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Photomicrographs of garnet and pyroxene from the Bakersville eclogite. (a) Inclusions of quartz and rutile in a garnet. The quartz displays no radial cracks. Uncrossed polars. (b) Crossed polars. Quartz inclusions are optically homogenous. (c) Quartz and amphibole needles in clinopyroxene. Quartz is more readily visibly because of its high relief. Uncrossed polars. (d) quartz and amphibole needles viewed approximately down the pyroxene c-axis (note the orthogonal pyroxene cleavages). High relief quartz " caps " terminate amphibole that is almost impossible to distinguish from the host clinopyroxene. BSE and X-ray images of the same fi eld of view are found in Figures 2b and 2c, labeled quartz and amphibole are the same in these fi gures. Uncrossed polars.
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Oriented quartz needles in clinopyroxene have become one of the diagnostic indicators of ultrahigh-pressure (UHP) metamorphism. The presence of apparently exsolved quartz is taken as evidence of decompression of a non-stochiometric Ca-Eskola component (Ca(0.5)square(0.5)AlSi(2)O(6), CaEs) that is presumed to be stable only at UHP conditions. Eclogi...
Citations
... The calculations were based on the numbers of cations per six oxygen anions. At first, all Fe was regarded as ferrous (Fe 2+ ), which minimises the cation deficiency from the ideal stoichiometric cation sum of four for CaEs-bearing pyroxene (Page et al., 2005). Subsequently, the CaEs component was assumed to reflect twice the deficiency of the cation sum from ideal stoichiometry: CaEs = 2 × (4 − cations), provided there is sufficient Al (CaEs < Al). ...
... With the exception of the Fjørtoftvika eclogite, these bimineralic inclusions are absent from the orthopyroxenefree samples, but they occur in all the orthopyroxene-bearing samples. This suggests that the formation of quartz needles with or without pargasite depends on the whole-rock chemistry and probably also on the hydrous content that was inherited from the precursor rocks rather than on the availability of a hydrous fluid derived from an external source (Page et al., 2005). Otherwise, the observed correspondence would be by chance. ...
The Western Gneiss Region (WGR) in western Norway exposes ultrahigh-pressure (UHP) eclogites that occur repeatedly, within an area of high-pressure (HP) eclogites, without evidence of being separated by tectonic shear or ductile flow structures. We studied 10 eclogites from two northern UHP areas and the interjacent HP area to evaluate the significance of this pattern. The orthopyroxene in orthopyroxene-bearing samples has low Al2O3 contents (0.17 wt %–0.37 wt %), provided its grain boundaries were unaffected by partial recrystallisation or replacement. Classical geothermobarometry based on element partitioning between coexisting mineral phases suggests metamorphic conditions within the diamond stability field for the samples from both the HP and UHP areas. The primary clinopyroxene in the associated orthopyroxene-free eclogites contains aligned inclusions of either needle-shaped quartz ± pargasite or lamellar albite, which are absent from the secondary (symplectic) clinopyroxene. Reconstructed mineral compositions of the primary clinopyroxene obtained from grain cross-section surfaces using a scanning electron beam or image processing are non-stoichiometric, and they have higher Ca-Eskola and lower Ca-Tschermak components than the inclusion-bearing host clinopyroxene. The molar ratios of these endmembers are consistent with the needles in the primary clinopyroxene being formed from vacancy-bearing precursor clinopyroxene by the exsolution reaction 2 Ca-Eskola = Ca-Tschermak + 3 quartz during early eclogite-facies retrogression. Further retrogression partially transformed the needle-shaped quartz to irregularly shaped albite within the clinopyroxene and partially transformed both clinopyroxene generations to amphibole that occasionally preserves the needles. The similarity of both the maximum metamorphic conditions and the mineral exsolution microstructures in the eclogites from UHP and HP areas indicates a shared metamorphic history within the stability field of diamond, but a history that diverged during retrogression. Consequently, the alternations of UHP and HP areas in the WGR may have formed by a process that allowed for spatial variations in retrogression efficiency, such as the localisation of strain (recrystallisation) or fluid flow (diffusion) or both, rather than by tectonic stacking of UHP and HP units. Evidence for the UHP metamorphism of WGR crustal rocks is now found from NE to SW along the entire coastal section that covers previously recognised UHP and interjacent areas.
