D2 Phase Research Articles

Tectonic evolution of the early Paleozoic intraplate orogen in the South China Block: Insights from Ductile Shear Zones in North Wuyishan

The ductile shear zones within the North Wuyishan domain are key to understand the tectonic evolution of the South China Block (SCB). This study integrates a multifaceted approach, including field observations, thin section analysis, quartz electron backscatter diffraction (EBSD), zircon U-Pb dating, and mica 40Ar/39Ar dating, to elucidate the deformation history of these shear zones in North Wuyishan. The research identifies a two-stage deformation process of early Paleozoic. The initial D1 phase is marked by a top-to-SW thrusting shear, with associated felsic veins dated to the Ordovician period (447 ± 10 Ma to 459 ± 4 Ma), correlating with high-grade metamorphism and partial melting. The later D2 phase is characterized by NE-striking foliation and lineation, indicative of sinistral strike-slip shear, dated to the early Devonian (412 Ma) and early Carboniferous (∼354 Ma). The D1 phase suggests early crustal thickening and melting within the Cathaysia block, while D2 indicates a transpressional regime during post-orogenic adjustment. The geological features of the SCB, notably the absence of arc magmatism, ophiolitic mélange, and high-pressure metamorphism, comply with an intracontinental rift closure model as previously proposed. This model supports the hypothesis that the Early Paleozoic intracontinental orogeny in the SCB was likely a far-field consequence of a continental collision between the SCB-North Vietnam and South Vietnam blocks near the east Gondwana supercontinent.

Gut microbial network signatures of early colonizers in preterm neonates with extrauterine growth restriction

BackgroundExtrauterine growth restriction (EUGR) represents a prevalent condition observed in preterm neonates, which poses potential adverse implications for both neonatal development and long-term health outcomes. The manifestation of EUGR has been intricately associated with perturbations in microbial and metabolic profiles. This study aimed to investigate the characteristics of the gut microbial network in early colonizers among preterm neonates with EUGR.MethodsTwenty-nine preterm infants participated in this study, comprising 14 subjects in the EUGR group and 15 in the normal growth (AGA) group. Meconium (D1) and fecal samples were collected at postnatal day 28 (D28) and 1 month after discharge (M1). Subsequently, total bacterial DNA was extracted and sequenced using the Illumina MiSeq system, targeting the V3-V4 hyper-variable regions of the 16S rRNA gene.ResultsThe outcomes of principal coordinates analysis (PCoA) and examination of the microbial network structure revealed distinctive developmental trajectories in the gut microbiome during the initial three months of life among preterm neonates with and without EUGR. Significant differences in microbial community were observed at the D1 (P = 0.039) and M1 phases (P = 0.036) between the EUGR and AGA groups, while a comparable microbial community was noted at the D28 phase (P = 0.414). Moreover, relative to the AGA group, the EUGR group exhibited significantly lower relative abundances of bacteria associated with secretion of short-chain fatty acids, including Lactobacillus (P = 0.041) and Parabacteroides (P = 0.033) at the D1 phase, Bifidobacterium at the D28 phase, and genera Dysgonomonas (P = 0.042), Dialister (P = 0.02), Dorea (P = 0.042), and Fusobacterium (P = 0.017) at the M1 phase.ConclusionOverall, the present findings offer crucial important insights into the distinctive gut microbial signatures exhibited by earlier colonizers in preterm neonates with EUGR. Further mechanistic studies are needed to establish whether these differences are the cause or a consequence of EUGR.

