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.     

Fifty-five-million-year history of oceanic subduction and exhumation at the northern edge of the Caribbean plate (Sierra del Convento mélange, Cuba)

Petrological and geochronological data of six representative samples of exotic blocks of amphibolite and associated tonalite-trondhjemite from the serpentinitic mélange of the Sierra del Convento (eastern Cuba) indicate counterclockwise P–T paths typical of material subducted in hot and young subduction zones. Peak conditions attained were ∼750 °C and 15 kbar, consistent with the generation of tonalitic partial melts observed in amphibolite. A tonalite boulder provides a U-Pb zircon crystallization age of 112.8 ± 1.1 Ma, and Ar/Ar amphibole dating yielded two groups of cooling ages of 106–97 Ma (interpreted as cooling of metamorphic/magmatic pargasite) and 87–83 Ma (interpreted as growth/cooling of retrograde overprints). These geochronological data, in combination with other published data, allow the following history of subduction and exhumation to be established in the region: (i) a stage of hot subduction 120–115 Ma, developed upon onset of subduction; (ii) relatively fast near-isobaric cooling (25 °C Myr⁻¹) 115–107 Ma, after accretion of the blocks to the upper plate lithospheric mantle; (iii) slow syn-subduction cooling (4 °C Myr⁻¹) and exhumation (0.7 km Myr⁻¹) in the subduction channel 107–70 Ma; and (iv) fast syn-collision cooling (74 °C Myr⁻¹) and exhumation (5 km Myr⁻¹) 70–60 Ma.     

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.     

Exhumation of the Central Alps: evidence from 40Ar/39Ar laserprobe dating of detrital white micas from the Swiss Molasse Basin

⁴⁰Ar/³⁹Ar single-grain laserprobe dating of detrital white micas from early Oligocene to middle Miocene (31–14 Ma) sedimentary rocks of the central Swiss Molasse basin reveals three distinct clusters of cooling ages for the hinterland. Two Palaeozoic age clusters reflect cooling after the Variscan orogeny with only limited reheating during the Alpine orogeny. The third Tertiary age cluster reflecting late Alpine cooling is restricted to sediments younger than 20 Myr old. Micas with cooling ages < 30 Myr are interpreted to originate from the footwall of the Simplon detachment fault, thus representing formerly exposed upper levels of the present-day Lepontine metamorphic dome. Erosion of these levels is reflected by an increase of low-grade metamorphic lithic grains in the sandstones. This interpretation puts constraints on the timing of exhumation as well as on the evolution of the drainage pattern of the Central Alps.     

Cretaceous high-temperature rapid loading and unloading in the Abukuma metamorphic terrane, Japan

The Cretaceous Abukuma metamorphic terrane consists of the oceanic Gosaisyo Series overthrust onto the terrigenous Takanuki Series. Although the dominant mineralogy defines one of the classic areas of andalusite–sillimanite type progressive metamorphism, there are several lines of evidence suggesting an earlier higher-pressure (up to c . 12  kbar) history of the Takanuki Series and the nearby Gosaisyo Series. These are: (1) the occurrence of rare although widespread relic kyanite in sillimanite+K-feldspar zone-grade pelitic rocks; (2) the high grossular content of garnet interiors (up to c . 30  mol %) overgrown by Ca-poor rims ( c . 2  mol % grossular) in pelitic rocks containing Al₂SiO₅ minerals (sillimanite±relic kyanite±retrograde andalusite), plagioclase and quartz; (3) the occurrence of rutile as inclusions in garnet in pelitic rocks; and (4) the occurrence of relic corundum+almandine association in silica-poor and Al–Fe-rich rocks. Garnet in the Takanuki Series pelitic rocks commonly shows textural sector zoning and preserves growth zoning despite the high metamorphic grade, suggesting rapid changes in P–T  conditions and a relatively short duration of high-temperature conditions. Combined with radiometric dating, these observations suggest that the Abukuma sillimanite+K-feldspar zone-grade rocks underwent a clockwise P–T  path with very fast (>4  mm  y⁻¹) average burial and exhumation rates.     

