40Ar/39Ar Constraints on the tectonic history and architecture of the ultrahigh-pressure Sulu orogen

New ⁴⁰Ar/³⁹Ar ages are presented from the giant Sulu ultrahigh-pressure (UHP) terrane and surrounding areas. Combined with U-Pb ages, Sm-Nd ages, Rb-Sr ages, inclusion relationships, and geological relationships, they help define the orogenic events before, during and after the Triassic collision between the Sino–Korean and Yangtze Cratons. In the Qinling microcontinent, tectonism occurred between 2.0 and 1.4 Ga. The UHP metamorphism occurred in the Yangtze Craton between 240 and 222 Ma; its thermal effect on the Qinling microcontinent was limited to partial resetting of K-feldspar ⁴⁰Ar/³⁹Ar ages. Subsequent unroofing at rates of 5–25 km Myr⁻¹ brought the UHP terrane to crustal levels where it underwent a relatively short amphibolite facies metamorphism. The end of that metamorphism is marked by ⁴⁰Ar/³⁹Ar ages in the 219–210 Ma range, implying cooling at crustal depths at rates of 50–200 °C Myr⁻¹. Ages in the 210–170 Ma range may reflect protracted cooling or partial resetting by Jurassic or Cretaceous magmatism. Jurassic 166–149 Ma plutonism was followed by cooling at rates of c. 15 °C Myr⁻¹, suggesting relatively deep crustal conditions, whereas Cretaceous 129–118 Ma plutonism was succeeded by cooling at rates of c. 50 C Myr⁻¹, suggesting relatively shallow crustal depths.     

Timing and exhumation of eclogite facies shear zones, Musgrave Block, central Australia

Timing constraints on shear zones can provide an insight into the kinematic and exhumation evolution of metamorphic belts. In the Musgrave Block, central Australia, granulite facies gneisses have been affected, to varying degrees, by mylonitic deformation, some of which attained eclogite facies. The Davenport Shear Zone is a dominant strike-slip system that formed at eclogite facies conditions ( T  ≈650  °C and P ≈12.0  kbar). Sm–Nd mineral isochrons obtained from equilibrated high-pressure assemblages, as well as ⁴⁰Ar–³⁹Ar data, show that the eclogite and greenschist facies high-strain overprints were coeval, at c .  550  Ma. Mylonitic processes do not appear to have reset the U–Pb system in zircon, but may have partially disturbed it. The thermal gradient in the Musgrave Block crust at c .  550  Ma was c .  16  °C  km⁻¹ and at c .  535  Ma was c .  18  °C  km⁻¹, based on P – T  estimates of eclogite and greenschist facies shear zones, respectively. These estimates are similar to present-day geothermal gradients in many stable continental shield areas, suggesting that the region did not undergo a significant transient perturbation of the geotherm. Therefore, in the Musgrave Block, cooling subsequent to eclogite facies metamorphism appears to have been controlled by exhumation, rather than by the removal of a heat source. Estimated exhumation rates in the range 0.2 to ≥1.5  mm year⁻¹ are comparable with other orogenic belts, rather than cratonic areas elsewhere.     

Tectonothermal evolution of high-alumina rocks within the Protogine Zone, southern Sweden

Abstract The Protogine Zone comprises a system of anastomosing deformation zones which approximately parallel the eastern boundary of the Sveconorwegian (1200–900 Ma) province in south-west Sweden. Ages of granulite facies metamorphism in the Sveconorwegian province require exhumation from c . 30 to 35 km crustal depths after 920–880 Ma. ⁴⁰Ar/³⁹ Ar cooling ages are presented for muscovite from high-alumina rocks formed by hydrothermal leaching associated with the Protogine Zone. Growth of fabric-defining minerals was associated with a ductile deformational event; muscovite from these rocks cooled below argon retention temperatures ( c . 375 ± 25° C) at c . 965–955 Ma. Muscovite from granofels in zones of intense alteration indicates that temperatures > 375 ± 25° C were maintained until c . 940 Ma. Textural relations of Al₂SiO₅ polymorphs and chloritoid suggest that dated fabrics formed during exhumation. The process of exhumation, brittle overprint on ductile structures and hydrothermal activity along faults within the Protogine Zone tentatively are interpreted as the peripheral effects of initial Neoproterozoic exhumation of the granulite region of south-western Sweden.
Muscovite in phyllonites associated with the 'Sveconorwegian thrust system'cooled below argon retention temperatures at c . 927 Ma. Exhumation associated with this cooling could have been related to extension and onset of brittle-ductile deformation superimposed on Sveconorwegian contraction.     

