Ultrahigh-pressure Metamorphic Terrane Research Articles

Evolution from Oceanic Subduction to Continental Collision: a Case Study from the Northern Tibetan Plateau Based on Geochemical and Geochronological Data

Two apparently distinct, sub-parallel, paleo-subduction zones can be recognized along the northern margin of the Tibetan Plateau: the North Qilian Suture Zone (oceanic-type) with ophiolitic melanges and high-pressure eclogites and blueschists in the north, and the North Qaidam Belt (continental-type) in the south, an ultrahighpressure (UHP) metamorphic terrane comprising pelitic and granitic gneisses, eclogites and garnet peridotites. Eclogites from both belts have protoliths broadly similar to mid-ocean ridge basalts (MORB) or oceanic island basalts (OIB) in composition with overlapping metamorphic ages (480–440Ma, with weighted mean ages of 464 6Ma for North Qilian and 457 7Ma for North Qaidam), determined by zircon U–Pb sensitive high-resolution ion microprobe dating. Coesite-bearing zircon grains in pelitic gneisses from the North Qaidam UHP Belt yield a peak metamorphic age of 423 6Ma, 40Myr younger than the age of eclogite formation, and a retrograde age of 403 9Ma. These data, combined with regional relationships, allow us to infer that these two parallel belts may represent an evolutionary sequence from oceanic subduction to continental collision, and continental underthrusting, to final exhumation. The Qilian–Qaidam Craton was probably a fragment of the Rodinia supercontinent with a passive margin and extended oceanic lithosphere in the north, which was subducted beneath the North China Craton to depths >100 km at c. 423Ma and exhumed at c. 403Ma (zircon rim ages in pelitic gneiss).

U–Pb SHRIMP geochronology of zircon in garnet peridotite from the Sulu UHP terrane, China: Implications for mantle metasomatism and subduction-zone UHP metamorphism

We studied the Zhimafang ultrahigh-pressure metamorphic (UHP) peridotite from pre-pilot drill hole PP-1 of Chinese Continental Scientific Drilling project in the Sulu UHP terrane, eastern China. The peridotite occurs as lens within quartofeldspathic gneiss, and has an assemblage of Ol + Opx + Cpx + Phl + Ti-clinohumite (Ti-Chu) + Grt (or chromite) ± magnesite (Mgs). Zircons were separated from cores at depths of 152 m (C24, garnet lhezolite), 160 m (C27, strongly retrograded phlogopite-rich peridotite) and 225 m (C50, banded peridotite), and were dated by SHRIMP mass spectrometer. Isometric zircons without inherited cores contain inclusions of olivine (Fo 91–92), enstatite (En 91–92), Ti-clinohumite, diopside, phlogopite and apatite. The enstatite inclusions have low Al 2O 3 contents of only 0.04–0.13 wt.%, indicating a UHP metamorphic origin. The weighted mean 206Pb / 238U zircon age for garnet lherzolite (C24) is 221 ± 3 Ma, and a discordia lower intercept age for peridotite (C50) is 220 ± 2 Ma. These ages are within error and represent the time of subduction-zone UHP metamorphism. A younger lower intercept age of 212 ± 3 Ma for a foliated wehrlite (C27) was probably caused by Pb loss during retrograde metamorphism. The source of zirconium may be partially attributed to melt/fluid metasomatism within the mantle wedge. Geochronological and geochemical data confirm that the mantle-derived Zhimafang garnet peridotites (probably the most representative type of Sulu garnet peridotites) were tectonically inserted into a subducting crustal slab and subjected to in situ Triassic subduction-zone UHP metamorphism.

