Tectono-metamorphic Evolution Research Articles

Geochemistry, Monazite (U–Pb–Th) Geochronology, and P-T Pseudosection Modelling of Two-Pyroxene Mafic Granulite from Sonapahar, Shillong Meghalaya Gneissic Complex, India: Implication for Tectono-Metamorphic Evolution and Global Pan-African Correlation

Geochemistry, Monazite (U–Pb–Th) Geochronology, and P-T Pseudosection Modelling of Two-Pyroxene Mafic Granulite from Sonapahar, Shillong Meghalaya Gneissic Complex, India: Implication for Tectono-Metamorphic Evolution and Global Pan-African Correlation

Tectono-metamorphic evolution of paragneiss in the Chengde metamorphic Complex, northern Paleoproterozoic Trans-North China Orogen

Metasedimentary rocks can provide important information for reconstructing tectonic evolution of an orogen. In this study, we report the newly identified metapelite and metagraywacke samples from the Chengde Metamorphic Complex, the Trans-North China Orogen in the North China Craton. The garnet-bearing paragneiss generally occurs as country rocks of mafic granulite and amphibolite, and displays compositional layering with distinct gneissosity. The metapelite mainly consists of garnet, sillimanite, biotite, muscovite, plagioclase, and quartz, while the metagraywacke commonly comprises garnet, plagioclase, K-feldspar, quartz, and biotite/chlorite. Application of conventional geothermobarometers and phase equilibrium modelling constrains diverse peak metamorphic pressure–temperature (P-T) conditions (CD35, ∼800 ℃/10.5 kbar determined by phase equilibrium modelling; CD37, 677−716 ℃/9.6−9.8 kbar determined by geothermobarometers; CD45, 665−691℃/9.1−9.6 kbar and ∼ 745 ℃/11.2 kbar determined by geothermobarometers and phase equilibrium modelling respectively). The metapelite sample CD35 also records two retrograde stages, defining a clockwise P-T path with isothermal decompression (ITD) process. U-Pb dating of zircon and monazite yields metamorphic ages of 1910−1796 Ma, implying the timing of subduction to the lower crust. Combining previous studies in the Chengde Metamorphic Complex, we conclude that this complex is composed of imbricate rocks with different peak metamorphic P-T conditions, metamorphic grades, metamorphic ages, protolith origins, or protolith ages, indicating that these rocks were subducted to different depths in the subduction channel and then exhumed together to shallower crustal levels.

Archean crustal growth and reworking in the Superior Province, Canada: Insights from whole-rock geochemistry and Nd isotopic data of the La Grande, Nemiscau and Opatica subprovinces

The southeastern Superior Province contains Meso- to Neoarchean (2952–2677 Ma) tonalite-trondhjemite-granodiorite (TTG) suites, volcanic (2773–2703 Ma) and sedimentary (<2724 Ma) supracrustal sequences, and granitoid intrusions (2708–2598 Ma). The region is a key crustal cross-section for understanding the growth, evolution, and stabilization of Archean cratons. Whole-rock geochemistry and Nd isotopic analyses have been performed on TTG suites, volcanic assemblages and granitoid plutons of the La Grande, Nemiscau and Opatica subprovinces. These analyses, combined with existing structural and U-Pb geochronological data, have been used to investigate the extend of the basement within the three subprovinces, their interaction, and the degree of crustal reworking in the southeastern Superior Province.Nd isotope data and zircon inheritance indicate that the TTG suites were generated from: (i) an old crustal domain with Nd model ages at 3.5–3.2 Ga that may represent the southern extension of the Langelier Complex, (ii) a ca. 3.0 Ga domain with isotopically juvenile signatures, and (iii) the reworking of < 3.0 Ga Nd model age TTG suites from the Opatica subprovince. The Nd juvenile signatures of the mafic to ultramafic volcanism at ca. 2773–2756 and ca. 2723–2703 Ma suggest continuous or multiple mantle plume events beneath the southeastern Superior Province, whereas interlayered felsic volcanic rocks were formed from anatectic melting at high pressure. Subsequent magmatism was related to sanukitoid suites (2704–2693 Ma) emplacement that record mantle-TTG melt interactions. Based on a common tectonometamorphic evolution of the three subprovinces from ca. 2756 Ma, previous studies argued that the Mesoarchean TTG suites exposed in the La Grande and Opatica subprovinces represent a single crustal block. The sedimentary belts of the area were deposited in sagduction-type basins formed at ca. 2724–2706 Ma and, within less than 15–20 myr, were buried and metamorphosed up to granulite facies at 2697–2685 Ma during dip-slip-dominated D2. The burial and the following exhumation of these rocks from ca. 2677 Ma may be attributed to the delamination of lower crustal residues, favoring interaction between mantle and TTG melts, and the formation of sanukitoids and anatectic S-type granites. Nd signatures of the 2646–2598 Ma I-type granitic intrusions indicate extensively reworking of TTGs due to crustal thickening during the D3-D4 events.Our study suggests that the Archean tectonic regime recorded in the southeastern Superior Province is the result of mantle plume tectonics. We conclude that the La Grande-Opatica boundary does not represent a major suture zone but rather but rather a fossil boundary within an older extensively reworked crustal block over which the metasedimentary rocks of the Nemiscau and La Grande subprovinces were deposited.

