Axial Plane Foliation Research Articles

Tectono-thermal Evolution of the Proterozoic Mahakoshal Belt along the Northern Margin of Central Indian Tectonic Zone: Constraints from Geochemistry, Phase Equilibria Modelling and in-situ Monazite Dating from a Metapelite Sequence around Jabalpur

ABSTRACT In this contribution, geochemical signatures and petrological evolution of a folded sequence of mica schist from the Jabalpur area of the Mahakoshal Belt in the northern Central Indian Tectonic Zone (CITZ) are described. Multiple tectonic discrimination diagrams incorporating both major and trace element concentrations have constrained back-arc settings for deposition of the sedimentary units. S1 foliation formation marks the beginning of the M1 metamorphic stage. In contrast, the later M2 metamorphic event resulted in garnet and andalusite formation during heating at 2-3 kbar, followed by staurolite-sillimanite, garnet-staurolite assemblages at peak P-T conditions of 5.4 kbar, 550-600°C. Textural evidence suggests that M2 is pre-syn tectonic to D2 deformation, resulting in formation of the NE-SW trending S2 axial planar foliation. From the isopleth thermobarometry, a clockwise P-T path with near-isothermal decompression has been determined for M2. Similar results have been obtained from conventional thermo-barometry performed on multiple samples. Monazite dating (EPMA) of one of the samples suggests 1.9 Ga, age for D1-M1 event. M2 is correlated with the younger age population of monazites yielding 1.5 Ga. A clockwise P-T path constrained for M2 suggests collisional tectonics along the northern margin of the CITZ, as recorded from its southern margin.

Devonian Andean‐type convergence in the southern Dunhuang block (NW China): Petro‐structural, metamorphic P–T and geochronological constraints

AbstractArchean to Palaeoproterozoic basement rocks exposed in the Dunhuang block in NW China were affected by Palaeozoic crustal reworking, as constrained by previous zircon U–Pb geochronological investigations. However, relationships between the Palaeozoic metamorphic ages, P–T evolution and deformational history of the region remain ambiguous. In order to address this issue, P–T–t–D paths of paragneisses from the basement of the Hongliuxia belt in the southern Dunhuang block were investigated. Inclusions in garnet and kyanite from the paragneisses are considered as vestiges of Palaeozoic M1 metamorphism corresponding to initiation of the prograde evolution. The earliest continuous metamorphic fabric is an originally steep N–S striking foliation S2. This fabric was reworked by vertical folds F3 associated with the development of a ubiquitous steep, mainly south‐dipping, E‐W striking axial planar foliation S3. The S2 foliation in paragneisses is mainly associated with Grt–St–Ky–Sil–Bt–Ms–Pl–Qz–Rt assemblages in samples from the western domain and with Grt–Ky–Sil–Bt–Kfs–Pl–Qz–Rt assemblages in samples from the northeastern domain of the Hongliuxia belt. The S3 foliation is associated with Grt–Sil–St–Bt–Ms–Pl–Qz–Ilm assemblages in the western domain and with Grt–Sil–Bt–Ms–Pl–Qz–Kfs–Ilm assemblages in the northeastern domain, followed by growth of chlorite in both domains. Early prograde stage (M1) from 4.0–6.5 kbar and 540–560°C to metamorphic peak (M2a) at 9–10 kbar and ~650–675°C is mainly recorded by paragneisses from the western domain. Subsequent decompression is initially accompanied by heating (M2b) constrained to 6.5–7 kbar and 675–710°C in the western domain, and to 6–6.5 kbar and ~730°C in the northeastern domain, followed by cooling (M3) through 4–6.5 kbar and 550–650°C till late chloritization (late M3). In situ U–Pb dating of monazite combined with monazite trace‐element compositions suggests that prograde evolution (M1) most likely started at c. 406 Ma, peak‐P conditions (M2a) were reached at 400–394 Ma, decompression associated with heating (M2b) took place at 393–391 Ma, and cooling (M3) during exhumation probably lasted from 380 to 354 Ma. The prograde metamorphism probably reflects burial during underthrusting of neighbouring continental basement (the Alxa block or an equivalent) below the Dunhuang block. This event culminated in pure shear thickening (D2a) of the whole supra‐subduction margin followed by minor heating and exhumation (D2b). The D3‐M3 event is interpreted as reflecting exhumation during orthogonal shortening of the system, possibly in response to an independent orogenic cycle. Combined with the available regional data, this study reveals the existence of a complex tectono‐metamorphic evolution for the Dunhuang block characterized by two distinct orogenic phases with (i) the thickening of a previously thinned arc‐back‐arc crust recorded in the northern and central belts at 420–410 Ma in the pro‐wedge side of the active margin (Sanweishan phase), followed by (ii) the 410–390 Ma thickening in the retro‐wedge side (Hongliuxia phase). Such a tectonic evolution of the whole Dunhuang block resembles Andean‐type migration of crustal thickening from the convergent front to hinterlands. The D3‐M3 event, potentially responsible for the juxtaposition of rocks from different geological occurrences and depths, is seemingly independent from this Andean‐type orogenic cycle.

