Ultrahigh-pressure Rocks Research Articles

Polyphase inclusions in the Shuanghe UHP eclogites formed by subsolidus transformation and incipient melting during exhumation of deeply subducted crust

Ultrahigh-pressure (UHP) layered eclogites from the Shuanghe area, Dabie Mountains display the peak assemblage garnet, Na-rich omphacite (Jd60), and minor amounts of coesite, rutile, Si-rich phengite (Si=3.58p.f.u) and apatite. Zr-in-rutile thermometry provides evidence for peak metamorphic conditions of ~650–700°C, which combined with garnet–clinopyroxene–phengite barometry yields a peak pressure of 3.5–4GPa. Three different types of polyphase inclusions that formed after peak conditions are identified in garnet. The first type consists of a symplectitic intergrowth of zoisite+quartz±amphibole associated with epidote and exhibits a rectangular shape. These inclusions are interpreted to have formed during decompression of the UHP rocks as pseudomorphs after prograde to peak lawsonite. The second type of polyphase inclusions contains approximately equal amounts of K-feldspar and quartz. These inclusions are usually surrounded by radial fractures and exhibit regular negative crystal shapes. The observed gradual breakdown of peak metamorphic phengite resulting in these types of inclusions suggests that phengite melting at an advanced stage of exhumation at ~1.5–2GPa and ~750–800°C occurred. The third type of polyphase inclusions displays irregular shapes and preserves the retrograde mineral assemblage amphibole+plagioclase+quartz+biotite+K-feldspar, which likely formed during late stage fluid influx that is also responsible for the replacement of phengite by biotite as well as omphacite by amphibole+plagioclase.The trace element composition of K-feldspar+quartz inclusions has been determined in-situ by LA-ICP-MS and is characterized by a strong enrichment in LILE and a moderate enrichment in LREE with respect to HREE and a depletion in HFSE. The presence of melts assisted in the recrystallization of the eclogites and resulted in the segregation of garnet+quartz-rich and omphacite-rich domains. Field observations show that the melting is very limited and no interconnected network of partial melts formed. Thus, the low degree of partial melting did not result in a chemical differentiation of the subducted rocks but likely influenced the rheology of the eclogites during exhumation.

The role of amphiboles in the metamorphic evolution of the UHP rocks: a case study from the Tso Morari Complex, northwest Himalayas

Amphiboles represent a crucial phase of the ultra-high-pressure (UHP) metamorphic rocks as their solid solution behavior reflects both bulk compositional and P–T changes. Three different types of amphibole have been reported from the UHP metamafic rocks of the Tso Morari Crystalline Complex, NW Himalayas: Na-rich (glaucophane); Na–Ca-rich (barroisite, taramite, winchite) and Ca-rich (tremolite, magnesio-hornblende, pargasite). The Na-amphibole is presented as a core of the zoned amphibole with Na–Ca-rich rim; Na–Ca-amphibole is presented as inclusion in garnets as well as in matrix, and Ca-amphibole is generally found in the matrix. The Na–Ca-amphibole is observed at two different stages of metamorphism. The first is pre-UHP, and the second is post-garnet–omphacite assemblage though with a significant difference in composition. The pressure–temperature estimations of the formation of these two sets of Na–Ca-amphiboles corroborate their textural associations. Ca-rich amphiboles are generally present in the matrix either as symplectite with plagioclase or as a pseudomorph after garnet along with other secondary minerals like chlorite and biotite. Two different types of zoning have been observed in the amphibole grains: (1) core is Na-rich followed by Na–Ca rim and (2) core of Na–Ca-amphibole is followed by Ca-rich rim. The pre-UHP (or the prograde P–T path) and post-UHP stages (or the retrograde P–T path) of Tso Morari eclogites are defined by characteristic amphibole compositions, viz. Na/Na–Ca-amphibole, Na–Ca-amphibole and Ca-amphibole and thus indicate their utility in inferring crustal evolution of this UHP terrain.

