Massive Eclogite Research Articles

ZIRCONS FROM MASSIVE ECLOGITES ON STOLBIKHA ISLAND, GRIDINO AREA, BELOMORIAN PROVINCE, FENNOSCANDIAN SHIELD

Isotope analysis has revealed three age groups of zircons in massive eclogites fromStolbikhaIsland: 1) short-prismatic igneous - 2752±16 Ma (AR I); 2) rounded metamorphic with Omp inclusions – 2683.1±7.8 Ma (AR II); and 3) Proterozoic prismatic, scarce rounded metamorphic – 1895.5±9.5 Ma (PR). The REE distribution spectra of AR 1 zircons are of igneous type and that of AR II zircons is comparable with that of eclogite. PR zircons were produced mainly by gradual and complete replacement of AR zircons, and their REE distribution spectrum displays an inherited pattern, which is due to proportional dilution provoked by the growth of additional crystals.

Metastability and Nondislocation‐Based Deformation Mechanisms of the Flem Eclogite in the Western Gneiss Region, Norway

AbstractThe eclogite in the Flem Gabbro from Flemsøya Island of the Western Gneiss Region in Norway contains atypical eclogitic minerals, such as olivine and orthopyroxene, and can be texturally divided into weakly deformed massive eclogite (MEC) and strongly deformed foliated eclogite (FEC). Based on phase equilibria modeling, peak metamorphic pressure‐temperature conditions of ~600–750 °C and ~1.0–2.5 GPa and ~700–820 °C and ~2.7–3.7 GPa are recorded in MEC and FEC, respectively. These different pressure‐temperature conditions between MEC (high‐pressure, HP) and FEC (ultrahigh‐pressure, UHP) in the same outcrop reflect deformation‐enhanced eclogitization metamorphism (HP to UHP transition) via the addition of external water during subduction/burial to early exhumation stages and metastable preservation of the HP MEC assemblage at UHP condition due to the local lack of deformation and fluid access at the deep subduction interface or around Moho beneath continental collision zone. Based on the mineral microstructures, nondislocation‐based creep mechanisms—such as diffusion creep, grain and phase boundary sliding, and rigid‐body‐like rotation—play dominant roles in governing the deformation features of FEC and developing the crystal preferred orientations of its major constituent minerals. These deformation mechanisms could considerably affect the interplate coupling at the subduction interface or the rheological strength of Moho beneath continental collision zones. Therefore, the effects of metastability‐based (i.e., preservation of low‐pressure assemblages at HP conditions) and contributions of nondislocation‐based creep mechanisms should be included in the future geodynamic and petrological simulations of subduction and collision processes.

Polyphase growth of garnet in eclogite from the Hong'an orogen: Constraints from garnet zoning and phase equilibrium

Abstract Major and trace element profiles as well as mineral inclusions were analyzed in garnets from massive and foliated eclogites from the low-T/high-P eclogite-facies zone in the Hong'an orogen, China. Garnets in the two types of eclogites show different core–rim zoning and mineral inclusions. At least two stages of garnet growth are evident for most garnet grains. Garnets in the massive eclogite contain abundant mineral inclusions such as quartz, chlorite, amphibole, rutile and phengite in cores, but only a few mineral inclusions such as quartz, rutile and titanite in rims. These garnet grains exhibit increasing MgO but decreasing CaO and REE contents from core to rim. Together with pseudosection calculations it is suggested that hydrous minerals such as chlorite, epidote and amphibole were probably the major reactants for the growth of garnet cores during prograde subduction, whereas the growth of garnet rims involves amphibole breakdown and/or dissolution of previously formed garnet. On the other hand, garnets in the foliated eclogite exhibit nearly constant MgO, decreasing MnO but increasing CaO and heavy REE contents (HREE) from core to rim. Along with pseudosection calculations it is inferred that the decomposition of epidote would consistently contribute to the growth of garnet core and rim. This is also supported by epidote zoning in matrix, with the occurrence of low-jadeite omphacite inclusions in the core and a few rutile inclusions in the rim. The enrichment of light REE (LREE) and depletion of HREE in the epidote rim relative to the core indicate that the epidote rim was recrystallized in equilibrium with the garnet rims. For protolith compositions, the foliated eclogite shows higher SiO2, Na2O, K2O and MgO contents but lower Al2O3, Fe2O3 and TiO2 contents than the massive eclogite. Along with phase equilibrium modeling, it is concluded that the differences in the protolith compositions of eclogites primarily dictate the differences in the garnet zoning patterns and the mineral assemblages of matrix and inclusions. The decomposition of hydrous minerals such as chlorite, amphibole and epidote is necessary for garnet growth during subduction-zone metamorphism.

