Ultrahigh-pressure Metamorphism Research Articles

Correlation of allochthonous terranes and major tectonostratigraphic domains between NW Iberia and the Bohemian Massif, European Variscan belt

The NW Iberian Allochthon and the Tepla-Barrandian and Moldanubian zones represent the internal parts of the Variscan belt in their respective domains. A correlation based on the lithological association, protolith ages, metamorphic evolution, detrital zircon age spectra and tectonic setting is attempted between the NW Iberian Massif and SE Bohemian Massif in order to check whether they could have formed part of the same allochthonous stack. The Galicia-Tras-os-Montes Zone of the Iberian Massif and the internal zones of the Bohemian Massif include from bottom to top a Parautochthon and Lower Allochthon representing the outer edge of the northern Gondwana margin, an oceanic Middle Allochthon with Cambro-Ordovician and Early Devonian ophiolites and an Upper Allochthon interpreted as a peri-Gondwanan terrane. Early Variscan, subduction-related high-pressure metamorphism characterizes many of the allochthonous units, with ages younging from the structurally upper to the lower units from 400–385 Ma to 370–360 Ma, respectively. High- and ultrahigh-pressure metamorphism occurred also in the Saxothuringian Autochthon at 360–340 Ma, but not in the NW Iberian Autochthon. The different behavior of the Autochthon in the Iberian and Bohemian massifs accounts for their distinct evolutions from 360 Ma onward. We conclude that the Upper Allochthon was a unique peri-Gondwanan terrane, whereas the Middle Allochthon represents units of the same peri-Gondwanan ocean, opened at the Cambro-Ordovician boundary, and having recorded localized renewed activity in the Silurian–Early Devonian. No other oceans separated the Lower Allochthon, Parautochthon and Autochthon.

Syn-exhumation magmatism during continental collision: Geochemical evidence from the early Paleozoic Fushui mafic rocks in the Qinling orogen, Central China

Mafic arc-like magmatism is rare during continental collision, but its products provide valuable information on the geochemical processes during plate convergence. Such rocks were found in the Fushui complex in the Qinling orogen, central China. In order to decipher their petrogenesis, we have carried an integrated study of geochronology and geochemistry. Zircon SIMS UPb dating yields consistent ages of 488 ± 4 to 490 ± 4 Ma for emplacement of mafic magmas. The ages are just slightly later than zircon UPb ages of 500–490 Ma for ultrahigh-pressure metamorphism of the continental crust in the North Qinling unit. Thus, the target rocks are the products of syn-exhumation magmatism during continental collision. These mafic igneous rocks exhibit arc-like trace-element patterns, such as enrichment in large ion lithophile elements (LILE), Pb, and light rare earth elements (LREE), and depletion in high field strength elements (HFSE). They show enriched whole-rock SrNd isotope compositions and variable zircon HfO isotope compositions, indicating involvement of the recycled oceanic basalt and terrigenous sediment in their mantle source. Such geochemical features suggest that the Fushui mafic magmas would originate from an enriched and fertile mantle source that was generated through the metasomatic reaction between the mantle wedge peridotite and the subducting Shangdan oceanic crust-derived liquid phase. With the tectonic transition from the lithospheric subduction for active compression to active extension for the crustal exhumation during the late Cambrian, the mantle source underwent decompression melting to produce mafic melts in synchronous with exhumation of the deeply subducted continental crust. Therefore, the syn-exhumation Fushui complex provides new geochemical insights into the nature of the orogenic lithospheric mantle and thus the tectonic evolution of this collisional orogen.

Decompressional equilibration of the Midsund granulite from Otrøy, Western Gneiss Region, Norway

Abstract The Western Gneiss Region (WGR) of the Scandinavian Caledonides is an archetypal terrain for high-pressure (HP) and ultrahigh-pressure (UHP) metamorphism. However, the vast majority of lithologies occurring there bear no, or only limited, evidence for HP or UHP metamorphism. The studied Midsund HP granulite occurs on the island of Otrøy, a locality known for the occurrence of the UHP eclogites and mantle-derived, garnet-bearing ultramafics. The Midsund granulite consists of plagioclase, garnet, clinopyroxene, relict phengitic mica, biotite, rutile, quartz, amphibole, ilmenite and titanite, among the most prominent phases. Applied thermodynamic modelling in the NCKFMMnASHT system resulted in a pressure–temperature (P–T) pseudosection that provides an intersection of compositional isopleths of XMg (Mg/Mg+Fe) in garnet, albite in plagioclase and XNa (Na/Na+Ca) in clinopyroxene in the stability field of melt + plagioclase + garnet + clinopyroxene + amphibole + ilmenite. The obtained thermodynamic model yields P–T conditions of 1.32–1.45 GPa and 875–970 °C. The relatively high P–T recorded by the Midsund granulite may be explained as an effect of equilibration due to exhumation from HP (presumably UHP) conditions followed by a period of stagnation under HT at lower-to-medium crustal level. The latter seems to be a more widespread phenomenon in the WGR than previously thought and may well explain commonly calculated pressure contrasts between neighboring lithologies in the WGR and other HP–UHP terranes worldwide.

