REGIONAL CARBONATE ALTERATION OF THE CRUST BY MANTLE-DERIVED MAGMATIC FLUIDS, TAMIL-NADU, SOUTH-INDIA

Regional carbonate alteration of the crust associated with major shear zones provides direct evidence for CO2-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. deltaC-13 values of ankerite are relatively homogeneous, ranging between - 6.5 and - 3.9 parts per thousand with a mean of - 5.3 parts per thousand. deltaO-18 is also fairly homogeneous, mostly ranging from + 7.4 to + 9.3 parts per thousand with a mean of + 8.5 parts per thousand. 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 approximately 500-degrees-C. The fine-grained calcite has a similar deltaC-13 to the ankerite (mean = - 4.5 parts per thousand), but deltaO-18 ranges to much higher values (up to +21.4 parts per thousand), consistent with the calcite having formed at much lower temperatures by recrystallization of ankerite, possibly during late-stage infiltration of H2O-rich fluids. The age-corrected Sr-87/Sr-86 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 isotopic composition of the ankerite is consistent with its being deposited from magmatic fluids released from these alkalic, mantle-derived magmas. Carbonate alteration of this type, often localized along major shear zones, may represent a lower temperature mid-crustal manifestation of the deeper level CO, infiltration proposed as a mechanism for granulite formation in this and other areas.