Rare earth element geochemistry and Rb-Sr geochronology of Archaean stromatolitic cherts of the Dharwar craton, south India

Abstract

Stromatolites associated with cherty dolomites of the Vanivilaspura Formation of the Archaean Dharwar Supergroup show a morphology indicative of the deposition of the latter in a intertidal to subtidal environment. The cherts are moderately high in their A1/A1 + Fe ratios but depleted in Fe203 and also most trace elements. Unlike most other Archaean cherts, the Vanivilaspur cherts exhibit significant negative Ce anomaly, which is interpreted to have resulted from contemporary manganese deposition. The Rb/Sr ratios in the cherts show a sufficient spread to define a linear correlation line in the Rb-Sr evolution diagram corresponding to an age of 2512 • 159 Ma and initial Sr ratio of 0.7128 • 0.0012 (2a). While this age is strikingly close to that of regional metamorphism in the Dharwar craton, the initial ratio is distinctly higher than that of the associated volcanics. Acid leaching experiments on the cherts suggest that they may have been isotopically equilibrated on a mm to cm scale about 500 Ma later than the time of regional metamorphism.

Manganiferous clays of Dharwar Craton, southern peninsular India: Insights on Archean weathering and ore formation processes

“…This Group is further divided into three formations including Vanivilas Formation with conglomerate, quartzite, carbonates, Fe–Mn arenite and argillite sequences followed by Ingaldhal Formation with predominant volcanic and pyroclastic rocks overlain by the uppermost Hiriyur Formation which is composed of greywacke–argillite suite intercalated with banded iron formations, metabasalts and conglomerates (Table 1; Swami Nath & Ramakrishnan, 1981; Ramakrishnan & Vaidyanathan, 2008). The U–Pb ages of the detrital zircons from the stromatolitic carbonates of the Vanivilas Formation were dated to be 3.4–2.6 Ga (Khelen et al, 2019) and the manganiferous cherts of the same formation are dated at 2.5 Ga (Srinivasan et al, 1997). The manganiferous clays sampled for the present study belong to the Bahadurgutta (BG) mine pit of the Vanivilas Formation from the Chitradurga Group (Figure 1d, Table 2).…”