Introduction Ophiolitic complexes—fragments of oceanic lithosphere tectonically emplaced onto continental margins—serve as crucial archives for reconstructing the formation and evolution of ancient ocean basins. The Sabzevar ophiolite in northeastern Iran represents one of the most significant remnants of Mesozoic Neotethyan oceanic lithosphere and provides a key window into the tectonic development of the northern Neotethys branch. Previous studies have assigned this ophiolite to diverse tectonic settings, including mid‑ocean ridge, supra‑subduction zone, and back‑arc environments (Khalatbari Jafari et al., 2013a, b; Rezaei et al., 2018; Jafari and Ghasemi, 2023). Nevertheless, the co‑occurrence of contrasting magmatic signatures within its volcanic sequences points to a polyphase magmatic history in a complex geodynamic framework (Omrani et al., 2018; Moghadam et al., 2025). This study investigates the pillow lavas and sheeted dikes exposed in the Sultanabad area, in the eastern sector of the Sabzevar ophiolite. Its primary aims are to characterize the geochemistry, constrain the crystallization ages of the volcanic units via zircon U–Pb geochronology, and assess their tectonomagmatic significance for the evolution of the Sabzevar oceanic basin. Geological Setting The Sabzevar ophiolite, situated in northeastern Iran, constitutes an integral component of the tectonic structure of the Central Iranian microcontinent (Alavi, 1994). It is widely interpreted as a relict of the northern Neotethyan oceanic realm, which formed within the Central Iranian terrane between Central Iran and the Alborz belt during the Mesozoic (Agard et al., 2005). The ophiolitic succession comprises ultramafic mantle rocks, layered ultramafic–mafic cumulates, sheeted dike complexes, pillow basalts, and pelagic sedimentary cover (e.g., Shojaat et al., 2003; Khalatbari Jafari et al., 2013a, b; Moghadam et al., 2025). The Sultanabad area hosts well‑preserved volcanic units, dominated by pillow basalts intercalated with sheeted dikes and minor massive lava flows. These rocks represent the uppermost levels of the oceanic crust. Field relations indicate that the pillow lavas erupted in a submarine setting and were subsequently affected by tectonic deformation during the Neotethyan closure and obduction of the ophiolitic complex onto the continental margin. Materials and Methods Representative samples of pillow basalts and sheeted dikes were collected from the Sultanabad area. Petrographic examination was performed using optical microscopy to characterize mineral assemblages and textural features. Whole‑rock major and trace element concentrations were determined via X‑ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP‑MS). Zircon grains were extracted from selected samples and subjected to U–Pb isotopic dating and trace element analysis using laser ablation‑inductively coupled plasma mass spectrometry (LA‑ICP‑MS). The geochemical data served to classify magma types, constrain magma sources, and discern tectonic affinities. Zircon trace element compositions were additionally examined to offer complementary constraints on magmatic processes and crystallization conditions. Results Petrographic observations reveal that the studied volcanic rocks consist predominantly of plagioclase, clinopyroxene, and secondary alteration minerals. The pillow lavas typically exhibit porphyritic to intersertal textures, whereas the sheeted dikes show fine‑grained to subophitic textures. Whole‑rock geochemical data indicate that the studied samples fall into three main magmatic series: alkaline, calc‑alkaline, and tholeiitic varieties. The alkaline basalts are enriched in incompatible elements and display pronounced LREE enrichment relative to HREE. Their trace element patterns are akin to those of ocean island basalts (OIB). The tholeiitic basalts and sheeted dikes exhibit relatively flat REE patterns and are marked by negative Nb and Ta anomalies, characteristic of magmas generated in supra‑subduction zone settings. Calc‑alkaline basalts show intermediate geochemical signatures between these two groups. Zircon U–Pb geochronology reveals that magmatic activity in the study area took place during the Cretaceous, yielding ages between ~113 and 90 Ma. The alkaline basalts record the oldest ages (~112–110 Ma), while the tholeiitic and calc‑alkaline rocks give slightly younger but overlapping ages of ~113–91 Ma. Trace element compositions of zircon grains show HREE enrichment and LREE depletion, consistent with a magmatic origin. Variations in elemental ratios such as Th/U and Eu/Eu* point to differences in magma composition and crystallization conditions across the studied rock types. Discussion The geochemical signatures of the studied rocks point to the involvement of multiple magma sources in generating the Sultanabad volcanic sequence. The alkaline basalts most likely originated from low‑degree partial melting of a garnet‑bearing enriched mantle source, consistent with an intraplate or seamount‑related affinity. By contrast, the tholeiitic basalts and sheeted dikes appear to have been derived from higher‑degree partial melting of a depleted mantle source metasomatized by slab‑derived fluids. The calc‑alkaline basalts exhibit geochemical features characteristic of subduction‑related magmatism and probably represent melts generated in a mantle wedge modified by slab‑derived fluids. The co‑occurrence of alkaline and supra‑subduction zone magmas within the same volcanic sequence implies that the Sabzevar oceanic basin underwent a complex tectonomagmatic history, encompassing both intraplate and subduction‑related processes. The zircon U–Pb ages obtained in this study show that magmatic activity persisted over a protracted interval during the Middle to Late Cretaceous. This prolonged magmatism likely reflects the progressive evolution of a supra‑subduction zone system linked to the initiation and advancement of subduction within the northern Neotethys. Conclusions Combined whole‑rock geochemistry and zircon U–Pb geochronology provide new constraints on the petrogenesis and tectonic evolution of the volcanic units in the eastern Sabzevar ophiolite. The investigated rocks fall into alkaline, calc‑alkaline, and tholeiitic suites, reflecting the contribution of multiple mantle sources and magmatic pathways. Zircon U–Pb dating restricts magmatism to ca. 113–90 Ma, i.e., the Middle–Late Cretaceous. Geochemical fingerprints suggest that the tholeiitic basalts and sheeted dikes formed in a supra‑subduction zone setting, whereas the alkaline basalts represent within‑plate‑related magmatism subsequently incorporated into the ophiolitic succession. Collectively, the results favor a model whereby the Sabzevar ophiolite developed within a complex supra‑subduction zone system during the evolution and final closure of the northern Neotethys Ocean.

