The mafic–ultramafic roots of a magmatic arc from the East Kunlun Orogen, NW China

Magmatic arcs are key contributors to continental growth. Here we investigate a mafic–ultramafic suite from Xiwanggou in the Eastern Kunlun Orogen, offering insights into a complete arc lower crust sequence within the Eastern Kunlun Orogenic Belt (EKOB). We present whole-rock geochemistry, electron probe microanalyses of olivine, pyroxene, and spinel, in-situ trace element analysis of pyroxene and apatite U-Pb isotopic data to gain insights into the crystallization mechanism and the crustal growth process of the EKOB. The Xiwanggou mafic–ultramafic rocks (XMURs) are mainly composed of wehrlite, clinopyroxenite, gabbro, and hornblende gabbro. LA-ICP-MS apatite U-Pb geochronology indicates that the wehrlite, clinopyroxenite, and gabbro formed coevally at 249.6 ± 6.2 Ma, 249.7 ± 2.6 Ma, and 247.8 ± 1.8 Ma, respectively. The hornblende gabbro is slightly younger with an age of 237.6 ± 1.9 Ma. Our data suggest that these rocks crystallized from a common parental magma and evolved through fractional crystallization according to the following sequence: Ol + Cpx → Cpx + interstitial Pl → Cpx (late stage) + Pl, and hydrous minerals (Phl and Hb). The crystallization sequence, calculated equilibrium melts, and elemental composition of clinopyroxenes indicate these rocks were produced during the transition stage from boninitic to island arc tholeiitic (IAT) magma. In conjunction with published data, we propose a crustal growth model for the EKOB in which mafic–ultramafic magmatism contributed mainly to the growth of the arc lower crust by layer cumulates, with a minor contribution to middle-upper crust.