Showing papers on "Metamorphism published in 2018"
TL;DR: In this article, thermal gradients of metamorphic rocks were analyzed for 456 localities from the Eoarchean to Cenozoic Eras to test the null hypothesis that thermal gradient through time did not vary outside of the range expected for each of these distinct plate tectonic settings.
Abstract: Abstract On the contemporary Earth, distinct plate tectonic settings are characterized by differences in heat flow that are recorded in metamorphic rocks as differences in apparent thermal gradients. In this study we compile thermal gradients [defined as temperature/pressure (T/P) at the metamorphic peak] and ages of metamorphism (defined as the timing of the metamorphic peak) for 456 localities from the Eoarchean to Cenozoic Eras to test the null hypothesis that thermal gradients of metamorphism through time did not vary outside of the range expected for each of these distinct plate tectonic settings. Based on thermal gradients, metamorphic rocks are classified into three natural groups: high dT/dP [>775 °C/GPa, mean ~1110 °C/GPa (n = 199) rates], intermediate dT/dP [775–375 °C/GPa, mean ~575 °C/GPa (n = 127)], and low dT/dP [<375 °C/GPa, mean ~255 °C/GPa (n = 130)] metamorphism. Plots of T, P, and T/P against age demonstrate the widespread occurrence of two contrasting types of metamorphism—high dT/dP and intermediate dT/dP—in the rock record by the Neoarchean, the widespread occurrence of low dT/dP metamorphism in the rock record by the end of the Neoproterozoic, and a maximum in the thermal gradients for high dT/dP metamorphism during the period 2.3 to 0.85 Ga. These observations falsify the null hypothesis and support the alternative hypothesis that changes in thermal gradients evident in the metamorphic rock record were related to changes in geodynamic regime. Based on the observed secular changes, we postulate that the Earth has evolved through three geodynamic cycles since the Mesoarchean and has just entered a fourth. Cycle I began with the widespread appearance of paired metamorphism in the rock record, which was coeval with the amalgamation of widely dispersed blocks of protocontinental lithosphere into supercratons, and was terminated by the progressive fragmentation of the supercratons into protocontinents during the Siderian–Rhyacian (2.5 to 2.05 Ga). Cycle II commenced with the progressive reamalgamation of these protocontinents into the supercontinent Columbia and extended until the breakup of the supercontinent Rodinia in the Tonian (1.0 to 0.72 Ga). Thermal gradients of high dT/dP metamorphism rose around 2.3 Ga leading to a thermal maximum in the mid-Mesoproterozoic, reflecting insulation of the mantle beneath the quasi-integral continental lithosphere of Columbia, prior to the geographical reorganization of Columbia into Rodinia. This cycle coincides with the age span of most anorogenic magmatism on Earth and a scarcity of passive margins in the geological record. Intriguingly, the volume of preserved continental crust of Mesoproterozoic age is low relative to the Paleoproterozoic and Neoproterozoic Eras. These features are consistent with a relatively stable association of continental lithosphere between the assembly of Columbia and the breakup of Rodinia. The transition to Cycle III during the Tonian is marked by a steep decline in the thermal gradients of high dT/dP metamorphism to their lowest value and the appearance of low dT/dP metamorphism in the rock record. Again, thermal gradients for high dT/dP metamorphism show a rise to a peak at the end of the Variscides during the formation of Pangea, before another steep decline associated with the breakup of Pangea and the start of a fourth cycle at ca. 0.175 Ga. Although the mechanism by which subduction started and plate boundaries evolved remains uncertain, based on the widespread record of paired metamorphism in the Neoarchean we posit that plate tectonics was established globally during the late Mesoarchean. During the Neoproterozoic there was a change to deep subduction and colder thermal gradients, features characteristic of the modern plate tectonic regime.
306 citations
TL;DR: In this paper, a unified and updated system of progressive shock metamorphism is proposed for impactites derived from silicate rocks and sediments, which are classified into four major classes: crystalline, mafic, chondritic, ultramafic and sedimentary.
