Felsic Veins Research Articles

Assessment of mineralogic composition of the radionuclide rich heavy beach sands in Western Anatolia (Çanakkale)

This study investigates the mineralogical composition of radionuclide-rich beach sands along the coast of the Miocene Ezine-Ayvacık magmatic complex including Kestanbol pluton, in Western Anatolia. The distribution of the radionuclides show variations along the coast with the highest recorded radioactivity at Hantepe Beach, where specific activities of the 238U-, and 232Th-series are significantly high, while 40K concentrations are moderate. In contrast, the southern beaches, dominated by material from the Kestanbol pluton, have higher 40K concentrations but lower 238U-, and 232Th-series specific activities than Hantepe beach. Using gamma spectrometry, bulk geochemical, and in-situ analyses, we provide a detailed assessment of the mineralogical composition of the sand samples and the primordial radionuclide concentrations of the different lithologies within the Kestanbol pluton, such as felsic veins, nepheline monzogabbro and tephriphonolitic dykes. We suggest that thorite group minerals control the high radioactivity at Hantepe beach, and pegmatitic veins of the Kestanbol pluton are the main source rock for thorite minerals. However, all the products of Miocene magmatism in the region exhibit elevated radioelement and radionuclide enrichments suggesting that other accessory and rock- forming minerals are the potential contributors to the high natural background radiation in the area.

Tectonic evolution of the early Paleozoic intraplate orogen in the South China Block: Insights from Ductile Shear Zones in North Wuyishan

The ductile shear zones within the North Wuyishan domain are key to understand the tectonic evolution of the South China Block (SCB). This study integrates a multifaceted approach, including field observations, thin section analysis, quartz electron backscatter diffraction (EBSD), zircon U-Pb dating, and mica 40Ar/39Ar dating, to elucidate the deformation history of these shear zones in North Wuyishan. The research identifies a two-stage deformation process of early Paleozoic. The initial D1 phase is marked by a top-to-SW thrusting shear, with associated felsic veins dated to the Ordovician period (447 ± 10 Ma to 459 ± 4 Ma), correlating with high-grade metamorphism and partial melting. The later D2 phase is characterized by NE-striking foliation and lineation, indicative of sinistral strike-slip shear, dated to the early Devonian (412 Ma) and early Carboniferous (∼354 Ma). The D1 phase suggests early crustal thickening and melting within the Cathaysia block, while D2 indicates a transpressional regime during post-orogenic adjustment. The geological features of the SCB, notably the absence of arc magmatism, ophiolitic mélange, and high-pressure metamorphism, comply with an intracontinental rift closure model as previously proposed. This model supports the hypothesis that the Early Paleozoic intracontinental orogeny in the SCB was likely a far-field consequence of a continental collision between the SCB-North Vietnam and South Vietnam blocks near the east Gondwana supercontinent.

Metamorphosed Plagiogranite Veins In Salma Eclogites, Belomorian Eclogite Province

The Salma subduction eclogites of the Meso-Neoarchean Belomorian Eclogite Province have compositions of N-MORB oceanic crust protoliths and contain felsic veins of plagiogranite compositions. Felsic veins consist of metamorphic minerals sush as garnet, quartz and kyanite, the earliest high-pressure inclusion of phengite and zoisite in garnet. Veins have geochemistry characteristics that are typical for island arc granites or adakites. They are compositionally comparable with eclogite-bearing tonalite-trondhjemite-granodiorite (TTG) gneisses, modern adakite, and Archaean andesite association of the Vedlozero–Segozero greenstone belt. The origin of these felsic veins can be represented as a product of melting of fluid-modified oceanic mafic slab at PT-conditions of eclogite metamorphism.Geothermometry and pseudosection calculations of metamorphic conditions in felsic veins, for eclogitic mineral assemblage of pyropic garnet, kyanite, high-silica phengite and quartz gave high pressures conditions (ca. 16–22 kbar) at relatively low temperatures (ca. 600–650 °C); for secondary decompression plagioclase ± biotite coronas between quartz, kyanite and garnet gave 14–11 kbar at 650–700 °C. This estimate corresponds “warm” subduction, which is most consistent with the parameters of the modern subduction of the base of the Cascadia plate . Mineral assemblages in eclogites, that included the felsic veins, give 15–11 kbar at 750–700 °C. Thermodynamic modelling indicates that the eclogites hosting the felsic veins did not reach partial melting conditions under the estimated PT conditions: exposure temperatures to the mafic rocks must have been significantly higher. A probable explanation for high-pressure parageneses in felsic veins may be the formation of plagiogranites as a result of the melting of the mafic slab under higher temperature and pressure conditions than were measured in this work. Thus, the high-pressure paragenesis of garnet, phengite, zoisite, and kyanite reflects the path of rock exhumation through eclogitic conditions.Zircon from the felsic veins gave several Archean and Paleoproterozoic ages ∼ 2.87, 2.78, ∼2.70 and, ∼1.9 Ga. The oldest age gave the fluid-modified oceanic zircon (∼2.9 Ga) from Fe-Ti eclogites directly near the veins. The oldest igneous zircons in the veins are represented by a population with the age of ∼ 2.87 Ga and correspond to the igneous age of some TTG gneisses, which do not contradict a hypothesis of the plagiogranite formation during the melting of the mafic slab in the subduction zone. A Neoarchean zircon population of ∼ 2.78 Ga is often present in eclogitized mafic rocks and surrounding TTG gneisses which can be associated both with the formation of TTG gneiss protoliths in the subduction zone (?) and with the recrystallization of older zircon as a result of fluid or thermal impact. The ɛHf of the older population (∼2.87 Ga) is + 3 to + 4, indicating a juvenile origin; the ∼ 2.78 Ga population has similar Hf-isotope compositions, suggesting that their ages reflect resetting (non-zero Pb loss) of the older zircons. The Sm-Nd model TDM ages of Grt-Ky veins have Meso-Neoarchean values (3.0–2.75 Ga) with positive εNd = 1.8–3.8 that may indicate a juvenile nature of sources of the plagiogranite melt. The eclogitized rocks of the Belomorian province generally underwent the granulite facies overprint at ∼ 2.74–2.64 Ga ago that is coincides with the age of granulite facies metamorphism of regional and global rank. The Paleoproterozoic evolution of the Salma eclogites and the East European Craton as a whole: at 2.5–2.4 Ga and 2.0–1.9 Ga with formation of the Lapland–Mid-Russia–South Baltia intracontinental and Svecofennian accretionary orogens was terminated in Paleoproterozoic time at ∼ 1.87–1.7 Ga. These age values correspond to closure of Pb-Pb and Sm-Nd isotopic systems of rock-forming metamorphic minerals of the felsic veins.