Combining traditional and quantitative multiscale structural analysis to reconstruct the tectono-metamorphic evolution of migmatitic basements: the case of the Valpelline Series, Dent-Blanche Tectonic System, Western Alps

Due to the ongoing development of new technologies, many instruments are available to assist geological investigations at different scales. These techniques, including 3D outcrop modelling from aerial photogrammetry and quantitative microstructural analysis are useful in crystalline basements studies. This contribution combines traditional and quantitative multiscale structural analysis techniques to the migmatitic rocks of the Valpelline Series (Dent-Blanche Tectonic System, Western Alps). Conventional structural analysis is integrated with the extraction of structural data from 3D models of representative smooth outcrops. Quantitative microstructural and mineral-chemical analyses are combined to link structural and metamorphic evolution. This approach allows identifying and correlating foliations that developed during three tectono-metamorphic stages. The first (D1) includes solid-state deformation associated with an early foliation (S1) preserved within metabasite boudins enclosed in migmatite gneiss. The second (D2) is related to the dominant foliation in migmatite gneiss (S2), coeval with the regional scale anatexis and growth of garnet and cordierite. The third (D3) is related to the late folding of S2 and the development of a sillimanite-rich axial plane foliation (S3) which wraps around garnet and cordierite. Finally, this work discusses pros and cons of each innovative methodology, still emphasising the importance of using manually acquired field data as ground control.

Tectono-Metamorphic Evolution of the Cretaceous Kluane Schist, Southwest Yukon

Abstract A wealth of information regarding the Mesozoic evolution of the Northern Canadian and Alaskan Cordillera is held within a series of variably metamorphosed and deformed rocks that formed in Jura-Cretaceous basins. Located at the interface between the pericratonic Intermontane and exotic Insular terranes, these basinal rocks are key to understanding the timing and tectonic style of Insular terrane accretion, a topic of longstanding debate. This study unravels the structural and metamorphic evolution of one of these basins, the Kluane Basin, within southwest Yukon Territory. The Kluane Schist is the primary assemblage of the Kluane Basin. It consists of metamorphosed and deformed low-Al pelites that were intruded by granodioritic plutons of the Paleocene Ruby Range batholith. Previous workers have suggested the variable metamorphic character of the Kluane Schist represents an extensive and static thermal aureole related to Ruby Range batholith emplacement. Our work, however, indicates that the Kluane Schist experienced Buchan-style metamorphism coeval with protracted deformation and can be divided into seven distinct petrologic zones, which, based on their unique combination of mineral assemblage and structure, are incompatible with static thermal metamorphism. Instead, we propose the Kluane Schist experienced two distinct metamorphic phases: (1) an early greenschist-facies phase that resulted in the development of a bedding-parallel chlorite-muscovite-titanite fabric, preserved by its lowest-grade units, and (2) a later amphibolite-facies phase that manifests as the progressive transposition of the earlier chlorite-muscovite-titanite fabric into a penetrative biotite-rich schistosity that transitions upgrade into a segregated gneissic fabric comprised of biotite-cordierite and plagioclase-quartz (± sillimanite-K-feldspar-melt). By integrating the results of detailed petrography and petrological modeling, we demonstrate that the second main metamorphic phase experienced by the Kluane Schist preserves a record of pressures and temperatures that align with other Buchan-style terranes worldwide. Our data defines a field gradient across the Kluane Schist ranging from 3.0–3.5 kbar at 375–400 °C to 4–4.5 kbar at 700–750 °C. This record of a coupled Buchan-style metamorphic-deformational evolution and tops-to-the SW non-coaxial shear structures is consistent with the override of the thermally mature Yukon-Tanana terrane as the principal driver of Kluane Schist metamorphism, with some limited heat likely contributed by the late-syn- to post-tectonic intrusion of the Ruby Range batholith.

