Ultrahigh-pressure Metamorphism Research Articles

Zircon U-Pb dating, Hf isotope characteristics of Late Jurassic granites in eastern Jiaodong Peninsula and their constraints on metallogenetic tectonic setting of gold deposit

胶东地区是目前我国最重要的金矿产地，已累积探明黄金储量接近5000t。金矿在胶东地区分布非常不均，主要集中在胶北地体，而胶东东部地区金矿产出较少。晚侏罗世的玲珑花岗岩是胶北金矿区最重要的赋矿围岩之一。虽然胶东东部同样有晚侏罗世花岗岩发育，但赋存金矿较少，因此前人对东部这些花岗岩体的研究相对较少。本文选取胶东东部的文登岩体和垛崮山岩体进行了锆石U-Pb定年和Hf同位素分析，同时收集整理了胶北地体的玲珑岩体数据，通过开展胶东东部及西北部同一时期岩体的差异性研究，为该区金成矿作用的研究提供重要线索。分析结果显示，文登岩体和垛崮山岩体的形成时代为晚侏罗世（~160Ma），与玲珑岩体时代一致。三个岩体均含有较多的三叠纪（~230Ma）和新元古代（~780Ma）的继承锆石，表明晚侏罗世花岗岩的岩浆源区主要为经历了超高压变质作用的扬子板块。不同的是，玲珑岩体中有更多的华北板块物源的年龄记录（~1400Ma、~1700Ma、~2500Ma），垛崮山岩体中这些年龄的继承锆石少于玲珑岩体，而文登岩体更少。文登岩体、垛崮山岩体的新生岩浆锆石ε_Hf（t）值范围分别为-30.7~-20.3和-27.5~-17.9，玲珑岩体的新生岩浆锆石ε_Hf（t）值的变化范围更广，在-29.3~-9.3之间，而且呈现出从文登岩体到垛崮山岩体到玲珑岩体，ε_Hf（160Ma）值逐渐增高的现象。其原因可能是扬子板块与华北板块碰撞后，幔源岩浆底侵并诱发加厚的地壳发生部分熔融，形成花岗岩。在此过程中胶北地体深部受到幔源物质的影响，从而对金及成矿相关元素产生一定的预富集作用，而同时期胶东东部地区未受到明显的地幔物质影响，可能是该区金矿产出较少的原因之一。;Jiaodong Peninsula is the most important gold producing province in China, with an explored reserve of nearly 5000t gold. However, the gold deposits are very unevenly distributed in this area, e.g., most in Jiaobei terrane but less in eastern Jiaodong Peninsula. Late Jurassic Linglong pluton is one of the important ore-hosting rocks in Jiaobei terrane. Although Late Jurassic granites also exist in eastern Jiaodong Peninsula, gold deposits are much less abundant in them. Thus, relatively few studies were focused on these granite plutons in the eastern Jiaodong Peninsula. Here, we carried out zircon U-Pb dating and Hf isotopic analysis on the Wendeng and Duogushan plutons in eastern Jiaodong Peninsula to compare them with those of the Linglong pluton in Jiaobei terrane to seek the differences of the gold mineralization in the Late Jurassic granites between these two areas. Our results show that the Wendeng and Duogushan plutons were formed at Late Jurassic (~160Ma), which is consistent with those of the Linglong pluton. The inherited zircons from the three plutons are all dominated by Triassic (~230Ma) and Neoproterozoic (~780Ma) ages, indicating their magmatic source mainly derived from crustal materials of Yangtze Craton, which had undergone ultrahigh-pressure metamorphism in Triassic. The difference among them is the number of inherited zircons relating to North China Craton (~1400Ma,~1700Ma,~2500Ma), which are most abundant in Linglong pluton, less in Duogushan pluton, and least in Wendeng pluton. Magmatic zircons from the two plutons in eastern Jiaodong Peninsula have variably negative ε_Hf(160Ma) values, from -30.7 to -20.3 for Wendeng pluton, and from -27.5 to -17.9 for Duotgushan pluton. Contrastively, the range of ε_Hf(160Ma) values of Linglong pluton is significantly wider, i.e., from -29.3 to -9.3. Moreover, the ε_Hf(160Ma) values also increase gradually from the Wendeng pluton to the Duogushan pluton, and then to the Linglong pluton. The possible interpretation of this trend is that after the collision of the Yangtze Craton and North China Craton in Triassic, the underplating of mantle material induced partial melting of the thickened crust as early as Late Jurassic. Meanwhile, the Jiaobei terrane was influenced by mantle materials, which promoted the pre-enrichment of gold. In contrast, the eastern Jiaodong Peninsula was lack of involvement of mantle material in Late Jurassic, which might be one of the reasons for fewer gold deposits in the area.

In situ laser ablation Lu–Hf geochronology of garnet across the Western Gneiss Region: campaign-style dating of metamorphism

