Garnet pyroxenite and eclogite in the Bohemian Massif: geochemical evidence for Variscan recycling...

Geol Rundsch (1995) 84:489-505 © Springer-Verlag 1995

L. G. MedariS • B. L. Beard • C. M. Johnson J. W. Valley • M. J. Spicuzza • E. Jelfnek • Z. Mfsftr

Garnet pyroxenite and eclogite in the Bohemian Massif: geochemical evidence for Variscan recycling of subducted lithosphere

Received: 10 October 1994 / Accepted: 4 March 1995

Abstract High-temperature, high-pressure eclogite and garnet pyroxenite occur as lenses in garnet peridotite bodies of the Gf0hl nappe in the Bohemian Massif. The high-pressure assemblages formed in the mantle and are important for allowing investigations of mantle compositions and processes. Eclogite is distinguished from garnet pyroxenite on the basis of elemental composition, with mg number <80, Na20>0.75 wt.%, Cr203<0.15 wt.% and Ni<400ppm. Considerable scatter in two-element variation diagrams and the common modal layering of some eclogite bodies indicate the importance of crystal accumulation in eclogite and garnet pyroxenite petrogenesis. A wide range in isotopic composition of clinopyroxene separates [eNd, +5.4 to --6.0; (87Sr/S6Sr)i, 0.70314-0.71445; 618OsMow, 3.8-5.8%0] requires that subducted oceanic crust is a component in some melts from which eclogite and garnet pyroxenite crystallized. Variscan Sm-Nd ages were obtained for garnet-clinopyroxene pairs from Dobego- vice eclogite (338 Ma), Uhrov eclogite (344 Ma) and Nov6 Dvory garnet pyroxenite (343 Ma). Gf0hl eclogite and garnet pyroxenite formed by high-pressure crystal accumulation (+ trapped melt) from transient melts in the lithosphere, and the source of such melts was subducted, hydrothermally altered oceanic crust, including subducted sediments. Much of the chemical variation in the eclogites can be explained by simple fractional crystallization, whereas variation in the py-

L. G. Medaris Jr (N~) • B. L. Beard • C. M. Johnson - J. W. Valley M. J. Spicuzza Department of Geology and Geophysics, University of Wisconsin, Madison, WI 53706, USA

E. Jelfnek Department of Geochemistry, Mineralogy & Crystallography, Charles University, 128 43 Prague 2, Czech Republic

Z. MfsSr Department of Geology, Charles University, 128 43 Praha 2, Czech Republic

roxenites indicates fractional crystallization accompanied by some assimilation of the peridotite host.

Key words Bohemian Massif • Variscan • Geochemistry • Garnet pyroxenite • Eclogite • Lithosphere

Introduction

The Bohemian Massif is ideally suited for the investigation of mantle processes and crust-mantle interactions during orogenesis because of the variety and abun- dance of high-pressure rocks there, including different types of eclogite, high-pressure granulite and orogenic spinel and garnet peridotites (solitary bodies previously referred to as alpine peridotites). Such rocks have formed from mantle and crustal protoliths as a result of continent-continent collision and tectonic insertion of upper mantle slices into the lower parts of the thick- ened continental crust, accompanied by recrystalliza- tion at elevated pressures. The orogenic peridotites and associated lenses of pyroxenite are especially important, because they allow in situ study of mantle compositions.

A geochemical investigation of garnet pyroxenite and eclogite layers in orogenic garnet peridotites in the Bohemian Massif has been carried out to evaluate mantle melting and crystallization processes. Elemental and Nd, Sr and O isotope compositions were determined to place constraints on the sources of melts (whether derived from asthenosphere, lithosphere or crust), crystallization mechanisms (involving crystal accumulation, trapping of melt and assimilation) and time of formation of the garnet pyroxenites and eclogites. The results are consistent with a significant contribution of subducted lithosphere to Variscan mantle melts in central Europe and confirm the importance of crystal accumulation processes in the petrogenesis of pyroxenitic layers in orogenic peridotites.

490

Geological setting It is generally agreed that the Variscan Orogen evolved in four main stages, including: (1) Cambro-Ordovician rifting of microplates from Gondwana; (2) Late Ordo- vician to Early Carboniferous amalgamation of the Gondwanan assembly, island arcs, and small ocean basins along the southern margin of Baltica; (3) culmina- tion of deformation, metamorphism and crustal anatex- is during Carboniferous collision of Gondwana with Baltica and the previously assembled collage; and (4) Late Carboniferous to Early Permian major collapse, accompanied by regional extension, strike-slip move- ment and post-orogenic volcanism, although many de- tails of this tectonic scenario remain unresolved (Behr et al. 1984; Lorenz and Nicholls 1984; Ziegler 1986; Franke 1989; Paris and Robardet 1990; Matte 1991; Meissner and Tanner 1993).

Franke (1989) and Matte et al. (1990) have proposed the existence of a number of terranes, or tectono-strati- graphic units, in the Bohemian Massif, and we have combined their concepts with previously published maps of Czechoslovakia (Fusfin et al. 1967; Miser et al. 1983) to divide the central part of the massif into the following six terranes (Fig. 1), which are in fault contact with each other: 1. The Moravian terrane consists of metasedimentary

rocks (gneiss, schist, calc-silicate schist, marble and quartzite), amphibolite and granitic orthogneiss, which have been thrust to the east over Cadomian basement.

Fig. 1 Terrane map of the central part of the Bohemian Massif (modified from Fusfin et aI. 1967 and Mfsa~" et al. 1983). Occurrences of garnet peridotite in the GfOhl terrane indicated by symbols: ~ garnet peridotite with spinel inclusions in garnet; R, garnet peridotite devoid of early spinel; and O, garnet peridotite not yet studied in detail. Lo- calities of investigated peridotite bodies containing garnet pyroxenite and eclogite: 1, Biskupice; 2, Nov6 Dvory; 3, Nihov; 4, i~hrov; 5, Dobe~o- vice; and 6, Be~vfiry

D Post-Carboniferous Deposits

~--~ Gf6hl Terrane; Gneiss & Granulite (Lines)

2. The M o n o t o n o u s and (3) Varied terranes (combined as one map unit in Fig. 1) include a thick sequence of middle-grade felsic paragneiss (Monotonous Group) and the same rock type with subordinate felsic orthogneiss, quartzite, marble and amphibolite (Varied Group).

4. The GfOhl terrane is characterized by two distinctive rock types, a high-pressure granulite (leptynite or Weiss-stein) and a migmatitic orthogneiss, which are shown separately in Fig. 1.

