Ultrahigh-pressure Metamorphism Research Articles

Tracing a remnant of subducted Indian felsic crust: insights from zircon studies

Zircon is a common mineral in nature that survives varied pressure and temperature conditions in the subduction process. It has excellent ability to reveal progressive metamorphic history and, hence, is useful in reconstructing the subduction tectonics in collisional orogenic belts. In the Tso Morari Gneiss of the Indus Suture Zone, Himalaya, eclogite boudins have registered the imprint of subduction-related ultrahigh-pressure (UHP) metamorphism; this imprint is, however, missing in the host gneisses. To search for the missing link, zircons of the gneisses were studied. The zircon overgrowth and the numerous mineral inclusions indicate the metamorphic responses of the gneisses. The Raman spectra of minerals show the cores of the zircon consist of apatite and quartz and the surrounding overgrowth preserves quartz–coesite, c-polymorphs and other metamorphic minerals. The distribution pattern of these minerals in the zircons is consistent with the Th/U ratios ranging from 0.30 to 0.01, recognizing the inner magmatic and outer metamorphic domains. The U–Pb ages from the inner magmatic (at c. 500 Ma) and from the outer metamorphic growth (at c. 45–42 Ma) suggest the former is the protolith age and the latter the metamorphic age of the gneisses. The tectonic interpretation reveals the subduction of Indian felsic crust to UHP depth (>100 km) at c. 45 Ma. Supplementary material : Raman spectra of C-inclusions in the zircon of Tso Morari Gneiss are available at https://doi.org/10.6084/m9.figshare.c.7168228 Thematic collection: This article is part of the Mesozoic and Cenozoic tectonics, landscape and climate change collection available at: https://www.lyellcollection.org/topic/collections/mesozoic-and-cenozoic-tectonics-landscape-and-climate-change

The Geodynamic Significance of Continental UHP Exhumation: New Constraints From the Tso Morari Complex, NW Himalaya

AbstractThe burial and exhumation of continental crust to and from ultrahigh‐pressure (UHP) is an important orogenic process, often interpreted with respect to the onset and/or subduction dynamics of continent‐continent collision. Here, we investigate the timing and significance of UHP metamorphism and exhumation of the Tso Morari complex, North‐West Himalaya. We present new petrochronological analyses of mafic eclogites and their host‐rock gneisses, combining U‐Pb zircon, rutile and xenotime geochronology (high‐precision CA‐ID‐TIMS and high‐spatial resolution LA‐ICP‐MS), garnet element maps, and petrographic observations. Zircon from mafic eclogite have a CA‐ID‐TIMS age of 46.91 ± 0.07 Ma, with REE profiles indicative of growth at eclogite facies conditions. Those ages overlap with zircon rim ages (48.9 ± 1.2 Ma, LA‐ICP‐MS) and xenotime ages (47.4 ± 1.4 Ma; LA‐ICP‐MS) from the hosting Puga gneiss, which grew during breakdown of UHP garnet rims. We argue that peak zircon growth at 47–46 Ma corresponds to the onset of exhumation from UHP conditions. Subsequent exhumation through the rutile closure temperature, is constrained by new dates of 40.4 ± 1.7 and 36.3 ± 3.8 Ma (LA‐ICP‐MS). Overlapping ages from Kaghan imply a coeval time‐frame for the onset of UHP exhumation across the NW Himalaya. Furthermore, our regional synthesis demonstrates a causative link between changes in the subduction dynamics of the India‐Asia collision zone at 47–46 Ma and the resulting mid‐Eocene plate network reorganization. The onset of UHP exhumation therefore provides a tightly constrained time‐stamp significant geodynamic shifts within the orogen and wider plate network.

Continental subduction in Paleoproterozoic: Insights from ∼ 1.9 Ga nepheline syenite in the North China craton

Subduction of continent is known as a feature of modern plate tectonics, and it is still unclear whether such a process also occurred in the early Precambrian. Despite of ultra-high pressure metamorphism of terrestrial rocks as a proxy of continental subduction, magmatism produced during continental subduction is also a potential alternative. The North China craton is well-known for the occurrence of 1.9 Ga retrograde eclogite, which was likely originated from the subducting oceanic crust along the Trans-North China Orogen (TNCO). Here a nepheline syenite (the Aohu pluton), the first of the kind from the TNCO, is discovered in Yunzhong Mts. It comprises nepheline (∼20 vol%), albite (50–55 vol%), K-feldspar (5–10 vol%), biotite (∼10 vol%), and calcite (∼5 vol%), with minor accessory minerals. No associated mafic or silicic rocks are outcropped in the pluton. SHRIMP zircon U-Pb analyses yield a crystallization age of 1893 ± 13 Ma and a metamorphic overprinting of 1871 ± 17 Ma. The rocks have medium SiO2 contents (51.9–58.9 wt%), low MgO (0.6–1.6 wt%) and Mg# (24–34), but high Al2O3 (20.0–22.9 wt%) and extremely high K2O + Na2O contents (10.0–13.6 wt%; Na2O/K2O = 1.3–2.6). They are enriched in light rare earth elements (REEs) and large ion lithophile elements, but relatively depleted in heavy REEs and high field strengthen elements. They have extremely low compatible elements (e.g., Ni and Cr). They show high (87Sr/86Sr)i ratios (0.706–0.707) but low εNd(t) values (−5.3 to −6.0; Nd T2DM = ca. 2.7 Ga). Their zircon in-situ εHf(t) values have a wide range (0 to −5.7; Hf T2DM = 2.5–2.85 Ga). The absence of mafic component and the extremely low-Mg feature, as well as their trace element patterns and initial Sr-Nd isotopes being compatible with melts derived from the Archean mafic crustal enclaves in the region, support a continental crust origin for the Aohu nepheline syenite. Based on comparison and modeling, we suggest that its generation was most likely under ultra-high pressure condition. It is thus proposed that continental subduction (into the mantle) might have occurred in the TNCO during the Paleoproterozoic.