Petro‐fabrics and seismic properties of blueschist and eclogite in the North Qilian suture zone, NW China: Implications for the low‐velocity upper layer in subducting slab, trench‐parallel seismic anisotropy, and eclogite detectability in the subduction zone

AbstractThe potential seismological contributions of metamorphosed and deformed oceanic crust in a subduction zone environment were studied in a detailed petro‐fabric analysis of blueschist and eclogite in the North Qilian suture zone, NW China. The calculated whole‐rock seismic properties based on the measured lattice preferred orientations of the constituting minerals show increasing P‐wave and S‐wave velocities and decreasing seismic anisotropies from blueschist to eclogite, mainly due to the decreasing volume proportion and deformation extent of glaucophane. The low velocity of the upper layer in the subducting oceanic crust can be explained by the existence of blueschist and foliated eclogite, which induces a 3–12% reduction in velocity compared to that induced by the surrounding mantle rocks. This low‐velocity layer may gradually disappear when blueschist and foliated eclogite are replaced by massive eclogite at a depth in excess of 60–75 km for the paleo North Qilian subduction zone. Trench‐parallel seismic anisotropy with a moderate delay time (0.1–0.3 s) can only effectively contribute to deformed blueschist and eclogite in a high‐angle (>45–60°) subducting slab, regardless of the direction of slab movement. The calculated reflection coefficients (Rc = 0.04–0.20) at the lithologic interfaces between eclogite and blueschist imply that it may be possible to detect eclogite bodies in shallow subduction channels using high‐resolution seismic reflection profiles. However, the imaging of eclogite bodies located in deep subduction zones could be challenging.

New data on the age (U-Pb, Sm-Nd) of garnetites from Salma eclogites of the Belomorian mobile belt

The origin of garnetites, which are quite abundant in highpressure metamorphic complexes, is still debatable. The idea about primary magmatic differen� tiation of basites to Fe-Ti (garnetite protolith) and Mg (protolith of metabasite complimentary to garnetite) parts is the most popular (1 and others). There are assumptions about the formation of garnetite as a result of metamorphic differentiation from active infiltration of fluid (2) and metasomatism with the for� mation of a metasomatic column (3). Extensive garnetization of eclogitic bodies as linear bands up to the appearance of garnetite containing up to 50% garnet and more was registered in Salma eclog� ites within the northwestern part of the Belomorian mobile belt (BMB). The authors studied in detail the body of massive eclogites (Sample 46) with a size of up to 10 m in diameter in the key area of Salma eclogites, in the KuruVaara deposit mine. This body occurs in migmatizes tonolite-trondhjemite gneiss intruded by numerous veins of ceramic pegmatites (4). Eclogites are strongly amphibolized at the contact with host gneiss with the formation of a garnet amphibolite rim (Sample 50) with a thickness of 1-2 m. The garnetite layer with a thickness of up to 60 cm (Sample 48) occurs between the amphibolite rim and the eclogite. Garnetite (Sample 48) contains garnet porphyro� blasts with a size of ~1 mm (up to 50%), intergranular brownishgreen amphibole (20%), andesine (14%), rutile, and ore mineral (5%). In contrast to eclogitic garnet (Sample 46), garnet from garnetite contains numerous poikilitic inclusions of prevailing quartz (10% of the whole rock volume), abundant horn� blende and rutile, and single grains of monoclinic pyroxene and biotite. Garnetite (Sample 48) and eclogite (Sample 46) located within the same body differ significantly in the chemical composition. Garnetite differs from eclogite by the high concentration of FeO* (18.0 and 12.1 wt %, respectively) and TiO2 (1.38 and 0.43 wt %) and the low concentrations of MgO (6.1 and 12.1 wt %) and CaO (11.1 and 13.4 wt %). Garnetite is significantly enriched in V (by a factor of 6) and depleted in Ni, Cr, and Ba by one order of magnitude in comparison with eclogite. The concentrations of Y, Zr, Hf, Th, and REE in garnetite are almost two times higher. A difference in major and minor elements is regu� larly observed in characteristic minerals of garnetite and eclogite as well. Garnet from garnetite differs from eclogitic garnet by the high concentrations of Fe, Ca, HREE, Y, and V and by low contents of Mg and Cr (4); amphibole and monoclinic pyroxene, by the high Fe#, Ti, and V contents; and rutile, by the high con� centrations of V, Zr, and Hf and the low contents of Cr and Nb. The contrasting chemical compositions of garnetite and eclogite do not result in qualitative change of the mineral association upon transforma� tion of eclogite to garnetite, but have an impact on the compositions of rockforming, as well as accessory, minerals.