40Ar-39Ar Thermochronological Constraints on the Exhumation of Ultrahigh-Pressure Metamorphic Rocks in the Sulu Terrane of Eastern China

In order to better understand the uplift history of the Sulu ultrahigh-pressure metamorphic terrane, 40Ar-39Ar thermochronological analysis was conducted on minerals extracted from both metamorphic (gneiss and retrograded eclogite) and igneous (pegmatite, granite, and mafic intrusion) rocks from this terrane. Samples of the major lithotectonic units were collected from key localities, including Rongcheng, Laoshan, Yangkou, Taohang, and Rizhao. The analyses of metamorphic rocks yielded ages for hornblende ranging from 222 to 186 Ma, biotite from 130 to 119 Ma, and K-feldspar from 112 to 108 Ma. Hornblende, biotite, and K-feldspar from igneous rocks yielded exclusively late Mesozoic ages from 137 to 97 Ma. A two-stage cooling path for the metamorphic rocks were revealed. A slow cooling (1.9 to 3.8°C/m.y.) during the period of ∼220 to ∼130 Ma was followed by fast cooling (6.8 to 16.7°C/m.y.) during ∼130 to ∼100 Ma. The time span and cooling rate of the second stage coincide with those of the analyzed post-collisional igneous rocks as well as with the formation of rift basins in the region. The study strongly indicates that extensional tectonics played an important role in the final stage of rapid exhumation of the ultrahigh-pressure rocks in the Sulu terrane.

Microdiamonds in Ultrahigh-Pressure Metamorphic Rocks

Since the first report of microdiamonds of metamorphic origin in crustal rocks of the Kokchetav Massif, northern Kazakhstan, diamonds have been described from several other ultrahigh-pressure (UHP) metamorphic terranes. In situ diamond is the best indicator of ultrahigh-pressure conditions (>4 GPa), and testifies to subduction of continental crust to depths within the diamond stability field followed by relatively rapid exhumation. In contrast to other UHP terranes, the Kokchetav Massif contains rocks with unusually abundant diamonds, particularly in the Kumdy-Kol region. Kumdy-Kol diamonds exhibit diverse morphologies, dependent upon the host rock. Raman and cathodoluminescence spectra and carbon isotope compositions differ between core and rim, indicating two distinct growth stages.

Evidence from ophiolites, blueschists, and ultrahigh-pressure metamorphic terranes that the modern episode of subduction tectonics began in Neoproterozoic time

Earth is the only known planet with subduction zones and plate tectonics, and this fact demonstrates that special conditions are required for this mode of planetary heat loss. Sinking of cold, dense lithosphere in subduction zones is the principal plate-driving force, so plate tectonics could not have begun until Earth cooled sufficiently to allow lithosphere to collapse into the underlying asthenosphere. Direct geologic evidence for when the modern episode of subduction tectonics began focuses on the first appearance of ophiolitic graveyards, blueschist facies metamorphic rocks, and ultrahigh-pressure metamorphic terranes. Ophiolites manifest two modes of lithospheric motion expected from subduction tectonics: seafloor spreading and obduction. High-pressure, low-temperature metamorphic blueschists and ultrahigh-pressure terranes indicate subduction and exhumation of oceanic and continental crust, respectively. These lines of evidence indicate that the modern style of subduction tectonics began in Neoproterozoic time. This revolution in the functioning of the solid Earth may have driven wild fluctuations in Earth’s climate, described under the ‘‘snowball Earth’’ hypothesis. These conclusions may be controversial, but suggest fruitful avenues for research in geodynamics and paleoclimate.

Role of partial melting in the evolution of the Sulu (eastern China) ultrahigh-pressure terrane

Strongly deformed potassium feldspar–rich dikes are widely distributed in the northern part of the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. The fact that the crystallization ages of these dikes overlap with the age of peak UHP metamorphic conditions implies the presence of melt during metamorphism. Sr isotopic ratios of the dikes are compatible with their origin as partial melts of the dominant felsic Sulu gneiss. Partial melting may be the key to solving several unusual features of the Sulu and other UHP terranes, such as the almost complete lack of mineralogical evidence for UHP conditions and the limited growth of zircon during UHP conditions in the dominant felsic gneiss. In addition, because partial melting will cause a drastic reduction in the strength of the UHP gneisses, the most likely exhumation mechanism is diapiric rise of a low-viscosity, partially molten mass containing entrained blocks of eclogite, and not a thin sheet as usually proposed.

