Petrotectonic Evolution of the Maksyutov Complex,Southern Urals,Russia: Implications for Ultrahigh-Pressure Metamorphism

The Maksyutov Complex consists of two juxtaposed lithotectonic units—Unit #1 of probable Late Proterozoic formation age, and Unit #2, apparently generated in Cambro-Ordovician time. The eclogite-facies metamorphism of Unit #1 occurred prior to 370-380 Ma, when this unit was subjected to blueschist-facies overprinting. Unit #2 displays the effects of a somewhat similar blueschist- or high-pressure greenschist-facies recrystallization, indicating that it may have been metamorphosed contemporaneously with Unit #1. Our field work and geochemical studies have focused on the Sakmara River area. Preliminary conclusions are as follows: (1) Unit #1 was subjected to metamorphic temperatures of 620 ± 70° C and minimum pressures of 1.5 GPa, or 2.7 GPa if the previously reported interpretation of coesite pseudomorphs from similar rocks exposed near the village of Shubino, 75 km to the south (Chesnokov and Popov, 1965), is correct. Peak metamorphic pressures would have reached at least 3.2 GPa if blocky graphite described in this report from a Sakmara River eclogitic mica schist has replaced neoblastic diamond; (2) Unit #2 experienced much lower maximum metamorphic pressures, on the order of 0.5 to 0.6 GPa; (3) Unit #2 was variably but intensely metasomatized, indicating the presence of an aqueous fluid during the Early Devonian blueschist/greenschist-facies metamorphism; (4) tectonic parallelism of the lithostratigraphic units and their bounding sutures, combined with P-T conditions of recrystallization, suggest assembly of the Maksyutov Complex in an intra-oceanic subduction zone. This process was followed by exhumation and suturing against the more easterly Middle Paleozoic unmetamorphosed ophiolitic (oceanic) basement and superjacent calc-alkaline Magnitogorsk island arc. The Late Proterozoic-Ordovician Mugodzhar and Ilmen microcontinents subsequently were thrust beneath the eastern edge of the Devonian Magnitogorsk Arc. Collision of the entire complex with the Ordovician-Lower Carboniferous continentalmargin Suvanjak-Sakmara accretionary complex, lying to the west on the Russian Platform, also occurred during Middle Paleozoic time. Finally, (5), the tectonic imbrication of the several units within and adjoining the Maksyutov Complex was itself truncated and deformed into N-S parallelism by postulated Late Paleozoic postcollisional strike-slip movement (Dobretsov et al., in review).     

Geology and geodynamic evolution of the high-P/low-T Maksyutov Complex, southern Urals, Russia

The high-pressure/low-temperature Maksyutov Complex is situated in the southern Urals between the Silurian/Devonian Magnitogorsk island arc and the East European Platform. The elongated N-S-trending complex is made up of two contrasting tectono-metamorphic units. Unit 1 consists of a thick pile of Proterozoic clastic sediments suggested to represent the passive margin of the East European Platform. The overlying unit 2, composed of Paleozoic sediments, volcanic rocks, and a serpentinite mélange with rodingites, is interpreted as a remnant of the Uralian Paleo-ocean. Devonian eastward subduction of oceanic crust beneath the Magnitogorsk island arc resulted in an incipient blueschist-facies metamorphism of unit 2 indicated by lawsonite pseudomorphs in the rodingites. While unit 2 was accreted to the upper plate, subduction of the continental passive margin caused the high-pressure metamorphism of unit 1. Buoyancy-driven exhumation of unit 1 into the forearc region led to its juxtaposition with unit 2 along a retrograde top-to-the-ENE shear zone. Further exhumation of the Maksyutov Complex into its present tectonic position was accomplished by later shear zones that were active as normal faults and are exposed along the margins of the complex. At the western margin a top-to-the-west shear zone juxtaposed a low-grade remnant of a Paleozoic accretionary prism (Suvanyak Complex) above the Maksyutov Complex. Along the eastern margin a top-to-the-east shear zone and the brittle Main Uralian Normal Fault emplaced the Maksyutov Complex against the Magnitogorsk island arc in the hanging wall.     

