Major and trace element chemistry and Sm–Nd age correlation of magmatic pegmatite garnet overprinted by eclogite-facies metamorphism

Chemical Geology 241 (2007) 4 – 22 www.elsevier.com/locate/chemgeo

Major and trace element chemistry and Sm–Nd age correlation of magmatic pegmatite garnet overprinted by eclogite-facies metamorphism G. Habler a,⁎, M. Thöni a , C. Miller b b

a Department of Lithosphere Research, University of Vienna, Althanstr. 14, A-1090 Vienna Austria Institute of Mineralogy and Petrography, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria

Accepted 17 January 2007

Abstract Meta-pegmatites are widespread in the polymetamorphic Koralpe–Saualpe basement, an Austroalpine unit with eo-Alpine highpressure overprint. Major and trace element data of coarse-grained garnet and equilibrium relationships with other pegmatite minerals indicate that garnet crystallized during pegmatite emplacement. Whole rock and garnet separates from the metapegmatites yield Sm–Nd isochrons with Late Permian–Early Triassic ages of 254 ± 2–239 ± 2 Ma. Surprisingly, the Cretaceous eclogite facies metamorphic overprint did not produce pervasive material exchange between garnet and matrix. Only the outermost 10–50 μm thick rims of cm-sized garnet crystals show diffusive major element re-equilibration. Detailed Sm–Nd investigations, using different separation techniques demonstrate isotopic equilibration of garnet, inclusions and the whole rock. High-precision age information is provided by garnet with extremely high 147Sm/144Nd ratios of 2−10, best-fit isochrons of multiple garnet magnetic fractions and reproducible results. However, age variations between different samples and between different crystals separated from single samples range outside analytical uncertainties. The Sm–Nd age results, covering a time-span of 18 Ma, are interpreted as reflecting long-term, though continuous garnet crystallization during pegmatite-emplacement. This was supposedly related to low pressure metamorphism with widespread formation of metamorphic andalusite in the metapelitic pegmatite hostrock. Protracted magmatic and metamorphic processes are in line with a tectonic scenario of crustal scale extension and initial rifting causing highly perturbed geothermal gradients, partial melting of metapelitic rocks and enhanced magmatic activity as postulated for the Austroalpine basement in Permian–Triassic time. © 2007 Elsevier B.V. All rights reserved. Keywords: Garnet; Sm–Nd geochronology; Meta-pegmatite; Koralpe; Permian; Re-equilibration

1. Introduction Garnet Sm–Nd dating may provide precise timeconstraints on magmatic, metamorphic and deformational processes and their rates, if microstructural and compositional data as well as P–T data can be corre⁎ Corresponding author. Tel.: +43 1 4277 53475; fax: +43 1 4277 9534. E-mail address: [email protected] (G. Habler). 0009-2541/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2007.01.026

lated with isotopic information. However, the interpretation of bulk multigrain mineral age data obtained from ID-TIMS analyses is often complicated by uncertainties concerning (i) bulk isotopic characteristics, mineral homogeneity, isotopic equilibration and the extent of REE fractionation, (ii) influence of polyphase or protracted mineral growth during long-lasting though coherent tectonometamorphic events and re-opening/ re-equilibration of isotopic systems during secondary

G. Habler et al. / Chemical Geology 241 (2007) 4–22

processes, and (iii) undetected inclusions in the analyzed mineral fractions with high trace element concentrations, variable mother–daughter ratios and closure temperatures strongly differing from those of garnet (e.g. Zhou and Hensen, 1995; DeWolf et al., 1996; Prince et al., 2000; Thöni, 2002). Some of these uncertainties can be minimized by different methods. Garnet Sm–Nd results may be significantly improved by testing data reproducibility using multiple magnetic and grain size fractions of single samples, by careful hand-picking and application of leaching techniques to eliminate impurities (Zhou and Hensen, 1995; Amato et al., 1999; Anczkiewicz and Thirlwall, 2003), and by correlation of isotopic, detailed microstructural, major and trace element data. Based on an investigation of meta-pegmatites from the Austroalpine Koralpe basement in the Eastern Alps (Fig. 1a), potentials and challenges of relating geochronological data with magmatic or metamorphic processes, as well as the uncertainties in assigning agevalues to specific geological processes are discussed. The wide scatter of available Permian age data from different Austroalpine basement units in the Eastern Alps has been discussed as due to a (i) long-term magmatic and metamorphic event, or (ii) to Carboniferous mineral relics or/and Cretaceous overprinting of isotopic systems and metamorphic mineral growth (Thöni and Miller, 2000; Schuster et al., 2001). In the present study, microstructures, mineral compositions and Sm–Nd isotopic data of meta-pegmatite garnet from the Koralpe are used to assign phase crystallization and compositional equilibration to either magmatic pegmatite-emplacement or to metamorphic processes during the subsequent Cretaceous eclogite-facies overprint (Miller et al., 2005). 2. Geological setting The Saualpe–Koralpe crystalline basement is part of the Austroalpine Units of the Eastern Alps east of the Penninic Tauern Window (Schmid et al., 2004). Garnetbearing micaschists and mylonitic kyanite-bearing paragneisses are its major lithologies, with intercalations of marbles, calc-silicates, amphibolites and eclogites (Fig. 1b). The first significant metamorphic imprint of the meta-sediments occurred at low pressure conditions during Permian–Triassic crustal extension and opening of oceanic domains (Habler and Thöni, 2001; Schuster et al., 2001). Remnants of magmatic rocks are represented by gabbros (Miller et al., 1988; Thöni and Jagoutz, 1992) and peraluminous pegmatites (Niedermayr and Göd, 1992; Heede, 1997; Thöni and Miller, 2000). PT-data of 600 ± 20 °C/0.4 ± 0.1 GPa (Habler and Thöni, 2001) and 650 °C/0.6–0.65 GPa (Tenczer et al.,

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2006) have been suggested for the regional Permian LP metamorphism in the Saualpe metapelites. Locally, partial melting of the metapelitic host rocks caused the generation of Permian–Triassic pegmatites (Morauf, 1981; Thöni and Miller, 2000). The main metamorphic and deformational imprint is due to the Cretaceous continent–continent collision, subduction and exhumation of the Saualpe–Koralpe Unit, a part of the eo-Alpine High Pressure Belt (Thöni and Jagoutz, 1993) close to the southern margin of the Austroalpine crystalline basement units. Based on Sm– Nd, Lu–Hf, U–Pb and Rb–Sr data on eclogite minerals (garnet, omphacite, zoisite, zircon, phengite) maximum burial was reached between 95 and 90 Ma ago (Miller et al., 2005; Thöni, 2006), followed by exhumation and cooling below 300 °C 70–80 Ma ago (Morauf, 1982; Thöni, 2002). Peak pressure conditions of eclogitefacies metamorphism were given as 625–730 °C/2.2– 2.8 GPa (Miller et al., 2005) for eclogites and from 700 ± 68 to 600 ± 63 °C/1.3–1.6 GPa (Tenczer and Stüwe, 2003) for metapelitic rocks from the Koralpe basement. Pervasive deformation occurred at eclogite facies conditions and during the earliest exhumation stages, whereas later amphibolite and greenschist facies deformation was partitioned into distinct shear zones. Pegmatites were deformed to dm-sized mylonitic layers. Only a few meta-pegmatite outcrops still have lithological contacts with the host rock that are discordant to the major foliation. The meta-pegmatites investigated in the present study are intercalated within Al-rich metapelites (so called “Disthen-Paramorphosenschiefer”). They were only weakly deformed during the Cretaceous high-pressure event in contrast to the mostly mylonitic Saualpe–Koralpe basement rocks. Geographic coordinates of the sampling sites are given in Table 1. 3. Petrographic features The investigated meta-pegmatites stem from the locality “Wirtbartl” in the S Koralpe about 10 km W of Schwanberg. They form several meters thick bodies that have intruded Al-rich metapelites and quartz-rich paragneisses. The coarse-grained magmatic assemblages were affected to some extent by metamorphic (re)crystallization and deformation. In samples WBK1, 04T25K and HS00704, euhedral primary phases predominate and syn-metamorphic deformation is confined to mm-scale shear zones. In contrast, sample WBK2 is strongly foliated and microstructurally resembles an orthogneiss. The meta-pegmatites display relic magmatic assemblages of centimeter–decimeter sized euhedral

