Formation and exhumation of blueschists and eclogites from NE Oman: new perspectives from Rb–Sr and 40Ar/39Ar dating

Seven eclogite facies samples from lithologically different units which structurally underlie the Semail ophiolite were dated by the ⁴⁰Ar/³⁹Ar and Rb–Sr methods. Despite extensive efforts, phengite dated by the ⁴⁰Ar/³⁹Ar method yielded saddle, hump or irregularly shaped spectra with uninterpretable isochrons. The total gas ages for the phengite ranged from 136 to 85 Ma. Clinopyroxene–phengite, epidote–phengite and whole‐rock–phengite Rb–Sr isochrons for the same samples yielded ages of 78 ± 2 Ma. We therefore conclude that the eclogite facies rocks cooled through 500 °C at c. 78 ± 2 Ma, and that the ⁴⁰Ar/³⁹Ar dates can only constrain maximum ages due to the occurrence of excess Ar inhomogeneously distributed in different sites. Our new results lead us to conclude that high‐pressure metamorphism of the Oman margin took place in the Late Cretaceous, contemporaneous with ophiolite emplacement. Previously published structural and petrological data lead us to suggest that this metamorphism resulted from intracontinental subduction and crustal thickening along a NE‐dipping zone. Choking of this subduction zone followed by ductile thinning of a crustal mass wedged between deeply subducted continental material and overthrust shelf and slope units facilitated the exhumation of the eclogite facies rocks from depths of c. 50 km to 10–15 km within c. 10 Ma, and led to their juxtaposition against overlying lower grade rocks. Final exhumation of all high‐pressure rocks was driven primarily by erosion and assisted by normal faulting in the upper plate.     

The Eocene unconformity of the Briançonnais domain in the French—Italian Alps,revisited (Marguareis massif,Cuneo); a hint for a Late Cretaceous—Middle Eocene frontal bulge setting

Abstract

The tectonic significance of the Eocene unconformity in the Briançonnais domain, classically regarded as recording a compressional event, is re-evaluated, based on field studies in the Marguareis massif, Maritime-Ligurian Alps. In this external, weakly metamorphic Briançonnais unit, we describe N-trending, folded paleo-normal faults. These paleofaults operated during the Late Cretaceous-Late Eocene, and control both the thickness of the Senonian-Paleocene calcschists and the distribution of the disconformable Middle Eocene-Early Priabonian formations, i.e. the channelised, resedimented Nummulitic limestones, associated with sandy turbidites, and the sandy- calcareous Lower Flysch Noir. The chaotic Upper Flysch Noir (Priabonian), which includes olistoliths from the Helminthoid Flysch nappes, disconformably overlies the Late Cretaceous-Middle/Late Eocene levels. At the scale of the whole Briançonnais domain of the French-Italian Alps, the superimposed Senonian-Eocene disconformities would indicate extensional faulting and block tilting, associated with a regional uplift which caused emersion of part of the domain (most internal Briançonnais, Corsica). Extension and coeval uplift would record the crossing of the frontal flexure (external bulge) of the European/Briançonnais lower plate situated west of the Alpine subduction zone between 80–70 Ma and ～40 Ma, i.e. before the subduction of the Briançonnais plateau around 38–35 Ma. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.     

Neotethys closure history of Anatolia: insights from 40Ar–39Ar geochronology and P–T estimation in high‐pressure metasedimentary rocks

The multiple high‐pressure (HP), low‐temperature (LT) metamorphic units of Western and Central Anatolia offer a great opportunity to investigate the subduction‐ and continental accretion‐related evolution of the eastern limb of the long‐lived Aegean subduction system. Recent reports of the HP–LT index mineral Fe‐Mg‐carpholite in three metasedimentary units of the Gondwana‐derived Anatolide–Tauride continental block (namely the Afyon Zone, the Ören Unit and the southern Menderes Massif) suggest a more complicated scenario than the single‐continental accretion model generally put forward in previous studies. This study presents the first isotopic dates (white mica ⁴⁰Ar–³⁹Ar geochronology), and where possible are combined with P–T estimates (chlorite thermometry, phengite barometry, multi‐equilibrium thermobarometry), on carpholite‐bearing rocks from these three HP–LT metasedimentary units. It is shown that, in the Afyon Zone, carpholite‐bearing assemblages were retrogressed through greenschist‐facies conditions at c. 67–62 Ma. Early retrograde stages in the Ören Unit are dated to 63–59 Ma. In the Kurudere–Nebiler Unit (HP Mesozoic cover of the southern Menderes Massif), HP retrograde stages are dated to c. 45 Ma, and post‐collisional cooling to c. 26 Ma. These new results support that the Ören Unit represents the westernmost continuation of the Afyon Zone, whereas the Kurudere–Nebiler Unit correlates with the Cycladic Blueschist Unit of the Aegean Domain. In Western Anatolia, three successive HP–LT metamorphic belts thus formed: the northernmost Tav?anl? Zone (c. 88–82 Ma), the Ören–Afyon Zone (between 70 and 65 Ma), and the Kurudere–Nebiler Unit (c. 52–45 Ma). The southward younging trend of the HP–LT metamorphism from the upper and internal to the deeper and more external structural units, as in the Aegean Domain, points to the persistence of subduction in Western Anatolia between 93–90 and c. 35 Ma. After the accretion of the Menderes–Tauride terrane, in Eocene times, subduction stopped, leading to continental collision and associated Barrovian‐type metamorphism. Because, by contrast, the Aegean subduction did remain active due to slab roll‐back and trench migration, the eastern limb (below Southwestern Anatolia) of the Hellenic slab was dramatically curved and consequently teared. It therefore is suggested that the possibility for subduction to continue after the accretion of buoyant (e.g. continental) terranes probably depends much on palaeogeography.     

