The petrology and geochemistry of Early Cretaceous ocean island volcanic rocks in the middle-western segment of Bangong Co-Nujiang suture zone

Zhonggang ocean island lies in the middle-western segment of Bangong Co-Nujiang suture zone. Previous researches mainly focused on the eastern segment of Zhonggang ocean island, with no attention paid to Dongco area in the middle segment. In this paper, the authors studied the rock association and whole rock geochemistry of Zhonggang ocean island and analyzed the source area and structural environment of the island volcanic rocks. Eight basalt and eight basaltic andesite geochemical samples were collected along a transect at the middle part of Zhonggang ocean island in the north of Dongco. The major element characteristics of these two kinds of rocks indicate that basalt is alkali basalt enriched in Ti, and the trace element characteristics show that both of them are enriched in Nb and Ta. The REE patterns and the trace element spider diagram are similar to features of typical OIB. Chemical compositions of Zhonggang ocean island basalts and basaltic andesites suggest that they probably came from the same magma source, and both of them had mantle source with OIB characteristics; the depletion of Cr, Ni indicates that fractional crystallization of olivine and augite occurred during the rockforming process. The ocean island was formed in an oceanic island tectonic environment on the base of the oceanic crust.