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Showing papers on "Metamorphism published in 2002"


Journal ArticleDOI
TL;DR: In this article, the trace element distribution coefficients between zircon and garnet were analyzed for trace elements using LA-ICP-MS and SHRIMP ion microprobe.

2,246 citations


Journal ArticleDOI
TL;DR: In this paper, the age of metamorphic zircon has been determined using trace element analysis and integrated cathodoluminescence, U-Pb isotope, trace element and electron backscatter diffraction pattern (EBSP) analyses.
Abstract: Protolith zircon in high-grade metagranitoids from Queensland, Australia, partially recrystallized during granulite-grade metamorphism. We describe the zircon in detail using integrated cathodoluminescence, U–Pb isotope, trace element and electron backscatter diffraction pattern (EBSP) analyses. Primary igneous oscillatory zoning is partially modified or obliterated in areas within single crystals, but is well preserved in other areas. A variety of secondary internal structures are observed, with large areas of transgressive recrystallized zircon usually dominant. Associated with these areas are recrystallization margins, interpreted to be recrystallization fronts, that have conformable boundaries with transgressive recrystallized areas, but contrasting cathodoluminescence and trace element chemistry. Trace element analyses of primary and secondary structures provide compelling evidence for closed-system solid-state recrystallization. By this process, trace elements in the protolith zircon are purged during recrystallization and partitioned between the enriched recrystallization front and depleted recrystallized areas. However, recrystallization is not always efficient, often leaving a ‘memory’ of the protolith trace element and isotopic composition. This results in the measurement of ‘mixed’ U–Pb isotope ages. Nonetheless, the age of metamorphism has been determined. A correlation between apparent age and Th/U ratio is indicative of incomplete re-setting by partial recrystallization. Recrystallization is shown to probably not significantly affect Lu–Hf ages. Recrystallization has been determined by textural and trace element analysis and EBSP data not to have proceeded by sub-grain rotation or local dissolution/re-precipitation, but probably by grain-boundary migration and defect diffusion. The formation of metamorphic zircon by solid-state recrystallization is probably common to high-grade terranes worldwide. The recognition of this process of formation is essential for correct interpretation of zircon-derived U–Pb ages and subsequent tectonic models.

1,456 citations


Journal ArticleDOI
TL;DR: In this article, the degree of organisation of carbonaceous material was systematically characterised by Raman microspectroscopy across several Mesozoic and Cenozoic reference metamorphic belts.
Abstract: Metasedimentary rocks generally contain carbonaceous material (CM) deriving from the evolution of organic matter originally present in the host sedimentary rock. During metamorphic processes, this organic matter is progressively transformed into graphite s.s. and the degree of organisation of CM is known as a reliable indicator of metamorphic grade. In this study, the degree of organisation of CM was systematically characterised by Raman microspectroscopy across several Mesozoic and Cenozoic reference metamorphic belts. This degree of organisation, including within-sample heterogeneity, was quantified by the relative area of the defect band (R2 ratio). The results from the Schistes Lustres (Western Alps) and Sanbagawa (Japan) cross-sections show that (1) even through simple visual inspection, changes in the CM Raman spectrum appear sensitive to variations of metamorphic grade, (2) there is an excellent agreement between the R2 values calculated for the two sections when considering samples with an equivalent metamorphic grade, and (3) the evolution of the R2 ratio with metamorphic grade is controlled by temperature (T). Along the Tinos cross-section (Greece), which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks, the R2 ratio is nearly constant. Consequently, the degree of organisation of CM is not affected by the retrogression and records peak metamorphic conditions. More generally, analysis of 54 samples representative of high-temperature, low-pressure to high-pressure, low-temperature metamorphic gradients shows that there is a linear correlation between the R2 ratio and the peak temperature [T(°C) = −445 R2 + 641], whatever the metamorphic gradient and, probably, the organic precursor. The Raman spectrum of CM can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism. Temperature can be estimated to ± 50 °C in the range 330–650 °C. A few technical indications are given for optimal application.

958 citations


Journal ArticleDOI
01 Jun 2002-Geology
TL;DR: In this article, it was shown that granulite terrains were too hot to have formed during continental collision and most formed in accretionary orogens during tectonic switching, when prolonged lithospheric extension was interrupted by intermittent, transient contraction.
Abstract: Many granulite terrains were too hot to have formed during continental collision. Rather, along with many high-grade metamorphic terrains that typify continental crust, most formed in accretionary orogens during tectonic switching, when prolonged lithospheric extension was interrupted by intermittent, transient contraction. Based on modern and ancient examples, tectonic switching occurs when slab retreat induces upper plate extension, causing arc splitting, formation of microcontinent slivers, and backarc basins; then intermittent arrival of buoyant oceanic plateaus induces transient flat subduction (or slab flip) and crustal thickening. During slab retreat, basaltic magmas produced from decompressed asthenosphere advect into the extending orogen, causing granulite facies metamorphism and granite generation, but subsequent thickening during flat subduction cools the region. Thickening is focused in the thermally softened backarc region and, if sediment filled, a hot, short-lived (~10 m.y.), narrow (50– 100 km) orogenic belt forms. Such thickening is often misleadingly ascribed to arc or microcontinent collision. Once slab-retreat mode is reestablished, lithospheric extension recommences and a new arc-backarc system forms, generally outboard. Arrival of another plateau will reverse the procedure, and another short-lived, hot orogen will form within the orogenic system. Cycles of tectonic switching efficiently produce continental crust.

