Showing papers on "Metamorphism published in 2003"
TL;DR: In this article, a fried ice cream model was proposed to explain the rapid processes of both plate subduction and exhumation, with a short residence time of the UHP slab at mantle depths.
767 citations
TL;DR: In this article, the authors synthesize the geology, geochronology, and tectonics of the Neo-archean through Mesoproterozoic evolution of the North China Craton.
763 citations
TL;DR: The Sanandaj-Sirjan Zone contains the metamorphic core of the Zagros continental collision zone in western Iran as discussed by the authors, and the zone has been subdivided into the following from southwest to northeast: an outer belt of imbricate thrust slices (radiolarite, Bisotun, ophiolite and marginal sub-zones, which consist of Mesozoic deep-marine sediments, shallow-marine carbonates, oceanic crust and volcanic arc, respectively) and an inner complexly deformed sub-zone (late Palaeozo
512 citations
TL;DR: More and more evidence is being discovered in Phanerozoic collision belts of the burial of crustal rocks to previously unsuspected (and ever increasing) depths, presently on the order of 150-200 km, and of exhumation from such depths as mentioned in this paper.
484 citations
TL;DR: In this article, the Luliang Movement is considered to have formed largely by the end of the Neoarchaean, with only local Proterozoic igneous and sedimentary activity recorded.
439 citations
TL;DR: The Saghand area of East-Central Iran exposes rocks that comprise the substratum of the Central Iranian continental terrane, as part of the larger Alpine-Himalayan orogenic system as discussed by the authors.
Abstract: The Saghand area of East-Central Iran exposes rocks that comprise the substratum of the Central Iranian continental terrane, as part of the larger Alpine-Himalayan orogenic system. Our new U-Pb ages and geochemical data from the magmatic, metamorphic and siliciclastic rocks of the Saghand area unravel three main episodes of orogenic activity in the latest Neoproterozoic-Early Cambrian, the Late Triassic, and the Eocene. Geologic events in the oldest episode include in chronological order, low- to medium-grade metamorphism, calc-alkaline plutonism, rhyolitic to andesitic volcanism, and widespread trondhjemite emplacement, from 547 Ma to 525 Ma. The Late Triassic event (approximately 220-213 Ma) is characterized by the emplacement of granite-tonalite plutons. The extensive, high-grade metamorphic rocks, migmatites and post-kinematic intrusions of Eocene age (47-44 Ma) occur in a distinct domain, in the western part of the Saghand area. These rocks previously were thought to represent the Precambrian basement of the Central Iranian Terrane. The terminal Neoproterozoic-Early Cambrian orogeny in central Iran was related to a broad-scale magmatic arc that developed along the Proto-Tethyan margin of the Gondwanaland supercontinent. The fragmented remains of that margin occur as displaced terranes, including the Central Iranian Terrane, now embedded within the Alpine-Himalayan orogenic system. The newly recognized Late Triassic intrusions of the Saghand area are indicative of a tectonomagmatic episode of possible collisional nature, in accord with the previously identified Early Kimmerian (Cimmerian) event in the region. The extensive Eocene metamorphic and magmatic activities correspond to the early Alpine Orogeny, which resulted from the convergence between Arabian and Eurasian plates, and the Cenozoic closure of the Tethys oceanic tract(s) by subduction.
398 citations
TL;DR: The conditions at which monazite and allanite were produced and destroyed during prograde metamorphism of pelitic rocks were determined in a Buchan and a Barrovian regional terrain and in a contact aureole, all from northern New England, USA as discussed by the authors.
Abstract: The conditions at which monazite and allanite were produced and destroyed during prograde metamorphism of pelitic rocks were determined in a Buchan and a Barrovian regional terrain and in a contact aureole, all from northern New England, USA. Pelites from the chlorite zone of each area contain monazite that has an inclusion-free core surrounded by a highly irregular, inclusion-rich rim. Textures and 208Pb/232Th dates of these monazites in the Buchan terrain, obtained by ion microprobe, suggest that they are composite grains with detrital cores and very low-grade metamorphic overgrowths. At exactly the biotite isograd in the regional terrains, composite monazite disappears from most rocks and is replaced by euhedral metamorphic allanite. At precisely the andalusite or kyanite isograd in all three areas, allanite, in turn, disappears from most rocks and is replaced by subhedral, chemically unzoned monazite neoblasts. Allanite failed to develop at the biotite isograd in pelites with lower than normal Ca and/or Al contents, and composite monazite survived at higher grades in these rocks with modified texture, chemical composition, and Th–Pb age. Pelites with elevated Ca and/or Al contents retained allanite in the andalusite or kyanite zone. The best estimate of the time of peak metamorphism at the andalusite or kyanite isograd is the mean Th–Pb age of metamorphic monazite neoblasts that have not been affected by retrograde metamorphism: 364.3±3.5 Ma in the Buchan terrain, 352.9±8.9 Ma in the Barrovian terrain, and 403.4±5.9 Ma in the contact aureole. Some metamorphic monazites from the Buchan terrain have ages partially to completely reset during an episode of retrograde metamorphism at 343.1±9.1 Ma. Interpretation of Th–Pb ages of individual composite monazite grains is complicated by the occurrence of subgrain domains of detrital material intergrown with domains of material formed or recrystallized during prograde and retrograde metamorphism.
