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1.
Controversy over the plate tectonic affinity and evolution of the Saxon granulites in a two‐ or multi‐plate setting during inter‐ or intracontinental collision makes the Saxon Granulite Massif a key area for the understanding of the Palaeozoic Variscan orogeny. The massif is a large dome structure in which tectonic slivers of metapelite and metaophiolite units occur along a shear zone separating a diapir‐like body of high‐P granulite below from low‐P metasedimentary rocks above. Each of the upper structural units records a different metamorphic evolution until its assembly with the exhuming granulite body. New age and petrologic data suggest that the metaophiolites developed from early Cambrian protoliths during high‐P amphibolite facies metamorphism in the mid‐ to late‐Devonian and thermal overprinting by the exhuming hot granulite body in the early Carboniferous. A correlation of new Ar–Ar biotite ages with published P–T–t data for the granulites implies that exhumation and cooling of the granulite body occurred at average rates of ~8 mm/year and ~80°C/Ma, with a drop in exhumation rate from ~20 to ~2.5 mm/year and a slight rise in cooling rate between early and late stages of exhumation. A time lag of c. 2 Ma between cooling through the closure temperatures for argon diffusion in hornblende and biotite indicates a cooling rate of 90°C/Ma when all units had assembled into the massif. A two‐plate model of the Variscan orogeny in which the above evolution is related to a short‐lived intra‐Gondwana subduction zone conflicts with the oceanic affinity of the metaophiolites and the timescale of c. 50 Ma for the metamorphism. Alternative models focusing on the internal Variscan belt assume distinctly different material paths through the lower or upper crust for strikingly similar granulite massifs. An earlier proposed model of bilateral subduction below the internal Variscan belt may solve this problem.  相似文献   

2.
Pre-kinematic greenschist facies metamorphism is often observed in granites and basement units of mountain belts, but rarely dated and accounted for in orogenic cycle reconstructions. Studying pre-kinematic alteration is challenging because of its usual obliteration by subsequent syn-kinematic metamorphism often occurring at conditions typical of the brittle–ductile transition. It is, however, to be expected that pre-kinematic alteration has major implications for the rheology of the upper crust. In the 305 Ma-old Variscan basement of the Bielsa massif (located in the Axial Zone of the Pyrenees), successive fluid–rock interaction events are recorded in granites below 350°C. Combined microstructural and petrographic analysis, low-T thermobarometry and in situ U–Th/Pb dating of anatase, titanite and monazite show extensive pre-orogenic (pre-Alpine) and pre-kinematic alteration related to feldspar sericitization and chloritization of biotite and amphibole at temperatures of 270–350°C at 230–300 Ma. This event is followed by a second fluid–rock interaction stage marked by new crystallization of phyllosilicates at 200–280°C and is associated with the formation of mylonitic shear zones and fractures parallel to the shear planes. U–Pb anatase and monazite ages as well as the microtextural relationships of accessory minerals suggest an age for this event at 40–70 Ma, consistent with independent regional geology constraints. The Variscan basement was therefore softened at late to post-Variscan time, at least 150–200 Ma before the main Alpine shortening while Alpine-age compression (c. 35–50 Ma) leads to the formation of a dense net of mylonites. The associated deformation, both distributed at the scale of the Bielsa massif and localized at decametric scale in mylonitic corridors, precedes the strain localization along the major thrusts of the Axial Zone. The Bielsa massif is a good example where inherited, pre-orogenic upper crustal softening controls the deformation patterns in granitic basement units through low-grade metamorphic reactions.  相似文献   

3.
The Windmill Islands region in Wilkes Land, east Antarctica, preserves granulite facies metamorphic mineral assemblages that yield seemingly comparable P–T estimates from conventional thermobarometry and mineral equilibria modelling. This is uncommon in granulite facies terranes, where conventional thermobarometry and phase equilibria modelling generally produce conflicting P–T estimates because peak mineral compositions tend to be modified by retrograde diffusion processes. In situ U–Pb monazite geochronology and calculated metamorphic phase diagrams show that the Windmill Islands experienced two phases of high thermal gradient metamorphism during the Mesoproterozoic. The first phase of metamorphism is recorded by monazite ages in two widely separated samples and occurred at c. 1,305 Ma. This event was regional in extent, involved crustally derived magmatism and reached conditions of ~3.2–5 kbar and 690–770°C corresponding to very high thermal gradients of >150°C/kbar. The elevated thermal regime is interpreted to reflect a period of extension or increased extension in a back‐arc setting that existed prior to c. 1,330 Ma. The first metamorphic event was overprinted by granulite facies metamorphism at c. 1,180 Ma that was coeval with the intrusion of charnockite. This event involved peak temperatures of ~840–850°C and pressures of ~4–5 kbar. A phase of granitic magmatism at c. 1,250–1,210 Ma, prior to the intrusion of the charnockite, is interpreted to reflect a phase of compression within an overall back‐arc setting. Existing conventional thermobarometry suggests conditions of ~4 kbar and 750°C for M1 and 4–7 kbar and 750–900°C for M2. The apparent similarities between the phase equilibria modelling and existing conventional thermobarometry may suggest either that the terrane cooled relatively quickly, or that the P–T ranges obtained from conventional thermobarometry are sufficiently imprecise that they cover the range of P–T conditions obtained in this study. However, without phase equilibria modelling, the veracity of existing conventional P–T estimates cannot be evaluated. The calculated phase diagrams from this study allow the direct comparison of P–T conditions in the Windmill Islands with phase equilibria models from other regions in the Musgrave–Albany–Fraser–Wilkes Orogen. This shows that the metamorphic evolution of the Wilkes Land region is very similar to that of the eastern Albany–Fraser Orogen and Musgrave Province in Australia, and further demonstrates the remarkable consistency in the timing of metamorphism and the thermal gradients along the ~5,000 km strike length of this system.  相似文献   

