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Plate tectonics plays a crucial role in the evolution of Earth's continents and oceans. However, the impact of oceanic subduction and continental collision on the continental lithospheric mantle within collisional orogenic belts is still a matter of debate. To address this issue, we performed a thorough analysis of mafic rocks in the Nixiong area within the central Lhasa Terrane, southern Tibet. The mafic rocks are composed of diabases and basalts formed during syn-collisional period (~50 Ma). The diabases have low SiO2 and moderate MgO contents, and are enriched in large ion lithophile elements and depleted in high field strength elements. They show high 87Sr/86Sri (0.711451–0.712084) and low εNd(t) (−7.9 to −6.44), as well as enriched Hf-Pb isotope compositions (εHf(t) = -7.58 to −6.59; 206Pb/204Pbi = 18.59–18.71; 207Pb/204Pbi = 15.71–15.72; 208Pb/204Pbi = 39.33–39.53). Geochemical modelling suggests that the diabases were formed through the partial melting of peridotite at high temperatures and shallow depths, with significant addition of subduction-related components to the primary melts. The diabases probably originated from a metasomatized lithospheric mantle due to subduction of Neo-Tethyan slab. The basalts display similar SiO2 but lower MgO contents compare with the diabases. They are also characterized by extremely high 87Sr/86Sri (0.713332–0.713448), low εNd(t) (−10.32 to −9.96), and significantly enriched Hf-Pb isotope compositions (εHf(t) = -12.30 to −12.05; 206Pb/204Pbi = 18.81–18.85; 207Pb/204Pbi = 15.74–15.75; 208Pb/204Pbi = 39.81–39.88), corresponding to an extremely enriched lithospheric mantle. Integrated with the previous studies, the Nixiong mafic rocks exhibit the most enriched isotopic compositions among the arc- or syn-collision-related mafic rocks in southern Tibet. These extremely isotopically enriched components were most likely derived from the ancient continental lithospheric mantle beneath the central Lhasa Terrane. Our results provided unambiguous evidence that the pre-existing ancient continental lithospheric mantle could persist locally during the prolonged oceanic subduction and subsequent continental collision. [ABSTRACT FROM AUTHOR]

Processes of magma generation and transportation in global mid‐ocean ridges are key to understanding lithospheric architecture at divergent plate boundaries. These magma dynamics are dependent on spreading rate and melt flux, where the SW Indian Ridge represents an end‐member. The vertical extent of ridge magmatic systems and the depth of axial magma chambers (AMCs) are greatly debated, in particular at ultraslow‐spreading ridges. Here we present detailed mineralogical studies of high‐Mg and low‐Mg basalts from a single dredge on Southwest Indian Ridge (SWIR) at 45°E. High‐Mg basalts (MgO = ∼7.1 wt.%) contain high Mg# olivine (Ol, Fo = 85–89) and high‐An plagioclase (Pl, An = 66–83) as phenocrysts, whereas low‐Mg basalts contain low‐Mg# Ol and low‐An Pl (Fo = 75–78, An = 50–62) as phenocrysts or glomerocrysts. One low‐Mg basalt also contains normally zoned Ol and Pl, the core and rim of which are compositionally similar to those in high‐Mg and low‐Mg basalts, respectively. Mineral barometers and MELTS simulation indicate that the high‐Mg melts started to crystallize at ∼32 ± 7.8 km, close to the base of the lithosphere. The low‐Mg melts may have evolved from the high‐Mg melts in an AMC at a depth of ∼13 ± 7.8 km. Such great depths of magma crystallization and the AMC are likely the result of enhanced conductive cooling at ultraslow‐spreading ridges. Combined with diffusion chronometers, the basaltic melts could have ascended from the AMC to seafloor within 2 weeks to 3 months at average rates of ∼0.002–0.01 m/s, which are the slowest reported to date among global ridge systems and may characterize mantle melt transport at the slow end of the ridge spreading spectrum. Plain Language Summary: Mid‐ocean ridges are divergent plate boundaries where asthenospheric mantle undergoes decompressional melting to form basaltic melts and lithospheric mantle. The southwest Indian ridge (SWIR) represents the slowest spreading class of ocean ridges. It is characterized by intermittent volcanism resulting in thin or missing oceanic crust. To better understand the magma dynamics of such specific ridges and their correlation with the spreading rates, we calculated the depth of mineral crystallization in high‐Mg and low‐Mg basalts collected together and constrained the residence time of individual crystals in the melt through Fe‐Mg diffusion profiles. We show that the high‐Mg melts started crystallization close to the base of the tectonic plate, and likely partially crystallized in a deep magma chamber in the mantle to form low‐Mg magmas prior to eruption to the seafloor. This study determined that the rate at which the melts flowed up through the mantle to the crust was the slowest yet found for any ocean ridges. The mineral dynamics of the ultraslow‐spreading SWIR therefore illustrate the critical role of the velocity at which the tectonic plates separate on the formation of magmas at mid‐ocean ridges. Key Points: Deep crystallization depths (∼32 ± 7.8 km) and a deep axial magma chamber (AMC) (∼13 ± 7.8 km) may exist under the ultraslow‐spreading Southwest Indian Ridge (SWIR)The ascent rate from the AMC to seafloor of the SWIR is ∼0.002–0.01 m/s, which is the slowest in global mid‐ocean ridge systemsMagma ascent rates correlate positively with the spreading rates in the global mid‐ocean ridge systems [ABSTRACT FROM AUTHOR]

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The lithosphere--asthenosphere boundary (LAB), which can be seismically detected, stabilizes plate tectonics. Several conflicting hypotheses have been proposed as the causes of LAB discontinuity, such as the contribution of hydrated minerals, mineral anisotropy, and partial melts. The petit-spot melts ascending from the asthenosphere, owing to subducting plate flexures, support the partial melting at the LAB. Here, we observed the lava outcrops of six monogenetic volcanoes formed by petit-spot volcanism in the western Pacific. Thereafter, we determined the 40Ar/39Ar ages, major and trace element compositions, and Sr, Nd, and Pb isotopic ratios of the petit-spot basalts. The 40Ar/39Ar ages of two monogenetic volcanoes were ca. 2.6 Ma (million years ago) and ca. 0 Ma, respectively. The isotopic compositions of the western Pacific petit-spot basalts suggest their geochemically similar melting sources. They were likely derived from a mixture of high-μ (HIMU) mantle-like and enriched mantle (EM) -1-like components related to carbonatitic/carbonated materials and recycled crustal components. A mass balance-based melting model implied that the characteristic trace element composition (i.e., Zr, Hf, and Ti depletions) of the western Pacific petit-spot magmas could be explained by the partial melting of garnet lherzolite with a small degree of carbonatite melt flux with crustal components. This result confirms the involvement of carbonatite melt and recycled crust in the source of petit-spot melts and provides an implication for the genesis of tectonic-induced volcanism with similar geochemical signatures to those of petit-spots. [ABSTRACT FROM AUTHOR]

The lithosphere–asthenosphere boundary (LAB), which can be seismically detected, stabilizes plate tectonics. Several conflicting hypotheses have been proposed as the causes of LAB discontinuity, such as the contribution of hydrated minerals, mineral anisotropy, and partial melts. The petit-spot melts ascending from the asthenosphere, owing to subducting plate flexures, support the partial melting at the LAB. Here, we observed the lava outcrops of six monogenetic volcanoes formed by petit-spot volcanism in the western Pacific. Thereafter, we determined the 40Ar/39Ar ages, major and trace element compositions, and Sr, Nd, and Pb isotopic ratios of the petit-spot basalts. The 40Ar/39Ar ages of two monogenetic volcanoes were ca. 2.6 Ma (million years ago) and ca. 0 Ma, respectively. The isotopic compositions of the western Pacific petit-spot basalts suggest their geochemically similar melting sources. They were likely derived from a mixture of high-μ (HIMU) mantle-like and enriched mantle (EM) -1-like components related to carbonatitic/carbonated materials and recycled crustal components. A mass balance-based melting model implied that the characteristic trace element composition (i.e., Zr, Hf, and Ti depletions) of the western Pacific petit-spot magmas could be explained by the partial melting of garnet lherzolite with a small degree of carbonatite melt flux with crustal components. This result confirms the involvement of carbonatite melt and recycled crust in the source of petit-spot melts and provides an implication for the genesis of tectonic-induced volcanism with similar geochemical signatures to those of petit-spots. [ABSTRACT FROM AUTHOR]

In magma‐rich rifts, normal faulting is commonly thought to be induced by dike intrusions. However, whether fault slip occurs purely tectonically is unclear. An earthquake sequence starting with a Mw 5.5 earthquake occurred in December 2022 in northern Afar, a continental rift near breakup. InSAR measurements show that seismicity was caused by normal faulting alone, without involvement of magma movements. Our best‐fit InSAR models show that conjugate faults ruptured during the seismic sequence with mainly normal dip‐slip and total deformation corresponding to a Mw 5.7 event, in agreement with local seismic recordings. Our models show that tectonic faulting accommodates 26 cm of extension corresponding to ∼30 years of plate spreading without any link to magma. Our observations point toward significant along‐rift variation in the proportion of extension from faulting, potentially caused by along‐rift variations in rate of extension and/or from a spatially and temporally segmented supply of magma. Plain Language Summary: The Earth's continents move away from each other forming a rift valley between the two separating tectonic plates. In mature stages of the continental rupture process, it is commonly accepted that plate separation occurs from the migration of molten rock (magma) toward the Earth's surface in narrow zones beneath the rift valley. Brittle faulting of the plate was thought to be less important. In this study we analyze the recent earthquake sequence of 26–28 December 2022 in the volcanically active northern Afar rift of Ethiopia. We show that, while the sequence occurred within the rift valley, it was caused by pure faulting without any involvement of magma migrations. These observations are unexpected and show that faults alone can assist plate separation even in mature and magma‐rich rift valleys. Key Points: A seismic sequence with a Mw 5.5 mainshock occurred in the Bada region of northern Afar rift between 26 and 28 December 2022InSAR models and seismicity revealed co‐seismic deformation along conjugate faults with no involvement of magma motionsOur observations show that extension can be achieved through purely tectonic processes in magma‐rich continental rifts [ABSTRACT FROM AUTHOR]

