EARLY PRECAMBRIAN Fe-RICH MANTLE RESERVOIR AND ITS SIGNIFICANCE: EVIDENCE FROM THE DONGARGARH BELT, CENTRAL INDIA

摘要

The early Proterozoic high Mg basalts (MgO -8-10 wt. percent) with komatiitic affinity in the Dongargarh volcanic-sedimentary sequence (Dongargarh Group) show primary Fe-rich (FeO ~ 10 wt. percent) character. The samples show similar REE pattern and have same level of Fe enrichment irrespective of Mg contents to the spatially unrelated lava (e.g. Dalma basalts, Mahakoshal basalts) of mantle plume origin of similar age occurring in different parts of the Indian Shield. When further compared with the late Archaean high-Mg basalt of the Southern Indian Craton (Sargur Group) and the Phanerozoic basalts (e. g. CFB, OIB and MORB), Fe in the mantle partial melts appears to have decreased in the younger lava implying Fe depletion in the mantle with time. It is shown that similarly high Fe level at variable Mg content in the suites can neither be attributed near surface processes nor by low pressure fractional crystallisaltion of the mantle derived melt. Olivine fractionation at higher pressure depleted the Dongargarh melt in Mg resulting into elevated FeO/MgO ratio, without significantly affecting original Fe concentration. It is suggested that primary melt to the Dongargarh high-Mg basalts may have been derived from a Fe-rich (with Ni and Cr) lower mantle source i.e. at least at a depth below 660 km. seismic discontinuity surface.