Tectonometamorphic evolution of the Atbashi high‐P units (Kyrgyz CAOB,Tien Shan): Implications for the closure of the Turkestan Ocean and continental subduction–exhumation of the South Kazakh continental margin

The South Tien Shan (STS) belt results from the last collision event in the western Central Asian Orogenic Belt (CAOB). Understanding its formation is of prime importance in the general framework of the CAOB. The Atbashi Range preserves high‐P (HP) rocks along the STS suture, but still, its global metamorphic evolution remains poorly constrained. Several HP units have been identified: (a) a HP tectonic mélange including boudins of mafic eclogites in a sedimentary matrix, (b) a large (>100 km long) high‐P metasedimentary unit (HPMU) and (c) a lower blueschist facies accretionary prism. Raman Spectroscopy on carbonaceous material combined with phengite and chlorite multiequilibria and isochemical phase diagram modelling indicates that the HPMU recorded homogeneous P–T conditions of 23–25 kbar and 560–570°C along the whole unit. ⁴⁰Ar/³⁹Ar dating on phengite from the HPMU ranges between 328 and 319 Ma at regional scale. These ages are interpreted as (re‐) crystallization ages of phengite during T_max conditions at a pressure range of 20–25 kbar. Thermobarometry on samples from the HP tectonic mélange provides similar metamorphic peak conditions. Thermobarometry on the blueschist to lower greenschist facies accretionary prism indicates that it underwent P–T conditions of 5–6 kbar and 290–340°C, highlighting a 17–20 kbar pressure gap between the HPMU‐tectonic mélange units and the accretionary prism. Comparison with available geochronological data suggests a very short time span between the prograde path (340 Ma), HP metamorphic peak (330 Ma), the T_max (328–319 Ma) and the final exhumation of the HPMU (303–295 Ma). Extrusion of the HPMU, accommodated by a basal thrust and an upper detachment, was driven by buoyant forces from 70–75 km up to 60 km depth, which directly followed continental subduction and detachment of the HPMU. At crustal depths, extrusion was controlled by collisional tectonics up to shallow levels. Lithological homogeneity of the HPMU and its continental‐derived character from the North Tien Shan suggest this unit corresponds to the hyper‐extended continental margin of the Kazakh continent, subducted southward below the north continental active margin of the Tarim craton. Integration of the available geological data allows us to propose a general geodynamic scenario for Tien Shan during the Carboniferous with a combination of (a) N‐dipping subduction below the Kazakh margin of Middle Tien Shan until 390–340 Ma and (b) S‐dipping subduction of remaining Turkestan marginal basins between 340 and 320 Ma.     

Dehydration and anatexis of UHP metagranite during continental collision in the Sulu orogen

Dehydration and anatexis of ultrahigh‐pressure (UHP) metamorphic rocks during continental collision are two key processes that have great bearing on the physicochemical properties of deeply subducted continental crust at mantle depths. Determining the time and P–T conditions at which such events take place is needed to understand subduction‐zone tectonism. A combined petrological and zirconological study of UHP metagranite from the Sulu orogen reveals differential behaviours of dehydration and anatexis between two samples from the same UHP slice. The zircon mantle domains in one sample record eclogite facies dehydration metamorphism at 236 ± 5 Ma during subduction, exhibiting low REE contents, steep MREE–HREE patterns without negative Eu anomalies, low Th, Nb and Ta contents, low temperatures of 651–750 °C and inclusions of quartz, apatite and jadeite. A second mantle domain records high‐T anatexis at 223 ± 3 Ma during exhumation, showing high REE contents, steeper MREE–HREE patterns with marked negative Eu anomalies, high Hf, Nb, Ta, Th and U contents, high temperatures of 698–879 °C and multiphase solid inclusions of albite + muscovite + quartz. In contrast, in a second sample, one zircon mantle domain records limited hydration anatexis at 237 ± 3 Ma during subduction, exhibiting high REE contents, steep MREE–HREE patterns with marked negative Eu anomalies, high Hf, Nb, Ta, Th and U contents, medium temperatures of 601–717 °C and multiphase solid inclusions of albite + muscovite + hydrohalite. A second mantle domain in this sample records a low‐T dehydration metamorphism throughout the whole continental collision in the Triassic, showing low REE contents, steep MREE–HREE patterns with weakly negative Eu anomalies, low Th, Nb and Ta contents, low temperatures of 524–669 °C and anhydrite + gas inclusions. Garnet, phengite and allanite/epidote in these two samples also exhibit different variations in texture and major‐trace element compositions, in accordance with the zircon records. The distinct P–T–t paths for these two samples suggest separate processes of dehydration and anatexis, which are ascribed to the different geothermal gradients at different positions inside the same crustal slice during continental subduction‐zone metamorphism. Therefore, the subducting continental crust underwent variable extents of dehydration and anatexis in response to the change in subduction‐zone P–T conditions.     

