Gelu Costin 1* , Elena Negulescu 2 , Gavril Sabau 2 , Peter Luffi 3 , René-Pierre Menot 4 1 Department of Geology, Rhodes University, South Africa; 2 Geological Institute of Romania, 3 Department of Mineralogy, University of Bucharest; 4 Department of Earth Sciences, “Jean Monnet” University *Corresponding author e-mail: g.costin@ru.ac.za Cr-Mg-staurolite: highest Cr 2 O 3 content in staurolite to date; example from quartz-free eclogites from Iezer Mts., South Carpathians, Romania Abstract Cr-Mg-staurolite occurs together with other Cr-rich phases (Cr-kyanite, Cr-phengite) in fine aggregates as rare and disperse mm-size, grass-green spots, within a fine grained eclogite near Leresti (Iezer Mountains, South Carpathians, Romania). The eclogitic bodies occur as flat lenses of as 0.3-1 m, parallel with the foliation of host metapelitic schists. The eclogites are quartz-free and show lack of deformation, having a granoblastic texture, with small (2-3 mm) garnets (Prp 41-56 Alm 24-36 Gross 15-23 Spess 0.5-1 ) floating in a mass of omphacite (Jd 20-45 ) and Ca-amphiboles. The Cr-aggregates developed as a fine greenish corona around a central ragged core represented by a relict chromite (Fe 2+ 0.71 Mg 0.16 Zn 0.13 ) ∑=1 (Cr 1.65 Al 0.35 ) ∑=2 O 4 . In few such aggregates, the BSE images show Cr-Mg-staurolite, Cr-kyanite (7-10.5% Cr 2 O 3 ) and rutile as replacement textures after chromite. Both Cr-Mg-staurolite and Cr-kyanite develop on chromite, invading it, sometimes as distinct lamellae in spinel, suggesting preferential replacement along distinct crystallographic planes or along former exsolution lamellae of the magmatic spinel. Cr-phengite (5.75% Cr 2 O 3 ) occurs in the external part of the Cr-rich aggregates. Within the Cr-aggregates and/or at the exterior of these, little grains of rutile are present. The amphibole and omphacite neighbours to Cr-rich aggregates are also anomalously rich in Cr. The Cr-Mg-staurolite studied here represents the staurolite with the highest Cr 2 O 3 content reported to date. The composition of Cr-Mg-staurolite (average of 10 EPMA analyses) show (wt%): SiO 2 =28.672, TiO 2 =0.535, Al 2 O 3 =46.710, Cr 2 O 3 =10.259, FeO=5.598, MnO=0.016, MgO=4.895, ZnO=1.259, Total=97.928. The Cr 2 O 3 range is 7.93-12.04 wt%. The X Mg =(Mg/(Mg+Fe 2+ ) range from is 0.583-0.706 with an average of 0.61, showing that it represents a high-pressure Cr-Mg-staurolite. The structural formulae of Cr-Mg-staurolite, when normalized to 23.5 Oxygen is: (Mg 1.034 Fe 2+ 0.663 Zn 0.135 □ 0.167 ) ∑=2.000 (Al 7.802 Cr 1.149 Ti 0.057 ) ∑=9.008 Si 4 O 23 (OH). The Cr-Mg-staurolite, together with Cr-kyanite and Cr-phengite resulted from high pressure breakdown of magmatic chromite (or possible a reaction chromite + opx), during (Variscan?) high-pressure metamorphism of an ultramafic or picritic protolith. Quartz-free eclogites, containg pyrope-garnet, Cr-omphacite, Cr- phengite, Cr-Mg-staurolite, Cr- kyanite Geological sketch of the Iezer-Leaota Mts. Introduction Up to date, the maximum content of Cr 2 O 3 in staurolite was reported by Ibarguchi & Mendia (1991) to be 6.43 wt. % in the metamorphosed ultrabasic rocks from Cabo Ortegal (Spain), while Nicollet (1986) reported staurolites with 2.2 wt. % Cr 2 O 3 in the metatroctolites from Madagascar. The highest measured values of Cr 2 O 3 in kyanite from eclogites in the South Carpathians were made by Negulescu & Sabau (1998) who found Cr 2 O 3 =11.71% within a Cr-eclogite from Fagaras Mts. Our measurements