355 citations
TL;DR: Using mineral inclusions in zircon and garnet and their rare earth element characteristics, it is possible to relate the ages to distinct stages of granulite facies metamorphism as discussed by the authors.
Abstract: Zircon from a lower crustal metapelitic granulite (Val Malenco, N-Italy) display inherited cores, and three metamorphic overgrowths with ages of 281 ± 2, 269 ± 3 and 258 ± 4 Ma. Using mineral inclusions in zircon and garnet and their rare earth element characteristics it is possible to relate the ages to distinct stages of granulite facies metamorphism. The first zircon overgrowth formed during prograde fluid-absent partial melting of muscovite and biotite apparently caused by the intrusion of a Permian gabbro complex. The second metamorphic zircon grew after formation of peak garnet, during cooling from 850 °C to c. 700 °C. It crystallized from partial melts that were depleted in heavy rare earth elements because of previous, extensive garnet crystallization. A second stage of partial melting is documented in new growth of garnet and produced the third metamorphic zircon. The ages obtained indicate that the granulite facies metamorphism lasted for about 20 Myr and was related to two phases of partial melting producing strongly restitic metapelites.
Monazite records three metamorphic stages at 279 ± 5, 270 ± 5 and 257 ± 4 Ma, indicating that formation ages can be obtained in monazite that underwent even granulite facies conditions. However, monazite displays less clear relationships between growth zones and mineral inclusions than zircon, hampering the correlation of age to metamorphism. To overcome this problem garnet–monazite trace element partitioning was determined for the first time, which can be used in future studies to relate monazite formation to garnet growth.
342 citations
TL;DR: In this article, the authors evaluate the behavior of trace elements during prograde metamorphism and dehydration using samples of high-pressure, low-temperature metamorphic rocks from New Caledonia.
Abstract: The transfer of fluid and elements from subducting crust to the overlying mantle wedge is a fundamental process affecting arc magmatism and the chemical differentiation of the Earth. While the production of fluid by breakdown of hydrous minerals is well understood, the liberation of trace elements remains generally unconstrained. In this paper, we evaluate the behaviour of trace elements during prograde metamorphism and dehydration using samples of high-pressure, low-temperature metamorphic rocks from New Caledonia. Samples examined include mafic and pelitic rock-types that range in grade from lawsonite blueschist to eclogite facies, and represent typical lithologies of subducting crust. Under lawsonite blueschist facies conditions, the low temperatures of metamorphism inhibit equilibrium partitioning between metamorphic minerals and allow for the persistence of igneous and detrital minerals. Despite this, the most important hosts for trace-elements include lawsonite, (REE, Pb, Sr), titanite (REE, Nb, Ta), allanite (LREE, U, Th), phengite (LILE) and zircon (Zr, Hf). At epidote blueschist to eclogite facies conditions, trace-element equilibrium may be attained and epidote (REE, Sr, Th, U, Pb), garnet (HREE), rutile (Nb, Ta), phengite (LILE) and zircon (Zr, Hf) are the major trace-element hosts. Chlorite, albite, amphibole and omphacite contain very low concentrations of the investigated trace elements. The comparison of mineral trace-element data and bulk-rock data at different metamorphic grades indicates that trace elements are not liberated in significant quantities by prograde metamorphism up to eclogite facies. Combining our mineral trace-element data with established phase equilibria, we show that the trace elements considered are retained by newly-formed major and accessory minerals during mineral breakdown reactions to depths of up to 150 km. In contrast, significant volumes of fluid are released by dehydration reactions. Therefore, there is a decoupling of fluid release and trace element release in subducting slabs. We suggest that the flux of trace elements from the slab is not simply linked to mineral breakdown, but results from complex fluid-rock interactions and fluid-assisted partial melting in the slab.
341 citations
01 May 2003
TL;DR: In this article, the authors present a new geological map together with cross-sections and lateral sections of the Everest massif and combine field relations, structural geology, petrology, thermobarometry and geochronology to interpret the tectonic evolution of the Himalaya.
