Geochemistry and origin of early proterozoic Dongargarh rapakivi granite complex, Central India - An example for magma mixing and differentiation

Early Proterozoic Dongargarh granite complex of Central India, intruding th e tonalitic to granodioritic Amgaon gneisses and the Nandgaon Group bimodal volcanic suite, comprises three different textural and compositional types , viz., porphyritic granodiorite (PG), coarse equigranular granite (EG) and microgranite(MG). Synplutonic mafic dykes are common in the granite comple x. The PG is characterised by rapakivi texture and the EG is the dominant f acies and exhibits sporadically developed rapakivi texture. Microgranular e nclaves are common in the EG while they are rare in PC. Major and trace ele ment geochemistry of PG shows marked I-type and some occasional A-type gran ite characters unusual for a rapakivi granite while the EG shows A-type gra nite signatures. The field, petrographic, chemical and isotopic data of the se granites suggest their derivation by mixing of mantle- derived basic mag ma with a crustal-derived partly crystalline granitic magma. Episodic mafic magma underplating caused the anatexis of the Archaean lower continental c rust in a continental margin tectonic setting resulting first in the format ion of the I-type granodiorite followed by A-type granite. The I-type grano diorite is mixed with the basic magma (synplutonic dykes) while the EG is f ormed by mingling of A-type granite magma and the intruding basic magma.