Showing papers on "Metamorphism published in 2006"
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TL;DR: Although metasedimentary successions are widely distributed across the North China Craton, those of Palaeoproterozoic age are restricted to three orogenic belts, namely: the Trans-North China Orogen, the Khondalite Belt and the Jiao-Liao-Ji Belt as mentioned in this paper.
506 citations
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TL;DR: The first occurrence of UHT granulite metamorphism in the record signifies a change in geodynamics that generated transient sites of very high heat flow, and the duality of metamorphic belts is the characteristic imprint of plate tectonics in the rock record as mentioned in this paper.
Abstract: Ultrahigh-temperature (UHT) granulite metamorphism is documented predominantly in the Neoarchean to Cambrian rock record, but UHT granulite metamorphism also may be inferred at depth in Cenozoic orogenic systems. The first occurrence of UHT granulite metamorphism in the record signifies a change in geodynamics that generated transient sites of very high heat flow. Many UHT granulite metamorphic belts may have developed in settings analogous to modern continental backarcs; on a warmer Earth, destruction of oceans floored by thinner lithosphere may have generated hotter backarcs than those associated with the modern Pacific ring of fire. Medium-temperature eclogite–high- pressure (EHP) granulite metamorphism is documented in the Neoarchean rock record and at intervals throughout the Proterozoic and Paleozoic record. EHP granulite metamorphic belts are complementary to UHT granulite metamorphic belts in that they are generally inferred to record subduction-to-collision orogenesis. Blueschists become evident in the Neoproterozoic rock record, but lawsonite blueschist–eclogite metamorphism (high pressure [HP]) and ultrahigh-pressure metamorphism (UHP) characterized by coesite or diamond are predominantly Phanerozoic phenomena. HP-UHP metamorphism registers the low thermal gradients and deep subduction of continental crust during the early stage of subduction-to-collision orogenesis. A duality of metamorphic belts—reflecting a duality of thermal regimes—appears in the record only since the Neoarchean Era. A duality of thermal regimes is the hallmark of modern plate tectonics, and the duality of metamorphic belts is the characteristic imprint of plate tectonics in the rock record. The occurrence of both UHT and EHP granulite metamorphism since the Neoarchean marks the onset of a “Proterozoic plate tectonics” regime, which evolved during a Neoproterozoic transition to the modern plate tectonics regime, characterized by colder subduction as chronicled by HP-UHP metamorphism.
417 citations
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TL;DR: Magmatic and metamorphic zircons have been dated from ductilely deformed gabbroic dykes defining a dyke swarm and signifying crustal extension in the northern part of the Hengshan Complex of the North China Craton.
383 citations
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TL;DR: The U-Pb and 4 0 Ar/3 9 Ar studies of a unique exposure of crystalline basement along the Jurassic-Early Cretaceous Bangong suture of central Tibet reveal previously unrecognized records of Mesozoic metamorphism, magmatism, and exhumation as discussed by the authors.
Abstract: The U-Pb and 4 0 Ar/ 3 9 Ar studies of a unique exposure of crystalline basement along the Jurassic-Early Cretaceous Bangong suture of central Tibet reveal previously unrecognized records of Mesozoic metamorphism, magmatism, and exhumation. The basement includes Cambrian and older orthogneisses that underwent amphibolite facies metamorphism coeval with extensive granitoid emplacement at 185-170 Ma. The basement cooled to ∼300 °C by 165 Ma and was exhumed to upper crustal levels in the hanging wall of a south-directed thrust system during Early Cretaceous time. We attribute Jurassic metamorphism and magmatism to the development of a continental arc during Bangong Ocean subduction, and Early Cretaceous exhumation to northward continental underthrusting of the Lhasa terrane beneath the Qiangtang terrane. We speculate that a Jurassic arc extended regionally along the length of the Bangong suture, but in all other places in Tibet has been buried, either depositionally or structurally, beneath supracrustal assemblages.
381 citations
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TL;DR: The Wilmington Complex as mentioned in this paper contains morphologically complex zircons that formed through both igneous and metamorphic processes during the development of an island-arc complex and suturing of the arc to Laurentia.
Abstract: High-grade rocks of the Wilmington Complex, northern Delaware and adjacent Maryland and Pennsylvania, contain morphologically complex zircons that formed through both igneous and metamorphic processes during the development of an island-arc complex and suturing of the arc to Laurentia. The arc complex has been divided into several members, the protoliths of which include both intrusive and extrusive rocks. Metasedimentary rocks are interlayered with the complex and are believed to be the infrastructure upon which the arc was built.