An extension of the Rooiberg Group and Upper Zone of the Bushveld Complex in the northernmost Waterberg segment

The Waterberg segment of the northern Bushveld Complex within the Southern Marginal Zone of the Limpopo Belt hosts an ∼ 850 m thick erosional remnant of a previously unrecognized Paleoproterozoic sequence of contact-metamorphosed basalt to basaltic andesite unconformably overlying Upper Zone ferrodiorite of the Rustenburg Layered Suite. The volcanic rocks are overlain by ∼ 1.5 km thick package of 1.95–2.05 Ga Waterberg Group sandstones. We report on data obtained from the NP1 borehole that intersects this sequence in the Waterberg segment north of the Hout River shear zone.The metabasalts are correlated with Dullstroom basalts and basaltic andesites of the Rooiberg Group based on their similar mineralogy, petrography, major, trace element compositions and supported by homogeneous and relatively non-radiogenic Sr-in-plagioclase (0.7042–0.7066, mean = 0.7052 at 2.056 Ga) with respect to more radiogenic Sr-in-plagioclase of 0.7064–0.7073 in underlying ferrodiorite, consistent with the known Upper Zone values. Both metabasalts and ferrodiorite show similar εNdt from − 5.4 to –5, typical of the Bushveld lineage, whereas anomalously low εHft from −13 to −18 could be a regional feature of the Waterberg segment. Contact-metamorphosed basalts enclose layers and xenoliths of metasedimentary hornfelses at the base. No intervening granite occurred between the Upper Zone and overlying volcanic rocks suggesting that numerous coarse-grained to granophyric fayalite-magnetite bearing felsic veins in the metavolcanics were derived from Upper Zone residual melts of monzonitic to granitic compositions. These felsic veins are rich in Zr-Hf and depleted in REE relative to cognate Upper Zone faylite-magnetite-pyroxene-plagioclase cumulates, while zircon from one of the topmost veins yielded an age of 2054.4 ± 3.7 Ma concordant with other ages for the Bushveld Complex within analytical uncertainty. Our study suggests a larger extension of Rooiberg basaltic volcanism than previously thought, supporting the comparable volumes of magmas for the volcanic and plutonic counterparts of the Bushveld Complex.

Effects of Metasomatism on Granite-Related Mineral Systems: A Boron-Rich Open Greisen System in the Highiş Granitoids (Apuseni Mountains, Romania)

Greisenization is typically linked with highly fractionated granites and is often associated with hydrothermal vein systems. Late to postmagmatic metasomatic processes involve the enrichment of volatile components such as boron and halogens as well as several metallic elements. The purpose of this study is to reveal the main metasomatic effects and paragenetic sequences of the related mineralizations in Highiş granitoids, Romania. In a natural outcrop, more than 30 samples were collected from granitoids, felsic veins, and country rocks. We carried out a detailed mineralogical and petrological characterization of carefully selected samples using X-ray powder diffractometry, electron microprobe analysis, and microscopic methods together with K–Ar ages of whole rocks and K-bearing minerals. Several characteristic features of albitization, sericitization, tourmalinization, epidotization, and hematitization were recognized in the studied samples. Crystallization of quartz, K-feldspar, and magnetite represents the first stage during the magmatic-hydrothermal transition. The mineral assemblage of albite, sericite, schorl, and quartz originates from the early and main stages of greisenization. While the subsequent mineral assemblages, which predominantly include dravite, specular hematite, and epidote, are closely related to the late vein-depositing stage. We propose that the study area could belong to a boron-rich open greisen system in the apical portion of Guadalupian A-type granite. Based on a new hypothesis, the previously published Permian crystallization ages (between ~272 Ma and ~259 Ma) could be homogenized and/or partially rejuvenated during the hydrothermal mineralization processes due to uraniferous vein minerals. Additionally, the Highiș granite-related system suffered a Cretaceous thermal overprint (between ~100 Ma and ~96 Ma). The results may help to understand the evolution of highly evolved granite intrusions worldwide and improve our knowledge of the effect of hydrothermal mineralization processes on the emplacement ages.