New geochemical data demonstrate the similarity between Neoarchaean TTGs found in the middle crust of the Zimbabwe Craton and the lower crust of the Northern Marginal Zone of the Limpopo Belt. Hence the Neoarchaean segment of the Zimbabwe Craton provides a complete crustal section—from lower crustal granulites to the supracrustal rocks of the greenstone belts. Mantle activity recorded in the basalts of the greenstone belts is interpreted as plume volcanism, setting a context for the process of crustal generation. However, published isotopic data and new trace element data reported here show that magma genesis in Zimbabwe Craton in the Neoarchaean incorporated up to 20% of older crust, indicating that not all the granitoids are juvenile. This paper seeks to identify the trace element composition of the parental melts to the TTGs of the Zimbabwe Craton, define the petrological processes by which they were generated and set limits on possible petrological models for the process of crust generation. The least contaminated TTGs have low SiO2 and moderately fractionated trace element patterns. Calculations indicate that they were derived by the partial melting of a garnet amphibolite source at ca 10 kb depth. Rare intrusive diorites represent a low fraction melt of an enriched mantle source derived from refertilised depleted mantle. These observations lead to a model in which the source material of the TTGs was basaltic and the thermal energy to melt the basalts came from continued basaltic and komatiitic magmatism. Partial melts of old felsic crust were incorporated into the TTG magmas. The resulting 'enriched' TTGs provided a fertile lower crust from which an extensive suite of late tectonic granites could be generated. These data do not support a modern-style plate tectonic model for the growth of the Zimbabwe Craton in the Neoarchaean. [ABSTRACT FROM AUTHOR]

Arc volcanoes are underlain by complex systems of molten‐rock reservoirs ranging from melt‐poor mush zones to melt‐rich magma chambers. Petrological and satellite data indicate that eruptible magma chambers form in the topmost few kilometres of the crust. However, very few chambers have ever been definitively located, suggesting that most are too short‐lived or too small to be imaged, which has direct implications for hazard assessment and modeling of magma differentiation. Here we use a high‐resolution technology based on inverting full seismic waveforms to image a small, high‐melt‐fraction magma chamber that was not detected with standard seismic tomography. The melt reservoir extends from ∼2 to at least 4 km below sea level (b.s.l.) at Kolumbo—a submarine volcano near Santorini, Greece. The chamber coincides with the termination point of the recent earthquake swarms and may be a missing link between a deeper melt reservoir and the high‐temperature hydrothermal system venting at the crater floor. The chamber poses a serious hazard as it could produce a highly explosive, tsunamigenic eruption in the near future. Our results suggest that similar reservoirs (relatively small but high‐melt‐fraction) may have gone undetected at other active volcanoes, challenging the existing eruption forecasts and reactive‐flow models of magma differentiation. Plain Language Summary: Arc volcanoes, which mark the curved boundaries between converging tectonic plates, host the most explosive events on Earth. The associated hazard depends on how much mobile magma is currently present shallow beneath a volcano. Standard tomographic methods used so far have relatively low resolution and give a blurred picture of only the largest molten‐rock bodies. In particular, they struggle to distinguish between mobile magma and melt spread between tightly packed mineral grains. This study, a first in volcanology, combines a next‐generation tomographic method with extraordinarily dense seafloor recordings of controlled marine sound sources. This state‐of‐the‐art experiment at Kolumbo volcano, offshore of Santorini allowed us to detect a body of mobile magma which has been growing at an average rate of 4 × 106 m3 per year since the last eruption in 1650 CE. This rate is large enough to counteract the effect of cooling and crystallization. Our results show that Kolumbo poses a serious threat and deserves a real‐time monitoring facility. Despite the excellent data coverage, the small magma body was missed by standard tomography. This suggests that applying next‐generation imaging methods to already‐well‐studied volcanoes may lead to similar discoveries. We envision that small‐volume, high‐melt‐fraction reservoirs may be more widespread than previously thought. Key Points: A shallow, very strong negative Vp anomaly imaged under the explosive, submarine Kolumbo volcano, Greece, using full‐waveform inversionThe high‐fidelity image and petrologic data indicate the anomaly is a small (∼0.6‐km wide, ∼2‐km deep), magma chamber with ∼42% of meltThe chamber was missed by travel‐time tomography indicating similar reservoirs may have gone undetected at other volcanoes [ABSTRACT FROM AUTHOR]

Sanukitoid rocks make up a complete magmatic series with distinct geochemical characteristics of TTG suites and granitoids from modern magmatic arcs and are regarded as markers of the transition from typically Archean geodynamics to modern plate tectonics. Although most known sanukitoid suites formed during the Neoarchean and Mesoarchean, numerous papers have characterized Paleoproterozoic magmatic arc granitoid rocks showing affinity with the sanukitoid series. This work presents new data from field, lithogeochemistry, zircon U-Pb geochronology, and Sm-Nd and Sr isotopic studies on granodioritic granulites with sanukitoid signatures from the Juiz de Fora Complex (JFC), one of the Paleoproterozoic tectonic components of the Minas-Bahia Orogenic System (MBOS), southern São Francisco Paleocontinent (southeastern Brazil). These rocks crystallized at -2175 Ma and present values of εNdt between -4.0 and +0.5, TDM between 2.57 and 2.12 Ga, and 87Sr/86Sri between 0.6937 and 0.7137. We interpret these rocks as the result of crystallization of magmas from a hybrid mantle source extensively contaminated by crustal material during protracted subduction. In southeastern São Francisco Paleocontinent, the association of sanukitoid rocks with other mag- matic arc rocks points to a complex and prolonged Rhyacian accretionary system similar to modern plate tectonics. [ABSTRACT FROM AUTHOR]

The existing data on the composition, petrogenesis, magma sources and geodynamic settings of Adakite Magmatism and its input in the formation of the Earth's continental crust as well as its association with the onset of plate tectonics are reviewed and summarized. A vast amount of data have been accumulated over 30-year period from the first use of the term "Adakite Magmatism" in 1990, showing the presence of adakites in numerous sites of both Cenozoic magmatism at the Pacific convergent margins and Mesozoic and Paleozoic magmatic complexes formed in various geodynamic settings. Adakites are represented by different subtypes and are found in spatial and temporal association with specific Nb-enriched basalts and Cu-Au mineralization. The adakites are compared with Precambrian tonalite-trondhjemite-granodiorite (TTG) series showing similarities and differences. Various petrogenetic models of Adakite Magmatism and relevant geodynamic settings are reviewed. This review shows that adakites may be found in a very wide range of geodynamic settings and that the presence of such rocks in magmatic series does not always suggest their origin from partial melting of basalts of subducting oceanic crust. [ABSTRACT FROM AUTHOR]

• Pearce and Reagan (2019) boninite discrimination scheme assessed. • Two western Superior Province Craton. • Previously report of komatiites incorrect. • Revision of plume stratigraphy in the Abitibi required. • Craton growth largely by subduction processes at craton margins. The growth of Archean cratons has been linked to either stagnant or mobile lid processes in models that ultimately depend on the accurate interpretation of high-Mg volcanism. Recognition of the diversity of such greenstone belt magmas highlights a need for accurate classification of the sub-types, which is potentially offered by the boninite discrimination scheme of Pearce and Reagan (2019). The utility of the scheme is assessed at two localities in the western Superior Craton where komatiites have been reported but not extensively studied: the Lake of the Woods greenstone belt of the Western Wabigoon terrane and the Shebandowan greenstone belt of the western Wawa-Abitibi terrane. It is then applied to the southern Abitibi belt where evidence for both mantle plume and subduction-style magmatism presents complex geodynamic challenges. In combination with previously recognized boninites or boninitic rocks in the Kidd-Munro Assemblage and Opatica Terrane, the new results support models of short-lived subduction events in the Archean and underlines their role in craton growth prior to the development of a global network of tectonic plates. [ABSTRACT FROM AUTHOR]

We have reappraised the age and composition of the mid-Proterozoic Häme dyke swarm in southern Finland. The dominant trend of the dykes of this swarm is NW to WNW. Petrographic observations and geochemical data indicate uniform, tholeiitic low-Mg parental magmas for all of the dykes. Nevertheless, the variability in incompatible trace element ratios, such as Zr/Y and La/Nb, provides evidence of changing mantle melting conditions and variable crustal contamination. Our ID-TIMS 207Pb/206Pb ages for four low-Zr/Y-type dykes indicate emplacement at 1639 ± 3 Ma, whereas the most reliable previously published ages suggest emplacement of the high-Zr/Y-type dykes at 1642 ± 2 Ma. We propose that the Häme dyke swarm, and possibly also the other mid- Proterozoic mafic dyke swarms in southern Finland, records a progressive decrease in Zr/Y values due to magma generation under developing areas of thinned lithosphere. We consider that the formation of mafic magmas was most probably associated with the upwelling of hot convective mantle in an extensional setting possibly related to the nearby Gothian orogeny. The generation of tholeiitic magmas below continental lithosphere was probably promoted by the elevated mantle temperature underneath the Nuna supercontinent. We speculate that the origin of most of the relatively small mid-Proterozoic mafic dyke swarms, anorthosites, rapakivi granites, and associated rocks found across Nuna was similarly triggered by extensional plate tectonics and the convection of anomalous hot upper mantle below the supercontinent. [ABSTRACT FROM AUTHOR]

• ∼2.77–2.70 Ga calc-alkaline basaltic-andesitic rocks from shallow and metasomatized mantle. • ∼2.70–2.68 Ga tholeiitic rocks mainly from much deeper and unmodified mantle. • Early Neoarchean plate subduction-induced deep mantle upwelling in Jiaobei terrane. • Global alternation of plate subduction and deep mantle upwelling processes. • A critical geodynamic transition regime for the early Neoarchean Earth. How Earth switched from any earlier regimes such as plume-lid tectonics to plate tectonics remains an unresolved issue in Earth sciences. We report early Neoarchean (∼2.77–2.68 Ga) metavolcanic rocks, including older calc-alkaline basaltic-andesitic rocks in the southwest but younger tholeiitic rocks in the northeast, from the Jiaobei terrane of North China Craton. Petrogenetic studies and thermodynamic and trace element modeling reveal that the tholeiitic magmas originated mainly from deep and unmodified mantle sources (∼1600 °C and ∼3.5 GPa), relative to the shallower and metasomatized depleted mantle sources (∼1450 °C and ∼2.2 GPa) of calc-alkaline magmas. Geochemical changes indicate that the mantle sources became isotopically enriched but less metasomatized from southwest to northeast. These data suggest an early Neoarchean plate subduction-induced deep mantle upwelling regime. We further depict a potential geodynamic framework for the early Neoarchean Earth involving active interaction of plate subduction and deep mantle upwelling, which possibly changed the thermal evolutionary trajectory of the Earth and accelerated the arrival of global plate tectonics. [ABSTRACT FROM AUTHOR]