In the Wilmington Complex rocks, both igneous and metamorphic zircons occur as elongate and equant forms. Chemical zoning, shown by cathodoluminescence (CL), includes both concentric, oscillatory patterns, indicative of igneous origin, and patchwork and sector patterns, suggestive of metamorphic growth. Metamorphic monazites are chemically homogeneous, or show oscillatory or spotted chemical zoning in backscattered electron images.
U-Pb geochronology by sensitive high resolution ion microprobe (SHRIMP) was used to date complexly zoned zircon and monazite. All but one member of the Wilmington Complex crystallized in the Ordovician between ca. 475 and 485 Ma; these rocks were intruded by a suite of gabbro-to-granite plutonic rocks at 434 ± 5 Ma. Detrital zircons in metavolcanic and metasedimentary units were derived predominantly from 0.9 to 1.4 Ga (Grenvillian) basement, presumably of Laurentian origin. Amphibolite to granulite facies metamorphism of the Wilmington Complex, recorded by ages of metamorphic zircon (428 ± 4 and 432 ± 6 Ma) and monazite (429 ± 2 and 426 ± 3 Ma), occurred contemporaneously with emplacement of the younger plutonic rocks. On the basis of varying CL zoning patterns and external morphologies, metamorphic zircons formed by different processes (presumably controlled by rock chemistry) at slightly different times and temperatures during prograde metamorphism. In addition, at least three other thermal episodes are recorded by monazite growth at 447 ± 4, 411 ± 3, and 398 ± 3 Ma.
362 citations
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TL;DR: In this paper, a large suite of samples representative of the lithologic and metamorphic variation in the Otago and Alpine schists was collected and analyzed for a comprehensive suite of elements.
Abstract: The Otago and Alpine schists of the South Island of New Zealand form a young ( A large suite of samples representative of the lithologic and metamorphic variation in the Otago and Alpine schists was collected and analyzed for a comprehensive suite of elements. The aim was to identify any rock type or metamorphic setting that may be depleted in the suite of ore-forming elements (Au, Ag, As, Sb, Hg, Mo, and W) relative to unmetamorphosed protoliths, perhaps representing the source for the enrichments observed in the Otago ore deposits. Gold, Ag, As, Sb, Hg, Mo, and W were found to have significantly lower concentrations in higher grade metamorphic rocks compared to unmetamorphosed protolith samples. These were the only elements in a suite of 12 major and 50 trace elements to show systematic depletions with metamorphic grade. Investigation of the trace element chemistry of sulfide minerals indicates that the whole-rock depletions are caused by the disappearance between greenschist and amphibolite facies conditions of pyrite, galena, sphalerite, and cobaltite, the major host phases for the ore-forming elements. More than 95 percent of upper greenschist and amphibolite facies samples are significantly depleted in the ore-forming elements. Such regional-scale depletions require pervasive, grain-boundary fluid flow throughout these rocks. The leaching is most likely to have been caused by metamorphic fluid produced by dehydration reactions at the greenschist-amphibolite boundary. The suite of elements depleted in mid- to high-grade Otago and Alpine schists is almost identical to those enriched in the orogenic gold deposits in Otago. Furthermore, the vertical zonation in depletions is similar to the vertical zonation in enrichments that occurs in the Otago deposits. Mass-balance calculations suggest that 2 metric tons (t) Au and 24,000 t As was leached from 1 km3 of amphibolite facies rock and that the Macraes deposit could have been formed by leaching of a 5- × 5- × 5-km cube of amphibolite facies rock. We propose that the orogenic gold deposits in Otago, such as Macraes, were formed directly from metal-rich metamorphic fluid produced during prograde metamorphism at depth. The contribution of other fluid and metal sources in the formation of these deposits, such as magmatic fluids, cannot be ruled out, but there is no direct evidence to support their involvement. Infiltration of meteoric water, such as occurs currently in the Southern Alps of New Zealand may have contributed to the formation of the late-stage deposits that formed at shallow level during uplift of the Otago schists.
331 citations
01 Jan 2006
TL;DR: In this paper, the authors summarize metamorphic changes in chondritic meteorites and discuss temperatures, oxidation states, and possible heat sources for metamorphism, and discuss the conditions under which chondrites originally formed.
Abstract: Thermal metamorphism has affected most chondritic meteorites to some extent, and in most ordinary chondrites, some carbonaceous chondrites, and many enstatite chondrites it has significantly modified the primary characteristics of the meteorites. Metamorphic grade, as described by the petrologic type, is one axis of the current two-dimensional system for classifying chondrites. Many changes are produced during thermal metamorphism, including textural integration and recrystallization, mineral equilibration, destruction of primary minerals, and growth of secondary minerals. Understanding these changes is critical if one hopes to infer the conditions under which chondrites originally formed. In addition to summarizing metamorphic changes, we also discuss temperatures, oxidation states, and possible heat sources for metamorphism.
