Crust–mantle interaction resulting from slab breakoff during the southward subduction of the Bangong–Nujiang Tethys: insights derived from Cretaceous Yanhu gabbros

Elucidating crust–mantle interaction processes during the southward subduction of the Bangong–Nujiang Tethyan Ocean (BNTO) is crucial for reconstructing the tectonic evolution of the Meso-Tethys Ocean. This study focuses on gabbroic dikes exposed in the Yanhu area of Geji County. Zircon U–Pb dating yields crystallization ages of 105.0 ± 2.8 Ma and 109.9 ± 3.1 Ma for the Yanhu gabbros. Geochemically, these mafic intrusions are enriched in large-ion lithophile elements and depleted in high-field-strength elements. They show low 87Sr/86Sr ratios (87Sr/86Sr(i) = 0.704158-0.705377), positive εNd(t) values (+1.29 to +5.89) and zircon εHf(t) values (+3.3 to +9.0), exhibiting the geochemical affinities of arc gabbros with continental sediment contamination. Geochemical modelling indicates that the mantle source of the Yanhu gabbros was derived from depleted mantle mixed with minor upper crustal and ocean island basalt (OIB) components, which subsequently experienced ~22% partial melting. We propose that the Yanhu gabbros formed in response to slab breakoff during the final stage of the southward subduction of the Bangong–Nujiang Ocean. Subduction processes account for the arc geochemical characteristics associated with the incorporation of upper-crustal material. OIB-type mantle materials originated from passive upwelling induced by slab breakoff. Mantle plume activity exerted lingering influences on the mantle beneath the Cretaceous Bangong–Nujiang oceanic basin.