Two fresh types of eclogites in the Dabie-Sulu UHP metamorphic belt, China: Implications for the deep subduction and earliest stages of exhumation of the continental crust

Two fresh types of eclogites, namely the massive eclogite and foliated eclogite, are discernible in large eclogite bodies surrounded by country rock gneisses from the Dabie (大别)-Sulu (苏鲁) UHP metamorphic zone. They are different in mineral assemblage, texture and structure at various scales. The massive eclogite has a massive appearance with a metamorphic inequigranular and granoblastic texture, which consists mainly of nominally anhydrous minerals such as garnet, omphacite, rutile with inclusions of coesite and rare microdiamond. Massive eclogites which formed at the peak UHP metamorphic conditions (∼3.1–4.0 GPa, 800±50 °C) within the coesite to diamond stability field recorded the deep continental subduction to mantle depths greater than 100 km during the Triassic (∼250–230 Ma). The diagnostic UHP minerals, mineral assemblages and absence of notable macroscopic deformation indicate the peak metamorphic’ forbidden-zone’ P-T conditions, an extremely low geothermal gradient (≤7 °C·km−1) and low differential stress. The foliated eclogite is composed of garnet+omphacite+rutile+phengite+kyanite+zoisite+talc+nyboite±coesite/quartz pseudomorphs after coesite. It is quite clear that the foliated eclogite bears relatively abundant hydrous mineral, and shows well-developed penetrative foliation carrying mineral and stretching lineation reflecting intense plastic deformation or flow of eclogite minerals. The foliated eclogite occurred at mantle levels and recorded the earliest stages of exhumation of UHP metamorphic rocks. At a map scale, the foliated eclogites define UHP eclogite-facies shear zones or high-strain zones. Asymmetric structures are abundant in the zones, implying bulk plane strain or dominant non-coaxial deformation within the coesite stability field. The earliest stages of exhumation, from mantle depths to the Moho or mantle-crust boundary layering, were characterized by a sub-vertical tectonic wedge extrusion, which occurred around 230-210 Ma. The three-dimensional relationship between the massive and foliated eclogites is well displayed a typical ‘block-in-matrix’ rheological fabric pattern indicating the partitioning of deformation and metamorphism in the UHP petrotectonic unit. The existing data support the now widely accepted concept of deep continental subduction/collision and subsequent exhumation between the Yangtze and Sino-Korean cratons. The pressure is a constitutive geological variable. The influence of tectonic overpresure on UHP metamorphism is rather limited.

Variation of mineral composition, fabric and oxygen fugacity from massive to foliated eclogites during exhumation of subducted ocean crust in the North Qilian suture zone, NW China