The & Aacute;lamo Complex, part of the Galician-Castilian Lineament within the Central Iberian Zone, lies between the Ollo de Sapo Domain and the Schist-Greywacke Complex. It comprises six tectonometamorphic sectors dominated by psammitic-pelitic metasediments (MTS), gneisses, migmatites, leucogranites and tourmaline-rich rocks. Zircon U-Pb dating identifies three Ediacaran partial melting events (similar to 628, 584 and 549 Ma) that occurred under high-pressure conditions within the kyanite stability field. These contrast with a low-pressure Variscan partial melting episode (similar to 310-315 Ma). Orthogneisses and leucogranites dated at similar to 482-465 Ma record Cambro-Ordovician magmatism, characterized by abundant inherited Ediacaran zircon cores, indicating significant crustal recycling. Petrographic and geochemical similarities, together with shared zircon inheritance patterns, link the & Aacute;lamo Complex with the Ollo de Sapo Domain and other segments of the Galician-Castilian Lineament, suggesting a common magmatic evolution. Tourmaline-rich rocks likely formed by boron metasomatism initiated during the Ediacaran and enhanced by recurrent partial melting. Variscan magmatism is represented by intrusive mafic and granitic bodies (similar to 307-311 Ma) and tourmaline-bearing leucogranites, reflecting continued reworking of Ediacaran crust into the Late Palaeozoic. These results shed light on the crustal evolution of Central Iberia.

Zircon studies in mafic and ultramafic rocks are particularly valuable because they can reveal deep-mantle petrogenetic and geodynamic processes. These studies, however, are hindered by the scarcity and uneven distribution of zircon in the mantle and mantle-derived rocks. Consequently, finding zircon in these rocks is inherently difficult and strongly influenced by the zircon abundance, grain size, volume, and the number of samples searched. Here, we describe a method for in-situ zircon identification and extraction for further analytical studies. To this end, we cut four representative 2 × 3 cm slabs, one from the granite and three from the gabbro, and carefully polished one side to enable automated large-area elemental mapping with SEM, EPMA, and µ-XRF using WDS and or EDS. Of all these, µ-XRF offers the best balance among acquisition time, surface coverage, spatial resolution and result quality. To overcome spectral interferences affecting the Zr signal, the method uses multidimensional analysis based on suitable ratios of the X-ray lines produced by the sample's major and, eventually, minor elements. Zircon-bearing pixels are then discriminated utilising a combination of global, local, and probabilistic statistical classifiers, such that zircon identification is accepted only where consistent statistical behaviour is reproduced across independent methods. Overall, the protocol enables reproducible detection and extraction of zircon down to 50 µm in grain size while minimising both false positives and false negatives. The same methodology can be used to locate other Zr-bearing (e.g., baddeleyite, zirconolite, srilankite) and Zr-lacking (e.g., apatite, xenotime monazite, rutile, chromite) accessory minerals that may be present in the rock sample.

