Showing papers on "Metamorphism published in 2021"

Early Mesoproterozoic evolution of midcontinental Laurentia: Defining the geon 14 Baraboo orogeny

Abstract: New geochronologic data from midcontinental Laurentia demonstrate that emplacement of the 1476–1470 Ma Wolf River granitic batholith was not an isolated igneous event, but was accompanied by regional metamorphism, deformation, and sedimentation. Evidence for such metamorphism and deformation is best seen in siliciclastic sedimentary rocks of the Baraboo Interval, which were deposited closely following the 1.65–1.63 Ga Mazatzal orogeny. In Baraboo Interval strata, muscovite parallel to slatey cleavage, in hydrothermal veins, in quartzite breccia, and in metamorphosed paleosol yielded 40Ar/39Ar plateau ages of 1493–1465 Ma. In addition, U–Th–total Pb dating of neoblastic overgrowths on detrital monazite gave an age of 1488 ± 20 Ma, and recrystallized hematite in folded metapelite gave a mean U/Th–He age of 1411 ± 39 Ma. Post-Baraboo, arkosic polymictic conglomerate, which contains detrital zircon with a minimum peak age of 1493 Ma, was intruded by a 1470 Ma granite porphyry at the northeastern margin of the Wolf River batholith. This episode of magmatism, regional deformation and metamorphism, and sedimentation, which is designated herein as the Baraboo orogeny, provides a midcontinental link between the Picuris orogeny to the southwest and the Pinware orogeny to the northeast, completing the extent of early Mesoproterozoic (Calymmian) orogenesis for 5000 km along the southern margin of Laurentia. This transcontinental orogen is unique among Precambrian orogenies for its great width (~1600 km), the predominance of ferroan granites derived from partial melting of lower continental crust, and the prevalence of regional high T-P metamorphism related to advective heating by granitic magmas emplaced in the middle to upper crust.

Late Neoarchean magmatic – metamorphic event and crustal stabilization in the North China Craton

Abstract: The ca. 2.5 Ga as the time boundary between the Archean and the Proterozoic eons is a landmark, indicating the most important continental crust evolving stage of the Earth, that is, the global cratonization or the formation of supercraton(s) that was unseen before and is unrepeated in the following history of the Earth9s formation and evolution. The North China Craton (NCC) is one of the best recorders of the ca. 2.5 Ga event, and therefore studies in the thorough understanding of early Precambrian continental evolution are continuous. The period from 2.8 to 2.6 Ga is the major crustal growth period of the NCC and formed seven micro-blocks. All the micro-blocks in the NCC were surrounded by 2.6 to 2.54 Ga greenstone belts. The clear geological presentations are as follows: (1) Archaic basement rocks in North China (various micro-blocks) experienced strong partial melting and migmatization. The granitoid rocks derived from crustal partial melting include potassium, TTG and monzonitic granitoids, which come, respectively, from continental crust (sedimentary rocks with TTG gneisses), juvenile crust (mafic rocks with TTG gneisses) or mixed crust; (2) the BIF-bearing supracrustal rocks are mainly distribute in greenstone belts. The lithologic associations in the greenstone belts within the NCC are broadly similar, belonging to volcano-sedimentary sequences, with common bimodal volcanic rocks (basalt and dacite) interlayered with minor amounts of komatiites in the lower part, and calc-alkalic volcanic rocks (basalt, andesite and felsic rocks) in the upper part; (3) nearly all old rocks of >2.5 Ga underwent ∼2.52 to 2.5 Ga metamorphism of amphibolite–granulite facies. Most metamorphosed rocks show high-temperature-ultra-high-temperature (HT–UHT) characteristics and record anticlockwise P–T paths, albeit a small number of granulites seemingly underwent high-pressure granulite facies metamorphism and record clockwise P–T paths; (4) ∼2.5 Ga mafic dikes (amphibolites), granitic dikes (veins) and syenitic–ultramafic dikes developed across these archaic basements and were strongly deformed or un-deformed; (5) the extensive 2.52 to 2.48 Ga low-grade metamorphic supracrustal covers has been recognized in eastern, northern and central parts of the NCC, which are commonly composed of bi-modal volcanic rocks and sedimentary rocks. The above mentioned ∼2.5 Ga geological rocks and their characters imply that the seven micro-blocks have been united through amalgamation to form the NCC. The metamorphosed late Neoarchean greenstone belts, as syn-formed mobile belts, welded the micro-blocks at the end of the Neoarchean. However, the metamorphic thermal grades of the greenstone belts are lower than those of the high-grade terranes within the micro-blocks, suggesting that the latter might have developed under a higher geothermal gradient than the former. Besides, the greenstone belts surround the various micro-blocks in the late Neoarchean when both the old continental crust and the oceanic crust were hotter. The subduction during the amalgamation, if it happened, must have been much smaller in scale as compared to those in the Phanerozoic plate tectonic regime, and all stages occurred at crust-scale instead of lithosphere-scale or mantle-scale. This is why most rocks record HT-UHT and anti-clockwise metamorphism, while only a few samples record high-pressure granulite facies metamorphism with clockwise P–T paths. The micro-block amalgamation was accompanied by extensive crust partial melting and granitization, which finally gave rise to the stabilization of the NCC. Except for the vast granitoid intrusions, mafic-syenitic dike swarms and sedimentary covers are also landmarks of cratonization. The ca. 2.5 Ga cratonization is a global epoch-making geological event, although the accomplishment of cratonization in various cratons is somewhat different in time. Cratonization declared the formation and stabilization of global-scale supercratons or cratonic groups coupling with lithosphere, which was followed by a “silent period” with rare tectonic-thermal action lasting 150 to 200 Ma (from 2.5 Ga – 2.3 or 2.35 Ga), and then followed by the Great Oxidation Event (GOE).

