Late Paleozoic magmatism set the stage for the present-day lower crust of West Tianshan, SW CAOB

The deep lithosphere influences mineral resource distribution and the planet’s habitability, yet probing its architecture remains challenging. The present-day lithospheric architecture is a terminal, time-integrated image shaped by long-term geological processes, among which magmatism plays a pivotal role. However, the exact relationship between ancient magmatic events and present-day lithospheric architecture has not been fully explored. In this study, we investigate Late Carboniferous to Middle Permian magmatic rocks in West Tianshan, SW Central Asian Orogenic Belt, using multi-proxy isotopic and elemental mapping and thermodynamic phase equilibrium modeling. Using radiogenic (Hf-Nd) isotope spatial imaging, we here show that felsic rocks constitute two distinct domains: an isotopically depleted domain (IDD) to the north and an isotopically enriched domain (IED) to the south. The two domains correspond well to differences in geophysical properties revealed by seismic and gravity data. The lower crust of IDD was built by magmatic differentiation of intermediate magmas sourced from an oceanic subduction-modified mantle. In contrast, the lower crust of IED was constructed through the differentiation of sanukitoid magmas derived from an ancient crust-metasomatized mantle, with variable-degree involvement of Tarim supracrustal relaminant. These findings directly link geophysical contrasts in the lithosphere to variations in deep-time magmatism, including mantle source heterogeneity and lower crustal differentiation. The study suggests that ancient magmatic rocks cannot only provide fresh insights into the dynamic processes that have shaped the lithosphere over geological time scales, but also be powerful proxies to understand present-day lithospheric architecture.