Petrogenetic evidence
Contamination of the basalts of the Baghmara Formation of the Sonakhan greenstone belt by continental crust during the magma ascent may be ruled out on the basis of their geochemical characteristics and the presence of the pillowed structure which is consistent with an oceanic, rather than a continental setting. There is also no field evidence to indicate that the Sonakhan greenstone basalts were emplaced in an older continental basement.
The REE patterns of the lower unit basalts of the Baghmara Formation of the Sonakhan greenstone belt are near-flat (slightly LREE-depleted to slightly LREE-enriched) (Fig. 8) and the rock types are subalkaline in composition (Fig. 5). These rocks have positive (to slightly negative) Nb-anomalies (Nb/Nb* = 0.93 – 1.48, mean 1.17, Table 2). All of these features are consistent with an oceanic plateau association generated by a mantle plume. The near-flat REE patterns may have resulted from the high degrees of partial melting in the mantle plume. Nb/Y ratio of the lower unit basalts ranging from 0.10 to 0.14 at the SiO2 level of 47.67 to 51.94%, indicates that the basalts were generated from the high percentage of melting at shallow depth (Greenough et al., 2005). The REE and incompatible multi-element patterns of these basalts are similar to those of many modern, Proterozoic and Neoarchaean ocean islands (Polat, 2009 and references therein). The multi-element patterns of the lower unit also exhibit distinct negative Zr-anomalies (Fig. 9). Their Nb/U ratio ranges from 10.27 to 46.39 which is chondritic to super-chondritic (Nb/U = 31 in chondrite). Similarly the La/U (11.36 – 51.26) and Nb/Th ratios (8.29- 18.59) are also chondritic to super-chondritic (La/U and Nb/Th values in chondrite are 30, 8.5 respectively). The Nb/La ratios (0.67 – 0.91) are sub-chondritic to chondritic (chondrite value is 1.0). These features are considered to be typical of ocean island basalt. Based on the geochemical and the field characteristics, it is proposed that the lower unit basalts of the Baghmara Formation represent an oceanic plateau erupted from a rising mantle plume. Type 1 felsic volcanic rocks with minor negative Nb-anomaly and positive Ti- anomaly may represent differentiated product of plume generated basaltic rock.
The upper unit basalts and the associated type 2 felsic volcanic rocks show chemical characteristics of the island arc volcanic association having larger variations in major and trace elements, ranging from sub-alkaline basalt through andesite, dacite to rhyolite (Fig. 5) forming a BADR (basalt-andesite-dacite-rhyolite) series. Similar patterns have recently been reported from the arc-related Gadwal greenstone belt of the Dharwar craton (Manikyamba and Khanna, 2007). Our samples of the upper unit basalts have distinct Th-Nb-REE systematics. The upper unit basaltic rocks and the type 2 felsic rocks are characterized by the positively fractionated REE patterns (Fig. 8). They also display strong depletion in Nb and Ti relative to Th, LREE and MREE (Fig. 9). Nb/U (2.46-3.77), Nb/Th (0.68 – 1.5) and Nb/La (0.11 – 0.24) ratios are sub-chondritic. They have higher Zr/Y ratios (7.76-10.51) compared to those of the lower unit basalt (0.41- 1.45) of the oceanic plateau association. The REE and the trace element patterns shown by the upper unit basalts and the associated type 2 felsic volcanics are very much similar to those of the volcanic rocks of the island arc setting. Such patterns have been reported from several Arcahean greenstone belts (Hollings and Kerrich, 2000; Wyman et al., 2000; Hollings, 2002; Sanderman et al., 2004).