The Iberian Variscan orogen is notorious for its important granitic magmatism. New field data, petrography, U-Pb zircon geochronology (sensitive high-resolution ion microprobe and chemical abrasion−isotope dilution−thermal ionization mass spectrometry), mineral and whole-rock geochemistry, and Sr-Nd isotope geochemistry were used to define two magmatic pulses in Northwest Iberia (Europe). The first magmatic event (324−315 Ma), caused by radiogenic heat production in a thickened crust, produced syntectonic G1 granitoids formed by I- ± S-type granites−granodiorites containing mafic enclaves. Lower crust metaigneous ± metasedimentary ± upper-mantle sources are inferred for these magmas. Simultaneously, important volumes of syntectonic S-type granites (G2) were produced. Neoproterozoic and early Paleozoic metasedimentary rocks and Cambrian−Ordovician orthogneisses were the likely protoliths. This initial melting event reduced the fertility of the middle−lower crust and conditioned the subsequent magmatic pulse (294−287 Ma) that produced post-tectonic G3 and G4 granitoids in a thinner crust. Lithospheric delamination and asthenospheric upwelling are the proposed heat sources in this case. G3 includes S-type granites derived from sources that preserved fertile components. Simultaneously, a significant volume of G4, including mainly I-type granitoids, was produced by the melting of residual and unmelted sources left behind after the first melting event. These sources comprised restitic metasedimentary rocks and juvenile mafic, intermediate metaigneous rocks. Small mafic enclaves attest to the minor involvement of mantle-derived melts. This model of Variscan magmatism shows that most S-type granites initially occurred due to the presence of fertile lithologies. The depletion of these fertile protoliths conditioned the subsequent melting event, resulting in the formation of a majority of I-type granites.

Subsolidus muscovite dehydroxylation represents a significant internal fluid source in felsic rocks. To investigate its effects, non-equilibrium differential-heating experiments were conducted at 1 bar and 750–1000 °C, together with piston–cylinder experiments at 800 °C and 500 MPa, using a two-mica orthogneiss and a biotite orthogneiss. The two lithologies show markedly different behaviour. In the biotite orthogneiss, only limited transformations occur. In contrast, muscovite dehydroxylation in the two-mica orthogneiss generates internally derived H2O fluids capable of reactivating mineral-replacement reactions without external fluid addition. Structurally bound OH released during dehydroxylation destabilises adjacent biotite, producing fine-grained orthopyroxene + spinel aggregates accompanied by the loss of H2O, K, Ti and F. The generated fluids also promote titanite growth and pervasive alkali metasomatism. Plagioclase is replaced by transitional Or–Ab feldspar, porous albite, and newly formed K-feldspar, producing textures comparable to those in natural hydrothermal and lower-crustal metasomatic systems. These microstructures indicate efficient fluid-mediated mass transfer even at extremely low fluid abundances (<1 wt%).The contrasting responses of the two orthogneisses highlight the role of muscovite in controlling fluid availability, alkali mobility, and subsolidus reaction pathways. A major consequence is the transfer of the K-feldspar component hosted in biotite into newly formed K-feldspar, shifting the bulk composition towards granite and potentially enhancing fertility during subsequent water-fluxed melting. These experiments demonstrate that mica-derived fluids can act as effective metasomatic agents during thermal shock events, linking dehydration reactions, feldspar re-equilibration, and the preconditioning of felsic rocks for anatexis.

The disturbance of the U-Pb isotopic system in zircon (ZrSiO4) is an important factor for U-Th-Pb geochronology, therefore understanding zircon nanostructure is crucial for reliable age determination. To understand the process of Pb mobilization in zircon, heating experiments on zircon grains from two samples of the Central Iberian Zone (CIZ), a Variscan tonalite and a Cambro-Ordovician orthogneiss, were performed. Samples were heated at 1400 degrees C for 30, 90 and 180 days in a horizontal furnace in N2 at 1 atm. Following the experiment, nano- and microstructural analyses were performed using Transmission Electron Microscopy (TEM). Mobilization of Pb was documented only in zircons from orthogneiss - these grains contain Pb nanospheres. Zircon grains from orthogneiss showed a significant influence of heating during the experiment on the microstructure. We observed Pb nanospheres (Pb0) and Pb nanoinclusions containing Pb in different oxidation states, namely Pb2+ and Pb4+. This is the first paper documenting nanospheres of metallic Pb existing together with Pb oxides (PbO, Pb2O3, Pb3O4 and PbO2) in one sample. As a result of heating, Pb was oxidized and after 6 months of heating, the spheres of metallic Pb were no longer found. The formation of metallic Pb nanospheres is explained by an annealing process at elevated temperature, that caused the concentration of Pb in noncrystalline, metamict domains of zircon. This study indicates that more than one mechanism can be responsible for the formation of nanospheres. In this case, chemical elements present in zircon as inclusions react with Pb causing its oxidation.