Electrical Conductivity of Omphacite as a Function of Water Content and Implications for High Conductivity Anomalies in the Dabie‐Sulu UHPM Belts and Tibet

AbstractMagnetotelluric surveys revealed high‐conductivity layers in the lower crust beneath Tibet and in the shallow part of the upper mantle relevant to continental collision extending to the Dabie‐Sulu ultrahigh‐pressure metamorphic (UHPM) belts of eastern China, which have been interpreted by the presence of aqueous fluids/partial melts or hydrous phases. However, these explanations are not consistent with their petrological features and seismic properties. Hydrogen‐bearing omphacite could be a probable candidate to explain such high‐conductivity anomalies due to its high water‐partitioning coefficient versus coexisting garnet in eclogite. In this study, we investigated electrical conductivities of Fe‐free and Fe‐bearing omphacite as a function of water content (0.005–0.122 wt.%) at 3 GPa and 500–1300 K. Our results show that water significantly enhances the electrical conductivity of omphacite, while iron facilitates conductivity by accelerating hydrogen diffusivity and lowering its activation enthalpy. Assuming a heat flow of 70 mW/m2, the high electrical anomalies observed beneath the Dabie‐Sulu UHPM belts and the Tibetan Plateau can be reasonably explained by omphacite containing 0.07 wt.% water since water content higher than 0.07 wt.% in omphacite was frequently reported in naturally collected eclogite.

Monazite behaviour during metamorphic evolution of a diamond-bearing gneiss: a case study from the Seve Nappe Complex, Scandinavian Caledonides

Abstract Monazite is a common mineral in metapelitic rocks including those which underwent ultra-high pressure (UHP) metamorphism. During metamorphic evolution monazite adapts its composition to the changing mineral assemblage, especially in its heavy rare earth element contents. We studied this process in diamond-bearing gneiss containing monazite, from Saxnäs in the Seve Nappe Complex of the Scandinavian Caledonides. Although the rock has been re-equilibrated under granulite facies and partial melting conditions, it still preserves minerals from the UHP stage: garnet, kyanite, rutile, and especially diamond. Microdiamonds occur in situ as inclusions in garnet, kyanite and zircon, either as single-crystals or polyphase inclusions with Fe-Mg carbonates, rutile and CO2. Both monazite and diamond occur in the rims of garnet showing the highest pyrope content and a secondary peak of yttrium. Such a position indicates thermally activated diffusion under high temperature at the end of prograde metamorphism. Monazite compositions show negative Eu anomalies, which we interpret to be inherited from the source rock, not reflecting the coexistence with plagioclase and/or K-feldspar which are unstable at UHP conditions. Our results suggest that the effect of whole-rock composition may be more important than that of coexisting phases. The UHP monazite was most likely formed from allanite during subduction and prograde metamorphism. The monazites included in garnet and kyanite are mostly unaltered, whereas those in the matrix show breakdown coronas consisting of apatite, REE-epidote/allanite and REE carbonate, likely formed due to pressure decrease and cooling. U-Th-Pb chemical age dating of monazites yields an isochron centroid age of 472 ±3 Ma. We interpret this age as monazite growth under UHP conditions related to subduction of the Baltica continental margin in Early Ordovician time.

