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The Neoarchean Qingyuan greenstone belt (QGB) is located at the northeastern margin of the North China Craton (NCC) and has experienced amphibolite- to granulite-facies metamorphism. The supracrustal rock assemblage is composed of biotite-hornblende (Bt-Hb) monzonitic gneiss, garnet-biotite-hornblende (Grt-Bt-Hb) monzonitic gneiss, Grt-Bt-Hb plagioclase gneiss, amphibolite and orthopyroxene-biotite-hornblende (Opx-Bt-Hb) plagioclase gneiss. This study involves thorough field and petrographic observations, whole-rock geochemistry and LA-ICP-MS zircon U‒Pb ages of these rocks to constrain the Neoarchean geodynamic setting of the QGB. Zircon U‒Pb dating indicates that these metavolcanic rocks formed during 2.57‒2.52 Ga and experienced subsequent regional metamorphism at 2.52‒2.47 Ga. Lithological and geochemical characteristics show that the QGB protoliths were tholeiitic to calc-alkaline basalts, andesites, dacites and rhyolites. The andesite-dacite-rhyolite assemblage exhibits high SiO2 and Mg# values, low Yb and Y contents, strongly fractionated chondrite-normalized REE patterns and depletion in Nb, Ta, Ti and P, resembling Phanerozoic adakites. These geochemical characteristics suggest an origin from the partial melting of a subducted slab interacting with mantle wedge material. The tholeiitic to calc-alkaline basalts exhibit slightly LREE-enriched chondrite-normalized REE patterns and negative Nb and Ti anomalies, like island arc basalts. These basalts might have originated by partial melting of a mantle source affected by metasomatism from subduction-derived fluids and melts. The lithological associations and geochemical characteristics imply that the Qingyuan greenstone belt developed in a continental margin arc setting. [ABSTRACT FROM AUTHOR]

The East Kunlun Orogenic Belt, an important part of the Central Orogenic Belt in China, preserved wealthy geological information for the tectonic evolution of the Proto- and Paleo-Tethys Ocean. Several Permian mafic-ultramafic rocks were identified in the Qimantagh area, EKOB, and zircon U-Pb dating, zircon Hf isotope, geochemistry and whole-rock Sr-Nd isotopes were studied in this paper. The results suggest that the formation age of the Xiyingzhaogou intrusion is 280 ± 4 Ma. The rocks show the characteristics of continental-arc calc-alkaline basalt. The whole-rock εNd(t) value of gabbro in Xiyingzhaogou intrusion is −4.2~-2.6 and the εHf(t) value of zircon is 2.77 ~ 5.82, indicating the characteristics of enriched or weakly depleted mantle; The formation age of Yingzhaogou intrusion is 263 ± 4 Ma. The geochemical characteristics of the Yinzhaogou rocks are consistent with those of continental-arc tholeiite. The whole-rock εNd(t) value of gabbro in Yingzhaogou intrusion is 0.73 ~ 0.92, and the εHf(t) value of zircon is 8.33 ~ 13.50, indicating the characteristics of depleted mantle. We believe that the Xiyingzhaogou intrusion was formed in the Early Permian and originated from the mantle wedge metasomatized by subduction fluid. The Yingzhaogou intrusion was formed in an extensional continental-arc environment caused by the retreat of the subduction plate. Asthenospheric mantle and fluid metasomatic lithospheric mantle were jointly involved in the mantle source. This study confirmed that the subduction of the Paleo-Tethys oceanic slab should had started before the Early Permian and a wide magma arc formed due to flat subduction. The retreat of the subduction plate led to the extension of the East Kunlun region, resulting in the significant enhancement of magmatism in the Middle Permian. [ABSTRACT FROM AUTHOR]

The Eldgjá eruption is the largest basalt lava flood of the Common Era. It has been linked to a major ice‐core sulfur (S) spike in 939–940 CE and Northern Hemisphere summer cooling in 940 CE. Despite its magnitude and potential climate impacts, uncertainties remain concerning the eruption timeline, atmospheric dispersal of emitted volatiles, and coincident volcanism in Iceland and elsewhere. Here, we present a comprehensive analysis of Greenland ice‐cores from 936 to 943 CE, revealing a complex volatile record and cryptotephra with numerous geochemical populations. Transitional alkali basalt tephra matching Eldgjá are found in 939–940 CE, while tholeiitic basalt shards present in 936/937 CE and 940/941 CE are compatible with contemporaneous Icelandic eruptions from Grímsvötn and Bárðarbunga‐Veiðivötn systems (including V‐Sv tephra). We also find four silicic tephra populations, one of which we link to the Jala Pumice of Ceboruco (Mexico) at 941 ± 1 CE. Triple S isotopes, Δ33S, spanning 936–940 CE are indicative of upper tropospheric/lower stratospheric transport of aerosol sourced from the Icelandic fissure eruptions. However, anomalous Δ33S (down to −0.4‰) in 940–941 CE evidence stratospheric aerosol transport consistent with summer surface cooling revealed by tree‐ring reconstructions. Tephra associated with the anomalous Δ33S have a variety of compositions, complicating the attribution of climate cooling to Eldgjá alone. Nevertheless, our study confirms a major S emission from Eldgjá in 939–940 CE and implicates Eldgjá and a cluster of eruptions as triggers of summer cooling, severe winters, and privations in ∼940 CE. Plain Language Summary: The eruption of Eldgjá in the tenth century is the largest lava flood in the history of Iceland. Although Eldgjá emitted immense volumes of ash, lava, and gas, the exact timing and duration of this eruption, as well as its environmental and climatic impact remain unclear. Here, we provide a comprehensive chemical analysis of Greenland ice‐core records spanning the period 936–943 CE. We identify volcanic ash from at least three different Icelandic eruptions and confirm that there was a major ash and gas emission from Eldgjá in 939 CE. Using tree ring temperature estimates we find strong evidence for Northern Hemisphere climate cooling in the summer of 940 CE. However, the variety of volcanic ash identified in the ice‐cores shows that several Icelandic and Northern Hemisphere arc volcanoes were also erupting in this period. While Eldgjá remains the prime candidate, these additional eruptions complicate the attribution of reported climate and societal changes to Eldgjá alone. Ultimately, our study sheds new light on a cluster of volcanic eruptions between 936 and 943 CE and highlights the challenges of disentangling the individual contributions of multiple eruptions on the environment and climate. Key Points: New analyses of Greenland ice‐core records of volcanism between 936 and 943 CEIcelandic eruptions from Grímsvötn and Bárðarbunga‐Veiðivötn detected between 936 and 941 CE, and major Eldgjá emission in 939–940 CEVarious silicic eruptions identified, including the Jala Pumice (Mexico), providing new and valuable trans‐continental tephra isochrons [ABSTRACT FROM AUTHOR]

A newly discovered population of Monetaria caputdraconis has been identified on Easter Island. This new population has a high occurrence of specimens expressing morphology that appear both melanistic and rostrate, and is named herein Monetaria caputdraconis form darkini. It is suggested that the geology of the island, which comprises Roiho basalt containing olivine tholeiite with relatively high concentrations of heavy metals, may contribute to this unique morphology. [ABSTRACT FROM AUTHOR]

The Palaeozoic tectonic evolution of the Western Kunlun Orogen plays an important role in deciphering the Tethyan tectonic evolution. This study provides new geochronology, geochemistry and Lu–Hf isotopic data of igneous rocks from the Omixia Complex in the eastern section of the northern Western Kunlun, as well as detrital zircon ages from turbidites and meta‐sediments north of it. From the Omixia Complex, the determined ages of five ultramafic–mafic rock samples are approximately 470, 456, 429, 401 and 382 Ma. Two samples of acidic rocks from the same complex yielded ages of approximately 438 and 378 Ma. One pegmatitic plagiogranite sample exhibits feature of accretionary arc granites, while other samples show geochemical characteristics of island arc tholeiite and E‐MORB. Additionally, four turbidite matrix and two limestone samples, with a major peak around ca. 480–500 Ma, have the youngest zircon ages ranging from ca. 481 to 387 Ma, paralleling the age range of igneous rocks in the ophiolitic mélange. The εHf(t) values of these samples reveal a broad spectrum of crustal and mantle processes. The youngest zircon ages of five meta‐sedimentary rock samples north of the Omixia Complex range from ca. 581 to 535 Ma, with peak ages concentrated around ca. 0.9–1.0 Ga. Their provenance characteristics differ from the turbidite matrix in the southern Omixia Complex and from the meta‐sediments in the northern Tiklik terrane with peak ages of ca. 0.8 Ga. A younger limestone sample yielded youngest zircon age of ca. 294 Ma, which is unconformably overlain the Omixia Complex and surrounding older rocks. Based on these new results, combined with previous data, we propose a new tectonic model for the eastern section of the northern Western Kunlun Orogen, suggesting a continuous evolution process of multi‐terrane subduction–accretion collage from the Early Ordovician to the Middle Devonian in the Paleo‐Tethys Ocean, which evolved into an Andean‐type active margin in the Early Permian, contributing to the substantial continental growth of the southern Tarim Craton. [ABSTRACT FROM AUTHOR]

