Geochronology and tectonic evolution of the Luobusa ophiolite in southern Tibet, China

This study presents detailed field observations, whole-rock geochemical analyses, high-precision zircon U-Pb dating, and Sr-Nd-Pb-O isotope data for the clinopyroxenite and gabbro dikes of the Luobusa ophiolite in the Tibetan Plateau. We further investigate granitoid breccias (Luobusa Group) and the granite of the Gangdese batholith, which are spatially and temporally associated. The clinopyroxenite studied yielded an age of 179.3 ± 2.7 Ma, indicating its formation in the late Early Jurassic. Geochemical analysis shows that the clinopyroxenite is notably depleted in light rare earth elements (LREEs) and displays negligible Eu anomalies, which is consistent with a cumulate origin. Moreover, zircon U-Pb dating identifies two periods of gabbro emplacement within the Luobusa ophiolitic massif: the late Middle Jurassic (ca. 165 Ma) and the Late Jurassic (ca. 152 Ma). The late Middle Jurassic gabbro dike, which intrudes into the Luobusa ophiolite, displays geochemical characteristics typical of normal mid-ocean-ridge basalt (N-MORB). In contrast, the Late Jurassic gabbro dike, which intrudes into Late Triassic flysch, exhibits arc-type geochemical features, enrichment in LREEs and large ion lithophile elements (LILEs), together with depletion in heavy rare earth elements (HREEs) and high field strength elements (HFSEs). Isotopically, the late Middle Jurassic gabbro dike exhibits a mean initial 87Sr/86Sr ratio of 0.703408 and a mean εNd(t) value of +8.06, indicative of a depleted mantle source. In contrast, the Late Jurassic gabbro dike displays more radiogenic isotope compositions, with a mean initial 87Sr/86Sr ratio of 0.707532 and a mean εNd(t) value of +1.29, suggesting an increasing contribution from subduction-related, enriched components. Based on the geochemical and isotope signatures, we infer that the late Middle Jurassic gabbro dike originated from the partial melting of spinel lherzolite, whereas the Late Jurassic gabbro dike was derived from the partial melting of spinel-garnet lherzolite. Furthermore, the late Middle Jurassic gabbro dike has higher zircon oxygen isotope values (mean δ18O = 7.56‰), suggesting low-temperature, water-rock interaction to some extent. Our systematic investigation revealed that the 183 Ma granite from the Gangdese batholith, which is spatiotemporally associated with the Luobusa ophiolitic massif, records evidence of the rollback of the Bangong-Nujiang oceanic lithosphere that promoted the opening of the Neo-Tethys Ocean. In contrast, the 134 Ma granitoid breccia of the Luobusa Group primarily reflects the northward subduction of the Neo-Tethyan oceanic lithosphere. Collectively, these results, combined with those of previous studies, indicate that the Luobusa ophiolite formed in the late Early Jurassic and underwent a two-stage evolutionary history. Initially, it formed in a typical N-MORB environment, but it subsequently evolved into a forearc suprasubduction zone setting during the late Middle−Late Jurassic.