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.
Variscan granitoids and associated mafic rocks exposed in the External Crystalline Massifs (ECM) of the Western Alps document the Variscan stages from the early Carboniferous collision to the early ...Permian post-collisional setting. Our study focuses on the Central part of the ECM, synthesizing newly acquired and existing geochronological, whole-rock geochemical and isotopic data. We identified two distinctive magmatic series: (i) high-K calc-alkaline granitoids, which range from magnesian (MgG) to ferro-magnesian (FeMgG) rocks; (ii) ultra-high-K metaluminous (UHKM) rocks (“durbachites”). These series were emplaced roughly simultaneously between ca. 350 and 300 Ma, with two main episodes during the Visean (ca. 348–335 Ma) and the late Carboniferous (305–299 Ma), with a more limited activity in between. A younger Permian event at ca. 280–275 Ma has also been identified in one granitoid pluton. Contemporaneous emplacement of these two series reflects concomitant crustal anatexis and melting of LILE–LREE-rich metasomatized lithospheric mantle. Trace elements and Nd–Sr isotopes reveal significant hybridization between these two magmatic end members, by magma mixing, or assimilation of crystallized mafic ultrapotassic enclaves in the high-K calc-alkaline granitoids. Granitoid composition evolves over time, especially SiO
2
, Mg#, Sr/Y, La/Yb and Nb/Ta, possibly explained by increasing differentiation of magmas over time, changes in the crust versus mantle sources mass-balance, and decrease in melting pressure due to the orogenic collapse. The εNd
i
values of both high-K calc-alkaline granitoids and durbachites decreases from − 3.8; − 2.9 to − 6.4; − 5.2 between 345 and 320 Ma, possibly indicating an increasing influence of subducted/relaminated crustal material contaminating the lithospheric mantle source. εNd
i
values then rise to − 3.7; − 0.5 during the late Carboniferous, possibly due to progressive exhaustion of the enriched mantle source, or advection of the asthenosphere during the post-collisional stage.
Graphic abstract
Possible geodynamic scenario along the central-eastern segment of the Variscan Belt, which may account for the temporal evolution of Variscan magmatism in the External Western Alps.
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.
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.
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.
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