Crystallisation and fast cooling of the (meta)gabbro from the Chenaillet ophiolite (Western Alps): In-situ U[sbnd]Pb dating of zircon, titanite, monazite and xenotime in textural context

The Chenaillet ophiolite, South of Montgenèvre (France), represents a preserved portion of the alpine ocean. The gabbros form lenticular bodies 50 to 200 m thick and a few hundreds of meters wide. They are intrusive in the serpentinites and often overlain by pillow-lavas. Plagiogranite/albitite veins are rare and volumetrically negligible. The petrology of the gabbro s.s. show great details of the progressive cooling of the massif, clinopyroxene are often rimmed by brown to green amphibole +/− ilmenite/titanite. Thermometry on these amphibole assemblages indicates a retrograde temperature evolution from late magmatic to subsolidus, between 950 and 900 °C, 800–750 °C, 600–500 °C. In a shear zone, a string of titanite associated with monazite and xenotime +/− ilmenite is located in a millimetric greenschist layer whose temperature is estimated at 600–550 °C. Accessories chemistry in this sample allowed in situ dating by LA-ICPMS on thin section in textural context. In the same sample, we performed UPb zircon dating, constraining the magmatic age, as well as UPb monazite, xenotime and titanite dating for the metamorphic evolution. In a Tera-Wasserburg diagram, monazite, xenotime and titanite yield 161.3 +/− 4.0 Ma, 161.5 +/− 2.4 Ma and 158.4 +/− 2.3 Ma lower intercept ages, respectively, while the zircon from the same sample indicates an age of 161.0 +/− 0.8 Ma. Finally, zircon dated in an albitite vein also yields a 161.8 ± 1.7 Ma age. These similar ages within the analytical uncertainties show that magmatism and metamorphism were concomitant. Thermal modelling predicts that a 150 m thick gabbroic massif crystallizes in a few hundred years and cools down in several thousand years. This is consistent with the ages reported in our study showing that the retrograde metamorphism in this massif records fast cooling. The development of this retrograde metamorphism required multiple fluid infiltrations during cooling. In the studied samples, 4 types of titanites could be distinguished based on their chemistries. 3 types variously depleted in REE are interpreted as related to water-driven fluids while another type has up to 10% REE, Y and Nb and suggest a possible felsic source. The two potential sources of water promoting this fast cooling are sea water-derived fluids and magmatic fluids exsolved from felsic veins.

Cyclicity of multistage anatexis of deeply subducted continental crust during the North Qaidam orogeny: Tracing the source, timescale, and evolution of pulsed melts

The most significant mass transfer processes at a convergent plate boundary are tectonic accretion and fluids/melts released from sites of generation to sites of accumulation. However, some crucial questions remain with regards to the source, timescale, and evolution of such anatectic processes, for example, single or multistage anatexis of deeply subducted continental crust. To better understand the processes involved in anatexis, we have quantified the timescale and nature of formation and evolution processes of multistage felsic veins within retrograde eclogite using zircon, monazite, and xenotime geochemistry and geochronology, whole-rock composition, and Sr-Nd-Hf isotope analysis from the Lüliangshan, North Qaidam orogen. The U–Pb dating of coexisting zircon, monazite, and xenotime gives three groups of ages at <i>ca.</i> 441 to 435, <i>ca.</i> 425, and <i>ca.</i> 413 to 409 Ma, respectively, which record at least three episodes of pulsed melts. The first stage of zircon formation is characterized by not only the absence of oscillatory zoning of different zircons in cathodoluminescence images but also some of them with flat HREE and without Eu anomalies, indicating they may form in anatectic melts under eclogite-facies conditions. The second and third phases of melts may have occurred under granulite- and amphibole-facies conditions during exhumation. Furthermore, two classes of felsic veins within the eclogite show wide variations of whole-rock composition, (⁸⁷Sr/⁸⁶Sr)_i, ε_Nd (t), and ε_Hf (t) values, which demonstrate that they were derived from fluid-present dehydration partial melting of different proportions of subducted gneiss and eclogite during different periods. These findings show that melts have systematic differences in chemical and isotopic signatures resulting from lithological diversity and depth of partial melting. Thus, small-scale melts released from the source can excellently explain the variability in whole-rock composition, accessory mineral growth zoning, and prominent isotope variability in syn-collisional heterogeneous granites. An additional implication is that as these melts escape their adjacent area of formation, they migrate and mix along channelized melt pathways, resulting in melt-rock and crust-mantle interaction and the triggering of syn- and post-collisional magmatism.