on kyanite from Leresti eclogite show a maximum value of 12.4 % Cr 2 O 3 which is the highest Cr 2 O 3 content in staurolite to date. Geological setting The main alpine structure of the South Carpathians is represented by a middle-Cretaceous to upper-Cretaceous nappe pile. It consists of four major Alpine units, which are, from top to base of the pile: (a) the Supragetic nappe, (b) the Getic nappe, (c) the Severin nappe and (d) the Danubian autochtonous unit (Fig. 1). The Iezer and Leaota Massifs represent the Eastern-most Getic unit which is overtrusted from NW to SE by the nappe system of the Supragetic unit from Fagaraş Massif. The basement of Iezer and Leaota Massifs (Fig. 2) consists of three main lithological units (from top to base of the pile): a) Călusu unit (Ordovician - datation based on palinological studies); it consists of fine grained granofels and schists showing a metamorphism within greenschists facies conditions (Iancu, 1999); b) Lereşti unit (Devonian – datation based on palinological studies). It consists of rocks metamorphosed within lower amphibolite facies conditions (Iancu, 1999). The most widespread rock type is the porphyroblastic, albite-mica-schists. C) Voineşti unit (proterozoic basement); it consists of various types of gneisses, (garnet bearing) amphibolites and micashists.A typical rock association is considered to be the augen gneiss and amphibolites (Iancu, 1999). Between the Lereşti and Voineşti units, a discontinous but relatively constant lithological level crops out. It was called “the Bughea level” or “the basal amphibolite” by Gherasi (1956). Iancu (1998) interpreted it as a shear zone and named it as “Bughea Shear Zone” (BSZ). BSZ mainly consists of amphibolites, metagranites (Albesti granite, cf. Dimitrescu, 1962), eclogites, micaschists, quartzites and Ms-Bt-Pl-gnaisses. It is to be noticed that all occurrences of Albesti metagranites and eclogites from Iezer and Leaota Massifs characteristically belong to BSZ Analytical techniques This study is based on microprobe analyses performed at “Blais Pascal” University of Clermont-Ferrand (5 spectrometers, 15 kV acceleration voltage, an electronic beam of 1 micron diameter at 20 seconds counting time) and at Rhodes University Grahamstown, South Africa, using a Jeol JXA 8230 Superprobe, with 4 WD spectrometers. Counting time 10 sec per peak and 5 sec per each background, except for Zn in staurolite where 30 sec per peak and 15 sec per background were used. Natural standards were employed for quant- ification using a ZAF correction matrix. For the hydrated phases a defocused beam of 20 microns was used in order to minimize loss of volatile components. Outcrop description of Cr- eclogites from Leresti Non-deformed and deformed granites (Albesti- type granites) crop out together with eclogites on the left side of the Raul Targului valley, at about 1 km North from Leresti locality and 100 m upstream from the confluence with the Dobriasu creek. Four eclogitic bodies occur as flat lenses parallel to the metamorphic foliation developed into the deformed granite and metapelites. Figure 1. Structural map of the South Carpathians with the localization of the studied area Figure 2. Detail of the Iezer-Leaota structural sketch after Iancu et al. (1999), with the ocurences of (quartz-bearing) eclogites and quartz-free eclogites from Leresti area. Figure 4. Scanned thin section with the contact between non-deformed eclogites and deformed granite (Albesti type granite); lengts