Thermal evolution of the orogenic lower crust during exhumation within a thickened Moldanubian root of the Variscan belt of Central Europe

At the eastern margin of the Bohemian Massif (Variscan belt of Central Europe), large bodies of felsic granulite preserve mineral assemblages and structures developed during the early stages of exhumation of the orogenic lower continental crust within the Moldanubian orogenic root. The development of an early steep fabric is associated with east–west-oriented compression and vertical extrusion of the high-grade rocks into higher crustal levels. The high-pressure mineral assemblage Grt-Ky-Kfs-Pl-Qtz-Liq corresponds to metamorphic pressures of ∼18 kbar at ∼850 °C, which are minimum estimates, whereas crystallization of biotite occurred at 13 kbar and ∼790 °C during decompression with slight cooling. The late stages of the granulite exhumation were associated with lateral spreading of associated high-grade rocks over a middle crustal unit at ∼4 kbar and ∼700 °C, as estimated from accompanying cordierite-bearing gneisses. The internal structure of a contemporaneously intruded syenite is coherent with late structures developed in felsic granulites and surrounding gneisses, and the magma only locally explored the early subvertical fabric of the felsic granulite during emplacement. Consequently, the emplacement age of the syenite provides an independent constraint on the timing of the final stages of exhumation and allows calculation of exhumation and cooling rates, which for this part of the Variscan orogenic root are 2.9–3.5 mm yr⁻¹ and 7–9.4 °C Myr⁻¹, respectively. The final part of the temperature evolution shows very rapid cooling, which is interpreted as the result of juxtaposition of hot high-grade rocks with a cold upper-crustal lid.     

Middle Miocene high-pressure metamorphism and fast exhumation of the Nevado-Filábride Complex, SE Spain

This study provides new constraints on fast cooling and exhumation rates of high-pressure metamorphic rocks in young active mountain belts. Ion microprobe (SHRIMP) U–Pb analysis of zircon in a pyroxenite layer of the Cerro del Almirez ultramafic rocks (Nevado-Filábride Complex, southern Spain) gave an age of 15.0 ± 0.6 Myr (95% c.l.). Mineral inclusions demonstrate that zircon formed close to the high-pressure peak. Combined with previous fission track data, the 15 Myr age suggests high cooling (˜ 80 °C Myr⁻¹) and exhumation (˜1.2 cm yr⁻¹) rates for the unit. The new results indicate that both the Nevado-Filábride Complex and the overlying Alpujárride Complex, with somewhat higher ages and exhumation rates, underwent similar metamorphic evolutions at different times. This implies that the Alpujárride rocks were exhumed when the Nevado-Filábride was subducting and that the same tectonic scenario propagated from one portion of the Betic Cordilleras to another.     

Apatite fission track analysis in the Argentera massif: evidence of contrasting denudation rates in the External Crystalline Massifs of the Western Alps

Apatite fission track dating from a central transect in the Argentera massif (southernmost External Crystalline Massif = ECM) yielded ages between 8.05 ± 0.6 and 2.4 ± 0.2 Myr, with a positive age/altitude correlation above 3 Ma, 1200 m. Recognising a thermal peak at c . 250°C, 33 Ma, based on stratigraphic, metamorphic and ³⁹Ar/⁴⁰Ar data, the present results suggest a slow cooling rate (8–5°C) for the Argentera massif during the Oligocene–early Pliocene. This rate compares with that from the Pelvoux massif, but contrasts with those observed in the northern ECM (Mont-Blanc and Aar: up to 14°C Myr⁻¹) for the same time interval. This can be related to the different location of the ECM within the collided European margin. At about 3–4 Ma, the denudation rate would have increased up to c . 1 mm yr⁻¹ in the Argentera massif, reaching the same value as in the Belledonne and northern ECM, likely a consequence of Penninic thrust inversion.     