The Allalin gabbro of the Zermatt-Saas meta-ophiolite consists of variably metamorphosed Mg- to Fe-Ti-gabbros, troctolites, and anorthosites, which are crosscut by basaltic dykes. Field relationships of the various rock types and petrographic studies together with bulk rock and mineral chemical composition data allow the reconstruction of the complete geological history of the Allalin gabbro. With increasing magmatic differentiation, the incompatible element content in clinopyroxene increases (e.g., REEs and Zr by a factor of 5), whereas the Mg# decreases (from 86.4 to 74.6) as do the compatible element contents (e.g., Cr and Ni by factors of 3.5 and 5, respectively). Exhumation to shallower depths led to subsolidus ductile deformation and cooling of the gabbro followed by the intrusion of fine-grained basaltic dykes, which display chilled margins. Bulk rock data of these dykes reveal strong similarities in fluid-immobile trace element patterns to tholeiitic pillow basalts of the Zermatt-Saas and nearby meta-ophiolites. The recalculated REE patterns of the melt in equilibrium with igneous clinopyroxene is very similar to the REE patterns of the mafic dykes, indicating a cogenetic origin of pillow basalts, dykes, and gabbros. Together with the previously determined Jurassic intrusion age of the gabbro, our observations demonstrate that the Allalin gabbro intruded as a tholeiitic magma in a slow spreading MOR environment of the Piemonte-Ligurian ocean of the Alpine Tethys. Subduction of the Allalin gabbro resulted in different eclogitization extents of the Mg-gabbros as a function of variable hydration degrees. Metagabbros with low extents of hydration record incomplete eclogitization; the magmatic mineralogy (olivine + clinopyroxene + plagioclase) is preserved together with disequilibrium textures in the form of reaction coronae surrounding mineral boundaries. Metagabbros with high extents of hydration are completely eclogitized and display pseudomorphic replacement textures of magmatic minerals by eclogite-facies mineral assemblages, which required significant major to trace element transport across mineral domains. The locally variable extents of hydration took place near the sea floor, as recorded by the presence of Cl-apatite (6.28 wt% Cl), and an increase in B concentrations of minerals pseudomorphically replacing olivine (e.g., chlorite with 0.20–0.31 µg/g B and omphacite with 0.22–0.25 µg/g B) compared to magmatic olivine (0.12–0.16 µg/g B). Moreover, the chemical zonation pattern of metamorphic garnet coronae is different in completely eclogitized gabbros and gabbros with relic igneous minerals, in agreement with a main hydration event prior to subduction. The Allalin gabbro therefore represents a classical example of an oceanic gabbro formed in a slow spreading setting in the mid Jurassic that experienced heterogeneous hydration near the sea floor. Paleogene subduction of the gabbro to 70–80 km depth produced variably equilibrated gabbroic eclogites. In eclogite-facies Mg-gabbros, the water-rich minerals chlorite, talc, and chloritoid pseudomorphing magmatic olivine remained stable to these depths, revealing the potential relevance of hydrated Mg-gabbros as a fluid source at subarc depths in subduction zones. [ABSTRACT FROM AUTHOR]

Lava flows have presented the greatest hazard to human property during the most recent eruptions of Hawaiian volcanoes, and lava fountains are a source of these lava flows. The height of Hawaiian lava fountains reflects the exsolved gas content of the magma that controls eruption intensity. However, fountain height is not always observed, so we sought a proxy to estimate fountain heights of eruptions that were older or otherwise unobserved. Here, methods are described to empirically derive a relationship between the modal diameter of vesicles within Pele's tears and spheres and lava fountain height, using samples of Pele's tears produced during the last eruptions of Kīlauea Iki (1959) and Mauna Ulu (1969). The tears used to develop these relationships were approximately 1 to 4 mm in diameter. Additionally, since lava fountains 50-580 m high were used, the relationships we describe may only describe lava fountains in this height range. The strongest empirical relation follows the trendline Hmax = -2575d + 820, where Hmax is maximum lava fountain height and d is modal vesicle diameter. This empirical relationship may be applied to sub-Strombolian eruptions of tholeiite basalt that were not directly measured or observed to assess long-term shifts in lava fountain heights and thus the exsolved gas contents of a volcanic system. While the same conceptual framework can be applied beyond Hawai'i, the quantitative empirical relation may be slightly different in different systems, depending on total dissolved volatiles, magma chemistry and other factors. [ABSTRACT FROM AUTHOR]

The discovery of eclogite outcrops in the East Kunlun Orogen Belt (EKOB) has confirmed the existence of an Early Paleozoic HP-UHP metamorphic belt. However, the protoliths and metamorphic histories of widespread metabasites remain poorly constrained. We collected three types of metabasites from the central part of EKOB. We present an integrated study of petrography, whole-rock geochemistry, Sr-Nd isotopes, estimated P–T conditions, and zircon U-Pb isotope ages. The results show that amphibolites and retrograde eclogites have clockwise P–T paths with peak conditions of, respectively, 11–12 kbar and 675–695 °C, and 21.5–22.2 kbar and 715–750 °C. Zircon dating of metabasites from Dagele yields Late Ordovician (~449 Ma) to Early Silurian (~440 Ma) protolith ages and Early Devonian (~414 Ma) amphibolite facies metamorphic ages. Retrograde eclogites from east Nuomuhong have a protolith age of ~902 Ma and metamorphic ages of ~418 Ma, consistent with other eclogites from East Kunlun. Our data suggest that the protoliths of Dagele metabasites represent arc-type magmatism during the subduction of a small back-arc oceanic basin. Instead, the protoliths of retrograde eclogites are Neoproterozoic tholeiitic basalts emplaced into continental crust and subsequently deeply subducted. We develop a new model for Early Paleozoic subduction and collision in the East Kunlun region, emphasizing the role of 'dominant' and 'secondary' suture boundaries. This model helps explain the ages and metamorphic histories of the metabasites studied here and offers new perspectives on the evolution of the Proto-Tethys Ocean. [ABSTRACT FROM AUTHOR]

Data collated from subalkaline lavas and dikes of Palaeogene ophiolites of the SW Pacific in Papua, New Guinea, New Caledonia, and the North Island, New Zealand interpreted as having formed first upon subduction initiation, are investigated to determine whether these are MORB like, similar to other subduction initiation ophiolites and the IBM forearc. SW Pacific ophiolite subalkaline lavas and dikes are mostly tholeiitic basalt; however, calc-alkaline lavas comprise ~ 25% of New Zealand samples. Major element abundances of first-formed subalkaline lavas and dikes of SW Pacific ophiolites are broadly similar to MORB, however incompatible element abundances range from MORB-like to more depleted than MORB and similar to the most enriched IBM forearc basalt (FAB). The reason for the more enriched incompatible-element nature of first-formed lavas and dikes of SW Pacific ophiolites relative to the IBM FAB may be that the sources of the former, unlike the latter, based on recent studies were not subjected to a pre-subduction initiation melt extraction event. Partial melting estimates for first-formed lavas and dikes of SW Pacific ophiolites range from 12–23% of an initial MORB-like source to a maximum of 18% partial melting of this source after 20% melt extraction. Partial melting was due to decompression of a MORB-like source that received little to no slab-derived contributions. Thermobarometry calculations of primitive lavas and dikes with ≥ 8 wt.% MgO and Mg# ≥ 60 show that the mantle source of SW Pacific ophiolites records potential temperatures and pressures of 1341–1431°C and 1.1–1.8 GPa, similar to primitive MORB. These temperatures are at least 30–70°C lower than those required for Central American and IBM first-formed lavas supporting previous studies suggesting a plume-induced subduction initiation origin for Central America and the IBM. [ABSTRACT FROM AUTHOR]

The termination of Neoproterozoic "Snowball Earth" glaciations is marked globally by laterally extensive neritic cap carbonates directly overlying glacial diamictites. The formation of these unique deposits on deglaciation calls for anomalously high calcium carbonate saturation. A popular mechanism to account for the source of requisite ocean alkalinity is the shallow-ridge hypothesis, in which initial spreading ridges surrounding fragments of Rodinia, assumed to be dominated by volcanic margins, were formed at sea level. The shallow ridges are inferred to have promoted widespread deposition and alteration of glassy hyaloclastite--a source of alkalinity. We test this hypothesis by quantifying the prevalence of shallow ridges along Pangea's passive continental margins, and by assessing Neoproterozoic reconstructions of tectonic plates. We find that the most frequently occurring depth range for incipient mid-ocean ridges is 2.1 ± 0.4 km. Ridges with initial elevations of approximately sea level are rare and have anomalous crustal thicknesses >14 km that only occur proximal to large igneous provinces (LIPs). Hyaloclastite is uncommon on mid-ocean ridges as it is generally restricted to water depths of <200 m for tholeiitic basalts, instead forming mostly on intraplate seamounts. Additionally, ocean drilling recently found hyaloclastite to be insignificant along the outer Vøring Plateau (offshore Norway)--an exemplar of a volcanic margin. Reconstructions of Rodinia and associated LIPs demonstrate that volcanic margins potentially hosting minor hyaloclastites were scarce during the late Neoproterozoic. We conclude that the shallow-ridge hypothesis fails to explain the formation of cap carbonates and suggest that other mechanisms such as enhanced continental weathering may be largely responsible. [ABSTRACT FROM AUTHOR]

