Fuping massifs where the belt is well exposed, allow us to draw a new tectonic map and crustal-scale cross sections. The available petrologic, radiometric, geochronologic data are integrated in a geodynamic evolution scheme for this orogen. The Low Grade Mafic Unit (LGMU) is interpreted as an ophiolitic nappe rooted in a suture zone located in the western part of the Lüliangshan. This ophiolitic nappe overthrusts to the SE upon the Orthogneiss-Volcanites Unit (OVU) that consists of a bimodal volcanic-sedimentary series metamorphosed under amphibolite facies conditions intruded by calcalkaline orthogneiss. The OVU is a composite Neoarchean-Paleoproterozoic magmatic arc developed during two stages (ca. 2500 and 2100 Ma) upon a continental basement corresponding to the western extension of the Neoarchean Fuping massif. The OVU overthrusts to the SE the Fuping massif along the Longquanguan shear zone. This stack of nappes, coeval with an amphibolite facies metamorphism, is dated at ca. 1880 Ma. Subsequently, the metamorphic series experienced a widespread migmatization at 1850 Ma and was intruded by post-orogenic plutons dated at 1800 Ma. The weakly to unmetamorphosed Hutuo Supergroup unconformably overlies the metamorphosed and ductilely deformed units (OVU and LGMU), but it is also involved in a second tectonic phase developed in subsurface conditions. These structural features lead us to question the ca. 2090 Ma age attributed to the Hutuo supergroup. Moreover, in the Fuping massif, several structural and magmatic lines of evidence argue for an earlier orogenic event at ca. 2100 Ma that we relate to an older west-directed subduction below the Fuping Block. The Taihangshan Fault might be the location of a possible suture zone between the Fuping Block and an eastern one. A geodynamic model, at variance with previous ones, is proposed to account for the formation of the TNCB. In this scheme, three Archean continents, namely from west to east, the Ordos, Fuping and Eastern Blocks are separated by the Lüliang and Taihang Oceans. The closure of the Taihang Ocean at ca. 2100 Ma by westward subduction below the Fuping Block accounts for the arc magmatism and the 2100 Ma orogeny. The second collision at 1900-1880 Ma between the Fuping and Ordos blocks is responsible for the main structural, metamorphic and magmatic features of the Trans-North China Belt.

In this contribution we present a reconstruction of the overall lithotectonic architecture, from inner zones to external ones, of the Paleoproterozoic Trans-North China Orogen, within the North China Craton. Moreover, forward thermobarometrical modelling on a kyanite-bearing gneiss yields a reliable prograde P-T-t-D path. In addition, 40 Ar/ 39 Ar dating on rocks from distinct litho-tectonic units helps us to distinguish several tectono-metamorphic events during the orogenic development. Considering these results, we propose a geodynamic model involving three cratonic blocks, namely the Western, Fuping and Eastern Blocks, separated by two oceans, the Lüliang and Taihang Oceans. The opening of oceanic basins occurred around 2.2-2.3 Ga. After the westward subductions of oceanic lithosphere, the Trans-North China Orogen was built up through a polyphase tectonic evolution within the period 1900-1800 Ma. The first event (D1) corresponded to the emplacement of lower and upper nappes herein called the Orthogneiss-and-Volcanite Unit (OVU) and the Low-Grade-and-Mafic Unit (LGMU), respectively. The syn-metamorphic D1 deformation (1880 ± 10 Ma) is characterized by a NW-SE stretching and mineral lineation with a top-to-the SE sense of shear. During ongoing compression of the thickening orogenic crust, a second deformation event D2 (1850 ± 10 Ma) was responsible for (1) syn-anatectic lateral flow and exhumation of the orogenic root and (2) folding of the middle and upper parts of the orogenic wedge that consequently acquired a fan-type geometry. The late D3 (1830 ± 10 Ma) and D4 (1810 ± 10 Ma) events are related to late-orogenic normal and strike-slip shearing, respectively. In our present state of knowledge, the Paleoproterozoic Trans-North China Orogen might be regarded as the assemblage of two continent-continent collisional belts, both of which are characterized by nappe stacking accommodated by top-to-the E/SE ductile shearing. Continental subduction, crustal thickening, partial melting of overthickened crust, exhumation of HP rocks and deposition of syn-orogenic detrital basins are typical features of modern collisional-type orogens.