Abstract: We reevaluate the systematics and geologic setting of terrestrial, lunar, Martian, and asteroidal “impactites” resulting from single or multiple impacts. For impactites derived from silicate rocks and sediments, we propose a unified and updated system of progressive shock metamorphism. “Shock-metamorphosed rocks” occur as lithic clasts or melt particles in proximal impactites at impact craters, and rarely in distal impactites. They represent a wide range of metamorphism, typically ranging from unshocked to shock melted. As the degree of shock metamorphism, at a given shock pressure, depends primarily on the mineralogical composition and the porosity of a rock or sediment sample, different shock classification systems are required for different types of planetary rocks and sediments. We define shock classification systems for eight rock and sediment classes which are assigned to three major groups of rocks and sediments (1) crystalline rocks with classes F, M, A, and U; (2) chondritic rocks (class C); and (3) sedimentary rocks and sediments with classes SR, SE, and RE. The abbreviations stand for felsic (F), mafic (M), anorthositic (A), ultramafic (U), sedimentary rocks (SR), unconsolidated sediments (SE), and regoliths (RE). In each class, the progressive stages of shock metamorphism are denominated S1 to Sx. These progressive shock stages are introduced as: S1–S7 for F, S1–S7 for M, S1–S6 for A, S1–S7 for U, S1–S7 for C, S1–S7 for SR, S1–S5 for SE, and S1–S6 for RE. S1 stands for “unshocked” and Sx (variable between S5 and S7) stands for “whole rock melting.” We propose a sequence of symbols characterizing the degree of shock metamorphism of a sample, i.e., F-S1 to F-S7 with the option to add the tabulated pressure ranges (in GPa) in parentheses.
252 citations
TL;DR: In the case of Archean and Paleoproterozoic deposits, the formation of orogenic gold deposits was one of the last events prior to cratonization.
Abstract: With very few exceptions, orogenic gold deposits formed in subduction-related tectonic settings in accretionary to collisional orogenic belts from Archean to Tertiary times. Their genesis, including metal and fluid source, fluid pathways, depositional mechanisms, and timing relative to regional structural and metamorphic events, continues to be controversial. However, there is now general agreement that these deposits formed from metamorphic fluids, either from metamorphism of intra-basinal rock sequences or de-volatilization of a subducted sediment wedge, during a change from a compressional to transpressional, less commonly transtensional, stress regime, prior to orogenic collapse. In the case of Archean and Paleoproterozoic deposits, the formation of orogenic gold deposits was one of the last events prior to cratonization. The late timing of orogenic gold deposits within the structural evolution of the host orogen implies that any earlier structures may be mineralized and that the current structural geometry of the gold deposits is equivalent to that at the time of their formation provided that there has been no significant post-gold orogenic overprint. Within the host volcano-sedimentary sequences at the province scale, world-class orogenic gold deposits are most commonly located in second-order structures adjacent to crustal scale faults and shear zones, representing the first-order ore-forming fluid pathways, and whose deep lithospheric connection is marked by lamprophyre intrusions which, however, have no direct genetic association with gold deposition. More specifically, the gold deposits are located adjacent to ∼10°–25° district-scale jogs in these crustal-scale faults. These jogs are commonly the site of arrays of ∼70° cross faults that accommodate the bending of the more rigid components, for example volcanic rocks and intrusive sills, of the host belts. Rotation of blocks between these accommodation faults causes failure of more competent units and/or reactivation and dilation of pre-existing structures, leading to deposit-scale focussing of ore-fluid and gold deposition. Anticlinal or antiformal fold hinges, particularly those of ‘locked-up’ folds with ∼30° apical angles and overturned back limbs, represent sites of brittle-ductile rock failure and provide one of the more robust parameters for location of orogenic gold deposits. In orogenic belts with abundant pre-gold granitic intrusions, particularly Precambrian granite-greenstone terranes, the boundaries between the rigid granitic bodies and more ductile greenstone sequences are commonly sites of heterogeneous stress and inhomogeneous strain. Thus, contacts between granitic intrusions and volcano-sedimentary sequences are common sites of ore-fluid infiltration and gold deposition. For orogenic gold deposits at deeper crustal levels, ore-forming fluids are commonly focused along strain gradients between more compressional zones where volcano-sedimentary sequences are thinned and relatively more extensional zones where they are thickened. World-class orogenic gold deposits are commonly located in the deformed volcano-sedimentary sequences in such strain gradients adjacent to triple-point junctions defined by the granitic intrusions, or along the zones of assembly of micro-blocks on a regional scale. These repetitive province to district-scale geometrical patterns of structures within the orogenic belts are clearly critical parameters in geology-based exploration targeting for orogenic gold deposits.
