D2 Event Research Articles

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.

Eo‐Variscan metamorphism in the Bohemian Massif: Thermodynamic modelling and monazite geochronology of gneisses and granulites of the Góry Sowie Massif, SW Poland

AbstractIn the Variscides of central Europe, the tectonothermal history of a few units jointly assigned to the Teplá‐Barrandian/Bohemian microplate had already terminated in the Devonian. One of these is the Góry Sowie Massif (GSM; Sudetes, SW Poland), mainly consisting of variably migmatitic gneisses with subordinate granulite and peridotite inserts. The eo‐Variscan evolution of the GSM was completed in five tectonothermal stages; however, the timing and pressure–temperature (P–T) conditions of the individual stages remain unclear because isotopic ages overlap within error, and previous P–T estimates have been determined without reference to these ages. Our thermodynamic modelling and LA–ICP–MS U–Pb and electron probe microanalysis (EPMA) U–Th–total Pb monazite geochronology provide new insights into the early Variscan multistage evolution of the GSM. The new P–T–time (t) data show that granulites were metamorphosed concurrently with gneisses but at different depths and temperatures. During subduction of Saxothuringia beneath Teplá‐Barrandia at 395–390 Ma, the granulites of crustal provenance experienced high‐P–high‐T conditions (740–750°C, 17–18 kbar) and came in contact with the overlying mantle. Slab break‐off gave rise to decompression from conditions of ~17 kbar/840°C to 11–12 kbar/850–860°C, which continued at 380–370 Ma when the granulites were sheared off and extruded as tectonic slices to lower–middle crustal depths, where gneisses were undergoing metamorphism and migmatization under amphibolite facies conditions. In the gneisses, the oldest monazite date of c. 390 Ma constrains the age of the early tectonometamorphic event D1. Two subsequent events (D2 and D3, sillimanite grade) occurred under conditions of 660–680°C and 5–9 kbar at 380–375 Ma, when, in various parts of the future massif, the gneisses were (re)folded and migmatized due to heat arising from underplated mantle. Isostatic exhumation was accompanied by pegmatite intrusions (D4) at 370 Ma and subsequent orogenic collapse (D5). Concurrently, high temperature conditions (~700°C) promoted a disturbance of the U–Pb system in monazite in some granulites.

An intracontinental orogen exhumed by basement-slice imbrication in the Longmenshan Thrust Belt of the Eastern Tibetan Plateau

AbstractThe Longmenshan Thrust Belt in Eastern Tibet resulted from a Mesozoic orogeny and Cenozoic reworking. It is generally believed that the Cenozoic tectonics along the Longmenshan Thrust Belt are mostly inherited from the Mesozoic. Reconstructing the Mesozoic tectonics of the Longmenshan Thrust Belt is therefore important for understanding its evolutionary history. On the basis of detailed structural analysis, we recognized a Main Central Boundary that divides the Longmenshan Thrust Belt into a Southeastern Zone and a Northwestern Zone. Both zones underwent a main D1 event characterized by D1E top-to-the-SE thrusting in the Southeastern Zone and D1W top-to-the-NW/N thrusting in the Northwestern Zone. In the Southeastern Zone, a D2 top-to-the-NW/N normal faulting that cuts the D1E structures is developed along the NW boundary of the basement complexes. Newly obtained and previous geochronological data indicate that the D1E and D1W events occurred synchronously at ca. 224–219 Ma, and the D2 top-to-the-NW/N normal faulting was episodically activated at ca. 166–160 Ma, 141–120 Ma, 81–47 Ma, and 27–25 Ma. Episodic and synchronously activated top-to-the-NW normal faulting and top-to-the-SE thrusting along the northwestern and southeastern boundaries of the basement complexes, respectively, leads us to propose that the basement slices were episodically imbricated to the SE during the Late Jurassic–Early Cretaceous and Late Cretaceous–earliest Paleocene. The D1 amphibolite facies metamorphic rocks above the basement complexes recorded fast exhumation during the Late Jurassic–Early Cretaceous. We propose that the early Mesozoic northwestward basement underthrusting along a crustal “weak zone” was responsible for the D1 double-vergent thrusting and amphibolite facies metamorphism. Subsequent basement-slice imbrications reworked the Longmenshan Thrust Belt and exhumed the amphibolite facies rocks. Our results highlight the importance of basement underthrusting and imbrication in the formation and reworking of the intracontinental Longmenshan Thrust Belt in Eastern Tibet.