568 citations


Journal ArticleDOI
TL;DR: In this article, the authors used U-Pb geochronology combined with cathodoluminescence (CL) imaging of zircon enables resolution of magmatic and metamorphic events that can be directed towards understanding the late Archean to Paleoproterozoic history of the Fuping Complex.
Abstract: The Fuping Complex is situated in the central part of the North China Craton and consists of four major lithological assemblages: Fuping tonalitic–trondhjemitic–granodioritic gneisses, Longquanguan augen gneisses, Wanzi supracrustal assemblage and Nanying granitic gneisses. SHRIMP U–Pb geochronology combined with U–Th and cathodoluminescence (CL) imaging of zircon enables resolution of magmatic and metamorphic events that can be directed towards understanding the late Archean to Paleoproterozoic history of the Fuping Complex. CL images reveal the coexistence of magmatic and metamorphic zircons in nearly all rock types of the Fuping Complex. The metamorphic zircons occur as either single grains or overgrowth (or recrystallization) rims surrounding and truncating oscillatory-zoned magmatic zircon cores, and are all characterized by nebulous zoning or being structureless, with extremely high luminescence and very low Th contents. These features make them distinct from magmatic zircons that are characterized by concentric oscillatory zoning, comparatively low luminescence and high Th and U contents. SHRIMP U–Pb analyses on magmatic zircons reveal that the tonalitic, trondhjemitic and granodioritic plutons of the Fuping gneisses were emplaced at 2523 ± 14 Ma, 2499 ± 10 Ma and 2486 ± 8 Ma, respectively; whereas the monzogranitic and granitic plutons of the Longquanguan augen gneisses were intruded, respectively, at 2510 ± 22 Ma and 2507 ± 11 Ma. Prismatic and oscillatory-zoned zircons dominate in the pelitic rocks of the Wanzi supracrustal assemblage and are interpreted as detritus from igneous source rocks. The concordant and discordant U–Pb ages of 2502 ± 5 Ma and 2507 ± 14 Ma obtained from two pelitic rock samples indicate these rocks must have been deposited no earlier than ∼2507 Ma ago. In addition, a zoned zircon grain in one pelitic rock sample has a near concordant age of 2109 ± 5 (1σ) Ma, which may provide a maximum depositional age for the Wanzi supracrustal rocks. SHRIMP results also reveal that granitic magmatism assigned to the Nanying granitic gneisses occurred over a protracted interval from ∼2077 ± 13 Ma to ∼2024 ± 21 Ma. The nebulously-zoned zircon grains and overgrowth/recrystallization zircon rims from different rocks yielded similar concordant 207Pb/206Pb ages in the range 1875 to 1802 Ma, interpreted as approximating the age of regional metamorphism of the Fuping Complex. Timing of primary zircon crystallization and regional metamorphism of the Fuping Complex is in general agreement with recent U–Pb zircon ion probe results for the Wutai and Hengshan Complexes that bound the Fuping Complex to the northwest. These areas are characterized by the emplacement of major granitoid bodies at around 2.50 Ga to 2.48 Ga ago, deposition of supracrustal rocks in the Paleoproterozoic, intrusion of Paleoproterozoic granitic bodies at 2.1 to 2.0 Ga, and regional metamorphism at 1.875 to 1.802 Ga. These data indicate that the Fuping and Hengshan Complexes do not represent an older crystalline basement to the Wutai Complex, as suggested in previous tectonic models but, together with the Wutai Complex, represent elements of a single late Archean to Paleoproterozoic magmatic arc system that has been subsequently tectonically disrupted and juxtaposed during the collision of the eastern and western North China blocks at ∼1.85 Ga, which resulted in the final assembly of the North China Craton.

404 citations


Journal ArticleDOI
TL;DR: In this article, four stages of the metamorphic history have been recognized in mineral assemblages based on inclusion, exsolution and reaction textures integrated with garnet zonation patterns as revealed by compositional maps and compositional profiles.
Abstract: High-pressure basic granulites are widely distributed as enclaves and sheet-like blocks in the Huaian TTG gneiss terrane in the Sanggan area of the Central Zone of the North China craton. Four stages of the metamorphic history have been recognised in mineral assemblages based on inclusion, exsolution and reaction textures integrated with garnet zonation patterns as revealed by compositional maps and compositional profiles. The P–T conditions for each metamorphic stage were obtained using thermodynamically and experimentally calibrated geothermobarometers. The low-Ca core of growth-zoned garnet, along with inclusion minerals, defines a prograde assemblage (M1) of garnet + clinopyroxene + plagioclase + quartz, yielding 700 °C and 10 kbar. The peak of metamorphism at about 750–870 °C and 11–14.5 kbar (M2) is defined by high-Ca domains in garnet interiors and inclusion minerals of clinopyroxene, plagioclase and quartz. Kelyphites or coronas of orthopyroxene + plagioclase ± magnetite around garnet porphyroblasts indicate garnet breakdown reactions (M3) at conditions around 770–830 °C and 8.5–10.5 kbar. Garnet exsolution lamellae in clinopyroxene and kelyphites of amphibole + plagioclase around garnet formed during the cooling process at about 500–650 °C and 5.5–8 kbar (M4). These results help define a sequential P–T path containing prograde, near-isothermal decompression (ITD) and near-isobaric cooling (IBC) stages. The clockwise hybrid ITD and IBC P–T paths of the HP granulites in the Sanggan area imply a model of thickening followed by extension in a collisional environment. Furthermore, the relatively high-pressures (6–14.5 kbar) of the four metamorphic stages and the geometry of the P–T paths suggest that the HP granulites, together with their host Huaian TTG gneisses, represent the lower plate in a crust thickened during collision. The corresponding upper-plate might be the tectonically overlying Khondalite series, which was subjected to medium- to low-pressure (MP/LP: 7–4 kbar) granulite facies metamorphism with a clockwise P–T path including an ITD segment. Both the HP and the MP/LP granulite facies events occurred contemporaneously at c. 1.90–1.85 Ga in a collisional environment created by the assembly process of the North China craton.