144 citations
TL;DR: Three-dimensional analysis of the morphology and orientation of the structures within the context of host rock fabrics, combined with texture-specific analyses of major and trace element chemistry, show that the ‘stromatolites’ are more plausibly interpreted as part of an assemblage of deformation structures formed in carbonate-altered metasediments long after burial.
Abstract: The Palaeoarchean supracrustal belts in Greenland contain Earth’s oldest rocks and are a prime target in the search for the earliest evidence of life on Earth. However, metamorphism has largely obliterated original rock textures and compositions, posing a challenge to the preservation of biological signatures. A recent study of 3,700-million-year-old rocks of the Isua supracrustal belt in Greenland described a rare zone in which low deformation and a closed metamorphic system allowed preservation of primary sedimentary features, including putative conical and domical stromatolites1 (laminated accretionary structures formed by microbially mediated sedimentation). The morphology, layering, mineralogy, chemistry and geological context of the structures were attributed to the formation of microbial mats in a shallow marine environment by 3,700 million years ago, at the start of Earth’s rock record. Here we report new research that shows a non-biological, post-depositional origin for the structures. Three-dimensional analysis of the morphology and orientation of the structures within the context of host rock fabrics, combined with texture-specific analyses of major and trace element chemistry, show that the ‘stromatolites’ are more plausibly interpreted as part of an assemblage of deformation structures formed in carbonate-altered metasediments long after burial. The investigation of the structures of the Isua supracrustal belt serves as a cautionary tale in the search for signs of past life on Mars, highlighting the importance of three-dimensional, integrated analysis of morphology, rock fabrics and geochemistry at appropriate scales.
92 citations
TL;DR: In this article, the trace element and U-Pb systematics of metamorphic apatite were investigated from a suite of 22 bedrock samples of diverse metamorphics grade and protolith type.
88 citations
TL;DR: Deep fluids are important for the evolution and properties of the lower continental and arc crust in tectonically active settings as discussed by the authors, and they comprise four components: H2O, nonpolar gases, salts, and roc...
Abstract: Deep fluids are important for the evolution and properties of the lower continental and arc crust in tectonically active settings. They comprise four components: H2O, nonpolar gases, salts, and roc...
88 citations
TL;DR: In this paper, a large-scale integrated structural-metamorphic study based on 8 profiles across eastern Nepal has been performed. And the results are integrated with previously published metamorphic studies.
83 citations
TL;DR: In this article, a global compilation of mafic igneous rocks demonstrates that this episode of magmatic quiescence was terminated about 2.2 billion years ago by a flare-up of juvenile magmatism.
Abstract: The geologic record exhibits periods of active and quiescent geologic processes, including magmatism, metamorphism and mineralization. This apparent episodicity has been ascribed either to bias in the geologic record or fundamental changes in geodynamic processes. An appraisal of the global geologic record from about 2.3 to 2.2 billion years ago demonstrates a Palaeoproterozoic tectono-magmatic lull. During this lull, global-scale continental magmatism (plume and arc magmatism) and orogenic activity decreased. There was also a lack of passive margin sedimentation and relative plate motions were subdued. A global compilation of mafic igneous rocks demonstrates that this episode of magmatic quiescence was terminated about 2.2 billion years ago by a flare-up of juvenile magmatism. This post-lull magmatic flare-up is distinct from earlier such events, in that the material extracted from the mantle during the flare-up yielded significant amounts of continental material that amalgamated to form Nuna — Earth’s first hemispheric supercontinent. We posit that the juvenile magmatic flare-up was caused by the release of significant thermal energy that had accumulated over some time. This flux of mantle-derived energy could have provided a mechanism for dramatic growth of continental crust, as well as the increase in relative plate motions required to complete the transition to modern plate tectonics and the supercontinent cycle. These events may also be linked to Palaeoproterozoic atmospheric oxygenation and equilibration of the carbon cycle. Earth experienced a lull in magmatic and tectonic activity about 2.3 billion years ago, followed by a flare-up of magmatism, according to a compilation of existing geologic data. These events might mark the transition to the supercontinent cycle.
82 citations
01 Jan 2018
TL;DR: The Dom Feliciano Belt as mentioned in this paper is an orogenic association that extends from southern Brazil to Uruguay parallel to the Atlantic coastline for over 1100 km, and is the result of interaction between the Rio de la Plata, Congo and Kalahari cratons, together with several microplates juxtaposed along major shear zones.
