Small Degrees Of Partial Melting Research Articles

Diachronous slab-driven Early Cretaceous crustal extension and pulsed magmatic response in southeastern China: Insight from A-type granite and related rocks

The Late Mesozoic magmatic belt in southeastern (SE) China, stretching over 1300 km, resulted from southwestward subduction of the paleo-Pacific plate. Our study focuses on the origin of Early Cretaceous syenite and A-type granite in SE China, utilizing zircon U-Pb and Hf isotopic analyses, bulk-rock compositions, and Nd-Hf isotopic compositions. Findings indicate that ∼ 142 Ma silica-unsaturated syenites in the southwest form through small degrees of partial melting of the metasomatized lithospheric mantle, while ∼ 142 Ma A-type granites originate from partial melting of juvenile felsic rocks. The ∼ 130 Ma A-type granites in the northeast result from mixing of ancient crustal-derived magma and mantle-derived magma. The southwest and northeast regions display distinct magmatic histories, with flare-ups and lulls occurring at different times. These variations suggest diverse extensional and compressional histories. A spatially migrating extensional regime is observed from the southwest (∼147 Ma) to the northeast (∼120 Ma) of SE China. Magmatic flare-ups in the southwest involve reworking juvenile crust, while in the northeast, they modify ancient crust, likely related to paleo-Pacific plate subduction and northeastward slab rollback. Slab subduction rate correlates strongly with observed geological events, indicating diachronous crustal extension, magmatic flare-ups, and lulls during ca. 147–110 Ma due to northeastward slab rollback and changing subduction rate.

Post-collisional ultrapotassic volcanic rocks and ultramafic xenoliths in the Eslamieh Peninsula, NW Iran: Petrological and geochemical constraints on mantle source and metasomatism

The Eslamieh Peninsula in NW Iran exposes Miocene potassic to ultrapotassic volcanic rocks that are often associated with a variety of ultramafic xenoliths. Based on geochemical features, the rocks investigated in this study can be subdivided into two main groups: 1) MgO-rich (8.0–13.9 wt%) basic (SiO2 = 46.1–50.9 wt%) rocks, which comprise mafic lamprophyres and pyroxenite xenoliths, and 2) MgO-poor (1.5–6.4 wt%) basic to intermediate (SiO2 = 48.2–61.2 wt%) rocks, mainly consisting of trachytes, felsic lamprophyres, and analcime-bearing tephriphonolites and phonotephrites. Rocks in both groups have variable but generally low Ni (10–115 ppm; mostly <60 ppm) and Cr (6–628 ppm; mostly <200 ppm), and display enrichment in LREE over HREE (e.g., La/Yb = 17–55), with a large overlap between the MgO-rich and MgO-poor types. These features are coupled with enriched LILE/HFSE ratios (e.g., Ba/Nb mostly 16–116), negative Nb-Ta-Ti anomalies and positive Pb peaks in primitive mantle-normalised diagrams (e.g., Nb/Nb* mostly <0.7). Both high-and poor-MgO samples have strongly radiogenic (87Sr/86Sr)i ratios (0.7078–0.7086), (143Nd/144Nd)i below the Chondritic Uniform Reservoir (CHUR) = 0.51235–0.51244, and ɛHfi ranging from −3.1 to −11.1‰, suggesting involvement of a lower continental crust component in their genesis. The major oxide and trace element contents the high-MgO and high-CaO rocks indicate derivation from a subcontinental lithospheric mantle source, variably metasomatised by fluids and melts released by a subducted slab in a mantle wedge. Primitive melts evolved via mafic mineral fractionation and possible assimilation of lower crust, generating the SiO2 higher and CaO-MgO-poorer group. We suggest that small degree partial melting of the enriched mantle sources could have been triggered by Neo-Tethys slab roll-back during the Arabian-Iranian collision.