An early Miocene age for a high-temperature event in gneisses from Zabargad Island (Red Sea,Egypt): mantle diapirism?

U–Pb zircon data from a felsic gneiss located at the contact zone with the central peridotite body of Zabargad Island (Red Sea, Egypt) provide an age of 23.2 ± 5.9 Myr consistent with the ²³⁸U–²⁰⁶Pb age of the youngest concordant grain (22.4 ± 1.3 Myr). Concordant grains indicate new zircon growth and/or resetting whereas slightly discordant analyses suggest participation of an older zircon component whose age cannot be defined precisely. SEM back-scattered imaging further reveals the occurrence of zoned domains almost completely erased by complex internal structures attributed to extensive recrystallization under metamorphic conditions. The 22.4 Myr Miocene age is thus interpreted as dating a high-temperature metamorphic event. The proximity between the studied sample and the peridotite supports previous conclusions which regard parts of the peridotites from Zabargad Island as an asthenopheric mantle diapir which intruded the thinned Pan-African continental crust during the early stages of the Red Sea opening.     

Evidence from U–Pb zircon and 40Ar/39Ar muscovite detrital mineral ages in metasandstones for movement of the Torlesse suspect terrane around the eastern margin of Gondwanaland

New U–Pb detrital zircon ages from Triassic metasandstones of the Torlesse Terrane in New Zealand are compared with ⁴⁰Ar/³⁹Ar muscovite data and together, reveal four main source components: (i) major, Triassic–Permian (210–270 Myr old) and (ii) minor, Permian–Carboniferous (280–350 Myr old) granitoids (recorded in zircon and muscovite data); (iii) minor, early middle Palaeozoic, metamorphic rocks, recorded mainly by muscovite, 420–460 Myr old, and (iv) minor, Late Precambrian–Cambrian igneous and metamorphic complexes, 480–570 Myr old, recorded by zircon only. There are also Proterozoic zircon ages with no clear grouping (580–1270 Myr). The relative absence of late Palaeozoic (350–420 Myr old) components excludes granitoid terranes in the southern Lachlan Fold Belt (Australia) and its continuation into North Victoria Land (East Antarctica) and Marie Byrd Land (West Antarctica) as a potential source for the Torlesse. The age data are compatible with derivation from granitoid terranes of the northern New England Orogen (and hinterland) in NE Australia. This confirms that the Torlesse Terrane of New Zealand is a suspect terrane, that probably originated at the NE Australian, Permian–Triassic, Gondwanaland margin and then (200–120 Ma) moved 2500 km southwards to its present New Zealand position by the Late Cretaceous (90 Ma). This sense of movement is analogous to that suggested for Palaeozoic Mesozoic terranes at the North American Pacific margin.     