Determining whether seismicity near volcanoes is due primarily to tectonic or magmatic processes is a challenging but critical endeavor for volcanic eruption forecasting and detection, especially at poorly monitored volcanoes. Global statistics on the occurrence and timing of earthquakes near volcanoes both within and outside of eruptive periods reveal patterns in eruptive seismicity that may improve our ability to discern magmatically driven seismicity from purely tectonic seismicity. In this paper, we catalog magnitude four and greater (M4+) earthquakes near volcanoes globally and compute statistics on their occurrence with respect to various eruptive and volcanic attributes, evaluating their utility as diagnostic indicators of eruptions. Using a 2‐week time window and a 30 km radius around the volcanoes, we find that 11% of eruptions are preceded by at least one M4+ earthquake, but only 1% of such earthquakes is followed by eruption. However, earthquakes located 5–15 km from the volcano, those with normal faulting mechanisms and/or large nondouble‐couple components, and those occurring as groups are more commonly associated with eruptions, providing significant forecasting utility in some cases. Similarly, certain volcanoes are more likely to exhibit such precursors, such as those with long repose periods. We illustrate the use of these data in eruption forecasting scenarios, including rapid identification of analogous earthquake sequences at other volcanoes. When integrated within the context of multiparametric, multidisciplinary probabilistic assessments of volcanic activity, global earthquake statistics can improve eruption forecasts, and our work provides a model for use on other rapidly expanding global volcanological databases. Plain Language Summary: Magnitude four and larger (M4+) earthquakes in volcanic regions are only rarely followed rapidly by volcanic eruptions. In fact, most such earthquakes are caused by normal tectonic activity in the regions hosting the volcanoes. However, quickly determining the cause of any such earthquake is difficult. In this paper, we compute statistics of M4+ earthquakes near volcanoes globally in order to reveal patterns in the earthquake data that might be useful for improving volcano eruption forecasting and detection. Using a 2‐week time window and a 30 km radius around the volcanoes, we find that 11% of eruptions are preceded by at least one M4+ earthquake, but only 1% of such earthquakes is followed by eruption. However, certain kinds of M4+ earthquakes are more likely to be directly caused by magma movement and certain kinds of volcanoes are more likely to experience these earthquakes prior to erupting than others. Our results provide significant forecasting utility in some cases and provide a model for future statistical analyses of other global volcano data sets, which promise to further improve volcano eruption forecasting and detection efforts. Key Points: Eruption forecasting and detection benefits from analyses of global earthquake data, including seismicity unassociated with eruptionsAnalysis of global magnitude four and greater (M4+) earthquakes near volcanoes reveals patterns in seismic activity precursory to eruptionsGlobally, 1% of M4+ earthquakes within 30 km of volcanoes are followed by eruption within 2 weeks [ABSTRACT FROM AUTHOR]

The Paleozoic evolution of Patagonia was the focus of controversies between its allochthonous or autochthonous origin. The arrival of plate tectonics supported new allochthonous alternatives and from an initial fixist resistance, different mobilistic hypotheses have made their way. There is currently some consensus about its allochthony, but there is no agreement on collision times or in the configuration of the continental blocks involved. Based on the present data an alternative is developed that fits better with existing information. The northeast-vergent deformation in Ventania System, the Hespérides Basin, its wide longitudinal and transverse distribution, show that collision occurred in the northern sector of Northern Patagonian Massif, and extended through the southern African Gondwanides. Their similar metamorphic and tectonic patterns identified a previous southward subduction with a Permian climax, characteristic of a continent-continent collision. The associated magmatic arc has been partially obliterated by slab-breakoff and delamination in the Late Permian-Triassic. The western magmatic belt along the Pacific margin is older, spanning from Devonian to mid-Carboniferous. The Chaitenia island arc collision in Upper Devonian produced an episode of exhumation and uplift. This western belt extends into Tierra del Fuego island and its contour allows tentatively to recognize a Southern Patagonian terrane. It is speculated that this block may have included the Antarctic Peninsula, although more data is needed to characterize its composition and areal development. However, it is concluded that the dimensions of this southern terrane cannot justify the broad regional deformation of the Gondwanides. [ABSTRACT FROM AUTHOR]

IODP Site U1437 is located in the Izu rear‐arc region, approximately 330 km west of the Izu‐Bonin trench axis. The oldest four units (Units IV through Unit VII) include volcaniclastic sediment and in situ hyaloclastites. They have ages of about 6–15 Ma, shortly after cessation of Shikoku backarc basin opening. Three magma types are identified by their distinct geochemistry; they are similar types to those found in the modern Izu arc (Rear Arc Seamount Chain [RASC]‐type, Rift‐type, and volcanic front [VF]‐type). RASC‐type has the most enriched Nd and Hf isotope and fluid‐immobile trace element ratios and dominates from 9 to 6 Ma. Rift‐type, dominant from 15 to 9 Ma, is similar to VF‐type in Nd‐Hf isotopes but has the least radiogenic Sr and Pb, and intermediate La/Yb and Nb/Yb, indicating a more fertile mantle source and less hydrous slab component than VF‐type. Less common and randomly distributed VF‐type sediments have the most radiogenic Sr and Pb, and the highest Ba/(Th, LREE [light rare earth element]) ratios, and are interpreted to be distally derived. The genesis of mafic Unit VII samples (~15 Ma) was modeled using the Arc Basalt Simulator. Results are most similar to those for basalts in the modern rift environment indicating the addition of ~1% of a melt‐rich slab component generated at ~125 km, to a Philippine Sea Plate ambient mantle that was more depleted than DMM (depleted MORB mantle). The initial post‐Shikoku basin magmatism in the Izu rear‐arc generated Rift‐type magmas for about 6 million years before the distinctive RASC‐type magmatism began, which then became increasingly enriched. Plain Language Summary: We explore the history of magma genesis in the Izu rear‐arc, Japan, by studying the geochemistry of volcaniclastic sediments that were obtained by ocean drilling. This paper presents their Sr‐Pb‐Nd‐Hf isotope ratios and numerical models of magma genesis. We identify the presence of three geochemical types that change through time. Rift‐type formed soon after cessation of seafloor spreading in the Shikoku Basin at ~15 Ma and came from a more fertile mantle source with a less hydrous slab component than at the contemporaneous volcanic front. This type is most like basalts from rifts behind the current volcanic front and was unknown before drilling. RASC‐type dominates from 9 to 6 Ma and has more enriched Nd and Hf isotope and fluid‐immobile trace element ratios that are like those of rocks dredged from the coeval rear‐arc seamount chains. VF‐type is uncommon, randomly dispersed, and interpreted as derived from the distal volcanic front. The results confirm that the arc front has migrated trenchward since the Miocene. Key Points: The geochemistry of the volcaniclastics and hyaloclastites of IODP Site U1437 is reportedTwo magma types and one magma type of distal origin existed in the Izu rear‐arc after spreading in the Shikoku Basin endedThe initial post backarc basin magmatism generated magmas similar to Quaternary Rift‐type and evolved to rear‐arc seamount chain type [ABSTRACT FROM AUTHOR]

Using ~25 years of GPS and InSAR observations and the time‐dependent inversion filter, we invert for the volume change history of three active volcanoes in the east central Alaska Aleutian arc and the secular velocity of the region. The inferred time‐dependent volume change shows (1) two exponentially decaying pulses followed by a nearly linear inflation period at Westdahl volcano, (2) multiple episodic inflation events at Akutan volcano, and (3) transient volcanic inflation and long‐term deflation signals at Makushin volcano. Comparing our regional velocity estimates with recently published block models, we find a small signal that can be explained by slip deficit on the megathrust, either from a shallow (<20 km depth) almost fully locked zone or a weakly locked zone confined to 25–50 km depth. Plain Language Summary: In the east central Alaska Aleutian arc, the surface is deforming due to magma supply beneath the active volcanoes, tectonic plate motion, and slip deficit on the plate interface (at some places where two tectonic plates cannot freely slide relative to each other). In this study, we separate and model those geophysical processes using measurements of surface deformation during the past 25 years and interpret the long‐term variations in magma supply at each volcano. Key Points: Inflation at Westdahl volcano shows two distinct exponentially decaying pulses followed by a long linear inflation periodAkutan volcano has experienced multiple episodic inflation events, and we observe long‐term transient volcanic signals at Makushin volcanoBlock motion and a small elastic strain signal from the subduction interface beneath the region can explain the steady nonvolcanic signal [ABSTRACT FROM AUTHOR]

The concentrations of Cu and Ag, both insoluble chalcophile elements, can be used to place tight constraints on the proportion of basalts in the upper continental crust (UCC) through time via analyses of fine-grained terrigenous sedimentary rocks. Copper and Ag concentrations in magmas are largely controlled by sulfide dissolution during melting and sulfide fractionation during differentiation. We show that Cu is high in basalts but low in komatiites and felsic rocks, making Cu useful for constraining the proportion of basalts in the UCC. Furthermore, Cu/Ag ratios are high in basalts and komatiites but decrease with differentiation. The fine-grained matrix of ancient glacial diamictites shows dramatic declines in both Cu concentrations and Cu/Ag ratios at 3.0–2.4 Ga, after which diamictite Cu concentrations and Cu/Ag ratios remain roughly constant. Mass-balance calculations using average Cu concentrations of Archean komatiites, basalts and felsic rocks require that a high proportion basalt (65–75%) was present in the UCC at ∼3.0 Ga, and this crust transitioned to a felsic-rock-dominated crust at ∼2.4 Ga ago and has remained basically unchanged since then. This conclusion is supported by the consistency between the Cu/Ag ratios in the diamictites and the weighted average Cu/Ag ratios based on the lithological proportions obtained using Cu mass-balance calculations. Our observations document the emergence of felsic continental crust in the late Archean, which, in turn, indicates a fundamental change in Earth's dynamic regime at that time. [ABSTRACT FROM AUTHOR]

