Geochemistry and metamorphic evolution of the Pohorje Mountain eclogites from the easternmost Austroalpine basement of the Eastern Alps (Northern Slovenia)

Lithos 78 (2004) 235 – 261 www.elsevier.com/locate/lithos

Geochemistry and metamorphic evolution of the Pohorje Mountain eclogites from the easternmost Austroalpine basement of the Eastern Alps (Northern Slovenia) Raffaele Sassi a,*, Claudio Mazzoli a,b, Christine Miller c, Ju¨rgen Konzett c a

Department of Mineralogy and Petrology, Universita` di Padova, Corso Garibaldi 37, 35137 Padua, Italy b Ist. Geoscienze Georisorse, CNR, C.so Garibaldi 37, 35137 Padua, Italy c Inst. fu¨r Mineralogie und Petrographie, Innrain 52, A-6020 Innsbruck, Austria Received 30 July 2003; accepted 17 May 2004 Available online 15 July 2004

Abstract Kyanite-rich and quartz-rich eclogites occur as lenses within amphibolite-facies quartzo-feldspathic gneisses in the Pohorje Mountains, Northern Slovenia, that form the easternmost Austroalpine basement. Major and trace elements indicate that the kyanite-rich eclogites were derived from plagioclase-rich gabbroic cumulates, whereas the quartz-rich eclogites represent more fractionated basaltic compositions. Both varieties are characterized by a LREE-depleted N-MORB type REE signature. Geothermobarometry and P – T pseudosections indicate that eclogites equilibrated at 1.8 – 2.5 GPa and 630 – 700 jC, consistently with the lack of coesite and with equilibration conditions of the chemically similar eclogites from the adjacent basement units at Koralpe and Saualpe type localities. Decompression reaction textures include (i) clinopyroxene – plagioclase intergrowths after omphacite, (ii) replacement of kyanite by corundum – plagioclase – spinel F sapphirine symplectites, (iii) breakdown of phengite to biotite – plagioclase sapphirine symplectites. The results of this study indicate that Koralpe, Saualpe and Pohorje high-pressure rocks represent former MORB-type oceanic crust that was subducted in the course of the late Cretaceous (approximately 100 Ma ago) collision between the European and the Apulian plates. D 2004 Elsevier B.V. All rights reserved. Keywords: Kyanite eclogite; Geochemistry; P – T conditions; Pohorje; Slovenia

1. Introduction Eclogites not only reveal the sites of former subduction or continental collision zones but may also retain the geochemical characteristics of the protoliths,

* Corresponding author. Tel.: +39-49-8272019; fax: +39-498272010. E-mail address: [email protected] (R. Sassi). 0024-4937/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2004.05.002

thus providing information on the geotectonic framework preceding the high-pressure (HP) metamorphic event. For kinetic reasons and others (fluids, plasticity), mafic rocks tend to preserve HP assemblages much better than non-mafic lithologies (e.g., Heinrich, 1982; Koons and Thompson, 1985) and, thus, are eminently suitable for reconstructing the tectonometamorphic history of a basement rock sequence. Eclogites are known from numerous localities in the Eastern Alpine basement extending from the

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¨ tztal-Stubai and Silvretta CrystalUlten Zone, the O line Basements in the West (e.g., Godard et al., 1996; Miller and Tho¨ ni, 1995; Magetti et al., 1987) to the eclogite type locality Kor- and Saualpe in the East (Hau¨y, 1822; Godard, 2001). All of them record a multistage evolution. Precise dating using Sm – Nd and/or U – Pb isotopic systems revealed ¨ tztal and Silvretta either a Variscan (Ulten Zone, O nappes: Miller and Tho¨ni, 1995; Tho¨ni, 1999; Ladenhauf et al., 2001) or a Eo-Alpine age (Korand Saualpe: Tho¨ni and Jagoutz, 1992; Miller and Tho¨ni, 1997) for the HP metamorphism thus invalidating earlier assumptions of a Caledonian (Purtsch-

eller and Sassi, 1975) or even late Proterozoic (Manby et al., 1988) eclogite-facies event. The eclogites from the Pohorje Mountains in Slovenia are the southeasternmost HP rocks known from the Austroalpine basement and are thought to represent the southern extension of the Koralpe – Saualpe eclogite zone. The Pohorje eclogites were first described by Ippen (1892) and later studied in more detail by Nikitin (1942), Hinterlechner-Ravnik (1982, 1987), Hinterlechner-Ravnik et al. (1991a,b) and Visona` et al. (1991). It has been suggested, albeit without adequate supporting data, that these rocks may represent oceanic crust which underwent a pre-

Fig. 1. Geological sketch map of the Pohorje Mountains (modified after Hinterlechner-Ravnik et al., 1991a) and location of the studied rock samples.

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Alpine, possibly Caledonian, eclogite-facies metamorphism (Hinterlechner-Ravnik et al., 1991b). In the present paper, an attempt was made to reconstruct the P – T metamorphic evolution of the Pohorje eclogites using conventional geothermobarometry in combination with multiequilibrium techniques, as well as to identify the eclogite protoliths using major and trace element geochemistry.

2. Geological setting The Pohorje massif is the southeasternmost part of the Austroalpine basement nappe system and forms the basement rim of the Neogene Styrian basin (Fig. 1). According to Hinterlechner-Ravnik (1971, 1973), it is mainly composed of amphibolite-facies orthogneisses and paragneisses with intercalations of micaschists, marbles and amphibolites. In addition, numerous lenses of eclogites and some metaperidotites are associated with amphibolites, micaschists and paragneisses, especially in the southeastern part of the massif (e.g., Hinterlechner-Ravnik and Moine (1977); Hinterlechner-Ravnik, 1982, 1987; HinterlechnerRavnik et al., 1991a,b; Visona` et al., 1991). In this area, the eclogite outcrops are rare and the relationships between eclogites and country rocks are difficult to assess due to poor exposure conditions. The metaperidotites (harzburgites and dunites) are invariably serpentinized and may reach sizes of up to 5
1 km (Hinterlechner-Ravnik, 1982). Garnet peridotites reported by Hinterlechner-Ravnik (1982, 1987) and Hinterlechner-Ravnik et al. (1991b) turned out to be metatroctolites based on new observations of the material collected by Hinterlechner-Ravnik (Seemann, pers. comm.). The basement rocks are intruded by the Oligocenic Pohorje tonalite/granodiorite and by Miocenic dacitic dikes (Faninger, 1970). According to Altherr et al. (1995), the Pohorje tonalite crystallized at a depth of approximately 20 km and was subsequently exhumed as part of the footwall of a normal fault zone where the Lavanttal line merges with the Insubric fault zone. It should be noted that the postulated Caledonian age of the Pohorje eclogites (Hinterlechner-Ravnik et al., 1991b) is in contrast to the well-established Cretaceous age of the type-locality eclogites in the Kor- and Saualpe (Tho¨ni and Jagoutz, 1992, 1993;

237

Miller and Tho¨ni, 1997; Heede, 1997) that form the northern continuation of the Pohorje basement unit. In fact, no ages older than 300 Ma have yet been established for this segment of the Austro-Alpine crystalline basement.

3. Petrography Sixty eclogite samples were collected in the Slovenska Bistrica area (Fig. 1). Quartz-rich and kyaniterich eclogites can be distinguished based on mineral assemblages and whole rock composition (Fig. 2a –c).

Fig. 2. Photomicrographs of thin sections (left column) and corresponding polished rock slices of the two Pohorje eclogite types (right column): (a) Qtz-rich eclogite with minor secondary alteration; (b) Qtz-rich eclogite with omphacite replaced by Di-Cam-Pl symplectites; (c) Ky-rich eclogite with texturally primary Ca-amphibole. Abbreviations after Kretz (1983). Planepolarized light.