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.

Transpression or polyphase deformation along craton margins: Insights from the Archean–Proterozoic boundary near Sudbury, Canada

Continental margins vary in composition and configuration, resulting in complex structural geometries during deformation. One example is the Archean–Proterozoic boundary near Sudbury, Canada, where the Creighton-Victoria Deformation Zone (CVDZ) contains both reverse dip-slip and dextral strike-slip kinematic indicators. To resolve if these structures formed during a single transpression event or multiple overprinting events, the orientation, relative timing and style of the CVDZ are compared to those of adjacent regional structures. Regional fold axes and their axial planar foliation are subparallel to the steeply-plunging stretching lineation and steeply-dipping foliation, respectively, along the CVDZ. Phase equilibria modelling of syn-tectonic staurolite-chloritoid assemblages indicates that the regional folds and foliation formed at 480–565 °C, similar to temperatures estimated for the formation of dip-slip quartz crystallographic preferred orientations (CPO) along the CVDZ. This suggests that the CVDZ, regional folds and foliation formed during the same regional folding and reverse shearing event. Major lateral lithological changes and inflections in the orientation of the craton margin controlled the formation of structures, and evidence for regional transpression, such as strike-slip kinematic indicators, steeply-plunging stretching lineation, and obliquity of regional folds with respect to major shear zones, resulted from polyphase deformation and transcurrent reactivation of the Superior craton margin.

Deformation Pattern of Well-Preserved High-Pressure Rocks (SE Syros, Cyclades)

New, detailed geological/structural mapping and field-based structural analysis were carried out to investigate the deformation pattern of well-preserved high-pressure rocks of the Blueschist Unit exposed in SE Syros (Cyclades, Greece). Geological mapping revealed the occurrence of extensive alternations between different rock groups, as well as interfingering patterns in map-scale that are possibly the result of folding. The earlier ductile deformation phase recognized in the mapped area is associated with the development of a penetrative foliation, which was formed at eclogite/blueschist-facies conditions under peak metamorphism. The subsequent main deformation phase occurred under blueschist facies conditions synchronous with the early stages of exhumation of the high-pressure rocks. This phase is mainly associated with the formation of WNW-trending folds and a pervasive axial planar foliation linked with ESE-directed shearing. The main deformation ceased under blueschist-facies conditions, and exhumation of the rocks to greenschist-facies conditions took place under very weak and localized deformation. Greenschist retrogression observed in the southwestern part of the mapped area seems to be controlled by fluids, rather than by intense deformation and formation of major syn-greenschist shear zones.