... 0.76) from core to rim (Fig. 5a, b). Numerous oriented Qz needles can be observed in the core of Cpx 1 (Fig. 3a, d), which usually coexists with Amp (Fig. 3e, f), referred to as ''hornblende with quartz 'caps"' by Page et al. (2005). Quartz exsolution in clinopyroxene commonly indicates that quartz was excluded from a non-stoichiometric Ca-Eskola or ''supersilipyx" component in clinopyroxene under UHP conditions (e.g., Liou et al., 1998Liou et al., , 2009Tsai and Liou, 2000;Katayama et al., 2000;Dobrzhinetskaya et al., 2002;Liati et al., 2002;Zhang et al., 2002bZhang et al., , 2003Zhang et al., , 2005cPage et al., 2005;Anderson and Moecher, 2007). ...
... Numerous oriented Qz needles can be observed in the core of Cpx 1 (Fig. 3a, d), which usually coexists with Amp (Fig. 3e, f), referred to as ''hornblende with quartz 'caps"' by Page et al. (2005). Quartz exsolution in clinopyroxene commonly indicates that quartz was excluded from a non-stoichiometric Ca-Eskola or ''supersilipyx" component in clinopyroxene under UHP conditions (e.g., Liou et al., 1998Liou et al., , 2009Tsai and Liou, 2000;Katayama et al., 2000;Dobrzhinetskaya et al., 2002;Liati et al., 2002;Zhang et al., 2002bZhang et al., , 2003Zhang et al., , 2005cPage et al., 2005;Anderson and Moecher, 2007). However, amphibole with quartz caps in the Cpx core has been proposed by Proyer et al. (2009) as forming during an open-system precipitation process within granulite-facies by the reaction of 2NaAlSi 2 O 6 + Ca 2+ = CaAl 2 SiO 6 + 2Na + + 3SiO 2 . ...
South Altyn Tagh contains ultrahigh-pressure (UHP) terranes that have been exhumed from ∼300 km mantle depth. Previous zircon U–Pb geochronology has yielded an eclogite-facies age of ca. 500 Ma and a high-pressure (HP) granulite-facies retrograde age of ca. 450 Ma in the Jianggalesayi area in the western segment of South Altyn Tagh. However, in the eastern segment (Yinggelisayi and Danshuiquan localities), an age range of 500–480 Ma has been determined, and it remains uncertain as to whether this age range represents the timing of the peak metamorphic stage or the retrograde overprint. Our study of newly discovered retrograde eclogite in the Danshuiquan locality shows that it underwent three stages of metamorphism, under eclogite-facies, HP granulite-facies and amphibolite-facies P–T conditions of 2.5–4.0 GPa and 870–1050 °C, 2.0–1.4 GPa and 830–940 °C, and 0.7–1.3 GPa and 704–880 °C, respectively. The decompression-dominated P–T path evolved mainly after crossing the solidus, indicating marked retrograde modification under melt-bearing conditions. LA–ICP–MS and SIMS zircon U–Pb dating yielded ca. 500 Ma eclogite-facies and ca. 484 Ma granulite-facies retrograde ages and a later retrograde age of ca. 452 Ma. The clockwise P–T–t path indicates rapid exhumation from eclogite-facies to granulite-facies within around 16 Myr, which is faster than that of the UHP rocks in the western segment. Thus, the HP–UHP rocks in South Altyn Tagh suggest a differential exhumation process for the eastern and western segments. The distinct HP–UHT metamorphism and rapid exhumation of (U)HP rocks in the eastern segment likely resulted from local mantle heating. The continuing P–T evolution of the (U)HP rocks under UHT conditions during exhumation led to a pervasive granulite-facies overprint in the eastern segment of the South Altyn Tagh. The rapid exhumation recorded in the eastern segment provides valuable insights into the exhumation mechanism of ultra-deep subducted UHP terranes.
... Crystallographically oriented silica rods exsolved in clinopyroxene, many coexisting with amphibole, are well-documented from ultrahigh-pressure (UHP) terranes around the world: (Bakun-Czubarow 1992;Smith 1984;Gayk et al. 1995;Wain 1997;Katayama et al. 2000;Schmadicke and Muller 2000;Tsai and Liou 2000;Dobrzhinetskaya et al. 2002;Zhu and Ogasawara 2002;Klemd 2003;Song et al. 2003;Janak et al. 2004;Sajeev et al. 2010;Dokukina and Konilov 2011;Konilov et al. 2011;Bi et al. 2018;Song et al. 2018). Although there is at present evidence of HP (and not UHP) clinopyroxenes possessing these crystallographically oriented silica precipitates in the Blue Ridge Mountains, U.S.A., the Kontum Massif, central Vietnam, and the Greek Rhodope (Page et al. 2005;Anderson and Moecher 2007;Nakano et al. 2007aNakano et al. , 2007bProyer et al. 2009;Faryad and Fisera 2015;Li et al. 2018), in the dearth of other UHP evidence the siliceous rods are often used as indicators of rocks experiencing UHP conditions. The breakdown textures of clinopyroxene and its chemical variations may help us to investigate pre-and post-peak rock evolution of the host rock. ...