Quantitative temperature-time information from retrograde diffusion zoning in garnet: constraints for the P–T–t history of the Central Black Forest, Germany

Garnet from a kinzigite, a high-grade gneiss from the central Black Forest (Germany), displays a prominent and regular retrograde diffusion zoning in Fe, Mn and particularly Mg. The Mg diffusion profiles are suitable to derive cooling rates using recent datasets for cation diffusion in garnet. This information, together with textural relationships, thermobarometry and thermochronology, is used to constrain the pressure–temperature–time history of the high-grade gneisses. The garnet–biotite thermometer indicates peak metamorphic temperatures for the garnet cores of 730–810  °C. The temperatures for the outer rims are 600–650  °C. Garnet–Al₂SiO₅–plagioclase–quartz (GASP) barometry, garnet–rutile–Al₂SiO₅–ilmenite (GRAIL) and garnet–rutile–ilmenite–plagioclase–quartz (GRIPS) barometry yield pressures from 6–9  kbar. U–Pb ages of monazite of 341±2  Ma date the low- P high- T metamorphism in the central Black Forest. A Rb/Sr biotite–whole rock pair defines a cooling age of 321±2  Ma. The two mineral ages yield a cooling rate of about 15±2  °C Ma⁻¹. The petrologic cooling rates, with particular consideration of the f O₂ conditions for modelling retrograde diffusion profiles, agree with the geochronological cooling rate. The oldest sediments overlying the crystalline basement indicate a minimum cooling rate of 10  °C Ma⁻¹.     

Evidence for high-temperature diffusional creep preserved by rapid cooling of lower crust (North Bohemian shear zone, Czech Republic)

ABSTRACT Volume diffusion and dislocation creep at T  ∼ 800 °C led to high finite strain in granulite and orthogneiss of the Ohře crystalline complex (North Bohemian shear zone). Intragranular creep by volume diffusion is indicated by (i) lobate phase boundaries between feldspar and quartz, and (ii) removal of perthite lamellae and precipitation of tiny, aluminium-rich needles at the margins of K-feldspar. The striking diffusional-creep structures imply high interfacial free energy that has been preserved from equilibration as a result of rapid cooling. U–Pb dating of monazite (342 ± 1 Ma) and ⁴⁰Ar–³⁹Ar dating of muscovite (341 ± 4 Ma) of Kadaň orthogneiss result in a cooling rate of 50 + 25/−17 °C Myr⁻¹. This high value is attributed to collapse-related 'elevator-style' movements along the North Bohemian shear zone, resulting in the juxtaposition of upper crustal rocks of the Tepla–Barrandian unit against lower crustal rocks of the Erzgebirge crystalline complex.     

Metamorphic evolution of the Naga Hills eclogite and blueschist, Northeast India: implications for early subduction of the Indian plate under the Burma microplate

Tectonic slices and lenses of eclogite within mafic and ultramafic rocks of the Early Cretaceous–Eocene Naga Hills ophiolite were studied to constrain the physical conditions of eastward subduction of the Indian plate under the Burma microplate and convergence rate prior to the India–Eurasia collision. Some of the lenses are composed of eclogite, garnet-blueschist, glaucophanite and greenschist from core to margin, representing a retrograde hydrothermal alteration sequence. Barroisite, garnet, omphacite and epidote with minor chlorite, phengite, rutile and quartz constitute the peak metamorphic assemblage. In eclogite and garnet-blueschist, garnet shows an increase in Mg and Fe and decrease in Mn from core to rim. In chlorite in eclogite, Mg increases from core to rim. Inclusions of epidote, glaucophane, omphacite and quartz in garnet represent the pre-peak assemblage. Glaucophane also occurs profusely at the rims of barroisite. The matrix glaucophane and epidote represent the post-peak assemblage. The Fe³⁺ content of garnet-hosted omphacite is higher than that of matrix omphacite, and Fe³⁺ increases from core to rim in matrix glaucophane. Albite occurs in late stage veins. P – T pseudosection analysis indicates that the Naga Hills eclogites followed a clockwise P – T path with prograde metamorphism beginning at ∼1.3 GPa/525 °C and peaking at 1.7–2.0 GPa/580–610 °C, and subsequent retrogression to ∼1.1 GPa/540 °C. A comparison of these P – T conditions with numerical thermal models of plate subduction indicates that the Naga Hills eclogites probably formed near the top of the subducting crust with convergence rates of ∼ 55–100 km Myr⁻¹, consistent with high pre-collision convergence rates between India and Eurasia.     

