Tectono-metamorphic Evolution Research Articles

Early Paleozoic Tectonothermal Evolution of the Khan-Khukhay Metamorphic Complex (Northern Mongolia)

A reconstruction of metamorphic stages of Moren and Erzin metamorphic Complexes in the Khan-Khuhay block of Nothern Mongolia presented in the paper. Based on it a general geodynamic history of Sangilen terrain of Tuva-Mongolia Massif discussed. Reconstructed “clock-wise” P-T trend demonstrated two stage of metamorphism, the first of which is due to collision metamorphism with P-T parameters of 9 kbar, 740°C, the second one is due to regional magmatic episode with P-T parameters of 6-7 kbar, 860–880°C. Geochemical and petrological peculiarities of rocks in the Khan-Khuhay block demonstrates resemblance with metamorphic block of Western Sangilen in South-East Tuva. In the first time quartz monzodiorite intrusive bodies founded within Khan-Khuhay block. Their geochemical peculiarities are similar to Ordovician gabbro-monzodiorite intrusives of Western Sangilen. Quartz monzodiorite intrusives of Khan-Khuhay are likely a small apophysis of a deep-crust intermediate magma chamber and can present probable thermal source of metamorphic stage M2 in the Khan-Khuhay block. Numerical thermo-mechanical model of metamorphism in the Khan-Khuhay block explain heating in the thickening crust during collision due to higher radiogenic heat source of 1.52 μW/m3. Using computer modeling was show that the P-T parameters of second metamorphic stage can realize only by means of magmatic heat from monzodiorite intrusion. Reconstruction of P-T parameters using mineral thermobarometry, similarity of geochemical characteristics, as well as the results of thermomechanical modeling allow us to conclude that the Erzin and Moren complexes have undergone a joint tectono-metamorphic evolution.

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 <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.

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. Although 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 c. 23 to 14 Ma, with initiation of late out-of-sequence thrusting after c. 14 Ma. These results are consistent with foreland thrust migration and juxtaposition compatible 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 : Supplementary tables and figures are available at https://doi.org/10.6084/m9.figshare.c.6420244

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).

Petrological, geochemical and geochronological evolution of massif type charnockite from the Eastern Ghats Province, India: Implications on the regional tectonics of the Rayner-Eastern Ghats orogeny

Massif type charnockite was emplaced in the lower crust of the Eastern Ghats Province and is now exposed over a large geographic area. The rock shows clear evidence of magmatic emplacement in lower crustal metasedimentary rocks, which occur as enclaves within the charnockite. Textural data suggest the magma underwent subsolidus cooling and subsequently metamorphosed to granulite facies (up to ∼ 910 °C, 9 kbar). Geochemical data show that the charnockite magma had variable chemistry which was acquired by differentiation and possible crustal contamination. The rock shows both high- and low-SiO2 types, with weakly peraluminous to metaluminous characters. Trace and REE fractionation trends suggest the magma had calcic to calc-alkaline affinities and was emplaced in a continental arc type collisional setting. Theoretical modelling suggests that such a magma could be generated by melting of a hydrated basaltic slab under CO2-rich fluid. U-Pb analysis on oscillatory zoned zircon domains from eight samples yields crystallization ages for the magma. While the majority of the samples show crystallization ages within ca. 980–940 Ma (978 ± 16 Ma, 968 ± 22 Ma, 951 ± 9 Ma, 954 ± 8 Ma, 951 ± 13 Ma and 939 ± 27 Ma), two samples yield crystallization ages of 1002 ± 13 Ma and 1020 ± 16 Ma. This implies two-phase emplacement of the charnockite magma which can be correlated with the tectonometamorphic evolution of the province. While the earlier pulse of charnockite magmatism is broadly synchronous with the first cycle (M1) of metamorphism, the later pulse followed when the lower crust was still hot. The two pulses of charnockite magmatism are broadly synchronous with those of the Mawson charnockite of the Rayner Province, East Antarctica. It is argued that the charnockite magmatism in the combined Rayner-Eastern Ghats Province was extensive and resulted from arc-continent accretion and collision between the India and East Antarctica during ca. 1030–900 Ma.