Episodic Crustal Extension and Contraction Characterizing the Late Mesozoic Tectonics of East China: Evidence From the Jiaodong Peninsula, East China

AbstractDuring the Late Mesozoic, East China is characterized by widespread magmatism, thrusting and folding, extensional doming, strike‐slip faulting, and block rotation. The Jiaodong Peninsula provides a key area located in East China to understand the episodic intracontinental extension and contraction and associated granitoid emplacement. Based on our structural analysis, magnetic fabric measurement, and gravity modeling, polyphase deformation and magma emplacement have been recognized within the Queshan‐Kunyushan‐Yuangezhuang‐Sanfoshan (QKYS) massif of the central Jiaodong Peninsula. A significant Late Jurassic D1 event, developed in the northern margin of the massif, was expressed by a high‐temperature, top‐to‐the‐NE shearing. Late Jurassic plutons display magnetic fabrics corresponding to the D1 structural fabrics and several NW–SE‐trending feeder zones at depth. These results link the syn‐kinematic emplacement of Late Jurassic plutons with regional NE–SW extensional tectonics. In the south of the massif, a lower‐temperature, top‐to‐the‐SW contractional deformation (D2) resulted from NE–SW contraction. The D3 shear zone with a top‐to‐the‐WNW kinematics is a rolling‐hinge type detachment fault that exhumed the massif, indicating NW–SE regional extension. Finally, Early Cretaceous plutons emplaced into the upper crust with a fast‐cooling rate and formed inverted drop shapes with concentric magnetic foliations and variably oriented magnetic lineations. In light of the previous geochronological results, the timing of these tectonic events is discussed. The tectonic evolution of the QKYS massif indicates a process from crustal thickening to lithospheric foundering in response to the Late Mesozoic plate convergences.

Blueschist mylonitic zones accommodating syn-subduction exhumation of deeply buried continental crust: the example of the Rocca Canavese Thrust Sheets Unit (Sesia\u2013Lanzo Zone, Italian Western Alps)

The Rocca Canavese Thrust Sheets Unit (RCTU) is a subduction-related mélange that represents the eastern-most complex of the Sesia–Lanzo Zone (SLZ), bounded by the Periadriatic (Canavese) Lineament that separates the Alpine subduction complex from the Southalpine domain. The RCTU is limited to the south by the Lanzo Massif (LM) and to the east by the Eclogitic Micaschists Complex (EMC). Particularly the tectonic contact area of the RCTU, adjacent to the neighbouring SLZ and the LM is characterised by a 100–200-m-thick mylonitic to ultra-mylonitic zone (MZ) that was active under blueschist-to greenschist-facies conditions. Despite the dominant mylonitic structure, some rocks (garnet-bearing gneiss, garnet-free gneiss and orthogneiss) still preserve pre-mylonitic parageneses in meter-sized domains. The scarcity of superposed structures and the small size of relicts impose a detailed microstructural analysis supported by chemical investigation to reconstruct the tectono-metamorphic history of the MZ. Therefore, we integrated the classical meso- and microstructural analysis approach with a novel quantitative technique based on the Quantitative X-Ray Map Analyzer (Q-XRMA), used to classify rock-forming minerals starting from an array of X-ray elemental maps, both at whole thin section and micro-domain scale, as well as to calibrate the maps for pixel-based chemical analysis and end-member component maps, relevant for a more robust conventional geothermobarometer application as well for calculating reliable PT pseudosections. Pre-Alpine relicts are garnet and white mica porphyroclasts in the garnet-bearing gneiss and biotite and K-feldspar porphyroclasts in garnet-free gneiss and orthogneiss, respectively, providing no PT constraints. The Alpine evolution of the MZ rocks, has been subdivided in three deformation and metamorphic stages. The first Alpine structural and metamorphic equilibration stage (D1 event) occurred at a pressure of ca. 1.25–1.4 GPa and at a temperature of ca. 420–510 °C, i.e. under blueschist-facies conditions. The D2 event, characterised by a mylonitic foliation that is pervasive in the MZ, occurred at ca. 0.95–1.1 GPa and ca. 380–500 °C, i.e. under epidote-blueschist-facies conditions. The D2 PT conditions in the MZ rocks are similar to those predicted for the blocks that constitute the RCTU mélange, and they overlap with the exhumation paths of the EMC and LM units. Therefore, the RCTU, EMC and LM rocks became coupled together during the D2 event. This coupling occurred during the exhumation of the different tectono-metamorphic units belonging to both continental and oceanic lithosphere and under a relatively cold thermal regime, typical for an active oceanic subduction zone, pre-dating Alpine continental collision.

Structural and geochemical evidence for a failed rift crustal evolution model in Western Dharwar Craton, South India

Detailed geological mapping along with structural and geochemical analysis were carried out in the Kibbanahalli Schist Belt (KSB) of the Bababudan Group in the Western Dharwar Craton (WDC). Repeated sequences of sedimentary rocks and volcanic rocks with conformable contact relations are the major lithological association in KSB. Sedimentary structures and field relations indicate that the Bababudan Group begins with basal conglomerate composed of debris (talus) and alluvial fan deposits or fluvial deposits. Volcanic rocks preserve features of explosive volcanism in an aqueous environment.Based on the field and microstructural observations five stages of deformations (D0, D1, D2, D3, and D4) are characterized from the study area. Among these deformation events, D2 is the major regional-scale event. East dipping NW–SE trending thrust faults and F2 parallel folds that are upright and isoclinal mark the D2 event. These F2 folds are sandwiched between D2 thrust faults. D2 structures suggest the temporal evolution of a fold-and-thrust belt by inversion of an intracratonic basin.Trace element geochemical analysis of the mafic to intermediate rocks from KSB and Chitradurga Schist Belt (CSB) shows three types of patterns. Type 1 has Nb–Ta negative anomalies, Type 2 is relatively flat trend and Type 3 has Zr–Hf positive anomalies. Type 1 has slightly enriched LREE trend while relatively flat LREE trend in Type 2 and 3. HREE trend is relatively flat in all samples, except for one sample from Type 3. The geochemical variations are related to the degree of crustal contamination experienced during their eruption in connection with the intracratonic extension.A pair of thrust faults and folded sequences of volcanic, volcanoclastic and sedimentary rocks summarize the major structural association in the study area. This association is identical to an inverted failed rift structure. Geochemical evidence also points to an intracratonic mafic to intermediate volcanism. Our study thus provides a well-preserved example of Archean failed rift. We thus propose the post-3.0 Ga crustal growth throughout the Western Dharwar Craton is therefore initiated by similar failed rifting events.