5. Muscovite-biotite paragneiss is predominant in the Kutnd Hora-Svratka terrane, but tectonic slices of the Gf0hl and Monotonous and Varied terranes (too small to be shown on the scale of Fig. 1) occur there as well (Synek and Oliveriov~i 1993).

6. The Tepld-Barrandian terrane consists of Cadomian low-grade metavolcanic and metasedimentary rocks that are unconformably overlain by folded, but un- metamorphosed, Cambrian to Devonian sedimentary rocks and subordinate volcanic rocks. The north- western corner of the Tepl~-Barrandian terrane contains the Marifinsk6 Lfizn6 metaophiolite complex (Kastl and Tonika 1984), to which Matte et al. (1990) have assigned separate terrane status. Garnet pyroxenite and eclogite of this investigation

are located in the Gf6hl terrane, which is a nappe sheet that is preserved in several large klippen (and numerous others that are too small to be shown in Fig. 1) thrust on the Monotonous and Varied, Moravian and Kutnfi Hora-Svratka terranes (Matte et aL 1990). The Ostrikeroot zone for this major structure may be lo-

[] Vafiscan Granitic []Tepl~_-BarrandianTerrane; ~Ku tnA Hora-Svratka Rocks Metaophiolite (Lines) [.z._zJ Terrane

[ ~ Monotonous & [ ] Moravian Terrane [ ] Cadomian Basement Varied Terranes

491

cated along the south-eastern margin of the Teplfi-Bar- randian terrane (Tollmann 1982), but has been largely obliterated by emplacement of the central Bohemian Pluton. The Gf6hl terrane is characterized by a distinctive association of high-pressure felsic granulite (16 kbar, 1000°C, Carswell and O'Brien 1993), high- temperature eclogite (765-1190 °C, ___12.4-19.8 kbar, Medaris et al. 1995) and garnet peridotite and is an important petrotectonic feature of the Variscides, with li- thological correlatives in the Polish Sudetes, German Schwarzwald and French Massif Central (Franke 1989; Bakun-Czubarow 1991; Brueckner et al. 1991; Matte 1991; Carswell and O'Brien 1993).

In the area covered by Fig. 1, spinel peridotites are widely distributed in the Monotonous and Varied terranes (Machart 1984, 1988), but garnet-bearing peridotites occur only in the Gf6hl terrane, where 30 localities have been recognized so far [note, however, that garnet peridotites also occur in the Saxothuringian Zone to the north-west of Fig. 1, in the T-7 borehole in north- western Bohemia (Fiala and Pad6ra 1977) and in the Saxonian Granulite Massif in Germany (Rost 1961)]. Two types of garnet-bearing peridotite bodies have been identified: those that are entirely garnet peridotite, with no petrographic evidence for pre-existing spinel, and those that are largely spinel peridotite, where garnet has formed locally at the expense of early spinel. These two types of peridotite are well illustrated by the Nov6 Dvory garnet peridotite and Mohelno spinel peridotite in western Moravia (Medaris et al. 1990). Garnet peridotites in both types of occurrence are distinctive in yielding temperatures of the order of 900-1200 ° C, well above those for garnet peridotites in the Norwegian Caledonides and the Lepontine Alps (Medaris and Carswell 1990).

Eclogite is rare in Gf6hl granulite or orthogneiss (Carswell 1991; Medaris et aL 1995), but eclogite and garnet pyroxenite layers and lenses are common in garnet peridotites of the Nov6 Dvory type; such layers and lenses record the passage of melts along mantle con- duits. Pyroxenite layers are also common in spinel peridotite of the Mohelno type, but these layers are generally garnet-free. Gf/3hl eclogites equilibrated at higher temperatures, > 850 ° C, than eclogites in the Monoton- ous and Varied terranes, 615-705 ° C, or eclogites in the Marifinsk6 Lfizn6 metaophiolite complex, 625-730°C (Medaris et al. 1995). In addition, garnet zoning patterns and mineral inclusions show that the high-temperature eclogites in the Gf6hl terrane have only a retro- grade history, whereas those in the Monotonous and Varied terranes and the Marifinsk6 Lfizn6 complex have prograde histories, modified by late-stage retro- gression.

Nomenclature, occurrence and sample descriptions

Garnet-pyroxene layers in garnet peridotites of the Nov6 Dvory type have been divided into eclogite and

garnet pyroxenite on the basis of rock chemistry and clinopyroxene composition. Eclogites have mg numbers generally less than 80, low Cr and Ni contents and contain omphacite (jadeite contents mostly greater than 25 tool.%); in contrast, garnet pyroxenites have mg numbers greater than 80, relatively high Cr and Ni contents and contain Cr-diopside. Both groups of rocks equilibrated at high pressures, and the clinopyroxene compositions are controlled largely by rock chemistry, with Na20 contents higher in eclogite than in garnet pyroxenite.

A similar two-fold division into Al-augite and Cr- diopside types has been applied to pyroxenite layers in the Ronda, Beni Bousera and Balmuccia massifs (Dick- ey 1970; Suen and Frey 1987; Shervais and Mukasa 1991; Pearson et al. 1993) and to pyroxenite xenoliths in alkali basalts and kimberlites (Wilshire and Shervais 1975). Gf6hl eclogites are similar in rock chemistry to Al-augite pyroxenites elsewhere, and Gf6hl garnet pyroxenites are similar to Cr-diopside pyroxenites (see following section on major and trace element geochemistry).

This investigation presents analyses of eclogite and garnet pyroxenite samples from the Dobegovice, Nihov, Nov6 Dvory and Uhrov garnet peridotite bodies (Fig. 1) and these results, combined with previous results from the Be~vfiry, Biskupice and Nov6 Dvory bodies (Beard et al. 1992) and from Austria (Scharbert and Carswell 1983; Carswell and Jamtveit 1990), provide a substantial geochemical database for eclogite and garnet pyroxenite in the GfShl terrane. In general, garnet pyroxenite layers range in thickness from milli- meters to centimeters, whereas eclogite layers are much larger, ranging from layers centimeters thick to bodies as large as 30 x 600 m, as at Nov6 Dvory. Locally, however, garnet pyroxenite and eclogite layers may have comparable thicknesses, as in the Be~vfiry garnet peridotite, where two parallel layers of garnet pyroxenite and eclogite, each 5 cm thick, occur 20 cm apart. Modal layering of garnet and clinopyroxene is common in larger bodies of eclogite.