Niobium, Critical Metal, and Progeny of the Mantle

Abstract Niobium is a critical metal in high demand because of technological advances and the supply risk created by the fact that over 90% of its production is by a single country (Brazil). In this paper, we review the geology of the deposits that are currently being mined and other potentially economic deposits as well as develop models for their genesis. With the exception of the Lovozero deposit (Russia), which is hosted by a layered silica-undersaturated alkaline igneous complex, all the deposits that are currently being mined for niobium are hosted by carbonatites, and most of the deposits with economic potential are also hosted by these rocks. Niobium owes its concentration in carbonatites and alkaline silicate rocks to its highly incompatible nature and the small degree of partial melting of the mantle required to generate the corresponding magmas. The primary control on the concentration of niobium to economic levels in alkaline silicate magmas is fractional crystallization, partly prior to but mainly after emplacement. In the case of silica-undersaturated magmas, the final residue saturates in minerals like eudialyte and loparite to form niobium-rich horizons in the layered complexes that crystallize from these magmas. The final residue, in the case of silica-saturated magmas, crystallizes the pegmatites that are the hosts to the economic niobium mineralization, which commonly takes the form of pyrochlore. In contrast, carbonatitic magmas undergo little to no fractional crystallization prior to emplacement. Moreover, fractional crystallization on emplacement has minimal impact on the concentration of niobium to economic levels. Instead, we propose that the metasomatic interaction of the carbonatitic magmas with their hosts to form rocks like phlogopitite (glimmerite) consumes much of the magma, leaving behind a phoscoritic residue from which pyrochlore crystallizes in amounts sufficient to form economic deposits. Although many niobium deposits display evidence of intense hydrothermal alteration, during which there can be major changes in the niobium mineralogy, the extremely low solubility of niobium in aqueous fluids at elevated temperature precludes significant mobilization and, thus, enrichment of the metal by hydrothermal fluids. However, weathering of carbonatite-hosted niobium deposits leads to supergene enrichment (due largely to the dissolution of the carbonate minerals) that can double the niobium grade and make subeconomic deposits economic. Pyrochlore is the principal niobium mineral in these laterite-hosted deposits, although its composition differs considerably from that in the primary mineralization. This paper evaluates the processes that appear to be responsible for the genesis of niobium ores and provides a framework that we hope will guide future in-depth studies of niobium deposits and lead to more effective strategies for their successful exploration and exploitation.

Age and origin of the West Kimberley lamproites, Western Australia

New high precision 40Ar/39Ar ages are presented which more tightly constrain the eruption ages of the West Kimberley lamproites of northwestern Western Australia. The data show a southward progression in ages of the lamproites over a distance of ∼135 km, from ∼22.5 Ma in the northernmost lamproites of the Ellendale Field to ∼19 Ma in the Noonkanbah Field in the south, excluding the Walgidee Hill lamproite. The large (∼490 ha) zoned Walgidee Hills lamproite at the northern edge of the Noonkanbah Field is the youngest of the West Kimberley lamproites and represents a later pulse of lamproite magmatism at 17.4 Ma, >1 Ma after the main lamproite magmatism.The southward progression of lamproite ages extends across a wide swath from the Proterozoic King Leopold Orogen at the southern margin of the Kimberley Craton to the Phanerozoic Fitzroy Trough in the Canning Basin. The magmatism does not appear to be the result of the migration of a mantle plume, but more likely reflects small degrees of partial melting of old, thick, metasomatised lithosphere extending from the craton margin beneath the basin, with melting triggered by regional uplift and/or small-scale mantle convection at the craton margin. Emplacement of the lamproites likely exploited ancient trans-lithospheric structures associated with the Proterozoic orogen at the craton margin and structures associated with the formation of the Canning Basin in the Phanerozoic.

Geochemistry and Petrogenesis of Shoshonitic Dyke Swarm in the Northeast of Meshkinshahr, NW Iran

The study area is located in Ardabil province in the northeast of Meshkinshahr city. More than 200 small and large Eocene-age dykes form outcrops in this area. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U–Pb zircon analyses yield a consistent age of 44.3 ± 1.8 Ma for the dyke swarms. These dykes include tephritic, andesitic and basaltic compositions, and show enrichment in LREEs (relative to HREEs) and are characterized by enrichment in LILEs and depletion in HFSEs. Petrological observations, along with major, rare earth and trace elements geochemistry, suggest that the dykes have a shoshonitic signature. All the rocks are highly enriched in incompatible trace elements and have variable Sr–Nd isotopes. Enrichment in incompatible elements and other geochemical features for the dyke swarm rocks suggest that a metasomatized subcontinental lithospheric mantle is the magma source. The negative Nb–Ta–Ti anomalies in the rocks are comparable with the features of subduction-related magmatism and contamination with ancient crustal components. The radiogenic 87Sr/86Sr isotopic values of the rocks imply the involvement of slab terrigenous sediments and/or a continental lithosphere. Isotopically, the volcanic rocks exhibit a binary trend, representing 1–5% mixing between the primary mantle and sediment melts. Our melting models suggest that there are residual garnet + spinel in the source, which are incompatible with the partial melting of amphibole- and/or phlogopite-bearing lherzolites. The geochronological, geochemical and isotopic data for the northeast Meshkinshahr dyke swarms suggest that these Late Eocene magmas were derived from a small degree of partial melting of a subduction-metasomatized lithospheric mantle source in a post-collisional setting.

Stable chromium isotopic variations in peridotite mantle xenoliths: Metasomatism versus partial melting