Mantle flow: The deep mechanism of large-scale growth in Tibetan Plateau

Abstract: The origin and growth of the Tibetan Plateau, located in the back lands of the Himalayan orogenic belt, have always been considered controversial. Based on data from previous studies and our own comprehensive research, we found that a giant high-heat flow zone thousands of kilometres long developed, spanning different tectonic units of the Qinghai-Tibet Plateau, encompassing the Kunlun, Qiangtang, Mangkang-Dali and Honghe-Ailao Mountain ranges. All these tectonic units show a tendency to have migrated from the inner plateau towards the northeast edge. Several rock groups are distributed along this giant high-heat flow zone in a continuous pattern: The potassic mafic rock-lamprophyre group (42–32 Ma) and potassic alkaline rock-carbonate rock (27–7 Ma) from partial melting of lithosphere and mantle, the ocean island basalt (16–1 Ma) from decompression melting of the asthenosphere, and the potassic feldspathic rock (40–0.3 Ma) from the melting of the middle-lower crust. The high temperature deep metamorphic zone, characterized by granulite facies metamorphism in the peak period, was accompanied by a large strike-slip fault zone (40–17 Ma). The metamorphic temperatures of the lower crust granulite xenoliths rose to 800°C. However, the mantle peridotite xenoliths show characteristics of vertical mantle flow. The six large low-speed anomalous bodies revealed by geophysical exploration are clustered, equidistant, and intermittently distributed. Our research proposes that the Indian continental lithospheric-mantle subduction triggered the Asian continental asthenosphere surge, which caused vertical upwelling along a number of mantle channels in the posterior continental region, thermally corroding and engulfing the mantle-lithosphere and reaching as far as the bottom of the crust. These “mantle flow channels” came from depths around 400 km. They began forming after the late (hard) collision 40 Ma, and not only provided deep heat to sustain the Tibetan Plateau uplift but also prepared new mantle source material for the plateau’s crustal growth. At the same time, this high-heat flow zone caused the plastic and lateral flow of the middle-lower crust, driving the lateral growth of the Tibetan Plateau to the northeast.