Ultra-deep (>5 GPa) subduction of paragneiss and metagranite in the Dabie UHP Terrane, central China: Constraints from zircon inclusions, phase equilibrium modelling and U-Pb geochronology

In the Dabie orogen, central China, the ultrahigh pressure metamorphism (UHPM) was best recorded from meta-mafic and ultramafic rocks occurring as lenses within voluminous amphibolite-facies metafelsic rocks. However, whether or not these metafelsic rocks experienced ultra-deep subduction (>5 GPa) has been rarely quantified because of the rare preservation of peak diagnostic assemblages. In this study, we reconstruct the P-T-t paths for paragneisses and metagranites from the Dabie region to better elucidate their metamorphic evolution based on detailed petrological, mineralogical and Raman micro-spectrometer analysis of zircon inclusion combined with conventional geothermobarometry, phase equilibrium modelling and U-Pb dating. At least three stages of metamorphic evolution are identified for these lithologies. The UHPM is represented by the assemblages of coesite + garnet Ia/Ib + jadeite + phengite + K-feldspar + aragonite for paragneiss and diamond + aragonite + magnesite + phengite + rutile + garnet for metagranite, respectively. The paragneiss experienced peak conditions of 4.5–6.0 GPa/690–720 °C, followed by a decompression heating to 3.5–4.6 GPa/750–850 °C. Also, the diamond + aragonite + magnesite inclusions in zircon from the metagranite provide the first direct mineralogical evidences for ultra-deep subduction. Furthermore, the minimum peak pressure was defined at 5.4 GPa based on pseudosection for metagranite. As a result, this study provides the first mineralogical and modelling evidences of ultra-deep subduction for the metafelsic rocks in the Dabie orogen. The dating results suggest that the UHPM, anatexis and amphibolite-facies overprint occurred at 236 ± 3 Ma, 226 ± 1 Ma and 195 ± 4 Ma, respectively. The estimated average exhumation rate is 5.8–7.2 km/Ma from peak UHP to amphibolite-facies stages. Moreover, a comparison of different collisional orogens suggests that the preservation of UHP minerals in metafelsic rocks strongly depends on exhumation rate.

Sedimentological Evidence for Pre‐Early Permian Continental Subduction in the Dabie Orogen, Central‐East China

AbstractHow to constrain the onset of continental subduction and prograde metamorphism in an orogen remains a fundamental question. The widespread Triassic ultrahigh‐pressure (UHP) metamorphic rocks in the Dabie orogen in central‐east China are generally attributed to continental subduction. The earliest Triassic peripheral basins (the Huangshi, Yueshan and Nanjing basins) around the Dabie orogen represent an ideal archive to reconstruct the early evolution of the orogen. Here we present a multidisciplinary provenance study on these Triassic strata, including framework petrography and heavy mineral analyses combined with U‐Pb age and trace‐element analyses of detrital zircon, rutile and apatite. The abundant metapelitic lithics and muscovite grains, a heavy mineral population dominated by metamorphic apatite, and a significant Permian (270–290 Ma) age peak from U‐Pb age spectra of detrital zircon, rutile and apatite imply an Early Permian medium‐high grade metamorphic source in the uppermost structural levels of the Dabie orogen. Our provenance data indicate that the onset of northward continental subduction of the South China Block commenced no later than the Early Permian (c. 290 Ma). This event is clearly earlier and distinct from the more widespread Triassic UHP metamorphism of the Dabie orogen, suggesting that continental subduction and mountain‐basin interaction were protracted processes (>55 Myr, c. 290–235 Ma). When combined with other geological evidence, our results show that the prolonged continental subduction is not always preceded by subduction of oceanic crust.

Retrogression of ultrahigh-pressure eclogite, Western Gneiss Region, Norway

Abstract. The Western Gneiss Region (WGR) in western Norway exposes ultrahigh-pressure (UHP) eclogites that occur repeatedly, within an area of high-pressure (HP) eclogites, without evidence of being separated by tectonic shear or ductile flow structures. We studied 10 eclogites from two northern UHP areas and the interjacent HP area to evaluate the significance of this pattern. The orthopyroxene in orthopyroxene-bearing samples has low Al2O3 contents (0.17 wt %–0.37 wt %), provided its grain boundaries were unaffected by partial recrystallisation or replacement. Classical geothermobarometry based on element partitioning between coexisting mineral phases suggests metamorphic conditions within the diamond stability field for the samples from both the HP and UHP areas. The primary clinopyroxene in the associated orthopyroxene-free eclogites contains aligned inclusions of either needle-shaped quartz ± pargasite or lamellar albite, which are absent from the secondary (symplectic) clinopyroxene. Reconstructed mineral compositions of the primary clinopyroxene obtained from grain cross-section surfaces using a scanning electron beam or image processing are non-stoichiometric, and they have higher Ca-Eskola and lower Ca-Tschermak components than the inclusion-bearing host clinopyroxene. The molar ratios of these endmembers are consistent with the needles in the primary clinopyroxene being formed from vacancy-bearing precursor clinopyroxene by the exsolution reaction 2 Ca-Eskola = Ca-Tschermak + 3 quartz during early eclogite-facies retrogression. Further retrogression partially transformed the needle-shaped quartz to irregularly shaped albite within the clinopyroxene and partially transformed both clinopyroxene generations to amphibole that occasionally preserves the needles. The similarity of both the maximum metamorphic conditions and the mineral exsolution microstructures in the eclogites from UHP and HP areas indicates a shared metamorphic history within the stability field of diamond, but a history that diverged during retrogression. Consequently, the alternations of UHP and HP areas in the WGR may have formed by a process that allowed for spatial variations in retrogression efficiency, such as the localisation of strain (recrystallisation) or fluid flow (diffusion) or both, rather than by tectonic stacking of UHP and HP units. Evidence for the UHP metamorphism of WGR crustal rocks is now found from NE to SW along the entire coastal section that covers previously recognised UHP and interjacent areas.