The role of polyphase deformation in the formation of iron deposits and gold mineralization in the paleoproterozoic nyong complex greenstone belt (Southwestern Cameroon)

The Toko-Nlonkeng area is situated in the Nyong Complex of the northwestern Congo craton. In this study, airborne geophysical data (radiometric and magnetic) and structural data (outcrops and core samples) were used to investigate the relationship between structural evolution, iron formation (IFs)-hosted iron ore deposits, and gold mineralization in the Paleoproterozoic Nyong complex greenstone belt in southwestern Cameroon. Magnetic and radiometric trends, combined with field data, show general NE-SW, NW-SE, and E-W directions that correlate with the main gold-bearing structures. The study area has undergone four (04) deformation phases relating to polycyclic metamorphic and magmatic events. The D1 and D2 phases are the result of progressive ductile and transpressional deformation; D3 is a brittle-ductile deformation; and D4 is essentially brittle with various types of fracture networks late to post-orogenic. The D3 deformation phase is characterized by constrictive deformation with C3 shears and meso‑ to mega-P3-folds, and the main stress axes show φσ2 > φσ1> φσ3, reflecting a shearing tectonic regime with a maximum E-W shortening. This polyphase deformation is derived from the Eburnean/Trans-Amazonian deformation related to the collision between the Congo and São Francisco cratons. The main strain in the structural style of the Nyong Complex appears to be transpressive tectonics. This study reveals that iron ore and gold mineralization were mostly found on the hinges of folds and confirms that folding shear zones controlled the iron ore and gold target in the Nyong Complex and Northeast Brazil. The study proposes an approach to optimize future exploration activity based on the integration of the data.

Celtic ‘Guardians’ of the Central Polish Section of the Amber Route

Abstract The paper aims to provide cultural and chronological analysis of the late La Tène period artefacts discovered in the Prosna and Warta interfluve. The authors attempted to categorise them and assess their character, rank and significance. As a result, some categories of items, e. g. brooches, stripe-painted pottery, mirrors or knotenringe have been found to be imports originating from the south, i. e. from the areas of dense Celtic settlement, while coinage and blacksmithing products are believed to be of local provenance. Gold coinage based on the Boii system was well attested by the finds in the Janków II settlement, where individual components of a minting workshop and numerous coins being its final product were discovered. In turn, local Celtic blacksmithing is documented by the discovery of rich graves of blacksmiths and many elements of armament occurring in the discussed area, such as double-edged long swords and spearheads decorated with cut edges and acid-etched ornaments. Additionally, research demonstrates the existence of armed warriors whose remains were discovered in the cemeteries in Wesółki and Ciecierzyn. All the above must lead to the conclusion that in the LT D2 phase, equivalent to the A3 phase of the Prosna and Warta interfluve (as evidenced by the dispersion of materials), there existed a thriving centre of power of a local Celtic rix, or successive rixes, protected by groups of armed warriors. We believe that their power and prosperity expressed in gold coinage may have stemmed from their control over a significant section of the amber route which generated sufficiently high profits.

Insights into the Subduction of the Ligure-Piemontese Oceanic Basin: New Constraints from the Metamorphism in the Internal Ligurian Units (Northern Apennines, Italy)

In the Northern Apennines, the Internal Ligurian Units are considered deformed and metamorphosed fragments of the Ligure-Piemontese oceanic basin. In this paper, we report on the temperature and pressure conditions of the metamorphic peak for four Internal Ligurian Units, estimated using different geothermometers and geobarometers based on the white mica and chlorite compositions. These minerals were formed during the D1 deformation phase in the pre-Oligocene. The results indicate that the Portello and Gottero units are both characterized by metamorphic conditions pertaining to low blueschists facies, while the Colli-Tavarone and Bracco-Val Graveglia Units show a lower metamorphic imprint that produces assemblages of prehnite-pumpellyite facies. The estimated geothermal gradient for the metamorphic peak achieved by the analyzed Internal Ligurian Units during the D1 phase is 7–15 °C/Km, which is indicative of deformation in a subduction setting. Under these conditions, the D1 phase developed in these units as a result of underplating at the base of the accretionary wedge during the closure of the Ligure-Piemontese basin. These data indicate a close geodynamic correlation among the Internal Ligurian Units and the ophiolite-bearing units of the Alps.