Eclogites from the North Qilian suture zone are high-pressure low-temperature metamorphic rocks of ocean crust protolith, and occur in both massive and foliated varieties as individual blocks of tens to hundreds of metres in size. The massive type is weakly deformed and shows granoblastic texture characterized by a coarse-grained peak mineral assemblage of Grt1 + Omp1 + Ph + Rt ± Lws (or retrograde Cz). In contrast, the foliated type is strongly deformed and shows a fine-grained retrograde mineral assemblage of Grt2 + Omp2 + Cz + Gln + Ph. Both total FeO and aegirine contents in omphacite, as well as XFe[=Fe3+/(Fe3+ + AlVI)] in clinozoisite/epidote, increase significantly from massive to foliated eclogites. Lattice preferred orientation (LPO) of omphacite, determined by electron back-scatter diffraction analysis, is characterized by weak and strong SL-type fabrics for massive and foliated eclogites, respectively. Clinozoisite/epidote also developed SL-type fabric, but different from the LPOs of omphacite in and axes, owing to their opposite crystallographic long and short axis definitions. The transition of deformation mechanism from dislocation creep to diffusive mass transfer (DMT) creep in omphacite and the concomitant retrograde metamorphism both are efficiently facilitated when the original coarse-grained Omp1 + Grt1 + Lws assemblage is dynamically recrystallized and retrogressed into the fine-grained Fe3+-rich assemblage of Omp2 + Grt2 + Cz + Gln. The DMT process with concomitant anisotropic growth assisted by fluids is considered to be an important deformation mechanism for most minerals in the foliated eclogite. P–T estimates yielded 2.3–2.6 GPa and 485−510 °C for the massive eclogite and 1.8–2.2 GPa and 450−480 °C for the foliated eclogite. The significant increase in total Fe and Fe3+ contents in omphacite and clinozoisite/epidote from massive to foliated eclogite suggests changes in mineral compositions accompanied by an increase in oxygen fugacity during ductile deformation associated with exhumation. The LPO transition of omphacite, clinozoisite and rutile from weak SL-type in massive eclogites to strong SL-type in foliated eclogites is interpreted to represent the increment of shear strain during exhumation along the ‘subduction channel’.

Meso- and micro-structures of foliated eclogites in Dabie-Sulu UHP belt and implications for the earliest stages of exhumation of UHP metamorphic rocks: An example from Taohang, southeastern Shandong, China

The sub-vertical meso- and micro-structures and fabrics developed in coesite-bearing foliated eclogites in the Taohang (桃行) area, southeastern Shandong (山东), China. The diagnostic structures and fabrics, including penetrative foliation or mylonitic foliation containing mineral and stretching lineations, as well as sheath-like folds, appear to be the development of anastomosing UHP eclogite-facies shear belt arrays hosting massive eclogites. Textural relationships and mineral assemblages indicate that the deformation of foliated eclogites developed closely after the formation of the massive eclogite, prior to the development of the granulite/amphibolite-facies symplectites and coronas, occurring over a very wide pressure range of (31−8)×102 MPa. It presents the structural records of the tectonometamorphic processes as being responsible for the earliest stages of exhumation of the UHP metamorphic rocks. Extensive regional field observations show that the meso- and micro-structures and fabrics recognized in the foliated eclogites at Taohang are remarkably similar or consistent in the whole Dabie (大别)-Sulu (苏鲁) UHP metamorphic belt. This article, thus, supports the idea that earliest stages of exhumation of the UHP metamorphic rocks, from mantle depths to the Moho or the mantle-crust boundary layering, may be attributed mainly to a sub-vertical extrusion and ductile flow along the subduction channel, belonging to a syn-collision exhumation at about 235 to 220 Ma.

In situ LA-ICP-MS zircon U-Pb age of ultrahigh-pressure eclogites in the Yukahe area, northern Qaidam Basin

Zircon grains were selected from two types of ultrahigh-pressure (UHP) eclogites, coarse-grained phengite eclogite and fine-grained massive eclogite, in the Yukahe area, the western part of the North Qaidam UHP metamorphic belt. Most zircon grains show typical metamorphic origin with residual cores in some irregular grains and sector, planar or misty internal textures on the cathodoluminescence (CL) images. The contents of REE and HREE of the core parts of grains range from 173 to 1680 μg/g and 170 to 1634 μg/g, respectively, in phengite eclogite, and from 37 to 2640 μg/g and 25.7 to 1824 μg/g, respectively, in massive eclogite. The core parts exhibit HREE-enriched patterns, representing the residual zircons of protolith of the Yukahe eclogite. The contents of REE and HREE of the rim parts and the grains free of residual cores are much lower than those for the core parts. They vary from 13.1 to 89.5 μg/g and 12.5 to 85.7 μg/g, respectively, in phengite eclogite, and from 9.92 to 45.8 μg/g and 9.18 to 43.8 μg/g, respectively, in massive eclogite. Negative Eu anomalies and Th/U ratios decrease from core to rim. Positive Eu anomalies are shown in some grains. These indicate that the presence of garnet and the absence of plagioclase in the peak metamorphic mineral assemblage, and the zircons formed under eclogite facies conditions. LA-ICP-MS zircon U-Pb age data indicate that phengite eclogite and massive eclogite have similar metamorphic age of 436±3Ma and 431±4Ma in the early Paleozoic and magmatic protolith age of 783–793 Ma and 748–759 Ma in the Neo-proterozoic. The weighted mean age of the metamorphic ages (434±2 Ma) may represent the UHP metamorphic age of the Yukahe eclogites. The metamorphic age is well consistent with their direct country rocks of gneisses (431±3 Ma and 432±19 Ma) and coesite-bearing pelitic schist in the Yematan UHP eclogite section (423–440 Ma). These age data together with field observation and lithology, allow us to conclude that the Yukahe eclogites were Neo-proterozoic igneous rocks and may have experienced subduction and UHP metamorphism with continental crust at deep mantle during the early Paleozoic, therefore the metamorphic age of 434±2 Ma of the Yukahe eclogites probably represents the continental deep subduction time in this area.

