Ultrahigh-pressure Minerals Research Articles

Mineral inclusions in zircons of para- and orthogneiss from pre-pilot drillhole CCSD-PP1, Chinese Continental Scientific Drilling Project

The pre-pilot drillhole CCSD-PP1, Chinese Continental Scientific Drilling Project (CCSD), with depth of 432 m, is located in the Donghai area in the southwestern Sulu terrane. The core samples are mainly comprised of paragneiss, orthogneiss and ultramafic rock with minor intercalated layers of eclogite and phengite-bearing kyanite quartzite. All analyzed paragneiss and orthogneiss samples were overprinted on amphibolite facies retrograde metamorphism. Coesite and coesite-bearing ultrahigh-pressure (UHP) mineral assemblages were identified by Raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from paragneiss, eclogite and phengite-bearing kyanite quartzite samples. In the paragneiss samples, UHP mineral inclusion assemblages mainly consist of Coe+Omp+Grt+Phe, Coe+Jd+Phe+Ap preserved in the mantles (M) and rims (R) of zircons. These UHP mineral inclusion assemblages yield temperatures of 814–852 °C and pressures of ≥28 kbar, presenting the P– T condition of UHP peak metamorphism of these country rocks. According to the mineral inclusions and cathodoluminescence images of zircons, the orthogneisses can be divided into two types: UHP (OG1) and non-UHP (OG2). In OG1 orthogneisses, low-pressure mineral inclusion assemblage, mainly consisting of Qtz+Phe+Ab+Ksp+Ap, were identified in zircon cores (C), while coesite or coesite-bearing UHP mineral inclusions were identified in the mantles (M) and rims (R) of the same zircons. These features suggest that the OG1 orthogneisses, together with the paragneisses, phengite-bearing kyanite quartzite and eclogite experienced widespread UHP metamorphism in the Sulu terrane. However, in the zircons of OG2 orthogneiss samples, no UHP mineral inclusions were found. Inclusions mainly comprised Qtz+Phe+Ap and were identified in cores (C), mantles (M) and rims (R) of OG2 zircons; the cathdoluminescence images of all analyzed zircons showed clear zonings from cores to rims. These features indicate that the OG2 orthogneisses in pre-pilot drillhole CCSD-PP1 did not experience UHP metamorphism. Therefore, we should not rule out the possibility that some orthogneisses in Sulu terrane might represent relatively low-pressure granitic intrusives emplaced after the UHP event.

Preface to thematic issue: Petrotectonic characteristics of the Kokchetav Massif, northern Kazakhstan

Preface to thematic issue: Petrotectonic characteristics of the Kokchetav Massif, northern Kazakhstan

High‐Si phengite, mineral chemistry and P–T evolution of ultra‐high‐pressure eclogites and calc‐silicates from the Dabie Shan, eastern China

AbstractA suite of coesite–eclogites and associated calc‐silicate rocks from the ultra‐high‐pressure (UHP) belt in the Dabie Shan (eastern China) was investigated petrologically. Field relations and the presence of UHP minerals such as coesite, omphacite and high‐Si phengite in the eclogites and the enclosing calc‐silicates testify to a common metamorphic evolution for these two lithologies. Except for one sample, all bear phengite with unusually high silica contents (Si up to 3.7 per formula unit). Phengite occupies various textural positions indicating that different metamorphic stages are reflected by these white micas, which correlate with distinct mineral zonation patterns. Using the latest thermobarometric calibrations for eclogite‐facies rocks, maximum pressure–temperature (P–T) conditions of 40–48 kbar at < 750°C were estimated for the peak‐metamorphic mineral assemblages. These P–T conditions were calculated for both eclogitic garnet porphyroblasts with diffusion‐controlled zoning as well as garnet porphyroblasts with prograde growth zonation patterns. Most samples were affected by a strong retrograde overprint mainly under eclogite‐ and amphibolite‐facies conditions. Thermobarometry using mineral sets from different textural positions reveals cooling and decompression of the UHP rocks down to < 20 kbar at < 600°C for the bulk of the samples. Decompression and heating indicated by a few samples is interpreted to result from mineral chemical disequilibrium or late thermal influence. These new data show that subduction of continental crust in the Dabie Shan was deeper than previously thought, and also that some cooling and decompression took place at upper‐mantle depths. Copyright © 2000 John Wiley & Sons, Ltd.

