Geochemistry, zircon U-Pb geochronology and Hf isotopes of granites in the Baoshan Block, Western Yunnan: Implications for Early Paleozoic evolution along the Gondwana margin

摘要

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 we present zircon U-Pb ages from four intrusions that show ages of 448-476 Ma suggesting that these rocks were emplaced during the Ordoviciaa The leucogranites are high-K calc-alkaline and strongly peraluminous, with K2O/Na2O > 1 and A/CNK = 1.12-1.54. These rocks are enriched in large-ion lithophile elements (ULEs) and light rare-earth elements (LREEs) [(La/Yb)_N = 1.13-32.4] and Pb, and are depleted in high field-strength elements (HFSEs). They show similar chondrite-normalized REE patterns, with negative Eu anomalies (Eu/Eu* = 0.03-0.46). A wide range of zircon ε_(Hf)(t) values (—9.6 to —2.6) and varying Hf-isotope crustal model ages (2.1-1.6 Ga) are also observed. The geochemical signatures indicate that the leucogranites are S-type granites derived mainly from the anatexis of ancient crustal materials. The ages, geochemistry and tectonics in the Baoshan Block and the Lhasa Terrane are closely comparable, suggesting that the Baoshan Block might represent part of an Early Paleozoic magmatic arc in the Gondwana continental margin facing the proto-Tethyan Ocean. The Pinghe granites of the early phase in the Baoshan Block which are coeval with the Cambrian magmatism (ca. 492 Ma) identified in the central and southern Lhasa subterranes can be interpreted as products of the slab break-off associated with the subduction of proto-Tethyan oceanic litho-sphere. However, the late leucogranite stocks are analogous to the North Himalayan leucogranites, which formed in a short-lived extensional setting caused by the slab break-off associated with the subduction-collision system.