An example of Phanerozoic continental crustal growth: The West Junggar Orogenic Belt, Northwest China

Abstract

It is generally accepted that continental crustal growth occurred principally during Precambrian, whereas the Phanerozoic growth seems insignificant. However, the West Junggar Orogenic Belt (WJOB), a key part of Central Asian Orogenic Belt in NW China, underwent significant crustal growth during Phanerozoic. A compilation of 683 whole-rock Sr–Nd isotopic data shows that WJOB has different crustal structures and/or compositions from adjacent areas. All igneous rocks in WJOB are characterized by depleted Sr–Nd isotopes, with initial 87Sr/86Sr ratios from 0.7022 to 0.7060, εNd (t) values from +3.0 to +9.2, and Nd model ages from 0.9 to 0.3 Ga. These features are obviously different from the large variations in Sr–Nd isotopes in adjacent Tianshan Orogenic Belt (Tianshan) to the south and Altay Orogenic Belt (Altay) to the north. The continental crustal growth in WJOB resulted from the transformation of pre-existing juvenile crust and mantle wedge during the successively southward subduction of oceanic lithosphere beneath the Ediacaran to Ordovician Tangbale-Mayile arc in southern WJOB, the Silurian to Devonian Boshchekul-Chingiz arc and Early Carboniferous Zharma-Saur arc in northern WJOB, followed by the addition of mantle-derived materials in a post-collisional setting throughout WJOB during Late Carboniferous to Permian, with little or no involvement of Precambrian crust. Such a pattern of continental crustal growth in WJOB is significantly different from those of the adjacent Tianshan and Altay as well as the Hercynian Orogenic Belt in French Massif Central, where Precambrian crustal materials are largely involved in. Thus, WJOB would probably be a good example of Phanerozoic continental crustal growth around the world.