Figure 10). A comparison of these feldspar built compositions with the positions of the isotherms on the ternary feldspar solvus suggests a minimum temperature of crystallization <750˚C, and crystallized at >900˚C. Furthermore, the figure indicates that exsolution occurred at temperatures <750˚C.

Figure 9. AFM triangular diagram, showing the demarcation between the tholeiitic and calc-al- kaline divisions.

Figure 10. Ab-An-Or ternary plot of anorthosite whole rocks (Fulrman & Lindsley, 1988).

5. Discussion

The formations of the study area show magmatic cross-bedding, graded bedding and cumulate structures. These field evidences indicate an undoubted igneous origin of the complex. This view is further corroborated by the twinning of plagioclase on Carlsbad and albite laws [43] . Layering is predominantly rhythmic type with alternating thin plagioclase-rich and hornblende rich layers. Layering involving different members of the complex i.e. anorthosite, gabbroic anorthosites, anorthositic gabbro, gabbros and ultramafic rock, is observed only at a few places of limited extent. The other layered complexes having cross bedding and graded bedding are skaergard intrusions and the Messina layered intrusion of Limpoo belt [23] . Opined that convection current was responsible for magmatic cross-bedding in skaergard. [35] considered the cross-stratifi- cation in chamber. [44] has discussed the role of the magmatic density currents in the cumulus processes in layered rocks.

Mose [45] stated that the magma will convict because it cools more rapidly at the top. It is therefore possible that crystals would form first at the top and those heavier than the liquid start settling downwards. The simplest convection pattern would be downward motion along the walls and inward convergence along the floor and outward dispersion along the roof. The convection, continuous or periodic, would influence the crystal deposition. Variation in the velocity of convection current will modify the rate between minerals of different settling velocities producing variations in the proportion of heavy and light minerals settling in the magma chamber. This would be a better working model to explain the rhythmic layering in the study area.

Batampudi complex has many characteristic features of the metamorphosed complexes. The plagioclase is strikingly rich in calcium with the anorthite molecular percentage ranging from An70 to An91 as in the case of most of the Archean anorthosites. Both orthopyroxene and clinopyroxene in different degrees of alteration are noticed. There are two types of hornblende viz; the pale green and olive green hornblende free from relict pyroxenes and blue green type with relict pyroxene and in some cases forming rims or coronas around pyroxenes could be of primary igneous origin. The whole rock chemistry reveals that the BAC is rich in CaO, Al2O3 and poor in alkalies, in conformity with the Archaean layered anorthosites. The AFM diagram plots show a pure calc-alkalin and amphiblites show tholiietic differentiation trend. This is also in conformity with the views of [14] that the differentiation trends in metamorphosed layered complexes are believed to be similar and display a calc-alkaline trend.

6. Conclusion

The Bethampudi complex is essentially a leucogabbro (gabbroic anorthosite and anorthositic gabbro) with minor volumes of anorthoistes, gabbro, amphibolites, pegmatites and quartz veins occur as concordant or discordant bodies in the country rock. The layered leucogabbros show magmatic cross-bedding, graded bedding and cumulate structures. These field evidences indicate an undoubted igneous origin of the complex. The geochemical data suggest that the anothosites are of calc-alkaline and amphibolites have tholeiitic affinity. The presence of well-developed rhythmic layering indicates that fractional crystallization is generated by gravity settling of successively crystallizing cumulus minerals from a primitive basaltic magma, clinopyroxenite, gabbro, anorthositic gabbro and anorthosite, in this order to build up the study area complex, BAC. The parent magma, in general is basaltic with high alumina and low silica and alkalies. The plots on SiO2 and MgO versus alumina and alkalis indicate the tholeiitic balsalt composition.

Conflicts of Interest

The authors declare no conflicts of interest.


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