Hallstatt period hoard from Somló Hill

In January 2023, the National Institute of Archaeology of the Hungarian National Museum launched a new research programme, the aim of which has been to explore the Somló Hill (Somló-hegy) (Veszprém County), a site neglected by systematic field research focusing on Late Bronze Age (LBA) and Early Iron Age (EIA) inhabitation. Somló Hill was often considered one of the most important EIA power centres in the western part of the Carpathian Basin. However, this notion was based mainly on funerary evidence partially unearthed from nearby burial mounds. Beyond doubt, these burials are associated with a prominent elite in the Ha C Period that maintained long-ranging contacts with communities in the (north)west and south. By contrast, virtually nothing is known about either the extent of the EIA settlement both in geographical and chronological terms, its inner structure, or the activity of the community associated with it. In this paper, we aim to show some of the first results of the field research conducted in the last months on the hill and to introduce the first documented EIA hoard of Somló Hill. The typo-chronological evaluation of the hoard suggests that it was deposited in the Late Hallstatt Period, i.e. the Ha D2–D3 phases.

Mapping and discrimination of the mineralization potential in granitoids from Banyo area (Adamawa, Cameroon), using Landsat 9 OLI, ASTER images and field observations

The Banyo area is situated in the west Cameroon domain of the Central African Fold Belt. In this study, field data and remote sensing conventional mapping techniques including Red-Green-Blue (RGB) colour combination, principal component analysis (PCA), Crosta-PCA, Band Ratios (BR), Constrained Energy Minimization (CEM), Minimum Noise Fraction (MNF), Spectral Angle Mapper (SAM) classification and Linear Spectral Unmixing (LSU) algorithms were used to process Landsat 9 OLI and ASTER data in order to discriminate various rock units such as biotite-amphibole granites, hornblende-biotite orthogneiss and map associated mineral deposits (e.g., iron-oxide, hydroxyl and corundum minerals). In addition, it was found that RGB combination (743) of OLI bands, PCA of OLI bands (PC123), BR of OLI bands (4/2, 6/7, 6/5), BR of ASTER (4/2, 7/6, 8/9), Crosta-PCA of OLI bands, MNF (123) of OLI bands, SAM, and LSU techniques applied to ASTER bands ascertain potential locations of granitoids and gneisses bearing iron and hydroxyl minerals. On the other hand, this study also revealed that corundum deposits are detected by applying the CEM method to OLI band ratio 6/7 highlighting red tones and processing SAM classification algorithm both using laboratory reference spectra. Automatic (using PCI Geomatica line tool) and manual extractions of lineaments revealed three major trends (N-S, E-W, and WNW-ESE to NW-SE) and an accessory trend (NE-SW to ENE-WSW) that surround hydrothermally altered rocks recognized as containing iron oxide, hydroxyl, and corundum mineral deposits. In the field, these lineaments may group to the tree deformation phases. The D1 phase (NW-SE); the D2 phase (NE-SW to ENE-WSW) and the D3 phase (N-S). Maximum Likelihood Calssification (MLC) and SAM classification methods showed to be more effective in discriminating rocks bearing Iron-oxide, hydroxyl and corundum minerals in the study area.This result permits to confirm that, the combination of remote sensing (Landsat 9 OLI and ASTER data in this case) and field data constitute a useful tool for mineral exploration in the shear zone. However, the accuracy assessment was relied on comparing the different maps produced from Landsat 9 OLI and ASTER data with the ground truth sample points and also, with the earliest geological map of Banyo area. Thus, quantitatively the results were accurate at 99%.

