Stable isotopic variation in the Siriwasan and Bakhatgarh carbonatites

Viladkar S.G.¹, Ramesh R.²

1-Carbonatite Research Centre, Amba Dongar, Kadipani, Gujarat, India

2. Physical Research Lab. Ahmedabad, Gujarat, India

 

The Siriwasan carbonatite-alkaline complex is located about 10 km north of the well known Amba Dongar complex in the Chhota Udaipur carbonatite-alkaline province.   Being on the periphery of flood basalt province the thickness of basalt is small and the inliers of the Cretaceous Bagh sandstone and limestone are exposed due to fault­ing and erosion.   The notable feature of the intrusion is its form; a sill which has penetrated the upper part of the Bagh sandstone under­lying the Deccan basalt flows.  This has resulted in formation of carbonatite breccia with lateral extent of 11 km long with an average width of 150 m. In spite of the carbonatite intrusion, the lower part the sediments are totally undisturbed thus preserving original strati­fication including current bedding while the upper part of the sill is highly brecciated. Due to only soaking the original depositional structures of the sandstone are well preserved at the base of the sill. Sövite also forms plugs of varying dimensions from few inches to more than one meter within the breccia exposures. Some of these are highly radioactive while others are rich in aegirine, magnetite, apatite and pyrochlore.

In Bhakhatgarh carbonatite forms dikes along fracture zones in Deccan basalts. These mostly monomineralic composed only of calcite.

The figure shows that some of the Siriwasan carbonatites plot within the mantle box. These are obviously primary carbonatites. There is also a fractionation trend shown by the solid line, as the magma evolved at a temperature greater than 700°C (Ramesh and Ray, 2000), Interestingly, most of the Bakhatgarh carbonatites also fall in the same fractionation trend. This implies that though their trace element concentrations are much lower than that of the Ambadongar carbonatites, their stable isotopic evolution is quite similar to that of the latter. This makes us conclude that these Bakhatgarh carbonatites are also primary and are of likely mantle origin. Furthermore, there is another trend shown by a long dashed line, indicating a possible, approximate mixing line between these primary carbonatites with the surrounding Bagh limestones. The isotopic values for this Bagh limestone end-member is derived from measurements reported by Tandon and Andrews, 2001.  Normal alteration of carbonatites by interaction with meteoric water would show a different trend (see e.g., Ray and Ramesh, 2000). If it is indeed an alteration trend either high temperature processes of alteration need to be envisaged (~500°C) of a fluid very rich in CO2 relative to the rock (Fc/Rc >>1) is required. As these are very unlikely, we choose the most parsimonious explanation that there has been a mixing of the erupting carbonatites magma with the overlying Bagh limestone. Such an effect is not seen at Ambadongar because, there the volume of the magma is much greater than the one that erupted at Siriwasan.

References:

Ray J.S. and Ramesh, R., 2000. Rayleigh fractionation of stable isotopes from a multicomponent source, Geochim. Cosmochim. Acta, 64,2, 299-306.

Sukheswala, R. N. and Borges S. M. 1975. The carbonatite injected sandstones of Siriwasan, Chhotaudepur, Gujarat. Indian J. earth Sci.2. 1-10

Tandon, S.K. and J.E. Andrews 2001.Lithofacies Associations and stable isotopes of palustrine and calcrete carbonates: examples from Indian Maastrichtian regolith. Sedimentology, 48, 339-355.