Sr, Nd and O isotope geochemistry of the mafic-ultramafic complexes in the southern margin of the Altay orogenic belt and discussion of their sources

The Sr, Nd and O isotopic compositions of the Kalatongke and Xibodu mafic-ultramafic complexes from the southern margin of the Altay orogenic belt show that they have similar isotopic compositions, characterized by low (⁸⁷Sr/⁸⁶Sr), and high ε _Nd(t) values (6.3–9.1). It suggests that they were derived from a depleted asthenospheric mantle. However, most of the samples have δ ¹⁸O values >6‰ (5.4‰–10.2‰), indicating crustal contamination. A combination of Sr and O isotopic data shows the incorporation of crustal materials into the depleted mantle. They were produced by the melting of depleted mantle by the incorporation of subducted oceanic crust, and this incorporation might have occurred in the Early Paleozoic in the light of their Nd model ages and regional tectonics. The Kalatongke complex might have undertaken the contamination of the upper crust whereas the Xibodu complex does not have any signature of contamination of the upper crust. In addition, the similarities of the sources of the two complexes, which were located at the northern and southern sides of the regional Irtysh fault zone respectively, suggest that this fault might not be the boundary between the Altay and Junggar orogenic belts. Translated from Geological Review, 2006, 52(1): 38–42 [译自: 地质论评]