Stable isotope geochemistry of marbles from the coesite UHP terrains of Dabieshan and Sulu, China

Marbles from Dabieshan and Sulu, China, suffered ultra high pressure (UHP) metamorphism in the coesite–eclogite facies at approximately 700°C and 30 kbars during Triassic continental collision and subduction. The marbles range in isotopic composition from +7 to +25 δ¹⁸O_VSMOW and from 0 to +6 δ¹³C_VPDB. High δ¹³C values are representative of unmodified protoliths and are similar to those of ¹³C-enriched Sinian carbonate rocks from the Yangtze craton. High oxygen isotope ratios reflect pristine protoliths but the low values may have been caused by infiltration of low ¹⁸O meteoric water during diagenesis and dolomitization, by fracture-controlled infiltration of water during subduction, by metamorphic mineral reactions, or by a combination of these processes. No evidence of regional isotopic transport during UHP metamorphism has been found. Sampling on scales of 1 to 100 m shows marbles to be inhomogeneous in both carbon and oxygen isotopes. Only samples separated by less than 10 cm have equilibrated oxygen and carbon isotope compositions. Limited isotopic equilibration between adjacent rocks is consistent with the preservation of unaltered UHP minerals and indicates that the metamorphic fluid–rock system was rock-dominated during and following peak metamorphism. A freely flowing, pervasive fluid phase was not present during UHP metamorphism. There is no evidence of isotopic exchange between marble and the upper mantle into which it was subducted. Correlation of geochemical similarities of UHP marbles with Sinian limestones implies that the subducted edge of the Yangtze craton extends at least as far north as the coesite–eclogite facies rocks of Dabieshan. Deposition of protolith carbonates may have taken place in a cold climate either preceding or following but not coincident with Neoproterozoic glaciation.     

Very fast exhumation of high-pressure metamorphic rocks with excess Ar and inherited Sr, Betic Cordilleras, southern Spain

In order to attempt to further constrain the age of the early Alpine tectonic evolution of the Mulhacén Complex and to explore the influence of inherited isotopes, micas from a small number of well-characterised rocks from the Sierra de los Filábres, with a penetrative tectonic fabric related to the exhumation of eclogite-facies metamorphic rocks, were selected for ⁴⁰Ar/³⁹Ar and Rb–Sr dating.

A single phengite grain from an amphibolite yielded an ⁴⁰Ar/³⁹Ar laser step heating plateau age of 86.9±1.2 Ma (2σ; 70% ³⁹Ar released) and an inverse isochron age of 86.2±2.4 Ma with an ³⁶Ar/⁴⁰Ar intercept within error of the atmospheric value. Induction furnace step heating of a biotite separate from a gabbro relic in an eclogite yielded a weighted mean age of 173.2±6.3 Ma (2σ; 95% ³⁹Ar released). These ages are diagnostic of excess argon (⁴⁰Ar_XS) incorporation, as they are older than independent age estimates for the timing of eclogite-facies metamorphism and intrusion of the gabbros. ⁴⁰Ar_XS incorporation probably resulted from restricted fluid mobility in the magmatic rocks during their metamorphic recrystallisation.

Rb–Sr whole-rock–phengite ages of graphite-bearing mica schists from Paleozoic rocks (Secano unit) show a dramatic variation (66.1±3.2, 40.6±2.6 and 14.1±2.2 Ma). An albite chlorite mica schist from the Mesozoic series of the Nevado–Lubrín unit has a whole-rock–mica–albite age of 17.2±1.9 Ma, which is within error of an ⁴⁰Ar/³⁹Ar plateau age published previously and of the youngest Rb–Sr age of the Paleozoic series obtained in this study. The significant spread in Rb–Sr ages implies that progressive partial resetting of an older isotopic system has occurred. The microstructure of the samples with pre-Miocene Rb–Sr ages reveals incomplete recrystallisation of white mica and inhibited grain growth due to the presence of graphite particles. This interpretation agrees with previously published, disturbed and slightly dome-shaped ⁴⁰Ar/³⁹Ar age spectra that may point similarly to the presence of an older isotope component. The progressively reset Rb–Sr system is a relic of Variscan metamorphism of the Paleozoic series of the Mulhacén Complex. In contrast, the origin of the ca. 17.2 Ma old sample from the Mesozoic series precludes any isotopic inheritance, in agreement with its pervasive tectono-metamorphic recrystallisation during the Miocene.

Exhumation of the eclogite-facies Mulhacén Complex occurred in two stages with contrasting rates of about 22.5 mm/year during the early phase and 9–10 mm/year during the late phase; the latter with a cooling rate in the order of 330 °C/Ma.     