Abstract: The Dom Feliciano Belt is an orogenic association that extends from southern Brazil to Uruguay parallel to the Atlantic coastline for over 1100 km. It was assembled in the Neoproterozoic, during the Brasiliano orogenic cycle, and is the result of interaction between the Rio de la Plata, Congo and Kalahari cratons, together with several microplates, juxtaposed along major shear zones. Along its extension, the Dom Feliciano Belt is exposed in three sectors: in the Brazilian states of Santa Catarina and Rio Grande do Sul, and in Uruguay. The blocks that acted as direct forelands to the belt in South America are smaller fragments to the main cratons: Luis Alves and Nico Perez. Three main lithotectonic domains are recognized in the belt, from east to west: a granitic batholith, a metasedimentary sequence and an association of foreland basins. Basement inliers are common, and evidence intense reworking and magmatism during the Neoproterozoic. Cryogenian to Ediacaran granitogenesis is widespread and voluminous, and usually displays an evolutionary tendency from medium- to high-K calc-alkaline, finishing with alkaline magmatism. The early evolution of the Dom Feliciano Belt is recorded in the Sao Gabriel Terrane, in which convergent tectonics is associated with intense juvenile magmatism, ophiolite complexes and accretion between 870 and 680 Ma. This is followed by two more deformational phases, identified in all three sectors. A convergent phase is associated with the deformation of the metavolcano-sedimentary complexes, shear zone nucleation and granitic magmatism associated with high-grade collisional metamorphism. This stage is constrained between c. 650–620 Ma in Santa Catarina and Rio Grande do Sul, and between c. 630–600 Ma in Uruguay. The last stage marks a transition to strike-slip deformation, with common shear zone reactivation associated with refolding in the metamorphic associations and widespread post-collisional granitic and volcanic magmatism. This phase is predominant from 610 to 550 Ma. The opening of the foreland basins was initiated during this period, probably associated with transtension along the main structures. Late-stage deformation and magmatism is common until 550–540 Ma. Abundant geochronological data have been added to the Dom Feliciano Belt in the last decades, leading to more precise time constraints for most of the geologic processes in the orogen. Details of its tectonic model, however, are still matters of debate, in terms of both the setting of its main units and its position into the assembly of southwestern Gondwana.
73 citations
01 Jan 2018
TL;DR: In this paper, a compilation of over 250 B-isotope analyses from about 90 localities of felsic igneous rocks in the continental crust shows a first-order distinction in composition between I-type and S-type magmas (derived from metasedimentary rocks).
Abstract: Boron is an incompatible lithophile element that is readily transported by granitic melts and hydrous fluids and therefore is concentrated in the continental crust relative to the mantle. The isotopic composition of boron in crystalline rocks of the continental crust (e.g., metamorphic and igneous lithologies) varies over a wide range of −20 to +10‰, depending on the B-isotope composition of the protoliths and on fractionation effects caused by phase transitions (metamorphic devolatilization reactions, fluid exsolution from magmas). Studies of progressive metamorphism and anatexis show that the behavior of boron and its isotopes depends heavily on the presence or absence of B-retentive minerals like tourmaline. In general, boron is prone to loss during devolatilization reactions, and metamorphic fluid preferentially removes the heavier isotope, but growth of tourmaline can minimize or prevent these effects. A new compilation of over 250 B-isotope analyses from about 90 localities of felsic igneous rocks in the continental crust shows a first-order distinction in composition between I-type magmas (subduction-related having meta-igneous sources) and S-type magmas (derived from metasedimentary rocks). Boron in I-type magmas is isotopically heavy (mean δ11B = −2‰, s.d. = 5) relative to unaltered MORB (mean δ11B = −7‰, s.d. = 1), presumably because of a greater contribution by subducted oceanic crust and pelagic sediments. Boron in S-type granitic rocks has a much lighter isotopic signature (mean δ11B = −11‰, s.d. = 4). The latter corresponds to the commonly cited B-isotope value of −10‰ for continental crust, but because much of Earth’s crust is derived from I-type magmas, its average B-isotope value is probably higher than previously thought. The dichotomy of B-isotope compositions in I- and S-type granitoids is also observed in their genetically related magmatic-hydrothermal ore deposits, as we demonstrate in a review of data from porphyry and Iron Oxide-Copper-Gold (IOCG) systems (I-type) and from Sn-W veins and granitic pegmatites (S-type). However, it is important to note that in all of these systems, there are significant and locally complex effects of isotopic fractionation due to magmatic fluid exsolution and to mixing of boron sourced from externally derived fluids.