Petrogenetic Evolution of the Neoproterozoic Igneous Rocks of Egypt

Abstract: The Late Neoproterozoic basement exposures in the Sinai Peninsula and Eastern Desert of Egypt represent the northern part of the Nubian Shield, which was contiguous with the Arabian shield before the opening of the Red Sea. These two Shields together form the Arabian-Nubian Shield (ANS). The ANS, formed during the Neoproterozoic Pan-African orogeny due to collision between East and West Gondwana, is the best-preserved juvenile continental crust of Neoproterozoic age on Earth. The Nubian shield evolved through a series of stages that can be characterized as (1) supercontinent rift and drift (1000–900 Ma), (2) subduction and consumption of intervening ocean basins (870–750 Ma), (3) continental collision and orogeny (750–630 Ma), and (4) post-orogenic extension and collapse (620–580 Ma plus minor later activity extending to 540 Ma). The Neoproterozoic (870–580 Ma) development of the juvenile continental crust specifically of the Egyptian Nubian Shield segment of the ANS, however, is most logically discussed in three main stages: pre-collisional, collisional, and post-collisional. The pre-collisional phase (~870–700 Ma) produced complete oceanic ophiolite assemblages coexisting with intra-oceanic island arcs. All the Egyptian Eastern Desert ophiolites are strongly deformed, metamorphosed, and affected by several types of alteration. They occur as dismembered, tectonized bodies and melanges of pillowed metabasalt, gabbro, and variably altered ultramafic rocks. Along shear zones, the ophiolitic ultramafics are highly altered into talc-carbonates, magnesite, and listvenite. The collisional stage (670–630 Ma) includes a subduction period that produced volcano-sedimentary island arc successions and calc-alkaline gabbro–diorite complexes, followed by development of Cordilleran-style calc-alkaline gabbros and granodiorites and their extrusive equivalents, ending with the merger of West and East Gondwana. All the units of the collisional stage are weakly deformed and experienced only low-grade metamorphism. The post-collisional phase (~620–580 Ma) was characterized by intracrustal melting that first generated calc-alkaline granitoids and then gradually shifted to a terminal stage that produced relatively small volumes of alkaline magma, preserved as both plutonic and volcanic units. Temporally, the later stages of post-collisional calc-alkaline magmatism and the alkaline magmatism overlapped. This chapter discusses the evolution of the Nubian from an igneous petrology perspective, including the plutonic (granitoid and mafic–ultramafic) and volcanic records. Granitoids were emplaced in Egypt during each phase of evolution of the Nubian Shield: pre-collisional granitoids include highly deformed trondhjemite, tonalite, and granodiorite; syn-collisional granitoids are weakly deformed granodiorite and less commonly granite; post-collisional granitoids include undeformed calc-alkaline and alkaline monzogranite, syenogranite, alkali feldspar granite, and alkaline/peralkaline granites. The Neoproterozoic mafic–ultramafic complexes in the Egyptian Nubian Shield include older and younger complexes. The older mafic–ultramafic complexes either form an integral part of obducted ophiolite sequences or constitute members of subduction-related, calc-alkaline gabbro–diorite complexes. The younger mafic–ultramafic complexes are mostly fresh, undeformed, and unmetamorphosed, and were emplaced in post-orogenic settings. Four major volcanic episodes have been recognized in the Neoproterozoic crust of Egypt, including ophiolitic metavolcanic rocks, island arc metavolcanic rocks, the Dokhan volcanic series, and post-collisional alkaline volcanics (Katherina Volcanics).

Geochemical and metamorphic record of the amphibolites from the Tuting–Tidding Suture Zone ophiolites, Eastern Himalaya, India: implications for the presence of a dismembered metamorphic sole

Abstract: The Tuting–Tidding Suture Zone (TTSZ), exposed along Dibang and Lohit river valleys in Arunachal Himalaya, NE India, is the easternmost continuation of the Indus–Tsangpo Suture Zone (ITSZ) and consists of ophiolites associated with metabasics and carbonates. Amphibolites, existing at the base of the ophiolite complex, were studied using whole-rock, mineral chemical analyses and pressure–temperature (P-T) pseudosection modelling to understand their metamorphic and petrogenetic history, and interpret the tectonic environment of their formation. They exhibit two-stage deformation, where D1 is depicted by polymineralic inclusion trails in former melt pools and the main foliation represents D2. Sub-alkaline tholeiitic character, high-field-strength element (HFSE) ratios and mid-oceanic ridge basalt (MORB) -like rare earth element (REE) patterns with negative Eu anomaly indicate that the protolith of these amphibolites originated in a spreading regime by extensive partial melting of a depleted mantle source at shallow depth. Petrography, mineral chemistry and P-T modelling indicate a three-stage metamorphic history for them. M1 is the prograde (c. 2.1 GPa, c. 450°C) defined by garnet centre compositions corresponding to the D1 event. The existence of former melts in the samples demarcates the M2 stage (1.4–1.8 GPa, c. 600°C). The rocks later underwent retrogression (M3: 0.8–1.0 GPa, 480–520°C), which corresponds to the D2 event. These observations suggest that the protolith of the TTSZ amphibolites originated in a mid-oceanic ridge setting, which accreted below a subduction zone where it underwent M1 metamorphism followed by M2 metamorphism, corresponding to partial melting of the rocks. Finally, the M3 event occurred during the obduction phase of the ophiolite complex, where the amphibolites were obducted as the metamorphic sole of the TTSZ ophiolites.

