Early Silurian Slab Break-Off and Crustal Reworking in the Southern Central Asian Orogenic Belt: Insights from Liuyuan A-Type Granites

The southern Central Asian Orogenic Belt (CAOB) underwent a major Early Paleozoic tectonic transition, yet its timing and mechanisms remain unclear. We present zircon U–Pb–Hf, whole-rock geochemical, and Sr–Nd isotopic data for newly identified Early Silurian (ca. 439–431 Ma) granitoids from the Liuyuan area of the southern Beishan Orogenic Belt. These high-silica, high-K calc-alkaline intrusions not only show arc-like trace-element patterns but also display elevated Ga/Al ratios and enriched Sr–Nd isotopic compositions ((87Sr/86Sr)i = 0.7158–0.7189; εNd(t) = −4.6 to −3.9), consistent with aluminous A2-type granites derived mainly from ancient crust. Their heterogeneous zircon εHf(t) values (−6.3 to +3.7) suggest a minor, localized input from mantle-derived mafic magmas superimposed on the dominant crustal signature. Integrating regional metamorphic constraints, we interpret this magmatism to have formed during the transition from oceanic subduction to incipient collision/continent involvement and subsequent post-subduction extension, plausibly triggered by slab break-off at the slab root (ocean–continent transition). Slab-window-related asthenospheric inflow and localized thermal perturbation could have promoted high-temperature crustal melting and facilitated Early Silurian crustal reworking in the southern CAOB.