Chromium isotope compositions have been determined for 42 mantle-derived xenoliths from Cameroon, France, Mongolia, Russia and the USA. The samples are mainly spinel and garnet lherzolites, or harzburgites, and yield δ53CrNIST979 from −0.33 to 0.43‰, within the range of previously reported compositions, −0.51 to 0.74‰ (Xia et al., 2017). Partial melting has been proposed as a process that alters the δ53Cr of the mantle residue. New non-modal melting models show that the compatible nature of Cr is inconsistent with large changes in the residue’s isotopic composition. The fractionation during melting is constrained by calculated Δ53Crmantle-melt-values <0.09 and the measured Δ53/52Cr mantle-melt ≈ 0.07‰, which is too small to generate the observed range in δ53Cr at low to medium degrees of melting. Fertile to melt-depleted, unmetasomatised peridotite xenoliths show no correlations between δ53Cr and indicators of partial melting. Almost all LREE-enriched samples studied here define a correlation between δ53Cr and (La/Yb)N. This is consistent with alteration of Cr isotopic compositions by some process that is related to interactions with low degree partial melts. The low Cr content of small degree partial melts renders metasomatism via such liquids a difficult explanation for the isotopic effects found. A possible explanation is some form of disequilibrium loss of isotopically light Cr into such liquids during their percolation through the mantle. This is similar to a previously proposed mechanism of kinetic isotopic fractionation of Cr associated with interactions between silicate melts and peridotitic mantle (Xia et al., 2017).

Sr‐Enriched Glassy Picrites From the Karoo Large Igneous Province are Evolved, not Primitive Magmatic Rocks

AbstractMagmatic products of the Karoo Large Igneous Province can be divided into a volumetrically dominant, compositionally uniform low‐Ti tholeiitic suite, and a subordinate, geographically restricted, compositionally diverse, incompatible‐rich high‐Ti suite. High‐Ti picrites contain up to 2,400 ppm Sr, 1,900 ppm Ba, and 550 ppm Zr, which seems unusual for olivine‐enriched rocks. We studied six Karoo picrites to determine the phase(s) in which Sr resides. Samples consist of 10–30% olivine phenocrysts in a groundmass of brown glass, augite, feldspar, ilmenite, and apatite. Glass compositions vary, but are generally evolved, ranging from basaltic trachyandesite to dacite. X‐ray intensity maps demonstrate that most of the Sr resides in the glasses, and to a lesser extent, in feldspars, if present. Some samples contain two texturally and compositionally distinct glasses, best modeled in terms of magma hybridization: variably accumulated olivine phenocrysts surrounded by evolved Sr‐rich (to 9,470 ppm) Type 2 melts formed by extensive olivine fractionation were infiltrated by chemically distinct Type 1 melts. Upon eruption, Type 2 melt quenched to minor glass around olivine phenocrysts, and the dominant Type 1 glass acquired its evolved composition by quench crystallization of groundmass mineral phases. Both glasses are rich in H2O (up to 3.8 wt. %) but are nearly devoid of CO2. Calculated parental melts have much higher K2O and incompatible trace elements (e.g., Sr or Ba &gt;1,200 ppm) relative to low‐Ti tholeiites. The unusual parental melt compositions imply derivation by small degrees of partial melting from SCLM mantle sources enriched in Sr and other incompatibles.

Sodic-Silicic Magmatism in Tadpatri Formation- A Study in Quartz Keratophyre and Albitite

Abstract The mafic sills and unmetamorphosed shales and siltstones of Tadpatri Formation in Cuddapah Supergroup are associated with thin flows of quartz keratophyre and an occasional albitite dyke. The textural characteristics of these quartz keratophyre essentially establish them as igneous extrusive rocks. A detailed study of texture and chemistry, coupled with comparison of available records in geological literature, suggests that these rocks were derived through very small degree of partial melting of metasomatized mantle at pressures more than plagioclase stability field and less than jadeite stability field. High alkali content and associated volatiles led to significant decrease in viscosity and emplacement of these lavas in an extensional tectonic set-up.

Olivine and Cr-spinel as indicators of the petrogenesis and partial melting conditions of the high-MgO ultramafic volcanic rocks from NW Ad Dhala Province—Yemen

The high-MgO ultramafic volcanic rocks in the NW Ad Dhala province are classified as meimechite according to the IUGS classification scheme. This province represents the southeastern outcrops of the Yemen Volcanic Group (YVG), which constitutes part of the Afro-Arabian continental large igneous province (LIP) and located within the boundary of the Afar mantle plume. In this study, we present the chemical compositions of olivine and Cr-spinel in meimechite rocks from Bagah Village in NW Ad Dhala province, aiming to characterize the genesis and partial melting conditions and to estimate the crystallization temperatures of these high-MgO rocks. Olivine crystals are characterized by high forsterite, ranges from Mg-rich core (up to Fo89.69) to relatively Fe-rich rim (down to Fo78.57), high CaO, MgO and MnO whereas Cr-spinel crystals have high TiO2 and Cr# values ranging from 0.49 to 0.63 which indicate that they are crystallized from primary magma and are typical of volcanic olivine and Cr-spinel that formed in intraplate tectonic setting. Olivine and Cr-spinel compositional data and primary melt composition (MgO ~ 23 wt.%) are compatible with the derivation of studied meimechite rocks from peridotite mantle source by small degrees of partial melting under conditions of high temperature and pressure at great depths, mostly within the garnet stability field. Also, these data provide a compelling indicator for the important role of upwelling Afar mantle plume in the genesis of these high-MgO ultramafic volcanic rocks. Finally, based on the different olivine-liquid equilibrium methods and Al-in-olivine thermometer approach the estimated crystallization temperature ranges from 1450 to 1490 °C, and mantle potential temperature (Tp) ranges from 1617 to 1677 °C, at high pressure (3–4.8 GPa). These high temperatures substantiate the existence of the Afar thermal mantle plume and its important role in the genesis of the studied meimechite rocks.