of the image is 5 cm. Figure 5. Microphotograph showing a sub-microscopic opaque aggregate surrounded by Cr-bearing silicates. Maxim size of the dark aggregate is ~ 1 mm. Figure 6. Correlations between various elements, showing A) non-ideal substitution of Cr – Al due to presence of Ti and possible Fe3+; B) Influence of measuring Zn (and Mn) on the analyses: 1= analyses from Ibarguchi and Mendia (1991) without Zn and 2= analyses from Leresti Cr-Mg-staurolite, with measured Zn and Mn. Mineralogy of Cr-eclogites The eclogites are quartz-free and the mineral assemblage consists of garnet (Prp 41-56 Alm 24-36 Gross 15-23 Spess 0.5-1 ) and omphacite (Jd 20-45 ), all in a widespread matrix of amphibole. They also contain small amounts of Cr-phases grouped in mm-sized nests. Toward the margins of the eclogitic bodies the diopside-plagioclase sympectites represent the widespread matrix in which fine idioblastic garnets floats. Here, neoformation of phlogopite-rich biotite is common. The eclogitic bodies has no amphibole mantle as usually develops between eclogites and schists as a retrogression reaction to amfibolite facies conditions. The massive, non-deformed eclogites are in direct contact with the deformed granite. Secondary chlorite, albite, epidote and/or calcite could develop, especially as filling cracks phases. There is a continuous transition from massive granite to deformed granite and then to muscovite-biotite-plagioclase schists (granitic ultramilonites or meta-pelites) as we get closer to the eclogitic bodies. The deformed granites and schists preserve obvious relicts of K-feldspar megacrystals (or pseudomorphs after them) as well as polygranular aggregates of grey-bluish quartz from the non-deformed granite. The thickness range of the eclogitic flat lenses is 0.4-1.5 m. The eclogites are very fine- to fine-grained non-deformed rocks. They have a granoblastic structure and the grain size decrease from the core of the lenses toward the contact with the highest-strained granite (Ms-Bt-Pl schists). The thinner bodies show a higher degree of retrogression to amphibolite facies conditions. In fresh samples, the eclogites present a grey to pale-brown-greenish colour. Fine secondary black and light-grey veinlets randomly crosscut the whole (and only) eclogitic bodies. The Cr-phases are all grouped in very rare and disperse 0.2-1 mm-sized green spots. At the contact with the deformed granite, the eclogites contains non-oriented small flakes of phlogopite-rich biotite Figure 3. A) Aggregate of Cr-staurolite (st) and Cr-spinel (sp) in a mass of omphacite (cpx). In between the omphacite and Cr-aggregate Cr-rich phengite developed (BSE image). B) Relict Opx and talc (tlc); Note: Cr-free ky lamellae on relict opx (En80); (BSE image); C) Zoned Cr-kyanite (7-10.5% Cr 2 O 3 ) with Cr-spinel inclusions, near a Cr-Mg-staurolite (Cr-Mg-st) (BSE image); D) Needles of Cr-kyanite in Cr-spinel (SE image) Cr-ky Cr-Mg- st sp Cr-ky Ky Sp Cr-phe phe st st phe cpx sp tlc grt opx ky grt cp x cpx A B C D Cr-phases in BSE and SE images Microscopically, these nests appear as an opaque ragged core (Cr-spinel) with a fine greenish corona (Cr-Mg-staurolite). In few such Cr-aggregates, the SEM images show graphic-like intergrowth between Cr-spinel and Cr-Mg-staurolite. In the external part of the corona Cr-kyanite and Cr-phengite could occur. Within the Cr-nests and/or at the exterior of these, little crystals of rutile are present. The amphibole and omphacite neighbours