Calculation of the CIPW norm: New formulas

A completely new set of formulas, based on matrix algebra, has been suggested for the calculation of the CIPW norm for igneous rocks to achieve highly consistent and accurate norms. The suggested sequence of derivation of the normative minerals greatly deviates from the sequence followed in the classical scheme. The formulas are presented in a form convenient for error-free implementation in computer programs. Accurate formulas along with the use of variable molecular weights for CaO and FeO; corrected formula weights for apatite, pyrite and fluorite; and suggested measures to avoid significant rounding off errors to achieve absolute match between the sum of the input weights of the oxides and the sum of the weights of the estimated normative minerals. Using an analogous procedure for determining the oxidation ratios of igneous rocks as used in the SINCLAS system of Verma et al (2002, 2003), the suggested calculation scheme exactly reproduces their results except for apatite for reasons explained in the text, but with a superior match between the totals for about 11,200 analyses representing rocks of a wide range of composition     

Grain‐scale pressure variations and chemical equilibrium in high‐grade metamorphic rocks

In the classical view of metamorphic microstructures, fast viscous relaxation (and so constant pressure) is assumed, with diffusion being the limiting factor in equilibration. This contribution is focused on the only other possible scenario – fast diffusion and slow viscous relaxation – and brings an alternative interpretation of microstructures typical of high‐grade metamorphic rocks. In contrast to the pressure vessel mechanical model applied to pressure variation associated with coesite inclusions in various host minerals, a multi‐anvil mechanical model is proposed in which strong single crystals and weak grain boundaries can maintain pressure variation at geological time‐scales in a polycrystalline material. In such a mechanical context, exsolution lamellae in feldspar are used to show that feldspar can sustain large differential stresses (>10 kbar) at geological time‐scales. Furthermore, it is argued that the existence of grain‐scale pressure gradients combined with diffusional equilibrium may explain chemical zoning preserved in reaction rims. Assuming zero net flux across the microstructure, an equilibrium thermodynamic method is introduced for inferring pressure variation corresponding to the chemical zoning. This new barometric method is applied to plagioclase rims around kyanite in felsic granulite (Bohemian Massif, Czech Republic), yielding a grain‐scale pressure variation of 8 kbar. In this approach, kinetic factors are not invoked to account for mineral composition zoning preserved in rocks metamorphosed at high grade.     

利用岩石化学成分计算花岗岩类实际矿物的新方法

介绍一种在三氏算法(戎嘉树等人于20世纪70年代初提出的一种用岩石化学成分计算花岗岩类实际矿物含量的方法)的基础上经长期实践并不断创新而发展起来的一种新的实际矿物含量的计算方法。该方法针对准铝质花岗岩类、铝过饱和、铝强过饱和花岗岩类分别设计了相应的计算方法,同时也兼顾了碱交代花岗岩及碱长花岗岩,不仅适用于新鲜岩石,也适用于部分蚀变的岩石。该方法在处理岩石化学分析数据的归纳和组合上尽量与实际存在的矿物相结合,力求计算结果与岩石中实际存在的矿物相符。计算过程中提出了长石的修正系数χ,用其修正岩石中钾长石和斜长石的含量,从而确保计算结果与实际相符。计算得到主要造岩矿物(Q、Or、Pl、Bi、Ms、Hb)的含量和反映岩石化学特征的两类参数(Ot、AnPl、K-Na、a、b、Mg)及相关图解。该方法不仅可以计算出岩石中主要造岩矿物的含量,同时还可以反映岩石化学特征、演化过程及其成矿专属性等。     

Raman spectrum of carbonaceous material: a possible metamorphic grade indicator for low-grade metamorphic rocks