Repeated Caledonian burial and ultrafast cooling and exhumation of high‐pressure granulite facies rocks from the Blåhø Nappe on the island of Fjørtoft, Western Gneiss Region, Norway

AbstractThe allochthonous Blåhø Nappe in the Nordøyane ultra high pressure (UHP) domain, Western Gneiss Region in Norway, acts as a window to examine geological processes occurring in continent–continent collisional zones, but many aspects regarding its tectonometamorphic evolution remain debated and elusive. In this contribution, an integrated study including major‐ and trace‐element zoning in garnet, phase equilibrium modelling and the simulation of cation diffusion in garnet was conducted on two high‐pressure (HP) granulite facies rocks from the Blåhø Nappe on the island of Fjørtoft. The results shed new light on the complex geodynamic processes that act in continent–continent collisional zones and finally shape collisional orogens. Phengite, biotite, amphibole, zoisite‐allanite and low‐Zr rutile enclosed in garnet likely attest to a prograde eclogite facies metamorphism for the studied rocks. Pressure–temperature (P–T) conditions of ~1.5–1.6 GPa and 615–670°C were retrieved for this stage. An extensive re‐equilibration under peak HP granulite facies conditions of ~1.5 GPa and 925 ± 50°C followed. Subsequently, the rocks were cooled and reburied to eclogite facies conditions of ~1.8–1.9 GPa and 805–825°C. This was followed by a final stage of decompression and cooling to amphibolite facies conditions of ~650–780°C and 0.5–1.0 GPa. Cooling and exhumation rates of >400°C/Ma and >75 km/Ma, respectively, indicating an ultrafast temperature and pressure decrease are estimated for this stage from simulations of cation diffusion in garnet. The anticlockwise P–T path obtained here is relatively complete and compatible with a repeated burial history during the Caledonian orogeny but not with UHP conditions proposed for the Blåhø Nappe. Our model proposes that the rocks later forming the Blåhø Nappe were buried to lower crustal depths of approximately 55 km equating to a geothermal gradient of ~13°C/km during the early Caledonian orogeny. Subsequent heating of these rocks to HP granulite facies conditions was likely driven by slab break‐off and hot mantle upwelling. Baltica underthrusting during the Scandian continent–continent collision cooled and transported the Blåhø Nappe to greater depths. The obtained cooling and exhumation rates indicate ultrafast exhumation, presumably in an exhumation channel.

Deformation history and processes during accretion of seamounts in subduction zones: The example of the Durkan Complex (Makran, SE Iran)

The Durkan Complex is a tectonic element of the Makran Accretionary Prism (SE Iran) that includes fragments of Late Cretaceous seamounts. In this paper, the results of map- to micro-scale structural studies of the western Durkan Complex are presented with the aim to describe its structural and tectono-metamorphic evolution. The Durkan Complex consists of several tectonic units bordered by mainly NNW-striking thrusts. Three main deformation phases (D1, D2, and D3) are distinguished and likely occurred from the Late Cretaceous to the Miocene–Pliocene. D1 is characterized by sub-isoclinal to close and W-verging folds associated with an axial plane foliation and shear zone along the fold limbs. This phase records the accretion of fragments of the seamount within the Makran at blueschist facies metamorphic conditions (160–300 °C and 0.6 – 1.2 GPa). D2 is characterized by open to close folds with sub-horizontal axial plane that likely developed during the exhumation of previously accreted seamount fragments. An upper Paleocene – Eocene siliciclastic succession unconformably sealed the D1 and D2 structures and is, in turn, deformed by W-verging thrust faults typical of D3. The latter likely testifies for a Miocene – Pliocene tectonic reworking of the accreted seamount fragments with the activation of out of sequence thrusts. Our results shed light on the mechanism of accretion of seamount materials in the accretionary prisms, suggesting that seamount slope successions favour the localization and propagation of the basal décollement. This study further confirms that the physiography of the subducting plates plays a significant role in the tectonic evolution of the subduction complexes.