Tectonic implications of P-T paths derived for garnet-bearing felsic gneisses from the Dabie and Sulu ultrahigh pressure terranes, east-central China

Felsic gneisses usually represent the vast majority of rocks in high-pressure (HP) and ultrahigh-pressure (UHP) terranes, but the derivation of metamorphic pressure-temperature (P-T) paths for these rocks have been rarely undertaken. We studied a felsic paragneiss with the mineral assemblage of plagioclase, quartz, phengite, biotite, garnet, epidote, K-feldspar, and rutile from the Chinese Continental Scientific Drilling main hole (depth: 775 m), located in the Sulu UHP terrane, eastern China, by analyzing minerals with the electron microprobe and computing P-T pseudosections with PERPLE_X. Garnet includes phengite, feldspar, quartz, and rutile and retains a Ca-rich core (XCa = 0.42). Towards the rim Ca contents decrease (XCa = 0.36) and Fe and Mg contents increase. This zoning points to a prograde P-T path at pressures lower than 1.3 GPa. In combination with Zr-in-rutile thermometry and garnet volume isopleths in the P-T pseudosection, peak P-T conditions of ca. 1.2 GPa and 610 °C were constrained which are below the wet granite solidus. K-feldspar formation at the expense of phengite indicates a nearly isothermal decompression accompanied by late fluid infiltration. A similar P-T path was additionally derived for a published felsic orthogneiss from the Dabie UHP terrane using P-T pseudosections. The prograde P-T paths obtained in this study imply that not all felsic gneisses from the Dabie and Sulu UHP terranes underwent UHP metamorphism. This is explained by tectonic juxtaposition of various felsic gneisses from different depths in an exhumation channel.

From magmatic generation to UHP metamorphic overprint and subsequent exhumation: A rapid cycle of plate movement recorded by the supra-subduction zone ophiolite from the North Qaidam orogen

The rapid subduction and exhumation of high-pressure (HP) and ultrahigh-pressure (UHP) rocks exposed in collisional belts play a critical role in understanding the evolution of geodynamics and plate tectonics in subduction zone environments. Herein, ophiolitic fragments, eclogites, and amphibolites, from a subduction channel exposed in the Dulan terrane, North Qaidam orogen were investigated. Garnet compositional zoning from the studied eclogites records only a single cycle of subduction and exhumation. New uranium–lead (U–Pb) zircon ages of the current study combined with previous rutile U–Pb ages from eclogites indicate a rapid cycle of events, starting with the generation of the magmatic protolith at ca. 446 Ma, its subsequent subduction reaching a depth of ca. 100–110 km at ca. 435 Ma, and its exhumation into shallow portions of the crust which occurred at ca. 416 Ma. Thus a rapid 30 million years (Myr) protolith generation–subduction–exhumation cycle could be recorded. According to an ophiolitic rock association with island arc basalt (IAB)-like and mid-ocean ridge basalt (MORB)- to ocean island basalt (OIB)-like trace element compositions, the slab-derived components determined by Sr–Nd–Hf isotopes and the short time span of only about 11 Myr between photolith generation and UHP metamorphism in eclogites, the protolith of eclogite is best explained as a supra-subduction zone ophiolite. This finding likely records the closure of the South Qilian ocean and the initiation of a continent-arc collision at ca. 446 Ma. Furthermore, starting with subduction erosion during continent-arc collision in the early Palaeozoic, ophiolitic and arc-related materials become subducted in the North Qaidam. It was inferred that such a rapid succession of events from protolith formation to UHP overprint and final exhumation, could be explained by movements of rock material in a subduction channel. Thus, this study provides a new view in understanding the mechanics of different episodes of subduction systems in North Qaidam UHP metamorphic belt.

Tracking Fe mobility and Fe speciation in subduction zone fluids at the slab-mantle interface in a subduction channel: A tale of whiteschist from the Western Alps

Fluids released by subducting slabs are important to the geochemical evolution of Earth’s crust-mantle system. However, it is still not well constrained for the mobility of redox sensitive elements (like Fe, C or S) in subduction zone fluids, and its effect on the redox property of the overlying mantle wedge and its derived arc magmas. Iron isotope fractionation is sensitive to the redox state and speciation of Fe in fluids, which can potentially provide clues on the oxygen fugacity (fO2) of slab-derived fluids. Whiteschists at the Dora-Maira Massif in the Western Alps have experienced ultrahigh-pressure metamorphism at subarc depths. They are rich in both SiO2 (mostly >65 wt.%) and MgO (4–10 wt.%), and mainly composed of pyrope, quartz/coesite, talc, phengite and kyanite. They were demonstrated to have a granite protolith, but metasomatized by serpentinite-derived Mg-rich fluids at the slab-mantle interface in a subduction channel. Thus these rocks provide an excellent target to explore the fluid-rock interaction by Fe isotopes. The whiteschists show extremely high δ56Fe values of +0.32 to +1.22‰, being much higher than those of +0.10 to +0.38‰ for the surrounding metagranites. The Fe isotope composition of whiteschists significantly deviates from the igneous differentiation trend defined by rocks with basaltic to rhyolitic compositions. The heavy Fe isotope compositions must be produced by a metasomatic process. The much lower Fe2O3 and FeOt contents and Fe3+/∑Fe ratios but higher δ56Fe values for the whiteschists relative to their protolith suggest the reduction of Fe3+ to Fe2+ and the loss of isotopically light Fe, probably in the form of Fe(II)-Cl and/or Fe(II)-(HS) complexes, through saline fluids and/or HS- bearing fluids. The reduction of Fe3+ to Fe2+ and the possible presence of HS- suggests the occurrence of relatively reduced fluids. Such reduced fluids were probably derived from serpentinite dehydration at the slab-mantle interface in the subduction channel. Therefore, the Fe isotope results demonstrate that the fO2 at the slab-mantle interface can be locally highly heterogeneous, leading to various oxidation states in the mantle wedge and arc magmas.

