Magmatic compositions and source terranes estimated from melt inclusions in detrital Cr-rich spinels: An example from mid-Cretaceous sandstones in the eastern Tethys Himalaya

Cr-rich spinel is a detrital component within turbidites from the well-exposed, mid-Late Cretaceous Tianba Formation sequence in the Nieru Valley, southern Tibet. About 5% of the spinels contain melt inclusions, most of which are partly crystallized. To homogenize the crystallized melt inclusions for subsequent analysis by electron microprobe, spinels were heated at 1200 °C and 1250 °C for 96 h at controlled oxygen fugacity (FMQ) and quenched. This technique differs significantly from those previously applied by other investigators to melt inclusions in spinels, in having oxidation fugacity and temperatures more closely resembling natural conditions. The compositions of the melt inclusions correlate well with those of host spinels. The geochemistry of the melt inclusions and the Cr-rich spinels in the Tianba Formation turbidites suggests that the source of the Cr-rich spinels was hotspot basalts. Based on palaeo-tectonic reconstructions, presence of mid-Late Cretaceous fossils in the strata, and the chemical compositions of melt inclusions and detrital spinel, we infer that the volcanics of Rajmahal, which are associated spatially and temporally with Kerguelen hotspot activity on India about 117 Ma ago, were the likely source of these Cr-rich spinels.