72 citations
TL;DR: The Pampean orogeny of northern Argentina resulted from Early Cambrian oblique collision of the MARA block, formerly attached to Laurentia, with the Gondwanan Kalahari and Rio de la Plata cratons.
TL;DR: In this article, an age range of 55'±'10'Ma is obtained to mark the collisional orogeny in the Early Cenozoic rather than throughout the Cenogene.
Abstract: The timing of continental collision between India and Asia has been controversial for a long time because of the difficulty in screening isotopic ages for different types of tectonothermal event along the convergent continental boundary. After distinguishing the collisional orogeny from the precollisional accretionary orogeny and the postcollisional rifting orogeny, an age range of 55 ± 10 Ma is obtained to mark the collisional orogeny in the Early Cenozoic rather than throughout the Cenozoic. This age range provides the resolution to the timing of tectonic reactivation not only for reworking of the marginal arc systems in the Early Cenozoic but also for overprinting of granulite facies metamorphism on eclogites in the Late Cenozoic. In particular, superimposition of the rifting orogeny on both accretionary and collisional orogens in the Late Cenozoic is the key to the reactivation of both Gangdese and Himalayan orogens for contemporaneous metamorphism and magmatism at high thermal gradients. Therefore, rise of the plateau may be caused by underplating of the asthenospheric mantle for rifting orogeny in the composite Himalayan–Tibetan orogens after foundering of their roots in the Late Cenozoic.
TL;DR: In this paper, the Kontum massif is subdivided from North to South into: (i) Kham Duc, (ii) Ngoc Linh, (iii) Kan Nack complexes.
TL;DR: In this article, newly discovered Neoproterozoic orthogneisses and early Paleozoic granitoids of the Mashan Complex from the Jiamusi Block were described.
TL;DR: In this article, the authors present evidence for decoupling in zircon from the highest grade metapelites taken along a continuous high-temperature metamorphic field gradient in the Ivrea Zone (NW Italy).
Abstract: Granulite facies rocks frequently show a large spread in their zircon ages, the interpretation of which raises questions: Has the isotopic system been disturbed? By what process(es) and conditions did the alteration occur? Can the dates be regarded as real ages, reflecting several growth episodes? Furthermore, under some circumstances of (ultra-)high-temperature metamorphism, decoupling of zircon U–Pb dates from their trace element geochemistry has been reported. Understanding these processes is crucial to help interpret such dates in the context of the P–T history. Our study presents evidence for decoupling in zircon from the highest grade metapelites (> 850 °C) taken along a continuous high-temperature metamorphic field gradient in the Ivrea Zone (NW Italy). These rocks represent a well-characterised segment of Permian lower continental crust with a protracted high-temperature history. Cathodoluminescence images reveal that zircons in the mid-amphibolite facies preserve mainly detrital cores with narrow overgrowths. In the upper amphibolite and granulite facies, preserved detrital cores decrease and metamorphic zircon increases in quantity. Across all samples we document a sequence of four rim generations based on textures. U–Pb dates, Th/U ratios and Ti-in-zircon concentrations show an essentially continuous evolution with increasing metamorphic grade, except in the samples from the granulite facies, which display significant scatter in age and chemistry. We associate the observed decoupling of zircon systematics in high-grade non-metamict zircon with disturbance processes related to differences in behaviour of non-formula elements (i.e. Pb, Th, U, Ti) at high-temperature conditions, notably differences in compatibility within the crystal structure.
TL;DR: Brown et al. as discussed by the authors interpreted a P-T evolution involving cooling at pressures of 8-9 kbar from >960°C to the solidus and late subsolidus decompression.