Mass balance during retrogression of eclogite-facies minerals in the Rongcheng eclogite, eastern Sulu ultrahigh-pressure terrane, China

Eclogite from Rongcheng in the easternmost part of the Sulu ultrahigh-pressure metamorphic terrane has broken down (at least partially) with the formation of coronae of spinel + anorthite around kyanite, symplectites of pargasite + plagioclase around garnet, and symplectites of diopside + plagioclase after omphacite. Textural evidence and microprobe analysis indicate that this breakdown was generated by discontinuous reactions. Quantification of material transfer and textural information suggest that the reaction mechanism involved local exchange of components among the three major minerals (omphacite, garnet, and kyanite) during dissolution and discontinuous precipitation, coupled with exchange between eclogites and their host rocks. The symplectite formed after omphacite gained Al and minor Si and Na from its surroundings, and lost Mg, Ca, and minor Fe to its surroundings. Formation of the symplectite around garnet was accompanied by gain of Ca, Al, Si, Na, and H 2 O, and by loss of Mg and Fe 2 + to its surroundings. Coronae around kyanite gained Ca, Fe, Mg, and minor Si, and provided Al to their surroundings. Material exchange between eclogite and host rocks occurred as well: eclogite gained minor Al, Na, and H 2 O from the host rocks, and provided Mg, Fe, and Ca to the host rocks. Pressure and temperature estimation using THERMOCALC yielded a lower temperature (655 ′ 70 °C) and higher pressure (1.3 ′ 0.2 GPa), which values contrast with estimates made by other authors. Strong deformation may have created a path for ingress of hydrous fluids, then resulted in a localized lower T and/or higher P.

Retrograded textures and associated mass transfer: evidence for aqueous fluid action during exhumation of the Qinglongshan eclogite, Southern Sulu ultrahigh pressure metamorphic terrane, eastern China

AbstractThe Qinglongshan eclogites in the Southern Sulu ultrahigh pressure metamorphic (UHPM) terrane show very different retrograded textures from their counterparts in the Northern Sulu terrane, implying a different thermal history. Scanning electron and optical microscope observations indicate that the peak assemblage of the Qinglongshan eclogite is anhydrous, composed of Grt + OmpI + Rt + (Ky + coesite). These primary minerals were replaced by second and third stage minerals, resulting in symplectite pseudomorphs or coronas. The following relationships are inferred: OmpI → OmpII + Ab + Fe‐oxide symplectite (type I) and Rt → Rt + Ilm intergrowth; and, Ky → Pg, OmpII (+Pl) → Amp (+Pl) symplectite (type II), and Grt → Prg (+Fe‐oxide). Mineral chemistry and mass‐balance demonstrate that the pseudomorphed textures were developed by metasomatism involving dissolution and precipitation intensified by fluids along grain boundaries. The formation of symplectite type I produced Fe, Mg and Na but consumed Ca and Si. The Mg and Fe diffused to garnet where exchange of (Mg, Fe) with Ca of the garnet resulted in compositional zonation with decreased Ca towards the edge of garnet grains where Ca was consumed during symplectite formation. The replacement of kyanite by paragonite consumed the extra Na. In the later stage, fluid infiltration partially transformed symplectite type I to type II, and narrow rims of pargasite resorbed garnet from their boundaries. Mass balance suggests that the transformation and resorption would have been coupled during fluid infiltration. In the latest stage, epidote and quartz were precipitated at very late stage as a result of fluid activity along microfractures. Tentative P–T conditions based on mineral reactions and thermocalc software suggest that the retrograded eclogite did not record the granulite facies retrograde evolution characteristic of eclogites from the Northern Sulu terrane. The difference in retrograde evolution between the Southern and Northern Sulu eclogites suggests a different exhumation history.