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The Sand River area is in the northeastern part of the Central Zone (CZ) of the Limpopo Belt, and represents a key terrane to reveal early Precambrian tectonic processes in southern Africa. Based on petrology, U–Pb zircon ages, and major and trace element data of high-pressure (HP) mafic granulites from the Sand River area, the tectonic evolution of the Central Zone of the Limpopo Belt is re-evaluated here that allows revision of previous interpretations. The protoliths of the HP mafic granulites and amphibolites are sub-alkaline tholeiitic basalts similar to those formed at a mid-ocean ridge and intruded at 2.8 Ga. Based on the distinct mineral assemblages, three metamorphic stages (M1–M3) are recognized in the studied samples: (1) a peak HP granulite facies stage M1 (~1.17–1.32 GPa/~825–870°C), (2) a post-peak near-isothermal decompression stage M2 (~0.70–0.73 GPa/~835–880°C), and (3) a later, independent metamorphic stage M3 (~0.53–0.69 GPa/~793–801°C). M1 and M2 define together a clockwise P–T path. It reveals important information related to the Neoarchean collisional orogeny, and its later overprinted by an independent orogeny with subsequent exhumation and cooling. U–Pb zircon dating of two mafic granulites samples yielded protolith formation of the mafic granulites is prior to ~2.72 Ga, and the two metamorphic age peaks of ~2.72 (M1) and ~2.0 Ga (M3), which present the peak and post-peak cooling stages, respectively. The metamorphic evolution of HP mafic granulites in the Sand River area as reconstructed in this study suggests the Neoarchean collision of the Kaapvaal and Zimbabwe cratons and a discrete Paleoproterozoic orogenic overprint followed by post-orogenic exhumation and cooling. [ABSTRACT FROM AUTHOR]

The paper considers data on the composition, age, and formation conditions of the Ulzutui and Kydzhimit sequences, which are ascribed to the Eravna Series of the Eravna Subzone of the Uda–Vitim Zone. The Eravna Series, at least 2400 m thick, is composed of tephroturbidites with a significant amount of lavas and tuffs of felsic and intermediate compositions, with which tectonic-gravitational mixtites are spatially and genetically related. The features of the structure and composition of the Eravna Series indicate the formation of sediments in an open deepening marine basin adjacent to the zone of active volcanism. Based on miospores, the time of formation of the Ulzutui Sequence was constrained, for the first time, to the Late Devonian–Early Carboniferous (late Famennian–early Tournaisian), the Kydzhimit Sequence is limited to the Early Carboniferous (Visean) age. At this time, the Kydzhimit volcanic zone was formed. The suprasubduction nature of the zone is evidenced by the following features of volcanic rocks: predominance of felsic varieties, affiliation of basalts to the tholeiitic series, and dacites and rhyodacites to the calc-alkaline series, the presence of high-K rock varieties corresponding to the K-Na alkalinity, very low MgO contents, low and moderate TiO2 contents, high Al2O3 and K2O, enrichment of the volcanogenic rocks in large-ion lithophilic elements relatively to high-field strength elements, Nb minimum, and negative εNd, their spatial association with monzonites (with suprasubduction signatures) and tephroturbidites, as well as metallogenic specialization (Cu and Zn) of the Eravna Series. We believe that the ore-bearing sequence for the pyrite–base metal Ulzutui and Ozernoe deposits is the Upper Devonian–Lower Carboniferous Eravna Series. [ABSTRACT FROM AUTHOR]

The Santos Basin comprises the most voluminous hydrocarbon accumulations in its 'Pre-salt' reservoirs. Magmatic rocks have been recorded in the basin mostly by the tholeiitic basalts (Camboriú Formation) within its rift megasequence at Hauterivian time. Recent studies have shown that magmatic rocks are also found interbedded with Aptian sedimentary rocks in the basin, related to its post-rift (mostly Aptian) megasequence. This paper presents petrological data obtained from sidewall core samples of magmatic rocks interbedded with Early Aptian coquina in a well drilled in the Santos Basin. The well comprises six magmatic sections of alkaline basalts being overlain and underlain by the Barra Velha limestones. The basalts are mostly plagioclase-phyric, amygdaloidal rocks but vary in texture and modal compositions. Magmatic compositions seem to have been slightly affected by alteration processes. Basalts comprise an alkaline, miaskitic series with relatively evolved compositions (MgO = 5.55 ± 1.50 wt%; Ni < 220 ppm; Cr < 400 ppm). The patterns in the Primitive Mantle normalized multielement diagram displays the typical Nb peak (0.3 < La/Nb(N)<0.7) of the OIB. Geochemical modelling showed that basalts within five magmatic sections are not cogenetic by differentiation, representing basaltic flows emitted from different vents. Modal batch partial melting model indicates that it is possible, although unlikely, to generate the differences in La/Yb(N) and Zr/Y ratios of the least evolved samples in the studied well by different partial melting degrees (0.5–11%) from the same mantle source. Binary mixing models using lithogeochemical datapoint to a role for the Tristan da Cunha plume tail in the petrogenesis of the alkaline basalts with no clear evidence for a possible role for the local subcontinental lithospheric mantle. The results of the petrogenetic models are consistent with a shallow MOHO and a high-temperature mantle below part of the Santos Basin in Early Aptian time. [ABSTRACT FROM AUTHOR]

Petrogeochemical data on basalts (lithophile elements and Sr–Nd–Pb isotopes, liquidus olivine and spinel compositions) from the transition zone of the Southwest Indian Ridge (SWIR) in the Du Toit–Andrew Bain fault zone indicate significant differences in their composition. Tholeiites enriched in Na and depleted in Fe (Na-TOR genetic type) are developed within the rift valley adjacent to the faults. Deep-type basalts (TOR-1) are present in the western wall of the Andrew Bain Fault. The outpouring of these magma types reflects a possible change in geodynamic setting during formation of this zone: from deep and high temperature to shallower magma generation conditions (Sushchevskaya et al., 2022). Differences in the primary melts of tholeiites from the rift valley and the Andrew Bain Transform Fault are also traced in the composition of liquidus olivine. The olivines from rift valley are similar to the typical Na-TOR olivines with Mg number of Fo88-87, low Ni and elevated Mn contents. On the contrary, olivines in tholeiite from the Andrew Bain Fault Zone are enriched in Ni and depleted in Mn, which may indicate the involvement of pyroxenite in melt generation. This component is either oceanic lithosphere recycled through the deep mantle or fragments of previously formed oceanic crust, which were subsequently involved in melting during the spreading axes jumping. A similar process is typical of the Bouvet Triple Junction, where the trace-element composition of olivine shows significant heterogeneity. The radiogenic composition of Pb and Sr of the Andrew Bain Fault tholeiites are similar to those of enriched magmas from such Indian Ocean rises as Crozet, Marion and Bouvet, but differ from those of the Conrad and Af. Nikitin rises. The source of such tholeiite melts is close in composition to the model HIMU type (with high U/Pb), which likely contains an admixture of EM-II component (with elevated Rb/Sr). [ABSTRACT FROM AUTHOR]

The ophiolitic Villa Clara serpentinite-matrix mélange, central Cuba, forms part of the large ophiolitic belt of the Greater Antilles. The composition of ultramafic and mafic rocks allows classifying them into two main groups, revealing a complex multi-stage formation of oceanic lithosphere in varied tectonic settings: i) group 1 matches fertile MORB-like mantle typical of abyssal/transform fault peridotites, while ii) group 2 shows characteristics of refractory mantle-wedge forearc peridotites. These features are consistent with moderate extent of partial melting (4–8%) of depleted mantle at a mid-ocean ridge setting that created residual abyssal peridotite (group 1) and intense remelting (14–18%) of the latter in a forearc setting that created highly refractory peridotite (group 2). The elemental and isotopic characteristics of groups 1 and 2 serpentinites denote the influx of fluids evolved from the subducting slab. Mafic rocks appear as variably sized tectonic blocks within serpentinite that underwent variable alteration at low pressure. Immobile element concentrations allow identifying two main groups: i) group 1 is consistent with forearc basalts formed during the second stage of melting of (abyssal) peridotite, while ii) group 2 has a compositional pattern like island-arc tholeiitic basalts. A geodynamic evolution from abyssal to forearc settings is presented to explain the nature of the oceanic lithosphere of the Villa Clara ophiolitic bodies and ensuing serpentinization and mélange formation. This model may explain other ophiolitic bodies of the Caribbean arc in Cuba, Dominican Republic and Puerto Rico and elsewhere formed in abyssal/transform fault or forearc settings. [ABSTRACT FROM AUTHOR]

We studied an Archean mafic dike in the Trans-North China Orogen of the North China Craton, which has a magmatic age of 2701 ± 83 Ma and is currently the oldest mafic dike in the North China Craton. Such an old dike is extremely rare in the world. The presence of mafic dikes indicates that the North China Craton was in a tensional tectonic environment at 2.7 Ga. Geochemical characteristics reveal that this mafic dike belongs to continental tholeiitic basalt. Results from Hf isotope analysis reveal that the mafic dike originates from a depleted mantle. The plate assembly in the North China landmass was realized during the Archean era (2.7 Ga), and a thick and stable continental crust was formed. Therefore, the first cratonization of the North China Craton was completed before 2.7 Ga. The intrusion of the 2.7-Ga-old mafic dike from the deep lithospheric mantle of the continent indicates that the North China Craton has undergone a period of extensional tectonic activity. This event marks a significant extensional event that occurred after the cratonization of the North China Craton. [ABSTRACT FROM AUTHOR]