328 citations
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TL;DR: In this article, a spinel+quartz equilibrium assemblage from these rocks in two textural settings is reported, with high ZnO (up to 14.47%wt.%) spinel with quartz as inclusions within garnet porphyroblasts defining pressure above 12-kbar and temperature of 900-°C.
311 citations
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TL;DR: In this paper, the maturity of the organic matter in matrix, determined by Raman spectroscopy, has been used as a powerful metamorphic tracer, independent of the mineralogical context and extent of aqueous alteration.
294 citations
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TL;DR: In this paper, a combined study of internal structure, U-Pb age, and Hf and O isotopes was carried out for metamorphic zircons from ultrahigh-pressure eclogite boudins enclosed in marbles from the Dabie orogen in China.
260 citations
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TL;DR: In this paper, the ages of inherited zircon cores from the giant Sulu ultrahigh pressure (UHP) terrane are presented, which are used to define the timing of peak recrystallization, the subsequent amphibolite-facies metamorphism, and the architecture of the Dabie-Sulu suture zone between the collided Sino-Korean and Yangtze cratons.
Abstract: [1] New U/Pb zircon and Th/Pb monazite ages are presented from the giant Sulu ultrahigh-pressure (UHP) terrane. Combined with Sm/Nd ages, Rb/Sr ages, inclusion relationships, and geologic relationships, they help define the timing of peak recrystallization, the timing of subsequent amphibolite-facies metamorphism, and the architecture of the Dabie-Sulu suture zone between the collided Sino-Korean and Yangtze cratons. The data indicate a ∼15 Myr record of UHP recrystallization, the first clearly documented for a giant UHP terrane; this requires that continental subduction in the Dabie-Sulu orogen involved multiple UHP tectonic or recrystallization events. A 244–236 Ma “precursor” UHP event, seen only in the Dabie Shan, was followed by a second, ∼230–220 Ma “main” UHP event, which was itself terminated by a 220–205 Ma amphibolite-facies overprint. Older eclogite-facies events seen in the Qinling segment of this orogenic belt raise the possibility that these rocks have undergone (U)HP metamorphism three or four times, but at present, there is no geochronological evidence in the Dabie-Sulu area to support this. The subduction of the lower, Yangtze plate did not proceed in a simple fashion: The ages of inherited zircon cores demonstrate that a ribbon continent of Yangtze affinity escaped subduction and became wedged against the Sino-Korean plate hanging wall.
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TL;DR: In this article, the authors show that left-lateral strike-slip shearing along the Red River shear zone started after 21 Ma, not at 35 Ma as previously thought, and the fault was purely a crustal structure.
Abstract: The 1000 km long NW–SE-striking, left-lateral Ailao Shan–Red River shear zone runs from the southeastern corner of Tibet to the Gulf of Tonkin and the South China Sea. It has been used as the prime example of a lithospheric-scale strike-slip fault that has accommodated between 500 and 1000 km of southeastwards extrusion of Indo-China away from the Indian plate indentor. Central to the model of continental extrusion is that such faults cut through the entire lithosphere, that shear heating resulted in high-grade metamorphism and local anatexis, and that the ages of sheared granites along the fault also date the timing of strike-slip shearing. However, structural data from the Red River shear zone clearly show that vertical strike-slip faulting post-dated metamorphism and granite emplacement. Most granites along the shear zone are mantle-related granodiorites or within-plate alkali granites formed prior to shearing along the Red River shear zone. Left-lateral kinematic indicators are ubiquitous within the Red River mylonites, but they are always lower-temperature fabrics, formed after peak sillimanite metamorphism and after granite crystallization. It is suggested that left-lateral strike-slip shearing along the Red River shear zone started after 21 Ma, not at 35 Ma as previously thought, and the fault was purely a crustal structure. None of the geological features used to propose the 500–1000 km offsets are robust, and the total finite offset remains unknown.
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TL;DR: The timing of ultra-high pressure (UHP) metamorphism has been difficult to determine because of a lack of age constraints on crucial events, especially those occurring on the prograde path as discussed by the authors.
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TL;DR: The Atlantis Massif (Mid Atlantic Ridge, 30°N) is an example of an oceanic core complex (OCC) exposed by a major detachment shear zone and development of this OCC.