Development of a synorogenic composite sill at deep structural levels of a magmatic arc (Odenwald, Germany). Part 2: Rheological inversion and mullion formation under bulk constriction

We present structural, microfabric and strain data of a spessartite sill and its quartzmonzodiorite host that were affected by bulk constriction at T = ca. 660 °C. Constrictional deformation led to (1) subvertical prolate grain-shape fabrics, (2) increase of the sill's dip, (3) doubling of the sill's thickness, (4) mullions with cusps pointing into the host, and (5) boudinage of felsic veins cutting through the mafic sill. The sigmoidal shape of the foliation/lineation within the sill was caused by sill rotation, which led to shear stresses along the contacts with the host resulting in antithetic rotation of the grain-shape fabric that is subvertical in its central part and subparallel along the margins. The shape of the mullions, the boudinage of the felsic veins and the sigmoidal foliation/lineation indicate that the mafic sill was incompetent and the felsic material of host and veins was competent. This inversion in rheology can be explained by phase-boundary diffusion that was more effective in the fine grained mafic sill than in the coarse grained felsic host. Evidence for phase-boundary diffusion in form of strongly lobate phase boundaries is widespread and concerns all of the constituent phases present within the sill, host and felsic veins.

Elevation of zircon Hf isotope ratios during crustal anatexis: Evidence from migmatites close to the eastern Himalayan syntaxis in southeastern Tibet

The initial Hf isotope composition of zircon in high-grade metamorphic rocks is an important tracer for protolith origin as well as for the growth and evolution of continental crust. However, less attention has been paid to the behavior of Lu-Hf isotopes in peritectic reactions during partial melting of the continental crust. In this paper, we present a combined study of zircon U-Pb ages, Lu-Hf isotopes and trace elements as well as petrology and whole-rock geochemistry for migmatites close to the eastern Himalayan syntaxis in southeastern Tibet. The results show that peritectic zircons in the migmatites have significantly higher 176Hf/177Hf ratios than their protolith zircons. More interestingly, the 176Lu/177Hf ratios and the contents of high field strength elements (such as Nb, Ta, and Hf) and heavy rare earth elements are significantly elevated in the peritectic zircons compared to the protolith zircons. This suggests that garnets with high Lu/Hf ratios and other minerals (such as titanite, ilmenite, amphibole, and biotite) with moderate Lu/Hf ratios were decomposed during crustal anatexis, contributing to the Hf isotope composition of peritectic zircons. This is confirmed by petrographic observations that garnet and biotite occur as residues or remnants in mesosomes (biotite gneiss) and some leucosomes (felsic veins), but are absent in the melanosomes of the target migmatites. Therefore, the peritectic zircons were produced together with anatectic melts through peritectic reactions and acquired the elevated 176Lu/177Hf ratios compared to the inherited zircons from the protoliths if no peritectic garnet was coevally produced during the crustal anatexis. In this regard, the greatly elevated Hf isotope compositions of peritectic zircons in anatectic granites cannot faithfully reflect the Hf isotope composition of parental rocks. As such, great care must be taken when using Hf isotope ratios to trace the nature of parental rocks with respect to magmatic processes and crustal evolution.

Petrogenesis of the Paleoproterozoic Näränkävaara layered intrusion, northern Finland, Part I: The northern peridotites and their relationships with the layered series and recharge events

The Paleoproterozoic Näränkävaara layered intrusion, northern Finland, has a surface area of 25 km x 5 km and a stratigraphic thickness of ~3 km. The main body of the intrusion includes a 1.5–2 km thick basal dunite series and a 1.3 km thick peridotitic-dioritic layered series, the latter with two peridotitic reversals related to magma recharge. In addition, a series of poorly known elongate poikilitic harzburgitic intrusions (the northern peridotites) are found along the NE contact between the intrusion and the granite-gneiss basement complex. We investigate new mineral and whole-rock geochemical data from the northern peridotites, with the aim of clarifying their petrogenetic relationship to the main layered body of the intrusion. The northern peridotites form a 200–400 m thick cumulate series grading from olivine orthocumulates (OC) at the northern basement complex contact to olivine-orthopyroxene heteradcumulates (HAC) towards the main intrusion body in the south. The OC show whole-rock and mineral chemical trends consistent with origin as rapidly cooled olivine-melt mixtures. The HAC have crystallized in situ from a relatively Cr- and SiO2-rich magma. Based on lithological and stratigraphical correlations, the northern peridotites are linked to the emplacement of the magma that caused the first reversal in the layered series: marginal orthocumulates were formed at the initial emplacement of a new pulse of LREE-enriched siliceous high-MgO basaltic (SHMB) magma into the Näränkävaara chamber, followed by heteradcumulate formation from a fractionating magma with added external SiO2 and fluid. Ubiquitous granite-gneiss xenoliths and felsic veins in drill core suggest assimilation may have been a local process. The northern peridotite parental magma shows undepleted metal ratios suggesting no sulfide saturation occurred prior to emplacement.