Characterization of the metasomatizing agent in the upper mantle beneath the northern Pannonian Basin based on Raman imaging, FIB-SEM, and LA-ICP-MS analyses of silicate melt inclusions in spinel peridotite

AbstractSilicate melt inclusions (SMI) containing several daughter minerals, residual glass, and a CO2 bubble were analyzed to constrain the composition and evolution of the metasomatic melt present in the upper mantle beneath the Nógrád-Gömör Volcanic Field (NGVF), northern Hungary to southern Slovakia. The SMI were analyzed with a combination of Raman spectroscopy, FIB-SEM, and LA-ICP-MS to identify phases and obtain their volume proportions and major- and trace-element geochemistry. Slicing through the entire volume of the inclusions and collecting geochemical information at each slice with FIB-SEM allowed us to model the 3D appearance of the phases within the SMI and to use this information to calculate bulk major-element compositions.The partially crystallized SMI are hosted in clinopyroxene in a lherzolite xenolith that shows evidence of a metasomatic event that altered the lherzolites to produce wehrlites. Based on bulk compositions, the SMI trapped the metasomatic melt linked to wehrlite formation in the NGVF. The melt is enriched in Fe and has an OIB-like trace-element pattern, which suggests an intraplate mafic melt similar to the host basalt, but with slightly different chemistry. Pre-entrapment evolution and reaction with the lherzolite wall rock produced an intermediate melt composition. Petrogenetic modeling indicates that the melt was generated as a result of a very small degree of partial melting of a garnet lherzolite source. Following entrapment, a volatile bubble exsolved from the residual melt during ascent to shallow depths as suggested by consistent densities of CO2 in vapor bubbles. Small crystals, including sulfates and mica, that formed at the boundary of the bubble and the glass indicate that the exsolved fluid originally contained S and H2O, in addition to CO2.

Chapter 5.4b Marie Byrd Land and Ellsworth Land: petrology

Abstract In Marie Byrd Land and Ellsworth Land 19 large polygenetic volcanoes and numerous smaller centres are exposed above the West Antarctic Ice Sheet along the northern flank of the West Antarctic Rift System. The Cenozoic (36.7 Ma to active) volcanism of the Marie Byrd Land Volcanic Group (MBLVG) encompasses the full spectrum of alkaline series compositions ranging from basalt to intermediate (e.g. mugearite, benmoreite) to phonolite, peralkaline trachyte, rhyolite and rare pantellerite. Differentiation from basalt is described by progressive fractional crystallization; however, to produce silica-oversaturated compositions two mechanisms are proposed: (1) polybaric fractionation with early-stage removal of amphibole at high pressures; and (2) assimilation–fractional crystallization to explain elevated 87 Sr/ 86 Sr i ratios. Most basalts are silica-undersaturated and enriched in incompatible trace elements (e.g. La/Yb N &gt;10), indicating small degrees of partial melting of a garnet-bearing mantle. Mildly silica-undersaturated and rare silica-saturated basalts, including tholeiites, are less enriched (La/Yb N &lt;10), a result of higher degrees of melting. Trace elements and isotopes (Sr, Nd, Pb) reveal a regional gradient explained by mixing between two mantle components, subduction-modified lithosphere and HIMU-like plume ( 206 Pb/ 204 Pb &gt;20) materials. Geophysical studies indicate a deep thermal anomaly beneath central Marie Byrd Land, suggesting a plume influence on volcanism and tectonism.

Olivine melilitites, mantle xenoliths, and xenocrysts of the Takarindiona district: Petrogenesis, magmatic evolution, and the sub-continental lithospheric mantle of east-central Madagascar

The olivine melilitites from the southern part of the 6.8 Ma-old Takarindiona volcanic field (Eastern Madagascar) are olivine ± chromite -phyric lavas, with zoned titanaugite, perovskite, melilite, nepheline, monticellite, Ba–Ti-mica and Fe–Ti oxides as microphenocrysts and groundmass phases. The rocks are very primitive, rich in incompatible trace elements (e.g., Ba = 1049 ± 153 ppm, Sr = 1050 ± 167 ppm, Nb = 98 ± 13 ppm; La/Ybn = 41 ± 5; La/Nb = 0.88 ± 0.05), and have restricted ranges of initial 87Sr/86Sr (0.70391–0.70410) and 143Nd/144Nd (0.51272–0.51282). The rocks follow a differentiation trend controlled by ab. 20% removal/addition of phenocryst olivine ± chromite. The olivine melilititic magmas are the product of small degrees of partial melting (1–3%) of a peridotitic source, enriched in highly incompatible trace elements by CO2-, F-, and H2O-rich melts, located within the garnet stability field (3–3.5 GPa and ~100 km depth) of sub-continental lithospheric mantle, where carbonates (dolomite) and possibly phlogopite were stable phases. Mantle xenoliths within the volcanics are mostly spinel harzburgites having mineral modes and chemical compositions suggesting variable degrees of “basalt” melt extraction. Based on textural and chemical evidence, and quantitative thermobarometric estimates, the xenoliths were incorporated at a pressure of ~1.1 GPa (~35–40 km depth), far shallower than the source of the melilititic magmas, and along a predictably cool geotherm beneath Archean continental lithosphere. Highly resorbed orthopyroxene xenocrysts mantled by augite indicate that the melilitites may have also entrained lower crustal materials or underplated subalkaline rocks. The mantle sources of the lavas and mantle xenoliths of the Takarindiona district indicate stratification of the lithospheric mantle, and help constraining the lithospheric features and the magmatic history of the Eastern Madagascar craton.