Biotite plays an important role in the geochemical cycle of Li, Rb, Cs, and Ba in the upper continental crust, as it is a significant carrier of Li and large-ion lithophile elements in felsic igneous rocks and high-grade detrital metasedimentary rocks. During its interaction with meteoric and hydrothermal fluids, biotite can be transformed into various types of clay minerals (mostly, interlayer-deficient biotite, vermiculites and smectites). These transformations can cause fractionation of the alkaline trace-element ratios Rb/Li, Cs/Li and Rb/Cs between biotite and its replacement products. This study examines the mineral transformations that occur when biotite interacts with aqueous and saline fluids and the poorly understood geochemical behaviour of the resulting phyllosilicates. For this purpose, we performed batch hydrothermal experiments of the interaction of biotite + quartz + graphite with ultrapure H2O, and 2 M NaCl, 2 M CaCl2 and 1 M NaF brine fluids at 170 degrees C and 10 bar using Teflon bombs, and at 550 degrees C and 800 to 1400 bar using autoclave apparatus. At lower-T conditions, biotite was replaced by 2:1 trioctahedral clay minerals (interlayer-deficient biotite, smectite, vermiculite, and other phyllosilicate species with higher interlayer charge) and Fe oxy-hydroxide minerals by coupled dissolution- precipitation mechanisms. At higher-T conditions, these mechanisms caused the transformation of biotite into the mineral assemblages (quartz + graphite): diopside + anorthite + titanite (CaCl2 brine experiments), albite + ilmenite + clay minerals (NaCl brine experiments), and cryolite + alkali feldspar with albite rimmed by Kfeldspar + Fe-oxides (NaF brine experiments). Therefore, a significant reduction of the clay mineral stability in the presence of NaF and CaCl2 brine fluids is inferred. The biotite replacements by phyllosilicates were mostly controlled by the ion exchange of K+ by H+ (or its hydrate state H3O+), hydrated Na+ and Ca2+, and NaF in the interlayer site. Conservation of the total mass and the Si, Al and Mg abundances occurred in most experimental phyllosilicates. However, in the products of the low-T NaF brine + graphite experiments, the total mass may have a gain of 5.3-11 % assuming Mg conservation. Sc, V, Nb and Ta abundances were also conserved, but a significant fractionation of the Rb/Li, Cs/Li, and Ba/Li ratios occurred in the experimental phyllosilicates. The experiments predict the generation of highly fractionated Rb/Li and Cs/Li phyllosilicates by replacement of biotite during interaction with aqueous fluids and, mostly, NaCl and NaF brine fluids at high-T and low-T conditions, respectively. This demonstrates a key role of biotite in the fractionation of Rb/Li, Cs/Li and Rb/Cs during the hydrothermal alteration of felsic igneous rocks. Conversely, a reversal in the mobility of Li with respect to Rb and Cs occurred in the phyllosilicate products when biotite interacted with NaCl or CaCl2 brine fluids at relatively low-T conditions. These experimental results highlight the key role of biotite-fluid interaction processes in controlling the budget of alkaline trace elements in the continental crust.

It is now widely known that the fluids play a major role in the formation and evolution of the Earth´s crust. Fluids and melts formed by dehydration-melting reaction of muscovite, biotite and amphibole during prograde metamorphism can have a profound effect on the trace element and isotope composition on the crustal sources of magmas.However, while the phase relations of these reactions as well as the textural and mineralogical evidence of fluid-rock interaction in high-grade metamorphic complexes are well studied, the mechanisms of the onset of fluid and melt generation by the dehydration melting along with the transport and geochemical impact of these mobile phases onto anhydrous minerals at the grain scale remain unclear.We investigated the mechanisms of local reactions involved in the incipient dehydration/melting processes at different temperatures, the sequence of reactions and the interaction of the fluid and/or melt with the anhydrous mineral phases, by conducting analogue heating experiments of rock cylinders in a vertical furnace. The rock samples used in the experiments were granitoids and gneisses from the Iberian Massif (Spain) with variable content of biotite and muscovite. They were cut into cylinders with dimensions of about 3x20 cm and placed into a vertical furnace. The experiments were done under thermal gradient at sub- and supersolidus conditions (600-1200oC) at ambient pressure in an inert atmosphere of N2 and last up to 8 days.First results show that significant compositional and textural changes in biotite and muscovite were produced in experimental runs at temperatures >850oC. Muscovite experienced dehydration melting breakdown to ultrabasic, very peraluminous melts with higher Na and lower K than the starting muscovite, small grains of aluminosilicates and large vesicles. Biotite underwent subsolidus dehydration, resulting in the formation of spinel and/or Fe-Ti oxides and alkali-rich aqueous fluid. Notably, K-feldspar did not nucleate at the dehydration site; instead, excess K and other incompatible elements (Li, Rb, Cs, Ba) were transported by fluids released from biotite and muscovite. These fluids subsequently induced metasomatic reactions in plagioclase, transforming it into K-feldspar. Additionally, Ca released from plagioclase contributed to the formation of titanite after ilmenite. The metasomatic changes were facilitated by fluid migration along micropores that were present in the starting plagioclase, highlighting the intricate processes involved in mica driven metamorphism and metasomatism. A second type of melt was generated with increasing temperature, characterized by higher silica and more granitic-like compositions, suggesting the involvement of quartz and feldspars in the melting reactions.Although the experimental pressure conditions are much lower than those inside the crust, these analogous experiments allow us to investigate the mechanism by which fluids and melts are segregated from the reaction sites and the influence of rock texture. In these analogue experiments the breakdown of hydrous minerals is enhanced because they are outside their P-T stability fields. Besides, the formation of gas is maximized since its solubility in the melt is very low. Therefore, it allows us to investigate the importance of vesiculation in the creation of pathways for fluid and melt migration.