Low-pressure metastable nanoscale crystals of a-cristobalite have been observed epitaxially exsolved in cores of UHP clinopyroxene from the Bohemian Massif, Czech Republic. SAED patterns and HRTEM images detail the close structural relationship between host clinopyroxene and a-cristobalite precipitate: [001]Di║[010]a, (010)Di ~║(101)a. TEM results indicate that a-cristobalite exsolved from host clinopyroxene. Non-crystalline Al-bearing silicate phases, also exsolved from UHP clinopyroxene, possesses Al/Si ratios close to eutectic compositions in the system NaAlSi3O8-SiO2-H2O system. The presence of glass exsolution suggests a high-temperature formation environment and presence of water. The a-cristobalite formed in a localized low-pressure, micro-environment formed through exsolution of vacancies and excess silica from the host pyroxene lattice. This micro-environment may be a result of negative density changes due to excess lower density silica exsolving from higher density pyroxene during an exsolution process that involved no localized volume change. Interface-controlled exsolution via lattice matching at the diopside/cristobalite interface, and stability changes and melting point depression due to nanoscale size effects contributed to the formation and persistence of this metastable phase. Amphibole in association with a-cristobalite and some non-crystalline silicate phases may be a clue to localized water quantities; silica exsolution with amphibole may have formed below the eutectic temperature and at a later stage than non-crystalline silicate phases without amphibole. Silica rods in Nové Dvory clinopyroxenes were previously thought to be quartz; however, our investigation reveals various low-pressure, high-temperature, and/or metastable phases greatly affected by the presence of vacancy and OH in clinopyroxenes. The results will help us better understand OH in the UHP pyroxene and even water release in the mantle.
... Thus, while Ca-eskolaite is clearly not an indicator of UHP conditions in the diamond stability field, it is distinctively high in the pressure window between coesite and diamond conditions (this study; Knapp et al., 2013;Konzett et al., 2008a;Page et al., 2005;Rosenthal et al., 2014). For modern MORB-like compositions (i.e. ...
... The breakdown of Caeskolaite is caused by the increased stability of garnet at the expense of clinopyroxene with depth, also increasing the excess coesite/ stishovite above 4-5 GPa (reactions (4) & (5)) (see above). We note that the onset of reactions (4) & (5) depend very much on the particular eclogitic bulk composition (this study; Aoki and Takahashi, 2004;Irifune et al., 1986;Knapp et al., 2013;Konzett et al., 2008a;Page et al., 2005;Zhao et al., 2011). The Ca-eskolaite component in clinopyroxene decreases up to 8-10 GPa, though more significantly above these pressures (Irifune et al., 1986;Knapp et al., 2013;Konzett et al., 2008a;Ono and Yasuda, 1996;Zhao et al., 2011). ...