Many recent studies conclude that plate tectonics started about 3 billion years ago in the mid Archean. The transition from a pre-subduction regime to modern plate tectonics is reported to be marked by changes in trace element ratios or isotopic compositions that monitor the rate of growth of the continental crust, the appearance of eclogitic inclusions in diamonds, or an apparent change in the composition of the upper crust. Behind most of these arguments is the hypothesis that, early in Earth history when the mantle was hotter, subduction was intermittent or impossible. If so, a mechanism other than subduction must have created the granitoids that dominate Archean continental crust. One alternative, commonly referred to as sagduction, proposes that the base of thick oceanic crust founders and partially melts to generate granitic magma. Here we evaluate the sagduction process, starting by discussing two crucial concepts: (1) thick oceanic crust is internally differentiated, with hydrated basalt being restricted to the uppermost layers, (2) the generation of granitic magma requires that water and basalt is present in the lower part of the crust or is taken deep into the mantle. We present the results of numerical modelling that demonstrates that when intrusion is taken into account, the lower portion of the crust is well above dehydration temperatures and therefore essentially dry. We show that any deformation within thick, differentiated crust is restricted to the lowermost layers of dry, infertile mafic-ultramafic cumulates that lack the ingredients essential for the generation of granitic magma. Given the implausibility of the sagduction process, we suggest that subduction was the main mechanism that generated granitoid magmas, in the Archean as today. [ABSTRACT FROM AUTHOR]

Magmatic intrusions play a vital role not only in accommodating extensional stresses in continental rifts but also in feeding volcanic systems. The location, orientation, and timescale of dike intrusions are dictated by the interaction of regional and local stresses, the effect of pre‐existing weaknesses, and the composition of magma. Observing active intrusions can provide important information regarding the interaction between magmatic processes and the tectonic stress field during continental rifting. We focus on a seismic swarm that occurred in 2015 to the northeast of Fentale volcano, in the Main Ethiopian Rift (MER), and use radar interferometry to study surface deformation associated with the seismic swarm. Interferograms show a pattern of dike‐induced deformation, with a model estimate of volume change of 33×106±0.6×106m3 at a depth range of 5.4 to 8 km. We use a small baseline subset algorithm to calculate line of sight time series and find that the displacements decay exponentially with a decay constant of ∼83 days. Coupled source‐sink models suggest that such slow dike intrusions require a high viscosity rhyolitic magma. The difference in behavior between Fentale and other caldera systems in the MER, which show multi‐year cycles of inflation and deflation, suggests fundamental differences in magma composition and architecture of the plumbing system. This is the first direct observation of a dike intrusion in the MER and provides new constraints on the temporal‐spatial patterns of stress and strain that occur during continental rifting. Whether this activity is transient or a long‐term feature associated with rift evolution is an open question. Key Points: We report the first directly detected dike intrusion to occur in the Main Ethiopian Rift (MER)The dike‐induced deformation decayed exponentially (τ= 83 days) consistent with a relatively high viscosity peralkaline rhyolite magmaMagmatism in the northern MER is currently tectonically controlled in contrast to caldera‐wide deformation in the Central MER [ABSTRACT FROM AUTHOR]

The occurrence of post-subduction magmatism in continental collision zones is a ubiquitous feature of plate tectonics, but its relation with geodynamic processes remains enigmatic. The nature of mantle sources in these settings, and their interaction with subduction-related components, are difficult to constrain using bulk rocks when magmas are subject to mixing and assimilation within the crust. Here we examine post-collisional magma sources in space and time through the chemistry of olivine-hosted melt inclusions and early-formed minerals (spinel, olivine and clinopyroxene) in primitive volcanic rocks from the Neogene–Quaternary East Carpathian volcanic range in Călimani (calc-alkaline; 10.1–6.7 Ma), Southern Harghita (calc-alkaline to shoshonitic; 5.3–0.03 Ma) and the Perșani Mountains (alkali basaltic; 1.2–0.6 Ma). Călimani calc-alkaline parental magma compositions indicate a lithospheric mantle source metasomatised by ~ 2% sediment-derived melts, and are best reproduced by ~ 2–12% melting. Mafic K-alkaline melts in Southern Harghita originate from a melt- and fluid-metasomatised lithospheric mantle source containing amphibole (± phlogopite), by ~ 5% melting. Intraplate Na-alkaline basalts from Racoș (Perșani) reflect small-degree (1–2%) asthenosphere-derived parental melts which experienced minor interaction with metasomatic components in the lithosphere. An important feature of the East Carpathian post-collisional volcanism is that the lithospheric source regions are located in the lower plate (distal Europe-Moesia), rather than the overriding plate (Tisza-Dacia). The volcanism appears to have been caused by (1) asthenospheric uprise following slab sinking and possibly south-eastward propagating delamination and breakoff, which induced melting of the subduction-modified lithospheric mantle (Călimani to Southern Harghita); and (2) decompression melting as a consequence of minor asthenospheric upwelling (Perșani). [ABSTRACT FROM AUTHOR]

We present paleostress analyses of dike swarms intruded during the subduction initiation in the northern Oman Ophiolite to understand the tectonomagmatic environment. Five swarms of subparallel dikes extending WNW-ESE are 1-5 km in width and are spaced every 5 km N-S. Each swarm has a core of 100% sheeted dikes 1-2 km in width, which emanated from the dunitewherlite- clinopyroxenite-gabbronorite-diorite- tonalite complexes below and intruded through V1 and into V2 extrusive rocks. Individual dike strikes are varied but generally subparallel to the overall trend of the swarm. Paleostress analyses indicate subvertical s1, ~s2, and subhorizontal s3 with high magma pressures, resulted in the mutually intrusive, extensional shear dikes and abrupt changes in dike strike at high angles. These occurrences suggest intrusions under a more compressive environment compared to the extensional stress field that formed the N-S-striking sheeted dikes of V1 spreading stage. Most E-W-striking dikes possess both boninitic and tholeiitic geochemistry. The latter resemble the V1 flows and dikes with affinities of mid-ocean ridge basalt. Some tholeiitic dikes strike N-S, which are mutually intrusive to E-W-striking dikes. Tholeiitic dikes are more intensely altered than boninite, suggesting their older ages. Conversion of the stress field from a N-S- running spreading axis to inextensional E-W-running rift zones associated with the change in magma geochemistry agree with the relatively compressive V2 arc above a forced subduction zone, which originated from intraoceanic thrusting caused by the clockwise rotation of a microplate including the future northern ophiolite. [ABSTRACT FROM AUTHOR]

Quantifying the global volcanic CO2 output from subaerial volcanism is key for a better understanding of rates and mechanisms of carbon cycling in and out of our planet and their consequences for the long‐term evolution of Earth's climate over geological timescales. Although having been the focus of intense research since the early 1990s, and in spite of recent progress, the global volcanic CO2 output remains inaccurately known. Here we review past developments and recent progress and examine limits and caveats of our current understanding and challenges for future research. We show that CO2 flux measurements are today only available for ~100 volcanoes (cumulative measured flux, 44 Tg CO2/year), implying that extrapolation is required to account for the emissions of the several hundred degassing volcanoes worldwide. Recent extrapolation attempts converge to indicate that persistent degassing through active crater fumaroles and plumes releases ~53–88 Tg CO2/year, about half of which is released from the 125 most actively degassing subaerial volcanoes (36.4 ± 2.4 Tg CO2/year from strong volcanic gas emitters, Svge). The global CO2 output sustained by diffuse degassing via soils, volcanic lakes, and volcanic aquifers is even less well characterized but could be as high as 83 to 93 Tg CO2/year, rivaling that from the far more manifest crater emissions. Extrapolating these current fluxes to the past geological history of the planet is challenging and will require a new generation of models linking subduction parameters to magma and volatile (CO2) fluxes. Key Points: Progress in determining subaerial volcanic CO2 flux has been significantChallenges remain with regard to extrapolations through time and global coverage of measurements and with regard to diffuse tectonic degassing and dynamic nature of volcanic degassingVolcanic and tectonic contributions are <2% of current anthropogenic contributions [ABSTRACT FROM AUTHOR]

Basal magma oceans develop in Earth and Venus after accretion as their mantles solidify from the middle outward. Fractional crystallization of the basal mantle is buffered by the core and radiogenic and latent heat in the magma ocean. Previous studies showed that Earth's basal magma ocean would have solidified after two or three billion years. Venus has a relatively hot interior that cools slowly in the absence of plate tectonics, which reduces heat flow through the solid mantle. Consequentially, the basal magma ocean could remain as thick as ~200–400 km today. Vigorous convection of liquid silicates could power a global magnetic field until recently while a core‐hosted dynamo is suppressed. The basal magma ocean may be a hidden reservoir of potassium and other incompatible elements. A high tidal Love number could reveal a basal magma ocean and would definitively establish that the core is at least partially liquid. Plain Language Summary: Venus is Earth's nearest neighbor but arguably the least‐studied planet in the inner solar system. Although there are no direct constraints on its deep structure, the mantle of Venus is assumedly solid by analogy to Earth's current condition. However, recent models of Earth focus on the prospect that a thick layer of melt called a "basal magma ocean" persisted in the lowermost mantle for billions of years. This layer cools orders of magnitude more slowly than a magma ocean near the surface because the solid mantle acts as an ~3,000‐km‐thick blanket. Moreover, the solid mantle itself remains hot in Venus compared to Earth because its surface is scorched and desiccated. This study argues that the lifetime of the basal magma ocean in Venus plausibly extends to the present. Detecting a thick, molten layer with future spacecraft missions would support the hypothesis that Venus and Earth formed under similarly energetic conditions. Key Points: Extensive melting of the deep mantle during Earth's accretion and differentiation was proposed to solve geochemical and geodynamic puzzlesHigh temperatures and slow mantle cooling relative to Earth naturally extend the predicted lifetime of a basal magma ocean in VenusA basal magma ocean in Venus would sequester incompatible elements such as potassium and argon and may have sustained a dynamo [ABSTRACT FROM AUTHOR]

