Abstract A mafic magmatic sequence of the Bhandara–Balaghat Granulite (BBG) Belt is represented by gabbroic rocks containing orthopyroxene (Opx)–clinopyroxene (Cpx)–plagioclase (Pl)–hornblende±quartz±garnet and showing tholeiitic affinity. These rocks are divided into two groups: (I) garnet-bearing; and (II) garnet-free. The garnet-bearing group is characterized by nearly flat REE patterns. In the multi-element plots, Sr, Zr and Ti show negative anomalies, indicating plagioclase, Ti-magnetite and apatite fractionation. The garnet-free rocks are geochemically subdivided into two subgroups: IIa and IIb. Subgroup IIa is marked by flat REE patterns; the LREE shows 20–30 times chondrite abundances and small positive Eu anomalies. Multi-element patterns show negative anomalies of Nb, P and Ti. Subgroup IIb is characterized by slightly enriched patterns; the LREE shows 10–60 times chondrite abundances. The REE patterns for the Subgroup IIb show moderately to highly fractionated LREE with flat HREE. Multi-element plots show negative anomalies in Nb, Ti and Zr. The Nd–Ce relationship suggests that mafic granulites of the BBGs are derived from higher degrees (Group I, c. 15–30%; Subgroup IIa, c. 20–40%; and Subgroup IIb, c. 18–35%) of partial melting of variably enriched mantle sources, followed by the evolution of the parental melt by fractional crystallization of Opx–Cpx–Pl. The geochemical signatures also suggest that the magma was further modified by crustal contamination during the course of its evolution. The Nd ( T DM ) model ages, which vary from 3.2 to 1.6 Ga, suggest a long-term evolution of the mafic granulites, possibly starting with overprinting of the isotope composition of their mantle source by crustal isotope signatures as a consequence of crustal recycling; evolving by emplacement and crystallization of the protolith at 2.7 Ga, as well as through later tectonotermal events up to granulite-facies metamorphism and exhumation of the BBG Belt during the collision of the Archaean Bundelkhand and Bastar cratons, and the formation of the Central Indian Tectonic Zone (CITZ) at 1.5 Ga.
Granulites are significant rocks that are eminent by distinct factors. One of the vital factors is that these rocks are counterparts of middle and lower crustal rocks because they are ready for fairly rational pressure-temperature estimation, existing on the borderline between melting and metamorphism. In the central Indian shield, the granulite belts commonly occur along the shear zones within the Central Indian Tectonic Zone (CITZ). The CITZ is a collage of different granulite belts, including Bhandara-Balaghat Granulite (BBG), Ramakona Katangi Granulite (RKG), and Makrohar Granulite (MKG) belts from south to north, which correspond to a multiphase orogenic evolution during Late Paleoproterozoic to Early Neoproterozoic time. Previously, numerous investigations were carried out in the central Indian shield, which are majorly refrained based on phase equilibria, thermobarometry, and metamorphic reactions. Geochemical and geochronological data sets may indicate that the granulite and granitic magmatism connected with the acidic magmatism during the Columbian crustal assembly
Geochemical and petrogenetic studies have been conducted on a set of basaltic rocks from Betul, amphibolites and dolerite dykes which intrude the Tirodi and Amgaon Gneissic Complex from Betul-Chindwara and Amgaon regions, respectively. Geochemically Betul mafic volcanics are classified as basalt to basaltic andesite with tholeiitic lineage in the form of Fe-enrichment trend. All the rocks are distinctly enriched in incompatible trace elements including light rare earth elements (LREE) and large ion lithophile elements (LILE), with continental signature in the form of negative Nb, P and Ti anomalies in their primitive mantle-normalised spidergrams. These chemical characteristics indicate their derivation from enriched mantle sources / sub-continental lithosphere. The dykes traversing the Tirodi Gneissic Complex (TGC) in Chindwara region have high abundances of Fe-oxides and alkali elements. These dykes are sub-alkaline, ranging in composition from basalt to basaltic andesite, with tholeiitic affinity. Doleritic dykes of TGC have enriched LREE-LILE with depleted high field strength elements (HFSE) patterns, which probably indicate their derivation by from enriched mantle sources, while the dykes of Amgaon Gneissic Complex (AGC) have lower LREE-LILE enrichment, which probably indicate higher degrees of partial melting of similarly enriched sources. Negative Sr-anomaly is conspicuous, probably indicating role of plagioclase fractionation in their genesis. Depleted mantle Sm-Nd model ages for the Betul mafic volcanics vary between 1951 Ma and 2320 Ma, indicating this period represents a major crustal evolution event in this region.