Garnet exsolution in garnet clinopyroxenite and clinopyroxenite xenoliths in early Cretaceous intrusions from the Xuzhou region, eastern China

AbstractExsolved garnet, sometimes associated with exsolved amphibole and zoisite, is abundant in aluminous clinopyroxene of garnet clinopyroxenite and clinopyroxenite xenoliths in the early Cretaceous dioriticmonzodioritic intrusions from the Xuzhou region, ∼100 km west of the Sulu ultrahigh-pressure (UHP) metamorphic terrane in eastern China. In addition to the mineral exsolutions, intergranular fine-grained garnet, zoisite, titanite, and sometimes amphibole are also present around the primary clinopyroxene grains. Clinopyroxene shows compositional gradients, with decreasing Al and increasing Si and Mg, adjacent to the garnet lamellae and intergranular garnet. In contrast, the exsolved and intergranular garnet is homogeneous in composition and typically more grossular-rich and pyrope- and almandinepoor than the coarse-grained primary garnet. The estimates of P and T for the precursor assemblage and garnet exsolution suggests a decrease of temperature from ∼950—1100ºC down to 620—780°C, and a nearly constant pressure from ∼15—20 kbar to 18 kbar, which are comparable with the metamorphic conditions of the associated eclogites. Therefore, garnet exsolution in garnet clinopyroxenite and clinopyroxenite xenoliths could be caused by high-pressure (HP) metamorphism. SHRIMP zircon U-Pb dating for the associated eclogite and gneiss xenoliths and Sm-Nd whole rock-garnet dating for the same eclogite demonstrate that parts of lower crust on the southeastern margin of the North China craton might be involved in the Triassic HP metamorphism due to the deep subduction of the Yangtze craton.

K-rich lamellar exsolution in clinopyroxene: Constraint on the depth of peridotite source at Zhimafang

The oriented lamellar K-rich exsolutions within clinopyroxene host were recognized in garnet peridotite massif at Zhimafang in the Sulu ultrahigh pressure metamorphic terrane, eastern China. EMP and TEM analyses revealed that the composition and the index of electron diffraction for this exsolution are corresponding with phlogopite, and the shape is the lamellae with 1 μm in width and 5 to 30 μm in length. Moreover, the topotaxy is that the (001) of K-rich exsolution parallels to (100) of the clinopyroxene host. The electron diffraction analyzing shows the exsolution is of a 1M polymorphism. The high-resolution lattice fringe image displays the stack features along [001], which only a layer mineral shows. We conclude that the clinopyroxene with 1M and K-rich exsolution was probably derived from mantle depths over 240 km, thus resulting from decompressional exsolution in the initial stage of slab exhumation.

Occurrence of “monalbite” in nature: a TEM study

Natural “monalbite” (MA) has been observed on a submicron-scale in jadeite along with high albite (HA) and low albite (LA) in jadeite quartzite in the Dabie ultrahigh-pressure (UHP) metamorphic terrane, China. Transmission electron microscope (TEM) observation of inclusions in jadeite crystals has revealed mineral phases with C2/ m and C1̄ structures. The cell parameters of the C2/ m crystal structure correspond to those of MA, whereas the cell parameters of C1̄ structure are identical to those of HA and LA. The existence of the C2/ m structure shows that the jadeite underwent partial change to MA on a submicron-scale during retrograde metamorphism. Albite transformation twinning of HA suggests the transformation of MA into HA. MA occurrence provides powerful evidence of high-temperature metamorphism during exhumation, probably above 930 °C. The preservation of MA may be due to the armouring by the jadeite and implies a very rapid cooling (quenching) during retrogression.

Dating of zircon from Ti-clinohumite–bearing garnet peridotite: Implication for timing of mantle meta-somatism: Comment and Reply

[Katayama et al. (2003)][1] dated zircon in some ultramafic rocks dominated by garnet and titanian clinohumite (hereafter Ti-clinohumite rocks) from the Kokchetav ultrahigh-pressure (UHP) metamorphic terrane, and interpreted that the zircon cores formed in the mantle in the Proterozoic and the

Discovery of metamorphic diamonds in central China: an indication of a > 4000‐km‐long zone of deep subduction resulting from multiple continental collisions

AbstractThe Central Orogenic Belt (COB) of China is a major continental collision zone that contains extensive outcrops of deeply subducted and exhumed rocks at both the eastern and the western end of the belt. Here we report discovery of microdiamonds from both eclogites and felsic gneisses in the North Qinling zone in the central portion of the COB. This discovery demonstrates that the country rocks of continental affinity shared in the ultra‐high‐pressure metamorphic (UHPM) event and provides a bridge connecting the two previously recognized UHPM terranes, thereby establishing the existence of a UHPM belt extending more than 4000 km. Geochronological dating yields Early Palaeozoic ages in the west and Early Mesozoic ages in the east, recording two separate continental collisions overprinted within the COB. Occurrence of UHP metamorphism during recurrent continental collision here and in the Alps suggests that deep subduction of continental material during such collisions is probably common rather than exceptional, with significant implications for processes of plate tectonic reorganization and mantle mixing over time.

