Eur. J. Mineral. 2002, 14, 355–360 Coesite in eclogites of the Lanterman Range (Antarctica): Evidence from textural and Raman studies BARBARA GHIRIBELLI1, MARIA -LUCE FREZZOTTI1 and ROSARIA PALMERI2 1 Dipartimento di Scienze della Terra, Università di Siena, Via Laterina 8, I-53100 Siena, Italy e-mail: frezzottiml@unisi.it 2Museo Nazionale dell’Antartide, Università di Siena, Via Laterina 8, I-53100 Siena, Italy Abstract: Quartz pseudomorphs after possible retrogression of coesite were recognized petrographically within garnets of mafic eclogites from the Lanterman Range (Antarctica). In one of the quartz pseudomorphs the presence of a pristine coesite is indicated by in situ Raman microprobe spectroscopy. The results of the Raman analyses show that the quartz inclusion in addition to the typical quartz vibrations has a weak band at 521 cm-1, which corresponds to the most intense fundamental vibration in coesite. This finding represents a first piece of evidence for ultrahigh-pressure metamorphism in Antarctica which was part of the Gondwana supercontinent affected by the Cambro-Ordovician orogenic cycle. Key-words: coesite, mafic eclogites, Raman spectroscopy, Lanterman Range, Antarctica. Introduction Ultrahigh-pressure (UHP) metamorphism, within the stability field of coesite, was first reported by Chopin (1984) in pyrope-quarzites from the Dora Maira Massif, Western Alps. Soon after this initial discovery, relics of coesite have been reported, generally within garnet, from several localities in high-pressure metamorphic belts, such as the Western Gneiss Belt of Norway (Smith, 1984); Dabie – Su Lu region, Eastern China (Wang & Liou, 1991); Northern Mali (Caby, 1994); Himalaya (O’Brien et al., 2001). Most coesite-bearing high-pressure metamorphic belts consist of crustal rocks, contain eclogite rocks s.s., and appear to be related to orogenies involving continental collision. The presence and preservation of UHP assemblages is not restricted to crustal rocks, UHP minerals have also been identified in ophiolites (Reinecke, 1991) and mantle peridotites. In this paper a first identification of relic coesite in one SiO2 inclusion within garnet of mafic eclogites from the Lanterman Range area is reported and discussed, based on microstructural and Raman evidence. Geological background The Lanterman Range pertains to Northern Victoria Land (Fig. 1), which consists of three tectonometamorphic terranes (Bradshaw & Laird, 1983): Wilson, Bowers and Robertson Bay. The Robertson Bay Terrane – a flysh type sequence of Cambro-Ordovician age – and the Bowers TerDOI: 10.1127/0935-1221/2002/0014-0355 rane – a Cambrian volcanic arc – experienced a very low- to low-grade metamorphic imprint (Buggish & Kleinschmidt, 1991), whereas the Wilson Terrane shows a more complex evolution. This terrane is characterised by an intermediatepressure belt at the boundary with the Bowers Terrane (Grew et al., 1984) and a low-pressure belt to the west of Aviator-Rennick Glaciers (Talarico et al., 1992). It was extensively intruded by the Granite Harbour Intrusive Complex, an orogenic association of magmatic-arc affinity (Armienti et al., 1990). The Lanterman Range occurs along the Antarctic palaeoPacific margin of Gondwana in Northern Victoria Land (Ricci et al., 1996) and consists of three metamorphic complexes (Talarico et al., 1998), from west to east (Fig. 1): the Edixon Metamorphic Complex, micaschists and gneisses carrying bands of Ca silicates showing a low-medium-pressure metamorphic imprint (Ghiribelli, 2000); the Bernstein Metamorphic Complex, sillimanite/kyanite- bearing micaschists and gneisses showing an intermediate-pressure metamorphism; the Gateway Hills Metamorphic Complex, a thin and discontinuous belt extending along the fault contact with the Bowers Terrane and consisting of mafic and ultramafic rocks including lenses and pods (centimetre to metre size) of eclogites within quartzo-feldspathic gneisses. Ricci et al. (1997) proposed that both formation and exhumation of the Lanterman eclogites occurred during the Cambro-Ordovician Ross orogenic cycle. Di Vincenzo et al. (1997) estimated temperatures up to @ 850°C and minimum pressures of ~ 1.5 GPa and a Sm-Nd and 238U-206Pb age of 500 Ma for the high-pressure event, and @ 600°C and @ 0.4 GPa for the final stages of the amphibolite-facies metamor0935-1221/02/0014-0355 $ 2.70 2002 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart 356 B. Ghiribelli, M.