冀西北怀安杂岩的年代学、地球化学、Nd-Hf-O同位素组成及其地质意义

冀西北地区怀安杂岩由变质表壳岩和变质深成岩组成,其中变质表壳岩的形成时代、怀安杂岩的构造背景以及其与孔兹岩带间的关系一直存在较大争议.本文对怀安杂岩的几处代表性露头进行了详细野外考察,对4件样品进行了岩石学、锆石SHRIMP U-Pb定年、同位素和元素地球化学分析.所有样品都给出了1.86~1.81 Ga的变质锆石年龄,进一步支持怀安杂岩广泛遭受到古元古代晚期变质作用改造的认识.侵入/包裹含BIF表壳岩组合的变质辉长岩（HB1425）和片麻状英云闪长岩（HB1426）分别给出了~2.5 Ga和2.55 Ga的形成年龄,限制表壳岩形成时代老于2.55 Ga,推测为新太古代表壳岩.浅粒岩（HB1431）和紫苏石榴黑云斜长片麻岩（HB1435）中最老的碎屑锆石分别为2.46 Ga和2.51 Ga,可能还存在古元古代的碎屑锆石,表明它们都为古元古代表壳岩.上述结果进一步确定了怀安杂岩中发育两期表壳岩组合.变质辉长岩和片麻状英云闪长岩的全岩ε_Nd（t）、T_DM1和T_DM2分别为+2.19~+3.06、2.67~2.75 Ga和2.67~2.69 Ga,表明其物源区不存在较大规模的古老陆壳物质,新太古代是怀安地区主要陆壳生长期.变质辉长岩中~1.82 Ga变质锆石中较多具有正的ε_Hf（t）值,最高可达11.1,最可能的解释是古元古代变质过程存在地幔添加作用.锆石的O同位素分析显示区域上可能存在低δ18O的岩石,在古元古代变质过程中,可能存在低δ18O流体对锆石的改造.怀安杂岩和西部孔兹岩带中不同类型岩石的比例明显不同,但两者都同样发育新太古代和古元古代的双层地壳结构,怀安杂岩或许代表孔兹岩带剥蚀更深而出露的深部地壳部分.      

Alpine reworking of Ordovician protoliths in the Western Carpathians: Geochronological and geochemical data on the Muráñ Gneiss Complex, Slovakia

Magmatic protoliths of Ordovician age have been identified in the metamorphic rocks of the Muráñ Gneiss Complex, Veporic Unit (Central Western Carpathians). Vapor digestion single zircon U–Pb dating yields an intrusion age of 464 ± 35 Ma (upper intercept) for the granite protolith. A lower intercept age of 88 ± 40 Ma records amphibolite-facies metamorphic overprint in the Cretaceous, during the Alpine orogeny. Geochemical and isotopic data suggest crustal origin of the orthogneiss. Nd_initial are between − 2.6 and − 5.0 and T_DM^Nd between 1.3 and 1.5 Ga (two-step approach). ⁸⁷Sr / ⁸⁶Sr_initial ratios vary between 0.7247 and 0.7120, and a steep REE pattern further constrains the crustal affinity of these rocks. Associated amphibolite bodies have Nd_initial values of 6.5, ⁸⁷Sr / ⁸⁶Sr_initial ratio of 0.7017, and a flat REE pattern. They are interpreted as MORB derived metabasites. Whole-rock Pb isotope analyses define a linear array in a ²⁰⁶Pb / ²⁰⁴Pb vs. ²⁰⁷Pb / ²⁰⁴Pb diagram with an age of ca. 134 Ma, consistent with intense Alpine metamorphism and deformation.

These basement rocks of the Central Western Carpathians are interpreted as Ordovician magmatic rocks intruded at an active margin of Gondwana. They represent the eastern prolongation of Cambro–Ordovician units of the European Variscides, which were part of the peri-Gondwana superterrane and accreted to Laurussia during the Variscan orogeny. Variscan metamorphic overprint is not recorded by the isotopic data of the Muráñ Gneiss Complex. Alpine metamorphism is the most dominant overprint.     

Metamorphic phase relations in orthopyroxene-bearing granitoids: implication for high-pressure metamorphism and prograde melting in the continental crust

In this work, the factors controlling the formation and preservation of high-pressure mineral assemblages in the metamorphosed orthopyroxene-bearing metagranitoids of the Sandmata Complex, Aravalli-Delhi Mobile Belt (ADMB), northwestern India have been modelled. The rocks range in composition from farsundite through quartz mangerite to opdalite, and with varying K₂O, Ca/(Ca + Na)_rock and FeO_tot + MgO contents. A two stage metamorphic evolution has been recorded in these rocks.
An early hydration event stabilized biotite with or without epidote at the expense of magmatic orthopyroxene and plagioclase. Subsequent high-pressure granulite facies metamorphism (∼15 kbar, ∼800 °C) of these hydrated rocks produced two rock types with contrasting mineralogy and textures. In the non-migmatitic metagranitoids, spectacular garnet ± K-feldspar ± quartz corona was formed around reacting biotite, plagioclase, quartz and/or pyroxene. In contrast, biotite ± epidote melting produced migmatites, containing porphyroblastic garnet incongruent solids and leucosomes.
Applying NCKFMASHTO T–M (H₂O) and P–T pseudosection modelling techniques, it is demonstrated that the differential response of these magmatic rocks to high-pressure metamorphism is primarily controlled by the scale of initial hydration. Rocks, which were pervasively hydrated, produced garnetiferous migmatites, while for limited hydration, the same metamorphism formed sub-solidus garnet-bearing coronae. Based on the sequence of mineral assemblage evolution and the mineral compositional zoning features in the two metagranitoids, a clockwise metamorphic P–T path is constrained for the high-pressure metamorphic event. The finding has major implications in formulating geodynamic model of crustal amalgamation in the ADMB.     

