Triassic high-Mg adakitic andesites from Linxi, Inner Mongolia: Insights into the fate of the Paleo-Asian ocean crust and fossil slab-derived melt–peridotite interaction

Abstract

Bulk element and isotopic compositions, single zircon U–Pb ages and trace element compositions of the Triassic high-Mg adakitic andesites (HMAs) from the Linxi area in the Inner Mongolia–Daxinganling orogenic belt (IMDOB) were studied to understand the area's petrogenesis and implications for Phanerozoic crustal growth. The Linxi HMAs are characterized by typical features of high-SiO2 adakite with high Mg# and high Cr and Ni contents. Coarse clinopyroxene (cpx) phenocrysts with reverse zoning were found. These cpx phenocrysts have cores with lower Mg# and Ni contents, and higher incompatible element contents (e.g., Zr and La) compared with their mantles and rims. Bulk rock Sr–Nd isotopic compositions (87Sr/86SrI=0.70382–0.70396 and εNd(t)=3.2–4.5) fall in the range of mid-ocean ridge basalts (MORB) and modern subduction-related adakites. Single zircon U–Pb dating by LA-ICP-MS suggests an eruption at ca. 238Ma. Combined with the tectonic setting and Precambrian zircon age spectrum, these features suggest that the Linxi HMAs could be derived from the subducted Paleo-Asian oceanic slab with sediments shed from the North China Craton and hybridized by peridotite in the mantle.Trace element-age variations of zircons indicate that the oceanic crust was formed during Carboniferous–early Permian times, and then subducted during ca. 270–260Ma. Melting of the subducted oceanic slab and hybridization by peridotite could have been initiated at ca. 250Ma. It is suggested that the southern accretionary zone between the North China Craton and the Solonker suture in the IMDOB, where the Linxi HMAs are located, could have been consolidated by Carboniferous–Permian times. This implies that the Linxi HMAs could have been derived from partial melting of a fossil oceanic slab after the subduction, and subduction-related melting might have been delayed if the slab was subducted under an old, cold craton. Although we are unsure of the true extent of the fossil oceanic slab, the genesis of the Linxi HMAs is essentially a snapshot of the melting of a subducted slab lagging behind subduction, and melting of the fossil oceanic slab could have played an important role in the Phanerozoic crustal growth in the IMDOB.