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3.1. Primary textures and mineral assemblages 3.1.1. Quartz-rich eclogites This rock type often shows a layered microstructure made up by alternating omphacite and garnet-rich domains. The primary mineral assemblage consists of omphacite (Omp) + garnet (Grt) + quartz (Qtz) + rutile (Rt) F zoisite/clinozoisite (Zo/Czo) F phengite (Phe) F minor kyanite (Ky) (Figs. 2a and 3g); apatite, zircon and pyrite may appear as accessories. A faint foliation is defined by elongated omphacite F clinoclinozoisite. Quartz forms polygonal grains in the matrix and is always monocrystalline when included in garnet, omphacite or kyanite. Radiating tensional cracks around quartz and zircon inclusions in garnet and omphacite were locally observed. Garnet crystals are frequently rounded and may contain inclusions of quartz, rutile, zircon, omphacite and clinozoisite. Omphacite contains inclusions of quartz, rutile and garnet. Bleb-textured and texturally late Ca-amphiboles may enclose any of the primary phases but clearly predate the symplectitic replacement textures discussed below. 3.1.2. Kyanite-rich eclogites Kyanite-rich eclogites are lighter in appearance due to the higher omphacite and kyanite contents. The primary mineral assemblage consists of omphacite (Omp) + garnet (Grt) + kyanite (Ky) + zoisite (Zo) F rutile (Rt) F quartz (Qtz) F Ca-amphibole (Cam) (Fig. 2c). Apatite, zircon, pyrite and pyrrhotite are accessories. Omphacite is anhedral and may contain inclusions of rutile, garnet, kyanite and quartz. Matrix kyanite is often euhedral and may display omphacite, garnet, zoisite and rutile inclusions. In rare cases, omphacite and garnet contain inclusions of amphibole, eastonite and paragonite. 3.2. Secondary replacement textures In both eclogite types, decompression has resulted in the growth of poikiloblastic Ca-amphibole (Fig. 3h) and in the formation of symplectite coronas and reaction rims around primary eclogite phases. Omphacite breaks down to symplectites consisting of low-Na clinopyroxene (Jd5 – 8) + plagioclase (Ab51 – 75), which are in turn replaced by tremolitic hornblende + plagioclase (Fig. 3a). Low-Na clinopyroxene may also

appear at the interface between the symplectites after omphacite and quartz (Fig. 3b). Garnet may react with omphacite to form coronas of aluminous amphiboles (pargasite, magnesiohornblende) F plagioclase F epidote (Fig. 3d). Kyanite is always mantled by a symplectitic intergrowth of corundum, calcic plagioclase (Ab89 – 97) and spinel (Fig. 3e and f). In rare cases, sapphirine appears as an additional phase in these kyanite symplectites. Similar kyanite breakdown reactions have been documented from the Saualpe eclogites by Miller (1990). Phengite shows partial or complete replacement by a very fine-grained symplectite consisting of biotite + sodic plagioclase F sapphirine F clinopyroxene (Fig. 3g). Primary zoisite is partially replaced by Ca-rich plagioclase and Fe-poor zoisite-II (Fig. 3c).

4. Major and trace element geochemistry Whole-rock major and trace element compositions of 14 eclogites are reported in Table 1. The low-K contents and AFM characteristics indicate a tholeiitic affinity (Fig. 4a). Screening whole rock compositions with the Al2O3 – TiO2 discrimination diagram (Pearce, 1983) shows that all kyanite-rich eclogites plot in the cumulate field, whereas the quartz-rich eclogites plot in the basalt field (Fig. 4b). This is confirmed by the CIPW normative mineralogy, yielding 0.2 – 11.5% normative quartz for quartz-rich eclogites and 9.7– 15.4% normative olivine for kyanite-rich eclogites. Modal kyanite is present in all samples with Al2O3> 15 wt.%. The wide range in Mg-numbers [Mg# = Mg/ (Mg + AFe)] of 0.40 –0.74 clearly indicates igneous fractionation. Additional support for an extensive fractionation is provided by the positive Cr – Mg# and Ni –Mg#-correlations (Fig. 4c and d). The primitive-mantle (PM) normalized trace element patterns of the quartz-rich eclogites are essentially depleted in the more mobile LIL elements with minor anomalies, the most prominent being the negative Sr anomaly (Fig. 5a). They are further characterized by LREE-depleted patterns (LaN/YbN = 0.56– 0.69) without noticeable Eu anomalies and flat HREE, ranging from 17 to 19 times chondrites, similar to NMORB (Fig. 5c). A depleted mantle origin is also supported by Ti/V, Zr/Y, Zr/Nb, La/Ta and Th – Hf – Ta systematics (Wood, 1980; Sun and McDonough,

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Fig. 3. Retrogressive microtextures: (a) Omp partially replaced by Di-Cam-Pl symplectite, (b) diopside-rim (Cpx II) at the boundary between a Qtz-grain and a symplectite, (c) alteration of primary zoisite to Ca-rich plagioclase and Fe-free zoisite (Zo II), (d) green Cam coronas around Grt in contact with symplectites after Omp, (e) fine-grained and (f) coarse-grained Crn-Pl-Spl symplectitic corona replacing Ky, (g) back scattered electron image of phengite with a Pl-Bt-Cpx-Spr symplectitic corona, (h) texturally late Cam poikiloblast. Abbreviations after Kretz (1983). Plane-polarized light (a – g) and cross-polarized light (h).

1989; Shervais, 1982). Erratic variations of the alkali elements (Rb, K) could be the result of mobilization during metamorphism.

Kyanite-rich eclogites are distinctly poorer in Fe, Ti and in incompatible trace elements compared with the quartz-rich varieties (Table 1). These low

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Table 1 Whole-rock compositions and CIPW norms of quartz- and kyanite-rich eclogites from the Pohorje Mountains, Slovenia Type

A (Qtz-E)

B (Ky-E)

Sample 3-1 (Pd)

H-15 (Pd)

H-16 (Pd)

H-6 (Pd)

H-8 (Pd)

CM15/01 CM16/01 CM20/01 2-1 (Inn) (Inn) (Inn) (Pd)

SiO2 TiO2 Al2O3 FeO Fe2O3 Fe2O3* MnO MgO CaO Na2O K2O P2O5 LOI Total Sc V Cr Co Ni Cu Zn Ga Rb Sr Y Zr Nb Ba U Pb Hf Ta Th La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

52.10 0.86 15.46 7.31 0.98

51.08 1.10 14.59 7.19 1.49

50.68 0.84 14.18 6.96 1.25

53.46 1.76 15.32 8.00 0.75

53.41 0.76 15.45 6.55 0.59

49.05 1.77 14.53 n.d. n.d. 12.54 0.16 0.17 0.18 0.18 0.14 0.18 7.62 8.17 8.80 6.53 6.69 7.57 11.76 11.79 12.15 10.68 12.95 11.70 2.61 2.27 2.10 1.36 1.77 2.66 0.03 0.34 0.31 0.12 0.09 0.11 0.07 0.07 0.02 0.19 0.07 0.22 0.44 1.06 1.97 0.99 1.02  0.38 99.39 99.32 99.43 99.34 99.49 99.84 15 18 17 16 14 46 310 289 273 273 282 325 110 311 328 230 300 180 40 48 43 31 29 44 73 106 100 80 103 68 40 58 62 66 50 57 58 67 68 53 67 67 2 10 8 6 7 18 2 5 14 4 6 0.97 165 138 120 194 263 57 20 23 18 29 22 37 48 40 28 89 45 107 3 4 3 5 3 2.1 18 47 52 26 24 9 0 0 0 0 0 0.29 14 11 9 0 0 1.2 n.d. n.d. n.d. n.d. n.d. 2.98 n.d. n.d. n.d. n.d. n.d. 0.20 0 0 2 0 0 0.18 0 0 0 0 8 3.33 13 18 1 8 0 10.47 1.82 0 11 8 14 8 10.47 n.d. n.d. n.d. n.d. n.d. 3.42 n.d. n.d. n.d. n.d. n.d. 1.28 n.d. n.d. n.d. n.d. n.d. 4.98 n.d. n.d. n.d. n.d. n.d. 0.90 n.d. n.d. n.d. n.d. n.d. 6.13 n.d. n.d. n.d. n.d. n.d. 1.35 n.d. n.d. n.d. n.d. n.d. 3.79 n.d. n.d. n.d. n.d. n.d. 0.61 n.d. n.d. n.d. n.d. n.d. 4.01 n.d. n.d. n.d. n.d. n.d. 0.59

49.57 1.56 14.40 n.d. n.d. 11.89 0.17 7.68 11.93 2.86 0.03 0.17  0.39 99.84 45 320 164 44 67 43 67 19 1.73 65 36 112 2.0 12 0.22 1.1 3.19 0.18 0.19 3.60 10.97 2.04 11.31 4.04 1.41 5.28 0.91 5.90 1.29 3.44 0.57 3.86 0.55