Late Cretaceous Tectono-Metamorphic Events in the Skyros Upper Metamorphic Unit (Olympus Mountain), Aegean Sea, Greece

Late Cretaceous metamorphic events are known in Crete and the Cyclades from klippen above the External Hellenides. This work extends their occurrence to the North Aegean area within the tectonic units of the Internal Hellenides. New 40Ar/39Ar white mica ages from garnet-bearing micaschists of the Upper Metamorphic Unit of Skyros Island, cropping out in the Skyrian Olympus Mountain, document a Late Cretaceous tectono-metamorphic evolution. Several mica generations have been distinguished using electron probe microanalyses and were dated using the 40Ar/39Ar method: a relict mica older than 96 Ma, followed by a foliation-forming mica of about 88–84 Ma and alteration phases ≤ 68 Ma were recognized. This Cretaceous tectono-metamorphic evolution falls between the closure of the internal Axios/Vardar oceanic basin in the Late Jurassic–Early Cretaceous, and the closure of the external Pindos–Cyclades oceanic basin in the Early Cenozoic. The position of the Upper Metamorphic Unit of Skyros was probably within the evolving Hellenic volcanic/magmatic arc during the continuous subduction of the African plate beneath the European plate. The present tectonic position of the units bearing the Late Cretaceous metamorphic event is the result of the Cenozoic tectonic emplacement onto the more external units across the Hellenides from the Pelagonian to the Pindos–Cyclades domain.

The effect of tectonic boudinage and folding in a subducted mélange of the Alpine orogenic belt (Zermatt–Saas Zone, Italian Western Alps)

The Zermatt–Saas Zone is an eclogite-facies meta-ophiolite unit representing the fossil oceanic lithosphere of the Jurassic Tethys. In the Italian Northwestern Alps, the Zermatt–Saas Zone includes a chaotic rock unit, or mélange, c. 40 m thick, interposed between serpentinites and calcschists. The mélange consists of decimetre-scale ultramafic layers and boudins embedded in a serpentine + carbonate-rich matrix showing a block-in-matrix fabric. The mélange has the same Alpine tectonometamorphic evolution as the surrounding rocks, starting with a prograde path developed under high-pressure (HP) conditions followed by a retrograde path during exhumation. The kinematic and metamorphic relations between inside- and outside-boudin foliations indicate that boudinage and shearing developed during the prograde HP path. Fluid–rock interaction enhanced shearing and focused ductile and brittle–ductile deformation along lithological contacts between rigid blocks and boudins and flowing carbonaceous matrix. Despite a pervasive orogenic evolution, the primary tectonosedimentary features of the mélange are still recognizable in some outcrops and are attributed to an intra-oceanic (Jurassic) setting characterized by mass-transport processes. The present-day fabric of the studied mélange unit thus results from the superimposition of the Alpine processes, responsible for fluid-assisted stratal disruption and mixing in the subduction channel, on the original stratigraphy formed during intra-oceanic gravitational processes. Supplementary material : Supplementary figures and tables are available at https://doi.org/10.6084/m9.figshare.c.6924080 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Structural and metamorphic evolution of a subducted passive margin: insights from the Briançonnais nappes of the Western Alps (Ubaye–Maira valleys, France–Italy)