Rocks from the mantle transition zone: majorite-bearing xenoliths from malaita, southwest pacific

Rocks containing high-pressure mineral assemblages derived from the mantle transition zone between depths of about 400 and 670 kilometers occur as xenoliths and megacrysts on the island of Malaita in the southwest Pacific on the Ontong Java Plateau. Observed ultrahigh pressure mineral chemistries include majorite, calcium- and magnesium-perovskite, aluminous silicate phases, and microdiamond. Based on an empirical barometer, majoritic garnets in these xenoliths record pressures of up to 22 gigapascal. The occurrence of material with perovskite chemistry and several enigmatic aluminous phases indicates pressures of up to 27 gigapascal. Samples were brought to the surface at about 34 million years ago by potassic ultramafic magmas, which evidently originated in the lower mantle.

Stable isotope geochemistry of marbles from the coesite UHP terrains of Dabieshan and Sulu, China

Marbles from Dabieshan and Sulu, China, suffered ultra high pressure (UHP) metamorphism in the coesite–eclogite facies at approximately 700°C and 30 kbars during Triassic continental collision and subduction. The marbles range in isotopic composition from +7 to +25 δ 18 O VSMOW and from 0 to +6 δ 13 C VPDB . High δ 13 C values are representative of unmodified protoliths and are similar to those of 13 C -enriched Sinian carbonate rocks from the Yangtze craton. High oxygen isotope ratios reflect pristine protoliths but the low values may have been caused by infiltration of low 18 O meteoric water during diagenesis and dolomitization, by fracture-controlled infiltration of water during subduction, by metamorphic mineral reactions, or by a combination of these processes. No evidence of regional isotopic transport during UHP metamorphism has been found. Sampling on scales of 1 to 100 m shows marbles to be inhomogeneous in both carbon and oxygen isotopes. Only samples separated by less than 10 cm have equilibrated oxygen and carbon isotope compositions. Limited isotopic equilibration between adjacent rocks is consistent with the preservation of unaltered UHP minerals and indicates that the metamorphic fluid–rock system was rock-dominated during and following peak metamorphism. A freely flowing, pervasive fluid phase was not present during UHP metamorphism. There is no evidence of isotopic exchange between marble and the upper mantle into which it was subducted. Correlation of geochemical similarities of UHP marbles with Sinian limestones implies that the subducted edge of the Yangtze craton extends at least as far north as the coesite–eclogite facies rocks of Dabieshan. Deposition of protolith carbonates may have taken place in a cold climate either preceding or following but not coincident with Neoproterozoic glaciation.

Petrology of ultrahigh-pressure eclogites from the ZK703 drillhole in the Donghai, eastern China

The 558-m-deep ZK703 drillhole located near Donghai in the southern part of the Sulu ultrahigh-pressure metamorphic belt, eastern China, penetrates alternating layers of eclogites, gneisses, jadeite quartzites, garnet peridotites, phengite–quartz schists, and kyanite quartzites. The preservation of ultrahigh-pressure metamorphic minerals and their relics, together with the contact relationship and protolith types of the various rocks indicates that these are metamorphic supracrustal rocks and mafic-ultramafic rock assemblages that have experienced in-situ ultrahigh-pressure metamorphism. The eclogites can be divided into five types based on accessory minerals: rutile eclogite, phengite eclogite, kyanite–phengite eclogite, quartz eclogite, and common eclogite with rare minor minerals. Rutile eclogite forms a thick layer in the drillhole that contains sufficient rutile for potential mining. Two retrograde assemblages are observed in the eclogites: the first stage is characterized by the formation of sodic plagioclase+amphibole symplectite or symplectitic coronas after omphacite and garnet, plagioclase+biotite after garnet or phengite, and plagioclase coronas after kyanite; the second stage involved total replacement of omphacite and garnet by amphibole+albite+epidote+quartz. Peak metamorphic P– T conditions of the eclogites were around 32 to 40 kbar and 720°C to 880°C. The retrograde P– T path of the eclogites is characterized by rapidly decreasing pressure with slightly decreasing temperature. Micro-textures and compositional variations in symplectitic minerals suggest that the decompression breakdown of ultrahigh-pressure minerals is a domainal equilibrium reaction or disequilibrium reaction. The composition of the original minerals and the diffusion rate of elements involved in these reactions controlled the symplectitic mineral compositions.