4.
Migmatites comprise a minor volume of the high‐grade part of the Damara orogen of Namibia that is dominated by granite complexes and intercalated metasedimentary units. Migmatites of the Southern Central Zone of the Damara orogen consist of melanosomes with garnet+cordierite+biotite+K‐feldspar, and leucosomes, which are sometimes garnet‐ and cordierite‐bearing. Field evidence, petrographic observations, and pseudosection modelling suggest that, in contrast to other areas where intrusion of granitic magmas is more important, in situ partial melting of metasedimentary units was the main migmatite generation processes. Pseudosection modelling and thermobarometric calculations consistently indicate that the peak‐metamorphic grade throughout the area is in the granulite facies (~5 kbar at ~800°C). Cordierite coronas around garnet suggest some decompression from peak‐metamorphic conditions and rare andalusite records late, near‐isobaric cooling to <650°C at low pressures of ~3 kbar. The inferred clockwise P–T path is consistent with minor crustal thickening through continent–continent collision followed by limited post‐collisional exhumation and suggests that the granulite facies terrane of the Southern Central Zone of the Damara orogen formed initially in a metamorphic field gradient of ~35–40°C/km at medium pressures. New high‐precision Lu–Hf garnet‐whole rock dates are 530 ± 13 Ma, 522.0 ± 0.8 Ma, 520.8 ± 3.6 Ma, and 500.3 ± 4.3 Ma for the migmatites that record temperatures of ~800°C. This indicates that high‐grade metamorphism lasted for c. 20–30 Ma, which is compatible with previous estimates using Sm–Nd garnet‐whole rock systematics. In previous studies on Damara orogen migmatites where both Sm–Nd and Lu–Hf chronometers have been applied, the dates (c. 520–510 Ma) agree within their small uncertainties (0.6–0.8% for Sm–Nd and 0.1–0.2% for Lu–Hf). This implies rapid cooling after high‐grade conditions and, by implication, rapid exhumation at that time. The cause of the high geothermal gradient inferred from the metamorphic conditions is unknown but likely requires some extra heat that was probably added by intrusion of magmas from the lithospheric mantle, i.e., syenites that have been recently re‐dated at c. 545 Ma. Some granites derived from the lower crust at c. 545 Ma are the outcome rather than the cause of high‐T metamorphism. In addition, high contents of heat‐producing elements K, Th, and U may have raised peak temperatures by 150–200°C at the base of the crust, resulting in the widespread melting of fertile crustal rocks. The continuous gradation from centimetre‐scale leucosomes to decametre‐scale leucogranite sheets within the high‐grade metamorphic zone suggests that leucosome lenses coalesced to form larger bodies of anatectic leucogranites, thereby documenting a link between high‐grade regional metamorphism and Pan‐African magmatism. In view of the close association of the studied high‐T migmatites with hundreds of synmetamorphic high‐T granites that invaded the terrane as metre‐ to decametre‐wide sills and dykes, we postulate that crystallization of felsic lower crustal magma is, at least partly, responsible for heat supply. Late‐stage isobaric cooling of these granites may explain the occurrence of andalusite in some samples.  相似文献   