Paradigms, new and old, for ultrahigh-pressure tectonism

Regional ultrahigh-pressure (UHP) metamorphic terranes exhibit a spectrum of lithological, structural and petrological characteristics that result from the geodynamic processes that formed and exhumed them. At least six geodynamic processes can be envisioned to have carried continental rocks to mantle depths: i) continental margin subduction, ii) microcontinent subduction, iii) sediment subduction, iv) intracontinental subduction, v) subduction erosion, and vi) foundering of a crustal root. Most of these processes have been investigated through numerical or analog models and most have been invoked for one or more specific occurrences of UHP rocks. At least six geodynamic processes can be envisioned to have exhumed UHP continental rocks: i) eduction, ii) microplate rotation, iii) crustal stacking, iv) slab rollback, v) channel flow, and vi) trans-mantle diapirs. Most of these processes have also been investigated through numerical or analog models and all have been invoked to explain the exhumation of at least one UHP terrane. More-detailed and systematic field investigations are warranted to assess the predictions of numerical models, and more-sophisticated and realistic numerical models are required to replicate and explain the petrological, structural, and chronological data obtained from UHP terranes.

Subduction along and within the Baltoscandian margin during closing of the Iapetus Ocean and Baltica-Laurentia collision

The recent discovery of ultrahigh-pressure (UHP) mineral parageneses in the far-transported (greater than 400 km) Seve Nappe Complex of the Swedish Caledonides sheds new light on the subduction system that dominated the contracting Baltoscandian margin of continental Baltica during the Ordovician and culminated in collision with Laurentia in the Silurian to Early Devonian. High-grade metamorphism of this Neoproterozoic to Cambrian rifted, extended, dike-intruded outer-margin assemblage started in the Early Ordovician and may have continued, perhaps episodically, until collision of the continents at the end of this period. The recent discovery of UHP kyanite eclogite in northern Jamtland (west-central Sweden) yields evidence of metamorphism at depths of 100 km. Although UHP rocks are only locally preserved from retrogression during the long-distance transport onto the Baltoscandian platform, these high-pressure parageneses indicate that deep subduction played an important role in the tectonothermal history of the complex. Based on existing isotopic age data, this UHP metamorphism occurred in the Late Ordovician, shortly before, or during, the initial collision between the continents (Scandian orogeny). In some central parts of the complex, migmatization and hot extrusion occurred in the Early Silurian, giving way to thrust emplacement across the Baltoscandian foreland basin and platform that continued into the Early Devonian. Identification of HP/UHP metamorphism at different levels within the Scandian allochthons, definition of their pressure-temperature-time paths, and recognition of their vast transport distances are essential for an understanding of the deeper structural levels of the orogen in the hinterland (e.g., the Western Gneiss Region), where the attenuated units were reworked together during the Early Devonian.

Protoliths and phase petrology of whiteschists

Whiteschists appear in numerous high- and ultrahigh-pressure rock suites and are characterized by the mineral assemblage kyanite + talc (+-quartz or coesite). We demonstrate that whiteschist mineral assemblages are well stable up to pressures of more than 4 GPa but may already form at pressures of 0.5 GPa. The formation of whiteschists largely depends on the composition of the protolith, which requires elevated contents of Al and Mg as well as low Fe, Ca, and Na contents, as otherwise chloritoid, amphibole, feldspar, or omphacite are formed instead of kyanite or talc. Furthermore, the stability field of the whiteschist mineral assemblage strongly depends on XCO2 and fO2: already at low values of XCO2, CO2 binds Mg to carbonates strongly reducing the whiteschist stability field, whereas high fO2 enlarges the stability field and stabilizes yoderite. Thus, the scarcity of whiteschist is not necessarily due to unusual P–T conditions, but to the restricted range of suitable protolith compositions and the spatial distribution of these protoliths: (1) continental sedimentary rocks and (2) hydrothermally and metasomatically altered felsic to mafic rocks. The continental sedimentary rocks that may produce whiteschist mineral assemblages typically have been deposited under arid climatic conditions in closed evaporitic basins and may be restricted to relatively low latitudes. These rocks often contain large amounts of the clay minerals palygorskite and sepiolite. Marine sediments generally do not yield whiteschist mineral assemblages as marine shales commonly have too high iron contents. Sabkha deposits may have too high CO2 contents. Protoliths of appropriate geochemical composition occur in and on continental crust. Therefore, whiteschist assemblages typically are only found in settings of continental collision or where continental fragments were involved in subduction. Our calculations demonstrate that whiteschists can form by closed-system metamorphism, which implies that the chemical and isotopic composition of these rocks provide constraints on the development of the protoliths.