Metamorphic evolution of a newly identified Mesoproterozoic oceanic slice in the Yuka terrane and its implications for a multi‐cyclic orogenic history of the North Qaidam UHPM belt

The North Qaidam Orogenic Belt (NQOB), lying at the northern margin of the Tibet Plateau, records two orogenic cycles: A Proterozoic cycle related to the amalgamation and breakup of the supercontinent Rodinia, and an Early Palaeozoic cycle including oceanic subduction and continental deep subduction. At present, the only information about the Proterozoic cycle is the concurrent c. 1,000–900 Ma magmatic and metamorphic events, which limited the understanding of the Proterozoic evolution of NQOB and the relationship between the Qaidam Block and other Rodinia fragments. In this study, a kyanite‐bearing eclogite was identified in Yuka terrane. It has positive‐slope chondrite‐normalized rare earth element distribution patterns, similar to present‐day N‐MORB. LA–ICP–MS zircon U–Pb dating obtained a protolith age of 1,273 Ma and an eclogite facies metamorphic age of 437 Ma, which is similar to the continental deep subduction age of the Yuka terrane. Zircon Lu–Hf analysis show that the magmatic zircon cores have high εHf(t) of 8.36–15.98 and T_DM1 of 1,450–1,131 Ma (M = 1,303 ± 55 Ma, consistent with its protolith age within error), indicating a juvenile crust protolith of the eclogite. The MORB‐like whole‐rock composition and zircon U–Pb and Lu–Hf analysis indicate that the protolith of the kyanite‐bearing eclogite was a Mesoproterozoic oceanic slice. P–T pseudosection analysis shows that the kyanite‐bearing eclogite experienced four metamorphic stages: (1) a prograde stage with the assemblage garnet+omphacite+talc+lawsonite+phengite+quartz at 22.4–23.2 kbar and 585°C; (2) a peak stage with the assemblage garnet+omphacite+lawsonite+phengite+coesite at 32.5 kbar and 670°C; (3) an early retrograde stage with the assemblage garnet+omphacite+kyanite+phengite+coesite/quartz±lawsonite at 27.1–30.0 kbar and 670–690°C; and (4) a late retrograde stage with the assemblage garnet+omphacite+epidote+hornblende+phengite+quartz at <18.0 kbar. The established clockwise P–T path is similar with other continental‐type eclogites in this area. On the basis of the geochemical and geochronological data, as well as the P–T path, we suggest that the protolith of the kyanite‐bearing eclogite was emplaced in the active margin of the Qaidam Block during the assembly of Rodinia and underwent continental deep subduction in the Early Palaeozoic. We conclude that (1) the Qaidam Block participated in the assembly of the Rodinia supercontinent. It was situated at or proximal to the margin of the supercontinent and probably close to India, east Antarctica and Tarim; and (2) both Mesoproterozoic and Early Palaeozoic oceanic crust slices occur in the NQOB. Thus, special caution is needed when using the metamorphic ages of oceanic affinity eclogites without protolith ages to constrain the evolution history of the North Qaidam UHPM belt.     

The geological significance of 40Ar/39Ar and Rb–Sr white mica ages from Syros and Sifnos,Greece: a record of continuous (re)crystallization during exhumation?

The Attic‐Cycladic crystalline belt in the central Aegean region records a complex structural and metamorphic evolution that documents Cenozoic subduction zone processes and exhumation. A prerequisite to develop an improved tectono‐metamorphic understanding of this area is dating of distinct P–T–D stages. To evaluate the geological significance of phengite ages of variably overprinted rocks, ⁴⁰Ar/³⁹Ar and Rb–Sr analyses were undertaken on transitional blueschist–greenschist and greenschist facies samples from the islands of Syros and Sifnos. White mica geochronology indicates a large age variability (⁴⁰Ar/³⁹Ar: 41–27 Ma; Rb–Sr: 34–20 Ma). Petrologically similar samples have either experienced greenschist facies overprinting at different times or variations in ages record variable degrees of greenschist facies retrogression and incomplete resetting of isotopic systematics. The ⁴⁰Ar/³⁹Ar and Rb–Sr data for metamorphic rocks from both islands record only minor, localized evidence for Miocene ages (c. 21 Ma) that are well documented elsewhere in the Cyclades and interpreted to result from retrogression of high‐pressure mineral assemblages during lower pressure metamorphism. Field and textural evidence suggests that heterogeneous overprinting may be due to a lack of permeability and/or limited availability of fluids in some bulk compositions and that retrogression was more or less parallel to lithological layering and/or foliation as a result of, possibly deformation‐enhanced, channelized fluid ingress. Published and new ⁴⁰Ar/³⁹Ar and Rb–Sr data for both islands indicate apparent age variations that can be broadly linked to mineral assemblages documenting transitional blueschist‐to‐greenschist‐ and/or greenschist facies metamorphism. The data do not record the timing of peak HP metamorphism, but may accurately record continuous (partial) resetting of isotopic systematics and/or (re)crystallization of white mica during exhumation and greenschist facies retrogression. The form of ⁴⁰Ar/³⁹Ar phengite age spectra are complex with the lowest temperature steps yielding Middle to Late Miocene ages. The youngest Rb–Sr ages suggest maximum ages of 20.6 ± 0.8 Ma (Syros) and 22.5 ± 0.6 Ma (Sifnos) for the timing of greenschist facies overprinting. The results of this study further accentuate the challenges of interpreting isotopic data for white mica from polymetamorphic terranes, particularly when mixing of populations and/or incomplete resetting of isotopic systematics occurs during exhumation. These data capture the full range of isotopic age variations in retrogressed HP rocks documented in previous isotopic studies, and can be interpreted in terms of the geodynamic evolution of the Aegean.     