5.
The Proterozoic belts that occur along the margins of the West Australian Craton, as well as those in intraplate settings, generally share similar geological histories that suggest a common plate‐margin driver for orogeny. However, the thermal drivers for intraplate orogenesis are more poorly understood. The Mutherbukin Tectonic Event records a protracted period of Mesoproterozoic reworking of the Capricorn Orogen and offers significant insight into both the tectonic drivers and heat sources of long‐lived intraplate orogens. Mineral assemblages and tectonic fabrics related to this event occur within a 50 km‐wide fault‐bound corridor in the central part of the Gascoyne Province in Western Australia. This zone preserves a crustal profile, with greenschist facies rocks in the north grading to upper amphibolite facies rocks in the south. The P–T–t evolution of 13 samples from 10 localities across the Mutherbukin Zone is investigated using phase equilibria modelling integrated with in situ U–Pb monazite and zircon geochronology. Garnet chemistry from selected samples is used to further refine the P–T history and shows that the dominant events recorded in this zone are prolonged D1 transpression between c. 1,320 and 1,270 Ma, followed by D2 transtension from c. 1,210 to 1,170 Ma. Peak metamorphic conditions in the mid‐crust reached >650°C and 4.4–7 kbar at c. 1,210–1,200 Ma. Most samples record a single clockwise P–T evolution during this event, although some samples might have experienced multiple perturbations. The heat source for metamorphism was primarily conductive heating of radiogenic mid‐ and upper crust, derived from earlier crustal differentiation events. This crust was thickened during D1 transpression, although the thermal effects persisted longer than the deformation event. Peak metamorphism was terminated by D2 transtension at c. 1,210 Ma, with subsequent cooling driven by thinning of the radiogenic crust. The coincidence of a sedimentary basin acting as a thermal lid and a highly radiogenic mid‐crustal batholith restricted to the Mutherbukin Zone accounts for reworking being confined to a discrete crustal corridor. Our results show that radiogenic regions in the shallow to mid crust can elevate the thermal gradient and localize deformation, causing the crust to be more responsive to far‐field stresses. The Mutherbukin Tectonic Event in the Capricorn Orogen was synchronous with numerous Mesoproterozoic events around the West Australian Craton, suggesting that thick cratonic roots play an important role in propagating stresses generated at distant plate boundaries.  相似文献   

6.
In this contribution, we highlight the importance of in-situ monazite geochronology linked to P−T modelling for identification of timescales of metamorphic processes. Barrovian-type micaschists, migmatites and augengneiss from the Gumburanjun dome in the southeastern extremity of the Gianbul dome, NW Himalaya, have been studied in order to correlate the early stages of Himalayan metamorphism at different crustal levels and infer the timing of anatexis. P−T−t paths are constrained through combined pseudosection modelling and in-situ and in-mount monazite and xenotime laser ablation–split-stream inductively coupled plasma-mass spectrometry. Petrography and garnet zoning combined with pseudosection modelling show that garnet-staurolite schists record burial from ~530 to 560°C and 5.5 kbar to ~630 to 660°C and 7 kbar; staurolite-kyanite schists from ~530 to 560°C and 5 kbar to ~670 to 680°C and 7−9 kbar; and garnet-kyanite migmatites from 540−570°C and 5 kbar to ~680 to 750°C and 7−10 kbar, probably also to >750°C and >9 kbar above the muscovite stability field. The decompression paths of garnet-staurolite schists indicate cooling on decompression, while garnet rim chemistry and local sillimanite growth point to a stage of re-equilibration at ~600 to 670°C and 4−6 kbar in some of the staurolite-kyanite schists, and at ~670 to 700°C and 6 kbar in garnet-kyanite migmatites. Some of the staurolite-kyanite schists and garnet-kyanite migmatites also contain andalusite or andalusite-cordierite. Monazite and xenotime were analysed in thin sections in garnet, staurolite and kyanite, and in the matrix; and in mounts. BSE images and compositional maps of monazite (xenotime was too small) show variable internal structures from homogeneous through patchy zoning with embayed to sharp boundaries. Two groups of samples can be identified on the basis of the presence or absence of c. 44 − 37 Ma ages. The first group of samples—two garnet-staurolite schists—recorded only c. 31 − 27 Ma ages in porphyroblasts and no c. 40 Ma ages. The second group (samples of staurolite-kyanite schist, garnet-kyanite migmatites, augengneiss) have both the older, c. 44 − 37 Ma monazite ages in porphyroblasts and younger ages down to c. 22 Ma. These significantly different ranges of ages from porphyroblasts of 44−37 Ma, and 31−27 Ma, are interpreted as the duration of prograde P−T paths in Eocene and Oligocene, and indicate diachronous two-stage burial of rocks. Early migmatization occurred at 38 Ma. The c. 29 Ma is interpreted as the time when rocks from the lower and middle crustal levels were partially exhumed and came in to contact with rocks that were downgoing at this time. Localized monazite recrystallization is as young as 26−24 Ma. The youngest ages of 23−22 Ma are related to leucogranite emplacement.  相似文献   