Diversity of potassium-bearing tourmalines in diamondiferous Kokchetav UHP metamorphic rocks: A geochemical recorder from peak to retrograde metamorphic stages

Chemical variations and inclusion mineralogy of tourmaline in diamond-bearing ultrahigh-pressure (UHP) gneisses and related siliceous rocks from the Kokchetav Massif, Kazakhstan are extensively investigated, in conjunction with a review of previously discovered K-bearing tourmaline with microdiamond inclusions. Tourmalines in these Kokchetav UHP rocks have a wide compositional range (dravite – “potassium-oxy-dravite” – schorl – uvite – feruvite – foitite, and some of their oxy-variants) with potassium content ranging from 0 to 0.576 atoms per formula unit (apfu, based on Y+Z+T cations=15) (or 0–2.76wt.% K2O) and decreasing from core to rim. The variation in potassium is spatially related to the inclusion mineralogy and is attributed to changes in metamorphic grade. Four stages of tourmaline growth in the Kokchetav UHP rocks are recognized based on the potassium contents and occurrences; in order of decreasing P and T: (1) K-dominant tourmaline (K>Na, Ca, and X-vacancy) with microdiamond and high-SiTi phengite inclusions and without quartz inclusions; (2) K-rich tourmaline (K: 0.2–0.05apfu) with graphite, phengite, kyanite and quartz inclusions; (3) K-poor tourmaline (K: 0.05–0.01apfu) with matrix quartz and biotite; and (4) K-free anhedral or overgrown tourmaline associated with low-P–T minerals such as amphibole or chlorite. Several lines of evidence also suggest that K-dominant tourmaline crystallized in the diamond stability field. Tourmalines in the Kokchetav UHP rocks recorded multi-stage growth history from peak UHP conditions through a granulite facies overprint event including partial melting and ultimately to the latest retrograde metamorphism during exhumation.

Synexhumation anatexis of ultrahigh-pressure metamorphic rocks: Petrological evidence from granitic gneiss in the Sulu orogen

Petrological evidence is provided for partial melting of ultrahigh-pressure (UHP) metamorphic granitic gneiss in the Sulu orogen. Petrographic observations show the occurrence of elongated, highly cuspate feldspars in grain boundaries, interstitial cuspate feldspars in triple junctions, felsic veinlets mainly consisting of K-feldspar+quartz, and feldspar crystal faces against quartz. These features indicate that the feldspar and quartz would have grown from anatectic melts in the granitic gneiss. Zircon domains grown from these melts were identified based on CL images, mineral inclusions and REE patterns. Some large zircon grains (>100μm) contain small relict domains of magmatic origin, suggesting nearly complete dissolution of the protolith zircon during the anatexis. All newly grown zircon domains are categorized into two groups based on the presence or absence of coesite inclusions. One group of domains contain no coesite inclusion and exhibit high U contents but low Th/U ratios (<0.1), steep REE patterns with strong negative Eu anomalies, and UPb ages of 217±2 to 224±2Ma. The other group of domains contain coesite inclusions and exhibit low U contents and very low Th/U ratios (<0.01), steep REE patterns with strong negative Eu anomalies, and UPb ages of 221±5 to 226±3Ma. The two groups of zircon domains are thus interpreted as growing from the anatectic melts at different pressures because they exhibit marked negative Eu anomalies that are absent for metamorphic zircons grown from aqueous fluids. The zircon UPb ages of 217±2 to 226±3Ma are close to, but slightly younger than, known ages for major UHP metamorphism in the Sulu orogen. Therefore, the UHP gneiss experienced incipient melting during the initial exhumation subsequent to the peak UHP metamorphism and extensive anatexis later at lower pressures. Muscovite relicts coexist with cuspate feldspars, suggesting that the anatectic melts originate from dehydration melting due to decompression breakdown of phengitic muscovite in the UHP granitic gneiss. As the degree of partial melting increases with temperature, significant fractions of the anatectic melts would be produced in the regional gneiss provided that the UHP rocks experienced “hot” exhumation. Such melts can be gathered together, being eventually emplaced as synexhumation granitic intrusions.