Quaternary Volcanism in Myanmar: A Record of Indian Slab Tearing in a Transition Zone From Oceanic to Continental Subduction

AbstractMagmatic processes that occur during the transition from oceanic to continental subduction and collision in orogens are critical and still poorly resolved. Oceanic slab detachment in particular is hypothesized to mark a fundamental change in magmatism and deformation within an orogen. Here, we report on two Quaternary volcanic centers of Myanmar that may help us better understand the process of slab detachment. The Monywa volcanic rocks are composed of low‐K tholeiitic, medium‐K calk‐alkaline, and high‐K to shoshonitic basalts with arc signatures, while the Singu volcanic rocks show geochemical characteristics similar to asthenosphere‐derived magmas. These volcanic rocks have low Os concentrations but extremely high 187Os/186Osi ratios (0.1498 to 0.3824) due to minor (&lt;4%) crustal contamination. The Monywa arc‐like rocks were generated by small degrees of partial melting of subduction‐modified asthenospheric mantle at variable depths from the spinel to garnet stability fields. Distinct from the Monywa arc‐like rocks (87Sr/86Sri = 0.7043 to 0.7047; εNdi = +2.3 to +4.7), the Singu OIB‐like rocks exhibit higher 87Sr/86Sri (0.7056 to 0.7064) and lower εNdi (+0.8 to +1.6) values. These isotopic characteristics indicate a large contribution of an isotopically enriched asthenosphere layer beneath the Burmese microplate, which possibly flowed from SE Tibet. We interpret that this short‐lived, small‐scale, and low‐degree melting Quaternary volcanism in Myanmar was triggered by its position above a slab window resulting from the tearing of the oceanic lithosphere from buoyant continental lithosphere of the Indian plate.

Geochemistry and petrogenesis of high-MgO ultramafic tertiary volcanic rocks from Bagah area, NW Ad Dhala Province, Yemen

The studied high-MgO ultramafic Tertiary volcanic rocks are recorded in Bagah Village at the NW Ad Dhala province within the southeastern exposures of the volcanic rocks of the late Oligocene-early Miocene Yemen Trap Series (YTS) that belong to the Yemen Volcanic Group (YVG) which constitute part of the Afro-Arabian rift-related continental large igneous province. These rocks are strongly porphyritic composed of olivine phenocrysts set in hypocrystalline groundmass composed of olivine, pyroxene, opaque, rarely plagioclase and glass materials. The Bagah high-MgO volcanic rocks characterized by their high MgO (23.98–25.47 wt.%) and TiO2 (1.32–1.48 wt.%) with low Na2O + K2O (1.15–1.60 wt.%) and SiO2 (43.71–44.26 wt.%) and are classified as meimechite, and this is the first reported meimechite rocks throughout the volcanic rocks of the western Yemen continental volcanic province and, upon our knowledge, throughout the Afro-Arabian continental flood province. These rocks are classified to be of the high-Ti type and assigned to HT1 type corresponding to the Ethiopian-Yemeni CFB zonal arrangement reported in the literatures. They have high Mg number (79.4–80.0) and high concentrations of Cr (up to 2976 ppm), Ni (up to 1115 ppm) and other compatible elements with overall enrichment in an incompatible and LREEs and depletion in Y and other HREEs with variable anomalies of LIL and HFS elements. These characteristics are similar to meimechite rocks in many publications particularly those from the Vestfjella area of the Karoo LIP and Yanbian Terrane of the Emeishan LIP, but slightly differ from those of Meimecha-Kotui region of the Siberian LIP. The genesis of these rocks implies the incorporation of four mantle sources closely related to the interaction of the Arabian lithospheric mantle with the upwelling thermal Afar plume beneath study area and include OIB, N-MORB and E-MORB sources with participation of HIMU source as an additional component. The OIB-like mantle source derived directly from Afar plume. The N-MORB and E-MORB sources are formed by partial melting of normal and metasomatized Arabian lithospheric mantle due to the continuous raising of Afar mantle plume, and these mantle lithosphere-derived magmas had introduced significant contributions to the genesis of Bagah ultramafic rocks than those derived from Afar plume. Accordingly, the geochemical characteristics of these rocks can be interpreted in terms of mixing between these mantle-derived magma sources and the conditions that were operated during the creation of such magmas. The Bagah ultramafic volcanic (supposed meimechite) rocks were formed by small degrees of partial melting of garnet-bearing peridotite mantle at great depths under high melting pressure and temperatures above the amphibole stability due to the thermal effects of Afar plume. Such small degrees of partial melting, under similar melting conditions of either normal and/or metasomatized mantle peridotite are suggested for the formation of meimechites rocks from the Meimecha-Kotui province of the Siberian LIP, the Vestfjella area of the Karoo LIP and the Yanbian Terrane of the Emeishan LIP. Finally, these meimechite-forming magmas are rapidly ascent through the Arabian lithosphere with no or negligible crustal assimilation in rout to the surface.