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G. Habler et al. / Chemical Geology 241 (2007) 4–22

oligoclase, quartz, K-feldspar, muscovite, garnet, tourmaline ± andalusite (Fig. 2a–c). The euhedral cm-sized garnets partly contain medium- or numerous very finegrained (10 to b 1 μm sized) inclusions of accessory phases (apatite, xenotime, ± zircon, monazite, Nb–Ta–Sn bearing rutile, sphalerite). Besides they are partly colour zoned, with dark core domains or sector zoning (Fig. 2d). Magmatic aluminosilicate is confined to quartz-rich pegmatite domains. Kyanite-paramorphs after magmatic andalusite show intergrowths with coarse-grained euhedral garnet, clearly documenting mutual magmatic growth of garnet and andalusite (Clarke et al., 2005). Presumably during Cretaceous high-pressure metamorphism andalusite has been completely replaced by aggregates of finegrained kyanite that still trace the euhedral andalusite-grain shapes (Fig. 2c). Microstructurally similar kyanite-aggregates also characterize the metapelitic host rock (“DisthenParamorphosenschiefer”). The Cretaceous eclogite facies metamorphic overprint caused granulation and re-equilibration of matrix minerals (plagioclase, K-feldspar, muscovite, quartz) and the replacement of andalusite by kyanite-aggregates. In some samples, growth of a second fine-grained garnet generation in the matrix was observed (WBK2), or garnet subgrain formation occurred in micro-scale high strain zones (WBK1). Initial stages of deformation zones within garnet are marked by 20–50 μm thick trails of 10–20 μm sized accessory phases (Ap, Xen, Rt, Ky) and rimmed by colourless garnet (Fig. 2d), whereas unaffected garnet core domains show b 1 μm sized inclusions (Ap, Xen and rare zircon, monazite, Nb–Ta–Sn bearing rutile and sphalerite). Garnet domains that were microstructurally affected by the metamorphic overprint are volumetrically subordinate to the weakly deformed magmatic phase. 4. Major element mineral compositions Major element mineral compositions were determined with a CAMECA SX 100 electron microprobe (EMP) at the Department of Geological Sciences, University of Vienna (Austria) and on a JEOL JXA8100 Superprobe (Institut für Mineralogie und Petrographie, University of Innsbruck). Standard measurement conditions were at 15 kV accelerating voltage, 10 to 20 nA beam current. Raw data were corrected for

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Table 1 Sample location (geodetic datum: WGS84; geographic coordinate system UTM, zone 33) Sample

Lithology

E

N

WBK1 04T25K WBK2 HS00704 HS00304

Meta-pegmatite Meta-pegmatite Pegmatitic gneiss Meta-pegmatite + Ky paramorphs

504,774 504,768 504,436 502,813

5,177,599 5,177,620 5,178,063 5,176,806

matrix effects by a PAP-routine (Pouchou and Pichoir, 1991). Natural and synthetic silicates and oxides were used as standard material. Data given in the text are mol% of the end-member component. Mineral abbreviations follow Kretz (1983), Xen = xenotime. Abbreviations of mineral phases are given with capitals, whereas phase components are given with lower case letters. Representative garnet and plagioclase analyses are listed in Table 2. 4.1. Garnet The analyzed coarse-grained garnets are essentially almandine–spessartine solid solutions, characteristic for magmatic pegmatite garnet (Whitworth and Feely, 1994). In all samples these two end-members make up more than 91 mol% of the garnet, pyrope constitutes up to 7.6 mol%, grossular is less than 2 mol%. Garnet compositions of samples WBK1, 04T25K and HS00704 are similar, whereas the microstructurally different sample WBK2 has significantly lower Mn (sps3–5) and higher Mg content (prp10–13). All coarse-grained garnets are continuously zoned with rims containing less Mn and more Fe and Mg relative to the core (Fig. 3). Only the outermost garnet rims (10–50 μm thick) and the boundaries with coarse-grained magmatic phases intergrown with garnet, as well as the fine-grained garnet subgrains show increasing Ca content (grs2–15) at constant Mn and Mg towards the rim (Fig. 4). Therefore the outermost garnet rim was probably affected by a Cretaceous metamorphic diffusional overprint. In contrast, newly grown metamorphic garnet (sample WBK2) has significantly lower Mn- and higher Mgcontents (prp33–37) (Figs. 3 and 5). No deviation from the primary magmatic composition was detected by

Fig. 1. a) Tectonic sketch map of the Eastern Alps modified after Frey et al. (1999) showing the main tectonic units and the location of published Permian–Triassic age data (Morauf, 1981; Schuster et al., 2001, with references; Thöni, 2002; Sölva et al., 2003, with references; Thöni and Miller, 2000). b) Geological overview of the S-Koralpe, modified after the “Geological map of Austria 1:50,000” published by the Geological Survey of Austria referring to the sheets 188 Wolfsberg (Beck-Mannagetta, 1980), 189 Deutschlandsberg (Beck-Mannagetta et al., 1991), 205 St. Paul i. Lavanttal (Kleinschmidt et al., 1989) and 206 Eibiswald (Beck-Mannagetta and Stingl, 2002). Crosses mark the wide-spread occurrences of mainly small-scale (a few meters sized) pegmatite bodies. The sampling location “Wirtbartl” is shown by enlarged open cross-symbols.

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quantitative EMP analysis in garnet core domains adjacent to inclusion trails of fine-grained accessory phases (Fig. 5). During the metamorphic overprint material exchange of magmatic garnet with the matrix is therefore confined to the outermost 10–50 μm of garnet.

4.2. Feldspar K-feldspar porphyroclasts are perthitic. Raster analyses yielded compositions of or70–78ab22–30, whereas point analyses of host K-rich feldspar indicate compositions of or88–89ab11–12 and a minimal anorthite

Fig. 2. Microstructural characteristics of the meta-pegmatites: a) A hand specimen (sample WBK1) shows coarse-grained K-feldspar and euhedral colour-zoned garnet in a quartz-domain, and a mm-sized shear zone deflected around the garnet grains. b) The coarse-grained magmatic assemblage consists of K-feldspar, quartz, garnet, tourmaline and kyanite-aggregates replacing euhedral andalusite (sample HS00704). c) Photomicrograph (crossed polarized light) documenting the common magmatic growth of garnet and andalusite, now replaced by kyanite + muscovite aggregates. d) Photomicrograph (plain polarized light) showing sector zoned garnet with dark and colourless domains. Note that garnet is bleached next to inclusion trails of 10–30 μm sized accessory phases (black and white arrows). e) Photomicrograph (plain polarized light) of a garnet domain from sample 04T25K. Very fine-grained (10 to b 1 micrometer sized) inclusions were identified by electron microprobe mainly as apatite, xenotime and rutile as well as subordinate zircon, albite and kyanite.

G. Habler et al. / Chemical Geology 241 (2007) 4–22

component (b 0.06 wt.% CaO). Fine-grained recrystallized K-feldspar in the matrix has a composition of or91–95ab5–9. Magmatic coarse-grained plagioclase is

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oligoclase (an10–17) with decreasing Ca-content towards the rim. Fine-grained inclusions in garnet and subgrains in the matrix are oligoclase or albite (an0–11).