Strain partitioning and preservation of Ar/Ar ages during Variscan exhumation of a subducted crust (Malpica–Tui complex, NW Spain)

The Malpica–Tui complex (NW Iberian Massif) consists of a Lower Continental Unit of variably deformed and recrystallized granitoids, metasediments and sparse metabasites, overridden by an upper unit with rocks of oceanic affinities. Metamorphic minerals dated by the ⁴⁰Ar/³⁹Ar method record a coherent temporal history of progressive deformation during Variscan metamorphism and exhumation. The earliest stages of deformation (D1) under high-pressure conditions are recorded in phengitic white micas from eclogite-facies rocks at 365–370 Ma. Following this eclogite-facies peak-metamorphism, the continental slab became attached to the overriding plate at deep-crustal levels at ca. 340–350 Ma (D2). Exhumation was accompanied by pervasive deformation (D3) within the continental slab at ca. 330 Ma and major deformation (D4) in the underlying para-autochthon at 315–325 Ma. Final tectonothermal evolution included late folding, localized shearing and granitic intrusions at 280–310 Ma.

Dating of high-pressure rocks by the ⁴⁰Ar/³⁹Ar method yields ages that are synchronous with published Rb–Sr and Sm–Nd ages obtained for both the Malpica–Tui complex and its correlative, the Champtoceaux complex in the French Armorican Massif. The results indicate that phengitic white mica retains its radiogenic argon despite been subjected to relatively high temperatures (500–600 °C) for a period of 20–30 My corresponding to the time-span from the static, eclogite-facies M1 peak-metamorphism through D1-M2 eclogite-facies deformation to amphibolite-facies D2-M3. Our study provides additional evidence that under certain geological conditions (i.e., strain partitioning, fluid deficiency) argon isotope mobility is limited at high temperatures, and that ⁴⁰Ar/³⁹Ar geochronology can be a reliable method for dating high pressure metamorphism.     

Transition of UHP eclogites to gneissic rocks of low-amphibolite facies during exhumation: evidence from the Dabie terrane, central China

The Shuanghe ultrahigh-pressure (UHP) slab in the Dabie Mountains consists of layered coesite-bearing eclogite, jadeite quartzite, marble and biotite gneiss, and is fault bounded against hosting orthogneiss. Representative assemblages of eclogite are Grt+Omp+Coe+Rt±Ky±Phn±Mgs; it formed at P>27 kbar and 680–720±50 °C. During exhumation, these UHP rocks experienced multistage retrograde metamorphism. Coesite was overprinted by quartz aggregates, phengite by biotite±muscovite and rutile by titanite. Garnet was successively replaced by a thin rim of Amp, Amp+Pl, and Amp+Ep±Bt+Pl (minor). Omphacite and kyanite were replaced by Amp+Pl±Cpx (or ±Bt) and by Zo+Pl+Ms±Mrg±Bt, respectively. Secondary calcite occurs as irregular pockets in some layers. An outcrop near the UHP slab border is composed of 20 thin, concordant layers of foliated eclogites, amphibolite and gneissic rocks of variable bulk composition. These layers exhibit mineral assemblages and textures transitional from less altered through extensively retrograded eclogite to gneissic rock of low-amphibolite facies through hydration, metasomatism and recrystallization. Retrograde metamorphism has caused oxygen and hydrogen isotope disequilibria between some of the minerals, but the fluid for retrograde reactions was internally buffered in the stable isotope compositions. Retrograde metamorphism of variable extent may be attributed to selective infiltration of retrograde fluids of CO₂-rich and low-salinity aqueous, intensity of deformation and mineral resistance to alteration. The fluid phase for retrogression may have occurred either as discontinuous flow along grain boundaries in completely retrograded eclogites, and/or as isolated pockets in extensive or less altered eclogite layers.     

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 _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 δ¹⁸O 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 (T)=−5.4 for an eclogite, 231±16 Ma and (T)=−6.2 for a garnet websterite, and 236±19 Ma and (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.     

Eclogites preserved as pebbles in Jurassic conglomerate, Dabie Mountains, China

Two types of eclogite pebbles were discovered in Middle to Upper Jurassic conglomerates from the Hefei Basin north of the Dabie ultrahigh-pressure (UHP) terrain, China. Type A eclogite pebbles are characterized by idioblastic garnet with well preserved chemical zonation. Si content in phengite is lower than 3.5 per formula unit (pfu). The maximum metamorphic pressure is lower than 2.5 GPa, and the temperature is below 600 °C. Type B eclogite pebbles contain coesite pseudomorphs in xenoblastic garnet. Si content in phengite is higher than 3.5 pfu. The maximum metamorphic pressure is 2.8–4.0 GPa at 700 °C indicating UHP metamorphism.

Types A and B eclogite pebbles are comparable with eclogites occurring in the southern portion of the Dabie UHP terrain. Based on the petrologic similarities and northeastwards directed paleocurrents, we infer that the eclogite pebbles were eroded from the Dabie UHP terrain. Sandstones containing detrital phengite with Si content higher than 3.5 pfu are also derived from UHP rocks. These petrologic and stratigraphic data place time constraints on exhumation and erosion history of the Dabie UHP terrain.     