335 citations


Journal ArticleDOI
01 Jun 2002-Lithos
TL;DR: In this article, a model for calc-alkaline Late Granite (Siluro-Devonian) genesis in the metamorphic Caledonian orogenic belt of Ireland and Scotland is presented.

304 citations


Journal ArticleDOI
TL;DR: Extensional accretionary orogens form by creation and destruction of large arc/back arc basin systems, generated by extension and sediment infilling during prolonged slab retreat, but episodically thickened by basin inversion during short-lived (∼10 Ma), orogenic contraction events.
Abstract: Extensional accretionary orogens form by creation and destruction of large arc/back arc basin systems, generated by extension and sediment infilling during prolonged slab retreat, but episodically thickened by basin inversion during short-lived (∼10 Ma), orogenic contraction events. They are characterised by widespread, syntectonic, silicic, and minor basaltic magmatism, regional low-P, variable-T metamorphism, and by the enigmatic development of rift basins throughout the peak orogenic history. These orogens have features associated with retreating subduction boundaries and contrast markedly with those formed by terrane accretion, such as the Canadian Cordillera. The Paleozoic Lachlan orogen example from eastern Australia shows that Silurian-Devonian synorogenic basalts and gabbros were intimately associated with rifting and granite emplacement, but they formed during a period of repeated orogenic contraction. Moreover, primitive basaltic compositions have oceanic affinities, indicating generation under lithosphere that was <30 km thick. Only in the final stages of orogeny (Middle Devonian), after at least three major crustal contraction events, did the lithosphere thicken to ∼80 km or more, leading to stabilization of the orogen. Extensional accretionary orogens grow by magmatic and sedimentary additions during extension, caused mainly by asthenospheric melting and rift basin formation/sedimentation, augmented by localized and repeated crustal thickening events. Orogenic contraction leaves an indelible structural imprint which may obliterate the prior-formed extensional structures. The orogen remains hot, despite repeated thickening events, because of ongoing extension, which promotes advective heat transfer into the crust by basalt injection and crustal melting. Rapid switching to contraction, possibly during intermittent arrival of buoyant oceanic plateaus, inverts the thermally softened basins and forms localized fold-thrust belts in which the penetrative foliations record the peak metamorphism. Their most diagnostic features are the presence of basaltic rocks and rift basins throughout the orogenic contraction history.

286 citations


Journal ArticleDOI
TL;DR: The Dabie Shan Ultrahigh-Pressure Metamorphic (UHPM) Belt has been used for dating of zircons and monazites in this paper.