Abstract: The Atlantis Massif (Mid-Atlantic Ridge, 30°N) is an example of an oceanic core complex (OCC) exposed by a major fault system. Our integrated field and analytical study of mafic and ultramafic rocks exposed on the south wall of the massif demonstrates the complex interplay of fluids, mass transfer, and metamorphism in strain localization associated with the evolution of a major detachment shear zone and development of this OCC. Extensive talc-amphibole-chlorite metasomatism as well as heterogeneous, crystal-plastic to cataclastic deformation characterize a strongly foliated, 100-m-thick zone of detachment faulting. The metasomatic fault rocks are key elements of this OCC and record a deformation and metamorphic history that is distinct from the underlying basement rocks. Talc-rich fault rocks preserve textural and geochemical characteristics of their ultramafic protoliths. Although primary textures and mineral parageneses are commonly obliterated in rocks dominated by amphibole, bulk rock data point to a mafic protolith. Major and trace elements indicate a complex mutual interaction between gabbroic and ultramafic rocks during metasomatism, which together with microstructures suggest localized circulation of oxidizing, Si-Al-Ca-rich fluids and mass transfer in high strain deformation zones. This type of flow was distinct from the more pervasive circulation that led to strongly serpentinized domains in the south wall. In contrast, cataclastic microfracturing is associated with a dominantly static metasomatism in less deformed domains, suggesting that a significant amount of metasomatism was controlled by diffuse flow and mass transfer associated with fractures that lack a strong preferred orientation. Distinct differences in lithologies, metamorphic overprinting, and degree of deformation between the south wall and central dome of the Atlantis Massif demonstrate the complex lateral and vertical heterogeneity in composition, alteration, and structure of this OCC.
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TL;DR: The formation, age and trace element composition of zircon and monazite were investigated across the prograde, low-pressure metamorphic sequence at Mount Stafford (central Australia) as mentioned in this paper.
Abstract: The formation, age and trace element composition of zircon and monazite were investigated across the prograde, low-pressure metamorphic sequence at Mount Stafford (central Australia). Three pairs of inter-layered metapelites and metapsammites were sampled in migmatites from amphibolite-facies (T 600 C) to granulite-facies conditions (T 800 C). Sensitive high-resolution ion microprobe U–Pb dating on metamorphic zircon rims and on monazite indicates that granulite-facies metamorphism occurred between 1795 and 1805Ma. The intrusion of an associated granite was coeval with metamorphism at 1802 ± 3Ma and is unlikely to be the heat source for the prograde metamorphism. Metamorphic growth of zircon started at T 750 C, well above the pelite solidus. Zircon is more abundant in the metapelites, which experienced higher degrees of partial melting compared with the associated metapsammites. In contrast, monazite growth initiated under sub-solidus prograde conditions. At granulite-facies conditions two distinct metamorphic domains were observed in monazite. Textural observations, petrology and the trace element composition of monazite and garnet provide evidence that the first metamorphic monazite domain grew prior to garnet during prograde conditions and the second in equilibrium with garnet and zircon close to the metamorphic peak. Ages from sub-solidus, prograde and peak metamorphic monazite and zircon are not distinguishable within error, indicating that heating took place in less than 20 Myr.
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TL;DR: In this paper, the SHRIMP zircon U-Pb dating of the Shangtaihua group of the North China Craton has been reported, showing that the group formed during the Paleoproterozoic (between 2.26 and 2.14 Ga).
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TL;DR: Garnet–albite-bearing mineral assemblages that record pressures of 1.2–1.5 GPa at temperatures of 600–650 °C from supracrustal amphibolites from the mid-Archaean Barberton granitoid-greenstone terrain represent metamorphic evidence for cold and strong lithosphere, as well as subduction-driven tectonic processes, during the evolution of the early Earth.
Abstract: A News Feature in Nature of 13 July aired a controversy about the 'age' of plate tectonics. Plate tectonics shaped the world we see today, but the question of when it started is one that divides geologists. In this issue Moyen et al. make a strong case for the existence of subduction processes (a key component of plate tectonics) over 3 billion years ago, at a time at which the very existence of plate tectonics has been questioned. The evidence in this case comes from signs of high pressure, low temperature metamorphism 3.2 billion years ago in an Archaean mountain-belt, the Barberton greenstone belt in South Africa. This is the first clear indication of this type of activity in the mid-Archaean. Although plate tectonics is the central geological process of the modern Earth, its form and existence during the Archaean era (4.0–2.5 Gyr ago) are disputed1,2. The existence of subduction during this time is particularly controversial because characteristic subduction-related mineral assemblages, typically documenting apparent geothermal gradients of 15 °C km-1 or less3, have not yet been recorded from in situ Archaean rocks (the lowest recorded apparent geothermal gradients4 are greater than 25 °C km-1). Despite this absence from the rock record, low Archaean geothermal gradients are suggested by eclogitic nodules in kimberlites5,6 and circumstantial evidence for subduction processes, including possible accretion-related structures2, has been reported in Archaean terrains. The lack of spatially and temporally well-constrained high-pressure, low-temperature metamorphism continues, however, to cast doubt on the relevance of subduction-driven tectonics during the first 1.5 Gyr of the Earth's history7. Here we report garnet–albite-bearing mineral assemblages that record pressures of 1.2–1.5 GPa at temperatures of 600–650 °C from supracrustal amphibolites from the mid-Archaean Barberton granitoid-greenstone terrain. These conditions point to apparent geothermal gradients of 12–15 °C—similar to those found in recent subduction zones—that coincided with the main phase of terrane accretion in the structurally overlying Barberton greenstone belt8. These high-pressure, low-temperature conditions represent metamorphic evidence for cold and strong lithosphere, as well as subduction-driven tectonic processes, during the evolution of the early Earth.
