GEOCHEMISTRY OF ADAKITES AND RHYOLITES FROM THE NEOARCHAEAN GADWAL GREENSTONE BELT, EASTERN DHARWAR CRATON, INDIA: IMPLICATIONS FOR SOURCES AND GEODYNAMIC SETTING

Abstract

Adakite and rhyolite volcanic flows with different petrographic and geochemical characteristics have been identified from the Neoarchaean Gadwal greenstone terrane of the eastern Dharwar craton, India. These are part of the bimodal basalt-felsic association that dominates the belt, which includes previously documented boninites and Nb-enriched basalts. Adakites plot in the MgO-SiO2 field of Cenozoic adakites, distinct from high-Mg andesites, and have low Yb (1.2 ppm) and fractionated rare-earth elements (REE) (LaZYbn = 16) of Cenozoic counterparts. They also possess the Cr/Ni (1.3-4.0), Nb/Ta (8.6-12.8), and Zr/Sm (33-58) ratios distinctive of adakites from recent oceanic arcs. Zero to positive Eu anomalies contrast with negative Eu present in older Dharwar cratonic crust, such that crustal contamination is unlikely, endorsing an intraoceanic setting. Cenozoic oceanic adakites may form by slab melting, then hybridizing to variable degree with wedge peridotite, and Gadwal adakites are also interpreted, to be slab melts. Rhyolites have greater SiO2, highly incompatible elements (Th, La, Zr), and higher Yb (2.41 ppm) contents than adakites, with fractionated REE and pronounced negative Eu anomalies; they are comparable to FI type rhyolites of other Archean greenstone belts, likely melts of thick mafic crust at ~40 km with residual garnet, in an extensional setting. Consequently, the switch from arc basalts and boninites to adakites, Nb-enriched basalt, and rhyolites in the Gadwal terrane signifies a transition from slab dehydration-wedge melting to slab melting-wedge hybridization, possibly triggered by ridge subduction or flattening of the slab, as well as crustal melting. These new observations endorse the emergence of complex arc magmatism in Neoarchean terranes. © 2007 NRC Canada.