Raman spectral analyses of carbonaceous material (CM) extracted from pelitic samples along two sections traversing the metamorphic belt of Taiwan were carried out in the present study. The results show similar spectral variations of CM with metamorphic grade as those documented in the literature. However, continuous sampling from zeolite facies through prehnite–pumpellyite facies to greenschist facies metamorphic rocks in the present study does reveal some interesting features on the Raman spectra of CM that were not noted before. Both the Raman D (disordered-)/O (ordered-) peak area (i.e. integrated intensity) ratio and the D/O peak width (i.e. full width at half maximum, FWHM) ratio of the CM decrease with progressive metamorphism, but the most prominent change in the D/O peak area ratio occurs in samples of lower greenschist facies metamorphic grade, while the most significant decrease in the D/O peak width ratio occurs in samples near the boundary of prehnite–pumpellyite facies and greenschist facies. This phenomenon is interpreted as a result of the decoupling of the changing rates of in-plane crystallite size and degree of defects of CM with progressive metamorphism. It is postulated that the Raman spectrum of CM can serve as a metamorphic grade indicator to distinguish samples of prehnite–pumpellyite facies metamorphic grade from those of greenschist facies metamorphic grade.     

Deformation during exhumation of medium‐ and high‐grade metamorphic rocks in the Variscan chain in northern Sardinia (Italy)

The Anglona and SW Gallura regions represent key places to investigate the tectonic evolution of medium‐ and high‐grade metamorphic rocks cropping out in northern Sardinia (Italy). From south to north we distinguish two different metamorphic complexes recording similar deformation histories but different metamorphic evolution: the Medium Grade Metamorphic Complex (MGMC) and the High Grade Metamorphic Complex (HGMC). After the initial collisional stage (D1 deformation phase), both complexes were affected by three contractional deformational phases (D2, D3 and D4) followed by later extensional tectonics. The D2 deformation phase was the most significant event producing an important deformation partitioning that produced localized shearing and folding domains at the boundary between the two metamorphic complexes. We highlight the presence of two previously undocumented systems of shear belts with different kinematics but analogous orientation in the axial zone of Sardinia. They became active at the boundary between the MGMC and HGMC from the beginning of D2. They formed a transpressive regime responsible for the exhumation of the medium‐ and high‐grade metamorphic rocks, and overall represent a change from orthogonal to orogen‐parallel tectonic transport. Copyright © 2008 John Wiley & Sons, Ltd.     

Polymetamorphism in high‐T metamorphic rocks: An example from the central Appalachians

Contact metamorphism associated with mafic intrusives is one of several mechanisms that has been invoked to produce extensive high‐temperature (HT) metamorphism and associated partial melting of the crust. Indisputable evidence for polymetamorphism in these settings can be difficult to decipher because both melt loss and retrogression (i.e. rehydration) can erase or obscure the records of earlier HT metamorphism by modifying HT mineral parageneses and compositions. Here, a combination of detailed field and petrographical observations, inverse mineral thermometry, and thermodynamic forward modelling is used to delineate the polymetamorphic history of migmatites from the Smith River Allochthon (SRA) in the central Appalachians. Bulk rock geochemical data suggest that some metapelitic samples lost a significant amount of melt during interpreted contact metamorphism with the Rich Acres gabbro, resulting in a residual bulk composition (<50 wt% SiO₂, ~30 wt% Al₂O₃). Garnet cores (Grt1) in SiO₂‐depleted samples are interpreted to grow during this HT contact metamorphism, with Fe‐Ti oxide thermometry on spinel inclusions in Grt1, cordierite–garnet thermometry, and thermodynamic forward modelling constraining peak P–T conditions during contact heating of the migmatites to ~800ºC and ～0.5 GPa. This is associated with an inferred peak assemblage prior to melt loss of crd+kfs+pl+grt+bt+spl (mag+usp+hc)+ilm+sil+qtz+melt. Garnet in SiO₂‐depleted samples has a distinct high‐Ca rim (Grt2), which appears to record a younger metamorphic event. A combination of substantial melt loss and later rehydration appears to be a major control on the ability of SiO₂‐depleted samples to faithfully record evidence for this polymetamorphism. The tectonic implications of this younger metamorphic event are not entirely clear, but it appears to record renewed burial and heating of the SRA sometime after the Taconic orogeny, which may be related to either the neo‐Acadian or Alleghanian orogenies.     