-L. Frezzotti, R. Palmeri Fig. 1. (a) Schematic structural map and main localities of Northern Victoria Land and (b) geological map of the Lanterman Range (modified after Talarico et al., 1998) with sample location. RTB, Robertson Bay Terrane; BT, Bowers Terrane; WT, Wilson Terrane; LR, Lanterman Range; SR, Salamander Range; MR, Mountaineer Range; DR, Dessent Ridge; DFR, Deep Freeze Range. phic event. On the base of 40Ar-39Ar ages on Ca-type amphibole the latter event was estimated to be 490-486 Ma, and a fast cooling with an average exhumation of 3-4 km/Ma was inferred for eclogites (Di Vincenzo & Palmeri, 2001). Petrography and SiO2 inclusion description Well-preserved eclogites are fine to medium-grained rocks with grano-nematoblastic texture, and consist of garnet, omphacite, rutile with accessory quartz, epidote, apatite, zircon and ilmenite. They are mainly foliated rocks with minor unfoliated medium- to coarse-grained rocks. Garnet usually has a small size ( 0.3 mm) and includes tiny crystals of rutile and zircon. Larger grains of garnet (up to » 0.6 mm) are present only in the less foliated samples. In larger grains inclusions are more abundant and of larger size and consist of rutile, zircon, quartz, epidote, apatite, and rare amphibole. Quartz inclusions, however, are extremely rare in both lithologies. The studied samples belong to the less foliated eclogites that contain larger garnet grains, in which besides rutile and zircon, inclusions of quartz, epidote, apatite, calcite and rare amphibole have been detected. An important amphibolite retrogression is evidenced by the widespread development of the symplectitic association of Na-poor clinopyroxene + plagioclase after omphacite, and of amphibole partly replacing clinopyroxene in the symplectite. However, garnet was only marginally altered offering the possibility to examine the nature of the inclusions, and in particular the quartz inclusions. Twenty samples with small and large garnets showing more abundant quartz inclusions (about 10 per section) have been selected and carefully investigated. Quartz inclusions in garnet always show sub-rounded to elliptical shapes and are present both as single crystals, and as polycrystalline aggregates (50 to 100 µm in size) (Fig. 2 a, b, d). Around single large mono- and polycrystalline quartz inclusions (> 30 µm), tensional cracks are typically developed radiating into garnet (Fig. 2 b), whereas only a few irregular joints are present around very small (< 10 µm) quartz inclusions. For other mineral inclusions fracturing of garnet is much less common. The observed textural characters in the quartz inclusions are indicated as decompression Coesite in eclogites of the Lanterman Range (Antarctica) 357 Fig. 2. Microphotographs and back-scattered electron images showing the microstructural sites of possible coesite and/or re-equilibrated quartz in mafic eclogites. a) Quartz inclusion in garnet of eclogite s.s. showing the typical fractures radiating through garnet host. The dark patch in the core is laser damage. This inclusion revealed the Raman coesite vibration. Sample GF7. b) Monocrystalline quartz inclusions in garnet of eclogite s.s. with well- developed radial texture. Sample TC12. c) Quartz inclusion and radial fractures in garnet of eclogite s.s.. Sample GF7. d) Polycrystalline quartz inclusion in garnet of quartzite in primary contact with eclogite s.s.. Sample G26. e) Back-scattered photograph of image d). features and are often considered as diagnostic for the identification of quartz pseudomorphs after coesite within a rigid host (Chopin, 1984). However, as suggested by Chopin & Sobolev (1995), the presence of such textural features should be treated with caution because they are not unique to coesite transformation. Coesite can be distinguished optically from quartz by its higher relief and lower birefringence. Gillet et al. (1984) and Wain et al. (2000) recently summarised the progressive stages of the transformation of coesite to quartz. They underline that the transformation produces distinct textural features: at the beginning of the re-equilibration (stage 1), coesite is surrounded by a thin rim of polycrystalline quartz with radial extinction (radial palisade structure). This texture is believed to form prior to decompression and hence to exhumation of the host rock. Further re-equilibration (stage 2) produces more diffused quartz, usually related to large ra- dial fractures in garnet. Continued recovery produces polycrystalline quartz aggregates forming a mosaic texture (stage 3). The size of the quartz grains progressively increases, and may form monocrystalline inclusions (stage 4). At these final stages radial fractures may be still present, but early fractures may have been annealed. Among the stages proposed by Gillet et al. (1984) and Wain et al. (2000), stage 2, 3 and 4 microtextures have been recognised in the Lanterman Range samples. Among all studied quartz inclusions in garnet, the majority is of the monocrystalline type (Fig. 2 b), some are of the polycrystalline type (2 to 5 grains per inclusion – Fig. 2 d, e), and only one within a large garnet of sample GF7 gives petrographic evidence that relic coesite might be present (Fig. 2 a). This inclusion (@ 80 µm), in fact, consists of a single colourless roundish grain (@ 20 µm) with high refractive index and low birefringence, rimmed (30 – 40 µm) by polycrystalline quartz grains showing a fibrous 358 B. Ghiribelli, M.