The southern Aegean is characterized by an ongoing process of oceanic subduction. In this study, 1,718 intermediate depth (>35 km) events recorded in the southern Aegean are used to calculate the peak delay times (tp). The tp are measured as the time difference between S wave onset and the peak of the S wave envelope in 2‐to 4‐, 4‐ to 8‐, 8‐ to 16‐, and 16‐ to 32‐Hz bands. The tp are also inverted to estimate the scattering parameters (κ and εparam) and power spectral density function (P (m)) at large wave numbers (m = 15 km−1 or ml) of the medium modeled as von Kármán type. κ controls frequency dependence of scattering while P (ml) represents the scattering strength in high frequencies for ɑml >> 1 (ɑ = correlation length of inhomogeneities). High P (ml) with low κ across Peloponnese in 0‐ to 20‐km depth may result from intermixing of oceanic material with continental rocks at different levels with a possibility of fluid activity and salt diapirism in northwest Peloponnese. Moderate P (ml) in 0‐ to 60‐km depth across Crete is likely due to a combination of factors including past megathrust earthquakes, sediment underplating, and flow of metamorphosed material in a subduction channel. Cyclades shows low κ in 0‐ to 40‐km depth, indicative of inhomogeneities produced by metamorphic core complexes. Very low κ in 60‐ to 80‐km depth is consistent with the location of mantle magma reservoir in the back‐arc region. Our results suggest that a significant portion of total S wave attenuation (Qs−1) in eastern Cyclades for 20‐ to 80‐km depth may be due to scattering losses. Key Points: Peak delay times of S waves are calculated to study the scattering strength in southern AegeanSpectral density of inhomogeneous medium is modeled as von Kármán type and its parameters are evaluated by inverting peak delay timesHigh inhomogeneity zones are observed in Cyclades, Crete, and upper crust of Peloponnese [ABSTRACT FROM AUTHOR]

One of the world's most notable intraplate volcanic regions lies on the eastern Australian plate and includes two age-progressive trails offshore (Tasmantid and Lord Howe seamount chains) and the world's longest continental hotspot trail (Cosgrove Track). While most studies agree that these chains formed by the rapid northward motion of the Australian plate over a slowly moving mantle source, the volcanic output along these trails, their plate–mantle interactions and the source of the magmatism remain unconstrained. A geophysical mapping and dredging campaign on the RV Southern Surveyor (ss2012_v06) confirmed the prolongation of the Lord Howe Seamount Chain to the South Rennell Trough, ∼200 km further north than previously sampled. Radiometric dating of these new samples at 27–28 Ma, together with previously published results from the southern part of the chain, indicate straightforward northward motion of the Australian plate over a quasi-stationary hotspot as predicted by Indo-Atlantic and Pacific hotspot models. A peak in Lord Howe Seamount Chain magmatism in late Oligocene time, also seen in the Tasmantid and Cosgrove trails, matches a 27–23 Ma slowdown of Australian plate motion. The average magma flux of the Lord Howe hotspot is estimated at 0.4 m3 s−1, similar to rates of crustal production at the South Rennell Trough prior to cessation of spreading in middle Oligocene time, supporting a potential genetic relationship to this spreading system. In addition, plate tectonic modelling suggests that the seamounts and plateaus in the Coral Sea may host the earliest evidence of plume activity in the area. [ABSTRACT FROM AUTHOR]

The ultraslow eastern Southwest Indian Ridge (SWIR) offers an opportunity to study the effect of magma supply on an ultraslow mid-ocean ridge starting from quasi-melt-free detachment-dominated spreading, and transitioning to volcanic spreading as one nears prominent axial volcanos. Detachments in the quasi-melt-free mode extend along-axis 60 to 95 km and have a lifetime of 0.6 to 1.5 myrs. They cut into their predecessor's footwall with an opposite polarity, causing part of the footwall lithosphere to experience further deformation, hydrothermal alteration, sparse magmatism and possibly thermal rejuvenation, in a hanging wall position. The accretion of the oceanic lithosphere in this context therefore occurs in two distinct stages over the lifetime of two successive detachment faults. We examine the transition from this nearly amagmatic detachment-dominated mode to the more common volcanic mode of spreading, showing that it occurs along-axis over distances ≤30 km. It involves a significant thinning of the axial lithosphere and a gradual decrease of the amount of tectonic displacement on faults, as the magmatic contribution to the divergence of the two plates increases. We develop a conceptual model of this transition, in which magma plays a double role: it fills the space between the diverging plates, thus reducing the need for displacement along faults, and it modifies the thermal state and the rheology of the plate boundary, affecting its thickness and its tectonic response to plate divergence. Based on a comparison of the ultraslow eastern SWIR, with the faster spreading Mid-Atlantic Ridge, we show that the activation of the volcanic, or of the detachment-dominated modes of spreading is connected with the volume of magma supplied per increment of plate separation, over a range of axial lithosphere thickness, and therefore over a range of the M ratio defined by (Buck et al., 2005) as the relative contribution of magma and faults to plate divergence (M is smaller, for a given volume of melt per increment of plate separation, if the plate is thicker). We therefore propose that M does not fully explain the variability in faulting styles observed at slow and ultraslow ridges and propose that rheological changes induced by magma also play a key role (melt itself is weak, hydrothermally altered gabbro-peridotite mixtures are weak, and melt heat sustains more vigorous hydrothermal circulation), resulting in contrasted potentials for strain localization, footwall flexure on faults and the development of detachment faults. • The Southwest Indian Ridge includes corridors of nearly amagmatic spreading. • New oceanic lithosphere there forms over lifetime of two successive detachment faults. • Nearly amagmatic corridors transition into more standard magmatic spreading in <30 km. • Melt supply impacts the thermal state and rheology of the axial lithosphere. • Melt supply is the main control on spreading mode at slow and ultraslow ridges. [ABSTRACT FROM AUTHOR]

Magmatic arcs typically exhibit non-steady-state evolution with episodic flare-ups and lulls, yet the main drivers remain contentious. Situated in the southwest margin of Southeast Asia, Sumatra records a long-lived magmatic arc that is still poorly constrained in age and tempo. Detrital zircon data from Sumatra delineate major arc magmatic pulses at ca. 212, 102-85, 52, and 22-11 Ma. The mid-Cretaceous to early Eocene zircons mostly yield high positive εHf(t) values, indicating magma derivation from juvenile sources and matching well with those of the Gangdese batholiths in the southern Lhasa terrane. These similarities substantiate an extended (~6000 km) Neo-Tethyan arc system from southern Tibet to Sumatra that exhibits concurrent magmatic lulls (ca. 150-105 and 85-65 Ma) and flare-ups (ca. 105-85 and 65-40 Ma). The Late Cretaceous magmatic lull coincided with a period of strong regional deformation and increasingly fast northward drift of India, likely attributable to Neo-Tethyan flat slab subduction. Periodic pulses of Neo-Tethyan arc magmatism most likely correlated with repeated steepening and shallowing of slab dip, rather than India-Eurasia convergence rates. [ABSTRACT FROM AUTHOR]

Volcano seismicity is an important tool used to monitor volcanic hazards, as seismic signals are commonly associated with fracturing and the movement of volcanic fluids. In investigating the relation between fluids and seismic signals, we record acoustic emissions (AEs) in the laboratory that result from fracturing and fluid depressurization through the fractured rock. We vary the fluid viscosity to create field analogues of hydrothermal and magmatic fluids. While fracturing causes high-frequency volcano-tectonic (VT)-type AE signals, fluid venting results in VT and long-period AEs with variable but overall lower dominant frequencies. The viscosity of the vented fluid correlates (1) inversely with the peak dominant frequency, (2) inversely with the number of induced AEs, (3) proportionally to the onset time of AEs, and (4) inversely with the initial rate of AE generation. Tremor was observed only in the early stages of low-viscosity fluid venting and thus may be more associated with gases and hydrothermal fluids than with magma. [ABSTRACT FROM AUTHOR]

The Early Cretaceous lithospheric mantle beneath the Jiaodong Peninsula has been recognized as ancient and enriched mantle with widespread mafic dike swarms. However, the enriching agent and the process of the lithospheric mantle remain under debate. In this study, we recovered the H2O contents of the primitive magmas of Early Cretaceous mafic dikes in the Jiaodong Peninsula by using the H2O contents of clinopyroxene (Cpx) phenocrysts and H2O partition coefficients D H 2 O cpx / melt . We also present whole-rock geochemical data and in situ major- and trace-element contents for Cpx phenocrysts from these mafic dikes. These dikes are characterized by low SiO2 contents, high MgO contents, enrichment of large-ion lithophile elements and light rare earth elements, depletion of high-field-strength elements, high (87Sr/86Sr)i ratios, and low εNd(t) values. These geochemical features imply an origin of enriched subcontinental lithospheric mantle. The calculated H2O contents of the primitive magmas range from 1.38 to 4.45 wt%, with associated H2O/Ce ratios varying from 237 to 322, suggesting that their mantle source was metasomatized by oceanic slab subduction–related fluids. In addition, covariations between H2O/Ce and incompatible elements indicate the addition of a dehydrated marine sediment component in the mantle source of the Jiaodong mafic dikes. In combination with the H2O contents of Early Cretaceous basaltic rocks in the southeastern North China Craton, the lithospheric mantle was metasomatized through fluids related to paleo–Pacific plate subduction. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • 2.4 Ga granitoids of the Quanji Massif show high-K I-type and A 2 -type affinities. • The magmas were derived from early Paleoproterozoic juvenile crust and ancient crust. • The plate tectonics did not shut down during the early Paleoproterozoic. Abstract The global plate tectonic regime in the early Paleoproterozoic period is highly debated. Granitoids bear key information to address such a debate. Petrological, geochemical and geochronological studies are conducted on two post-collisional granitoid plutons in the Quanji Massif, northwestern China, to investigate the tectonic regime during this period. The granitoids are composed of syenogranite, monzogranite, and granodiorite, with minor tonalite, which intruded into the Delingha and Hudesheng regions at ca. 2.39–2.37 Ga. These plutons are high-K I-type granitoids with variable Ga/Al ratios, showing some characteristics of A 2 -type granitoids. They are characterized by enrichment of LILEs and LREEs and depletion of Sr, P, Ti, and Eu. They show depleted Nd and Hf isotope signatures with whole rock ε Nd (t) = +0.7 to +4.8 and zircon ε Hf (t) = −1.0 to +7.8, indicating a juvenile crustal growth event at ca. 2.44–2.37 Ga. Our new results together with other coeval post-collisional granitoids in the Quanji Massif suggest that the protracted post-collisional magmatism at ca. 2.39–2.34 Ga occurred just after a short interval of subduction and generation of juvenile magmas before or around ca. 2.4 Ga. Collectively, the formation of these ca. 2.4–2.3 Ga granitoids in the Quanji Massif may correlate with coeval granitoids in the Tarim and North China cratons, and is broadly coeval with magmatism in several other cratons in the West African and Canadian Shields. Thus, the globally well-documented magmatism at early Paleoproterozoic, or Siderian, provides further information for filling up the age gap of the so-called plate tectonic "shutdown" in the early Paleoproterozoic period worldwide. The geologic record therefore suggests no Siderian shutdown of plate tectonics, but instead, continuous global subduction and generation of juvenile magmas from the Archean through the Paleoproterozoic. [ABSTRACT FROM AUTHOR]

