Origin of granitoids of the Nilgiri-Kaptipada area, south-eastern part of Singhbhum Craton, Odisha: Implications for crustal evolution

Sahoo, Manoj Kumar (2025) Origin of granitoids of the Nilgiri-Kaptipada area, south-eastern part of Singhbhum Craton, Odisha: Implications for crustal evolution. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

The fundamental mechanism behind the formation of the Archean crust is one of the most debated topics in early Earth research worldwide. Different researchers provide various opinions on the geodynamic processes associated with the formation and growth of the Archean crust. Researchers have identified two primary geodynamic processes related to the formation of the Archean crust: Archean-style tectonics (vertical tectonics) and modern-style tectonics (horizontal tectonics). Some researchers suggested that vertical tectonics is the most dominant factor for Archean crust formation. At the same time, another group argued that horizontal tectonics (modern-style tectonics) is the key factor for Archean crustal formation, and a few groups also suggested that both processes have a mutual role in the Archean crust formation process. Granitoids form the major lithodemic components of Archean cratons which provide crucial information related to the geodynamic setting and mechanism for the crust formation processes. In the eastern part of India, in the Singhbhum Craton, formation and diversification of granitoids hold a long record of the extended history of crust formation, maturation and stabilization from early Paleoarchean to early Neoarchean. The well-exposed and well- preserved granitoids are a key factor in testing the models for early Earth crustal evolution. The present work mainly focused on the granitoid units exposed along the southeastern part of the Singhbhum Craton in the Nilgiri-Kaptipada area. This work incorporates field geology (mapping and sampling), petrographic study, whole rock geochemistry (major and trace elements), zircon U-Pb dating, along with Hf isotope and trace element chemistry to understand their petrogenesis, role in crustal evolution, and implications for the evolution of crust-mantle system during the Paleoarchean. Fieldwork helped with the preparation of a geological map of the study area with a scale of 1:50,000 which was followed by petrochronological study. Whole-rock major elements are used for the classification of these granitoids whereas the trace elements are used to understand the petrogenesis. U-Pb dating is used to determine the emplacement age of these granitoids, and the Hf isotope system is used to determine the nature of the crust (juvenile or reworking of older crust). This work also addresses the following research issues: (a) What was the reason for granitoids diversity in early Archean? (b) Did the mantle depletion start early in the history of the Earth (at least during the early Paleoarchean)? (c) What does blocks with disparate evolutionary histories within a craton imply for the early continental crust formation? The granitoids of the study area are divided into six lithodemes, such as Nilgiri granite, Kalakad granitoid, Kaptipada TTG, Thikirbil tonalite, Sarat tonalite and Leucogranites, based on the field appearances, texture, structure, nature of deformations, migmatisation, and presence of enclaves. In the study area, the geochemical data along with the U-Pb dating suggests episodic reworking of crust. These includes the first appearances of a high-HREE TTG at 3.50 Ga, followed by the generations of high- HREE TTGs, low-HREE TTGs and K-rich granites over 3.33 ̶ 3.28 Ga. This history is described below: The ~3.50 Ga Kalakad granitoids exposed in the western part of area represent the oldest dated granitoid in the Singhbhum Craton. These granitoids vary in composition from tonalite, trondhjemite to granite. They show wide variations in major and trace element contents, including a broad range of SiO₂ values (65–75 wt.%) and K₂O/Na₂O (0.25–1.85). Most of the samples display flat HREE patterns (GdN/YbN = 1.23–3.21), suggesting shallow (low-pressure) melting of the source. Low Sr/Y and Y, flat HRRE patterns with the compositional variation suggest a heterogeneous source, including tonalite and low-potassic mafic rocks. The positive zircon εHft values (+1.6 to +4.4) indicate these granitoids were derived from the juvenile crust. The ~3.33 Ga Thikirbil granitoids are tonalitic in composition characterized by elevated SiO₂, Na₂O, low K₂O, K₂O/Na₂O and low total ferromagnesian element contents (Fe₂O₃T+ MgO + MnO + TiO₂ = 5.2–3.6 wt.%). They show depletion in Y, Yb and Nb, high Sr/Y, and moderate to high LaN/YbN. These granitoids show a depleted HREE pattern with a positive Eu anomaly. These characteristics are typical of high-pressure TTG. The zircon positive εHf values (+3.3 to +1.3) indicate a short crustal residence time of the mafic source after its derivation from a depleted mantle. The geochemical data suggested that the Thikirbil tonalite is derived from the partial melting of low-potassic mafic rock in equilibrium with a plagioclase- free, garnet-pyroxene (±rutile) bearing residue. The ~3.32 Ga Kaptipada granitoids are characterized by high silica (71–73 wt.