Cryogenian-Ediacaran crustal growth and evolution of the active margin of the Dahomeyide belt, Ghana

AbstractThe study presents detailed petrographical, geophysical, structural and geochemical data of the internal nappes zone to establish the deformational history, origin and tectonic setting and constrain the crustal growth and evolution of the active margin of the Dahomeyide belt. Two main lithological units, (i) deformed meta-granitoids (migmatites and gneisses) and (ii) undeformed granitoids, dominate the internal nappes zone. The granitoids are generally I-type, metaluminous to weakly peraluminous, low-K tholeiite to high-K calc-alkaline and of tonalite, granodiorite and granite affinity. The overall trace element patterns of the studied granitoids characterized by the enriched LILE and depleted HFS, with negative peaks of Nb-Ta, Sr, P and Ti, are indications of arc-related magmatism. Structural analysis reveals four deformation phases (D1-D4). D1 represents Northwest-Southeast (NW-SE) Pan African shortening associated with a continent-continent collision, resulting in westward nappe stacking. Progressive NW-SE shortening resulted in D2 and D3 top-to-the-NW dextral and sinistral thrusting events during the Pan-African orogeny. D4 is an extensional event likely associated with the orogenic collapse phase. The gneisses and migmatites, with dominant axial planar foliations, point to their formation in a collisional setting or influence by the Pan-African collisional processes. Continental-arc signatures in these rocks imply continental subduction during their protolith formation. The intrusive granitoid and pegmatite are undeformed, meaning late- to post-orogenic emplacement. These findings suggest that the internal nappes zone archived the subduction-collision and post-collisional phase of the Pan-African orogeny and recorded large-scale migmatization and granitoid emplacement due to partial melting of thickened lower crust between Mid-Cryogenian and late Ediacaran.

Silurian‐Devonian Lithospheric Thinning and Thermally Softening Along the Northern Margin of the Tarim Craton: Geological Mapping, Petro‐Structural Analysis and Geochronological Constraints

AbstractWhile the western part of northern Tarim Craton has long been considered as a Paleozoic passive margin, a pronounced Silurian‐Devonian magmatism developed on eastern part of this margin may indicate different but active margin setting. In this contribution, detailed structural mapping, petro‐structural analysis, and geochronological investigations were conducted in the Korla area, eastern part of northern Tarim Craton. Three main generations of fabrics were recognized. The earliest pervasive fabric is an originally sub‐horizontal metamorphic S1 foliation that is in part associated with migmatization characterized by high temperature/low pressure metamorphic mineral assemblages, interpreted as reflecting crustal extension. S1 foliation was affected by D2 contraction forming regional‐scale F2 upright folds associated with sub‐vertical axial planar foliation S2. D3 is marked by development of NW‐SE oriented dextral fault, asymmetric mega‐folding of S2 and spaced NW‐SE‐striking S3 foliation, likely in response to dextral transpression. Geochronological data indicate that D1 extension occurred from ca. 420 to 410 Ma, D2 contraction started around 410 Ma and lasted till 400 Ma or later, and D3 transpression was ongoing around ∼370 Ma. Integrated with regional data, an updated geodynamic model is proposed by interpreting the Central Tianshan, South Tianshan and NE Tarim Craton as an early Paleozoic supra‐subduction system. We suggest that the Silurian‐Devonian event reflects thermal softening and horizontal stretching of the supra‐subduction crust, resulting in drifting of the Central Tianshan continental arc from the proto Tarim Craton in association with opening of the South Tianshan back‐arc basin in‐between.

Tonian Continental Arc Magmatism of the Porongos Complex, Dom Feliciano Belt, Southern Brazil

This study provides new information on the deformation, metamorphism, and tectonic setting of the metavolcanic rocks of the southern portion of the Porongos Complex, southern Brazil. The structural analysis enabled the identification of three deformational phases, formed under ductile to semi-ductile conditions. D1 progressed through deformation partitioning, comprising closed to isoclinal folds and an axial plane foliation. Then, simple shear prevailed, resulting in isoclinal folds, sigma-type porphyroclasts, foliation transposition, and mylonitic rocks. D2 comprises open to gentle folds and an axial plane cleavage. The formation of shear bands is ascribed to the semi-ductile D3. The mineral assemblage represented by phengite + chlorite + clinozoisite-epidote + actinolite + albite + quartz associated with the temperatures obtained through the chlorite geothermometer (316°C) indicates greenschist facies conditions. The protoliths of these metavolcanic rocks are geochemically discriminated as calc-alkaline, dacite-to-rhyodacite, with peraluminous compositions. The bulk trace element compositions show enrichment in large-ion lithophile elements and light rare-earth elements and depletion in heavy rare-earth elements. Also, negative Nb, P, Ti, and Ta anomalies are observed in the multielement diagram. All these geochemical features are typical of rocks formed in continental magmatic arcs. Geochemical comparison with other pre-collisional Tonian orthometamorphic rocks from the Dom Feliciano Belt demonstrates strong similarities, which corroborates the prior interpretation of a continental arc setting for the origin of these Tonian rocks.

