Petrology, geochemistry, and geological significance of the Nadong ocean island, Banggongco-Nujiang suture, Tibetan plateau

We studied oceanic mafic igneous rocks of the Mesozoic Banggongco-Nujang suture zone in western Tibet to constrain the tectonic evolution of these rocks and the region as a whole. Two transects were accomplished. Seven basalt samples from the base of the Nadongshan transect (N1 basalts) have flat chondrite-normalized rare earth element (REE) and primitive-mantle-normalized trace element variation diagrams that are similar to MORB. Two basalt samples from the base of the Nadongshan transect (N1 basalts), ten gabbro samples from the middle of Nadongshan transect (N2 gabbros), four basalt samples from the bottom of Tanjiuxiama transect (T1 basalts), and four basalt samples from the top of the Tanjiuxiama transect (T2 basalts) are alkali basalts and have light rare earth element (LREE)-enriched chondrite-normalized REE patterns, and have primitive-mantle-normalized trace element variation diagrams that are enriched in highly incompatible elements, similar to OIB. LREE concentrations increase from N1 basalts to the T1 and T2 basalts, which have (La/Yb)(N) up to 16 and have even higher (Ce/Sm)(N). These data indicate that the Nadong ocean island is an Azores-type ocean island that formed during the mature stage of development of the Banggongco-Nujiang Ocean. The conformable nature of the Nadong ocean island with the Mugagangri Group flysch indicates that the Banggongco-Nujiang Ocean was never a large ocean.