Abstract: Handling editor: Michael Brown Abstract In this study, sapphirine-bearing granulites and sapphirine-absent garnet–sillimanite gneisses from the Tuguiwula area in the eastern segment of the Khondalite Belt, North China Craton (NCC) are interpreted to show a P–T evolution involving cooling at pressures of 8–9 kbar from >960°C to the solidus (~820°C) and late subsolidus decompression. This interpretation is based on the sequence of mineral appearance and thermodynamic modelling of phase equilibria. Sapphirine is observed to coexist with spinel within the peak assemblages. This observation conflicts with the traditional view that spinel generally appears prior to sapphirine and thus indicates pre-Tmax compression. For ultrahigh-temperature (UHT) metapelites at Tuguiwula, a clockwise P–T path may be more likely, which would be consistent with the clockwise P–T evolution of the extensive “normal” granulites (Tmax <900°C) and UHT granulites at other localities in the eastern segment of the Khondalite Belt. At Tuguiwula, for UHT metapelites with low bulk-rock Mg/ (Mg+Fe), the oxidation state/Fe content is interpreted to be a significant factor in controlling the mineral assemblages. We find that these compositions tend to contain sapphirine under oxidized conditions but spinel (without sapphirine) under reduced conditions. This difference may account for the simultaneous presence of both sapphirine-bearing UHT granulites and sapphirine-absent garnet–sillimanite UHT gneisses at Tuguiwula. LA-ICP-MS U–Pb dating of metamorphic zircon in the UHT metapelites yields mean Pb/Pb ages of c. 1.92 Ga (two samples), which are interpreted to record the timing of cooling of the UHT rocks to the solidus. The UHT metamorphism is interpreted to have been generated by mantle upwelling and emplacement of mafic magmas within a post-orogenic setting.
TL;DR: The Shirokaya Salma eclogite-bearing complex is located in the Archean-Paleoproterozoic Belomorian Province (Russia).
Abstract: The Shirokaya Salma eclogite-bearing complex is located in the Archean-Paleoproterozoic Belomorian Province (Russia). Its eclogites and eclogitic rocks show multiple clinopyroxene breakdown textures, characterized by quartz-amphibole, orthopyroxene, and plagioclase lamellae. Representative samples, a fresh eclogite, two partly retrograded eclogites, and a strongly retrograded eclogitic rock, were collected for this study. Two distinct mineral assemblages – (1) omphacite+garnet+quartz+rutile±amphibole and (2) clinopyroxene+garnet+amphibole+plagioclase +quartz+rutile+ilmenite±orthopyroxene – are described. Based on phase equilibria modelling, these assemblages correspond to the eclogite- and granulite-facies metamorphism that occurred at 16-18 kbar, 750-800 °C and 11-15 kbar, 820-850 °C, respectively. The quartz-amphibole lamellae in clinopyroxene formed during retrogression with water ingress, but do not imply UHP metamorphism. The superfine orthopyroxene lamellae developed due to breakdown of an antecedent clinopyroxene (omphacite) during retrogression that was triggered by decompression from the peak of metamorphism, while the coarser orthopyroxene grains and rods formed afterwards. The P-T path reconstructed for the Shirokaya Salma eclogites is comparable to that of the adjacent 1.9 Ga Uzkaya Salma eclogite (Belomorian Province), and those of several other Paleoproterozoic high-grade metamorphic terranes worldwide, facts allowing us to debate the exact timing of eclogite-facies metamorphism in the Belomorian Province.
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TL;DR: In this article, the textural and trace element evolution of pyrite during greenschist facies metamorphism has been described using a variety of techniques including laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) on fifty-four samples.
TL;DR: The Wulonggou Pluton is located in the eastern segment of the Eastern Kunlun Orogenic Belt, NW China, and consists of mainly alkali-feldspar granites covering an area of about 150 km2.
TL;DR: The Cassiterita Orthogneiss is the oldest pluton so far found within the Mineiro belt as discussed by the authors, with an in situ zircon U-Pb ages of between 2472 and 2414.
TL;DR: A comprehensive and updated review on the gneissic Ollo de Sapo Formation (OSF) in the Iberian Massif can be found in this article, where the authors discuss multiple aspects (lithostratigraphy, petrography, geochemistry, geochronology, and partial melting relationships) of its evolution through time.
TL;DR: In this article, the authors reported new SIMS zircon U-Pb dates along with ZIRcon O isotope and trace element analyses, to constrain the origin of these zircons and the interpretation of Zircon dates.
TL;DR: It is shown that the associated formation of Caledonian eclogites at the expense of Proterozoic granulites was related to local pressure perturbations rather than burial, and that the HP metamorphism resulted from fluid-induced weakening of an initially dry and highly stressed lower crust when thrust upon the hyperextended margin of the Baltic shield.