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.

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.

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

P–T–t–D records of Early Palaeozoic Andean‐type shortening of a hot active margin: The Dunhuang block in NW China

AbstractHigh‐pressure (HP) granulites form either in the domain of the subducted plate during continental collision or in supra‐subduction systems where the thermally softened upper plate is shortened and thickened. Such a discrepancy in tectonic setting can be evaluated by metamorphic pressure–temperature–time‐deformation (P–T–t–D) paths. In the current study, P–T–t–D paths of Early Palaeozoic HP granulite facies rocks, in the form of metabasic lenses enclosed in migmatitic metapelite, from the Dunhuang block, NW China, are investigated in order to constrain the nature of the HP rocks and shed light on the geodynamic evolution of a modern hot orogenic system in an active margin setting. The rocks show a polyphase evolution characterized by (1) relics of horizontal or gently dipping fabric (S1) preserved in cores of granulite lenses and in garnet porphyroblasts, (2) a N‐S trending sub‐vertical fabric (S2) preserved in low‐strain domains and (3) upright folds (F3) associated with a ubiquitous steep E‐W striking axial planar foliation (S3). Garnet in the granulites preserves relics of a prograde mineral assemblage M1a equilibrated at ~11.5 kbar and ~770–780°C, whereas the matrix granulite assemblage (M1b) from the S1 fabric attained peak pressure at ~13.5 kbar and ~850°C. The granulites were overprinted at ~8–11 kbar and ~850–900°C during crustal melting (M2) followed by partial re‐equilibration (M3) at ~8 kbar and ~625°C. A garnet Lu–Hf age of 421.6 ± 1.2 Ma dates metamorphism M1, while a garnet Sm–Nd age of 385.3 ± 4.0 Ma reflects M3 cooling of the granulites. The mineral assemblage, M1, of the host migmatitic metapelite formed at ~9–12.5 kbar and ~760–810°C, partial melting and migmatization (M2) occurred at ~7 kbar and ~760°C and re‐equilibration (M3) at ~5–6 kbar and ~675°C. A garnet Lu–Hf age of 409.7 ± 2.3 Ma dates thermal climax (M2) and a garnet Sm–Nd age of 356 ± 11 Ma constrains M3 for the migmatitic metapelites. The timing of this late phase is also bracketed by an emplacement age of syntectonic granite dated at c. 360 Ma. Decoupling of M1 and M2 P–T evolutions between the mafic granulites and migmatitic metapelites indicates their different positions in the crustal column, while the shared pressure–temperature (P–T) evolution M3 suggests formation of a mélange‐like association during the late stages of orogeny. The high‐pressure event D1‐M1 is interpreted as a result of Late Silurian–Early Devonian moderate crustal thickening of a thermally softened and thinned pre‐orogenic crust. The high‐temperature (HT) re‐equilibration D2‐M2 is interpreted as a result of Mid‐Devonian shortening of the previously thickened crust, possibly due to ‘Andean‐type’ underthrusting. The D3‐M3 event reflects Late Devonian supra‐subduction shortening and continuous erosion of the sub‐crustal lithosphere. This tectono‐metamorphic sequence of events is explained by polyphased Andean‐type deformation of a ‘Cascadia‐type’ active margin, which corresponds to a supra‐subduction tectonic switching paradigm.

Superimposed structures, incremental strain and deformation path from field data to modelling: A case study from the Alpi Apuane metamorphic core complex (NW Tuscany, Italy).