Mineral chemistry and deformation in a temperature gradient in the Sierras Pampeanas of Córdoba (Argentina): The Chicamtoltina Tonalite–Trondhjemite Orthogneiss

On the eastern margin of the Sierra Chica, in a gneissic host rocks metamorphosed under medium to high amphibolite facies (‘Sil-in’ isograd) during the Neoproterozoic–Cambrian M2-D2 event, a body of tonalitic-trondhjemitic composition was discordantly emplaced, cutting the S2 metamorphic foliation and the ‘Sil-in’ isograd. A strong deformation, with along the temperature gradient of the host rocks, transformed the igneous rocks into an orthogneiss during a M3-D3 event. Based on textural, microstructural and mineral chemistry changes along the mentioned metamorphic transition and age determination, the orthogneissification process is related to the deformational and thermal history in the Sierras Pampeanas of Córdoba. The M3-D3 event would have reworked the S2 foliation, generating an S3 foliation associated with folding and boudinage in the gneissic host rocks, forming a major intrafoliar fold in the tonalitic-trondhjemitic body, with its flanks on both sides of the ‘Sil-in’ isograd. The M3-D3 event is recorded in the textural and mineral chemistry variations, following a medium-high temperature gradient during deformation, from the southeast to the northwest of the body. In that direction, the ‘core and mantle’ texture in plagioclase increased the proportion of the recrystallized mantle, which was also enriched in anorthite content. Quartz textures evolved from polycrystalline ribbons, with internal deformation bands, to dismembered ones, with internal chessboard pattern. Recrystallization of the entire plagioclase-quartz aggregate ended in an interlobate to amoeboid texture, implying high-temperature conditions (~650-750 °C) along the mentioned temperature gradient during the M3-D3 event. Regarding micas, the primary biotite progressively increased in Mg and decreased in Fe towards the northwest, without appreciable textural changes. Concomitantly, the primary Ti-rich ferrimuscovite increases its content in Mg and becomes poor in Ti, at the same time that it developed symplectitic quartz-plagioclase intergrowths. During a final D4 deformational event, also under high ductility conditions, asymmetric intrafolial folds (drag folds, with geometrical S4 axial planes) were developed, associated with imbrication and folding of boudins within the S3 (// S2) foliation. The D2, D3, and D4 deformation events are part of a continuum (progressive deformation) in a combined simple shear and flattening regimen (transcompression = convergent oblique shear). The contrasting rheology between the stiffer Chicamtoltina igneous body and its gneissic host rocks explain the different behavior during high ductility deformation. Internally, this small igneous body, with plagioclase as stress-supporting mineral framework (plagioclase-quartz ratio 2:1), mirrored the rheological behavior of the middle to lower crust deformation. A concordia age of 535.81 ± 0.79 Ma (U–Pb LA-ICP-MS on zircon) obtained for the Chicamtoltina orthogneiss is interpreted as the crystallization age of the tonalitic-trondhjemitic protolith (igneous event Ig1). By correlation with the tonalitic orthogneisses from the Sierra Chica and Sierra Norte of Córdoba, this protholith is linked to the Pampean magmatic arc, with an age range of 541–515 Ma and a peak of magmatic activity at 530–535 Ma.

U–Pb geochronology of the meta-volcanic rocks from Sierra de Calcatapul: Implications for the Middle Triassic syn-rift volcanism and tectonic evolution of northern extra-Andean Patagonia

This contribution deals first new ICP-MS U–Pb zircon ages that indicate Middle- and Late Triassic magmatic episode and tectonometamorphic events, respectively, for the central Patagonia region, instead of a Paleozoic history as previously estimated. The Calcatapul Formation consists of a (meta-) volcano-sedimentary syn-rift sequence deposited unconformably, shortly after the extensional Gondwanide orogenic collapse, over a basement composed of Permian granites. Two 206Pb/238U ages yielded 245.1 ± 2.8 and 244.8 ± 4.1 Ma to magmatic crystallization of ignimbrites, along with another weighted mean of 226.7 ± 4.4 Ma interpreted as the age of the tectonometamorphic event D1-M1 affecting the Calcatapul sequence. U–Pb ages open new insights into the tectonomagmatic evolution of the area because the syn-rift wedge deposited in the Uribe and Yancamil half-grabens under the extensional tectonic regime was then inverted along the basin-bounding faults like the Yancamil fault. Inversion tectonics involved reverse fault-related uplift against a buttress of the rigid granitic block, and under dynamic greenschist-to-amphibolite facies metamorphic conditions, before the Late-Triassic granitic magmatism of the Lipetrén Suite took place. The pre-rift granitic basement represents the southern extension of the main magmatic phase of Choiyoi Province in Patagonia, whereas the Middle Triassic recorded the extensional, post-Choiyoi magmatism. The local, compressive D1 event punctuated an overall widespread lithospheric extension that leads to Gondwana breakup since the earliest Jurassic times.