Petrogenesis of the Maowu pyroxenite–eclogite body from the UHP metamorphic terrane of Dabieshan: chemical and isotopic constraints

The Maowu eclogite–pyroxenite body is a small (250×50 m) layered intrusion that occurs in the ultra-high-pressure (UHP) metamorphic terrane of Dabieshan, China. Like the adjacent Bixiling complex, the Maowu intrusion was initially emplaced at a crustal level, then subducted along with the country gneisses to mantle depths and underwent UHP metamorphism during the collision of the North and South China Blocks in the Triassic. This paper presents the results of a geochemical and isotopic investigation on the metamorphosed Maowu body. The Maowu intrusion has undergone open system chemical and isotopic behavior three times. Early crustal contamination during magmatic differentiation is manifested by high initial 87Sr/ 86Sr ratios (0.707–0.708) and inhomogeneous negative ε Nd( T) values of −3 to −10 at 500 Ma (probable protolith age). Post-magmatic and pre-UHP metamorphic metasomatism is indicated by sinusoidal REE patterns of garnet orthopyroxenites, lack of whole-rock (WR) Sm–Nd isochronal relationship, low δ 18O values and an extreme enrichment of Th and REE in a clinopyroxenite. Finally, K and Rb depletion during UHP metamorphism is deduced from the high initial 87Sr/ 86Sr ratios unsupported by in situ Rb/Sr ratios. Laser ICP-MS spot analyses on mineral grains show that (1) Grt and Cpx attained chemical equilibrium during UHP metamorphism, (2) Cpx/Grt partition coefficients for REE correlate with Ca, and (3) LREE abundances in whole rocks are not balanced by that of the principal phases (Grt and Cpx), implying that the presence of LREE-rich accessory phases, such as monazite and apatite, is required to account for the REE budget. Sm–Nd isotope analyses of minerals yielded three internal isochrons with ages of 221±5 Ma and ε( T)=−5.4 for an eclogite, 231±16 Ma and ε( T)=−6.2 for a garnet websterite, and 236±19 Ma and ε( T)=−6.9 for a garnet clinopyroxenite. The Cpx/Grt chemical equilibrium and the consistent mineral isochron ages indicate that the metasomatic processes mentioned above must have occurred prior to the UHP metamorphism. These Sm–Nd ages agree with published zircon and monazite U–Pb ages and constrain the time of UHP metamorphism to 220–236 Ma. The Maowu and Bixiling layered intrusions are similar in their in situ tectonic relationship with their country gneisses, but the two bodies are distinguished by their magma-chamber processes. The Bixiling magmas were contaminated by the lower crust, whereas the Maowu magmas were contaminated by the upper crustal rocks during their emplacement and differentiation. The two complexes represent two distinct suites of magmatic rocks, which have resided in the continental crust for about 300–400 Ma before their ultimate subduction to mantle depths, UHP metamorphism and return to the crustal level.