Abstract: This paper presents a new geological map together with cross-sections and lateral sections of the Everest massif. We combine field relations, structural geology, petrology, thermobarometry and geochronology to interpret the tectonic evolution of the Everest Himalaya. Lithospheric convergence of India and Asia since collision at c. 50 Ma. resulted in horizontal shortening, crustal thickening and regional metamorphism in the Himalaya and beneath southern Tibet. High temperatures (>620 °C) during sillimanite grade metamorphism were maintained for 15 million years from 32 to 16.9 ± 0.5 Ma along the top of the Greater Himalayan slab. This implies that crustal thickening must also have been active during this time, which in turn suggests high topography during the Oligocene–early Miocene. Two low-angle normal faults cut the Everest massif at the top of the Greater Himalayan slab. The earlier, lower Lhotse detachment bounds the upper limit of massive leucogranite sills and sillimanite–cordierite gneisses, and has been locally folded. Ductile motion along the top of the Greater Himalayan slab was active from 18 to 16.9 Ma. The upper Qomolangma detachment is exposed in the summit pyramid of Everest and dips north at angles of less than 15°. Brittle faulting along the Qomolangma detachment, which cuts all leucogranites in the footwall, was post-16 Ma. Footwall sillimanite gneisses and leucogranites are exposed along the Kharta valley up to 57 km north of the Qomolangma detachment exposure near the summit of Everest. The amount of extrusion of footwall gneisses and leucogranites must have been around 200 km southwards, from an origin at shallow levels (12–18 km depth) beneath Tibet, supporting models of ductile extrusion of the Greater Himalayan slab. The Everest–Lhotse–Nuptse massif contains a massive ballooning sill of garnet + muscovite + tourmaline leucogranite up to 3000 m thick, which reaches 7800 m on the Kangshung face of Everest and on the south face of Nuptse, and is mainly responsible for the extreme altitude of both mountains. The middle crust beneath southern Tibet is inferred to be a weak, ductile-deforming zone of high heat and low friction separating a brittle deforming upper crust above from a strong (?granulite facies) lower crust with a rheologically strong upper mantle. Field evidence, thermobarometry and U–Pb geochronological data from the Everest Himalaya support the general shear extrusive flow of a mid-crustal channel from beneath the Tibetan plateau. The ending of high temperature metamorphism in the Himalaya and of ductile shearing along both the Main Central Thrust and the South Tibetan Detachment normal faults roughly coincides with initiation of strike-slip faulting and east–west extension in south Tibet (
333 citations
TL;DR: In situ Th-Pb ion microprobe dating of monazite inclusions in garnets allows direct assessment of the timing of amphibolite-grade metamorphism and synchronous left-lateral shearing as discussed by the authors.
Abstract: [1] Exposures of high-grade, midcrustal rocks within the Red River shear zone (RRSZ), which separates the Indochina and South China blocks, exhibit clear evidence of left-lateral, ductile deformation. Assuming that the South China Sea represents a pull-apart basin formed at the southeastern termination of the RRSZ, it has been argued that seafloor magnetic anomalies constrain the timing of sinistral slip accommodated by the RRSZ between ∼32 and 17 Ma at a rate of ∼4 cm/yr. While 40Ar/39Ar thermochronometry indicates that left-lateral slip occurred along the RRSZ between 25 and 17 Ma, the timing of earlier high-temperature deformation has not been directly constrained. In situ Th-Pb ion microprobe dating of monazite inclusions in garnets allows direct assessment of the timing of amphibolite-grade metamorphism and synchronous left-lateral shearing. Results from northern segments of the RRSZ in Yunnan, China, indicate that synkinematic garnet growth occurred between 34 and 21 Ma and are the first to document late Oligocene metamorphism and left-lateral shearing. Data from the southern RRSZ within Vietnam are complicated by Tertiary overprinting of rocks that experienced amphibolite facies metamorphism during the Indosinian orogeny (∼220 Ma). The period during which sinistral deformation is now constrained to have occurred along the RRSZ (i.e., 34–17 Ma) is essentially coincident with spreading of the South China seafloor (32–17 Ma). This temporal and kinematic link between left-lateral shearing along the RRSZ and opening of the South China Sea supports the view that Indochina was extruded from Asia as a block along lithospheric-scale strike-slip faults.
TL;DR: In-situ U-Th-Pb analyses by ion-microprobe on zircon in intact textural relationships are combined with backscatter and cathodoluminescence analysis and trace element analyses to provide evidence for growth episodes as mentioned in this paper.
