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1. Magmatism during the closure of the Mongol-Okhotsk Ocean: a tale of a sediment-rich collision

   Henriquez, S. (January 2021, AGU Fall Meeting Abstracts)

   The Mongol-Okhotsk suture in NE Mongolia and SE Russia is the last vestige of the Mongol-Okhotsk Ocean, a basin that separated the Siberian from the North China Craton. Although the exact location of the suture is cryptic and the timing and kinematics of closure remains debated, magmatism north and south of the suture zone provide insight into the characteristics of the subduction system during the closure of this ocean. Magmatic provinces recording pre-, syn- and post-collisional processes (and potentially deeper mantle sources) along the Mongol Okhotsk Belt were emplaced from the Permian to Jurassic. Our compilation of geochemical, isotopic, and geochronologic data from 51 published studies shows that the age of peak magmatism and subsequent magmatic lull decreases eastward. We interpret this to record a shift in the locus of collision as the ocean zipped close. Most mafic rocks within the suture zone show the influence of lower crustal components and both spinel and garnet peridotites in the source. However, with one exception, their compositions are generally not consistent with a depleted mantle source (usually associated with MgO-TiO2-rich, isotopically fertile, and OIB components). Additionally, although significant geochemical variability due to crustal petrogenetic processes is observed, the data show a ubiquitous subduction signature characterized by enrichment in large ion lithophile elements, light rare earth elements, and La/Nb ratios, as well as by depletion in high field strength elements and heavy rare earth elements. We interpret the consistent peraluminous nature of the magmatic rocks before, during, and after collision, together with their crust-like Nb/U content, high (Th/Yb)/(Ba/La) and (Th/Nb)/(Ba/Nb) ratios, to reflect the strong influence of sediment melts throughout the collision process. This requires not only unusually high geothermal gradients (ca. 200 °C higher than a normal mantle wedge) that can partially melt sedimentary rocks along the whole subduction system, but also persistence of underplated, assimilated, and/or metasomatically incorporated sediments in the magma source during the continental closure. Together, the compiled data paint a picture of a sediment-rich magmatic engine above a hot, dynamic, subduction system with largely intact slabs. 

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2. Tectonic evolution of the Beishan orogen in central Asia: Subduction, accretion, and continent-continent collision during the closure of the Paleo-Asian Ocean

   https://doi.org/10.1130/B36451.1  

   Li, Jie; Wu, Chen; Chen, Xuanhua; Zuza, Andrew V.; Haproff, Peter J.; Yin, An; Shao, Zhaogang (July 2022, GSA Bulletin)

   The Beishan orogen is part of the Neoproterozoic to early Mesozoic Central Asian Orogenic System in central Asia that exposes ophiolitic complexes, passive-margin strata, arc assemblages, and Precambrian basement rocks. To better constrain the tectonic evolution of the Beishan orogen, we conducted field mapping, U-Pb zircon dating, whole-rock geochemical analysis, and Sr-Nd isotopic analysis. The new results, when interpreted in the context of the known geological setting, show that the Beishan region had experienced five phases of arc magmatism at ca. 1450−1395 Ma, ca. 1071−867 Ma, ca. 542−395 Ma, ca. 468−212 Ma, and ca. 307−212 Ma. In order to explain the geological, geochemical, and geochronological data from the Beishan region, we present a tectonic model that involves the following five phases of deformation: (1) Proterozoic rifting that separated the North Beishan block from the Greater North China craton that led to the opening of the Beishan Ocean, (2) early Paleozoic north-dipping subduction (ca. 530−430 Ma) of the Beishan oceanic plate associated with back-arc extension followed by collision between the North and South Beishan microcontinental blocks, (3) northward slab rollback of the south-dipping subducting Paleo-Asian oceanic plate at ca. 450−440 Ma along the northern margin of the North Beishan block that led to the formation of a northward-younging extensional continental arc (ca. 470−280 Ma) associated with bimodal igneous activity, which indicates that the westward extension of the Solonker suture is located north of the Hongshishan-Pengboshan tectonic zone, (4) Late Carboniferous opening and Permian north-dipping subduction of the Liuyuan Ocean in the southern Beishan orogen, and (5) Mesozoic-Cenozoic intracontinental deformation induced by the final closure of the Paleo-Asian Ocean system in the north and the Tethyan Ocean system in the south. 

