Most of Earth’s volcanism occurs at tectonic plate boundaries associated with subduction or rifting processes. The mantle plume hypothesis is an important supplement to plate tectonics for explaining some high-volume intraplate volcanic fields. However, many intraplate magmatic provinces occur as low-volume, monogenetic basaltic-suite fields that are neither associated with plate-boundary processes nor attributable to mantle plumes, and the origin of such magmatism has long been debated. Identification of their source characteristics and possible mechanisms that trigger mantle melting will provide essential insights into Earth’s mantle heterogeneity and also develop our knowledge of tectonic plate movement through time. Here, we report new geochronology, mineral chemistry (especially olivine), and whole-rock chemical and Sr-Nd-Pb-Hf isotopic compositions on Cenozoic intracontinental alkaline basalts from the northwestern Tarim craton (central Asia), aiming to better assess the origin of Earth’s low-volume effusive intraplate volcanic fields. The basalts (ca. 42 Ma) have olivine (e.g., mean Ni abundances of ∼2250 ppm, mean Mn/Zn ratios of 13.7) and whole-rock chemistry consistent with their derivation from a mixed peridotite-pyroxenite source. Moderately depleted Sr-Nd-Pb-Hf isotopes (87Sr/86Sr = 0.7039−0.7053; εNd = +4.0 to +5.5; 206Pb/204Pb = 18.247−18.535; εHf = +8.1 to +8.7) require a young (ca. 500 Ma) oceanic crust recycled into the source, possibly related to subduction events during the assembly of Pangea. Estimated thermal-chemical conditions indicate that the original melting occurred in a relatively dry (H2O = 1.4 ± 0.9 wt%) and reduced (logfO2 ΔFMQ = −0.97 ± 0.21, where FMQ is fayalite-magnetite-quartz) asthenosphere under a mantle potential temperature of ∼1420 °C and a pressure of ∼3.7 GPa (corresponding to a depth of ∼120 km). Combining these data with regional tectonic history and geophysical data (high-resolution P-wave tomography), we propose that the long-lasting India-Eurasia collision triggered asthenospheric upwelling, focusing melts along translithospheric zones of weakness; this model provides a robust explanation for the observed Cenozoic intracontinental volcanism in central Asia. The integrated geochemical and geophysical evidence reveals that plate subduction−induced mantle upwelling represents a likely mechanism for the generation of many regions of plume-absent intraplate magmatism within continents.
Oceanic subduction is an important trigger for mantle heterogeneity, which further increases melt production and controls the compositions of intraplate basalts. Such a role played by the (Paleo-) Pacific subduction have been extensively studied and well constrained based on the widespread mantle xenoliths and intraplate magmatism in the eastern North China Craton. By contrast, the recycled materials from other Phanerozoic subducted slabs beneath the craton are relatively poorly recognized. Here, a reappraisal is made to the recently reported peridotite xenoliths and ~89 Ma host basalts from the Langshan area in the northwest North China Craton and regional data on xenoliths and basalts, with the emphasis on the mass transfer in the mantle wedge from the subducted Paleo-Asian oceanic slab. The Langshan peridotites are fertile in composition and record complex melt extraction and metasomatism. One episode of metasomatism is likely induced by silicate melts with concomitant enrichments in large ion lithophile elements and high field strength elements and positive Eu anomaly, suggestive of the contribution from recycled materials. This metasomatism should take place in Paleozoic according to the diffusion modelling. The host basalts are comparable with partial melts of pyroxenite under 2–3 GPa and have oceanic island basalts-like trace-element compositions. Positive Sr and Eu anomalies, low Rb/Sr and high Ba/Rb ratios, and the moderated depleted but slightly decoupled Sr–Nd isotopes suggest the involvement of subducted oceanic crustal materials in the mantle source. The Pb isotopic compositions are best modeled by the mixing between depleted mantle, altered oceanic crust with minor young (<500 Ma) sediments. These basalts are interpreted to be partial melts of pyroxenite-bearing asthenosphere containing slab-derived materials. Collectively, the mantle wedge beneath the northwest part of the craton is pervasively modified by slab-derived melts. The infiltrating melts were gradually consumed by interaction with country peridotites of the melt conduit during the migration from the slab-asthenosphere contact. This mass transfer process triggered the formation of pyroxenite in the asthenosphere at high melt/rock ratio and metasomatism in the lithospheric mantle at low melt-rock ratio. Considering this Paleozoic melt infiltration, the regional geological records and the tectonic locality, the recycled materials in the mantle wedge beneath northwest part of the craton are likely derived from the subducted Paleo-Asian oceanic slab.