Anticlockwise metamorphic paths at ca. 890–790 Ma from the NE Baidrag block, Mongolia, indicate back-arc compression at the Rodinia periphery

The processes leading to the assembly of the Rodinia supercontinent through Grenvillian collisional orogeny are relatively well known. In contrast, accretionary orogenic processes occurring at the supercontinent periphery following Rodinia assembly are poorly understood. To fill this gap, we have identified metamorphic rocks in the Mongolia collage of the Central Asian Orogenic Belt, where numerous data testify for Meso- to Neoproterozoic magmatic reworking. The tectono-metamorphic evolution of the peri-Siberian tract of the Central Asian Orogenic Belt is mainly characterized by the late Proterozoic–early Cambrian (Baikalian) cycle. However, we document here a Tonian age metamorphism at the northern part of the Precambrian Baidrag block, previously considered as a typical example of the Baikalian metamorphic belt. This study incorporates zircon and in-situ monazite geochronology linked to P-T modelling of Grt-Sil-Ky migmatite gneiss and Grt-St micaschist. Grt-Sil-Ky gneiss records initial burial to the sillimanite stability field at ∼720 °C and 6.0 kbar followed by further burial to the kyanite stability field at ∼750 °C and ∼9 kbar and decompression to ∼650 °C and ∼8 kbar. The Grt-St schist records initial burial to the staurolite stability field at ∼620 °C and 6 kbar, followed by further burial to ∼590 °C and 8.5 kbar. The monazite data yield a continuum of 207Pb-corrected 238U/206Pb dates of ca. 926–768 Ma in the Grt-Sil-Ky gneiss, and ca. 937–754 Ma in the Grt-St schist. Based on monazite textural positon, internal zoning, and REE patterns, the time of prograde burial to 6.0 kbar under a thermal gradient of 27–32 °C/km is estimated at ca. 890–853 Ma. It is not clear whether such high-grade conditions prevailed until a phase of further burial under a geothermal gradient of 18–22 °C/km dated at ca. 835–815 Ma. The late monazite recrystallization at ca. 790 Ma is related to decompression. Additionally, monazite with dates of ca. 568–515 Ma occur as whole grains or as rims with sharp boundaries on Tonian monazite in Grt-St schist suggesting a minor Baikalian overprint. Metamorphic zircon rims with Th/U ratios of ∼ 0.01–0.06 in Grt-Sil-Ky gneiss with 877 ± 7 Ma age, together with lower intercepts of detrital zircon discordia lines in both Grt-Sil-Ky gneiss and Grt-St schist further support the Tonian age of high-grade metamorphism. The anticlockwise P-T evolution is interpreted as a result of thickening of a supra-subduction extensional and hot edifice – probably of back-arc or arc type. This kind of prograde metamorphism has so far only been described on the northern part of the Tarim block and was interpreted to be a result of initiation of peri-Rodinian subduction of the Mirovoi Ocean. The geodynamic consequences of a unique discovery of Tonian metamorphism are discussed in terms of tectonic switch related to initiation of peri-Rodinian oceanic subduction during supercontinent assembly, followed by strong mechanical coupling potentially related to onset of Rodinia dispersal.

Deciphering the tectonometamorphic history of subducted metapelites using quartz‐in‐garnet and Ti‐in‐quartz (QuiG–TiQ) geothermobarometry—A key for understanding burial in the Scandinavian Caledonides