Textures of eclogite and garnet pyroxenite are fine- to medium-grained, inequigranular (garnet typically coarser than clinopyroxene) and massive to slightly fol- iated. Garnet to clinopyroxene ratios vary widely from sample to sample, ranging from 70:30 to 20:80, and a similar range in modal ratio occurs in individual samples of layered eclogite. Eclogite commonly contains accessory rutile and some types may also contain small amounts of quartz, kyanite, orthopyroxene, ilmenite or apatite. Garnet pyroxenite includes both garnet clinopyroxenite and garnet websterite, depending on the ratio of clinopyroxene to orthopyroxene. Many samples have been extensively retrograded, eclogite more so than garnet pyroxenite, with symplectitization of omphacite, kelyphitization of garnet and growth of amphi- bole.

492

Analytical methods

Major element abundances for rocks were determined by X-ray fluorescence wavelength dispersive spectros- copy using a Phillips instrument at Carleton College, Northfield, Minnesota, USA. The precision of major element abundances is estimated to be 5%, based on replicate analyses. Rare earth element (REE) and other trace element contents for rocks were obtained by instrumental neutron activation analysis (INAA) at Washington University, St Louis, Missouri, USA following the procedures described by Korotev (1987). Precision for Sc and Co is 1%, for Ni 2-10%, depending on concentration, and for the REEs 2-5%, depending on element and concentration, except for Ce, the precision for which is 10-20%.

Mineral separates for Sr and Nd isotopic analysis were acid-leached in either 2.5 M HC1 (procedure I) or 2.5 M HC1 followed by 5% HF (procedure II). We do not believe that these different leaching procedures have biased the results because garnet and clinopyroxene from sample CZ-12B (leached using procedure I) are the same as those reported by Brueckner et al. (1991) for a split of this sample. The leaching procedure followed by Brueckner et al. (1991) is the same as our procedure II leaching protocol.

Sr and Nd isotopic compositions were determined at the University of Wisconsin-Madison and follow those reported in Beard et al. (1992, in press). Sr isotope ratios were measured during two periods when two different values for the 87Sr/86Sr ratio of NBS-987 were determined; during procedure I (Table 3) measured 87Sr/86Sr ratios for NBS-987 were 0.71022___1 (2o-, n=23), and during procedure II (Table 3) measured 87Sr/86Sr ratios for NBS-987 were 0.71034_+2 (2o-, n =20). All data are reported in Table 3 relative to an 87Sr/86Sr ratio for NBS-987 of 0.71022. In Table 3 we include isotopic data for samples that were originally reported in Beard et al. (1992); Rb contents for these samples were incorrectly reported in Beard et al. (1992) and the correct values are given here.

Nd isotope data are reported relative to a 143Nd/ 144Nd ratio of 0.511859 for the La Jolla Nd standard and an internal laboratory Nd standard, UW Ames Nd, which has a la3Nd/144Nd ratio of 0.512143; during the current study, 33 analyses of the La Jolla standard were made and 54 analyses of the Ames Nd standard. Al- though some small shifts in Nd isotope ratios occurred for these standards during the study, as fully discussed in Beard et al. (in press), the difference in 143Nd/144Nd ratios between the La Jolla and Ames standards has been consistently 5.5_+0.2 eyd units, indicating that these small shifts can be accurately corrected. The 143Nd/144Nd ratios for these two standards produce an 143Nd/144Nd ratio of 0.512619 for the BCR-1 standard, which we take to be equal to the present day C H U R value (Wasserburg et al. 1981).

Mineral separates of clinopyroxene and garnet were hand-picked for oxygen isotope analysis from crushed

1 kg samples. One to three milligram samples were fluorinated as fine sand using a CO2 laser and BrF> All analyses were duplicated with an average precision of +0.08%0 (lo-) and values are reported in the standard per mil notation relative to V-SMOW. Fourteen analyses of the UW Gore Mountain garnet standard #1 were made on the three days of sample analysis with an average 3180=6.02__0.08%o (lo', n=14). The value of this standard, normalized to 6180 = 9.6%0 for NBS28, is believed to be 6.2 +0.2%oSMOW (Kohn et al. 1993).

Major and trace element geochemistry

Elemental abundances for eclogite and garnet pyroxenite whole rocks are given in Tables 1 and 2 and illustrated in Figures 2-4, in which previously published results from the Gf0hl terrane are shown for complete- ness, including Czech eclogites and garnet pyroxenites (Beard et al. 1992) and Austrian garnet websterites and garnet clinopyroxenites (Scharbert and Carswell 1983; Carswell and Jamtveit 1990). Four thin (0.5 cm) pyroxenite and eclogite layers from Nov6 Dvory have relatively high loss on ignition (LOI) values (1.80-3.70%; Table 1) due to the presence of thin serpentine veinlets originating from enclosing peridotite.

Within the entire suite of analyzed samples, there is appreciable overlap in SiO2, A1203 and CaO, but eclogite generally contains less Cr203 and more Na20 and TiO2 than does garnet pyroxenite (Fig. 2). A notable feature of the oxide variation diagrams is the considerable degree of scatter in the data, both within the entire sample population and within samples from individual massifs, reflecting in a large part modal variations in garnet and clinopyroxene. Garnet-rich samples have higher contents of A1203 and clinopyroxene-rich samples have higher contents of SiO2, CaO and Na20. The scatter in data for the oxide variation diagrams pre- cludes the possibility that eclogite or garnet pyroxenite simply represent fractionated basaltic melts. The scatter is consistent with the common occurrence of modal layering and indicates that crystal accumulation played a significant part in the petrogenesis of both rock types. If so, there is no significance to normative calculations or to compositional discriminant diagrams for the pur- poses of classification and tectonic interpretation.