Protracted garnet growth in high‐P eclogite: constraints from multiple geochronology and P–T pseudosection

Understanding convergent margin processes requires determination of the onset and the termination of subduction, the duration of subduction‐zone metamorphism, and the subduction zone polarity. Garnet growth and intracrystalline zonation can be used to constrain the timing, duration and kinetics of tectonometamorphic processes. An eclogite from the Huwan shear zone in the Hong'an orogen was investigated with combined pseudosection analysis and multiple geochronologies. The pseudosection analysis illustrates that garnet growth is continuous and along an early near‐isothermal trajectory followed by a near‐isobaric heating path from 1.9 GPa/500 °C to 2.4 GPa/575 °C and subsequent near‐isothermal decompression. ⁴⁰Ar/³⁹Ar dating of an amphibole inclusion in garnet from the eclogite yielded an age of 310 ± 5 Ma, which is consistent with a U–Pb age of 305 ± 3 Ma for the metamorphic zircon within uncertainty. Garnet core and rim material produced Lu–Hf ages of 296.9 ± 3.8 and 256.9 ± 3.9 Ma respectively; the latter is consistent with its Sm–Nd age of 254.3 ± 4.6 Ma for the same aliquots. Similarly, limited zircon U–Pb ages of c. 257 Ma were obtained in zircon rims with garnet inclusions. These ages were interpreted to bracket the period of garnet growth and the difference of up to c. 40 Ma is best explained by protracted garnet growth. We propose that the rocks represent detachment of part of the downgoing slab and remained free of significant compression/decompression or heating/cooling close to the subduction channel, most likely underplating the mantle wedge, for a long time. These rocks were incorporated into the following subduction channel due to the successive entry of the buoyant materials, and exhumed at some time later than c. 254 Ma. The increasing observations of protracted garnet growth and long‐lived subduction in various orogens worldwide demand more sophisticated geodynamic models.     

Ocean floor and subduction record in the Zermatt‐Saas rodingites,Valtournanche, Western Alps

Multiscale structural analysis and petrological modelling were used to establish the pressure‐peak mineral assemblages and pressure–temperature (P–T) conditions recorded in the rodingites of the upper Valtournanche portion of the oceanic Zermatt‐Saas Zone (ZSZ; Western Alps, northwestern Italy) during Alpine subduction. Rodingites occur in the form of deformed dykes and boudins within the hosting serpentinites. A field structural analysis showed that rodingites and serpentinites record four ductile deformation stages (D1–D4) during the Alpine cycle, with the first three stages associated with new foliations. The most pervasive fabric is S2 that is marked by mineral assemblages in serpentinite indicating pressure‐peak conditions, involving mostly serpentine, clinopyroxene, olivine, Ti‐clinohumite and chlorite. Three rodingite types can be defined: epidote‐bearing, garnet–chlorite–clinopyroxene‐bearing and vesuvianite‐bearing rodingite. In these, the pressure‐peak assemblages coeval with S2 development involve: (i) epidote_II + clinopyroxene_II + Mg‐chlorite_II + garnet_II ± rutile ± tremolite_I in the epidote‐bearing rodingite; (ii) Mg‐chlorite_II + garnet_II clinopyroxene_II ± vesuvianite_II ± ilmenite in the garnet–chlorite–clinopyroxene‐bearing rodingite; (iii) vesuvianite_II + Mg‐chlorite_II + clinopyroxene_II + garnet_II ± rutile ± epidote in vesuvianite‐bearing rodingite. Despite the pervasive structural reworking of the rodingites during Alpine subduction, the mineral relicts of the pre‐Alpine ocean floor history have been preserved and consist of clinopyroxene porphyroclasts (probable igneous relicts from gabbro dykes) and Cr‐rich garnet and vesuvianite (relicts of ocean floor metasomatism). Petrological modelling using thermocalc in the NCFMASHTO system was used to constrain the P–T conditions of the S2 mineral assemblages. The inferred values of 2.3–2.8 GPa and 580–660 °C are consistent with those obtained for syn‐S2 assemblages in the surrounding serpentinites. Multiscale structural analysis indicates that some ocean floor minerals remained stable under eclogite facies conditions suggesting that minerals such as vesuvianite, which is generally regarded as a low‐P phase, could also be stable in favourable chemical systems under high‐P/ultra‐high‐pressure (HP/UHP) conditions. Finally, the reconstructed P–T–d–t path indicates that the P/T ratio characterizing the D2 stage is consistent with cold subduction as estimated in this part of the Alps. The estimated pressure‐peak values are higher than those previously reported in this part of ZSZ, suggesting that the UHP units are larger and/or more abundant than those previously suggested.     

P–T evolution and episodic zircon growth in barroisite eclogites of the Lanterman Range,northern Victoria Land,Antarctica