Abstract: In-situ U-Th-Pb analyses by ion-microprobe on zircon in intact textural relationships are combined with backscatter and cathodoluminescence
imaging and trace element analyses to provide evidence for growth episodes of zircon. This approach helps: (a) to unravel
the polymetamorphic history of aluminous migmatitic and granitoid gneisses of the regional contact aureole around the Rogaland
anorthosite-norite intrusive complex; and (b) to constrain the age of M 2 ultrahigh-temperature (UHT) metamorphism and the subsequent retrograde M 3 event. All samples yield magmatic inherited zircon of c. 1035 Ma, some an additional group at c. 1050 Ma. This suggests that loss of Pb by volume diffusion in non-metamict zircon is not an important factor even under extreme
crustal conditions. Furthermore, the identical inheritance patterns in aluminous (garnet, cordierite ± osumilite-bearing)
migmatites and orthogneisses indicate a metasomatic igneous instead of a sedimentary protolith for the migmatite. Results
for the M 1 metamorphic event at c. 1000 Ma BP are consistent in all samples, including those from outside the orthopyroxene-in isograd. The latter do not show evidence
for zircon growth during the M 2 metamorphic episode.
Zircon intergrown with or included within M 2 metamorphic minerals (magnetite, spinel, orthopyroxene) give an age of 927 ± 7 Ma (2 σ, n = 20). The youngest observed results are found in zircon outside M 2 minerals, some overgrown by M 3 mineral assemblages (late garnet coronas, garnet + quartz and orthopyroxene + garnet symplectites) and yield a slightly younger
pooled age of 908 ± 9 Ma (2 σ, n = 6). These textures are relative time markers for the crystallization of zircon overgrowths during discrete stages of the
UHT event. These youngest age groups are consistent with the emplacement age of the Rogaland intrusive complex and the last
magmatic activity (Tellnes dyke intrusion), respectively. This is direct and conclusive evidence for UHT metamorphism in the
regional aureole being caused by the intrusions, and corrects earlier notions that the events are not linked. Trace element
behaviour of zircon (Tb/U and Y content) has been tracked through time in the samples and shows variations both within and
between samples. This heterogeneous behaviour at all scales appears to be common in metamorphic rocks and precludes the use
of ‘rules of thumb’ in the interpretation of zircon chemistry, but chemical tracers are useful for recognition of zircon growth
or recrystallization during metamorphism.
TL;DR: In this paper, the main east-west ductile shear zones are dextral and coeval with an eastward decreasing greenschist retrograde metamorphism, and the main motion is sinistral and occurred along the Erqishi zone at 280-290 Ma.
Abstract: New structural studies and 40Ar/39Ar dating in northwest China provide information about late Paleozoic strike-slip motions subsequent to accretional events, which built eastern central Asia during the Paleozoic. Two principal areas were affected by these large transcurrent motions. First, in the Tianshan range, main east-west ductile shear zones are dextral and coeval with an eastward decreasing greenschist retrograde metamorphism. Associated biotites give ages ranging from 290 Ma to 245 Ma. The earlier N110 shearing occurred in western Tianshan, while the last one, dextral in whole Tianshan, occurred 250–245 Myr ago. Second, in the Chinese Altay region several NW-SE shear zones structured the area. The main motion is sinistral and occurred along the Erqishi zone at 280–290 Ma. It is followed by a complex succession of dextral and sinistral shearing episodes, leading to the northwestward structuring, dated at 245 Ma, of a metamorphic zone that was folded during a compressive event.
TL;DR: In this paper, structural, petrographic, geothermobaro- metric and thermochronological data demonstrate that two major tectonometamorphic events charac- terize the evolution of the GHS.
Abstract: The metamorphic core of the Himalaya in the Kali Gandaki valley of central Nepal corresponds to a 5-km-thick sequence of upper amphibolite facies metasedimentary rocks. This Greater Himalayan Sequence (GHS) thrusts over the greenschist to lower amphibolite facies Lesser Himalayan Sequence (LHS) along the Lower Miocene Main Central Thrust (MCT), and it is separated from the overlying low-grade Tethyan Zone (TZ) by the Annapurna Detachment. Structural, petrographic, geothermobaro- metric and thermochronological data demonstrate that two major tectonometamorphic events charac- terize the evolution of the GHS. The first (Eohimalayan) episode included prograde, kyanite-grade metamorphism, during which the GHS was buried at depths greater than c. 35 km. A nappe structure in the lowermost TZ suggests that the Eohimalayan phase was associated with underthrusting of the GHS below the TZ. A c. 37 Ma 40Ar/39Ar hornblende date indicates a Late Eocene age for this phase. The second (Neohimalayan) event corresponded to a retrograde phase of kyanite-grade recrystallization, related to thrust emplacement of the GHS on the LHS. Prograde mineral assemblages in the MCT zone equilibrated at average T=880 K (610°C) and P=940 MPa (=35 km), probably close to peak of meta- morphic conditions. Slightly higher in the GHS, final equilibration of retrograde assemblages occurred at average T=810 K (540°C) and P=650 MPa (=24 km), indicating re-equilibration during exhumation controlled by thrusting along the MCT and extension along the Annapurna Detachment. These results suggest an earlier equilibration in the MCT zone compared with higher levels, as a consequence of a higher cooling rate in the basal part of the GHS during its thrusting on the colder LHS. The Annapurna Detachment is considered to be a Neohimalayan, synmetamorphic structure, representing extensional reactivation of the Eohimalayan thrust along which the GHS initially underthrust the TZ. Within the upper GHS, a metamorphic discontinuity across a mylonitic shear zone testifies to significant, late- to post-metamorphic, out-of-sequence thrusting. The entire GHS cooled homogeneously below 600-700 K (330-430°C) between 15 and 13 Ma (Middle Miocene), suggesting a rapid tectonic exhumation by movement on late extensional structures at higher structural levels.