Alpine structural evolution of the Internal Piedmont Zone in the Upper Viù Valley (Lanzo Valleys Ophiolite, Western Alps)

New structural data have been collected in the high-pressure meta-ophiolites exposed in the Lanzo Valleys (i.e., the Lanzo Valleys Ophiolite, LVO) and tectonically overlying the continental crust of the Dora-Maira and Gran Paradiso massifs in the Western Alps. Detailed geological mapping and structural analysis carried out in the south-western sector of the LVO (i.e., the Upper Viù Valley) allow to distinguish four ductile deformation phases, corresponding to the subduction-related D1 phase, the early exhumation-related D2-phase and the late exhumation-related D3 and D4 phases. The geometry of structures and kinematics have been reconstructed, and their regional tectonic meaning is discussed in the frame of the Alpine tectonic evolution of the Internal Piedmont Zone (i.e., the remnants of the Alpine Tethyan oceanic lithosphere) in the Western Alps.

Late‐Orogenic Evolution of the Southern European Variscan Belt Constrained by Fabric Analysis and Dating of the Camarat Granitic Complex and Coeval Felsic Dykes (Maures–Tanneron Massif, SE France)

AbstractThe Camarat Granitic Complex (CGC), emplaced in the migmatitic Internal Zone of the Maures–Tanneron Massif (MTM), SE Variscides, consists of the Gigaro granodiorite and the composite Camarat granite. U‐Pb dating of the latter gives crystallization ages of 304.5 ± 3.3 Ma (zircon date) and 303.5 ± 4.0 Ma (monazite date). Two representatives of late felsic dykes cutting across the MTM Internal Zone have 39Ar/40Ar muscovite ages of 302.43 ± 2.62 Ma and 298.11 ± 2.38. Magmatic lineations revealed by anisotropy of magnetic susceptibility (AMS) measurements and image analysis, aplite dykes, dyke‐like bodies of cordierite microgranite and joints in the Camarat granite, as well as the late dykes all have orientations consistent with subhorizontal, NNE‐SSW‐trending lineations in the migmatitic country rocks representing the stretching direction of a late‐Variscan transpression phase (D3). The CGC and the late dykes are therefore witness to a thermal event that affected the MTM between ∼305 and ∼298 Ma (late Pennsylvanian–earliest Permian times), at the end of D3 which initiated at ∼325 Ma. Grabens related to post‐Variscan, Permian rifting (D4 phase), which cut across the MTM are WNW‐ESE‐trending, indicating a NNE‐SSW direction of extension, parallel to the previous (D3) lateral horizontal flow. The present results and a comparison with AMS data published for the Corsica–Sardinia Batholith reveal that evolution in the SE Variscides from Devonian–early Carboniferous contraction to Permian extension, through late Carboniferous transpression is characterized by the persistence of a ca. N‐S stretching direction, supporting a strong horizontal, orogen‐parallel crustal flow.

Structural Study of the Precambrian Basement of Kaele (Far North Cameroon): Contribution of Remote Sensing (Application of Landsat 8 OLI/TIRS Images) and Field Data

The Kaele is located in the northern domain of the Pan-African fold belt of Central Africa in Cameroon, north of Mayo- Kébbi domain. The structural study carried out in this area is based on Landsat 8 OLI/ TIRS image processing techniques and traditional geological prospection methods. The objective is to evaluate the contribution of landsat 8 OLI/TIRS image processing and field data in the structural mapping of the Kaélé region. The application of the 7×7 directional filters of the Sobel type in the N-S, E-W, NE-SW and NW-SE directions, to the ETM+3 channels gives better results. The rose diagram of the lineament map (obtained from image processing) shows a major E-W direction (N100E) and a secondary direction N-S (N10E). The studied area is affected by a polyphase deformation D1 to D4. The first phase of deformation (D1) is tangential and has set up the sub-horizontal dipping S1 schistosity which bears a composite L1 lineation (stretching, mineral). The D2 phase is constrictional and responsible for the establishment of the subvertical S2 axial plane foliation of the P2 folds, carrying the composite L2 lineation (stretching, mineral) and the B2 boudinage. The dextral-moving C2 shear cuts the S1 foliation, the P2 folds are asymmetrical isopach and anisopach. The third phase D3 is a constriction and set up the S3 schistosity which appears in syn -tectonic granite and highlight the C3 shear planes. The C3 shear is dextral and the P3 folds are isopach, anisopach, intrafolial and sheath fold. The last phase of deformation D4 is brittle and has set up faults and joints. There exist similarities between orientation of lineament from image processing and direction of D2, D3 and D4 deformational phases structures.