Trace Elements of Multi‐stage Minerals and Titanite U‐Pb Dating for the Gneisses from Liansan Island, Sulu UHPM Belt

AbstractGneisses with anatectic characteristics from the Liansan island in the Sulu UHPM (ultra‐high pressure metamorphic) belt were studied for petrography, titanite U‐Pb dating and mineral geochemistry. Three origins of garnets are distinguished: metamorphic garnet, peritectic garnet and anatectic garnet, which are formed in the stages of peak metamorphism, retrograde anatexis and melt crystallization, respectively. The euhedral titanite has a high content of REE and high Th/U ratios, which is interpreted as indicating that it was newly‐formed from an anatectic melt. The LA‐ICP‐MS titanite U‐Pb dating yields 214–217 Ma ages for the titanite (melt) crystallization. The distribution of trace elements varies in response to the different host minerals at different stages. At the peak metamorphic stage, Y and HREE are mainly hosted by garnet, Ba and Rb by phengite, Sr, Nb, Ta, Pb, Th, U and LREE by allanite and Y, U and HREE by zircon. During partial melting, Y, Pb, Th, U and REE are released into the melt, which causes a dramatic decline of these element contents in the retrograde minerals. Finally, titanite absorbs most of the Nb, U, LREE and HREE from the melt. Therefore, the different stages of metamorphism have different mineral assemblages, which host different trace elements.

Garnet–Quartz Inclusion Thermobarometry and Lu–Hf Chronology Detail the Pre-Ultra-High Pressure Metamorphic History of the Grapesvare Nappe, Scandinavian Caledonides

ABSTRACT The subduction–exhumation history of the Grapesvare nappe in the northern Seve Nappe Complex (Scandinavian Caledonides) is recorded by late Cambrian/Early Ordovician ultra-high pressure (UHP) and subsequent amphibolite facies metamorphic events. Records of these events obscured earlier metamorphic episodes that are important for understanding the tectonics of the orogen. To extract the pre–UHP metamorphic records, garnet Lu–Hf geochronology, Titanium-in-Quartz thermobarometry, and Quartz-in-Garnet elastic thermobarometry were applied to garnet porphyroblasts in metasedimentary rocks and eclogite. Metasedimentary rocks contain chemically homogeneous garnet (Grt-M1) with shape-matured quartz inclusions. In some rocks, these garnets are overgrown by garnet with bell-shaped Mn-zoning (Grt-M2) containing irregularly-shaped quartz inclusions. This evolution is interpreted as partial dissolution of Grt-M1 and subsequent growth of Grt-M2. Garnet in the eclogite is volumetrically dominated by eclogite-facies garnet (Grt-E1) that envelope remnants of an older, chemically distinct generation (Grt-E0) with highly irregular and diffuse boundaries. Shape-matured quartz inclusions are present within both garnet generations and define a zoning pattern that is not reflective of the chemical zoning. Collectively, these characteristics are interpreted as replacement of Grt-E0 by Grt-E1 via interface-coupled dissolution–reprecipitation, with the latter inheriting the shape-matured quartz inclusions of the former. Pressure–temperature (P–T) conditions extracted from the quartz inclusions in Grt-M1 and Grt-E0/E1 are 1.08 to 1.21 GPa at 645°C to 695°C and 0.94 to 1.03 GPa at 605°C to 640°C, respectively. These conditions are interpreted as cooling of the rocks from a high temperature metamorphic history, altogether preceding subduction of the Grapesvare nappe. The quartz inclusions in Grt-M2 record 1.04 to 1.21 GPa at 620°C to 675°C, interpreted as prograde metamorphic growth of Grt-M2 during subduction at 495.7 ± 3.2 Ma. Subsequent eclogite-facies metamorphism was responsible for the formation of Grt-E1 at the expense of Grt-E0. The collective results indicate a prolonged polymetamorphic history of the Grapesvare nappe prior to UHP metamorphism that has not been recognized previously.