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TL;DR: In this article, the authors introduce the term Himalayan Unconformity to distinguish it from high strain zones in the Himalayan Metamorphic Front, and suggest the name; High Himal Thrust.
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TL;DR: The Kathmandu thrust sheet consists of Upper Proterozoic through mid-Paleozoic rocks that were emplaced over Lesser Himalayan strata (part of India's cratonal cover) during middle to late Tertiary time as mentioned in this paper.
Abstract: The Kathmandu thrust sheet consists of Upper Proterozoic through mid-Paleozoic rocks that were emplaced over Lesser Himalayan strata (part of India's cratonal cover) during middle to late Tertiary time. Primary components of the thrust sheet include Upper Proterozoic metasedimentary rocks of the Bhimphedi Group, Cambrian-Ordovician granite bodies, and Ordovician (through Devonian?) conglomerate, sandstone, shale, and limestone of the Phulchauki Group. Deformation, metamorphism, uplift, and erosion accompanied Tertiary emplacement of the thrust sheet, but there is also evidence of widespread early Paleozoic tectonism, such as: (1) Rocks of the Bhimphedi Group were metamorphosed at ca. 490 Ma, as indicated by Th-Pb ages of monazite inclusions in garnet crystals. (2) Bhimphedi rocks are interpreted to have been imbricated along a south-vergent(?) thrust system during early Paleozoic time, with U-Pb ages recording ductile deformation through ca. 484 Ma but ending (at least locally) by ca. 473 Ma. (3) Cambrian-Ordovician granite bodies may have been generated by crustal melting during thrust loading, and at least some are interpreted to have been emplaced as syntectonic sills along the early Paleozoic thrust faults. (4) Ordovician conglomeratic strata were shed from the uplifted Bhimphedi Group and Cambrian-Ordovician granite bodies and are interpreted to have accumulated in a foreland basin setting with respect to the early Paleozoic thrust system.
Our findings, together with evidence for early Paleozoic ductile deformation and metamorphism in adjoining regions, indicate that early Paleozoic tectonism has played an important role in shaping the Himalayan orogen. This early Paleozoic tectonism has been overlooked in most current models for the formation of the Himalayan orogen.
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TL;DR: In this article, major and trace-element data on the constituent minerals of garnet peridotite xenoliths hosted in early Paleozoic kimberlites and Neogene volcanic rocks within the North China Craton are compared with those from the pre-pilot hole of the Chinese Continental Scientific Drilling Project (CCSD-PP1) in the tectonically exhumed Triassic ( 220Ma) Sulu ultrahighpressure (UHP) terrane along its southern margin.