Polyphase deformation-controlled giant Renli Nb–Ta deposit, South China: Constraints from systematic structural analysis

The Early Cretaceous Renli Nb–Ta deposit was recently discovered near the southern margin of the Late Jurassic–Early Cretaceous Mufushan composite granitic batholith, central Jiangnan Orogenic Belt, and represents the largest known rare metal deposit in South China. Substantial breakthroughs have been made in prospecting the pegmatite-dike-type of Nb–Ta mineralization in the deposit, especially from the perspective of geochemical exploration. However, there is still a lack of systematic structural studies in this area which has seriously hindered the in-depth understanding of the emplacement of pegmatite dikes and the distribution patterns of Nb–Ta ore bodies. In order to shed new light on this key issue, this study presents new and detailed structural data from the Mufushan region. The results indicate that the Renli and adjacent areas have experienced several deformational events, including D1 Neoproterozoic NE–SW contraction and upright-folding, D2 latest Late Jurassic to Early Cretaceous regional extension accompanied by granitic magma intrusion, and D3 Early Cretaceous brittle normal faulting and fracturing with granitic magma emplacement and Nb–Ta mineralization. The D1 event, corresponding to the Jiangnan Orogeny, formed regional-scale NW–SE-striking upright folds with ubiquitous subvertical S1 cleavages and subhorizontal L1 crenulation lineations. The D2 event represented orogenic exhumation and produced a domal structure cored primarily by gneissic/mylonitic rocks and granites and mantled by schists, which dominated the bulk structural architecture of the Mufushan area. This was followed by the D3 event which controlled the formation of numerous extension fractures in the study area. The Nb–Ta-bearing magmatic-hydrothermal fluids migrated along the pre-existing D2 shear zone and subsequently filled in the widespread D3 extension fractures to form pegmatite dikes. The D2 to D3 phases can be considered as a progressive exhumation event from plastic to brittle regimes. The D2 and D3 structures dictate the spatial distribution of Nb–Ta ore bodies.

Fouimba and Goma Mounts Greenstone Belts Litho-Structural Analysis Related to Côte D’Ivoire Birimian Geodynamic Setting and Implying in West-Africa Craton Gold Deposits

Mount Fouimba and Mount Goma (Seguela) greenstone belts petro-structural studies combine remote sensing, geophysics, petrography and structural analysis. In view of establishing mapping details of paleoproterozoic geological formations, geological setting rocks observed are essentially magmatic formations, such as two-mica granite, granodiorites, and porphyritic basalts; and a few metamorphics which are metatonalite, amphibolites and amphibo-lopyroxenites. Remote sensing, such as Landsat 8 OLI satellite imagery and geophysical data, has been combined to show regional NNE-SW shear zone. Tectonic structures and microstructures have enabled to identify two main deformation phases: D1 phase corresponding to compression, and D2 is a transpression phase. Mechanisms responsible for deformations are respectively flattening and transpression. Geological formations derived from mantle origin but contain crustal components, and their tectonic setting occurred during subduction.

Litho-structural mapping and structural evolution of the Bocaranga pluton, northwest Adamawa-Yadé domain, Central African Republic