TL;DR: In this article, the authors presented the first evidence of late Pan-African magmatism in the Jiamusi Massif of northeastern China using SHRIMP U-Pb zircon techniques.
TL;DR: In this article, it was shown that fundamental aspects of the Himalayan orogen originated in an early Paleozoic thrust belt and are unrelated to Tertiary IndiaAsia collision.
Abstract: Research by many workers in various regions of the Himalaya, combined with our recent geologic and geochronologic studies in Nepal, indicate that fundamental aspects of the Himalayan orogen originated in an early Paleozoic thrust belt and are unrelated to Tertiary IndiaAsia collision Manifestations of early Paleozoic tectonism include ductile deformation, regional moderate- to highgrade metamorphism, large-scale southvergent thrusting, crustal thickening and the generation of granitic crustal melts, uplift and erosion of garnet-grade rocks, and accumulation of thick sequences of synorogenic strata Determining the relative contributions of early Paleozoic versus Tertiary tectonism constitutes a significant challenge in understanding the Himalayan orogen
TL;DR: Laouni et al. as discussed by the authors showed that the terranes from Central Hoggar (LATEA) belonged to a single old passive margin, to which they gave the acronym name LATEA.
TL;DR: The metamorphic peak conditions of the coesite-bearing eclogites are estimated to be 27-32 kbar and 700-770 C, using garnet-pyroxene-phengite geobarometry and garnet pyroxene geothermometry, respectively as mentioned in this paper.
Abstract: Coesite relics were discovered as inclusions in clinopyroxene in eclogite and as inclusions in zircon in felsic and pelitic gneisses from Higher Himalayan Crystalline rocks in the upper Kaghan Valley, north- west Himalaya. The metamorphic peak conditions of the coesite-bearing eclogites are estimated to be 27-32 kbar and 700-770 � C, using garnet-pyroxene-phengite geobarometry and garnet-pyroxene geothermometry, respectively. Cathodoluminescence (CL) and backscattered electron (BSE) imaging distinguished three different domains in zircon: inner detrital core, widely spaced euhedral oscillatory zones, and thin, broadly zoned outermost rims. Each zircon domain contains a characteristic suite of micrometre-sized mineral inclusions which were identified by in situ laser Raman microspectroscopy. Core and mantle domains contain quartz, apatite, plagioclase, muscovite and rutile. In contrast, the rim domains contain coesite and minor muscovite. Quartz inclusions were identified in all coesite-bearing zircon grains, but not coexisting with coesite in the same growth domain (rim domain). 206 Pb ⁄ 238 U zircon ages reveal that the quartz-bearing mantle domains and the coesite-bearing rim were formed at c. 50 Ma and 46.2 ± 0.7 Ma, respectively. These facts demonstrate that the continental materials were buried to 100 km within 7-9 Myr after initiation of the India-Asia collision (palaeomagnetic data from the Indian oceanic floor supports an initial India-Asia contact at 55-53 Ma). Combination of the sinking rate of 1.1-1.4 cm year )1 with Indian plate velocity of 4.5 cm year )1 suggests that the Indian continent subducted to about 100 km depth at an average subduction angle of 14-19� .
TL;DR: In this article, a review of the rock sequences and evolution of the eastern and central terranes of Hoggar is presented, focusing on the Neoproterozoic subduction-related evolution and collision stages in the central part of the Tuareg shield.
TL;DR: In this article, it was shown that the exhumation of the MCT zone from 35 to 45 km to around 18 km, occurred from 18 to 16 to c. 13 Ma.
