Regional Carbonate Alteration of the Crust by Mantle-Derived Magmatic Fluids, Tamil Nadu, South India

TLDR: In this paper, carbon, oxygen, and strontium isotope analyses of samples of ankerite and calcite, together with some of the coexisting silicate minerals, have been used to constrain the conditions of formation of the carbonate alteration and the origin of the fluids involved.

Abstract: Regional carbonate alteration of the crust associated with major shear zones provides direct evidence for $CO_{2}$-rich fluid mobility. A good example occurs within the Attur lineament, one of numerous Proterozoic shear zones which crosscut charnockitic gneisses and other lithologies of the southern Indian craton. At this locality, widespread carbonate alteration of silicate rocks has involved growth of ankerite and other carbonate minerals which replace preexisting silicates. Some of the ankerite was subsequently recrystallized to fine-grained calcite and iron oxide, accompanied by sericitization of feldspar. Carbon, oxygen, and strontium isotope analyses of samples of ankerite and calcite, together with some of the coexisting silicate minerals, have been used to constrain the conditions of formation of the carbonate alteration and the origin of the fluids involved. $\delta^{13}C$ values of ankerite are relatively homogeneous, ranging between -6.5 and -3.9 ‰ with a mean of -5.3 ‰. $\delta^{18}O$ is also ...