7.
To better understand the evolution of deep‐seated crust of the Variscan orogen in the Sardinia‐Corsica region, we studied garnet‐bearing micaschists which were sampled 3 km east and 15 km northeast of Porto Vecchio, south‐eastern Corsica. After a careful investigation of the textural relations and compositions of minerals, especially of zoned garnet, a P–T path was reconstructed using contoured P–T pseudosections. U–Th–Pb dating of monazite in the micaschists was undertaken with the electron microprobe. The micaschists from both localities were formed along similar anticlockwise P–T paths. The prograde branch of these paths starts at 3 kbar close to 600°C in the P–T field of sillimanite and reaches peak conditions at 7 kbar and 600 (15 km NE of Porto Vecchio) to 630°C (3 km E of Porto Vecchio). The metamorphism at peak P–T conditions happened c. 340 Ma based on low‐Y (<0.65 wt% Y2O3) monazite. Ages of monazite with high‐Y contents (>2 wt% Y2O3), which probably have formed before garnet, scatter around 362 Ma. The retrograde branch of the P–T paths passes through 4 kbar at ~550°C. We conclude that the micaschists belong to a common metasedimentary sequence, which extends over the Porto Vecchio region and is separated from other metamorphic rock sequences in the north and the south by major tectonic boundaries. This sequence had experienced peak pressures which are lower than those determined for metamorphic rocks, such as micaschist and gneiss, from north‐eastern Sardinia. At present, we favour a continent–continent collisional scenario with the studied metasedimentary sequence buried during the collisional event as part of the upper plate. The contemporaneous high‐P metamorphic rocks from NE Sardinia were part of the upper portion of the lower plate. The addressed rocks from both plates were exhumed in an exhumation channel.  相似文献   

8.
Small oval‐shaped, unshielded monazite grains found in a Variscan garnet–muscovite‐bearing mylonitic paragneiss from the Liegendserie unit of the Münchberg Metamorphic Complex in the northwestern Bohemian Massif, central Europe, yield only pre‐Variscan ages. These ages, determined with the electron microprobe, have maxima at c. 545, 520 and 495 Ma and two side‐maxima at 455 and 575 Ma, and are comparable with previously determined ages of detrital zircon reported from paragneisses elsewhere in the NW Bohemian Massif. The pressure (P)–temperature (T) history of this mylonitic paragneiss, determined from contoured P–T pseudosections, involved an initial stage at 6 kbar/600 °C, reaching peak P–T conditions of 12.5 kbar/670 °C with partial melting, followed by mylonitization and retrogression to 9 kbar/610 °C. The monazite, representing detrital grains derived from igneous rocks of a Cadomian provenance between 575 and 455 Ma, has survived these Variscan metamorphic/deformational events unchanged because this mineral has probably never been outside its P–T stability field during metamorphism.  相似文献   

9.
In Rogaland, South Norway, a polycyclic granulite facies metamorphic domain surrounds the late‐Sveconorwegian anorthosite–mangerite–charnockite (AMC) plutonic complex. Integrated petrology, phase equilibria modelling, monazite microchemistry, Y‐in‐monazite thermometry, and monazite U–Th–Pb geochronology in eight samples, distributed across the apparent metamorphic field gradient, imply a sequence of two successive phases of ultrahigh temperature (UHT) metamorphism in the time window between 1,050 and 910 Ma. A first long‐lived metamorphic cycle (M1) between 1,045 ± 8 and 992 ± 11 Ma is recorded by monazite in all samples. This cycle is interpreted to represent prograde clockwise P–T path involving melt production in fertile protoliths and culminating in UHT conditions of ~6 kbar and 920°C. Y‐in‐monazite thermometry, in a residual garnet‐absent sapphirine–orthopyroxene granulite, provides critical evidence for average temperature of 931 and 917°C between 1,029 ± 9 and 1,006 ± 8 Ma. Metamorphism peaked after c. 20 Ma of crustal melting and melt extraction, probably supported by a protracted asthenospheric heat source following lithospheric mantle delamination. Between 990 and 940 Ma, slow conductive cooling to 750–800°C is characterized by monazite reactivity as opposed to silicate metastability. A second incursion (M2) to UHT conditions of ~3.5–5 kbar and 900–950°C, is recorded by Y‐rich monazite at 930 ± 6 Ma in an orthopyroxene–cordierite–hercynite gneiss and by an osumilite gneiss. This M2 metamorphism, typified by osumilite paragenesis, is related to the intrusion of the AMC plutonic complex at 931 ± 2 Ma. Thermal preconditioning of the crust during the first UHT metamorphism may explain the width of the aureole of contact metamorphism c. 75 Ma later, and also the rarity of osumilite‐bearing assemblages in general.  相似文献   