Abstract: Geothermometry and mineral assemblages show an increase of temperature structurally upwards across the Main Central Thrust (MCT); however, peak metamorphic pressures are similar across the boundary, and correspond to depths of 35-45 km. Garnet-bearing samples from the uppermost Lesser Himalayan sequence (LHS) yield metamorphic conditions of 650-675 ! C and 9-13 kbar. Staurolite-kyanite schists, about 30 m above the MCT, yield P-T conditions near 650 ! C, 8-10 kbar. Kyanite-bearing migmatites from the Greater Himalayan sequence (GHS) yield pressures of 10-14 kbar at 750-800 ! C. Top-to-the- south shearing is synchronous with, and postdates peak metamorphic mineral growth. Metamorphic monazite from a deformed and metamorphosed Proterozoic gneiss within the upper LHS yield U⁄Pb ages of 20-18 Ma. Staurolite-kyanite schists within the GHS, a few metres above the MCT, yield monazite ages of c. 22 ± 1 Ma. We interpret these ages to reflect that prograde meta- morphism and deformation within the Main Central Thrust Zone (MCTZ) was underway by c. 23 Ma. U⁄Pb crystallization ages of monazite and xenotime in a deformed kyanite-bearing leucogranite and kyanite-garnet migmatites about 2 km above the MCT suggest crystallization of partial melts at 18- 16 Ma. Higher in the hanging wall, south-verging shear bands filled with leucogranite and pegmatite yield U⁄Pb crystallization ages for monazite and xenotime of 14-15 Ma, and a 1-2 km thick leuco- granite sill is 13.4 ± 0.2 Ma. Thus, metamorphism, plutonism and deformation within the GHS con- tinued until at least 13 Ma. P-T conditions at this time are estimated to be 500-600 ! C and near 5 kbar. From these data we infer that the exhumation of the MCT zone from 35 to 45 km to around 18 km, occurred from 18 to 16 to c. 13 Ma, yielding an average exhumation rate of 3-9 mm year )1 . This process of exhumation may reflect the ductile extrusion (by channel flow) of the MCTZ from between the overlying Tibetan Plateau and the underthrusting Indian plate, coupled with rapid erosion.
TL;DR: In this paper, the first sensitive high-resolution ion microprobe U-Pb in zircon geochronology for the Mogok metamorphic belt (MMB) in Myanmar has been used.
Abstract: Situated south of the eastern Himalayan syntaxis at
the western margin of the Shan-Thai terrane the highgrade
Mogok metamorphic belt (MMB) in Myanmar occupies a key position in the tectonic evolution of Southeast Asia The first sensitive high-resolution ion microprobe U-Pb in zircon geochronology for the MMB shows that strongly deformed granitic orthogneisses near Mandalay contain Jurassic (~170 Ma) zircons that have partly recrystallized during ~43 Ma high-grade metamorphism A hornblende syenite
from Mandalay Hill also contains Jurassic zircons with
evidence of Eocene metamorphic recrystallization
rimmed by thin zones of 309 plus or minus 07 Ma magmatic
zircon The relative abundance of Jurassic zircons in
these rocks is consistent with suggestions that southern
Eurasia had an Andean-type margin at that time Mid-
Cretaceous to earliest Eocene (120 to 50 Ma) I-type
granitoids in the MMB, Myeik Archipelago, and Western Myanmar confirm that prior to the collision of India, an up to 200 km wide magmatic belt extended along the Eurasian margin from Pakistan to Sumatra
Metamorphic overgrowths to zircons in the orthogneiss
near Mandalay date a period of Eocene (~43 Ma)
high-grade metamorphism possibly during crustal
thickening related to the initial collision between
India and Eurasia (at 65 to 55 Ma) This was
followed by emplacement of syntectonic hornblende
syenites and leucogranites between 35 and 23 Ma
Similar syntectonic syenites and leucogranites intruded
the Ailao Shan-Red River shear belt in southern China
and Vietnam during the Eocene-Oligocene to Miocene,
and the Wang Chao and Three Pagodas faults in
northern Thailand (that most likely link with the MMB)
were also active at this time The complex history of
Eocene to early Miocene metamorphism, deformation,
and magmatism in the MMB provides evidence that it
may have played a key role in the network of
deformation zones that accommodated strain during
the northwards movement of India and resulting
extrusion or rotation of Indochina
TL;DR: Wang et al. as discussed by the authors showed that the Dulan region is a UHP metamorphic terrane, and the peak conditions of the NDB eclogites are P = 2.9-3.2 GPa, and T = 631-687°C.