Grenvillian and early Paleozoic polyphase metamorphism recorded by eclogite and host garnet mica schist in the North Qaidam orogenic belt

Abstract: The North Qaidam orogenic belt (NQOB) is generally considered to be an early Paleozoic ultrahigh pressure metamorphic belt, but increasing reports of the Neoproterozoic magmatic and metamorphic events indicate that the NQOB probably also experienced the assembly of the Rodinia. However, the Neoproterozoic evolution of the NQOB is not well constrained due to the sparse records and ambiguous nature of the Neoproterozoic metamorphism. In order to reveal the multi-orogenic history of the NQOB, an integrated study of petrology, phase equilibrium modelling and geochronology was conducted on an epidote eclogite and host garnet mica schist from the Yuka–Luofengpo terrane. New zircon and monazite U–Pb ages show that the protolith of the garnet mica schist was deposited during 994–920 Ma and experienced Neoproterozoic (920–915 Ma) and early Paleozoic (451–447 Ma) polyphase metamorphism together with the enclosed eclogite. Relic omphacite inclusions were first identified in garnet and early Paleozoic zircon domains from the garnet mica schist, which provide solid evidence for the early Paleozoic eclogite facies metamorphism of the mica schist. Similar early Paleozoic peak P–T conditions of >27.4 kbar/613–670 °C and 30.2–30.8 kbar/646–655 °C were obtained for the garnet mica schist and enclosed eclogite, respectively, indicating that eclogites and their host paragneisses in this region underwent continental deep subduction as a coherent metamorphic terrane in early Paleozoic. The peak P–T conditions of the Neoproterozoic metamorphism were roughly constrained at 7.7–12.0 kbar and 634–680 °C for the garnet mica schist, based on stability field of mineral inclusions in Neoproterozoic zircons domains in P–T pseudosection, the relic garnet core composition and Ti-in-zircon thermometer. The high thermal gradients (16–37 °C/km) defined by presently our and previously reported P–T conditions indicate that the Neoproterozoic metamorphism likely occurred in continental collision setting at >945–890 Ma. Since the Grenvillian syn-orogenic granitic magmatism and metamorphism (ca. 1.0–0.9 Ga) in the NQOB are much younger than the Grenvillian orogeny in the central part of Rodinia, the Qaidam Block was probably located at the north margin of Rodinia in Neoproterozoic.

Basement-Cover Decoupling During the Inversion of a Hyperextended Basin: Insights From the Eastern Pyrenees

Abstract: Deformation processes related to early stages of collisional belts, especially the inversion of rifted systems remain poorly constrained, partly because evidence of these processes is usually obliterated during the subsequent collision. The Pyrenean belt resulting from the inversion of a Cretaceous hyperextended rifted margin associated with a HT/LP metamorphism in the Internal Metamorphic Zone (IMZ), is a good example for studying the early stage of orogenic deformation. This study is focused on the Eastern Pyrenees where the relation between inverted Mesozoic rifted basins and their basement are well-preserved. By using maximum temperatures (Tmax) estimated by the Raman Spectroscopy of Carbonaceous Materials geothermometer and structural data, we describe the spatial distribution of the various tectono-metamorphic units. Tmax recorded in the sedimentary cover exposed to the north and to the south of a Paleozoic basement block (Agly massif), exceed 550°C, while the Paleozoic metasediments and their autochthonous Mesozoic cover show Tmax <350°C. The metamorphic sedimentary cover is affected by ductile deformation, while the basement is only affected by brittle deformation. Post-metamorphism breccias are observed between the basement and the metamorphic Accepted Article This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as