The Indian passive margin has preserved several pulses of magmatism during and after the disassembly of Gondwana since the late Paleozoic, providing valuable insights into the long-term magmatic evolution of various passive margins, including the Indian passive margin. In the Yumai Complex, eastern Tethyan Himalaya, a pulse of Late Triassic alkaline volcanism (ca. 227-216 Ma) is evident. The Late Triassic volcanic rocks are mildly alkaline to tholeiitic basalts with minor ultrabasic rocks, similar in geochemistry to within-plate flood basalts. The TiO2 contents (1.46-3.38 wt%, mainly >2 wt%), (La/Yb)N values (4.05-7.50), εNd(t) values (+4.86 to +6.98), and results from partial-melt modeling suggest that the basalts likely originated from garnet peridotite. Elemental and Sr-Nd systematics of magmatic rocks emplaced during the Triassic indicate oceanic island basalt (OIB) components in the magma source, interpreted as enriched mantle components rather than crustal contamination products. Spatiotemporal and geochemical patterns of magmatism reveal that the bulk compositions of the basalts changed from enriched OIB-like to depleted mid-ocean-ridge basalt (MORB)-like compositions. This phenomenon likely resulted from the evolving nature of the rifting basin, changing from a nascent continental setting to a mature ocean basin. The Triassic magmatism in the Tethyan Himalaya can be attributed to remnant lithospheric instability arising from the prolonged rifting of eastern Gondwana, leading to the formation of a magmatically passive margin. [ABSTRACT FROM AUTHOR]

Tholeiitic basalts and trachytes of the bimodal association of Wangtian'e volcano are studied. It is shown that trachytes formed with the leading role of crystal fractionation of the parental tholeiitic magma. It has been identified from the melt inclusion study that hedenbergite and plagioclase phenocrysts of trachytes from Wangtian'e volcanic neck crystallized in the temperature range of 1080–1100 and 1050–1060°C, respectively. After heating experiments with melt inclusions in plagioclase of trachytes, in some cases relics of carbonates and CO2 were observed in the inclusions. The trachyte phenocrysts host water-bearing Fe-rich globules and CO2 inclusions with carbonate phases. The Fe-rich silicate globules are often intergrown with titanomagnetite and are covered by amorphous carbon films. A model is suggested explaining the formation of water-bearing Fe-rich globules and CO2 inclusions with carbonate relics as a result of silicate–silicate and silicate–carbonate liquid immiscibility, that was caused by the evolution of the parental basaltic melt. Upon the trachytic melt ascending to the surface, the ferrocarbonate liquid was decomposed on magnetite, carbon, and carbon dioxide. [ABSTRACT FROM AUTHOR]

The Yeongnam Massif is located in the southern part of the Korean Peninsula and holds key elements in understanding the Palaeoproterozoic tectonic evolution of north‐east Asia during the amalgamation of the Columbia supercontinent. In this study, we have conducted geochronological and geochemical studies on the mafic intrusive along the northern margin of the Yeongnam Massif. Sensitive high‐resolution ion microprobe (SHRIMP) U–Pb zircon dating on two amphibolite and one gabbro samples in the Damyang area show that the amphibolites crystallized at ~1.87 Ga and the gabbro crystallized at ~87.6 Ma. The 1.87 Ga Damyang amphibolites are simultaneous with the Muju and Okbang amphibolites along the northern margin of the Yeongnam Massif. They also show similar geochemical characteristics, including tholeiitic basalt composition, enriched Light Rare Earth Elements (LREE) and large ion lithosphile elements (LILEs), depleted high field strength elements (HFSEs), and slightly enriched but various Hf isotopes, indicating that they originated from an enriched but heterogeneous mantle source with fluid addition. Thus, the 1.87 Ga amphibolites along the northern margin of the Yeongnam Massif were probably emplaced in an arc‐related setting. Combined with the 2.02–1.96 Ga magmatism, it is proposed that the subduction might have lasted during the 2.02–1.85 Ga interval and mantle‐derived magma rose up due to the roll‐back process between 1.88–1.85 Ga. [ABSTRACT FROM AUTHOR]

The Sabzevar ophiolite is the most extensive and best-exposed section of oceanic lithosphere in NE Iran. In this study, we examined the mantle section of the eastern part of the Sabzevar ophiolite, where mantle peridotites are the most widespread rock type and are crosscut by pyroxenite veins and mafic dykes. Major and trace element compositions (minerals and whole-rock) of lherzolites are akin to those of abyssal peridotites from mid-ocean ridges (MOR), whereas those of harzburgites evidence formation in a supra-subduction zone (SSZ) setting. Rare-earth element (REE) patterns of whole-rock and clinopyroxene of lherzolites and harzburgites suggest polybaric melting, with initial melting in the garnet peridotite field (~5–6%), followed by ~5–15% melting for lherzolites and ~10–20% for harzburgites in the spinel peridotite field. Harzburgites show LREE enrichment incompatible with partial melting models, indicating significant interaction with SSZ-related melts, whereas analysed lherzolites experienced limited melt-rock interaction. Pyroxene dissolution and olivine precipitation in residual peridotites resulting from reaction with island arc tholeiite and/or boninite melts led to the formation of dunites. These melts eventually intruded as dykes and veins in the mantle and crustal sections of the ophiolite. In addition, boninite melts are inferred to be responsible for forming olivine pyroxenite veins within the harzburgites. We conclude that the mantle section of the Sabzevar ophiolite formed in an intra-oceanic arc setting and experienced a geochemical evolution from a MOR-like to SSZ setting. The intra-oceanic subduction of the Neotethys oceanic lithosphere and shifting conditions of melting during the development of subduction beneath the incipient island arc were responsible for the geochemical heterogeneity of mantle peridotites and the generation of SSZ-related magmas within the Sabzevar oceanic lithosphere. [ABSTRACT FROM AUTHOR]

The Indus Suture Zone preserves the ophiolitic remnants of the eastern part of the Neo-Tethys Ocean. Parts of these ophiolitic remnants are exposed along the Khangral-Chiktan and Dras-Kargil road sections of western Ladakh. The observed rock types include ultramafic-mafic cumulates, gabbros, and volcanics emplaced as faulted blocks over the Mesozoic Dras arc complex. Geochemically, these rocks show sub-alkaline tholeiitic characteristics with basalt to basaltic-andesite compositional variation. Based on modal mineralogy, the ultramafic cumulates classify as olivine-websterite, the mafic cumulates as olivine-norite, and the gabbros as norite to hornblende-gabbro. The ultramafic cumulates depict depleted chondrite normalized REE patterns [(La/Yb)N = 0.6–1.1] while the mafic cumulates display depleted to enriched REE patterns [(La/Yb)N = 0.6–3.2]. Fractionated patterns are observed in gabbros [(La/Yb)N = 1.6–4.1], while flat and enriched chondrite normalized REE-patterns in volcanics [(La/Yb)N = 1.0–12.3] similar to NMORB, and EMORB. The multi-element patterns depict subduction-related geochemical characteristics such as enriched LILE and depleted HFSE compared to the primitive mantle. Presence of Mg-rich olivines, orthopyroxenes, and clinopyroxenes while Ti-poor clinopyroxenes in ultramafic-mafic cumulates reflect their derivation from previously depleted mantle sources at high-pressure and temperature comparable to the base of modern intra-oceanic island arc tholeiitic sequences. The studied rocks exhibit close similarity to an intra-oceanic subduction system contemporaneous to the Dras-Shergol-Suru-Thasgam ophiolitic slices of western Ladakh. [ABSTRACT FROM AUTHOR]

Deposition of terrestrial carbonate (i.e. travertine) and silica sinter in geothermal areas is always closely linked to the presence of relevant crustal structures which enhance the permeability in the upper crust favouring the upflow of fluids to the surface. Most travertine deposits are originated by geothermal fluids stored within carbonate reservoirs in continental crust, and their morphology and areal distribution provide useful information on geometry, kinematics, and age of faults which control the fluid flow. In this paper, we focus on the tectonic control on a travertine depositional system, associated with silica sinter, developing in oceanic crust, in the Lýsuskarð volcano-geothermal area on the Snæfellsnes Peninsula, West Iceland. The presence of HCO3 and Ca in the geothermal fluids is related to chemical-physical processes linked to fluid–rock interaction that occurs in the geothermal reservoir made up of tholeiitic basalt, by fluids enriched in CO2 of deep origin (i.e. magmatic). Travertine deposits consist of mounds and slopes, which are still in formation and overlie the silica sinter deposits. Both sinter and travertine deposits derive from thermal springs aligned according to two trends: NNE-SSW and WNW-ESE. This evidence suggests the occurrence of buried, permeable, and thus active faults, which control the circulation of fluids containing HCO3 and Ca. Faults are near parallel to those which define the rift systems and transform zones in Iceland, and probably comprise the still active sector of the abandoned transform zone that, starting with the Snæfellsnes Peninsula, interrupts eastward the northern continuation of the western and eastern rift-zones in the south-central Iceland. In this setting, we refer, for the first time, travertine and silica sinter deposition to an oceanic active crustal structure. [ABSTRACT FROM AUTHOR]