Petrogenetic constraints on the felsic vein magmatism in northern Portugal based on petrological and geochemical data

Petrogenetic constraints on the felsic vein magmatism in northern Portugal based on petrological and geochemical data

Mobilization and fractionation of Ti-Nb-Ta during exhumation of deeply subducted continental crust

The behavior of Ti-Nb-Ta is crucial to reveal the genesis of island arc magmatism. However, mobilization and fractionation of Ti-Nb-Ta in subduction zone settings remain poorly understood. The discovery of felsic veins rich in coarse-grained rutile within retrograde eclogite of the North Qaidam UHP metamorphic belt provides a unique and novel opportunity to study age variation during rutile formation and alteration, as well as Ti-Nb-Ta mobility and fractionation during fluid/melt-rock interaction. Rutile high-resolution elemental mapping, and U-Pb bulk grain (ID-TIMS), and in-situ U-Pb geochronology have been utilized to focus on the properties of rutile in both, felsic vein and retrograde eclogite host to gain insight into possible similarities and differences. Three groups of rutile were distinguished according to its host rock, trace elements signature, and genetical connection to ilmenite: eclogite-hosted rutile (Rt-1), felsic vein-hosted rutile not associated with ilmenite (Rt-2a), and associated with ilmenite (Rt-2b). Field evidence and rutile trace elements characteristics document the source of vein-hosted rutile to be mainly derived from the eclogite during fluid/melt-rock interaction. Principal Component Analysis reveals that Nb, Ta, Sn, and W are more enriched in Rt-2a compared to Rt-1; Rt-2b has higher Nb, U, and Hf than Rt-2a. High-resolution mapping across large rutile grains shows the enrichment of high field strength elements (HFSEs) in rutile near to ilmenite, which indicates a HFSEs back diffusion from the rutile–ilmenite boundary during the replacement of rutile by ilmenite. The Nb/Ta ratios of Rt-2a are lower than those of Rt-1, which result from different partition coefficients of Nb and Ta during fluid/melt-rock interaction. The diffusion-influenced rutile exhibits suprachondritic Nb/Ta ratios and demonstrates that diffusion of Nb in rutile is higher than that of Ta under identical P-T conditions. Rutiles Rt-1 and Rt-2a yield consistent 206Pb/238U ages of 426–423 Ma, which is similar to the 433 ± 3 Ma determined by ID-TIMS results of bulk rutile grains. This indicates that Ti-Nb-Ta must have been mobilized during the exhumation of deeply subducted continental crust. However, the diffusion-influenced rutile shows a large variation of ages compared to the rutile not associated with ilmenite, demonstrating that the back-diffusion may affect the U-Pb system in rutile. Therefore, when rutile is partially altered into ilmenite or titanite, its dating should be used with caution. Thus, this study demonstrates volume diffusion is a very important geological process to result in extreme HFSEs fractionation and age variation of rutile on the mineral scale. The rutile aggerates that occur in the felsic veins in 3–5 m distance to the adjacent retrograde eclogite suggest that Ti-Nb-Ta-rich melts/fluids were transported over a distance of at least several meters and that rutile does not represent a residual phase of the Na-Si-Al-, F- and CH4-bearing fluid/melt environment that formed during anatexis of the subducted continental crust. The formation of rutile-rich aggregates during the generation, transport, and crystallization of subducted continental crust-derived melts/fluids in the deep roots of orogenic belts may be a critical trigger for the depletion of HFSEs in arc magmatic rocks during the formation of the continental crust.

Active Basalt Alteration at Supercritical Conditions in a Seawater‐Recharged Hydrothermal System: IDDP‐2 Drill Hole, Reykjanes, Iceland

AbstractThe 4.5 km deep IDDP‐2 drill hole was drilled at Reykjanes, Iceland, in an active seawater‐recharged hydrothermal system on the landward extension the Mid‐Atlantic Ridge. Drilling targeted a well‐defined hydrothermal up‐flow zone feeding the Reykjanes geothermal reservoir, which produces fluids compositionally equivalent to basalt‐hosted deep sea hydrothermal vents. Spot cores recovered from depths between 3,648 to 4,659 m depths consist of pervasively altered sheeted diabase dikes. The shallowest core has an alteration mineral assemblage similar to the producing geothermal reservoir but is being overprinted by the underlying Hornblende Zone assemblage dominated by labradorite and hornblende. The deepest cored section (4,634–4,659 m) retains a diabasic texture, but all primary minerals are replaced or have changed composition. Plagioclase ranges from An30 to An99 and igneous augite is replaced by intergrown hornblende, clinopyroxene, and orthopyroxene. Hydrothermal biotite and potassium feldspar formed locally due to reaction with a phase‐separated brine, indicated by co‐existing vapor‐rich and salt + vapor‐rich fluid inclusions. Seawater‐derived supercritical hydrothermal fluid entering the bottom of the bore hole actively phase separates due pressure drop controlled by the fluid levels in the drill hole. Felsic veins, present in trace amounts, record a continuous transition from magmatic to hydrothermal conditions, including incipient hydrous melting. The vertical changes observed in mineralogy and mineral chemistry indicate that fluids from the deep high temperature reaction zone can undergo significant cooling and reaction with host rocks along their path to the seafloor.

