Summary Several mafic-ultramafic carbonatite complexes have been discovered in the Shillong plateau and its surroundings in northeastern India. The aeromagnetic anomalies over known carbonatite plugs in the Sung valley of Shillong plateau exhibit a predominant magnetic low flanked by a feeble high towards north, indicating the presence of remnant magnetism. These conspicuous aeromagnetic signatures led to the discovery of another carbonatite complex near Jasra. A re-look in to the old aeromagnetic data flown in 1956 for oil exploration revealed two such signatures unnoticed till now. As the area is inaccessible, geological confirmation could not be made.
However, the Surface Radar Topographic image (SRTM image by USGS) could reveal a circular feature over one of the newly suspected bodies, confirming the presence of a plug like source. As the magnetic characteristics of these sources are much similar, it is assumed that these carbonatites are contemporaneous, emplaced at about 110 Ma. The cross-sectional area of these plugs exceeds 25 km2.
Introduction In northeastern of India, the Shillong plateau and the Mikir hills (Figure 1) represent a block-uplifted plateau to a height of 600-800 m above mean sea level. The area is occupied by Archean-Proterozoic gneissic complex with acid, basic and alkaline intrusives. The carbonatites occur as isolated annular plugs along a NE trending zone from Mawpyut in Jaintia hills district to Barpung in the Mikir hills of the Karbi Anglong district of Assam, through Sung valley, Jasra, Samchampi, Sam Taran and Tarpung area (Nandy, 1999). Carbonatites are considered to be igneous intrusions occurring mostly in the form of either plugs or veins, much similar to kimberlites. These groups of rocks are very important due to their association with certain rare earth elements. Krishnamurthy et. al., (2000) discussed the economic aspects of Indian carbonatites. Many carbonatite occurrences known in India were emplaced over a wide span of the geological time scale (Krishnamurthy, 1988).
The carbonatites in northeastern India are considered to be much younger (about 110 Ma) when compared to those occurring in other parts (except Ambadongar) of India (Krishnamurthy, 1988 and Heaman et. al., 2002). Due to their association with magnetizable minerals such as pyrite and magnetite, and due to large areal extent, carbonatite complexes produce strong magnetic anomalies, which can be observed even at greater flight heights (1000 m above ground). The aeromagnetic signatures of the Sung valley carbonatite complex provided a clue for the discovery of another complex near Jasra by Mamallan et al. (1994). A close examination of the old aeromagnetic data acquired for oil exploration revealed the presence of two more carbonated complexes in the Mikir hills area. One of them coincides with a circular feature on the Shuttle Radar Topographic Map (SRTM image by USGS), confirming the presence of a plug-like feature. Therefore, the present study emphasizes the utility of old aeromagnetic data to unravel some of the mineral deposits hitherto unknown.
Aeromagnetic data and its interpretation In 1977, the National Geophysical Research Institute conducted aeromagnetic surveys over parts of Assam and Meghalaya (NGRI, 1978).