The tungsten-gold veins of Bonnac (French Massif central): new constraints for a Variscan granite-related genesis

In the Brioude-Massiac district (French Massif Central: FMC), a network of W-As-Bi-Au quartz veins constitutes the Bonnac deposit, where tungsten is the major economic element, together with high-grade gold (up to 15 g/t Au). The evolution of this mineralization has been divided into 3 stages: (i) an early deep-seated wolframite-löllingite stage formed between 12 to 9 km, at up to 400 °C; (ii) a ductile/brittle deformation stage associated with scheelite and arsenopyrite deposition, with an estimated temperature of 480–300 °C; (iii) a late stage controlled by fluid-overpressure potentially triggered by fault-valve mechanism, at a depth of 7 to 5 km, and a temperature estimated between 266 to 240 °C, is marked by micro-fracturing infilled by native bismuth, bismuthinite, hedleyite, electrum, pyrite and base-metals. Structural analysis and apatite LA-ICP-MS U/Pb dating demonstrate a spatial and temporal link between the emplacement of the peraluminous leucogranitic dykes and the Bonnac mineralization. In more details, the mineralization was deposited between 321–316 Ma, during, or just after, the emplacement of the peraluminous dykes estimated around 329–315 Ma, suggesting a magmatic-hydrothermal transition for the ore-forming process. In the proposed model, the cooling of a hidden two-mica granitic pluton could have generated a magmatic fluid, and acted as the heat source responsible for fluid flow towards inherited permeability zones. The magmatic fluid could have then re-equilibrated at high temperature by fluid-rocks interaction. The sharp changes in pressure, associated with the decrease of the temperature, and sulfide-fugacity generated by a late input of meteoric fluid were responsible for the deposition of the late gold-stage. At the regional scale, the tungsten-gold event is ascribed to an early hydrothermal stage, dissociated from the formation of the antimony event in the district. The leucogranitic dykes and Bonnac quartz veins are controlled by a NW-SE stretching direction, interpreted as an expression of the Serpukhovian-Bashkirian syn-orogenic extension (D4 event of the FMC). These new data provide evidence for an early tungsten and gold metallogenic event in the FMC, prior the “Or300” event. The genetic classification of the Bonnac mineralization is equivocal. The W-As-Bi-Au-quartz veins exhibit the features of both an “orogenic gold” deposit at a relatively deep emplacement level (mesozonal), and an Intrusion-Related-Gold-Deposit (IRGD) type with a spatial-temporal link with the peraluminous intrusion emplacement. We propose that the Bonnac deposits represent an intermediate type between a typical orogenic-gold deposit and an IRGD. We argue that the presence of economic high-grade gold content in tungsten vein-type, and more generally the IRGD deposits, have been underestimated in the Variscan French Massif Central.

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

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

Geology and in-situ sulfur and lead isotope analyses of the Jinlongshan Carlin-type gold deposit in the Southern Qinling Orogen, China: Implications for metal sources and ore genesis

The Jinlongshan gold deposit, Shaanxi Province, central China, is a large deposit (>150 t Au) in the Southern Qinling Orogen. Gold mineralization is hosted in Upper Devonian to Lower Carboniferous carbonaceous clastic-carbonate sequences. Four stages of pyrite and paragenetic sulfides have been identified based on morphology and paragenesis. Laser-ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) and laser ablation multi-collector inductively coupled plasma mass spectrometry (LA–MC–ICP–MS) analyses were conducted to determine the chemical and S–Pb isotopic composition of the sulfides. Pre-ore stage pyrites (Py1 and Py2) contain relatively high concentrations of Mn, Co, Ni, Cu, Zn, Sb, and Pb exhibiting a wide range of δ34S values, ranging from –29.9‰ to +48.4‰. These two pyrites were formed by bacterial and thermochemical sulfate reduction of Upper Devonian marine sulfate, respectively. Main-ore stage pyrite (Py3) and arsenopyrite (Apy) contain high concentrations of Au, As, Cu, Sb, Hg, and Tl with restricted ranges of δ34S values from 9.6‰ to 15.3‰, which is interpreted as resulting from the underlying low-grade metamorphic Paleozoic sedimentary rocks by the decomposition of pyrite and/or inorganic reduction of sulfate. Late- or post-ore stage pyrite (Py4) is characterized by low concentrations of Au, Ag, Sn, Hg, and Bi and exhibited variable δ34S values, ranging from –4.4‰ to +27.7‰, that may reflect the mixing of metamorphic fluids with meteoric water. Similarly, the Pb isotope composition of ore-stage sulfides resembles those of underlying low-grade metamorphic Paleozoic sedimentary rocks whereas differs from Precambrian metamorphic basement rocks and granitoids around the basin, indicating that the metals of the Jinlongshan gold deposit were sourced from the underlying Paleozoic sedimentary rocks.Detailed fieldwork revealed that the deposit has mainly undergone four deformation events. The ore bodies developed in NE-trending dextral oblique-slip reverse faults of D3 that crosscut WNW-trending shear zones of D2. The timing and corresponding stress regimes of the D3 event suggest that it formed in a local extensional environment of Yanshanian intracontinental orogeny of Qinling orogenic belt. Gold metallogenesis of the Jinlongshan gold deposit occurred during the transition of tectonic setting from compression to extension, which reactivated the shear zones and migrated deep ore-bearing fluids to NE-trending brittle faults. The geological and geochemical features of the Jinlongshan gold deposit are more analogous to the Carlin-type deposits than other orogenic gold deposits worldwide; therefore, we propose it is a typical Carlin-type gold deposit in South Qinling Orogen.

