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The widespread Cenozoic alkaline magmatism within and around the Tibetan Plateau offers a prime opportunity to probe the nature of the mantle at the depths where basalt magmas originate. The close temporal and spatial relationship between volcanism and regional strike‐slip fault systems also helps better understand the geodynamics of outward growth of the plateau in response to the continued India‐Asia convergence. We present a comprehensive study of the deeply sourced alkaline basalts formed along the Kunlun strike‐slip fault with the aim of understanding their petrogenesis and the composition of mantle sources beneath the northeastern Tibetan Plateau. High Nb/U and Ce/Pb ratios and relatively depleted bulk‐rock Sr‐Nd‐Pb isotope compositions corroborate the mantle origin of these alkaline basalts. Homogeneous and low 87Sr/86Sr of clinopyroxene indicates negligible crustal contamination during magmatic evolution. Low δ26Mg in the alkaline basalts and positive correlations with Hf/Sm and Ti/Ti* indicate that the basalts were derived from mantle that was metasomatized by melts derived from sedimentary carbonates during the Paleo‐Tethyan seafloor subduction. Based on 40Ar/39Ar dating results, it appears that the alkaline basaltic magmatism in the northeastern Tibetan Plateau occurred simultaneously with Kunlun strike‐slip faulting. These observations suggest that the India‐Asia convergence must have reactivated ancient subduction plate boundaries and resulted in strike‐slip faulting along these suture zones within and around the Tibetan Plateau. The eruption of low‐volume and deeply rooted alkaline basalts may have been controlled by fractures associated with the strike‐slip fault systems. Plain Language Summary: Small volcanoes that erupted basalt lava with alkaline compositions about 11.5 million years old occur along the northeast margin of the Tibetan Plateau and are close to a major fault system, the Kunlun Fault. Based on their chemical compositions, these basalts were melted from the upper mantle and provide information on the composition of the mantle beneath the plateau. Our chemical, geochronological, and isotopic analyses of the lava indicate that the sub‐plateau mantle was indeed affected by ancient oceanic slab subduction, including subduction of seafloor sediments that resulted in the addition of carbonate‐rich melts to the sub‐plateau mantle, making the mantle more susceptible to melting and more likely to produce alkaline basalt when it did melt. The age of the basalt lava overlaps the age of movement along the Kunlun fault, which suggests that the magma may have been produced from the same tectonic movements that produced the fault and risen through the crust along fractures associated with the fault. Key Points: The light Mg isotopes of the studied alkaline basalts highlight the occurrence of mantle carbonatite metasomatism via Paleo‐Tethyan oceanic slab subductionAlkaline basalts in northeastern Tibetan Plateau erupted coevally with Kunlun strike‐slip faulting that gave rise to mantle upwelling and decompression meltingLithospheric extrusion induced by India‐Asia collision reactivated pre‐existing lithospheric weakness and triggered mantle melting beneath the NE and SE plateau [ABSTRACT FROM AUTHOR]

Abstract The widespread Cenozoic alkaline magmatism within and around the Tibetan Plateau offers a prime opportunity to probe the nature of the mantle at the depths where basalt magmas originate. The close temporal and spatial relationship between volcanism and regional strike‐slip fault systems also helps better understand the geodynamics of outward growth of the plateau in response to the continued India‐Asia convergence. We present a comprehensive study of the deeply sourced alkaline basalts formed along the Kunlun strike‐slip fault with the aim of understanding their petrogenesis and the composition of mantle sources beneath the northeastern Tibetan Plateau. High Nb/U and Ce/Pb ratios and relatively depleted bulk‐rock Sr‐Nd‐Pb isotope compositions corroborate the mantle origin of these alkaline basalts. Homogeneous and low 87Sr/86Sr of clinopyroxene indicates negligible crustal contamination during magmatic evolution. Low δ26Mg in the alkaline basalts and positive correlations with Hf/Sm and Ti/Ti* indicate that the basalts were derived from mantle that was metasomatized by melts derived from sedimentary carbonates during the Paleo‐Tethyan seafloor subduction. Based on 40Ar/39Ar dating results, it appears that the alkaline basaltic magmatism in the northeastern Tibetan Plateau occurred simultaneously with Kunlun strike‐slip faulting. These observations suggest that the India‐Asia convergence must have reactivated ancient subduction plate boundaries and resulted in strike‐slip faulting along these suture zones within and around the Tibetan Plateau. The eruption of low‐volume and deeply rooted alkaline basalts may have been controlled by fractures associated with the strike‐slip fault systems.

