Paper No. 6
Presentation Time: 9:15 AM
TECTONIC IMPLICATIONS OF CA. 1000 MA MAGMATIC EVENTS IN CENTRAL INDIA
BICKFORD, M.E., Earth Sciences, Syracuse University, 204 Heroy Geology Laboratory, Syracuse, NY 13244-1070 and BASU, Abhijit, Department of Geological Sciences, Indiana Univ, 1001 E 10 Street, Bloomington, IN 47408, mebickfo@syr.edu
New SHRIMP U-Pb data from zircons from two tuff beds in the Tarenga Fm, Chhattisgarh Basin, India, indicate formation at ca. 994±8 Ma, essentially coeval with the formation of the 980-1020 Ma Sukhda-Sapos tuffs in the eastern part of the basin. If these rhyolitic tuffs, both of which may be up to 300 m thick and occur over about 200 km in strike-length, are parts of the same stratigraphic unit, they must be deposits of a major volcanic event or events in central India at ca. 1000 Ma. Further evidence for a major ca. 1000 Ma thermal/volcanic event is found in the isotopic patterns of detrital zircons from the Singhora tuff near the base of the Chhattisgarh succession. Many of the detrital zircons plot on a discordia line indicating an upper intercept age of 2484±16 Ma, with a lower intercept at 275±130 Ma; these zircons were derived from the basement of the Bastar Craton. Other grains, however, plot on a well-defined discordia giving an upper intercept of 2530±41 Ma with a lower intercept at 950±74 Ma. The data probably indicate Pb-loss in some of the detrital zircon grains at ca. 1000 Ma, presumably in response to a major thermal event.
Other examples of ca. 1000 Ma igneous activity in India include emplacement of kimberlites, lamproites, and alkaline dikes. The last magmatism and metamorphism of the Chhotanagpur granite-gneiss in the Singhbhum Craton has been estimated at ca. 1000 Ma, and 800 to 1100 Ma ages of migmatites and pegmatites have also been reported.
The tectonic significance of the ca. 1000 Ma event remains speculative. The best evidence at hand indicates that the Chhattisgarh Basin opened later than ca. 1400 Ma, but the other major Purana basins opened much earlier (e.g., Vindhyan Basin, ca. 1750 Ma). Significantly, however, ca. 950-1100 Ma ages of charnockites and related rocks in the Orissa region of the Eastern Ghats mountains to the south have been interpreted to record the time of docking of east Antarctica with India during the final assembly of Rodinia. If correct, this event might be expected to have had major far-field effects in the orogenic foreland.
