Discovery of Ultra-Depleted Melt Inclusion in Late Cretaceous Intracontinental Basaltic Andesites in South China: Implications for Recycling of Lower Oceanic Crust

Incompatible element ultra-depleted melt (UDM) has occasionally been found as olivine-hosted inclusion in plume-related oceanic island basalts and mid-oceanic ridge basalts and is genetically linked with the involvement of lower oceanic gabbroic crust. However, it has rarely been reported in intracontinental magmatism, and the contribution of such a recycled component to the origin of intracontinental magmas remains unclear. Here, we first report UDM inclusions hosted in olivine from Late Cretaceous intracontinental basaltic andesites in South China. These UDM inclusions are characterized by low SiO2 (46.4-47.9 wt%) and extremely high Al2O3 (18.47-21.79 wt%) contents, positive Sr-Eu anomalies, and low 207Pb/ 206Pb and 208Pb/206Pb ratios. Such geochemical features resemble those of high-pressure melts derived from young lower oceanic gabbro. These results, in combination with geochemical modeling, plate reconstruction, and seismic topography, suggest that tearing or fragmentation of the subducted paleo-Pacific slab during Late Cretaceous may trigger melting of the refractory lower oceanic cumulates. Our results demonstrate that lower oceanic gabbroic crust could also play an important role in the generation of intracontinental magmatism, which may previously have been underestimated. Plain Language Summary The diversity of recycled oceanic slabs, including marine sediments, upper oceanic crust, lower oceanic gabbro, and oceanic mantle, plays an important role in producing chemical heterogeneity of Earth's mantle. Melt inclusions trapped in olivine crystals are isolated from external melt and have the potential to preserve information on the heterogeneous composition of mantle-derived magmas, which is completely or partially homogenized in bulk rocks. Incompatible element ultra-depleted melts (UDMs) have been occasionally found as olivine-hosting inclusions in plume-related oceanic island basalts and mid-oceanic ridge basalts and are considered as evidence for the involvement of lower oceanic gabbroic crust in origin. Here, we are the first to document olivine-hosted UDM inclusions from Late Cretaceous intracontinental basaltic andesites in South China. Their geochemical features resemble those of high-pressure melts by partial melting of a young lower oceanic gabbroic crust. Partial melting of the refractory lower oceanic gabbro was likely attributed to slab tearing or fragmentation, as revealed by seismic tomographic images that the torn paleo-Pacific slabs are still stagnated between 1,000 and 1,500 km in the lower mantle. Our results reveal the role of lower oceanic crust in producing the geochemically heterogeneous mantle even beneath the intracontinental regions.