Geochemical evidence from coesite-bearing jadeite quartzites for large-scale flow of metamorphic fluids in a continental subduction channel

Metamorphic fluids produced by dehydration of subducting crust transport mass and energy at the slab-mantle interface in subduction channels. It is commonly assumed that fluid flow is significant in oceanic subduction channels but insignificant in continental subduction channels. This assumption is challenged by a combined study of whole-rock geochemistry, Mg and O isotopes, zircon U-Pb ages and trace elements in coesite-bearing jadeite quartzites from the Dabie orogen, China. Although the target samples were collected from different outcrops in an area of ∼50 km2, zircon U-Pb dating yields similar discordia lines with not only consistent upper intercept ages of 1.9–2.0 Ga but also consistent lower intercept ages of 224–235 Ma. This indicates the same Paleoproterozoic protolith and the same Triassic metamorphism for these jadeite quartzites. The O isotope analysis of mineral separates and whole-rock yields variable δ18O values from 6.3‰ to 9.4‰, indicating involvement of supracrustal components. Except for one outlier at −0.43‰, all the other rocks give variable δ26Mg values from −0.16‰ to 0.61‰, much higher than normal mantle values. The whole-rock Mg isotopes show significant positive correlations not only with MgO contents but also with Rb/La, Rb/Gd and Rb/Nb ratios, but a negative correlation with Na2O contents. These observations indicate that the middle Paleoproterozoic protolith of jadeite quartzites was weathered to produce a kind of sedimentary rocks in a passive continental margin and then underwent significant metasomatism by metamorphic fluids with high δ26Mg values during the continental subduction in the Triassic. The metamorphic fluids were produced by the breakdown of biotite in the metasedimentary rocks during their subduction to subarc depths for ultrahigh-pressure metamorphism. They would have acquired their geochemical compositions not only from the biotite breakdown but also through leaching reaction with the TTG provenance. In view of the spatial occurrences of the target samples, the metamorphic fluids would have flowed inside the continental subduction channel on a large scale. This is the first report of the large-scale fluid flow in the continental subduction zone and therefore demonstrates that fluid flow can be significant in continental subduction channels.

Geochemical characteristics of the organic matter in UPPMRs and the implication for fluid–rock exchange due to the retrograde metamorphism in the Dabie–Sulu orogenic belt, North China

The samples were collected from ultrahigh-pressure para-metamorphic rocks (UPPMRs) around 500–2000 m deep by Chinese Continental Scientific Drilling. The combined method of ultrasonic disintegrator and Soxhlet was strictly conducted to extract the indigenous hydrocarbons from the UPPMRs, to obtain non-contaminated and entire organic matter to reveal the geochemical characteristics and origin of hydrocarbons in the UPPMRs. Through gas chromatography–mass spectrometry analysis, the ratios of Pr/Ph in the soluble hydrocarbon were from 0.04 to 0.87. It was inferred that the precursor of extracts would be deposited in the anoxic setting. Based on the relative content among C27, C28 and C29 steranes, it was found that the main source of organic matter was marine algae. But the isomer ratios of C31 hopane 22S/(22S + 22R) were 0.39–0.69, and the distribution range of C29 sterane 20S/(20S + 20R) was 0.41–0.63, both of which reflected the equivalent maturity of organic matter was no more than 1% (Ro%). Therefore, the immature organic matter derived from algae obviously conflicted with its host rock, which experienced ultrahigh-pressure metamorphism. Therefore, this sort of immature organic matter probably is of secondary origin. Furthermore, the chromatography of n-alkane almost assumes bimodal distribution, and the data of Tmax ranges from 394 to 565°C, as S1/total organic carbon (TOC) ratios systematically increase with the corresponding Tmax. Therefore, it could be further proved that the organic matter in the UPPMRs probably is of mixed origin and is mainly derived from immigrant hydrocarbons during the stages of subduction and retrograde metamorphism. The emergence of Zr-in-rutile also indicates that the geological temperature was lower than that during the peak of metamorphism. Therefore, it could be inferred that the kerogen associated with rutile might go through diagenesis regularly at the stage of retrograde metamorphism. It was also shown that the δ18O value suddenly reduced up to −10‰ corresponding to the highest TOC (1820 μg/g) around the subduction fault. The relationship between the TOC and δ18O value indicated that the fluid–rock exchange reaction was the main reason for the immature organic matter present in the para-metamorphic rock. The returned subduction and retrograde metamorphism resulted in the activity of the formation fluid, which could prominently impact the geochemical characteristics of the para-metamorphic rock and should also be considered in the geodynamics research of the metamorphic orogenic belt.