The development of in situ laser ablation Lu–Hf geochronology of apatite, xenotime and garnet has opened avenues to quickly and directly date geological processes. We demonstrate the first use of campaign-style in situ Lu–Hf geochronology of garnet across the high- to ultrahigh-pressure Western Gneiss Region in Norway. Mafic eclogites from this region have been the focus of much work, and were clearly formed during continental subduction during the Caledonian Orogeny. However, abundant quartzofeldspathic and pelitic lithologies record a more complex history, with some preserving polymetamorphic age data, and most containing no indication of high-pressure mineral assemblages formed during subduction. Twenty metapelitic and felsic samples spanning 160 lateral kilometres across the Western Gneiss Region have been analysed using garnet Lu–Hf geochronology. The results reveal Caledonian ages for the majority of the garnets, suggesting that some quartzofeldspathic and metapelitic lithologies were reactive and grew garnet during high- to ultrahigh-pressure metamorphism. However, two ultrahigh-pressure eclogite locations, Verpeneset and Fjørtoft, preserve both Caledonian and Neoproterozoic-aged garnets. Despite significant uncertainties on some of the Lu–Hf geochronological ages, laser ablation Lu–Hf efficiently identifies the polymetamorphic history of parts of the Western Gneiss Region, illustrating the effectiveness of this novel analytical method for rapid mapping of metamorphic ages. Supplementary material: All laser ablation Lu–Hf geochronological data for the garnets analysed in this study are available at https://doi.org/10.6084/m9.figshare.c.5715453 Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/cc/caledonian-wilson-cycle

Solidus of carbonated phlogopite eclogite at 3–6 GPa: Implications for mantle metasomatism and ultra-high pressure metamorphism

The interaction of natural eclogite (Ecl) with synthetic hydrous carbonate melts with Na:K = 0:1 (KH2) and 1:1 (NKH2) was studied in multianvil experiments at 3–6 GPa and 850–1250 °C. The interaction with KH2 consumes garnet and clinopyroxene producing phlogopite and calcite-dolomite solid solution. Besides, the interaction yields a decrease in the jadeite component of clinopyroxene, evolving eclogite toward pyroxenite. This is consistent with a metasomatic alteration of eclogite xenoliths, manifested as Na-poor “spongy” clinopyroxene, replacing primary omphacite, and kelyphitic rims around garnet, containing phlogopite and carbonates. The interaction with NKH2 also produces phlogopite and carbonate, but the latter is more magnesian and represented by magnesite, above the solidus, and magnesite + dolomite below the solidus. The interaction with NKH2 increases the jadeite component in clinopyroxene and grossular component in garnet, evolving eclogite Group A to eclogite Group B.The studied systems have H2O/K2O = 2, like that in phlogopite, and therefore correspond to carbonated phlogopite eclogite under fluid-absent conditions. Based on the obtained results its solidus is situated near 1050 °C at 3 GPa and decreases to 950 °C at 6 GPa. Thus, hydrous K- and Na-K-carbonatite melts can coexist with eclogite in SCLM at depths exceeding 120–170 km, and solidify as temperature decreases below 950–1050 °C according to the following solidus reactions: pyrope + diopside + melt → phlogopite + dolomite, below 6 GPa, and pyrope + diopside + melt → phlogopite + magnesite + grossular, at 6 GPa.The melting reaction, involving phlogopite and dolomite, suggests the partial melting at the peak of ultrahigh-pressure metamorphism (UHPM) during continent–continent plate collision. The prograde P-T path of UHPM crosses the solidus of clinopyroxene + garnet + phlogopite + dolomite assemblage at 4.7–5.2 GPa and 970–990 °C and yields the formation of hydrous K-carbonatite melt-fluid in situ. This melt could be responsible for the formation of K-bearing clinopyroxenes and microdiamonds in the UHPM marbles in the Kokchetav massif, Kazakhstan. The retrograde P-T path intersects the solidus that has a negative Clapeyron slope in the diamond stability field. Thus, the hydrous K-carbonatite melt should disappear soon after the peak of metamorphism reacting with garnet to produce Ca-Mg carbonates and phlogopite.

Multi-Stage Metamorphism of the South Altyn Ultrahigh-Pressure Metamorphic Belt, West China: Insights into Tectonic Evolution from Continental Subduction to Arc–Backarc Extension

Abstract The South Altyn ultrahigh-pressure (UHP) metamorphic belt is claimed to host the deepest subducted continental crust based on the discovery of former stishovite, and thus can provide unique insights into the tectonic evolution from deep continental subduction and exhumation to arc–backarc extension. In this article, we present detailed studies of petrography, mineral chemistry, phase equilibria modelling and zircon U–Pb dating for three representative samples involving garnet amphibolite (A1531 & A1533) and associated garnet-biotite gneiss (A1534) from the UHP belt. Three phases of metamorphism are inferred for the rocks. The first phase high-pressure (HP)–UHP-type eclogite facies is represented by the mineral assemblages of garnet and phengite inclusions in zircon and garnet cores with the high grossular (XGrs = 0.33–0.34). The Si contents of 3.40–3.53 and 3.24–3.25 p.f.u. in phengite inclusions yield pressure conditions of >1.7–2.3 GPa for A1533 and 2.5–2.55 GPa for A1534 at a fixed temperature of 770°C. The second phase medium-pressure (MP)-type overprinting of garnet amphibolite facies shows P–T conditions of 0.8–1.2 GPa/750–785°C based on the stability fields of corresponding mineral assemblages, the measured isopleths of Ti contents in biotite and amphibole cores, and XGrs in garnet. The third phase low-pressure (LP) type overprinting includes early-stage heating to peak granulite facies followed by cooling towards a late-stage amphibolite facies. The peak granulite facies is represented by the high Ti amphibole mantle, high Zr titanite and the intergrowths of clinopyroxene + ilmenite in A1533 & A1531, with P–T conditions of 800–875°C/0.80–0.95 GPa. The late-stage is defined by the solidus assemblages, giving P–T conditions of 0.5–0.7 GPa/720–805°C. U–Pb geochronology on metamorphic zircons from A1533 and A1534 gives three ages of c. 500 Ma, c. 482 Ma and c. 460 Ma. They are interpreted to represent the HP-UHP, MP and LP types of metamorphism respectively, based on cathodoluminescence (CL) images, mineral inclusions and trace element patterns. Combining the regional geology and metamorphic evolution from the Altyn Orogen, a tectonic model is inferred, including the following tectonic scenarios. The small Altyn Microcontinent (AM) was subducted to great mantle depths with dragging of the surrounding vast oceanic lithosphere to undergo the HP-UHP eclogite facies metamorphism during the early subduction stage (c. 500 Ma) of the Proto-Tethys Ocean. Then, the subducted slabs were exhumed to a thickened crust region to be overprinted by the MP-type assemblages at c. 482 Ma. Finally, an arc–backarc extension was operated within the thickened crust region due to the retreat of subduction zones. It caused evident heating and the LP-type metamorphic overprinting at c. 460 Ma, with a fairly long interval of 30–40 Myr after the HP-UHP metamorphism, distinct from the short interval of <5–10 Myr in the Bohemian Massif.

