Ultrahigh-pressure Metamorphism Research Articles

Two phases of granulite-facies metamorphism superimposied on retrograde eclogite: Constraints on the early Paleozoic tectonic evolution of the Qinling Orogenic Belt, central China

Existing studies provide adequate petrological evidences on ca. 500 Ma ultra-high pressure (UHP) metamorphism in the North Qinling Orogenic Belt (NQOB) in central China, but the genesis of 470–420 Ma multi-phase granulite-facies metamorphism in the NQOB and their relationship with the ca. 500 Ma UHP metamorphism remain controversial, resulting in the early Paleozoic evolution of the Qinling Orogenic Belt (QOB) highly debatable. In this study, we present mafic granulites and host felsic gneisses with a “red-eye socket” texture from the Shuanglong area, eastern NQOB, which recorded two phases of granulite-facies metamorphism superimposing on former eclogite-facies metamorphism. The former eclogite-facies metamorphism is indicated by eclogite-facies zircon trace element patterns and 496–495 Ma zircon ages, which are the same with those of the HP–UHP eclogite-facies metamorphic rocks in NQOB. The first granulite-facies metamorphism occurred at 460–448 Ma is characterized by coarse-grained minerals in matrix. Compositions and zonings of these minerals define an anticlockwise P–T path involving a prograde stage (751–763 °C), a high-temperature peak stage (9.2 kbar and 864 °C), and a near-isobaric cooling retrograde stage (8.3 kbar and 818 °C). The second granulite-facies metamorphism occurred at 422–421 Ma is represented by coronal garnet and coexisting fine-grained mineral aggregates. Coronal garnet compositional zonings suggest a clockwise P–T path consisting of a high-pressure peak stage (9.5–11.2 kbar and 748–783 °C) and a decompressing and heating retrograde stage (9.2–9.5 kbar and 789–800 °C). Combining dating results of leucosomes in these rocks and existing data, we proposed a new model for early Paleozoic tectonic evolution of the NQOB. The North Qinling Terrane (NQT), probably separated from the South China Block (SCB) during the breakup of Rodinia, drifted northwards and underwent UHP metamorphism at 500 Ma and then rapidly exhumed to crust level. Later, the Shangdan Ocean subducted northwards beneath the exhumed NQT at 470–440 Ma, resulting in the first granulite-facies metamorphism and contemporaneous migmatization and magmatism. Finally, the closure of the Shangdan Ocean led to collision between the NQT and South Qinling Terrane/SCB and the second granulite-facies metamorphism and anatexis at 422–418 Ma.

Ultrahigh-pressure to high-pressure eclogite in Cuban ophiolitic mélange reveals proto-Caribbean spreading ridge subduction

Proto-Caribbean oceanic crust produced during ocean-floor spreading between diverging North and South American plates was subsequently subducted beneath the Caribbean plate. However, the timing and spatial configuration of proto-Caribbean spreading ridge subduction remain subjects of debate. High-pressure (HP) basaltic metamorphic rocks, representing relics of the subducted proto-Caribbean oceanic crust, commonly occur in Cuban ophiolitic mélanges. In this study, an integrated set of petrological, geochemical, and geochronological data is presented for eclogite from the Las Villas mélange, central Cuba. The typical geochemical signature of mid-ocean-ridge basalt (MORB) indicates that the protolith of eclogite formed at the proto-Caribbean spreading ridge. Based on pressure-temperature (P-T) estimates obtained by pseudosection analysis as well as Zr-in-rutile and Ti-in-zircon thermometry, the following P-T paths for representative samples can be derived: a prograde path from 24−25 kbar and 510−520 °C to peak conditions of 29−31 kbar and 525−575 °C, and a complex retrograde path initially following almost isothermal exhumation to 25−27 kbar, followed by near-isobaric heating to 610−640 °C before final exhumation. This is the first documentation of prograde oceanic ultrahigh-pressure (UHP) metamorphism in the northern Caribbean area. U-Pb dating of magmatic zircon with steep heavy rare earth element (HREE) patterns and negative Eu anomalies yielded a protolith age of 126.3 ± 0.7 Ma. In contrast, metamorphic zircon with flat HREE patterns and without an Eu anomaly yielded a weighted mean age of 118.6 ± 1.6 Ma. The short time interval of >8 m.y. between MORB magmatism and UHP metamorphism suggests that the oceanic crust was subducted to great depth (∼100 km) shortly after generation in an oceanic ridge, which provides robust evidence for subduction of the proto-Caribbean spreading ridge. Furthermore, this work demonstrates high potential to trace ancient spreading ridge subduction by joint petrological, geochemical, and geochronological study of oceanic eclogite.

A subduction-dismembered Neoproterozoic large igneous province in the Qinling Orogenic Belt, China: Implications for 850 Ma–initiated mantle plume activity in the greater Yangtze Block