Titanian clinohumite–garnet–pyroxene rock from the Su-Lu UHP metamorphic terrane, China: chemical evolution and tectonic implications

A garnet–pyroxene rock containing abundant Ti-clinohumite (ca. 40 vol.%) occurs along with eclogites as small blocks in quartzo-feldsparthic gneiss in the southern end of the Chinese Su-Lu ultrahigh-pressure (UHP) metamorphic terrane. It consists of three aggregates: (1) Ti-clinohumite-dominated aggregate with interstitial garnet and pyroxene, (2) garnet+pyroxene aggregate with Ti-clinohumite inclusions, and (3) Ti-clinohumite-free aggregate dominated by garnet. Apatite, phlogopite, zircon, hematite, pentlandite, and an unknown Ni-Fe-volatile-Si (NFVS) mineral, which is replaced by Ni-greenalite, occur as accessories. Serpentine is the major secondary mineral. Garnet (Prp 63.9–64.6Alm 25.8–26.9Grs 1.4–7.9Uva 0.5–7.6Sps 1.0) in all three aggregates is pyrope-rich with very low grossular component, with that in the aggregate (2) most enriched in Cr (Cr 2O 3=2.55 wt.%). Orthopyroxene is depleted in Al (Al 2O 3=0.16 wt.% in the cores) and Ca (CaO=0.06–0.09 wt.% in the cores), with X Mg (Mg/(Mg+Fe)) values at ca. 0.900. Clinopyroxene is chromian diopside with Fe 3+≥Fe 2+. Matrix clinopyroxene has a lower X Mg (0.862) than that (0.887) included in Ti-clinohumite. The rock contains modest amount of heavy rare earth elements (HREE) (10 to 12×C1 chondrite), with significant enrichment in Cr, Co, Ni, V, Sr, and light rare earth elements (LREE) (22 to 33×C1 chondrite). The clinopyroxene is very enriched in Cr (Cr 2O 3 is up to 2.09 wt.% in the cores) and Sr (ca. 350 ppm) and LREE (Ce N/Yb N=157.7). Ti-clinohumite is enriched in Ni (1981 ppm), Co (123 ppm), and Nb (85 ppm). While it is possible to enrich ultramafites in incompatible elements in a subducted slab, the high Al, Fe, Ti, and low Si, Ca, and Na contents in the Ti-clinohumite rock are difficult to account for by crustal metasomatism of an ultramafite. On the other hand, the similarity in major and trace element compositions and their systematic variations between the Ti-clinohumite-garnet-pyroxene rock of this study and those of Mg-metasomatised Fe–Ti gabbros reported in the literature suggest that crustal metasomatism occurred in a gabbroic protolith, which resulted in addition of Cr, Co, Ni, and Mg and removal of Si, Ca, Na, Al, and Fe. This implies that the rock was in contact with an ultramafite at low pressure. During subsequent subduction, the metagabbro was thrust into the country gneiss, where gneiss-derived hydrous fluids caused enrichment of Sr and LREE in recrystallised clinopyroxene. P–T estimates for the high-pressure assemblage are ca. 4.2 GPa and ca. 760 °C, compatible with those for the eclogites and gneisses in this terrane. It is possible that the Ti-clinohumite-garnet-pyroxene rock and associated eclogites represent remnants of former oceanic crust that was subducted to a great depth.

Petrology, geochemistry and isotopic ages of eclogites from the Dulan UHPM Terrane, the North Qaidam, NW China

The Dulan eclogite–gneiss region is located in the eastern part of the North Qaidam eclogite belt, NW China. Widespread evidence demonstrates that this region is a typical ultrahigh-pressure (UHP) metamorphic terrane. Eclogites occur as lenses or layers in both granitic and pelitic gneisses. Two distinguished sub-belts can be recognized and differ in mineralogy, petrology and geochemistry. The North Dulan Belt (NDB) has tholeiitic protoliths with high TiO 2 and lower Al 2O 3 and MgO contents. REE patterns and trace element contents resemble those of N-type and E-type MORB. In contrast, eclogites in the South Dulan Belt (SDB) are of island arc protoliths with low TiO 2, high Al 2O 3 and show LREE-enriched and HFSE-depleted patterns. Sm–Nd isotope analyses give isochron ages of 458–497 Ma for eclogite-facies metamorphism for the two sub-belts. The ages are similar to those of Yuka and Altun eclogites in the western extension of the North Qaidam-Altun eclogite belt. The Dulan UHP metamorphic terrane, together with several other recently recognized eclogite-bearing terrenes within the North Qaidam-Altun HP-UHP belt, constitute the key to the understanding of the tectonic evolution of the northern Tibetan Plateau. The entire UHP belt extends for more than 1000 km from the Dulan UHP terrane in the southeast to the Altun eclogite–gneiss terrane in the west. This super-belt marks an early Paleozoic continental collision zone between the Qaidam Massif and the Qilian Massif.