Chapter 3.1a Antarctic Peninsula and South Shetland Islands: volcanology

Abstract: The Antarctic Peninsula contains a record of continental-margin volcanism extending from Jurassic to Recent times. Subduction of the Pacific oceanic lithosphere beneath the continental margin developed after Late Jurassic volcanism in Alexander Island that was related to extension of the continental margin. Mesozoic ocean-floor basalts emplaced within the Alexander Island accretionary complex have compositions derived from Pacific mantle. The Antarctic Peninsula volcanic arc was active from about Early Cretaceous times until the Early Miocene. It was affected by hydrothermal alteration, and by regional and contact metamorphism generally of zeolite to prehnite–pumpellyite facies. Distinct geochemical groups recognized within the volcanic rocks suggest varied magma generation processes related to changes in subduction dynamics. The four groups are: calc-alkaline, high-Mg andesitic, adakitic and high-Zr, the last two being described in this arc for the first time. The dominant calc-alkaline group ranges from primitive mafic magmas to rhyolite, and from low- to high-K in composition, and was generated from a mantle wedge with variable depletion. The high-Mg and adakitic rocks indicate periods of melting of the subducting slab and variable equilibration of the melts with mantle. The high-Zr group is interpreted as peralkaline and may have been related to extension of the arc.

High Ba–Sr adakitic charnockite suite from the Nagercoil Block, southern India: Vestiges of Paleoproterozoic arc and implications for Columbia to Gondwana

Abstract: The Nagercoil block is the southernmost crustal segment of the Southern Granulite Terrane (SGT) in India and is mainly composed of charnockitic rocks and felsic gneisses (charnockite suite). In this study, we present petrologic, geochemical, zircon U–Pb, REE, and Hf isotopic studies on the charnockites and leucogneiss from the Nagercoil block. Based on field investigations and petrologic studies, the charnockites can be divided into garnet-bearing and garnet-absent anhydrous granulite facies rocks with orthopyroxene. The charnockites and leucogneiss show transition from adakites to non-adakitic magmatic rocks, with enrichment in LREEs (light rare earth elements) and LILEs (large ion lithophile elements), and depletion in HREEs (heavy rare earth elements) and HFSEs (high field strength elements). Some of the charnockites and the leucogneiss show typical HSA (high silica adakite) characters, (high SiO2, Al2O3, Ba–Sr, La/Yb, and Sr/Y). The HSA is considered to have formed from the interaction of slab derived melts and peridotitic mantle wedge. The high Ba–Sr features were possibly inherited from subducted oceanic crust melting under high thermal gradient during Precambrian. The magmas were underplated and subjected to fractional crystallization. Zircon grains from the charnockite and leucogneiss show zoned magmatic cores surrounded by structureless metamorphic rims. Magmatic zircon grains from the charnockites show ages ranging from 1983 ± 8.8 Ma to 2046 ± 14 Ma, and the metamorphic domains show an age range of 502 ± 14 Ma to 547 ± 8.7 Ma. Zircon from the leucogneiss yielded magmatic and metamorphic ages of 1860 ± 20 Ma and 575.6 ± 8.8 Ma. Both charnockites and leucogneiss show two prominent age peaks at 1987 Ma and 568 Ma. The REE data of the zircon grains show LREE depletion and HREE enrichment, with the metamorphic grains showing more depletion in HREE. Zircon Hf isotopic data of the magmatic cores of zircon grains from the charnockite yielded eHf(t) values from −1.17 to 0.46 with TDM and TDMC and age peaks at 2392 Ma and 2638 Ma, suggesting Neoarchean to Paleoproterozoic juvenile sources. We suggest that the high Ba–Sr adakitic charnockite suite from the Nagercoil block formed in a Paleoproterozoic magmatic arc setting during the assembly of the Columbia supercontinent, and underwent high-grade metamorphism associated with the amalgamation of the Gondwana supercontinent during the late Neoproterozoic–Cambrian. Our study provides new insights into the vestiges of Columbia fragments within the Gondwana assembly with two distinct cycles of crustal evolution.