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3. Persistent High-Pressure Magma Storage beneath a Near-Ridge Ocean Island Volcano (Isla Floreana, Galápagos)

   https://doi.org/10.1093/petrology/egaf031  

   Gleeson, Matthew; Wieser, Penny E; DeVitre, Charlotte L; Shi, Sarah C; Millet, Marc-Alban; Muir, Duncan D; Stock, Michael J; Lissenberg, Johan (May 2025, Journal of Petrology)

   Abstract Volcanic evolution in ocean island settings is often controlled by variations in the chemistry and volumetric flux of magma from an underlying mantle plume. In locations such as Hawaiʻi or Réunion, this results in predictable variations in magma chemistry, the rate of volcanic activity, and the depth of magma storage with volcanic age and/or distance from the centre of plume upwelling. These systems, however, represent outliers in global plume volcanism due to their high buoyancy flux, frequent eruptions, and large distance from any plate boundary. Most mantle plumes display clear interaction with nearby plate boundaries, influencing the dynamics of solid plume material in the upper mantle and the distribution of melt across regions of active volcanism. Yet, the influence of plume–ridge interaction and plume–ridge distance on the structure, characteristics, and evolution of magma storage beneath ocean island volcanoes remains under constrained. In this study, we consider the evolution of magmatic systems in the Galápagos Archipelago, a region of mantle plume volcanism located 150–250 km south of the Galápagos Spreading Centre (GSC), focusing on the depth of magma storage during the eastward transport of volcanic systems away from the centre of plume upwelling. Geochemical analysis of gabbro xenoliths from Isla Floreana in the southeastern Galápagos suggest that they formed at ~2–2.5 Ma, when the island was located close to the centre of plume upwelling. These nodules, therefore, provide rare insights into the evolution of volcanic systems in the Galápagos Archipelago, tracking variations in the magma system architecture as the Nazca plate carried Isla Floreana eastwards, away from the plume centre. Mineral thermobarometry, thermodynamic modelling, and CO2 fluid inclusion barometry reveal that Isla Floreana’s plume-proximal stage of volcanic activity—recorded in the gabbro xenoliths—was characterized by the presence of high-pressure magma storage (>25 km), below the base of the crust. In fact, we find no petrological evidence that sustained, crustal-level magma storage ever occurred beneath Isla Floreana. Our results contrast with the characteristics of volcanic systems in the western Galápagos above the current centre of plume upwelling, where mid-crust magma storage has been identified. We propose that this change in magmatic architecture of plume-proximal volcanic centres in the Galápagos—from high-pressure mantle storage at 2.5 Ma to mid-crustal storage at the present day—is controlled by the variations in plume–ridge distance. Owing to the northward migration of the GSC, the distance separating the plume stem and GSC is not constant, and was likely <100 km at 2.5 Ma, significantly less than the current plume–ridge distance of 150–250 km. We propose that smaller plume–ridge distances result in greater diversion of plume-material to the GSC, ‘starving’ the eastern Galápagos islands of magma during their initial formation and restricting the ability for these systems to develop long-lived crustal magma reservoirs. 

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4. Geochemical and geochronological records of tectonic changes along a flat-slab arc-transform junction: Circa 30 Ma to ca. 19 Ma Sonya Creek volcanic field, Wrangell Arc, Alaska

   https://doi.org/10.1130/GES02114.1  

   Berkelhammer, Samuel E.; Brueseke, Matthew E.; Benowitz, Jeffrey A.; Trop, Jeffrey M.; Davis, Kailyn; Layer, Paul W.; Weber, Maridee (September 2019, Geosphere)