Origin and magmatic evolution of the Quaternary syn-collision alkali basalts and related rocks from Salmas, northwestern Iran

Quaternary alkali basalts and related rocks are located in the northern portion of the Sanandaj–Sirjan zone in Iran (SaSZ). These rocks, also known as the Salmas Quaternary volcanic rocks (SQVR), consist primarily of basalts and trachy basalts (mafic parts) and basaltic trachy andesites and trachy andesites (intermediate parts), which are highly sodic alkaline and high-K calc-alkaline in character, respectively. These mafic to intermediate samples contain variable proportions of pyroxene, olivine and plagioclase, alongside ubiquitous accessory levels of biotite, amphibolte and oxides. Based on the negative Nb, Ta, Zr, and Hf, and also positive Th and U anomalies in some samples, the SQVR formed in a widely-recognized syn-collision environment following the termination of oblique Neotethys subduction beneath the Central Iranian microplate. Following slab break-off of the remnants of the oceanic lithosphere under the Arabia-Eurasia collision zone, a combination of asthenospheric upwelling, small-scale mantle convection, and localized crustal extension promoted magmatism. We model that decompression melting occurred on the boundary of the lithosphere and asthenosphere at around 65–70 km. The magma of the SQVR resulted from relatively small degrees of partial melting (<5%) of a metasomatized amphibole-bearing garnet-spinel lherzolite. Fractional crystallization, combined with a small amount of assimilation and fractional crystallization occurred during transfer to the surface and resulted in the evolution of basaltic parent magmas towards intermediate compositions.

Age and compositional evolution of Sahand volcano in the context of post-collisional magmatism in northwestern Iran: Evidence for time-transgressive magmatism away from the collisional suture

Sahand volcano along with the two neighbouring volcanic systems of Saray and Sabalan are three Late Miocene-Quaternary volcanoes that formed in the wake of the collision between the Arabian and Eurasian continental plates along the Neo-Tethyan suture zone in northwestern Iran. Sahand volcano is an isolated, extensive (3707 m elevation; >3000 km2 area) stratovolcanic complex, which is located ~165 km north of the Neo-Tethyan suture zone and was constructed over ~45 km thick continental crust. Andesitic to dacitic pyroclastic deposits, ignimbrites, and lava flows form the now strongly eroded Paleo- and Meso-Sahand edifice. Paleo-Sahand rocks are mostly restricted to the central part the edifice, where they underlie Meso-Sahand deposits. By contrast, Neo-Sahand volcanic rocks intruded older volcanic successions within and outside a largely eroded caldera margin, which likely formed during the Meso-Sahand stage. Neo-Sahand units comprise several basaltic andesitic to rhyolitic domes in the center of the complex as well as small parasitic cones along with subvolcanic dikes. High spatial resolution and sensitivity U-Pb geochronology using secondary ionization mass spectrometry (SIMS) define the formation of Paleo-, Meso-, and Neo-Sahand to ca. 8 to 3 Ma, ca. 1 Ma, and ca. 600 to <173 ka, respectively, with dating mostly focusing on Neo-Sahand. Antecrystic zircon crystals in Sahand volcanic deposits also indicate that post-collisional magmatism initiated locally at about the same time or slightly earlier than at Saray volcano to the west of Sahand. The initiation of volcanic activity subsequently migrated to the northeast over a distance of ~300 km away from the Neo-Tethyan suture, where activity started at Sabalan volcano at ca. 4.5 to 2.9 Ma. This time corresponds to a magmatic lull of Sahand before the Meso-Sahand eruptive activity resumed at ca. 1 Ma. Subsequently, Neo-Sahand and Neo-Sabalan erupted synchronously throughout the middle and possibly into the late Pleistocene. Paleo- and Neo-Sahand volcanic rocks are characterized by enrichments of large-ion lithophile (LILE) and light rare earth elements (LREE) relative to high-field strength and heavy rare earth elements (HREE), respectively, and prominent negative Ti, Nb, and Ta anomalies relative to primitive mantle compositions. The least evolved magmas at Sahand have formed by 1–5% partial melting of garnet- and spinel-peridotite and were subsequently overprinted through assimilation – fractional crystallization (AFC) processes acting in the continental crust. Although Sahand and Sabalan volcanic rocks share many geochemical characteristics indicative of a continental subduction setting, Sahand is less potassic and less enriched in incompatible trace elements compared to Sabalan. Another difference is that Sahand primitive magmas were formed by smaller degrees of partial melting in the mantle than is the case for Sabalan. Both effects likely result from variable distances from the suture zone and crustal thickening away from the suture towards the northeast. Overall, the apparent age progression and geochemical variation of the post-collisional volcanism, lack of asthenospheric mantle components, and the subduction-affinity of Sahand and Sabalan volcanic rocks supports a scenario where mantle melting was triggered by slab roll back and break-off shortly after continental collision, followed by migration of the slab edge towards the northeast at plate-like velocities.