Coesite micro-inclusions and the U/Pb age of zircons from the Hareidland Eclogite in the Western Gneiss Region of Norway

We have identified by laser micro-Raman spectroscopy that inclusions of coesite occur together with other eclogite-facies mineral phases within metamorphic zircons separated from the large eclogite body at Ulsteinvik–Dimnøy on Hareidland. This is the first identification of coesite from this portion of the northwestern Western Gneiss Region (WGR) and supports continuity of ultrahigh-pressure (UHP) metamorphism between the documented coesite occurrences on Stadlandet to the south and the microdiamond and coesite pseudomorph localities on Fjørtoft in the Nordøyene to the north. The zircons, first analysed by U–Pb TIMS in 1973, have been re-analysed and have yielded a much more precise age of 401.6±1.6 Ma, that overlaps with the previously determined age. Our discovery of coesite and the indication of a close to 402 Ma formation age add to a growing number of mid–late Early Devonian ages that signal that the UHP metamorphism in this part of west Norway occurred relatively late in the Caledonian orogenic cycle. These observations should be incorporated in geodynamic models for the exhumation of these rocks and for the metastable preservation of eclogite-facies mineralogies.     

Franciscan eclogite revisited: Reevaluation of the P–T evolution of tectonic blocks from Tiburon Peninsula, California, U.S.A.

Summary High-grade blocks in the Franciscan complex at Tiburon, California, record relatively low temperature eclogite-facies metamorphism and blueschist-facies overprinting. The eclogite-facies mineral assemblage contains prograde-zoned garnet + omphacite + epidote ± hornblende (katophoritic and barroisitic Ca–Na amphibole) ± glaucophane + phengite (∼3.5 Si p.f.u.) ± paragonite + rutile + quartz. The blueschist-facies mineral assemblage contains chlorite + titanite + glaucophane + epidote ± albite ± phengite (∼3.3 Si p.f.u.). Albite is not stable in the eclogite stage. New calculations based on garnet-omphacite-phengite thermobarometry and THERMOCALC average-P–T calculations yield peak eclogite-facies P–T conditions of P = 2.2–2.5 GPa and T = 550–620 °C; porphyroclastic omphacite with inclusions of garnet and paragonite yields an average-P–T of 1.8 ± 0.2 GPa at 490 ± 70 °C for the pre-peak stage. The inferred counterclockwise hairpin P–T trajectory suggests prograde eclogitization of a relatively “cold” subducting slab, and subsequent exhumation and blueschist-facies recrystallization by a decreasing geotherm. Although an epidote-garnet amphibolitic assemblage is ubiquitous in some blocks, P–T pseudosection analyses imply that the epidote-garnet amphibolitic assemblage is stable during prograde eclogite-facies metamorphism. Available geochronologic data combined with our new insight for the maximum pressure suggest an average exhumation rate of ∼5 km/Ma, as rapid as those of some ultrahigh pressure metamorphic terranes.     

Tracing the protolith, UHP metamorphism, and exhumation ages of orthogneiss from the SW Sulu terrane (eastern China): SHRIMP U–Pb dating of mineral inclusion-bearing zircons

Orthogneisses are the major country rocks hosting eclogites in the Sulu UHP terrane, eastern China. All of the analyzed orthogneiss cores from the main drilling hole of the Chinese Continental Scientific Drilling Project (CCSD-MH) have similar major and trace element compositions and a granite protolith. These rocks have relatively high LREE/HREE ratios, strong negative Eu anomalies (Eu/Eu*=0.20–0.39), and negative Ba anomalies (Ba/Ba*=0.25–0.64). Coesite and coesite-bearing UHP mineral assemblages are common inclusions in zircons separated from orthogneiss, paragneiss, amphibolite, and (retrograded) eclogite of the CCSD-MH. This suggests that the eclogite, together with its country rocks, experienced in situ ultrahigh-pressure (UHP) metamorphism. Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircons from the orthogneisses are zoned and that they have distinct mineral inclusions in the different zones. Most zircons retain early magmatic cores with abundant low-pressure mineral inclusions, which are mantled with metamorphic zircon-containing inclusions of coesite and other UHP minerals. The outermost rims on these grains contain low-pressure mineral inclusions, such as quartz and albite. SHRIMP U–Pb dating of the zoned zircons gives three discrete and meaningful groups of ages: Proterozoic ages for the protolith, 227±2 Ma for the coesite-bearing mantles, and 209±3 Ma for the amphibolite facies retrograde rims. The widespread occurrence of UHP mineral inclusions in zircons from the Sulu metamorphic belt dated at about 227 Ma suggests that voluminous continental crust experienced late Triassic subduction to depths of at least 120 km and perhaps more than 200 km. Eighteen million years later, the terrane was rapidly exhumed to midcrustal levels, and the UHP rocks were overprinted by amphibolite facies metamorphism. The exhumation rate deduced from the zircon age data and previously obtained metamorphic P–T data is estimated to be 5.6–11.0 km/Ma. Such rapid exhumation of the Sulu UHP terrane may be due to the buoyancy forces produced by subduction of low-density continental material into the deep mantle.     