48.33 1.61 14.69 n.d. n.d. 12.40 0.18 8.06 12.12 2.60 0.01 0.20  0.37 99.83 n.d. 340 209 48 71 58 67 19 < 62 36 114 1.9 33 < 1.2 2.97 0.16 0.11 3.99 12.53 2.07 11.09 3.86 1.47 5.19 0.93 6.23 1.35 3.71 0.59 3.89 0.61

46.53 0.07 23.73 3.84 0.17

47.00 0.06 23.40 3.49 0.18

47.05 0.21 20.46 n.d. n.d. 4.71 0.06 0.05 0.06 10.26 10.58 11.66 12.40 12.22 14.10 1.39 1.40 1.39 0.04 0.04 0.01 0.02 0.02 0.09 0.89 0.89 0.16 99.39 99.33 99.9 3 5 n.d. 26 23 102 187 195 1242 49 52 36 225 232 298 41 39 48 26 23 27 8 10 12 2 2 < 211 221 52 0 0 5 8 5 9.6 2 1 < 33 25 < 0 0 < 39 42 1.2 n.d. n.d. 0.34 n.d. n.d. < 0 0 < 0 0 0.43 0 0 1.32 0.23 0 0 1.31 n.d. n.d. 0.50 n.d. n.d. 0.26 n.d. n.d. 0.74 n.d. n.d. 0.13 n.d. n.d. 0.87 n.d. n.d. 0.20 n.d. n.d. 0.53 n.d. n.d. 0.09 n.d. n.d. 0.55 n.d. n.d. 0.09

48.16 0.21 17.23 n.d. n.d. 5.50 0.10 11.02 15.09 2.07 0.02 0.08 0.37 99.85 n.d. 160 557 37 193 46 30 12 1.02 116 7 6.0 < 9.8 < 1.4 0.22 < < 0.38 1.12 21 1.26 0.57 0.37 0.96 0.16 1.17 0.26 0.71 0.11 0.74 0.11

50.08 0.74 15.68 n.d. n.d. 9.92 0.17 8.43 12.79 2.15 < 0.10  0.23 99.83 n.d. 223 157 40 77 53 67 17 < 120 15 24 0.3 4.5 < 1.2 0.84 0.03 < 1.02 3.45 0.58 3.27 1.33 0.71 1.97 0.35 2.39 0.52 1.45 0.24 1.52 0.24

49.26 0.21 20.14 n.d. n.d. 3.61 0.04 9.35 12.80 3.19 0.01 0.06 0.25 98.92 n.d. 101 1076 25 228 19 41 14 < 25 3 25 0.3 5.6 < 1.4 0.68 0.03 < 0.23 0.30 0.12 0.62 0.32 0.15 0.52 0.09 0.56 0.12 0.32 0.05 0.36 0.06

CIPW Q Or

1.18 0.18

0.95 2.02

0.18 1.90

11.51 6.99 0.71 0.53

3.14 0.18

2.32 0.06

0.00 0.24

0.00 0.21

0.00 0.12

4.29 0.00

0.00 0.06

3.73 0.65

1-3 (Pd)

CM25/01 CM42/01 CM48/01 RS01/01 (Inn) (Inn) (Inn) (Inn)

0.00 0.06

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241

Table 1 (continued ) Type

A (Qtz-E)

B (Ky-E)

Sample 3-1 (Pd)

H-15 (Pd)

H-16 (Pd)

H-6 (Pd)

H-8 (Pd)

CM15/01 CM16/01 CM20/01 2-1 (Inn) (Inn) (Inn) (Pd)

1-3 (Pd)

CM25/01 CM42/01 CM48/01 RS01/01 (Inn) (Inn) (Inn) (Inn)

CIPW Ab An Di Hy Ol

19.51 29.17 24.25 19.62 0.00

18.21 29.08 26.39 20.68 0.00

11.67 35.95 13.65 21.55 0.00

15.23 34.47 24.70 15.58 0.00

22.51 27.38 18.50 10.27 0.00

12.03 58.38 2.45 12.85 13.65

11.76 49.56 15.03 8.25 9.69

22.31 30.72 22.61 19.77 0.00

24.20 26.37 20.97 9.40 0.00

22.00 28.36 19.88 10.86 0.00

11.92 59.29 2.42 10.35 15.36

17.06 37.67 28.29 0.00 10.04

18.29 33.14 21.61 10.98 0.00

24.93 40.61 17.07 0.00 10.77

Below detection limit ( < ), not determined (n.d.), (Pd) = XRF, (Inn) = K2O determined by AAS, Sc determined by ICP-OES.

concentrations are consistent with a cumulate origin of their protoliths, because incompatible elements tend to reside in the intercumulus liquid. In contrast

with the quartz-rich eclogites, the kyanite-rich varieties always have a positive Sr anomaly in a PMnormalized trace element diagram (Fig. 5b). Kya-

Fig. 4. (a) Bulk composition of representative eclogite samples in an AFM diagram and (b) in the TiO2 vs. Al2O3 discrimination diagram after Pearce (1983). (c) Cr vs. Mg# and (d) Ni vs. Mg# variation diagrams. H-R (1982): Hinterlechner-Ravnik (1982).

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lation, suggesting crystal fractionation at low to moderate pressures ( P < 0.9 GPa; e.g., Bender et al., 1978). The LREE depleted nature and the collinear array of data for the Pohorje, Kor- and Saualpe quartz- and kyanite-rich eclogites on a Ce – Yb plot (Fig. 6a) with an intersection near the origin suggest that these rocks are genetically comparable and that garnet was not present in the source. In summary, the major and trace element characteristics of the kyanite-rich eclogites are similar to those of cumulate gabbros from midocean ridges (e.g., Meyer et al., 1989), and the chemical trends displayed by both eclogite types (Fig. 6b –d) suggest a mafic rock suite evolving through fractional crystallization of olivine, clinopyroxene and plagioclase.

5. Mineral chemistry

Fig. 5. (a) Primitive-Mantle normalized (Sun and McDonough, 1989) trace element patterns of Qtz-rich and (b) Ky-rich eclogites; (c) chondrite-normalized (Boynton, 1984) REE patterns of Qtz-rich and Ky-rich eclogites.

nite-rich eclogites also show LREE depleted patterns (LaN/YbN = 0.35 –0.53), however, with considerably lower absolute REE concentrations that do not exceed 10 times chondrite (Fig. 5c). In addition, the kyanite-rich eclogites have positive Eu anomalies with Eu/Eu* = 1.12 – 1.53. Both positive Eu and Sr anomalies are explained by plagioclase accumu-

Electron microprobe analyses were carried out at the Department of Mineralogy and Petrology, University of Padova (Cameca Camebax), and at the Institut fu¨r Mineralogie, University of Innsbruck (ARL-SMQII) using standard analytical procedures (accelerating voltage: 15 kV; beam current: 10 nA). Clinopyroxene: omphacitic clinopyroxene in quartz-rich eclogites contains 30 –38 mol.% jadeite along with 2– 4 mol.% aegirine calculated on the basis of Jd = AlVI/(Na + Ca) and Aug=(Na – AlVI)/ (Na + Ca), respectively. Omphacite in kyanite-rich eclogites has a lower jadeite content of 19 – 32 mol.% combined with higher XMg and very little Aug, reflecting the different bulk composition (Fig. 7). During retrogression, omphacite broke down to sodium-poor clinopyroxene with 5– 8 mol.% jadeite (Table 2). Garnet coexisting with omphacite (Fig. 7b) usually shows a flat compositional zoning with little variation on a thin section scale. However, garnet composition is strongly dependent upon whole rock composition: garnet from Qtz-rich eclogites has pyrope and grossular contents of 25 –36 and 22– 27 mol.% respectively, whereas garnet from Ky-rich eclogites is characterized by much higher pyrope and lower grossular contents of 39– 57 and 17 –22 mol.% respectively (Table 3, Figs. 7b– 9).

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243

Fig. 6. Trace element variation diagrams for the Pohorje eclogites. H-R (1982): Hinterlechner-Ravnik (1982).

Fig. 7. (a) Chemistry of clinopyroxenes from quartz- and kyanite-rich eclogites; secondary clinopyroxene is marked by (s) in the legend. (b) Tie lines connect Grt-Omp from the two rock types. Dashed line represents omphacite compositions from Hinterlechner-Ravnik et al. (1991a,b).