256 citations


Journal ArticleDOI
TL;DR: The Jiaodong gold province of China as discussed by the authors is one of the largest gold deposits in the world, containing >900 tonnes of gold and gold-bearing quartz veins.
Abstract: The Jiaodong gold province, along the southeastern margin of the North China craton, has combined production and resources of >900 tonnes of gold. More than 95% of the gold in this province is hosted in Mesozoic granitoids, which intruded Late Archean metamorphic basement rocks episodically at ca. 160–150 and 130–126 Ma. The deposits of the Jiaodong Peninsula form one of the largest provinces of granitoid-hosted lode-gold deposits in the world. The Jiaodong gold deposits occur as massive auriferous quartz veins hosted in subsidiary second- or third-order faults, and as disseminated- and stockwork-style mineralization hosted in alteration zones along major regional-scale faults. Mineralization took place during brittle reactivation (D3) of pre-existing D2 ductile shear zones. Wall rock alteration is dominated by K-feldspar and sericite, with also sulfide minerals and extensive silicification closest to the orebodies. The alteration was accompanied by the introduction of volatiles, Fe, Mg, K, Rb, Ag, As, Au, Bi, and Sb. Enrichment of base metals in the gold lodes is generally low, although locally significant. Gold deposition took place at temperatures of 250 to 350 °C for most gold deposits, and the ore fluids are CO2-bearing and have calculated salinities of 6–13 wt% NaCl equiv. Deposits from the Zhao–Ye, Xixia, and Muping–Rushan belts have similar, if not identical, structural controls, alteration patterns, ore mineral assemblages, and stable isotope signatures. Lead isotope data indicate that the ore-stage leads were buffered by their host rocks. The granitoid-hosted gold deposits in the Zhao–Ye belt are most radiogenic, the greenstone-hosted deposits in the Xixia belt is the least radiogenic and the granitoid-hosted deposits in the Muping–Rushan belt have intermediate compositions. Gold mineralization in the Zhao–Ye, Xixia, and Muping–Rushan belts in the Jiaodong gold province appears to be broadly contemporaneously at ca. 130–120 Ma, as constrained by SHRIMP U–Pb zircon dating on host rocks and a post-gold dike, as well as by Ar–Ar, Rb–Sr, and K–Ar dating of hydrothermal alteration minerals and ores. This Early Cretaceous age is about 100 million years younger than the peak of ultra-high pressure metamorphism in the Qinling–Dabie–Sulu orogen between the North China and Yangtze cratons, and about 500 km south of the gold province. However, it overlaps with the extension and exhumation stage of the collisional orogeny, and the subduction of the Pacific plate beneath the Eurasian continental crust. Whereas the majority of the Jiaodong gold deposits occur on the margin of the North China craton, some also occur in the suture zone between the two cratons. They are significantly landward relative to the subduction zone of the Pacific Plate compared with typical orogenic lode-gold deposits globally. Lode-gold deposits of ca. 130–120 Ma occurred on both sides of the Tan-Lu fault zone. Available data favor a temporal link of gold mineralization with Pacific Plate subduction.

231 citations


Journal ArticleDOI
TL;DR: The Fuping Complex as discussed by the authors comprises three major lithological assemblages: the ∼2.50 Ga trondhjemite-tonalite-granodiorite (TTG) gneisses, the Wanzi paragneisses and the 2.02 Ga Nanying granitic gneises.

Journal ArticleDOI
TL;DR: In this paper, the authors used numerical simulations of coupled intergranular and intracrystalline diffusion processes in coronal textures around partially resorbed garnet crystals from the Llano Uplift, U.S.A.
Abstract: Mounting evidence suggests that partial disequilibrium—meaning disequilibrium for some elements, but not for others—may be a common but rarely detected phenomenon during metamorphic mineral growth, even in ordinary prograde reactions that progress to completion. Detailed examination of compositional variations in garnet crystals over a range of metamorphic grade suggests distinctly different scales of equilibration for common elements, with strong temperature dependence. Under lower greenschist-facies conditions, Fe and Mg may equilibrate at hand-sample scale, whereas Mn and Ca may not equilibrate even at millimeter-scale. Although Mn may achieve hand-sample-scale equilibration under upper greenschist-facies conditions, Ca and many trivalent cations (e.g., REEs) may not do so until temperatures exceed those of the middle amphibolite facies. Even in the lower granulite facies, some elements (e.g., Y, Yb) show indications of disequilibrium at sub-centimeter-scales during garnet growth. Analysis and numerical modeling of undisputed disequilibrium textures demonstrate that the most common impediment to equilibration during metamorphism is the sluggishness of intergranular diffusion, the same mechanism known to govern porphyroblast crystallization in many metamorphic environments. Despite this importance to petrology, few quantitative determinations exist of inter-granular diffusion rates under metamorphic conditions. Using numerical simulations of coupled intergranular and intracrystalline diffusion processes in coronal textures around partially resorbed garnet crystals from the Llano Uplift, U.S.A., a very precise and relatively accurate estimate is obtained for the rate of intergranular diffusion of Al in fluid-undersaturated systems. This result and an earlier estimate for fluid-saturated systems provide bracketing values for Al diffusivity during metamorphism.

Journal ArticleDOI
01 Jul 2002-Geology
TL;DR: In this paper, a slab breakoff model was proposed to explain the Eclogites, Miocene partial melts, and late Eocene K-rich magmas in southeastern Tibet, and that metamorphic and plutonic ages help define the timing and rates of breakoff and extrusion.
Abstract: The Greater Himalayan Sequence is the metamorphic core of the Himalaya and has been a focus of considerable study, yet its petrologic evolution remains controversial. Pre-Oligocene metamorphism was nearly obliterated by Miocene metamorphism and melting, which many workers ascribe to shear heating, unusually high concentrations of radioactive elements accompanying burial, and/or several kilometers of Miocene exhumation due to extensional faulting. Sparse Oligocene to Eocene ages are often assigned to a tectonically unspecified event. An alternative slab-breakoff model (subduction of Greater Himalayan rocks to ∼100 km depth in the Eocene followed by buoyant extrusion due to decoupling of the oceanic lithosphere) has also been proposed based on rare Eocene eclogites, but without explanation of Miocene melting or metamorphic petrogenesis. We argue that slab breakoff readily explains the Eocene eclogites, Miocene partial melts, and late Eocene K-rich magmas in southeastern Tibet, and that metamorphic and plutonic ages help define the timing and rates of breakoff and extrusion. This model implies that (1) much of the Greater Himalayan Sequence was subducted to depths greater than commonly considered, (2) fluid-absent, decompression melting at 30-35 km depth was the consequence of as much as 100 km of extrusion, rather than radioactive heating or a smaller, crustal level extensional or erosional event, and (3) eruption of Eocene, K-rich Tibetan Plateau lavas has no implication for topography of the Tibetan Plateau.