This contribution analyses the structural architecture and tectono-metamorphic evolution of Briançonnais units in the southern French-Italian Western Alps. The studied area extends from a virtually non-metamorphic area adjacent to the Helvetic-Dauphinois External Domain in the west to the Monviso-Queyras ocean-derived units in the east, where metamorphism increases up to eclogite-facies. Mapping at the scale 1:10.000 of a mountainous part of the Ubaye-Maira transect was performed, which portrays in detail the Briançonnais units over ~ 100 km2. The lithologies include meta-andesite, meta-siliciclastics, marbles, turbiditic calcschists and flysch whose age spans from Late Carboniferous to Eocene. Metamorphism is low-grade greenschist-facies in the west but reaches the blueschist-facies to the east. Structures related to four phases of deformation are identified. The first two generations of structures (D1–D2), related to the original top-to-the-west nappe stacking, are associated with conditions close to the peak of Alpine metamorphism. Previous tectonic surfaces are transposed by the dominant deformation structures (D3), developed under retrograde/decompression conditions. This D3 corresponds to a backfolding and back-thrusting event with a top-to-the-E transport. A fourth phase (D4) developed during late low-grade metamorphic conditions and deforms the previous surfaces by locally developing a crenulation cleavage, followed by brittle tectonics. An updated metamorphic map is presented, backed on published estimates and new thermometric data obtained by Raman Spectroscopy of Carbonaceous Material (RSCM). The TRSCM values range from ~ 295 °C to > 350 °C, moving from the most external Briançonnais unit to the internal Queyras “Schistes Lustrés” units. Suspected Upper Cretaceous palaeofaults have been documented, allowing us to group the classic Aiguille de Mary and Ceillac (sub-) units into a single tectonic unit, here referred to as Maurin unit. The palaeogeographical reconstruction of the evolution of the studied transect highlights the importance of the Marinet and Maurin axial units in the feeding of the Late Cretaceous-Eocene breccias deposited on the more internal Acceglio-type units.

Magmatism during late Ordovician-early Silurian accretion of the Caledonides of Arctic Scandinavia: the Halti–Guolasjávri area revisited

In the Scandinavian Caledonides, evidence of syn-collisional magmatism related to extensional basin development immediately prior to late Silurian Baltica–Laurentia collision was considered restricted to exotic terranes until late Silurian ages were obtained from the Halti Igneous Complex (HIC), hosted by a thrust sheet (Corrovarre Nappe, CN) of continental affinity. Various orogenic models for the extension and magmatism, a. o. subduction flip, slab roll-back, and ridge subduction have been proposed. Crucial factors include the affinity (Baltican or exotic) of the CN, and the nature of the debated unconformity at the base of the overlying exotic Vaddas Nappe (Köli). This study reexamines a critical tectonostratigraphic section and reports U–Pb zircon ages (441–436 Ma) of palingenetic granitic dykes generated by the HIC. We reinterpret the CN as a slice of the continental margin and accreted with some other nappes of the Seve Nappe Complex (SNC) which decoupled from the continent-ocean transition (COT) at an early stage of subduction of the margin. The lower part of the bipartite Vaddas Nappe, composed of a very dense dolerite dyke swarm with screens of quartzite, marble and subordinate black schist, is reinterpreted as a continuation northwards of the uppermost nappe of the SNC in Indre Troms, derived from the Ediacaran outermost continental margin. The contact with the overlying telescoped succession of conglomerates, marbles, volcanics and turbidites of the Vaddas Nappe is tentatively interpreted as a tectonised unconformity. To a classical model of back-arc spreading outboard of Laurentia during closure of the Iapetus Ocean, we add the arrival of Baltica, with a prism of decoupled slices of the COT and exhumed/exhuming UHP nappes. The spreading centre of the basin provides a plausible setting for gabbros and pillow lavas of the Vaddas Nappe, and the shores of the basin for a Seve–Köli unconformity. Future PTt studies are required in order to be able to relate the proposed shallow, syn-exhumation HIC magmatism to the tectonometamorphic evolution of the HP–UHP nappes of the SNC.

Evidence for a pre-Himalayan metamorphism in the High Himalayan Crystalline of the Miyar Valley (NW India)