Field observations, magnetic fabrics and microstructures evidences of syn-kinematic emplacement of the Numba granitic pluton (western Cameroon domain)

The N–S elongated Pan-African Numba granitic pluton (NGP), hosted in banded biotite hornblende gneiss and amphibolite country-rocks, belongs to the Western Domain of the Pan-African Belt of Central African in Cameroon. It consists of biotite hornblende granite and fine-grained biotite granite crosscut by gabbroic dykes. The western border of the granite is mylonitized. Structures and microstructures recorded in granite reveal magmatic, submagmatic and high- to moderate- temperature solid-state deformations. Magnetic characterisation of the NGP reveals its ferromagnetic behaviour due to the presence of multi-domain automorphic or xenomorphic pure magnetite grains. Field and AMS measurements testify that the study area recorded three deformation events: (1) a D1 event of ENE-WSW to E-W flattening deformation; (2) a D2 event of sinistral top-towards south sense of shear movement characterised by low to moderate dips (i) NE-SW strike foliation in country-rocks; (ii) NNE-SSW to NE-SW strike of magmatic foliation marked by the preferred orientation of K-feldspar megacrysts in granite and (iii) low plunges NE-SW mineral stretching lineation in biotite hornblende gneiss parallel to the axes of SW vergence of "S" shape F2 folds and (3) a D3 event of dextral top-towards east sense of shear movement that developed narrow NNE-SSW strike mylonite corridor. The NGP emplaced in a fracture within an amphibo-gneissic country-rock and has recorded opposite sinistral top-towards south (D2 Pan-African event) to dextral top-towards east senses of shear movements (D3 Pan-African event). It can be inferred that the emplacement and the deformation of the NGP was initiated during regional D2 deformation (dated at 613-585 Ma) when its magnetic fabric was acquired, and it was completely achieved during regional D3 deformation (dated ca. 585-540 Ma). Tj vs. Pj diagram associated with Flinn diagram suggests an emplacement in dominant flattening strain regime with a considerable shear component. The different data recorded in the NGP imply that its emplacement mechanism points to a ballooning. Indeed, the eastern domain of NGP with elliptical shape represents the inner part of the architecture while its western domain represents the outer part. The moderate to high dips observed in the inner part of the elliptical domain suggest the probable location of a feeder zone, and the lineation pointing away from the centre of the intrusion in the outer part may be developed related to the magmatic flow. This concentricity is easily associated with internal microstructures that show moderate-temperature solid-state to submagmatic and magmatic microstructures from the outside of the pluton towards the centre. The magma was then emplaced by ballooning in a gneissic country-rock that was subjected to sinistral shearing. Afterwards, the deformation characterised by a dextral shearing contributed to a generation of a NNE-SSW mylonitic band at the western border of the pluton.

Tectonic evolution of the intra-uplift Vigodi-Gugriana-Khirasra-Netra Fault System in the seismically active Kachchh rift basin, India: Implications for the western continental margin of the Indian plate

The E-W striking Kachchh Rift Basin (KRB) at the western continental margin of Indian plate is located in the northern part of Gujarat state, India. KRB has undergone polyphase deformation owing to changing stress-field since Late Triassic. We establish the kinematic framework along the NW striking intra-uplift Vigodi-Gugriana-Khirasra-Netra Fault System (VGKNFS) located at the western part of the KRB. The VGKNFS consists of 0.25–50 km long, sub-parallel, striated, reactivated dip-slip faults. A total of 1150 fault-slip data consisting of fault planes and striations attitude were recorded from 66 sites (including a single site with systematic joints). Slip-sense of faults was determined by documenting kinematic indicators. Paleostress analysis was carried out using Win-Tensor, T-Tecto, FaultKin and SG2PS to compare and validate the results. The chi-square statistical analysis was undertaken to evaluate how relevant are paleostress analysis results to the present-day compressional stress field. Geophysical investigations were conducted at selected sites using Ground Penetrating Radar (GPR) to understand the shallow subsurface geometry and trace the continuity of faults buried below patchy alluvial cover. 15–17 m long, 2D GPR cross-sections were recorded across faults using monostatic antenna of 200 MHz frequency.Two major deformation events – D1 and D2 are determined in the present study. The extension event D1 shows W and NW–NNW extension during Late Triassic to Late Cretaceous. It occurred during the rift phase of KRB in response of the break-up of Gondwanaland. The latest event D2 shows NNE–NE compression from Late Cretaceous up to the present. It is interpreted to have developed during post-rift inversion phase, induced by collision of the Indian plate with the Eurasian plate at ~55 Ma.

