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In this contribution, we present detailed field, petrography, mineral chemistry, and geochemistry of newly identified high-Si high-Mg metavolcanic rocks from the southern part of the ~3.3 Ga Holenarsipur greenstone belt in the western Dharwar craton, India. The rocks occur as conformable bands that were interleaved with the mafic-ultramafic units. The entire volcanic package exhibits uniform foliation pattern, and metamorphosed under greenschist to low grade amphibolite facies conditions. The rocks are extremely fine grained and exhibit relict primary igneous textures. They are composed of orthopyroxene and clinopyroxene phenocrysts with serpentine, talc, and amphibole (altered clinopyroxene). Cr-spinel, rutile, ilmenite, and apatite occur as disseminated minute grains in the groundmass. The mineralogical composition and the geochemical signatures comprising of high SiO2 (~53 wt. %), Mg# (~83), low TiO2 (~0.18 wt. %), and higher than chondritic Al2O3/TiO2 ratio (~26), reversely fractionated heavy rare earth elements (REE) (GdN/YbN ~ 0.8), resulting in concave-up patterns, and positive Zr anomaly, typically resembled with the Phanerozoic boninites. Depletion in the high field strength elements Nb, and Ti relative to Th and the REE in a primitive mantle normalized trace element variation diagram, cannot account for contamination by pre-existing Mesoarchean continental crust present in the study area. The trace element attributes instead suggest an intraoceanic subduction-related tectonic setting for the genesis of these rocks. Accordingly, the Holenarsipur high-Si high-Mg metavolcanic rocks have been identified as boninites. It importantly indicates that the geodynamic process involved in the generation of Archean boninites, was perhaps not significantly different from the widely recognized two-stage melt generation process that produced the Phanerozoic boninites, and hence provides compelling evidence for the onset of Phanerozoic type plate tectonic processes by at least ~3.3 Ga, in the Earth’s evolutionary history. [ABSTRACT FROM AUTHOR]

We report on the bulk-rock geochemistry and Lu-Hf isotope systematics, and infer the petrogenesis of the ultramafic rocks in the Neoarchean Veligallu greenstone terrane, eastern Dharwar craton, India. The ultramafics along with the basalts and adakites yield a bulk-rock Lu-Hf isochron age of 2.696 ± 0.054 Ga, consistent with the available SIMS zircon U-Pb age of 2.697 ± 5 Ma, reported for the felsic volcanics, in the literature. The rocks have positive initial εHf (2.696 Ga) = +3.0 to +6.5, consistent with an origin from a long term depleted mantle source relative to a chondritic reservoir at ∼2.7 Ga. Geochemically, the ultramafic rocks are characterized by high MgO = 24–34 wt.%, Mg# ∼ 89, Cr = 2290–3855 ppm and Ni = 604–966 ppm contents; moderate to low SiO 2 = 52–49 wt.%, Al 2 O 3 = 9.2–4.7 wt.%, and TiO 2 = 0.30–0.14 wt.%. To first order, these chemical compositions broadly resemble komatiites. The rocks exhibit higher than chondrite Al 2 O 3 /TiO 2 (21–37) ratio, depletions in their middle-REE relative to the heavy-REE i.e. Gd N /Yb N < 1, and negative Nb, Ti, and positive Zr (Hf) anomalies in a primitive mantle normalized trace element variation diagram; attributes indicating a “boninite-like affinity”. Hence, the Veligallu ultramafics lack the key geochemical characteristics that would indicate a plume origin typically attributed to komatiites. Post-magmatic low grade metamorphism, hydrothermal alteration and element mobility, and/or contamination by Archean upper continental crust cannot be the cause of these geochemical patterns. Present study suggests that these ultramafic rocks are not melts sensu stricto as they have significantly high Mg-number and MgO contents. Rather, these rocks are most likely to be the products of fractional crystallization of orthopyroxene ± olivine from a basaltic melt, and hence interpreted as cumulates. The spatial and temporal association of these ultramafics having “boninite-like affinity” with the associated arc magmatic sequence comprising of basalts, andesites and adakites in the Veligallu greenstone terrane essentially indicates that the ultramafic rocks originated in a Neoarchean subduction-related intraoceanic setting, therefore they represent arc cumulates. [ABSTRACT FROM AUTHOR]

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% SiO 2 , ∼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 (La N /Yb N = 0.76–1.0), in contrast to the samples with Nb/Th < 8, which display slight enrichments in light-REE (La N /Yb N = 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 SiO 2 (∼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 (La N /Sm N ∼ 1.7) and heavy-REE depleted (Gd N /Yb N ∼ 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 SiO 2 ∼ 73 wt%, Na 2 O ∼ 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 (La N /Yb N ∼ 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, an [ABSTRACT FROM AUTHOR]