Most of the tectonic, metamorphic and geochronological data suggest that the Himalaya is essentially the consequence of a single orogenic cycle associated with the India-Asia collision during the Cenozoic era. Therefore, metamorphic assemblages and tectonic structures across the Himalayan range are systematically considered as post-collisional geological records. However, over the last decades, several observations arguing for geological events predating the continental collision have become increasingly recurrent in the literature. Nevertheless, although some of these arguments are thoroughly documented, they are unduly ignored in the construction of models drawing the tectono-metamorphic evolution of the Himalayan range. Yet, the occurrence of a pre-Himalayan history would have considerable consequences on the classical models for the building of the Himalaya. The recent discovery of inclusions of staurolite crystals in greenschist facies garnets from the Miyar Valley in Upper Lahul region (Himachal Pradesh; NW India) revives the debate on the existence of a pre-Himalayan metamorphism. Indeed, the occurrence of high-temperature staurolites included in greenschist facies garnets suggests that the High Himalayan Crystalline rocks experienced an amphibolite facies metamorphism prior the predominant Himalayan greenschist facies metamorphism observed in this part of the range. In this study, phase petrology, microtectonic investigations combined with preexisting geochronological data infer that the crystallization of the included staurolite predates the growth of Himalayan garnets. These original data bring new arguments to bear on the long lasting debate of the existence of a Pre-Himalayan orogenic cycle. They lead to the conclusion that the growth of staurolite predates the continental collision between India and Asia and reflects a metamorphic event that belongs to a pre-Himalaya orogenic cycle.

A petrochronological study of Fe[sbnd]Ti oxides in rodingites of the Western Carpathians, Slovakia

Rodingitization is the metasomatic transformation of mafic lithologies into rodingite, a Ca-rich and Si-poor rock associated with serpentinized ultramafic bodies. Serpentinization and rodingitization occur simultaneously but it remains challenging to obtain absolute ages on these events in order to identify the geodynamic setting of their formation. In this study, we investigate rodingites from an occurrence in the Western Carpathians with the aim to define the timing of rodingite formation during the tectono-metamorphic evolution of the Carpathian Orogen. The rodingites consist mainly of vesuvianite, diopside, garnet and chlorite. They are noticeably characterized by the presence of FeTi rich aggregates composed of ilmenite, rutile, titanite and calcite. Textural relationships and chemical compositions of the investigated FeTi minerals document their hydrothermal-metamorphic origin. These minerals are characterized by considerable rare earth element (REE) concentrations with prevailing positive Eu anomaly, which presumably derives from the plagioclase-rich protolith of the investigated rodingites. Moreover, the similarity of REE patterns between the analysed phases suggests that the REE concentrations in FeTi minerals are controlled by the immobility and availability of these elements in the system. A particular lamellae texture composed of rutile and calcite originated from titanite destabilization. The titanite destabilization was triggered by cooling, silica leaching and influx of CO2-rich fluids. In situ UPb dating of rutile, which have formed by retrograde breakdown of titanite during rodingitization, yields a crystallization age of 102.6 ± 19.9 Ma. This age is in good agreement with Cretaceous ages of the regional alpine metamorphic evolution reported in the literature and suggests that the rodingitization and serpentinization processes took place during the exhumation and cooling of the accretionary wedge resulting from the closure of the Meliata Ocean.

Metamorphic Remnants of the Variscan Orogeny across the Alps and Their Tectonic Significance

Lithospheric slices preserving pre-Alpine metamorphic imprints are widely described in the Alps. The Variscan parageneses recorded in continental, oceanic, and mantle rocks suggest a heterogeneous metamorphic evolution across the Alpine domains. In this contribution, we collect quantitative metamorphic imprints and ages of samples that document Variscan tectonometamorphic evolution from 420 to 290 Ma. Based on age distribution and metamorphic imprint, three main stages can be identified for the Variscan evolution of the Alpine region: Devonian (early Variscan), late Devonian–late Carboniferous (middle Variscan), and late Carboniferous–early Permian (late Variscan). The dominant metamorphic imprint during Devonian times was recorded under eclogite and HP granulite facies conditions in the Helvetic–Dauphinois–Provençal, Penninic, and eastern Austroalpine domains and under Ep-amphibolite facies conditions in the Southalpine domain. These metamorphic conditions correspond to a mean Franciscan-type metamorphic field gradient. During the late Devonian–late Carboniferous period, in the Helvetic–Dauphinois–Provençal and central Austroalpine domains, the dominant metamorphic imprint developed under eclogite and HP granulite facies conditions with a Franciscan field gradient. Amphibolite facies conditions dominated in the Penninic and Southalpine domains and corresponded to a Barrovian-type metamorphic field gradient. At the Carboniferous–Permian transition, the metamorphic imprints mainly developed under amphibolite-LP granulite facies conditions in all domains of the Alps, corresponding to a mean metamorphic field gradient at the transition between Barrovian and Abukuma (Buchan) types. This distribution of the metamorphic imprints suggests a pre-Alpine burial of oceanic and continental crust underneath a continental upper plate, in a scenario of single or multiple oceanic subductions preceding the continental collision. Both scenarios are discussed and revised considering the consistency of collected data and a comparison with numerical models. Finally, the distribution of Devonian to Triassic geothermal gradients agrees with a sequence of events that starts with subduction, continues with continental collision, and ends with the continental thinning announcing the Jurassic oceanization.

