Abstract
The Archaean Eastern Indian Craton consists of several granitoid batholiths intruding greenstone-like sequences. The Bonai pluton, one such body in the central sector of the craton, consists of a dominant porphyritic and a subordinate equigranular variety ranging in composition from trondhjemite to granite. Xenoliths of massive trondhjemite, migmatite and quartzite occur in both phases. The granitoid xenoliths represent remnants of a sialic crust and are the oldest recognizable element in the Bonai pluton. These are high-Al 2O 3 trondhjemites with a highly fractionated rare earth element (REE) pattern. A Pb-Pb isochron age of 3369 ± 57 Ma has been obtained from a few spatially related xenoliths. The porphyritic and equigranular granitoids show similar petrographic characteristics and most of the samples show a calc-alkaline trondhjemitic association. A few granite samples, however, belong to a normal calc-alkaline association. The trondhjemitic samples are of the low-Al 2O 3 type with fractionated light REE (LREE), flat heavy REE (HREE) and a marked negative Eu anomaly. Granites of the calc-alkaline association have an identical REE pattern. Pb isotopic data for all samples together yield a Pb-Pb isochron age of 3163 ± 126 Ma. The REE characteristics of the two trondhjemite suites can be generated by partial melting of a mafic source with variable residual phases at different depths. On the other hand, the normal calc-alkaline granite may be generated by partial melting of trondhjemite similar in composition to that represented by the 3369-Ma xenoliths. Isotopic ages and the chemical nature of the Bonai pluton are similar to those identified in the adjacent Singhbhum Granite Complex. It is suggested that the entire Eastern Indian Craton has a common mid-Archaean history. The earliest sialic crust formed at ∼ 3400 Ma, and was followed by trondhjemitic addition at ∼ 3300 Ma. During this phase, restricted reworking of the older sialic crust produced calc-alkaline granites.
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