Abstract:

Shiduo volcanic rocks in the western segment of Gangdise, Tibet belong to high potassium calc-alkali series and calc-alkali series. They are divided into two stages of volcanicism:Paleocene(59.7Ma) and Miocene（17.2Ma）. In time and position, the formor corresponds to Dianzhong Formation volcanic rocks of Linzizong Group in eastern Gangdise magmatic belt. Both of ∑REE(58.83x10^-6)and degree of fractionation between LREE and HREE ((La/Yb)_N=3.44 in the Paleocene volcanic rocks are much less than those in the Dianzhong Formation volcanic rocks. ¹⁴³Nd/¹⁴⁴Nd(0.512 275 to 0.512 420) and ε_Nd(t)(-6.7 to -4.0) in the Paleocene volcanic are also smaller than those in the Dianzhong Formation volcanic rocks. These geochemical characteristics imply that volcanicism had some differences between western and eastern Gangdise magmatic belt in the same period. Based upon the Pb-Nd-Sr insotopes composition character and tectonic setting discrimination of trace element geochemistry, Shiduo volcanic rocks are formed in the environment of continental margin-arc-continental collision orogenic, and volcanicism orginated from the transitional process from the subduction of Neo-Tethyan oceanic plate to continental collision orogenics. Minerogenesis of Shiduo iron-copper deposit is related to Paleocene volcanism.

Key words: volcanic rocks, geologic and geochemical characteristics, Gangdise, Tibet