Tectonic Evolution of the Dabie-Sulu UHP and HP Metamorphic Belts, East-Central China: Structural Record in UHP Rocks

The complex structural history of the Dabie-Sulu terrane is deduced from various scales of structural features studied in UHP metamorphic units combined with metamorphic and thermal data. Excluding pre-UHP events, the following sequence of distinct tectonometamorphic stages is suggested: (1) The first deformation, D1, produced weak foliation and lineation in massive eclogite in the P-T stability field of coesite/diamond under low differential stress. (2) The D2 event is mainly inferred from a dominant coesite eclogite-facies texture characterized by a stretching mineral lineation, mesoscale sheath-like folds, and a network of ductile shear zones. (3) D3 structures and fabrics developed shortly after the formation of granulite/amphibolite—facies symplectites. These structures are characterized by a regional, steeply dipping foliation and heterogeneous compositional layering, eclogite boudinages of various dimensions, intrafolial folds, and ductile shear zones forming an anastomosing pattern, leading to tectonic juxtaposition or nappe-like structures defining shear zone-bounded crustal slices. The D3 deformation event was associated with decompressional partial melting and intense retrogressive metamorphism. (4) Post-collisional ductile crustal thinning and extension affected the D3 foliation and compositional layering, producing a regional, gently dipping D4 main amphibolite-facies foliation and stretching lineation, dome- and arc-shaped structures, and low-angle and extensional detachments typified by different stretching directions. All of these structures formed prior to intrusion of Late Mesozoic plutons, faulting, basin development, and tectonic unroofing at shallow crustal levels (D5). The first two stages of ductile structures (D1 and D2) were related to subduction and collision between the Sino-Korean and Yangtze cratons at UHP (>27 kbar) and HP metamorphic conditions at 230-250 Ma. In contrast, D3 and D4 stages of Late Triassic to Jurassic (~230-170 Ma) ductile deformation accompanied exhumation of UHP and HP metamorphic rocks to crustal levels, initially driven by compression and later by extension. D4 structures dominate the map pattern of most UHP and HP metamorphic rocks in the orogen. The final D5 deformation largely controls the present-day geomorphology of the Dabie-Sulu region. A modified tectonic evolution model for the UHP and HP metamorphic belts is proposed. It involves continental subduction and collision between the Yangtze and Sino-Korean cratons, and subsequent polyphase exhumation of the UHP and HP metamorphic rocks.

A counter‐clockwise P–T path for the Voltri Massif eclogites (Ligurian Alps, Italy)