Does ternary feldspar constrain the metamorphic conditions of high‐grade meta‐igneous rocks? Evidence from orthopyroxene granulites,Bohemian Massif

The presence of ternary feldspar in high‐grade meta‐igneous rocks, and the recognition of the thermometric significance of this mineral, has led recent researchers to postulate peak metamorphic temperatures in excess of 1000 °C. However, it needs to be established that such ternary feldspar is not in fact a survivor of the original high‐temperature crystallization of the igneous protolith. After exsolution, the host and lamellae in the ternary feldspar grains may be stable throughout subsequent history as long as recrystallization does not occur. Such a history may involve rehydration and metamorphism, including H₂O‐saturated conditions, with the compositions and proportions of the host and lamellae being modified to reflect the P–T conditions experienced. In the case of the high‐grade meta‐igneous rocks from the Moldanubian of the Bohemian Massif, some samples that contain ternary feldspar preserve a substantial measure of their igneous heritage. Orthopyroxene‐bearing granulites not only include types that are barely affected by the metamorphism, but also others that have undergone hydration of the igneous protolith prior to the development of a metamorphic overprint. A key to establishing the igneous origin of the ternary feldspar grains is their preservation in garnet that is either itself igneous, or of a relatively low‐temperature metamorphic origin. Applying the logic to the other ternary feldspar‐bearing meta‐igneous rocks deprives the Moldanubian of its ultrahigh temperature (UHT) metamorphic status.     

Preservation of high‐P rocks coupled to rock composition and the absence of metamorphic fluids

Eclogites, blueschists and greenschists are found in close proximity to one another along a 1‐km coastal section where the Cyclades Blueschist Unit (CBU) is exposed on SE Syros, Greece. Here, we show that the eclogites and blueschists experienced the same metamorphic history: prograde lawsonite blueschist facies metamorphism at 1.2–1.9 GPa and 410–530°C followed, at 43–38 Ma, by peak blueschist/eclogite facies metamorphism at 1.5–2.1 GPa and 520–580°C. We explain co‐existence of eclogites and blueschists by compositional variation probably reflecting original compositional layering. It is also shown that the greenschists record retrogression at 0.34 ± 0.21 GPa and T = 456 ± 68°C. This was spatially associated with a shear zone on a scales of 10–100‐m and veins on a scale of 1–10‐cm. Greenschist facies metamorphism ended at (or shortly after) 27 Ma. We thus infer a period of metamorphic quiescence after eclogite/blueschist facies metamorphism and before greenschist facies retrogression which lasted up to 11–16 million years. We suggest that this reflects an absence of metamorphic fluid flow at that time and conclude that greenschist facies retrogression only occurred when and where metamorphic fluids were present. From a tectonic perspective, our findings are consistent with studies showing that the CBU is (a) a high‐P nappe stack consisting of belts in which high‐P metamorphism and exhumation occurred at different times and (b) affected by greenschist facies metamorphism during the Oligocene, prior to the onset of regional tectonic extension.     

Early Cretaceous exhumation of high‐pressure metamorphic rocks of the Sistan Suture Zone,eastern Iran

The Sistan Suture Zone (SSZ) of eastern Iran is part of the Neo‐Tethyan orogenic system and formed by convergence of the Central Iranian and Afghan microcontinents. Ar Ar ages of ca. 125 Ma have been obtained from white micas and amphibole from variably overprinted high‐pressure metabasites within the Ratuk Complex of the SSZ. The metabasites, which occur as fault‐bounded lenses within a subduction mélange, document peak‐metamorphic conditions in eclogite or blueschist facies followed by near‐isothermal decompression resulting in an epidote–amphibolite‐facies overprint. ⁴⁰Ar/³⁹Ar step heating experiments were performed on a phengite + paragonite mixture from an eclogite, phengites from two amphibolites, and paragonite from a blueschist; ‘best‐fit’ ages from these micas are, respectively, 122.8 ± 2.2, 124 ± 13, 116 ± 19 and 139 ± 19 Ma (2σ error). Barroisite from an amphibolite yielded an age of 124 ± 10 Ma. The ages are interpreted as cooling ages that record the post‐epidote–amphibolite stage in the exhumation of the rocks. Our results imply that both the high‐pressure metamorphism and the epidote–amphibolite‐facies overprint occurred prior to 125 Ma. Subduction of oceanic lithosphere along the eastern margin of the Sistan Ocean had therefore begun by Barremian (Early Cretaceous) times. Copyright © 2008 John Wiley & Sons, Ltd.     