Table 2 Major element composition of garnet normalized to 12 oxygens Sample

WBK2 WBK2

WBK2

Comment c, core c, /Ab-incl f, aggregate matrix c, rim/matrix c, core

WBK1 WBK1

WBK1

WBK1

04T25K 04T25K

c, rim/25 μm c, rim/5 μm c, core

04T25K

WBK2

c, rim

f, core

f, rim

Grt 1/2

Grt1

Grt1

Grt1

Grt1

Grt1

Grt1

Grt1

Grt1

Grt1

Grt2

Grt2

Analysis

6/95

5/4

6/127

3/2

3a/90

3a/10

3a/6

78/46

76/3

88/7

88/2

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Total Si Al Ti Fe3+ Fe2+ Mn Mg Ca Z(3) Y(2) X(3) alm sps prp grs

36.41 36.63 0.005 0.015 20.67 20.73 28.31 31.20 13.31 9.80 0.836 1.237 0.316 0.909 99.9 100.5 2.995 2.999 2.005 2.000 0.000 0.001 0.000 0.000 1.945 2.090 0.927 0.679 0.103 0.151 0.028 0.080 3.00 3.00 2.00 2.00 3.00 3.00 64.8 69.7 30.9 22.6 3.4 5.0 0.9 2.6

36.88 0.013 20.91 30.92 8.96 1.368 1.332 100.4 2.998 2.002 0.001 0.000 2.102 0.616 0.166 0.116 3.00 2.00 3.00 70.1 20.6 5.5 3.8

37.11 0.032 20.96 31.44 8.85 1.423 1.009 100.8 3.003 1.999 0.002 0.000 2.127 0.606 0.172 0.087 3.00 2.00 2.99 71.1 20.3 5.7 2.8

36.47 36.70 0.066 0.017 20.63 20.78 28.36 31.94 13.32 8.62 0.805 1.556 0.302 0.515 100.0 100.1 2.998 2.998 1.998 2.001 0.004 0.001 0.000 0.000 1.947 2.169 0.927 0.597 0.099 0.189 0.027 0.045 3.00 3.00 2.00 2.00 3.00 3.00 64.9 72.3 30.9 19.9 3.3 6.3 0.7 1.4

36.95 0.053 20.95 28.83 7.75 1.277 4.323 100.1 2.995 2.002 0.003 0.000 1.939 0.532 0.154 0.375 3.00 2.00 3.00 64.6 17.7 5.1 12.4

36.89 0.000 21.08 36.80 1.84 2.846 0.524 100.0 2.988 2.012 0.000 0.000 2.491 0.126 0.344 0.045 3.00 2.00 3.01 82.9 4.2 11.4 1.5

37.08 38.10 38.03 0.000 0.000 0.007 21.14 21.69 21.59 33.73 31.65 31.62 1.59 0.30 0.34 2.386 6.818 6.022 3.751 1.715 2.619 99.7 100.3 100.2 2.993 2.992 2.995 2.011 2.008 2.004 0.000 0.000 0.000 0.000 0.000 0.000 2.277 2.042 2.052 0.108 0.020 0.023 0.287 0.798 0.707 0.324 0.144 0.221 3.00 3.00 3.00 2.00 2.00 2.00 3.00 3.00 3.00 76.0 68.0 68.3 3.6 0.7 0.8 9.6 26.6 23.6 10.8 4.8 7.3

Major element composition of plagioclase (normalized to 8 oxygens) Sample

WBK1

WBK1

WBK1

04T25K

04T25K

04T25K

WBK2

WBK2

HS00704

Comment

c, core matrix

f, in grt

f, recrystallized in matrix

c, core matrix

f, in grt

f, rim recrystallized

c, core

c, rim matrix

recrystallized in matrix

Analysis

6a/21

5a/5

6a/51

3b/58

3b/8

3b/45

87/4

86/2

10a/53

SiO2 Al2O3 CaO Na2O K2O Total Si Al Ca Na K Z(4) X(1) an ab or

65.70 21.66 2.539 10.13 0.224 100.2 2.881 1.119 0.119 0.861 0.013 4.00 0.99 12.1 86.7 1.3

67.99 20.15 0.565 11.35 0.200 100.3 2.959 1.033 0.026 0.958 0.011 3.99 1.00 3.0 95.9 1.1

66.55 21.27 2.068 10.36 0.255 100.5 2.907 1.095 0.097 0.878 0.014 4.00 0.99 9.8 88.8 1.4

65.71 21.35 2.486 9.94 0.397 99.9 2.892 1.107 0.117 0.848 0.022 4.00 0.99 11.9 85.7 2.3

67.74 20.03 0.645 11.14 0.290 99.9 2.958 1.031 0.030 0.944 0.016 3.99 0.99 3.3 95.0 1.6

65.93 21.24 2.359 9.94 0.285 99.8 2.902 1.102 0.111 0.848 0.016 4.00 0.98 11.4 86.9 1.6

66.00 21.47 2.250 10.19 0.350 100.3 2.894 1.109 0.106 0.866 0.020 4.00 0.99 10.7 87.3 2.0

66.72 21.20 1.726 10.78 0.122 100.5 2.912 1.090 0.081 0.912 0.007 4.00 1.00 8.1 91.2 0.7

67.70 20.37 1.027 11.06 0.100 100.3 2.954 1.048 0.048 0.936 0.006 4.00 0.99 5.0 94.5 0.6

Fe2O3/FeO were calculated with Petrakakis and Dietrich (1985, version 2.0a, 2002).

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4.3. Muscovite Muscovite from samples WBK1 and WBK2 shows some (ferro)aluminoceladonite substitution as well as Fe3+ content, whereas sample HS00704 contains pure end-member muscovite. 5. Trace element composition of garnet In situ trace element analyses of individual mineral phases were carried out using laser ablation inductively coupled plasma spectrometry at the CNR-IGG, Unit of Pavia, Italy. The laser probe consists of a Q-switched Nd:YAG laser, model Quantel (Brilliant), whose fundamental emission in the near-IR region (1064 nm) is

converted to 266 or 213 nm by harmonic generators. Spot diameter was typically 40–60 μm. The ablated material was analyzed by using, alternatively: (i) a double focussing sector-field analyser Element I (ThermoFinnigan MAT), in which the standard field regulator power stage of the magnet and the ICP torch were upgraded to those of the Element II model; (ii) a Perkin Elmer Elan DRC-e mass spectrometer. Helium was used as carrier gas and mixed with Ar downstream of the ablation cell. NIST SRM 610 was used as external standard, with Ca or Si used as internal standard. Precision and accuracy were assessed from repeated analyses of the BCR-2 or NIST SRM 612 reference glasses and results were usually better than 7% (1σ) and 10% (at ppm level), respectively. Detection limits were typically

Fig. 3. (a) Almandine- and (b) grossular- versus spessartine-content of meta-pegmatite garnet. Magmatic garnet is characterized by increasing Fewith decreasing Mn-contents, whereas garnet affected by the metamorphic overprint shows decreasing Fe with increasing Ca. Samples WBK1 and 04T25K have similar compositions. Data from sample HS00704 scatter more widely, and garnet from sample WBK2 has very low spessartine contents. The garnet core of WBK2 is homogeneous (dashed circle), whereas the microstructurally different fine-grained garnet generation newly formed in the matrix has variable grossular contents that depend on the local growth domain (dotted circle).

G. Habler et al. / Chemical Geology 241 (2007) 4–22

in the range of 100–500 ppb for Sc, 10–100 ppb for Sr, Zr, Ba, Gd and Pb, 1–10 ppb for Y, Nb, La, Ce, Nd, Sm, Eu, Dy, Er, Yb. Hf and Ta, and usually b 1 ppb for Pr, Th and U. A detailed description of instrumental parameters and quantification procedure is given in Tiepolo et al. (2003). A subset of analyses is given in Table 3.