Zircon U–Pb age, trace element and Hf isotope composition of Kongling terrane in the Yangtze Craton: refining the timing of Palaeoproterozoic high-grade metamorphism

U–Pb age, trace element and Hf isotope compositions of zircon were analysed for a metasedimentary rock and two amphibolites from the Kongling terrane in the northern part of the Yangtze Craton. The zircon shows distinct morphological and chemical characteristics. Most zircon in an amphibolite shows oscillatory zoning, high Th/U and ¹⁷⁶Lu/¹⁷⁷Hf ratios, high formation temperature, high trace element contents, clear negative Eu anomaly, as well as HREE-enriched patterns, suggesting that it is igneous. The zircon yields a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2857 ± 8 Ma, representing the age of the magmatic protolith. The zircon in the other two samples is metamorphic. It has low Th/U ratios, low trace element concentrations, variable HREE contents (33.8 ≥ Lu_N≥2213; 14.7 ≤ Lu_N/Sm_N ≤ 354) and ¹⁷⁶Lu/¹⁷⁷Hf ratios (0.000030–0.001168). The data indicate that the zircon formed in the presence of garnet and under upper amphibolite facies conditions. The metamorphic zircon yields a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2010 ± 13 Ma. These results combined with previously obtained Palaeoproterozoic metamorphic ages suggest a c. 2.0 Ga Palaeoproterozoic collisional event in the Yangtze Craton, which may result from the assembly of the supercontinent Columbia. The zircon in two samples yields weighted mean two-stage Hf model ( T _DM2) ages of 3217 ± 110 and 2943 ± 50 Ma, respectively, indicating that their protoliths were mainly derived from Archean crust.     

Late Miocene–Pliocene eclogite facies metamorphism, D'Entrecasteaux Islands, SE Papua New Guinea

The D'Entrecasteaux Islands of south‐eastern Papua New Guinea are active metamorphic core complexes that formed within a region where the plate tectonic regime has transitioned from subduction to rifting. While rapid, post 4 Myr exhumation and cooling of amphibolite and greenschist facies rocks that constitute the footwall of the crustal scale detachment fault system have been previously documented on Fergusson and Goodenough Islands of the D'Entrecasteaux chain, the timing of eclogite facies metamorphism in rocks of the footwall was unknown. Recent work revealed that at least one of the eclogite bodies formed during the Pliocene. We present combined in situ ion microprobe U–Pb age analyses of zircon from five variably retrogressed eclogite samples from Fergusson and Goodenough Islands that document Late Miocene–Pliocene (8–2 Ma) eclogite formation on these islands. Textural relationships and zircon–garnet rare earth element partition coefficients indicate that U–Pb ages constrain zircon crystallization under eclogite facies conditions in all samples. Results suggest westward younging of eclogite facies metamorphism from Fergusson to Goodenough Island. Present‐day exposure of Late Miocene–Pliocene eclogites requires exhumation rates > 2.5 cm yr^?1.     

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.     