10.
The Anita Peridotite is a ~20 km long by 1 km wide exhumed fragment of spinel facies sub‐arc lithospheric mantle that is enclosed entirely within the ≤4 km wide ductile Anita Shear Zone, and bounded by quartzofeldspathic lower crustal gneisses in Fiordland, south‐western New Zealand. Deformation textures, grain growth calculations and thermodynamic modelling results indicate the mylonitic peridotite fabric formed during rapid cooling, and therefore likely during extrusion. However, insights into the exhumation process are gained through examination of aluminous garnet‐bearing meta‐sedimentary gneisses also enclosed within the shear zone. P–T calculations indicate that prior to mylonitization the gneisses enclosing the peridotite equilibrated at 675–746 °C in the sillimanite stability field (stage I), before being buried to near the base of thickened arc crust (stage II; ~686 ± 26 °C and 10.7 ± 0.8 kbar). From this point on, the peridotite unit and the quartzofeldspathic rocks share a deformation history involving extensive recrystallization (stage III) within the Anita Shear Zone. Coupled exhumation of these portions of lower crust and upper mantle occurred during regional thinning of over‐thickened lithosphere at c. 104 Ma (U–Pb zircon). Our favoured model for the exhumation process involves heterogeneous transpressive deformation within the translithospheric Anita Shear Zone, which provided a conduit for ductile extrusion through the crust.  相似文献   

11.
The Connecticut Valley–Gaspé (CVG) trough represents a major, orogen-scale Silurian–Devonian basin of the Northern Appalachians. From Gaspé Peninsula to southern New England, the CVG trough has experienced a contrasting metamorphic and structural evolution during the Acadian orogeny. Along its strike, the CVG trough is characterized by increasing strain and polyphase structures, and by variations in the intensity of regional metamorphism and contrasting abundance of c. 390–370 Ma granitic intrusions. In southern Quebec and northern Vermont, a series of NW–SE transects across the CVG trough have been studied in order to better understand these along-strike variations. Detailed structural analyses, combined with phase equilibria modelling, Raman spectrometry, and muscovite 40Ar/39Ar dating highlight a progressive and incremental deformation involving south–north variation in the timing of metamorphism. Deformation evolves from a D1 crustal thickening event which originates in Vermont and progresses to southern Québec where it peaked at 0.6 GPa/380°C at c. 375 Ma. This was followed by a D2 event associated with continuous burial in Vermont from 378 to 355 Ma, which produced peak metamorphic conditions of 0.85 GPa/380°C and exhumation in Quebec from 368 to 360 Ma. The D3 compressional exhumation event also evolved from south to north from 345 to 335 Ma. D1 to D3 deformation events form part of a continuum with an along-strike propagation rate of ~50 km/Ma During D1, the burial depth varied by more than 15 km between southern Quebec and Vermont, and this can be attributed to the occurrence of a major crustal indenter, the Bronson Hill Arc massif, in the New England segment of the Acadian collision zone.  相似文献   

12.
Evidence for ultrahigh‐pressure metamorphism (UHPM) in the Rhodope metamorphic complex comes from occurrence of diamond in pelitic gneisses, variably overprinted by granulite facies metamorphism, known from several areas of the Rhodopes. However, tectonic setting and timing of UHPM are not interpreted unanimously. Linking age to a metamorphic stage is a prerequisite for reconstruction of these processes. Here, we use monazite in diamond‐bearing gneiss from Chepelare (Bulgaria) to date the diamond‐forming UHPM event in the Central Rhodopes. The diamond‐bearing gneiss comes from a strongly deformed, lithologically heterogeneous zone (Chepelare Mélange) sandwiched between two migmatized orthogneiss units, known as Arda‐I and Arda‐II. Diamond, identified by Raman micro‐spectroscopy, shows the characteristic band mostly centred between 1332 and 1330 cm?1. The microdiamond occurs as single grains or polyphase diamond + carbonate inclusions, rarely with CO2. Thermodynamic modelling shows that garnet was stable at UHP conditions of 3.5–4.6 GPa and 700–800 °C, in the stability field of diamond, and was re‐equilibrated at granulite facies/partial melting conditions of 0.8–1.2 GPa and 750–800 °C. The texture of monazite shows older central parts and extensive younger domains which formed due to metasomatic replacement in solid residue and/or overgrowth in melt domains. The monazite core compositions, with distinctly lower Y, Th and U contents, suggest its formation in equilibrium with garnet. The U–Th–Pb dating of monazite using electron microprobe analysis yielded a c. 200 Ma age for the older cores with low Th, Y, U and high La/Nd ratio, and a c. 160 Ma age for the dominant younger monazite enriched in Th, Y, U and HREE. The older age of c. 200 Ma is interpreted as the timing of UHPM, whereas the younger age of c. 160 Ma as granulite facies/partial melting overprint. Our results suggest that UHPM occurred in Late Triassic to Early Jurassic time, in the framework of collision and subduction of continental crust after the closure of Paleotethys.  相似文献   