Metamorphic evolution of the coesite‐bearing ultrahigh‐pressure terrane in the North Qaidam, Northern Tibet, NW China

AbstractWidespread evidence for ultrahigh‐pressure (UHP) metamorphism is reported in the Dulan eclogite‐bearing terrane, the North Qaidam–Altun HP–UHP belt, northern Tibet. This includes: (1) coesite and associated UHP mineral inclusions in zircon separates from paragneiss and eclogite (identified by laser Raman spectroscopy); (2) inclusions of quartz pseudomorphs after coesite and polycrystalline K‐feldspar + quartz in eclogitic garnet and omphacite; and (3) densely oriented SiO2lamellae in omphacitic clinopyroxene. These lines of evidence demonstrate that the Dulan region is a UHP metamorphic terrane. In the North Dulan Belt (NDB), eclogites are characterized by the peak assemblage Grt + Omp + Rt + Phn + Coe (pseudomorph) and retrograde symplectites of Cpx + Ab and Hbl + Pl. The peak conditions of the NDB eclogites areP= 2.9–3.2 GPa, andT= 631–687 °C; the eclogite shows a near‐isothermal decompressionP–Tpath suggesting a fast exhumation. In the South Dulan Belt (SDB), three metamorphic stages are recognized in eclogites: (1) a peak eclogite facies stage with the assemblage Grt + Omp + Ky + Rt + Phn atP= 2.9–3.3 GPa andT= 729–746 °C; (2) a high‐pressure granulite facies stage with Grt + Cpx (Jd < 30) + Pl (An24–29) + Scp atP= 1.9–2.0 GPa,T= 873–948 °C; and (3) an amphibolite facies stage with the assemblage Hbl + Pl + Ep/Czo atP= 0.7–0.9 GPa andT= 660–695 °C. The clockwiseP–Tpath of the SDB eclogites is different from the near‐isothermal decompressionP–Tpath from the NDB eclogites, which suggests that the SDB was exhumed to a stable crustal depth at a slower rate. In essence these two sub‐belts formed in different tectonic settings; they both subducted to mantle depths of around 100 km, but were exhumed to the Earth's surface separately along different paths. This UHP terrane plays an important role in understanding continental collision in north‐western China.

Chromian dissakisite-(Ce) in a garnet lherzolite from the Chinese Su-Lu UHP metamorphic terrane: Implications for Cr incorporation in epidote-group minerals and recycling of REE into the Earth’s mantle

Chromian dissakisite-(Ce), a Cr-enriched Mg analogue of allanite-(Ce), is found in a garnet lherzolite from the Su-Lu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. Major minerals of the lherzolite are olivine [ X Mg = Mg/(Mg + Fe) = 0.92], pyrope-rich garnet (Prp70.1–73.2Alm11.3–13.4Grs6.9–8.3Uvr3.6–5.1Anr0–0.4Sps0.7–1.7), orthopyroxene ( X Mg = 0.92–0.93), and clinopyroxene (XMg = 0.92–0.95). The chromian dissakisite-(Ce) occurs as a single-crystal inclusion in a clinopyroxene neoblast that also contains several irregularly shaped olivine grains. It is partly metamict, and there is intensive radial cracking in the surrounding clinopyroxene. Its composition is represented by the averaged formula: (Ca0.86Na0.03)0.89(REE0.86Th0.04)0.90(Mg0.98Fe2+0.20Fe3+0.21Cr0.36Al1.39Ti0.04)Si3.03O12(OH), with Ce2O3, MgO and Cr2O3 contents up to 13.1, 7.3, and 5.4 wt%, respectively. Because the M3 site in the chromian dissakisite-(Ce) is fully occupied by Mg and Fe2+, Fe3+ and Cr can only occupy the smaller M1 site. Similar site occupancy is noted for a chromian allanite from the Sanbagawa metamorphic belt, Japan, which contains up to 0.547 Cr atoms per formula unit (apfu). Such compositional characteristics of the chromian dissakisite/allanite and existing polarized absorption data on Cr-bearing epidote suggest the existence of possible end-members, “CeCa(Mg,Fe)CrAlSi3O12(OH),” for the epidote-group minerals, which may form solid solution with Ca2Al3Si3O12(OH) through coupled substitutions of REEMgCa−1Al−1, REEFeCa−1Al−1, and CrAl−1. The rare earth elements (REE), Ca, and Na total 1.79 apfu, implying that Mn and Fe2+ are also present on the nine-coordinated A1 site. The high REE abundance, the presence of minor Na, and the Mg and Cr-enriched characteristics of the mineral suggest that it was formed as a result of reaction between the host lherzolite and crust-derived melt/fluid at UHP conditions. Textural relations suggest that the chromian dissakisite-(Ce) was unstable in the garnet lherzolite and reacted with olivine to form clinopyroxene. Dissakisite-(REE) in subducted ultramafic rocks can transport rare earth elements into the Earth’s mantle.