Abstract: Widespread evidence for ultrahigh-pressure (UHP) metamorphism is reported in the Dulan eclogite-bearing terrane, the North Qaidam–Altun HP–UHP belt, northern Tibet. This includes: (1) coesite and associated UHP mineral inclusions in zircon separates from paragneiss and eclogite (identified by laser Raman spectroscopy); (2) inclusions of quartz pseudomorphs after coesite and polycrystalline K-feldspar + quartz in eclogitic garnet and omphacite; and (3) densely oriented SiO2 lamellae in omphacitic clinopyroxene. These lines of evidence demonstrate that the Dulan region is a UHP metamorphic terrane. In the North Dulan Belt (NDB), eclogites are characterized by the peak assemblage Grt + Omp + Rt + Phn + Coe (pseudomorph) and retrograde symplectites of Cpx + Ab and Hbl + Pl. The peak conditions of the NDB eclogites are P = 2.9–3.2 GPa, and T = 631–687 °C; the eclogite shows a near-isothermal decompression P–T path suggesting a fast exhumation. In the South Dulan Belt (SDB), three metamorphic stages are recognized in eclogites: (1) a peak eclogite facies stage with the assemblage Grt + Omp + Ky + Rt + Phn at P = 2.9–3.3 GPa and T = 729–746 °C; (2) a high-pressure granulite facies stage with Grt + Cpx (Jd < 30) + Pl (An24–29) + Scp at P = 1.9–2.0 GPa, T = 873–948 °C; and (3) an amphibolite facies stage with the assemblage Hbl + Pl + Ep/Czo at P = 0.7–0.9 GPa and T = 660–695 °C. The clockwise P–T path of the SDB eclogites is different from the near-isothermal decompression P–T path from the NDB eclogites, which suggests that the SDB was exhumed to a stable crustal depth at a slower rate. In essence these two sub-belts formed in different tectonic settings; they both subducted to mantle depths of around 100 km, but were exhumed to the Earth's surface separately along different paths. This UHP terrane plays an important role in understanding continental collision in north-western China.
TL;DR: In this article, the authors discuss the question of whether the late Mesoproterozoic and early Neoproteerozoic rocks of eastern, central and southern Africa, Madagascar, southern India, Sri Lanka and South America have played any role in the formation and dispersal of the supercontinent Rodinia, believed to have existed between about 1000 and 750 Ma ago.
TL;DR: In this paper, a kinematic model for the Himalayan thrust belt is presented that satisfies structural and metamorphic data and explains recently reported late Miocene-Pliocene geochronologic and thermochronology ages from rocks in the Main Central thrust zone in central Nepal.
Abstract: We present a kinematic model for the Himalayan thrust belt that satisfies structural and metamorphic data and explains recently reported late Miocene‐Pliocene geochronologic and thermochronologic ages from rocks in the Main Central thrust zone in central Nepal. At its current exposure level, the Main Central thrust juxtaposes a hanging-wall flat in Greater Himalayan rocks with a footwall flat in Lesser Himalayan rocks of the Ramgarh thrust sheet, which is the roof thrust of a large Lesser Himalayan duplex. Sequential emplacement of the Main Central (early Miocene) and Ramgarh (middle Miocene) thrust sheets was followed by insertion of thrust sheets within the Lesser Himalayan duplex and folding of the Main Central and Ramgarh thrusts during late Miocene‐ Pliocene time. Thorium-lead (Th-Pb) ages of monazite inclusions in garnets from central Nepal record the timing of coeval, progressive metamorphism of Lesser Himalayan rocks in the footwall of the Main Central thrust. Although this model does not rule out minor, late-stage reactivation of the Main Central thrust, major late Miocene reactivation is not required.
TL;DR: In this article, the authors reported 40Ar/39Ar ages from various tectonic units in the Aegean and westernmost Turkey, and they proposed that distinct stages of high pressure metamorphism were controlled by the underthrusting of fragments of mainly thinned continental crust and that these punctuated events were superposed on progressive slab retreat.
Abstract: [1] We report 40Ar/39Ar ages from various tectonic units in the Aegean and westernmost Turkey. On the basis of published geochronologic data and our 40Ar/39Ar ages we propose that the Aegean is made up of several high-pressure units, which were successively underplated from the Late Cretaceous until the Miocene. Ages for high-pressure metamorphism range from 80–83 Ma in parts of the Vardar-Izmir-Ankara suture zone in the north to 21–24 Ma for the Basal unit in the Cyclades and the external high-pressure belt on Crete in the south. Published seismic data suggest that high-pressure metamorphism is currently occurring underneath Crete. Younging of high-pressure metamorphism in a southerly direction mimics the southward retreat of the Hellenic subduction zone. We propose that distinct stages of high-pressure metamorphism were controlled by the underthrusting of fragments of mainly thinned continental crust and that these punctuated events were superposed on progressive slab retreat. By far most of the exhumation of the high-pressure units occurred early during the orogenic history in a forearc position.
TL;DR: In this article, a comprehensive structural and kinematic map and a general cross-section of the Dabieshan including its foreland fold belt and the Northern Dabieshans Domain (Foziling and Luzenguang groups) are presented.