Origin and evolution of the Escambray Massif (Central Cuba): an example of HP/LT rocks exhumed during intraoceanic subduction

High‐pressure (HP) metabasites from the Sancti Spiritus dome (Escambray massif, Central Cuba) have been studied in order to better understand the origin and evolution of the Northern Caribbean boundary plate during the Cretaceous, in a global subduction context. Geochemical and petrological studies of these eclogites reveal two groups with contrasting origins and pre‐subduction metamorphic histories. Eclogites collected from exotic blocks within serpentinite (mélange zone) originated from a N‐MORB type protolith, do not record pre‐eclogitic metamorphic history. Conversely eclogites intercalated in Jurassic metasedimentary rocks (non‐mélange zone) have a calc‐alkaline arc‐like origin and yield evidence for a pre‐subduction metamorphic event in the amphibolite facies. However, all the studied Escambray eclogites underwent the same eclogitic peak (around 600 °C at 16 kbar), and followed a cold thermal gradient during their exhumation (estimated at around 13.5 °C km^?1), which can suggest that this exhumation was coeval with subduction. Concordant geochronological data (Rb/Sr and Ar/Ar) support that the main exhumation of HP/LT rocks from the Sancti Spiritus dome occurred at 70 Ma by top to SW thrusting. The retrograde trajectory of these rocks suggests that the north‐east subduction of the Farallon plate continued after 70 Ma. The set‐off to the exhumation can be correlated with the beginning of the collision between the Bahamas platform and the Cretaceous island arc that induced a change of the subduction kinematics. The contrasting origin and ante‐subduction history of the analysed samples imply that the Escambray massif consists of different geological units that evolved in different environments before their amalgamation during exhumation to form the present unit III of the massif.     

Contact metamorphism in the Malashan dome, North Himalayan gneiss domes, southern Tibet: an example of shallow extensional tectonics in the Tethys Himalaya

Combined petrographic, structural and geochronological study of the Malashan dome, one of the North Himalayan gneiss domes, reveals that it is cored by a Miocene granite, the Malashan granite, that intruded into the Jurassic sedimentary rocks of Tethys Himalaya. Two other granites in the area are referred to as the Paiku and Cuobu granites. New zircon SHRIMP U-Pb and muscovite and biotite ⁴⁰Ar-³⁹Ar dating show that the Paiku granite was emplaced during 22.2–16.2 Ma (average 19.3 ± 3.9 Ma) and cooled rapidly to 350–400 °C at around 15.9 Ma. Whole-rock granite chemistry suggests the original granitic magma may have formed by muscovite dehydration melting of a protolith chemically similar to the High Himalayan Crystalline Sequence. Abundant calcareous metasedimentary rocks and minor garnet-staurolite-biotite-muscovite ± andalusite schists record contact metamorphism by three granites that intruded intermittently into the Jurassic sediments between 18.5 and 15.3 Ma. Two stages of widespread penetrative ductile deformation, D1 and D2, can be defined. Microstructural studies of metapelites combined with geothermobarometry and pseudosection analyses yield P – T conditions of 4.8 ± 0.8 kbar at 550 ± 50 °C during a non-deformational stage between D1 and D2, and 3.1–4.1 kbar at 530–575 °C during syn- to post-D2. The pressure estimates for the syn- to post-D2 growth of andalusite suggest relatively shallow (depth of ∼15.2 km) extensional ductile deformation that took place within a shear zone of the South Tibetan Detachment System. Close temporal association between intrusion of the Malashan granite and onset of D2 suggests extension may have been triggered by the intrusion of the Malashan granite.     

