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

Abstract

The Ulungur River granites from North Xinjiang, China, display two petrological trends: (1) 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 ϵ Nd( T) 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 87Sr/ 86Sr ratios cannot be determined precisely by the whole-rock isochron technique mainly because most samples are highly radiogenic and have high Rb/Sr 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: (1) 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-derived magmas into the differentiating magma chamber and followed by fractional crystallisation with retention of hydrous phases (hornblende and biotite). The petrological evolution of the Ulungur River granites is thus from peralkaline to calc-alkaline, and not from calc-alkaline to peralkaline as more often observed elsewhere. The Ulungur River granites, in common with the voluminous granitoids occurring in the Central Asian Orogenic Belt, represent a significant juvenile addition of the continental crust in the Phanerozoic.