Formation of positive εNd(T) granitoids from the Alataw Mountains, Xinjiang, China, by mixing and fractional crystallization:: implication for Phanerozoic crustal growth

Syn-collisional granitic plutons intruded into Devonian and Carboniferous sedimentary and volcanic rocks along the southern slope of the Alataw Mountains in the North Tianshan fold belt. These intrusions are 307-290 Ma in age. Granitoids in the eastern part of the mountain show characteristics similar to I-type affinity while those in the western part show S-type affinity. Carboniferous volcanic rocks (andesite to rhyolite) cover the eastern part of the region; they share many common geochemical characteristics with the eastern granitoids. Intermediate and acid igneous rocks show positive epsilon (Nd)(T) values ranging from +7 to about 0 and initial Sr isotopic ratios of 0.702-0.711. Carboniferous basalts exposed in the region give Sr-87/Sr-86(T) 0.701 -0.704 and epsilon (Nd)(T) + 7- + 12. Country rock and basement gneisses show Sr-87/Sr-86(T) 0.707-0.715 and epsilon (Nd)(T) -5 to -8. The positive epsilon (Nd)(T) suggests that mantle-derived material play an important role in petrogenesis of the Alataw granitoids. A common mantle-derived component is much likely to be involved in the petrogenesis of the granites from northern Xinjiang since the positive epsilon (Nd)(T) granites with early Paleozoic ages are widespread in the entire Central Asia-Mongolia granitic belt. Our data provide a constraint on the Sr. acid Nd isotopic ratios of this mantle-derived mafic component, which may be similar to those of the Calboniferous basalts, but can also have lower epsilon (Nd)(T) Of +3- + 7. Positive epsilon (Nd)(T) values suggest large mantle input, representing a contribution of mantle-derived materials to the continental crust. It is very likely that this event of continental growth took place in very late Proterozoic or early Paleozoic. (C) 2000 Elsevier Science B.V. All rights reserved.