The leucogranites in the Baoshan Block of the Tethyan belt in western Yunnan,are composed mainly of two-mica granite with subordinate muscovite granite. Here wepresent zircon U-Pb ages from four intrusions that show ages of 448 ~ 476Ma suggestingthat these rocks were emplaced during the Ordovician. The leucogranites are high-Kcalc-alkaline and strongly peraluminous, with K2O/Na2O > 1 and A/CNK = 1.12 ~ 1.54.These rocks are enriched in large-ion lithophile elements (LILEs) and light rare-earthelements (LREEs) [(La/Yb)N = 1.13 ~ 32.4] and Pb, and are depleted in high field-strengthelements (HFSEs). They show similar chondrite-normalized REE patterns, with negativeEu anomalies (Eu/Eu* = 0.03 ~ 0.46). A wide range zircon Hf(t) values (?9.6 ~ ?2.6) andvarying Hf-isotope crustal model ages (2.1 ~ 1.6 Ga) are also observed. The geochemicalsignatures indicate that the leucogranites are S-type granites derived mainly from anatexisof ancient crustal materials. The ages, geochemistry and tectonics in the Baoshan Blockand the Lhasa Terrane are closely comparable, suggesting that the Baoshan Block mightrepresent part of an Early Paleozoic magmatic arc in the Gondwana continental marginfacing the proto-Tethyan Ocean. The Pinghe granites of the early phase in the BaoshanBlock which are coeval with the Cambrian magmatism (ca. 492 Ma) identified in thecentral and southern Lhasa subterranes can be interpreted as products of the slab break-offassociated with the subduction of proto-Tethyan oceanic lithosphere. However, the lateleucogranite stocks are analogous to the North Himalayan leucogranites, which formed in ashort-lived extensional setting caused by the slab break-off associated with thesubduction-collision system.