Geochemistry, zircon U-Pb ages and Sr-Nd-Hf isotopes of an Early Triassic appinitic complex in southeastern Inner Mongolia, China: implications for the late tectonic evolution of the Paleo-Asian Ocean

Abstract

Appinite commonly occurs in convergent plate tectonic settings and thus can constrain the tectonic evolution of ancient orogens. Geochemical and geochronological analyses were carried out on a newly identified Early Triassic appinitic complex in southeastern Inner Mongolia in the eastern segment of the Central Asian Orogenic Belt. Petrographically, the Luotuochang complex can be divided into two zones: an outer zone of intermediate rocks and inner zone dominated by mafic rocks. A monzonite sample from the outer zone yielded weighted mean zircon 206Pb/238U ages of 246 ± 1.6 Ma, whereas the gabbro from the inner zone yielded an age of 243 ± 1.2 Ma. The inner zone mafic rocks have SiO2 contents of 45.56 to 54.27 wt% with high MgO, Cr, Ni and Sr contents, elevated Ba/Nb, Ba/Zr, Rb/Y and Th/Zr, and low TiO2, Nb/Zr and Nb/Y. These features suggest that the metasomatized lithospheric mantle is the primitive magma source. SiO2 contents of the outer zone intermediate rocks range from 57.6 to 63.69 wt% and K2O, Ba and Sr contents are enriched; the eNd(t) (+2.2 to +4.0) and eHf(t) (+8.4 to +13.5) values are also high. These characteristics suggest that the magma of these rocks mainly derived from the mantle with possible juvenile lower crust involvement. Based on these geochemical data and results from regional geological investigations, we propose that the Luotuochang appinitic complex was formed in a post-orogenic extensional setting. Its formation was likely the result of lithospheric delamination, upwelling of new mantle material and partial melting of the overlying lower crust during crustal compression and thickening soon after the closure of the restricted Paleo-Asian Ocean basin.