Metamorphic solid salt (KCl-NaCl) in quartzo-feldspathic polyphase inclusions in the Sulu ultrahigh-pressure eclogite

Unusual polyphase inclusions of K-feldspar + quartz + titanite + solid salt and K-feldspar + albite + quartz + epidote with textures similar to the other K-feldspar + quartz inclusions were found in omphacite grains from the Sulu ultrahigh pressure (UHP) eclogites. One of these inclusions contain square to round solid salt inclusions of KCl-NaCl composition. Such a mineral assemblage within K-feldspar-bearing inclusions hosted by UHP metamorphic phases suggests that (1) potassium granitic melts enriched in Cl components were presented during UHP metamorphism or at the early stage of rapid exhumation of deeply subducted continental slab; (2) they were resulted from reactions between the incoming granitic melts and quartz (or coesite); and (3) solid salt inclusions of NaCl-KCl were derived from dehydration and desiccation of Cl-bearing melts. Our new observations further demonstrate that during the tectonic evolution of UHP rocks, fertile components within deeply subducted continental materials could undergo partial melting, leading to the formation of Cl-bearing potassium granitic melts and substantial migration of fluid-conservative elements (e.g. Ti, Hf) within the UHP slab.

Oriented kokchetavite compound rods in clinopyroxene of Kokchetav ultrahigh-pressure rocks

Two microdiamond-bearing samples, a dolomite marble and a garnet-clinopyroxene rock, from the Kokchetav ultrahigh-pressure metamorphic terrane were selected in the present study to explore the possible origin of KAlSi3O8 rod inclusions oriented along the c-axis of clinopyroxene host.The KAlSi3O8 rod inclusions at clinopyroxene cores, where K2O content is high in the range of 0.5–1.0wt.%, are mostly fine-grained with a rod width less than 1μm. AEM studies showed that the KAlSi3O8 phase in most rod inclusions is kokchetavite. K-feldspar is present only in a few cases, probably the result of phase transformation/recrystallization from kokchetavite during rock exhumation. Electron diffractions further showed that kokchetavite rods are oriented parallel to clinopyroxene [001] direction and they exhibit the same epitaxial relation with the clinopyroxene host in both samples with the (Al, Si)O4 tetrahedra chains along the hexagonal a-axis of kokchetavite parallel to the single SiO4 chain along the c axis of clinopyroxene; i.e., [12¯10]Ko//[001]Cpx and (0001)Ko//(100)Cpx. It is interesting to note that kokchetavite is always in association with phengite, tremolite, β-cristobalite, Si-rich (Al, K, Ca-bearing) low crystallinity phase, ±Si–Ca (Cl, As) phase, ±calcite, ±apatite, ±lollingite (FeAs2), forming compound rods. Furthermore, all these phases are also present within submicron-scale polyphase inclusion pockets in garnet within garnet-clinopyroxene rock sample. These kokchetavite compound rods are therefore most likely to have resulted from melt/fluid-clinopyroxene interactions leading to epitaxial deposition rather than exsolution sensu stricto from the clinopyroxene host. The suggested melt/fluid would have an “external” and/or an “internal” origin related to rock partial melting involving phengite breakdown.Discrete phlogopite and phengite needle-like inclusions with a needle width less than 1μm, as well as phlogopite–phengite and kokchetavite-mica intergrowth needles, are also not uncommon in clinopyroxene cores. There are specific crystallographic orientation relationships among phases. These micas might have formed earlier than kokchetavite and have exsolved from clinopyroxene host, although the mass balance issue during exsolution and the temporal relation between phlogopite and phengite needles remain to be settled.Clinopyroxene rims generally contain mica needles only. Domains near fractures with coarse-grained kokchetavite compound rods or with coarse-grained phlogopite+quartz needles are not uncommon. These clinopyroxene rims/domains are low in K2O (<0.5wt.%) and their inclusions are most probably a result of precipitation/recrystallization from late-stage(s) infiltrated fluid-clinopyroxene interactions.

