GEOCHEMICAL AND ISOTOPIC STUDIES OF THE SEDIMENTARY AND GRANITIC ROCKS OF THE ALTAI OROGEN OF NORTHWEST CHINA AND THEIR TECTONIC IMPLICATIONS

Abstract

The Altai orogen (northwest China) represents the southwestern margin of the Central Asian Orogenic Belt. Geochemical and Nd3.4 to 1.8 Ga, and ISr = 0.7106 to 0) and ISr (0.7110.7051). Voluminous syn-orogenic granitoids have [varepsilon]Nd(T) = +2.1 to 0.714 and TDM = 0.7–1.6 Ga. They were not derived by melting of local metasedimentary rocks as suggested by previous workers, but by melting of a more juvenile source at depth. Post-orogenic granites have higher [varepsilon]Nd(T) ([similar] +4.4) than the syn-orogenic granitoids, indicating their derivation from a deeper crustal level where juvenile crust may predominate.