Despite the large amount of compositional scatter, eclogite can be distinguished from garnet pyroxenite on the basis of mg number. All but two Czech eclogite samples have mg numbers less than 80, and all but one Czech garnet pyroxenite have mg numbers greater than 80 (Fig. 2). The slight overlap in mg number in the two rock groups is due to modal variations in layered samples and the strong partitioning of Fe and Mg between garnet and clinopyroxene; for example, a range in mg number from 71.1 to 81.7 occurs in garnet- and clinopyroxene-rich layers, respectively, in an eclogite from Bis- kupice. The apparent absence of eclogites with mg numbers between 60 and 70 may be due to sampling

Table I Major element chemical compositions of garnet pyroxenite and eclogite in the Gf6hl terrane

493

Locality: Dobegovice l)hrov

Sample: CZ12A CZ12B CZ12C CZl2D CS-UR-1 CS-UR-2 CS-UR-3 CS-UR-4 CZ 9C CZ 9D Lithology: Ky Ecl Ky Ky Qtz-Ky Ecl Ecl Ecl Qtz-Ky Qtz-Ky

Ecl Eel Ecl Ecl Ecl Ecl

SiO2 47.26 48.61 46.40 47.24 49.50 45.11 44.71 43.77 47.68 49.93 TiO2 0.11 0.92 0.15 0.19 0.13 1.71 1.47 1.91 0.16 0.09 AIaO3 16.19 9.51 15.44 14.82 17.29 15.71 15.29 15.79 19,61 16.89 Cr203 0.08 0.18 nd 0.07 nd nd nd nd 0.05 0.06 Fe203 7.67 9.32 8.51 7.99 6.71 12.64 13.92 15.60 6.29 6.13 MnO 0.18 0.19 0.17 0.15 0.13 0.19 0.25 0.27 0.11 0.14 MgO 13.45 15.89 12.73 13.15 10.21 9.06 9.07 8.47 9.25 9.94 CaO 11.55 14.00 13.19 13.91 14.20 13.06 13.27 12.93 14.90 14.67 Na20 2.85 1.18 2.16 2.17 1.03 2.07 1.77 1.54 1.29 1.27 K20 0.02 0.23 0.03 0.03 0.17 0.23 0.04 0.04 0.17 0.32 P205 0.01 0.02 0.02 0.02 0.02 0.07 0.03 0.07 0.02 0.01 LOI 0.80 0.36 0.52 0.62 0.31 nd nd nd 0.42 0.51 Sum 100.16 100.41 99.32 100.36 99.70 99.85 99.82 100,39 99.95 99.96

Mg number 77.7 77.2 74.8 76.5 75.1 58.7 56.3 51.8 74.4 76.3

Locality: N~ov Nov6 Dvory

Sample: CZ 4A CZ 4B CZ 2H CND 1D CND 5B CZ 2D E-7 E-9 E-11 Lithology: Grt Grt Grt Grt Grt Ecl Ecl Ecl Ecl

Web Web Cpxite Cpxite Web

SiO2 51.37 51.94 43.61 43.05 45.43 43.97 42.31 43.54 44.13 TiO2 0.26 0.19 0.24 0.10 0.19 0.50 1.81 0.38 0.35 A1203 6.72 5.65 15.13 15.48 7.22 15.74 14.37 17.69 17.02 CrzO3 0.56 0.50 0.20 0.38 0.70 0.08 nd nd nd Fe203 6.23 7.26 9.26 7.58 8.18 9.13 16.09 9.30 9.35 MnO 0.16 0.15 0.18 0.25 0.19 0.16 0.27 0.13 0.11 MgO 22.09 28.10 18.36 20.87 26.43 15.32 12.20 14.45 14.90 CaO 11.01 4.75 9.16 6,95 6.50 12.04 11.45 12.34 11.92 Na20 0.57 0.23 0.55 0.31 0.52 1.80 1.09 1.38 1.31 K20 0.04 0.02 0.02 0,01 0.05 0.03 0.13 0.16 0.10 P205 0.01 0.01 0.01 0.01 0.03 0.03 0.00 0.00 0.00 LOI 0.65 0.21 2.49 3.21 3.70 1.80 nd nd nd Sum 99.67 99.01 99.21 98,20 99.14 100.60 99.72 99.37 99.19

Mg number 87.5 88.5 79.7 84.5 86.5 76.9 60.0 75.5 75.9

Abbreviations: Ky Ecl, kyanite eclogite; Qtz-Ky Ecl, quartz kyanite eclogite; Grt Web, garnet websterite; and Grt Cpxite, garnet clinopyroxenite, nd, Not determined.

bias; no such gap exists in elemental plots versus wt.% MgO.

The parental M O R B melt of Jaques and Green (1980) is intermediate in major e lement chemistry with respect to GfOhl eclogite and garnet pyroxenite, but average N - M O R B has a lower mg number than most Gf0hl eclogite samples (Fig. 2). The scatter and range of composit ions of Gf0hl eclogite and garnet pyroxenite are similar to those of Al-augite and Cr-diopside layers in the Beni Bousera peridoti te in the Rif Moun- tains, Morocco (Pearson et al. 1993), the Ronda peridotite in the Betic Cordillera, Spain (Suen and Frey 1987) and the Balmuccia peridoti te in the Iv rea-Verbano Zone, I taly (Shervais and Mukasa 1991).

A plot of Na20-TiO2-Cr203 (Fig. 3) is especially useful in discriminating be tween eclogite and garnet pyroxenite and emphasizes the different proport ions of Cr203 in the two rock types. Of particular interest in

this figure is the tendency for garnet pyroxenite to plot be tween the eclogite field and the average composit ion for Nov6 Dvory garnet peridotite. Triangular plots that utilize Ni are equally effective in distinguishing between the two rock types and show analogous trends, as described in the Discussion.

The R E E contents and pat terns are widely variable in Gf0hl eclogite and garnet pyroxenite, but all samples have REEN values between about 0.3 and 20, normalized to the primitive mantle of Sun and McDonough (1989) (Fig. 4; Table 2). No obvious correlation exists be tween R E E pat tern and mg number or garnet-clinopyroxene ratio. Unfortunately, R E E analyses have not been obtained f rom separate garnet- and clinopyroxene-rich layers in individual samples, where a correlation between mode and R E E pat tern would be expected due to the different partit ioning of L R E E s and H R E E s in garnet and clinopyroxene. However , several

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bearing eclogite has a relatively flat pattern, with either slight enrichment or slight depletion in LREEs (CZ12A, D; c z g c , D; UR1). Prominent Eu anomalies, both positive and negative, occur in garnet clinopyroxenite, eclogite and kyanite eclogite, indicating the in- fluence of plagioclase at some stage in the petrogenesis of these rocks.

Isotope geochemistry

Nd and Sr isotope ratios

Values of end and initial 87Sr/86Sr ratios for clinopyroxene separates (and bulk rocks for comparison) from

Na20

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Fig. 3 Proportions of Na20, TiO2 and CrzO3 in Czech and Aus- trian garnet pyroxenite and eclogite. Symbols as in Fig. 2, except N, average composition for Nov6 Dvory garnet peridotite

Gf6hl eclogite and garnet pyroxenite are negatively correlated and range from near Hart's (1988) depleted MORB mantle component (DMM) to beyond his enriched mantle II component (EMII) (Table 3; Fig. 5). Analyses for most samples lie within or close to the field for oceanic mantle (calculated at 350 Ma), but two samples depart significantly: clinopyroxene from kyanite eclogite at Dobegovice (CZ12A) has ENd =--6.0 and (SYSr/S6Sr)i=0.71445, and clinopyroxene from eclogite at Uhrov (UR4) has end = + 5.4 and (87Sr/ 86Sr)i =0.70883. The wide range in Nd and Sr isotope compositions of the Gf6hl samples is evidence for the contribution of one or more crustal components to the melts from which eclogite and garnet pyroxenite crystallized, as considered in more detail in the Discus- sion.