Large igneous provinces (LIP) provide invaluable clues for recognizing the growth, breakup, and cycles of supercontinents and constraining the activity of ancient mantle plumes (or superplumes). The widespread intraplate magmatism is considered as a possible fingerprint of ancient plume activities and LIPs. Here, we present the results of an integrated study on protoliths of eclogites and amphibolite-facies bimodal volcanic rocks from North Qinling Ultrahigh-pressure Metamorphic (UHPM) Terrane, Qinling Orogen, and identify a dismembered Neoproterozoic LIP. These rocks share similar protolith ages of ca. 850–800 Ma and experienced high- to ultrahigh-pressure metamorphism at ca. 500–480 Ma. The mafic rocks are mainly tholeiitic and display enrichment of LILEs and HFSEs, similar to the E-MORB/OIB characters. They also have typical features of continental flood basalts (CFBs) with low CaO/Al2O3 (0.68–0.87), Lu/Yb (0.14–0.15), Zr/Nb (8–13), high Nb/La (0.87–1.24, mean 1.07), La/Nb (0.80–1.15), La/Ta (11–20), Nb/Yb (3.0–6.2) ratios, and positive εNd(t) (+4.3 to + 5.4). Their occurrence, geochemical features, age data, and high mantle potential temperature (1500–1550 °C) suggest that they are remnants of Neoproterozoic CFBs with mantle plume origin. Based on the lateral distribution length (ca. 300 km) of meta-basaltic rocks and the maximum subduction depths (ca. 100–200 km) recorded by UHP eclogites, the identified ca. 850 Ma-initiated CFBs in the North Qinling terrane should represent an LIP, covering more than 100,000 km2. This Neoproterozoic LIP, together with the coeval volcanic sequences in the South Qinling terrane and the northern Yangtze, would have occupied a maximum landmass of 300,000 km2, which lends support for the onset of a mantle plume at ca. 850 Ma within the Rodinia supercontinent. We conclude that the Qinling Block is one of the fragments of the Rodinia supercontinent with a passive margin covered by CFBs, which was subducted to mantle depths in the Early Paleozoic.

Exhumation of ultra-high pressure (UHP) rocks modulated by rifted margin-subduction feedback: Implications for their preservation in old collisional orogens

In continental subduction, rifted margins can be carried to mantle depths (> 90 km) where ultra-high pressure (UHP) metamorphism above coesite stability is attained. Although different exhumation mechanisms for UHP rocks have been discussed, none of them integrate the recent understanding of rifted continental margins. Here, we perform high-resolution thermomechanical numerical experiments to demonstrate that segments of magma-poor rifted margins that reach UHP conditions can efficiently exhume back to shallower levels, while segments of magma-rich rifted margins cannot. This is because the thick layer of rocks with a basaltic composition in magma-rich margins becomes negatively buoyant during metamorphism, preventing their exhumation. This new concept might be pivotal for explaining why exhumed UHP rocks, a key feature of modern-style continental orogens, only appeared and became common late in Earth's history. We suggest that higher mantle potential temperatures and fertility in Earth's early history favored magma-rich rifted margins, making exhumation of UHP crust inefficient. Conditions for magma-poor rifted margins may have arisen during Earth's middle age (1.5–0.8 Ga) due to a colder, more refractory mantle that limited melting and magmatism. We argue that at the end of Neoproterozoic, these colder and positively buoyant magma-poor rifted margins were then subducted and efficiently exhumed to form the large collisional orogens of Gondwana where Earth's oldest coesite-bearing UHP rocks have been unequivocally found. Since then, UHP rocks have become a key and ubiquitous feature of continental orogens.

Passive continental margin subducted to mantle depths: Coesite-bearing metasedimentary rocks from the Neoproterozoic Brasília Orogen, West Gondwana margin

In this contribution, we investigate the ultra-high pressure metamorphism (UHPM), and subsequent pressure–temperature-time (P-T-t) exhumation path recorded by granulites (rutile-kyanite-garnet-Kfeldspar and rutile-titanite-hornblende-garnet gneisses) of a coherent thrust package (Três Pontas-Varginha Nappe) of the southernmost edge of the Brasília Orogen. The Brasília Orogen, along with Northern Borborema Province, Dahomey, and Hoggar Belts, comprise a widespread linear system of orogens developed during the West Gondwana assembly, recording the diachronic closure of the Tonian-Ediacaran long-lived Goiás-Pharusian Ocean. Optical petrography, Raman spectroscopy, mineral chemistry, Ti-in-zircon and Zr-in-rutile thermometry, and U-Pb dating (LA-ICP-MS) of zircon, monazite, and rutile were combined to identify UHP minerals and textures and to track the decompression/exhumation path. Garnet porphyroblasts preserve UHPM chemical domains and are often radially fractured around rounded quartz inclusions. Remnants of microcoesite were detected by Raman spectroscopy within these inclusions, identified by the diagnostic bands at approximately 170 cm−1, 270 cm−1, and 520 cm−1. Coesite remnants were identified across the nappe stack along with 60° angle-oriented rutile needle inclusions in garnet, which suggests exsolution from Ti-bearing UHP garnets. Soccer ball zircon crystals of Ky-bearing granulites, with flat HREE pattern and negative Eu anomaly, yielded a U-Pb age of around 620 Ma. This age is interpreted as the record of the granulite facies metamorphism (750–805 °C and 15 kbar) post-dating the UHPM from the subduction channel. Zircon and monazite record exhumation under high temperatures around 595–590 Ma, and rutile crystals record the final stages of the decompression path from 590 to 585 Ma. Monazite crystallization occurred until 570 Ma. The new data combined with previously reported metamorphic paths, record a time interval of about 25 m.y. for the exhumation of the Tres Pontas-Varginha Nappe under subsolidus conditions. Our findings represent one of the oldest reports of Neoproterozoic coesite-forming UHPM recorded in metasedimentary rocks buried to mantle depths in the subduction of the passive continental margin.

Geochemistry and Zircon U-Pb and Hf Isotopes of Early Devonian Hardawu Granites in the Eastern Segment of the Ultrahigh-Pressure Metamorphic Belt, Northern Qaidam Basin

Geochemistry and Zircon U-Pb and Hf Isotopes of Early Devonian Hardawu Granites in the Eastern Segment of the Ultrahigh-Pressure Metamorphic Belt, Northern Qaidam Basin

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.

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.

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.

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.