REGIONAL CARBONATE ALTERATION OF THE CRUST BY MANTLE-DERIVED MAGMATICFLUIDS, 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 ex ample occurs within the Attur lineament, one of numerous Proterozoic s hear 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 a nkerite was subsequently recrystallized to fine-grained calcite and ir on oxide, accompanied by sericitization of feldspar. Carbon, oxygen, a nd strontium isotope analyses of samples of ankerite and calcite, toge ther 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 re latively homogeneous, ranging between - 6.5 and - 3.9 parts per thousa nd with a mean of - 5.3 parts per thousand. deltaO-18 is also fairly h omogeneous, 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 ank erite was deposited from fluids with fairly uniform carbon and oxygen isotopic composition over a narrow temperature range. DELTA(quartz-pla gioclase) and DELTA(quartz-ankerite) are uniform and low in all sample s, and quartz-plagioclase fractionations are very similar to those obs erved in pristine, uncarbonated high-grade gneisses throughout souther n India. The ankerite probably formed close to equilibrium with the si licate minerals, at temperatures at least as high as approximately 500 -degrees-C. The fine-grained calcite has a similar deltaC-13 to the an kerite (mean = - 4.5 parts per thousand), but deltaO-18 ranges to much higher values (up to +21.4 parts per thousand), consistent with the c alcite having formed at much lower temperatures by recrystallization o f ankerite, possibly during late-stage infiltration of H2O-rich fluids . The age-corrected Sr-87/Sr-86 ratios of bulk carbonate samples mostl y fall between 0.704 to 0.710, extending to significantly higher value s than the range of initial ratios of Late Proterozoic carbonatite, sy enite, and alkali gabbro plutons that are found as small intrusions al l along the Attur lineament. However, ankerite samples have almost ide ntical carbon, oxygen, and strontium isotopic composition to these plu tons. The igneous-like isotopic composition of the ankerite is consist ent with its being deposited from magmatic fluids released from these alkalic, mantle-derived magmas. Carbonate alteration of this type, oft en localized along major shear zones, may represent a lower temperatur e mid-crustal manifestation of the deeper level CO, infiltration propo sed as a mechanism for granulite formation in this and other areas.