Cambrian ages for metavolcanic rocks in the Lower Köli Nappes, Swedish Caledonides: implications for the status of the Virisen arc terrane

The Köli Nappe Complex (KNC) of the Scandinavian Caledonide orogen originated as oceanic terranes within the Iapetus Ocean. These terranes have characteristics of magmatic arcs and associated forearc or back-arc basins and underwent several periods of rifting and magmatism prior to their accretion to the Baltican margin. We present new U–Pb zircon ages from the Lower Köli Ankarede Volcanite Formation in Västerbotten, Sweden. U–Pb ages of magmatic zircon grains from metamorphosed dacitic to andesitic rocks show ages of 512 ± 3.5, 497 ± 2, 491 ± 1 and 488 ± 4 Ma. The three younger ages fit with previous ages for Lower Köli volcanic rocks, but the 512 Ma age is older than any previous age for this unit. These dates constrain the age of magmatism in an ensimatic arc system within Iapetus. We compare this evolution with published information from the other Köli nappes. Magmatic ages within the KNC overlap with ages for an early episode of ultrahigh-pressure (UHP) metamorphism within the underlying Seve Nappe Complex (SNC), supporting the hypothesis that attributes UHP metamorphism within the SNC to subduction beneath the island arc now preserved within the Lower Köli Nappes. Supplementary material: Photographs of sampled exposures (S1), BSE images of selected zircons (S2), charts of zircon trace elements (S3) and analyses of zircon (S4, S5) are available at https://doi.org/10.6084/m9.figshare.c.6843787 Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wilson-cycle

From divergent to convergent plate boundary: A ca. 200 Ma Wilson cycle recorded by ultrahigh-pressure eclogites in the Dora-Maira Massif, Western Alps

Sparse eclogite exposure in accretionary-to-collisional orogens cannot only reveal the sites of ancient subduction zones and plate boundaries, but also elucidate the multi-stage tectonic evolution in the Wilson cycle. The southern part of the Dora-Maira Massif in the Western Alps is well known for its ultrahigh-pressure (UHP) rocks and contains small occurrences of petrographically distinct types of eclogite. Due to the lack of detailed geochronological and geochemical data on these rocks, the nature of their protolith and related geodynamic setting has remained unknown. In the current study, 33 samples from 13 localities were studied by whole-rock major- and trace-element analysis, and a selection of the samples were studied using Sr-Nd-Hf isotopes, as well as high-resolution elemental mapping and U-Pb geochronology of zircon. According to macroscopic appearance, petrography, chemical composition, and principal-component analysis of multi-elements, two major types are distinguished. Light-colored eclogite is phengite-rich, commonly foliated, bears kyanite and quartz/coesite, shows relatively high MgO, K2O, SiO2, εNd(t) (−2.2 to +1.4), and εHf(t) (+3.5 to +7), with a protolith geochemically similar to an enriched-type mid-oceanic-ridge basalt (E-MORB). Dark eclogite generally is massive, rutile-rich, shows higher values of Ti, Fe, P, Nb, and Zr, εNd(t) values between −2.8 and −0.8, εHf(t) values between −6.1 and −3.2, with an oceanic island basalt (OIB)-type protolith composition. Magmatic zircon cores, which are characterized by steep heavy rare earth element (HREE) patterns and negative Eu anomalies, yield consistent protolith ages of ca. 253−252 Ma in both eclogite types. Metamorphic zircon domains with flat HREE patterns and insignificant Eu anomalies yield a younger mean age of ca. 34 Ma, which is the age commonly assigned to eclogite-facies UHP metamorphism. Considering the various geochemical signatures of E-MORB- to OIB-like eclogites and the regional tectonic evolution, their protolith is best explained as rocks, which crystallized from rift-related basaltic magma, associated with the break-up of the Pangea Supercontinent that eventually resulted in the birth of the Piemonte-Liguria Ocean (Alpine Tethys Ocean) between the future Eurasia and Adria plates. Therefore, the UHP eclogites in the Dora-Maira Massif likely fingerprint a multi-stage tectonic evolution from divergent (continental extension, rifting) to convergent (subduction zone) plate boundary, corresponding to the beginning and end of a Wilson cycle. Incidentally, they reveal that mafic rocks in the Alpine basement units may not be polymetamorphic, but may actually consist of post-Variscan products that underwent only Alpine metamorphism.

Exhumation of ultrahigh-pressure eclogite and metarodingite within UHP serpentinites from the Chinese southwestern Tianshan

Extensive research over the past two decades suggests that serpentinite loses its ability to effectively facilitate the exhumation of high-density high-pressure (HP) eclogites at depths of 60–80 km under average subduction zone geotherms. However, at Changawuzi, the western termination of Chinese southwestern Tianshan, eclogite and eclogitic metarodingite are commonly enclosed within ultrahigh-pressure Ti-chondrodite serpentinites (510–530 °C, 37 ± 7 kbar). This metamorphic mafic–ultramafic association, which has undergone a complex evolution from oceanic alteration to deep subduction and subsequent exhumation, presents a unique opportunity to study the incompletely understood mechanisms leading to the exhumation of ultrahigh-pressure (UHP) eclogite in UHP serpentinite.The Changawuzi serpentinite wraps three different rock types: retrograded eclogite, Rodingite I and Rodingite II. Field observations and petrological analyses indicate that the precursor of Rodingite I intruded as vein into the serpentinite protolith prior to subduction, sharing the evolution towards ultrahigh-pressure conditions. According to its composition, the serpentinite originated from a depleted oceanic mantle peridotite, with only minor enrichment in fluid-mobile elements (FME) during oceanic serpentinization. The eclogites have N-MORB like bulk compositions, with lower Si but higher Mg content compared to other eclogites in the Tianshan area, and appear to have been relatively unaffected by oceanic alteration processes. The Changawuzi serpentinite and eclogites likely evolved within a slab setting with limited interaction with seawater. Petrological studies and phase equilibria modeling suggest that the eclogites experienced peak UHP metamorphism at 28–34 kbar and around 460–465 °C. Density measurements yielded relatively low densities of 2.66–2.97 g/cm3 for the host serpentinites, but 3.27–3.63 g/cm3 for Rodingite I. The similarity in the low temperature/ultrahigh pressure conditions, as well as the geochemical characteristics shared by the serpentinite and eclogites, further support the connection between the Changawuzi serpentinites and the interior of the subducting slab. According to our data, two types of UHP eclogitic mafic rocks exhumated with UHP serpentinite. (1) High density Rodingite I veins are embedded within serpentinite and share a common evolution and exhumation history with the host serpentinite. (2) The eclogite and serpentinite tectonically detached from the interior of the subducting slab at different depths and ascended to the plate interface above the slab (subduction channel). These two rock types experienced separate exhumation processes within the subduction channel during the early stages of exhumation. Subsequently, the buoyant serpentinite captured the denser retrograded eclogite at a depth of approximately 80 km, and together they underwent further retrograde metamorphism as they ascended towards the surface. During a later exhumation stage, external fluid induced a second serpentinization process, leading to the formation of Rodingite II within the subduction channel. This study presents a novel exhumation mechanism specific to the Tianshan eclogites and highlights the significant role of deeply subducted serpentinite in the exhumation of eclogites and eclogitic metarodingites from sub-arc depths within an oceanic subduction zone.