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Analyzed euhedral garnets from samples WBK1 and 04T25K contain low to extremely low abundances of Ti (27–416 ppm), V (b 0.22 ppm), Cr (b1.6–4.7 ppm), Co (1.6–2.7 ppm), Ni (b 0.48 ppm), Sr (b0.09 ppm), Rb (b 0.6 ppm), Nb (N 0.48 ppm), Ta (0.02–1 ppm), Pb (b0.35 ppm), Th (b 0.01 ppm), U (b 1.2 ppm), La

Fig. 4. Major element composition of garnet from sample WBK1: a) Rim and core profile of a garnet grain with a diameter of 15 mm. b–c) Back scattered electron (BSE) images showing subgrain formation of garnet between two adjoining magmatic garnet-grains. Point analyses of marked profiles are given in e. d) Inclusions of albite in coarse-grained garnet, with garnet compositions along profile EF and at marked points shown in e. e) Garnet compositional profiles marked with capital letters in b–d.

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(b 0.03 ppm), Ce (b 0.37 ppm), Pr (b 0.05 ppm), Eu (b0.06 ppm), and moderate concentrations of Sc (42–219 ppm), Y (89–464 ppm), Zr (2–20 ppm) and Hf (0.1–0.7 ppm). In addition, garnet contains 42–131 ppm Li, 89–357 ppm Na and 144–238 ppm Zn. Chondrite-normalized rare earth element (REE) data (Fig. 6) show that garnets are characterized by a wide range of HREE contents (LuN = 38–309), a strong enrichment of HREE over LREE (YbN/CeN = 135 to 4470) especially in the garnet core domains and pronounced negative Eu anomalies (mean Eu/Eu* = 0.019). Fig. 6 also shows some within-grain variation, with garnet rims containing slightly lower REE contents compared to cores and/or steeper HREE profiles. Similarly, Ycontents are significantly higher in the garnet cores (211–350 ppm) than in the rims (88–126 ppm). As an exception, the rim analysis 1.2 of sample 04T25K positioned close to an anhedral garnet grain boundary

has HREE contents similar to the core, but MREE, LREE and Y contents similar to the idiomorphic rim domains. 6. Sm–Nd isotopic data of garnet 6.1. Sample preparation and analytical methods After cleaning and crushing of the kg-sized samples, whole rock splits were taken. Subsequently, grain sizes of 0.16–0.25 (0.3) and 0.25–0.42 mm were extracted from the remaining whole rock crushate by sieving. Single, cm-sized garnet crystals (samples WBK1 and 04T25K) were individually isolated from the matrix and the rims were carefully abraded using a microsaw. Grain size/magnetic fractions were extracted by step-wise crushing and sieving, split into different aliquots using a Frantz magnetic separator, and repeated washing in

Fig. 5. a–b) Photomicrograph (plain polarized light) and BSE images of coarse-grained magmatic garnet from sample WBK2. Note that fine-grained metamorphic garnet is present in the matrix adjacent to magmatic garnet, but does not form overgrowths. c) Garnet compositions along profiles AB, CD and EF.

G. Habler et al. / Chemical Geology 241 (2007) 4–22

acetone. From this concentrate, optically clean garnet fragments were handpicked under a binocular microscope. Before decomposition, garnet fractions were again repeatedly rinsed, using acetone and deionised water in an ultrasonic bath and then washed for a few minutes in warm (70 °C) 2.5 N HCl. No further leaching experiment was applied to any of the samples listed in Table 4. Samples were digested in tightly screwed Savillex® beakers using an ultrapure 5:1 mixture of HF and HClO4 for 2 weeks at 105 °C on a hot plate. After evaporating the acids, repeated treatment of the residue using 5.8 N HCl resulted in clear solutions for all samples. Upon cooling, between 10 and 18% of the sample solution were split off and spiked for Sm and Nd concentration

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determination by isotope dilution (ID) using a mixed REE tracer (147Sm–150 Nd spike). The REE fraction was extracted using AG® 50W-X8 (200–400 mesh, BioRad) resin and 4.0 N HCl. Nd and Sm were separated from the REE fraction using teflon-coated HdEHP, and 0.24 N and 0.8 N HCl, respectively, as elution media. Total procedural blanks for Sm and Nd during the period of element separation were between 2 and 12 pg. Considering sample weights and concentrations, these contributions are negligible both for the 147 Sm/144 Nd and the 143 Nd/144Nd ratio. Sm and Nd ID fractions were loaded as nitrates and measured as metals from an Re double filament, using a Finnigan® MAT262; Nd IC fractions were run in static mode on a

Table 3 LA-ICP-MS trace element data [ppm] of magmatic garnet from Koralpe meta-pegmatites Sample

WBK1

WBK1

WBK1

WBK1

WBK1

04T25K

04T25K

04T25K

04T25K

04T25K

04T25K

04T25K

Grt 1.2

Grt 1.4

Grt 1.3

Grt 1.5

Grt 1.1

Grt 1.1

Grt 1.6

Grt 1.5

Grt 1.4

Grt 1.2

Grt 2.1

Grt 2.2

Rim

Rim

Core

Rim

Core

Rim

131.4 3.0 104.7 96.6 348.0 0.2 3.4 2.6 b0.34 159.2 b0.03 b0.02 348.9 16.9 0.1 b0.01 b0.08 0.012 b0.01 b0.01 0.161 2.740 b0.01 19.580 8.260 59.740 10.930 30.950 5.810 50.560 6.740 0.623 0.421 b0.04 0.003 b0.09

106.0 2.1 357.5 134.3 396.0 0.2 2.6 1.6 b0.19 161.9 0.1 0.0 392.5 19.2 0.2 b0.01 0.1 b0.01 0.327 0.008 0.169 2.770 0.039 21.780 9.130 69.000 13.170 39.560 7.970 71.050 9.980 0.711 1.050 b0.03 b0.01 0.601

95.1 b2.36 119.5 58.9 150.0 b0.21 2.0 2.5 0.2 161.3 0.7 b0.02 112.2 11.0 0.0 b0.01 b0.07 b0.01 0.147 0.011 b0.06 0.685 0.019 7.390 3.120 22.750 3.720 9.920 1.960 14.790 2.170 0.328 0.074 b0.05 b0.01 0.160

89.5 1.7 164.9 47.2 191.0 b0.24 2.3 2.4 b0.33 169.6 b0.03 b0.02 121.8 11.5 0.0 b0.01 b0.09 0.012 b0.01 b0.01 0.142 1.160 b0.02 8.180 3.970 25.070 3.430 8.750 1.475 12.940 1.480 0.394 0.051 0.026 b0.01 0.033

99.4 3.7 235.0 69.0 397.0 b0.21 b1.62 2.2 b0.22 153.1 b0.03 0.0 297.2 19.9 0.1 b0.01 b0.05 0.029 0.010 0.010 0.197 2.030 0.021 15.950 7.800 60.410 9.300 23.640 3.860 32.270 3.930 0.685 0.132 b0.04 b0.01 0.178

87.0 3.0 199.0 56.9 139.0 b0.19 3.3 2.4 0.5 147.3 0.6 0.0 211.7 14.9 0.5 b0.01 0.0 b0.01 0.366 0.054 0.090 1.710 0.024 11.600 5.550 38.600 6.740 19.450 3.830 33.750 4.780 0.391 0.895 0.042 b0.01 0.053

108.3 5.3 226.3 61.2 416.0 b0.25 2.9 1.7 b0.29 153.3 0.0 0.1 267.0 18.7 0.3 b0.02 0.1 b0.02 0.039 b0.01 0.088 1.580 0.058 13.840 6.590 46.790 8.760 25.080 4.810 45.090 6.510 0.669 0.988 0.349 0.011 1.236