Multi-stage metamorphism and deformation of the North Qinling Orogenic Belt: Constraints from petrology, geochronology, and structural analysis of the Qinling Complex

The Qinling Complex is the only continental nucleus that outcrops in the North Qinling Orogenic Belt (NQOB) and preserves valuable evolutionary information of NQOB during the early Paleozoic. There are many high-pressure (HP)–ultrahigh-pressure (UHP) metamorphic rocks distributed in the northern and southern parts of the Qinling Complex. Whether the central part (namely the Qihe tectonic slab) underwent a similar HP–UHP metamorphism and what the relationship among different tectonic slabs of Qinling Complex is, are the key to revealing the formation and exhumation process of these HP–UHP rocks. Four episodes of deformation (D1–4) have been recognized in the Qinling Complex. The metamorphic pressure‐temperature conditions of syn-D3 amphibolite in the Qihe tectonic slab are limited to 3.3–6.4 Kbar and 479–538 ℃ based on equilibrium modeling, corresponding to an extensional tectonic setting. The U-Pb dating of the amphibolite yielded a protolith age of 714 ± 46 Ma, an eclogite-facies metamorphic age of 514 ± 4 Ma, and two stages of retrograde metamorphism at 462 ± 3 Ma and 418 ± 5 Ma. The parametamorphic rocks of different tectonic slabs have similar detrital zircon age spectra, which is a very different pattern from adjacent areas of the Qinling Complex. The consistency in the protolith ages, peak metamorphic ages, two stages of retrograde metamorphism, and detrital zircon age from different tectonic slabs in the Qinling Complex indicate that HP–UHP metamorphic rocks from Qinling Complex formed during the same tectonic event. Therefore, the Qinling Complex evolved in deep subduction and underwent eclogite facies metamorphism, followed by superimposed deformation structures and retrograde metamorphism during the two stages of exhumation.

Microtectonic control of 40Ar/39Ar white mica age distributions in metamorphic rocks (Erzgebirge, N-Bohemian Massif): Constraints from combined step heating and multiple single grain total fusion experiments

Felsic rocks of the Erzgebirge in the Central European Variscides experienced ultra-high pressure metamorphism at ∼340 Ma, followed by nearly isothermal decompression during channel like rapid exhumation within ∼10 Ma. Despite the general time frame of exhumation and emplacement within mid-crustal levels is known, available geochronological data do not provide a detailed timescale of individual stages of the entire process. Addressing this problem we combined white mica 40Ar/39Ar ages obtained from multi-grain step heating and multiple single grain total fusion experiments with mineral chemical, structural and tectono-metamorphic constraints. Gneisses of the channel center are characterized by E-W elongated white mica exclusively aligned in the continuous foliation. The rocks of the channel contact zone and in the hanging and footwall contain NW-SE stretched white mica parallel to the foliation, but also components of ductile to brittle-ductile shear bands truncating the main foliation. Well defined weighted plateau ages of all samples range between 340.0 ± 1.1 Ma and 327.9 ± 1.3 Ma, confirming previously published data, but represent presumably mixed ages due to multi-phase deformation. Single grain age distributions, in contrast, exhibit two statistically significant age peaks at 338.6 ± 0.2 Ma and at 332.8 ± 0.3 Ma. Rocks without shear bands exclusively contain white mica belonging to the older age fraction, whereas rocks with shear bands show a broad age scatter including both age fractions. We interpret these age distributions as being independent of closure temperature, but reflecting different dynamic recrystallization events. The older age fraction reflects the formation of the main foliation during west-directed emplacement of the ultra-high pressure rocks in mid-crustal levels at ∼339 Ma, whereas the shear band related neo- or recrystallization is responsible for the younger age fraction dating the final transport in the upper crust at ∼333 Ma.

Fe and O isotopes in coesite-bearing jadeite quartzite from the Western Alps record multistage fluid-rock interactions in a continental subduction zone