International audienceA structural and petrochronological study was carried out in the southern part of the Belledonne crystalline massif. A first tectonometamorphic event, Dx, corresponds to the eastward thrusting of the Chamrousse ophiolitic complex characterized by a low-temperature–moderate-pressure metamorphism reaching 0.535 ± 0.045 GPa and 427.5 ± 17.5 °C. A subsequent D1 deformation is defined by a penetrative S1 foliation that mostly dips toward the west and displays an E–W- to NE–SW-trending mineral and stretching lineation L1. D1 is associated with a top-to-the east shearing and is responsible for the crustal thickening accommodated by the eastward nappe stacking and the emplacement of the Chamrousse ophiolitic complex upon the Rioupéroux-Livet unit. This event is characterized by an amphibolite facies metamorphism (0.58 GPa ± 0.06; 608 ± 14 °C) that attains partial melting at the base of the nappe pile (0.78 ± 0.07 GPa; 680.5 ± 11.5 °C). LA-ICP-MS U-Pb dating of monazite grains from the mica schists of the Rioupéroux-Livet unit constrain the age of D1 to 337 ± 7 Ma. The D2 tectono-metamorphic event is characterized by NE–SW trending, upright to NE-verging synfolial folding. Folding associated with D2 is pervasively developed in all lithotectonic units with the development of a steeply-dipping S2 foliation. In particular, D2 involves the uppermost weakly metamorphosed Taillefer unit. LA-ICP-MS U-Pb dating performed on detrital zircon grains shows that the Taillefer conglomerates was deposited during the Visean. A zircon SIMS U-Pb age of 352 ± 1 Ma from a plagioglase-rich leucocratic sill of the Rioupéroux-Livet unit is interpreted as the age of magmatic emplacement. Our results suggest that the D2 event took place between 330 Ma and 310 Ma. We propose a new interpretation of the tectonometamorphic evolution of the southern part of the Belledonne massif, focusing on the Middle Carboniferous stages of the Variscan orogeny

In order to constrain the finite deformation pattern of the Variscan basement of the Agly massif, a detailed structural analysis over the whole Agly massif was performed. Our investigation combined geological mapping, reappraisal of published and unpublished data completed with our own structural work. Results are provided in the form of new tectonic maps and series of regional cross-sections through the Agly massif. At variance from previous studies, we identified three deformation fabrics named D1, D2 and D3. The D1 deformation is only relictual and characterized by a broadly northwest-southeast striking and eastward dipping foliation without any clear mineral and stretching lineation direction. D1 might be attributed to thickening of the Variscan crust in a possible orogenic plateau edge position. The D2 deformation is a heterogeneous non-coaxial deformation, affecting the whole massif, that produced a shallowly dipping S2 foliation, and an anastomosed network of C2 shear zones that accommodated vertical thinning and N20 directed extension. D2 is coeval with LP-HT metamorphism and plutonism at ca. 315–295 Ma. D2 corresponds to the extensional collapse of the partially molten orogenic crust in a global dextral strike-slip at the scale of the whole Variscan belt. The D2 fabrics are folded and steepened along a D3 east-west trending corridor, called Tournefort Deformation Zone (TDZ), where the Saint-Arnac and Tournefort intrusives and surrounding rocks share the same NE-SW to E-W subvertical S3 foliation. Along the D3 corridor, the asymmetrical schistosity pattern and kinematic criteria suggest a D3 dextral kinematics. The D3 deformation is a record of E-W striking dextral shearing that facilitated and localized the ascent and emplacement of the diorite and granitic sheet-shaped plutons. D3 outlasted D2 and turned compressional-dominated in response to the closure of the Ibero-Armorican arc in a transpressional regime. The progressive switch from D2 thinning to D3 transpression is attributed to the lessening of gravitational forces at an advanced stage of extensional collapse that became overcome by ongoing compressional tectonic forces at the southern edge of the Variscan orogenic plateau.