UHP metamorphism recorded by coesite-bearing metapelite in the East Kunlun Orogen (NW China)

AbstractThe East Kunlun Orogen (EKO) is the NW part of the Central China Orogenic Belt, which records the evolutionary history of the Proto- and Palaeo-Tethys Oceans from the Cambrian to the Triassic. An Early Palaeozoic eclogite belt has been recognized in recent years, which extends discontinuously for ∼500 km as three eclogite-bearing terranes. In this study, we report an integrated study of zircon grains from mica-schists accompanying the eclogites, in terms of mineral inclusions, U–Pb age systematics andP–Tconditions. The presence of coesite is identified, as inclusions within the metamorphic domain of zircons, which provides unambiguous evidence for subducted terrigenous clastic rocks of the Proto-Tethys Ocean exhumed from coesite-forming depths. U–Pb dating of the metamorphic zircons yields a concordia age of 426.5 ± 0.88 Ma, which is likely to be the time of ultrahigh-pressure metamorphism in the Kehete terrane.P–Tcalculations suggest that metapelite may have experienced a clockwiseP–Tpath with peakP/Tconditions of 685 ± 41 °C and >28 kbar, and equilibrated at 482–566 °C and 5.6–8.9 kbar during subsequent exhumation. The high-pressure – ultrahigh-pressure (HP-UHP) metamorphic belt within the EKO may have formed by collision between the Qaidam Block and the South Kunlun Block, as a consequence of the closure of the Proto-Tethys Ocean.

Basement evolution of the Sulu orogenic belt: Constraints on zircon U–Pb ages and trace elements from the Weihai gneisses

Early Paleoproterozoic rocks occur sporadically in the Sulu ultrahigh‐pressure (UHP) belt. The tectonic affinity and evolution of these rocks are still disputed. Two magmatic events (Early Paleoproterozoic and Neoproterozoic) and two metamorphic events (Late Paleoproterozoic and Triassic) were revealed by the zircon U–Pb ages and trace elements from the Weihai gneisses, which provide a key clue to understand the evolution of the Sulu orogen. The Early Paleoproterozoic zircons occur as residual zircons or zircon‐inherited cores in the two gneisses. They generally show oscillation zoning and enriched HREE patterns. The weighted mean age of the upper intercept age of these zircons is ~2.46 Ga, suggesting that the protolith of the ancient gneisses formed at ~2.46 Ga. The Late Paleoproterozoic zircons occur as metamorphic zircon core or mantle domains in the two gneisses and a quartzite. They generally show planar zoning or no zoning, with relative lower Th/U ratios. These zircons yield metamorphic ages of 1.8–1.84 Ga, which are consistent with the age of the Paleoproterozoic ultrahigh‐temperature metamorphism of the northern Sulu orogenic belt. The Neoproterozoic zircons occur as magmatic cores or mantles in the zircon grains of the two gneisses and their wall rocks. They show typical oscillatory zoning and enriched HREE patterns, yielding magmatic ages of ca. 740–787 Ma. The Neoproterozoic igneous rocks are the dominant basement rocks in the Sulu orogenic belt. Triassic UHP metamorphism has been well studied in the Sulu orogenic belt. Two Triassic age groups were also recognized here: 228–229 and 208 Ma, respectively. The Triassic zircons occur as metamorphic rims of the two ancient gneisses and their wall rocks. These two age ranges correspond to the time of subduction and exhumation of the the South China Craton (SCC) during the Triassic orogeny. This study provides solid evidence that the Weihai ancient gneisses have a strong affinity with the Yangtze Block. The protoliths of them are Paleoproterozoic (ca. 2.46 Ga) magmatic rocks. They were subsequently suffered from ca. 1.8‐ to 1.84‐Ga metamorphism, new crustal materials overgrowth during ca. 740‐ to 787‐Ma magmatism, and then experienced Triassic continental deep subduction and exhumation process to build the Sulu orogenic belt.