Abstract The Central Indian Tectonic Zone (CITZ) comprises northern and southern Indian cratonic blocks and is a tectonic window that is suitable for investigating the Proterozoic crustal evolution because of the presence of a wide variety of lithologies. Geochemical and geochronological data on mafic granulites by previous workers do not ascertain the possibility of mafic protoliths and their coeval link to other CITZ units. Thus, determining the precise timing of the formation of mafic granulites may indicate a connection between metamorphism and fragmentation of the Columbian supercontinent. This study presents zircon U–Pb ages, Nd isotopes and the geochemistry of mafic granulites to evaluate their genesis and timing of metamorphism. The results show the tholeiitic affinity and primary magmatic differentiation of the parental melt. Depletion of Nb, P, Zr and Ti and positive enrichment of Ba, U and Pb indicate the derivation of mafic granulites from a variably enriched subcontinental lithospheric mantle (SCLM) source. The zircon U–Pb ages (1564 ± 8 to 1598 ± 9 Ma) are interpreted as a period of granulite-facies metamorphism. The T DM (depleted-mantle) model ages (2.9–3.4 Ga) of mafic granulites indicate the timing of mafic protolith extraction. The mineral isochron age c. 1.0 Ga indicates that these rocks underwent some events during an early Neoproterozoic period. Protolith of mafic granulites could be related to the evolution of melts derived from metasomatized SCLM through fractional crystallization processes.
The Central Indian Tectonic Zone (CITZ) is major E-W trending suture zone between Northern and Southern Indian crustal blocks. The southern portion of the CITZ comprises three litho-tectonic units: Tirodi Gneissic Complex (TGC), Sausar Mobile Belt (SMB) and Bhandara-Balaghat Granulite Belt (BBGB). Elemental and isotopic data are used to constrain the genesis of granitoids and their protoliths, which may help us to understand the Proterozoic crustal evolution in CITZ. Geochemical and isotopic results are consistent with previous studies that these granitoid plutons are linked to the felsic magmatism of the Columbian crustal assembly in India, North America and North China. Granitoids varies from tonalite to granite, alkalic to calcic, metaluminous to peraluminous composition. Normalized elemental ratios of La/Sm, La/Yb, La/Lu, and Gd/Yb depict variable LREE enrichments and varying degrees of partial melting of heterogeneous crustal/lithospheric sources. The studied rocks are characterized by positive anomalies for Pb and negative anomalies for Nb, Sr, P, Ti, which indicate the influence of subduction-zone fluids in the source regions. Negative anomalies for K, Sr, and Ti for SMB and BBGB granitoids may also be attributed to K-feldspar, plagioclase, and Fe-Ti oxide fractionation. However, TGC porphyritic leucogranites display K, Ba and Eu positive anomalies, probably related to the accumulation of K-feldspar phenocrysts. Nd-Sr data presents initial ratios of 143Nd/144Nd t=1.6 Ga ranges between 0.509961 and 0.510300; εNd t=1.6 Ga ranges from -5.3 to -11.9 with TDM ages ranging from 2.20 to 2.78Ga for TGC granitoid. The ratios of 143Nd/144Nd t=1.6 Ga ranges between 0.510232 and 0.510985; εNd t=1.6 Ga ranges from +0.2 to +8.2 and TDM ages varies from 1.5 to 3.0 Ga. The initial 87Sr/86Sr t=1.6 Ga ratios ranges between 0.699834 and 0.797151 for SMB granitoid. However, BBGB granitoids show the ratios of 143Nd/144Nd t=1.6 Ga ranges between 0.509752- and 0.510910; εNd t=1.6 Ga ranges from +6.7 to -16, and TDM ages range from 1.51 to 3.29 Ga. The initial 87Sr/86Sr (t=1.6 Ga) ratios varies between 0.705096 and 0.717440. These ranges of εNd (t) and TDM values possibly indicate their derivation from enriched and heterogeneous crustal/lithospheric sources, and minor components from depleted lithospheric sources.
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