Geochronology and geochemistry of precambrian granitic rocks of goa, sw india

Abstract

The Precambrian rock succession in Goa, representing the NW extremity of the South Indian Precambrian Craton, includes: (1) a low grade supracrustal sequence known as the Goa Group; and (2) several acid granitic and gneissic bodies occurring in close spatial association with the supracrustals. The Anmod Ghat trondhjemitic gneisses of NE Goa, which form the basement to the Goa Group, yield a Rb-Sr whole-rock isochron age of 3400 ± 140 Ma. The pervasive late-Archaean acid plutonism post-dates evolution of the Goa Group supracrustal sequence and spans syn- to post-tectonic regimes with respect to the last major penetrative folding in the segment. The Rb-Sr whole-rock ages of the granitic plutons and migmatites (Chandranath granodiorite-granite: 2650 ± 100 Ma; Dudhsagar granite: 2565 ± 95 Ma; Londa migmatites; 2650 ± 130 Ma; Canacona graniteporphyry: 2395 ± 390 Ma) suggest a minimum age of the Goa Group of c. 2.6 Ga and confirm the correlation of the Goa Group with the Chitradurga Group, which occurs to the south of the craton. Geochemical data presented bring out essential differences between the c. 3.4 Ga old basement trondhjemitic gneisses and the younger group of granites (c. 2.6-2.4 Ga). The early Archaean trondhjemitic gneisses show low Rb/Sr (0.20), Th/U (1.45–1.93), highSr/Ba ( >0.5), low ΣREE (24.14–52.81) with a slightly fractionated REE pattern moderately depleted in HREE (CeN/YbN=2.1–6.5). Compared with the early Archaean gneisses, the younger granites are distinctly potassic with higher Rb/Sr (0.50–0.87), Th/U (4.05–14.91), lower Sr/Ba (0.22–0.43), a LREE enriched pattern (CeN/YbN=15.1–26.6) and much higher ΣREE (132.06–180.84).