Petrology and geochemistry of greywackes from Goa-Dharwar sector, western Dharwar Craton: Implications for volcanoclastic origin

Abstract

Late Archaean Supracrustals of the Goa-Dharwar sector (GDS) are composed of a thick sequence of greywacke sequence with narrow intercalations of quartzite, BIF and carbonates. Mafic volcanics occupy the base of the sequence. The greywackes are predominantly tuffacious containing chlorite-sericite and hornblende. Arkosic variations containing biotite dominate the western part of the sector. Fine-grained variations occur as isolated narrow lenses within other types of greywackes. The conglomeratic greywackes are localized along the western and the eastern margins of the sector. All of the greywackes are all typically immature containing coarser clasts of mostly plagioclase (18-23%) and quartz (32-34%). Lithic fragments of felsic volcanic rocks are common. The matrix is dominated by mafic material. Biotite and amphibole are related to metamorphic recrystallization. Chlorite, sericite, epidote, carbonate and chert are products of the interplay of diagenesis and low-grade metamorphism. Fe-Ti oxide, sphene, apatite and zircon are usual accessories. But for slight enrichment in K 2 O, the major element chemistry of the GDS greywackes is similar to the chemistry of Late Archaean greywackes. They also compare in respect of V, Co, Hf contents, K Nd ratios, steep REE patterns with distinct LREE enrichment and HREE depletion. The GDS greywackes however are distinctly enriched in Rb, Ba, Sr, Th, U, Cu, Zr, Ce/Ce* and depleted in Cr, Ni, and Zn. The conglomeritic and biotite bearing verities contain considerable proportions of clasts derived from the basement tonalitic/granitic terrain. The common tuffacious greywackes containing hornblende and biotite-sericite however include only volcanic clasts and bear evidence of derivation from submarine weathering of predominantly felsic volcanics erupted on a large scale to form a magmatic arc in the later stages of geosynclinal deposition. Geochemical data suggest that the GDS greywackes were laid down in progressively changing basin geometry from a passive to active continental margin and island arc setting.

The geology of Goa Group: Revisited

“…The recent age data (e.g. Devaraju et al 2010) suggest that these gneisses are distinctly younger that the basement trondhjemite gneisses from the Dharwar craton. They form the basement for the Sanvordem Formation of the Ponda Group.…”

“…The Chandranath granite gneiss was considered to be syntectonic with the first phase of folding, the Tamdi felspathic gneiss and the Dudhsagar granite were considered to be synkinematic with the second phase, whereas the Canacona granite was considered to be post-tectonic (Gokul et al 1985). In the light of the recent work (Dhoundial et al 1987;Devaraju et al 2007Devaraju et al , 2010Dessai et al 2009) especially on the granitic gneisses, greenstones and the intrusive granites from representative sections, it is proposed to revisit the classification of the greenstones from this area. The Goa Group of Gokul et al (1985) needs to be integrated (see also Devaraju et al 2010) with the contiguous Dharwar Supergroup of WDC, as otherwise it would give an erroneous impression of forming a separate stratigraphic entity within the Dharwar craton.…”

“…In the light of the recent work (Dhoundial et al 1987;Devaraju et al 2007Devaraju et al , 2010Dessai et al 2009) especially on the granitic gneisses, greenstones and the intrusive granites from representative sections, it is proposed to revisit the classification of the greenstones from this area. The Goa Group of Gokul et al (1985) needs to be integrated (see also Devaraju et al 2010) with the contiguous Dharwar Supergroup of WDC, as otherwise it would give an erroneous impression of forming a separate stratigraphic entity within the Dharwar craton. The Goa Group (e.g Gokul et al 1985) denotes a local classification confined to Goa and is here devided into two groups namely the Barcem Group and the Ponda Group which correspond respectively to the Bababudan and Chitradurga Groups of the Dharwar Supergroup in the unified regional classification.…”

“…These layered intrusives were placed stratigraphically higher than the 'intrusive' Chandranath granitic gneisses (Gokul et al 1985). However, according to Devaraju et al (2010) the complex is enclosed within the supracrustal rocks and as such was emplaced during the early stages of the basin formation and prior to the deposition of the supracrustals.…”

“…Geological map of Goa (revised after GSI, 1996, by integrating arguments based on the field, petrological, geochemical and isotope data following the works of Dhoundial et al 1987;Devaraju et al 2007;2010;Dessai et al 2009). …”