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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]

To assess how chemical weathering processes in large high-relief river systems respond to climatic variability, we studied seasonal changes in radiogenic strontium (87Sr/86Sr) and stable calcium (δ44/40Ca) and magnesium (δ26Mg) isotopes in the Jinsha and Yalong rivers, which drain the southeastern Tibetan Plateau. During the low-runoff season, with discharge (Q) < 2000 m3/s, the river waters reflect the Sr, Ca, and Mg isotope signatures of recharge meltwaters, with additional isotope fractionation signals for Ca and Mg related to secondary mineral precipitation, which might imply that meltwater flushes soil solutions from the soil. During medium-runoff intervals (2000 m3/s < Q < 4000 m3/s), the Sr, Ca, and Mg isotope signatures in the Jinsha river waters are similar to those of the headwaters, which are influenced by evaporite dissolution, while the Yalong is affected by greater carbonate weathering relative to silicate weathering. In both rivers, bedrock dissolution governs the chemical composition of the river waters. During the high-runoff season (Q > 4000 m3/s), storms generate rapid overland flow, which transfers large volumes of soil into the rivers, such that soil weathering plays an important role in regulating riverine chemical compositions. At these times, the riverine Ca and Sr isotope evolution is influenced by secondary mineral dissolution and sediment–water cation exchange. Overall, this study highlights the potential of combining multiple isotope systems (Sr, Ca, Mg) to trace the dynamics of water–rock interaction under variable hydrological conditions. [ABSTRACT FROM AUTHOR]