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The Miocene Galatian Volcanic Province (GVP) is one of the largest volcanic provinces in central-western Anatolia, with an extent of ~ 8,900 km2. The volcanic activity is extended from 22.5 to 7.5 Ma. The volcanic compositions straddle the alkaline-subalkaline fields, from basic to acid compositions and mostly transitional to sodic affinity. Major oxides show good correlation with SiO2 indicating prolonged effects of fractional crystallization. Primitive mantle-normalized multi-element patterns indicate overall similarities among the different samples of the three geographic sectors, sharing strong negative anomalies in Nb–Ta–Ti, strong positive peaks at Cs and K, coupled with a common, albeit not always present, positive anomaly at Pb. Mineral-melt geothermobarometric estimates indicates ~1070–1235°C and ~7–19 kbar for melting conditions of basaltic compositions and ~1000–1150°C and ~3–12 kbar for andesitic-dacitic rocks. The absence of correlation between radiogenic isotopes and SiO2 and MgO is here interpreted as consequence of assimilation-fractional-crystalization processes involving lower continental crust as contaminant. The GVP parental magmas are generated from ~2% to 10% partial melting of a lherzolitic mantle with high spinel/garnet ratio based on intra-REE fractionation constraints. The subduction-related metasomatism inferred for the GVP mantle sources based on their chemistry is interpreted to be linked to the northward subduction of the northern branch of the Neo-Tethys slab. Successive slab retreat resulted in extension for the critical stress distribution through the Cyprus slab, favouring magma propagation for the GVP volcanic region. The eventual break-off of the slab after the continent-continent collision of Arabian and Eurasian plates could have caused a toroidal mantle flow, favouring the widely distributed 15–16 Ma alkaline magmatism in the eastern GVP, associated with passive hot asthenospheric upwelling imaged by teleseismic P-wave tomography. [ABSTRACT FROM AUTHOR]

Northeast China was formed during the progressive closure of the Paleo‐Asian Ocean before the Early Mesozoic, and was then later affected by the westward subduction of the Paleo‐Pacific Plate and the west‐to‐east closure of the Mongolia‐Okhotsk Ocean. Huge volumes of granites were emplaced across this region during the Mesozoic, and widespread Cenozoic volcanism is still active in the Quaternary. To study past and present magmatic processes in this region, two parallel high‐resolution magnetotelluric profiles were collected to produce lithospheric resistivity models. Northwest of the Greater Xing'an Range, the lithosphere has a relatively low resistivity. The resistivity of the upper mantle low‐resistivity anomaly (C2) is consistent with the presence of a few percent of basaltic melt. This feature connects with a shallower low‐resistivity zone (C1) that extends into the crust. Its properties are consistent with 5–10% shoshonitic melt. The lithosphere in the southeast part is highly resistive. The upper mantle high‐resistivity zones (R1 and R2) are interpreted as regions of thickened lithosphere that may be undergoing delamination. The lower crust exhibits localized low‐resistivity zones (C3, C4, and C5), and they are interpreted as partial melt which may have originated in the ongoing upwelling. The overall pattern of magmatism appears to be driven by asthenosphere upwelling which might have been triggered by the Paleo‐Pacific Plate retreat, but the upper mantle low‐resistivity zone originates and rises from the northwest. Down‐welling in the southeast might be the combined effect of the subduction and retreat of the Paleo‐Pacific Plate and the instability of thickened lithosphere. Plain Language Summary: Northeast China is located in the southeastern Central Asian Orogenic Belt. The crust and mantle of this area experienced the multistage closure of the Paleo‐Asian Ocean in the Paleozoic, and the subduction and retreat of the Paleo‐Pacific Plate in the Mesozoic. These tectonic events have led to extensive magmatism and volcanism that continues today. Magnetotellurics (MT) is a geophysical method that uses natural radio signals to image the electrical resistivity structure of the Earth. Two high‐resolution magnetotelluric profiles were collected in Northeast China to study the deep structure of the lithosphere. The data were inverted to produce resistivity models, that were interpreted to map the distribution of partially molten rock in the crust and upper mantle. These models show that surface volcanism can be related to regions of upwelling in the asthenosphere. Key Points: Low‐resistivity bodies (C2 and C1) in the northwest part are interpreted as partial melt, and the melt supply is moving northeastLow‐resistivity bodies beneath the Greater Xing'an Range (C3 and C4) may be caused by partial melt associated with delaminationThe tectonics of this region is characterized by zones of upwelling causing volcanism, and down‐welling caused by thickened lithosphere [ABSTRACT FROM AUTHOR]