AbstractThe Seve Nappe Complex is a subduction‐related high‐grade metamorphic unit that was emplaced onto the margin of Baltica during Caledonian orogenesis. In this paper, the tectonometamorphic evolution of the Lower Seve Nappe in the Scandinavian Caledonides was characterized with the help of the continuous Collisional Orogeny in the Scandinavian Caledonides (COSC‐1) drill core, using a combination of various P–T estimation techniques based on garnet–quartz mineral pairs (quartz‐in‐garnet and Ti‐in‐quartz [QuiG–TiQ]), conventional thermobarometry and thermodynamic modelling of phase equilibria. This multi‐method approach yields complementary results and delivers critical data to constrain a comprehensive pressure–temperature–deformation–time (P–T–D–t) evolutionary path for the metasedimentary rocks of the Lower Seve Nappe. In the garnetiferous metasedimentary rocks, quartz inclusions in garnet preserve the P–T conditions of three distinct garnet growth stages corresponding to three metamorphic stages Ms1 to Ms3, including prograde and peak metamorphic conditions. Ms1 and Ms2 stages were constrained via quartz inclusions in garnet core and mantle. They are relatively close in the P–T space and could be considered as one single continuous prograde event occurring at epidote–amphibolite facies conditions of 460–520°C and 0.6–0.85 GPa. The growth of the garnet outermost rim defines the Ms3 stage at amphibolite facies conditions of 590–610°C and 1.13–1.18 GPa and corresponds to the peak metamorphic conditions. The microstructural analysis shows that the finite ductile strain pattern of the Lower Seve Nappe results from the superposition of four deformation phases. The initial phase D1 is defined by the S1 foliation that is still preserved as a curved inclusion trail in the garnet core. The D2 phase initiated contemporaneously with garnet core growth and the development of muscovite–biotite–plagioclase S2 foliation. Garnet outermost rim growth marks the end of the prograde path and peak metamorphic conditions. This stage is overprinted by the D3 phase and Ms4 stage associated with the development of the main regional metamorphic and mylonitic fabric S3 associated with C′‐type shear bands along the retrograde path. Ms4 stage, which was constrained using traditional thermobarometric techniques, corresponds to the chemical re‐equilibration of the metasedimentary minerals and occurred under amphibolite facies conditions at ~570–610°C and 0.78–1.00 GPa. The D3 phase is then generally weakly to strongly overprinted by later lower grade deformation D4 phase at greenschist facies conditions (Ms5). 40Ar/39Ar ages of syn‐kinematic white mica and biotite indicate that the final stage of the thrusting of the Lower Seve Nappe and thus the timing of its emplacement onto the Offerdal Nappe occurred at c. 423 Ma. Collectively, these results are consistent with previous estimates of the timing and conditions of metamorphism derived from the Lower Seve Nappe especially in west‐central Jämtland. However, application of QuiG–TiQ thermobarometry demonstrated that quartz inclusions in garnet can preserve different aspects of garnet growth, which are not accessible by traditional methods especially in complex terranes, and therefore provided new significant insights into the Lower Seve prograde evolution.

Defining the Timing, Extent, and Conditions of Paleozoic Metamorphism in the Southern Appalachian Blue Ridge Terranes of Tennessee, North Carolina, and Northern Georgia

AbstractThe tectonometamorphic evolution of the southern Appalachians, which results from multiple Paleozoic orogenies (Taconic, Neoacadian, and Alleghanian), has lacked a consensus interpretation regarding its thermal‐metamorphic history. The Blue Ridge terranes have remained the focus of the debate, with the interpreted timing of regional Barrovian metamorphism and associated deformation ranging from early (Taconic) to late Paleozoic (Alleghanian). New monazite U‐Pb geochronology and thermobarometric data are integrated with previously reported geo‐ and thermochronology to delimit the Paleozoic thermal‐metamorphic evolution of these terranes. Monazite compositional, textural, and U‐Pb age systematics are remarkably consistent for all samples, yielding a single dominant age mode for each sample. The western, central, and eastern Blue Ridge terranes yield weighted mean monazite U‐Pb ages of 450–441, 459–457, and 458–453 Ma, respectively. Thermodynamic modeling using mineral assemblages yields peak conditions of 600°C–650°C and 5.8–8.9 kbar for staurolite and kyanite grade western Blue Ridge units, including the stratigraphically youngest unit in the Murphy syncline, which also yields a weighted mean monazite U‐Pb age of 441 Ma. The Taconic metamorphic core of the central Blue Ridge yields peak conditions of 775°C and ∼11.5 kbar. Combined, these ages indicate that the relatively intact Barrovian metamorphic progression mapped across the Blue Ridge of Tennessee, North Carolina, and northern Georgia is solely of Ordovician (Taconic) age. Synthesis of this new data with existing geo‐ and thermochronology support a model of Barrovian metamorphism resulting from construction of a Taconic accretionary wedge and subduction complex, followed by post‐Taconic unroofing during Neoacadian and Alleghanian thrusting.