The Nd and Sr isotope array defined by all the Gf6hl samples encompasses that for clinopyroxene from Cr-diopside layers at Balmuccia (Shervais and Mukasa 1991) and is similar to that for clinopyroxene from Al-augite and Cr-diopside layers from Beni Bou- sera (Pearson et al. 1993). Clinopyroxene from the garnet websterite at N~ov, which plots close to EMII and has an extreme isotopic composition for pyroxene from mantle-derived ultramafite, is similar isotopically to clinopyroxene in some of the garnet websterites at Beni Bousera.

Sm-Nd geochronology

Sm-Nd isotope analyses of garnet-clinopyroxene pairs in kyanite eclogite from Dobesovice (CZ12A), eclogite from Uhrov (UR4) and garnet clinopyroxenite from Nov6 Dvory (ND3B) yield ages of 338-+ 8, 344-+ 6 and 343 _+ 17 Ma, respectively (Table 3; Fig. 6). In addition, an eclogite from Dobegovice (CZ12B) that was previously analyzed by Brueckner et al. (1991) was re-analyzed to provide an interlaboratory comparison between the University of Wisconsin and Lamont-Doher-

ty; the two ages are the same within error (333 + 11 Ma, UW; 338 + 6, LD). The Sm-Nd ages for Gf6hl eclogite are comparable with U-Pb zircon ages for Gf6hl granulite: 345_+5 Ma (van Breemen et al. 1982), 338+1 Ma (Aftalion et al. 1989) and 347+9_10Ma (Kr6ner et al. 1988). In contrast, eclogite and granulite in the area of Winklarn (Oberpfalz, north-east Bavaria) both have an Sm-Nd age of 424 Ma (von Quadt and Gebauer 1993), indicating Silurian high-pressure metamorphism in the Monotonous and Varied terranes.

Altogether, 11 Sm-Nd ages have now been determined for eclogite and garnet pyroxenite in the GfOhl terrane (Table 4) and the ages fall into two groups, a younger group averaging 339-+ 6 Ma (lo-, n =8) and an older group averaging 373 -+ 3 Ma (lo-, n = 3). The older age group is similar to those determined for the Mtinchberg Gneiss Massif (Gebauer and Griinenfelder 1979; Stosch and Lugmair 1990) and the Mari~insk6 L~zn6 complex (Beard et al. in press) and may reflect Givetian to Frasnian subduction processes and high- pressure metamorphism. The younger age group may be related to Vis6an exhumation and quenching of high-temperature rocks, but it remains a puzzle as to why similar rock types within the GfOhl terrane should record these two different ages. However, enough Sm- Nd ages are now available to conclude that garnet pyroxenites yield both ages, whereas all eclogites dated so far give only Vis6an ages. Rock types from the same massif, such as at Nov6 Dvory, yield the same age (Ta- ble 4).

Oxygen isotope compositions

Ratios for 180/160 have been determined by laser fluo- rination for clinopyroxene and garnet separates from six samples of eclogite and three of garnet pyroxenite (Table 3). Clinopyroxene and garnet are in high-temperature oxygen isotope equilibrium, with ACpx- art =--0.06 + 0.29%0 (1 O', n = 9); values of 618OsMow for clinopyroxene range from 3.8 to 5.8%o in eclogite and from 4.8 to 5.7%0 in garnet pyroxenite (Fig. 7). The two extreme 6180(Cpx) values occur in eclogite from Dobe- govice (CZ12A, 5.8%o) and lQhrov (UR4, 3.8%o), which are also the most extreme samples with respect to Nd and Sr isotopes, but no correlation exists between oxygen and Nd and Sr isotopes in other samples. Com- pared with an expected 5180 value of 5.6%0 for mantle clinopyroxene, based on analyses of over 50 anhydrous spinel and garnet peridotite xenoliths (Mattey et aL 1993), clinopyroxene from GfOhl eclogite and garnet pyroxenite is isotopically light, with six samples cluster- ing around 4.7-5.0%o. Clinopyroxene from Beni Bou- sera garnet pyroxenite (Al-augite layers) also has a wide range in 5lso values, but is isotopically heavy compared with Gf6hl samples and mantle clinopyroxene, with values as high as 7.5%o (Fig. 7; Pearson et al. 1991, 1993). The range in 8iso values for GfOhl and Beni Bousera samples taken together (3.8-9.3%o) is as

100

Nihov Be~vfiry

10 CZ4A CZ6D . . . . . O- O

0.1

La Ce SmEu To Yb Lu

100

497

Nov6 Dvory

M5 . . . . - A - . •

, , . . - "

CND5B - , ........... ,~...& A - " • . .......... ~k ...................

....... .k-'" CND1D

~ t , . o"

0 . 1 I I I I I I I I I I I I I I I

La Ce SmEu Tb YbLu

100

10

Biskupice

CZ1411

CZ14F

0 . 1 i , , , i i , i i , , i i , i

La Ce SmEu Tb Yb Lu

100

10

Nov4 Dvory

CZ2E

. - - ' _..o-...._ _ CZ2G

o...o. -~ .e ,-j'- . ~ " ~ ~ ....... • . . . . . . ""o CZ2D

ND2

0.1 I I I I I I I I l I I I i I I

La Ce Sm Eu Tb YbLu

100

10

a, 1

0 . 1 I I I I I I I I I I I I

La Ce

Dobegovice 100

. . . . . O - . O

O" ' : . . . . . . CZ12A ~ 10 • o-o o . , cz! -

......... o .......... o...o .o ..... CZ12D ~ 1

o M

I I I

SmEuGdTbDy Er YbLu

Uhrov

UR3

~ . - o .o. .CZ9C UR1 O"

0.1 u I I I I ~ t I t I I I I L I

La Ce Sm EuGdTb Dy Er Yb Lu

Fig. 4 R E E abundances in Gf6hl garnet pyroxenite and eclogite, (1989). Symbols: x , orthopyroxene eclogite; • and ©, eclogite; • normalized to the primitive mantle of McDonough and Sun and A, garnet pyroxenite