244

Table 2 Representative electron microprobe analyses of clinopyroxene Type

A (Qtz-E)

Sample 3-1

3-6d

B (Ky-E) H-8

H-8

CM16/01 CM20/01 H-17

3-6d

2-1

2-2a

2-4

CM22/01 CM25/01 CM46/01 RS1/01 2-2

CM22/01

PX37

PX35

PX50

OMPH4

S31

S1

PX13

PX39

PX3

Notes

Omp

Omp

Omp

Omp

Omp

Omp

Cpx2

Cpx2

Omp

Omp

Omp

Omp

Omp

Omp

Omp

Cpx2

Cpx2

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O Total

55.83 0.14 10.77 0.02 3.41 0.00 9.38 15.22 5.59 100.37

55.52 0.14 9.06 0.00 3.40 0.02 10.87 17.39 4.48 100.89

55.33 0.15 9.76 0.03 3.30 0.03 10.42 16.86 4.87 100.74

55.05 0.12 9.44 0.00 3.43 0.02 10.63 17.14 4.99 101.00

54.52 0.14 10.18 0.03 4.69 0.03 9.13 15.33 5.88 99.93

54.65 0.06 9.62 0.09 4.60 0.06 9.41 16.28 5.26 100.03

54.50 0.03 0.60 0.06 5.67 0.00 15.15 24.16 0.53 100.78

54.96 0.04 0.67 0.00 5.60 0.04 14.99 24.19 0.71 101.29

55.39 0.10 6.91 0.06 1.99 0.03 13.30 20.17 3.11 101.06

55.21 0.09 6.78 0.45 1.99 0.00 13.37 20.07 3.00 100.97

54.69 0.11 7.31 0.41 2.01 0.03 12.72 19.44 3.30 100.02

54.63 0.11 9.07 0.02 3.07 0.00 11.20 17.33 4.34 99.77

55.06 0.00 6.06 0.04 2.03 0.00 13.51 20.96 2.83 100.49

54.96 0.00 7.75 0.06 2.40 0.07 12.25 18.39 3.92 99.80

54.63 0.17 8.19 0.17 2.17 0.00 11.32 18.03 4.22 98.90

53.54 0.42 3.54 0.41 2.43 0.00 15.29 24.50 0.74 100.87

53.64 0.02 0.65 0.07 5.02 0.00 15.13 24.25 0.41 99.19

Cations per six oxygens Si 1.983 1.971 Ti 0.004 0.004 Al 0.451 0.379 Cr 0.001 0.000 Fe3 + 0.000 0.000 Fe2 + 0.101 0.101 Mn 0.000 0.001 Mg 0.497 0.575 Ca 0.579 0.661 Na 0.385 0.308 CAT 4.000 3.999 Jd 0.38 0.31 Ac 0.00 0.00 Aug 0.61 0.69

1.962 0.004 0.408 0.001 0.000 0.098 0.001 0.551 0.641 0.335 4.000 0.33 0.00 0.66

1.948 0.003 0.394 0.000 0.046 0.056 0.001 0.561 0.650 0.342 4.000 0.30 0.05 0.66

1.947 0.004 0.429 0.001 0.077 0.063 0.001 0.486 0.586 0.407 4.001 0.33 0.08 0.59

Fe3 + was estimated in order that Acat. = 4 and O = 6.

2.000 0.000 0.333 0.001 0.015 0.083 0.007 0.568 0.644 0.349 4.000 0.33 0.02 0.65

1.991 0.001 0.026 0.002 0.027 0.146 0.000 0.825 0.946 0.038 4.000 0.01 0.03 0.96

1.996 0.001 0.028 0.000 0.027 0.143 0.001 0.812 0.941 0.050 4.000 0.02 0.03 0.95

1.964 0.003 0.289 0.002 0.000 0.059 0.001 0.703 0.766 0.214 3.999 0.21 0.00 0.79

1.962 0.002 0.284 0.013 0.000 0.059 0.000 0.708 0.764 0.207 4.000 0.19 0.00 0.79

1.960 0.003 0.309 0.012 0.000 0.060 0.001 0.680 0.747 0.229 4.000 0.22 0.00 0.77

S20

1.957 0.003 0.383 0.001 0.000 0.092 0.000 0.598 0.665 0.301 4.000 0.30 0.00 0.70

1.966 0.000 0.255 0.001 0.007 0.053 0.000 0.719 0.802 0.196 4.000 0.19 0.01 0.80

1.967 0.000 0.327 0.002 0.010 0.062 0.002 0.653 0.705 0.272 4.000 0.26 0.01 0.73

1.971 0.005 0.348 0.005 0.000 0.065 0.000 0.609 0.697 0.295 3.995 0.29 0.00 0.71

1.932 0.011 0.150 0.012 0.003 0.070 0.000 0.823 0.947 0.052 4.000 0.04 0.00 0.95

1.987 0.001 0.028 0.002 0.025 0.131 0.000 0.835 0.962 0.029 4.000 0.00 0.03 0.97

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245

Table 3 Representative electron microprobe analyses of garnet Type

A (Qtz-E)

Sample 3-1a

3-1a

B (Ky-E) H-8

H-8

Point

GRT1 GRT11 GR18 GRT7

Notes

Rim

Rim

2-1a

2-1a

2-4

2-4

CM22/ CM25/ CM25/ CM42/ CM46/ RS1/ 01 01 01 01 01 01

GRT1 GRT3 GRT8 GR3 GR7

Core

Rim

Rim

Rim

Rim

Core Rim

SiO2 40.11 TiO2 0.02 22.11 Al2O3 Cr2O3 0.00 FeO 20.15 MnO 0.47 MgO 9.93 CaO 8.77 Total 101.58

39.77 39.95 0.00 0.05 22.00 22.53 0.00 0.02 20.19 19.91 0.51 0.36 9.72 9.74 8.68 9.35 100.88 101.91

39.88 0.06 21.78 0.01 18.59 0.35 10.03 9.74 100.44

39.14 0.00 21.95 0.00 22.53 0.52 6.64 9.57 100.35

39.07 0.00 21.90 0.00 20.90 0.59 7.62 9.45 99.53

40.71 0.00 22.65 0.22 13.92 0.41 14.86 7.14 99.92

41.32 41.16 0.10 0.03 22.70 22.71 0.05 0.06 12.05 12.47 0.31 0.23 16.22 15.67 7.21 7.81 99.95 100.13

40.53 0.04 22.60 0.08 13.21 0.17 15.29 7.34 99.24

41.01 0.01 22.62 0.33 13.02 0.22 14.84 7.80 99.85

40.34 41.36 41.34 41.03 0.00 0.00 0.00 0.00 22.75 23.26 23.34 23.11 0.00 0.04 0.03 0.00 17.27 12.57 12.31 14.76 0.24 0.26 0.37 0.62 11.63 15.41 14.57 13.87 7.95 7.27 8.42 7.16 100.18 100.17 100.38 100.55

40.37 0.00 22.49 0.03 15.16 0.30 13.19 7.65 99.19

40.66 0.06 23.03 0.15 15.14 0.38 12.92 8.05 100.39

Cations Si Ti Al Cr Fe3 + Fe2 + Mn Mg Ca CAT Alm Sps Grs Prp

oxygens 2.979 1.942 0.000 0.000 0.079 1.186 0.032 1.085 0.697 8.000 0.40 0.01 0.23 0.36

2.986 1.922 0.003 0.001 0.088 1.077 0.022 1.120 0.781 8.000 0.36 0.01 0.26 0.37

2.998 1.982 0.000 0.000 0.021 1.423 0.034 0.758 0.785 8.000 0.47 0.01 0.26 0.25

2.995 1.979 0.000 0.000 0.026 1.314 0.038 0.871 0.776 8.000 0.44 0.01 0.26 0.29

2.978 1.953 0.000 0.013 0.057 0.795 0.025 1.620 0.560 8.000 0.26 0.01 0.19 0.54

2.993 1.938 0.005 0.003 0.060 0.670 0.019 1.752 0.560 8.000 0.22 0.01 0.19 0.58

2.972 1.953 0.002 0.005 0.068 0.742 0.011 1.671 0.577 8.000 0.25 0.00 0.19 0.56

2.996 1.948 0.001 0.019 0.036 0.759 0.014 1.616 0.611 8.000 0.25 0.00 0.20 0.54

2.998 1.992 0.000 0.000 0.010 1.064 0.015 1.288 0.633 8.000 0.35 0.01 0.21 0.43

2.999 1.969 0.000 0.002 0.030 0.912 0.019 1.461 0.609 8.000 0.30 0.01 0.20 0.49

2.989 1.995 0.003 0.009 0.004 0.927 0.024 1.416 0.634 8.000 0.31 0.01 0.21 0.47

per 12 2.981 1.937 0.001 0.000 0.081 1.172 0.030 1.100 0.698 8.000 0.39 0.01 0.23 0.37

Core

CM16/ CM20/ 2-1a 01 01

2.958 1.966 0.003 0.001 0.072 1.161 0.023 1.075 0.742 8.000 0.39 0.01 0.25 0.36

Core

2.984 1.940 0.002 0.003 0.070 0.686 0.014 1.694 0.607 8.000 0.23 0.00 0.20 0.56

Rim

3.000 1.988 0.000 0.002 0.010 0.753 0.016 1.666 0.565 8.000 0.25 0.01 0.19 0.56

Core

3.000 1.996 0.000 0.002 0.002 0.745 0.023 1.576 0.655 7.999 0.25 0.01 0.22 0.53

Rim

2.998 1.990 0.000 0.000 0.012 0.890 0.038 1.511 0.561 8.000 0.30 0.01 0.19 0.50

Rim

Fe3 + was calculated assuming full octahedral site occupancy.