Magmatic processes leading to granitoid formation are of relevance to the evolution of continental crust and its mineralization. A comprehensive study of field observations with whole-rock and in situ mineral chemical and isotopic compositions was carried out on granitoids, mafic microgranular enclaves (MME) and country-rock xenoliths (CRX) from Tonglu, South China to constrain the magmatic processes operating. Zircon U–Pb geochronology indicates that the MME and granitoids formed coevally at ∼130 Ma. Petrographic observations suggest that the MME are quenched mafic clots formed during incomplete magma mixing. The different zircon Hf isotopic compositions of the MME (εHf(t) = -4·0 to -0·7) and the host granitoids (εHf(t) = -8·1 to -1·7) indicate mingling between mafic and felsic magmas from different sources. The CRX are composed of fresh cores and dark rims. The pyroxene-rich fresh cores are depleted in Rb, Ba and K while the biotite-rich dark rims show obvious enrichments in Rb, Ba and K, indicating modification by hydrous K-rich felsic melts or fluids from the host magma. In contrast, some large CRX have embayed structures and are surrounded by several small, biotite-rich CRX, suggesting disaggregation and modification of large CRX into the host magma. The occurrence of abundant felsic magmatic veinlets in the CRX implies that they could have experienced brittle deformation in the cold shallow crust, which agrees with an emplacement depth of about 5 km estimated using Al-in hornblende geobarometry. The high 87Sr/86Sri (0·7129) and low εNd(t) (-10·2) values imply that these CRX were derived from the upper continental crust. All these features suggest a typical reactive bulk assimilation process. Microanalysis of 87Sr/86Sr ratios in plagioclase from the Tonglu granitoids vary over a large range (0·7073–0·7137) with complex rim-core-rim variations, which resulted from open-system processes. Given the variation in Sr isotopes, four types of plagioclase were identified. Type I plagioclase are homogeneous in terms of 87Sr/86Sr, suggesting normal crystal fractionation. Recharge of mafic magma injecting into felsic magma resulted in the core–mantle variations of type II plagioclases. Albitic cores with high 87Sr/86Sr (up to 0·7092) indicate felsic magma with highly radiogenic Sr (87Sr/86Sr > 0·7092). Influx of mafic magma with less radiogenic Sr (87Sr/86Sr < 0·7080) resulted in a decrease in 87Sr/86Sr and an abrupt increase of An contents (∼An30 to ∼An60) from core to mantle. Type III plagioclase are distinguished by lower 87Sr/86Sr ratios in the core (0·7084–0·.7086) and significantly more radiogenic Sr at the rim (0·7097–0·7112), which is attributed to the assimilation of the country rocks. Core-mantle-rim variations in type IV plagioclase not only record magma recharge events, but also crustal assimilation. Increasing An values and decreasing 87Sr/86Sr ratios (down to 0·7075) from core to mantle and significantly more radiogenic Sr (up to 0·7117) in the outer rim suggest that the recharge event took place prior to the assimilation of ancient crustal components. Recharge of hot mafic magma into a pre-existing magma chamber will not only increase the temperature of the magma in the chamber, but may also induce intense shattering of the brittle country-rocks, both of which could have enhanced the process of reactive bulk assimilation. [ABSTRACT FROM AUTHOR]