Zircon from Orogenic Peridotite: An Ideal Indicator for Mantle-Crust Interaction in Subduction Zones

The orogenic peridotites can be subdivided into crust- and mantle-derived types. They record complex geological processes in subduction and collision zones. The crust-derived peridotites are derived from cumulates crystallized from ultramafic-mafic magmas in deep continental crust, an early mantle-crust interaction, prior to subduction. The mantle-derived orogenic peridotites are originated from subcontinental lithospheric mantle (SCLM) wedge and other mantle domains, and are later involved in the subduction channel and orogenic system. The mantle-derived peridotites usually record complex metasomatism, ultra-high pressure (UHP) metamorphism and mantle-crust interaction during the orogenic processes. Zircons are rarely found in orogenic peridotites. These zircons in orogenic peridotites are generally formed during metasomatism, they can be divided into old zircons (mainly the cores of residual magmatic and recrystallized) and newly grown zircons. Three key factors for zircon formation in orogenic peridotites are that: (1) zircon has strong crystallization ability, and Zr is easier to exchange Si in zircon crystal structure than other elements in the mantle; (2) metamorphic destruction of Zr-bearing minerals and precipitation of intergranular melts during the high-grade metamorphism can nucleate zircon under sub-solidus conditions; (3) the melts/fluids released from the subducted crust can metasomatize the mantle wedge to form zircons. In-situ studies on zircons and zircon inclusions enclosed in mantle minerals indicate that zircon can be an ideal indicator for mantle- crust interaction in subduction zones. The inclusions in zircons and Hf-O isotope of zircons are effective to reflect the composition of the melts/fluids, source properties, and the physical and chemical conditions. Dating of the zircons has been widely used in the studies of lithospheric evolution and crust-mantle interaction. During the complex processes of plate convergence, the orogenic peridotites can be subjected to the melt/fluid metasomatism, modifying the mineral and elemental compositions of peridotites. Thus, zircon is very useful to unravel the history of specific lithospheric mantle and the relationship between continental cratonic cores and their margins.

Metamorphic Evolution and Tectonic Implications of the Granulitized Eclogites from the Luliangshan Terrane in the North Qaidam Ultrahigh Pressure Metamorphic Belt, NW China: New Constraints from Phase Equilibrium Modeling

The granulitized eclogites from the Luliangshan terrane of the North Qaidam UHP metamorphic belt occur as lenses within pelitic gneisses and orthogneisses. Combined petrologic data and phase equilibrium modeling indicate a multi-stage metamorphic history of the granulitized eclogites: (1) an earlier eclogite facies metamorphism (P>18.5 kbar, T> 830 °C) is deduced from omphacite relics in the matrix and rare omphacite inclusions within garnet. The possible assemblage is garnet+omphacite+rutile+ quartz; (2) the early stage of high pressure granulite facies assemblages (garnet+clinopyroxene+ plagioclase+rutile+quartz+liquid) developed in the early decompression process has a P-T regime of 17.5 kbar and 852–858 °C, constrained by plagioclase and clinopyroxene inclusions in garnet. The late stage of high pressure granulite assemblages (garnet+clinopyroxene+amphibole+plagioclase+rutile+quartz+liquid) records an isothermal decompression process with the pressure successively declining from 17.5 to 14.7 kbar and to 11.3 kbar at 858 °C; (3) the later medium pressure granulite facies assemblage (garnet+ orthopyroxene+ clinopyroxene+amphibole+plagioclase+ilmenite+liquid+quartz) indicates a drop in pressure and rise in temperature at P-T conditions of 7.6–7.7 kbar and 878–883 °C; (4) retrogressive amphibolite facies stage, which is represented by amphibole+plagioclase kelyphitic rims around garnet, formed under conditions of <5 kbar and <650 °C. The preservation of medium pressure granulite facies assemblage and the garnet composition feature constrain a following isobaric cooling path during late exhumation. This process suggests a clockwise P-T path and indicates that the granulitized eclogites record a high grade “Barrovian” metamorphic overprint at the middle-lower crust during exhumation. The present data show that the Luliangshan terrane is a “hot” HP-UHP terrane.