Unravelling the D1 event: evidence for early granite-up, greenstone-down tectonics in the Eastern Goldfields, Western Australia

The early tectonic history of the Eastern Goldfields Terrane (EGT) is poorly understood, but in places ca 2800 Ma mafic–ultramafic sequences are conformably overlain by ca 2720–2670 Ma sequences (e.g. Leonora and Laverton districts), suggesting minimal early deformation. The first significant angular unconformities occur at the base of the ca 2670–2655 Ma late basins and indicate that deformation was contemporaneous with deposition of the late basins. These basins mark the end of the volcano-sedimentary record in the EGT and typically grade upwards from polymictic mafic-dominated conglomerates to more siliciclastic compositions with abundant granitic clasts. The clastic sequences record the uplift and exhumation of granite-cored domes. There is a distinct lack of clasts with internal deformation fabrics (e.g. schist or gneiss). The timing of late basin formation overlaps with the ca 2672–2660 Ma D1 event, which has been variously described as extensional or compressional. D1 structures comprise a bedding-parallel S1 that is axial planar to F1 folds. The late basins and early D1 fabrics are overprinted by upright north-trending F2 folds and a sub-vertical S2. A period of crustal thickening was achieved by autochthonous processes with deposition of greenstone sequences into local basins between ca 2720 and 2670 Ma. Partial convective overturn (or granite-up, greenstone-down tectonics) developed as a result of gravitational instability. As the large granite–gneiss bodies rose, solid-state extensional D1 shears developed around the granites with a radial pattern of L1 extension lineations. Areas of compression developed within the sinking greenstone sequences. A final phase of subsidence in the central parts of the greenstone belts produced depocentres (late basins). A shift from dominantly vertical tectonics to horizontal tectonics at ca 2655–2650 Ma is marked by the onset of east–west D2 compression and the change in granite compositions from high-Ca to low-Ca granites. KEY POINTS Older ca 2800 Ma mafic–ultramafic sequences are conformably overlain by ca 2720–2670 Ma sequences suggesting minimal early deformation in the EGT. The first significant angular unconformities are at the base of the ca 2665 Ma late basin sequences. Late basin sequences record the uplift and exhumation of granite-cored domes and sedimentation is contemporaneous with the regional ca 2672–2660 Ma D1 event. The D1 event represents granite-up, greenstone-down tectonics (D1 event) that developed as a result of gravitational instability arising from autochthonous crustal thickening from ca 2720–2670 Ma. Granite-up, greenstone-down tectonics precludes an accretionary model for the development of the EGT.

Evolution of structures and hydrothermal alteration in a Palaeoproterozoic supracrustal belt: Constraining paired deformation–fluid flow events in an Fe and Cu–Au prospective terrain in northern Sweden

Abstract. An approximately 90 km long Palaeoproterozoic supracrustal belt in the northwestern Norrbotten ore province (northernmost Sweden) was investigated to characterize its structural components, assess hydrothermal alteration–structural geology correlations, and constrain a paired deformation–fluid flow evolution for the belt. New geological mapping of five key areas (Eustiljåkk, Ekströmsberg, Tjårrojåkka, Kaitum West, and Fjällåsen–Allavaara) indicates two major compressional events (D1 and D2) have affected the belt, with each associated with hydrothermal alteration types typical for iron oxide–apatite and iron oxide Cu–Au systems in the region. Early D1 generated a regionally distributed, penetrative S1 foliation and oblique reverse shear zones that show a southwest-block-up sense of shear that formed in response to NE–SW crustal shortening. Peak regional metamorphism at epidote–amphibolite facies broadly overlaps with this D1 event. Based on overprinting relationships, D1 is associated with regional scapolite ± albite, magnetite + amphibole, and late calcite alteration of mafic rock types. These hydrothermal mineral associations linked to D1 structures may form part of a regionally pervasive evolving fluid flow event but are separated in this study by crosscutting relationships. During D2 deformation, folding of S0–S1 structures generated F2 folds with steeply plunging fold axes in low-strain areas. NNW-trending D1 shear zones experienced reverse dip-slip reactivation and strike-slip-dominated movements along steep, E–W-trending D2 shear zones, producing brittle-plastic structures. Hydrothermal alteration linked to D2 structures is a predominantly potassic–ferroan association comprising K-feldspar ± epidote ± quartz ± biotite ± magnetite ± sericite ± sulfides. Locally, syn- or post-tectonic calcite is the main alteration mineral in D2 shear zones that intersect mafic rocks. Our results highlight the importance of combining structural geology with the study of hydrothermal alterations at regional to belt scales to understand the temporal–spatial relationship between mineralized systems. Based on the mapping results and microstructural investigations as well as a review of earlier tectonic models presented for adjacent areas, we suggest a new structural model for this part of the northern Fennoscandian Shield. The new model emphasizes the importance of reactivation of early structures, and the model harmonizes with tectonic models presented by earlier workers based mainly on petrology of the northern Norrbotten area.