Abstract: Majorand trace-element data on the constituent minerals of garnet peridotite xenoliths hosted in early Paleozoic (457–500Ma) kimberlites and Neogene (16–18Ma) volcanic rocks within the North China Craton are compared with those from the pre-pilot hole of the Chinese Continental Scientific Drilling Project (CCSD-PP1) in the tectonically exhumed Triassic ( 220Ma) Sulu ultrahighpressure (UHP) terrane along its southern margin P–T estimates for the Paleozoic and Neogene peridotite xenoliths reflect different model geotherms corresponding to surface heat flows of 40mW/m (Paleozoic) and 80mW/m (Neogene) Garnet peridotite xenoliths or xenocrysts from the Paleozoic kimberlites are strongly depleted, similar to peridotites from other areas of cratonic mantle, with magnesium olivine (mean Fo927), Cr-rich garnet and clinopyroxene with high La/Yb Garnet (and spinel) peridotite xenoliths hosted in Neogene basalts are derived from fertile mantle; they have high Al2O3 and TiO2 contents, low-Mg-number olivine (mean Fo895), low-Cr garnet and diopside with flat rare earth element (REE) patterns The differences between the Paleozoic and Neogene xenoliths suggest that a buoyant refractory lithospheric keel present beneath the eastern North China Craton in Paleozoic times was at least partly replaced by younger, hotter and more fertile lithospheric mantle during Mesozoic– Cenozoic times Garnet peridotites from the Sulu UHP terrane have less magnesian olivine (Fo915), and lower-Cr garnet than the Paleozoic xenoliths The diopsides have low heavy REE (HREE) contents and sinusoidal to light REE (LREE)-enriched REE patterns These features, and their high Mg/Si and low CaO and Al2O3 contents, indicate that the CCSD-PP1 peridotites represent a moderately refractory mantle protolith Details of mineral chemistry indicate that this protolith experienced complex metasomatism by asthenosphere-derived melts or fluids in Mesoproterozoic, and subsolidus re-equilibration involving fluids/melts derived from the subducted Yangtze continental crust during UHP metamorphism in the early Mesozoic Tectonic extension of the subcontinental lithospheric mantle of the North China Craton and exhumation of the Sulu UHP rocks in the early Mesozoic induced upwelling of the asthenosphere Peridotites sampled by the Neogene basalts represent newly formed lithosphere derived by cooling of the upwelling asthenospheric mantle in Jurassic–Cretaceous and Paleogene time JOURNAL OF PETROLOGY VOLUME 47 NUMBER 11 PAGES 2233–2256 2006 doi:101093/petrology/egl042
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TL;DR: In the Namaqua-Natal belt, the age of the granitoid fabric is known to be at least 1.03-1.04-Ga.
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01 Jan 2006TL;DR: In this article, the age of the Sino-Korean craton-Yangtze craton collision has been established by recognizing that there was a Paleozoic collision between the Qinling unit and the South Korean craton.
Abstract: There are three sutures in the Qinling-Dabie-Sulu orogen in the Tongbai–Xinxian (northern Hong’an)–northern Dabie area: the Silurian Sino-Korean craton–Erlangping intra-oceanic arc suture, the Silurian Erlangping arc–Qinling unit (microcontinent) suture, and the Early Triassic Qinling unit–Yangtze craton suture. We resolve the controversy regarding the age of the Sino-Korean craton–Yangtze craton collision by recognizing that there was Paleozoic collision between the Qinling unit and the Sino-Korean craton and Mesozoic collision between the Qinling unit and the Yangtze craton. The Qinling unit constitutes a long and narrow microcontinent that extends through the Qinling-Dabie area and probably into the Sulu area. Its common characteristics are the Mesoproterozoic (ca. 1.0 Ga) Jinningian orogeny, ca. 0.8–0.7 Ga arc formation and rifting, and Late Silurian–Early Devonian (ca. 400 Ma) arc magmatism with concomitant regional contact metamorphism up to granulite-facies conditions (peak: 680–740 °C at 0.9–1.1 GPa). A common Proterozoic history links the Qinling microcontinent to the Yangtze craton. Its 400 Ma arc, forearc basin, and its separation from the Yangtze craton by the partly oceanic Huwan melange make the Qinling microcontinent distinct. The forearc basin sits on the southern part of the 400 Ma arc and underlying Proterozoic continental basement, and detrital geochronology ties it to the Qinling microcontinent basement and its arc. The Huwan melange is a subduction-accretion complex containing elements of the Qinling microcontinent and its arc, the Paleotethyan ocean floor, and possibly the Yangtze craton.
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TL;DR: Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircon from Sulu-Dabie dolomitic marbles is characterized by distinctive domains of inherited (detrital), prograde, ultrahigh pressure (UHP) and retrograde metamorphic growths.