Prograde P–T–t paths of eclogites are often ambiguous owing to high variance of mineral assemblages, large uncertainty in isotopic age determinations and/or variable degree of retrograde equilibration. We investigated these issues using the barroisite eclogites from the Lanterman Range, northern Victoria Land, Antarctica, which are relatively uncommon but free of retrogression. These eclogites revealed three stages of prograde metamorphism, defining two distinctive P–T trajectories, M_1–2 and M₃. Inclusion minerals in garnet porphyroblasts suggest that initial prograde assemblages (M₁) consist of garnet+omphacite+barroisite/Mg‐pargasite+epidote+phengite+paragonite+rutile/titanite+quartz, and subsequent M₂ assemblages of garnet+omphacite+barroisite+phengite+rutile±quartz. The inclusion‐rich inner part of garnet porphyroblasts preserves a bell‐shaped Mn profile of the M₁, whereas the inclusion‐poor outer part (M₂) is typified by the outward decrease in Ca/Mg and X_Fe (=Fe²⁺/(Fe²⁺+Mg)) values. A pseudosection modelling employing fractionated bulk‐rock composition suggests that the eclogites have initially evolved from ~15 to 20 kbar and 520–570°C (M₁) to ~22–25 kbar and 630–650°C (M₂). The latter is in accordance with P–T conditions estimated from two independent geothermobarometers: the garnet–clinopyroxene–phengite (~25 ± 3 kbar and 660 ± 100°C) and Zr‐in‐rutile (~650–700°C at 22–27 kbar). The second segment (M_3A–B) of prograde P–T path is recorded in the grossular‐rich overgrowth rim of garnet. Apart from disequilibrium growth of the M_3A garnet, ubiquitous overgrowth of the M_3B garnet permits us to estimate the P–T conditions at ~26 ± 3 kbar and 720 ± 80°C. The cathodoluminescence (CL) imaging of zircon grains separated from a barroisite eclogite revealed three distinct zones with bright rim, dark mantle and moderately dark core. Eclogitic phases such as garnet, omphacite, epidote and rutile are present as fine‐grained inclusions in the mantle and rim of zircon, in contrast to their absence in the core. The sensitive high‐resolution ion microprobe U–Pb dating on metamorphic mantle domains and neoblasts yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 515 ± 4 Ma (tσ), representing the time of the M₂ stage. On the other hand, overgrowth rims as well as bright‐CL neoblasts of zircon were dated at 498 ± 11 Ma (tσ), corresponding to the M₃. Average burial rates estimated from the M₂ and M₃ ages are too low (<2 mm/year) for cold subduction regime (~5–10°C/km), suggesting that an exhumation stage intervened between two prograde segments of P–T path. Thus, the P–T–t evolution of barroisite eclogites is typified by two discrete episodes with an c. 15 Ma gap during the middle Cambrian subduction of the Antarctic Ross Orogeny.     

HP/LT metamorphism in the Volparone Breccia (Northern Corsica, France): evidence for involvement of the Europe/Corsica continental margin in the Alpine subdution zone

The Alpine belt in Corsica (France) is characterized by the occurrence of stacked tectonic slices derived from the Corsica/Europe continental margin, which outcrop between two weakly or non‐metamorphic tectonic domains: the ‘autochthonous’ domain of the Hercynian basement to the west and the Balagne Nappe (ophiolitic unit belonging to the ‘Nappes supérieures’) to the east. These slices, including basement rocks (Permian granitoids and their Palaeozoic host rocks), Late Carboniferous–Permian volcano‐sedimentary deposits, coarse‐grained polymict breccias (Volparone Breccia) and Middle Eocene siliciclastic turbidite deposits, were affected by a polyphase deformation history of Alpine age, associated with a well‐developed metamorphic recrystallization. This study provides new quantitative data about the peak of metamorphism and the retrograde P–T path in the Alpine Corsica: the tectonic slices of Volparone Breccia from the Balagne region (previously regarded as unmetamorphosed) were affected by peak metamorphism characterized by the phengite + chlorite + quartz ± albite assemblage. Using the chlorite‐phengite local equilibria method, peak metamorphic P–T conditions coherent with the low‐grade blueschist facies are estimated as 0.60 ± 0.15 GPa and 325 ± 20 °C. Moreover, the retrograde P–T path, characterized by a decrease of pressure and temperature, is evidence of the first stage of the exhumation path from the peak metamorphic conditions to greenschist facies conditions (0.35 ± 0.06 GPa and 315 ± 20 °C). The occurrence of metamorphic peak at high‐pressure/low‐temperature (HP/LT) conditions is evidence of the fact that these tectonic slices, derived from the Corsica/Europe continental margin, were deformed and metamorphosed in the Alpine subduction zone during their underplating at ～20 km of depth into the accretionary wedge and were subsequently juxtaposed against the metamorphic and non‐metamorphic oceanic units during a complex exhumation history.     

Expression of the Toarcian Oceanic Anoxic Event: New insights from a Swiss transect

A sedimentological, biostratigraphical and geochemical (stable isotopes and Rock‐Eval parameters) analysis was performed on four Swiss successions, in order to examine the expression of the Toarcian Oceanic Anoxic Event along a north–south transect, from the Jura through the Alpine Tethys (Sub‐Briançonnais and Lombardian basins). The locations were selected to represent a range of palaeoceanographic positions from an epicontinental sea to a more open marine setting. The Toarcian Oceanic Anoxic Event was recognized by the presence of the characteristic negative carbon‐isotope excursion in carbonate (ca 2 to 4‰) and organic matter (ca 4 to 5‰) at the base of the falciferum ammonite Zone (NJT6 nannofossil Zone). The sedimentary expression of the Toarcian Oceanic Anoxic Event varies along the transect from laminated mudstone rich in total organic carbon (≤11 wt.%) in the Jura, to thin‐bedded marl (≤5 wt.% total organic carbon) in the Sub‐Briançonnais Basin and to hemipelagic reddish marly limestone (total organic carbon <0·05 wt.%) in equivalent levels from the Lombardian Basin. The carbon‐isotope excursion is thus independent of facies and palaeoceanographic position. The low nannofossil abundance and the peak in Calyculaceae in the Jura and the Sub‐Briançonnais Basin indicate low salinity surface waters and stratified water masses in general. Sedimentological observations (for example, obliquely‐bedded laminae and homogeneous mud layers containing rip‐up clasts) indicate the presence of dynamic conditions, suggesting that water mass stratification was episodically disrupted during the Toarcian Oceanic Anoxic Event. The proposed correlation highlights a stratigraphic gap and/or condensed interval between the Pliensbachian–Toarcian boundary and the Toarcian Oceanic Anoxic Event interval (most of the tenuicostatum ammonite Zone is missing), which is also observed in coeval European sections and points to the influence of sea‐level change and current dynamics. This transect shows that the sedimentary expression of the Toarcian Oceanic Anoxic Event is not uniform across the Alpine Tethys, supporting the importance of local conditions in determining how this event is recorded across different palaeoceanographic settings.     