TL;DR: The discovery of microdiamonds from both eclogites and felsic gneisses in the North Qinling zone in the central portion of the Central Orogenic Belt (COB) of China is reported in this article, which demonstrates that the country rocks of continental affinity shared in the ultra-high pressure metamorphic (UHPM) event and provides a bridge connecting the two previously recognized UHPM terranes.
Abstract: The Central Orogenic Belt (COB) of China is a major continental collision zone that contains extensive outcrops of deeply subducted and exhumed rocks at both the eastern and the western end of the belt. Here we report discovery of microdiamonds from both eclogites and felsic gneisses in the North Qinling zone in the central portion of the COB. This discovery demonstrates that the country rocks of continental affinity shared in the ultra-high-pressure metamorphic (UHPM) event and provides a bridge connecting the two previously recognized UHPM terranes, thereby establishing the existence of a UHPM belt extending more than 4000 km. Geochronological dating yields Early Palaeozoic ages in the west and Early Mesozoic ages in the east, recording two separate continental collisions overprinted within the COB. Occurrence of UHP metamorphism during recurrent continental collision here and in the Alps suggests that deep subduction of continental material during such collisions is probably common rather than exceptional, with significant implications for processes of plate tectonic reorganization and mantle mixing over time.
TL;DR: In this article, an interval of melting and melting of orogenic middle crust followed by emplacement of a syn-extensional pluton during decompression and cooling are demonstrated for a metamorphic core complex in central Turkey.
TL;DR: In this paper, a large-scale crustal thinning of the Alpine orogen in the westernmost Mediterranean (Alboran Sea and surrounding regions) was rapid, simultaneous over an area of 60,000 km(2), and took place similar to 25 m.y. after the main crustal thickening event.
Abstract: Large-scale crustal thinning of the Alpine orogen in the westernmost Mediterranean (Alboran Sea and surrounding regions) was rapid, simultaneous over an area of 60,000 km(2), and took place similar to25 m.y. after the main crustal thickening event. Crustal thinning, which locally exposed subcontinental mantle, was accompanied by high-grade metamorphism and partial melting. Both U-Pb dating of zircon and thermal modeling of cooling histories indicate that the thermal peak was reached between 23 and 21 Ma over the entire region. Final exhumation and cooling followed immediately. Possible explanations for this dramatic late orogenic extension include subduction rollback, slab detachment, lithospheric delamination, and convective removal of subcontinental lithosphere. Of these processes, only convective removal of subcontinental lithosphere predicts a significant time lapse between crustal thickening and the onset of extension, the simultaneity of extension over the whole region, and heating of rock during rapid exhumation to produce the high-grade metamorphic event.
TL;DR: In this paper, a 200km-long and up to 40km-wide area of the Zambezi Belt in central Zambia is interpreted to represent part of a late Precambrian suture zone, with the mafic rocks being relics of subducted oceanic crust.
Abstract: Gabbros and eclogites occur closely associated in a 200-km-long and up to 40-km-wide area of the Zambezi Belt in central Zambia. This area is interpreted to represent part of a late Precambrian suture zone, with the mafic rocks being relics of subducted oceanic crust. Gradual stages of prograde transformation from gabbro to eclogite are preserved by disequilibrium textures of incomplete reactions. This resulted in kyanite-omphacite-bearing assemblages for eclogites that have Al-poor bulk compositions. Undeformed eclogites typically preserve features of a former gabbroic texture, reflected by replacements of plagioclase and magmatic pyroxene by eclogite facies minerals. Textures of deformed eclogites range from sheared porphyroclastic to porphyroblastic. Relics of magmatic pyroxene are common and complete eclogitisation occurred only in millimetre to centimetre-scale domains in most of the rocks. No evidence for prograde blueschist or amphibolite facies mineral assemblages was found in eclogites. In contrast, the fine grained intergrowth of omphacite, garnet, kyanite and quartz, which replace former plagioclase or was formed in the pressure shadow of magmatic pyroxene relics, indicates that eclogitisation might have affected the gabbroic protoliths directly without any significant intervening metamorphic reactions. Eclogitisation took place under P-T conditions of 630-690 degreesC and 26-28 kbar, suggesting a large overstepping (>10 kbar) of reaction boundaries. Eclogitisation was initialised and accompanied by a channelised fluid flow resulting in veins with large, subhedral grains of omphacite, kyanite and garnet. The gabbro-to-eclogite transformation was enhanced by a fluid which allowed the necessary material transport for the dissolution-precipitation mechanism that characterises the metamorphic mineral replacements. The process of eclogitisation was limited by reaction kinetics and dissolution-precipitation rates rather than by the metamorphic P-T conditions. Even though ductile deformation occurred and equilibrium phase boundaries were overstepped, the infiltration of fluids was necessary for triggering the gabbro-to-eclogite transformation.