Journal ArticleDOI
TL;DR: In this article, the Cretaceous granitoids from the Dabie orogenic belt have been compared to the basement rocks of the Nd-Nd-Pb isotopic compositions.

Journal ArticleDOI
TL;DR: In this paper, coupled thermal-mechanical models are used to investigate interactions between metamorphism, deformation and exhumation in large convergent orogens, and the implications of coupling and feedback between these processes for observed structural and metamorphic styles.
Abstract: Coupled thermal-mechanical models are used to investigate interactions between metamorphism, deformation and exhumation in large convergent orogens, and the implications of coupling and feedback between these processes for observed structural and metamorphic styles. The models involve subduction of suborogenic mantle lithosphere, large amounts of convergence (≥ 450 km) at 1 cm yr−1, and a slope-dependent erosion rate. The model crust is layered with respect to thermal and rheological properties — the upper crust (0–20 km) follows a wet quartzite flow law, with heat production of 2.0 μW m−3, and the lower crust (20–35 km) follows a modified dry diabase flow law, with heat production of 0.75 μW m−3. After 45 Myr, the model orogens develop crustal thicknesses of the order of 60 km, with lower crustal temperatures in excess of 700 °C. In some models, an additional increment of weakening is introduced so that the effective viscosity decreases to 1019 Pa.s at 700 °C in the upper crust and 900 °C in the lower crust. In these models, a narrow zone of outward channel flow develops at the base of the weak upper crustal layer where T≥600 °C. The channel flow zone is characterised by a reversal in velocity direction on the pro-side of the system, and is driven by a depth-dependent pressure gradient that is facilitated by the development of a temperature-dependent low viscosity horizon in the mid-crust. Different exhumation styles produce contrasting effects on models with channel flow zones. Post-convergent crustal extension leads to thinning in the orogenic core and a corresponding zone of shortening and thrust-related exhumation on the flanks. Velocities in the pro-side channel flow zone are enhanced but the channel itself is not exhumed. In contrast, exhumation resulting from erosion that is focused on the pro-side flank of the plateau leads to ‘ductile extrusion’ of the channel flow zone. The exhumed channel displays apparent normal-sense offset at its upper boundary, reverse-sense offset at its lower boundary, and an ‘inverted’ metamorphic sequence across the zone. The different styles of exhumation produce contrasting peak grade profiles across the model surfaces. However, P–T–t paths in both cases are loops where Pmax precedes Tmax, typical of regional metamorphism; individual paths are not diagnostic of either the thickening or the exhumation mechanism. Possible natural examples of the channel flow zones produced in these models include the Main Central Thrust zone of the Himalayas and the Muskoka domain of the western Grenville orogen.

Journal ArticleDOI
TL;DR: In this paper, the Fuping Complex and the adjoining Wutai and Hengshan Complexes are located in the central zone of the North China craton and the dominant rock types are high-grade tonalitic-trondhjemitic-granodioritic (TTG) gneisses, with minor amounts of mafic granulites, syntectonic granitic rocks and supracrustal rocks.
Abstract: The Fuping Complex and the adjoining Wutai and Hengshan Complexes are located in the central zone of the North China craton. The dominant rock types in the Fuping Complex are high-grade tonalitic–trondhjemitic–granodioritic (TTG) gneisses, with minor amounts of mafic granulites, syntectonic granitic rocks and supracrustal rocks. The petrological evidence from the mafic granulites indicates three stages of metamorphic evolution. The M1 stage is represented by garnet porphyroblasts and matrix plagioclase, quartz, orthopyroxene, clinopyroxene and hornblende. Orthopyroxene+plagioclase symplectites and clinopyroxene+plagioclase±orthopyroxene coronas formed in response to decompression during M2 following the peak metamorphism at M1. Hornblende+plagioclase symplectites formed as a result of further isobaric cooling and retrograde metamorphism during M3. The P–T estimates using TWQ thermobarometry are: 900–950 °C and 8.0–8.5 kbar for the peak assemblage (M1), based on the core compositions of garnet, matrix pyroxene and plagioclase; 700–800 °C and 6.0–7.0 kbar for the pyroxene+plagioclase symplectites or coronas (M2); and 550–650 °C and 5.3–6.3 kbar for the hornblende+plagioclase symplectites (M3), based on garnet rim and corresponding symplectic mineral compositions. These P–T estimates define a clockwise P–T path involving near-isothermal decompression for the Fuping Complex, similar to the P–T path estimated for the metapelitic gneisses. The inferred P–T path suggests that the Fuping Complex underwent initial crustal thickening, subsequent exhumation, and finally cooling and retrogression. This tectonothermal path is similar to P–T paths inferred for the Wutai and Hengshan Complexes and other tectonic units in the central zone of the North China craton, but different from anti-clockwise P–T paths estimated for the basement rocks in the eastern and western zones of the craton. Based on lithological, structural, metamorphic and geochronological data, the eastern and western zones of the craton are considered to represent two different Archean to Paleoproterozoic continental blocks that amalgamated along the central zone at the end of Paleoproterozoic. The P–T paths of the Fuping Complex and other tectonic units in the central zone record the collision between the eastern and western zones that led to the final assembly of the North China craton at c. 1800 Ma.