498

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© -d

0

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2

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r.~

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cQ e~ ¢Q cq ~ eq

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¢-,.I ~1- C<l c.q cq ~ -

~ i I ~oo.....~.~.~oo ~-~~'1 I

I I I I

Cq

+14-1 +1 -I-I -H +1 +1 +1 +I-H I '~ IL'~ I~- ~ ~- ¢Q CQ CQ ¢q P~ I~- ~ eQ Cq eQ ¢Q e~ ¢Q ¢Q Cq ¢Q ~'- ~ I~'- ~ "~- Cq ¢-,1C~ CQ Cq C~ Cq ¢',1 ~'~ eQ eq ¢Q e~ ¢Q ~ I"Q eQ eQ C~ eq ~ ~ eQCQ

-FI +1 -H +1 -H +1 +1 -H +1 +1 +1 +1 + 1 + 1 4 - 1 + 1

d o t e d o c ~ d c ~ c ~ d c ~ oct" " ' " c ~

I , . ~ , - q ~ c ~ o ~ ~ - c ~ o ~ ~ " " ' " ~ -

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cq cq 00 ~ ~ ©

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499

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15

10

0

0.5140

0.5135

0.5130

0.5125

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338 ..'1:8 Ma / Grt

. Z

' t '"

0,2 0.4 0.6 0.8 1

-5

-1C 0.702 0.704 0.706 0.708 0.710 0.712 0.714

(87Sr/86Sr)i Fig. 5 end versus (87Sr/a6Sr)i for GfOhl garnet pyroxenite and eclogite; elinopyroxene (closed symbols) and whole rocks (open symbols). Initial isotope ratios calculated at the ages defined by Sm-Nd garnet-clinopyroxene isochrons. Data for mantle components (#), DMM, HIMU, EMI and EMII from Hart (1988), calculated at 350 Ma; field for oceanic mantle from White and Hof- mann (1982), O'Nions et aL (1977), Hawkesworth et al. (1979), and Dosso and Murthy (1980), calculated at 350 Ma and based on derivation from a CHUR reservoir of 4.5 Ga; data for Beni Bou- sera pyroxenites from Pearson et al. (1993). Endmember compositions used in mixing calculations: mantle (eNa,+ 10; Nd, i ppm; 87Sr/a6Sr, 0.70226; St, 8 ppm); arc detritus (eNd, +4; Nd, 10 ppm; a7Sr/86Sr, 0.7108; Sr, 200 ppm); and oceanic clay (eNa, -10; Nd, 30 ppm; 87Sr/86Sr, 0.7260; Sr, 50 ppm)

large as that reported for all eclogite xenoliths (2.2-7.9%0, Kyser 1990).

D i s c u s s i o n

A high-pressure cumulate origin for eclogite and garnet pyroxenite layers in garnet peridotite bodies of the Gf6hl terrane is indicated by the presence of modal layering, a large degree in scatter in oxide variation diagrams, the correlation between elemental compositions and modes and the absence of any prograde metamorphic features. In contrast, eclogite bodies in gne i s s

t e r ranes represent low-pressure basaltic melts that were subsequently metamorphosed at high pressure (Griffin 1987; Kornprobst et al. 1987).

Decreasing end with increasing initial 87Sr/86Sr, as well as 6180(Cpx) values that lie both above and below mantle values, strongly indicate that subducted oceanic crust contributed to the melts that crystallized to form the eclogite and garnet pyroxenite of this study. More-

"O 0.5145 Z • r-...,,, 0.5140

"O z

0.5135 ¢O

0.5130

0.5125

0.5140

0.5135

0.5130

0.5125

CS-UR-4

337.-I; 6 Ma ~ , ~ n

t ........ W R 1 / "'"',

• I Eu Ce Nd Sm Od Dy Er Yb

! ! ! i

0.2 0.4 0.6 0.8

ND-3B

327 ± 1 G 9 ~ Grt

j "...

I Cpx Eu I.

Ce Nd $m Gd Dy Yb

0.2 0.4 0.6 0.8

143 Nd/147Sm

Fig. 6 Sm-Nd mineral isochrons for samples of Dobegovice kyanite eclogite (CZ12A), I)hrov eclogite (UR4) and Nov6 Dvory garnet clinopyroxenite (ND3B). Insets: normalized REE patterns for minerals and rocks, determined by Isotope dilution. A147Sm =6.54 x 10-12y 1

over, most samples have low 6180(Cpx) values, sug- gesting that the subducted component belonged to the deeper, high-temperature sections of subducted oceanic crust. A contribution from subducted oceanic crust has also been proposed for equivalent rocks in the Ronda (Suen and Frey 1987), Beni Bousera (Pearson et al.

1991, 1993) and Balmuccia (Shervais and Mukasa 1991) peridotite massifs.

500

Table 4 Sm-Nd age determinations for garnet pyroxenite and eclogite, Gf6hl terrane

Locality Sample Lithology Age (Ma) Source

Czech Republic Becv~iry Biskupice Dobe~ovice

Nl"hOV Nov6 Dvory

Uhrov

Austria Mitterbachgraben Karlstetten

CZ6C Garnet clinopyroxenite 377 + 20 CZ14F Eclogite 324 + 5 CZ12A Kyanite eclogite 338+8 CZl2B Eclogite 338 + 6 CZ4A Garnet websterite 373 + 7 CZ2E Eclogite 341 + 7 ND2 Eclogite 336 + 16 ND3B Garnet clinopyroxenite 343 + 17 UR4 Eclogite 344 + 6

M1 Garnet clinopyroxenite 370 + 15 M186 Garnet websterite 344 + 10

Beard et al. (1992) Brueckner et al. (1991) This investigation Brueckner et al. (1991) Brueckner et al. (1991) Brueckner et al. (1991) Beard et al. (1992) This investigation This investigation

Carswell and Jamtveit (1990) Carswell and Jamtveit (1990)

Elemental composition of the source

Although eclogite and garnet pyroxenite in peridotite massifs are cumulate rocks, the compositions of the melts from which they crystallized can be calculated from the compositions of mineral separates. The REE contents of melts for one sample of garnet clinopyroxenite, four of eclogite and two of kyanite eclogite have been calculated from the compositions of garnet and clinopyroxene (Fig. 8) using the D-values compiled by Miller et al. (1992), except for D-values of 0.002, 0.005 and 0.050 for La, Ce and Nd, respectively, in garnet, which were chosen to be consistent with the results from D-values for these elements in clinopyroxene and which are within the range given by Frey et al. (1978).

