Formation of Proterozoic tholeiite intrusives in and around Cuddapah Basin, South India and their Gondwana counterparts in East Antarctica; and compositional variation in their mantle sources
- Nilanjan Chatterjee, Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, U.S.A.
- Somdev Bhattacharji, Department of Geology, Brooklyn College, City University of New York, Brooklyn, New York 11210, U.S.A.
Mid-late Proterozoic dikes of tholeiites and their differentiates intrude the
Archean crust around the intracratonic Cuddapah basin of South India. Inside the
basin, mid-Proterozoic tholeiitic flows and sills related by fractional crystallization
of olivine, clinopyroxene and plagioclase, occur within the lower strata. Some
of the sills inside the basin contain gabbroic xenoliths and mafic xenocrysts.
The bulk composition of these sills indicate that they formed by accumulation
of fractionated crystals from associated xenolith and xenocryst-free tholeiites.
Olivine-melt and two-pyroxene (in xenoliths) equilibria indicate that fractionation
and crystal-accumulation occurred at a depth of about 18 km (~5 kb). Calculated
olivine compositions (Fo78-79) in equilibrium with the tholeiites at 5 kb and
1142-1154oC agree well with the olivine in the xenoliths (Fo78-82). The temperature
range is also in agreement with temperatures (1019-1148oC at 5 kbar) obtained
from two-pyroxene thermometry of the xenoliths. The calculated pressure and temperature
support the existence of a mid-level magma chamber under Cuddapah basin, postulated
by previous workers. New trace element data on tholeiites, both inside and outside
the basin, indicate that their mantle source was metasomatically enriched in Ba,
Rb and K compared to mid oceanic ridge-basalt. Comparison with East Antarctic
and other Proterozoic Gondwana tholeiites indicates that the metasomatic enrichment
event may have occurred at ~2.9-2.7 Ga. Differences in trace element contents
and ratios (e.g., Ti/Y, Ti/Zr, Y/Nb, Zr/Nb and Ba/Rb) of the temporally related
Cuddapah tholeiites indicate that the sills inside the basin may have originated
from a different mantle source than the dikes outside the basin. A comparison
of major element fractionation relations, trace element ratios and published initial
87Sr/86Sr indicates that the tholeiitic sills inside the Cuddapah basin and the
contemporary tholeiitic dikes of Napier Complex, Enderby Land, East Antarctica
(studied by other workers) had a similar magmatic history and may have originated
from a similar but not same mantle source. These observations support Proterozoic
reconstruction studies of Gondwanaland. The Cuddapah-Enderby Land region mostly
contains low P2O5-TiO2 basalts. The Jurassic division of high P2O5-TiO2 basalts
and low P2O5-TiO2 basalts into two geographic domains of Gondwanaland does not
apply to these Proterozoic basalts and the Jurassic high P2O5-TiO2 domain may
have resulted from a change in the mantle source between the Proterozoic and the
Jurassic.