Chemical compositions and tectonic significance of chrome-rich spinels in the Tianba Flysch, southern Tibet

Turbiditic sandstones from the mid-Late Cretaceous Tianba Flysch sequence are well exposed near Tianba in the north Nieru Valley, southern Tibet, and contain significant amounts of chrome-rich spinels. Microprobe results show that the spinels have a well-developed Fe-Ti trend and have Cr/(Cr + Al) 0.4-0.65, Mg/(Mg + Fe2+) 0.3-0.9, and TiO2 >1%. The occurrence of melt inclusions, and well above 0.2% TiO2 contents, shows that a volcanic suite of rocks was the source of these Cr-rich spinels. Specifically, these spinels compositionally resemble spinels from intraplate basalts (e.g., Hawaii and Disko Island, western Greenland) rather than from ophiolitic ultramafic or gabbroic material expected in an intraoceanic subduction system associated with the closing of the Neo-Tethys. On the basis of paleotectonic reconstruction, the presence of mid-Late Cretaceous fossils in the strata, and the limited range of chemical compositions, we suggest that 117-Ma volcanics of the Rajmahal-Kerguelen hot spot were the source for the chrome-rich spinels in the Tianba Flysch.