Journal ArticleDOI
TL;DR: In this paper, the authors used magnetostratigraphy to determine the sediment sources, their metamorphic grade and exhumation rates in the Himalayan palaeo-hinterland.

Journal ArticleDOI
TL;DR: Zircon cores from the Huwan shear zone have Ordovician to Devonian ages, flat to steep heavy-REE patterns, negative Eu anomalies, and in some cases plagioclase inclusions, indicative of derivation from North China Block igneous and low pressure metamorphic source rocks.
Abstract: SHRIMP U–Pb dating and laser ablation ICP-MS trace element analyses of zircon from four eclogite samples from the north-western Dabie Mountains, central China, provide evidence for two eclogite facies metamorphic events Three samples from the Huwan shear zone yield indistinguishable late Carboniferous metamorphic ages of 312 ± 5, 307 ± 4 and 311 ± 17 Ma, with a mean age of 309 ± 3 Ma One sample from the Hong'an Group, 1 km south of the shear zone yields a late Triassic age of 232 ± 10 Ma, similar to the age of ultra-high pressure (UHP) metamorphism in the east Qinling–Dabie orogenic belt REE and other trace element compositions of the zircon from two of the Huwan samples indicate metamorphic zircon growth in the presence of garnet but not plagioclase, namely in the eclogite facies, an interpretation supported by the presence of garnet, omphacite and phengite inclusions Zircon also grew during later retrogression Zircon cores from the Huwan shear zone have Ordovician to Devonian (440–350 Ma) ages, flat to steep heavy-REE patterns, negative Eu anomalies, and in some cases plagioclase inclusions, indicative of derivation from North China Block igneous and low pressure metamorphic source rocks Cores from Hong'an Group zircon are Neoproterozoic (780–610 Ma), consistent with derivation from the South China Block In the western Dabie Mountains, the first stage of the collision between the North and South China Blocks took place in the Carboniferous along a suture north of the Huwan shear zone The major Triassic continent–continent collision occurred along a suture at the southern boundary of the shear zone The first collision produced local eclogite facies metamorphism in the Huwan shear zone The second produced widespread eclogite facies metamorphism throughout the Dabie Mountains–Sulu terrane and a lower grade overprint in the shear zone

01 Jan 2002
TL;DR: In this paper, the degree of organisation of carbonaceous material was systematically characterised by Raman microspectroscopy across several Mesozoic and Cenozoic reference metamorphic belts.
Abstract: Metasedimentary rocks generally contain carbonaceous material (CM) deriving from the evolution of organic matter originally present in the host sedimentary rock. During metamorphic processes, this organic matter is progressively transformed into graphite s.s. and the degree of organisation of CM is known as a reliable indicator of metamorphic grade. In this study, the degree of organisation of CM was systematically characterised by Raman microspectroscopy across several Mesozoic and Cenozoic reference metamorphic belts. This degree of organisation, including within-sample heterogeneity, was quantified by the relative area of the defect band (R2 ratio). The results from the Schistes Lustres (Western Alps) and Sanbagawa (Japan) cross-sections show that (1) even through simple visual inspection, changes in the CM Raman spectrum appear sensitive to variations of metamorphic grade, (2) there is an excellent agreement between the R2 values calculated for the two sections when considering samples with an equivalent metamorphic grade, and (3) the evolution of the R2 ratio with metamorphic grade is controlled by temperature (T). Along the Tinos cross-section (Greece), which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks, the R2 ratio is nearly constant. Consequently, the degree of organisation of CM is not affected by the retrogression and records peak metamorphic conditions. More generally, analysis of 54 samples representative of high- temperature, low-pressure to high-pressure, low-temperature metamorphic gradients shows that there is a linear correlation between the R2 ratio and the peak temperature (T(� C) ¼ )445 R2 + 641), whatever the metamorphic gradient and, probably, the organic precursor. The Raman spectrum of CM can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism. Temperature can be estimated to ± 50 � C in the range 330-650 � C. A few technical indications are given for optimal application.