Diamond and coesite inclusions in detrital garnet of the Saxonian Erzgebirge, Germany

Abstract Local occurrences of coesite- and diamond-bearing rocks in the central Erzgebirge (northwestern Bohemian Massif, Germany) reveal ultrahigh-pressure (UHP) metamorphic conditions during the Variscan orogeny. Although UHP metamorphism supposedly affected a wider area, implying that rocks that equilibrated under UHP conditions occur dispersed in large volumes of high-pressure country-rock gneisses, mineralogical evidence is scarce. Here we have applied the new concept of capturing the distribution and characteristics of UHP rocks by analyzing inclusions in detrital garnet. Out of 700 inclusion-bearing garnets from seven modern sand samples from creeks draining the UHP area around the Saidenbach reservoir, we detected 26 garnets containing 46 mainly monomineralic coesite inclusions and 22 garnets containing 41 diamond inclusions. Combining these results with geochemical classification of the host garnets, we show (1) that coesite-bearing rocks are common and comprise eclogites as well as felsic gneisses, (2) that small inclusion size is a necessary precondition for the preservation of monomineralic coesite, and (3) for the first time, that diamond-bearing crustal rocks can be detected by analyzing the detrital record. Our results highlight the potential of this novel application of sedimentary provenance tools to UHP research, and the necessity of looking at the micrometer scale to find evidence in the form of preserved UHP minerals.

The geodynamic setting of Dulan eclogite-type rutile deposits in the North Qaidam orogen, western China

Abundant eclogite-type rutile deposits have recently been discovered along the North Qaidam ultrahigh-pressure (UHP) metamorphic belt in China. The identified titanium reserves exceed 3 million tons and range in grade from 1.0% to 4.5%. The rutile deposits of economic interest are found within two types of eclogite. However, the origin and original geodynamic setting of the two types of rutile-bearing eclogites remain controversial. This study therefore investigates ore and mineralization bodies belonging to the two types by way of detailed field work and geochronological, geochemical and Sr–Nd–Hf isotopic compositional analysis, taking the Dulan eclogite-type rutile deposit as its case study. Type-1 rutile-bearing eclogites enclosed in granitic gneisses are found to have Neoproterozoic protolith ages of 845 ± 4 Ma and an eclogite-facies metamorphic age of ca. 433 Ma. Their high TiO2, TFe2O3, and P2O5 contents cannot have been produced by rutile-formation during UHP metamorphism but could have arisen through the accumulation of ilmenite and apatite in a high-Ti protolith. They have ocean-island basalt (OIB)-like trace element patterns, with enrichment in light rare-earth elements (LREEs) and high field strength elements (HFSEs) (Nb, Ta, Zr, and Hf). The Hf isotopic compositions of the residual magmatic zircon cores in Type-1 rutile-bearing eclogites (εHf (845) = +1.4 to +6.0, TDM = 1.2–1.5 Ga) indicate that their parental magmas were derived from the Proterozoic mantle. However, they show initial 87Sr/86Sr values of 0.71059–0.71326 and εNd (845) values of −1.5 to −2.5, suggesting significant crustal contamination. Thus, the Type-1 rutile-bearing eclogites may have originated through partial melting of an OIB-type mantle plume source during the Neoproterozoic rifting of the Rodinia supercontinent. In contrast, the Type-2 rutile-bearing eclogites, which are hosted in a mafic–ultramafic ophiolitic complex, have protolith ages of 449 ± 3 Ma and eclogite-facies metamorphic ages of ca. 435–433 Ma. Correlations between the whole-rock MgO and the major and trace element contents of Type-2 rutile-bearing eclogites suggest that they are derived from cumulates with alternating olivine and pyroxene-rich and ilmenite-rich layers. They have mid-ocean-ridge basalt (MORB) and OIB-like trace element patterns, indicating extensive fractional crystallization and the possibility that there were multiple injections of magma during the formation of the protolith. Meanwhile, the +0.6 to +4.3 εNd (449) values and −1.8 to +11.8 εHf (449) values of the Type-2 rutile-bearing eclogites indicate that the protoliths may have been generated by partial melting of a mantle wedge source metasomatized by slab-derived components. The short interval (ca. 14 my) between magma generation and eclogite-facies metamorphism, in combination with the type of rock assemblage within which they are found, indicates that the Type-2 rutile-bearing eclogites originated in a supra-subduction zone (SSZ) ophiolite. Type-1 and Type-2 rutile-bearing eclogites have similar eclogite-facies metamorphic ages of ca. 435–433 Ma, indicating that the Type-1 and SSZ-related Type-2 protoliths were incorporated into a subduction mélange that was dragged down the subduction channel and underwent eclogite-facies metamorphism via subduction of the Qaidam Block, eventually forming eclogite-type rutile deposits.

