Petrogenesis of low-Ti and high-Ti basalt, adakite and rhyolite association in the Peddavuru greenstone belt, eastern Dharwar craton, India: A Neoarchean analogue of Phanerozoic-type back-arc magmatism

摘要

The petrogenetic record, available in the literature, illustrating Neoarchean arc magmatic processes in a back-arc setting is rare. In this contribution, we present a comprehensive account of field, petrography and geochemistry of the Neoarchean bimodal mafic - felsic metavolcanic rock association in the Peddavuru greenstone belt, eastern Dharwar craton, India. Basalt is the predominant rock type. Felsic volcanic rocks are interleaved with the basalts, in the central part. 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. On the basis of TiO2 contents, the basalts can be classified into low-Ti (< 1 wt. % TiO2) and high-Ti (> 1 wt. % TiO2) geochemical subgroups. The two subgroups, however, are consanguineous in nature. They display slightly depleted (La/Sm similar to 0.89) and mildly enriched (La/Sm similar to 1.44) chondrite normalized light rare earth element patterns (REE), and slightly depleted heavy REE (Gd/Yb similar to 1.2). On a primitive mantle normalized trace element variation diagram, irrespective of low- or high-Ti, the basalts display negative Nb and Ti anomalies, and zero to negative Zr-Hf anomalies relative to the neighbouring REE. 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 (La-N/Yb-N similar to 30 vs. 10) with negligible to slightly positive Eu anomaly (Eu/Eu* = 0.9-1.2 vs. 0.6). On a primitive mantle normalized trace element variation diagram they exhibit negative Nb and Ti anomalies, similar to the rhyolites, but contrasting positive Zr-Hf peaks and high Zr/Sm ratio (73 +/- 18 vs. 24). 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 indicate that interaction with the Archean upper continental crust, magma mixing and/or assimilation and fractional crystallization processes cannot be the cause of these geochemical patterns. The attributes instead 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. Accordingly, the Peddavuru greenstone belt presents a Neoarchean analogue of Phanerozoic-type back-arc magmatism.