1. Introduction
Geoscience researchers have been fascinated by the progressive metamorphic changes in the Eastern Ghats Mobile Belt (EGMB) for many years. Ultrahigh temperature (UHT) metamorphism (900–1100°C, 7–13 kbar) of continental crust has gained recognition as a widespread occurrence in orogenic belts formed along continental collision zones. This phenomenon is particularly evident where subduction-collision tectonics, linked with the amalgamation of supercontinent assemblies, supplied heat and volatiles necessary to stabilize UHT mineral assemblages (Harley Reference Harley1998, Kelsey et al. Reference Kelsey, Clark and Hand2008, Santosh et al. Reference Santosh, Tsunogae, Li and Liu2007, Santosh and Kusky Reference Santosh and Kusky2010).
A comprehensive range of geological, geochronological, tectonic and geophysical investigations (Biswal et al. Reference Biswal, De Waele and Ahuja2007, Biswal and Sinha Reference Biswal and Sinha2004, Bose and Dasgupta Reference Bose and Dasgupta2018, Chetty Reference Chetty2001, Chetty and Murthy Reference Chetty and Murthy1994, Dasgupta et al. Reference Dasgupta, Bose and Das2013, Dobmeier and Raith Reference Dobmeier and Raith2003, Kumar and Leelanandam Reference Kumar and Leelanandam2008, Ramakrishnan et al. Reference Ramakrishnan, Nanda and Augustine1998, Ranjan et al. Reference Ranjan, Upadhyay, Abhinay, Pruseth and Nanda2018, Rickers et al. Reference Rickers, Mezger and Raith2001) provide constraints on the tectono-metamorphic evolution of this area. The various domains and provinces of the Eastern Ghats Belt (EGB) are associated with different metamorphic events. Dispersed mobile belts offer insights into the shifting configurations of continents over time, particularly during the assembly and fragmentation of large continental masses throughout Earth’s history (Condie Reference Condie2005). The occurrence of UHT metamorphism, following an isothermal decompression (ITD) path observed in certain granulite terrains, may result from the presence of exceptionally large continents (Yoshida et al. Reference Yoshida, Jacobs, Santosh and Rajesh2003).
The research site (Junagarh Sector), located in the north-western part of the EGB (Fig. 1a), belongs to the Rampur domain of the Eastern Ghats Province (EGP) (Dobmeier and Raith, Reference Dobmeier and Raith2003). It is distinguished by the presence of high-grade metamorphic rocks and rare mineral assemblage. Multiple metamorphic events have been proposed for the EGB. However, the Rampur domain, positioned centrally, has not been extensively documented. This study aims to explore comprehensively the tectono-metamorphic evolution of the Rampur domain and the broader EGB pressure–temperature–time. This will be supported by petrographic observations, geothermobarometric estimations, phase equilibria modelling combined with textural analysis, bulk-rock compositional analysis via X-ray fluorescence (XRF) and geochronological data obtained from granulites. This study primarily investigates the presence of sapphirine-bearing granulite, characterized by a mineral assemblage that includes sapphirine, spinel, sodic-gedrite, orthopyroxene, calcic-hornblende, plagioclase and biotite. An important aspect of this study is its documentation of various metamorphic stages in the studied granulites, including a unique mineral assemblage featuring sapphirine within an anorthite matrix. Generally, sapphirine in the pelitic assemblage is found within cordierite matrix, whereas the sapphirine in anorthite matrix association is not common in the Indian subcontinent, though, a similar assemblage has been reported from Madagascar and only a few occurrences are reported from Sri Lanka, Greenland, Antarctica and North America (Schumacher and Robinson, Reference Schumacher and Robinson1987; Lund et al. Reference Lund, Piazolo and Harley2006). Moreover, it highlights the rare occurrence of the mineral assemblage sodic-gedrite and calcic-amphibole, which has received comparatively less attention than pelitic granulites in previous studies on the EGB. Gedrite is recognized as a prevalent mineral in Mg-Al-rich granulites that have undergone metamorphism ranging from amphibolite to granulite facies. It has been identified in various magnesium-aluminium-rich rocks from UHT metamorphic terrane (Harley Reference Harley1985, Ouzegane and Boumaza, Reference Ouzegane and Boumaza1996, Dasgupta et al. Reference Dasgupta, Sengupta and Ehl1999). Gedrites in UHT rocks typically have elevated levels of Na2O NaA (greater than 0.5), referred to as papikeite (Hawthorne et al., Reference Hawthorne, Oberti, Harlow, Maresch, Martin, Schumacher and Welch2012). The process of cooling during metamorphism is documented through the observation of plagioclase and biotite symplectite in the petrographic analysis of thin sections. McDonald et al. (2017) documented microtextural evidence of the breakdown of phengite to biotite and plagioclase symplectites during the decompression path, explaining the association between biotite and plagioclase as a result.