Electron probe petrochronology of monazite- and garnet-bearing metamorphic rocks in the Saxothuringian allochthonous domains (Erzgebirge, Granulite and Münchberg massifs)

Abstract In the Saxothuringian Zone, a unique assemblage of high- to ultra-high-pressure and ultra-high-temperature metamorphic units is associated with medium- to low-pressure and temperature rocks. The units were studied in a campaign with garnet and monazite petrochronology of gneisses, micaschists and phyllites, and monazite dating in granites. P–T path segments of garnet crystallization were reconstructed by geothermobarometry and interpreted in terms of the monazite stability field, EPMA Th–U–Pb monazite ages and garnet Y + HREE zonations. One can recognize (1) Cambrian plutonism (512–503 Ma) with contact metamorphism in the Münchberg Massif. Subordinate monazite populations may indicate a (2) widespread but weak Silurian (444–418 Ma) thermal event. A (3) Devonian (389–360 Ma) high-pressure metamorphism prevails in the Münchberg and Frankenberg massifs. In the ultra-high-pressure and high-pressure units of the Erzgebirge the predominant (4) Carboniferous (336–327 Ma) monazites crystallized at the decompression paths. In the Saxonian Granulite Massif, prograde–retrograde P–T paths of cordierite-garnet gneisses can be related to monazite ages from 339 to 317 Ma. A (5) local hydrothermal overprint at 313–302 Ma coincides partly with post-tectonic (345–307 Ma) granite intrusions. Such diverse monazite age pattern and P–T time paths characterize the tectono-metamorphic evolution of each crustal segment involved in the Variscan Orogeny.

Tectonic evolution of a Paleoproterozoic diffuse cryptic suture zone in northern Fennoscandia and the correlation of northern Fennoscandia and southern Greenland

The boundary between the Karelia and Norrbotten continental blocks in Fennoscandia is a diffuse cryptic suture zone recording c. 200 Ma of tectono-metamorphic evolution between 1.92 and 1.72 Ga. Syn-collisional deposition at 1.92–1.91 Ga was followed by post-collisional deposition at 1.88–1.87 and 1.85–1.84 Ga. The post-collision structural overprint in the suture starts with sinistral movements due to aulacogen inversion (D2) east of the suture. Late-D2 and D orocline extensional stages are followed by NE-vergent basin inversion (D3) of &lt;1.88 Ga rocks and transpressional dextral shear in the suture zone. Nearly orthogonal D4 and D5 deformation events overprint earlier fabrics at c. 1.83 and c. 1.78 Ga, respectively. The youngest (D6, c. 1.73 Ga) structures include sinistral brittle–ductile movements. Northern Fennoscandia and southern Greenland are linked by coeval continent–continent collisions (D1, 1.92–1.89 Ga). The D orocline stage ( c. 1.87 Ga), responsible for oroclinal buckling in central Fennoscandia, is conceivably related to west-directed subduction and subsequent regional shortening in western Greenland. The D3–D4 events in Fennoscandia are comparable to, and the D5–D6 events correlate well with, tectonic events in the suture zone in the eastern Nagssugtoqidian orogen in Greenland. Supplementary material: Analytical methods, data, interpretations of U–Pb data, and structural data on detailed study areas and key locations are available at https://doi.org/10.6084/m9.figshare.c.6675191

Polyphase W-Sn mineralization and rare metal magmatism in relation to the late-Variscan tectono-metamorphic evolution of the southeastern French Massif Central

Polyphase W-Sn mineralization and rare metal magmatism in relation to the late-Variscan tectono-metamorphic evolution of the southeastern French Massif Central

Cenozoic evolution of the Tripolitza carbonate platform in the Tethyan realm: new age constraints on deposition, diagenesis, metamorphism and nappe emplacement based on U-Pb and Rb-Sr dating (External Hellenides, Crete)