Oligo-Miocene granitic magmatism in central Vietnam and implications for continental deformation in Indochina

Two granitoid intrusions within the Bu Khang extensional complex in central Vietnam have been dated by U–Pb and Rb–Sr geochronology. A monazite U–Pb age of 26.0 ± 0.2 (2σ) Myr was obtained for the Bu Khang pluton and 23.7 + 1.6/–1.7 Myr for monazite, allanite and zircon from the Dai Loc intrusion. These ages date crystallization of magmas previously assigned Precambrian to Devonian. Rb–Sr analyses of K-feldspar and biotite fractions from the samples yield ages of 19.8 ± 0.6 (2σ) Myr and 19.6 ± 0.5 Myr, respectively. The thermal history recorded by the different geochronometers implies an average exhumation rate of ∼2 mm yr⁻¹ corresponding to ∼9 km of unroofing. Magmatism was either (i) induced passively by lithospheric thinning driven by changes in regional tectonic stresses, or (ii) triggered actively by an ascending plume. Tertiary exhumation and magmatism documented elsewhere in Indochina (e.g. Ailao Shan-Red River and Wang Chao shear zones) favours a regional tectonic cause for extension and granitoid magmatism in the Bu Khang complex. On the other hand, the presence of an upwelling thermal anomaly since at least 35 Ma, causing mantle melting below Indochina, is supported by shear-wave velocity variations in the mantle, and source geochemistry of both the Bu Khang plutons and the Red River belt intrusions. In either case, Tertiary exhumation of the Bu Khang complex can account for previously undocumented NE–SW-directed extension, which is required in northern Vietnam to account for structural changes related to the opening of the South China Sea.     

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.     

Late Proterozoic tectonic events in southern Finland, constrained by 40Ar/39Ar incremental heating and single spot fusion experiments on K-feldspars

The post-Svecofennian tectonic development of southern Finland is controlled by intrusion of rapakivi granites (and associated rocks), reactivation of Svecofennian wrench zones, formation of sedimentary basins and successive intrusion of olivine dolerite dykes and sills. Relative age determinations have revealed that fault reactivation acted before, simultaneously and after intrusion of the rapakivi granites. Results of ⁴⁰Ar/³⁹Ar geochronometry of the Porkkala–Mäntsälä fault (30 km west of Helsinki) reveal ages predominantly in the range 950–1300 Myr. These ages are all significantly younger than the intrusion age of the rapakivi granites. It is suggested that these ages represent tectonic events related to the intrusion of olivine dolerite dykes and sills in SW Finland and the Sveconorwegian Orogeny active further west. ⁴⁰Ar/³⁹Ar ages of a sample taken from the Obbnäs granite (U–Pb zircon ages of 1645 ± 5 Myr) show ages predom-inantly in the range of 1400–1550 Myr. These ages are suggested to represent either cooling ages of the granite or ages associated with the formation of the sedimentary grabens.     

Pressure–temperature conditions and retrograde paths of eclogites, garnet–glaucophane rocks and schists from South Sulawesi, Indonesia

High-pressure metamorphic rocks exposed in the Bantimala area, c . 40  km north-east of Ujung Pandang, were formed as a Cretaceous subduction complex with fault-bounded slices of melange, chert, basalt, turbidite, shallow-marine sedimentary rocks and ultrabasic rocks. Eclogites, garnet–glaucophane rocks and schists of the Bantimala complex have estimated peak temperatures of T  =580–630 °C at 18  kbar and T  =590–640 °C at 24  kbar, using the garnet–clinopyroxene geothermometer. The garnet–omphacite–phengite equilibrium is used to estimate pressures. The distribution coefficient K _D1=[( X _pyr)³( X _grs)⁶/( X _di)⁶]/[(Al/Mg)_M2,wm (Al/Si)_T2,wm]³ among omphacite, garnet and phengite is a good index for metamorphic pressures. The K _D1values of the Bantimala eclogites were compared with those of eclogites with reliable P–T  estimates. This comparison suggests that peak pressures of the Bantimala eclogites were P =18–24  kbar at T  =580–640 °C. These results are consistent with the P–T  range calculated using garnet–rutile–epidote–quartz and lawsonite–omphacite–glaucophane–epidote equilibria.     