We decipher late-orogenic crustal flow characterized by feedback relations between partial melting and deformation in the Variscan Montagne Noire gneiss dome. The dome shape and finite strain pattern of the Montagne Noire Axial Zone (MNAZ) result from the superimposition of three deformations (D1, D2 and D3). The early flat-lying S1 foliation is folded by D2 upright ENE-WSW folds and transposed in the central and southern part of the MNAZ into steep D2 high-strain zones consistent with D2 NW-SE horizontal shortening, in bulk contractional coaxial deformation regime that progressively evolved to noncoaxial dextral transpression. The D2 event occurred under metamorphic conditions that culminated at 0.65 ± 0.05 GPa and 720 ± 20°C. Along the anatectic front S1 and S2 foliations are transposed into a flat-lying S3 foliation with top-to-NE and top-to-SW shearing in the NE and SW dome terminations, respectively. These structures define a D3 transition zone related to vertical shortening during coaxial thinning with a preferential NE-SW to E-W directed stretching. Depending on structural level, the metamorphic conditions associated with D3 deformation range from partial melting conditions in the dome core to subsolidus conditions above the D3 transition zone. We suggest that D2 and D3 deformation events were active at the same time and resulted from strain partitioning on both sides of the anatectic front that may correspond to a major rheological boundary within the crust.

International audienceUnravelling the detailed pressure–temperature–time-deformation (P–T–t-D) evolution of magmatic and metamorphic rocks provides essential insights into the timing and duration of partial melting and related plutonism during crustal flow and migmatitic dome formation. The Montagne Noire Axial Zone (MNAZ) is a migmatitic dome located within the Variscan orogen in the southern French Massif Central. The timing of the main thermal event that was responsible for intense partial melting is still highly debated. In this study we present new laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) age data on micaschists, migmatites and granites that clarify the P–T–t-D evolution of the MNAZ. Structurally controlled samples were collected in order to constrain the timing of metamorphism, migmatization and plutonism regarding the main structural pattern D1, D2 and D3. D1 and D2 correspond to nappe stacking and dextral transpression, respectively. D3 is related to vertical shortening and coaxial thinning with a preferential NE–SW- to E–W-directed stretching. LA-ICP-MS analyses on the syntectonic Anglès, Soulié and Martys granites yielded U–Th/Pb monazite ages of 305 ± 1.5, 306 ± 1.9 and 314 ± 2 Ma, respectively. Five migmatitic rocks sampled in the eastern and central Espinouse area yielded in situ ages ranging between 312 ± 2 and 301 ± 2 Ma. Along the dome envelope, two garnet–staurolite-bearing micaschists near Saint-Pons-de-Thomières village gave in situ U–Th–Pb ages of 312.1 ± 2.1 and 309.0 ± 3.1 Ma. A fine-grained gneiss with a D3 fabrics in the eastern dome envelope yield a 208Pb/232Th mean age at 305.7 ± 3.9 Ma. All ages obtained in this study for the micaschists, migmatites and granites range between 315 and 301 Ma. We interpret this time span as the record of the high thermal event responsible for intense crustal partial melting within the lower and middle crust. The onset of partial melting occurred at ca. 315 Ma that marked the beginning of transpressional deformation D2. Based on structural and petrological studies, our new U–Th–Pb results suggest that (1) partial melting may have started at ca. 315 Ma and lasted 15–10 Myr and (2) D2 et D3 developed between 315 and 300 Ma and were synchronous. D1 deformation ended at 315 Ma. The onset and duration of D1 related to nappe stacking and crustal thickening is still uncertain