In this study, superimposed folds and antitaxial strain fringes are investigated to unravel the deformation path of the core zones of the Alpi Apuane metamorphic complex (northwest Tuscany Italy). Detailed field mapping combined with microstructural analyses is used together with computer-modelling of a complex strain fringe system to disclose the main steps of the Alpi Apuane syn-metamorphic deformation history. A comparison between natural investigated fibres and those produced by paraffine wax experiments provides details of the relative core-object-fringe rotation allowing new insight on the growth kinematics of strain fringes. The results of our analyses show that our composite fringe system is characterized by a first set of fibres to be related with the main composite axial planar foliation of regional scale isoclinal non-cylindric folds. This first set of fibre fringes records a local non-coaxial strain with an absolute core-object rotation (relative to an external reference frame represented by the main foliation) of 140° and a calculated shear strain (γ) of about 4. The second set of fringes, developed after the tilting of the main foliation and the first fringe-set in a sub-vertical orientation, records a lower absolute rotation of the core-object (25°), a lower shear strain (γ) of about 1.4 and a coaxial strain associated with the vertical shortening. This part of the composite fringe system and the related structures may relate to the switch from the crustal scale contractional regime to the extensional one, which characterized the development of the inner northern Apennines orogen.

Implications of the dominant LP–HT deformation in the Guanhães Block for the Araçuaí West-Congo Orogen evolution

The Araçuaí-West Congo Orogen is the result of convergence between the São Francisco and Congo cratons during the assembly of West Gondwana. Within this orogen, the Araçuaí Belt in SE Brazil reflects the western external domain. The Araçuaí Belt is characterized by major east-dipping structures and high- to medium pressure metamorphic rocks developed during crustal thickening at ca. 585–560 Ma and are juxtaposed along crustal-scale structures to the crystalline core of the orogen.The Guanhães Block in the southern Araçuaí Belt is characterized by higher metamorphic grade and dominantly shallowly west-dipping structures compared to the neighboring tectonic domains. While several studies have focused on the evolution of the axial crystalline core of the Araçuaí orogen and on the early collisional stages in the Araçuaí Belt, little attention has been given to the structural and metamorphic history of the poorly studied Guanhães Block.The supracrustal rocks in the central Guanhães Block record two tectono-metamorphic stages: D2/M2 and D3 /M3, with potential D1/M1 relicts rarely preserved within D2/M2 low-strain domains. The D2/M2 overprinting stage is associated with the development of a dominant, shallow axial planar foliation (S2) of isoclinal F2 folds under low-pressure and high-temperature (LP–HT) conditions (ca. 6 kbar and 750 °C). During this D2/M2 stage continuous segregation of syn-kinematic partial melting and generation of granitoid rocks occurred. The 530 Ma, syn- D2/M2 granitoid has negative εHf(i) at ca. −32 which indicates the source of magma from melting of older, isotopically evolved, Archaean or Paleoproterozoic crust.U–Pb geochronology on monazite, titanite and hydrothermal zircon constrain the retrograde D3/M3 event at ca. 510 Ma. The Guanhães Block uniquely records dominant deformation under LP–HT metamorphic conditions during the late-orogenic stages and its evolution could have important tectonic implications for the concurrent models suggested for the late Neoproterozoic Araçuaí-West Congo Orogen.

Formation of the Hengshanli granitic gneiss dome in the Paleoproterozoic Jiao-Liao-Ji Belt, North China Craton