AbstractWe present kinematic, radiometric, geochemical and PT data, which help to constrain the tectonometamorphic evolution of the Tripolitza Unit (TPU). The age of both the metamorphic peak (P = 0.4 ±0.2 GPa, T = ca. 310 °C) and top-to-the WNW mylonitic thrusting, attributed to the emplacement of the hanging Pindos nappe, has been constrained at 19 ±2.5 Ma using Rb-Sr on synkinematic white mica of a basal mylonite of NW Crete. This early tectonic event is also documented by the oldest generation of veins, which cut through less metamorphic (T = 240 ±15 °C) late Bartonian/Priabonian Nummulite limestone exposed as olistolith in TPU flysch of central Crete. Calcite of these veins yielded a similar U-Pb age at 20 ±6 Ma. U-Pb dating of matrix calcite, on the other hand, reflect the time of sedimentation (38.4 ±5.7 Ma and 37.6 ±1.2 Ma), which is in line with the faunal content of the black limestone. Geochemical data and U-Pb calcite ages of fibres of the Nummulite test (32.3 ±3.1 Ma and 34.6 ±0.9 Ma) suggest unexpected pseudomorphic fibre replacement during late Priabonian/early Rupelian diagenesis. Additional calcite veins, which developed at ca. 10–11 and 7 – 9 Ma (U-Pb on calcite), are attributed to top-to-the S thrusting and subsequent extension, respectively. The resulting anticlockwise rotation of the shortening direction within the TPU from WNW-ESE at ca. 20 Ma to N-S at ca. 10 Ma has significant implications for the geodynamic evolution of the External Hellenides.

Tectono-metamorphic evolution of a post-variscan mid-crustal shear zone in relation to the Tethyan rifting (Ivrea-Verbano Zone, Southern Alps)

The Ivrea-Verbano Zone, in the Italian Southern Alps, samples a complete section of middle to lower continental crust, which records the transition between the Variscan and the Alpine Tethys rift-related tectonics. We present a structural-metamorphic characterization of one of the most important structures, the Forno-Rosarolo shear zone which drove the post-Variscan tectonic evolution of the Ivrea-Verbano Zone. The Forno-Rosarolo shear zone is a NE-SW-oriented, subvertical shear zone, mainly made of metapelites, amphibolites, calc-silicates and granulites involved in anastomosed proto- to ultra-mylonite layers enveloping weakly deformed lenses. Mylonitic deformation developed on an already folded and sheared metamorphic (Variscan) complex but predates brittle fracturing and faulting, locally associated with pseudotachylites.In present day orientation, the kinematic indicators point to a sinistral sense of shear. Removing the Alpine ∼90° tilt of the IVZ crustal section, this kinematic results in a former extensional shear zone. Investigations on the kinematic of the flow in the mylonites revealed a non-coaxial deformation with a major component of pure shear acting together with simple shear. Metamorphic conditions of the wall rocks vary from upper amphibolite (SE, footwall) to granulite facies (NW, hanging wall). Estimations from the mineral assemblages within the mylonites point to amphibolite facies conditions during deformation (∼650 °C and ∼5.5 kbar).In absence of precise geochronological constraints, we discuss the role of the Forno-Rosarolo shear zone within the Tethyan rifting system in comparison with recent reconstructions. Kinematic data and metamorphic conditions allow to constrain the development of the Forno-Rosarolo shear zone mylonitic deformation during the intermediate phase of the Tethyan rift known as “thinning mode”. This stage was characterized by general shear conditions (pure shear between 70% and 50%) suggesting a phase of transition from a symmetric to an asymmetric configuration of rift.

Tectono-metamorphic evolution and strain partitioning along the Mugesse mega shear zone, Ubendian Belt (Northern Malawi)