火星陨石和火星表面泥岩中蚀变过程对比研究

火星次生含水蚀变矿物是火星地质历史时期水环境和气候演变历史的真实记录,一直以来都是火星探测、火星陨石研究的重点,是认识火星环境特征和气候演化的重要研究对象。文中对比研究了表土角砾岩NWA7034、火成堆晶岩MIL03346等两块最富蚀变矿物火星陨石,以及Gale撞击坑出露的Sheepbed泥岩3种岩石类型的蚀变程度及其蚀变矿物类型和组合,分析了层状硅酸盐、铁氧化物/氢氧化物、钙硫酸盐等蚀变矿物的成因及环境指示意义。发现这3类岩石的蚀变作用各不相同。火星陨石NWA7034的蚀变作用以氧化和加热作用为主,无蒸发盐类矿物。火星陨石MIL03346的蚀变程度最低,为后期水溶液进入缝隙而引发的,蚀变作用以橄榄石的伊利石化、裂隙和缝隙中填充次生矿物细脉为主。而火星Sheepbed泥岩经历了后期的等化学风化过程(isochemical weathering),次生过程包括成岩蚀变和成岩后蚀变两个阶段。其中,成岩过程中的蚀变以橄榄石蚀变为铁氧化物和蒙皂石矿物为主,成岩后以形成蒸发盐类矿物硫酸钙为主。以上3种岩石蚀变矿物组成差异反映了火星上不同地质背景中、不同气候条件下蚀变过程的复杂性。文中对火星含水矿物及部分典型矿物的形成条件和过程进行系统总结,这对于理解未来火星探测任务、识别含水矿物的形成、揭示火星水环境和地质历史具有重要指导意义。     

Applying the box plot to the recognition of footwall alteration zones related to VMS deposits in a high-grade metamorphic terrain, South Africa, a lithogeochemical exploration application

Alteration zones (more commonly foot wall alteration zones) are related to volcanic-hosted massive sulfide (VMS) deposits and represent unique features that may be targeted during exploration. Of these, the chloritic foot wall alteration pipe is the most extensive and characteristic of VMS deposits. This feature is geochemically identified by a strong relative enrichment in aluminium and magnesium and a coupled depletion in calcium and sodium, giving rise to chloritic rocks in the primary environment of formation. During high grade regional metamorphism such chloritic precursor rock types are replaced by an unusual mineral paragenesis, typically containing magnesium rich cordierite, phlogopite, orthoamphiboles or orthopyroxenes and aluminium rich minerals such as sillimanite and corundum. This suggests that the unusual geochemical features of the alteration zone, retained during the deformation and metamorphism, should be recognisable in lithogeochemical exploration.The massive sulfide deposit in the eastern part of the metamorphic Namaqua Province, South Africa, at Areachap, Kantienpan and the defunct Prieska Cu–Zn Mine are hosted by a Mid-Proterozoic volcano sedimentary succession known as the Areachap Group. These deposits were affected by a complex deformation and metamorphic history and represent examples of upper amphibolite to granulite grade metamorphosed VMS deposits.The application of the known lithogeochemical methods is especially complicated where the geology is not well understood, due to the poor rock exposure of complexly deformed and metamorphosed areas, such as in the eastern part of the Namaqua Province.The box plot presents a more readily applicable lithogeochemical method to characterize and identify the alteration process, but it was designed for relatively un-metamorphosed environments. It is demonstrated here that the box plot may also be applied to high-grade metamorphic terrains and that the mineral phases used in defining the boxplot in low grade metamorphic environments may be replaced by their equivalents in high grade metamorphic terrains. The compositional trends of the metamorphic minerals themselves may be used in defining the boxplot for high grade metamorphic terrains. These include the transition of: annite to phlogopite; grossular to almandine or pyrope, augite to enstatite or clinoenstatite, and hornblende to gedrite or cummingtonite. Close to the ore zone, the relative Mg content of pyroxene, cordierite and biotite are higher than further away from this zone. It could be demonstrated that the changes in the mineral compositions are gradational when comparing unaffected rocks with progressively more altered wall rocks.Conclusions based on an application of the isocon method demonstrate that primary footwall alteration zones in the Areachap Group's VMS deposits are characterized by elemental depletion of Na₂O, CaO, Sr, Ni, V and La and enrichment of MgO, Fe₂O_3(total), S, Zn, SiO₂, Co and F. It is shown that the whole rock compositions of rocks that were independently identified as the metamorphic equivalents of altered rocks, using the isocon method, plot in the correct place in the box plot for high grade regionally metamorphosed terrains. This establishes the box plot as an effective and practical tool for lithogeochemical exploration for VMS deposits in complexly deformed high grade metamorphosed terrains.     