Strain in magmatic rifts is accommodated by both faulting and dike intrusion, but little is known of the frequency of dike intrusions in early‐stage rifts. We use a new earthquake data set from a dense temporary seismic array (2013–2014) in the ~7‐Myr‐old Magadi‐Natron‐Manyara section of the East African Rift, which includes the carbonatitic Oldoinyo Lengai volcano that erupted explosively in 2007–2008. Full moment tensor analyses were performed on M > 3.4 earthquakes (0.03‐ to 0.10‐Hz band) that occurred during the intereruptive cycle. We find two opening crack‐type and various non‐double‐couple earthquake source mechanisms and interpret these as fluid‐involved fault rupture. From waveform analysis on the nearest permanent seismic station, we conclude that similar rupture processes probably occur over eruptive and intereruptive cycles. The repeated and dynamically similar fluid‐involved seismicity, along with intrabasinal localization of active deformation, suggests that significant and persistent strain is accommodated by magmatic processes, modulated by tectonic cycles. Plain Language Summary: How do Earth's plates deform between volcanic eruptions and after plate tectonic stresses have been released in large earthquakes? Simulations of earthquake rupture in rift zones where the plates are diverging tell us that opening + shearing cracks created several of the moderate earthquakes. The opening may be due to magmatic fluids and gases from crustal magma reservoirs. We also compare the earthquake signals to those from the past 20 years and find that these magma‐related earthquakes are frequent occurrences at least in the past 10 years. Magmatic processes contribute to the short time scale cycles of plate boundary deformation in rift zones. Key Points: Non‐double‐couple earthquakes are observed in an early‐stage rift and interpreted as fluid‐involved faulting above a sill complexEarthquakes in 2013–2014 have similar driving mechanisms as the fault‐dike‐eruption episode in 2007–2008 but did not rupture the same faultsMagma‐involved faulting persistently accommodates strain over intereruptive cycles [ABSTRACT FROM AUTHOR]

The Neoproterozoic tectonic evolution of the Jiangnan Orogen is controversial, with one of the issues being whether the ca. 850-820-Ma granitoids were generated by mantle plumes or the collision between the Yangtze and Cathaysia blocks. This paper tackles this problem by examining the age and petrogenesis of one of the granitoids, the Getengling pluton in the central Jiangnan Orogen, and through comparison with a regional geochronological-geochemical database compiled from previous studies. The Getengling pluton is characterized by high A/CNK values (~1.5), slight negative whole-rock εNd(t) values (−2.8 to −3.4), and positive zircon εHf(t) values (0.7 ± 1.1), suggesting S-type granite affinities with juvenile contributions. Rb/Sr, Rb/Ba, and high CaO/Na2O ratios indicate psammitic sources with both clay-rich and clay-poor characters. These geochemical characteristics are distinct from those of the granitoids (typically of A type) associated with mantle plumes. The zircon laser ablation-inductively coupled plasma-mass spectrometry U-Pb age of 845 ± 4 Ma obtained in this study, together with other ca. 835-820 Ma ages of S-type granites in the Jiangnan Orogen, indicates that the felsic magmatism in the Jiangnan Orogen lasted for ca. 25 Ma, which is longer than typical plume-related felsic magmatism. In addition, the mafic rocks in the Jiangnan Orogen and elsewhere in the South China Block are geochemically distinct from the coeval mantle plume-related ones in Australia and west Laurentia. In geochemical diagrams diagnostic of tectonic settings, the Getengling pluton and other ca. 850-820 Ma intrusions plot in the syn- and post-collisional fields, whereas the pre-850 and post-820-Ma igneous rocks plot in the arc and within-plate settings, respectively. This sequential tectonic evolution from plate subduction through collision to within-plate environments further supports the hypothesis that the ca. 850-820-Ma granitoids in the Jiangnan Orogen resulted from the Yangtze-Cathaysia collision rather than from mantle pluming. [ABSTRACT FROM AUTHOR]

There is a growing concern about the potential eruption of the Changbaishan volcano (CBV) in recent years, but the magmatic system beneath this volcano remains hotly debated. In this study, we construct a high‐resolution 3‐D crust and upper mantle S wave velocity (Vs) model beneath the CBV and adjacent regions combining ambient noise and earthquake surface wave tomography. Distinct low Vs anomalies are revealed in the crust and upper mantle beneath the CBV. The middle‐lower crustal low‐velocity body is interpreted to be the main magma chamber that may feed the surface volcanism and hydrothermal activities. The upper mantle low‐velocity zone is representative of upwelling asthenosphere. The lower crust beneath the Longgang volcano and Jingpohu volcano is also characterized by low Vs anomalies, maybe indicating low‐degree partial melting. We propose that the intraplate volcanism of the CBV as well as the Longgang volcano and Jingpohu volcano is driven by the decompression melting of upwelling asthenosphere, which could originate from the mantle transition zone as suggested by previous large‐scale tomographic studies. Moreover, the geologically identified Solonker‐Xar Moron‐Changchun‐Yanji suture zone is delineated by a prominent high Vs anomaly in the uppermost mantle, which may be explained by the fossil slab remnants or deformational fabrics resulted from the final closure of the Paleo‐Asian Ocean in the Late Paleozoic to Early Mesozoic. Plain Language Summary: The concerns over the instability of the Changbaishan volcano located at the border between China and North Korea have drawn much public attention in recent years worldwide. However, the shallow detailed magmatic system underlying this volcano still remains hotly debated. Since seismic S wave velocity can be used as an indicator for existence of magma bodies in the Earth's interior, we constructed a 3‐D high‐resolution S wave velocity model in the Changbaishan volcano and adjacent regions by combining seismic ambient noise and earthquake surface wave tomography. We found that the S wave velocity beneath the Changbaishan volcano is significantly lower than the surrounding regions throughout the crust and upper mantle. Based on previous geophysical and geochemical investigations, our imaging results suggest that there might be a middle‐lower crustal magma chamber residing beneath the Changbaishan volcano, which is responsible for the surface volcanism and hydrothermal activities. This crustal magma chamber is likely fed by mantle‐derived magma resulted from decompression melting within the upwelling asthenosphere. In addition, we observed a band‐shaped high‐velocity anomaly along the Solonker suture zone where the Paleo‐Asian Ocean is supposed to complete its final closure. We suspect that this high‐velocity anomaly may reflect the traces of fossil subduction caused by continental collision during the final closure of the Paleo‐Asian Ocean. Key Points: A high‐resolution 3‐D Vs model beneath the Changbaishan volcano and adjacent regions is constructedCrustal and upper mantle low‐velocity bodies are imaged beneath the Changbaishan volcanoThe Solonker suture zone is delineated by an elongated high velocity in the uppermost mantle [ABSTRACT FROM AUTHOR]

A-type magmatism is commonly generated in extensional settings. Cenozoic postcollisional extensional structures and magmatism widely occur in the well-known Himalayan-Tibetan orogen (HTO). So far, however, no Cenozoic A-type magmatism has been identified in the orogen. Here, we report on newly identified 24-23 Ma felsic lavas (trachytes + rhyolites) in the Konglong area of the Lhasa block, southern Tibet. They have diagnostic minerals (sodalites) and geochemical characteristics (high total-alkali, FeO/MgO, Zr, and Ga/Al, low CaO, and strongly negative Eu, Sr, and Ba anomalies), demonstrating their close affinity with A-type granitoids. Generation of the A-type lavas required high-temperature melting (up to 944 °C) of crustal source rocks in the pressure range of 0.5-1.2 GPa. Our study not only identifies the first postcollisional A-type magmatism in the HTO, but it also indicates an extension event contemporaneous with the onset of the Main Central thrust and South Tibetan detachment system, and development of the Kailas basin and ultrapotassic magmatism of the southern HTO. These events straddled the Himalaya and Lhasa blocks and can be reconciled with foundering of the subducted Indian plate. Thus, we suggest a tectonic model of Indian plate flat subduction and foundering beneath the western Lhasa block for the postcollisional evolution of the southern HTO. [ABSTRACT FROM AUTHOR]

The origin of Iberia's topography is examined by combining gravity, magmatic, topographic, and seismological observations with geomorphic considerations. We have four principal results. First, the highest coherence between free‐air gravity and topography is at wavelengths ≲250 km where admittance indicates that elastic thickness of Iberia's plate is 20 ± 3 km. These results imply that flexural and subplate support of Iberian topography could be expressed at wavelengths of O(100) km. Second, P‐to‐S receiver functions and simple isostatic calculations indicate that while crustal thickness variations and flexural loading (e.g., as a result of plate shortening) partially explain the elevation of Pyrenean, Betics, Cantabrian, Spanish Central System, and Iberian Chain topography, they fail to explain the elevation of large parts of Iberia. Third, a new full waveform shear wave tomographic model and velocity to temperature conversions suggest that the asthenosphere beneath Iberia is anomalously slow and has excess temperatures of up to 162 ± 14 °C. Simple isostatic calculations indicate asthenospheric support of topography of up to 1 km. Neogene‐Recent (∼23–0 Ma) extrusive magmatism (e.g., Calatrava, Catalan) sit atop many of the slow shear wave velocity anomalies. Finally, biostratigraphic data, combined with inversion of 3,217 river profiles, show that most of Iberia's topography grew during the last ∼30 Ma at rates of up to 0.3 mm/year. Best‐fitting theoretical rivers have a low residual root‐mean‐square misfit (=0.96) and calculated uplift is consistent with an independent inventory of stratigraphic and biostratigraphic observations. We suggest that Neogene‐Recent growth of most of central Iberia's topography was a result of asthenospheric support. Key Points: Calculated history of dynamic support of IberiaExcess asthenospheric temperatures support up to 1 km of topographyMarine terraces and geomorphology suggest growth of support in last ∼30 Ma [ABSTRACT FROM AUTHOR]