A group of granitic gneiss domes that may provide constraints on the tectonic evolution of early Precambrian orogenic belts are well preserved in the LiaoJi segment (or LJOB-Liaoji orogenic belt) of the Jiao-Liao-Ji Orogenic Belt (JLJOB), eastern North China Craton. To deeply understand the genesis of the domes and how the crustal mass flowed in collisional orogenic belt, we carried out structural analysis of the Hengshanli dome in the western part of the LJOB. Detailed analyses of deformation structures, microstructures and fabrics, and geochronology of syntectonic granitic dykes demonstrate that three episodes of structural deformation contributed to the formation of the dome structure. During D1, a pure shear-dominated deformation, penetrative foliations (S1), symmetrical lenses or boudinages, intrafolial folds (F1) and mineral lineations (L1) developed in the interlayered rocks of viscosity contrasts. The D2, the WNW-ESE orientated sub-horizontal simple shear-dominated deformation, is characterized by asymmetrical A-type folds (F2), stretching lineations (L2) , mylonitic foliations (S2), localized ductile shear zones, axial planar foliations, rotated porphyroblasts, L>S tectonites and S-C fabrics that indicate high-strain simple shear deformation. A D3 brittle deformation is evidenced by E-W directed open folds (F3) by flexural folding mechanism. LA-ICP-MS U-Pb dating of zircons from the syntectonic granitic dykes reveals that the main deformation event occurred from ca. 1946 to 1844 Ma. The dating also reveals that the Liaoji granites at the core were emplaced at ca. 2147 Ma and experienced anatexis at ca. 1864 Ma and 1846 Ma. It is suggested that the mechanism of doming is a combination of middle and lower crustal sub-horizontal flow, diapirism and folding superposition. Convergent flow of remobilized magma at depth and sub-horizontal simple shearing of metasedimentary rocks triggered middle and lower crustal flow in association with diapirism during the doming. The granitic gneiss domes are the products of dominant lateral and subsidiary vertical crustal mass flow under E-W directed compressional regime in the LJOB. We suggest that the orogenic belt experienced peak deformation at ca. 1946-1844 Ma, accompanied with syntectonic metamorphism and magmatism at nearly the same time.

Kinematics and Ar–Ar illite age of deformation in the late Paleozoic Chicomuselo Fold-Thrust Belt (CFTB), Chiapas, Mexico and tectonic implications

The Chicomuselo fold-thrust belt (CFTB) is perhaps the less known cover orogen in Mexico. It is exposed in southernmost Mexico, Guatemala and Belize, along a fringe of about 500 km long and 50–100 km wide, on the southern edge of the Maya tectonostratigraphic terrane. The CFTB is constituted by a succession of sedimentary late Paleozoic marine rocks that include the Santa Rosa Inferior, the Santa Rosa Superior, the Grupera and the Paso Hondo formations. Based on the fossil content of the Santa Rosa Inferior and Paso Hondo formations, the age of these units was constrained from the Mississippian to the late Wolfcampian (early Permian). These successions are folded and thrusted, affecting the two more competent upper formations, whereas the two finer-grained lower units are also highly foliated. Folds, thrusts, and axial-plane foliation (S1) are more consistent with a NE-SW shortening direction and the asymmetry indicates a vergence towards the NE. Such structures are unconformably covered by red-beds of the Middle Jurassic Todos Santos Formation. No angular unconformity was found among the Paleozoic formations, as reported in previous works, which indicates that shortening deformation may have occurred in an orogenic cycle that took place between the latest Permian and Triassic. Structures related to such deformation are cut by kink bands and lateral faults (both dextral and sinistral), spaced across the belt. In order to constrain the thermal conditions of deformation in the CFTB, about 53 samples of phyllite, slate and shale were collected, and their illite crystallinity index (ICI) determined along the belt axis. Calibrated ICI's indicate that deformation occurred between low epizone to the upper diagenetic zone (Kübler index between 0.2 and 1 Δ°2θ angle), with a slow increase in crystallite size from the lower to the upper units, along and across the belt. Deformation-related illite from the Santa Rosa Inferior and Paso Hondo formations was dated with the 40Ar-39Ar method. In the first sample, mica fish of tens of microns dominate the fabric, though a finer-grained illite parallel to the axial plane of late kink bands overgrows such fabric. In the second sample, only one generation of micrometric size illite is visible under the SEM. However, the characterization and dating of several clay size fractions (0.2–0.5, 0.5–1 and 1–2 μm, in addition to 2–4 μm in the first sample) indicate that both samples contain the same two end-member generations of illite, one formed during a first event of intense shortening at 252.3 ± 8.1 Ma, and another associated to spaced faulting formed at 175.1 ± 6.5 Ma. All these data indicate that all sedimentary units within the CFTB were shortened simultaneously during an orogeny that started in the Permian-Triassic threshold and were presumably affected by Middle Jurassic faulting during the opening of the Gulf of Mexico.