石英包体拉曼偏移压力计——一种变质压力计算方法

矿物或流体包体可以有效保存寄主矿物生长过程中的化学环境、温度、压力等信息。在高压变质过程中,石榴子石中石英包体的分子振动与环境压力之间具有良好的相关性。根据这一特性,前人提出了石英包体拉曼偏移压力计,其原理是利用显微激光拉曼光谱仪分析石英包体在室温常压下保留的残余压力,再结合石英与寄主矿物的弹性物理特性,恢复石英包体被捕获时的压力,属于矿物物理光谱学压力计,本文介绍了利用石英包体的拉曼位移计算变质压力的基本原理和方法,并对不同方法的适用条件和使用局限做了简要分析。石英包体拉曼偏移压力计是恢复变质岩石形成温度和压力条件的有效方法,具有广泛的应用前景。     

Discriminating four tectonic settings: Five new geochemical diagrams for basic and ultrabasic volcanic rocks based on log — ratio transformation of major-element data

We present five new discriminant function diagrams based on an extensive database representative of basic and ultrabasic rocks from four tectonic settings of island arc, continental rift, ocean-island, and mid-ocean ridge. These diagrams were obtained after log_e-transformation of concentration ratios of major-elements — a technique recommended for a correct statistical treatment of compositional data. Higher % success rates (overall values from ∼ 83 to 97%) were obtained for proposing these new diagrams as compared to those (∼82 to 94%) obtained from the discriminant analysis of the raw major-element concentration data (i.e., without the log_e-transformation and without taking ratios of the compositional data, but using exactly the same database to provide an unbiased comparison), suggesting that such a data transformation constitutes a statistically correct and recommended technique. The new diagrams also resulted in less mis-classification of basic and ultrabasic rocks from known tectonic settings than the diagrams obtained from the raw data. The use of these highly successful new discriminant function diagrams is illustrated using Miocene to Recent basic and ultrabasic rocks from three areas of Mexico with complex or controversial tectonic settings (Mexican Volcanic Belt, Los Tuxtlas volcanic field, and Eastern Alkaline Province), as well as older rocks from three areas (Deccan, Malani, and Bastar) of India. Additionally, the major-element data from two ‘known’ continental arc settings are used to show that they are similar to those from the island arc setting. Continental rift setting is inferred for all Mexican cases and for one cratonic area of India (Malani) and an IAB setting for the Bastar craton. The Deccan flood basalt province of India is used to warn against an indiscriminate use of those discrimination diagrams that do not explicitly include the likely setting of the area under evaluation. An Excel template is also provided for an easy application of these new diagrams for discriminating the four settings considered in this work.     

Evidence for a mid‐Pleistocene shift of ice‐drift pattern in the Nordic seas

Sediment proxy records from a continuous, 1.5 million year long deep‐sea sediment core from a site in the western Norwegian Sea were used to obtain new insights into the nature of palaeoceanographic change in the northern North Atlantic (Nordic seas) during the climatic shift of the Mid‐Pleistocene Revolution (MPR). Red‐green sediment colour and magnetic susceptibility records both reveal significant differences in their mean values when comparing the intervals older than 700 000 yr (700 ka) with those from the past 500 kyr. The timing and duration of these changes indicates that the MPR in the Nordic seas is characterised by a gradual transition lasting about 200 kyr. Together with further sedimentological evidence this suggests that the mid‐Pleistocene climate shift was accompanied by a general change in ice‐drift pattern. It is further proposed that prior to the onset of the major late Pleistocene glaciations in the Northern Hemisphere a significant proportion of the ice in the eastern Nordic seas originated from a southern provenance, whereas later it dominantly came from the surrounding landmasses. Copyright © 2003 John Wiley & Sons, Ltd.     