Fault rock petrology exerts an important influence on the permeability structure and mechanical properties of fault zones. Slip‐related deformation on upper‐crustal faults in basaltic rocks is closely associated with fluid‐rock interaction, altering the distribution of physical properties within the fault. Here, we present quantitative descriptions of the geochemical and petrological evolution of basalt‐derived fault rocks from three passively exhumed fault zones in the Faroe Islands. Fault‐rock petrology is determined by optical petrography and automated phase identification based on micrometer‐scale chemical maps from scanning electron microscope X‐ray spectroscopy. Geochemical evolution is assessed from major and trace element composition measured by X‐ray fluorescence. The fault rocks show intense fluid‐mediated alteration from a tholeiitic basalt protolith in the damage zones, and mechanical mixing in the fault cores. Pervasive alteration occurs early during fault zone evolution, with incipient fault damage increasing permeability and allowing along‐fault percolation of carbonated meteoric water, increasing fluid‐rock ratios. Our results suggest that the only mobile species within the fault zones are Ca, Si, and Al, which are leached during the hydrolysis of volcanic glass and plagioclase, and CO2, which is added by percolating waters. These species are transported from the damage zones into the fault cores, where they precipitate as zeolite and calcite cement in veins and hydrothermal breccias. We propose that solutes are replenished by cement dissolution through pressure‐solution during cataclastic creep, during repeated cycles of hydrofracture and cementation. The fault zones are natural reactors for fluid‐mediated alteration by CO2 and water, while other species are redistributed within the fault zones. Key Points: Basalt‐hosted fault petrology records progressive stages of fluid‐rock alteration followed by dissolution‐precipitation cycles of cementThe main mobile elements are Ca, Si, Al, and CO2, which are largely reprecipitated as Ca‐zeolites and calcite within the fault zoneAlternating calcite–zeolite mineralization highlights carbonate activity and metal cation availability feedback during fluid‐rock reaction [ABSTRACT FROM AUTHOR]

The ~3033 Ma Stella layered intrusion is hosted by supracrustal rocks of the Kraaipan–Madibe greenstone terrane, South Africa. The studied portion of the intrusion consists mainly of magnetite leucogabbro and magnetite anorthosite, as well as several massive magnetite layers. The intrusion hosts a laterally continuous, ~60-m-thick, PGE mineralized interval, with total resources amounting to 108t Pt + Pd + Au, constituting one of the oldest known PGE reef-style mineralizations on Earth. The richest reef, with a grade of 4.4 ppm Pt+Pd over a width of 5–8 m, occurs in semi-massive magnetitite. It is suggested that the mineralized oxide and silicate layers formed through a combination of primary magmatic, late magmatic, and hydrothermal processes, including granular flow and phase sorting of a magnetite- and sulfide-bearing gabbroic crystal mush that crystallized from a tholeiitic basalt, as well as remobilization of S and metals by late magmatic and hydrothermal fluids that led to crystallization of platinum-group minerals. [ABSTRACT FROM AUTHOR]

In the Mississippian Matachel small volcanic field of the Ossa-Morena Zone (southern Iberian Massif) outpoured basic-intermediate lavas exhibit geochemical characteristics of Low-Ti continental tholeiites and calc-alkaline lavas. Low-Ti continental tholeiites integrate two contrasting groups of rocks: basalts (Mg#: 54 to 70; Ti/Zr: 61-79; LaN/LuN: 1.6-2.9; εNdi: +4.0-+6.6; "Group #1"), and basalts and basaltic andesites (Mg#: 43 to 66; Ti/Zr: 36-58; LaN/LuN: 2.5-5.9; εNdi: -0.2-+3.5; "Group #2"). Primitive Group #1 tholeiitic magmas were generated by partial melting of a garnet-free lherzolite from an enriched lithospheric mantle, near the lithosphere-asthenosphere thermal boundary layer (with a very limited asthenosphere melting input). Progressive interaction of these magmas with crustal alkali igneous rocks resulted in the formation of the petrological evolutionary trends observed, to a larger extent in the case of Group #2 Low-Ti tholeiites. Further assimilation of amphibole-rich calc-alkaline metaigneous rocks might have originated the basalts and basaltic andesites with calc-alkaline signature (Mg#: 33 to 56; Ti/Zr: 25-78; LaN/LuN: 2.0-5.6; εNdi: +2.8-+4.8). These exhibit a "Cordilleran-type" evolutionary trend, though are unrelated to plate convergence. The magmas with calc-alkaline signature attest to a closed-system differentiation process controlled by the fractionation of plagioclase, clinopyroxene, magnetite and ilmenite. It is proposed that Mississippian lithospheric-scale intra-continental wrenching, unrelated to coeval mantle plume upwelling, reworked complexly docked mantle domains and triggered mantle melting. Enduring mid-upper crustal processes (magma storage in mid-crustal chambers and crustal assimilation) likely shaped the latest petrologic and geochemical aspects of the Matachel Low-Ti tholeiites and related rocks with calc-alkaline signature. [ABSTRACT FROM AUTHOR]

The Cretaceous High Arctic Large Igneous Province (HALIP) in Canada involved extrusion of continental flood basalts (CFBs) at 130-120 Ma and 100-95 Ma and emplacement of an extensive sill and dike network that intersected the Carboniferous to Paleogene Sverdrup Basin. In this paper, we present new 40Ar/39Ar ages, major and trace elements, and Sr-Nd-Pb isotope ratios for HALIP lava, dikes, and sills from Bukken Fiord, NW Axel Heiberg Island, Canadian Arctic Islands. Our best constrained 40Ar/39Ar ages yield a weighted average of 124.1 ± 1 (2σ) Ma, coincident with the first pulse of tholeiitic CFB magmatism in the Arctic-wide HALIP as exemplified by Isachsen Formation flood basalts on Axel Heiberg Island. The Bukken Fiord samples are plagioclase and clinopyroxene-phyric tholeiitic basalts, are relatively evolved (3.2-6.5 wt% MgO), and share similar major and trace element compositions to typical HALIP tholeiites. Initial 143Nd/144Nd ranges from 0.51260 to 0.51291 and initial 87Sr/86Sr ranges from 0.70362 to 0.70776, while measured 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb range from 18.614 to 19.199, 15.534 to 15.630, and 38.404 to 39.054, respectively. The most primitive sample in this study has Sr-Nd-Pb isotope signatures that suggest an enriched plume-derived mantle source for HALIP tholeiites. Most samples, however, possess relatively radiogenic isotope signatures that can be explained by moderate degrees of assimilation of Sverdrup Basin sedimentary rocks. Magma-crust interaction in the HALIP plumbing system was likely widespread and may have increased the environmental impact of the HALIP, particularly if crustal carbon was volatilized. [ABSTRACT FROM AUTHOR]

The study presents detailed petrographical, geophysical, structural and geochemical data of the internal nappes zone to establish the deformational history, origin and tectonic setting and constrain the crustal growth and evolution of the active margin of the Dahomeyide belt. Two main lithological units, (i) deformed meta-granitoids (migmatites and gneisses) and (ii) undeformed granitoids, dominate the internal nappes zone. The granitoids are generally I-type, metaluminous to weakly peraluminous, low-K tholeiite to high-K calc-alkaline and of tonalite, granodiorite and granite affinity. The overall trace element patterns of the studied granitoids characterized by the enriched LILE and depleted HFS, with negative peaks of Nb-Ta, Sr, P and Ti, are indications of arc-related magmatism. Structural analysis reveals four deformation phases (D1-D4). D1 represents Northwest-Southeast (NW-SE) Pan African shortening associated with a continent-continent collision, resulting in westward nappe stacking. Progressive NW-SE shortening resulted in D2 and D3 top-to-the-NW dextral and sinistral thrusting events during the Pan-African orogeny. D4 is an extensional event likely associated with the orogenic collapse phase. The gneisses and migmatites, with dominant axial planar foliations, point to their formation in a collisional setting or influence by the Pan-African collisional processes. Continental-arc signatures in these rocks imply continental subduction during their protolith formation. The intrusive granitoid and pegmatite are undeformed, meaning late- to post-orogenic emplacement. These findings suggest that the internal nappes zone archived the subduction-collision and post-collisional phase of the Pan-African orogeny and recorded large-scale migmatization and granitoid emplacement due to partial melting of thickened lower crust between Mid-Cryogenian and late Ediacaran. [ABSTRACT FROM AUTHOR]

The 2020–2022 field works gave results that were used as a basis for describing the geological structure and whole-rock compositions for Kronotsky Volcano, one of the least known eruption centers in the East Volcanic Belt (EVB), Kamchatka. The stratovolcano edifice was estimated to be 350 km3 in volume, which is much above the respective figures for the other stratovolcanoes in the EVB frontal zone. The rocks are low-K and high-Fe tholeiitic basalts and basaltic andesites (SiO2 = 47.04‒53.15 wt %; K2O = 0.24‒0.65 wt %; FeO*/MgO = 1.2‒2.89). The basalts have extremely low concentrations of silica, potassium, titanium, and phosphorus compared with the rocks of the other frontal volcanoes in Kamchatka. The petrochemical differences thus identified were used to more accurately classify structures situated at the junction between the nearby Kronotsky and Krasheninnikov volcanoes. The acquired data will make the basis for further petrologic and geochemical characterization of the volcano, as well as being possibly used for reconstructing the sequence of volcanic events in the area of study, including a more accurate history of the formation of Lake Kronotsky. [ABSTRACT FROM AUTHOR]