Fluid is a key agent for the mass transfer between the subducting slab and the mantle wedge, which greatly affects the evolution of the crust-mantle system at convergent plate boundaries. A geochemical study was carried out for coesite-bearing jadeite quartzite and its country rock granitic gneiss from the Dora Maira Massif in the Western Alps. The results provide new insights into the composition of metamorphic fluids and fluid-rock interaction processes in the continental subduction zone. Coesite inclusions are found for the first time in metamorphic zircons of the granitic gneiss. The ultrahigh-pressure (UHP) metamorphism was dated to occur at 34.7 ± 0.3 Ma, confirming that the country rock experienced the UHP metamorphism synchronously with the whiteschist and jadeite quartzite. The jadeite quartzite occurs as layers and boudins within the coesite-bearing whiteschist, and shows similar whole-rock REE distribution patterns to both granitic gneiss and whiteschist. In addition, relict domains of magmatic zircon in the three types of UHP metamorphic rocks exhibit similar U-Pb ages and δ18O values, indicating that they have the same protolith of granites. However, metamorphic zircons in the jadeite quartzite show significantly lower δ18O values of 6.1–7.3‰ than the relict magmatic domains of 9.4–10.6‰. Furthermore, the jadeite quartzite has whole-rock δ18O values of 7.9–8.7‰, significantly lower than those of 9.5–10.9‰ for the granitic gneiss but higher than 6.5–7.9‰ for the whiteschist. These differences suggest that the jadeite quartzite was probably formed through metasomatism of metagranite by external fluids with relatively low δ18O values. The jadeite quartzite shows high δ56Fe values of 0.69–0.87‰, considerably higher than those of 0.18–0.38‰ for the granitic gneiss, but falling within the range of 0.32–1.03‰ for the whiteschist from the same outcrop. The Fe isotope modeling results suggest that the metasomatic fluids responsible for the formation of jadeite quartzite were possibly derived from the host whiteschist under UHP metamorphic conditions. Such fluids are enriched in Si, Al and Mg, and have high δ56Fe but low δ18O values. Therefore, the jadeite quartzite records the action of metamorphic supercritical fluids at the subarc depth. If the metasomatized rocks in the continental subduction zone would dehydrate to generate fluids with variable geochemical compositions due to multistage fluid-rock interactions, the fluids could further infiltrate the surrounding rock and possibly the mantle wedge to result in further geochemical transfer from the subducting slab to the mantle wedge.

Halogens in Eclogite Facies Minerals from the Western Gneiss Region, Norway

Ultra-high-pressure (UHP) eclogites and ultramafites and associated fluid inclusions from the Western Gneiss Region, Norwegian Caledonides, have been analysed for F, Cl, Br and I using electron-probe micro-analysis, time-of-flight secondary ion mass spectrometry and neutron-irradiated noble gas mass spectrometry. Textures of multi-phase and fluid inclusions in the cores of silicate grains indicate formation during growth of the host crystal at UHP. Halogens are predominantly hosted by fluid inclusions with a minor component from mineral inclusions such as biotite, phengite, amphibole and apatite. The reconstructed fluid composition contains between 11.3 and 12.1 wt% Cl, 870 and 8900 ppm Br and 6 and 169 ppm I. F/Cl ratios indicate efficient fractionation of F from Cl by hydrous mineral crystallisation. Heavy halogen ratios are higher than modern seawater by up to two orders of magnitude for Br/Cl and up to three orders of magnitude for I/Cl. No correlation exists between Cl and Br or I, while Br and I show good correlation, suggesting that Cl behaved differently to Br and I during subduction. Evolution to higher Br/Cl ratios is similar to trends defined by eclogitic hydration reactions and seawater evaporation, indicating preferential removal of Cl from the fluid during UHP metamorphism. This study, by analogy, offers a field model for an alternative source (continental crust) and mechanism (metasomatism by partial melts or supercritical fluids) by which halogens may be transferred to and stored in the sub-continental lithospheric mantle during transient subduction of a continental margin.

The contribution of elastic geothermobarometry to the debate on HP versus UHP metamorphism.

Characterizing the pressure and temperature (P–T) histories of eclogite facies rocks is of key importance for unravelling subduction zone processes at all scales. Accurate P–T estimates provide constraints on tectonic and geochemical processes affecting subduction dynamics and help in interpreting the geophysical images of present‐day converging plates. Conventional equilibrium geothermobarometers are challenged in ultra high pressure (UHP) metamorphic terranes, as minerals may undergo re‐equilibration along their exhumation path. Elastic geobarometry applied to host‐inclusion systems is a complementary method to determine P–T conditions of metamorphism independent from chemical equilibrium. Because only a single measurement, the inclusion strain, is made, only a line in P–T space of possible entrapment conditions, the entrapment isomeke, can be determined. Thus, the entrapment pressure along an isomeke can only be determined if the entrapment temperature is known. An alternative is to calculate entrapment conditions for two types of inclusions that are believed, from petrological evidence such as being in the same garnet growth zone, to have been entrapped at the same time. The intersection between the two sets of isomeke calculated on multiple quartz and zircon inclusions demonstrates that measuring different inclusion phases trapped inside a single host allows unique P–T conditions for the host rock to be determined. Here, we combine Zr‐in‐Rutile thermometry and thermodynamic modelling with micro‐Raman measurements on quartz and zircon inclusions trapped in garnet to obtain pressures and temperatures of equilibration of a quartz–garnet vein from the Proterozoic Ulla gneiss basement and of garnet–kyanite gneiss from the Caledonian Blåhø nappe, both in the Fjørtoft UHP terrane, Norway. We find that the quartz–garnet vein formed at high pressure (1.5–2.5 GPa and 750–800°C) and recrystallized at ~1.2 GPa and 880°C. In contrast, the garnet–kyanite gneiss followed an anticlockwise path with peak P–T at 1.2 GPa and 880°C: these estimates are consistent with previous thermodynamic modelling and suggest that the Ulla gneiss and the Blåhø nappe came into contact at these last conditions. We also discuss a new method to detect hydrostatic versus Non‐hydrostatic stresses near quartz and zircon inclusions in garnet.