P–T–t paths and post-metamorphic exhumation of the Day Nui Con Voi shear zone in Vietnam

The Day Nui Con Voi belt in Vietnam is the southeasternmost part of the Red River shear zone in Asia. It is a narrow high-grade metamorphic core complex consisting of garnet–sillimanite–biotite gneisses, mylonite bands, amphibolite layers and migmatites. Geothermobarometric study of the complex revealed that the peak metamorphism took place under amphibolite-facies conditions of 690₋₆₀⁺³⁰°C and 0.65±0.15 GPa and the subsequent mylonitization occurred under greenschist-facies conditions of 480°C and under 0.3 GPa. Fifteen synkinematic hornblende and biotite separates from gneisses, amphibolites and mylonites were dated with the K/Ar method. Hornblende separates from the Day Nui Con Voi give K–Ar ages of 26.4–28.5 Ma, and the biotite separates do give 24.5–24.7 Ma. Combination of thermobarometric and geochronological data yields the cooling history of 500°C at 28 Ma and 300°C at 24 Ma with a cooling rate of 70–110°C Ma⁻¹, and 23 km post-metamorphic exhumation of the core complex. The first 16 km exhumation from the peak of metamorphism (at probably 31 Ma) to 28 Ma was triggered by the left-lateral strike-slip displacement of the Red River shear zone.     

Petrochemical constraints for dual origin of garnet peridotites from the Dabie-Sulu UHP terrane, eastern-central China

Garnet peridotites occur as lenses, blocks or layers within granulite–amphibolite facies gneiss in the Dabie-Sulu ultra-high-pressure (UHP) terrane and contain coesite-bearing eclogite. Two distinct types of garnet peridotite were identified based on mode of occurrence and petrochemical characteristics. Type A mantle-derived peridotites originated from either: (1) the mantle wedge above a subduction zone, (2) the footwall mantle of the subducted slab, or (3) were ancient mantle fragments emplaced at crustal depths prior to UHP metamorphism, whereas type B crustal peridotite and pyroxenite are a portion of mafic–ultramafic complexes that were intruded into the continental crust as magmas prior to subduction. Most type A peridotites were derived from a depleted mantle and exhibit petrochemical characteristics of mantle rocks; however, Sr and Nd isotope compositions of some peridotites have been modified by crustal contamination during subduction and/or exhumation. Type B peridotite and pyroxenite show cumulate structure, and some have experienced crustal metasomatism and contamination documented by high ⁸⁷Sr/⁸⁶Sr ratios (0.707–0.708), low ε_Nd( t ) values (−6 to −9) and low δ¹⁸O values of minerals (+2.92 to +4.52). Garnet peridotites of both types experienced multi-stage recrystallization; some of them record prograde histories. High- P–T  estimates (760–970 °C and 4.0–6.5±0.2 GPa) of peak metamorphism indicate that both mantle-derived and crustal ultramafic rocks were subducted to profound depths >100 km (the deepest may be ≥180–200 km) and experienced UHP metamorphism in a subduction zone with an extremely low geothermal gradient of <5 °C km⁻¹.     

Hot contacts of garnet peridotites in middle/upper crustal levels: new constraints on the nature of the late Variscan high-T/low-P event in the Moldanubian (Central Vosges/NE France)

P–T  paths based on parageneses in the immediate vicinity of former high-temperature contact zones between mantle peridotites and granulitic country rocks of the Central Vosges (NE France) were derived by applying several conventional thermometers and thermobarometric calculations with an internally consistent dataset. The results indicate that former garnet peridotites and garnet–spinel peridotites were welded together with crustal rocks at depths corresponding to 1–1.2 GPa. The temperature of the crustal rocks was about 650–700 °C at this stage, whereas values of 1100 °C (garnet peridotites) and 800–900 °C (garnet–spinel peridotites) were calculated for the ultramafic rocks. After emplacement of the mantle rocks, exhumation of the lower crust took place to a depth corresponding to 0.2–0.3 GPa. The temperatures of the incorporated peridotite slices were still high (900–1000 °C) at this stage. This is indicated by the presence of high- T  /low- P parageneses ( c . 800 °C, 0.2–0.3 GPa) in a small (1–10 m) contact aureole around a former garnet peridotite. Crustal rocks distant to the peridotites equilibrated in the same pressure range at lower temperature (650–700 °C). High cooling rates (10²–10³ °C Ma⁻¹) were calculated for a garnet–biotite rock inclusion in the peridotites and for the crustal rocks at the contact by applying garnet–biotite diffusion modelling. Minimum rates of 0.75–7.5 cm a⁻¹ are required for vertical ascent of rock units (30 km vertical distance) derived from the crust–mantle boundary, resulting in a late Variscan (340 Ma) high- T  /low- P event.     