U–Pb ages and trace elements in metamorphic zircon and titanite from UHP eclogite in the Dabie orogen: constraints on P–T–t path

Laser ablation inductively coupled plasma mass spectrometry analyses of U–Pb isotopes and trace elements in zircon and titanite were carried out on epoxy mounts and thin sections for ultrahigh‐pressure (UHP) eclogite in association with paragneiss in the Dabie orogen. The results provide a direct link between metamorphic ages and temperatures during continental subduction‐zone metamorphism. Zircon U–Pb dating gives two groups of concordant ages at 242 ± 2 to 239 ± 5 Ma and 226 ± 2 to 224 ± 6 Ma, respectively. The Triassic zircon U–Pb ages are characterized by flat heavy rare earth element (HREE) patterns typical of metamorphic growth. Ti‐in‐zircon thermometry for the two generations of metamorphic zircon yields temperatures of 697 ± 27 to 721 ± 8 °C and 742 ± 19 to 778 ± 34 °C, respectively. We interpret that the first episode of zircon growth took place during subduction prior to the onset of UHP metamorphism, whereas the second episode in the stage of exhumation from UHP to HP eclogite facies regime. Thus, the continental subduction‐zone metamorphism of sedimentary protolith is temporally associated with two episodes of fluid activity, respectively, predating and postdating the UHP metamorphic phase. The significantly high Ti‐in‐zircon temperatures for the younger zircon at lower pressures indicate the initial ‘hot’ exhumation after the peak UHP metamorphism. There are two types of titanite. One exhibits light rare earth element (LREE) enrichment, steep MREE–HREE patterns and no Eu anomalies, and yields Zr‐in‐titanite temperatures of 551 to 605 °C at 0.5 GPa, and the other shows LREE depletion and flat MREE–HREE patterns, and gives Zr‐in‐titanite temperatures of 782–788 °C at 2.0 GPa. The former is amenable for U–Pb dating, yielding a discordia lower intercept age of 252 ± 3 Ma. Thus, the first type of titanite is interpreted to have grown in the absence of garnet and plagioclase and thus in the early stage of subduction. In contrast, the second one occurs as rims surrounding rutile cores and thus grew in the presence of garnet during the ‘hot’ exhumation. Therefore, there is multistage growth of zircon and titanite during the continental subduction‐zone metamorphism. The combined studies of chronometry and thermobarometry provide tight constraints on the P–T–t path of eclogites during the continental collision. It appears that the mid‐T/UHP eclogite facies zone would not only form by subduction of the continental crust in a P–T path slightly below the wet granite solidus, but also experience decompression heating during the initial exhumation.     

Mineralization events in the Xiaokele porphyry Cu (–Mo) deposit,NE China: Evidence from zircon U–Pb and K‐feldspar Ar–Ar geochronology and petrochemistry

The Great Xing'an Range (GXR), Northeast (NE) China, is a major polymetallic metallogenic belt in the eastern segment of the Central Asian Orogenic Belt. The newly discovered Xiaokele porphyry Cu (–Mo) deposit lies in the northern GXR. Field geological and geochronological studies have revealed two mineralization events in this deposit: early porphyry‐type Cu (–Mo) mineralization, and later vein‐type Cu mineralization. Previous geochronological studies yielded an age of ca. 147 Ma for the early Cu (–Mo) mineralization. Our ⁴⁰Ar/³⁹Ar dating yielded ⁴⁰Ar/³⁹Ar plateau ages of 124.8 ± 0.4 to 124.3 ± 0.4 Ma on K‐feldspar in altered Cu‐mineralized diorite porphyrite dikes that represent the overprinting vein‐type Cu mineralization, consistent with zircon U–Pb ages of the diorite porphyrite (126.4 ± 0.5 to 125.0 ± 0.5 Ma). The Cr and Ni contents and Mg^# of the Xiaokele diorite porphyrites are high. The diorite porphyrites at Xiaokele are enriched in light rare‐earth elements (REEs), and large‐ion lithophile elements (e.g., Rb, Ba, and K), are depleted in heavy REEs and high‐field‐strength elements (e.g., Nb, Ta, and Ti), and have weak negative ε_Hf(t) values (+0.29 to +5.27) with two‐stage model ages (T_DM2) of 1,164–845 Ma. Given the regional tectonic setting in Early Cretaceous, the ore‐bearing diorite porphyrites were likely formed in an extensional environment related to lithospheric delamination and asthenospheric upwelling induced by subduction of the Paleo‐Pacific Plate. These tectonic events caused large‐scale magmatic activity, ore mineralization, and lithospheric thinning in NE China.     