This study investigates the phase and melting relations of nominally ‘dry’ residual eclogites (Res2 and Res3), with varying bulk CaO/Na2O ratios (4 and 12, respectively), from ~160 (5 GPa) to ~90 km (3 GPa) depth. Garnet, clinopyroxene and minor quartz/coesite are subsolidus phases in both compositions. In contrast to Res2, in Res3, the proportions of garnet always exceeding those of clinopyroxene. This also leads to higher modal quartz/coesite in Res3 relative to Res2. In modelling melting along a near-adiabatic upwelling path with a mantle potential temperature of ~1360 °C, at 5 GPa, near-solidus andesitic Res3 partial melts are much less siliceous and sodic, and are more calcic and magnesian than the incipient dacitic melts of Res2. Continuously self-fluxed melting increases considerably from 4 to 3 GPa due to the increased breakdown of Ca-Eskolaite solid solution component in clinopyroxene along the adiabat. This causes a steepening of the solidus, but more-so for Res2 than for Res3. At 3 GPa, the near exhaustion of residual clinopyroxene causes higher melt productivity for Res3 (~60%) than for Res2 (~30%), despite both melts being of basaltic-andesite composition. Resulting Res3 melts are therefore significantly more calcic and magnesian, and less sodic than those of Res2 melts. As Res3 undergoes a higher degree of melting relative to Res2 during adiabatic ascent, Res3 eclogitic residues become significantly more refractory; with relatively higher Mg# and grossular in garnet, higher Mg# and Ca-tschermaks, and lower jadeite components of clinopyroxene, and higher garnet/clinopyroxene ratios than eclogitic Res2 residuals. In upwelling heterogenous mantle domains, the siliceous eclogitic melts formed within a body of eclogite will react with encapsulating mantle peridotite, effectively refertilising it and producing hybrid pyroxene- and garnet-rich rocks. Subsequent melting of these sources may lead to compositionaly diverse primitive mantle-derived magmas, with high Ca/Al and low Na/Ca signatures indicators of preferential melting of a heterogeneous mantle, previously refertilised by recycled Ca-rich oceanic crustal material, and primitive magmas with low Ca/Al and high Na/Ca derived from melting of mantle with a ‘normal recycled crustal material signature’. Thus, compositional magma diversity may directly reflect precursor compositions of the mantle source region.
... To explore the likely composition of the clinopyroxene that existed before the breakdown, we carried out a simplified reintegration in the manner employed in many other studies (e.g. Dobrzhinetskaya, Schweinenage, Massonne, & Green, 2002;Gayk, Kleinschrodt, Langosch, & Seidel, 1995;Katayama et al., 2000;Page et al., 2005;Zhang et al., 2005). Lamellae proportion estimation, based on high resolution BSE images, was carried out with the javabased software ImageJ (ver. ...
... Their CaEs content is usually <10 mol.%, and retrogression with water ingress is commonly considered to account for the lamellae formation (e.g. Anderson & Moecher, 2007;Gayk et al., 1995;Page et al., 2003Page et al., , 2005Proyer et al., 2009). The amphibole lamellae in the clinopyroxene of the Shirokaya Salma eclogites indeed display subtle mineral-chemical discrepancies (see Figure 7). ...
... However, experimental studies also show that although the CaEs component can be incorporated in omphacite, its solubility does not increase with pressure at the same temperature (Konzett et al., 2008). The exsolution of quartz in omphacite, which is the result of CaEs breakdown, is not a diagnostic evidence of UHP metamorphism (Page et al., 2005). Therefore, unless direct indicators of UHP metamorphism-coesite or microdiamonds-are found, the P-T conditions of the eclogite facies metamorphism constrained above remain reasonable. ...
The rare occurrence of Precambrian eclogite has been used to argue when plate tectonics initiated. Well-preserved kyanite-bearing eclogites were collected in the Kuru-Vaara quarry, northern Belomorian Province, Fennoscandian Shield. Textural observations and phase equilibria modeling were applied to constrain their P-T evolution. The eclogite is characterized by the peak pressure assemblage garnet + omphacite + kyanite + amphibole + quartz. The garnet has inclusions of zoisite, omphacite and kyanite. This omphacite with high jadeite (up to 33 mol%) and 5% Ca-Eskola content bears oriented rods of quartz, amphibole, and rutile. Omphacite was replaced by diopside + plagioclase ± orthopyroxene symplectites and garnet surrounded by plagioclase coronas. Kyanite was mantled by the assemblage sapphirine + spinel + plagioclase ± scapolite. The symplectites of diopside + plagioclase ± orthopyroxene are further replaced by amphibole. P-T estimates of peak eclogite facies conditions are c. 18-20 kbar and 720-820 °C. The granulite facies assemblages formed at c. 11-13 kbar and 870-915 °C. Zircon U-Pb dating reveals the protolith ages of 2802-2929 Ma for the eclogites. The c. 1896 Ma zircon ages correspond to the time of prograde metamorphism in eclogite facies. The age of 1885 ± 7 Ma for a corundum-bearing mica pegmatite vein that intruded into the eclogites reflects the infiltration of fluid at amphibolite facies conditions. Thus, the Kuru-Vaara eclogite was metamorphosed during the Paleoproterozoic, not the Archean. The P-T path of Kuru-Vaara eclogites was characterized by eclogite facies metamorphism overprinted by granulite facies and retrograde amphibolite facies metamorphism within c. 10 Ma. This process reflects the fast exhumation of the eclogites. The increase in temperature from eclogite to granulite facies in the HT-UHT domain suggests a hot Precambrian tectonic environment.
