The nature of the basement in the Archaean Dharwar craton of southern India and the age of the Peninsular Gneiss

Abstract

The Archaean Peninsular Gneiss of southern India is considered by a number of workers to be the basement upon which the Dharwar supracrnstai rocks were deposited. However, the Peninsular Gneiss in its present state is a composite gneiss formed by synkinematic migmatization during successive episodes of folding (DhFI, DhFI, and DhF2) that affected the Dharwar supracrustal rocks. An even earlier phase of migniatization and deformation (DhF,) is evident from relict fabrics in small enclaves of gneissic tonalites and amphibolites within the Peninsular Gneiss. We consider these enclaves to represent the original basement for the Dharwar supracrustal rocks. Tonalitic pebbles in conglomerates of the Dharwar Supergroup confirm the inferenoe that the supracrustal rocks were deposited on a gneissic basement. Whole rock Rb-Sr ages of gneisses showing only the DhF1 structures fall in the range of 310(O3200 Ma. Where the later deformation (DhF 2) has been associated with considerable recrystallization, the Rb-Sr ages are between 2500 Ma and 2700 Ma. Significantly, a new Rb-Sr analysis of tonafitic gneiss pebbles in the Kaldurga conglomerate of the Dharwar sequence is consistent with an age of ,,-2500 Ma and not that of 3300 Ma reported earlier by Venkatasubramanian and Narayanaswamy (1974). Pb-Pb ages based on direct evaporation of detrital zircon grains from the metasedimentary rocks of the Dharwar sequence fall into two groups, 3300-3100Ma, and 2800-3000 Ma. Stratigraphic, structural, textural and geochronologic data, therefore, indicate that the Peninsular Gneiss of the Dharwar craton evolved over a protracted period of time ranging from > 3300 Ma to 2500 Ma.

Pb–Pb zircon ages of Archaean metasediments and gneisses from the Dharwar craton, southern India: Implications for the antiquity of the eastern Dharwar craton

“…The zircon ages of the WDC basement gneisses and the metasediments obtained in this study are consistent with those reported earlier for gneisses from Holenarasipur, Gorur-Hassan and Chikmagalur regions of the Dharwar craton (Beckinsale et al 1980;Taylor et al 1984;Meen et al 1992;Naha et al 1993;Peucat et al 1993). A span of crystallization ages ranging from 3.0 to 3.3 Ga for the orthogneiss protoliths are evident in the data presented here and those reported earlier suggests that episodic emplacement of the TTG suite in the WDC over a time span of few hundred million years.…”

“…Zircons from the three orthogneissic samples from the Nuggihalli schist belt area and the orthogneissic clast from the Kaladurga conglomerate yielded ages ranging from 3.0 to 3.26 Ga. Ages of 3.2 Ga for the core region in heterogeneous zircons in these samples are comparable to the reported 207 Pb-206 Pb zircon and Rb-Sr whole rock ages of 3.0 to 3.3 Ga for the gneisses from the Gorur-Hassan region in the WDC (Beckinsale et al 1980;Naha et al 1993;Peucat et al 1993). Concordant ages seen in both individual zircons and in multiple analyses within single zircons (table 2) and the well-preserved core-overgrowth age relationship validates the identification of the magmatic component in our dataset for these three gneissic samples and indicates that the inferred ages of 3.1 Ga is close to their formation age.…”

“…They found preponderance of 3.3-3.4 Ga zircons and less than 5% zircons older than 3.5 Ga in these quartzites. From the foregoing it is inferred that the antiquity of the WDC goes beyond 3.5 Ga and the Sargur Group in the WDC accumulated prior to 3 Ga. Rb-Sr, Pb-Pb, Sm-Nd and SHRIMP U-Pb isotope dating led to identification orthogneisses with ages of 3.3 to 3.0 Ga in the WDC (Beckinsale et al 1980;Naha et al 1993;Peucat et al 1993).…”

“…The supracrustal rocks of the greenstone belts and the gneisses grade into a granulite complex in the southern part of the craton. According to Naha et al (1993) the Peninsular Gneiss is a polyphase migmatite-gneiss complex that evolved over a protracted period between 3.5 and 2.6 Ga. Ramakrishnan and Vaidyanathan (2008) state that the Peninsular Gneiss is a complex, which evolved in three distinct episodes: 3.4-3.3 Ga, 3.0-3.1 Ga and 2.5-2.6 Ga.…”

Basement–cover structural relationships in the Kaladgi Basin, southwestern India: Indications towards a Mesoproterozoic gravity gliding of the cover along a detached unconformity

“…3.3 to 2.5 Ga (Beckinsale et al, 1980;Beckinsale and Reeves-Smith, 1982;Jayananda et al, 2000). The PGC of Western Dharwar craton represents one of the oldest rocks in India and dates back to 3.3 to 3.4 Ga as determined by Rb-Sr and Pb-Pb methods (Bhaskar Rao et al, 1991;Naha et al, 1993). U-Pb zircon ages ranging from 3.5 to 3.6 Ga have also been published (Meert et al, 2010).…”