Sm/Nd, Rb/Sr, and 40Ar/39Ar Isotopic Systematics of the Ultrahigh-Pressure Metamorphic Rocks in the Dabie-Sulu Belt, Central China: A Retrospective View

Because of a complicated metamorphic history, the isotopic systematics of the ultrahigh-pressure (UHP) metamorphic rocks in the Dabie-Sulu belt, east China, appear to be rather different from what were expected. Depending on the degree of retrograde metamorphism and on the retentivity of isotopes, the radiogenic isotopic systematics in the UHP metamorphic rocks yielded a wide range of radiometric ages. Some of these ages are geologically meaningful, but others may not be. In some fine-grained UHP metamorphic rocks, Sm/Nd isotopic systematics appear to be in equilibrium among the UHP phases, showing the best estimate for the age of peak metamorphism at 226 ± 3 Ma. On the other hand, retrograde overprinting often makes the interpretation of isotopic data more difficult. It is common to find that the Sm/Nd and Rb/Sr isotopic systematics among the UHP phases and retrograde phases are not in equilibrium. Regression of isotopic data involving both UHP and retrograde minerals in isotopic correlation diagrams often yields geologically meaningless ages. Although 40Ar/39Ar dating of UHP metamorphic rocks has been reported not to be very helpful in establishing the thermal history because of the presence of excess argon, a good correlation between excess argon and rock type in the Dabie-Sulu belt would provide a criterion in identifying the possible sources of excess argon. By taking all the possible effects into consideration, a T-t path with two rapid cooling stages for UHP metamorphic rocks from the Dabie-Sulu belt can be postulated. An initial rapid cooling stage in the period from 226 to 219 Ma may have resulted from rapid exhumation of UHP metamorphic rocks immediately after the peak metamorphism. The second rapid cooling stage, from 450°C to 300°C, may have been caused by the exhumation of the entire terrane, including UHP metamorphic units and their host gneisses, during the period from 180 to 167 Ma.

Exsolution Lamellae in Minerals from Ultrahigh-Pressure Rocks

Exsolution microstructures have recently been identified in ultrahigh-pressure (UHP) ultramafic and eclogitic minerals from several orogenic belts, including the Western Gneiss Region, Norway; Alpe Arami, Switzerland; the Bohemian massif, central Europe; the Kokchetav massif, Kazakhstan; the Dabie-Sulu UHP terrane, east-central China; and in central Sulawesi, Indonesia. They include ilmenite rods and magnetite lamellae in olivine; K-feldspar, Mg-Al-Cr titanomagnetite and/or ilmenite, quartz, and garnet rods in clinopyroxene; pyroxene and rutile lamellae in garnet; and lamellae of monazite and an unknown phase in apatite. Although the mechanisms for exsolution in some UHP minerals are poorly known, most exsolution microstructures, such as K-feldspar and quartz in clinopyroxene and pyroxene in garnet, may have resulted from nearly isothermal decomposition of very high pressure precursor phases at mantle depths. Detailed characterization and experimental investigations regarding exsolution microstructures of these and other minerals in UHP rocks will improve our understanding of mantle processes and continental subduction.

Magnetite lamellae in olivine and clinohumite from Dabie UHP ultramafic rocks, central China

Ultrahigh-pressure (UHP) minerals of the Maowu mafic-ultramafic complex in the Dabieshan, east-central China exhibit many exsolution textures. Magnetite lamellae are common in olivines and Ti clinohumites from harzburgite and garnet pyroxenite. Monazite-(Ce) lamellae occur in apatites from the garnet pyroxenite. Independent P-T estimates suggest that these ultramafic rocks formed at P > 5 GPa and T = 700 ± 50 °C. The lamellae-bearing minerals are believed to preserve an earlier, higher P-T record prior to exsolution. Compositions and unit-cell parameters of the magnetite and host olivine and intergrowth relations were determined using a newly developed X-ray diffraction microprobe technique employing synchrotron radiation. The host olivine and magnetite lamellae bear a topotaxial relation with [220] Mag | | [200] Ol, [111]Mag | | [3 3 1]Ol, [11 1] Mag | | [331] Ol, [242]Mag | | [2 2 0]Ol. The recalculated composition of primary olivine may contain up to 1.5 wt% Fe 2O3. Four hypotheses may explain the observed intergrowths of oriented magnetite lamellae in olivine: (1) oxidation of olivine; (2) decomposition of Fe 3+ -bearing olivine formed at >6 GPa; (3) exsolution of a spinel (wadsleyite) solid-solution Fe 3O4-(Fe,Mg)2SiO4 during decompression; and (4) breakdown of phase A [Mg7Si2(OH)6] + enstatite. The third hypothesis appears to be the most likely inasmuch as no additional silicate phase occurs as associated inclusions in the olivine host. However, the actual mechanism for exsolution of magnetite from olivine remains to be studied experimentally.