   The Sonya Creek volcanic field (SCVF) contains the oldest in situ volcanic products in the ca. 30 Ma–modern Wrangell Arc (WA) in south-central Alaska, which commenced due to Yakutat microplate subduction initiation. The WA occurs within a transition zone between Aleutian subduction to the west and dextral strike-slip tectonics along the Queen Charlotte–Fairweather and Denali–Duke River fault systems to the east. New 40Ar/39Ar geochronology of bedrock shows that SCVF magmatism occurred from ca. 30–19 Ma. New field mapping, physical volcanology, and major- and trace-element geochemistry, coupled with the 40Ar/39Ar ages and prior reconnaissance work, allows for the reconstruction of SCVF magmatic evolution. Initial SCVF magmatism that commenced at ca. 30 Ma records hydrous, subduction-related, calc-alkaline magmatism and also an adakite-like component that we interpret to represent slab-edge melting of the Yakutat slab. A minor westward shift of volcanism within the SCVF at ca. 25 Ma was accompanied by continued subduction-related magmatism without the adakite-like component (i.e., mantle-wedge melting), represented by ca. 25–20 Ma basaltic-andesite to dacite domes and associated diorites. These eruptions were coeval with another westward shift to anhydrous, transitional-tholeiitic, basaltic-andesite to rhyolite lavas and tuffs of the ca. 23–19 Ma Sonya Creek shield volcano; we attribute these eruptions to intra-arc extension. SCVF activity was also marked by a small southward shift in volcanism at ca. 21 Ma, characterized by hydrous calc-alkaline lavas. SCVF geochemical compositions closely overlap those from the <13 Ma WA, and no alkaline lavas that characterize the ca. 18–10 Ma eastern Wrangell volcanic belt exposed in Yukon Territory are observed. Calc-alkaline, transitional-tholeiitic, and adakite-like SCVF volcanism from ca. 30–19 Ma reflects subduction of oceanic lithosphere of the Yakutat microplate beneath North America. We suggest that the increase in magmatic flux and adakitic eruptions at ca. 25 Ma, align with a recently documented change in Pacific plate direction and velocity at this time and regional deformation events in southern Alaska. By ca. 18 Ma, SCVF activity ceased, and the locus of WA magmatism shifted to the south and east. The change in relative plate motions would be expected to transfer stress to strike-slip faults above the inboard margin of the subducting Yakutat slab, a scenario consistent with increased transtensional-related melting recorded by the ca. 23–19 Ma transitional-tholeiitic Sonya Creek shield volcano between the Denali and Totschunda faults. Moreover, we infer the Totschunda fault accommodated more than ~85 km of horizontal offset since ca. 18 Ma, based on reconstructing the initial alignment of the early WA (i.e., 30–18 Ma SCVF) and temporally and chemically similar intrusions that crop out to the west on the opposite side of the Totschunda fault. Our results from the SCVF quantify spatial-temporal changes in deformation and magmatism that may typify arc-transform junctions over similar time scales (>10 m.y.). 

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5. Detrital zircon record of Phanerozoic magmatism in the southern Central Andes

   https://doi.org/10.1130/GES02346.1  

   Capaldi, T.N.; McKenzie, N.R.; Horton, B.K.; Mackaman-Lofland, C.; Colleps, C.L.; Stockli, D.F. (May 2021, Geosphere)

   null (Ed.)

   Abstract The spatial and temporal distribution of arc magmatism and associated isotopic variations provide insights into the Phanerozoic history of the western margin of South America during major shifts in Andean and pre-Andean plate interactions. We integrated detrital zircon U-Th-Pb and Hf isotopic results across continental magmatic arc systems of Chile and western Argentina (28°S–33°S) with igneous bedrock geochronologic and zircon Hf isotope results to define isotopic signatures linked to changes in continental margin processes. Key tectonic phases included: Paleozoic terrane accretion and Carboniferous subduction initiation during Gondwanide orogenesis, Permian–Triassic extensional collapse, Jurassic–Paleogene continental arc magmatism, and Neogene flat slab subduction during Andean shortening. The ~550 m.y. record of magmatic activity records spatial trends in magma composition associated with terrane boundaries. East of 69°W, radiogenic isotopic signatures indicate reworked continental lithosphere with enriched (evolved) εHf values and low (<0.65) zircon Th/U ratios during phases of early Paleozoic and Miocene shortening and lithospheric thickening. In contrast, the magmatic record west of 69°W displays depleted (juvenile) εHf values and high (>0.7) zircon Th/U values consistent with increased asthenospheric contributions during lithospheric thinning. Spatial constraints on Mesozoic to Cenozoic arc width provide a rough approximation of relative subduction angle, such that an increase in arc width reflects shallower slab dip. Comparisons among slab dip calculations with time-averaged εHf and Th/U zircon results exhibit a clear trend of decreasing (enriched) magma compositions with increasing arc width and decreasing slab dip. Collectively, these data sets demonstrate the influence of subduction angle on the position of upper-plate magmatism (including inboard arc advance and outboard arc retreat), changes in isotopic signatures, and overall composition of crustal and mantle material along the western edge of South America. 

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