8

7

t:

~ 5 © OO

4

O

°1

3 4 5 6 7 8

81So (Cpx) Fig. 7 61SO(garnet) versus 61SO(clinopyroxene) for Gf6hl eclogite, 0, and garnet pyroxenite, A, compared with data from Beni Bousera, O (Pearson et aL 1991). 61so for mantle clinopyroxene from Mattey et al. (1993)

Calculated melt compositions for eclogite have steep LREE-enriched patterns, with normalized Ce/Yb ratios of 124 in ND2 and 40-44 in the other three (neglecting the decrease in Ce in CZ12B and UR4). The two kyanite eclogite samples have lower Ce/Yb ratios of 7 and 26, and garnet clinopyroxenite, CZ6C, has a REE pattern almost identical to that of kyanite eclogite UR1. No correlation exists between Ce/Yb ratios and total REE contents among the analyzed samples or between Ce/Yb ratios and mg number.

The steep REE patterns and low Yb contents for the calculated melt compositions are unlike those for MORB melts and require different petrogenetic processes. The REE features of the calculated melt compositions can be produced by single-stage partial fusion of a garnet peridotite source, but a two-stage melting model, as described in the following, provides a better comparison with the compositions of eclogite and garnet pyroxenite cumulates, as well as those of calculated melts, and is necessary to account for the Nd, Sr and O isotope data.

Trace element contents were used to place constraints on a two-stage scenario for eclogite and garnet pyroxenite petrogenesis, in which garnet-pyroxene rocks are assumed to have formed as cumulates from melts of subducted ocean crust, which in turn had formed by partial melting of a mantle source. Although modelling is very open-ended because of the multitude of parameters involved in such a two-stage process, REE contents have been calculated for melts and garnet-clinopyroxene cumulates (Fig. 8) based on the following parameters: the primitive mantle composition of Sun and McDonough (1989); 5% nonmodal batch melting of spinel peridotite (estimated from incompatible and compatible trace element contents of average Nov6 Dvory peridotite and using the conditions cited by Ot- tonello et al. 1984); no fractionation of melt to form oceanic crust; 25% modal batch melting of subducted and eclogitized oceanic crust composed of 30% garnet and 70% clinopyroxene (a common mode for eclogite), and finally, fractional crystallization of 30% garnet and 70% clinopyroxene (initial cumulate composition is

501

Grt Clinopyroxenite Ky Eclogite 100 100

~10 ~10 ~ A

1 ~ 1 N ~ UR1

Ce Nd S m F a G d Dy Er Yb Ce Nd SmEuGd Dy Er Yb

Eclogite 100 100

cD

"Z~

N

0.1

~ C Z ~ z 1 2 B

I I I I I I I i I I I I I i I

Ce Nd SmEuGd Dy Er Yb

~D

10

0.1

Model REE Contents ]Plelt 2

®"0 .......... "Q.., Melt 1 ....... ~ " o / - ~ - - A I

~ 1 ° " ° "'"" ....................... ..

~ \ .... e

. ~ - . ~ " * ... . ~esidualManfle

I~" I I I I I I I I I I I I I I

LaCe Nd SmF~uGd Dy Y b L u

Fig. 8 Normalized REE contents of melts from which Gf6hl garnet clinopyroxenite, eclogite and kyanite eclogite accumulated, calculated from the compositions of garnet and clinopyroxene separates and averaged for each sample• Shown for comparison are normalized REE contents of residual solid, melts and cumulates calculated from a two-stage melting model, starting with a primitive spinel peridotite mantle. See text for discussion

plotted in Fig. 8 at F=0.99, where F is the fraction of melt). If a garnet peridotite source, rather than spinel peridotite, is used, the model results are similar, except that the HREE contents of melts and cumulates are lower due to a greater retention of HREEs in the source.

Despite the numerous uncertainties involved in the calculation, good agreement has been obtained between calculated melt and cumulate compositions and those for a number of eclogite samples. The calculated REE composition of second-stage melt (melt 2, Fig. 8) is similar in terms of normalized Ce/Yb ratio (25) and

REE contents (2-60 times primitive mantle) to that for eclogite samples CZ2E, CZ12B and UR4 (Fig. 8). Larger Ce/Yb values for calculated melt are obtained with either smaller amounts of ocean crust melting (Ce/ Yb =50, 10% melting) or increased fractionation (Ce/ Yb=54, F=0.80). There is also close similarity between REE composition for the calculated cumulate and a number of eclogite samples, both in terms of a convex upward pattern and overall concentrations from about three to nine times primitive mantle (cumulate 2, Fig. 8; Fig. 4). The effect of fractionation on the cumulate REE pattern is to increase the LREEs, decrease the HREEs, and shift the apex of the convex-upward pattern towards Nd; incorporation of trapped melt will transform the convex-upward pattern to one with a negative slope; and greater melting of oceanic crust will produce HRE-enriched patterns like those of CZ6D, UR3 and UR4 in Fig. 4.

However, a simple two-stage melting model cannot account fully for the variability in REE contents of

502

La N

, - , \ ...... , 2 .... \ , e%, , . ; ......... ..... \

O ...... °tP \

Sm N YbN b

Ni N

Co N Sc N

Fig. 9a Plot of normalized La-Sm-Yb for Gf0hl garnet pyroxenite (A, Czech; A, Austria) and eclogite (0, Czech; ©, Austria). Dotted arrows: calculated cumulate compositions (30% garnet, 70% clinopyroxene) resulting from first-stage 5% batch melting, followed by fractional crystallization from 1 to 99% (i.e. the fraction of remaining liquid, F, from 0.99 to 0.10, with nodes at intervals of 0.10), for 0 and 25% trapped melt. Solid arrows: the same, except resulting from second-stage 25% batch melting, b Plot of normalized Ni-Co-Sc for GfOhl garnet pyroxenite, eclogite, primitive mantle (~r) and average Nov4 Dvory garnet peridotite (N). Dotted arrow: calculated second-stage cumulate compositions (30% garnet, 70% clinopyroxene) for combined assimilation and fractional crystallization (AFC), with F from 0.99 to 0.40 and nodes at intervals of 0.10 from 0.90 to 0.60. Solid arrow: the same for fractional crystallization alone (FC), except with F from 0.99 to 0.30 and, for clarity, nodes shown only at 0.50 and 0.40. See text for discussion

eclogite and garnet pyroxenite. In particular, the mag- nitude of Eu anomalies, both positive and negative, in some samples requires the crystallization of plagioclase at some stage in the evolution of these rocks. Most like- ly, stage one mantle-derived melts crystallized plagioclase at low pressures, producing oceanic crust with Eu- poor differentiates and complementary Eu-rich cumulates. A wide variation in the composition of stage two melts could then be produced by derivation from different reservoirs in heterogeneous, subducted oceanic crust. In addition, isotopic data are consistent with a sedimentary component in some eclogite and garnet pyroxenite samples, as discussed in the following, further broadening the range of source compositions from which stage two melts were derived.