Journal ArticleDOI
TL;DR: In this article, the ages of composite zircon segments from the aureole of the Rogaland anorthosite-norite intrusive complex were characterized using electron backscattered electron and cathodoluminescence images plus trace element analysis.
Abstract: SHRIMP U–Pb ages have been obtained for zircon in granitic gneisses from the aureole of the Rogaland anorthosite–norite intrusive complex, both from the ultrahigh temperature (UHT; >900 °C pigeonite-in) zone and from outside the hypersthene-in isograd. Magmatic and metamorphic segments of composite zircon were characterised on the basis of electron backscattered electron and cathodoluminescence images plus trace element analysis. A sample from outside the UHT zone has magmatic cores with an age of 1034 ± 7 Ma (2σ, n = 8) and 1052 ± 5 Ma (1σ, n = 1) overgrown by M1 metamorphic rims giving ages between 1020 ± 7 and 1007 ± 5 Ma. In contrast, samples from the UHT zone exhibit four major age groups: (1) magmatic cores yielding ages over 1500 Ma (2) magmatic cores giving ages of 1034 ± 13 Ma (2σ, n = 4) and 1056 ± 10 Ma (1σ, n = 1) (3) metamorphic overgrowths ranging in age between 1017 ± 6 Ma and 992 ± 7 Ma (1σ) corresponding to the regional M1 Sveconorwegian granulite facies metamorphism, and (4) overgrowths corresponding to M2 UHT contact metamorphism giving values of 922 ± 14 Ma (2σ, n = 6). Recrystallized areas in zircon from both areas define a further age group at 974 ± 13 Ma (2σ, n = 4). This study presents the first evidence from Rogaland for new growth of zircon resulting from UHT contact metamorphism. More importantly, it shows the survival of magmatic and regional metamorphic zircon relics in rocks that experienced a thermal overprint of c. 950 °C for at least 1 Myr. Magmatic and different metamorphic zones in the same zircon are sharply bounded and preserve original crystallization age information, a result inconsistent with some experimental data on Pb diffusion in zircon which predict measurable Pb diffusion under such conditions. The implication is that resetting of zircon ages by diffusion during M2 was negligible in these dry granulite facies rocks. Imaging and Th/U–Y systematics indicate that the main processes affecting zircon were dissolution-reprecipitation in a closed system and solid-state recrystallization during and soon after M1.

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TL;DR: In this paper, a new structural map was prepared using Landsat TM ratio images and SIR-C/X SAR imagery, which can distinguish and map the relationships between rock units, structures, and alteration zones associated with mineral deposits along the Allaqi suture of Egypt's SE Desert.

Journal ArticleDOI
TL;DR: The Eastern Nigeria terrane belongs to the 3000 km-long Trans-Saharan belt which was formed in the Neoproterozoic, between 750 and 500 Ma by continental collision between the converging West African craton, Congo craton and East Saharan block as discussed by the authors.

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TL;DR: In this paper, the stratigraphical and structural continuity of the Late Proterozoic Dalradian rocks of the Scottish Highlands is re-examined in the light of new U-Pb zircon ages on the tuffs belonging to the Tayvallich Volcanic Formation (601 ± 4 Ma), and on the late Grampian ‘Newer Gabbros’ (470
Abstract: The stratigraphical and structural continuity of the Late Proterozoic Dalradian rocks of the Scottish Highlands is re-examined in the light of new U–Pb zircon ages on the tuffs belonging to the Tayvallich Volcanic Formation (601 ± 4 Ma), and on the late Grampian ‘Newer Gabbros’ (470 ± 9 Ma) of Insch and Morven–Cabrach in Aberdeenshire. These age data, together with the existing 590 ± 2 Ma age for the Ben Vuirich Granite, provide key radiometric constraints on the evolution of the Dalradian block, and the implications arising from these ages are critically assessed. Three main conclusions are drawn. (1) The entire Caledonian orogeny, although short-lived, is unlikely to have affected sediments of Arenig age and a break probably occurs between those Dalradian sediments of late Proterozoic ( (2) A period of crustal thickening probably affected some Dalradian rocks prior to 590 Ma. Such an event is indicated by both the polymetamorphic histories of the lower parts of the Dalradian pile and the contact metamorphic assemblages within the aureole of the Ben Vuirich Granite, which are incompatible with sedimentary thicknesses. (3) Age constraints on global Late Proterozoic glacial activity also suggest that the Dalradian stratigraphy is broken into discrete smaller units. Models involving continuous deposition of Dalradian sediments from pre-750 Ma to 470 Ma are rejected.

Journal ArticleDOI
TL;DR: In this paper, high-resolution ion microprobe (SHRIMP) zircon analyses have been used to place time constraints on the Proterozoic reworking of Archaean rocks during the Rayner Structural Episode in the Oygarden Group of islands, Kemp Land, East Antarctica.

Journal ArticleDOI
TL;DR: The Nagssugtoqidian Orogen of West Greenland represents a belt of Palaeoproterozoic deformation and metamorphism between the North Atlantic Craton of South Greenland and a northern, lesser known co...
Abstract: The Nagssugtoqidian Orogen of West Greenland represents a belt of Palaeoproterozoic deformation and metamorphism between the North Atlantic Craton of South Greenland and a northern, lesser known co...

Journal ArticleDOI
TL;DR: The amphibolite facies Moyar, Bhavani and Palghat shear zones separate the South Indian Dharwar Craton (DC) from the Nilgiri and Madurai high-grade crustal provinces as discussed by the authors.