Structural evolution and U-Pb geochronology of the metasedimentary Nemiscau subprovince, Canada: implications for Archean tectonics in the Superior Province

The Nemiscau subprovince is a metasedimentary rocks-dominated sequence of the Archean eastern Superior Province. It is bounded by the gneissic and tonalite-trondhjemite-granodiorite (TTG) rocks-dominated La Grande and Opatica subprovinces. The Nemiscau consists of variably migmatized metasedimentary rocks and felsic to intermediate gneisses and plutonic suites. Mafic-to-ultramafic metavolcanic rocks occur along its northern and southern boundaries. Previous structural and metamorphic studies suggested that it was the result of subduction-related, accretionary and collisional tectonics with adjacent plutonic terranes during the Kenorean orogeny. This study integrates various sets of structural, metamorphic, and U-Pb geochronological data suggesting a long-lasting tectonometamorphic evolution between ca. 2843 and 2598 Ma. Four tectonometamorphic events have been recognized. The first event (D1) is only locally preserved and occurred shortly after the deposition of the oldest volcanic sequences of the Nemiscau ( ca. 2756–2736 Ma) under amphibolite facies metamorphic conditions (M1). The kinematic analysis of shear zones bounding the supracrustal sequences of both the Nemiscau and La Grande subprovinces suggests their relative crustal sinking as compared to TTG of the La Grande and Opatica subprovinces, shortly after deposition (at ≤2724 and ≤2706 Ma, respectively). Sedimentation was followed by regional dip-slip-dominated (D2) deformation between ca. 2704 and 2671 Ma, coeval with extensive high-grade granulite facies metamorphism (M2) and anatexis in the Nemiscau subprovince from ca. 2697 to 2685 Ma, followed by exhumation between ca. 2677 and 2671 Ma. This D2 event was followed by regional-scale dextral strike-slip shearing (D3) from ca. 2658 until 2621 Ma at amphibolite facies metamorphism (M3). The youngest deformation event, D4, was accompanied by strain localization along brittle-to-ductile conjugated shear zones and waning crustal cooling from amphibolite to greenschist facies conditions at ca. 2598 Ma and younger. It is suggested that the Nemiscau, La Grande, and Opatica subprovinces represent a single composite terrane, and that their mutual boundaries do not correspond to “collisional sutures” between different crustal blocks or microcontinents. The Nemiscau subprovince is interpreted as a sedimentary sequence unconformably overlying a ca. 2760–2756 Ma (and older) basement made up of volcanic and crosscutting TTG. In terms of tectonic evolution, the overall structural architecture and isotopic ages of the Nemiscau, La Grande, and Opatica subprovinces support a tectonic model in which the vertical transfer of crustal material occurred during the early stages (D2) of regional deformation, which evolved into a predominant lateral crustal flow of the ductile crust during later stages (D3–D4). A non-uniformitarian tectonic model for the Archean more adequately accounts for synchronous vertical and horizontal tectonism as preserved in the Nemiscau basin.

Thermobaric Conditions for Exhumation of Ti-clinohumite Garnetites of the Kokchetav Subduction-Collision Zone (Northern Kazakhstan)

Abstract —A new variety of Ti-clinohumite garnetites (‘garnet peridotites’) of the diamond-bearing Kumdy Kol terrane in the Kokchetav massif has been studied. An ultrahigh-pressure phase, Ti-chondrodite, orthopyroxene and contrast-zoned garnets of retrograde zoning have been found in these rocks for the first time. Ti-chondrodite and orthopyroxene were not described in these rocks earlier, and garnet composition was supposed to be homogeneous. Based on the character of chemical zoning of garnet and orthopyroxene, the peak and three retrograde parageneses for these minerals have been distinguished. Using the garnet–orthopyroxene geothermobarometry, for the Kumdy Kol terrane, based on the quantitative P–T estimates of Ti-clinohumite garnetites, a detailed retrograde P–T trend has been constructed for the first time; this trend includes the UHP peak stage (55–57 kbar, 1075–1120 °С), and three stages of retrograde metamorphism under UHP (32 kbar, 830 °С), HP (21 kbar, 750 °С), and MP (12 kbar, 740 °С) conditions. For the Kumdy Kol terrane this is the most detailed Р–Т trend indicating subisothermal decompression during crustal rock exhumation from the mantle depths (~170 km) to the lower-crust conditions (~36 km). The above-mentioned mineralogical findings allowed reconstructing all the described metamorphism stages, using the same rock and various generations of the same minerals (garnet and orthopyroxene) formed at different stages of tectono–metamorphic evolution of the Kumdy Kol terrane. Preservation of the relics for mineral zoning of the early metamorphic stages seemed to be due to high uplift velocities resulted in rapid decrease in P–T parameters and short duration of regressive metamorphism. The virtually complete identity of the obtained P–T trend for the Kumdy Kol terrane and the P–T trend for the Barchi terrane supports the idea of the common tectono–metamorphic evolution of the whole western (diamond-bearing) part of the Kokchetav massif.