of the Cr-rich aggregates are also anomalous rich in Cr. Cr-Mg-staurolite The Cr-Mg-staurolite described herein shows the highest Cr 2 O 3 content compared to other natural Cr-staurolites ever quoted wide world. For these reasons, the mineralogy of Cr-Mg-staurolite from Leresti is discussed by comparison with the Mg-staurolite and Cr-staurolite described in the high pressure ultrabasic rocks from Cabo Ortegal (Spain) by Ibarguchi & Mendia (1991). The-Cr-Mg-staurolite in Leresti eclogites always appears together with the Cr-spinel and rutile within Cr-phases micro-aggregates. Two types of relationships between Cr-Mg-staurolite and Cr-spinel were observed: a) Cr- spinel as relict and corodated-like by Cr-Mg-staurolite and b) intimate crystallographic intergrowths. At the exterior of the Cr-Mg-staurolite and Cr-spinel aggregates, Cr- phengite is common. It is to be mentioned that phengite was observed only within these microtextural circumstances. Within few aggregates, phengite seems to replace the staurolite. The composition of Cr-Mg-staurolite is relatively constant, with the 7.85- 12.4 wt. % range of Cr 2 O 3 (Table 1). The structural formula of Cr-Mg-staurolite was calculated for 23.5 oxygens. The X Mg =(Mg/(Mg+Fe2) range in staurolite from Leresti is 0.583-0.706 with an average of 0.61. This proves that the staurolite from Leresti is a high-pressure Mg-staurolite. As Ibarguchi and Mendia (1991) noticed for the Cr-Mg staurolite from the Cabo Ortegal (Spain), the aluminium substitution by chromium within the octahedral sites in staurolite is revealed by a good correlation between the two elements, as seen in figure 6. Even so, the relationship between the Cr (cation proportions) and Cr occupancy proves no ideal substitution (Cr:Al=1:1) between Cr and Al (Fig. 6A). This non-ideal substitution could have the following causes: • the substitution of Al by Ti within the octahedral positions which imply the deformation of the octahedrons; • different oxidation state of Cr during the formation of staurolite (less probable); • the substitution of Al by Fe 3+ ; even the Fe 3+ is not common in staurolites, this possibility is to be considered due to the difficulties of measuring Fe 3+ ; No correlation was observed beween Cr and X Mg . As the Zn cation occupies the same structural positions as Mg and Fe 2+ , we consider that the estimation of X Mg is not always simple, due to the Zn content of staurolites. It is to mention here that we performed analyses with and without measured ZnO. The ZnO content in Leresti eclogites is up to 1.45 wt. %, whereas, within the high-pressure ultrabasic rocks from Cabo Ortegal, the ZnO was apparently not measured or is absent. As seen in figure 13, the X Mg* (Mg/(Mg+Fe2+Mn+Zn) vs X Fe* (Fe2/(Mg+Fe+Mn+Zn) is highly influenced by the presence of Zn, as well as of Mn (Fig. 6B). Not measuring these elements will overestimate the X Mg or X Fe . Conclusions The Cr-Mg-staurolite studied here represents the staurolite with the highest Cr 2 O 3 content reported to date. The composition of Cr-Mg-staurolite (average of 10 EPMA analyses) show (wt%): SiO 2 =28.672, TiO 2 =0.535, Al 2 O 3 =46.710, Cr 2 O 3 =10.259, FeO=5.598, MnO=0.016, MgO=4.895, ZnO=1.259, Total=97.928. The Cr 2 O 3 range is 7.93-12.04 wt%. The structural formulae of Cr-Mg-staurolite, when normalized to 23.5 Oxygen is (Mg 1.034 Fe 2+ 0.663 Zn 0.135 □ 0.167 ) ∑=2.000 (Al 7.802 Cr 1.149 Ti 0.057 ) ∑=9.008 Si 4 O 23 (OH) The relationship between the Cr (cation proportions) and