Post-collisional mafic magmatism: Insights into orogenic collapse and mantle modification from North Qaidam collisional belt, NW China

Post-collisional magmatism contains important clues for understanding processes of orogenic belts, potentially including unrooting and collapse. Here we report new geochronological and geochemical data for a suite of post-collisional mafic dykes in the North Qaidam ultrahigh-pressure metamorphic (UHPM) belt. Two groups of magmatic rocks can be distinguished: (1) Middle Devonian (392–375 Ma) basic-intermediate dykes; (2) Upper Devonian (~360 Ma) ultrabasic dykes. Whole-rock geochemistry and zircon Hf isotopes reveal that the intermediate-basic dykes are derived from a lithospheric mantle source, whereas the ultrabasic dykes are melting of asthenosphere mantle. We suggest that such mantle-derived mafic magmatism is the critical indicator for the start of post-collisional magmatism and orogen unrooting and collapse. We propose a geodynamic model explaining the activities of the lithospheric and asthenospheric mantle during the post-collisional stage, which reveals a ~ 35 million year unrooting process, from slow lithospheric mantle erosion between ~395–375 Ma, to final collapse by lithosphere delamination and asthenosphere upwelling at ~360 Ma. Addition of juvenile mantle materials to the crust by the post-collisional mafic magmatism suggests that the post-collisional stage is an important period for continental growth in Earth's history.

Isua (Greenland) ~3700 Ma meta-serpentinite olivine Mg# and δ18O signatures show connection between the early mantle and hydrosphere: Geodynamic implications

The ~ 3700 Ma Inner Arc Group of the Isua supracrustal belt (Greenland) contains a 10 km long strip of ultramafic schists with two ≤ 1 km long meta-dunite lenses, preserving relict olivine + antigorite + titano-clinohumite and titano-chondrodite ~ 2.6 GPa ultra-high-pressure (UHP) assemblages. There are two distinct relict meta-peridotite variants: The southern lens ‘A’ variant is dominated by an aggregate of Fo91-92 olivine with δ18OVSMOW of + 5.4 ± 0.1‰ (95% confidence) in which are rare small clinopyroxene inclusions, plus rare interstitial accessory chrome spinel partly altered to magnetite. The olivine grains are bounded by serpentine domains, which are intergrown with the olivine margins with a marginally more iron-rich composition of Fo90.5. The northern lens ‘B’ contains coarser-grained Fo96-98 olivine which encloses magnetite as clusters and trails, some arranged with 60°-120° conjugate intersections. The Fo96-98 olivine has δ18OVSMOW of + 3.2 ± 0.2‰ and is in equilibrium with high-Al antigorite and the accessory UHP Ti-rich minerals.We interpret that the Fo90-92 versus the Fo96-98 olivine assemblage variants are caused by a varying degree of fluid influx during ca. 3700 Ma serpentinisation, prior to peak UHP metamorphism. The Fo90-92 variant was a rock-dominated system with a postulated early serpentine + ferroan brucite alteration assemblage, with abundant relict mantle-like δ18OVSMOW + 5.4‰ olivine. The Fo96-98 variant was a fluid-dominated system, where obliteration of the protolith olivine formed serpentine + magnetite + brucite ± magnesite. During prograde UHP metamorphism consumption of brucite by reaction with serpentine gave rise to a lower δ18OVSMOW Fo96-98 olivine + serpentine assemblage. Both varieties have similar bulk chemistry, interpreted as depleted mantle that was enriched in LILE and LREE by a melt or fluid, prior to varying degrees of serpentinisation and superimposed UHP metamorphism.In Phanerozoic abyssal peridotites similar textural, chemical and isotopic variations are the result of rock versus fluid-dominated serpentinisation and variable alteration temperatures. Analogously, the Fo96-98 Isua peridotites are interpreted as mantle peridotite entirely serpentinised by high temperature seawater in a fluid-dominated environment and then metamorphosed under UHP conditions at a convergent plate boundary. This indicates lateral lithosphere motions facilitating hydrosphere - mantle communication early in Earth’s history.

Stability conditions and compositional variations of deerite in high-pressure meta-ironstone during subduction–exhumation processes (SW Tianshan, China)

Index minerals in metamorphic rocks, such as coesite, glaucophane, or lawsonite, are of great importance to understand high-pressure (HP) to ultrahigh-pressure (UHP) metamorphism, the thermal gradient, and the geodynamics of subduction zones. Deerite is thought to be diagnostic for blueschist-facies metamorphism in cold subduction environments and a sensitive indicator of the oxygen fugacity (fO2) in Fe-rich rocks. However, little is known concerning its stability and compositional variation during subduction and exhumation cycles. In this study, we provide the first detailed investigation of deerite in eclogite-facies meta‑ironstones (deerite–garnet riebeckitite) from the Southwestern Tianshan (U)HP metamorphic belt. Petrological observation and textural evidence suggest that the meta‑ironstones –enclosing eclogite blocks– form part of an initially coherent volcano-sedimentary sequence that underwent a common tectono-metamorphic evolution, including HP metamorphism and subsequent exhumation. Two types of deerite, which display a different texture (inclusion vs. matrix) and composition (TiO2 content), occur in the deerite–garnet riebeckitite. Dee_1 and aegirine–augite appear exclusively as fine-grained inclusions in garnet, and together with garnet, riebeckite, quartz and magnetite form the peak metamorphic mineral assemblages at P–T conditions of ~510 °C and 2.6 GPa. Matrix deerite (Dee_2), on the other hand, mainly occurs as coarse-grained rods and acicular needle-like grains in quartz, and rarely as inclusions in riebeckite and garnet intimately in the riebeckitite, representing the blueschist-facies retrogression of the meta‑ironstones during exhumation. The garnet–clinopyroxene oxybarometer and thermodynamic calculations show that the fO2 of the deerite–garnet riebeckitite increases from ΔFMQ–0.64 at the peak eclogite-facies stage to ΔFMQ+1.5 at the blueschist-facies retrogression. This fO2 variation corresponds to the chemical variation of deerite, whereby the TiO2 content increases gradually from <0.1 wt% in the Dee_1 in garnet to about 5 wt% in the matrix Dee_2. Our study reveals the detailed metamorphic P–T–fO2 evolution of the prograde eclogite- and retrograde blueschist-facies deerite-bearing meta‑ironstones during subduction–exhumation processes. This study further emphasizes the potential of deerite to reveal the P–T–fO2 evolution of subduction-related high-pressure rocks.