A comprehensive mineralogical, geochemical and isotopic review of six ultramafic-alkaline-carbonatite magmatic intrusions of the Shillong Plateau (Sung Valley, Jasra, Swangkre-Rongjeng, and Mawpyut) and Mikir Hills (Samchampi-Samteran and Barpung) is presented here, using the published data. These intrusions emplaced ca. 115–102 Ma ago, thus are significantly younger than the tholeiitic flood basalts erupted in Rajmahal-Sylhet province (ca. 118–115 Ma). The intrusive lithologies vary from ultramafic (dunites, clinopyroxenites, melilitolites) to mafic (ijolites, gabbros sensu lato, shonkinites), to felsic (syenites, nepheline syenites) and carbonatites (mostly calcite-rich varieties). The volcanic-subvolcanic facies (lamprophyres, phonolites) are not abundant. The range of chemical compositions of the magmatic phases in the various assemblages is notable; the intrusive rocks are thus the result of crystallization of magmas from variably evolved, independent liquid-lines-of descent, generally of alkaline/strongly alkaline lineages and sodic-to-potassic in affinity. The large variations of the Sr–Nd isotopic ratios of the silicate intrusive rocks (sensu lato) suggest a role of shallow-level crustal contamination during their formation. The carbonatites of the Sung Valley and Samchampi-Samteran have different isotope ratios than the associated silicate rocks, have some isotopic affinity with the Group I tholeiitic basalts of Rajmahal Traps and have an ultimate genesis in a carbonate-bearing lithospheric mantle. [ABSTRACT FROM AUTHOR]

Ignimbrite flare-ups are rare periods of intense silicic volcanism during which the pyroclastic volume and eruptive frequency is more than an order of magnitude higher than background activity. Investigating the compositional differences between flare-up and steady-state magmas provides critical constraints on the petrogenetic causes for the event and can offer unique opportunities to investigate the role of large-scale tectonic or geodynamic processes in arc magmatism. In this study, we focus on the bimodal Deschutes Formation ignimbrite flare-up of Central Oregon, which erupted unusually high volumes of pyroclastic material 6.25–5.45 Ma from a new axis of volcanism in the Cascades arc. This episode is marked by increased eruption rates and eruption of more silicic compositions relative to the Quaternary Cascade arc, which rarely erupts rhyolites. Ignimbrites are crystal-poor (<10%) dacite to rhyolites (mostly 65–77 wt.% SiO2) with anhydrous mineral assemblages and higher FeO/MgO, Y, Eu/Eu*, MREE and Zr/Sr, indicating drier magmatic evolution compared to the Quaternary arc, and are more similar to those from the rear-arc High Lava Plains (HLP) province that lies to the east. Magnetite-ilmenite oxybarometry indicates that Deschutes Formation felsic magmas tend to be hotter and more reduced (NNO-1 to NNO) than the Quaternary arc (NNO to NNO + 1.5). Rhyolite-MELTS geobarometry suggests complex storage of diverse Deschutes Formation magmas within the shallow crust (50–250 MPa), and the common co-eruption of multiple plagioclase populations, pumice compositions, and compositionally banded pumice suggest variable degrees of mixing and mingling of distinct magmas. Deschutes magmas also have low δ18Oplagioclase values that indicate partial melting and assimilation of hydrothermally altered shallow crust. Trace element systematics and rhyolite-MELTS modeling suggests that felsic pumice cannot be produced by simple fractionation of co-erupted mafic pumice or basaltic lavas, and requires a crustal melting origin, and trace elements and Pb isotopes suggest that young mafic crust may have been the primary protolith. We suggest that partial melting produced low-Si rhyolite melt (~72 wt.%) that acted as both a parent for the most evolved rhyolites, and as a mixing endmember to create the dacite to rhyodacite magmas with heterogenous plagioclase populations. Unlike the predominantly calc-alkaline basalts erupted in the Quaternary Cascade arc, Deschutes Formation primary basalts are mostly low-K tholeiites, indicative of decompression melting. These are similar to the compositions erupted during a contemporaneous pulse of low-K tholeiite volcanism across the whole HLP that reached into the Cascades rear-arc. We suggest that intra-arc extension focused decompression melts from the back-arc into the arc and that tensional stresses allowed this high flux of hot-dry-reduced basalt throughout the crustal column, causing partial melting of mafic protoliths and the production of hot-dry-reduced rhyolite melts. Depletion of incompatible elements in successive rhyolites implies progressive depletion in fertility of the protolith. Extension also allowed for the establishment of a robust hydrothermal system, and assimilation of hydrothermally-altered rocks by magmas residing in a shallow, complex storage network lead to low δ18O melts. Our findings suggest the integral role that extensional tectonics played in producing an unusual ignimbrite flare-up of hot-dry-reduced rhyolite magmas that are atypical of the Cascades arc and may be an important contributor to flare-ups at arcs worldwide. [ABSTRACT FROM AUTHOR]

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The Amer Belt, hosting eleven informal formations of the Amer supergroup, is proposed as type area for four regional Paleoproterozoic sequences (Ps1–Ps4) in central Rae Craton, western Churchill Province. The ca. 1.9–1.865 Ga Snowbird orogeny (DP1) affected only Ps1–Ps3, whereas the ca. 1.87–1.81 Ga Hudsonian orogeny (DP2) affected all four. Sequence Ps1 Ayagaq Lake formation (<2.3 Ga quartzite) initiates as schistose basal polymict orthoconglomerate unconformably overlying paleoweathered Neoarchean rocks. It transitions upward to lower Ps2 Resort Lake formation: gossanous, recessive, graphitic, pyritic-metalliferous phyllite (Re–Os age: 2126 ± 24 Ma). Ps2 conductors beneath the <1.74 Ga Thelon formation have been drilled for unconformity-related uranium. Gradationally overlying siliceous dolomitic marble (Aluminium River formation) completes Ps2. The gradationally overlying lower Ps3 is either foliated calcareous tholeiitic basalt (Five Mile Lake formation) or grey phyllite (Three Lakes formation). These lateral equivalents host strong linear aeromagnetic markers. The Oora Lake formation foliated feldspathic calcareous sandstone gradationally overlies only the Three Lakes formation and underlies the upper Ps3 Showing Lake formation (interbedded phyllite and foliated calcareous arkose) whose two arkose members host laterally continuous disseminated uraninite + magnetite forming linear aeromagnetic markers. Pristine sequence Ps4 molasse, deposited and weakly deformed during DP2, comprises four formations of rhythmically interbedded, deep maroon to pink and green, feldspathic-lithic arenite, conglomerate, and mudstone that unconformably overlie and include clasts of DP1-deformed Ps1–Ps3 + Neoarchean basement. Ps4 detrital zircon (3.05–1.90 Ga) suggests local + distal provenance from south Rae and Slave Cratons, and Taltson–Thelon magmatic zone. [ABSTRACT FROM AUTHOR]

The St. Ignace Island complex in Northern Ontario is a package of dominantly felsic rocks emplaced within the upper portions of the Osler Volcanic rocks of the ~1.1 Ga Midcontinent Rift System. The Osler volcanic rocks are predominantly tholeiitic basalts intercalated with rare interflow sediments and rhyolites. The St. Ignace Island complex is an ~26-km2 stock with a felsic core of quartz–feldspar–phyric rhyolites and dacites and an outer ring of anorthosite and gabbro. Textures at a variety of scales within the rocks of the complex show clear evidence of the mingling and mixing of partially crystallized mafic and felsic liquids. Two multigrain (zircon/baddeleyite) fractions from a sample of the gabbro define a Discordia line with an upper intercept date of 1107 ± 8.9 Ma. The core of the complex consists of dacites and rhyolites with similar REE abundances with negative Nb anomalies, whereas the surrounding mafic rocks are gabbros to monzogabbros that are less LREE-enriched than the felsic rocks but with similar HREE. Felsic units have a narrow range of 87Sr/86Sri (0.7032–0.7045) and 143Nd/144Ndi (0.51051–0.51057), whereas the mafic end members have similar 87Sr/86Sri (0.7040–0.7061) but more radiogenic 143Nd/144Ndi (0.51067–0.51085). Very well-preserved silicate melt inclusions (MI), many completely glassy, were observed in quartz, clinopyroxene, and some plagioclase phenocrysts from the complex. These represent some of the oldest unrecrystallized silicate melt inclusions described to date. Melt inclusions within quartz from the felsic volcanics are broadly rhyolitic in composition, whereas MI from plagioclase in the mafic volcanics range from basalt to basaltic andesite; these felsic and mafic melt compositions are interpreted to represent the end-member liquids in the system and bulk rock analyses affirm mixtures of the two. Concentrations of Cu and Ag (in both mafic and felsic MI), and Mo (in felsic MI), are up to an order of magnitude higher in the mafic and felsic MI than in continental crust. Bulk rock metal concentrations are also significantly lower than in the MI, suggesting that the melt inclusions may preserve pre-eruptive metal tenors that were subsequently modified by sulfide saturation, degassing, or post-solidus hydrothermal alteration. The whole rock and MI geochemistry of the St. Ignace complex are broadly similar to the Central Osler Group and, given the broad similar ages, suggests they may have been derived from a similar mantle source, but distinct from the source of rhyolites in the Black Bay Peninsula. The negative Nb anomalies and negative εNd values for the St. Ignace complex are consistent with mixing with older continental crust during ascent and emplacement. The rocks of the St. Ignace Island complex likely formed as the result of emplacement of a large mafic magma chamber at the base of the Osler volcanic pile that triggered partial melting to generate the rhyolite end members. The felsic melts ascended to shallower levels in the crust where they mixed with mafic magmas derived directly from the deeper chamber. Generally, melt inclusions in the complex have very high Cu and Ag contents, similar to those observed in arc-related and extremely oxidized early rift-related rocks and may account for the world-class volcano-sediment-hosted Cu–(Ag) deposits within the rift and the presence of small porphyry-style deposits. [ABSTRACT FROM AUTHOR]