This contribution reports a detailed study on in situ Sr isotope analyses, along with textural and compositional characteristics, of plagioclase phenocrysts occurring in the rhyodacitic dome-lavas and associated mafic enclaves, erupted during the last magmatic activity at Nisyros volcano (Greece). Dome-lavas and enclaves have a paragenesis dominated by plagioclase. We recognize five different types of plagioclase based on their specific textures and composition. Dome-lava plagioclases (Type-1) are mainly large (1-5 mm), subhedral, clear, and poorly zoned crystals with low An content (An). The plagioclase phenocrysts (Type-4 and Type-5) and groundmass microlites crystallizing in the enclaves, and found in dome-lavas as xenocrysts, have high An content (An). In both dome-lavas and enclaves, two other types of plagioclase do also occur: (1) plagioclase phenocrysts with size and core composition similar to those of Type-1 having a dusty sieve zone (DSZ) at the rims (Type-2); (2) plagioclases with a DSZ affecting the entire crystal but a thin rim (Type-3). The drilled plagioclases have Sr/Sr negatively correlated with their An content. Low An cores of Type-1 and Type-2 have quite homogeneous Sr/Sr (0.7044-0.7046), whose values are more radiogenic than their host magmas (0.70403-0.70408) and similar to those of the previous Upper Pumice (UP) rhyolite magma (0.70438-0.70456). The DSZs of Type-2 and Type-3 show lower and scattered Sr/Sr (0.70397-0.70426) with intermediate and variable An content. High An cores of Type-4 and Type-5 have the least radiogenic Sr isotope composition (0.70379) in equilibrium with that measured in the enclaves (0.70384-0.70389). We demonstrate that Type-1 plagioclase crystallizes in the previous UP rhyolitic magmas representing the silica-rich magma from which the dome-lava melts derived by open system evolutionary processes (e.g., mixing, mingling, and crystal migration), caused by successive refilling of mafic enclave-forming magma. The Type-2 plagioclase derives from entrainment of Type-1 into the still molten enclave magma. The DSZs originated in response to the interaction between the low An plagioclase and the enclave mafic melt in which dissolution and re-crystallization acted together as function of the interaction time. Type-1 and Type-2 plagioclases record, therefore, a long-lived timescale of events starting from their crystallization in the UP rhyolite. Instead, the different width of DSZs (Type-2 and Type-3) seems to indicate short different interaction timescales between the single crystals and the enclave melt (from few hours to some 40 days). These microanalytical data contribute to the understanding of the origin of the rhyodacitic dome-lavas at Nisyros volcano and to set robust constraints on the dynamics of mingling/mixing processes in terms of crystal exchange pathways and enclave disaggregation. [ABSTRACT FROM AUTHOR]

In situ trace-element and isotopic (87Sr/86Sr) data and whole-rock Sr–Nd–Hf data on 12 gabbro xenoliths from the Hyblean Plateau (south-eastern Sicily) illustrate the complex petrogenetic evolution of this lithospheric segment. The gabbros formed by precipitation of plagioclase + clinopyroxene from a HIMU-type alkaline melt, then were cryptically metasomatized by a low-Rb, high-87Sr/86Sr fluid, and finally infiltrated by an exotic, late Fe–Ti-rich melt with 87Sr/86Sr ~ 0.7055, carrying high concentrations of Sr, Rb and HFSE. The geochemical and isotopic features of both the metasomatizing fluid and the Fe–Ti-rich melt are compatible with their common derivation by the progressive melting of an amphibole–phlogopite–ilmenite metasomatic domain (MARID-type?) that probably resided within the subcontinental lithospheric mantle. Therefore, both the astenosphere and the lithosphere underneath the Hyblean Plateau contributed to the petrogenesis of the gabbros. Sm–Nd dating yields an age of 253 ± 60 Ma for the cumulitic pile, roughly coinciding with a hydrothermal event recorded by crustal zircons in the area. We suggest that the Hyblean Plateau suffered a thermal event—probably related to lithospheric thinning and upwelling and melting of the asthenosphere—in Permo-Triassic time (the opening of the Ionian Basin?). The induced perturbation in the lithosphere caused consequent melting of some previously metasomatised portions. [ABSTRACT FROM AUTHOR]