Petrogenesis of basalt–high-Mg andesite–adakite in the Neoarchean Veligallu greenstone terrane: Geochemical evidence for a rifted back-arc crust in the eastern Dharwar craton, India

• We document the role of arc magmatic processes in the Veligallu greenstone terrane.
• Based on the geochemical attributes we characterize the basalts as back-arc basalt.
• We report orthopyroxene bearing high-Mg andesite (HMA) in the Dharwar craton.
• The felsic volcanic rocks are adakitic slab melts.
• We propose evolution of the Veligallu greenstone belt in a rifted back-arc setting.

The ∼2.7 Ga Veligallu greenstone terrane in the eastern Dharwar craton is one of the least studied Neoarchean greenstone belts in India. To our knowledge, there is no published dataset that provides a detailed account of the petrogenetic processes, which culminated into the evolution of this greenstone belt. This paper presents a comprehensive account of field, petrography and geochemistry of the basalt–high-Mg andesite–adakitic volcanic rock association in the Veligallu greenstone belt.The basalts are tholeiitic in nature and are characterized by primitive geochemical compositions with ∼49 wt% SiO2, ∼9 wt% MgO, Mg# 54, Cr ∼ 197 ppm, Ni ∼ 122 ppm, Y ∼ 27 ppm, Yb ∼ 2.7 ppm; Zr/Y < 3 and Ce/Yb < 5. They do not plot on a mixing line between Mesoarchean TTG and N-MORB, therefore, inconsistent with any interaction with the upper continental crust. Accordingly, they are interpreted to have erupted in an oceanic setting. On the basis of Nb/Th ratio the Veligallu basalts are divided onto two groups. The basalts with Nb/Th > 8 display systematically flat to light-REE depleted chondrite normalized REE patterns (LaN/YbN = 0.76–1.0), in contrast to the samples with Nb/Th < 8, which display slight enrichments in light-REE (LaN/YbN = 1.0–1.6). The depleted basalts are characterized by primitive mantle normalized Nb concentrations lower than La but equal to Th, in contrast to the enriched basalts, which are characterized by slight depletions of Nb relative to both La and Th. Collectively, as a group, they display Nb/Yb ratio higher than the N-MORB. The enrichment in Nb is attributed to moderate to low degree (∼10%–4%) partial melting of a depleted MORB source. The trace, REE and their interelement ratios in the Veligallu basalts exhibit transitional characteristics between the average N-MORB and the typical Phanerozoic island arc basalts. Overall, the geochemical characteristics of the Veligallu basalts are identical to basalts erupted in the Phanerozoic back-arc basins.The Veligallu high-Mg andesites are orthopyroxene-bearing calc-alkaline andesites and are characterized by moderate SiO2 (∼56 wt%), high MgO (∼8 wt%), Mg# (61–67), Cr (174–330 ppm) and Ni (173–242 ppm) contents; low Y (∼13 ppm) and Yb (∼1.2 ppm) concentrations, relative to typical island arc andesites. They exhibit light-REE enriched (LaN/SmN ∼ 1.7) and heavy-REE depleted (GdN/YbN ∼ 1.9) fractionated REE patterns. On a primitive mantle normalized trace element variation diagram they exhibit negative Nb and Ti, and contrasting positive Zr anomalies relative to the neighboring REE. The La–Yb systematics suggest that these high-Mg andesites were generated by hydrous partial melting of a slab-melt metasomatized mantle wedge in the sub-arc mantle. The felsic volcanic rocks are characterized by high SiO2 ∼ 73 wt%, Na2O ∼ 4.5 wt%, and significantly low concentrations of MgO (∼0.72 wt%), Cr ∼ 19 ppm, Ni ∼ 23 ppm, Y ∼ 5.9, and Yb ∼ 0.44 ppm; strongly fractionated chondrite normalized REE patterns (LaN/YbN ∼ 30) with occasionally minor positive Eu anomaly, and primitive mantle normalized negative Nb and Ti, and contrasting positive Zr anomaly. The geochemical attributes of the Veligallu felsic volcanic rocks are analogous to the Cenozoic high silica-type adakites that are produced by partial melting in the sub-arc mantle, and presumably without any significant interaction of the slab-melts with the overlying mantle wedge.The juxtaposed occurrence of basalts with the banded iron formations, and their close spatial association with the high-Mg andesites, in the Veligallu greenstone belt, indicates that the petrogenesis of these volcanic rocks is consistent with their eruption in an intraoceanic setting. The close proximal association of the back-arc type basalts, and the high-Mg andesites and adakites within the Veligallu greenstone belt potentially suggests that this greenstone belt represents a rifted back-arc crust. This event appears to have significantly contributed to the peak of crustal growth activity in the eastern Dharwar craton at 2.7 Ga.