Kyanite contains Cr and Fe as main impurities with Cr2O3 = 0.03 – 0.32 wt.% and Fe2O3 = 0.18 – 0.39 wt.%. Zoisite/clinozoisite contains 11 –38 mol.% pistacite component in the Qtz-rich eclogites and 6– 12 mol.% in the ky-rich varieties. Small secondary zoisite crystals associated with Ca-rich plagioclase replacing primary zoisite in some Qtz-rich eclogites (Zo-II in Fig. 3c) are Fe-free (Table 4). Micas: Phengite was exclusively found in Qtzrich eclogites (Fig. 3g) and shows Si contents of 3.24 –3.31 apfu. A slight zoning may be present with higher Si contents preserved in the phengite cores. Ti content ranges between 0.59 and 0.82 wt.% TiO2. Paragonite and eastonite were only identified as inclusions in garnet from kyanite-rich eclogites. Paragonite is close to end member composition with

1– 2 mol.% muscovite and 1 –3 mol.% margarite components (Table 5). Amphiboles are calcic amphiboles according to Leake et al. (1997) and vary in composition depending upon the microstructural site (Table 6, Fig. 9). Amphiboles included in omphacite and garnet are magnesio-hornblende as well as texturally late poikiloblastic amphibole and amphibole within symplectites. Large pale-green secondary calcic amphibole is edenite and colourless amphibole at garnet rims is pargasite (Fig. 9). Plagioclase composition (Table 7) depends upon the local chemical environment: XAb in plagioclase within symplectites after omphacite and garnet ranges from 0.54 to 0.99. Within plagioclase + corundum symplectites after kyanite, plagioclase composition varies from XAb = 0.69 in the outermost portions of

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6. P – T estimates 6.1. Geothermobarometry

Fig. 8. Compositional profiles across garnet from quartz- and kyanite-rich eclogites.

the symplectite to XAb = 0.03 at the contact to kyanite. In addition, nearly pure anorthite (XAn = 98 – 99%) may be present as a breakdown product of primary zoisite (Fig. 3c).

Eclogitic stage: Peak metamorphic temperatures were estimated using the Fe –Mg exchange between garnet and omphacite based on the calibrations by Krogh Ravna (2000), Krogh (1988), Powell (1985) and Ellis and Green (1979). Assuming charge balance (four cations + six oxygens in Cpx formula) the following temperature ranges were obtained at 2 GPa: 602 – 769 jC (Krogh Ravna, 2000), 631 – 804 jC (Krogh, 1988), 650 – 812 jC (Powell, 1985) and 760 – 830 jC (Ellis and Green, 1979) (Table 8). It should be noted that the Krogh Ravna (2000) algorithm yields temperatures which are 42 – 122 jC lower than those obtained with the Powell (1985) calibration and agree well with oxygen isotope temperatures in the range 575– 606 jC reported by Tennie (1996) for Saualpe kyanite eclogites. In addition, the Fe– Mg exchange between garnet and phengite calibrated by

Fig. 9. Classification of amphiboles observed in the eclogites after Leake et al. (1997). Dashed contours denote compositions by HinterlechnerRavnik et al. (1991a,b).

R. Sassi et al. / Lithos 78 (2004) 235–261

247

Table 4 Representative electron microprobe analyses of zoisite – clinozoisite Type

A (Qtz-E)

B (Ky-E)

Sample 3-1

H-8

H-9

H-9

CM16/ CM20/ H-9 01 01

H-9

2-1

2-1

2-2

CM22/ CM23/ CM42/ CM48/ 01 01 01 01

Point

EP1

ZO1

H91

3.1

ZO19

ZO20

ZO3

ZO4

ZO9

1.6 core

1.40

2.24 core

1.15 core

Zo II

Zo II

44.91 0.00 34.84 0.00 0.07 0.04 0.00 20.85 0.40 0.02 100.72

44.81 0.01 35.12 0.04 0.06 0.00 0.00 20.99 0.46 0.02 101.03

39.52 0.02 31.83 0.02 1.12 0.00 0.04 25.00 0.00 0.01 97.55

39.42 0.04 31.53 0.05 1.08 0.00 0.07 25.03 0.00 0.00 97.21

39.67 0.10 32.29 0.31 0.86 0.05 0.11 25.30 0.00 0.00 98.69

39.39 0.00 32.52 0.00 1.32 0.00 0.03 24.47

39.40 0.00 32.56 0.09 1.20 0.00 0.00 24.51

39.33 0.25 33.03 0.00 0.60 0.00 0.00 24.56

39.26 0.10 31.94 0.00 1.84 0.00 0.02 24.39

97.73

97.76

97.77

97.55

ZO8

1.1

Notes SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Total

39.76 0.03 32.27 0.04 1.79 0.02 0.10 24.94 0.03 0.00 98.93

39.55 0.09 31.51 0.00 1.68 0.00 0.09 25.01 0.01 0.03 97.94

Cations per 25 equivalent Si 6.023 6.055 Ti 0.003 0.011 Al 5.761 5.686 Cr 0.004 0.000 Fe3 + 0.227 0.216 Mn 0.003 0.000 Mg 0.022 0.021 Ca 4.048 4.103 XFe 0.11 0.11

38.91 0.04 31.21 0.00 1.82 0.04 0.08 24.87 0.02 0.00 96.98 oxygens 6.028 0.005 5.698 0.000 0.236 0.006 0.018 4.128 0.12

40.60 0.01 31.06 0.05 1.65 0.05 0.09 26.27 0.03 0.00 99.77

6.120 0.001 5.517 0.006 0.207 0.006 0.019 4.242 0.12

38.58 0.13 28.64 0.12 5.44 0.00 0.08 23.93

38.68 0.00 28.61 0.15 5.95 0.00 0.09 24.06

96.92

97.54

6.001 0.015 5.252 0.015 0.707 0.000 0.019 3.988 0.36

5.987 0.000 5.221 0.018 0.770 0.000 0.021 3.991 0.38

6.458 0.000 5.905 0.000 0.009 0.004 0.000 3.213 0.00

Green and Hellman (1982) was used for additional T constraints yielding 724– 779 jC at a pressure of 2 GPa (Table 8). Paragonite, rarely trapped as inclusion in garnet, was postdated by the higher-pressure assemblage omphacite + kyanite, indicating a minimum pressure of 1.6 GPa, based on the equilibrium paragonite = jadeite + kyanite + quartz + H 2 O (Holland, 1979). Peak conditions therefore must have exceeded 1.6 GPa because garnet continued to grow far beyond entrapment. For phengite-bearing samples 3-1 and CM15/01, assuming a temperature of 650 jC, pressures of 1.69 and 1.85 GPa are obtained by means of the updated version of the Phe-Omph-Grt geobarometer (Waters and Martin, 1993, http://www.earth.ox.ac.uk/ ~davewa/research/ecbar.html), with the empirical corrections recommended by Carswell et al. (1997). P –T conditions for phengite-bearing quartz eclogite CM15/01 were also estimated using the TWQ program (Berman, 1991) in combination with the