The effects of crystallization and assimilation in the petrogenesis of GfOhl eclogite and garnet pyroxenite can be evaluated using ternary plots of normalized trace elements, as described by Ottonello et al. (1984). Much of the variability in proportions of La, Sm and Yb (normalized to the primitive mantle of Sun and McDonough 1989) in eclogite and garnet pyroxenite can be accounted for by different degrees of one- and two-stage melting, when the effects of fractional crystallization and trapped melt are included (Fig. 9A).

The transition elements, Ni, Co and Sc, are not effective in discriminating between melt and cumulate compositions in the model considered here, but they are useful in distinguishing between the effects of fractional crystallization alone and those of combined assimilation and fractional crystallization (DePaolo 1981). Eclogite compositions, in terms of normalized Ni, Co and Sc, agree closely with those calculated for cumulates for up to about 55% fractional crystallization (i.e. F = 0.45) and garnet pyroxenite compositions cor- respond to those calculated for combined assimilation and up to about 30% fractional crystallization (F= 0.70), using an average composition of Nov6 Dvo- ry peridotite for the assimilant and a value of 0.5 for the ratio (rate of assimilation)/(rate of crystallization) (Fig. 9B). The different trends for eclogite and garnet pyroxenite cannot be due to different amounts of trapped melt in the two rock types because the proportion of Ni in the Ni-Co-Sc triad is negligibly affected by trapped melt. The divergent trends for eclogite and garnet pyroxenite in terms of NazO, TiO2 and Cr203 (Fig. 3) are also probably due to predominantly fractional crystallization in eclogite, but combined assimilation and fractional crystallization in garnet pyroxenite.

Isotopic constraints on subducted components

Nd and Sr isotopes indicate that subducted oceanic crust, i.e. altered basalt and high-87sr/g6Sr sedimentary rocks, contributed to the melts from which Gf0hl eclog- ire and garnet pyroxenite accumulated. As shown in Fig. 5, eNa and (SVSr/86Sr)i for eclogite and garnet pyroxenite range from near DMM to EMII and beyond. The EMIl mantle component has been ascribed to subducted sedimentary material (Hart 1988). The extreme isotopic composition of eclogite CZ12A [~Nd=--6.0, (87Sr/S6Sr)i =0.71445] may be generated by mixing depleted mantle with 17% oceanic clay and that of eclogite UR4 [eNd = +5.4, (87Sr/86Sr)i=0.70883] by mixing

503

depleted mantle with 15% island arc detritus (Fig. 5; Beard 1992). However , such large amounts of sedimentary endmembers seem to be excessively high for pure mixing processes (Othman et al. 1989), and oxygen isotope data also argue against this simple model, unless some mechanism can be found to lower their 6180 values.

A mixture consisting of 17% oceanic clay of 6180=15-20%o (e.g. Muehlenbachs 1986) and 83% pure mantle component of 6180=5.2%o would have 618Omixture ~- 6.9-7.7%0, inconsistent with the mantle-like 6180 values for minerals from CZl2A. Because the very high initial 87Sr/86Sr ratio for CZ12A is well con- strained by low-Rb clinopyroxene and is unlikely to have been generated by seawater Sr isotopic exchange in a Phanerozoic ocean (Burke et al. 1982), we propose that the original sedimentary 6*80 values were low- ered. We envision selective melting of subducted sedimentary components that had previously undergone high-temperature oxygen isotope exchange during subduction, which would have decreased the 6180 values of the sediment component . The very low 6*80 values of UR4 (3.8%0 in Cpx) provide strong evidence for a subducted component that has undergone high-temperature oxygen isotope exchange with seawater, and the moderately high initial 87Sr/86Sr ratio, which lies within the range of early Phanerozoic seawater (Burke et al. 1982), is consistent with this interpretation.

The comparatively high 6180 values for Beni Bou- sera pyroxenite (Fig. 7; Pearson et al. 1991) imply derivation from melts of the low-temperature, upper sections of hydrothermally altered ocean crust, whereas the lower 6180 values repor ted here for Gf6hl eclogite and garnet pyroxenite suggest their derivation from the high-temperature, lower sections of hydrothermally altered ocean crust, perhaps indicating delamination of the upper and lower sections of subducted oceanic crust.

Conclusions

Eclogite and garnet pyroxenite layers in garnet peridotite bodies in the GfOhl terrane do not represent melt compositions, but rather formation by high-pressure crystal accumulation ( + t r a p p e d melt) from transient melts in the lithosphere, and are similar in composition and origin to Al-augite and Cr-diopside layers in peridotite massifs elsewhere. Eclogite is the product of simple fractional crystallization from basaltic magmas, whereas garnet pyroxenite is the result of reaction between fractionating melts and wall-rock peridotite, The source of melts from which eclogite and garnet pyroxenite crystallized was subducted, hydrothermally altered oceanic crust, including subducted sedimentary material such as abyssal clay or island-arc detritus.

Three Sm-Nd ages from garnet pyroxenite, averaging 373 Ma, may record an episode of crystallization from melts generated during Middle to Late Devonian

subduction and the assembly of Gondwanan continental fragments, island arcs and small ocean basins along the southern margin of Baltica. Eight other Sm-Nd ages from eclogite and garnet pyroxenite, averaging 338 Ma, may reflect the time that these rocks were quenched during rapid exhumation related to Carboniferous extension and collapse. However , the possibility that melts might have been generated by lithospheric delamination and asthenospheric upwelling, immediately before Carboniferous collapse, should be investigated further.

Acknowledgements We are indebted to Arnogt Dudek and Kar- el Pad6ra, who supervised the collection of many samples and provided valuable insights into the geology of the Bohemian Mas- sif; to Shelby Boardman, who directs the XRF laboratory at Car- leton University; to Randy Korotev, who performed INAA analyses in Larry Haskin's laboratory at Washington University; and to Hannes Brueckner and an anonymous reviewer, who construc- tively critiqued the original manuscript. This research was sup- ported by NSF grants EAR-9004795 to LGM and CMJ, EAR- 9105966 and EAR-9106271 to CMJ and EAR-9304372 to JWV.

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Garnet pyroxenite and eclogite in the Bohemian Massif: geochemical evidence for Variscan recycling...

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