Fluid inclusions in granulites, granulitized eclogites and garnet clinopyroxenites from the Dabie–Sulu terranes, eastern China

Minor granulites (believed to be pre-Triassic), surrounded by abundant amphibolite-facies orthogneiss, occur in the same region as the well-documented Triassic high- and ultrahigh-pressure (HP and UHP) eclogites in the Dabie–Sulu terranes, eastern China. Moreover, some eclogites and garnet clinopyroxenites have been metamorphosed at granulite- to amphibolite-facies conditions during exhumation. Granulitized HP eclogites/garnet clinopyroxenites at Huangweihe and Baizhangyan record estimated eclogite-facies metamorphic conditions of 775–805 °C and ≥15 kbar, followed by granulite- to amphibolite-facies overprint of ca. 750–800 °C and 6–11 kbar. The presence of (Na, Ca, Ba, Sr)-feldspars in garnet and omphacite corresponds to amphibolite-facies conditions. Metamorphic mineral assemblages and P–T estimates for felsic granulite at Huangtuling and mafic granulite at Huilanshan indicate peak conditions of 850 °C and 12 kbar for the granulite-facies metamorphism and 700 °C and 6 kbar for amphibolite-facies retrograde metamorphism. Cordierite–orthopyroxene and ferropargasite–plagioclase coronas and symplectites around garnet record a strong, rapid decompression, possibly contemporaneous with the uplift of neighbouring HP/UHP eclogites.

Carbonic fluid (CO₂-rich) inclusions are predominant in both HP granulites and granulitized HP/UHP eclogites/garnet clinopyroxenites. They have low densities, having been reset during decompression. Minor amounts of CH₄ and/or N₂ as well as carbonate are present. In the granulitized HP/UHP eclogites/garnet clinopyroxenites, early fluids are high-salinity brines with minor N₂, whereas low-salinity fluids formed during retrogression. Syn-granulite-facies carbonic fluid inclusions occur either in quartz rods in clinopyroxene (granulitized HP garnet clinopyxeronite) or in quartz blebs in garnet and quartz matrices (UHP eclogite). For HP granulites, a limited number of primary CO₂ and mixed H₂O–CO₂(liquid) inclusions have also been observed in undeformed quartz inclusions within garnet, orthopyroxene, and plagioclase which contain abundant, low-density CO₂±carbonate inclusions. It is suggested that the primary fluid in the HP granulites was high-density CO₂, mixed with a significant quantity of water. The water was consumed by retrograde metamorphic mineral reactions and may also have been responsible for metasomatic reactions (“giant myrmekites”) occurring at quartz–feldspar boundaries. Compared with the UHP eclogites in this region, the granulites were exhumed in the presence of massive, externally derived carbonic fluids and subsequently limited low-salinity aqueous fluids, probably derived from the surrounding gneisses.     

Low-T eclogite in the Dabie terrane of China: petrological and isotopic constraints on fluid activity and radiometric dating