The Dabie Extensional Tectonic System: Structural Framework

A previous study of the Dabie area has been supposed that a strong extensional event happened between the Yangtze and North China blocks. The entire extensional system is divided into the Northern Dabie metamorphic complex belt and the south extensional tectonic System according to geological and geochemical characteristics in our study. The Xiaotian-Mozitan shear zone in the north boundary of the north system is a thrust detachment, showing upper block sliding to the NNE, with a displacement of more than 56 km. However, in the south system, the shearing direction along the Shuihou-Wuhe and Taihu-Mamiao shear zones is tending towards SSE, whereas that along the Susong-Qingshuihe shear zone tending towards SW, with a displacement of about 12 km. Flinn index results of both the north and south extensional systems indicate that there is a shear mechanism transition from pure to simple, implying that the extensional event in the south tectonic system could be related to a magma intrusion in the Northern Dabie metamorphic complex belt. Two 40Ar-39Ar ages of mylonite rocks in the above mentioned shear zones yielded, separately, ~190 Ma and ~124 Ma, referring to a cooling age of ultrahigh-pressure rocks and an extensional era later.

Carbon isotope heterogeneity in metamorphic diamond from the Kokchetav UHP dolomite marble, northern Kazakhstan

We analysed isotopic compositions of metamorphic microdiamond secondary ion mass spectrometry. Typical microdiamonds in this dolomite marble show star-shaped morphologies (S-type) consisting of single-crystal cores and polycrystalline rims. Four S-type microdiamonds and two R-type microdiamonds (single crystals with rugged surfaces) were analysed using a 5 μm diameter ion beam. S-type microdiamonds have heterogeneous carbon isotopic compositions even in a single grain. Analysis of a typical S-type microdiamond (no. xx01-1-13) revealed clear difference in δ13C between core and rim. The rim shows lighter isotopic compositions ranging from −17.2‰ to −26.9‰, whereas the core is much heavier, with δ13C ranging from −9.3‰ to −13.0‰. The δ13C values of R-type microdiamonds fall into narrow ranges from −8.3‰ to −14.9‰ for no. xx01-1-10 and from −8.3‰ to −15.3‰ for no. xx01-1-16. These δ13C values are similar to those of the S-type microdiamond cores. The R-type probably formed at the same stage as the core of the S-type, whereas rim growth at a second stage did not occur or occurred very weakly in R-type microdiamonds. These carbon isotopic data support the two-stage growth of microdiamonds in the Kokchetav ultrahigh-pressure host rock. To explain the second stage growth of S-type microdiamonds, we postulate a simple fluid infiltration of light carbon from neighbouring gneisses into the dolomite marble.

Melting of metasedimentary rocks at ultrahigh pressure—Insights from experiments and thermodynamic calculations

Diamondiferous quartzofeldspathic rocks from the Kokchetav Massif, Kazakhstan, and the Erzgebirge, central Europe, are rare witnesses of melting of sedimentary material at great depths. To better understand the melting process, two powdered samples from these localities were objects of experiments conducted in a piston-cylinder apparatus at pressures of 3–5 GPa. In addition, thermodynamic calculations in the system Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O using a haplogranitic melt model yielded isochemical phase equilibria diagrams (i.e., pseudosections) at ultrahigh pressure. The solidus for both rocks was located close to 1000 °C and 1100 °C at 3 GPa and 5 GPa, respectively, in the experiments. Initial potassic to ultrapotassic melts form by phengite breakdown. At ∼200 °C above the solidus, clinopyroxene disappears from the restite assemblage, and coexisting melts are granitic. The restite consists of garnet + coesite (±kyanite) up to temperatures close to the liquidus, which occurs at a temperature ∼350 °C above the solidus. The results of the thermodynamic calculations approximate the pressure-temperature conditions and phase relations around the solidus but increasingly deviate from the experimental results with rising temperature. According to the experiments, the melt that crystallized to diamondiferous saidenbachite from the Erzgebirge should have been as hot as 1400 °C, whereas the ultrahigh-pressure rocks from the Kokchetav Massif experienced temperatures of at least 1200 °C. These high melting temperatures are derived for rocks with no free water, which is the most likely scenario for continental crust taken to mantle depths where diamond forms.