-L. Frezzotti, R. Palmeri Table 1. Raman active modes (in cm-1) of the analysed SiO2 inclusion in garnet from eclogites of the Lanterman Range compared to characteristic frequencies in the region 50-800 cm-1 of coesite and quartz from natural samples and literature data. Studied inclusion 207 Coe(1) Coe(2) 117 78 118 149 174 202 242 151 177 204 244 269 312 323 352 378 271 Qtz(1) Qtz(3) 128 128 206 206 261 350 264 326 356 379 391 397 462 Fig. 3. a) Raman spectra of the analysed inclusion. b) Coesite and c) quartz spectra in inclusions in garnet of the Dabie Shan UHP metamorphic rocks are also reported for comparison. 521 610* 635* 424 439 458 427 519 521 637* 654* 805 Raman analyses Raman spectroscopic analyses have been performed using a Labram microspectrometer (Jobin Yvon, Ltd) at the Earth Sciences Department of Siena, Italy. Both polished and covered 30-µm thin sections were used in this study. A polarised Ar+-ion laser operating at the 514.5 nm wavelength at 300 to 500 mW of incident power was used as the excitation source. The laser spot size was focussed to 1-2 µm. Accumulation times varied between 10 and 60 sec. The spectrograph used an 1800 groove/mm holographic grating; the estimated spectral resolution is 1.5 cm-1. Calibration was performed using the 1332 cm -1 diamond band. Polarised spectra of the SiO2 inclusions in garnet of the Lanterman mafic eclogites within the 300-900 cm -1 region are shown in Fig. 3. The scaling for each spectrum plotted 354 393 398 393 397 464 464 510 637* 693 782 radial extinction, although individual grains are too small to be resolved optically. This inclusion is surrounded by radial cracks extending into the host garnet. The latter and other mono- and polycrystalline inclusions were analysed with a scanning electron microscope and the attached microanalysis system (SEM-Edax, Philips 515). Analyses reveal the presence of pure SiO2 as demonstrated by the backscattered electron image of Fig. 2e. This excludes the presence of quartz K-feldspar aggregates as observed in eclogitic garnet from other localities (Massonne et al., 2000). 466 354 694 696 805 808 787 (1) Raman spectra of SiO 2 inclusions from Dabie Shan eclogites; (2) Literature data (Liu et al., 1997); (3) Literature data (McMillan et al., 1992). Bold: main coesite vibrations; Italic: main quartz vibrations. * = vibrations due to section resin mount. was relative to the strongest feature in this spectrum. The relative intensities of single bands depend on the orientation of single crystals. The Raman spectra of all SiO2 inclusions in garnet show the presence of the typical quartz vibrations at 805, 696, 464, 397, 391 and 352 cm -1. The SiO2 inclusion that contains a high-relief grain surrounded by polycrystalline quartz shows a supplementary feature. In the spectrum of the inner phase, there is an additional weak band at 521 cm -1 (Fig. 3 a). This 521-cm -1 peak does not correspond to any Raman mode of quartz, and could be associated with the presence of a high-pressure polymorph of SiO2, such as coesite (Table 1). The 521-cm -1 band is, in fact, the most intense fundamental vibration in coesite and it has been unambiguously assigned by many authors to the s (Si-O-Si) stretching mode, correlating with the 464-cm -1 vibration in quartz (cf. Table 1). Polarised Raman spectra of the surrounding polycrystalline quartz grains are characterised only by typical quartz bands that are identical to the Raman spectra collected from of other SiO2 inclusions in quartz. Note that vibrations at 635 and 610 cm -1 (Table 1) are artefacts due to thin-section resin mount. Coesite in eclogites of the Lanterman Range (Antarctica) 359 Evidence for coesite in the Lanterman Range Raman evidence for relic coesite in a quartz inclusion in garnet from eclogites of the Lanterman Range is based on the presence of an additional band at 521 cm -1 in the spectrum, corresponding to the most intense fundamental vibration in coesite. We did not observe the appearance of other bands which could be assigned to coesite (i.e. 151, 177, 244, 269, 427 and 787 cm-1; Liu et al., 1997; McMillan et al., 1992;), while all the Raman mode frequencies for quartz are present (Table 1). It is well known that fundamental coesite and quartz frequencies can be present in single coesite spectra, and it has been reported from many natural coesite samples (Boyer et al., 1985). This can be explained because the grains are small