Abstract: Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircon from Sulu-Dabie dolomitic marbles is characterized by distinctive domains of inherited (detrital), prograde, ultrahigh-pressure (UHP) and retrograde metamorphic growths. The inherited zircon domains are dark-luminescent in CL images and contain mineral inclusions of Qtz + Cal + Ap. The prograde metamorphic domains are white-luminescent in CL images and preserve a quartz eclogite facies assemblage of Qtz + Dol + Grt + Omp + Phe + Ap, formed at 542–693 °C and 1.8–2.1 GPa. In contrast, the UHP metamorphic domains are grey-luminescent in CL images, retain the UHP assemblage of Coe + Grt + Omp + Arg + Mgs + Ap, and record UHP conditions of 739–866 °C and >5.5 GPa. The outermost retrograde rims have dark-luminescent CL images, and contain low-P minerals such as calcite, related to the regional amphibolite facies retrogression. Laser ablation ICP-MS trace-element data show striking difference between the inherited cores of mostly magmatic origin and zircon domains grown in response to prograde, UHP and retrograde metamorphism. SHRIMP U-Pb dating on these zoned zircon identified four discrete 206Pb/238U age groups: 1823–503 Ma is recorded in the inherited (detrital) zircon derived from various Proterozoic protoliths, the prograde domains record the quartz eclogite facies metamorphism at 254–239 Ma, the UHP growth domains occurred at 238–230 Ma, and the late amphibolite facies retrogressive overprint in the outermost rims was restricted to 218–206 Ma. Thus, Proterozoic continental materials of the Yangtze craton were subducted to 55–60 km depth during the Early Triassic and recrystallized at quartz eclogite facies conditions. Then these metamorphic rocks were further subducted to depths of 165–175 km in the Middle Triassic and experienced UHP metamorphism, and finally these UHP metamorphic rocks were exhumed to mid-crustal levels (about 30 km) in the Late Triassic and overprinted by regional amphibolite facies metamorphism. The subduction and exhumation rates deduced from the SHRIMP data and metamorphic P–T conditions are 9–10 km Myr−1 and 6.4 km Myr−1, respectively, and these rapid subduction–exhumation rates may explain the obtained P–T–t path. Such a fast exhumation suggests that Sulu-Dabie UHP rocks that returned towards crustal depths were driven by buoyant forces, caused as a consequence of slab breakoff at mantle depth.
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TL;DR: A tectonic model for the Palaeoproterozoic Lapland-Kola orogen (LKO) in the northern Fennoscandian Shield is proposed in this article.
Abstract: A tectonic model is proposed for the Palaeoproterozoic Lapland-Kola orogen (LKO) in the northern Fennoscandian Shield. Although long regarded as an Archaean craton, integrated geological, geochemical and geophysical observations show that the Lapland-Kola orogen is a Palaeoproterozoic collisional belt containing both Archaean terranes and an important component of juvenile Palaeoproterozoic crust. Rifting, from 2.5 to 2.1 Ga, began under the influence of a mantle plume (> 1000 km diameter), related to the break-up of the Kenorland supercontinent. Two linear suture zones within the orogeriic core mark the sites of continental separation, ocean formation and closure. One of these is identified as a belt of 1.98-1.91 Ga juvenile crust of both arc magmatic and sedimentary origin, marked by the Lapland Granulite, Umba and Tersk terranes. Palaeomagnetic data and ancient sedimentary detritus within these terranes suggest limited oceanic separation. Collision of juvenile terranes with the surrounding Archaean took place mainly between 1.93 and 1.91 Ga, resulting in a Himalayan-scale mountain belt, manifest by a thick-skinned region of high- P granulite-facies metamorphism, including the classical Lapland Granulite Belt and a broad zone of compressional deformation extending southwards into the Belomorian Mobile Belt. Protracted cooling and exhumation, possibly related to the buttressing effect of surrounding lithosphere, culminated in the intrusion of 1.80-1.77 Ga post-tectonic granites.
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TL;DR: The Legs Lake shear zone is a crustal-scale thrust fault system in the western Canadian Shield that juxtaposes high pressure (1.0+GPa) granulite facies rocks against shallow crustal (0.5) amphibolite facia rocks as mentioned in this paper.
Abstract: The Legs Lake shear zone is a crustal-scale thrust fault system in the western Canadian Shield that juxtaposes high-pressure (1.0+ GPa) granulite facies rocks against shallow crustal (< 0.5 GPa) amphibolite facies rocks. Hangingwall decompression is characterized by breakdown of the peak assemblage Grt + Sil + Kfs + Pl + Qtz into the assemblage Grt + Crd + Bt ± Sil + Pl + Qtz. Similar felsic granulite occurs throughout the region, but retrograde cordierite is restricted to the immediate hangingwall of the shear zone. Textural observations, petrological analysis using P–T/P–MH2O phase diagram sections, and in situ electron microprobe monazite geochronology suggest that decompression from peak conditions of 1.1 GPa, c. 800 °C involved several distinct stages under first dry and then hydrated conditions. Retrograde re-equilibration occurred at 0.5–0.4 GPa, 550–650 °C. Morphology, X-ray maps, and microprobe dates indicate several distinct monazite generations. Populations 1 and 2 are relatively high yttrium (Y) monazite that grew at 2.55–2.50 Ga and correspond to an early granulite facies event. Population 3 represents episodic growth of low Y monazite between 2.50 and 2.15 Ga whose general significance is still unclear. Population 4 reflects low Y monazite growth at 1.9 Ga, which corresponds to the youngest period of high-pressure metamorphism. Finally, population 5 is restricted to the hydrous retrograded granulite and represents high Y monazite growth at 1.85 Ga that is linked directly to the synkinematic garnet-consuming hydration reaction (KFMASH): Grt + Kfs + H2O = Bt + Sil + Qtz. Two samples yield weighted mean microprobe dates for this population of 1853 ± 15 and 1851 ± 9 Ma, respectively. Subsequent xenotime growth correlates with the reaction: Grt + Sil + Qtz + H2O = Crd. We suggest that the shear zone acted as a channel for fluid produced by dehydration of metasediments in the underthrust domain.