UHP metamorphism recorded by kyanite-bearing eclogite in the Seve Nappe Complex of northern Jämtland, Swedish Caledonides

The first evidence for ultrahigh-pressure (UHP) metamorphism in the Seve Nappe Complex of the Scandinavian Caledonides is recorded by kyanite-bearing eclogite, found in a basic dyke within a garnet peridotite body exposed close to the lake Friningen in northern Jämtland (central Sweden). UHP metamorphic conditions of ~3GPa and 800°C, within the stability field of coesite, are constrained from geothermobarometry and calculated phase equilibria for the peak-pressure assemblage garnet+omphacite+kyanite+phengite. A prograde metamorphic evolution from a lower P–T (1.5–1.7GPa and 700–750°C) stage during subduction is inferred from inclusions of pargasitic amphibole, zoisite and kyanite in garnet cores. The post-UHP evolution is constrained from breakdown textures, such as exsolutions of kyanite and silica from the Ca-Eskola clinopyroxene. Near isothermal decompression of eclogite to lower crustal levels (~0.8–1.0GPa ) led to formation of sapphirine, spinel, orthopyroxene and diopside at granulite facies conditions. Published age data suggest a Late Ordovician (460–445Ma) age of the UHP metamorphism, interpreted to be related to subduction of Baltoscandian continental margin underneath an outboard terrane, possibly outermost Laurentia, during the final stages of closure of the Iapetus Ocean. The UHP rocks were emplaced from the hinterland collision zone during Scandian thrusting of the nappes onto the Baltoscandian foreland basin and platform. The record of P–T conditions and geochonological data from UHP rocks occurring within the allochthonous units of the Scandinavian Caledonides indicate that Ordovician UHP events may have affected much wider parts of the orogen than previously thought, involving deep subduction of the continental crust prior to final Scandian collision between Baltica and Laurentia.

Apatite fission track and (U–Th)/He ages from the Higher Himalayan Crystallines, Kaghan Valley, Pakistan: Implications for an Eocene Plateau and Oligocene to Pliocene exhumation

Apatite fission track and apatite and zircon (U–Th)/He ages were obtained from high- and ultra high-pressure rocks from the Kaghan Valley, Pakistan. Four samples from the high altitude northern parts of the valley yielded apatite fission track ages between 24.5±3.7 and 15.6±2.1Ma and apatite (U–Th)/He ages between 21.0±0.6 and 5.3±0.2Ma. These data record cooling of the formerly deeply-subducted high-grade metamorphic rocks induced by denudation and exhumation consistent with extension and back sliding along the reactivated, normal-acting Main Mantle Thrust. Overlap at around 10Ma between fission track and (U–Th)/He ages is recognised at one location (Besal) showing that fast cooling occurred due to brittle reactivation of a former thrust fault. Widespread Miocene cooling is also evident in adjacent areas to the west (Deosai Plateau, Tso Morari), most likely related to uplift and unroofing linked to continued underplating of the Indian lower crust beneath Ladakh and Kohistan in the Late Eocene to Oligocene. In the southernmost part of the study area, near Naran, two significantly younger Late Miocene to Pliocene apatite fission track ages of 7.6±2.1 to 4.0±0.5Ma suggest a spatial and temporal separation of exhumation processes. These younger ages are best explained by enhanced Late Miocene uplift and erosion driven by thrusting along the Main Boundary Thrust.

Collision and rotation of the South China block and their role in the formation and exhumation of ultrahigh pressure rocks in the Dabie Shan orogen

Terra Nova, 24, 339–350, 2012AbstractStudies of regional plate‐scale processes that may influence the exhumation of ultra‐high pressure (UHP) metamorphic rocks are few despite their potential importance. Here, we review and summarize available age constraints along with the Qinling‐Dabie Shan orogen, which separates the North China Block (NCB) from the South China Block (SCB). We find that collision‐related events in the west and extension‐related events in the east, where the large Dabie Shan UHP terrane is located, were contemporaneous. Available palaeomagnetic data indicate clockwise rotation of the SCB relative to the NCB during this time. Previous work indicated that the collision propagated from east to west where indentation of the SCB into the Qinling‐Dabie Shan orogen was active in the Hannan Dome area at 165 ± 3 Ma as constrained by new 40Ar/39Ar dates on synkinematic sericite in an indentation‐related mylonitic shear zone. While this contraction was occurring in the west, exhumation of the UHP terrane was occurring in the Dabie Shan in the east. We propose that the collision of the SCB with the NCB occurred not only in a scissor‐like fashion, as previously suggested, but also involved a later rotation about a pole west of the Dabie Shan during the closure of the Mianlue Ocean. As a consequence of this rotation, extension occurred between the South China Block and North China Block east of the pole of rotation leading to extensional exhumation of the Dabie Shan orogen. This microcontinent‐scale rotation of the SCB may have provided an important component in the exhumation of the largest UHP terrane in the world.