Cr occupancy proves no ideal substitution (Cr:Al=1:1), due to Cr replacement by Ti and Fe3 +. . The X Mg =(Mg/(Mg+Fe 2+ ) range from is 0.583-0.706 with an average of 0.61, showing that it represents a high-pressure Mg-staurolite. The Cr-Mg- staurolite, together with Cr-kyanite and Cr-phengite resulted from high pressure breakdown of magmatic chromite, during (Variscan?) high-pressure metamorphism of an ultramafic or picritic protolith. The absence of quartz, the relict ortopyroxene and talc, as well as the transformations on the Cr-spinel suggest an ultramafic composition of the protolith, or aat least a picritic composition. The high Mg/Fe ratio within the Cr-phases, the missing Ca-phases within the Cr-aggregates suggest the the initial spinel didn’t react with a clinopyroxene to form the staurolite but with a ferro-magnesian phase with low Al and eventually hydrated (or anhydrous but in the presence of a hydrous fluid with low activity). Consequently, we consider the reactant mineral to be the ortopyroxene (XMg=0.84) with high Al-ortopyroxene molecule. Sample 1 10 13 14 18 42 47 48 9 SiO2 28.25 28.54 28.69 28.50 28.37 28.99 29.39 28.64 28.64 TiO2 0.48 0.53 0.51 0.47 0.47 0.68 0.68 0.64 0.48 Al2O3 47.82 45.66 46.44 46.27 46.02 47.10 48.42 48.80 45.95 Cr2O3 8.92 10.99 10.86 10.51 10.92 10.06 8.69 7.85 12.42 FeO 5.48 5.99 5.76 5.79 5.34 5.40 5.14 5.25 5.89 MnO 0.02 0.00 0.00 0.01 0.01 0.06 0.03 0.05 0.00 MgO 4.94 5.03 4.98 5.04 4.83 4.62 4.60 4.76 5.12 CaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ZnO 1.24 1.40 1.30 1.25 1.44 1.15 0.87 1.13 1.44 Na2O 0.01 0.01 0.01 0.04 0.01 0.02 0.03 0.00 0.03 Total 97.14 98.14 98.55 97.87 97.40 98.08 97.84 97.11 99.98 Cation based on 23.5 oxigens Ti 0.051 0.057 0.054 0.050 0.050 0.072 0.072 0.068 0.051 Al 8.015 7.655 7.732 7.753 7.747 7.833 8.004 8.132 7.591 Cr 1.003 1.235 1.213 1.182 1.233 1.122 0.964 0.877 1.377 Fe 0.651 0.712 0.681 0.688 0.638 0.637 0.603 0.621 0.691 Mn 0.002 0.000 0.000 0.001 0.001 0.007 0.004 0.006 0.000 Mg 1.047 1.067 1.049 1.068 1.029 0.973 0.961 1.003 1.070 Ca 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Si 4.018 4.059 4.053 4.052 4.053 4.091 4.122 4.049 4.015 Zn 0.133 0.150 0.138 0.134 0.155 0.123 0.092 0.120 0.153 Na 0.004 0.004 0.002 0.010 0.004 0.004 0.007 0.000 0.008 Total cat 14.924 14.940 14.922 14.937 14.909 14.862 14.827 14.878 14.955 Table 1. Electron microprobe analyses of Cr-Mg-staurolite from Leresti. 14 15 16 17 18 0 1 2 3 4 Al Cr Ideal substitution Cr for Al Non-ideal Cr = Al+Fe 3+ (?) References -Iancu V., (2000) Teză de Doctorat ; Biblioteca Catedrei de Mineralogie, Universitatea Bucuresti -Jose I. Jil Ibarguchi, Miren Mendia (1991) – Mg- and Cr-rich staurolite and Cr-rich kyanite in high pressure ultrabasic rocks (Cabo Ortegal, nortwestern Spain), American Mineralogist,vol. 76, p 501-511 -Messiga,B., Kienast, J.R., Rebay, G., Riccardi, M.P. et Tribuzio, R., (1999)-Cr-rich magnesiochloritoids- eclogites from the Monviso ophiolites (westernAlps, Italy). J. Metam. Geol., 17, 287-299. -Mevel, C. and J.R Kienast (1980) – Chromian jadeite, phengite, pumpellyite and lawsonite in a high-pressure metamorphosed gabbro from the French Alps, Mineralogical Magazine,vol.43, p. 979- 984 -Negulescu Elena, Săbău Gavril (1998) - Chromium-rich Minerals in the eclogites from the south Făgăraş Mountains. Carpathian-Balkan Geological Association, XVI Congress Abstracts, Geol. Surv. of Austria, 418 ek 0.2 0.3 0.4 0.5 0.5 0.6 0.7 Fe2/(Mg+Fe2+Mn+Zn) Mg/(Mg+Fe2+Mn+Zn) 1 2