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TL;DR: In the Trans-North China Orogen, a Paleoproterozoic orogenic belt along which two discrete continental blocks, referred to as the Eastern and Western Blocks, were amalgamated to form the North China Craton as mentioned in this paper.
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TL;DR: In this article, pressure and temperature estimates for the primary eclogite assemblage are consistent with minimum pressures of 20-25kbar at 482 to 625°C.
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TL;DR: In the Central Pontides region of Central Asia, a flexural Foreland basin developed in front of the south-vergent high-pressure-low-temperature (HP-LT) metamorphic thrust sheet as discussed by the authors, and the biostratigraphy of the foreland basin constrains the exhumation of the HP-LT rocks to the lbronian-Coniacian.
Abstract: Biostratigraphic, isotopic, and petrologic data from the Central Pontides document major southward growth of the Eurasian continental crust by subduction-accretion during the Cretaceous and Triassic Periods. A major part of the accreted material is represented by a crustal slice, 75 km long and up to 11 km thick, consisting of metabasite, metaophiolite, and mica schist that represent underplated Tethyan oceanic crustal and mantle rocks. They were metamorphosed at 490 degrees C and 17 kbar in mid-Cretaceous time (ca. 105 Ma). The syn-subduction exhumation occurred in a thrust sheet bounded by a greenschist facies shear zone with a normal sense of movement at the top and a thrust fault at the base. A flexural Foreland basin developed in front of the south-vergent high-pressure-low-temperature (HP-LT) metamorphic thrust sheet; the biostratigraphy of the foreland basin constrains the exhumation of the HP-LT rocks to the lbronian-Coniacian, similar to 20 m.y. after the HP-LT metamorphism, and similar to 25 m.y. before the terminal Paleocene continental collision. The Cretaceous subduction-accretion complex is tectonically overlain in the north by oceanic crustal rocks accreted to the southern margin of Eurasia during the latest Triassic-earliest Jurassic. The Triassic subduction-accretion complex is made up of metavolcanic rocks of ensimatic arc origin and has undergone a high pressure, greenschist facies metamorphism with growth of sodic amphibole. Most of the Central Pontides consists of accreted Phanerozoic oceanic crustal material, and hence is comparable to regions such as the Klamath Mountains in the northwestern United States or to the Altaids in Central Asia.
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TL;DR: In this paper, a geodynamic model of plume-craton interaction is proposed to explain the geological and geochemical characteristics of the Taishan greenstone belt, which consists of an association of komatiites, pillow basalts, banded iron formations, conglomerates, and greywacke sandstones.
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TL;DR: In this paper, the authors review the evolution of the Eastern Mediterranean-Balkan region with special reference to Anatolia, and provide new isotopic data on the Palaeozoic magmatic and metamorphic rocks.
Abstract: We review the Palaeozoic-Early Mesozoic evolution of the Eastern Mediterranean-Balkan region with special reference to Anatolia, and provide new isotopic data on the Palaeozoic magmatic and metamorphic rocks. The pre-Alpide evolution of the region involves episodic growth of Laurussia by accretion of oceanic terranes and Gondwana-derived microcontinents. Terrane accretion, associated with deformation, magmatism and regional metamorphism, took place in the Late Ordovician-Early Silurian, Carboniferous, Late Triassic-Early Jurassic and Mid-Jurassic. The Late Ordovician-Early Silurian accretion is inferred from strati-graphic and faunal records in the Pontides; other evidence for it is buried under young cover on the northern margin of the Black Sea. The Carboniferous orogeny is related to southward subduction and continental collision on the southern margin of Laurussia. It is marked in the Pontides by high-grade regional metamorphism, north-vergent deformation and post-orogenic latest Carboniferous-Early Permian plutonism. The latest Triassic-Early Jurassic Cimmeride orogeny involved the collision and amalgamation of an oceanic plateau to the southern margin of Laurasia. It is represented by voluminous accretionary complexes with Late Triassic blues-chists and eclogites. Late Jurassic regional metamorphism and deformation is confined to the Balkans, and is the result of continental collision between the Rhodope-Serbo-Macedonian and Strandja blocks in the Late Jurassic. The Palaeozoic geological history of the Balkans and the Pontides resembles that of Central Europe, although the similarities end with the Mesozoic, as a consequence of the formation of Pangaea.