Detailed textural and compositional study of calc-alkaline lamprophyres and minettes from Zeneta, SE Spain Neogene Volcanic Province (NVP) are used to unravel the magma sources and differentiation processes involved in their formation. The presence of xenocrysts of various origins indicates a hybrid nature involving mantle-derived alkaline lamproitic and continental crust-derived granitic parental magmas. A new U-Th-Pb zircon age of Zeneta minettes allows contextualizing their generation in time and space during regional lamproite magma intrusion and crustal anatexis in the NVP. Interaction and mixing of these compositionally contrasted magmas resulted in the formation of hybrid calc-alkaline lamprophyre minette. This process is reflected by the phases that crystallized from the hybrid magma, particularly phlogopite, and by the whole-rock composition of the less fractionated rocks. The calculated contribution of lamproitic and crustal-derived magma end-members in minette formation are 30–40% and 60–70%, respectively. Mixing of end-member magmas of contrasting rheological properties was possible only in a calculated thermal-window of 1025–1125°C, after cooling of intruding lamproitic magma and heating of host granitic anatectic region. The calculated thermal window fits with the estimated rheological properties of Zeneta minettes and the crystallization temperature of early minette-derived phlogopite. Furthermore, fractional crystallization was identified for the first time in the Zeneta minettes, based on the observed whole-rock line of descent from the less evolved mixed magmas through intermediate to felsic minettes and associated textural and compositional features of late crystallized phases. Polytope Vector Analysis allowed an integrated quantitative characterization of magma differentiation, including magma mixing and subsequent fractional crystallization. The identification of mixing and fractionation processes in calc-alkaline minettes genesis, which otherwise does not show spatial and temporal association in the field with granitic bodies, opens a new scenario to be considered in understanding the petrogenesis of these rocks.

Underplated mafic intrusions ponded at the base of the lower continental crust in extensional settings can experience ultra-high-temperature (UHT) granulite-facies metamorphism during tens of My due to slow cooling rates. These intrusions are also the source of heat and carbonic fluids for regional high-temperature (HT) granulite-facies metamorphism in the continental crust. This work analyses the fluid-melt-rock interaction processes that occurred during the magmatic to HT-UHT-granulite- and amphibolite-facies metamorphic evolution of high-grade mafic rocks from the Eastern Ediacaran Adrar-Suttuf Metamafic Complex (EASMC) of the Oulad Dlim Massif (West African Craton Margin, Southern Morocco). P-T conditions were determined using Ti-in-amphibole thermometry, two-pyroxene and amphibole-plagioclase thermobarometry, and phase diagram calculations. The thermobarometric study reveals the presence of tectonically juxtaposed lower- and mid-crustal blocks in EASMC that experienced decompression-cooling paths from, respectively, UHT and HT granulite-facies conditions at ca. 1.2 +/- 0.28 GPa and 975 +/- 50 degrees C, and ca. 0.82 +/- 0.15 GPa and 894 +/- 50 degrees C, to amphibole-facies conditions at ca. 0.28 +/- 0.28 GPa and 787 +/- 45 degrees C (precision reported for the calibrations at 1 s level). An age for the magmatic to UHT granulite-facies metamorphic transition of 604 Ma was constrained from published SHRIMP Th-U-Pb zircon ages of the igneous protoliths. An amphibole Ar-40-Ar-39 cooling age of 499 +/- 8 Ma (precision at 2 s level) was obtained for the lower-crustal blocks. Amphibole Ar-40-Ar-39 closure temperatures of 520-555 degrees C were obtained for an age range of 604-499 Ma and an average constant cooling rate of 4.2 degrees C/My, suggesting that the lower-crustal blocks cooled down to the greenschist-amphibolite facies transition in ca. 100 My. During the high-temperature stage, interstitial hydrous melts assisted textural maturation of the rock matrix and caused incongruent dissolution melting of olivine and pyroxenes, and, probably, development of An-rich spikes at the grain rims of plagioclase, and local segregation of pargasite into veins. Subsequent infiltration of reactive hydrous metamorphic fluids along mineral grain boundaries during cooling down to amphibolite-facies conditions promoted mineral replacements by coupled dissolution-precipitation mechanisms and metasomatism. Ubiquitous dolomite grains, with, in some cases, evidence for significant textural maturation, appear in the granoblastic aggregates of the high-grade mafic rocks. However, calculated phase relationships reveal that dolomite could not coexist with H2O-CO2 fluids at HT-UHT granulite- and low-medium P amphibolite-facies conditions. Therefore, it is proposed that it may have been generated from another CO2-bearing phase, such as an immiscible carbonatitic melt exsolved from the parental mafic magma, and preserved during cooling due to the prevalence of fluid-absent conditions in the granoblastic matrix containing dolomite. The lower-crustal mafic intrusions from EASMC can represent an example of a source of heat for granulitisation of the mid crust, but a sink for carbon due to the apparent stability of dolomite under fluid-absent conditions.