13.
The time‐scales and P–T conditions recorded by granulite facies metamorphic rocks permit inferences about the geodynamic regime in which they formed. Two compositionally heterogeneous cordierite–spinel‐bearing granulites from Vizianagaram, Eastern Ghats Province (EGP), India, were investigated to provide P–T–time constraints using petrography, phase equilibrium modelling, U–Pb geochronology, the rare earth element composition of zircon and monazite, and Ti‐in‐zircon thermometry. These ultrahigh temperature (UHT) granulites preserve discrete compositional layering in which different inferred peak assemblages are developed, including layers bearing garnet–sillimanite–spinel, and others bearing orthopyroxene–sillimanite–spinel. These mineral associations cannot be reproduced by phase equilibrium modelling of whole‐rock compositions, indicating that the samples became domainal on a scale less than that of a thin section, even at UHT conditions. Calculation of the P–T stability fields for six compositional domains within which the main rock‐forming minerals are considered to have attained equilibrium suggests peak metamorphic conditions of ~6.8–8.3 kbar at ~1,000°C. In most of these domains, the subsequent evolution resulted in the growth of cordierite and final crystallization of melt at an elevated (residual) H2O‐undersaturated solidus, consistent with <1 kbar of decompression. Concordant U–Pb ages obtained by SHRIMP from zircon (spread 1,050–800 Ma) and monazite (spread 950–800 Ma) demonstrate that crystallization of these minerals occurred during an interval of c. 250 Ma. By combining LA‐ICP‐MS U–Pb zircon ages with Ti‐in‐zircon temperatures from the same analysis sites, we show that the crust may have remained above 900°C for a minimum of c. 120 Ma between c. 1,000 and c. 880 Ma. Overall, the results suggest that, in the interval 1,050 to 800 Ma, the evolution of the Vizianagaram granulites culminated with UHT conditions from c. 1,000 Ma to c. 880 Ma, associated with minor decompression, before further zircon crystallization at c. 880–800 Ma during cooling to the solidus. However, these rocks are adjacent to the Paderu–Anantagiri–Salur crustal block to the NW that experienced counterclockwise P–T–t paths, and records similar UHT peak metamorphic conditions (7–8 kbar, ~950°C) followed by near‐isobaric cooling, and has a similar chronology during the Neoproterozoic. The limited decompression inferred at Vizianagaram may be explained by partial exhumation due to thrusting of this crustal block over the adjacent Paderu–Anantagiri–Salur crustal block. The residual granulites in both blocks have high concentrations of heat‐producing elements and likely remained hot at mid‐crustal depths throughout a period of relative tectonic quiescence in the interval 800–550 Ma. During the Cambrian Period, the EGP was located in the hinterland of the Denman–Pinjarra–Prydz orogen. A later concordant population of zircon dated at 511 ± 6 Ma records crystallization at temperatures of ~810°C. This age may record a low‐degree of melting due to limited influx of fluid into hot, weak crust in response to convergence of the Crohn craton with a composite orogenic hinterland comprising the Rayner terrane, EGP, and cratonic India.  相似文献   

14.
New phase equilibrium modelling, combined with U–Th/Pb petrochronology on monazite and xenotime, and 40Ar/39Ar geochronology on white mica, reveal the style of deformation and metamorphism near the southern tip of the extruded Himalayan metamorphic core (HMC). In the Jajarkot klippe, west Nepal foreland, greenschist to lower amphibolite facies metamorphism is entirely constrained to the Cenozoic Himalayan orogeny, in contrast with findings from other foreland klippen in the central Himalaya. HMC rocks exposed in the Jajarkot klippe yield short‐lived, hairpin pressure–temperature–time–deformation paths that peaked at 550–600°C and 750–1,200 MPa at 25 Ma. The Main Central thrust (MCT) and the South Tibetan detachment (STD) bound the base and the top of the HMC, respectively, and were active simultaneously for at least part of their deformation history. The STD was active at c. 27–26 Ma and possibly as late as c. 19 Ma, while the MCT may have been active as early as 27 Ma and was still active at c. 22 Ma. The tectonometamorphic conditions in the Jajarkot klippe are characteristic of crustal thickening and footwall accretion of new material at the tip of the extruding metamorphic orogenic core. Our new results reveal that collisional processes active in the middle to late Miocene at the base of the HMC now exposed in the hinterland were also active earlier, during the Oligocene, at the tip of the southward‐extruding middle crust.  相似文献   

15.
Zircon UPb dating by SIMS of the Mont-Louis granite yields an age of 305±5 Ma, intrepreted to reflect the igneous emplacement age of the massif. It is in agreement with the Hercynian syntectonic character of Pyrenees granite. 40Ar/39Ar on hornblende, biotite and K-feldspar permit, to estimate the massif cooling. A rapid temperature decrease (≈30 °C/Ma) is revealed from Westphalian to Late Stephanian, coeval with the emplacement of a laccolithe in the upper crust. Then, the cooling rate decreases to ≈1 °C/Ma. This would be consistent with a long time residence for the pluton from the Late Palaeozoic to the Early Cainozoic at 6–8 km depth. To cite this article: O. Maurel et al., C. R. Geoscience 336 (2004).  相似文献   