Abstract A diffuse magmatic province covering central‐eastern Asia continent displays a compositional transition at 120–100 Ma and probably reflects melting initiation in isotopically enriched lithospheric mantle, followed by melting of the asthenosphere. However, the cause for the transition across such a vast landmass remains poorly constrained. Here, analyses of newly found Chaoge basalts (∼95 Ma, central Asia) and compiled data from across the basaltic province are combined to reveal the factors controlling the basalt dichotomy. The Chaoge basalts are considered to originate from a hot pyroxenite‐bearing asthenosphere with potential temperatures of ∼1,450°C, overlapping the source thermochemical conditions for most post‐transition basaltic rocks. The asthenosphere in 120–100 Ma is suggested to be hotter and to have controlled the compositional transition in the studied basaltic province. We suggest that asthenospheric warming resulted from prolonged continental thermal blanketing and can account for other diffuse igneous provinces with similar compositional variations and tectonic histories.
Abstract The Eastern South China Block (SCB) has experienced complex Mesozoic‐Cenozoic tectonism and intraplate volcanism. However, due to a lack of exhaustive exploration of the upper mantle's thermochemical structure, it is difficult to determine the extent of the lithospheric modification and the mechanisms by which the volcanism generate. Here, we jointly invert Rayleigh wave dispersion, surface heat flow, geoid height, and elevation data to provide a comprehensive thermal and compositional structure of the upper mantle beneath eastern SCB and infer regions of partial melting. Our model reveals widespread lithospheric thinning in the eastern SCB and large variations of lithospheric composition with a more fertile eastern Lower Yangtze lithosphere than the lithosphere elsewhere, suggesting the lithosphere of the eastern Lower Yangtze is more severely modified than the rest of the SCB. Moreover, two high‐temperature anomalies are revealed: one beneath the eastern Lower Yangtze and the other beneath the Pearl River Delta region, associated with the Pacific plate subduction and Hainan plume, respectively. The high‐degree partial melting (∼6%) in the asthenosphere beneath the Lower Yangtze is responsible for the strong ongoing lithospheric modification and the young intraplate volcanism in the Nvshan and Subei areas. Small‐scale upper mantle convections triggered by the large mantle upwellings created a low value of ∼3% melts, possibly responsible for the intraplate volcanism in the coastal CB and less severe lithospheric modification. We demonstrate that the lithospheric thickness and its thermochemical state are the key factors that influence the composition and evolution of intraplate volcanism in the eastern SCB.
Abstract Adiabatic decompressional melting of asthenosphere under spreading centers has been accepted to produce vertical compositional variations of oceanic lithospheric mantle. However, theoretical estimates of the compositional gradients are much smaller than those observed from ophiolites, clearly requiring additional processes. Here we conduct systematic high-density sampling and whole-rock and mineral compositional analyses of harzburgites in a Tibetan ophiolitic mantle section (~2 km thick), which shows a primary upward depletion (~12% difference over ~2 km) and local depleted anomalies. Thermodynamic modeling demonstrates that these features cannot be produced by decompressional melting or proportional compression of residual mantle. Instead, they can be explained by reaction between silica-undersaturated melts and peridotite with lateral melt/rock variations in the topmost asthenospheric upwelling column, showing stronger depletion in its melt-focusing center and local zones. This column will split from the center into two parts, which rotate in the mantle flow to become horizontal, thus forming the oceanic uppermost lithospheric mantle characterized by vertical depletion and local anomalies within a sub-spreading-center regime.
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