6.427 0.001 5.936 0.005 0.008 0.000 0.000 3.226 0.00

6.056 0.003 5.748 0.003 0.143 0.000 0.010 4.104 0.08

6.064 0.005 5.717 0.005 0.139 0.000 0.016 4.125 0.07

6.011 0.011 5.767 0.037 0.109 0.006 0.026 4.108 0.06

5.990 0.000 5.819 0.004 0.194 0.000 0.007 3.992 0.10

5.997 0.000 5.843 0.011 0.152 0.000 0.000 3.997 0.08

5.977 0.027 5.918 0.000 0.077 0.000 0.000 3.999 0.04

5.999 0.011 5.753 0.000 0.235 0.000 0.005 3.993 0.12

thermodynamic data and phengite solution model by Massonne (1997). This approach yielded 600 jC and 1.78 GPa. As the sample also contains minor kyanite, a stable intersection at T = 622 jC and P = 2.31 GPa involving the following equilibria was calculated with THERMOCALC v3.1 (Powell et al., 1998), using activities obtained by the AX.01-program: 3 Di þ 2 Ky ¼ Prp þ Grs þ 2 Qtz;

ð1Þ

3 Ms þ 6 Di ¼ Prp þ 2 Grs þ 3 Cel;

ð2Þ

Grs þ 3 Cel þ 2 Ky ¼ 3 Ms þ 3 Di þ 2 Qtz;

ð3Þ

3 Cel þ 4 Ky ¼ Prp þ 3 Ms þ 4 Qtz:

ð4Þ

(Equilibria (1), (2) and (4) were also used by Krogh Ravna and Terry (2001) as a geothermobarometer for phengite –kyanite– quartz eclogites. When applied to

248

R. Sassi et al. / Lithos 78 (2004) 235–261

Table 5 Representative electron microprobe analyses of micas Type

A (Qtz-E)

Sample

3-1

3-1

3-1

3-1

3-1

3-1

3-1

3-1

CM15/01

CM42/01

B (Ky-E) CM42/01

Point

MS1

MS2

MS3

MS4

MS5

MS6

MS7

MS8

I

2.17

2.6

Matrix Phe

Incl Par

Incl East

49.45 0.82 28.96 0.00 2.05 0.00 2.81 0.00 0.37 11.15 95.61

46.50 0.12 39.49 0.00 0.49 0.00 0.22 0.48 7.58 0.17 95.05

36.70 0.00 22.66 0.00 6.29 0.04 19.97 0.27 2.08 7.39 95.40

Notes SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Total

49.59 0.73 29.90 0.00 1.16 0.03 3.23 0.00 0.97 9.70 95.31

Cations per Si Ti Al Cr Fe2 + Mn Mg Ca Na K CAT Fe + Mg

11 equivalent oxygens 3.284 3.293 0.036 0.035 2.334 2.307 0.000 0.001 0.064 0.078 0.002 0.002 0.319 0.319 0.000 0.000 0.124 0.121 0.819 0.847 6.984 7.002 0.38 0.40

48.81 0.69 29.01 0.02 1.37 0.04 3.17 0.00 0.92 9.84 93.88

48.58 0.60 28.83 0.00 1.04 0.00 3.11 0.00 0.51 10.58 93.26

3.303 0.031 2.310 0.000 0.059 0.000 0.316 0.000 0.067 0.918 7.003 0.37

48.31 0.70 29.84 0.04 1.11 0.00 2.97 0.00 0.49 10.51 93.97

3.260 0.035 2.373 0.002 0.062 0.000 0.299 0.000 0.064 0.904 7.001 0.36

48.64 0.59 29.00 0.00 1.23 0.00 3.15 0.00 0.54 10.20 93.34

3.299 0.030 2.318 0.000 0.070 0.000 0.318 0.000 0.072 0.883 6.989 0.39

Qtz-rich eclogite samples 3-1 and CM15/01, their formulation yields P – T conditions of 2.3 GPa at 710 jC and 2.4 GPa at 692 jC, respectively. Secondary stage: During decompression, omphacite is replaced by sodium-poor clinopyroxene F calcalcic amphibole and plagioclase. Using sodium-poor clinopyroxene and plagioclase compositions from sample CM20/01 and assuming temperatures of 550 – 600 jC for the symplectite stage, a pressure close to 0.8 GPa can be calculated using the TWQ program by Berman (1991). Further P – T constraints on the retrogressive evolution of kyanite-bearing eclogites can be placed using the breakdown reaction: 2 Ky þ 2 Zo ¼ Crn þ 4 An þ H2 O:

ð5Þ

Using mineral compositions from sample CM46/01, pressures < 0.6 – 0.7 GPa at 550 –600 jC are obtained

49.38 0.69 31.35 0.02 0.95 0.03 2.84 0.01 0.44 10.46 96.17

49.06 0.59 29.10 0.00 1.04 0.05 3.21 0.02 0.40 10.33 93.79

3.244 0.034 2.428 0.001 0.052 0.002 0.278 0.000 0.056 0.877 6.973 0.33

3.308 0.030 2.313 0.000 0.059 0.003 0.322 0.000 0.052 0.888 6.975 0.38

49.36 0.67 29.21 0.00 1.06 0.04 3.21 0.02 0.59 10.48 94.64

3.304 0.034 2.304 0.000 0.060 0.002 0.320 0.000 0.077 0.895 6.995 0.38

3.302 0.041 2.280 0.000 0.114 0.000 0.280 0.000 0.048 0.950 7.015 0.394

2.979 0.006 2.982 0.000 0.026 0.000 0.021 0.033 0.941 0.014 7.002 0.047

2.596 0.000 1.889 0.000 0.372 0.002 2.105 0.020 0.285 0.667 7.936 2.477

with the TWQ program (Berman, 1991) for equilibrium (5). For plagioclase – Ca – amphibole assemblages appearing in retrogressive domains associated with cracks, temperatures of 520– 560 jC at 0.2– 0.6 GPa are obtained, using the geothermobarometer devised by Plyusnina (1982). 6.2. P – T pseudosections P – T pseudosections for both quartz-rich eclogites [sample 3-1 (Fig. 10a)] and kyanite-rich eclogites [sample 2-1 (Fig. 10b)] were calculated using the Vertex computational approach of Connolly (1990) on the basis of the bulk rock composition (Fig. 10a and b). For calculations involving kyanite-rich eclogites, a NCFMASH system was used with the following phases (database of Holland and Powell, 1998) and solution models (in brackets): garnet (Holland and Powell, 1998), clinopyroxene (Gasparik,

Table 6 Representative electron microprobe analyses of amphiboles A (Qtz-E)

Sample

H-9

H-9

H-17

B (Ky-E)

Point

A10

A20

A31

Notes

In sym (pale green)

In sym (green)

Large (pale green)

Large (pale green)

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2 O Total

48.29 0.26 7.55 0.09 12.52 0.24 13.71 12.37 1.04 0.19 96.27

45.35 0.48 10.64 0.10 13.72 0.27 12.46 12.18 1.51 0.24 96.95

46.96 0.37 12.18 0.06 10.48 0.11 15.61 9.86 2.15 0.20 98.00

47.55 0.52 13.28 0.00 6.81 0.26 15.59 10.37 3.43 0.08 97.89

Cations per 23 equivalent oxygens Si 7.123 6.717 Ti 0.029 0.053 Al 1.312 1.857 Cr 0.011 0.012 Fe2 + 1.545 1.700 Mn 0.030 0.034 Mg 3.016 2.752 Ca 1.955 1.933 Na 0.298 0.433 K 0.036 0.045

6.709 0.040 2.051 0.007 1.252 0.014 3.324 1.510 0.595 0.036

CM23/01

6.661 0.059 2.189 0.000 0.800 0.034 3.258 1.558 0.935 0.017

2-1

2-1

2-1b

2-1b

2-1b

2-1b

A13

A14

A5

A5

AM1

AM2

In Omph (colourless)

In Omph (colourless)

Large (colourless)

Large (colourless)

Rim on Grt (colourless)

53.74 0.15 7.38 0.00 2.95 0.01 20.50 11.49 1.71 0.35 98.28

53.25 0.13 7.34 0.01 3.03 0.00 20.68 11.37 1.73 0.26 97.81

52.28 0.12 7.50 0.05 5.30 0.06 18.85 12.15 0.83 0.11 97.24

51.75 0.10 8.23 0.01 4.64 0.09 19.73 11.61 0.96 0.16 97.28

41.06 0.06 20.81 0.00 7.81 0.09 12.40 10.62 2.27 0.02 95.14

7.359 0.015 1.190 0.000 0.338 0.002 4.185 1.685 0.453 0.062

7.331 0.014 1.191 0.001 0.349 0.000 4.244 1.676 0.462 0.046

7.309 0.012 1.236 0.005 0.620 0.007 3.928 1.820 0.225 0.020

7.211 0.010 1.351 0.001 0.541 0.011 4.099 1.734 0.258 0.029

5.989 0.006 3.578 0.000 0.953 0.011 2.696 1.659 0.642 0.005

CM42/01

CM23/01

Rim on Grt (colourless)