Evidence of chemical heterogeneity within lunar anorthosite parental magmas

Lunar anorthosites are known for displaying a limited range of plagioclase An content (∼An 94 to 98). Here we demonstrate that plagioclase trace-element variations from Apollo ferroan anorthosites (FAN) samples (collected by the Apollo 15 and 16 missions) display more significant chemical heterogeneity (e.g., chondrite-normalized [La/Sm] 0.33–5.42) than previously reported. We report mineral (plagioclase, pyroxene, and olivine) major- and trace-element abundances for a suite of Apollo FAN samples, in addition to, anorthositic clasts within Apollo 16 regolith breccias. This suite of data extends the compositional range currently reported for Apollo anorthosites and for anorthositic clasts previously found within lunar meteorites. Petrological classifications of the regolith breccia clasts (e.g., anorthosite versus noritic anorthosites) cannot always be accurately assessed due to the limited size (<1 cm) these rock fragments, however, the overlap in chemistry with the FAN suite highlights a genetic link with the FAN bedrock source. This observation emphasizes the usefulness of clasts and mineral fragments within regolith breccias, offering important insights into potentially unsampled bedrock lithologies from the Apollo 16 landing site. Melts in equilibrium with plagioclase can be used to assess parental melt compositions of the lunar magma ocean (LMO), from which anorthosites are generally agreed to have crystallized. In general, melts in equilibrium with the anorthosites reported here display slight light rare earth (LREE) depletions to LREE enrichments ([La/Sm]CI 0.87–2.5). The observed range of LREE enrichments from this suite, together with variations in ratios of other incompatible trace-elements (e.g., Th/Sm = 0.002–0.19) cannot be accounted for by fractional crystallization alone. We propose that the observed trace-element enriched anorthosites are related to overturn processes in the lunar mantle. During mantle overturn, the act of exhuming deep mafic-rich cumulates to the base of the lunar crust will trigger decompression melting. These are likely to be small degree (<10%) partial melts, which are typically enriched in incompatible elements. Variable mixing between such melts, KREEP, and overlying plagioclase-saturated residual melts or plagioclase-rich lithologies will result in lunar anorthosites that display variable incompatible element enriched signatures. This is similar to the proposal of Floss et al. (1998) that suggested infiltration of local LMO magmas occurred by more evolved liquids through a process of metasomatism. By understanding the petrogenesis of these lunar anorthosites, we are able to constrain some of the complexities associated with the solidification of a magma ocean. This in turn, has important implications for understanding the timing and formation mechanisms of the Moon’s crust.

Geodynamic setting of Upper Miocene to Quaternary alkaline basalts from Harrat al ‘Uwayrid (NW Saudi Arabia): Constraints from K[sbnd]Ar dating, chemical and Sr-Nd-Pb isotope compositions, and petrological modeling

The volcanic field of Harrat ar Rahah-’Uwayrid (NW Saudi Arabia) consists of an older plateau basalt sequence that overlies Cambrian sandstone and younger cinder cones with smaller flows that are concentrated in the central part of this field. Petrographic and whole rock geochemical data indicate that the older plateau basalts are alkali olivine basalts, while the younger volcanic products are basanites and tephrites with some phonotephrites and tephriphonolites. KAr model ages on 22 bulk-rock samples were obtained for variable grain size fractions (125–250, 250–500 μm). These dates range from 8.2 to 0.34 Ma, whereby three phases of volcanic activity during the Upper Miocene, Pliocene and Quaternary can be distinguished. Sr-Nd-Pb isotopic ratios are similar for all age groups with 87Sr/86Sr = 0.70307–0.70324, 143Nd/144Nd = 0.512912–0.512957, 206Pb/204Pb = 19.360–19.717, 207Pb/204Pb = 15.603–15.633 and 208Pb/204Pb = 39.083–39.521 (present-day ratios are indistinguishable from calculated initial ratios) suggesting that chemical differences among the lavas were probably produced by different degrees of partial melting rather than by different source compositions. Trace element ratios indicate an origin of all volcanics by small degrees of partial melting of amphibole-spinel-garnet peridotite. Geochemical and isotopic data of the Harrat ar Rahah-’Uwayrid are similar to those from the small volcanic fields of Harrat Kura (about 200 km SSE of Harrat al ‘Uwayrid) and of Wadi Jizan in SW Saudi Arabia, but clearly more enriched than those of the volcanic fields that are located in the more central parts of the eastern shoulder of the Red Sea Rift. The lithospheric thickness underneath Harrat ar Rahah-’Uwayrid is estimated to about 60 km based on published results from seismic tomography. Our data support the model that the metasomatized lithospheric mantle was the primary source of the magmas, while the asthenospheric mantle or a plume-type component played an unsignificant role. The composition of the lithospheric mantle source was similar to prevalent mantle (PREMA). There is no need to postulate the participation of a HIMU component and/or enriched mantle components (EM-1, EM-2) in the source of the investigated magmas.