The Bocaranga pluton, in the northern domain of the Central African Orogenic Belt (CAOB) in the Central African Republic (CAR), is located in the Adamawa-Yadé domain, near the Mbéré Shear Zone (MBSZ). This study provides new results of the satellite image processing, the detailed structural and petrographic studies of Bocaranga pluton which helps to understand the emplacement mechanism of this pluton. Image processing of the satellite data and field showed the occurrence and spatial distribution of (1) amphibole-pyroxenites, (2) garnet-biotite rich amphibolite, (3) migmatite, (4) hornblende-biotite and hornblende-garnet gneiss, (5) hornblende-biotite granite (HBG), (6) two micas granite (TMG), (7) fine-grained biotite granite (FBG), and (8) coarse-grained biotite granite (CBG) rock types. The mapping of regional lineaments was carried out by automatic and manual lineament extraction methods. NE-SW direction corresponds to the direction of S2 and S3 foliations, faults, mineral lineation, quartz vein, dyke (aplitic granitic or pegmatitic). NW-SE direction also corresponds to the direction of S1 foliation, F1 folds, faults, quartz veins, dykes (aplitic granitic or pegmatitic). In the absence of absolute geochronological data in study area, it has been possible to propose a polyphase structural evolution, deduced from the relative chronology of the Bocaranga granitoid. Three phases of deformation can be defined in this study. D1 phase is represented by early NW-SE trending foliation (S1) that has been progressively rotated parallel to S2 overprinting foliation occurring in gneiss, amphibolites and hornblende-biotite granite (HBG) intrusion as well. D2 phase of ductile sinistral sense from NE-SW to ENE-WSW is responsible for an emplacement of Bocaranga granitic pluton. The scarcity of structure orientation in the TMG and FBG (except the contact zones), would imply that these granites would have been crystallized in the absence of any stress. D3 is a phase of brittle deformation with three main directions corresponding to the main direction of the shear zones in the Northern domain of the CAR Pan-African fold belt.The mylonitic deformation observed in study area illustrates the intensity of the deformation. The dominance of ellipsoidal structures observed in Bocaranga area suggests that the shear mechanism is the main controlling structure of Bocaranga granite emplacement, which can be correlated with the emplacement of Tamkoro-Bossangoa granitoid in CAR and the Adamawa-Yadé domain in Cameroon, Chad and NE Brazil.These results imply a syntectonic emplacement of Bocaranga pluton during sinistral simple shear regime related to deformation along the Central Cameroon Shear Zone (CCSZ).

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.

P-T path from garnet zoning in pelitic schist from NE Sardinia, Italy: Further constraints on the metamorphic and tectonic evolution of the north Sardinia Variscan belt

Mylonitic micaschists in the south-eastern sector of the Posada-Asinara Shear Zone in the Axial Zone of the Sardinia Variscan chain were investigated for the reconstruction of their metamorphic evolution and P-T history. Micaschists underwent polyphase ductile deformation consisting of an old D1 deformation (~345–340 Ma) associated to shearing and folding and to a penetrative S1 axial plane foliation. The S1 foliation is progressively transposed by the D2 phase, which is associated with upright up to NE verging folds and dextral shear zones. Micaschists are characterized by abundant centimetric garnet crystals with strong compositional zoning. The garnet porphyroblasts (~15 vol%) are associated with plagioclase, quartz, biotite, staurolite, white mica, chloritoid and retrograde chlorite. Garnet presents an internal foliation identified by the isoorientation of quartz inclusions sometimes arranged into a sigmoidal pattern suggesting rotation of the garnet during growth, discordant respect to the external S2 foliation. The S2 foliation is identified by the preferred orientation of micas and chlorite and by the alternance between quartzo-feldspathic and micaceous layers. The garnet core contains several small inclusions of quartz, rutile, apatite, and minor monazite and zircon. Additional inclusions, observed in the garnet domain around the core are ilmenite, chloritoid, staurolite and white mica. EMPA analyses reveal an even more complex chemical zoning consisting of garnet core, garnet mantle, and rim. The compositional isopleths that match the composition of the garnet core intersect with isopleths for the highest Si contents in K-white mica at about T 430–490 °C, P 1.3–1.4 GPa. Garnet rim isopleths and isopleths for the lowest Si contents in K-white mica indicate P-T conditions close to 560–630 °C/0.6–1.1 GPa. The resulting P-T path is clockwise, subdivided into two separate stages. The first stage is a prograde segment suggested by the garnet core to mantle compositional variation whereas the second reflects garnet rim growth during exhumation. • Garnet crystals up to 1 cm are characterized by core, mantle, rim, and outer rim • Clockwise P-T path shows a prograde segment up to pressure peak (1.7GPa) • High – pressure conditions pre-date Barrovian metamorphism in Sardinia

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.