Origin of Erzgebirge ultrahigh‐pressure garnetite: Formation from a basaltic protolith by serpentinization‐assisted metasomatism?

AbstractErzgebirge ultrahigh‐pressure (UHP) garnet peridotite includes scarce layers of garnet pyroxenite, nodules of garnetite and, very rarely, of eclogite. Peridotite‐hosted eclogite shows the same subalkali‐basaltic bulk rock composition, mineral assemblage and peak conditions as gneiss‐hosted eclogite present in the same UHP unit. Garnetite has considerably more Mg, moderately enhanced Ca and Fe and significantly lower contents of Na, Ti, P, K and Si than eclogite, whereas Al is very similar. In addition, the compatible trace elements (Ni, Co, Cr, V) are elevated and most incompatible elements (Zr, Hf, Y, Sr, Rb and rare Earth elements [REE]) are depleted in garnetite relative to eclogite. In contrast to other large ion lithophile elements (LILEs), Pb (+121%) and Ba (+83%) are strongly enriched. The REE patterns of garnetite are characterized by depletion of light and heavy REE and a medium REE hump indicative of metasomatism, features being absent in eclogite. An exceptional garnetite sample shows an REE distribution similar to that of eclogite. Garnetite is interpreted to have formed from the same, but metasomatically altered, igneous protolith as eclogite. Except for Ba and Pb, the chemical signature of garnetite is explained best by metasomatic changes of its basaltic protolith caused by serpentinization of the host peridotite. Garnetite is chemically similar to basaltic rodingite/metarodingite. Although rodingite is commonly more enriched in Ca, there are also examples with moderately enhanced Ca matching the composition of Erzgebirge garnetite. Limited Ca metasomatism is attributed to the preservation of Ca in peridotite during hydrous alteration. This can be explained by incomplete serpentinization favouring metastable survival of the original clinopyroxene. In this case, most Ca is retained in peridotite and not available for infiltration and metasomatism of the garnetite protolith. This inescapable consequence is supported by the fact that clinopyroxene is part of the garnet peridotite UHP assemblage, which would not be the case if Ca had been removed from the protolith prior to high‐pressure metamorphism. The enrichment of compatible elements in garnetite is attributed to decomposition of peridotitic olivine (Ni, Co) and spinel (Cr, V) during serpentinization. Enrichment of Ba and Pb contrasts the behaviour of other LILEs and is ascribed to dehydration of the serpentinized peridotite (deserpentinization). This requires two separate stages of metasomatism: (1) intense chemical alteration of the basaltic garnetite precursor, together with serpentinization of peridotite at the ocean floor or during incipient subduction; and (2) prograde metamorphism and dehydration of serpentinite during continued subduction, thereby releasing Pb–Ba‐rich fluids that reacted with associated metabasalt. Finally, subduction to >100 km and UHP metamorphism of all lithologies led to formation of garnetite, eclogite and garnet pyroxenite hosted by co‐facial garnet peridotite as observed in the Erzgebirge.

UHP metamorphism, decompression anatexis and retrogression of garnet-bearing metagranite and granitic gneiss from the Dabie orogen during continental collision