... Oriented lamellae of a Fe-Mg-rich phase (b 0.2 μm) also exsolved from the matrix hornblende (Fig. 6E). The quartz rods and Fe-Mg-rich lamellae in the matrix hornblende resemble textures found in clinopyroxene from hightemperature eclogites (e.g., Anderson and Moecher, 2007;Groppo et al., 2015;Page et al., 2005). Hornblende replaced the clinopyroxene in the symplectites during exhumation, and the hornblendes in both matrix and symplectites were partially replaced by actinolite during further retrogression (Fig. 5C). ...
Mafic lenses hosted by felsic paragneiss in a Taconic thrust slice (Canaan Mountain Formation) from northwest Connecticut (New England, USA) contain relict mineral assemblages and decompression textures indicative of high pressure (HP) precursors. Symplectic intergrowths consisting mostly of diopside + plagioclase or biotite + plagioclase are pseudomorphous after omphacite and phengite, respectively. Pseudosection analysis and thermobarometry demonstrate that the inferred peak assemblage of garnet + clinopyroxene + phengite formed at > 14 kbar and ~ 710 °C in the eclogite facies. Bulk-rock geochemistry and field relations indicate that the protoliths of the mafic gneisses were likely rifting-related mafic intrusions. Zircon U-Pb dating by ion probe yields a 456 ± 4.6 Ma (2σ) metamorphic age for the mafic gneiss. The zircons from the felsic host rocks have an identical 456 ± 11 Ma metamorphic rim age and Grenvillian detrital cores. The HP metamorphism and the coeval arc magmatism reflect the collision between the Laurentian passive margin and a Taconic arc complex over an east-dipping subduction zone that was active until ~ 456 Ma in Connecticut (southern New England). The P- T path is characterized by post-peak- T compression, suggesting that the eclogite-facies metamorphism was associated with the deformation of the collision zone after the initial continent-arc collision. After the culmination of collision the subduction polarity switched; metamorphic ages decrease southward along the orogen suggesting that this reversal occurred ~ 10 Myr later in New England than in Newfoundland. Nearly all mafic rocks in the study area crop out as fairly ordinary-looking amphibolites, so it is reasonable to speculate that HP metamorphism was more extensive than currently recognized in New England but has been obscured by thermal overprinting and retrogression.
... The CaEs component fraction was then calculated by subtracting twice the CaTs, the jadeite, and the K-cpx fraction from the total Al content (X CaEs = Al tot À 2X CaTs À X jad À X K-cpx ). Alternatively, the total cation deficiency from a theoretical four cations per six oxygens should equal the fraction of the defect-bearing CaEs component (i.e., X CaEs = 2 × (4 À ∑cations) [Page et al., 2005]). All Na was set as jadeite (X jad = Na tot ), and all K was set as K-cpx component (X K-cpx = K tot ). ...
We have experimentally tested the possibility that the coesite-stishovite transition in eclogite bodies is responsible for the X discontinuity, a locally observed, low-impedance jump in seismic wave velocities at 260–330 km depth. We determined phase relations and free SiO 2 abundances in three natural-analog eclogite compositions that simulate different subduction scenarios in terms of pressure-temperature conditions and whether or not melt extraction occurred. Eclogitic compositions representing residues after either shallow or deep melting contain either no coesite or else too little (<4 wt %) to produce the observed impedance contrast for the X discontinuity. Only an unmodified mid-ocean ridge basalt (MORB) composition was found to contain just enough coesite (6–8 wt %) to be consistent with the expected impedance contrast when it transforms to stishovite. However, we assert that MORB cannot remain compositionally unmodified during subduction down to ~300 km. Fluid loss due to dehydration reactions during the transformation from basalt to eclogite lowers bulk SiO 2 content. In addition, the MORB wet solidus intersects the coesite-stishovite boundary at ~290 km, implying that at greater depths a melt phase should be present before stishovite stability is reached. Our data indicate that melt generation is an efficient means of lowering the free SiO 2 content in the mineral assemblage. This study also confirms previous work indicating that exsolution of SiO 2 from the Ca-Eskola (Ca 0.5 AlSi 2 O 6) component in clinopyroxene is not a feasible mechanism for producing significant stishovite upon reaching its stability field. We conclude that the coesite-stishovite transition in eclogite bodies is not a viable petrological explanation for the X discontinuity.