16.
This study uses zircon and apatite fission‐track (FT) analyses to reveal the exhumation history of the granitoid samples collected from the Lesser Hinggan Mountains, northeast China. A southeast to northwest transect across the Lesser Hinggan Mountains yielded zircon FT ages between 89.8 ± 5.7 and 100.4 ± 8.6 Ma, and apatite FT ages between 50.6 ± 13.8 and 74.3 ± 4.5 Ma with mean track lengths between 11.7 ± 2.0 and 12.8 ± 1.7 µm. FT results and modelling identify three stages in sample cooling history spanning the late Mesozoic and Cenozoic eras. Stage one records rapid cooling from the closure temperature of zircon FT to the high temperature part of the apatite FT partial annealing zone (∼210–110 °C) during ca. 95 to 65 Ma. Stage two records a period of relative slow cooling (∼110–60 °C) taking place between ca. 65 and 20 Ma, suggesting that the granitoids had been exhumed to the depth of ∼1−2 km. Final stage cooling (60–20 °C) occurred since the Miocene at an accelerated rate bringing the sampled rocks to the Earth's surface. The maximum exhumation is more than 5 km under a steady‐state geothermal gradient of 35 °C/km. Integrated with the tectonic setting, this exhumation is possibly led by the Pacific Plate subduction combined with intracontinental orogeny associated with asthenospheric upwelling. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

17.
New petrographic and microstructural observations, mineral equilibria modelling and U/Pb (monazite) geochronological studies were carried out to investigate the relationships between deformation and metamorphism across the Rehamna massif (Moroccan Variscan belt). In this area, typical Barrovian (muscovite to staurolite) zones developed in Cambrian to Carboniferous metasedimentary rocks that are distributed around a dome‐like structure. First assemblages are characterized by the presence of locally preserved andalusite, followed by prograde evolution culminating at 6 kbar and 620 °C in the structurally deepest staurolite zone rocks. This Barrovian sequence was subsequently uplifted to supracrustal levels, heterogeneously reworked at greenschist facies conditions, which was followed locally by static growth of andalusite, indicating heating to 2.5–4 kbar and 530–570 °C. The 206Pb/238U monazite age of 298.3 ± 4.1 Ma is interpreted as minimum age of peak metamorphic conditions, whereas the ages of 275.8 ± 1.7 Ma and 277.0 ± 1.1 Ma date decompression and heating at low pressure, in agreement with previous dating of Permian granitoids intruding the Rehamna massif. The prograde metamorphism occurred during thickening and associated horizontal flow in the deeper crust (S1 horizontal schistosity). The horizontally disposed metamorphic zones were subsequently uplifted by a regional scale antiform during ongoing N–S compression. The re‐heating of the massif follows late massive E–W shortening, refolding and retrograde shearing of all previous fabrics coevally with regionally important intrusions of Permian granitoids. We argue that metamorphic evolution of the Rehamna massif occurred several hundred kilometres from the convergent plate boundaries in the interior of continental Gondwanan plate. The tectonometamorphic history of the Rehamna massif is put into Palaeozoic plate tectonic perspective and Late Carboniferous reactivation of (Devonian)–Early Carboniferous basins formed during stretching of the north Gondwana margin and formation of the Palaeotethys Ocean. The inherited heat budget of these magma‐rich basins plays a role in the preferential location of this intracontinental orogen. It is shown that rapid transition from lithospheric stretching to compression is characterized by specific HT type of Barrovian metamorphism, which markedly differs from similar Barrovian sequences along Palaeozoic plate boundaries reported from Variscan Europe.  相似文献   

18.
The Red River shear zone (RRSZ) is a major left‐lateral strike‐slip shear zone, containing a ductilely deformed metamorphic core bounded by brittle strike‐slip and normal faults, which stretches for >1000 km from Tibet through Yunnan and North Vietnam to the South China Sea. The RRSZ exposes four high‐grade metamorphic core complexes along its length. Various lithologies from the southernmost core complex, the Day Nui Con Voi (DNCV), North Vietnam, provide new constraints on the tectonic and metamorphic evolution of this region prior to and following the initial India–Asia collision. Analysis of a weakly deformed anatectic paragneiss using P–T pseudosections constructed in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (MnNCKFMASHTO) system provides prograde, peak and retrograde metamorphic conditions, and in situ U–Th–Pb geochronology of metamorphic monazite yields texturally controlled age constraints. Tertiary metamorphism and deformation, overprinting earlier Triassic metamorphism associated with the Indosinian orogeny and possible Cretaceous metamorphism, are characterized by peak metamorphic conditions of ~805 °C and ~8.5 kbar between c. 38 and 34 Ma. Exhumation occurred along a steep retrograde P–T path with final melt crystallizing at the solidus at ≥~5.5 kbar at ~790 °C. Further exhumation at ~640–700 °C and ~4–5 kbar at c. 31 Ma occurred at subsolidus conditions. U–Pb geochronological analysis of monazite from a strongly deformed pre‐kinematic granite dyke from the flank of the DNCV provides further evidence for exhumation at this time. Magmatic grains suggest initial emplacement at 66.0 ± 1.0 Ma prior to the India–Asia collision, whereas grains with metamorphic characteristics indicate later growth at 30.6 ± 0.4 Ma. Monazite grains from a cross‐cutting post‐kinematic dyke within the core of the DNCV antiform provide a minimum age constraint of 25.2 ± 1.4 Ma for the termination of fabric development. A separate and significant episode of monazite growth at c. 83–69 Ma is suggested to be the result of fluid‐assisted recrystallization following the emplacement of magmatic units.  相似文献   