Figure 1. (a) Regional geological map showing the major tectonic shear zones, lithological correlations modified after Ramakrishnan et al. (Reference Ramakrishnan, Nanda and Augustine1998) and various isotopic domains identified by (Rickers et al. Reference Rickers, Raith and Dasgupta2001a; Dobmeier and Riath, Reference Dobmeier and Raith2003) of the Eastern Ghats Mobile Belt. Abbreviations used are VSZ – Vamshadhara Shear Zone, MSZ – Mahanadi Shear Zone, NSZ – Nagavalli Shear Zone, KSZ – Koraput-Sonepur Shear Zone, SSZ – Sileru Shear Zone and EGBSZ – Eastern Ghats Boundary Shear Zone; (b) Geological map delineating major litho-units of the study area (Junagarh), Kalahandi, Odisha, India.
The approximated pseudosection constraints have been derived by sequentially removing the sapphirine and spinel compositions from the initial bulk data (used for the estimation of peak metamorphic assemblage) to obtain the bulk composition data for the retrograde assemblage to contemplate the lower P-T assemblage as observed under the petrographic study. The T-X (H2O) pseudosection model has been constrained to understand the role of water in both the peak and retrograde metamorphic assemblages. Another significant aspect emphasized in this study is the monazite dating conducted with the EPMA CAMECA SX-Five instrument. The geochronological data obtained is crucial for establishing the time frame, thereby achieving the primary objective of delineating the pressure-temperature-time (P-T-t) path for the evolution of UHT sapphirine-bearing granulites of the Junagarh sector which has not been well documented to date. Monazite EPMA chemical dating reveals ages spanning from the Neoproterozoic to Mesoproterozoic periods (∼846 Ma to ∼959 Ma) at a moderate pressure of ∼6.5 kbar. Thus, this study aims to provide insights into the processes that governed the evolution and exhumation of rocks from the lower continental crust, contributing to the formation of the Junagarh sector within the EGB.
2. Geological background
The area under investigation lies within the Junagarh Sector of the EGP (Dobmeier and Raith Reference Dobmeier and Raith2003) and is delineated on the Survey of India’s toposheet number 65-M1. This study focuses on a specific region that represents UHT metamorphism, marked by the presence of anorthite- and orthoamphibole-bearing granulites located in the north-western portion of the EGMB (Fig. 1a).
The area under investigation lies within the Junagarh Sector of the EGP (Dobmeier and Raith, Reference Dobmeier and Raith2003) and is delineated on the Survey of India’s toposheet number 65-M/1. This study focuses on a specific region that represents UHT metamorphism, marked by the presence of anorthite- and orthoamphibole-bearing granulites located in the north-western portion of the EGMB (Fig. 1a).
2. a. Regional geology and its relation with the study area
The first zonation of the EGMB, proposed by Nanda and Pati (Reference Nanda and Pati1989), was refined by Ramakrishnan et al. (Reference Ramakrishnan, Nanda and Augustine1998), primarily based on lithological characteristics without incorporating the metamorphic and geochronological histories of the belt. The principal zones include: the Western Charnockite Zone (WCZ), the Western Khondalite Zone (WKZ), the Central Migmatite Zone (CMZ) and the Eastern Khondalite Zone (EKZ). The study area is located in the WKZ (Fig. 1a), which is predominantly composed of mafic granulites, charnockites and enderbites, with sparse occurrences of amphibolites.