Metamorphosed granitic rocks are commonly volumetrically dominant in continental collisional terranes, which are crucial targets for resolving the metamorphic evolution of deeply subducted continental crust. However, their metamorphic processes in ultrahigh pressure (UHP) belts are often poorly understood, because they are commonly intensely retrogressed and presently dominated by amphibolite-facies minerals with very rare petrological evidences of UHP metamorphism. It is known that, in these lithologies, zircon can potentially preserve evidence of the UHP history; therefore, the P-T-t path of metafelsic rocks may be estimated using a multidisciplinary approach which combines zircon microanalysis with phase equilibria modelling. In the central Dabie UHP metamorphic zone (CDZ) (China), two types of metamorphosed garnet-bearing granitic rocks, i.e. metagranite and granitic gneiss, whose massive vs. gneissic structure reflects a different degree of deformation, are closely associated with eclogites. Opposite to the eclogites, their detailed metamorphic evolution is poorly understood. In this study, a three-stage metamorphic evolution is reconstructed for these lithologies, based on field observation, bulk-rock elemental geochemistry, sensitive high-resolution ion microprobe zircon UPb dating, Raman micro-spectroscopy analysis of zircon inclusion, mineral chemistry, conventional thermobarometry and phase equilibria modelling. The first UHP stage is represented by the assemblage coesite-rutile-garnet-phengite preserved in the metamorphic domains of zircon and by garnet core in the matrix, revealing minimum peak pressures of 44 kbar and 30 kbar for granitic gneiss and metagranite, respectively, at 231 ± 4 Ma. The second stage is represented by anatexis at 18–20 kbar, 725–870 °C and 222 ± 3 Ma, evidenced by leucosomes at the outcrop scale and by quartzofeldspathic films with low dihedral angles, melt inclusions preserved in peritectic garnet and quartz/K-feldspar/plagioclase- garnet intergrowth at the micro-scale. The third stage is represented by amphibolite-facies overprint at 590–700 °C, 9–14 kbar, at 214–219 Ma, recorded by biotite-plagioclase-quartz symplectites developed at the expenses of phengite, re-equilibration of the peritectic garnet rim, and amphibole-quartz (biotite-titanite) inclusions in zircon overgrowth rims. The resulting P-T-t path overlaps with previous results estimated from other CDZ UHP rocks. Besides, the protoliths of both metagranite and granitic gneiss were emplaced during the mid-Neoproterozoic (683–786 Ma) period, as already documented throughout the Dabie orogen. Compared with previously published data for meta-granitic rocks in the CDZ, the weakly deformed metagranite does not appear to represent low-grade metamorphic rocks. Instead, this study (i) suggests that the two groups of lithologies experienced a coeval metamorphic evolution; (ii) reconstructs a multistage P-T-t history based on zircon microanalysis and pseudosections; (iii) reveals that anatexis, retrogression and deformation facilitate the elimination of (U)HP mineralogical records of metagranitoids. Overall, these results provide new constraints on the metamorphic evolution and exhumation processes of deeply subducted granitic rocks during continental collision.

Some thoughts about eclogites and related rocks

Abstract. The past 40 years have been a golden age for eclogite studies, supported by an ever wider range of instrumentation and enhanced computational capabilities, linked with ongoing developments in thermobarometry and geochronology. During this time, we have made robust estimates of pressure–temperature (P–T) conditions; determined ages related to the prograde, metamorphic peak and retrograde stages; and calculated time-integrated rates of cooling and exhumation for eclogites and related rocks, including blueschists, from orogenic belts worldwide. Improvements to single mineral thermometers and new developments in elastic barometry using inclusions of one mineral in another (e.g. quartz and/or zircon in garnet), coupled with ongoing innovations in petrochronology and diffusion modelling, presage a new age for eclogite studies in which detailed quantification of metamorphic conditions and timescales will be linked to an improved understanding of processes at all scales. Since the turn of the century, numerical modelling of subduction zone and rock exhumation processes has become increasingly important. As a result, subduction and exhumation are quite well understood, but the volume of continental crust subducted to and returned from mantle conditions and the amount lost to the mantle are largely unknown. We have generated sufficient data to investigate the spatiotemporal distribution of metamorphism and secular change but not without controversy in relation to the rare occurrence of orogenic eclogites and the absence of blueschists prior to the late Neoproterozoic and the emergence of plate tectonics on Earth. Since the turn of the century, the assumption that metamorphic pressure is lithostatic has come under increasing scrutiny. Whether local variations in stress extrapolate to the crustal scale and, if so, whether the magnitude of the calculated deviations from lithostatic pressure can be generated and sustained in mechanically heterogeneous rock units remains contentious. Could the paradigm of subduction of continental lithosphere to mantle depths be simply an artefact of the lithostatic assumption? Fluid cycling in subduction zones and understanding the role of fluids in the generation of intermediate-depth earthquakes remain important topics of current research. Dry (H2O-absent) conditions are unlikely around the peak of ultrahigh-pressure (UHP) metamorphism or during exhumation, due to dehydroxylation of nominally anhydrous minerals and breakdown of hydrous minerals at P–T conditions in the realm of supercritical fluid and hydrous melt. Indeed, the presence of melt may be necessary to facilitate the exhumation of HP and UHP tectonometamorphic rock units. Finally, our ability to interrogate inclusions in superdeep diamonds should lead to a better understanding of how the deep interior and surface are linked in the context of Earth as a fully coupled system.

Detrital garnet petrology challenges Paleoproterozoic ultrahigh-pressure metamorphism in western Greenland

Abstract. Modern-style plate tectonics is characterised by the global operation of cold and deep subduction involving blueschist facies and ultrahigh-pressure metamorphism. This has been a common process since the Neoproterozoic, but a couple of studies indicate similar processes were active in the Paleoproterozoic, at least on the local scale. Particularly conspicuous are extreme ultrahigh-pressure conditions of ∼ 7 GPa at thermal gradients < 150 ∘C GPa−1 proposed for metamorphic rocks of the Nordre Strømfjord shear zone in the western part of the Paleoproterozoic Nagssugtoqidian Orogen of Greenland. By acquiring a large dataset of heavy minerals (n = 52 130) and garnet major-element composition integrated with mineral inclusion analysis (n=2669) from modern sands representing fresh and naturally mixed erosional material from the metamorphic rocks, we here intensely screened the area for potential occurrences of ultrahigh-pressure rocks and put constraints on the metamorphic evolution. Apart from the absence of any indications pointing to ultrahigh-pressure and low-temperature–high-pressure metamorphism, the results are well in accordance with a common Paleoproterozoic subduction–collision metamorphic evolution along a Barrovian-type intermediate temperature and pressure gradient with a pressure peak at the amphibolite–granulite–eclogite-facies transition and a temperature peak at medium- to high-pressure granulite-facies conditions. In addition, we discuss that all “evidence” for ultrahigh-pressure metamorphism proposed in the literature for rocks of this area is equivocal. Accordingly, the Nordre Strømfjord shear zone is not an example of modern-style plate tectonics in the Paleoproterozoic or of very low thermal gradients and extreme pressure conditions in general.