HP–UHT granulites in the East Kunlun Orogen, NW China: Constraints on the transition from compression to extension in an arc setting of the Proto‐Tethys Ocean

AbstractThe East Kunlun Orogen (EKO) of the northwest part of the Central China Orogenic belt records long‐lived orogenic events from the Cambrian to the Triassic, involving the tectonic evolution from Proto‐Tethys to Palaeo‐Tethys. Although high‐ and ultra high pressure (HP‒UHP) metamorphism has been recognized in the EKO, other metamorphic events associated with the orogenic history are little known. The metamorphic history of high‐ and ultra high temperature granulites and their pressure–temperature–time (P‒T–t) path is keys to reconstructing tectonic events associated with a subduction‐to‐collision transition. In this contribution, we present P‒T–t results for the mafic and pelitic granulite facies metamorphic rocks in the Qingshuiquan region of the EKO, determined using phase equilibrium modelling and zircon geochronology. The Jingshuikou granulite terrane records a clockwise P–T path with high‐P and ultra high temperature (HP–UHT) conditions at Pmax ~900 to 950°C at 13 kbar, at Tmax of ~950°C at 9‒12 kbar, and a final stage of 800‒895°C at 6‒10 kbar. LA‐ICP‐MS U–Pb dating of zircons provides systemic constraints on a long‐lived metamorphic age span of 530‒465 Ma, with a peak of c. 503 Ma. The HP–UHT metamorphism of the Qingshuiquan granulite terrane reveals a complete process from arc compression at the early stage of subduction to extension on the active continental margin related to rollback of the subducted slab in the Early Palaeozoic, driven by heat transfer from upwelling of the sub‐arc hot asthenosphere. The timing and P‒T conditions of the high geothermal gradient metamorphism can be used to understand the subduction and collision process of the Proto‐Tethys Ocean, including the relationship between high‐P and high‐ to ultra‐temperature metamorphism in the East Kunlun and other orogenic belts.

HP–UHP eclogites in the East Kunlun Orogen, China: P–T evidence for asymmetric suturing of the Proto-Tethys Ocean

The Early Paleozoic eclogite belt extends discontinuously for ~500 km within the East Kunlun Orogen (EKO) and records the subduction history of the Proto-Tethys Ocean. This eclogite belt, therefore, is an excellent site for studying the first-order transition in convergent-margin tectonic systems, and the tectonic events associated with subduction and collision can be determined by reconstructing the high-pressure (HP)–ultrahigh-pressure (UHP) P–T–time path. In this contribution, we present P–T results from phase equilibrium modeling and Zr-in-rutile/titanite thermometry for eclogites located on the east and west sides of the EKO. In the eastern EKO, MT–UHP eclogite records peak conditions of ~30 kbar/650–720 °C, retrograde conditions of 10 kbar at ~650–700 °C, and then further retrograde conditions at 3–6 kbar and 500–580 °C. In the western EKO, LT–HP eclogite records peak conditions of ~27 kbar at 570 °C and then retrograde conditions at 10 ± 2 kbar/470–560 °C. These two types of eclogite record oceanic subduction (west) and continental collision (east), respectively. The EKO, therefore, contains two types of eclogites that indicate two end-member subduction complexes within the orogenic belt, further indicating that there are differences in the subduction-depth and collision-time from east to west. This finding is consistent with asymmetric suturing of the subducted Proto-Tethys Ocean within the EKO. Together, the P–T conditions and time-scales of the HP–UHP metamorphism can be applied to understand the process from subduction to collision between the Qaidam Block and the South Kunlun Block, as a consequence of asymmetric closure of the Proto-Tethys Ocean.

Isotope-Geochemical Evidence of the Nature of the Protoliths of Diamondiferous Rocks of the Kokchetav Subduction–Collision Zone (Northern Kazakhstan)

Abstract —The isotope-geochemical features of diamondiferous metamorphic rocks of the Kokchetav subduction–collision zone (KSCZ) show that both the basement rocks and the sediments of the Kokchetav massif were their protoliths. A whole-rock Sm–Nd isochron from the diamondiferous calc-silicate, garnet–pyroxene rocks and migmatized granite-gneisses of the western block of the KSCZ yielded an age of 1116 ± 14 Ma, while an age of 1.2–1.1 Ga was obtained by U–Pb dating of zircons from the granite-gneiss basement of the Kokchetav microcontinent. Based on these data, we assume that the protoliths of the calc-silicate, garnet–pyroxene rocks and the granite-gneisses of the KSCZ were the basement rocks sharing an initially single Nd source, which was not influenced by high- to ultrahigh-pressure metamorphism (~530 Ma). Therefore, their geochemical features are probably not directly related to ultrahigh-pressure metamorphism. The corresponding rock associations lack isotope-geochemical evidence of partial melting that would occur during ultrahigh-pressure metamorphism, which suggesting that they were metamorphosed under granulite-facies conditions. At the same time, the high-alumina diamondiferous rocks of the Barchi area (garnet–kyanite–mica schists and granofelses), which were depleted to different degrees in light rare-earth elements (REE) and K, have yielded a Sm–Nd whole-rock isochron age of 507 ± 10 Ma indicating partial melting of these rocks during their exhumation stage. The close ɛNd (1100) values of the basement rocks and garnet–kyanite–mica schist with geochemical characteristics arguing against its depletion during high-pressure metamorphism indicate that the basement rocks were a crustal source for high-alumina sediments.

