Geochemistry of ultramafic–mafic rocks of the Madawara Ultramafic Complex in the southern part of the Bundelkhand Craton, Central Indian Shield: Implications for mantle sources and geodynamic setting

Abstract

Detailed field, petrography and whole‐rock geochemical study was carried out in order to constrain the mantle sources and geodynamic setting of the Madawara Ultramafic Complex (MUC) of the Bundelkhand Craton. Studies reveal that there are two types of ultramafic rocks: (a) high‐Mg ultrabasic/basic rocks and (b) undeformed ultramafic–mafic plutonic rocks. The high‐Mg ultrabasic/basic rocks have undergone severe low‐grade (greenschist) metamorphism and are characterized by stringer and veinlet structures of talc–tremolite–actinolite schists with alternate layers of serpentinites showing comparatively higher SiO2 (46.1–49.4 wt%), lower MgO (24.6–26.2 wt%), and higher Al2O3 (4.58–7.06 wt%) and CaO (2.72–6.77 wt%) compared to the undeformed ultramafic rocks. The undeformed ultramafic rocks (mainly harzburgite, lherzolite, and olivine websterite) are characterized by globular structures and have lower SiO2 (40–44.1 wt%), higher MgO (30.4–38 wt%) and lower Al2O3 (1.84–4.03 wt%) and CaO (0.16–3.14 wt%). The undeformed mafic rocks (mainly gabbro) occur as small pockets within the undeformed ultramafic rocks as well as independent outcrops. Limited variation in Nb/Th against Nb/Yb along with negative Nb‐Ti anomalies of all the rock types in the multi‐element diagram reveals the significant role of the metasomatized mantle in their genesis. All the rocks show enrichment in light rare earth element and large‐ion lithophile elements compared to heavy rare earth elements and high‐field strength elements. The geochemical characteristics coupled with Ce/Yb versus Ce variation of the rocks of MUC point towards two different sources for their genesis. The high‐Mg ultrabasic/basic rocks are derived from partial melting of metasomatized mantle at shallow depth, while the undeformed ultramafic rocks were formed as a result of asthenospheric upwelling from a greater depth that induced the melting in the overlying lithosphere. Gabbro rocks represent the last and most evolved phase of the complex. Geochemical signatures suggest that the rocks of MUC were formed in a continental arc setting.