The earliest subduction initiation of the Neo-Tethys Ocean recorded by a crustal sequence within the Yarlung-Zangbo suture zone, China

Determining the initiation timing of Neo-Tethys oceanic subduction represents a critical scientific priority for unravelling the geological evolution and metallogenic process of the Neo-Tethys region. This study presents the first documented occurrence of Late Triassic supra-subduction zone−type oceanic crustal rocks in the Luobusa ophiolite from the Yarlung-Zangbo suture zone, southern Tibet, China. The Luobusa ophiolite’s crustal sequence predominantly comprises websterite, gabbronorite, and layered gabbro. Petrographic examination reveals characteristic igneous layering and mesocumulate textures, with mineral crystallization sequences of olivine → orthopyroxene (Opx) → clinopyroxene (Cpx) in websterite, Opx → Cpx → plagioclase (Pl) in gabbronorite, and Cpx → Pl in layered gabbro. Notably, the clinopyroxenes exhibit significant Mg# variability among websterite (82.5−91.0), gabbronorite (78.2−86.1), and layered gabbro (72.4−78.5). The plagioclases show marked calcium enrichment features in gabbronorite (An = 61.8−95.2) and layered gabbro (An = 63.5−96.4). Trace element compositions of the equilibrium melt of clinopyroxene in the lower oceanic crustal rocks are characterized by low concentration of rare earth elements and depletions of high field strength elements (e.g., Ti) and enrichments of large ion lithophile elements (e.g., Ba), similar to those of subduction-related magmatism. Thermodynamic modeling reveals that the mineral crystallization of high-Ca boninitic magmas is Opx → Cpx → Pl, identical to that of the Luobusa lower oceanic crustal rocks. Trace element calculation of the crystallization process corroborates that the Luobusa lower oceanic crustal rocks likely formed through fractional crystallization (13%−54%) of high-Ca boninitic magmas. U-Pb geochronological analysis of zircon and apatite from these cumulate rocks yields a coherent formation age range of 217−216 Ma. Compared to the Jurassic to Late Cretaceous subduction-related magmatic and metamorphic records, these rocks potentially represent the earliest intra-oceanic subduction activity within the Neo-Tethys Ocean. We propose that these Late Triassic Luobusa lower oceanic crustal rocks originated through the crystallization of high-Ca boninitic magmas within a forearc basin environment during subduction initiation. This discovery potentially identifies the earliest documented intra-oceanic subduction event within the Neo-Tethys ocean.