The Trans-North China Belt (TNCB) is a Paleoproterozoic collisional orogen (ca. 1.9-1.8 Ga) responsible for the amalgamation of the North China Craton. Detail field works in Lüliangshan, Hengshan, ...Wutaishan and Fuping massifs where the belt is well exposed, allow us to draw 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 volcanicsedimentary 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 nappe, 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.
∼3.5-2.8 Ga granitoids from the Pilbara Craton in Western Australia are one of the most ancient and best-preserved records of early processes of continental crust generation. A number of recent ...studies have focused on the nature of the mantle source from which Pilbara granitoids derived, yet no consensus has been reached on whether the mantle was chondritic or depleted in the Eo/Palaeoarchaean. Here we present integrated whole-rock (major and trace elements) and zircon (U-Pb and Lu-Hf isotopes) data for 10 granitoids sampled across the Mount Edgar Dome, which recorded four main magmatic events between 3.47 and 3.23 Ga. Whole-rock major and trace element analyses suggest that the samples belong to two distinct petrogenetic groups. The first group is akin to the tonalite-trondhjemite-granodiorite (TTG) suite, representing highly fractionated magmas initially formed by partial melting of a basaltic crust. The second group, here classified as granites, is best interpreted by the remelting of a basaltic crust and the addition of more evolved material, and it is striking that TTG-like and granitic magmas occurred coevally in time and space. Overall, both groups were formed through intense intracrustal differentiation processes that lead to the loss of significant geochemical information about their original sources. High-precision Lu-Hf analyses in zircon allow to obtain such information and to trace back the isotopic composition of the Palaeoarchaean mantle. A clear change from superchondritic to subchondritic Hf isotope compositions is observed between 3.47 and 3.23 Ga. The superchondritic Hf isotope composition of the 3.47 Ga old granitoids substantiates derivation from a depleted mantle source that separated from the chondritic mantle prior to 3.8 Ga. The presence of ca. 3.5 Ga old inherited zircons in younger magmas suggests that crustal remelting processes were involved in their generation. We propose that all granitoids investigated in this study had their crustal sources originated from a single mantle-crust differentiation event that occurred at 3.50 Ga. This event resulted in the differentiation, from the same original mantle, of two distinct crustal reservoirs, i.e., a mafic reservoir with a 176Lu/177Hf ratio of 0.023, and a reservoir of intermediate/felsic composition (176Lu/177Hf=0.013). 3.32-3.31 Ga-old granitoids were produced by remelting of the mafic reservoir, whereas 3.43 and 3.23 Ga granitoids derived from the intermediate/felsic reservoir. Overall, our data suggest that protracted intracrustal remelting processes and differentiation have played a key role in the formation, evolution, and maturation of the building blocks of continents during the Palaeoarchaean.
IODP Hole U1309D (Atlantis Massif, Mid-Atlantic Ridge 30°N) is the second deepest hole drilled into slow spread gabbroic lithosphere. It comprises 5.4% of olivine-rich troctolites (~
>
70% olivine), ...possibly the most primitive gabbroic rocks ever drilled at mid-ocean ridges. We present the result of an
in situ trace element study carried out on a series of olivine-rich troctolites, and neighbouring troctolites and gabbros, from olivine-rich intervals in Hole U1309D. Olivine-rich troctolites display poikilitic textures; coarse-grained subhedral to medium-grained rounded olivine crystals are included into large undeformed clinopyroxene and plagioclase poikiloblasts. In contrast, gabbros and troctolites have irregularly seriate textures, with highly variable grain sizes, and locally poikilitic clinopyroxene oikocrysts in troctolites. Clinopyroxene is high Mg# augite (Mg# 87 in olivine-rich troctolites to 82 in gabbros), and plagioclase has anorthite contents ranging from 77 in olivine-rich troctolites to 68 in gabbros. Olivine has high forsterite contents (82–88 in olivine-rich troctolites, to 78–83 in gabbros) and is in Mg–Fe equilibrium with clinopyroxene. Clinopyroxene cores and plagioclase are depleted in trace elements (e.g., Yb
cpx ~
5–11
×
Chondrite), they are in equilibrium with the same MORB-type melt in all studied rock-types. These compositions are not consistent with the progressively more trace element enriched (evolved) compositions expected from olivine rich primitive products to gabbros in a MORB cumulate sequence. They indicate that clinopyroxene and plagioclase crystallized concurrently, after melts having the same trace element composition, consistent with crystallization in an open system with a buffered magma composition. The slight trace element enrichments and lower Cr contents observed in clinopyroxene rims and interstitial grains results from crystallization of late-stage differentiated melts, probably indicating the closure of the magmatic system. In contrast to clinopyroxene and plagioclase, olivine is not in equilibrium with MORB, but with a highly fractionated depleted melt, similar to that in equilibrium with refractory oceanic peridotites, thus possibly indicating a mantle origin. In addition, textural relationships suggest that olivine was in part assimilated by the basaltic melts after which clinopyroxene and plagioclase crystallized (impregnation). These observations suggest a complex crystallization history in an open system involving impregnation by MORB-type melt(s) of an olivine-rich rock or mush. The documented magmatic processes suggest that olivine-rich troctolites were formed in a zone with large magmatic transfer and accumulation, similar to the mantle-crust transition zone documented in ophiolites and at fast spreading ridges.