Paleoproterozoic metamorphosed calc-alkaline dikes of the southwestern Río de la Plata craton, Tandilia belt of Argentina, record a prograde high-pressure, medium-temperature evolution

Two metamorphosed calc-alkaline dikes, a metadacite and metarhyolite, and their country-rock from El Quebracho hill in the Tandilia belt (TB), Rio de la Plata craton, were studied in detail. Both dikes contain chemically zoned garnet porhyroblasts with the following compositions: Alm35GrosAnd36.5Pyr0.5Spes28 (core), Alm41GrosAnd41Pyr1Spes17 (rim) in the metadacite; Alm37GrosAnd38Pyr0Spes25 (core), Alm50.8GrosAnd32Pyr0.2Spes17 (rim) in the metarhyolite. We calculated P-T pseudosections in the system Na-K-Ca-Fe-Mn-Mg-Al-Si-Ti-H-O and contoured them for chemical and modal parameters of interest with the PERPLE_X computer software to model these metamorphosed dikes. Molar fractions in garnet and mineral assemblages signalize a prograde metamorphic evolution along an apparent geothermal gradient of ~14 °Ckm−1. Syn-kinematic growth of garnet at a NE-SW compressional D1 event during burial to depths of ~43 km at around 600 °C trapped titanite, epidote, and quartz (Si) at crustal depths of 35–40 km at 535–540 °C. Afterwards, the rocks were exhumed in a continuously lasting compressional regime with S1 formation delineated by mafic minerals in the metadacite (Ca-amphibole + biotite) and metarhyolite (biotite). Biotite with Mg# contents of ~0.2 in the metadacite and Ca-rich plagioclase in both dikes point to exhumation accompanied by cooling. Local mylonitization (D2) with the development of an Sm - foliation occurred between 600-500 °C, prior to the late post-kinematic blastesis of muscovite (Si ~3.15 pfu) in the metarhyolite. LA-ICP-MS U-Th-Pb dating of zircon and its trace-element signature indicate intrusion ages of 2130 ± 10 Ma (MSWD = 4.9) for the metarhyolite and 2205 ± 7 Ma (MSWD= 1.4) for the country-rock (metagranodiorite). Based on the regional background, the metamorphic overprint is hypothesized to have taken place still in the Paleoproterozoic.

Geological anatomy of the Upper Triassic sequence in southeastern Tibet: Implication for tectonic evolution of the eastern Himalayan Orogen

Located south of the Yarlung‐Tsangpo Suture Zone (YTSZ), the Upper Triassic rocks of the Tethyan Himalayan Sequence (THS) in the Shannan area, southeastern Tibet, serve as an ideal geologic entity to investigate the post‐collisional evolution of the eastern Tethyan Himalayan realm. Detailed field observations suggest that D2 north‐verging folds and thrusts dominate the deformational pattern of the newly named Luolin Group, while D1 south‐verging imbricated structures characterize the Langjiexue Group and the Nieru Unit. Illite crystallinity measurements in pelitic samples indicate that the metamorphic grade decreases from epizonal in the Luolin Group to anchizonal facies in the Langjiexue‐Nieru Unit. Fluid inclusions from two stages of syn‐deformational quartz veins are characterized by moderate homogenization temperature (~260°C) and low salinity (5% NaCleqv). Electron microprobe analyses (EMPA) of garnet amphibolites show metamorphic temperature of ~510°C. The age of D1 event is determined by 40Ar–39Ar dating of phyllite sample at ca. 47 Ma. Correlations with regional deformational events through the THS demonstrate that the D2 event occurred between ca. 30 and 22 Ma. These data imply that the Upper Triassic sequence is part of the fold‐thrust belt of the Himalayan Orogen, rather than the accretionary mélange within the YTSZ.

Emplacement of the giant Kunene AMCG complex into a contractional ductile shear zone and implications for the Mesoproterozoic tectonic evolution of SW Angola

Massif-type anorthosites are voluminous, plagioclase-dominated batholiths that occur worldwide, but only during the Proterozoic Eon. Granitoids are closely associated with the anorthosites, and they collectively may form an AMCG suite (i.e., anorthosite, mangerite, charnockite, granite). Many fundamental questions regarding the origin of the AMCG complexes remain unresolved, including their tectonic setting and mode of emplacement. Here, we present new data from reconnaissance structural mapping, U–Pb LA-ICP-MS zircon dating, and 40Ar/39Ar amphibole and mica dating from a 500 km2 area straddling the western margin of one of the world’s largest (≥ 15,000 km2), but also least studied AMCG suites, the Kunene Complex (KC) in Angola. The oldest recognized (pre-Kunene) deformation event D1 produced a steep WNW–ESE-striking S1 fabric in supracrustal rocks, migmatite and Paleoproterozoic (1800–1780 Ma) granodiorite before ~ 1680 Ma. This was folded by the D2 event, resulting in steep, N–S-striking S2 leucosomes and gneissosity in the rocks surrounding the KC. Concordant magmatic layering and lamination, and high- to low-temperature gneissic foliation in the marginal Kunene anorthosite and coeval megacrystic granite are parallel with S2 fabrics in the Paleoproterozoic country rocks. Mineral stretching lineations L2 in all rock types across intrusive contacts are steep and associated with west-side-up (thrust) shearing. Overlapping U–Pb zircon/baddeleyite ages from the Kunene megacrystic granite and Kunene anorthosite, with 40Ar/39Ar mica ages from the mantling Paleoproterozoic country rocks, attest to progressive contraction, thrusting and exhumation of the Paleoproterozoic country rocks onto the Kunene anorthosite in the interval 1410–1380 Ma. We suggest that thermal softening of the country rocks caused by heat dissipation from the earliest phase (1500–1410 Ma) of ponding of hot anorthositic magma in the deep crust localized D2 deformation at the margin of the KC. Our data point to a tectonic model whereby anorthosite sheets were emplaced at a high angle to the shortening direction and along host-rock S2 anisotropy deep in the crust and cooled during progressive ductile thrusting and exhumation of the country rocks. These results question the current view that Proterozoic AMCG suites are emplaced in extensional environments as large diapiric bodies and favor formation in a continental arc setting.