41.6 5.0 93.8 219.1 27.0 b0.18 4.7 2.2 b0.26 204.1 0.0 0.0 126.5 2.3 b0.01 0.1 0.4 b0.01 0.015 0.012 0.058 0.177 b0.01 1.500 1.016 14.510 4.740 19.760 5.030 51.950 7.760 0.095 0.025 1.135 0.018 0.026

85.7 4.0 224.4 41.9 431.0 b0.19 4.7 2.0 0.3 219.4 0.1 b0.01 218.5 16.7 0.1 0.1 b0.07 0.024 0.044 b0.01 0.068 1.830 b0.02 14.390 5.620 39.260 7.200 22.070 4.100 41.030 6.030 0.258 0.514 0.119 b0.06 0.326

84.6 b1.2 207.6 82.2 70.0 b0.19 4.5 2.1 b0.34 237.9 0.0 0.0 88.8 10.9 0.5 b0.01 b0.09 b0.01 0.041 0.244 0.064 0.567 b0.01 5.020 2.250 17.050 3.050 8.990 1.730 14.730 2.190 0.370 0.051 b0.02 b0.01 0.013

Rim Li B Na Sc Ti V Cr Co Ni Zn Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U

78.9 5.5 89.4 59.0 114.0 b0.19 2.7 2.5 b0.34 179.1 b0.04 0.0 98.6 9.0 b0.02 b0.01 0.1 b 0.01 0.025 b 0.01 b0.06 0.566 0.016 3.710 1.990 15.220 3.040 9.710 1.940 18.110 2.380 0.177 0.020 b0.04 b0.01 0.031

Core 113.9 3.4 221.1 78.4 288.0 b0.21 4.2 2.4 b0.34 143.8 b0.04 0.0 464.2 17.6 b0.01 b0.01 0.1 b0.01 b0.01 0.008 0.125 1.920 0.030 15.340 8.560 77.880 15.170 44.040 8.190 74.380 9.230 0.627 0.057 0.031 b0.02 0.078

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G. Habler et al. / Chemical Geology 241 (2007) 4–22

Fig. 6. Chondrite-normalized (Boynton, 1984) REE patterns of magmatic garnet from Koralpe meta-pegmatite samples 04T25K and WBK1. Solid symbols represent core domains, open symbols are rim domains.

ThermoFinnigan® Triton TIMS machine. A 143 Nd/144Nd ratio of 0.511850 ± 3e− 6 (n = 58) was determined for the La Jolla (Nd) international standard during the 2 years period of investigation. Within-run mass fractionation was corrected for 146 Nd/144Nd = 0.7219. Uncertainties on the Nd isotope ratio are quoted as 2 sigma of the mean (2σm). Maximum errors on the 147 Sm/ 144 Nd ratio are given as ± 1.0%; regression calculation is based on these uncertainties;

age errors are given at the 2σ level. Isochron calculation follows Ludwig (2003). Ages are based on a decay constant of 6.54 × 10− 12 a− 1 for 147Sm (Lugmair and Marti, 1978). 6.2. Sm–Nd results The meta-pegmatite samples WBK1, WBK2 and 04T25K were used for garnet Sm–Nd isotopic

Table 4 Sm–Nd whole rock (WR) and garnet data of meta-pegmatites Sample fractions

Sm [ppm]

Nd [ppm]

147

Sm/144Nd a

143

Nd/144Nd

±2σ

04T25Kc Fmx 04T25Kc Grt xx-1 1MF 04T25Kc Grt xx-1 2MF 04T25Kc Grt xx-1 3MF WBK1 Grt-1 bulk (.16–.45) WBK1 wr WBK1 Grt xx-1 b0.15 WBK1 Grt xx-1 (.16–.42) WBK1 Grt xx-2/1 (.16–.42) WBK1 Grt xx-2/2 (.16–.42) WBK2 wr WBK2 Grt bulk (.16–.42)

0.286 2.175 1.962 2.038 1.608 2.072 2.227 1.937 2.222 2.242 3.361 0.479

0.926 0.401 0.606 0.127 0.120 6.120 1.739 0.140 0.141 0.140 10.220 0.047

0.18660 3.28086 1.95949 9.76604 8.14794 0.20466 0.77436 8.41025 9.56365 9.74899 0.19882 6.16919

0.512211 0.517053 0.515004 0.527173 0.525095 0.512237 0.513145 0.525589 0.527787 0.528157 0.512271 0.521925

6.2e−6 8.0e−6 5.8e−6 10.0e−6 12.8e−6 6.0e−6 17.7e−6 19.0e−6 10.2e−6 13.4e−6 3.5e−6 32.2e−6

Age [Ma]

±2σ [Ma]

ε(t)Nd

239.4

1.5

−8

247.6

1.7

− 8.1

254.3

1.8

−8

247.1

2.6

− 7.2

Geographic coordinates of sampling locations (UTM projection zone 33; geodetic datum WGS84) are: 04T25K (N 5,177,620, E 504,768); WBK1 (N 5,177,599, E 504,774); WBK2 (N 5,178,063, E 504,436). a Errors on the 147Sm/144Nd ratio were assumed as 1% of the value.

G. Habler et al. / Chemical Geology 241 (2007) 4–22

investigation. Two euhedral garnet grains (Grt-xx1 and Grt-xx2) with a diameter of c. 12 mm each were cut from sample WBK1. After mechanical abrasion of the garnet rims, a garnet core fraction (0.15–0.42 mm) and a fine-grained sieve fraction (b 0.15 mm) were separated from Grt-xx1. From the second grain Grt-xx2, two identical garnet fractions (0.15–0.42 mm) were handpicked from the garnet core separate. In addition, the rock sample was crushed and another garnet fraction (0.16–0.45 mm) was isolated from the bulk crushate. From sample WBK2, the whole rock (WR) and a handpicked garnet fraction (0.16–0.45 mm) were analyzed. From sample 04T25K a single cm-sized garnet crystal was separated, crushed and split into three magnetic fractions. The resulting separates were hand-picked and analyzed together with the WR. Sm–Nd analytical results are given in Table 4.

15

has the highest 147Sm/144Nd ratio, in line with the garnet major element composition showing continuous zoning with highest Mn- and lowest Fe-contents in the core. Core domains are therefore expected to have lower magnetic susceptibilities than fractions mainly containing material from the rim domains. However, all data from sample 04T25K show perfect Nd isotopic equilibration for garnet and the whole rock (MSWD b 1). 6.5. Sample WBK2 This sample yielded a garnet-whole rock age at 247.1 ± 1.8 Ma (εNd(247) = − 7.2; Initial 143 Nd/144Nd = 0.511949 ± 5.9e− 6) (Fig. 7b). The garnet fraction has relatively low Sm and Nd contents, but a high

6.3. Sample WBK1 All garnet fractions of sample WBK1 show extremely high 147Sm/144Nd ratios between 8 and 10 (Fig. 7a). The core domain of one single grain (Grt-xx1 in Fig. 7a) gave nearly identical Sm–Nd data as the garnet fraction derived from the bulk crushate. Together with the finegrained sieve fraction (b0.15 mm), containing all inclusion phases, and the WR a regression age of 247.6 ± 1.7 Ma (MSWD = 1.3; n = 4; εNd(248) = − 8.1; Initial 143 Nd/144Nd = 0.511904 ± 7.2e− 6) was obtained. Two fractions of a second single garnet grain (Grt-xx2) gave within error identical results, but lie somewhat above the isochron defined by all other fractions. An age calculation using data from the second single grain and the WR yielded 254 ± 1.8 Ma (MSWD = 0.29; n = 3; εNd (254) = − 8, Initial 143 Nd/144Nd = 0.511897 ± 7.3e− 6). The two “age groups” within sample WBK1 differ outside the analytical uncertainties but are internally reproducible (defined by two garnet fractions each). Using all garnet fractions, their inclusions (sieve fraction b 0.15 mm) and the WR, resulted in a mean age of 252.3 ± 7.4 Ma (MSWD = 7.9; n = 6; εNd(252) = − 8.9; initial 143 Nd/144Nd = 0.51186 ± 3.4e− 4). 6.4. Sample 04T25K Three magnetic fractions of one garnet grain and the whole rock define a best-fit isochron at 239.4 ± 1.5 Ma (MSWD = 0.69; n = 4; εNd(239) = − 8; Initial 143 Nd/144Nd = 0.511919 ± 7.3e− 6) (Fig. 7b). The garnet magnetic fractions show high, but strongly variable 147 Sm/144Nd ratios between 2 and 10 (Table 4). The garnet fraction with the lowest magnetic susceptibility