The onset of the Tan–Lu fault movement in eastern China: constraints from zircon (SHRIMP) and 40Ar/39Ar dating

In eastern China, the Dabie Shan–Su–Lu orogenic belt has been separated by the Tan–Lu sinistral strike–slip fault. Mylonites are exposed along the strike–slip fault system in the southern segment, and along the eastern margin of the Dabie Shan orogenic belt. The country rocks of the mylonites are retrograde UHP eclogites, gneissic granites, muscovite granites and gneisses. The ductile strike–slip shear zone trends 30–40°N (NE30–40°‐trending) and exhibits stretching lineations and nearly vertical, SE‐dipping foliations. Most of the zircon grains separated from mylonites have a weighted average radiometric age of 233 ± 6–225 ± 6 Myr. These data constrain the onset of the Tan–Lu sinistral strike–slip movement and imply that the Tan–Lu sinistral strike–slip motion developed after retrograde UHP metamorphism. The related phengite within the eclogite rocks on the western side of the Tan–Lu fault, with ⁴⁰Ar/³⁹Ar plateau ages of c. 182–190 Myr, is also deformed and aligned parallel to the almost NE trending stretching lineations. Non‐metamorphosed granites exhibit sinistral strike–slip shearing and indicate that the Tan–Lu fault initially developed after 182–190 Myr. Muscovite collected from the mylonite yields ⁴⁰Ar/³⁹Ar plateau ages of 162 ± 1–156 ± 2 Myr. The zircon SHRIMP age data, the muscovite ⁴⁰Ar/³⁹Ar plateau ages, together with structural and petrological field information support the interpretation that the Tan–Lu strike–slip fault was not related to the Yangtze–north China plates collision, but corresponded to the formation of a NE‐trending tectonic framework in eastern China starting c. 165–160 Ma.     

Jadeite–chloritoid–glaucophane–lawsonite blueschists in north-west Turkey: unusually high P/T ratios in continental crust

Sodic metapelites with jadeite, chloritoid, glaucophane and lawsonite form a coherent regional metamorphic sequence, several tens of square kilometres in size, and over a kilometre thick, in the Orhaneli region of northwest Turkey. The low‐variance mineral assemblage in the sodic metapelites is quartz + phengite + jadeite + glaucophane + chloritoid + lawsonite. The associated metabasites are characterized by sodic amphibole + lawsonite ± garnet paragenesis. The stable coexistence of jadeite + chloritoid + glaucophane + lawsonite, not reported before, indicates metamorphic pressures of 24 ± 3 kbar and temperatures of 430 ± 30 °C for the peak blueschist facies conditions. These P–T conditions correspond to a geotherm of 5 °C km^?1, one of the lowest recorded in continental crustal rocks. The low geotherm, and the known rate of convergence during the Cretaceous subduction suggest low shear stresses at the top of the downgoing continental slab.     

Early Cretaceous exhumation of high‐pressure metamorphic rocks of the Sistan Suture Zone,eastern Iran

The Sistan Suture Zone (SSZ) of eastern Iran is part of the Neo‐Tethyan orogenic system and formed by convergence of the Central Iranian and Afghan microcontinents. Ar Ar ages of ca. 125 Ma have been obtained from white micas and amphibole from variably overprinted high‐pressure metabasites within the Ratuk Complex of the SSZ. The metabasites, which occur as fault‐bounded lenses within a subduction mélange, document peak‐metamorphic conditions in eclogite or blueschist facies followed by near‐isothermal decompression resulting in an epidote–amphibolite‐facies overprint. ⁴⁰Ar/³⁹Ar step heating experiments were performed on a phengite + paragonite mixture from an eclogite, phengites from two amphibolites, and paragonite from a blueschist; ‘best‐fit’ ages from these micas are, respectively, 122.8 ± 2.2, 124 ± 13, 116 ± 19 and 139 ± 19 Ma (2σ error). Barroisite from an amphibolite yielded an age of 124 ± 10 Ma. The ages are interpreted as cooling ages that record the post‐epidote–amphibolite stage in the exhumation of the rocks. Our results imply that both the high‐pressure metamorphism and the epidote–amphibolite‐facies overprint occurred prior to 125 Ma. Subduction of oceanic lithosphere along the eastern margin of the Sistan Ocean had therefore begun by Barremian (Early Cretaceous) times. Copyright © 2008 John Wiley & Sons, Ltd.     

Constraining peak P–T conditions in UHP eclogites: calculated phase equilibria in kyanite‐ and phengite‐bearing eclogite of the Tromsø Nappe,Norway