AbstractIntegrated petrological and structural investigations of eclogites from the eclogite zone of the Voltri Massif (Ligurian Alps) have been used to reconstruct a complete Alpine P–T deformation path from burial by subduction to subsequent exhumation. The early metamorphic evolution of the eclogites has been unravelled by correlating garnet zonation trends with the chemical variations in inclusions found in the different garnet domains. Garnet in massive eclogites displays typical growth zoning, whereas garnet in foliated eclogites shows rim‐ward resorption, likely related to re‐equilibration during retrogressive evolution. Garnet inclusions are distinctly different from core to rim, consisting primarily of Ca‐, Na/Ca‐amphibole, epidote, paragonite and talc in garnet cores and of clinopyroxene ± talc in the outer garnet domains. Quantitative thermobarometry on the inclusion assemblages in the garnet cores defines an initial greenschist‐to‐amphibolite facies metamorphic stage (M1 stage) at c. 450–500 °C and 5–8 kbar. Coexistence of omphacite + talc + katophorite inclusion assemblage in the outer garnet domains indicate c. 550 °C and 20 kbar, conditions which were considered as minimum P–T estimates for the M2 eclogitic stage. The early phase of retrograde reactions is polyphase and equilibrated under epidote–blueschist facies (M3 stage), characterized by the development of composite reaction textures (garnet necklaces and fluid‐assisted Na‐amphibole‐bearing symplectites) produced at the expense of the primary M2 garnet‐clinopyroxene assemblage. The blueschist retrogression is contemporaneous with the development of a penetrative deformation (D3) that resulted in a non‐coaxial fabric, with dominant top‐to‐the‐N sense of shear during rock exhumation. All of that is overprinted by a texturally late amphibolite/greenschist facies assemblages (M4 & M5 stages), which are not associated with a penetrative structural fabric. The combined P–T deformation data are consistent with an overall counter‐clockwise path, from the greenschist/amphibolite, through the eclogite, the blueschist to the greenschist facies. These new results provide insights into the dynamic evolution of the Tertiary oceanic subduction processes leading to the building up of the Alpine orogen and the mechanisms involved in the exhumation of its high‐pressure roots.

Anisotropy of magnetic susceptibility of eclogites: Mineralogical origin and correlation with the tectonic fabric (Cabo Ortegal, Spain)

The fabric and the anisotropy of magnetic susceptibility of the Cabo Ortegal eclogite (NW Spain) are studied. These mafic rocks were metamorphosed and deformed under high pressures and temperatures between 390 and 370 Ma in a subduction/collision tectonic setting. Massive eclogite slices and deformed eclogite in shear zones have bulk magnetic susceptibilities of 31 to 82 · 10 −5 S.I. and 28 to 75 · 10 −5 S.I., respectively. The paramagnetic mineral fraction is the principal magnetic susceptibility carrier. This fraction includes notably garnet and clinopyroxene as matrix minerals, and ilmenite and rutile as accessory constituents. Though magnetic anisotropy degree varies between 3.1 % and 6.6%, variations of this parameter in each rock type are marked. In the deformed eclogite, magnetic lineation (K max) and the pole to the magnetic foliation (K min) are coaxial and coincident with macroscopic petrofabric elements (foliation and lineation). In the massive eclogite, the magnetic fabric is dispersed along the principal structural planes and inversions are associated with samples with small degrees of anisotropy. The anisotropy of magnetic susceptibility is interpreted as being due to the crystallographic preferred orientation and spatial organisation of the polymineralic aggregate. Relating the evolution of the symmetry of magnetic fabric to the symmetry of petrofabric or deformation is rather precluded since susceptibility has multiple origins and bulk magnetic fabric is due to minerals of different symmetry.

Anisotropy of magnetic susceptibility of eclogites: mineralogical origin and correlation with the tectonic fabric (Cabo Ortegal, Spain)

The fabric and the anisotropy of magnetic susceptibility of the Cabo Ortegal eclogite (NW Spain) are studied. These mafic rocks were metamorphosed and deformed under high pressures and temperatures between 390 and 370 Ma in a subduction/collision tectonic setting. Massive eclogite slices and deformed eclogite in shear zones have bulk magnetic susceptibilities of 31 to 82·10−5 S.I. and 28 to 75·10−5 S.I., respectively. The paramagnetic mineral fraction is the principal magnetic susceptibility carrier. This fraction includes notably garnet and clinopyroxene as matrix minerals, and ilmenite and rutile as accessory constituents. Though magnetic anisotropy degree varies between 3.1 % and 6.6 %, variations of this parameter in each rock type are marked. In the deformed eclogite, magnetic lineation (Kmax) and the pole to the magnetic foliation (Kmin) are coaxial and coincident with macroscopic petrofabric elements (foliation and lineation). In the massive eclogite, the magnetic fabric is dispersed along the principal structural planes and inversions are associated with samples with small degrees of anisotropy. The anisotropy of magnetic susceptibility is interpreted as being due to the crystallographic preferred orientation and spatial organisation of the polymineralic aggregate. Relating the evolution of the symmetry of magnetic fabric to the symmetry of petrofabric or deformation is rather precluded since susceptibility has multiple origins and bulk magnetic fabric is due to minerals of different symmetry. © Elsevier, Paris