An alternative domain-based classification was proposed by Rickers et al. (Reference Rickers, Mezger and Raith2001), relying on isotopic datasets including Sm–Nd and Rb–Sr whole rock data, as well as Pb–Pb isotopic data from feldspars in ortho- and para-gneissic rocks. This model identifies five distinct isotopic domains (1A, 1B, 2, 3 and 4), each characterized by unique protolith ages and tectono-metamorphic evolution. These domains can be broadly correlated with the earlier zonal classifications (Ramakrishnan et al., Reference Ramakrishnan, Nanda and Augustine1998; Chetty and Murthy, Reference Chetty and Murthy1994), based on structural and shear zone associations. However, isotopic data and P-T-t evolutionary paths remain sparse for the north-western segment of the EGMB. Specifically, the Rampur domain lacks data that would integrate it into Rickers et al.’s (Reference Rickers, Mezger and Raith2001) isotopic framework.
Chetty (Reference Chetty2001) and Chetty et al. (Reference Chetty, Vijay, Narayana and Giridhar2003) offered yet another structural classification, dividing the EGMB into nine distinct terranes. This approach emphasizes structural markers such as stretching lineations, shear zones, fold geometries and axial plane orientations formed during early deformation. Within this framework, the Rampur domain constitutes one of several tectonic blocks and is bounded by three significant shear zones: Koraput–Sonepur Shear Zone (KSZ) to the north and west, Vamsadhara Shear Zone (VMS) to the south and Nagavali Shear Zone (NSZ) to the east.
The Eastern Ghats are interpreted as a collage of crustal blocks, each with a unique geological evolution. Dobmeier and Raith (Reference Dobmeier and Raith2003) termed the crustal masses as a ‘Province’ and classified these into four provinces: EGP, Rengali Province, Jeypore Province and Krishna Province. The EGP includes isotope domains 2 and 3 of Rickers et al. (Reference Rickers, Mezger and Raith2001), while the Krishna Province encompasses the Ongole Domain, correlating with isotope domain 1A. Domains 1B and 4 correspond to the Jeypore and Rengali Provinces, respectively.
The Koraput Alkaline Complex in the Jeypore Province was studied by Nanda et al. (Reference Nanda, Gupta and Hacker2018) using U–Pb zircon and titanite geochronology. Their findings revealed a Columbian-age signature (∼1500 Ma) – the earliest magmatic thermal imprint in the Eastern Ghats – predating the Grenvillian UHT metamorphic event (∼1130–930 Ma; Korhonen et al., Reference Korhonen, Clark, Brown, Bhattacharya and Taylor2013). In addition, they identified an unusual zircon growth episode at ∼800 Ma, which may have partially reset the ∼1.0 Ga zircons linked to peak UHT metamorphism, likely redistributing isotope systematics (Mezger and Cosca, Reference Mezger and Cosca1999; Gupta and Bose, Reference Gupta and Bose2004).
In the present study, monazite dating from the Rampur domain in the Junagarh Sector shows ages ranging from 799 Ma to 1196 Ma, capturing a broad spectrum of tectono-metamorphic events. The Koraput–Sonepur intracontinental shear zone demarcates the boundary between the Rampur domain and the adjacent Jeypore Province. Strategically situated in the north-western portion of the EGMB, the Rampur domain presents a critical opportunity to combine geothermobarometry, isotopic data and petrographic analyses. This integration could yield significant insights into the geodynamic evolution of the EGMB and its correlation with regional tectono-thermal events.
2. b. Lithological relationship of the study area
The investigated terrane of the Junagarh sector, trending usually NNE-SSW is characterized basically by the presence of extensive and irregular bands of granulite facies rocks viz., mafic granulite, pelitic granulite, leptynites with substantial occurrences of upper amphibolite facies rocks. The high-grade Mg-Al sapphirine granulites are exposed in multiple localities in the research area. However, establishing its precise boundaries with the different rock types in the terrane is difficult due to very poor continuity of the exposures and widespread soil cover, including agricultural land. An attempt has been made to prepare the geological map of the study area (Fig. 1b). Macroscopically, these granulites are observed as massive bodies with weak foliation, commonly exposed in excavation pits (Fig. 2). The granulites found in the study area are composed mainly of the mineral assemblage sapphirine-spinel-orthopyroxene-biotite-plagioclase-gedrite-hornblende, with their mineralogy, textures and interpretations discussed in the following sections.
Figure 2. (a) Field photograph showing the occurrence of mafic granulite in pits (b) Close view of sampling mafic granulite from a pit near Jilindara area, Junagarh.
