Continental OIB magmatism reveals asthenospheric heterogeneity driven by deep carbon recycling in postcollisional southeastern Tibet

The formation of the Tibetan Plateau and the subsequent postcollisional convergence have left profound imprints on the lithosphere, yet how these processes have modified the underlying upper mantle remains poorly constrained due to the lack of direct magmatic records. Continental ocean-island-type basalts, although volumetrically minor, provide an exceptional window into such compositional overprinting by deep geodynamic processes. Here, we probe heterogeneity in the postcollisional asthenosphere by integrating mineralogical, elemental, and Mg-Zn-Sr-Nd isotopic constraints from continental basalts in the Tongguan region, southeastern Tibetan Plateau. These basalts erupted during the Quaternary (ca. 1 Ma; zircon U-Pb ages) and exhibit typical ocean-island basalt (OIB)–type trace element signatures with depleted Sr-Nd isotopic compositions (εNd ≤ +8.6). Mineral and whole-rock thermometry, hygrometry, and oxybarometry collectively indicate a relatively hydrous (H2O ≤ 3.5 wt%) yet low-fO2 (FMQ+0.1) primary magma that formed under normal mantle thermal conditions (<1368 °C). Olivine chemistry and whole-rock elemental systematics consistently require derivation from a dominantly Stage 2 pyroxenitic asthenospheric mantle with minor peridotite involvement. The basalts display systematically light δ26Mg (−0.60‰ to −0.49‰) and heavy δ66Zn (+0.34‰ to +0.42‰) values yet lack elemental signatures of carbonate metasomatism. This marked isotope–element decoupling compellingly traces recycled marine carbonates that underwent deep subduction-induced decarbonation, leaving a “ghost carbonate” imprint in the mantle source. Together with geophysical evidence, these features directly link Tongguan basaltic magmatism to dehydration of stagnant oceanic slabs in the mantle transition zone (MTZ). The Mg-Zn isotopic variations further delineate a clear regional imprint of recycled sedimentary carbonates in the asthenospheric mantle south of ~26°N as a consequence of deep slab stagnation, in stark contrast to the relatively noncarbonated asthenosphere to the north, where slab tearing dominates. Taken together, our study demonstrates that continental OIB-type magmatism in the southeastern Tibetan Plateau records pronounced postcollisional asthenospheric heterogeneity dictated by spatially variable subduction dynamics. More broadly, this case highlights the pivotal role of stagnant slabs in the MTZ in governing deep carbon recycling and generating asthenosphere-derived continental OIB-type magmatism.