Nitrogen storage capacity of phengitic muscovite and K-cymrite under the conditions of hot subduction and ultra high pressure metamorphism

Nitrogen storage capacity and partitioning between main N hosts have been investigated in N-bearing Al-rich natural pelite at 3.0–7.8 GPa, 825–1070 °C and oxygen fugacity (fO2) from NNO (Ni-NiO buffer)+0.3 to NNO-4.1 log units. Under the conditions of hot subduction, pelite converts into a phase assemblage typical of ultrahigh-pressure (UHP) eclogites. Phengitic muscovite in this assemblage is in equilibrium with a volatile-rich granite-like melt at 3.0 GPa and with supercritical fluid at 5.5–7.8 GPa, and contains, respectively, 115–135 and 759–962 wt ppm NH4+ at fO2 values close to NNO. Both melt and supercritical fluid have H2O and CO2 as predominant volatiles and the NH3/(NH3 + N2) ratio from 0.04 to 0.12. The pattern of nitrogen partitioning between its main hosts in pelite (DNH4Ms-Melt = 0.41–0.58 and DNH4Ms-Fluid = 0.63–2.4) proves that the ammonium exhibits moderately incompatible to compatible behavior within a hot oxidized slab in the pressure range from 3.0 GPa to 7.8 GPa. Therefore, even relatively oxidized sediments containing phengitic muscovite can efficiently transport nitrogen to the mantle at sub-arc depths under the hot subduction conditions. At the same time, the changeover of the N host from biotite to muscovite at the arc depths in combination with subsequent pelite melting and an abrupt decrease in phengitic muscovite abundance in pelite may lead to an avalanche-like N outgassing of the slab. The incorporation of an additional nitrogen source of (NH3 + N2) into pelite reduces fO2 to NNO-3.1 – NNO-4.1 log units and increases both the NH3/(NH3 + N2) ratio in the fluid (up to 0.44–0.86) and the NH4+ concentration in phengitic muscovite (up to 3750–3820 wt ppm). K-cymrite was produced in pelite at P ≥ 6.3 GPa and the bulk nitrogen content 3.2–5.9 wt%. K-cymrite possesses exceptional nitrogen storage capacity: it hosts 1.4–1.6 wt% NH4+, up to 0.5 wt% NH3, and 4–6 wt% N2. The DN-NH4Cym-Ms coefficient is as high as 20. Being stable in sediments subducted to mantle depths, K-cymrite, with its extremely high storage capacity, can act as a huge hidden redox-insensitive nitrogen reservoir in the mantle and thus can be involved in the deep nitrogen cycle.

Fluid effect on zircon O and U-Pb isotopes during ultrahigh-pressure metamorphism: Insights from the Dora-Maira Massif of the Western Alps

Zircon geochemistry such as U-Pb and O isotopes have been widely used in dating and tracing complex geological processes. However, it still remains unclear how fluid action affects zircon geochemistry during metamorphic and metasomatic processes in subduction zones. Here a systematic study on zircon U-Pb dating, O isotopes and trace elements as well as whole-rock O isotopes was carried out for the coesite-bearing whiteschists, jadeite quartzites and granitic gneisses from the Dora-Maira Massif, Western Alps. Whole-rock and zircon geochemistry supports a common protolith, i.e., Permian S-type granites, for the above three types of rocks and an intense fluid metasomatism during the Alpine orogeny to form whiteschists and jadeite quartzites. Zircon cores in all samples have nearly identical δ18O values (9‰–11‰), whereas their apparent 206Pb/238U ages show a greater variability due to Pb loss during metamorphism. Zircon rims formed in the late Eocene to early Oligocene can be categorized into two types. Type-I rims occur in granitic gneisses and jadeite quartzites. They have high δ18O values consistent with zircon cores, but much lower contents of P and Y as well as lower Th/U ratios than the cores. Their growth can be attributed to internal metamorphic fluid action at the UHP metamorphic stage. Type-II rims occur in whiteschists and jadeite quartzites. They have remarkably lower δ18O values (5‰–8‰) and Th/U ratios (<0.01), compared with zircon cores and Type-I rims. Their growth can be ascribed to external fluids during the metasomatic process. Some zircon domains in whiteschists and jadeite quartzites show a positive correlation between δ18O values and apparent 206Pb/238U ages, which suggest the simultaneous impacts on U-Pb-O isotopes during external fluid metasomatism. This process can be attributed to the fluid-assisted dissolution and recrystallization of protolith zircons. Especially, coesite inclusions that would have been expected to occur only in young zircon rims formed during UHP metamorphism are also observed in the relict magmatic zircon cores, indicating that the fluid-related metasomatism at the UHP metamorphic conditions also affected these pre-existing protolith-related cores. Therefore, fluid action in subduction zones reveals significant impacts on both the U-Pb and O isotope systems of zircon, especially when external metasomatic fluids are involved. Therefore, a detailed study on zircon, including microstructure, mineral inclusion and geochemical data of different growth and recrystallization domains, is needed in order to unravel continental crustal evolution based on zircon U-Pb ages and O isotope compositions.

