ELSEVIER Chemical Geology 138 (I 997) 135-159 CHEMICAL GEOLOGY ISOTOPE GEOSCIENCE Depleted-mantle source for the Ulungur River A-type granites from North Xinjiang, China: geochemistry and Nd-Sr isotopic evidence, and implications for Phanerozoic crustal growth Bao-fu Han a, Shi-guang Wang a, Bor-ming Jahn b,*, Da-wei Hong c Hiroo Kagami d, Yuan-lin Sun a a Department of Geology, Peking UniL'ersity, Beijing, 100871, China b Geosciences Rennes, Universite de Rennes I, Avenue du General Leclere, 35042, Rennes Cedex, France C Lithosphere Research Center, Chinese Academy of Geological Sciences, Beijing, 100037, China d 1nstitutefor Study of the Earth's 1nterior, Okayama Unil'ersity, 827 Yamada, Misasa, Tottori, 682-02, Japan Received 13 July 1996; accepted 18 December 1996 Abstract The Ulungur River granites from North Xinjiang, China, display two petrological trends: (I) a peralkaline trend, composed of alkali amphibole- and alkali pyroxene-bearing granites and dykes, and (2) an aluminous trend, represented by alkali granites, monzogranites and aluminous granite dykes within the peralkaline granites. Rb-Sr whole-rock isochron dating yielded an age of 300 Ma for the emplacement of the peralkaline granites and 270 Ma for the aluminous granites. The Ulungur River granites share all the geochemical features common to A-type granites of the world, The Ulungur River granites are post-orogenic and emplaced in a post-collisional extensional environment. The peralkaline and aluminous rocks have indistinguishable Nd isotopic compositions. They have unusually positive and relatively uniform ENiT) values, ranging from + 5.1 to + 6.7 for the peralkaline rocks at 300 Ma, and from + 5.5 to + 5.9 for the aluminous rocks at 270 Ma. The initial 87 Sr I 86Sr ratios cannot be determined precisely by the whole-rock isochron technique mainly because most samples are highly radiogenic and have high RblSr ratios. In addition, the large variation in Sr isotopic compositions can partly be explained by crustal contamination during magma differentiation. The Nd isotopic data clearly indicate that both the peralkaline and aluminous rocks were derived from a long-lived depleted mantle reservoir. The formation of the Ulungur River A-type granites is hypothesized as follows: (I) a depleted mantle was first metasomatised, then subjected to partial melting leading to the generation of basic magmas now represented by the Kalatongke mafic-ultramafic complex in the Ulungur River area; (2) the basic magmas have undergone high degrees of fractional crystallisation with separation of biotite, hornblende, plagioclase, potassium feldspar, and other accessory phases such as apatite and ilmenite, with the residual magmas forming the peralkaline granites. During the magmatic differentiation, the liquids were probably contaminated by crustal materials as evidenced by Nb depletion and Nd-Sr isotopic systematics. On the other hand, the formation of aluminous granites involved a replenishment of new mantle-de- , Corresponding author. 0009-2541/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. Pll S0009-254J(97)00003-X