Nd–Sr isotopic mapping of the Chinese Altai and implications for continental growth in the Central Asian Orogenic Belt

Abstract

The Chinese Altai orogen is a typical domain of the Central Asian Orogenic Belt and in previous syntheses was divided into six tectonic units (or terranes). In this paper, 41 new and 36 published whole-rock Nd and Sr isotopic data for granitoid intrusions and mafic dykes within units 2 and 3 of the central Altai and units 4 to 6 of the southern Altai were used for isotopic mapping. The age of the samples used is known from U–Pb zircon dating. ε Nd( t) values range from − 4 to + 2 in the central Altai units 2 and 3, and from + 1.4 to + 6 in the southern Altai units 4 and 5 and up to + 8.4 in unit 6. Correspondingly, Nd model ages ( T DM) decrease from 1.6–1.1 Ga in the central Altai to 1.0–0.5 Ga in the southern Altai, except for a continental fragment in unit 4. I Sr is elevated in the central units, decreasing to < 0.705 in the southern units 5 and 6. These results demonstrate that the central Altai contains widespread old crustal components in its basement, whereas the southern Altai comprises a higher proportion of juvenile crust. When integrated with published U–Pb zircon ages, our data show that new crust grew southwards in the Chinese Altai. In addition, our data suggest that units 2 and 3 are a single tectonic unit, whereas the southern Altai (unit 4), which previously was regarded as a single young accretionary terrane, should be subdivided into two units: a younger arc and an older continental fragment. The granitoids in the Chinese Altai can be divided into syn- and post-/non-orogenic groups. The syn-orogenic granitoids have ca. 50–80% mantle-derived component, whereas the post/non-orogenic granitoids contain ca. 70–90%. The generation of syn- and post-/non-orogenic granitoids represents horizontal (lateral) and vertical crustal growth, which contributed ca. 18–28% and 7–8% respectively to the area of the Chinese Altai. This work shows that isotopic mapping is an effective approach to elucidate terrane composition in the Central Asian Orogenic Belt and thereby better constrain Palaeozoic crustal growth.