The distribution of Mg isotopes in minerals is becoming increasingly relevant in Earth science. Usually, they are determined by dissolving mineral concentrates and, after purifying Mg with ion exchange resins, analysing the resulting solutions by TIMS or, most often, MC‐ICP‐MS. When applied to individual minerals, these methods are slow and prone to contamination from impurities in the concentrates, inconveniences that may be avoided using spot analysis techniques such as LA‐MC‐ICP‐MS or SIMS, albeit at the price of a large instrumental mass fractionation (IMF) and isobaric interferences, most prominent in the former. Here, we studied the potential of the multi‐collector SHRIMP II ion microprobe for measuring Mg isotopes in Fe‐Mg silicates and oxides. We found that, when corrected for the divergence of the Mg ion paths within the sample chamber caused by the Earth's magnetic field, the SHRIMP's IMF overwhelmingly depends on the mineral species, and the effects of variable chemical composition are negligible. We propose that the IMF is caused by the force constant difference , ∆F, between "hard" and "soft" bonds linking the ions of the studied element to the mineral lattice. Given that ∆F is a constant for each mineral species, we calculated IMF‐correction factors for the most common Mg‐bearing minerals. The thus‐calculated correction factors permit the analysis in the same session, and with reasonable accuracy (within ~ 0.3‰ of the δ 26 Mg determined by SN‐MC‐ICP‐MS analyses of concentrates), of samples from different mineral species, facilitating the application of Mg isotopes to terrestrial studies.

Inherited zircons found in mantle-derived rocks are increasingly relevant for characterizing the history and petrogenetic processes of these rocks. However, the occurrence of inherited zircons in mafic igneous rocks poses an enigma because, as shown by numerous experiments combined with modelling studies, zircon, e.g., 150 μm size, is expected to dissolve extremely swiftly, e.g., c. 1 day, in the presence of mafic melts. Slow dissolution kinetics may explain zircon survival in fast ascending magmas that scavenged zircons near their final emplacement, but it cannot explain the persistence of inherited zircons that dwelled in the magma-bearing mantle for more extended periods. To understand how these zircons can survive over long times in contact with mafic melts, we performed a series of experiments, including some novel nano-scale experiments, to explore the behaviour of zircons in contact with different volumes of melt. Our results demonstrate that if melt occurs in narrow, spatially restricted domains around zircon grains, they get rapidly saturated with Zr. From this point onwards, the melt acts as a protective layer preventing further dissolution. These results carry important implications for the mode of melt transfer in mantle segments from which inherited zircons have been found: zircon crystals in partially molten mantle systems can only survive long in mush-like zones in which the melt migrates through porous flow, the wetting behaviour of basaltic melt on zircon would appear to further facilitate such survival. Zircon crystals would be rapidly consumed in high melt fraction regions of channelised (e.g., dike) flow. Such regions are therefore constrained to be short-lived (weeks to months) and pulsed in nature. Thus, the presence of zircons in mantle-derived rocks helps to place constraints on modes and timescales of melt transfer in the respective mantle segments.