While extensive studies have demonstrated fluid release during subduction of oceanic crust, little attention has been paid to fluid activity during subduction and exhumation of continental crust. Abundant occurrence of quartz veins within eclogites in the Dabie-Sulu orogenic belt of China provides us with an opportunity to study the origin and role of vein-forming fluids with respect to heat and mass transfer during ultrahigh pressure (UHP) metamorphism and its relevant processes. This study focuses on kyanite-quartz vein that occurs as polycrystalline aggregates within the low-T eclogite in the Dabie terrane, which are interpreted as pseudomorphs after former porphyroblasts of lawsonite. Coesite pseudomorphs were found for the first time in eclogite garnet, resulting in a revised estimate of peak P–T conditions at 670°C and 3.3 GPa for the eclogite and thus upgrading the high-P unit to an UHP unit. On the basis of the relationship between calculated P–T path and metamorphic reactions as well as the absence of foliation texture, and undulose extinction of quartzes in the vein, we conclude that lawsonite breakdown into kyanite–quartz–zoisite assemblage took place at the onset of exhumation subsequent to peak pressure. Retrograde metamorphism caused O and H isotope disequilibria between some of the minerals, but the fluid for retrograde reactions was internally buffered in stable isotope compositions. Zircon U–Pb dating and whole-rock Nd–Sr isotope analyses indicate that eclogite protolith is the paleoceanic basalt that was derived from the depleted mantle by magmatism at about 1.8 to 1.9 Ga but experienced hydrothermal alteration by surface waters. The altered basalt underwent UHP metamorphism in the Triassic that caused fluid release for zircon growth/overgrowth not only at about 242±3 Ma prior to the onset of peak pressure but also at about 222±4 Ma during decompression dehydration by lawsonite breakdown and hydroxyl exsolution in the low-T/UHP eclogite. Consistent ages of 236.1±4.2 Ma and 230±7 Ma were obtained from mineral Sm–Nd and Rb–Sr isochron dating, respectively, indicating attainment and preservation of Nd and Sr isotope equilibria during the Triassic UHP eclogite-facies metamorphism. Ar–Ar dating on paragonite from the eclogite gave consistent plateau and isochron ages of 241.3±3.1 Ma and 245.5±9.8 Ma, respectively, which are interpreted to date paragonite crystallization during the prograde eclogite-facies metamorphism. The timing of peak UHP metamorphism for the low-T eclogite is constrained at sometime prior to 236.1±4.2 Ma. Thus the termination age of peak UHP metamorphism may be different in different slices of deep-subducted slab.     

Cretaceous isochron ages from K–Ar and Ar / Ar dating of eclogitic rocks in the Tso Morari Complex, western Himalaya, India

The Tso Morari Complex, which is thought to be originally the margin of the Indian continent, is composed of pelitic gneisses and schists including mafic rock lenses (eclogites and basic schists). Eclogites studied here have the mineral assemblage Grt + Omp + Ca-Amp + Zo + Phn + Pg + Qtz + Rt. They also have coesite pseudomorph in garnet and quartz rods in omphacite, suggesting a record of ultrahigh-pressure metamorphism. They occur only in the cores of meter-scale mafic rock lenses intercalated with the pelitic schists. Small mafic lenses and the rim parts of large lenses have been strongly deformed to form the foliation parallel to that of the pelitic schists and show the mineral assemblages of upper greenschist to amphibolite facies metamorphism. The garnet–omphacite thermometry and the univariant reaction relations for jadeite formation give 13–21 kbar at 600 °C and 16–18 kbar at 750 °C for the eclogite formation using the jadeite content of clinopyroxene (X_Jd = 0.48).

Phengites in pelitic schists show variable Si / Al and Na / K ratios among grains as well as within single grains, and give K–Ar ages of 50–87 Ma. The pelitic schist with paragonite and phengite yielded K–Ar ages of 83.5 Ma (K = 4.9 wt.%) for paragonite–phengite mixture and 85.3 Ma (K = 7.8 wt.%) for phengite and an isochron age of 91 ± 13 Ma from the two dataset. The eclogite gives a plateau age of 132 Ma in Ar/Ar step-heating analyses using single phengite grain and an inverse isochron age of 130 ± 39 Ma with an initial ⁴⁰Ar / ³⁶Ar ratio of 434 ± 90 in Ar/Ar spot analyses of phengites and paragonites. The Cretaceous isochron ages are interpreted to represent the timing of early stage of exhumation of the eclogitic rocks assuming revised high closure temperature (500 °C) for phengite K–Ar system. The phengites in pelitic schists have experienced retrograde reaction which modified their chemistry during intense deformation associated with the exhumation of these rocks with the release of significant radiogenic ⁴⁰Ar from the crystals. The argon release took place in the schists that experienced the retrogression to upper greenschist facies metamorphisms from the eclogite facies conditions.     

A transitional eclogite- to high pressure granulite-facies overprint on coesite–eclogite at Taohang in the Sulu ultrahigh-pressure terrane, Eastern China

A transitional eclogite- to high-pressure granulite-facies paragenesis (Omp+Pl+Qtz±Grt) after peak coesite–eclogite facies metamorphism and predating the later amphibolite-facies overprint is identified in coesite–eclogite from the Taohang area of the Sulu ultrahigh-pressure (UHP) terrane in eastern China. These minerals were equilibrated at 17 kbar and 820°C. This reveals that fluid infiltration might activate retrograde recrystallisation even at a deep level during the exhumation process of the UHP rocks. The tectono-metamorphic significance of the unusually high pressure overprint is also discussed.     