Barium isotope behavior during interaction between serpentinite-derived fluids and metamorphic rocks in the continental subduction zone

Serpentinites in subduction zones are important fluid reservoirs for crust-mantle interaction and arc magma formation. The serpentinite-derived fluids can carry fluid-mobile elements such as Ba and potentially affect the geochemical composition of the metamorphic rocks in the subduction zones. However, the Ba isotope behavior during interaction between serpentinite-derived fluids and metamorphic rocks is still poorly understood. The whiteschists of the Dora-Maira Massif in the Western Alps have experienced fluid infiltration by serpentinite-derived fluids during deep subduction. Here we report the Ba isotope data of the whiteschist and its protolith metagranite to investigate the Ba isotope behavior during fluid-rock interaction. The metagranites, which have experienced ultrahigh-pressure metamorphism, have a small variation of δ138Ba (−0.25 to 0.26‰), while the whiteschists display a large Ba isotope variation from −0.99‰ to 0.48‰. Such a large variation in whiteschists cannot be explained by chemical weathering, protolith heterogeneity, or kinetic effects. Instead, considering the negative correlations of δ138Ba with Ba and other fluid-mobile elements (e.g., Sr, Cs, Zn, and Cu) and the geochemical composition features of serpentinite-derived fluids, the large Ba isotope variation in whiteschists is inferred to be induced by the interaction between serpentinite-derived fluid and metagranite during intense fluid infiltration. This is consistent with the observation that the whiteschists with low Ba contents tend to exhibit heavier Mg-Fe isotope compositions but lighter Li isotope compositions, which indicates that the related fluids were derived from the dehydration of mantle wedge serpentinite. The fluid infiltrating process is well explained by a numerical model with high fluid-rock ratios of 4 to 12. Our modeling further suggests that serpentinite-derived fluid has a distinctly higher δ138Ba value than the depleted upper mantle. During fluid-rock interaction, heavy Ba isotopes prefer to be infiltrated into fluids with an apparent fractionation factor (Δ138Barock-fluid) of mostly −0.5‰ to −0.1‰ but can be down to −0.8‰ to −0.7‰ required for a few samples. A two-stage dehydration and metasomatism model at different depths can well explain the formation of Dora-Maira whiteschists. Whereas the Ba isotope composition of mantle wedge serpentinite is mainly dictated by crust-derived fluids from the subducting slab, an interaction between the serpentinite-derived fluids and the metagranite protolith resulted in a large Ba isotope variation of whiteschists. In case of a general involvement of serpentinites in subduction zones, serpentinite-derived fluids would significantly affect the Ba isotope compositions of subarc mantle peridotites and also arc lavas.

Methane in Carbonate Melt Inclusions in the Rock-Forming Minerals of Calc-Silicate Rocks of the Kokchetav Massif

Secondary inclusions of the carbonate melt in garnets and clinopyroxenes from ultrahigh-pressure calc-silicate rocks were studied. Secondary carbonate inclusions are located in the healed cracks confined to large (100 µm–3 mm) primary carbonate inclusions in garnet. In some cases, the healed cracks form plates that intersect both garnet and inclusions of potassium-bearing clinopyroxene in garnet. The high content of K2O (0.64 wt %) in the healed cracks in clinopyroxene inclusions suggests that their formation and healing took place during the stage of ultrahigh pressures. Interpretation of the Raman spectra of the secondary inclusions of the carbonate melt in garnet and clinopyroxene confirmed the presence of the following phases: methane, graphite, calcite, dolomite, muscovite, and phlogopite. The data obtained allowed us to recognize the stage of ultrahigh pressure metamorphism in the calc–silicate rocks of the Kokchetav massif, during which methane accumulation in the carbonate melt took place, for the first time.

Magnetotelluric constraints on the tectonic evolution beneath the Luotian dome in the North Dabie complex zone, Central China

The Dabie orogen, formed by a Triassic continent-continent collision between the Yangtze and the North China craton, contains one of the largest ultrahigh-pressure (UHP) metamorphic terranes in the world. However, compared to the widespread UHP metamorphic rocks in the Central Dabie, little evidence shows the North Dabie Zone (NDZ) has experienced UHP metamorphism. To understand the cause of these variances and constrain the post-collision evolution of the NDZ, we conducted a magnetotelluric profile across the Luotian dome, southwestern NDZ. Our electrical resistivity model exhibits intriguing post-collision evolution-related characteristics. On both sides of the profile, the resistive features extend to ∼10 km, suggesting the UHP metamorphic or magmatic rocks are mainly concentrated in the upper crust; the underlying crustal high conductive features are interpreted to be the graphite from the mantle-derived or related magma, which were interconnected by the shearing. The middle resistive features extend deeper to ∼20 km, which may represent the magmatic rocks that contributed to the formation of the Luotian dome during the early Cretaceous. The lithosphere mantle generally behaves as high conductive features related to graphite, indicating a fertile mantle. Combining geological and geophysical observations previously, we suggest the Luotian region experienced doming in response to the hot asthenosphere upwelling resulting from the delamination of the lithospheric keel of the Dabie orogen during the early Cretaceous as triggered by synchronous tectonic events like the development of the Tan-Lu fault. These processes may fertilize the lithosphere, uplift and erode the Luotian or the NDZ, resulting in the observed resistivity structures and the lack of UHP metamorphic rocks.