and the laser beam includes some of the surrounding quartz, or because of coesite transformation to quartz, although in some cases this effect can be induced by the laser itself. Based on the petrographic and micro-Raman observations, we propose that the inner high-relief phase present in one inclusion in garnet represents a coesite high-pressure phase transformed to quartz on decompression. In this hypothesis, the weak and relatively broad band might indicate amorphisation of the crystalline coesite lattice. Raman spectra further suggest that the re-equilibration is at an advanced stage, as also supported by the high amount of polycrystalline quartz aggregates present within the SiO2 inclusion. We see no other evidence for relic coesite in our Raman spectra in all analysed polycrystalline SiO2 inclusions, although decompression features around polycrystalline quartz inclusions are ubiquitous. According to Liou et al. (1997), the preservation of coesite relics depends on many factors including the rigidity of the host mineral, the P-T conditions and path of metamorphic crystallisation, the rate of exhumation, and the presence of fluids during retrogression. The nearly isothermal P-T path followed by the studied eclogites (Fig. 4), which maintained very-high-temperature conditions during the early stages of uplift, together with the abundant fluid circulation during the amphibolite retrogression which transformed eclogites to amphibolites, may explain the extremely rare preservation of coesite. The early retrograde P-T history of the Lanterman eclogites is, in fact, characterised by a nearly isothermal decompression path (Fig. 4), which allowed to retain eclogite-facies equilibration temperatures (750-850°) down to lower crustal levels ( 0.7 GPa); the following evolution is marked by both temperature and pressure decrease (Fig. 4). In the study of Gillet et al., (1984) – where coesite-bearing rocks and their P-T path are modelled to explain the cause of coesite preservation, or partial to total transformation into quartz – this type of exhumation path can be compared with the “Type 1 path”. In such a P-T path, characterised mainly by isothermal decompression, the difference between internal and external pressure of garnet reaches its maximum at the end of isothermal decompression, and garnet fracturing would occur at high temperatures (700-800°C), leading to a complete breakdown of coesite. Moreover, field and petrographic observations suggest that during retrogression eclogites were Fig. 4. P-T path of the mafic eclogite from the Lanterman Range modified after Di Vincenzo et al., (1997). Jadeite isopleths (Jd 45, 30) are after Holland (1983). Chlorite + plagioclase + quartz + calcite = hornblende+ zoisite + CO 2 + vapour and Al tot content of hornblende are after Plyusnina (1982). Kyanite = sillimanite curve after Holdaway (1971). Coesite = quartz curve after Bohlen & Boettcher (1982). Mineral symbols according to Kretz (1983). invaded by fluids, which caused the intense retrogression of eclogite boudins to amphibolites. In conclusion, the possible presence of relic coesite in eclogites s.s. of the Lanterman Range suggests that rocks affected by UHP metamorphism are present in this area. The presence of this high-pressure SiO2 polymorph, in fact, implies that host rocks have undergone to pressures > 2.9 GPa, at temperatures of 850°C. These pressure conditions are indeed much higher than those calculated on the base of the jadeitic content of omphacite (minimum pressure of 1.5 GPa; Di Vincenzo et al., 1997). The possible presence of UHP rocks in the Lanterman Range represents a first piece of evidence for UHP metamorphism, not only in Antarctica but also in all areas of the Gondwana supercontinent affected by the Cambro-Ordovician orogenic cycle named Ross, Delamerian and Sierra Pampeanas orogenies in Antarctica, Australia and south America, respectively. Acknowledgements: The authors are grateful to C. Ghezzo and C.A. Ricci for useful suggestions and comments. Reviews by E.A.J. Burke and H.J. Massonne have improved the paper considerably. Raman microprobe facilities are financially supported by P.N.R.A., the Italian Organisation for Scientific Research in Antarctica. This work was supported by P.N.R.A., C.N.R. project “L’Agenzia 2000” and 360 B. Ghiribelli, M.-L. Frezzotti, R. Palmeri by local projects from the University of Siena (M.L.F.). Dr. Bin Fu is thanked for providing a coesite sample from Dabie Shan. 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(1991): Regional ultrahigh-pressure coesite bearing eclogitic terrane in central China: evidence from country rocks gneiss, marble, and metapelite. Geology, 19, 119-131. Received 29 January 2001 Modified version received 16 July 2001 Accepted 5 October 2001