It is generally considered that the IAT and boninite related magmas were probably derived from a subduction modified hydrous mantle source that lack any additional trace element evidence for OIB magmas. Mondal and Raza (2009) proposed a subduction modified oceanic plateau environment for the SGB volcanic sequences. They considered that the pillow basalts of SGB as the lower unit (LB-1) and proposed that the this sequence is plume derived, whereas the upper mafic and felsic volcanic rocks are related to an Island arc. However, Deshmukh et al, (2017), proposed that the pillow basalts and meta basalts of the SGB (LB-2) exhibit low Nb/La (0.45), Ti/Y (252.3), Ti/V (20.5), TiO2 (0.77), Ta (0.52) and Nb (1.3) values and are akin to an N-MORB related magma source which was modified by dehydration of the subducting slab in a SSZ environment. The OIB magmas are generally characterized by high HFSE abundances (e.g. Nb=20-200ppm and positive Nb anomalies in the Primitive mantle normalized diagrams; see Pfänder et al, 2007). In the present case, whereas, the Nb concentrations of TH-1 (0.16-1.6 ppm), TH-2 (0.13-0.84 ppm), LB-1 (2-4 ppm) and LB-2 () are significantly lower than the OIB magmas. If significant contributions from OIB source/OIB like components were involved in the genesis of SGB rocks, it would have resulted in a pronounced enrichment in Nb and Ta components. However, the observed Nb-Ta characters are inconsistent to the OIB components. The Nb/Ta values of the TH-1 (5.0-13.4), TH-2 (1.8-6.3) LB-1 (6.2-11.5) and LB-2 () are lower than the reported values of OIB magmas (15-18) (see Pfänder et al, 2007). Thus, involvement of an OIB like deep plumes are unlikely in the case of SGB. Further, similar rock associations (IAT and boninite-like) in Archean SSZ environments have been reported from many greenstone terranes which include the ?3.8 Ga Isua supracrustal belt in Greenland (Polat et al., 2002; Furnes et al., 2009), Gadwal greenstone belt of Dharwar craton (Manikyamba et al, 2005), Whundo and Mallina greenstone belts of Pilbara craton (Smithies et al, 2004), Abitibi greenstone terrane of Superior province (Kerrich et al, 1998) and Iron ore group of Singhbhum craton (Manikyamba et al, 2015), suggesting that subduction processes were likely operating even at early stages at about 3.7–3.8 Ga.
Considering the geochemical signatures of TH-1 and TH-2 samples along with the mineral chemistry clearly point towards a subduction related magmatism.
The overall geochemical characters of TH-1 samples and mineral chemistry of associated clino-pyroxenes clearly attributes an Island arc signature (Fig. __).The whole rock geochemistry, mineral chemistry of clinopyroxenes and chromites along with the parental melt calculations in the chromites of TH-2 magma depict a boninite magmatic signature and a SSZ environment in the SGB (Fig. ). In addition, if we invoke a plume model () for the boninite related rocks (TH-2) an OIB like plume would need to rise and infiltrate the refractory mantle wedge, between IAT and boninite related melting. Considering the indistinguishable temporal relation between the IAT and boninitic rocks of both Phanerozoic and Archean terranes, propagation of a plume prior to the boninite related magmatism (TH-2) is unlikely, especially in the case of SGB rocks. Recent studies from the SGB (see Deshmukh et al, 2016, Manu Prasanth et al, 2017) are also point towards subduction and subsequent dehydration processes in a SSZ setting.
In the Th/Yb vs. Nb/Yb discrimination diagram (Fig. ), the TH-1 and TH-2 samples plot close to the fore arc field, where as the majority LB-1 samples plots in the mantle array region between N-MORB and E-MORB and the LB-2 show considerable enrichment in the TH/Yb values and plot in the arc-forearc field. The Nb/Yb values in the basalts are considered as a monitor of the degree of depletion (Pearce 2008) (low Nb/Yb, MORB-like) or enrichment (high Nb/Yb, alkaline OIB-like) of the mantle source.
We interpret the lower basalts are moreover related to MORB like magmas rather than the OIB. In addition, the lower basalts exhibit relatively lower TiO2 (0.83-1.27) and higher SiO2 () values than the typical N-MORB (TiO2= 1.27; Sun and MC; SiO2 49.51). Slightly lower TiO2 and higher SiO2 contents of these basalts exhibit a similarity to the MORB-like melts which marks the initiation of a subduction zone. These types of rocks are commonly found as the lower basaltic sequence of many ophiolites (Stern et al, 2012).MP1 As the subduction process initiate in an intra oceanic subduction regime, MORB related lavas incorporate minor fluids from the subducting plate, which will be reflected by the slightly elevated Th/Yb and Nb/Yb values than the typical N-MORB magmas. In the phanerozoic ophiolite record, majority of the Kizildag and Oman lavas plot with in the mantle array between N-MORB and E-MORB in the Th/Yb-Nb/Yb discriminant diagram (see Dilek and Furnes 2009). Whattam and Stern (2011) proposed that the most of the Tethyan ophiolites (like Mirdita, Pindos, Troods and Semali) exhibit subduction initiation relationships with their MORB-related lower lavas and arc-related upper lavas. The geochemical signatures of LB-1, LB-2, TH-1 and TH-2 collectively indicate subduction initiation relationship and a fore arc extensional regime, where heterogenous mantle source and a intra oceanic subduction zone with multiple melt extraction episodes accompanied by fluid fluxing and metasomatism of the overlying mantle wedge attributed to the compositional diversity of the SGB magmas. The shallow subduction of the young and hot oceanic lithosphere during Neoarchean and subsequent melt extraction episodes are markedly identical to that observed in many Phanerozoic supra subduction zones. (). The SSZ characteristics of SGB rocks are evident from the Fig. and Fig. Pearce et al. (1984) introduced the term supra subduction zone for the ophiolites formed in an intra oceanic immature arc systems composed of crustal components along with the subduction modified oceanic lithosphere. Later on, Hawkins and Evans (1983) and Hawkins et al. (1984) expanded the term SSZ by incorporating all components that were formed in the subduction system ranging from arc, fore arc to back. Further, recent developments in the understanding of tectono-magmatic characteristics of the ophiolites and related rocks led to the identification of SSZ ophiolites or ophiolite related volcanic sequences in many Archean greenstone belts (e.g., Dilek and Polat, 2008; Dilek and Furnes, 2011; Kusky et al., 2011; Dilek and Furnes, 2014). Presence of boninite related lavas at the top of the fore-arc basalts is considered as a major indication of subduction initiation setting (refe). In a recent study, Turner et al (2014) noticed the chemo-stratigraphic similarities between Eoarchean Nuvvuagittuq supracrustal belt in northern Quebec, Canada, and the Izua-Mariana-Forearc and proposed a possible occurrence of forearc environment since Hadean or Eoarchean. The ____ and ___ relations clearly point to a fore arc tectono-magmatic environment for the lower SGB rocks. As illustrated in the Fig . an intra oceanic subduction zone initiated in the oceanic environment of SGBMP2 .
Dehydration of the subducting plate and upwelling of the MORB related melts in a fore arc spreading environment resulted the formation of lower basalts of SGB. Further, continued subduction, slab dehydration and hydrous fluxing have liberated the LREE and LILE enriched fluids, which induced partial melting of the depleted mantle wedge. The TH-1 samples of the SGB represents partial melting of hydrous lherzolite source with hydrous fluxing in a proto-forearc spreading environment. The initial melt extraction episodes and subsequent depletion and subduction induced partial melting along with the hydrous components account for the formation of boninitic dykes.