Abstract The ~1100 Ma CC2 and P13 lamproite dykes in the Wajrakarur Kimberlite Field (WKF), Eastern Dharwar Craton, and ~65 Ma Kodomali and Behradih lamproite diatremes in the Mainpur Kimberlite Field (MKF), Bastar Craton share a similar mineralogy, although the proportions of individual mineral phases vary significantly. The lamproites contain phenocrysts, macrocrysts and microcrysts of olivine set in a groundmass dominated by diopside and phlogopite with a subordinate amount of spinel, perovskite, apatite and serpentine along with rare barite. K-richterite occurs as inclusion in olivine phenocrysts in Kodomali, while it is a late groundmass phase in Behradih and CC2. Mineralogically, the studied intrusions are classified as olivine lamproites. Based on microtextures and compositions, three distinct populations of olivine are recognised. The first population comprises Mg-rich olivine macrocrysts (Fo89–93), which are interpreted to be xenocrysts derived from disaggregated mantle peridotites. The second population includes Fe-rich olivine macrocrysts (Fo82–89), which are suggested to be the product of metasomatism of mantle wall-rock by precursor lamproite melts. The third population comprises phenocrysts and overgrowth rims (Fo83–92), which are clearly of magmatic origin. The Mn and Al systematics of Mg-rich olivine xenocrysts indicate an origin from diverse mantle lithologies including garnet peridotite, garnet–spinel peridotite and spinel peridotite beneath the WKF, and mostly from garnet peridotite beneath the MKF. Modelling of temperatures calculated using the Al-in-olivine thermometer for olivine xenocrysts indicates a hotter palaeogeotherm of the SCLM beneath the WKF (between 41 and 43 mW/m2) at ~1100 Ma than beneath the MKF (between 38 and 41 mW/m2) at ~65 Ma. Further, a higher degree of metasomatism of the SCLM by precursor lamproite melts has occurred beneath the WKF compared to the MKF based on the extent of Ca Ti enrichment in Fe-rich olivine macrocrysts. For different lamproite intrusions within a given volcanic field, lower Fo olivine overgrowth rims are correlated with higher phlogopite plus oxide mineral abundances. A comparison of olivine overgrowth rims from the two fields shows that WKF olivines with lower Fo content than MKF olivines are associated with increased X Mg in spinel and phlogopite and vice versa. Melt modelling indicates relatively Fe-rich parental melt for WKF intrusions compared to MKF intrusions. The Ni/Mg and Mn/Fe systematics of magmatic olivines indicate derivation of the lamproite melts from mantle source rocks with a higher proportion of phlogopite and/or lower proportion of orthopyroxene for the WKF on the Eastern Dharwar Craton compared to those for the MKF on the Bastar Craton. This study highlights how olivine cores provide important insights into the composition and thermal state of cratonic mantle lithosphere as sampled by lamproites, including clues to elusive precursor metasomatic events. Variable compositions of olivine rims testify to the complex interplay of parental magma composition and localised crystallisation conditions including oxygen fugacity variations, co-crystallisation of groundmass minerals, and assimilation of entrained material. Highlights • Olivines in the studied lamproites exhibit three distinct populations: xenocrystic, metasomatic, and magmatic. • Xenocrystic olivines indicate a hotter palaeogeotherm of the SCLM beneath the Dharwar Craton than beneath the Bastar Craton. • Metasomatic olivines resulted from interaction of precursor lamproite melts with the mantle wall rock. • Magmatic olivines indicate relatively Fe-rich parental melt for Dharwar lamproites compared to Bastar lamproites. [ABSTRACT FROM AUTHOR]

Abstract We present stratigraphic, geochemical (CNS elemental analysis), and micropaleontological (ostracode assemblages) evidence for the 2011 Tohoku-oki tsunami impact in Matsushima Bay, northeastern Japan, and examine the recovery process after the impact to the bay environment using short sediment cores from two different topographic settings at a water depth of approximately 4 m. At both sites, tsunami deposits are composed of two sedimentary layers that recorded the first tsunami run-up and backwash. At the western site near the channel that connects the bay with the Pacific Ocean, the first tsunami wave eroded the bay floor sediments and left the tsunami deposits composed of medium sand rich in shell fragments, but with sparse numbers of meiobenthic ostracodes. By contrast, at the eastern site, which is surrounded by many small islands, the first wave eroded very little of the bay floor sediments and left tsunami deposits consisting of sandy mud, rich in exotic ostracodes, such as phytal species, shallow marine sand dwelling species, and brackish water species. Overlying post-tsunami deposits are composed mainly of organic-rich mud in which organic matter was derived primarily from marine plankton. Ostracode assemblages in the tsunami deposits lack the offshore species that live in water depths >50 m and are dominated by the species from Matsushima Bay and its adjacent nearshore, upper sublittoral areas, and brackish water environments. The distance from deep-water offshore areas to the core sites is too far to transport ostracode valves by tsunami waves. Highlights • The larger first tsunami wave was the main source of the tsunami deposit. • Ostracode assemblages in the tsunami deposits lack the offshore species. • Ostracode assemblages had not yet recovered from the 2011 tsunami impact. • The post-tsunami deposit contained organic-rich muds attributed to marine planktons. [ABSTRACT FROM AUTHOR]

Abstract A new high-resolution bathymetric map combined with a regional Digital Elevation Model (DEM) analysis reveal the modalities of occurrence and emplacement of post-spreading magmatism along the NNE-SSW oriented, 1000 km long Tore-Madeira Rise (TMR) as well as its relationship with the activity of major fault systems including the Estremadura Fault System (ESF) and the Azores-Gibraltar Fracture Zone (AGFZ). Morphological and structural analysis of the bathymetric map were performed to map volcanic features such as eruptive cones, vents and fissures together with faults along the TMR. The new bathymetric map shows that the main NNW-SSE seamount alignment is formed by three structurally distinct volcanic massifs, the Tore, the Josephine and the Southern Volcanic Groups. The majority of the volcanoes of each group emplaced within or along specific portion of pre-existing faults (ESF and AGFZ) including splay fault, releasing bend, fault tips and interaction zones between different segments. Magmas were channelled into sub-vertical pre-existing lithospheric faults that acted as preferential pathways for the vertical magma ascent. Migration and final eruption of magma are controlled by the local stress variation induced by complex fault geometries, change in plate kinematics as well as strong shear zone anisotropy as suggested by the emplacement within localised areas of transtension. We conclude that post-spreading magma emplacement in the southern part of the Iberia margin was related to the development of a transtensional plate boundary between the Iberian and African Plate during the Late Cretaceous. More generally, our findings emphasize that the distribution of volcanism as the expression of the interaction between shallow plate tectonic and mantle processes should be included in plate kinematic reconstruction. This study also demonstrates that the accurate mapping of oceanic seafloor is pivotal to better understand tectono-magmatic evolution of volcanic seamount chains and geological processes in oceanic domains. Highlights • New MBES dataset reveals seafloor structures in the northern Central Atlantic. • Post-spreading deformation processes are associated with volcanic emplacement. • TMR consists of three volcanic fields connected to major lithospheric faults. • Local stress variations due to complex fault geometry controlled magma migration. • Volcanism recorded plate kinematic reorganisation in Middle Cretaceous time. [ABSTRACT FROM AUTHOR]

To understand the stability of hydrous phases in mafic oceanic crust under deep subduction conditions, high-pressure and high-temperature experiments were conducted on two hydrous basalts using a Kawai-type multi-anvil apparatus at 17–26 GPa and 800–1200 °C. In contrast to previous studies on hydrous basalt that reported no hydrous phases in this pressure range, we found one or two hydrous phases in all run products at or below 1000 °C. Three hydrous phases, including Fe-Ti oxyhydroxide, Al-rich phase D and Al-rich phase H, were present at the investigated P-T conditions. At T ≤ 1000 °C, Fe-Ti oxyhydroxide is stable at 17 GPa, Al-rich phase D is stable at 18–23 GPa, and Al-rich phase H is stable at 25–26 GPa. Our results, in combination with published data on the stability of hydrous phases at lower pressures, suggest that a continuous chain of hydrous phases may exist in subducting cold oceanic crust (≤1000 °C): lawsonite (0–8 GPa), Fe-Ti oxyhydroxide (8–17 GPa), Al-rich phase D (18–23 GPa), and Al-rich phase H (>23 GPa). Therefore, in cold subduction zones, mafic oceanic crust, in addition to peridotite, may also carry a substantial amount of water into the mantle transition zone and the lower mantle. [ABSTRACT FROM AUTHOR]

Olivine in kimberlites can provide unique insights into magma petrogenesis, because it is the most abundant xenocrystic phase and a stable magmatic product over most of the liquid line of descent. In this study we examined the petrography and chemistry of olivine in kimberlites from different tectonic settings, including the Slave craton, Canada (Ekati: Grizzly, Koala), the Brasilia mobile belt (Limpeza-18, Tres Ranchos-04), and the Kaapvaal craton, South Africa (Kaalvallei: Samada, New Robinson). Olivine cores display a wide range of compositions (e.g., Mg# = 78-95). The similarity in olivine composition, resorption of core zones and inclusions of mantle-derived phases, indicates that most olivine cores originated from the disaggregation of mantle peridotites, including kimberlite-metasomatised lithologies (i.e. sheared lherzolites and megacrysts). Olivine rims typically show a restricted range of Mg#, with decreasing Ni and increasing Mn and Ca contents, a characteristic of kimberlitic olivine worldwide. The rims host inclusions of groundmass minerals, which implies crystallisation just before and/or during emplacement. There is a direct correlation between olivine rim composition and groundmass mineralogy, whereby high Mg/Fe rims are associated with carbonate-rich kimberlites, and lower Mg/Fe rims are correlated with increased phlogopite and Fe-bearing oxide mineral abundances. There are no differences in olivine composition between explosive (Grizzly) and hypabyssal (Koala) kimberlites. Olivine in kimberlites also displays transitional zones and less common internal zones, between cores and rims. The diffuse transitional zones exhibit intermediate compositions between cores and rims, attributed to partial re-equilibration of xenocrystic cores with the ascending kimberlite melt. In contrast, internal zones form discrete layers with resorbed margins and restricted Mg# values, but variable Ni, Mn and Ca concentrations, which indicates a discrete crystallization event from precursor kimberlite melts at mantle depths. Overall, olivine exhibits broadly analogous zoning in kimberlites worldwide. Variable compositions for individual zones relate to different parental melt compositions rather than variations in tectonic setting or emplacement mechanism. [ABSTRACT FROM AUTHOR]