Tectonometamorphic evolution of the Trivandrum and Southern Madurai blocks in the Southern Granulite Terrane, south India: correlation with south-central Madagascar

AbstractDetailed structural investigations across the Achankovil terrane boundary shear zone (AKSZ) system show distinct differences in the geometry of superposed fold structures between the Trivandrum (TB) and Southern Madurai (SMB) blocks separated by the AKSZ. The metamorphic temperatures and pressures estimated from the SMB and TB have a similar range, that is, 600–880°C and 5–8 kbar. The similar clockwiseP-Tpaths retrieved by phase equilibrium modelling from both the blocks represent the last deformation and metamorphism shared between them during their accretion along the AKSZ. The distinct evolutionary history of the SMB and TB prior to their amalgamation is supported by the contrasting structural, metamorphic and chronological patterns, particularly the lack of prominent middle Neoproterozoic ages and the presence of Palaeoproterozoic ages in the TB, and vice versa in the SMB. The prominent 600–500 Ma monazite ages in the TB, SMB and AKSZ attest to their timing of accretion along the AKSZ. The study corroborates the S-directed subduction model proposed for terrane accretion along the AKSZ, and provides further insight into the subduction–accretion–collision tectonics associated with the late Neoproterozoic – Cambrian evolutionary history of this region.The tectonothermal histories and geochronology of the SMB and TB are compared to the once adjoined crustal domains of Madagascar within Gondwanaland. It is suggested that the TB is equivalent to the Androyan and Anosyan domains of southern Madagascar, the SMB is equivalent to the Antananarivo and Itremo–Ikalamavony blocks of the central Madagascar, and the sinistral AKSZ is contiguous with the Ranotsara Shear Zone in southern Madagascar.

The Maira-Sampeyre and Val Grana Allochthons (south Western Alps): review and new data on the tectonometamorphic evolution of the Briançonnais distal margin

Here we describe the structure, the high-pressure, low-temperature (HP-LT) metamorphism and tectonic evolution of the Briançonnais distal margin units from the south Western Alps. The studied area extends southwest of the Dora-Maira (U)HP basement units and east-southeast of the classical Briançonnais nappes. A new structural map accompanied by geological profiles shows the thrusting of the oceanic nappes (Monviso and Queyras units) onto the distal Briançonnais units (D1 and D2 late Eocene deformation phases) under blueschist-facies conditions. Subsequent deformation during the Early Oligocene (D3 deformation phase) took place under greenschist-facies conditions and was associated with back-folding and -thrusting in the units overlying the Dora-Maira massif and with exhumation related to normal reactivation of former thrusts within the latter massif. Two large cover units, detached from their former distal Briançonnais basement, are redefined as the Maira-Sampeyre and Val Grana Allochthons (shortly: Maira-Grana Allochthons = MGA) including, (i) the Val Maira-Sampeyre unit involving Lower and Middle Triassic formations, seemingly detached from the Dora-Maira units during the subduction process, and (ii) the Val Grana unit with Middle-Upper Triassic and Early-Middle Jurassic formations, which was probably detached from the Maira-Sampeyre unit and correlates with the “Prepiemonte units” known from the Ligurian Alps to the Swiss Prealps. Three major shear zones involving tectonic mélanges of oceanic and continental rocks at the base of the Val Grana, Maira-Sampeyre and Dronero units testify to an early phase of exhumation within the subduction channel in front of the Adria plate. We present a new metamorphic map based on published and new petrological data, including new thermometric data obtained by Raman spectroscopy of carbonaceous material (RSCM). The TRSCM values range from ~ 400 °C to > 500 °C, going from the most external Val Grana unit and overlying Queyras schists to the uppermost Dora-Maira unit. During the Late Triassic, the width of the Briançonnais s.l. domain can be restored at ~ 100 km, whereas it reached ~ 150 km after the Jurassic rifting. A significant, second rifting event affected the Briançonnais domain during the Late Cretaceous-Paleocene, forming the Longet-Alpet chaotic breccias, which deserve further investigations.