The hunting of the snArc

Volcanic and intrusive rocks with geochemical signatures typical of modern continental or oceanic arcs are uncommon in the Archaean and the archetypal Archaean granite-greenstone dome-and-keel architecture has no modern analogue. Proposed Archaean ophiolites, Atlantic-style passive margins, overprinting thrust and fold belts, blueschists, ultra high-pressure rocks, paired metamorphic belts, orogenic andesites, and subduction-zone mélanges that typify Phanerozoic plate margins are rare to absent. Since there is little evidence for Archaean subduction zones, we consider Archaean arcs to be ‘Snarks’, imaginary constructs with no objective existence. Some Archaean folds and upper-mantle structures interpreted as evidence for regional thrusting may equally well have developed during horizontal extension of low-viscosity crust. Nonetheless, there are clearly many Archaean terrains exhibiting fabrics formed by bulk shortening and some cratons contain terranes with contrasting histories. Given the absence of evidence for Archaean subduction, what could be a plausible driving force for compression and terrane accretion? Cratonic mobilism in response to mantle convection currents offers a possible solution to this paradox. Once a proto-craton develops a deep high-viscosity mantle keel, it would become subject to pressure from mantle currents and could drift. Immature cratons or oceanic plateaux would not have a strong mantle keel and so would be static. So, contrary to conventional wisdom, we consider that Archaean cratons are not immobile nucleii along whose margins ‘mobile belts’ form by subduction-zone accretion. Instead, we propose that Archaean cratons were the active tectonic agents, accreting and subcreting basaltic plateaux, other proto-cratons, and heterogeneous mantle domains as they drifted. In this model, accreted terranes and structures indicating bulk shortening would be concentrated at the cratonic leading edge, with oblique and strike-slip shear zones at the sides, extension and possible seafloor-spreading in the lee, and major oblique-slip shear zones in the interior developed as a result of the imposed stress field. Overridden oceanic plateaux would be thrust (subcreted) deep enough to melt in the garnet field and generate syntectonic pulses of tonalite–trondhjemite–granodiorite (TTG), contributing to craton growth and stabilization.

Metamorphic chemical geodynamics in continental subduction zones

Chemical geodynamics is an integrated discipline that studies the geochemical structure and tectonic evolution of geospheres with the aim of linking tectonic processes to geochemical products in the Earth system. It was primarily focused on mantle geochemistry, with an emphasis on geochemical recycling in oceanic subduction zones. It has been extended to geochemical reworking and recycling under high-pressure (HP) to ultrahigh-pressure (UHP) conditions in all convergent plate margins. In particular, UHP terranes, along with UHP metamorphic minerals and rocks in continental subduction zones, represent natural laboratories for investigating geochemical transport and fluid action during subduction and exhumation of continental crust. As a result of this extension, the study of UHP terranes has significantly advanced our understanding of tectonic processes in collisional orogens. This understanding has principally benefited from the deciphering of petrological and geochemical records in deeply subducted crustal rocks that occur in different petrotectonic settings. This review focuses on the following issues in continental subduction zones: the time and duration of UHP metamorphism, the origin and action of metamorphic fluid/melt inside UHP slices, the element and isotope mobilities under HP to UHP conditions during continental collision, the origin of premetamorphic protoliths and its bearing on continental collision types, and the crustal detachment and crust–mantle interaction in subduction channels. The synthesis presented herein suggests that the nature of premetamorphic protoliths is a key to the type of collisional orogens and the size of UHP terranes. The source mixing in subduction channels is a basic mechanism responsible for the geochemical diversity of continental and oceanic basaltic rocks. Therefore, the geochemical study of HP to UHP metamorphic rocks and their derivatives has greatly facilitated our understanding of the geodynamic processes that drive the tectonic evolution of convergent plate margins from oceanic subduction to continental collision. Consequently, the study of chemical geodynamics has been developed from oceanic subduction zones to continental collision zones, and it has enabled important contributions to development of plate tectonic theory.