Complex deformation history of the Keszthely Hills, Transdanubian Range, Hungary

Abstract We have investigated the deformation history of the Keszthely Hills (Transdanubian Range, W Hungary), which belongs to the uppermost slice of the Austroalpine nappe system. This Upper Triassic to Upper Miocene sedimentary rock sequence documented the deformation of the upper crust during repeated rifting and inversion events. We investigated the structural pattern and stress field evolution of this multistage deformation history by structural data collection and evaluation from surface outcrops. Regarding the Mesozoic deformations, we present additional arguments for pre-orogenic (Triassic and Jurassic) extension (D1 and D2 phases), which is mainly characterized by NE–SW extensional structures, such as syn-sedimentary faults, slump-folds, and pre-tilt conjugate normal fault pairs. NW–SE-striking map-scale normal faults were also connected to these phases. The inversion of these pre-orogenic structures took place during the middle part of the Cretaceous; however, minor contractional deformation possibly reoccurred until the Early Miocene (D3 to D5 phases). The related meso- and map-scale structures are gentle to open folds, thrusts and strike-slip faults. We measured various orientations, which were classified into three stress states or fields on the basis of structural criteria, such as tilt-test, and/or superimposed striae on the same fault planes. For this multi-directional shortening we presented three different scenarios. Our preferred suggestion would be the oblique inversion of pre-orogenic faults, which highly influenced the orientation of compressional structures, and resulted in an inhomogeneous stress field with local stress states in the vicinity of inherited older structures. The measured post-orogenic extensional structures are related to a new extensional event, the opening of the Pannonian Basin during the Miocene. We classified these structures into the following groups: immediate pre-rift phase with NE–SW extension (D6), syn-rift phase with E–W extension (D7a) and N–S transpression (D7b), and post-rift phase with NNW–SSE extension (D8).

Petrography and structural features of the Precambrian basement rocks in the Benin-Nigerian Shield, NW Nigeria: Implications for their correlation with South Atlantic Precambrian terranes

<abstract> <p>This study investigated the petrographic and structural features of the Precambrian (Neoproterozoic) basement rocks of the Benin-Nigerian Shield that crop out in northwestern Nigeria within Kanoma and its environs to give an insight into the evolution and deformational episodes that pervaded them. The major rock types in the area are schists and quartzites, which have been intruded by granitic rocks that appear to be metamorphosed. The origin of these rocks is attributed to the Eburnean Precambrian orogenic episode and the Pan-African orogeny, which started and ended with the intrusion of the granite suites. The dominant mineralogy associated with the rock types includes quartz, orthoclase, plagioclase, microcline, biotite, chlorite, and very few accessory minerals. The schist shows the dominance of quartz, feldspars (alkali and plagioclase), biotite, muscovite, chlorite, and opaque minerals. The quartzite is typically dominated by quartz that appears recrystallized in places, whereas the meta-granite contains quartz, feldspars (alkali and plagioclase), biotite, and opaque minerals. Structural features such as joints, quartz veins with minor folds, and faults observed in the lithological units have a predominant N-S trend and are the imprints of the last tectonic event (Pan-African orogeny). The level of deformation in Kanoma led to the development of N to NNE trending moderately (S<sub>1</sub>) to steeply (S<sub>2</sub>) dipping foliations in the schist. The evolution of these deformational mechanisms from moderately dipping foliations to steeply dipping foliations along the N to NNE- trend is associated with late orogenic uplift and exhumation following oblique convergence during the Pan-African orogeny. Structural overprinting relations recognized within Kanoma and its environs allow us to decipher the geologic structures into three successive Pan-African deformational events (D1–D3). D1 fabrics are manifested by simple anticline micro folds in the schist. The D2 structures are the predominant ones in the area comprising the N-S directional joints and faults. The D3 phase of deformation is a progressive one, which started as N-S high angle thrusts and thrust-related folds that resulted from the NE–SW contraction during the orogenic episodes. The studied rocks can be correlated with the Pan-African and Brasiliano belts based on their overlapping features.</p> </abstract>