%), high Na₂O (4.1–5.5 wt.%), moderate CaO (2.41–3.34 wt.%) and K2O (0.96–3.07 wt.%), low MgO, Mg# (31–41), Cr and Ni with K₂O/Na₂O (0.18–0.74). The rocks show well-fractionated LREE, moderate to well-fractionated HREE and weakly negative to positive Eu anomaly. It shows PM-normalized enrichment of LILE and negative Nb, P and Ti anomaly. The positive εHft values +0.5 to +4.4 of zircons suggest the short crustal residence time of the source. The wide-range of pressure-sensitive element contents and ratios suggests that Kaptipada lithodeme contains all TTG pressure groups. Thus, samples with low Y and Yb contents, high Sr/Y and LaN/YbN ratios, positive Eu anomaly, and depleted HREE pattern belong to the high-pressure TTG generated by partial melting at a greater depth (15–20 kb) with a plagioclase-free, garnet pyroxene (± rutile) bearing residue. Conversely, the samples with higher Y and Yb contents, low Sr/Y and LaN/YbN ratios, negative Eu anomalies, and flat HREE pattern (lowpressure TTGs) represent shallow melting (10–12 kb) with plagioclasepyroxene- amphibole residue. The geochemical data suggest that these granitoids are TTG in nature and are formed by the partial melting of the low-potassic mafic rock at variable depths. The ~3.29 Ga Nilgiri granite is rich in silica (71–75 wt.%) and K₂O (3.5–4.7 wt.%). It shows ferroan nature with high Fe₂O₃T (3.0–5.7 wt.%)along with low MgO and Mg# (3–10). The Nilgiri granite is also enriched in HFSE (Zr, Hf, Nb and Ta), U, Th, REE, Zn and Y. LILE (Rb, Ba and Pb) concentrations are moderate to high. The Sr contents and Sr/Y ratios are low. The rock shows well fractionated LREE, flat HREE and prominent negative Eu anomaly. The rock shows an affinity towards anorogenic (A2- type) granite with high values of Y/Nb ratio (>1.2). The εHf isotopic values range from +0.83 to +2.57. These data suggest that the Nilgiri granite formed through a reworking of crustal material with short residence time. The whole- rock zircon saturation temperatures are high (avg. 800°C). It is suggested that the Nilgiri granite was produced by high-temperature, shallow dehydration melting of a TTG source. The ~3.29 Ga Sarat granitoids are moderately silicic (SiO₂ = 68–71 wt.%), high- alumina, dominantly tonalites with moderate MgO, Fe₂O₃ and Mg#, and low K₂O and K₂O/Na₂O. The REE patterns are characterized by fractionated LREE, depleted HREE and mostly positive Eu anomalies. The samples display low Y and Yb, and high Sr contents, resulting in high Sr/Y and moderate to high LaN/YbN ratios, suggesting high-HREE TTG. Primitive mantle normalized plots show the enrichment of incompatible elements with distinct negative anomalies at U, Nb, Ta, P and Ti. The positive zircon εHf values (+0.5 to +2.8) indicate that the Sarat tonalite is derived from the partial melting of the mafic juvenile crust at shallow depths. The ~3.28 Ga leucogranites are rich in silica (SiO₂ = 72–75 wt.%), K2O (4.8–6.9 wt.%) and K₂O/Na₂O (1.5–2.7). They also have higher concentrations of Ba, Rb, Th, Y and Yb and are poor in MgO, FeO and Mg#. The REE patterns are also distinct, being characterized by a strong negative Eu anomaly (Eu/Eu* = 0.21–0.37) with a flat HREE pattern. The leucogranite samples display primitive mantle normalized highest enrichment of incompatible elements with strong negative U, Nb, Ta, Sr, P and Ti anomalies. The rocks show low Sr/Y and LaN/YbN values, which indicate shallow pressure melting of source rock. The positive to slightly negative zircon εHf values (+4.1 to –0.7) suggest that the leucogranites were derived mostly from the reworking of ~3.33–3.29 Ga juvenile tonalites with slight involvement of older crust. Form the above information two conclusions are made: (a) the mantle was depleted, at least partially, at ~3.5 Ga and (b) shallow (lowpressure) crustal melting was the main process of felsic magma generation during this time in the Singhbhum Craton. The present work also links this Paleoarchean granitoid diversification with crustal thickening allowing intracrustal melting and granitoid generation at variable depths. All the granitoids display suprachondritic to chondritic initial zircon epsilon values. This is possibly related to a unique geodynamic setting in Paleoarchean which facilitated continued juvenile addition of mafic crust from depleted mantle and its quick differentiation into zircon-bearing felsic crust. A synthesis of craton-wide data is also made which suggests that the Paleoarchean granitoid crust in the Singhbhum Craton can be divided into several blocks bordered by greenstone belts. These blocks show disparate temporal evolutionary history in term of granitoid diversification. This work suggests that the spatial and temporal evolution of Paleoarchean granitoids may be related to the craton-scale variation in crustal thickness and its compositional heterogeneity.

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