Fig. 7. Garnet Sm–Nd isochron plots of meta-pegmatites: a) WBK1, b) WBK2 and 04T25K. All garnet fractions have high to very high 147 Sm/144Nd ratios. Data give a Late Permian to Early Triassic age of magmatic garnet crystallization, but internal regression ages from three different samples scatter outside the analytical uncertainties (254.3 ± 1.8–239.4 ± 1.5 Ma; see discussion in the text).

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147

Sm/144Nd ratio of 6.2. Despite of the microstructural and compositional differences, the age result from sample WBK2 is identical to the younger age group of sample WBK1 (Table 4). 7. Discussion 7.1. Correlation of meta-pegmatite microstructures, mineral composition and age information The microstructural characteristics of the investigated meta-pegmatites clearly correlate with the mineral compositional data. Coarse-grained euhedral major phase assemblages suggest a common magmatic crystallization from a peraluminous melt. The composition of the euhedral garnets and their REE patterns (Fig. 6) are characteristic features of Mn–Fe-rich garnets crystallized from silicic melts and of garnets in pegmatites (e.g. Harris et al., 1992; Bea, 1996; Zhang et al., 2001; Thöni and Miller, 2004). Pressure and temperature conditions of the magmatic crystallization are constrained by the presence of andalusite with an upper P-limit at 0.3 GPa based on the thermodynamic data by Pattison (1992) for the andalusite–sillimanite equilibrium (Fig. 8). The solidus constrains a minimum temperature for magmatic crystallization at c. 650 °C (Holland and Powell, 2001). In large parts of the Saualpe–Koralpe basement, the Cretaceous eclogite facies metamorphic overprint obliterated the pre-Cretaceous assemblages (Miller and Thöni,

1997; Habler and Thöni, 2001). Although the analyzed meta-pegmatites only show a weak syn-metamorphic deformation, the Cretaceous event has locally induced metamorphic processes that are clearly identified by microstructural and compositional characteristics of the mineral phases. Thus, phase re-equilibration has caused compositional changes of the outermost (10–50 μm) rims of the magmatic garnet and within subgrains. Complete breakdown of andalusite to kyanite did not only occur in the matrix, but also when enclosed in garnet. In addition, inclusion trails in garnet suggest that locally minute inclusions of apatite, xenotime and rutile have recrystallized. In order to minimize the influence of the metamorphic overprint, garnet fractions used for the geochronological investigation were carefully separated and different separation techniques were applied. Correlation of micro-structural and compositional data with the Sm–Nd results indicates that the garnet ages document Permian pegmatite crystallization between 256 and 238 Ma. The time range of 18 Ma suggested by the new data provides a closer constraint on the pegmatite emplacement age, compared with the scattering age data in the literature (Schuster et al., 2001; Thöni, 2002). There is no evidence of any pre-Permian mineral relics in the meta-pegmatites, and thus the hypothesis of Carboniferous pegmatites giving Carboniferous–Cretaceous “mixing ages” may be discarded for the investigated material. However, the results of garnet Sm–Nd geochronology vary outside the analytical uncertainties both between different grains of one sample as well as between different samples of an outcrop-scale area. 7.2. Variations of garnet age results outside the analytical error The observed variations of the high-precision garnet Sm–Nd age data could reflect (1) differences in the crystallization age of different garnet domains, or rather long-term garnet-growth processes, (2) contamination of the garnet fractions by undetected LREE-rich inclusions that are not in isotopic equilibrium with garnet, and (3) contributions of metamorphic garnet to the dominantly magmatic garnet fractions, or partial isotopic resetting of the magmatic garnet system during the metamorphic overprint. Arguments confirming or excluding these possibilities are discussed below.

Fig. 8. The PT-grid of the NKASH system shows the hypothetical maximum stability field of magmatic andalusite in peraluminous granitic melt systems (Joyce and Voigt, 1994; Clarke et al., 2005). Phase equilibria were calculated using THERMOCALC 3.25 (Holland and Powell, 1998 updated 2003; Powell et al., 1998).

7.2.1. True garnet crystallization ages The extremely high Sm/Nd ratios of all garnet fractions suggest robust age information, and thus favor the interpretation that distinct garnet domains have different crystallization ages. Different garnet magnetic fractions

G. Habler et al. / Chemical Geology 241 (2007) 4–22

17

from 04T25K, as well as a single grain of sample WBK1 together with the bulk garnet fraction and the finegrained sieve fraction containing inclusions define bestfit isochrons strongly suggesting primary Nd isotopic equilibration. In addition, the reproducibility of the data from a single crystal of sample WBK1 (xx-2/1, xx-2/2, Table 4) indicates homogeneous garnet separates. Continuous Fe–Mn-zoning and decreasing REE contents towards the rim of coarse-grained magmatic garnet document that significant diffusive major element homogenization did not occur during or after garnet growth. Core and rim domains of magmatic garnet are therefore expected to yield a somewhat different age. However, abrasion of the outermost rim domains of garnet from samples WBK1 and 04T25K has no significant influence on the age result compared with the bulk garnet fractions. The Permian event is thought to represent a longlasting thermal event related to crustal extension and thinning (Thöni and Miller, 2000; Schuster et al., 2001). Available age data for the Permian magmatic activity in the wider study area scatter between 280 and 220 Ma (Thöni, 2002), providing a scenario for polyphase pegmatite-generation and long-lasting crystallization processes. Fig. 9 shows that successively more negative ε(t)Nd values are correlated with younger pegmatite emplacement ages in the Saualpe–Koralpe basement, suggesting a polyphase Permian magmatic evolution or long-term magmatic processes. Isotopic variations may have been caused either by fractionation processes, or diachronous melting of metapelitic source material with heterogeneous bulk composition and hence somewhat

different melting temperatures. Polyphase pegmatite generation and emplacement may explain age differences between different pegmatite bodies, but a time span of ≥15 Ma for pegmatite crystallization processes within single samples is difficult to understand. For the material investigated in the present study, polyphase pegmatiteintrusions causing multiple magmatic garnet crystallization seem unlikely, as at least samples 04T25K and WBK1 show identical garnet composition indicating crystallization from the same bulk composition. Furthermore there is no micro- and mesoscopic evidence for the presence of multiple pegmatite generations in the outcrop area. All samples were collected within a small area in order to avoid sampling pegmatites from different tectonic units. Reopening of the whole rock system can also be excluded because major and trace elements indicate continuous garnet growth in a closed system, and the WR data points fit the internal garnet isochrons (Fig. 7). Whereas the literature data from the wider study area may reflect heterochronously emplaced pegmatite-generations, the new data indicate that significant age variations also result from material of a regionally limited, compositionally homogeneous area. Still, effective garnet crystallization processes lasting for more than 15 Ma in a closed magmatic system seem quite unrealistic, as high-temperature and low-pressure conditions are required for andalusite-bearing melt systems (Fig. 8). Over an extended period, high temperatures would cause intense melt generation, breakdown of white mica and growth of metamorphic Kfs in the metapelitic host rocks. Alternative causes for the age-variations implied by the new data are therefore discussed.