Neoproterozoic U‐Th‐REE‐bearing Pegmatites in Wadi Ibib, South Eastern Desert, Egypt: Structural and Geochemical Measures for a Syn‐Tectonic Anatectic Model of Formation

AbstractThe Wadi Ibib area is situated in the northern part of the Neoproterozoic Hamisana Shear Zone (HSZ), which is a high strain zone evolved during the late stages of the Pan‐African orogeny, likely as a tectonic escape structure. Amphibolite facies pelitic metasedimentary windows crop out in the axial parts of the HSZ and are noticeably associated with numerous N‐trending pegmatite dikes. Whole‐rock geochemistry of the pegmatites reveals a peraluminous (S‐type) affinity, with low K/Rb ratios and elevated concentrations of U, Th, REE, Rb, Li, Cs, Y, Nb and Ta. Structurally, the pegmatite sets intrude along the shear plane of the HSZ, corresponding to the regional N‐trending tectonic fabrics, such as axial planar foliation and dextral‐shearing in the metasedimentary host rock. Field relationships, including structural context, coupled with geochemical characteristics of the Wadi Ibib pegmatites, do not support their formation as a complementary part of evolved granitic magmas. Space‐localized decompression‐induced partial melting of peraluminous garnet‐bearing metapelites was alternatively the underlying process for formation of these pegmatites. Such decompression was associated with regional escape tectonics and stress axes permutations during the late deformation stage (D3) in the evolution of the south Eastern Desert terrane, due to end‐orogeny system pressure‐release.

Transpression in the Sanandaj-Sirjan Zone (Zagros Orogen, Iran) during the Jurassic and Early Cretaceous: Evidence from the North Shahrekord Shear Zone

Deformation style associated with the early stage of subduction of the Neo-Tethys under Iranian microcontinent is poorly understood. Utilizing field observations, microscopic thin-section interpretation, and recently published 40Ar-39Ar ages, we conducted a structural analysis of the North Shahrekord Shear Zone. The study area is located in the central Sanandaj-Sirjan Zone in western Iran, in an area that includes high-grade eclogite-bearing and low-grade metamorphic units of the North Shahrekord Metamorphic Complex. Two generations of deformation were observed in the study area, including D1 and D2. D1 structures are rare and involve folds and axial-plane foliations (S1), which have been almost obliterated by D2. D2 is characterized by a dextral transpressional zone, where strain is partitioned into distinct simple shear and pure shear domains. The pure shear domain is observed mainly in low-grade zone and some high-grade metamorphic rocks, and is characterized by NW-striking close to isoclinal folds (F2) and axial-plane foliations (S2). Folds are mostly inclined and their vergence is towards the southwest and northeast in the northern and southern parts of the study area, respectively. In the central part of the area, F2 includes approximately upright folds. These mesoscopic folds are associated with map-scale folds. The simple shear domain is restricted to the high-grade metamorphic rocks of the North Shahrekord Shear Zone, defined by NW-striking steeply dipping mylonitic foliation with low-angle stretching lineation. Kinematic indicators, such as quartz sigmoids, feldspar porphyroclasts, mica fish, and C- and C′-type shear bands, reveal a dextral strike-slip movement with a minor reverse component along the North Shahrekord Shear Zone. Recrystallized phengite from mylonitic eclogite and metagranite have been recently dated (40Ar-39Ar) at 178–170 Ma and ~110 Ma, respectively, indicating that the Sanandaj-Sirjan Zone was dominated by dextral transpression phases in the Early-Middle Jurassic and Early Cretaceous. The transpressional phases are interpreted to have occurred in response to the oblique subduction of the Neo-Tethys Ocean under the Iranian microcontinent.

Devonian–Carboniferous regional deformation in the northeastern Lachlan Orogen, southeastern Australia