In Grt (colourless)

In Grt (colourless)

41.35 0.02 19.77 0.00 8.03 0.03 13.20 11.45 2.30 0.08 96.23

42.60 0.08 17.14 0.37 9.96 0.14 13.36 10.85 3.13 0.21 97.84

40.15 0.00 19.12 0.10 13.06 0.11 11.06 9.75 3.88 0.11 97.34

5.995 0.002 3.377 0.000 0.974 0.003 2.854 1.779 0.647 0.015

6.052 0.009 2.868 0.043 1.186 0.017 2.825 1.647 0.862 0.034

5.785 0.000 3.248 0.009 1.576 0.017 2.373 1.507 1.083 0.017

R. Sassi et al. / Lithos 78 (2004) 235–261

Type

249

250

Table 7 Representative electron microprobe analyses of plagioclase Type

A (Qtz-E)

B (Ky-E)

Sample 3-1B-1

3-6d-0

H-8

Point

S8

Pl01

S34

Notes

Sym Sym Sym Sym Zo 2 + Pl Ky = Pl + Crn Ky = Pl + Crn Sym Pl + Crn Sym Pl + Crn Ky-Sympl Sym Pl + Crn Sym (Amph + Pl) (Amph + Pl) (Amph + Pl) (Amph + Pl) (Ep rim) (Ep rim) (Ky rim) (Ky rim) (Ky rim) (Ky ext rim) (Amph + Pl)

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2 O Total

61.94 0.03 15.46 0.03 1.67 0.05 5.19 9.92 6.44 0.00 100.74

Cations for eight oxygens Si 2.795 3.013 Ti 0.000 0.000 Al 0.823 1.005 Cr 0.000 0.000 Fe3 + 0.063 0.000 Mn 0.003 0.000 Mg 0.350 0.000 Ca 0.480 0.005 Na 0.563 0.915 K 0.000 0.008 CAT 5.075 4.945 Ab 54.00 98.68 Or 0.00 0.68 An 46.00 0.64

64.85 0.04 20.20 0.00 0.59 0.00 1.19 4.92 9.34 0.07 101.19

2.850 0.003 1.045 0.000 0.023 0.000 0.078 0.233 0.795 0.003 5.028 77.19 0.35 22.45

62.84 0.00 23.07 0.00 0.46 0.00 0.16 4.94 8.76 0.00 100.23

2.776 0.000 1.202 0.000 0.017 0.000 0.011 0.234 0.750 0.000 4.990 76.22 0.00 23.78

H-10

H-9-2

H-9-3

2-1b-1

2-1b-1

ZO7

PL11

PL14

PL2

PL3

43.25 0.04 35.66 0.00 0.00 0.00 0.00 20.71 0.11 0.01 99.79

2.013 0.000 1.955 0.000 0.000 0.000 0.000 1.033 0.010 0.000 5.013 0.98 0.08 98.94

54.28 0.01 29.03 0.08 0.09 0.00 0.01 11.76 5.17 0.06 100.49

2.443 0.000 1.540 0.003 0.003 0.000 0.000 0.568 0.450 0.003 5.013 44.15 0.32 55.53

45.44 0.00 34.63 0.00 0.06 0.00 0.00 18.36 1.12 0.01 99.64

2.103 0.000 1.888 0.000 0.003 0.000 0.000 0.910 0.100 0.000 5.005 9.96 0.08 89.96

45.41 0.02 34.79 0.00 0.06 0.00 0.00 19.16 0.77 0.02 100.21

2.093 0.000 1.888 0.000 0.003 0.000 0.000 0.945 0.068 0.000 4.998 6.75 0.10 93.15

46.93 0.00 34.15 0.00 0.02 0.00 0.00 17.97 1.36 0.00 100.43

2.148 0.000 1.843 0.000 0.000 0.000 0.000 0.880 0.120 0.000 4.993 12.01 0.00 87.99

CM46/01

2-1b-1

CM46/01

PL4

4 3.61 0.00 36.17 0.19 0.27 0.00 0.00 19.68 0.31 0.00 100.23

2.015 0.000 1.970 0.007 0.010 0.000 0.000 0.974 0.028 0.000 5.004 2.79 0.00 97.21

58.61 0.01 26.19 0.00 0.05 0.00 0.00 8.60 6.82 0.00 100.28

2.613 0.000 1.375 0.000 0.003 0.000 0.000 0.410 0.590 0.000 4.993 58.94 0.00 41.06

61.00 0.00 24.44 0.00 0.23 0.00 0.08 6.07 8.18 0.00 100.00

2.710 0.000 1.280 0.000 0.009 0.000 0.005 0.289 0.705 0.000 4.998 70.93 0.00 29.07

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70.57 0.00 19.97 0.00 0.00 0.00 0.00 0.13 11.04 0.12 101.82

CM20/01

R. Sassi et al. / Lithos 78 (2004) 235–261

251

Table 8 Summary of the geothermobarometric estimates Rock

Sample

Eclogitic stage

Amph. stage

T (jC) (at 2.00 GPa)

P (GPa) (at 650jC)

Cpx – Grt

Qtz-rich Qtz-rich Qtz-rich Qtz-rich Qtz-rich Qtz-rich Qtz-rich Qtz-rich Qtz-rich Qtz-rich Qtz-rich Qtz-rich Qtz-rich Qtz-rich Qtz-rich Qtz-rich Qtz-rich Qtz-rich Qtz-rich Ky-rich Ky-rich Ky-rich Ky-rich Ky-rich Ky-rich Ky-rich Ky-rich Ky-rich Ky-rich Ky-rich Ky-rich Ky-rich Ky-rich Ky-rich

3-1a 3-1a 3-1a 3-1a 3-1a 3-6 H-8 H-8 H-9 H-9 H-9 H-9 H-9 H-9 CM15/01 CM16/01 CM16/01 CM20/01 SKP2 2-1 2-2a 2-2a 2-2 2-4 2-4 2-4 CM22/01 CM23/01 CM25/01 CM42/01 CM46/01 CM48/01 RS01/01 RS03/01

T (jC)

Grt – Phe

Phe – Omph – Grt

Pl – Hbl

E and G 79

P 85

K 88

KR 00

G and E 82

W and M 93

P 82

829

810

791

768

733 779 740 724 745

1.57 1.69 1.58 1.69 1.36

817 807 830

798 789 812

782 776 804

748 743 769 520 550 530 520 520 560

764 784 771 789 760 789 798

650 676

631 658

721 701 742 763 750 768 739 769 778 720 736 751

705 681 703 732 718 741 707 735 753 693 702 720

730 729 755 714

695 700 731 679

602 621 623 662 635 650 671 657 682 621 679 689 649 614 636 639 626 621 653 613

P (GPa)

0.25 0.20 0.20 0.60 0.40 0.45

1.63

E and G 79 (Ellis and Green, 1979); P 85 (Powell, 1985); K 88 (Krogh, 1988); KR 00 (Krogh Ravna, 2000); G and E 82 (Green and Hellman, 1982); W and M 93 (Waters and Martin, 1993); P 82 (Plyusnina, 1982).

1985, modified Connolly, pers. comm.), amphibole (Powell and Holland, 1999), plagioclase (ideal), kyanite, zoisite, chloritoid (Holland and Powell, 1998), lawsonite, quartz, H2O. For the quartz-rich eclogites, phengite (nonideal hybrid solution model for K –Na micas by Chatterjee and Froese, 1975, with ideal celadonite substitution) and biotite (Holland and Powell, 1998) were included in the calculations. The bulk water content was assumed to be equal to the loss on ignition (LOI).

