Early Triassic mid-ocean-ridge basalt (MORB)-like diabase swarms in the southeastern segment of the Central Asian Orogenic Belt: A response to lithospheric extension triggered by Paleo-Asian oceanic slab break-off

Mafic dike swarms in paleo-subduction zones generally serve as important indicators of tectono-magmatic processes and provide a different perspective on mantle source and geodynamic evolution. The geodynamic evolution of the southeastern segment of the Central Asian Orogenic Belt (CAOB) during the Late Permian−Early Triassic remains unclear, including oceanic ridge subduction, slab break-off, slab roll-back, and post-collision extension. To address these disputes, we carried out a comprehensive study of petrography, whole-rock major and trace elements, mineralogy, secondary ion mass spectrometry (SIMS) zircon U-Pb dating, and in situ zircon Hf isotope and apatite Sr isotope analyses for the newly discovered early Mesozoic diabase swarms in the southern CAOB. High-precision SIMS zircon U-Pb dating of these diabases yielded concordant ages ranging from 250.8 ± 3.8 Ma to 248.2 ± 2.4 Ma, representing the Early Triassic magmatic activities. These diabase samples belong to sub-alkaline series with low SiO₂ and K₂O contents and relatively high TiO₂, MgO, and transition metal element contents (e.g., Cr, Ni). They are enriched in large-ion lithophile elements and depleted in high field strength elements, with Nb-Ta negative anomalies, indicating typical arc-style geochemical affinities. They show weakly fractionated rare earth elements (REE) with flat chondrite-normalized REE patterns similar to enriched mid-ocean-ridge basalt (MORB) and relatively homogeneous and highly depleted isotopic compositions, with zircon ε_Hf(t) values of +2.63 to +7.97 and significantly low apatite ⁸⁷Sr/⁸⁶Sr values of 0.703656−0.704147. These diabase samples were originated by ∼5%−10% decompression melting of a mantle wedge (spinel-garnet lherzolite) metasomatized by subduction-related melts, with minor contributions of mid-ocean-ridge basalt (MORB)-like asthenospheric materials. These Early Triassic diabase swarms, together with coeval magmatic flare-ups of high-Mg andesites and Nb-enriched mafic rocks and adakites, confirm a lithosphere extension-related “hot” environment accompanied by the melting of oceanic slab and the associated melt-mantle wedge interactions in the southeastern segment of the CAOB. It is suggested that the Early Triassic diabase swarms were formed in a slab window, triggered by slab break-off of the Paleo-Asian oceanic slab. The upwelling of asthenospheric materials induced decompression melting of the lithospheric mantle, which was favorable for the emplacement of the Early Triassic diabase swarms.