Contrasting metamorphic histories of lenses of high‐pressure rocks and host migmatites with a flat orogenic fabric (Bohemian Massif,Czech Republic): a result of tectonic mixing within horizontal crustal flow?

Migmatites with sub‐horizontal fabrics at the eastern margin of the Variscan orogenic root in the Bohemian Massif host lenses of eclogite, kyanite‐K‐feldspar granulite and marble within a matrix of migmatitic paragneiss and amphibolite. Petrological study and pseudosection modelling have been used to establish whether the whole area experienced terrane‐wide exhumation of lower orogenic crust, or whether smaller portions of higher‐pressure lower crust were combined with a lower‐pressure matrix. Kyanite‐K‐feldspar granulite shows peak conditions of 16.5 kbar and 850 °C with no clear indications of prograde path, whereas in the eclogite the prograde path indicates burial from 10 kbar and 700 °C to a peak of 18 kbar and 800 °C. Two contrasting prograde paths are identified within the host migmatitic paragneiss. The first path is inferred from the presence of staurolite and kyanite inclusions in garnet that contains preserved prograde zoning that indicates burial with simultaneous heating to 11 kbar and 800 °C. The second path is inferred from garnet overgrowths of a flat foliation defined by sillimanite and biotite. Garnet growth in such an assemblage is possible only if the sample is heated at 7–8 kbar to around 700–840 °C. Decompression is associated with strong structural reworking in the flat fabric that involves growth of sillimanite in paragneiss and kyanite‐K‐feldspar granulite at 7–10 kbar and 750–850 °C. The contrasting prograde metamorphic histories indicate that kilometre‐scale portions of high‐pressure lower orogenic crust were exhumed to middle crustal levels, dismembered and mixed with a middle crustal migmatite matrix, with the simultaneous development of a flat foliation. The contrasting P–T paths with different pressure peaks show that tectonic models explaining high‐pressure boudins in such a fabric cannot be the result of heterogeneous retrogression during ductile rebound of the whole orogenic root. The P–T paths are compatible with a model of heterogeneous vertical extrusion of lower crust into middle crust, followed by sub‐horizontal flow.     

Direct dating of mid‐crustal shear zones with synkinematic allanite: new in situ U‐Th‐Pb geochronological approaches applied to the Mont Blanc massif

Dating the timing of motion on crustal shear zones is of tremendous importance for understanding the assembly of orogenic terranes. This objective is achieved in this paper by combining petrological and structural observations with novel developments in in situ U‐Th‐Pb geochronology of allanite. A greenschist facies shear zone within the Mont Blanc Massif is documented. Allanite is synkinematic and belongs to the mylonitic assemblage. LA‐ICP‐MS U‐Th‐Pb isotope analyses of allanite reveal high contents and highly radiogenic isotopic compositions of the common‐Pb component. The use of measured Pb‐isotope compositions of associated minerals (feldspars and chlorite) is critical for accurate common‐Pb correction, and provides a powerful mechanism for linking allanite growth to the metamorphic assemblage. A mean ²⁰⁸Pb/²³²Th age of 29.44 ± 0.95 Ma is accordingly taken for synkinematic allanite crystallisation under greenschist facies conditions. This age reflects the timing of the Mont Blanc underthrusting below the Penninic Front and highlights the potential of directly dating deformation with allanite.     