19.
A Late Palaeozoic accretionary prism, formed at the southwestern margin of Gondwana from Early Carboniferous to Late Triassic, comprises the Coastal Accretionary Complex of central Chile (34–41°S). This fossil accretionary system is made up of two parallel contemporaneous metamorphic belts: a high‐pressure/low temperature belt (HP/LT – Western Series) and a low pressure/high temperature belt (LP/HT – Eastern Series). However, the timing of deformation events associated with the growth of the accretionary prism (successive frontal accretion and basal underplating) and the development of the LP/HT metamorphism in the shallower levels of the wedge are not continuously observed along this paired metamorphic belt, suggesting the former existence of local perturbations in the subduction regime. In the Pichilemu region, a well‐preserved segment of the paired metamorphic belt allows a first order correlation between the metamorphic and deformational evolution of the deep accreted slices of oceanic crust (blueschists and HP greenschists from the Western Series) and deformation at the shallower levels of the wedge (the Eastern Series). LP/HT mineral assemblages grew in response to arc‐related granitic intrusions, and porphyroblasts constitute time markers recording the evolution of deformation within shallow wedge material. Integrated P–T–t–d analysis reveals that the LP/HT belt is formed between the stages of frontal accretion (D1) and basal underplating of basic rocks (D2) forming blueschists at c. 300 Ma. A timeline evolution relating the formation of blueschists and the formation and deformation of LP/HT mineral assemblages at shallower levels, combined with published geochronological/thermobarometric/geochemistry data suggests a cause–effect relation between the basal accretion of basic rocks and the deformation of the shallower LP/HT belt. The S2 foliation that formed during basal accretion initiated near the base of the accretionary wedge at ~30 km depth at c. 308 Ma. Later, the S2 foliation developed at c. 300 Ma and ~15 km depth shortly after the emplacement of the granitoids and formation of the (LP/HT) peak metamorphic mineral assemblages. This shallow deformation may reflect a perturbation in the long‐term subduction dynamics (e.g. entrance of a seamount), which would in turn have contributed to the coeval exhumation of the nearby blueschists at c. 300 Ma. Finally, 40Ar–39Ar cooling ages reveal that foliated LP/HT rocks were already at ~350 °C at c. 292 Ma, indicating a rapid cooling for this metamorphic system.  相似文献   

20.
The Upper Units of the allochthonous complexes of the NW Iberian Massif constitute a terrane with continental affinity. They represent the vestiges of a Cambrian magmatic arc developed in the periphery of Gondwana (West African Craton) which was involved in the Devonian Variscan collision, undergoing high-P, high-T metamorphism. This includes ultramafic rocks, high-P mafic rocks (eclogites and granulites) and high-P migmatitic paragneisses. The latter rocks show an extensive migmatization with the leucosomes oriented parallel to the regional foliation. The migmatitic paragneisses are composed of garnet, kyanite, biotite, quartz, plagioclase, K-feldspar, rutile and Ti-hematite. Thermodynamic modelling using the measured bulk composition in the NCKFMASTHO system indicates metamorphic peak conditions of ~15 kbar and ~800 to 835°C, followed by a significant cooling. The prograde evolution is assessed by means of a melt-reintegration approach, using the composition of the garnet and its inclusions. An appropriate composition of liquid is added to the measured bulk composition to emulate the pre-melting bulk composition. Modelling of this melt-reintegrated composition allows to identify a colder high-P episode below ~500°C. Zircon crystals extracted from the leucosomes show overgrowths crystallized from the partial melt at c. 389 Ma (U–Pb system). The P–T–t path proposed reveals a subduction of the peri-Gondwanan arc-derived section down to mantle depths. An isobaric heating stage occurred as a result of residence at great depths and/or inception of a transient oceanic basin at c. 395 Ma. The ensuing near-isothermal exhumation occurred due to the extension related to the inception of the basin, reaching the thermal peak shortly before c. 389 Ma. Subsequent cooling is related to the underthrusting of colder oceanic and transitional crust below the HP-HT Upper Units.  相似文献   

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