Early Cenozoic Igneous Rocks in the Tibetan Plateau

Vast amounts of Paleocene-Eocene volcanic rocks and Eocene intrusions had developed as a large approximately west-east plutonic-volcanic belt in southern Tibet, forming the Gangdese Magmatic Belt (GMB). The Paleocene-Eocene volcanic rocks consist mainly of Linzizong volcanic succession (LVS), which is lithologically intermediate-rhyolitic lavas and pyroclastic rocks with interlayers of sedimentary rocks. The LVS sequence has been well characterized in total thickness of 5200 m thickness in the Linzhou basin about 30 km north of the Lhasa city and can be divided into three formations upwards, i.e., the Dianzhong, the Nianbo and the Pana formation with continuous between-formation relation contacts and a clear unconformity over the highly deformed Cretaceous meta-sedimentary rocks. Geochemically, the LVS volcanic rocks are mostly calc-alkaline although some are more alkalic, and can be further classified into several rock types in TAS diagram with variable SiO2 content (SiO2 = 47–79 wt.%). Detail chronology on the andesite and rhyolite places the LVS eruptions in the Paleocene to Early Eocene with age of 63.5–43.9 Ma, suggesting a continuous volcanic activity for ~20 Myrs. The Gangdese magmatic belt (GMB) contains numerous granitoid batholiths and can be roughly divided into three sub-belts, which are getting younger southwards. The South Gangdese Batholith (SGB) is dominated by 60–40 Ma composite plutons, extends E-W for up to 2000 km, and accounts for ~45% of all the batholiths exposed in the Gangdese Belt. The lithologies of SGB are diverse, ranging from mafic to felsic rocks, including granodiorite, quartz diorite, quartz monzonite and monzogranite with abundant mafic micro-granular enclaves (MME) in these granites, indicating magma mixing process and interaction between mafic and felsic magmas. They are alkalic to calc alkalic and mostly plot in the field of shoshonitic rock series. These granitic intrusions are coeval with the LVS, and were previously interpreted as resulting from Tethyan seafloor subduction beneath the Andean-type Eurasian continental margin. It is accepted that the Cenozoic magmatic rocks have formed as a result of northward subduction of the neo-Tethyan seafloor beneath the Asian continent and the subsequent India-Asia continental collision and provide constraints on the timing and processes of the collision.