Abstract The Ushuaia pluton (UP) and the Ushuaia Peninsula Andesites (UPA) are intrusive units of a Late Cretaceous rear-arc in the Southernmost Andes. We report new LA-ICP-MS zircon ages from ∼71 to 75 Ma for the UP, and of ∼84 Ma for the UPA, which allow to differentiate these two magmatic events; consistent with their distinct petrography and geochemistry. The UP is a composite, incrementally assembled pluton in upper crustal levels. Successive injections of magma batches define two main sections, each in turn constructed by several smaller pulses: an early ultramafic-mafic section consists mainly of hornblende pyroxenite, hornblendite and gabbro/diorite with reverse concentric zonation, and a later intermediate section built up mainly by monzodiorite. Mingling and layering between mafic and intermediate facies along the contact point to simultaneous emplacement at the end of one main pulse and the beginning of the other. Mineralogy and geochemistry suggest the rock sequence in the UP is cogenetic. In part it resulted by in-situ magmatic processes that included crystal-melt fractionation during injection of magma mushes, mingling, local melt segregation and mixing with crustal partial melts. The country rock is affected by penetrative ductile deformation and regional metamorphism developed during mid-Cretaceous Andean main orogenic phase. It also shows an overimposed pluton-margin parallel foliation and a post-kinematic contact metamorphic aureole. On the other hand, the UP has a concentric igneous foliation parallel to pluton margins interpreted as a primary magmatic feature. The above evidences, along with absence of subsolidus fabrics and metamorphism, indicate the pluton post-date peak regional metamorphism and ductile structures in the country rock. The new zircon ages constrain the top age of early Andean deformation, as well as the magmatic gap registered for the end of the Cretaceous, and confirm an emplacement during a recorded compressive regime that transmitted shortening to the foreland. Highlights • Field, petrological, chemical and chronological data are given for the Ushuaia pluton. • Pluton emplacement was incrementally in the upper crust. • Country rock plastic deformation and partial melting point to a high thermal regime. • Magma diversification from ultramafic to felsic lithologies occurred partly in-situ. • Emplacement post-dates peak regional contractional deformation and metamorphism. [ABSTRACT FROM AUTHOR]

Anorthosite-bearing layered intrusions are unique to the Archaean rock record and are abundant in the Archaean craton of southern West Greenland and the Superior Province of Canada. These layered intrusions consist mainly of ultramafic rocks, gabbros, leucogabbros and anorthosites, and typically contain high-Ca (>An70) megacrystic (2-30 cm in diameter) plagioclase in anorthosite and leucogabbro units. They are spatially and temporally associated with basalt-dominated greenstone belts and are intruded by syn-to post-tectonic granitoid rocks. The layered intrusions, greenstone belts and granitoids all share the geochemical characteristics of Phanerozoic subduction zone magmas, suggesting that they formed mainly in a suprasubduction zone setting. Archaean anorthosite-bearing layered intrusions and spatially associated greenstone belts are interpreted to be fragments of oceanic crust, representing dismembered subduction-related ophiolites. We suggest that large degrees of partial melting (25-35%) in the hotter (1500-1600 °C) Archaean upper mantle beneath rifting arcs and backarc basins produced shallow, kilometre-scale hydrous magma chambers. Field observations suggest that megacrystic anorthosites were generated at the top of the magma chambers, or in sills, dykes and pods in the oceanic crust. The absence of high-Ca megacrystic anorthosites in post-Archaean layered intrusions and oceanic crust reflects the decline of mantle temperatures resulting from secular cooling of the Earth. [ABSTRACT FROM AUTHOR]

Abstract Potassic high Ba-Sr granitoids are widely distributed over collisional orogenic belts worldwide, however, their magma sources and petrogenetic processes are still debated. To better understand them, we collected a suite of Paleogene high Ba-Sr granitoids in the Jianchuan Basin, southeast Tibet. Based on the mineralogical compositions, major and trace element geochemistry, and age of formation, we subdivide them into two groups: intermediate series and granitic series. LA-MC-ICP-MS zircon U-Pb analyses show that the intermediate series crystallized at 33.8–36.9 Ma, similar to or slightly earlier than the granitic rocks with crystallization ages of 33.1–34.0 Ma. New geochemical and zircon Hf isotopic results, in combination with previous results, support an enriched mantle source for the high Ba-Sr intermediate rocks, and a mixed source of enriched mantle and lower crust for the granitic ones. We suggest that partial melting of enriched mantle with fractional crystallization of mafic minerals produced the potassic high Ba-Sr intermediate series. Meanwhile, the juvenile heat from such magma induced melting of thickened lower crustal materials which together with magma from mantle accounted for source of the granite series. The high Ba-Sr intermediate rocks have nearly identical major and trace element, and isotopic signatures as Archean sanukitoids, thus offer a potential constraint on the formation mechanism for the sanukitoids. Our study supports that the emergence of the sanukitoids requires a sediment-metasomatized mantle and thus indicates the onset of crustal recycling into the mantle. Highlights • A suite of high Ba-Sr rocks emplaced in Jianchuan Basin in the late Eocene. • High Ba-Sr intermediate series represent magma derived from an enriched mantle. • High Ba-Sr granites were mixed products of from mantle as well as from lower crust. • Sanukitoids indicate the onset of crustal recycling into the mantle. [ABSTRACT FROM AUTHOR]

Transitions in eruptive style are common at volcanoes. Understanding how and why these transitions occur remain open questions. The 2012 eruption of the submarine Havre volcano in the Kermadec arc (southwest Pacific Ocean) produced a raft of floating pumice followed by a pair of domes from the same vent. Here, we used measurements on erupted magmas and constraints on the eruption rate, combined with a model for magma ascent, to identify the dominant controls on the transition in eruption style. During the raft-forming stage, magma ascent was fast enough that little gas was lost. Magma reached the seafloor with great enough vesicularity to be buoyant and produce clasts that could float. As the eruption waned, the eruption rate decreased, and the conduit narrowed. Sufficient gas was then lost to the surrounding country rocks during ascent such that the erupted magma was no longer buoyant relative to seawater. Most of the original dissolved water in the magma was lost to the crust surrounding the conduit during the dome-forming stage. [ABSTRACT FROM AUTHOR]

La Gloria Pluton (LGP) in central Chile is a shallow, north-northwest- elongated granitoid body of 18 km length and 4-6 km width, belonging to a regional north-south trend of Miocene plutons, from the San Francisco Batholith and porphyries in the north, to Mesón Alto, and San Gabriel in the south. New U-Pb zircon ages of the LGP indicate that crystallization occurred mostly within an interval between 11.3-10.2 Ma, with a pattern of decreasing ages along the pluton axis from south to north. The progression of zircon ages can be explained either by gradual northwestward migration of the feeder zone that supplied magma to the shallow pluton or, more likely, by shallow lateral magma propagation southeastward from a fixed feeder zone located beneath the northern margin of the pluton. The age progression, together with existing data of subhorizontal mineral and magnetic lineations in the LGP parallel to the pluton axis, indicate lateral propagation of magma during reservoir construction along the hinge of an anticline of the volcanic host sequences. In addition to controlling the position of possible volcanic output, such horizontal migration of silicic magmas in the upper crust significantly increases the surface footprint over which fluids are exsolved and outgas, strongly decreasing the potential for magmatic-hydrothermal ore formation above laterally emplaced laccoliths in the shallow crust. [ABSTRACT FROM AUTHOR]

Geological characteristics and geochemical analyses are reported for the early Carboniferous pillow basalt in the Huilvshan region (west Junggar, Northwest China), with the aim to indicate its petrogenesis, magma source characteristics, and tectonic implication. This pillow basalt consists of clinopyroxene and plagioclase with trace amounts of magnetite, apatite, and chromian spinel. It is tholeiitic in composition with low concentrations of Na2O + K2O (1.52–4.74 wt.%). Similar to the N-MORB, the samples of this pillow basalt have nearly flat chondrite-normalized REE patterns ((La/Yb)N = 0.87–1.47) with insignificant Eu anomalies (Eu/Eu* = 0.84–1.18), and show no obvious enrichments of LILEs and insignificant depletions in HFSEs. Petrology and geochemical characteristics suggest that this pillow basalt is the product of MORB-like magma derived from a depleted mantle corresponding to ≤4% partial melting of spinel lherzolite. SIMS analysis of the zircons separated from tuff interlayered with basalt gives a weighted average U–Pb age of 328 ± 3 Ma (MSWD = 1.4), which represents the magma eruption time in the Huilvshan region. From these observations, in combination with the previous work, we conclude that an extensional tectonic regime dominated the tectonic activity of west Junggar during early Carboniferous. [ABSTRACT FROM AUTHOR]

Abstract As shown by geological, mineralogical, and isotope geochemical data, trachybasaltic-trachytic-trachyrhyolitic (TTT) rocks from the Nyalga basin in Central Mongolia result from several eruptions of fractionated magmas within a short time span at about 120 Ma. Their parental basaltic melts formed by partial melting of mantle peridotite which was metasomatized and hydrated during previous subduction events. Basaltic trachyandesites have high TiO 2 and K 2 O, relatively high P 2 O 5 , and low MgO contents, medium 87Sr/86Sr(0) ratios (0.70526-0.70567), and almost zero or slightly negative ε Nd (T) values. The isotope geochemical signatures of TTT rocks are typical of Late Mesozoic basaltic rocks from rift zones of Mongolia and Transbaikalia. The sources of basaltic magma at volcanic centers of Northern and Central Asia apparently moved from a shallower and more hydrous region to deeper and less hydrated lithospheric mantle (from spinel to garnet-bearing peridotite) between the Late Paleozoic and the latest Mesozoic. The geochemistry and mineralogy of TTT rocks fit the best models implying fractional crystallization of basaltic trachyandesitic, trachytic, and trachyrhyodacitic magmas. Mass balance calculations indicate that trachytic and trachydacitic magmas formed after crystallization of labradorite-andesine, Ti-augite, Sr-apatite, Ti-magnetite, and ilmenite from basaltic trachyandesitic melts. The melts evolved from trachytic to trachyrhyodacitic and trachyrhyolitic compositions as a result of prevalent crystallization of K-Na feldspar, with zircon, chevkinite-Ce, and LREE-enriched apatite involved in fractionation. Trachytic, trachyrhyodacitic, and trachyrhyolitic residual melts were produced by the evolution of compositionally different parental melts (basaltic trachyandesitic, trachytic, and trachyrhyodacitic, respectively), which moved to shallower continental crust and accumulated in isolated chambers. Judging by their isotopic signatures, the melts assimilated some crustal material, according to the assimilation and fractional crystallization (AFC) model. [ABSTRACT FROM AUTHOR]