreme alteration. Seismic anisotropy was not detected in the COOLE I and ICSSP studies, but is predicted based on the

results from this laboratory study. Metapelitic xenoliths exhibit a moderate but significant V p anisotropy of up to 14%, which is related to modal mineralogy, especially the sillimanite content that conspicuously defines the lineation. Anisotropy is probably attributable to deformation associated with the closure of the Iapetus ocean as well as subsequent extension. It is difficult to constrain the regional extent and orientation of the bulk strain ellipsoid in the lower crust. However, the lineation is likely to have a NE–SW and sub-horizontal orientation given the sinistral strikeslip nature of the late Caledonian and subsequent Acadian deformation (Soper et al., 1992; Dewey and Strachan, 2003; Soper and Woodcock, 2003). U–Pb dating of zircons from migmatitic leucosomes within the xenoliths shows that high grade metamorphism and melting continued after the Caledonian and Acadian events, and are as young as latest Devonian, probably associated with extensional deformation (Van den Berg, 2005). Geometrically, these later leucosomes were controlled by the orientation of the preexisting mineral fabrics, which date from the Caledonian collision.

6. Conclusions Measured seismic velocities of granulite-facies metapelitic xenoliths from central Ireland (mean V p = 7.09 F 0.4 km s
1; mean V s = 4.1 F 0.15 km s
1) correlate strongly with mineralogy, especially the modal abundance of garnet and sillimanite. With the exception of strongly heterogeneous samples, the velocities calculated from modal mineralogy also agree with experimentally-determined values. While these velocities are higher than those measured for the lower crust along the ICSSP, COOLE I and VARNET lines, they are in good agreement when corrected for temperature. In addition, metamorphic pressures from metapelite thermobarometry (550–700 MPa) correspond well with the calculated values for the upper of two lower crustal layers detected beneath central Ireland at a depth of 22–26 km. These results, together with the absence of compositional zoning in the metamorphic minerals and the fact that no significant tectonism has occurred in the region of the xenolith localities since the Lower Carboniferous, support the view that the high-grade xenoliths are

R. van den Berg et al. / Tectonophysics 407 (2005) 81–99

effectively present-day samples of the lower crust beneath Ireland. Metapelitic xenoliths from central Ireland exhibit moderate but significant anisotropy (A p up to 14%), which is related to modal mineralogy, especially sillimanite content which conspicuously defines the lineation. This anisotropy is probably attributable to deformation associated with the closure of the Iapetus ocean as well as subsequent extension. It is difficult to predict the regional extent or orientation of the bulk strain ellipsoid but lineation is likely to be NE–SW and sub-horizontal given the strike-slip nature of the late Caledonian deformation.

Acknowledgements Velocity measurements were carried out at Dalhousie University, Halifax, Nova Scotia with assistance from Robert Iuliucci. We thank Phil Meredith and Phil Benson of University College London for the use of drilling and lapping equipment and assistance with drilling the minicores. We are grateful to Tom Culligan (UCD) for preparation of thin sections and Damien Gagnevin for useful comments on an early draft of the paper. This project was supported by Enterprise Ireland Basic Research Grant SC/1998/524 and a University College Dublin President’s Research Award, both awarded to JSD.

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