The Pressure‐Temperature‐time‐deformation history of the Beni Mzala unit (Upper Sebtides, Rif belt, Morocco): Refining the Alpine tectono‐metamorphic evolution of the Alboran Domain of the Western Mediterranean

Abstract: The structural and thermal relaxation overprint associated with the Neogene Alboran rifting have obscured the early Alpine tectono‐metamorphic evolution of the Alboran Domain, representing the metamorphic core of the Betic–Rif orogen of the western Mediterranean region. This study focuses on the Beni Mzala unit, forming the lower and deeper structural level of the Alpine metamorphic nappe stack (Upper Sebtides) in the Moroccan Rif. Meso‐ and micro‐scale structural investigations are carried out on high‐P aluminum silicate (Ky‐bearing)‐quartz segregations that occur as boudins within the main retrogressive syn‐greenschist foliation (S2/D2) and assumed to preserve the early M1 HP metamorphism associated with the Alpine orogenic construction in the Alboran Domain. These boudins host an early crenulated high‐P foliation (S1, D1/M1) made of quartz–kyanite–white mica–rutile. A large spread in white mica composition is documented, with the highest Si content per formula unit (up to 3.18 apfu) preserved along the S1 foliation and the lower Si content observed in the white mica marking the S2 retrogressive foliation (D2/M2) and the rim of S1 mica. Microtextural evidence documents post‐tectonic andalusite growth and static recrystallization of the quartz microlithons. Inverse (Zr‐in‐Rt thermometry) and forward modelling thermobarometry are integrated with Ar–Ar white mica geochronology to define the peak and exhumation pressure–temperature–time (P–T–t) path of the Beni Mzala unit. Minimum thermo‐baric estimates for the M1 event are ~1.4 GPa and 600°C, corresponding to a metamorphic gradient of ~11°/km, consistent with subduction zone metamorphism. Exhumation is constrained by re‐equilibration of the white mica composition (from high to low celadonite) between c. 29 and 22 Ma, during a nearly isothermal retrogressive path, with final equilibration at high‐T/low‐P conditions within the andalusite stability field (~0.2–0.3 GPa and 500°C). A minimum late Oligocene age is proposed for the Alpine D1 tectono‐metamorphic stage in the Rif, suggesting as feasible the previously proposed Eocene timing for the subduction‐zone metamorphism of the Alboran Domain. Conclusive evidence is provided to link the early Miocene tectono‐metamorphic event to a late thermal perturbation that affected the Alboran Domain at shallow crustal conditions, post‐dating the almost complete exhumation of the deep roots of the Alpine belt in the western Mediterranean.

The Lavrion Mines: A Unique Site of Geological and Mineralogical Heritage

Abstract: The Lavrion area corresponds to the western part of the Attic-Cycladic metamorphic belt, in the back-arc region of the active Hellenic subduction zone. Between the Eocene and the Miocene, metamorphic rocks (mainly marbles and schists) underwent several stages of metamorphism and deformation due to collision and collapse of the Cycladic belt. Exhumation during the Miocene was accommodated by the movement of a large-scale detachment fault system, which also enhanced emplacement of magmatic rocks, leading to the formation of the famous Lavrion silver deposits. The area around the mines shows the stacking of nappes, with ore deposition mainly localized within the marbles, at marble-schist contacts, below, within, or above the detachment. The Lavrion deposit comprises five genetically-related but different styles of mineralization, a feature never observed in another ore deposit elsewhere, containing the highest number of different elements of any known mining district. The local geology, tectonic, and magmatic activity were fundamental factors in determining how and when the mineralization formed. Other key factors, such as the rise and the fall of sea level, which resulted from climate change over the last million years, were also of major importance for the subsequent surface oxidation at Lavrion that created an unmatched diversity of secondary minerals. As a result, the Lavrion deposit contains 638 minerals of which Lavrion is type-locality for 23 of them, which is nearly 12% of all known species. Apart from being famous for its silver exploitation, this mining district contains more minerals than any other district on Earth. The unique geological, mineralogical, and educational (mining, archaeological, and environmental) features suggest that it is highly suitable to be developed as a future UNESCO Global Geopark.