The timing of regional deformation of the turbiditic Silurian–Devonian Hill End Trough in the northeastern Lachlan Orogen has been a contentious issue with one view ascribing the regional north–south folds and axial planar foliation to Middle Devonian basin inversion. Alternatively, it has been argued that given the low-angle discordance between Lower and Upper Devonian units on the Capertee and Molong highs, and the development of a dominant episode of folding mapped from the Hill End Trough into the adjoining highs, the major regional deformation is latest Devonian to early Carboniferous and predates intrusion of Bathurst-type granites at 358–314 Ma. We have approached the problem by analysing the gentle, upright, southeast-trending folds and deformation patterns in Devonian units of the northern Capertee High (Cudgegong area), drawing a cross-section across the Hill End Trough and reviewing the structure of two areas in the Molong High (west of Orange, and south of Wellington). Upper Devonian units on the highs are affected by fold patterns that extend into the underlying successions and have been interpreted as continuous into the central Hill End Trough, confirming the extent of the latest Devonian to early Carboniferous regional deformation in the northeastern Lachlan Orogen. Constraints from timing of deposition and radiometric ages are ambiguous and imply deformation was overlapping with sedimentation, as has also been inferred from sedimentary features of the Upper Devonian succession. Steeply dipping faults along the highs predated the Late Devonian and were probably formed in extension during formation of the Hill End Trough and were reactivated during basin inversion and potentially during Upper Devonian sedimentation. Our conclusion is that the simplest hypothesis is that the regional east–west shortening was of latest Devonian to early Carboniferous age, although poorly understood basin inversion occurred in the Middle to Late Devonian. KEY POINTS The timing of the development of regional north–south folds in the northeastern Lachlan Orogen has been a contentious issue and is difficult to resolve, given the broad stratigraphic constraints and ambiguous radiometric ages. The simplest hypothesis remains that the regional deformation was of latest Devonian to early Carboniferous timing. Middle to Late Devonian deformation included reactivation of basin-forming steeply dipping faults during basin inversion and ongoing sedimentation.

Geology and geochemistry of deformed granites from the Querari Complex, Rio Negro Province, NW-Amazonian Craton, Brazil

The Querari Complex corresponds to the basement rocks from Uaupes Domain and represents one of the least studied units in the northwest of the Rio Negro Province, Amazonian Craton. In this paper, we present new field geology, petrographic, and geochemical data of deformed granites from the Querari Complex. Based on our results, we suggest the differentiation of the Querari Complex into two magmatic facies. The Pana-Pana Facies comprise mainly porphyritic syeno to monzogranites with biotite as the main mafic mineral, and titanite, epidote, and allanite as accessory phases. The rocks of the Pana-Pana Facies have calc-alkaline and metaluminous affinities. The Matapi Facies have a syenogranite composition, coarse to porphyritic texture with magmatic muscovite and biotite, normative corundum higher than 1% (1.85-2.06), and mostly a peraluminous character. Both facies are generally enriched in silica and have a small spectrum of variation. (67.5 to 70.1 wt.% Pana-Pana Facies - 71.1 to 73 wt.% Matapi Facies), but they differ by the high Al2O3, Fe2O3, MgO, CaO, Sr, and Ba contents in the Matapi Facies. Geological and geochemical compositions suggest that the magmatic precursors of these rocks of Matapi Facies may be derived from partial melting of metasedimentary sources, whereas the Pana-Pana Facies are probably derived from a meta-ignea source of intermediate composition, such as tonalitic gneisses. The rocks of the Querari Complex would have been generated in the collisional tectonic settings during the Paleoproterozoic (between 1778 and 1740 Ma). During this 1.78-1.74 Ga tectono-magmatic event (D1), the magmatic fabric (S0) and the parallel S1 foliation have been developed, with NE-SE orientation. A subsequent tectonic event (D2) was responsible for the folding S1 foliation, creating the S2 axial planar foliation with an approximate orientation E-W. This D2 event is thought to represent a Calymmian (1.52 - 1.48 Ga) tectono-metamorphic event, associated with widespread I- and S-type granite emplacement in the Rio Negro Province. The deformation textures, as well as the mineral paragenesis for the Pana-Pana and Matapi facies, reveal temperature conditions at upper amphibolite facies, up to 700oC. Finally, the D3 tectono-metamorphic event comprises the reactivation of many NE-SW ductile shear zones with sinistral kinematic, creating a mylonitic foliation (S3) in the rocks of the Querari Complex. This D3 tectono-metamorphic event is probably linked to the intracratonic K´Mudku tectono-metamorphic event (between 1317 and 1198 Ma) of the Amazonian Greenvillian-Sunsas continental collision.