Fluid evolution and exhumation history of ultrahigh-pressure rocks at Lanshantou, Sulu terrane, Eastern China

Fluid inclusions in coesite-bearing eclogites and eclogite-facies quartz veins at Lanshantou in the Sulu ultrahigh-pressure metamorphic (UHPM) terrane, Eastern China, have preserved remnants of peak metamorphic and exhumation fluids. Three types of fluids have been distinguished: (1) mixed H 2O CO 2 ± N2 fluid, (2) hypersaline aqueous fluid and (3) low salinity H 2O fluid. Mixed H 2O CO 2 ± N2 fluids were introduced during or just after peak metamorphism. The hypersaline fluids might be products of dehydration melting during early stage of exhumation. Low salinity H 2O inclusions can be related to the latest evolutionary stage of the high-pressure veins during exhumation.

Relict edenite in a garnet lherzolite from the Chinese Su-Lu UHP metamorphic terrane: Implications for metamorphic history

Study of mineral inclusions in garnet in orogenic peridotites provides important information on their metamorphic history. Similar study of mineral inclusions in pyroxene is also encouraging. Edenite is found as relict inclusions in phlogopite-clinopyroxene intergrowths forming the cores of clinopyroxene porphyroblasts in a garnet lherzolite at Zhimafang in the Chinese Su-Lu ultrahighpressure metamorphic terrane. Texture shows that the phlogopite and clinopyroxene were products of the breakdown of edenite. The composition of the edenite is represented by K-Ed 6.0 Ed 70.7 Fe- Ed 6.6 Ts 8.4 Fe-Ts 1.8 Tr 1.2 Cm 5.4 . Those of the clinopyroxene and phlogopite are, Ur 4.5 Jd 3.7 Ae 6.5 CaTs 2.2 Di 81.1 En 1.9 and Phl 86.0 Na-Phl 13.3 Na-Ann 0.7 , respectively. The edenite breakdown is accompanied by gains of Ca, Si, and Cr, and losses of H, Mg, Al, Fe, Na, and K. The hypothetical composition of clinopyroxene resulting from isochemical breakdown of edenite is higher in Jd, CaTs, and En but lower in Di components than the cores of clinopyroxene porphyroblasts. The calculated mode of phlogopite is much higher than measured. The gain of Si and loss of H can be explained by involvement of SiO 2 -enriched fluids during formation of the Cpx+Phl intergrowth, whereas the other gains and losses suggest the existence of an older generation clinopyroxene slightly more enriched in Ca and Cr and depleted in Na, Mg, and Al, which homogenized with the clinopyroxene produced by the breakdown of edenite. Interaction with fluids also increased the Ca/Mg ratio of the system. It is suggested that, after having been exhumed to a shallow level for the first time, the peridotite was infiltrated by crust-derived fluids resulting in the replacement of clinopyroxene by edenite. During subduction, the edenite was replaced by clinopyroxene + phlogopite. Relics of edenite in clinopyroxene were able to survive ultrahigh-pressure metamorphism.