[Display omitted] • Electrical resistivity model across the Late Cenozoic Abaga-Dalinor volcanic field (ADVF) reveals some striking features related to intraplate volcanism. • A remarkably high-conductivity zone lying in the middle-lower crust indicates the presence of saline fluids and a northwest-southeast trending fault. • A moderately highly-conductivity feature detected in the uppermost mantle indicates the presence of a low degree of partial melting. • The pre-existing tectonic weak zones such as ancient sutures, and major faults control the Late Cenozoic volcanism in the ADVF. • Late Cenozoic intraplate volcanism in the ADVF may be attributed to the decompression melting in a local asthenospheric upwelling. The Abaga-Dalinor volcanic field (ADVF) in Inner Mongolia, China, located to the west of the North-South Gravity Lineament (NSGL), is the largest and less known area of Late Cenozoic intraplate volcanism in Northeast China. Knowledge of the subsurface structure beneath the ADVF will improve our understanding of its evolution process and formation mechanisms. New broadband magnetotelluric (MT) data were collected along a profile of about 300 km that crosses the major basaltic areas exposed at the surface in the ADVF and a series of east-north-east (ENE) trending faults. The electrical resistivity structure of the crust and uppermost mantle across the ADVF was generated by 3-D inversion of full impedance tensor and tipper data. This model reveals a thick high-resistivity layer (>1000 Ωm) in the upper crust that may represent the Precambrian basement and Late Paleozoic-Mesozoic granitic plutons. In contrast, high conductivity anomalies dominate the middle-lower crust. The northward-dipping high-conductivity feature at depths of 20–40 km below sea level under the Chagan Obo fault is suggested to be the involvement of the Early Paleozoic subducted oceanic crust. A remarkably high-conductivity zone (1–20 Ωm) exists in the middle-lower crust beneath the Abaga and Dalinor volcanic areas. It is explained to be a saline fluid zone of ∼0.45%–0.48% that exsolved from the partial melts as magma ascent rapidly. The impermeable upper crust traps these fluids in the middle-lower crust. A moderately high-conductivity anomaly in the uppermost mantle below near the Solonker Suture Zone (SSZ) is attributed to a minimum partial melt of ∼3–4% associated with the localized asthenospheric upwelling, which feeds the Abaga and Dalinor volcanos as a common magma source. Integrating our resistivity model with previous geophysical and geochemical studies, we suggest that intraplate volcanism in the ADVF is mainly controlled by the pre-existing tectonic weak zones. The lithosphere within the SSZ is susceptible to thermal perturbation from the upwelling mantle, providing the preconditions for melt generation. A set of ENE trending faults and an inferred NW-SE trending fault jointly dominated the vertical transport of magma and lateral extension along the faults, which may have influenced the spatial distribution of basalts and volcanic cones at the surface in the ADVF. In addition, we tend to suggest that Late Cenozoic intraplate volcanism in the ADVF could be the result of decompressing melting within the localized small-scale asthenospheric upwelling induced by the edge-driven convection near the NSGL, which is indirectly related to the westward subduction and retreat of the (Paleo-)Pacific plate during the Late Mesozoic to Cenozoic. [ABSTRACT FROM AUTHOR]

Abstract: Felsic volcanic rocks (mainly rhyolites) and basalts found in northeast Hokkido, Japan, result from intense volcanism during the Middle Miocene (14–9Ma), Late Miocene (8–6Ma) and Pliocene to Quaternary (5–2Ma). Rhyolites were examined to determine any genetic relationship to coeval basalts, high magnesian andesite (HMA), lower crustal rocks and mantle peridotite. Rhyolites have initial Sr and Nd isotopic ratios (SrI and NdI) which overlap with coeval north Hokkaido basaltic rocks and HMA. Previously published Sr and Nd isotopic data show that most Middle Miocene to Quaternary (14–0Ma) basaltic rocks from north Hokkaido have a relatively narrow SrI- and NdI- range (SrI 0.70299 to 0.70400, and NdI 0.51281 to 0.51311), with no temporal variation in either SrI or NdI. Basalt and HMA from three locations are, however, more undepleted in terms of both SrI and NdI than other north Hokkaido basaltic rocks. Some of rhyolites (termed undepleted rhyolites here) have similar SrI and NdI to some north Hokkaido basalts and HMA. Rhyolites with similar SrI and NdI to other north Hokkaido basaltic rocks are termed depleted rhyolites. Although the similarity of SrI and NdI between rhyolites and coeval basalts and HMA can be accounted for by fractional crystallization, this process is inconsistent with the REE chemistry of basalts, HMA and rhyolites, and with the results of fractional crystallization modeling. However, a few rhyolites may result from the fractional crystallization of basaltic and HMA magmas with assimilation of some metasedimentary rocks. Small degrees of partial melting of a metazomatized mantle peridotite is an unlikely mechanism to explain the genesis of rhyolites according to REE chemistry and partial melt modeling of an amphibole bearing spinel lherzolite source. Gabbros of the Hidaka metamorphic belt are a possible source for isotopically depleted rhyolites, as both the rhyolites and gabbros have similar SrI and NdI. I-type tonalite and some gabbros in the Hidaka metamorphic belt are possible source rocks for isotopically undepleted rhyolites based on similarity in SrI and NdI. These hypotheses are supported by partial melt modeling of olivine gabbro and I-type tonalite respectively. A possible tectono-magmatic model for the production of post-Middle Miocene rhyolites from NE Hokkaido involves upwelling of the asthenosphere during the Middle Miocene, associated with the spreading of the Kurile back-arc basin and Japan Sea back-arc basin. This would have resulted in thinning of the overlying lithosphere beneath north Hokkaido, and the production of asthenosphere-derived basaltic rocks with low SrI and high NdI throughout north Hokkaido since the Middle Miocene, but less common production of lithosphere-derived basaltic rocks and HMA (with high SrI and low NdI). Basaltic magmas formed since the Middle Miocene either erupted, or caused melting of the crust, resulting in the generation of rhyolitic magma. [Copyright &y& Elsevier]

