Geochemical and Nd isotope constraints on petrogenesis of granitoids from NW part of the eastern Dharwar craton: Possible implications for late Archaean crustal accretion

Geochemical and Nd isotope data on granitoids of the NW part of the late Archaean eastern Dharwar craton are presented to elucidate their petrogenesis and role in crust formation. The granitoids are divided into three suites viz. trondhjemite–granodiorite gneisses, biotite monzogranites and porphyritic biotite granodiorites. The gneisses are pre- to syn-kinematic (with respect to deformation in the adjacent Hungund–Kushtagi schist belt), which show variable SiO2 and Al2O3, enriched LREE and depleted HREE with slightly negative to no Eu anomalies. They display unusual chemistry in having higher FeO(T), K2O, Ba, Cr and Ni compared to the typical Archaean tonalite–trondhjemite–granodiorite (TTG). The biotite monzogranites are mostly syn- to late-kinematic and exhibit evolved calc-alkaline compositions with high SiO2, K2O, LILE and LREE, depleted to undepleted HREE and strongly negative to no Eu anomalies. The porphyritic granodiorites show syn- to late-kinematic calc-alkaline, sanukitoid-like character with a wide range of SiO2, higher TiO2, P2O5, Sr, Ba, Cr and Ni, and lower Rb. They, however, uniquely display higher K2O, ΣREE and Th than typical sanukitoids. The trondhjemite–granodiorite gneisses are interpreted as product of melting of a subducted basaltic slab followed by slight contamination from the overlying metasomatized mantle wedge. Subsequent melting of the extremely metasomatized mantle wedge resulted in formation of the parental magma of the porphyritic granodiorites. Intrusion of the latter triggered melting of the TTG crust accreted earlier and generated the evolved monzogranites. The monzogranites occurring east of the Hungund–Kushtagi schist belt show higher εNd but lower TDM ages than those occurring to the west, indicating that terranes with different histories were juxtaposed by lateral accretion.

► The evolution of granitoids during the growth of NW part of eastern Dharwar craton is elucidated. ► Subduction-related accretion followed by slab break-off was the main process of crust formation. ► Initially slab melting enriched the overlying mantle wedge as well as produced TTG-like rocks. ► Then sanukitoid-like porphyritic granodiorites were produced from the enriched mantle wedge. ► Intrusion of these granodiorites triggered melting of crust producing biotite monzogranites.