Trace elements in calcareous organisms have been widely used for paleoclimatic studies. However, the factors controlling their incorporation into mollusc shells are still unclear. We studied here the ...Sr, Mg, Ba and Mn serial records in the shells of two aragonitic marine bivalve species:
Mesodesma donacium and
Chione subrugosa from the Peruvian Coast. The elemental concentrations were compared to local temperature and salinity records. The relationships with crystal growth rate
G were investigated thanks to well defined periodic growth structures providing a precise shell chronology. Our results show that for both species, environmental parameters only have minor influence, whereas crystal growth rate strongly influences trace elements concentrations, especially for Sr (explaining up to 74% of the variance). The relationship between
G and Sr/Ca exhibits variability among the shells as well as inside the shells. For a same growth rate value, Sr/Ca values are higher in more curved shell sections, and the growth rate influence is stronger as well. We show that intercellular and Ca
2+-pump pathways cannot support the calcification Ca
2+ flux, leading us to propose an alternative mechanism for ionic transport through the calcifying mantle, implying a major role for calcium channels on mantle epithelial cell membranes. In this new calcification model, Sr/Ca shell ratios is determined by Ca
2+-channel selectivity against Sr
2+, which depends (i) on the electrochemical potential imposed by the crystallisation process and (ii) on the Ca
2+-channel density per surface unit on mantle epithelia.
A new data set combining thermobarometry, geo‐thermochronology, chronostratigraphic, and structural analyses highlights the tectono‐thermal evolution of the St. Martin granodiorite from its ...emplacement to its surface exposure. The described vertical motions in this part of the upper plate of the Lesser Antilles subduction zone since 30 Myrs are linked to the migration of the Lesser Antilles volcanic arc toward the plate interior. Results suggest that the St. Martin granodioritic pluton emplaced at 4–5 km depth and underwent a four‐step history: (a) 30–27 Ma, emplacement along N20–40° transtensive structures oblique to the trench followed by, (b) 27–24 Ma, rapid post‐emplacement cooling and exhumation (∼0.6 mm/yr) controlled by perpendicular to the trench N45° trending structures, (c) 24–9 Ma, slow subsidence (0.01 mm/yr) and development of carbonate platforms associated to tectonic quiescence, westward migration of the arc, and subsequent cooling of the crust, (d) 9 Ma to present‐day, exhumation (∼0.25 mm/yr) and uplift of Neogene carbonate platforms mainly along N45° faults that likely accommodate the progressive trench curvature since 30 Ma. Pecube forward modeling using this scenario reproduces both the observed present‐day geometry and thermochronometric ages. A similar sequence of events is observed in the Virgin Islands. Along with our new data, this suggests a southward migration of the deformation associated with the opening of the Anegada Trough.
Key Points
The tectono‐thermal history of the St. Martin Oligocene granodiorite is reconstructed
Exhumation occurred during two periods: between 27 and 24 Ma and since 9 Ma, along N45° trending normal faults
Regionally, exhumation rates suggest a southward migration of the deformation associated with the opening of the Anegada Trough
Diamond-bearing UHP metamorphic rocks witness for subduction of lithospheric slabs into the mantle and their return to shallow levels. In this study we present U–Pb and trace elements analyses of ...zircon and rutile inclusions from a diamond-bearing garnet megacryst collected in a mélange unit exposed on the northern margin of Africa (Edough Massif, NE Algeria). Large rutile crystals (up to 300 μm in size) analyzed in situ provide a U–Pb age of 32.4±3.3 Ma interpreted as dating the prograde to peak subduction stage of the mafic protolith. Trace element analyses of minute zircons (≤30 μm) indicate that they formed in equilibrium with the garnet megacryst at a temperature of 740–810 °C, most likely during HP retrograde metamorphism. U–Pb analyses provide a significantly younger age of 20.7±2.3 Ma attributed to exhumation of the UHP units. This study allows bracketing the age of UHP metamorphism in the Western Mediterranean Orogen to the Oligocene/early Miocene, thus unambiguously relating UHP metamorphism to the Alpine history. Exhumation of these UHP units is coeval with the counterclockwise rotation of the Corsica–Sardinia block and most likely resulted from subduction rollback that was driven by slab pull.