Fig. 9. Correlation of ε(t)Nd with garnet-WR Sm–Nd ages of metapegmatites from the Saualpe–Koralpe basement. Solid symbols represent data from this study, open circles are data from the literature (Thöni and Miller, 2000; Habler and Thöni, 2001; Thöni, 2002).

7.2.2. Effects of mineral inclusions in garnet Sm–Nd age variations of garnet may be due to LREE-rich inclusions that (i) crystallized earlier than garnet and did not isotopically re-equilibrate during the magmatic event, or (ii) were isotopically reopened during Cretaceous metamorphism. On the other hand, if the inclusions were in Nd isotopic equilibrium with garnet during magmatic crystallization, their REE budget would only influence the Sm/Nd ratio, but not the age result (Thöni, 2002). In general, contamination of garnet fractions by very fine-grained inclusions can never be excluded, as inclusions with grain sizes of a few microns cannot be detected by optical means. Analytical evidence of LREE-rich phases pre-dating garnet is absent. Rare c. 20 μm sized zircon (sample WBK1) and monazite inclusions (sample HS00704, not used for Sm–Nd dating) are the only candidates for containing relic material inherited from the source rock. Other accessory phases presumably are part of the magmatic

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G. Habler et al. / Chemical Geology 241 (2007) 4–22

pegmatite assemblage or have recrystallized during the metamorphic overprint. In several samples, extremely fine-grained (b 1 μm sized) xenotime and apatite inclusions in garnet possibly contribute to the REE budget and thus influence the Sm–Nd garnet data (Sha and Chappell, 1999). Re-crystallization processes produced trails of 20–30 μm sized inclusions in garnet (Fig. 2d). Mineral inclusions with a lower closure temperature than garnet could have reequilibrated even if the garnet system itself remained closed (cf. Blichert-Toft and Frei, 2001). Garnet domains containing re-crystallized inclusions do not show a significant change of the magmatic major element composition (Fig. 10). Material exchange with the matrix is confined to the outermost 10–50 μm of the

garnet rim, but did not affect the internal garnet domains. Therefore, intragranular domains affected by recrystallization and subgrain formation should have preserved the inherited magmatic Nd isotopic composition. The fact that all garnet fractions have extremely high 147 Sm/144 Nd ratios of 6–10 and low Nd contents of 0.12–0.6 ppm is a strong argument against an essential contribution of accessory LREE rich phases. A significant rotation of the garnet isochrons is unlikely unless the Sm–Nd budget was dominated by the composition of the inclusions. This seems not relevant for the material under investigation as the garnet fractions consist of optically clean hand-picked grains. For sample WBK-1, a comparison of the range of Sm and Nd concentrations obtained by LA-ICP-MS (Sm:

Fig. 10. a) Photomicrograph (plain polarized light) of a colour-zoned magmatic garnet from meta-pegmatite 04T25K. Note the deflection of the mylonitic matrix foliation. b) BSE image of the garnet core with trails of fine-grained inclusions (subvertical in this figure, marked by white arrows). White spots are 10–30 μm sized xenotime. c) BSE image of the garnet rim. Only the outermost 20 μm was affected by the metamorphic overprint. d) Garnet composition along numbered profiles.

G. Habler et al. / Chemical Geology 241 (2007) 4–22

0.57–2.77 ppm; Nd: b 0.06–0.17 ppm) with those obtained by ID-TIMS on hand-picked garnet fractions (Sm: 1.61–2.24 ppm; Nd: 0.12–0.141 ppm; Table 4) shows that the in-situ analyses are consistent with the concentrations obtained by ID-TIMS, indicating high sample purity of the hand-picked fractions. Only the impure sieve fraction Grt-xx1 b 0.15 that yielded the youngest age of 243.5 ± 5.9 Ma shows significant differences between the results obtained by different analytical methods. LA-ICP-MS analyses of two garnets from sample 04T25K yielded low Nd and Sm concentrations (0.06– 0.28 and 0.57–2.19 ppm, respectively) and high Sm/Nd ratios (3.1–26.9). Again, Sm contents and Sm/Nd ratios obtained by ID are within the range given by LA-ICPMS, whereas Nd concentrations of garnet fractions 1MF and 2MF are approximately 1.4 and 2 times the maximum obtained by in situ analysis. As the garnet fraction 3MF yielded equal Nd-contents both by ID-TIMS (0.13 ppm) and LA-ICP-MS analysis, and all garnet fractions together with the whole rock gave a best-fit isochron, the influence of potential inclusions on the age result seems to be negligible. The perfect data reproducibility argues against a significant influence of un-equilibrated inclusions, because only homogeneously distributed very fine-grained material could affect the Sm–Nd characteristics of several garnet fractions to the same extent, and therefore a larger scatter of the Sm/Nd data would result from unequilibrated inclusions. 7.2.3. Influence of metamorphism on the garnet Sm–Nd system Cretaceous eclogite facies metamorphism could theoretically have caused reopening of the magmatic Sm–Nd system and/or growth of a metamorphic garnet generation. Deformation zones within garnet are presumably related to the main deformation of the matrix. Re-crystallization of ≤ 1 μm sized accessory inclusions (xenotime, apatite, rutile, aluminosilicate and corundum) to grains with diameters of 10–30 μm, as well as the replacement of andalusite by kyanite and white mica aggregates occurred also within internal garnet domains. Therefore, a contribution of isotopically reequilibrated LREE rich accessory phases or an influence on the Sm–Nd system of the host mineral should be taken into account. Peak T-conditions of Cretaceous high pressure metamorphism in the Koralpe basement were given at approximately 625–730 °C for eclogites (Miller et al., 2005) and 700 ± 68 to 600 ± 63 °C for micaschists and paragneisses (Tenczer and Stüwe, 2003), thus possibly hardly approaching the closure

19

temperature for the Sm–Nd system in garnet (Ganguly et al., 1998). And indeed, the minor parts of metamorphic garnet observed could have contaminated those garnet fractions that were obtained from the bulk crushate of the meta-pegmatites. In other pegmatiteoccurrences of the Koralpe–Saualpe basement, Cretaceous garnet crystallization formed pronounced overgrowths on relic magmatic garnet (Thöni and Miller, 2000; Habler and Thöni, 2001). A fine-grained metamorphic garnet generation is present in the white mica rich matrix domains of sample WBK2 but is compositionally different from magmatic garnet affected by metamorphic diffusion processes and does not form overgrowths on magmatic garnet. Concerning the potential isotopic re-equilibration of garnet, any major element exchange between garnet and matrix phases or re-equilibrated inclusions (plagioclase, kyanite-paramorphs after andalusite) is restricted to 10– 50 μm thick rim domains of garnet (Figs. 5 and 10). The garnet domain affected by volume diffusion is volumetrically minor (≤3 vol.% of the garnet content, assuming 10–50 μm thick rims of garnet grains with 10–12 mm diameter modelled as spheres), even for those garnet fractions that were derived from the crushed bulk rock. Because no deviation from the magmatic garnet core composition was detected by quantitative EMP analysis, even in immediate contact with recrystallized inclusion trails (Fig. 10), material exchange between internal garnet domains and the matrix did not occur. Therefore, intragranular recrystallization processes should not affect the garnet Sm–Nd system significantly provided that the Sm and Nd diffusivities are lower than the major element diffusivities in garnet (Tirone et al., 2005; Carlson, 2006). This is documented by the fact that the single garnet crystal from sample WBK1 from which the marginal domains were abraded is in perfect isotopic equilibrium with the garnet fraction separated from the crushed bulk, its sieve fraction and the WR. Therefore, Cretaceous re-equilibration processes obviously did not modify the isotopic system of garnet. 7.3. Geologic implications Age data from Permian–Triassic (meta)pegmatites range between 208 and 280 Ma as documented by Rb–Sr (muscovite and whole rock isochrons), Sm–Nd (garnet) and U–Pb (zircon) geochronology of widespread pegmatite occurrences in Austroalpine and South Alpine basement units (Morauf, 1981; Sanders et al., 1996; Heede, 1997; Habler and Thöni, 2001; Schuster et al., 2001, with references; Thöni and Miller, 2000; Thöni, 2002; Sölva et al., 2003). The age scatter may be

