CRUSTAL DERIVATION OF 1000 MA RHYOLITIC TUFF, CHHATTISGARH AND INDRAVATI BASINS, INDIA: EVIDENCE FROM CHEMISTRY AND HF ISOTOPIC COMPOSITIONS

1000 Ma rhyolitic tuffs that occur near the top of the Mesoproterozoic Chhattisgarh Basin, peninsular India, include the Sukhda and Dhamda tuffs. A similar 1000 Ma tuff occurs near the top of the Indravati Basin. These rhyolites are thought to result from far-field effects of the ca. 1000 Ma collision of East Antarctica with cratonic India during assembly of Rodinia. Sukhda, Dhamda, and Indravati tuff beds are siliceous (SiO₂ ~72-77 wt. percent), but geochemically diverse (Na₂O/K₂O = 0.01 to 38; Rb/Sr = 0.01 to 18; Nb/Y = 4 to 1.5). Sukhda and Dhamda tuffs have elevated LREE and strongly depleted HREE. Most show a pronounced Eu anomaly, but two Sukhda tuff analyses do not. Indravati tuff is similar, but with less depleted HREE. These compositions and REE patterns are typical of magmas derived by partial melting of granitic crust.

Hf isotopic data from zircons from the Dhamda tuff yield a two-stage “crustal” model age of 2520 ± 77 Ma. Most of Bastar Craton, basement to the Chhattisgarh Basin, is likely about 2.5 Ga, for, although some 3.56 Ga rocks occur, Bickford et al. (2011) found only 2.5 Ga detrital zircons in the basal Lohardih Sandstone, in which Das et al. (2009) also found a peak in detrital monazite ages at 2.51 ± .036 Ma. Bickford et al. (2011) also found an almost unimodal 2.5 Ga peak in detrital zircons from the overlying Kansapathar Sandstone. Further, a sample of Dongargarh granite, a major pluton in the Bastar Craton, yielded a SHRIMP U- Pb age of 2520 ± 11 Ma. Thus, the model age of Dhamda tuff is consistent with derivation from ca. 2.5 Ga cratonic rocks by partial melting. Hf isotopic data from zircons from the Indravati rhyolitic tuff yield a less precise two-stage model age of 1949 ± 256 Ma. The age of Indravati Basin basement is poorly constrained, but the Hf data clearly indicate derivation from older crustal rocks.

The 1000 Ma rhyolitic volcanism discussed here coincides with the closure of the Indravati and Chhattisgarh basins in the Bastar craton. Indirect evidence, although controversial, indicates that the Vindhyan, Bhima and Kurnool basins, Bundelkhand and Dharwar cratons, also closed near that time. It is likely that the 1000 Ma collision of East Antarctica with cratonic India was marked by rhyolitic volcanism and the inversion and closure of “Purana” basins that had been receiving sediments over several hundred million years.