•In situ U–Pb analyses of zircon and rutile inclusions in a diamond-bearing garnet.•UHP metamorphism in the Western Mediterranean is bracketed between 32.4 and 20.7 Ma.•Retrograde metamorphism and exhumation of UHP units occurred at 20.7 Ma.•Exhumation of UHP units is coeval with rotation of the Corsica–Sardinia block.•This process resulted from subduction rollback driven by slab pull.
The Bonikro gold deposit and satellite Hiré open pits are located at the southern tip of the Oumé-Féttékro granite–greenstone belt in Côte d’Ivoire. Country rocks in the region have undergone ...polyphase deformation and prolonged arc magmatism during the Paleoproterozoic Eburnean orogeny. Intrusive host rocks at Hiré and Bonikro have been dated at 2180 ± 6 and 2086 ± 4 Ma (U–Pb on zircon), respectively. These plutonic bodies acted as favourable sites for fluid flow due to their brittle rheological characteristics. Gold mineralisation at Bonikro is hosted by (i) a sheeted quartz vein array characterised by an Au–W–Bi–Te–Ag metal association, in the cupola of a porphyritic granodiorite; and (ii) an overprinting swarm of fault-fill smoky quartz–(molybdenite) veins. Gold mineralisation at Hiré is shear-hosted and shares a similar relative timing with the latter set of auriferous veins. This second phase of gold mineralisation occurred during a period of transcurrent tectonics late in the Eburnean tectono-magmatic history, soon after the cessation of compressional deformation. It is responsible for the bulk of gold resources in the region (e.g. Bonikro, Hiré, Agbahou) and has been dated at 2074 ± 16 Ma (Re–Os on molybdenite). The integration of structural–paragenetic relationships with high precision dating of magmatic and hydrothermal events highlights the superposition of late Eburnean orogenic gold mineralisation on an earlier intrusion-related gold system linked to the local emplacement of a porphyritic granodiorite at Bonikro and illustrates the genetic diversity of Paleoproterozoic granitoid-hosted gold mineralisation in the West African Craton. It also further supports that gold mineralisations in the West African Craton occurred diachronously throughout the Eburnean orogeny through a variety of deposit types including Au-skarn, intrusion-related Au, orogenic Au, and porphyry Cu–Au.
Based on new structural, petrological and U-Th-Pb geochronological data, a reappraisal of the Variscan tectono-metamorphic history of the Pelvoux Massif (External Crystalline Massif, French Alps) is ...proposed with the aim to understand the flow pattern and kinematics of the Variscan partially molten crust and the Eastern Variscan Shear Zone. The Pelvoux Massif consists of high-grade metamorphic rocks of middle to lower crust, mostly migmatites, that record a prominent syn-metamorphic deformation event (D2) characterized by a pervasive NE-SW striking, steeply dipping, S2 foliation, and a network of anastomosed NS and NW-SE trending shear zones, the kinematics of which indicates a sinistral transpression. Relics of an early syn-metamorphic event (D1/M1) related to crustal thickening and top-to-the-east nappe stacking are also reported. Both the D1 and D2 features are interpreted as reflecting a NW-SE shortening event, firstly marked by dominant nappe stacking, and secondly overprinted by a sinistral transpression that started at peak metamorphism with the onset of crustal partial melting at ca. 650°C during the late Visean (ca. 335-330 Ma). Ongoing sinistral D2 transpression in the partially molten middle-lower crust of the Pelvoux involved strain partitioning between C and C' shear zones and horizontal longitudinal flow in the range 330-300 Ma. Along the anatectic front, vertical shortening and top-to-the-NW shearing (D3) is coeval with D2 and argue for southeastward motion of the partially molten crust. The contemporaneity between NW-SE directed transpressional flow and vertical shortening is supported by our radiometric data of D2 and D3 and attests for strain partitioning between the suprastructure and infrastructure during horizontal crustal flow under transpressive regime. The exhumation of deep-seated rocks during sinistral transpression followed a near isothermal (ca. 700°C) evolution down to pressure of ca. 0.5 GPa in the period 325-306 Ma. The sinistral transpression recorded in the Pelvoux Massif might corresponds to an antithetic shear zone coeval with the dextral East-Variscan Shear Zone, proposed for this part of the Variscan orogen.