Abstract: Volcanism occurring away from ridges and subduction zones does not have an obvious plate tectonic explanation, but instead must arise from sub-lithospheric processes that generate upwelling flow and decompression melting. Several convective processes, such as mantle plumes, convective instability, edge-driven convection, and Richter rolls, produce upwelling via the action of gravity on density heterogeneities in the mantle. Here we investigate an alternative mechanism, the shear-driven upwelling (SDU), which instead generates upwelling solely through the action of asthenospheric shear flow on viscosity heterogeneity. Using a numerical flow model, we examine the effect of viscosity heterogeneity on viscous shear flow induced within an asthenospheric layer. We demonstrate that for certain geometries and viscosity ratios, circulatory flow develops within a “cavity” or “step” embedded into the lithospheric base, or within a low-viscosity “pocket” embedded within the asthenospheric layer. For asthenosphere shearing at 5cm/yr, we estimate that SDU can produce upwelling rates of up to ∼0.2cm/yr within a continental rift, ∼0.5cm/yr along the vertical edge of a craton, or ∼1.0cm/yr within a “pocket” of low-viscosity asthenosphere. In the last case, the pocket must feature an aspect ratio of more than 5, occupy ∼20–60% of the asthenosphere''s thickness, and be at least 100 times less viscous than the surrounding asthenosphere. Such viscosity heterogeneity may be associated with thermal, chemical, melting, volatile, or grain-size anomalies, and is consistent with tomographic constraints on asthenospheric variability. We estimate that SDU may generate up to 2.5km/Myr of melt that is potentially eruptible as surface volcanism; this is faster than eruption rates observed at some locations of continental basaltic volcanism. We conclude that SDU could provide an explanation for intraplate volcanism occurring above rapidly shearing asthenosphere, for example in the Basin and Range region of North America. [Copyright &y& Elsevier]

Abstract: The post-collisional Cenozoic magmatic activity in the Biga Peninsula (NW Anatolia) started in the Middle Eocene (45.3±0.9 Ma) and lasted continuously until the Late Miocene (8.32±0.19 Ma) with minor punctuations, producing widespread volcanoplutonic complexes. These magmatic suites are characterized by calc-alkaline, high-K calc-alkaline, shoshonitic, mildly alkaline, and alkaline series. The Middle Eocene to Middle Miocene lavas have high LILE/LREE, LILE/HFSE and 87Sr/86Sr(i) (0.7046–0.7087) and low ɛNd(i) (+1.2 to −6.4) values. By contrast, the Late Miocene lavas have the lowest 87Sr/86Sr(i) (0.7030–0.7033) and the highest ɛNd(i) (+2.6 to +6.7) values. Geochemical features of these rocks indicate two episodes of enrichment processes during the evolution of the Eocene–Early Miocene volcanism in the Biga Peninsula. The first stage, source enrichment metasomatism, occurred within the continental lithospheric mantle as a result of the previous subduction event(s), and produced a heterogeneously enriched source. The second stage, crustal contamination, resulted from interaction between the metasomatized mantle-derived melts and those magmas derived from the Sakarya crustal basement during their ascent to the surface. Slab breakoff-induced asthenospheric upwelling caused the melting of metasomatized mantle lithosphere beneath the Sakarya continent and produced the Middle Eocene calc-alkaline lavas. Geochemical, isotopic and age relationships suggest increasing amounts of crustal contamination and a decreasing subduction signature during the evolution of magmas from the Eocene through the Early Miocene. Crustal input was peaked during the Early Miocene volcanism, coinciding with the exhumation and rapid uplift of the Kazdag metamorphic massif. Asthenospheric upwelling beneath the Kazdag core complex contributed to the generation of hybrid magmas formed from mixing of mantle and crustal melts during this stage. The Middle Miocene (15.2 Ma) mildly alkaline rocks show less pronounced enrichment in LILE and LREE and display intermediate major and trace element compositions between the Early Miocene and Late Miocene lavas. The Cenozoic volcanism in Biga Peninsula waned briefly during 15–11 Ma, and was subsequently rejuvenated (Late Miocene, 11.0±0.4–8.32±0.19 Ma) producing mainly OIB-type alkaline lavas. Advanced extensional tectonics throughout NW Anatolia in the Late Miocene led to upwelling of the asthenospheric mantle and its decompressional melting that collectively resulted in the eruption of short-lived episodes of mainly OIB-type alkaline lavas. This tectonomagmatic evolution of the Cenozoic continental volcanism in western Anatolia presents a well-documented case study of an idealized post-collisional magmatism in most orogenic belts around the world. [Copyright &y& Elsevier]