GF:Geochronology and petrogenesis of the mafic dykes from the Purang ophiolite: Implications for evolution of the western Yarlung-Tsangpo suture zone, southwestern Tibet
The >2000 km Indus-Yarlung Tsangpo suture zone (IYSZ) is composed of Neotethyan oceanic remnants, flysch units and related continental rocks, which has been regarded as the boundary between the Eurasian and Indian terranes. Among the ophiolitic mélanges, the Purang ophiolite is the biggest massif in the IYSZ, and many studies have been conducted on this ophiolite. However, previous studies have mainly focused on harzburgite, clinopyroxenite and dunite. Field observations show that mafic dykes were emplaced within the Purang ophiolite. However, petrogenetic evolutions of those mafic dykes are poorly understood. In this study, we present new LA-ICP-MS zircon U-Pb dating results, whole-rock geochemistry and Sr-Nd-Hf isotope analyses for microgabbro, gabbro and dolerite dykes from the Purang Ophiolite of the southwestern IYSZ, respectively. Three samples yielded zircon U-Pb ages of 144.2 ± 2.1 Ma, 127.9 ± 2.3 Ma and 126.5 ± 0.42 Ma, suggesting two different phases of magmatic activities distinctly. Whole-rock geochemical results suggest that the gabbro samples show alkaline features marked by enrichments of light rare earth elements (LREE) and large-ion lithophile elements (LILE), as well as Nb-Ta elements, suggesting an oceanic island basalt-like (OIB-like) geochemical affinity. However, the dolerite and microgabbro samples demonstrate sub-alkaline characteristics with normal mid-oceanic ridge basalt-like (N-MORB-like) geochemical features. Three distinct mafic dykes show significant Rb element depletion. The geochemical data and Sr-Nd-Hf isotopic features suggest that the microgabbro and gabbro rocks were derived from a depleted mantle that had been metasomatized by partial melts of sediments and enriched slab-derived fluids. The dolerite was also originated from a depleted mantle marked by significantly depleted Sr-Nd-Hf compositions, which is not influenced by enriched slab-derived fluids and sediments contamination during subsequent evolution. The isotope and geochemical data and tectonic diagrams suggest a tectonic transition from a within-plate to a mid-oceanic ridge basalt-like (MORB-like) setting during the period from ca. 144 Ma to 127 Ma. Combined with regional background and this study, we propose that these mafic dykes were formed in an oceanic back-arc basin setting. Additionally, integrated with previous studies, we suggest that the geodynamic evolution of the southwestern and central parts of the Neo-Tethys oceanic basin is comparable in Early Cretaceous.