Ultrahigh Pressure Metamorphism and Tectonic Evolution of Southwestern Tianshan Orogenic Belt, China: A Comprehensive Review

AbstractRecently, a huge ultrahigh‐pressure (UHP) metamorphic belt of oceanic‐type has been recognized in southwestern (SW) Tianshan, China. Petrological studies show that the UHP metamorphic rocks of SW Tianshan orogenic belt include mafic eclogites and blueschists, felsic garnet phengite schists, marbles and serpentinites. The well‐preserved coesite inclusions were commonly found in eclogites, garnet phengite schists and marbles. Ti‐clinohumite and Ti‐chondrodite have been identified in UHP metamorphic serpentinites. Based on the PT pseudosection calculation and combined U‐Pb zircon dating, the P‐T‐t path has been outlined as four stages: cold subduction to UHP conditions before ∼320 Ma whose peak ultrahigh pressure is about 30 kbar at 500oC, heating decompression from the Pmax to the Tmax stage before 305 Ma whose peak temperature is about 600oC at 22kbar, then the early cold exhumation from amphibolite eclogite facies to epidote‐amphibolite facies metamorphism characterized by ITD PT path before 220 Ma and the last tectonic exhumation from epidote amphibolite facies to greenschist facies metamorphism. Combining with the syn‐subduction arc‐like 333‐326 Ma granitic rocks and 280‐260 Ma S‐type granites in the coeval low‐pressure and high‐temperature (LP‐HT) metamorphic belt, the tectonic evolution of Tianshan UHP metamorphic belt during late Cambrian to early Triassic has been proposed.

The Metamorphic Evolution and Tectonic Significance of the Eclogite in the Sumdo Complex, Tibet, China

AbstractAs the main tectonic component of the Himalayan–Tibetan orogen, the Lhasa terrane has received much attention as it records the entire history of the orogeny. The occurrence of high pressure eclogite in the Sumdo complex in central Lhasa terrane has a significant bearing on the understanding of the Paleo‐Tethys subduction and plate itineration processes in this area. The petrological, geochemical and geochronological data from eclogite and associated blueschist and garnet‐bearing mica schist from Sumdo, Jilang and Bailang area have been briefly review to explore the origin and metamorphic evolution of this suture. Eclogites from the Sumdo complex have experienced low temperature, high pressure to ultrahigh pressure metamorphism, revealing a fast subduction and exhumation process in a typical oceanic subduction zone. The large P‐T span between different eclogites in the literature may be affected by the big error of unappropriated using geothermobarometry and may also because of slices of subducted blocks derived from different depths juxtapose together during exhumation. By summarizing the U‐Pb, Lu‐Hf and Sm‐Nd ages of eclogites, the eclogite facies metamorphism is likely to occur in early Triassic during 245‐225 Ma, but not the previously accepted late Permian at ca. 260 Ma by the reinterpretation of the former geochronological data from literature. The opening of Paleo‐Tethys Ocean between the Lhasa terrane initiate prior to ca. 280 Ma and ultimate closure to integrate the Lhasa terrane was no earlier than 225 Ma and may triggered by the initial subduction of Bangong‐Nujiang Tethys Ocean in the north.

About this title - HP–UHP Metamorphism and Tectonic Evolution of Orogenic Belts

High pressure (HP) and ultrahigh pressure (UHP) metamorphic rocks play a key role in understanding the tectonic evolution of orogenic belts. They have typically experienced complex changes during subduction and exhumation processes arising from recrystallization, deformation, fluid–rock interactions and even partial melting, and may therefore carry valuable records of evolving geodynamic systems in an orogenic belt. This special publication addresses the current work on HP–UHP metamorphism and its relation to the tectonic evolution of orogenic belts. This special publication contains fifteen papers covering the important orogenic belts of the Himalaya, Dabie–Sulu, Tian Shan, North Qaidam and others that have been grouped into three parts: (I) new developments in the determination of metamorphic pressure–temperature (PT) conditions and their timing, (II) overview papers of well-known HP–UHP metamorphic belts and (III) research papers for some newly discovered HP–UHP belts.

The Secret Life of Granite: A Story of Emplacement, Alteration, Subduction and Metamorphism

Ultra-high pressure (UHP) metamorphism has long been associated with deep subduction. UHP minerals, such as coesite and microdiamond, are often found in discrete mafic units hosted in larger scale ...