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

Abstract 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 εHf(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.