Source of coesite inclusions within inherited magmatic zircon from Sulu UHP rocks, eastern China, and their bearing for fluid–rock interaction and SHRIMP dating

The 5-km deep Chinese Continental Scientific Drilling Main Hole penetrated a sequence of ultrahigh pressure (UHP)-metamorphic rocks consisting mainly of eclogite, gneiss and garnet-peridotite with minor schist and quartzite. Zircon separates taken from thin layers of schist and gneiss within eclogite were investigated. Cathodoluminescence images of zircon grains show that they have oscillatory zoned magmatic cores and unzoned to patchy zoned metamorphic rims. Zircon rims contain rare coesite and calcite inclusions whereas cores contain inclusions of both low- P minerals (e.g. feldspar, biotite and quartz) and coesite and other eclogite-facies minerals such as phengite and jadeite. The zircon cores give highly variable ²⁰⁶Pb/²³⁸U ages ranging from 760 to 431 Ma for schist and from 698 to 285 Ma for gneiss, and relatively high but variable Th/U ratios (0.16–1.91). We suggest that the coesite and other eclogite facies mineral inclusions in zircon cores were not magmatic but formed through metasomatic processes caused by fluids during UHP metamorphism, and that the fluids contain components of SiO₂, Al₂O₃, K₂O, FeO, MgO, Na₂O and H₂O. Metasomatism of the Sulu UHP rocks during continental subduction to mantle depths has partly altered magmatic zircon cores and reset isotopic systems. This study provides key evidence that mineral inclusions within magmatic zircon domains are not unequivocal indicators of the formation conditions of the respective domain. This finding leads us to conclude that the routine procedure for dating of metamorphic events solely based on the occurrence of mineral inclusions in zoned zircon could be misleading and the data should be treated with caution.     

滇西澜沧江构造带中-南段澜沧群变质变形期次及Ar-Ar年代学约束

澜沧群变质变形过程的厘定有助于揭示澜沧江构造带的演化历史.通过对双江-惠民地区出露的中-低绿片岩相变质岩系与高压变质岩系的"构造-变质"双重解析,发现澜沧群变质岩系:（1）以挤压体制下的褶劈理S₂为区域性面理,浅变质岩系中石英脉条带代表的面理S₁与层理S₀产状一致,并在第2期强烈压扁后与S₂近平行;（2）构造样式与变质变形期次在研究区域内基本一致,表现为二叠纪-三叠纪两期次中深构造层次透入性变形为侏罗纪-新生代浅-表层次变形所叠加;（3）高压变质岩记录了两个期次的深层次构造变形,电气石与石榴石变斑晶具有明显的两阶段生长模式,核部（M_1aD_1a）可能代表了峰期榴辉岩相变质作用,边部代表折返过程中的蓝片岩相（M_1bD_1b）阶段.高压变质岩与浅变质岩第2期（M₂D₂）构造层次及变形样式一致,代表折返后的挤压拼贴过程,两者的剪切型褶劈理S₂均指示上盘S向的剪切滑动.结合新生代三江地区发生的近90°块体旋转,恢复第2期为上盘指向E的逆冲剪切作用.通过对幸福变质石英砂岩白云母、小黑江多硅白云母和蓝闪石的40Ar-39Ar定年,以及对前人小黑江蓝闪石40Ar-39Ar年龄的重新解释,共厘定出澜沧群4个构造阶段:（1）高压变质岩第1期晚期（D_1b）蓝片岩相变质作用（~250 Ma）;（2）第2期（M₂D₂）中层次挤压（215~214 Ma）;（3）浅变质岩系第3期早期（D_3a）N-S向纵弯褶皱（111.9~103.7 Ma）与晚期（D_3b）逆冲（~82.28 Ma）;（4）第4期近E-W向纵弯褶皱（新生代,很可能晚于59.18 Ma）.