3. Textural relations and interpretation
This section of the study interprets the preserved textural relations observed under the petrographic investigation. The occurrence of a unique set of the studied metamorphic assemblage containing sapphirine, spinel, orthopyroxene, gedrite, hornblende, plagioclase and biotite is the first of its kind in the Indian subcontinent. In fact, sapphirine is usually found to occur in cordierite matrix or in association with orthopyroxene and spinel (Osanai et al. Reference Osanai, Hamamoto, Maishima and Kagami1998; Kruckenberg and Whitney Reference Kruckenberg and Whitney2011, Prakash et al. Reference Prakash, Singh, Singh, Tewari, Arima and Frimmel2015, Singh et al. Reference Singh, Prakash, Kharya and Sachan2023). In the current study, sapphirine has been observed occurring within anorthite, analogous to occurrences reported by Raith et al. (Reference Raith, Rakotondrazafy and Sengupta2008), who described this association as a distinct rock suite termed ‘sakenites’ from southern Madagascar. However, the assemblage reported by Raith et al. (Reference Raith, Rakotondrazafy and Sengupta2008) is different from the one reported in the present study in regard to the presence of corundum and the absence of gedrite in the already reported ‘sakenite’ suite of rock.
The textural records of the peak metamorphic stage have been dominantly overprinted by the imprints of post-peak and retrograde stages of metamorphism, as the occurrence of high-grade minerals like sapphirine and orthopyroxene occurs only as relics in the observed thin sections. The transition from amphibolite to granulite facies is noted from the formation of orthopyroxene at the expense of the amphiboles both hornblende and gedrite.xxx. This can be understood from the textural evidence as shown in Fig. 3a and Fig. 3b corresponding to the following metamorphic reaction:
Figure 3. Photomicrographs in plane polarized light) of the representative sample (J-3, J-6) showing textural evidences of prograde and peak stages of UHT metamorphism.
$${\rm{Hornblende}} + {\rm{Gedrite}} = {\rm{Orthopyroxene}} + {\rm{Plagioclase}} + {\rm{Water}} \ldots .$$
$$\eqalign{
& {\rm{C}}{{\rm{a}}_{\rm{2}}}{\rm{M}}{{\rm{g}}_{\rm{4}}}{\rm{A}}{{\rm{l}}_{\rm{2}}}{\rm{S}}{{\rm{i}}_{\rm{8}}}{{\rm{O}}_{{\rm{22}}}}{\left( {{\rm{OH}}} \right)_{\rm{2}}}\,\,\,\,{\rm{M}}{{\rm{g}}_{\rm{5}}}{\rm{A}}{{\rm{l}}_{\rm{4}}}{\rm{S}}{{\rm{i}}_{\rm{6}}}{{\rm{O}}_{{\rm{22}}}}{\left( {{\rm{OH}}} \right)_{\rm{2}}}\,\,\,\,{\rm{5M}}{{\rm{g}}_{\rm{2}}}{\rm{S}}{{\rm{i}}_{\rm{2}}}{{\rm{O}}_{\rm{6}}} \cr
& {\rm{2CaA}}{{\rm{l}}_{\rm{2}}}{\rm{S}}{{\rm{i}}_{\rm{2}}}{{\rm{O}}_{\rm{8}}}\;\;\;{\rm{4}}{{\rm{H}}_{\rm{2}}}{\rm{O}} \cr} $$
Silica (SiO2) is balanced in the above reaction through internal redistribution among minerals. Typically, hornblende and gedrite have lower silica content relative to orthopyroxene and plagioclase. Therefore, the transformation from amphiboles (amphibole and gedrite) to orthopyroxene and plagioclase requires additional silica. This additional silica is usually derived through the breakdown of amphiboles themselves, possibly involving the release or reorganization of silica structurally bound within the amphibole crystal structure. As the studied high-grade rock experiences further extremes of P-T conditions under the granulite facies of metamorphism, the formation of sapphirine from the reaction between low aluminous phases such as orthopyroxene and hornblende