This study presents new U-Pb zircon ages for the Neoproterozoic granitoids of the Bou Azzer inlier at the central part of the Anti-Atlas chain (Morocco), in the northwestern edge of the West African Craton (WAC). Analyzes of zircon grains yield the following ages: 658 +/- 3 Ma for Ait Ahmane pluton, 647 +/- 5 Ma for Bou-Izbane intrusion, 656 +/- 3 Ma for Bou Azzer quartz-diorites, 583 +/- 12 Ma in the Oustrate intrusion and 644 +/- 4 Ma for the Ait Ahmane leucogranite. These ages confirm a major magmatic event between 640 and 660 Ma contemporaneous with the obduction of oceanic crust marking the formation of the ophiolites of Bou Azzer and Sirwa. The emplacement of granitoids would have led to a decrease in the density of the oceanic crust and triggered the process of obduction. The granitoids display the geochemical features of volcanic arc magmas and have positive initial epsilon Nd (+4.67 to +6.57) and low initial 87Sr/86Sr (0.7002-0.7055). The quartz-diorite group displays young Depleted Mantle Model (TDM) ages that range from 904 to 1065 Ma. The leucogranites have younger TDM of 869-884 Ma. These new data attest to their juvenile nature and the absence of any crustal contribution. The NNW-SSE extended inlier band of Bou Azzer -Sirwa constitutes an exotic terrane of Neoproterozoic age and of a juvenile nature wedged between the SW and NE domains, respectively, formed by or surmounting a Meso-Paleoproterozoic or even Archean basement. The Bleida granodiorite, dated at 559 +/- 4, displays older U-Pb ages from inherited zircon cores at 1234, 1748 and 2085 Ma. The Bleida granodiorite was emplaced during a later magmatic episode that belongs to the Ouarzazate Group, generated during the post-collisional metacratonic evolution of the northern edge of the West African craton.

Nd model ages are valuable tools for studying the structure and evolution of the continental crust. However, their calculation is severely affected by the 147Sm/144Nd lack of constancy during crustal evolution because it results in spurious Nd model ages that may induce wrong geodynamic and geochemical interpretations. An extensive database of granite analyses shows that 147Sm/144Nd may vary significantly in the high-SiO2 range of granite rocks and that these variations are far more common in peraluminous than in metaluminous varieties. Only a tiny fraction of the latter has anomalously low ratios, likely resulting from apatite, perhaps titanite, fractionation. In contrast, many high-SiO2 peraluminous granites also show abnormally high, even superchondritic 147Sm/144Nd ratios that likely resulted from monazite fractionation. A Sm and Nd residence study in peraluminous sources revealed that Sm/Nd ratios are lower in monazite and the feldspars than in the bulk rock but higher in apatite and garnet. Monazite carries 90% of Nd and 80% of Sm, apatite carries about 3–5% of Nd and up to 16% of Sm, and garnet may concentrate up to 27% of Sm. Accordingly, the interplay among monazite, apatite and garnet determines the 147Sm/144Nd of resulting magmas, obscuring the role of the other phases. Garnet is expected to be primarily a residual or early crystallized phase, although some can be entrained in melts. Apatite may also behave as a residual phase in high-SiO2 metaluminous systems. However, it is highly soluble in peraluminous systems. Monazite may dissolve partially, remaining in the residua of peraluminous melts and crystallizing early from these, as seen in leucosome-melanosome pairs from the Ivrea-Verbano Zone, NW Italy, and Peña Negra, Central Iberia, and the upwards differentiated Pedrobernardo granite sill, also in Central Iberia. Garnet or apatite retention in residua, or segregated from a crystallizing magma, produces melts with low 147Sm/144Nd that may cause anomalously young Nd model ages, as exemplified by the Roccapietra granite of the Ivrea-Verbano zone, NW Italy. Monazite retention or crystallization, in contrast, produces high 147Sm/144Nd melts that yield anomalously old Nd model ages. The effects of intracrustal 147Sm/144Nd variations may be softened or eliminated by employing the two-stage Nd model age calculation method (DePaolo et al., 1991) with a Sm/Nd evolution model adapted to each specific crustal segment.