Deciphering the sub-ice geology in the Wilkes Subglacial Basin region is important for understanding solid earth-ice sheet evolution and for assessing geological ties between East Antarctica and formerly contiguous Australia. We analyse marine sediment samples derived from drill site U1359 of Integrated Oceanic Drilling Program Expedition 318. Our study reports for the first time that the inland sediment source area comprises a complex mafic igneous terrain and a metamorphosed Precambrian subglacial basement. Pyroxene geochemical analyses confirm the presence of tholeiitic to calc-alkaline basalts. The high-grade part of the subglacial terrain contains upper amphibolite to granulite facies rocks that are comparable to Archaean to Palaeoproterozoic rocks exposed in the Terre Adélie Craton and the formerly adjacent Gawler Craton in Australia. Chemical Th-U-total Pb isochron method (CHIME) ages extracted from a subhedral monazite grain associated with the low-grade biotite-muscovite schist rock fragment provide a unimodal age of 799 ± 13 Ma. Rare occurrences of 800 Ma age in the Terre Adélie Craton and/or George V Coast provide evidence for the presence of at least one late Neoproterozoic magmato-metamorphic event in the interior of Wilkes Land. The affinity of the unexposed geological domains of Wilkes Land, East Antarctica, with their Australian counterparts is discussed in the context of the Rodinia supercontinent. [ABSTRACT FROM AUTHOR]

In Large Igneous Provinces (LIPs), sills are a key component of the plumbing system responsible for magma transport through the crust where magma fractionation takes place. In this work, we investigate four dolerite sills related to the Paraná-Etendeka LIP from Cerro do Coronel region, southern Brazil. They are basaltic andesites composed of plagioclase, augite+pigenonite, Ti-magnetite, and rare olivine, divided into a high- and a low-MgO group. The former is more depleted in SiO2, alkalis, and trace elements, with lower FeOt/MgO ratios, higher Eu/Eu*(N), less radiogenic 87Sr/86Sr and more radiogenic 143Nd/144Nd. The composition of Cerro do Coronel dolerites resemble those of evolved rocks from Morungava Intrusions, the silica saturated tholeiite dykes in southern Brazil, and the Cuaró Dyke Swarm dolerites from Uruguay. They evolved by fractional crystallization from primitive Gramado-type basalts under FMQ-2 fO2 conditions at low pressures (<3 kbar) and underwent assimilation of variable upper and lower crust, representing shallow-depth equilibration of magmas sourced from deeper crustal levels. The studied sills share some geochemical features with the Vale do Sol Fm. lavas, although we cannot assume they have the same age, and are part of a major plumbing system related to the low-Ti magmas of the Paraná-Etendeka LIP. [ABSTRACT FROM AUTHOR]

The extensive exposure of the Archean continental crust in southern West Greenland makes it an important window into the tectonic evolution of early Earth. Here, we provide a comprehensive geochemical data set for tholeiitic amphibolites (meta-basalts), calc-alkaline leucoamphibolites (meta-andesites), and ultramafic rocks (meta-cumulates) for the Mesoarchean Bjørneøen Supracrustal Belt, Nuuk region, SW Greenland. This data helps constrain the geodynamic setting in which these rocks formed. The volcanic rocks display two distinct geochemical trends in terms of their La/Sm ratios. Nonetheless, both tholeiitic amphibolites and leucoamphibolites have negative Nb-Ta-Ti anomalies and thus geochemical features associated with apparent island arc or crust contamination processes. Uranium-lead zircon dating of a leucoamphibolite yields an age of 3077 ± 6 Ma, which is older than regional orthogneisses. A series of models for both major element variation (thermodynamics-based) and trace element variation (partition coefficient-based) implies that fractional crystallization of tholeiitic basalt can effectively produce the observed ultramafic rocks. Such ultramafic cumulates had low degrees of crystallinity, reflecting open system magmatic process at shallow depths likely representing magma conduits in a volcanic pile. The geochemical features of the andesites are distinct from the basalts and our modeling excludes a connection via fractional crystallization or crustal assimilation of the two suites. Instead, the andesites formed via high degrees of mixing between basaltic and felsic endmember magmas, requiring elevated temperatures in the mid- to lower-crust. The introduction of felsic components could be derived from partial melting of mafic lower crust, for example by mafic underplating or via some other process that achieves such anatexis, or alternatively by the addition of rhyolitic melt from extensive fractional crystallization. Mixing and homogenization of basaltic and felsic endmembers to produce andesites may occur in modern-style subduction environments, although this could also be feasible in other geodynamic settings in a hotter early Earth. [ABSTRACT FROM AUTHOR]

This paper reports petrogenetic modelling and geodynamic evolution of felsic volcano-plutonic rocks of Riwasa and Nigana areas from Tusham Ring Complex (TRC), NW Indian shield and also discusses their geochemical characteristics to understand the possible magmatic source. These granitoids are identified with hypersolvus, high-K calc-alkaline to shoshonitic series, peraluminous, ferroan, typical A-type affinity and anorogenic genesis. The investigated rocks are enriched in SiO2, Na2O + K2O, REEs (except Eu), LILE + HFSE, halogens (F and Cl), depleted in CaO, MgO, Sr, Cr, Ni, P, Ti, V and Eu abundances and have elevated Fe/Mg, Ga/Al, Th/U, A/CNK ratios. The elemental geochemistry in conjunction with petrogenetic modelling (batch melting plus fractional crystallization) suggests that the felsic volcano-plutonic rocks of TRC were derived from a source similar to 12% partial melting of Nathwara tholeiitic basalt leaving a residue with (80% plagioclase, 5% orthoclase, 5% clinopyroxene, 5% orthopyroxene and 5% hornblende), 9% partial melting of Kundal basalt leaving a residue with (67% plagioclase, 13% orthoclase, 8% clinopyroxene, 7% orthopyroxene and 5% hornblende), 5% partial melting of Mesoproterozoic plagioclase amphibolite of southwestern Yangtze Block leaving a residue with (79% plagioclase, 4% orthoclase, 4% clinopyroxene, 9% orthopyroxene and 4% hornblende) and 15% fractional crystallization of least evolved felsic volcanic rocks leaving a residue with (60% plagioclase, 20% clinopyroxene, 5% hornblende and 5% magnetite). In this study, we also propose that the tectonic-magmatic activities that occurred in TRC are closely related to Neoproterozoic magmatism of NW India (Malani Igneous Suite-MIS), South China (SW Yangtze Block), NW Vietnam (Phan Si Pan Zone), SE Pakistan (Nagarparkar Igneous Complex), Seychelles and Madagascar. [ABSTRACT FROM AUTHOR]

The Kab Amiri area in the Central Eastern Desert (CED) of Egypt comprises ophiolitic rocks, island arc metasediments, and granitic rocks. This study deals with the petrography and geochemistry of the ophiolitic and island arc rocks to understand their petrogenesis and geodynamic evolution of the CED ophiolitic belt. The Kab Amiri ophiolite is dismembered, comprising serpentinites and metabasalt (spilite). Serpentinites have low contents of Al2O3 (1.14 wt%) and CaO (0.65 wt%), suggesting they are depleted peridotite protoliths (e.g., depleted harzburgites to dunites). The high L.O.I. value (13.7 wt%) of serpentinite rocks indicates intense hydration and serpentinization during alteration processes. Petrographic and geochemical studies suggest that serpentinites were likely formed after depleted peridotites in a supra-subduction zone (SSZ) setting (e.g., a fore-arc setting). Spilitic basalt shows a tholeiitic affinity of the depleted mantle source. The arc-related metasediments are represented mainly by schists and slate. Many samples of metasediments are relatively low in alumina (Al2O3 < 15%), suggesting a low clay content and formation in an island arc setting. In contrast, protoliths of island-arc metabasalts and meta-andesites crystallized from calc-alkaline magmas in the immature oceanic arcs. [ABSTRACT FROM AUTHOR]

The mafic volcanic rocks from the eastern felsic volcanic terrane (EFVT) of the Neoarchean Sandur greenstone belt of Western Dharwar Craton (WDC) are studied to evaluate their geochemical characteristics and tectonic implications. Characterised as subalkaline, tholeiitic basalts, they display moderate SiO2 (49.82–53.74 wt.%), Al2O3 (9.08–15.91 wt.%), TiO2 (0.25–0.66 wt.%) and total alkalies (Na2O+K2O= 0.60–3.97 wt.%) with Mg# ranging from 37–55 at low Cr (7–33 ppm), Ni (8–24 ppm) and V (252–612 ppm). Their rare earth and trace element patterns are coherent exhibiting moderate fractionation (La/Yb=0.16–1.09), slight negative Eu anomalies indicating limited plagioclase fractionation (Eu/Eu*= 0.69–0.96), negative Nb, Ta, Ti and Zr-Hf anomalies revealing their arc signatures. The Nb-Th and Zr-Y relationships of these basalts suggest their derivation from the partial melting of depleted mantle in an intra-oceanic arc setting. Trace element modelling of these basalts indicates their genesis by ∼15% partial melting of depleted MORB-mantle and depletion of the source is attributed to previous melting events. These basalts are geochemically similar with the depleted arc and forearc basalts of the Phanerozoic Tonga and Izu-Bonin-Mariana arcs emphasising the operation of analogous tectonic processes during the Archean era. [ABSTRACT FROM AUTHOR]