Role of compaction in melt extraction and accumulation at a slow spreading center: Microstructures of olivine gabbros from the Atlantis Bank (IODP Hole U1473A, SWIR)

The exposure of gabbroic sequences at Oceanic Core Complexes (OCC) along ultraslow- to slow-spreading ridges permits the study of the processes forming the lower oceanic crust. On top of the Atlantis Bank OCC along the ultraslow-spreading Southwest Indian Ridge, IODP Expedition 360 drilled Hole U1473A, mainly composed of primitive olivine gabbros interspersed with more evolved Ti-Fe oxide-bearing gabbros and minor felsic veins. These rocks record a complex history of protracted magmatism during continuous uplift and deformation of the gabbroic sequence. Extensive crystal-plastic deformation is dominantly recorded in the shallower sections of the drillhole, whereas the deeper sections better preserve primary magmatic features. We focus on microstructures, including intra-crystalline deformation of rock-forming minerals, and plagioclase crystallographic preferred orientations of olivine gabbros lacking evidence for exhumation-related crystal plastic deformation, to gain insights on the relationship between compaction, melt migration and melt accumulation during the early magmatic history of this section of lower oceanic crust. Olivine gabbros are characterized by ubiquitous grain-size variations, from coarse- to fine-grained intervals. Minerals in coarse-grained intervals show intra-crystalline deformation, while fine-grained crystals lack internal strain. Bent coarse-grained plagioclase associated with weak magmatic foliation and lack of lineation suggest that the coarse-grained intervals were deformed under weak compaction. On the other hand, crystallographic preferred orientations of undeformed fine-grained plagioclase show weak lineations, likely indicative of non-coaxial strain. We thereby infer that the coarse-grained intervals underwent ongoing weak compaction from the stage of olivine + plagioclase ± clinopyroxene crystal mush to the melt-poor stage, and that this process likely aided melt extraction and accumulation in discrete melt-rich zones where crystals orientated in the direction of magmatic flow. Crystallization of melts in the melt-rich zones ultimately formed the fine-grained intervals at different depths in Hole U1473A. This indicates that processes of compaction can lead to local chemical and grain-size heterogeneities in a lower crustal section, while had a minor role in the melt movement at larger scales (e.g., the whole crystal mush) within the oceanic crust.

Late Paleozoic magmatic flare-up in the Nuoergong-Langshan Belt, Alxa Block: Insights into tectonic evolution of the southern Paleo-Asian Ocean

Magmatic flare-up events are widely recognized in continental arcs and have played a critical role in constructing arc crust. However, the geodynamic mechanism that triggers the Late-Paleozoic magmatic flare-ups in the Nuoergong-Langshan Belt (NLB) is still debated. We have identified a Late-Paleozoic magmatic flare-up in the Nuoergong-Langshan Belt (NLB), Alxa Block, based on new and published data. Zircon UPb dating shows that the diorites, intermediate dikes, and their host granites in the NLB crystallized at ca. 275–279 Ma, indicating contemporaneous emplacement of these rocks. Geochemical analyses, together with field observations, suggest that the intermediate dikes were probably formed through mixing of felsic and mafic magmas. The diorites have high MgO (3.2–4.0 wt%), Cr (70–80 ppm) contents, and enriched isotopic compositions and were probably derived from a lithospheric mantle source modified by subducted materials. The host granite of the intermediate dikes have similar isotopic compositions with the gabbro and diorite in the NLZ and were mainly generated by remelting of newly-formed mafic rocks and contaminated by ancient crustal components. Syntectonic folded veins and felsic veins intruding a fold core indicate a crustal shortening and thickening event in the early Carboniferous and early Permian in the NLZ. Our geochronological and geochemical data, coupled with published data, reveal that the magmatic flare-up events were simultaneous with crustal thickening in the southern CAOB, implying that flare-ups in continental arcs were probably related to crustal thickening. The temporal and spatial distribution of the late Paleozoic magmatic rocks in the northern Alxa Block reveals a retreating accretionary orogeny of the Paleo-Asian Ocean (PAO).