... Indirect lines of evidence include coesite inclusions from paragneiss zircons in the Dulan terrane Song et al., 2003a), diamond inclusion preserved in a zircon crystal, exsolution textures preserved in garnet and olivine from the Luliangshan garnet peridotite (Song et al., 2004;Song et al., 2005), and coesite pseudomorphs and quartz exsolutions in eclogite (Song et al., 2003a;Zhang et al., 2005b;Yang et al., 2005). These observations, however, remain controversial (Hermann et al., 2005;Page et al., 2005;Smith, 2005). The coesite inclusions reported here represent unequivocal evidence for UHP metamorphism for the North Qaidam eclogite and deep subduction of oceanic lithosphere (Zhang et al., 2008) and continental crustal materials (Song et al., 2005b;Zhang et al., 2008). ...
... Oriented silica precipitates in clinopyroxene have been reported widely in mafic and ultramafic rocks from high pressure (HP) and ultrahigh pressure (UHP) metamorphic terranes around the world. The host rocks can be classified into four types: UHP metamorphic eclogite (Dobrzhinetskaya et al., 2002;Katayama et al., 2000;Liu et al., 2011;Smith, 1984;Tsai and Liou, 2000;Zhang et al., 2002), HP metamorphic eclogite (Miller et al., 2005;Page et al., 2005), eclogite xenolith in kimberlite pipe (Smyth, 1980;Sobolev et al., 1968) and HP granulite (Gayk et al., 1995). The oriented silica phases in clinopyroxene are generally α-quartz (Dobrzhinetskaya et al., 2002;Katayama et al., 2000;Proyer et al., 2009;Schmädicke and Müller, 2000;Smith, 1984;Smith and Cheeney, 1980;Z.M. Zhang et al., 2011), rarely coesite (Zhang et al., 2005) and keatite (Hill et al., 2013). ...
... They sometimes coexist with calcic amphibole, phlogopite, phengite and fluid inclusions. As a common observation in rocks from UHP terranes, the origin of oriented quartz needles/rods in clinopyroxene has been controversial with the crux of dispute being whether it can be regarded as an UHP-indicator (Dobrzhinetskaya and Faryad, 2011;Page et al., 2005). ...
... However, whether oriented silica precipitations in clinopyroxene can be treated as an indisputable UHP-indicator has been debated (e.g., Klemd, 2003;Page et al., 2005). Evidence against an UHP origin includes oriented quartz needles/rods within clinopyroxene from rocks which have experienced only HP but no UHP conditions (Gayk et al., 1995;Konzett et al., 2008b;Miller et al., 2005;Page et al., 2005;Proyer et al., 2009). ...
Abundant oriented silica precipitates of α-quartz (4.0 ± 1.0 vol.%), in part coexisting with calcic amphiboles (< 0.2 vol.%), have been identified in clinopyroxenes of eclogites from the Weihai area, Sulu ultrahigh pressure terrane, eastern China. Electron backscatter diffraction (EBSD) analyses demonstrate that the majority (97%) of quartz precipitates have topotactic relationships with their host clinopyroxenes. Three types of crystallographic topotactic relationship have been identified between quartz and host clinopyroxene: (1) 52% quartz precipitates share the same orientation for the c-axes with [0001]qz//[001]cpx; (2) 34% quartz precipitates share the same orientation for the a-axes with [11 0]qz//[001]cpx; (3) 11% quartz precipitates share the same orientation for the s-planes with (11 1)qz//(100)cpx. Other quartz axes and planes disperse in large or small girdles around the shared axes or planes. Many quartz rods/needles are elongated parallel to the [001]cpx with the long axes of quartz being either [0001]qz or [11 0]qz. Amphibole precipitates have also a strong crystallographic relationship with host clinopyroxene, i.e., (100)amp//(100)cpx, [010]amp//[010]cpx, and [001]amp//[001]cpx. These results provide quantitative microstructural evidence supporting an exsolution origin for oriented quartz needles/rods in clinopyroxene and demonstrate that the exsolution of quartz from clinopyroxene occurred within the stability field of α-quartz rather than coesite. The oriented precipitates of α-quartz, in part coexisting with calcic amphiboles, in host clinopyroxene are probably promoted by supercritical fluid or partial melting during the early exhumation of eclogites. Our results suggest that oriented quartz precipitates in clinopyroxene cannot be used as an indisputable UHP-indicator.