The Qinglongshan oxygen and hydrogen isotope anomaly near Donghai in Jiangsu Province, China

The Qinglongshan isotope anomaly has unusually low values of both δ 18O and δD. Grnets from coesite-bearing eclogite are as low as −11‰ (VSMOW) and rutiles are −15‰. Phengites have δD of −120‰ (VSMOW). The low values were acquired in an ancient geothermal system prior to subduction during Triassic continental collision. New data shows that depleted isotope values in different rock types extend over distances of 20 × 40 km demonstrating that the geothermal system retained structural coherence throughout subduction and exhumation. The finding of structural coherence suggests that not only eclogites, with their characteristic ultra-high pressure mineral assemblages, but also gneisses, meta-granites, and meta-sediments were all subjected to metamorphism in the coesite-eclogite facies. Oxygen isotope fractionation between quartz and garnet gives estimates of peak metamorphic temperatures at 754–893°C. The spatial extent of oxygen isotopic equilibration between different rock types was limited to tens of centimeters. The persistence of hydrous minerals such as epidote and phengite approaching high temperature oxygen isotope exchange equilibrium with ultrahigh pressure minerals in coesite-bearing rocks shows that mineral-bound water can be subducted into the upper mantle.

Buoyancy-driven, rapid exhumation of ultrahigh-pressure metamorphosed continental crust.

Preservation of ultrahigh-pressure (UHP) minerals formed at depths of 90-125 km require unusual conditions. Our subduction model involves underflow of a salient (250 +/- 150 km wide, 90-125 km long) of continental crust embedded in cold, largely oceanic crust-capped lithosphere; loss of leading portions of the high-density oceanic lithosphere by slab break-off, as increasing volumes of microcontinental material enter the subduction zone; buoyancy-driven return toward midcrustal levels of a thin (2-15 km thick), low-density slice; finally, uplift, backfolding, normal faulting, and exposure of the UHP terrane. Sustained over approximately 20 million years, rapid ( approximately 5 mm/year) exhumation of the thin-aspect ratio UHP sialic sheet caught between cooler hanging-wall plate and refrigerating, downgoing lithosphere allows withdrawal of heat along both its upper and lower surfaces. The intracratonal position of most UHP complexes reflects consumption of an intervening ocean basin and introduction of a sialic promontory into the subduction zone. UHP metamorphic terranes consist chiefly of transformed, yet relatively low-density continental crust compared with displaced mantle material-otherwise such complexes could not return to shallow depths. Relatively rare metabasaltic, metagabbroic, and metacherty lithologies retain traces of phases characteristic of UHP conditions because they are massive, virtually impervious to fluids, and nearly anhydrous. In contrast, H2O-rich quartzofeldspathic, gneissose/schistose, more permeable metasedimentary and metagranitic units have backreacted thoroughly, so coesite and other UHP silicates are exceedingly rare. Because of the initial presence of biogenic carbon, and its especially sluggish transformation rate, UHP paragneisses contain the most abundantly preserved crustal diamonds.

