Tectonic setting
Pearce (2008) has suggested that the E-MORB and OIB sources are enriched relative to N-MORB sources that are revealed by the high Ta/Yb and Th/Yb ratios yielding a MORB-OIB array (Fig. 10). Crust input, either by magma-crust interactions or by crustal recycling into magma sources, raises the Th/Yb ratios that plot above this MORB-OIB array. On the Ta/Yb vs Th/Yb diagram (Fig. 10), the lower unit basalts plot close to E-MORB within the array. On the other hand, the upper unit basalts and the felsic volcanics (except for the two felsic volcanic samples which plot close to E-MORB) plot mainly within the volcanic arc field.
Figure 10. Ta/Yb vs. Th/Yb diagram
Ta/Yb vs. Th/Yb diagram for the volcanic rocks of the Baghmara Formation. Data for N-MORB, E-MORB, OIB are after Sun and McDonough (1989). Fields after Pearce (1982, 2003). CA, Calc-alkaline; TH, tholeiite.
It is proposed that the Sonakhan greenstone belt contains rocks of both the oceanic plateau association and the island arc association. It is noteworthy that the lower unit pillowed oceanic plateau basalt and the upper unit basalt of the island arc association are not spatially related with unconformity in between them, although both the rock types were emplaced in different tectonic settings. The lower unit pillowed basalt and the upper unit basalt along with felsic volcanics occur even without any obvious structural discontinuity. This indicates that the relationship between these two lithotypes is primary and may indicate tectonic juxtaposition of the island arc association over the oceanic plateau association. This, in turn, may indicate co-existence of contemporaneous plume and arc magmatism in the Sonakhan greenstone belt. The plume derived plateau basalts occupy the lower part of the Baghmara Formation and predate the upper unit calc-alkaline basalt, andesite, dacite and rhyolite series of the Baghmara Formation of the island arc association. Thus, the Bhagmara Formation, as a whole, represents both plume type and island arc type volcanism (Fig. 11).
Figure 11. Geodynamic evolution of the Sonakhan Greenstone belt
Cartoon showing the geodynamic evolution of the Sonakhan Greenstone belt. A. Eruption of the plume-related basalt forming the oceanic plateau (lower unit basalt of the Baghmara Formation). B. Subduction of the oceanic plate under the oceanic plateau resulting in arc-related magma.
The operation of the subduction process in this part of the Indian shield is also indicated by geochemical characteristics of the Baya gneisses (Hussain et al., 2004; Mondal et al., 2006). It appears that partial melting of the mantle wedge which was metasomatized by slab released – fluids has given rise to rocks of the basalt-andesite-dacite-rhyolite association of the upper unit basalt of the Baghmara Formation. Lithospheric extension in the overriding plate may have occurred in response to subduction slab-rollback resulting in the formation of an intra-arc basin where the clastics of the Arjuni Formation were deposited. In the absence of geochronological data, the timing of these processes remains speculative. However, based on the lithological make-up and the stratigraphic positions, a broad age band of Neoarchaean to Paleoproterozoic may be assigned to the Sonakhan greenstone belt. Hollings and Kerrich (2006) reported the presence of such compositionally diverse, spatially associated basaltic flows in the 2.7 Ga St. Joseph greenstone belt, Canada.