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

• Published in: The Journal of geology 1994-07, Vol.102 (4), p.379-398
• Main Authors: Wickham, S. M., Janardhan, A. S., Stern, R. J.
• Format: Article
• Language: English
• Subjects: Calcite; Carbon dioxide; Carbonates; Fractionation; Geology; Gneiss; Isotopes; Plate tectonics; Quartz; Rocks; Shear zones; Silicates
• ISSN: 0022-1376; 1537-5269
• DOI: 10.1086/629681

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 fairly homogeneous, mostly ranging from + 7.4 to +9.3‰ with a mean of +8.5 ‰. These values suggest that the ankerite was deposited from fluids with fairly uniform carbon and oxygen isotopic composition over a narrow temperature range.$\Delta_{quartz-plagioclase}$and$\Delta_{quartz-ankerite}$are uniform and low in all samples, and quartz-plagioclase fractionations are very similar to those observed in pristine, uncarbonated high-grade gneisses throughout southern India. The ankerite probably formed close to equilibrium with the silicate minerals, at temperatures at least as high as ~500°C. The fine-grained calcite has a similar$\delta^{13}C$to the ankerite (mean = - 4.5‰), but$\delta^{18}O$ranges to much higher values (up to +21.4 ‰) consistent with the calcite having formed at much lower temperatures by recrystallization of ankerite, possibly during late-stage infiltration of$H_{2}O$-rich fluids. The age-corrected$^{87}Sr/^{86}Sr$ratios of bulk carbonate samples mostly fall between 0.704 to 0.710, extending to significantly higher values than the range of initial ratios of Late Proterozoic carbonatite, syenite, and alkali gabbro plutons that are found as small intrusions all along the Attur lineament. However, ankerite samples have almost identical carbon, oxygen, and strontium isotopic composition to these plutons. The igneous-like isotopie composition of the ankerite is consistent with its being deposited from magmatic fluids released from these alkalic, mant