Abstract: Studies of mineral equilibria in metamorphic rocks have given valuable insights into the tectonic processes operating at convergent plate margins during an orogeny. Geodynamic models simulating orogenesis and crustal thickening have been constrained by temperature and pressure estimates inferred from the mineral assemblages of the various lithologies involved along with age constrains from increasingly precise geochronological techniques. During such studies it is assumed that the pressure experienced by a given rock is uniquely related to its depth of burial. This assumption has been challenged by recent studies of high pressure (HP) and ultrahigh pressure (UHP) rocks. Here, we describe an example of Caledonian HP metamorphism from the Bergen Arcs in western Norway, and show that the associated formation of Caledonian eclogites at the expense of Proterozoic granulites was related to local pressure perturbations rather than burial, and that the HP metamorphism resulted from fluid-induced weakening of an initially dry and highly stressed lower crust when thrust upon the hyperextended margin of the Baltic shield.
TL;DR: In this article, the authors presented a systematic study of petrology, geochemistry and geochronology of paragneisses and intermediate to basic intrusions from the Hualong Group, which represents the lower part of Precambrian basement of the block.
TL;DR: In this article, the authors reported new field observations and zircon U-Pb ages and Hf isotopic compositions of representative rocks from the so-called Precambrian basement in the northeastern Pamir, i.e., the Bulunkuole Group.
TL;DR: Recently, a previously unrecognized central Dharwar Craton block (Central-Dharwar-Craton; CDC) within the craton has been revealed as mentioned in this paper, which requires revision of this model and reinterpretation of metamorphic and magmatic age data.
Abstract: Numerous tectonic scenarios have been proposed for terrane growth and accretion within the Archean Dharwar Craton, southern Peninsular India. Previously accepted interpretations involving a two-terrane model—comprising a Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC) block—have invoked west-dipping subduction and ocean closure, leading to arc magmatism and accretionary orogeny in the WDC, followed by metamorphic overprinting and collisional orogeny in the EDC. However, recent field investigations have revealed the existence of a previously unrecognized ‘central’ block (Central Dharwar Craton; CDC) within the craton, which requires revision of this model and reinterpretation of metamorphic and magmatic age data. Five samples of high-pressure, upper amphiboliteand granulite-facies meta-igneous and metasedimentary rocks from the southern portion of the Chitradurga Suture Zone, which divides the WDC and CDC, record minimum peak metamorphic conditions of 820–875 C at 10 kbar, indicating equilibration at the base of thickened continental crust. U–Pb zircon and Pb–Pb monazite geochronology indicates crystallization of parent mafic magmas at c. 2 61–2 51 Ga and subsequent regional metamorphism of these intrusions to garnet-amphibolite and garnet-granulite facies at c. 2 48–2 44 Ga, bracketing the timing of microblock accretion to the Archean–Proterozoic boundary. Light rare earth element enrichment within these zircon grains indicates magma generation in a suprasubduction-zone environment. In addition, detrital magmatic zircon grains with ages of c. 3 10–3 03 Ga and c. 2 97– 2 86 Ga imply contamination of these magmas with Mesoarchean material sourced from the Western Dharwar Craton continental nucleus. Comparison of these metamorphic and magmatic age data with those recorded in the EDC shows that westward-directed subduction is implausible, and that all three terranes (the WDC, CDC, and EDC) must have accreted synchronously, driven by two separate eastward-dipping ocean–continent convergent plate margins. These data further support a recent abundance of observations from the geological record supporting the hypothesis that subduction-driven plate tectonics had initiated on Earth before c. 2 5 Ga, as opposed to a Neoproterozoic onset (c. 0 8–0 9 Ga) reported by numerous studies.
TL;DR: In this paper, the authors present U/Pb ages of metamorphic zircon from several Adria-derived continental units now situated in the Western Alps, defining a range between 286 and 266 ǫMa.