This paper presents and discusses new geochronological and petrological data on a suite of calc-alkaline plutons composed predominantly of diorites and tonalites from the West Massif Central. Their ...petrochemical fingerprints are compatible with partial melting of a hydrous mantle wedge followed by fractional crystallization of amphibole and plagioclase before final emplacement between 5 and 8 kbar within the continental upper plate of a subduction system. In situ U-Pb zircon dating on tonalites yields a fairly narrow age range of 365−354 Ma (including uncertainties) for igneous crystallization. These calc-alkaline plutons imply active margin magmatism near the Devonian-Carboniferous boundary and are contemporaneous with the back-arc magmatism and HP metamorphism as dated by recent studies. However, such isolated igneous bodies do not form a transcrustal magmatic arc but rather represent dispersed plutons emplaced within less than 30 Myr when all data from the Variscan belt of France are considered. In Limousin, they intrude migmatitic paragneisses and retrogressed eclogites from the Upper Gneiss Unit (UGU), suggesting that the high pressure rocks were already exhumed at 19−30 km depth before 365 Ma. Moreover, the diorites and tonalites are never found within units below the UGU. It therefore suggests that these tectono-metamorphic units of the Western French Massif Central were piled up after 354 Ma. Altogether these results support the monocyclic model for Variscan geodynamics in the French Massif Central, with the transition between oceanic subduction and continental collision taking place between Upper Devonian and Lower Carboniferous.
Cette étude apporte de nouvelles contraintes géochronologiques et pétrologiques sur une série de plutons calco-alcalins composés principalement de diorites et de tonalites et situés Massif central occidental. Leur signature pétro-géochimique implique la fusion partielle d’un coin mantellique hydraté suivie de la cristallisation fractionnée d’amphibole et de plagioclase avant la mise en place finale entre 5 et 8 kbar dans la plaque continentale supérieure d’un système de subduction. La datation U-Pb in situ des zircons provenant des tonalites donne une gamme d’âge de 365 à 354 Ma (erreurs incluses) interprétée comme représentant la cristallisation magmatique des zircons. Ces plutons calco-alcalins proviennent d’un magmatisme de marge active à la limite Dévonien-Carbonifère et sont synchrones du magmatisme d’arrière-arc et du métamorphisme de haute pression datés par des études récentes. Ces intrusions ne forment cependant pas un arc magmatique d’échelle crustale mais représentent plutôt des plutons dispersés qui se sont mis en place en moins de 30 millions d’années si l’on considère toutes les données disponibles, réévaluées et robustes pour la chaîne varisque en France. Dans le Limousin, ces plutons intrudent notamment les paragneiss migmatitiques et les éclogites rétromorphosées de l’Unité Supérieure des Gneiss, ce qui suggère que les roches de haute pression étaient déjà exhumées à une profondeur de 19-30 km avant 365 Ma. En outre, les diorites et les tonalites n’ont pas été observées dans les unités situées sous l’Unité Supérieure. Cela indique que les unités tectoniques du Massif central occidental ont été empilées après 354 Ma. Ces résultats soutiennent donc le modèle monocyclique de la géodynamique varisque dans le Massif central français où la transition subduction océanique/collision continentale se produit entre le Dévonien supérieur et le Carbonifère inférieur.
The Sassandra-Cavally (SASCA) domain (SW Côte d'Ivoire) marks the transition between the Archean Kenema-Man craton and the Paleoproterozoic (Rhyacian) Baoule-Mossi domain. It is characterized by the ...tectonic juxtaposition of granulite-facies and amphibolite-facies rocks. Migmatitic grey gneisses, garnet-cordierite-sillimanite migmatitic paragneisses and garnet-staurolite-bearing micaschists reached peak pressure conditions ranging from ∼ 6.6 kbar at 620 °C to ∼ 10 kbar at 820 °C. These conditions are associated with the first recorded deformation D1 and correspond to a Barrovian geothermal gradient of ∼ 25 °C/km. Subsequent exhumation, associated with a second deformation D2, was marked by decompression followed by cooling along apparent geothermal gradients of ∼ 40 °C/km. A D3 deformation phase is marked by folding and local transposition of the regional S1/S2 foliation into E-W trending shear zones. LA-ICP-MS U-Pb dating of monazite, which displays complex internal structures, reveals four age groups correlated to textural position of monazite grains and analytical points: (1) rare relictual zones yield dates at the Archean-Paleoproterozoic transition (ca. 2400-2600 Ma); (2) a cluster of dates centered at ca. 2037 Ma on grains aligned along the S2 foliation of the migmatitic grey gneiss, attributed to D2; (3) a cluster of dates centered at ca. 2000 Ma, and (4) dates spreading from ca. 1978 to 1913 Ma, documented for the first time in the West African Craton monazites. The ages of the latter two groups are similar to the ones identified in the Guiana Shield, and could be attributed to a disturbance by fluids, to a periodic opening of U-Pb system or to an episodic crystallization of monazite during slow cooling lasting several tens of Myrs. These data allow to propose a model for the tectonic evolution of the SASCA domain at the contact between the Rhyacian Baoule-Mossi domain and the Archean Kenema-Man nucleus whereby crustal thickening is achieved by crustal-scale folding and is followed by and concomitant with lateral flow of the thickened partially molten crust accommodated by regional transcurrent shear zones. This combination of crustal thickening controlled by tectonic forces and gravity-driven flow leads to the juxtaposition of granulite- and amphibolite-facies rocks.
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