Journal ArticleDOI
TL;DR: The timing of garnet growth during metamorphism associated with the Grampian Orogeny in the sillimanite zone of the Barrovian type-locality in Glen Clova, Scotland, was determined by Sm/Nd geochronology as discussed by the authors.
Abstract: The timing of garnet growth during metamorphism associated with the Grampian Orogeny in the sillimanite zone of the Barrovian type-locality in Glen Clova, Scotland, was determined by Sm/Nd geochronology. Two high precision garnet-whole-rock ages were achieved by employing HF partial dissolution of garnet separates to optimize purity. Multiple garnet growth generations were identified on the basis of the geochronology and detailed textural and chemical data: an early stage, at 472.9 ± 2.9 Ma, during D 2 deformation under garnet zone conditions (c. 500–550 °C), and a later stage, at 464.8 ± 2.7 Ma, during or slightly after D 3 deformation mostly under sillimanite zone conditions (peak temperature of c. 660 °C), but possibly including some growth during kyanite zone conditions. When combined with recently published garnet ages from the kyanite and garnet zones the data suggest that peak metamorphic temperatures in at least these three of Barrow9s zones were achieved roughly contemporaneously. The difference between garnet zone and sillimanite zone peak temperature attainment is 2.8 ± 3.7 Ma. The near contemporaneity of peak temperature attainment in different metamorphic zones requires an additional source of heat beyond thermal relaxation of a variably over-thickened crust. We suggest that local igneous intrusions, with synmetamorphic ages, provided that additional heat.

Journal ArticleDOI
01 Oct 2002-Lithos
TL;DR: In this paper, an Eocene fold-and-thrust belt with Triassic eclogite and blueschist facies metamorphism was studied along the Imagezmir-Ankara suture.

Journal ArticleDOI
TL;DR: The metamorphic sole along the base of the Semail ophiolite in Oman records the earliest thrust slice subducted and accreted to the Base of the ophiola mantle sequence.
Abstract: The metamorphic sole along the base of the Semail ophiolite in Oman records the earliest thrust slice subducted and accreted to the base of the ophiolite mantle sequence. In the Bani Hamid area (United Arab Emirates) a c. 870 m thick thrust slice of granulite facies rocks includes garnet + diopside amphibolites, enstatite + cordierite + sillimanite + spinel ± sapphirine quartzites, alkaline mafic granulites (meta-jacupirangites) quartzo-feldspathic gneisses and calc-silicates. The latter contain garnet + diopside + scapolite + plagioclase ± wollastonite. P–T conditions of granulite facies metamorphism are in the range 800–860 °C and 10.5 ± 1.1 kbar to 14.7 ± 2.8 kbar. Garnet + clinopyroxene + hornblende + plagioclase amphibolites from the metamorphic sole record peak P–T conditions of 840 ± 70 °C and 11.6 ± 1.6 kbar (THERMOCALC average P–T mode) and 840–870 °C and 13.9–11.8 kbar (conventional thermobarometry) with low degrees of partial melting producing very small melt segregations of tonalitic material. Pressure estimates are equivalent to depths of 57–46 km beneath oceanic crust, much deeper than can be accounted for by the thickness of the ophiolite. 40Ar/39Ar hornblende ages from the amphibolites range from 95–93 Ma, synchronous with formation of the plagiogranites in the ophiolite crustal sequence (95 Ma), eruption of the Lasail (V2) volcanic sequence and deposition of Cenomanian–Turonian radiolaria in metalliferous sediments between the Geotimes (V1) and Lasail (V2) lavas. Protoliths of the metamorphic sole were Triassic–Jurassic and early Cretaceous Haybi volcanic rocks, Exotic limestones and quartzites and were clearly not equivalent to the Semail ophiolite rocks, showing that initiation of subduction could not have occurred at the ridge axis. Heat for metamorphism was derived from the mantle sequence harzburgites and dunites which were at or around 1100–1500 °C. All data from the sub-ophiolite metamorphic sole in Oman and the United Arab Emirates indicate that the ophiolite was formed in a Supra-Subduction zone setting and that obduction occurred along a NE-dipping high-temperature subduction zone during Late Cretaceous times.

Journal ArticleDOI
TL;DR: The most extensive, well preserved, early Archaean (>3500 Ma) structures occur in the northern end of the 3850-3560 Ma Itsaq Gneiss Complex, southern west Greenland, an area which includes the 3800-3700 Ma Isua supracrustal belt.
Abstract: [1] The world's most extensive, well preserved, early Archaean (>3500 Ma) structures occur in the northern end of the 3850–3560 Ma Itsaq Gneiss Complex, southern west Greenland, an area which includes the 3800–3700 Ma Isua supracrustal belt These provide a unique opportunity to study early tectonics The area is dominated by ∼3800–3650 Ma orthogneisses containing migmatized inclusions of ≥3800 Ma metavolcanic, sedimentary, ultramafic, and gabbroic rocks The thickest and most extensive of these inclusions forms the southern side of the Isua supracrustal belt Also present are panels of less migmatized 3710–3690 Ma volcanic and sedimentary rocks, in the northern side of the Isua supracrustal belt, and probably also in the Tussaap supracrustal belt in the south of the area Combined mapping and geochronology has identified mylonites which were active before regional early Archaean folding and 3580–3560 Ma metamorphism Sensitive high-resolution ion microprobe dating of premylonitization, synmylonitization, and postmylonitization granite sheets indicates some mylonites were active between 3650 and 3600 Ma During this period it is proposed that large horizontal motions and interleaving occurred, results of which included juxtaposition of supracrustal sequences in the Isua supracrustal belt with different depositional ages The tectonic interleaving was most likely due to horizontal motions in the vicinity of plate margins The new combined field observations and geochronology support hypotheses that some form of plate tectonics at least contributed to early Archaean sialic crustal development