Excess Ar-free phengite in ultrahigh-pressure metamorphic rocks from the Lago di Cignana area, Western Alps

Ar/Ar analyses of phengites and paragonites from the ultrahigh-pressure metamorphic rocks (zoisite–clinozoisite schist, garnet–phengite schist and piemontite schist) in the Lago di Cignana area, Western Alps were carried out with a laser probe step-heating method using single crystals and a spot dating method on thin sections. Eight phengite and two paragonite crystals give the plateau ages of 37–42 Ma with 96–100% of ³⁹Ar released. Each rock type also contains mica crystals showing discordant age spectra with age fractions (20–35 Ma) significantly younger than the plateau ages. Phengite inclusions in garnet give ages of 43.2 ± 1.1 Ma and 44.4 ± 1.5 Ma, which are significantly older than the spot age (36.4 ± 1.4 Ma) from the matrix phengites, and the plateau ages from the step-heating analyses. Inclusion ages (43 and 44 Ma) are consistent with a zircon SHRIMP age (44 ± 1 Ma) in this area. These results suggest that the oceanic materials that underwent a simple subduction related UHPM, form excess ⁴⁰Ar-free phengite and that the peak metamorphism is ca. 44 Ma or little older. We suggest that matrix phengites experienced a retrogression reaction changing their chemistry contemporaneously with deformation related to the exhumation of rocks releasing significant radiogenic ⁴⁰Ar from the crystals. This has lead to the apparent ages of the matrix phengites that are significantly younger than the inclusion age.     

Permian to Cretaceous polymetamorphic evolution of the Stewart River region, Yukon-Tanana terrane, Yukon, Canada: P–T evolution linked with in situ SHRIMP monazite geochronology

Textural relationships and the trace element chemistry of accessory minerals and garnet can provide the linkage between in situ SHRIMP ages and quantitative pressure–temperature data that is required to decipher complex polymetamorphic and polydeformational histories. Application of these methods to lower amphibolite facies rocks of the Stewart River area, Yukon (Canada) yields robust new constraints on the tectonic evolution of central Yukon Tanana Terrane (YTT).
A TIMS U/Pb titanite age of 365–350 Ma is interpreted to date low- P metamorphism (M1) and D1 deformation associated with arc plutonism above an east-dipping subduction zone. Monazite inclusions in garnet porphyroblasts record a transition from low to high pressure (∼9 kbar and 600 °C) at c . 239 Ma. These data help to establish a c . 260–240 Ma tectonometamorphic event (M2–D2) reflecting intra-arc thickening during west-dipping subduction of Slide Mountain Ocean. Another transition from low- to high- P (M3–D3; 7.8 kbar and 595 °C), dated by c . 195–187 Ma monazite, is interpreted to reflect the change from regional contact metamorphism during arc plutonism to internal duplication of YTT during initial collision of YTT with the North American craton.
The Mt Burnham (north-eastern) region records a different history because of its proximity to later plutons and its late exhumation via extensional faulting. Monazite growth at 146 Ma dates ∼9 kbar metamorphism (M4), interpreted to reflect a previously unrecognized period of plutonism associated with auriferous quartz veins in the Klondike region. Monazite growth at 114–107 Ma reflects low- P (<4.6 kbar) contact metamorphism (M5) accompanying regional plutonism and extension.     

Intracontinental subduction: a possible mechanism for the Early Palaeozoic Orogen of SE China

The Early Palaeozoic Orogen of SE China consists of three litho-tectonic elements, from top to bottom: a sedimentary Upper Unit, a metamorphic Lower Unit and a gneissic basement. The boundaries between these units are flat lying, south directed, ductile decollements. The lower one is coeval with an amphibolite facies metamorphism (M1). The belt is reworked by migmatite–granite domes, high-temperature metamorphism (M2) and granitic plutons related to post-orogenic crustal melting. We date here the syn-M1 ductile shearing at 453 ± 7 Ma by U-Th/Pb method on monazite. Previous ages and our new ⁴⁰Ar/³⁹Ar ages of biotites and muscovites show that the metamorphic rocks experienced syn-M2 exhumation from 440 to 400 Ma. The Early Palaeozoic Orogen of SE China is an intracontinental belt in which decollements accommodated the north-directed subduction of the Cathaysian continent. This orogen is an example of intracontinental subduction that was not preceded by oceanic subduction.     