In this contribution we focus on a portion of the Mugesse mega shear zone (MGSZ) in the Ubendian Belt, for which details on kinematics and strain partitioning remains poorly documented. We present a field-based structural work, a petrological and thermobarometrical study of mylonites and LA-ICP-MS in-situ U-Pb dating on monazite and zircon. The detailed field mapping highlights that the whole study area shows the same sinistral strain pattern with deformation being partitioned within S-C and C-C’ structures. Syn-kinematic metamorphic mineral assemblages indicate that the MGSZ mylonites recorded a clockwise P-T evolution with peak temperature conditions at upper amphibolite metamorphic facies of 11–15 kbar and 760–790 °C. Retrogression and cooling reached P-T conditions of 1–7 kbar and 330–480 °C. LA-ICP-MS in-situ U-Pb monazite dating suggests that the main activity of the MGSZ was Pan-African, at ca. 547 ± 5 and 545 ± 4 Ma. Zircon dates at 1985 ± 44 Ma and 1127 ± 29–988 ± 10 Ma are interpreted as inherited ages of potentially detrital origin related to the Ubendian and Irumide orogeny, respectively. A 1146 ± 14 Ma date obtained on zircons from the Lwakwa mylonitic orthogneiss is interpreted as the crystallisation age of the granitic protolith that intruded the Paleoproterozoic basement. Strain partitioning and strain softening during the growth of the MGSZ was strongly controlled by feedback relationships between deformation and retrograde metamorphism enhanced by fluid ingress and recrystallisation.

Metamorphic evolution of the UHT granulites from Mandikota, Eastern Ghats Mobile Belt, India: Constraints from phase equilibria modelling, fluid inclusions and monazite U-Th-total Pb geochronology

The Eastern Ghats Mobile Belt (EGMB) had been subjected to extreme thermal metamorphic history at various sections of its terrane. One such terrane (Mandikota) located near the western part of the EGMB has been speculated based on asserted reaction textures, conventional geothermobarometry, geochronology, fluid inclusions, and pseudosection modelling. The present integrated study aims to understand the intricacies of the diversified and controversial tectono-metamorphic evolution of these polycyclic granulite terranes. The stable occurrence of sapphirine + quartz in contact is rare in its own and is a diagnostic feature of Ultra-High Temperature (UHT) metamorphism. Conventional geo-thermobarometeric tools applied, suggests an ordered series of reactions for the different stages of metamorphism experienced by the investigated granulites. Also, the P-T estimated by conventional methods lies in sound correlation with the peak P-T condition (in excess of 950 °C at nearly 9 kbar) attained in the constrained pseudosection model. The exhumation trajectory of the examined UHT terrane reflects P-T path responsible for the thrusting mechanism with isothermal decompression of 3 kbar followed by late-stage isobaric cooling. This study also presents the primary report for the systematic fluid inclusion study of the UHT granulites and its correlation with the evolved P-T trajectory. In addition, monazite dating using Electron Micro Probe Analysis (EPMA) is procured to assess the time frame in understanding the P-T-t evolution of the high-grade terrane. The geochronological study further revealed a new age (∼600 Ma) associated with the Visakhapatnam Domain which adheres to the Pan-African orogeny and marks the reworking phase of the widely reported the Grenvillian event in the EGMB province. As such, the results from the studied fluid inclusions displaying monophase inclusions of CO2-rich carbonic fluid, and its corroboration with the P-T pseudosection model interpreting the phase equilibrium conditions prevailing during the Neoproterozoic time (∼900 Ma).

Tectonometamorphic evolution of the Himalayan metamorphic core in the Makalu-Arun region, eastern Nepal

The metamorphic core of the Himalayan orogen, once viewed as a structurally contiguous unit, is now known to be divided into multiple packages by a series of structural discontinuities. While these structures have been identified based on distinct metamorphic and kinematic histories of the bounding packages, the number of structures identified across the exhumed former midcrustal core varies throughout the Himalaya. The scarcity of field evidence for these structures not only impedes their identification, but also hinders our understanding of their role in the development of the Himalaya. This study characterizes the metamorphic and geochronological history of garnet, sillimanite, and kyanite bearing gneisses in Makalu-Arun region, Nepal. Phase equilibria modelling coupled with monazite U/Th-Pb petrochronology delineates four rock packages with distinct metamorphic histories separated by at least three thrust-sense structures. The earliest thrust activity and subsequent in-sequence thrusting lasted from ca. 23-14 Ma, with initiation of late out-of-sequence thrust after ca.14 Ma. These results are consistent with foreland thrust migration and juxtaposition consistent with models that incorporate underplating, metamorphism and exhumation of midcrustal rocks during orogenesis. Moreover, the demonstrated complex evolution of the metamorphic core in Makalu region is inconsistent with models that incorporate singular, orogenic-wide structures in the Himalaya. Supplementary material: https://doi.org/10.6084/m9.figshare.c.6420244