Tearing on the southward subducting Kelameili oceanic lithosphere in the early Devonian: Evidence from the magmatism in the Harlik arc, southern Altaids

Abstract

<p>The coexistence of adakites, high-Mg andesites (HMAs) and Nb-enriched basalts (NEBs) records special subduction geodynamics, for instance, subduction initiation, subduction of oceanic ridges or slab tearing, which are widespread in the circum-Pacific subduction zone. The Altaids in Central Asia shared similar tectonic evolution to the western Pacific. However, slab tearing has not been widely reported, which hinders a better comparison between the Altaids and the western Pacific. In this study, we report the contemporaneous early Devonian granodiorites and mafic microgranular enclaves (MMEs) in the Sidingheishan area of the Harlik arc, southern Altaids. The granodiorites have high Sr (534-683 ppm) but low Y (7.79-10.2 ppm) and Yb (0.71-1.68 ppm) contents, with corresponding high Sr/Y values (29.78-76.93). In combination with their juvenile isotopes (initial 87Sr/86Sr = 0.70333-0.70356, & epsilon;Nd(t) = +7.4 - +8.01) and high Mg# numbers (50.9-53), they indicate that their primitive magmas were derived from slab melting. The MMEs show similarly high Sr contents (507-696 ppm), and identically juvenile isotopic signatures (initial 87Sr/86Sr = 0.70346-0.70356, & epsilon;Nd(t) = +7.14 - +7.5) to their host granodiorites. However, their relatively low silica contents (55-60 wt%), evolved Mg# numbers (44.8-55.2) and high Nb contents (12.7-24.7 ppm) demonstrate that they are the fractionation products of NEBs, which were produced by the partial melting of the mantle that was previously metasomatized by these adakitic magmas. Considering all the available evidence, we propose a slab tearing model in which the southward subducting Kelameili oceanic lithosphere underneath the Harlik arc underwent rollback in the early Devonian and its differential rollback rates caused slab tearing. Subsequent asthenosphere upwelling stimulated the Kelameili oceanic lithosphere to melt, producing the adakitic magmas, some of which intruded the crust after limited interaction with the mantle peridotites and underwent crystallization of dominant amphibole in the crust. The rest reacted completely with the mantle peridotites, which underwent further melting to produce the Nb-enriched basaltic magmas. They intruded into the adakitic magma chamber after fractional crystallization of olivine, clinopyroxene and garnet in the deep, forming the MMEs and promoting the remobilization of those hornblendites. This study reveals the development of slab tearing that are closely associated with slab rollback in the Altaids, providing further evidence that argues for the similarity between the Altaids and the western Pacific.<br></p>