Constraining the Timing of Evolution of Shear Zones in Two Collisional Orogens: Fusing Structural Geology and Geochronology

In recent decades, constraining the timing of shear activity has been one of the main topics of research about the tectono-metamorphic evolution of orogenic belts. We present a review of a combined structural and geochronological approach to two major ductile regional shear zones, in two collisional orogens: the first one affecting the Variscan basement in northern Sardinia (Italy) and the External Crystalline Massifs of the Alps (East Variscan Shear Zone; EVSZ), and the second one deforming the medium- to high-grade rocks of the metamorphic core of the Himalaya (High Himalayan Discontinuity). High-resolution, texturally and chemically controlled monazite geochronology applied in separated shear zones of the Variscan belt allowed recognizing a similar timing of activity ranging between c. 340–330 and 300 Ma. This approach led to a better understanding of the evolution of the EVSZ, supporting a model where several branches were active according to a growth by linkage model. Following a similar approach, in situ U-Th-Pb analysis of monazite constrained the timing of top-to-the-S/SW shearing of a regional-scale High Himalayan Discontinuity in the Himalayan belt to between c. 28 Ma and 17 Ma. Earlier exhumation of the hanging wall was triggered by shear zone activity, whereas at the same time, the footwall was still experiencing burial with increasing P-T conditions. The timing of shearing of this shear zone fits with an in-sequence shearing tectonic model for the exhumation of the Himalayan mid-crust.

Basement domes in a greenschist facies context: Tectono-metamorphic evolution of the southern Quadrilátero Ferrífero, southeast Brazil

New petrostructural data are presented from the southeastern Quadrilátero Ferrífero, a distinct polyorogenic domain of the São Francisco Craton characterized by granite-gneiss domes and keel structures. The Minas Supergroup of Siderian age that hosts a huge volume of ferriferous sediments was deposited on the flat and eroded surface of the Archean basement. The domes of Archean gneisses and granites represent semi-rigid extrusions around which the Minas Supergroup accumulated in deep synclines with common triple junctions. This study focuses first on the petro-structural context of low-pressure thermal aureoles observed towards the dome margins. Pressure-temperature estimates on these mineral assemblages indicate nearly static burial to ≥ 10 km depths of the Minas Supergroup cover, followed by decompression and heating during the intrusion of pegmatites and granitoids of late Rhyacian age in basement and cover. The Archean basement and the thermal aureoles were severely overprinted by synkinematic greenschist facies metamorphism portrayed by chloritoid, chlorite, pyrophyllite, kyanite, phengitic mica and secondary biotite. This deep greenschist facies metamorphism (T ca. 420°C and P = 0.4 to 0.45 GPa) is regionally observed in all units from the domes and the keels ranging in age from Archean to Ediacaran. It is concluded that this major tectono-metamorphic event relates to tectonic burial of the southern Quadrilatero Ferrifero below the west-verging nappes of the Araçuaí belt during the Early Paleozoic, as documented by 40Ar/39Ar ages of 490-485 Ma obtained on syn-kinematic metamorphic micas [Chauvet et al., 2001]. It is proposed that building up of the dome and keel tectonic style is first a consequence of the high-density contrast estimated around 0.5 g/cm3 between the crystalline basement and the Minas Supergroup, the final architecture being achieved during the Cambro-Ordovician stage of the E-W Late Neoproterozoic collision.