20

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explained as due to either i) polyphase or protracted magmatic processes during a long-term tectonic event, or ii) effects of the variable and partly intense Cretaceous metamorphic overprint. Our new detailed investigations on a regionally very restricted area were designed to minimize the latter effects by careful sample selection and preparation as well as by major and trace element mineral compositional investigations, thus focusing on the magmatic crystallization ages. The time-range of 256–238 Ma (Upper Permian to lower Triassic) for pegmatite emplacement in the investigated area represents a relatively limited period with respect to the literature data. The time-span of 18 Ma is obviously not related to multiple magmatic pulses of pegmatite intrusions but documents continuous mineral growth in a closed system. Pegmatite emplacement therefore was not an “instantaneous” process caused by the intrusion of a granitic melt into relatively “cold” host rock. However, long-term magmatic crystallization processes are in line with the interpretation of the Permian event as due to pervasive crustal extension triggering heat input by mantle upwelling and high thermal gradients, thus causing partial melting of the metapelitic rocks (Habler and Thöni, 2001; Schuster et al., 2001). Different pegmatite occurrences may well represent heterochronously generated and emplaced intrusions caused by polyphase magma generation, spatial variations in the thermal structure of the crust or by bulk composition dependent variations in the melting-temperatures. This might explain some of the age scatter documented in the literature with data mainly derived from single samples. However, the present study demonstrates a period of approximately 20 Ma even for single-phase magmatic mineral growth. Long-term magmatic crystallization requires regionally elevated metamorphic T-conditions in the metapelitic host rocks. A correlation of pegmatite-generation and low-pressure metamorphism has been discussed by Habler and Thöni (2001) and Schuster et al. (2001). Still, the exact timing of metamorphic mineral crystallization in the host rock and the metamorphic T-peak with respect to the period of pegmatite generation and emplacement is not unequivocally clear. First Sm–Nd garnet data from metapelitic rocks in the footwall and the hanging wall of the eclogite facies metamorphic portion of the Saualpe–Koralpe basement yielded somewhat older – although still Permian – ages for metamorphic garnet growth at 262 ± 4 Ma (Thöni, 2002) and 269 ± 4 Ma (Schuster and Thöni, 1996). Although this uncertainty clearly requires additional geochronological data from metapelites representing the immediate pegmatite host rock, one might assume that metamorphic

garnet crystallization in the metapelites occurred during the prograde (heating) path of Permian metamorphism, but prior to the main stage of pegmatite generation at the T-peak of metamorphism. In any case, the new results definitely constrain extended pegmatite formation by local melting of pelite sources in the period of 260– 235 Ma. On the other hand, evidence of prolonged magmatic crystallization processes requires protracted metamorphic crystallization in the metasedimentary host rock. Low-pressure metamorphism in the metapelites is certainly related to the pegmatite generation/ emplacement process, but may well have covered an even wider time-span than the magmatic stage. 8. Conclusions The detailed investigation of garnet-bearing metapegmatites combining major and trace element compositions of garnet with microstructures and Sm–Nd isotopic data highlights the problem of assigning ages to magmatic and/or metamorphic processes. The sample material is well suited for this purpose due to the strong Sm vs Nd fractionation in garnet, the simple bulk composition, the clear-cut phase relationships and the well-constrained magmatic and metamorphic evolution. Best-fit Sm–Nd isochron regressions of multiple fractions based on reproducible data-sets from different samples treated with different mechanical separation methods provide robust magmatic garnet crystallization ages during pegmatite-emplacement. The results, ranging between 254 ± 2–239 ± 2 Ma, provide excellent ageconstraints for the Permian–Triassic magmatic event and thus also for the low pressure metamorphic event in the metapelitic host rock, which formed up to dm-sized andalusite crystals (now represented by the fine-grained Ky-paramorphs) within the well-known “Disthen-Paramorphosenschiefer” of the Koralpe (Beck-Mannagetta, 1980). Surprisingly, the Cretaceous eclogite facies metamorphic overprint did not significantly affect the major and trace element characteristics of garnet and the whole rock system. However, high-precision ID-TIMS analyses yield reproducible age variations between different grains of single samples and between different samples that are outside the analytical errors. These variations are interpreted to represent a real range in garnet ages, documenting protracted magmatic crystallization processes. Alternatively, they could be an effect of very finegrained, homogeneously distributed LREE-rich inclusions in garnet, preferentially apatite or xenotime. When affected by metamorphic recrystallization, these inclusions could have caused a rotation of Sm–Nd

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isochrons. In this case, even best-fit isochrons and reproducible multifraction data would not be unequivocal indicators of isotopic equilibrium. Even though major and trace element compositions and microstructural data exclude metamorphic reopening and resetting of a magmatic system, comprehensive tests are required for an unequivocal geologic interpretation of the ages. Acknowledgements This work was supported by the Austrian Science Fund (FWF) grant P15644-N06. EMP and ID-TIMS facilities were provided by the Department of Geological Sciences at the University of Vienna and the Institute of Mineralogy and Petrography at the University of Innsbruck. We thank A. Zanetti for the LA-ICPMS analyses and Theo Ntaflos for providing assistance with EMP analyses. Ralf Schuster is thanked for directing us to the outstanding Wirtbartl pegmatite/kyaniteparamorph schist localities. Harold Stowell and Thomas Lapen deserve special thanks for their constructive reviews that significantly improved the manuscript. The authors also thank Stefan Jung for his efforts in producing this special volume. References Amato, J.M., Johnson, C.M., Baumgartner, L., Beard, B.L., 1999. Rapid exhumation of the Zermatt-Saas ophiolite deduced from high-precision Sm–Nd and Rb–Sr geochronology. Earth and Planetary Science Letters 171, 425–438. Anczkiewicz, R., Thirlwall, M.F., 2003. Improving precision of Sm– Nd garnet dating by H2SO4 leaching: a simple solution to the phosphate inclusion problem. In: Vance, D., Müller, W., Villa, I.M. (Eds.), Geochronology: Linking the Isotopic Record with Petrology and Textures. Geological Society Special Publication, London, pp. 83–91. Bea, F., 1996. Residence of REE, Y, Th and U in granites and crustal protoliths — implications for the chemistry of crustal melts. Journal of Petrology 37, 521–552. Beck-Mannagetta, P., 1980, 188 Wolfsberg, Geological Map of Austria 1:50000, Geological Survey of Austria, Vienna. Beck-Mannagetta, P., Stingl, K., 2002, 206 Eibiswald, Geological Map of Austria 1:50000, Geological Survey of Austria, Vienna. Beck-Mannagetta, P., Eisenhut, M., Ertl, V., Homann, O., 1991, 189 Deutschlandsberg, Geological map of Austria 1:50000, Geological Survey of Austria, Vienna. Blichert-Toft, J., Frei, R., 2001. Complex Sm–Nd and Lu–Hf isotope systematics in metamorphic garnets from the Isua supracrustal belt, West Greenland. Geochimica et Cosmochimica Acta 65 (18), 3177–3187. Boynton, W.V., 1984. Cosmochemistry of the rare earth elements: meteorite studies. In: Henderson, P. (Ed.), Rare Earth Element Geochemistry. Elsevier, Amsterdam, pp. 63–114. Carlson, W.D., 2006. Rates of Fe, Mg, Mn, and Ca diffusion in garnet. American Mineralogist 91 (1), 1–11.

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