Alkaline magmas in zones of continental convergence: The Tezhsar volcano-intrusive ring complex, Armenia

Alkaline igneous rocks are relatively rare in settings of tectonic convergence and little is known about their petrogenesis in these settings. This study aims to contribute to a better understanding of the formation of alkaline igneous rocks by an investigation of the Tezhsar volcano-intrusive alkaline ring complex (TAC) in the Armenian Lesser Caucasus, which is located between the converging Eurasian and Arabian plates. We present new petrological, geochemical and SrNd isotope data for the TAC to constrain magma genesis and magma source characteristics. Moreover, we provide a new 40Ar/39Ar age of 41.0 ± 0.5 Ma on amphibole from a nepheline syenite that is integrated into the regional context of ongoing regional convergence and widespread magmatism.The TAC is spatially concentric and measures ~10 km in diameter representing the relatively shallow plumbing system of a major stratovolcano juxtaposed by ring faulting with its extrusive products. The plutonic units comprise syenites and nepheline syenites, whereas the extrusive units are dominated by trachytic-phonolitic rocks. The characteristic feature of the TAC is the development of pseudomorphs after leucite in all types of the volcanic, subvolcanic and intrusive alkaline rocks.Whole-rock major element data show a metaluminous (Alkalinity Index = 0–0.1), alkalic and silica-undersaturated (Feldspathoid Silica-Saturation Index <0) character of the TAC. The general trace element enrichment and strong fractionation of REEs (LaN/YbN up to 70) indicate a relatively enriched magma source and small degrees of partial melting. All TAC rocks show a negative NbTa anomalies typical of subduction zone settings. The initial 87Sr/86Sr ratios (0.704–0.705) and positive εNd values (+3 to +5) indicate an isotopically depleted upper mantle and lack of significant crustal influence, which in turn suggests the TAC magma has formed via differentiation from lithospheric mantle melts.Regionally, the age of ~41 Ma places the TAC amid a Lesser Caucasian Eocene period of dominantly calc-alkaline magmatism. The TAC's arc-like geochemical signatures are interpreted to result from prior subduction of the Tethyan slab beneath the Eurasian continental margin. The alkaline character, distinct from regional trends, is attributed to Neotethyan slab rollback causing extension and inducing small degrees of decompression melting of metasomatised lithospheric mantle.

The Neoproterozoic arc-type and OIB-type mafic-ultramafic rocks in the western Jiangnan Orogen: Implications for tectonic settings

We report zircon U–Pb ages, zircon Hf isotopic data, mineral chemistry and the whole rock geochemistry of three representative Neoproterozoic mafic–ultramafic intrusions (Jinche gabbros, Huaqiao clinopyroxenite and Maodu gabbroic rocks) from the western Jiangnan Orogen in this manuscript, to decipher their petrogenesis and tectonic setting, and further provide constraints on the reconstruction of the Rodinia. The Jinche gabbros with a zircon SHRIMP U–Pb age of 806.4 ± 4.6 Ma, and it is suggested that the Huaqiao clinopyroxenites emplaced nearly synchronously as indicated by field geological evidences. They are typical tholeiitic arc rocks, characterized by the enrichment of light rare earth elements ([La/Yb]N = 2.27–3.64), significantly negative Nb, Ta and Ti anomalies, and positive εNd(t) (0.7–3.4) and εHf(t) (7.4–12.0) values. They are inferred to be derived from high-degree 5–10% partial melting of mantle wedge in the subduction setting from the garnet to spinel stability field. The Maodu gabbroic rocks yielded a zircon LA–ICP–MS U–Pb age of 773.6 ± 4.1 Ma. They are of alkaline series, and display ocean island basalt (OIB)-like geochemical signatures, with the enrichment of LREE and large-ion lithophile element relative to heavy rare earth elements ([La/Yb]N = 11.28–13.42), and have negative εNd(t) (−1.4 to −3.6) and positive εHf(t) (3.5–8.6) values. They are suggested to be derived from small degree (2%) partial melting of the upwelling asthenospheric mantle in an extensional setting in the garnet stability field. Based on comprehensive analyses of the Neoproterozoic mafic-ultramafic rocks reported in both this study and previous publications and regional geology in the western Jiangnan Orogen, we infer that the opening of the Nanhua rift was resulted from the back-arc extension related to the roll-back of the subducted slab, and the South China Block may be located at the margin of the Rodinia, as part of the Neoproterozoic Circum-Rodinia Subduction System.