Nature of petrological, geochemical, geochronological settings and evolution of bundelkhand greenstone complexes in the Bundelkhand Craton, India

Abstract

This doctoral thesis describes the geology, petrography, geochemistry, mineralogy, Sm–Nd isotope geochemistry and geochronology of mafic–ultramafic rocks, TTG (tonalite-trondhjemite-granodiorite) gneisses and high-K granitoids to decipher the nature and origin of the geodynamic changes that took place between ~3.59 and 2.50 Ga in the Central Bundelkhand Greenstone Complex (CBGC), Bundelkhand Craton (BC), India. The CBGC consists of mainly two greenstone belts: (1) the Babina greenstone belt (2) the Mauranipur greenstone belt. The petrography and mineral assemblages of these mafic–ultramafic volcanic rocks indicate that they experienced greenschist to amphibolite facies metamorphism. The mafic volcanic rocks from the Babina belt are characterized by SiO2 = 43.9–51.2 wt%, MgO = 5.4–11.0 wt% and Mg# = 44–77 [Mg# = 100 × (Mg2+/(Mg2+ + Fe2+))], whereas those from the Mauranipur belt are characterized by higher silica (51.8–55.6 wt%), MgO = 6.9–9.5 wt% and Mg# = 59–70. The ultramafic rocks of the Babina and Mauranipur belts contain SiO2 = 46.9–50.3 wt%, MgO = 20.2–21.1 wt% and Mg# = 77–82. The Babina mafic rocks show a nearly flat REE and HFSE profile [(La/Yb)CN = 0.87–1.40] with negative Nb (Nb/Nb* = 0.13–0.77) and positive Pb anomalies that could be attributed to subduction-related metasomatic agents. On the other hand, the Mauranipur mafic rocks have total REE (26.0–40.7 ppm) and display flat chondrite normalized LREE [(La/Yb)CN = 1.1–1.7; (La/Sm)CN = 1.1–2.0] with no Eu anomalies (Eu/Eu* = 0.89–1.0) and negative Nb anomalies (Nb/Nb* = 0.13–0.77), which are most probably the effects of crustal contamination. Additionally, the whole-rock isotopic (Sm–Nd) data of the mafic–ultramafic volcanic rocks from the Babina belt yield an isochron age of ~3.44 Ga, which represents the first estimation of the age of these rocks. Isotopic and geochemical characteristics of mafic–ultramafic volcanic rocks of the CBGC reveal that they were generated from a mantle source with long-term depletion. The outcrops mafic–ultramafic volcanic rocks in the Babina and Mauranipur belts are remnants of oceanic crust possibly emplaced in a subduction-related setting. LA-SF-ICP-MS zircon dating reveals that TTG magmatism mainly developed around ~3.51–3.20 Ga at regular intervals of 100 Myr although it has also found evidence of younger Neoarchean TTG magmatism at 2.71–2.67 Ga. In addition, TTG and mafic–ultramafic magmatisms suggest contemporary felsic plutonism and mafic volcanism at ~3.44 Ga. Subsequently, high-K granitoids magmatism (sanukitoids and anatectic granites) occurred during the two discrete episodes during Neoarchean between ~2.58 and 2.50 Ga and suggest synchronous emplacement. These high-K granitoids commonly represent the last Archean magmatic event represented by large scale crustal melting (anatexis) that seems to be related to the final stabilization of the craton. The TTG gneisses and high-K granitoids contain inherited zircons, which suggest the participation of ancient crust in their origin. In addition, the combined zircon CL images and U–Pb data allowed unraveling different metamorphic events/overprint at ~3.31–3.10 Ga and 2.64 Ga that may have formed mainly by recrystallization in the BC. Paleoarchean TTG gneisses display SiO2 (64–73 wt%) and Neoarchean TTG gneisses contain SiO2 (66–74 wt%). In contrast, sanukitoids show a range of SiO2 contents from 61 to 71 wt.% and anatectic granites display high content of SiO2 from 70 to 78 wt.%. The Paleoarchean TTG gneisses are characterized by moderate to high total REE (78–319 ppm) and moderately fractionated REE patterns [(La/Yb)CN = 3.88–41.12] with variable negative Eu anomalies. In contrast, Neoarchean TTG gneisses contain moderate total REE (107–142 ppm) and strongly fractionated REE patterns [(La/Yb)CN = 9.83–26.44] without any significant Eu anomalies signifying the role of garnet in source residue. Sanukitoids are characterized by enrichment in the LREE compared with the HREE and show a clear LREE/HREE fractionation, whereas anatectic granites display enrichment in the LREE with respect to the HREE with strong to moderate negative Eu anomalies. In the Primitive Mantle (PM) normalized incompatible elements diagram, Paleoarchean TTG gneiss samples show enrichment in Ba, U, K and Pb and slightly elevated values for Sr, and Zr, with large negative Nb and Ti anomalies and weak negative Ce, P and Ta anomalies. A similar pattern also recognized in Neoarchean TTG gneisses and high-K granitoids, which suggest for subduction-related origin. The values of εNd(t) ranging from +4.3 to +0.3 (excepting one sample showing –0.5) along with their whole-rock geochemical nature indicate that the low-HREE type TTGs were produced through melting of subducted oceanic arc crust at different depths which probably interacted variably with felsic crustal melts in a stability fields of garnet and ilmenite, probably plagioclase-free source. On the other hand, high-HREE type TTG suggest a likely originated from a plagioclase and garnet-amphibolite at low pressure. Further, a Neoarchean TTG gneiss shows εNd(t) values of +4.5 belonging to the low-HREE group, formed by the partial melting of deep-seated mafic crust in the garnet stability field with trivial involvement of ancient felsic crustal material in a subduction setting. Sanukitoids show εNd(t) values between −3.6 to −1.6 along with their whole-rock geochemical characteristics reveal that they were formed by the mixing of mantle melts with anatectic melts followed by homogenization at shallow intrusion levels within a subduction environment. In contrast, anatectic granites display highly negative εNd(t) values (−3.4 to −8.7) which indicate that they were formed purely by partial melting of crustal materials (anatexis) in a collisional setting. Finally, the amalgamation of diverse micro-blocks occurred by arc-continent collision and repeated slab break-off between north and south terranes of the BC during late Archean. These pieces of evidence suggest that the BC was not assembled until ~2.50 Ga. The anatectic granites were generated by intensive partial melting of the continental crust in an arc-continent collision, representing the final stabilization of the BC.