Pan-African eclogite facies metamorphism of ultramafic rocks in the Shackleton Range, Antarctica

In the Shackleton Range of East Antarctica, garnet-bearing ultramafic rocks occur as lenses in supracrustal high-grade gneisses. In the presence of olivine, garnet is an unmistakable indicator of eclogite facies metamorphic conditions. The eclogite facies assemblages are only present in ultramafic rocks, particularly in pyroxenites, whereas other lithologies – including metabasites – lack such assemblages. We conclude that under high-temperature conditions, pyroxenites preserve high-pressure assemblages better than isofacial metabasites, provided the pressure is high enough to stabilize garnet–olivine assemblages (i.e. ≥18–20 kbar). The Shackleton Range ultramafic rocks experienced a clockwise P–T path and peak conditions of 800–850 °C and 23–25 kbar. These conditions correspond to ∼70 km depth of burial and a metamorphic gradient of 11–12 °C km⁻¹ that is typical of a convergent plate-margin setting. The age of metamorphism is defined by two garnet–whole-rock Sm–Nd isochrons that give ages of 525 ± 5 and 520 ± 14 Ma corresponding to the time of the Pan-African orogeny. These results are evidence of a Pan-African suture zone within the northern Shackleton Range. This suture marks the site of a palaeo-subduction zone that likely continues to the Herbert Mountains, where ophiolitic rocks of Neoproterozoic age testify to an ocean basin that was closed during Pan-African collision. The garnet-bearing ultramafic rocks in the Shackleton Range are the first known example of eclogite facies metamorphism in Antarctica that is related to the collision of East and West Gondwana and the first example of Pan-African eclogite facies ultramafic rocks worldwide. Eclogites in the Lanterman Range of the Transantarctic Mountains formed during subduction of the palaeo-Pacific beneath the East Antarctic craton.     

福建紫金山矿田深部似斑状花岗闪长岩锆石SHRIMP U-Pb和单矿物~(40)Ar/~(39)Ar年龄及其地质意义

通过对福建紫金山矿田深部与成矿作用有关的主期似斑状花岗闪长岩3组锆石SHRIMP U-Pb和2组角闪石、钾长石~(40)Ar/~(39)Ar测年,获得锆石~(206)Pb/~(238()U加权平均年龄为101.8±1.5 Ma(n=34,MSWD=1.0),代表紫金山矿田深部与成矿作用有关的主期似斑状花岗闪长岩的成岩年龄;同时获得角闪石~(40)Ar/~(39)Ar冷却年龄为100±11 Ma、102.2 Ma,钾长石的~(40)Ar/~(39)Ar冷却年龄为96.3±1.7 Ma、98.5 Ma。依据矿物封闭温度理论,估算紫金山矿田深部与成矿作用有关的主期似斑状花岗闪长岩由锆石结晶至角闪石40Ar/39Ar体系封闭、再到钾长石~(40)Ar/~(39)Ar体系封闭的岩石冷却速率分别是40.7~67.1℃/Ma、116.9~216.3℃/Ma,显示岩石的冷却速率较大;由古地温梯度推算主期似斑状花岗闪长岩结晶(101.8±1.5 Ma)至钾长石~(40)Ar/~(39)Ar体系封闭(96.3±1.7 Ma)期间岩体隆升剥露了约3 km,暗示地壳在这一时期发生了快速隆升剥蚀作用。紫金山矿田深部似斑状花岗闪长岩锆石206Pb/238U年龄佐证了紫金山矿田深部存在一个大岩基,并约束了紫金山矿田斑岩型矿床的成矿时代,单矿物的~(40)Ar/~(39)Ar年龄为矿区的隆升剥露研究提供新资料。     

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.