In the Peddavuru greenstone belt, eastern Dharwar craton, basalt is the predominant rock type with interleaved felsic volcanic rocks. The basalts are fine grained, aphyric and essentially composed of amphibole and plagioclase with rutile, magnetite and ilmenite as accessory Fe-Ti oxide phases. The felsic volcanic rocks exhibit porphyritic texture. Based on the composition of the phenocryst type i.e. feldspar or quartz, two variants have been recognized. Biotite is present in subordinate amounts while, apatite and magnetite are the accessory phases. Basalts are tholeiitic in composition, whereas the felsic volcanic rocks are calc-alkaline in nature. The basalts can be classified into low-Ti (< 1 wt. % TiO2) and high-Ti (> 1 wt. % TiO2) geochemical subgroups. They display slightly depleted (La/Sm ∼ 0.89) and mildly enriched (La/Sm ∼ 1.44) chondrite normalized light rare earth element patterns (REE), and slightly depleted heavy REE (Gd/Yb ∼ 1.2). The felsic volcanic rocks are characterized by contrasting geochemical compositions. On the basis of high field strength element systematics, they are classified as dacites and rhyolites. Compared to the rhyolites, the dacites are characterized by high Mg# (39 ± 9 vs. 24 ± 5), low Nb (≤ 5 ppm vs. 14 ppm), Y (5.9 ± 1.8 ppm vs. 19 ppm) and Yb (0.47 ± 0.17 ppm vs. 1.72 ppm) contents. Further, the dacites exhibit comparatively steep chondrite normalized REE patterns (LaN/YbN ∼ 30 vs. 10) with negligible to slightly positive Eu anomaly (Eu/ Eu* = 0.9–1.2 vs. 0.6). The trace and rare earth element attributes of these dacites are identical to the adakitic rocks that have been recognized from the Phanerozoic intraoceanic arcs. On the contrary, rhyolites are the partial melt products involving minor plagioclase fractionation that are generated beneath the arc crust under extension. Overall, the chemical compositions of the mafic and felsic volcanic rocks in the Peddavuru belt reflect primary mantle source characteristics. MORB-like trace element signatures in combination with arc-like geochemical affinity in the Peddavuru basalts, provides compelling evidence of their origin in an intraoceanic back-arc setting. Thus it presents a Neoarchean analogue of Phanerozoic-type back-arc magmatism.
Tarun C. Khanna and V.V. Sesha Sai (2020). Geochemistry 80 (2020) 125606