The Oulad Dlim Massif, adjacent to the Reguibat Shield in South Morocco was considered up to now as part of the Variscan belt (Mauritanides) with a polyphase geologic history and a complex geodynamic evolution implicating oceans closures and accretion of exotic terranes (Avalonian and Meguman) during the Variscan - Alleghanian orogeny. The use of modern technology to characterize the petrology, the geochemistry and the geochronology of the lithological units forming this region, combined to field surveys has led to an updated geological architecture and different geological history. The Oulad Dlim Massif is mainly a deformed Archean terrane, as recorded by its eastern and western sectors, dominated in its central part by a bimodal felsic-mafic magmatism forming the Ediacaran sector. The study of these magmatic complexes supports strongly the intracontinental origin of this bimodal magmatism vs. the oceanic origin published before in literature. The exploration of this massif conducted also to the identification of a Silurian - Devonian sector in the western part. Therefore, up to date, different magmatic events lasting from the Meso-Archean to the Cretaceous are recorded in the Oulad Dlim Massif rocks, among them different generations of granitoids are reported. New data on granitoids from the Ediacaran sector are presented in this paper. This recent data demonstrates that Oulad Dlim Massif has been affected by the main Ediacaran-Cambrian extensional event widely documented in other structural domains of Morocco and other parts of North Gondwana. Additionally, the study of the Silurian-Devonian sector rocks highlighted the presence of a Caledonian tectonic event challenging the ideas about the paleogeography of this part of northwestern Africa and its geological evolution during the Paleozoic. However, despite the significant contribution of this extensive survey and the abundance of data on the Oulad Dlim Massif, more studies are required to reconstruct the puzzle at plate tectonic scale.

Zircon is a ubiquitous accessory phase that carries important isotopic and geochemical information. Experimental work indicates that zircon solubility in silicate melts increases exponentially with decreasing silica; accordingly, crystallizing mafic magmas would only precipitate zircon from the last drops of residual melts, likely of granophyric composition. However, this view is inconsistent with the abundance of syn-magmatic zircons in many mafic rocks, in which zircon often occupies textural positions compatible with early crystallization. Given that factors other than temperature and magma composition negligibly affect the solubility of zircon, its precipitation from mafic magmas must involve the formation of small zircon-saturated transient zones. Here we explored that possibility using 2D finite elements to model the crystallization of MORB melts confined in pores. We found that zircon-saturated volumes may form locally at the growing mineral-melt interfaces if the growth rate of a low KdZr mineral (<0.2) is much faster than the diffusion rate of the rejected Zr4+ away into the melt, thus leading to the precipitation of zircon in low-Zr mafic magmas. Local saturation close to interfaces growing in confined pores also explains the wide range of crystallization temperatures displayed by zircons of mafic rocks, the variety of their inclusions and textures, and in the particular case of the Atlantic seafloor, why syn-magmatic zircons are more common in troctolites and olivine gabbros than in the more evolved clinopyroxene or amphibole-rich gabbros. This mechanism can also account for the formation of other accessory phases composed of elements scarcely partitioned in major minerals.

Zircon derived from crustal rocks can survive dissolution into hot basalts during magma hybridization and rock assimilation if it is shielded as an inclusion phase in early-formed phenocrysts or in minerals from non-disaggregated xenoliths. Under these conditions, zircon can be thermally shocked, triggering recrystallization of metamict domains and reaction with its hosted mineral inclusions. This work simulates this process by performing thermal annealing experiments on zircon grains with variable degrees of metamictization. These were embedded in cristobalite powder under a N2 atmosphere at 1 bar and 1300 °C. The thermal annealing produces recrystallization of metamict domains, melting of multi-phase mineral inclusions, nanopore formation, and microcrack propagation by thermo-elastic stress. The porosity enhances intracrystalline melt mobility, leaching out trace-element and mineral impurities. Baddeleyite was formed at temperatures below the thermal decomposition of pure zircon by two mechanisms: (i) recrystallization of metamict domains assisted by silica migration from the reaction site and (ii) incongruent zircon dissolution into molten mineral inclusions with a high CaO/SiO2 ratio. Highly metamict zircons with elevated common Pb and radiogenic Pb loss, which were impossible to date with SHRIMP, lost all their common Pb and some radiogenic Pb upon annealing, producing well-fitted discordias with a significant upper intercept age.

This paper synthesizes available and original U-Pb geochronological and hafnium and oxygen isotope data on zircon from gabbro and peridotites in the oceanic core complexes (OCC) of the Mid-Atlantic Ridge (MAR) extending for 2000 miles along its crest zone. We attempted to reproduce the evolution of MAR magmatism and to determine the geochemical and geodynamic nature of zircon protolith in OCC. We show that the relicts of old continental lithosphere have been preserved locally beneath the axial ridge zone and were involved in the partial melting of a shallow mantle during the entire magmatic evolution of the MAR rift valley. Age variations of zircon from plutonic rocks of the oceanic basement of fracture zones at some distance from the rift valley suggest young magmatism that differs in age from established magnetic anomalies. During the geological history of the Atlantic Ocean, the evolution of the melt originated in the rift valley of MAR, with zircon crystallization at the final stages, was influenced by aqueous (or aqueous–saline) fluid. Obtained conclusions confirm the fundamental significance of the interaction between hydrothermal and magmatic systems in the slow-spreading mid-ocean ridges.