Development of a synorogenic composite sill at deep structural levels of a continental arc (Odenwald, Germany). Part 1: Sederholm-type emplacement portrayed by contact melt in shrinkage cracks

We present geochronological, thermobarometric and computertomographic data from a composite sill exposed in the Weschnitz pluton of the Variscan Odenwald. The wall rock consists of quartzmonzodiorite, which intruded at deep structural levels (P = ca. 0.50 GPa, ca. 18 km depth) in a continental arc setting. Zircons and titanites of the quartzmonzodiorite yielded similar UPb ages at 344.3 ± 0.6 and 343.2 ± 2.1 Ma, respectively, reflecting fast initial cooling (≥76 °C/m.y.) until the solidus was attained at ca. 680 °C. Under these conditions, the quartzmonzodiorite was cut by a spessartite sill, which yielded a UPb titanite age at 342.0 ± 1.0 Ma. The short period between wall rock and sill emplacement (≤3.9 m.y.) suggests that both are genetically related.The quartzmonzodiorite situated adjacent to the spessartite sill is enriched in quartz and plagioclase and depleted in hornblende. As geothermobarometric data of these felsic parts yielded T = ca. 710 °C and P = 0.42 GPa (ca. 15 km depth), they are interpreted as contact melt, which invaded into shrinkage cracks of the cooling sill resulting in thin felsic veins. The migration of contact melt from the wall rock into the shrinkage cracks was suction-induced because of volume gain by contact melting in the wall rock and volume loss by cooling of the sill.The formation of such Sederholm-type composite veins seems to be very fast. Titanites of the felsic veins yielded a UPb age at 341.8 ± 1.5 Ma, which is the same like that of the sill. Moreover, results of thermal modelling indicate a maximum period of 4 months between sill emplacement and the time when both the quartzmonzodiorite and the sill had attained the ambient temperature. The modelling further explains, why Sederholm-type veins, described from several places around the world, are restricted to deep crustal levels.

Silica‐Rich Vein Formation in an Evolving Stress Field, Atlantis Bank Oceanic Core Complex

AbstractDrilling 809‐m Hole U1473A in the gabbro batholith at the Atlantis Bank Oceanic Core Complex (OCC) found two felsic vein generations: late magmatic fractionates, rich in deuteric water, hosted by oxide gabbros, and anatectic veins associated with dike intrusion and introduction of seawater‐derived volatiles. Microtextures show a change from compressional to tensional stress during vein formation. Temperatures and oxidation state were obtained from amphibole‐plagioclase and oxide pairs in the adjacent gabbros. Type I veins generally have reverse shear‐sense, with restricted ΔFMQ, high Mt/Ilm ratios, and low‐amphibole Cl/F indicating deuteric fluids. They formed during percolation and fractionation of Fe‐Ti‐rich melts into the primary olivine gabbro. Type II veins are usually hosted by olivine gabbro, occur at dike contacts and the margins of normal‐sense shear zones. They are undeformed or weakly deformed, with highly variable ΔFMQ, low Mt/Ilm ratios, and high‐amphibole Cl/F, indicating seawater‐derived fluids. The detachment fault on which the gabbro massif was emplaced rooted near the base of the dike‐gabbro transition beneath the rift valley. The ingress of seawater volatiles began at &gt;800°C and penetrated at least ~590 m into the lower crust during extensional faulting in the rift valley and adjacent rift mountains. The sequence of the felsic vein formation likely reflects asymmetric diapiric flow, with a reversal of the stress regime, and a transition from juvenile to seawater‐derived volatiles. This, in turn, is consistent with fault capture leading to the large asymmetries in spreading rates during OCC formations and heat flow beneath the rift mountains.

Subduction-related hybridization of the lithospheric mantle revealed by trace element and Sr-Nd-Pb isotopic data in composite xenoliths from Tallante (Betic Cordillera, Spain)

Ultramafic xenoliths are rarely found at convergent plate margins. A notable exception is in the Betic Cordillera of southern Spain, where the eruption of xenolith-bearing alkaline basalts during the Pliocene post-dated the Cenozoic phase of plate convergence and subduction-related magmatism. Mantle xenoliths of the monogenetic volcano of Tallante display extreme compositional heterogeneities, plausibly related to multiple tectono-magmatic episodes that affected the area. This study focuses on two peculiar composite mantle xenolith samples from Tallante, where mantle peridotite is crosscut by felsic veins of different size and mineralogy, including quartz, orthopyroxene, and plagioclase. The veins are separated from the peridotite matrix by an orthopyroxene-rich reaction zone, indicating that the causative agents were alkali-rich hydrous silica-oversaturated melts, which were likely related to recycling of subducted continental crust components. The present study reports new and detailed major and trace elements and Sr-Nd-Pb analyses of the minerals in the composite Tallante xenoliths that confirm the continental crust derivation of the metasomatic melts, and clarifies the mode in which subduction-related components are transferred to the mantle wedge in orogenic areas. The particular REE patterns of the studied minerals, as well as the variation of the isotopic ratios between the different zones of the composite xenoliths, reveal a complex metasomatic process. The distribution of the different elements, and their isotope ratios, in the studied xenoliths are controlled by the mineral phases stabilised by the interaction between the percolating melts and the peridotitic country rock. The persistence of marked isotopic heterogeneities and the lack of re-equilibration suggest that metasomatism of the sub-continental lithospheric mantle occurred shortly before the xenolith exhumation. In this scenario, the studied xenoliths and the metasomatic processes that affected them may be representative of the mantle sources of mafic potassic to ultrapotassic magmas occurring in post-collisional tectonic settings.