Experimental constraints on phase relations in subducted continental crust

Synthesis piston cylinder experiments were carried out in the range 2.0–4.5 GPa and 680–1,050 °C to investigate phase relations in subducted continental crust. A model composition (KCMASH) has been used because all major ultrahigh-pressure (UHP) minerals of the whole range of rock types typical for continental crust can be reproduced within this system. The combination of experimental results with phase petrologic constraints permits construction of a UHP petrogenetic grid. The phase relations demonstrate that the most important UHP paragenesis consists of coesite, kyanite, phengite, clinopyroxene, and garnet in subducted continental crust. Below 700 °C talc is stable instead of garnet. As most of these minerals are also stable at much lower pressure and temperature conditions it is thus not easy to recognize UHP metamorphism in subducted crust. A general feature, however, is the absence of feldspars at H2O-saturated conditions. Plagioclase is never stable at UHP conditions, but K-feldspar can occur in H2O-undersaturated rocks. Mineral compositions in the experiments are fully buffered by coexisting phases. The Si content of phengite and biotite increase with increasing pressure. At 4.0 GPa, 780 °C, biotite contains 3.28 Si per formula unit, which is most probably caused by solid solution of biotite with talc. Above 800 °C, the CaAl2SiO6 component in clinopyroxene buffered with kyanite, coesite and a Mg-phase increases with increasing temperature, providing a tool to distinguish between 'cold' and 'hot' eclogites. Up to 10% Ca-eskolaite (Ca0.5[]0.5AlSi2O6) in clinopyroxene has been found at the highest temperature and pressure investigated (>900 °C, 4.5 GPa). Garnet buffered with coesite, kyanite and clinopyroxene displays an increase of grossular component with increasing pressure for a given temperature. Although the investigated system represents a simplification with respect to natural rocks, it helps to constrain general features of subducted continental crust. The observed phase relations and phase compositions demonstrate that at pressures >3.0 GPa and temperatures >800 °C continental crust can retain significant amounts of H2O (>1 wt%), whereas K-free mafic or ultramafic rocks are dry at these conditions. UHP parageneses are only preserved if the whole exhumation path is situated within the stability field of phengite, i.e. if there is cooling during exhumation or if the whole exhumation occurred at T <700 °C. In contrast, break down of phengite and concomitant partial melting in terranes that show isothermal decompression may lead to a complete recrystallization of the subducted crust during exhumation. The density of UHP rocks can be estimated on the basis of the established phase relations. Pelitic rocks are likely to have a density close to mantle rocks (3.3 g/cm3) because of significant amounts of dense garnet and kyanite whereas granitic rocks are less dense (3.0 g/cm3). Hence, subducted average continental crust is most probably buoyant with respect to mantle rocks and tends to get exhumed as soon as it is detached from the down-going slab. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00410-001-0336-3.

Refolded syn-ultrahigh-pressure thrust sheets in the south Dabie complex, China: Field evidence and tectonic implications

The exhumation mechanisms of ultrahigh-pressure (UHP) rocks have been a controversial issue ever since the discoveries of coesite and diamond in metamorphic rocks from several orogenic belts. The lack of consensus principally reflects the limited number of combined structural, petrologic, and geochronologic studies that have been carried out in great detail within these terrains. In this study, we derive a complex structural history for the UHP South Dabie terrain based on detailed (1:10000) mapping in the eastern Dabie Shan, and use this to place new constraints on the exhumation process. Structural analysis demonstrates that syn-UHP south-vergent thrusting (D 1 ) was followed by north-vergent folding under amphibolite facies conditions (D 2 ) and a third generation of folding at shallower level (D 3 ). This supports a two-stage exhumation model in which initially rapid exhumation from mantle depth to lower crustal levels is followed by slower exhumation from the lower crust level to the Earth surface. The remarkable structural concordance between lithological units containing UHP mineral assemblages and the enclosing gray gneiss units implies that the apparent difference in peak pressures recorded by different lithologies is not a consequence of tectonic juxtaposition, but a result of differential reequilibration during decompression.

地球の分化過程と内部構造

The phase transitions in the peridotite composition can account for the observed density and seismic wave velocity profiles of the upper mantle and the transiton zone. On the other hand, the lower mantle is not well constrained by the present experimental data, because of large uncertainties in the physical properties of the lower mantle minerals, and the temperature distribution in the lower mantle. Both a homogeneous mantle with a peridotite composition, and a stratified mantle with a peridotite upper mantle and a silica rich lower mantle can be consistent with the observed seismic data. The source upper mantle of the Al-depleted komatiite observed in the late Archean (2.5 Ga old) shows chondritic abundances of some key refractory lithophile elements compatible with the ultrahigh pressure minerals such as majorite and Mg-perovskite. The Al-depleted and Al-enriched komatiites generally observed in the early Archean (3.5-3.8 Ga old) show evidence of the majorite fractionation, melting at the depths of the transition zone. Recent Hf isotopic data of Al-depleted komatiites imply that the source mantle of the komatiites shows no evience of chemical layering in the early Archean; the source mantle of such komatiites was chondritic in terms of some refractory lithophile elements, such as Al, Ti, Sc, Hf, HREE. Solid state convection might have homogenized the stratification formed by global melting during the accretional stage of the Earth.