Low pressure-high temperature (LPHT) metamorphism, with geothermal gradients in the order of 50-100°C/km, is a common feature of the late evolution of collisional orogens. These abnormal thermal conditions may be the results of complex interactions between magmatism, metamorphism and deformation. The Agly massif, in the French Pyrenees, preserves the metamorphic footprints of the late Variscan thermal structure of an almost continuous section from the upper and middle continental crust. The upper crust is characterized by a very high geothermal gradient of ~55°C/ km, evolving from greenschist to amphibolite facies, while the middle crust, exposed in a gneissic core, exhibits granulite facies conditions with a near isothermal geothermal gradient (<8°C/km) between 740 and 790°C. The abnormal and discontinuous crustal geothermal gradient, dated at c. 305 Ma on syn-granulitic monazite by LA-ICP-MS, is interpreted to be the result of magmatic intrusions at different structural levels in the crust: the Ansignan charnockite (c. 305 Ma) in the deepest part of the gneissic core, the Tournefort granodiorite (c. 308 Ma) at the interface between the gneissic core and the upper crust and the Saint-Arnac granite (c. 304 Ma) in the upper section of the massif. The heat input from these magmas combined with the thermal buffering effect of the biotite dehydration-melting reaction resulted in the near isothermal geothermal gradient in the gneissic core (melt-enhanced geotherm). The higher geothermal gradient (>50°C/km) in the upper crust is only due to conduction between the hot middle crust and the Earth's surface. The estimated maximum finite pressure range suggests that ~10 to 12 km of crust are exposed in the Agly massif while the present-day thickness does not exceed 5-6 km. This pressure/depth gap is consistent with the presence of several normal mylonitic shear zones that could have contributed to the subtraction of ~5 km of the rock pile. Monazite U-Th-Pb ages carried out on monazite overgrowths from a highly mylonitized sample suggest that this vertical thinning of the massif occurred at c. 296-300 Ma. This later Variscan extension might have slightly perturbed the 305 Ma geothermal gradient, resulting in an apparent higher conductive geothermal gradient in the upper crust. Although the Agly massif has been affected by Cretaceous extension and Eocene 800 | SIRON et al.

Abstract:The Paleoproterozoic tectono-metamorphic evolution of the pre-Athabasca basement (ϳ1.7 Ga) within the WollastonMudjatik Transition Zone (WMTZ) (Saskatchewan, Canada) has been characterized using both exposed basement and drill cores from the Wolly-McClean exploration drilling project. The finite ductile strain pattern of the WMTZ results from the superposition of two tectono-metamorphic events M1-D1 and M2-D2. M1-D1 is associated with the development of a gently dipping foliation striking N90°-N100°and a southward decrease in peak pressures from up to 10 kbar (1 kbar = 100 MPa) in the Cochrane River area down to 6 kbar in the Wolly-McClean exploration drilling project. The M2-D2 event is responsible for the main northeasterly trend of the WMTZ that developed in a sinistral transpressional tectonic regime during the final oblique collision of the Trans-Hudson Orogeny. Thermobarometric estimations on M2-D2 assemblages show that the studied area was reequilibrated at about 4-5 kbar and 750-825°C. The basement has thus been affected by a differential isothermal decompression event between D1 and D2 that allowed the juxtaposition of the deepest northeastern domains and the Wolly-McClean exploration drilling project, at the same structural level. These results suggest that the basement exposed to the northeast of the Athabasca Basin is not an analog of the basement located beneath the eastern Athabasca Basin where uranium-enriched granitic pegmatites and granites are known. We also suggest that uranium-enriched melts produced during the early M1-D1 stage of partial melting in the deep crust were transferred to the midcrust, owing to D2 shear zones, where they have differentiated to produce uranium-bearing pegmatites.Résumé : L'évolution tectono-métamorphique Paléoprotérozoique du socle pré-Athabasca (ϳ1.7 Ga) de la Zone de Transition Mudjatik-Wollaston (ZTMW) (Saskatchewan) a été définie à partir du socle affleurant et des forages du projet Wolly-McClean. Le champ de déformation finie ductile résulte de la superposition de deux évènements tectono-métamorphique M1-D1 et M2-D2. M1-D1 est associé au développement d'une foliation peu inclinée orientée N90°-N100°et à un pic de pression décroissant vers le sud avec plus de 10 kbar (1 kbar = 100 MPa) à la rivière Cochrane, jusqu'à 6 kbar dans la zone de forage du projet Wolly-McClean. L'évènement M2-D2 est responsable de la structuration majeure Nord-Est de la ZTMW qui s'est développée dans un régime transpressif senestre au cours de la phase finale de la collision oblique Trans-Hudsonienne. Les estimations thermobaromé-triques sur les assemblages M2-D2 mettent en évidence un rééquilibrage à 4-5 kbar et 750-825°C. Le socle a donc été affecté entre D1 et D2 par un épisode de décompression isotherme, ramenant au même niveau structural, la partie nord-est la plus profonde de la zone d'étude et la zone de forage Wolly-McClean. Ces résultats suggèrent que le socle affleurant au Nord-Est du bassin de l'Athabasca n'est pas l'analogue du socle situé sous le bassin là où les...