Genesis of metasomatic gold mineralization in the Pahardiha-Rungikocha gold deposit, eastern India: Constraints from trace element signatures in chromite-cored magnetite and bulk rock geochemistry

The Pahardiha- Rungikocha gold deposit is located south of the Dalma volcano-sedimentary belt within the North Singhbhum Mobile Belt (NSMB), in the eastern part of India. The rocks of this area have undergone three phases of deformation, viz. D1, D2, D3. While the D1 and D2 events signify progressive regional metamorphism, the D3 stage was a consequence of continued N-S compression resulting in E-W trending displacement fractures. This D3 deformation event was responsible for the mobilization of auriferous fluids within the host quartz-chlorite-schist and subsequent gold precipitation. Here, gold occurs as native grains either within the cracks or along grain boundaries of euhedral pyrite. Detailed mineral paragenesis reveals that euhedral pyrite associated with gold is coeval with abundant chromite-cored magnetite. The chromite-cored magnetite shows a complex paragenetic history, i.e., an initial chromite alteration event, followed by epitaxial growth of magnetite rims on the chromite cores, as revealed by LA-ICP-MS trace element data and EPMA X-ray compositional maps. The paragenetic stages of the chromite-cored magnetite is synchronous with the successive deformation events of the host rock. The final phase of deformation is responsible for the formation of the chromite-cored magnetite rims which is coeval with the gold mineralizing event. These magnetite rims formed as a result of metasomatism brought about by the interaction of hydrothermal fluids with the wall rock. The chromite-cored magnetite trace element signatures resemble the high temperature Ca-Fe alteration and high temperature K-Fe alteration facies, reported from various IOCG and IOA type deposits. There is a distinct enrichment of mafic elements such as Ni, Cr, V, Mg, V, Cu within magnetite rims. A similar pattern of enrichment in mafic elements is also seen in the bulk rock geochemical data. Major element discrimination diagrams reveal that the chlorite schists evolved from a mixed provenance of felsic and mafic/ultramafic rocks. Consequently, the D3 induced metasomatism most likely resulted in preferential stripping of the mafic elements, which is well reflected in the analyzed magnetite rims. Thus, we propound that the bulk rock chemical composition of the host chlorite-schist had a first order control on the chemistry of the chromite-cored magnetite, coeval with the gold mineralizing event. Given the fact that there are no causative igneous intrusions underlying the host chlorite schists, we infer that the gold mineralizing fluids were intrabasinal in nature and were mobilized due to reactivation of pre-existing faults and fractures during the D3 deformation event.

The Vaasa Migmatitic Complex (Svecofennian Orogen, Finland): Buildup of a LP‐HT Dome During Nuna Assembly

AbstractTectonic evolution of the Paleoproterozoic Vaasa migmatitic complex (VMC) in the central part of the Svecofennian accretionary orogen is deciphered using aeromagnetic and gravity maps, deep seismic and magnetotelluric profiles, and structural and metamorphic data. The VMC is a semicircular structure with migmatitic rim and granitic core composed of several subdomes. It evolved in three main tectonic events (D1–D3). The D1 event (ca. 1.89–1.88 Ga) corresponds to the stacking of supracrustal rocks and the formation of an inverted metamorphic gradient. Anatexis at LP‐HT metamorphic conditions enabled the material to flow. The D2 event (ca. 1.88–1.87 Ga) corresponds to large‐scale folding of the partially molten crust within an orocline. It is marked by folds with an E‐W vertical axial planar foliation. The late D3 event resulted from mass redistribution owing to mechanical instabilities within the hinge of the orocline. It is marked by vertical shearing (ca. 1.87–1.85 Ga) in the marginal parts of the complex and along the granitoid subdomes. The seismic reflection profile (FIRE 3a) and magnetotelluric profiles (MT‐PE and MT‐B2) image large‐scale D1 stacking structures within an accretionary prism. Near vertical breaks in crustal‐scale reflectivity and conductivity models are interpreted as D3 shear zones. The VMC is an example of early mass and heat transfer within a collage of hot supracrustal rocks in an accretionary belt. Partial melting enhanced the flow of material, the production, and rise of magma as well as exhumation, marked by magmatic domes in the hinge of the orocline.