Metapelite from the high‐ to ultrahigh‐pressure terrane of the Eastern Alps (Pohorje Mountains, Slovenia)—New pressure, temperature and time constraints on a polymetamorphic rock

AbstractThe Austroalpine nappe stack of the Pohorje Mountains (Mts.) in northeastern Slovenia comprises a suite of eclogite facies metamorphic rocks that were partially assigned to Eo‐Alpine ultrahigh‐pressure metamorphism (UHPM). We selected a micaschist, which was previously related to this metamorphism, for a detailed study including the chemical zonation of garnet and potassic white mica, the identification of mineral inclusion assemblages, pseudosection modelling with PERPLE_X, and monazite in‐situ dating with the electron microprobe. Polymetamorphism was revealed by (at least) two generations of garnet and phengite and four populations of monazite yielding ages of 283.6 ± 6.1 (2σ), 94.1 ± 3.7, 47.9 ± 10.8 and 26.2 ± 2.8 Ma. The Permian monazite population is characterized by relatively high Y contents (~1.15 wt% Y) and low La/Gd mass ratios (8.7) indicating its formation before the growth of porphyroblastic garnet. The Eo‐Alpine population, however, grew synchronously with garnet based on low Y contents (~0.05 wt%) and high La/Gd ratios (21.4). The older Tertiary population (48 Ma) shows also high Y contents (1.1 wt%) and low La/Gd ratios (10.6) whereas the younger Tertiary population is characterized by low Y contents. The Permian P–T conditions of 7.5–10 kbar at 600–650°C were obtained using the inclusion assemblage of staurolite+rutile+biotite in porphyroblastic garnet. High pressure (HP) but no UHPM was reconstructed for both Eo‐Alpine coarse phengite (Si = 3.22 per formula unit = pfu) and small Tertiary garnet+fine‐grained phengite (Si = 3.27 pfu) at peak pressures ~16 kbar and 18.5–23 kbar respectively. Maximum temperatures close to 650°C were likely reached during the Eo‐Alpine HP event, whereas those of the Tertiary HP event were probably ~580°C. These HP metamorphic events suggest that the Pohorje Mts. experienced both an Eo‐Alpine and a Tertiary subduction–exhumation history, the latter of which was mainly reported for underlying Penninic nappes so far.

Late Triassic contractional tectonics in the overriding plate of the Sulu orogenic belt, Eastern China

It is well known that the Dabie-Sulu orogenic belt resulted from the northward continental subduction of the South China Block (SCB) beneath the North China Block (NCB) during the Triassic. Correlating the deformation events recorded in the subducted and overriding plates is essential to understand its tectonic evolution, but related deformation is poorly documented in the overriding plate. In this contribution, we present detailed structural analysis, anisotropy of magnetic susceptibility (AMS), and geochronological data in the Linsishan unit of the Jiaobei massif, the overriding plate of the Sulu orogenic belt. The Linsishan unit preserves a significant ductile deformation characterized by NNW-, NE-, or SE-dipping foliations with NE–SW trending mineral lineations. Coupling mesoscopic and magnetic fabrics augment the NE–SW linear structures. The NE-dipping monoclinal structures with top-to-SW kinematics could be interpreted as a SW-directed thrusting, the deformation temperature of which is 350–450 °C according to the quartz c-axis fabrics. The timing of deformation is constrained to be 223–221 Ma based on 40Ar/39Ar dating of the deformed rocks and linked with a syn-collisional stage in the Sulu orogenic belt. Combining these new results with previous works, we propose a three-stage evolution model for the Sulu orogenic belt, namely (1) subducted SCB experienced ultra-high pressure (UHP) metamorphism before 230 Ma; (2) SW-directed thrusting was developed in the overriding plate, coeval with the syn-collisional exhumation of UHP rocks during 230–220 Ma; (3) post-collisional migmatization, magmatism, extension, and UHP rocks exhumation took place at 220–205 Ma.

Geochronological and geochemical constraints on the non-ultrahigh-pressure metasedimentary blocks of North China affinity within the Sulu ultrahigh-pressure belt, eastern China, and tectonic implications

ABSTRACT The Sulu ultrahigh-pressure (UHP) belt has been proven to form by deep northward subduction of the Yangtze Block (YB) beneath the North China Block (NCB) in the Triassic. The basement rocks originated from the YB and experienced Triassic UHP metamorphism. In this contribution, we identify for the first time tens of non-UHP metasedimentary blocks enclosed in the regional UHP granitic gneiss in the Weihai terrane in the northeastern Sulu UHP belt. They are mainly composed of paragneiss, marble and calc-silicate rocks, in which UHP-related minerals have not been identified. Detrital zircons from the non-UHP metasedimentary rocks are mostly of magmatic origin and yield U-Pb ages mainly from 2842 to 1792 Ma with two clusters of 2580–2460 Ma and 1900–1800 Ma. This zircon U-Pb age spectrum is typical of the NCB but not the YB, and matches the tectonothermal/magmatic history of the southwestern end of the Jiao-Liao-Ji belt (JLJB), eastern NCB, which lies immediately adjacent to the Weihai terrane. Furthermore, their whole-rock geochemical compositions show great resemblance to those of the Paleoproterozoic metasedimentary rocks within the JLJB. In particular, metamorphic zircons from the non-UHP metasedimentary rocks indicate widespread late Triassic-early Jurassic (213–194 Ma) metamorphic overprinting of the Paleoproterozoic (1850–1820 Ma) metamorphism that characterizes the JLJB. This means that the enclosed non-UHP metasedimentary rocks with NCB signatures in the Weihai terrane were involved in the collision-subduction process during the Triassic. They have not been carried to mantle depths during deep continental subduction. These lines of evidence prompt us to establish a Triassic tectonic erosion model to explain these new discoveries and pre-existing problems along the Sulu UHP orogenic belt.