Comparative geology and metamorphic evolution of the Luswishi Dome, Copperbelt, Zambia: Implications for exploration targeting

The Luswishi Dome forms part of the Neoproterozoic Lufilian Arc which hosts copper-cobalt ores of the Central African Copperbelt. We report new data from a study that combines observations from drill core logging, petrographic and chemical analyses, geothermobarometry and phase equilibria modelling. Drill core logging identifies lithostratigraphic units which are partially correlated with the Katanga Supergroup based on the occurrence of regional markers. The Nguba Group constrained by the presence of glacial tillite includes variably deformed, carbonaceous shales, and carbonates. An underlying sequence of quartzites, metapelites, calc-silicates, meta-sandstones, shales, dolomites and gabbros is partially correlated with the Roan Group. Overprinting relationships of structures in drill core provide evidence for possible polyphase deformation. Early deformation generated folds which were overprinted by faults and later by brecciation caused by hydrofracturing during later deformation, and subsequently metasomatism. Petrographic examinations reveal porphyroblast-inclusion-matrix relationships which indicate that the area experienced at least two penetrative ductile deformation events, D1 and D2, which are in turn overprinted by brittle structures at D3 and later phases. Thermobarometry calculations identify peak metamorphic conditions of 580–600 °C with maximum pressures of ~7 ± 1 kbar. Integrating thermobarometry with pseudosection modelling allows reconstruction of clockwise P-T-d paths consistent with consumption of epidote, chlorite, muscovite, quartz and low Ca-plagioclase to form biotite, garnet, high Ca-plagioclase and aluminosilicates. These observations suggest Barrovian intermediate P/T-type metamorphism which can be attributed to a tectonic scenario of burial under geothermal gradients of ~20–25 °C/km prior to uplift and erosion. These data complement previous models of regional metamorphism by crustal thickening during the Lufilian orogeny.

EVOLUÇÃO ESTRUTURAL DO DOMÍNIO INTERNO DA FAIXA BRASÍLIA MERIDIONAL NA REGIÃO DE ESTRELA DO SUL, MINAS GERAIS, BRASIL

Na região de Estrela do Sul – MG, o domínio interno da Faixa Brasília é formada pelas unidades Goiandira E e Goiandira W, Complexo Monte Carmelo e Grupo Araxá. Compreende dois compartimentos estruturais: SW, que evidencia quatro fases deformacionais (D1, D2, D3, D4) e é caracterizado por uma foliação principal (S2) com alto ângulo de mergulho para SW, associada a uma lineação mineral N-S, com baixo caimento. A fase D3, responsável pelo desenvolvimento da foliação S3, de baixo mergulho, predominantemente para SE, associada a lineação mineral E-W com baixo ângulo de caimento, é responsável pela formação da foliação S3 e estruturação geral da área. O compartimento NE apresenta estruturas semelhantes às da fase D3 no compartimento SW, com uma foliação principal de baixo ângulo de mergulho associada a lineação mineral E-W de baixo ângulo de caimento, entretanto, as estruturas de alto ângulo não foram identificadas nesse compartimento. Essas características evidenciam uma evolução tectônica conjunta dos domínios estruturais a partir da fase D3. Paragêneses minerais das rochas do Grupo Araxá e do Ortognaisse Goiandira E evidenciam metamorfismo principal no compartimento SW associado à fase D2, de fácies anfibolito, zona da estaurolita (Gr. Araxá) e sillimanita.