West Singhbhum Research Articles

Geochemistry of the Chakradharpur granite—Gneiss complex—A Precambrian trondhjemite body from West Singhbhum, Eastern India

The Chakradharpur Granite—Gneiss complex (CKPG) is exposed as an elliptical body within the arcuate metamorphic belt sandwiched between the Singhbhum Granite in the south and the Chotonagpur Granite—Gneiss to the north. It consists of an older bimodal suite of grey gneiss and amphibolites, intruded by a younger unit of pegmatitic granite. The bimodal suite represents the basement to the enveloping metasediments. The average major-element chemistry of the grey gneiss conforms to the definition of trondhjemite and includes both low-Al 2O 3 and high-Al 2O 3 types. The amphibolites can be grouped into a low-MgO and a high-MgO type. Rocks of the younger unit range in composition from granodiorite to quartz monzonite. The two granitic units also differ significantly in their Rb, Sr and Ba contents, and in the REE distribution pattern. The grey gneiss shows a highly fractionated REE pattern and a distinct positive Eu anomaly, with Eu/Eu * values increasing with increase in SiO 2 %. In samples of the younger granite, the REE pattern is less fractionated, with higher HREE abundance relative to the grey gneiss and usually shows a negative Eu anomaly. The two types of REE patterns in amphibolites are interpreted to represent the two broad groups identified on the basis of major element chemistry. On the basis of chemical data, a two-stage partial melting model for the genesis of grey gneiss is suggested, involving separation of hornblende and varying amounts of plagioclase in the residual phase. Varying amounts of plagioclase in the residuum result in the wide range of Al 2O 3 content of the partial melt from which the trondhjemites crystallised. Residual hornblende produces the highly fractionated REE pattern, but a relatively higher HREE content of the trondhjemites compared to those produced by separation of garnet in the residual phase. The high level of Ba together with moderate levels of Sr in the trondhjemites can also be explained by plagioclase in the residue, whose effectiveness in partitioning Ba compared to Sr is lower. Of the two groups of amphibolites, the low-MgO type shows relative depletion of LREE compared to the high-MgO type. It contains varying amounts of plagioclase and is tentatively suggested to represent the residue. The other group, with a slightly fractionated REE pattern (Ce N/ Yb N = 2.01), is generally considered to represent the source material for the trondhjemites. This may have been generated by 5–15% partial melting of mantle peridotites, containing higher concentrations of LIL elements than those which produced average Precambrian tholeiites. This first phase of partial melting resulted in the slightly fractionated REE pattern of these amphibolites. Derivation of the younger granitic unit from the trondhjemites can be ruled out on the basis of REE data alone. REE data suggest partial melting of metasediments to be the origin of these rocks. It is also possible that deeply buried volcanic rocks, similar to calc-alkaline components of greenstone belts, are the parent for this component.