Subcratonic and tectonic evolution of pyroxenite and eclogite with lamellar inclusions in garnet, Western Gneiss Region, Norway

Abstract Oriented lamellar inclusions of pyroxene and rutile in mantle garnet often serve as evidence for majoritic and titaniferous precursor garnets, respectively. We investigated ten new such microstructure-bearing samples from six orogenic peridotite bodies in SW Norway, which originated in the E Greenland mantle lithosphere, petrologically and thermobarometrically. All pyroxenite (nine) and eclogite (one) samples have large (mainly porphyroclastic) garnet containing silicate and oxide inclusions with shape-preferred orientation relationship. These inclusions vary—dependent on their size—systematically in shape (acicular to subprismatic), width (∼50 μm to submicron size), spacing (several 100 to ∼10 μm) and phase (pyroxene to Ti-oxide ± pyroxene). Smaller inclusions can fill the space between larger inclusions, which support the idea of consecutive generations. The larger, early formed lamellae occur least frequent and are most poorly preserved. A younger generation of other inclusions decorates healed cracks cutting across cores but not rims of garnet. These inclusions comprise oxides, silicates, carbonates (aragonite, calcite, magnesite) and fluid components (N2, CO2, H2O). The older, homogeneously distributed inclusions comply texturally and stoichiometrically with an origin by exsolution from excess Si- and Ti-bearing garnet. Their microstructural systematic variation demonstrates a similar early evolution of pyroxenite and eclogite. The younger inclusions in planar structures are ascribed to a metasomatic environment that affected the subcratonic lithosphere. The microstructure-bearing garnets equilibrated at ∼3.7 GPa (840 °C) and ∼3.0 GPa (710 °C), at a cratonic geotherm related to 37–38 mW m−2 surface heat flow. Some associated porphyroclastic grains of Mg-rich pyroxene have exsolution lamellae of Ca-rich pyroxene and vice versa that indicate a preceding cooling event. Projected isobaric cooling paths intersect isopleths for excess Si in garnet at ∼1550 °C, if an internally consistent thermodynamic data set in the system Na2O–CaO–MgO–Al2O3–SiO2 (NCMAS) is applied (or ∼1600 °C if using CMAS). This temperature may confine the crystallisation of the unexsolved garnets at 100–120 km depths of the E Greenland subcratonic lithosphere. Tectonism is indicated in coastal and hinterland samples by porphyroclastic orthopyroxene with Al2O3 concentrations showing W-shaped profiles. Cores of associated large (&amp;gt;200 μm) recrystallised grains have low Al2O3 contents (0.18–0.23 wt.%). Both characteristics typify relatively short intracrystalline Al diffusion lengths and a prograde metamorphism into the diamond stability field. We assign this event to subduction during the Scandian orogeny. Porphyroclastic orthopyroxene in other samples shows U-shaped Al2O3 concentration profiles paired with long Al diffusion lengths (several 100 μm) that exceed the radius of recrystallised grains. Their cores contain high Al2O3 contents (0.65–1.16 wt.%), consistent with a diffusional overprint that obliterated prograde and peak metamorphic records. Unlike Al2O3, the CaO content in porphyroclastic orthopyroxene cores is uniform suggesting that early exhumation was subparallel to Ca isopleths in pressure–temperature space. The depth of sample origin implies that rock bodies of Scandian ultra-high pressure metamorphism occur in nearly the entire area betweenNordfjord and Storfjord and from the coast towards ∼100 km in the hinterland, i.e. in a region much larger than anticipated from crustal eclogite.

Ultra-High Pressure Metamorphism and Geochronology of Garnet Clinopyroxenite in the Paleozoic Dunhuang Orogenic Belt, Northwestern China

Ultra-high pressure (UHP) metamorphism is recorded by garnet clinopyroxenite enclaves enclosed in an undeformed, unmetamorphosed granitic pluton, northeastern Paleozoic Dunhuang orogenic belt, northwestern China. The protoliths of the garnet clinopyroxenite might be basic or ultrabasic volcanic rocks. Three to four stages of metamorphic mineral assemblages have been found in the garnet clinopyroxenite, and clockwise metamorphic pressure–temperature (P-T) paths were retrieved, indicative of metamorphism in a subduction environment. Peak metamorphic P-T conditions (790–920 °C/28–41 kbar) of garnet clinopyroxenite suggest they experienced UHP metamorphism in the coesite- or diamond-stability field. The UHP metamorphic event is also confirmed by the occurrence of high-Al titanite enclosed in the garnet, along with at least three groups of aligned rutile lamellae exsolved from the garnet. Secondary ion mass spectrometry (SIMS) U-Pb dating of metamorphic titanite indicates that the post-peak, subsequent tectonic exhumation of the UHP rocks occurred in the Devonian period (~389–370 Ma). These data suggest that part of the Paleozoic Dunhuang orogenic belt experienced UHP metamorphism, and diverse metamorphic facies series prevailed in this Paleozoic orogen. It can be further inferred that most of the UHP rocks in this orogen remain buried.