Hydrogen and oxygen isotope evidence for fluid–rock interactions in the stages of pre- and post-UHP metamorphism in the Dabie Mountains

Hydrogen and oxygen isotope studies were carried out on high and ultrahigh pressure metamorphic rocks in the eastern Dabie Mountains, China. The δ¹⁸O values of eclogites cover a wide range of −4.2 to +8.8‰, but the δD values of micas from the eclogites fall within a narrow range of −87 to −71‰. Both equilibrium and disequilibrium oxygen isotope fractionations were observed between quartz and the other minerals, with reversed fractionations between omphacite and garnet in some eclogite samples. The δ¹⁸O values of −4 to −1‰ for some of the eclogites represent the oxygen isotope compositions of their protoliths which underwent meteoric water–rock interaction before the high to ultrahigh pressure metamorphism. Heterogeneous δ¹⁸O values for the eclogite protoliths implies not only the varying degrees of the water–rock interaction before the metamorphism at different localities, but also the channelized flow of fluids during progressive metamorphism due to rapid plate subduction. Retrograde metamorphism caused oxygen and hydrogen isotope disequilibria between some of the minerals, but the fluid for retrograde reactions was internally buffered in the stable isotope compositions and could be derived from structural hydroxyls dissolved in nominally anhydrous minerals.     

Isotopic constraints on age and duration of fluid-assisted high-pressure eclogite-facies recrystallization during exhumation of deeply subducted continental crust in the Sulu orogen

Fluid availability during high‐grade metamorphism is a critical factor in dictating petrological, geochemical and isotopic reequilibration between metamorphic minerals, with fluid‐absent metamorphism commonly resulting in neither zircon growth/recrystallization for U‐Pb dating nor Sm‐Nd isotopic resetting for isochron dating. While peak ultra‐high pressure (UHP) metamorphism is characterized by fluid immobility, high‐pressure (HP) eclogite‐facies recrystallization during exhumation is expected to take place in the presence of fluid. A multichronological study of UHP eclogite from the Sulu orogen of China indicates zircon growth at 216 ± 3 Ma as well as mineral Sm‐Nd and Rb‐Sr reequilibration at 216 ± 5 Ma, which are uniformly younger than UHP metamorphic ages of 231 ± 4 to 227 ± 2 Ma as dated by the SHRIMP U‐Pb method for coesite‐bearing domains of zircon. O isotope reequilibration was achieved between the Sm‐Nd and Rb‐Sr isochron minerals, but Hf isotopes were not homogenized between different grains of zircon. The HP eclogite‐facies recrystallization is also evident from petrography. Thus this process occurred during exhumation with fluid availability from decompression dehydration of hydrous minerals and the exsolution of hydroxyl from nominally anhydrous minerals. This provides significant amounts of internally derived fluid for extensive retrogression within the UHP metamorphosed slabs. Based on available experimental diffusion data, the consistent reequilibration of U‐Pb, Sm‐Nd, Rb‐Sr and O isotope systems in the eclogite minerals demonstrates that time‐scale for the HP eclogite‐facies recrystallization is c. 1.9–9.3 Myr or less. This provides a maximum estimate for duration of the fluid‐facilitated process in the HP eclogite‐facies regime during the exhumation of deeply subducted continental crust.     

Oxygen and carbon isotopic constraints on the development of eclogites, Holsnøy, Norway

Eclogite formation on the island of Holsnøy required the addition of water to anhydrous granulite-facies protoliths. In order to assess this process, oxygen and carbon isotope ratios of whole rock powders and mineral separates from eclogites and granulites have been measured. Whole rock oxygen isotope ratios range from 7.3 to 6.0%. SMOW in granulites (average = 6.38%.) and 7.2 to 6.1%. in eclogites (average = 6.55%.). Field relations permit identification of the granulite protolith of eclogites. Oxygen isotope measurements show shifts of up to 0.5%. between some eclogites compared to their corresponding granulite protoliths, indicating open system and locally heterogeneous fluid behavior. Mineral pair fractionations in the eclogites show disequilibrium, are incompatible with slow cooling and diffusive exchange between phases, and suggest that open system fluid movement continued after eclogite-facies metamorphism. Carbonate is also present in some of the eclogites as a primary mineral (dolomite) and as part of a retrograde assemblage (calcite). Textural evidence suggests that carbonate formation occurred during and after eclogite formation, however all measured carbonate is out of isotopic equilibrium with eclogite facies minerals, due to the influx of retrograde fluids. Massive calcite marble pods, containing amphibolite facies cale-silicate minerals, have average δ¹⁸O of 9.5 ± 0.6%., while calcite in retrograded eclogites has δ¹⁸O 17.7 ± 2.7%., The δ¹³C (≈ −4 ± 0.8%.) is indistinguishable between these two groups.