Tectono-magmatic Interplay and Related Metasomatism in Gabbros of the Chenaillet Ophiolite (Western Alps)

Abstract The Jurassic Chenaillet ophiolite in the Western Alps consists of a gabbro–mantle association exhumed to the seafloor through detachment faulting and partly covered by basaltic lavas. One of the Chenaillet gabbroic bodies includes mylonites that are transected by a network of felsic veins, thereby testifying to the interplay of ductile shearing and magma emplacement. The deformed gabbros preserve clinopyroxene porphyroclasts of primary magmatic origin, which are typically mantled by amphibole (titanian edenite) and minor secondary clinopyroxene. Titanian edenite and secondary clinopyroxene also occur as fine-grained syn-kinematic phases locally associated with fine-grained plagioclase. The felsic veins are made up of anorthite-poor plagioclase and minor titanian edenite. Geothermometric investigations document that the ductile gabbro deformation and the crystallization of the felsic veins occurred at 765 ± 50 °C and 800 ± 55 °C, respectively. With respect to undeformed counterparts, the deformed gabbros are variably enriched in SiO2 and variably depleted in Mg/(Mg + Fetot2+) and Ca/(Ca + Na). In addition, the deformed gabbros show relatively high concentrations of incompatible trace elements such as rare earth elements (REE), Y, Zr and Nb. The felsic veins are characterized by low Mg/(Mg + Fetot2+) and Ca/(Ca + Na), high SiO2 and high concentrations of incompatible trace elements. Relict clinopyroxene porphyroclasts from the deformed gabbros display a rather primitive, mid-ocean ridge-type geochemical signature, which contrasts with the trace element fingerprint of titanian edenite from both the deformed gabbros and the felsic veins. For instance, titanian edenite typically has relatively high REE abundances, with chondrite-normalized REE patterns characterized by a pronounced negative Eu anomaly. A similar trace element signature is shown by secondary clinopyroxene from the deformed gabbros. Amphibole from both the deformed gabbros and the felsic veins displays high F/Cl values. We show that the SiO2-rich hydrous melts feeding the felsic veins were involved in the high-temperature gabbro deformation and that melt–gabbro reactions led to major and trace element metasomatism of the deforming gabbros.

Silurian anorogenic basic and acidic magmatism in Northwest Turkey: Implications for the opening of the Paleo-Tethys

The Sakarya Zone (northern Turkey) is a Gondwana-derived continental block accreted to northern Laurussia during the Carboniferous, and is regarded as the eastward extension of Armorica. Timing of its detachment from the northern margin of Gondwana, thus opening of the Paleo-Tethys, is poorly known. Here, we report on metagranite and amphibolite with Silurian igneous crystallization ages from the Early Carboniferous high-temperature/middle to low-pressure amphibolite-facies metamorphic rocks of the Sarıcakaya Massif within the Sakarya Zone (NW Turkey). The metagranite-amphibolite complex is exposed mainly along the southern margin of the Sarıcakaya Massif over an area of ca. 12 km by 1.5 km. The metagranite contains preserved domains of porphyric texture, indicative of derivation from a former granite porphyry. The amphibolite is devoid of any relict igneous texture. Both the metagranite and amphibolite are crosscut by late up to 50 cm thick felsic veins. Uranium-Pb dating on igneous zircons from both metagranite and amphibolite yielded Silurian ages of ca. 419 ± 6 to 434 ± 7 Ma (2σ), and on those from a felsic vein an age of 319 ± 5 Ma (2σ) (Late Carboniferous). Geochemically, amphibolite displays anorogenic transitional tholeiitic to alkaline signatures. Initial εHf values of the igneous zircons from both metagranite and amphibolite show a large variation with medial values of −16 to −9 and + 3 to +6, respectively. Thus, the protoliths of amphibolite were derived from melts of depleted mantle, and those of the metagranite, on the other hand, from melts of reworked crustal material. We suggest that the Silurian anorogenic magmatism is related to a rifting event at the northern margin of Gondwana leading to the detachment of the Sakarya Zone and hence placing an age on the initial opening of the Paleo-Tethys. This interpretation is based on (i) the presence of Late Silurian to Devonian deep-sea sedimentary blocks in the Paleo-Tethyan accretionary complexes, and (ii) the resemblance of the UPb age spectra of detrital zircons in the metaclastic sequence of the Sarıcakaya Massif to those of Cambro-Ordovician sandstones in Jordan (Gondwana), and (iii) the local occurrence of anorogenic A-type granites of Late Ordovician-Silurian age in the Anatolide-Tauride Block, a continental block which rifted from Gondwana during the Early Triassic. Wholly anorogenic nature of the Late Ordovician to Silurian igneous rocks in the Sarıcakaya Massif and reported in literature does not support the opening of the Paleo-Tethys as back-arc ocean, as suggested in most paleogeographic reconstructions.