Kyanite‐ and phengite‐bearing eclogites have better potential to constrain the peak metamorphic P–T conditions from phase equilibria between garnet + omphacite + kyanite + phengite + quartz/coesite than common, mostly bimineralic (garnet + omphacite) eclogites, as exemplified by this study. Textural relationships, conventional geothermobarometry and thermodynamic modelling have been used to constrain the metamorphic evolution of the Tromsdalstind eclogite from the Tromsø Nappe, one of the biggest exposures of eclogite in the Scandinavian Caledonides. The phase relationships demonstrate that the rock progressively dehydrated, resulting in breakdown of amphibole and zoisite at increasing pressure. The peak‐pressure mineral assemblage was garnet + omphacite + kyanite + phengite + coesite, inferred from polycrystalline quartz included in radially fractured omphacite. This omphacite, with up to 37 mol.% of jadeite and 3% of the Ca‐Eskola component, contains oriented rods of silica composition. Garnet shows higher grossular (X_Grs = 0.25–0.29), but lower pyrope‐content (X_Prp = 0. 37–0.39) in the core than the rim, while phengite contains up to 3.5 Si pfu. The compositional isopleths for garnet core, phengite and omphacite constrain the P–T conditions to 3.2–3.5 GPa and 720–800 °C, in good agreement with the results obtained from conventional geothermobarometry (3.2–3.5 GPa & 730–780 °C). Peak‐pressure assemblage is variably overprinted by symplectites of diopside + plagioclase after omphacite, biotite and plagioclase after phengite, and sapphirine + spinel + corundum + plagioclase after kyanite. Exhumation from ultrahigh‐pressure (UHP) conditions to 1.3–1.5 GPa at 740–770 °C is constrained by the garnet rim (X_Ca^Grt = 0.18–0.21) and symplectite clinopyroxene (X_Na^Cpx = 0.13–0.21), and to 0.5–0.7 GPa at 700–800 °C by sapphirine (X_Mg = 0.86–0.87) and spinel (X_Mg = 0.60–0.62) compositional isopleths. UHP metamorphism in the Tromsø Nappe is more widespread than previously known. Available data suggest that UHP eclogites were uplifted to lower crustal levels rapidly, within a short time interval (452–449 Ma) prior to the Scandian collision between Laurentia and Baltica. The Tromsø Nappe as the highest tectonic unit of the North Norwegian Caledonides is considered to be of Laurentian origin and UHP metamorphism could have resulted from subduction along the Laurentian continental margin. An alternative is that the Tromsø Nappe belonged to a continental margin of Baltica, which had already been subducted before the terminal Scandian collision, and was emplaced as an out‐of‐sequence thrust during the Scandian lateral transport of nappes.     

Geochemistry,Zircon U–Pb Analysis,and Biotite 40Ar/39Ar Geochronology of the Maoling Gold Deposit,Liaodong Rift,NE China

The Maoling gold deposit is located in the southwestern part of the Liaodong rift, NE China, and has estimated reserves of 25 t. In this paper we present the results of an investigation into the geochronology and geodynamic mechanisms of igneous activity and metallogenesis within the Maoling gold deposit. New zircon U–Pb age data, biotite ⁴⁰Ar/³⁹Ar age data, whole‐rock geochemistry, and Hf isotopic analyses are presented in order to constrain the petrogenesis and mineralization of the deposit. Zircon U–Pb dating of the Wolongquan biotite monzogranite and Maoling biotite granite yielded mean ages of 194.0 ± 1.1 Ma and 196.1 ± 1.1 Ma, respectively. All the granites are characteristically high‐K calc‐alkaline, enriched in light rare earth elements and large ion lithophile elements, and depleted in high field strength elements, which is consistent with the geochemical characteristics of arc‐type magmas. The Hf isotope characteristics indicate that the granites formed by partial melting of early Paleoproterozoic crustal material. In addition, biotite ⁴⁰Ar/³⁹Ar dating indicates that the Maoling gold deposit formed at 188.9 ± 1.2 Ma, implying that the mineralization was related to both the granite intrusions. Taking into account previous data on S–Pb–O–H isotopes and our new age data, the Maoling deposit can be classified as an intrusion‐related gold deposit. Taking into account the geology of the study area and adjacent regions, we propose that the Maoling gold deposit and its associated granitic intrusions formed in a geodynamic setting that was dominated by subduction of the Paleo‐Pacific Plate beneath the Eurasian continent.     

Exhumation of eclogite and blueschist (Cyclades,Greece): Pressure–temperature evolution determined by thermobarometry and garnet equilibrium modelling

High‐P rocks such as eclogite and blueschist are metamorphic markers of palaeo‐subduction zones, and their formation at high‐P and low‐T (HP–LT) conditions is relatively well understood since it has been the focus of numerous petrological investigations in the past 40 years. The tectonic mechanisms controlling their exhumation back to the surface are, however, diverse, complex and still actively debated. Although the Cycladic Blueschist Unit (CBU, Greece) is among the best worldwide examples for the preservation of eclogite and blueschist, the proposed P–T evolution followed by this unit within the Hellenic subduction zone is quite different from one study to another, hindering the comprehension of exhumation processes. In this study, we present an extensive petrological data set that permits refinement of the shape of the P–T trajectory for different subunits of the CBU on Syros. High‐resolution quantitative compositional mapping has been applied to support the thermobarometric investigations, which involve semi‐empirical thermobarometry, garnet equilibrium modelling and P–T isochemical phase diagrams. The thermodynamic models highlight the powerful use of reactive bulk compositions approximated from local bulk compositions. The results are also combined with Raman spectrometry of carbonaceous material (RSCM) to retrieve the metamorphic peak temperature distribution at the scale of the island. A major result of this study is the good agreement between all the independent thermobarometric methods, permitting reconstruction of the prograde and retrograde P–T trajectories. Garnet compositional zoning was used to retrieve prograde, peak and retrograde growth stages in line with the results of the P–T isochemical phase diagrams, RSCM temperature and peak‐pressure crystallization of the garnet–omphacite–phengite assemblage. Our results are consistent with previous thermobarometric estimates from other occurrences of CBU rocks (Tinos, Andros), suggesting a multistage exhumation process with (1) early syn‐orogenic exhumation within the subduction channel, (2) isobaric heating at mid‐crustal depths (~10–12 kbar) following thermal re‐equilibration of the lithosphere from a cold syn‐orogenic regime in the subduction zone to a warmer post‐orogenic regime in the back‐arc domain and (3) exhumation and cooling related to a post‐orogenic phase of extension following slab retreat. Expanding to the general aspects of subduction zones, we suggest that such metamorphic evolution of HP–LT units should be regarded as a characteristic feature of exhumation driven by slab rollback.