Abstract: During the late Palaeozoic, lithospheric thinning in part of the Alpine realm caused high-temperature low-to-medium pressure metamorphism and partial melting in the lower crust. Permian metamorphism and magmatism has extensively been recorded and dated in the Central, Eastern, and Southern Alps. However, Permian metamorphic ages in the Western Alps so far are constrained by very few and sparsely distributed data. The present study fills this gap. We present U/Pb ages of metamorphic zircon from several Adria-derived continental units now situated in the Western Alps, defining a range between 286 and 266 Ma. Trace element thermometry yields temperatures of 580–890 °C from Ti-in-zircon and 630–850 °C from Zr-in-rutile for Permian metamorphic rims. These temperature estimates, together with preserved mineral assemblages (garnet–prismatic sillimanite–biotite–plagioclase–quartz–K-feldspar–rutile), define pervasive upper-amphibolite to granulite facies conditions for Permian metamorphism. U/Pb ages from this study are similar to Permian ages reported for the Ivrea Zone in the Southern Alps and Austroalpine units in the Central and Eastern Alps. Regional comparison across the former Adriatic and European margin reveals a complex pattern of ages reported from late Palaeozoic magmatic and metamorphic rocks (and relics thereof): two late Variscan age groups (~330 and ~300 Ma) are followed seamlessly by a broad range of Permian ages (300–250 Ma). The former are associated with late-orogenic collapse; in samples from this study these are weakly represented. Clearly, dominant is the Permian group, which is related to crustal thinning, hinting to a possible initiation of continental rifting along a passive margin.
TL;DR: In this article, structural analysis and U-Pb geochronological study on zircons from the southern Chinese Altai (the Kalasu area, SE of the Altai city) show that the Cambro-Ordovician accretionary wedge underwent four major geological events: 1) emplacement of Early Devonian magmas (ca. 410-400) associated with formation of a volcano-sedimentary cover, 2) major Middle Devonian tectono-metamorphic event, 3) Late Devonian-Early Carboniferous folding without apparent metamorph
TL;DR: In this article, the authors demonstrate the potential of a new approach of multiple single grain fusion dating, which provides a comprehensive inventory of the range of ages present in different rocks and at different scales, which results in a more refined understanding of argon retention and isotopic closure of phengite and the geological significance of the ages.
TL;DR: In this paper, the authors conducted a series of piston cylinder "sandwich" experiments, testing the hydrothermal fluid hypothesis, and a newly suggested process of carbonatite metasomatism.
Abstract: Nolans-type ore deposits contain REE and Th mineralization hosted in fluorapatite veins. These veins intrude granulite facies rocks and are surrounded by a diopside selvage. Nolans-type deposits are thought to form by REE, F and P-rich hydrothermal fluids derived from alkali or carbonatitic intrusions. However, REE are not effectively transported in F and P-rich systems. REE ore deposits are commonly hydrothermally overprinted, possibly obscuring the igneous nature of the primary mineralization. We conducted a series of piston cylinder “sandwich” experiments, testing the hydrothermal fluid hypothesis, and a newly suggested process of carbonatite metasomatism. Our results confirm theoretical predictions that REE are hydrothermally immobile in these systems and the experimental phase assemblage is not compatible with the natural rocks. Our results show that fluorapatite can only host several weight percent levels of REE at temperatures higher than ∼600 °C. Below that temperature, a miscibility gap exists between REE-poor fluorapatite and REE-rich silicates such as britholite or cerite. In contrast, experiments reacting P and REE-rich carbonatite with silicate rock above 700 °C closely resemble natural rocks from Nolans-type deposits. Selvage mineralogy is sensitive to the MgO content of the carbonatite. A diopside selvage formed at carbonatite MgO/(CaO+MgO) ≈ 0.2 while wollastonite and forsterite formed at lower and higher ratios, respectively. Phosphate solubility in carbonatites decreases with decreasing MgO contents. As diopside formed, REE-rich fluorapatite preferentially crystallized from the selvage inwards. Thus, carbonatites are effective at simultaneously mobilizing REE, F and P to the site of deposition. Nolans-type deposits are the cumulate residue of this reaction, with the carbonatite liquid migrating elsewhere. At temperatures below 700 °C the carbonatite–silicate reaction additionally formed monticellite, cuspidine and magnesioferrite, resembling a skarn assemblage. Whereas skarns form by infiltration of silicate magmas or related fluids to carbonate rocks, our experiments are the opposite: intrusion of carbonatite into silicate rock. These mid-crustal skarn-like rocks may host elevated ore elements of carbonatitic affinity, such as F, P, Y, REE, Th, Ba, Sr, and Nb. We propose the term “antiskarn” to describe such systems, and suggest they trace the migration of carbonatite liquids through the crust. Hydrothermal reworking, retrogression, or metamorphism of antiskarns may obscure the carbonatitic genesis of the rocks. These metasomatic zones are the crustal equivalent of wehrlites that form by peridotite–carbonatite reaction at mantle depths.