Both whole rock and carbonate stable isotope data are interpreted as indicating a continued history of fluid infiltration during and after peak eclogite facies metamorphism. The most probable source of fluids are from dewatered sediments tectonically juxtaposed during the Caledonian orogeny.     

Prograde high- to ultrahigh-pressure metamorphism and exhumation of oceanic sediments at Lago di Cignana, Zermatt-Saas Zone, western Alps

Pelagic metasediments and MORB-type metabasalts of the former Tethyan oceanic crust at Cignana, Valtournanche, Italy, experienced UHP metamorphism and subsequent exhumation during the Early to Late Tertiary. Maximum PT conditions attained during UHP metamorphism were 600–630 °C, 2.7–2.9 GPa, which resulted in the formation of coesite-glaucophane-eclogites in the basaltic layer and of garnet-dolomite-aragonite-lawsonite-coesite-phengite-bearing calc-schists and garnet-phengite-coesite-schists with variable amounts of epidote, talc, dolomite, Na-pyroxene and Na-amphibole in the overlying metasediments. During subduction the rocks followed a prograde HP/UHP path which in correspondance with the Jurassic age of the Tethyan crust reflects the thermal influence of relatively old and cold lithosphere and of low to moderate shear heating. Inflections on the prograde metamorphic path may correspond to thermal effects that arise from a decrease in shear heating due to brittle-plastic transition in the quartz-aragonite-dominated rocks, induced convection in the asthenospheric mantle wedge and/or heat consumption by endothermic reactions over a restricted PT segment during subduction. After detachment from the downgoing slab some 50–70 Ma before present, the Cignana crustal slice was first exhumed to ca. 60 km and concomitantly cooled to ca. 550 °C, tracing back the UHP/HP prograde path displaced by 50–80 °C to higher temperatures. Exhumation at this stage is likely to have occurred in the Benioff zone, while the subduction of cool lithosphere was going on. Subsequently, the rocks were near-isothermally exhumed to ca. 30 km, followed by concomitant decompression and cooling to surface conditions (at < 500 °C, < 1 GPa). During this last stage the UHPM slice arrived at its present tectonic position with respect to the overlying greenschist-facies Combin zone. In contrast to the well-preserved HP/UHPM record of the coesite-glaucophane eclogites, the HP/UHP assemblages of the metasediments have been largely obliterated during exhumation. Relics from which the metamorphic evolution of the rocks during prograde HP metamorphism and the UHP stage can be retrieved are restricted to rigid low-diffusion minerals like garnet, dolomite, tourmaline and apatite.     

Mineral and fluid inclusions in zircon of UHP metamorphic rocks from the CCSD-main drill hole: A record of metamorphism and fluid activity

The Chinese Continental Scientific Drilling (CCSD) main drill hole (0–3000 m) in Donghai, southern Sulu orogen, consists of eclogite, paragneiss, orthogneiss, schist and garnet peridotite. Detailed investigations of Raman, cathodoluminescence, and microprobe analyses show that zircons from most eclogites, gneisses and schists have oscillatory zoned magmatic cores with low-pressure mineral inclusions of Qtz, Pl, Kf and Ap, and a metamorphic rim with relatively uniform luminescence and eclogite-facies mineral inclusions of Grt, Omp, Phn, Coe and Rt. The chemical compositions of the UHP metamorphic mineral inclusions in zircon are similar to those from the matrix of the host rocks. Similar UHP metamorphic P–T conditions of about 770 °C and 32 kbar were estimated from coexisting minerals in zircon and in the matrix. These observations suggest that all investigated lithologies experienced a joint in situ UHP metamorphism during continental deep subduction. In rare cases, magmatic cores of zircon contain coesite and omphacite inclusions and show patchy and irregular luminescence, implying that the cores have been largely altered possibly by fluid–mineral interaction during UHP metamorphism.

Abundant H₂O–CO₂, H₂O- or CO₂-dominated fluid inclusions with low to medium salinities occur isolated or clustered in the magmatic cores of some zircons, coexisting with low-P mineral inclusions. These fluid inclusions should have been trapped during magmatic crystallization and thus as primary. Only few H₂O- and/or CO₂-dominated fluid inclusions were found to occur together with UHP mineral inclusions in zircons of metamorphic origin, indicating that UHP metamorphism occurred under relatively dry conditions. The diversity in fluid inclusion populations in UHP rocks from different depths suggests a closed fluid system, without large-scale fluid migration during subduction and exhumation.