This paper presents the chemical composition (including H2O and CO2) of matrix glass, melt inclusions in phenocrysts, and their host minerals in eruptive products from Aso caldera. We found a group of melt inclusions with clearly lower potassium (0.6–2 wt % K2O at 50–70 wt % SiO2) than previously reported high-K2O whole-rock compositions (3–5 wt % K2O at 55–70 wt % SiO2). While most of the high-K2O intermediate to felsic melt inclusions are vapor undersaturated and show the features of H2O, CO2, and K2O accumulation, the low-K2O basaltic melt already has higher H2O and CO2. We reconcile this discrepancy with a model in which (1) the volatile-rich basalt magmas degas near the surface, (2) migrate back to depths of |$\ge$| 12 km in the crust, and (3) crystallize feldspar and quartz to produce high-K2O felsic melt, (4) magma mixing among the undegassed, degassed, and evolved magmas. By crystallization, about five times as much low-K2O basaltic magma is required to produce high-K2O felsic magma. The quantity of felsic magma ejected from 270 ka (Aso-1) to 90 ka (Aso-4) requires a basalt magma supply rate of 18 to 31 km3/ka. This magma supply rate is comparable to or less than the present-day production rate of degassed magma (73 km3/ka) at an active center of Aso, Nakadake. These findings suggest volcanic gas flux monitoring has the potential to be a 'basalt usage meter' during the dormant period of caldera volcanoes. [ABSTRACT FROM AUTHOR]

The c. 66 Ma Girnar Complex of the Deccan Traps consists of various mafic to silicic rocks and provides a unique setting for understanding the dynamics of magma chamber processes. The intrusive alkaline rocks in Girnar are diorites/monzodiorites and contain syenite/nepheline syenite veins. Several (alkaline) mafic dykes cut the alkaline rocks. These rocks are surrounded by intrusive tholeiitic gabbro and extrusive basaltic flows and a silicic (SiO2 > 65 wt%) ring dyke at the outer margin of the complex. The tholeiitic gabbro and basaltic lavas have relatively flat light rare earth element (REE) patterns (La/SmN ~ 0.8 to 1.8) with positive Eu anomalies of 1.1–1.3. The tholeiitic basalts formed by 3–5% partial melting of an incompatible element depleted mid-ocean ridge basalts (MORB)-like source. Accumulating c. 20–30% of (olivine, orthopyroxene, clinopyroxene, and plagioclase) crystals from the tholeiitic melt generated the tholeiitic gabbro. The alkaline rocks display enriched light REE patterns (La/SmN ~ 5.0 to 7.8) consistent with (~9%) partial melting of a spinel lherzolite source, different from the MORB-like source for tholeiitic rocks. The subsequent 58–72% fractional crystallization of olivine, clinopyroxene, plagioclase, orthoclase, spinel, and apatite mineral assemblage formed the alkaline rocks. Reverse and oscillatory zoning in plagioclase (An30 to An50) phenocrysts of the silicic rocks suggest that convective movement of crystals in a magma chamber led to initial crystallization of high-An plagioclase in the hotter zone of the magma chamber, followed by addition of low-An plagioclase in cooler regions of the magma chamber. Pressure estimates from shattered (xenocryst or antecryst) clinopyroxene crystals in silicic rocks give a minimum value of 450 MPa. Ti-in-quartz geothermometry gives a crystallization temperature between 690°C and 1080°C. The distinct isotopic composition of the silicic rocks (87Sr/86Sri = 0.7204–0.7275, εNd(i) = −6.8 to −7.3, 206Pb/204Pbi = 18.74–19.02, 207Pb/204Pbi = 15.76–15.79, 208Pb/204Pbi = 39.63–40.03, εHf(i) = −6.0 to −7.3) further indicates that the silicic rocks assimilated 7 to 9% granitic basement rocks. Overall, therefore, the Girnar Complex started to form with the emplacement of mafic rocks (both alkaline and tholeiitic) in the central part along a reactivated fault lineament and silicic rocks along the concentric marginal fault. Petrography, mineral chemistry, and whole-rock geochemical and Sr-Nd-Pb-Hf isotope ratios of the Girnar Complex rocks indicate that despite the close spatial association of tholeiitic gabbro and basalt, alkaline rocks, and silicic rocks, these rocks originated from multiple sources and evolved in a complex magma drainage and storage network in a continental setting of the Deccan Large Igneous Province (LIP). [ABSTRACT FROM AUTHOR]

The subduction initiation and the early‐stage tectonics of the southern Paleo‐Asian Ocean have rarely been discussed. To address these problems, we report new geochemical and geochronological data of arc‐related volcanic rocks in the Huaniushan arc of the southern Beishan. Basaltic andesites and high‐Mg rocks yield zircon U‐Pb ages of 449–422 Ma and 460–437 Ma, respectively. The basaltic andesite is a continental arc‐related tholeiite with moderate amounts of TiO2; and is enriched in light rare earth elements (LREE), such as Rb, Ba, Th and Sr; it is also depleted in Nb, Ta and Eu; and has moderate εHf (t) (−0.3 to +5.5) and εNd (t) (−3.48 to +0.03) values; and high (87Sr/86Sr)i (0.7066–0.7079) values. The high‐Mg andesite and dacite are sanukitic‐type high‐Mg rocks, which have high MgO (Mg# = 49–62) contents, A/CNK (0.82–2.36) and (La/Sm)N values, low Sr/Y (3–15) and Ba/Th (11–182) ratios, moderate to low εHf (t) (−16.7 to +7.6) and εNd (t) (−3.74 to +0.05), and high (87Sr/86Sr)i values. All geochemical data indicate that they were sourced from mixing between mantle‐peridotite‐derived melts and silicic melts generated by the subducting oceanic slab and/or sediments in a hot subduction setting. After integrating the Sanukitic high‐Mg andesites (HMAs) with previously reported Nb‐enriched basalt, volumes of arc‐related tholeiitic to calc‐alkaline volcanic rocks, I‐ to S‐type granites, and HT eclogites, we conclude that the subduction initiation was one of the most likely geodynamic process for the metamorphism and magmatism in the Middle Ordovician to early Silurian. Key Points: The Huaniushan high‐Mg rocks (460–437 Ma) are Sanukitic type high magnesium andesites (HMAs)The magmatic assemblages suggest a hot subduction in Middle Ordovician to early Silurian in the southern Beishan of the southern AlaidsMiddle Ordovician subduction initiation resulted volumes of Sanukitic HMAs, Nb‐enriched basalt, HT eclogite, and arc‐related magmatism [ABSTRACT FROM AUTHOR]

The Hannuoba basalt, located in the northern margin of the North China Craton, is a typical intra-continental basalt with ocean island basalt-like geochemical features and has been extensively studied. However, its origin and deep processes, such as magma mixing and crystallization conditions, are still unclear. To further understand the mechanisms leading to the compositional heterogeneity and magmatic processes of Hannuoba basalt at crustal and/or mantle depth, in situ major element, trace element, and 87Sr/86Sr compositional heterogeneity of four representative plagioclase crystals in three Hannuoba tholeiite samples, as well as whole-rock major and trace element data, are reported. According to the petrographic characteristics, the basalts are divided into fine-grained and coarse-grained groups. The anorthite content in plagioclase of samples varies in a small range (56–64%), but the content of trace elements in plagioclase from the coarse-grained samples is generally higher than that of the fine-grained samples. Clinopyroxene-melt equilibrium thermobarometer and plagioclase-clinopyroxene magnesium and rare earth element exchange thermometers show that the magma for the two types of basalt was stored and crystallized at a similar depth, and crystallized within a 20 °C (fine-grained basalt) and 50 °C (coarse-grained basalt) temperature window, which may be a reason for the grain size diferences between the two types of basalts. We found that 87Sr/86Sr of all the studied plagioclase crystals varied from 0.70333 ± 0.00018 (2SE) to 0.70556 ± 0.00031 (2SE), a much large range than the whole rock of Hannuoba basalts reported previously and consistent with that of Cenozoic basalts in North China. Therefore, at least two kinds of melts with significant differences in isotope and minor heterogeneity in major and trace elements are injected into each magma plumbing system. The content of trace elements in the Hanuoba tholeiite is between the Hanuoba alkaline basalt and the lower crust, which may be explained by the mixing of the alkaline basalt and the lower crust, but the low 87Sr/86Sr (<0.704) characteristics of plagioclase cannot be derived from alkaline basalts, because trace element abundances in the plagioclase are not in equilibrium with the alkaline basalt. Therefore, we believe that the compositional heterogeneity of Hannuoba tholeiitic basalt is caused by the mixing of heterogeneous lower crust rather than different mantle-derived melts. In turn this indicates that the contribution of the continental lower crust to the continental basalt is more complicated than previously recognized. [ABSTRACT FROM AUTHOR]