coupled with the reaction between spinel and plagioclase takes place (Fig. 3c and Fig. 3d). Spinel or other Al-rich phases serve as aluminium sources. Alternatively, if the reaction is occurring in an Al-rich chemical domain, the breakdown of earlier Al-rich minerals (e.g., spinel or aluminosilicates) releases aluminium into the local chemical environment, making it available for the formation of sapphirine and plagioclase. This stage of metamorphism represents the peak P-T condition attained by the studied assemblage. The probable metamorphic reactions leading to the formation of sapphirine are as below:
$${\rm{Orthopyroxene}} + {\rm{Hornblende}} = {\rm{Sapphirine}} + {\rm{Plagioclase}} + {\rm{Water}}{\rm{.}}$$
$${\rm{3M}}{{\rm{g}}_{\rm{2}}}{\rm{S}}{{\rm{i}}_{\rm{2}}}{{\rm{O}}_{\rm{6}}}\;\;{\rm{C}}{{\rm{a}}_{\rm{2}}}{\rm{M}}{{\rm{g}}_{\rm{4}}}{\rm{A}}{{\rm{l}}_{\rm{2}}}{\rm{S}}{{\rm{i}}_{\rm{8}}}{{\rm{O}}_{{\rm{22}}}}{\left( {{\rm{OH}}} \right)_{\rm{2}}}\;\;{\rm{M}}{{\rm{g}}_{\rm{7}}}{\rm{A}}{{\rm{l}}_{{\rm{12}}}}{\rm{Si}}{{\rm{O}}_{{\rm{24}}}}\;\;{\rm{CaA}}{{\rm{l}}_{\rm{2}}}{\rm{S}}{{\rm{i}}_{\rm{2}}}{{\rm{O}}_{\rm{8}}}{{\rm{H}}_{\rm{2}}}{\rm{O}}$$
$${\rm{Spinel}} + {\rm{Plagioclase}} = {\rm{Sapphirine}} + {\rm{Hornblende}} + {\rm{Gedrite}}$$
$$\eqalign{
& {\rm{4}}\left( {{\rm{MgA}}{{\rm{l}}_{\rm{2}}}{{\rm{O}}_{\rm{4}}}} \right)\;\;\;{\rm{2CaA}}{{\rm{l}}_2}{\left( {{\rm{AlSi}}} \right)_2}{{\rm{O}}_{{\rm{10}}}}\;\;\;{\rm{3}}{\left( {{\rm{Mg}},{\rm{Al}}} \right)_8}{\left( {{\rm{Al}},{\rm{Si}}} \right)_{\rm{6}}}{{\rm{O}}_{{\rm{20}}}}\,\,\, \cr
& {\left( {{\rm{Ca}},{\rm{Na}}} \right)_2}{\left( {{\rm{Mg}},{\rm{Fe}}} \right)_5}{\left( {{\rm{Al}},{\rm{Si}}} \right)_{\rm{8}}}{{\rm{O}}_{{\rm{22}}}}\;\;{\left( {{\rm{Al}},{\rm{Fe}}} \right)_2}{\left( {{\rm{Si}},{\rm{Al}}} \right)_{\rm{4}}}{{\rm{O}}_{{\rm{10}}}}{\left( {{\rm{OH}}} \right)_2} \cr} $$
Now, the initiation of exhumation processes could be responsible for the dissociation of sapphirine in the presence of hornblende to form gedrite and plagioclase at comparatively lower P-T conditions. This step marks the post-peak stage of metamorphism (Fig. 3e and Fig. 3f). The asserted reaction of metamorphism for the dissociation of sapphirine is mentioned below:
$${\rm{Hornblende}} + {\rm{Sapphirine}} = {\rm{Gedrite}} + {\rm{Plagioclase}}$$
$$\eqalign{
& {\rm{C}}{{\rm{a}}_2}{\rm{M}}{{\rm{g}}_{\rm{4}}}{\rm{A}}{{\rm{l}}_{\rm{2}}}{\rm{S}}{{\rm{i}}_{\rm{7}}}{{\rm{O}}_{{\rm{22}}}}{\left( {{\rm{OH}}} \right)_{\rm{2}}}\;\;{\rm{2M}}{{\rm{g}}_{\rm{7}}}{\rm{A}}{{\rm{l}}_{\rm{4}}}{\rm{S}}{{\rm{i}}_{\rm{3}}}{{\rm{O}}_{{\rm{20}}}}\;\;{\rm{3M}}{{\rm{g}}_{\rm{5}}}{\rm{A}}{{\rm{l}}_{\rm{4}}}{\rm{S}}{{\rm{i}}_{\rm{6}}}{{\rm{O}}_{{\rm{22}}}}{\left( {{\rm{OH}}} \right)_{\rm{2}}} \cr
& {\rm{CaA}}{{\rm{l}}_{\rm{2}}}{\rm{S}}{{\rm{i}}_{\rm{2}}}{{\rm{O}}_{\rm{8}}} \cr} $$
Fig. 4a and Fig. 4b show the photomicrographs (both in ppl and xpl, respectively,) of the unique occurrence of the arrested relicts of sapphirine within plagioclase melt possibly during later stages of metamorphism. Such a rare occurrence of sapphirine within plagioclase is significant in understanding the UHT metamorphic conditions of the mafic granulites and is being reported for the first time through this communication. Fig. 4c represents the occurrence of sapphirine within plagioclase under the BSE image while the data obtained from the graph shown in Fig. 4d confirms the presence of sapphirine within the plagioclase matrix.