The juxtaposition of eclogite and mid‐crustal rocks in the Orlica–Śnieżnik Dome,Bohemian Massif

Eclogite, felsic orthogneiss and garnet–staurolite metapelite occur in a 5 km long profile in the area of Mi?dzygórze in the Orlica–?nie?nik dome (Bohemian Massif). Petrographic observations and mineral equilibria modelling, in the context of detailed structural work, are used to document the close juxtaposition of high‐pressure and medium‐pressure rocks. The structural succession in all lithologies shows an early shallow‐dipping fabric, S1, that is folded by upright folds and overprinted by a heterogeneously developed subvertical foliation, S2. Late recumbent folds associated with a weak shallow‐dipping axial‐plane cleavage, S3, occur locally. The S1 fabric in the eclogite is defined by alternation of garnet‐rich (grs = 22–29 mol.%) and omphacite‐rich (jd = 33–36 mol.%) layers with oriented muscovite (Si = 3.26–3.31 p.f.u.) and accessory kyanite, zoisite, rutile and quartz, indicating conditions of ～19–22 kbar and ～700–750 °C. The assemblage in the retrograde S2 fabric is formed by amphibole, plagioclase, biotite and relict rutile surrounded by ilmenite and sphene that is compatible with decompression and cooling from ～9 kbar and ～730 °C to 5–6 kbar and 600–650 °C. The S3 fabric contains in addition domains with albite, chlorite, K‐feldspar and magnetite indicating cooling to greenschist facies conditions. The metapelites are composed of garnet, staurolite, muscovite, biotite, quartz, ilmenite and chlorite. Chemical zoning of garnet cores that contain straight ilmenite and staurolite inclusion trails oriented perpendicular to the external S2 fabric indicates prograde growth, from ～5 kbar and ～520 °C to ～7 kbar and ～610 °C, during the formation of the S1 fabric. Inclusion trails parallel with the S2 fabric at garnet and staurolite rims are interpreted to be a continuation of the prograde path to ～7.5 and ～630 °C in the S2 fabric. Matrix chlorite parallel to the S2 foliation indicates that the subvertical fabric was still active below 550 °C. The axial planar S2 fabrics developed during upright folding are associated with retrogression of the eclogite under amphibolite facies conditions, and with prograde evolution in the metapelites, associated with their juxtaposition. The shared part of the eclogite and metapelite P–T paths during the development of the subvertical fabric reflects their exhumation together.     

Delamination and ultra‐deep subduction of continental crust: constraints from elastic wave velocity and density measurement in ultrahigh‐pressure metamorphic rocks

Thirty‐three samples, including 22 eclogites, collected from the Dabie ultrahigh‐pressure (UHP) metamorphic belt in eastern China, have been studied for seismic properties. Compressional (V_p) and shear wave (V_s) velocities in three mutually perpendicular directions under hydrostatic pressures up to 1.0 GPa were measured for each sample. At 1.0 GPa, V_p (7.5–8.4 km s^?1), V_s (4.2–4.8 km s^?1), and densities (3.2–3.6 g cm^?3) in the UHP eclogites are higher than those of UHP orthopyroxenite (7.3–7.5 km s^?1, 4.1–4.3 km s^?1, 3.2–3.3 g cm^?3, respectively) and HP eclogites (7.1–7.9 km s^?1, 4.0–4.5 km s^?1, 3.1–3.5 g cm^?3, respectively). Kyanitites (with 99.5% kyanite) show extremely high velocities and density (9.37 km s^?1, 5.437 km s^?1, 3.581 g cm^?3, respectively). The eclogites show variation of V_p‐ and V_s‐anisotropy up to 9.70% and 9.17%, respectively. Poisson’s ratio (σ) ranges from 0.218 to 0.278 (with a mean of 0.255) for eclogites, 0.281–0.298 for granulites and 0.248 to 0.255 for amphibolites. The σ values for serpentinite (0.341) and marble (0.321) are higher than for other lithologies. The elastic moduli K, G, E of kyanitite were obtained as 163, 102 and 253 GPa, respectively. The V_p and density of representative UHP metamorphic rocks (eclogite & kyanitite) were extrapolated to mantle depth (15 GPa) following a reasonable geotherm, and compared to the one dimension mantle velocity and density model. The comparison shows that V_p and density in eclogite and kyanitite are greater than those of the ambient mantle, with differences of up to ΔV_p > 0.3 km s^?1 and Δρ > 0.3–0.4 g cm^?3, respectively. This result favours the density‐induced delamination model and also provides evidence in support of distinguishing subducted high velocity materials in the upper mantle by means of seismic tomography. Such ultra‐deep subduction and delamination processes have been recognized by seismic tomography and geochemical tracing in the postcollisional magmatism in the Dabie region.