The peak metamorphic assemblage for quartz-rich eclogites is Grt + Omp + Phe + Qtz F Ky (Fig. 10a) over a wide temperature range between 520 and 740 jC and at pressures >1.7 GPa, consistent with the estimates based on Krogh Ravna and Terry (2001). Ca-amphibole is not stable at peak metamorphic conditions consistent with textural observations from quartz-rich eclogite. For kyanite-rich eclogites, the peak metamorphic assemblage is Grt + Omp + Ky + Zo F Cam F Qtz

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(Fig. 10b). The presence in the peak assemblage of Ca-amphibole is consistent with the occurrence of amphibole in textural equilibrium with garnet and omphacite. The P –T stability field of the eclogitic assemblage is limited at low pressure by the plagio-

clase-in reaction (1.4 GPa at 650 jC in Fig. 10b), at high pressure by the amphibole-out reaction (2.5 GPa at 650 jC) and at low temperature by the breakdown reactions of chloritoid that was never observed in these rocks.

Fig. 10. (a) P – T grid for Qtz-rich eclogite 3-1 and (b) for Ky-rich eclogite 2-1 calculated with the Vertex software package (Connolly, 1990). Numbered reactions in panel (a): (1) Amph-in, (2) Pl-in, (3) Bt-in. Numbered reactions in panel (b): (1) Pl-in, (2) Zo-out, (3) Cpx-out. See text for discussion.

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253

Fig. 10 (continued).

In addition to the Vertex approach, pseudosections in the KNCFMASH system were calculated with DOMINO (De Capitani, 1994; database of Berman, 1988); and the following solid solution models: garnet (Berman, 1990), phengite (Massonne and Szpurka, 1997), omphacite and amphibole (Meyre et al., 1997), assuming a H2O content equal to loss

on ignition. The peak metamorphic assemblage Grt + Omp + Phe + Ky + Qtz observed in quartz-rich eclogites [samples 3-1 (Fig. 11a) and CM15/01 (Fig. 11b)] is stable at temperatures above approximately 590 jC at pressures above 1.9 GPa. The absence of Cam and Zo/Czo in the peak metamorphic assemblage is consistent with textural observa-

254

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tions indicating a relatively late and postkinematic formation of these two phases. It should be noted that the peak metamorphic assemblage in sample CM 15/01 shows a more restricted PT-field compared to sample 3-1 due to the absence of kyanite at T>620 –700 jC and P < 2.5 GPa (Fig. 11b).

Kyanite-rich eclogites (samples 2-1 and CM42/01) show a PT-field for the assemblage Grt + Omp + Ky + Zo + Cam that is large for sample 2-1 (Fig. 11c) and very restricted for sample CM42/01 covering a PT interval of only of 600– 680 jC at pressures between 2.0 and 2.2 GPa (Fig. 11d).

Fig. 11. (a) P – T grid for Qtz-rich eclogites 3-1 and CM15/01 (b), and for Ky-rich eclogites 2-1 (c) and CM42/01 (d), calculated with the DOMINO software package (De Capitani, 1994).

R. Sassi et al. / Lithos 78 (2004) 235–261

255

Fig. 11 (continued).

It should be noted that garnet compositions calculated with DOMINO markedly differ from the analysed values: at 640 jC and 2.1 GPa, the calculated grossular component of 31 mol.% of bulk composition CM42/01 is distinctly higher than the 18 mol.% measured. Likewise, the calculated pyrope

component of 30 mol.% is significantly lower than the analysed value of 51 mol.%. For quartz-rich eclogites, the differences are far less pronounced with 26 mol.% (calc.) –22 mol.% (meas.) grossular and 25 mol.% (calc.) –28 mol.% (meas.) pyrope. In contrast, calculated and analysed jadeite contents of

256

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Fig. 11 (continued).

clinopyroxene in both rock-types are very similar: 39 (calc.) vs. 40 (meas.) mol.% in quartz-rich eclogites; 21 (calc.) vs. 22 (meas.) mol.% for kyanite-rich eclogites. These inconsistencies indicate that the thermodynamic data and/or solution models used by DOMINO are not yet sufficiently sophisti-

cated to accurately model complex natural chemical systems. A comparison between pseudosections calculated with VERTEX and DOMINO clearly indicates P – T conditions within the quartz stability field, thus limiting pressures to < 2.5 – 2.8 GPa. For quartz-rich eclo-

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257

Fig. 11 (continued).

gite 3-1 there is a fair agreement in the P – T stability fields of the observed peak assemblages (Figs. 10a and 11a). In case of the Ky-rich eclogites, both computational approaches predict an absence of quartz at approximately 2 GPa. This would be consistent with the absence of quartz from samples 2-1 and CM42/01.

7. Discussion and conclusions The whole-rock geochemical data define a Feand Ti-enrichment trend indicating a tholeiitic magma for the Pohorje eclogite precursor rocks and a simple differentiation trend, apparently controlled

258

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by olivine, clinopyroxene and plagioclase fractionation/accumulation at intermediate to shallow depths. The Ky-rich eclogites are interpreted as plagioclase-rich gabbroic cumulates recrystallized during HP metamorphism, whereas the Qtz-rich eclogites may represent metabasaltic compositions. Both eclogite varieties have N-MORB characteristics similar to those of the type-locality Koralpe and Saualpe eclogites and gabbroic precursor rocks (Miller and Tho¨ni, 1997; Miller et al., 1988; Tho¨ni and Jagoutz, 1992), indicative of partial melting of an asthenospheric, depleted mantle source. However, the major and trace element composition of the eclogites does not necessarily imply an origin of their protoliths at a midocean spreading ridge. MORB-type magmas could also be generated in embryonic narrow ocean and back-arc basins or even in continental rift systems (e.g., Desmurs et al., 2002). A transitional continental– oceanic rift system could perhaps best explain the association with metapelites. The range of temperature and pressure estimates obtained using the approaches discussed above shows that it is difficult to precisely constrain peak metamorphic conditions of the Pohorje kyanite- and quartz-eclogites. Nevertheless, an important outcome of this study is the consistency of pressure in the range 1.8 – 2.5 GPa and temperature in the range 640 – 740 jC derived from various approaches based both on conventional methods and multiequilibrium approaches. This is important in view of the recent suggestion by Janak et al. (2003) that the Pohorje eclogites equilibrated in the coesite stability field at pressures of approximately 3.3 – 3.6 GPa and 760 – 870 jC. This assumption was based on the Krogh Ravna and Terry (2001) thermobarometric approach and the presence of quartz inclusions with radial cracks in garnet and omphacite. An application of the Krogh Ravna and Terry (2001) approach to samples of the present study (see above), however, yielded pressures at least 1 GPa below those derived by Janak et al. (2003) in agreement with all other barometers applied. In addition, radial cracks around quartz may result from the delation of a-quartz without a phase transformation (cf. Whitney et al., 2000; Klemd, 2003). Therefore, mineral assemblages and mineral compositions of the Pohorje eclogites are

consistent with subduction to a depth not exceeding approximately 60 km. Only the identification of coesite would require a revision of this pressure estimate. The replacement of omphacite, kyanite and phengite by complex symplectites essentially reflects the instability of these phases during decompression. Breakdown of kyanite to spinel + plagioclase F sapphirine bearing assemblages is often interpreted as evidence for high-temperature granulite facies conditions during retrogression of kyanite eclogites (Mo¨ller, 1999; Elvevold and Gilotti, 2000). However, these assemblages often form in microchemical domains and therefore represent local equilibria that cannot be used to infer granulite facies temperature conditions (700 –800 jC), especially when evidence for granulite-facies metamorphism is lacking in the country rocks. This is also consistent with results obtained by Simon and Chopin (2001) and Nakamura (2002). The results of this study show that Koralpe, Saualpe and Pohorje rocks represent former MORB-type mafic crust that was subducted in the course of the complex collision between the European and the Apulian plates. As far as the timing of subduction and HP metamorphism is concerned, a common history of the Koralpe – Saualpe – Pohorje crustal segment is indicated by Cretaceous Sm –Nd mineral isochron ages in the range 108– 91 Ma for Koralpe – Saualpe eclogites and metapelites (Tho¨ni and Jagoutz, 1992; Tho¨ni and Miller, 1996, Miller and Tho¨ni, 1997) and by more recent Sm – Nd mineral ages on Pohorje metapelites of 93 –87 Ma (Tho¨ni, 2002), the latter representing the eclogite country rocks.

Acknowledgements We are indebted to T. Dolenec and A. Hinterlechner-Ravnik (University of Lubljana) for advice with the field-work, to J.A.D. Connolly (ETH Zu¨rich) for help in using the Vertex program, and to G. Godard and an anonymous referee for their critical reading and suggestions. Discussions with F.P. Sassi and G. Hoschek are gratefully acknowledged. Financial support was provided by 2001 ex-60% MIUR funds and by the University of Innsbruck.

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