Elastic and Seismic Properties of Dabie‐Sulu Ultrahigh Pressure Metamorphic Rocks

Abstract:Lamé modulus (λ) and shear modulus (μ) are among the most important, intrinsic, elastic constants of rocks. Using λ and μ could be much more advantageous than using P‐ and S‐wave velocities (Vp and Vs). Here we quantified these equivalent isotropic elastic moduli for 115 representative rocks from the ultrahigh pressure (UHP) metamorphic terrane of the Dabie‐Sulu erogenic belt (China) and their variations with pressure (P), temperature (T), density (p), Vp, Vs and minera logical composition. Both moduli increase nonlinearly and linearly with increasing pressure at low (&lt;200–300 MPa) and high (&gt;200–300 MPa) pressures, respectively. In the regime of high pressures, λ and μ decrease quasi‐linearly with increasing temperature with temperature derivatives dλ/dT and dμ/dT generally in the range of –10 × 10–3 to –1 × 10–3 GPa/°C. Dehydration of water‐bearing minerals such as serpentine in peridotites and chlorite in retrograde eclogites results in an abrupt drop in λ while μ remains almost unchanged. In λ‐ρ, μ‐ρ and λ‐μ plots, the main categories of UHP rocks can be characterized. Serpentinization leads to significant decreases in μ and λ as serpentine has extremely low values of λ, μ and ρ. Eclogites, common mafic rocks (mafic gneiss, metagabbro and amphibolite), and felsic rocks (orthogneiss and paragneiss) have high, moderate and low μ and λ values, respectively. For pyroxenes and olivines, λ increases but μ decreases with increasing Fe/Mg ratios. For plagioclase feldspars, both λ and μ exhibit a significant positive correlation with anorthite content. SiO2‐rich felsic rocks and quartzites are deviated remarkably from the general trend lines of the acid‐intermediate‐mafic rocks in Vs‐ρ, μ‐ρ, λ‐Vp, λ‐Vs and μ‐λ diagrams because quartz has extremely low λ (∼8.1 GPa) and p (2.65 g/cm3) but moderate μ (44.4 GPa) values. Increasing the contents of garnet, rutile, ilmenite and magnetite results in a significant increase in the λ and μ values of the UHP metamorphic rocks. However, either λ or μ is insensitive to the compositional variations for pyralspite (pyrope‐almandine‐spessartine) solution series. The results provide potentially improved constraints on characterization of crustal composition based on the elastic properties of rocks and in situ seismic data from deep continental roots.

Experimental investigation on low-degree dehydration partial melting of biotite gneiss and phengite-bearing eclogite at 2 GPa

The ultrahigh-pressure (UHP) eclogite and gneiss from the Dabie (大别)-Sulu (苏鲁) oro- gen experienced variable degrees of partial melting during exhumation. We report here dehydration partial melting experiments of biotite gneiss and phengite-bearing eclogite at 2 GPa and 800-950 ℃. Our results show that the partial melting of gneiss is associated with the breakdown of biotite into almandine-rich garnet starting at 900 ℃. About 10% granitic melt can be produced at 950 ℃. In con- trast, the partial melting of phengite-bearing eclogite exists at slightly lower temperatures (800-850 ℃). The melt fraction is in general more in biotite gneiss than in phengite-bearing eclogite under similar pressure and temperature conditions. Both melts are rich in silica and alkali, but poor in FeO, MgO and CaO. These results suggest that low-degree partial melting of gneiss and eclogite is often associated with dehydration of hydrous mineral, such as micas. The dehydration temperature and melt composi- tion can place important constraints on the partial melting phenomena (granitic leucosome and multi-phase mineral inclusions) recorded in UHP rocks.

Feedback between rifting and diapirism can exhume ultrahigh-pressure rocks

The processes by which crustal rocks are buried to tremendous depths and subsequently exhumed to Earth's surface remain controversial. Rapid exhumation of Earth's youngest (ultra-) high-pressure (UHP) rocks in the Woodlark Basin, Papua New Guinea, is occurring within an active rift, in contrast to more common scenarios of UHP exhumation during plate convergence. We use 2D and 3D thermo-mechanical models to demonstrate that UHP exhumation can result from feedback between rifting and the diapiric rise of a previously subducted continental fragment through the lithosphere. We infer that this feedback is responsible for the exhumation of the UHP rocks in gneiss domes in the Woodlark Basin. Our models successfully reproduce UHP exhumation paths and rates, and geological structures within the gneiss domes. We show that UHP exhumation by diapirism is mechanically consistent in post-collisional rifts. Our models highlight the complex feedback between diapiric ascent and extension.