Figure 4. Photomicrographs in plane polarized light) of the representative sample (J-3, J-6) showing the rare occurrence of sapphirine within anorthite matrix along with the textural evidences of retrograde and symplectite stages of UHT metamorphism.
During the retrograde stage of metamorphism, the phases such as hornblende and orthopyroxene become unstable and form gedrite, biotite and plagioclase. Hornblende is a mineral that exhibits stability over a broad spectrum of metamorphic conditions and only destabilizes during pronounced retrograde metamorphism at lower pressures and temperatures, transitioning into phases such as gedrite, biotite and plagioclase. Later, as the rocks are further cooled and uplifted, the formation of symplectites of biotite and plagioclase is evident (Fig. 4e and Fig. 4f). The ascribed metamorphic reaction showing the retrograde and cooling stage is given below:
$$\eqalign{
& {\rm{Hornblende}} + {\rm{Orthopyroxene}} + {\rm{K - Feldspar}} = \cr
& {\rm{2C}}{{\rm{a}}_{\rm{2}}}{\rm{M}}{{\rm{g}}_{\rm{4}}}{\rm{Al}}\left( {{\rm{S}}{{\rm{i}}_{\rm{7}}}{\rm{Al}}} \right){{\rm{O}}_{{\rm{22}}}}{\left( {{\rm{OH}}} \right)_{\rm{2}}}\;\;\;{\rm{2M}}{{\rm{g}}_{\rm{2}}}{\rm{S}}{{\rm{i}}_{\rm{2}}}{{\rm{O}}_{\rm{6}}}\;\;\;{\rm{KAlS}}{{\rm{i}}_{\rm{3}}}{{\rm{O}}_{\rm{8}}} \cr
& {\rm{Gedrite}} + {\rm{Biotite}} + {\rm{Plagioclase}} + {\rm{Water}} \cr
& {\rm{M}}{{\rm{g}}_{\rm{5}}}{\rm{A}}{{\rm{l}}_{\rm{4}}}{\rm{S}}{{\rm{i}}_{\rm{6}}}{{\rm{O}}_{{\rm{22}}}}{\left( {{\rm{OH}}} \right)_{\rm{2}}}\;\;{\rm{KM}}{{\rm{g}}_{\rm{3}}}{\rm{AlS}}{{\rm{i}}_{\rm{3}}}{{\rm{O}}_{{\rm{10}}}}{\left( {{\rm{OH}}} \right)_{\rm{2}}}\;\;{\rm{2CaA}}{{\rm{l}}_{\rm{2}}}{\rm{S}}{{\rm{i}}_{\rm{2}}}{{\rm{O}}_{\rm{8}}}\;\;{{\rm{H}}_{\rm{2}}}{\rm{O}} \cr} $$
In most geological contexts, the dominant mechanism is fluid-mediated metasomatism (metamorphic fluid influx), or the breakdown of pre-existing potassium-rich minerals such as K-feldspar. Thus, the main source of potassium for biotite formation in the above reaction is generally an external source, typically through metasomatic fluids or the breakdown of existing K-bearing minerals.
4. Mineral chemistry
The studied minerals present in the assemblage of sapphirine-bearing granulites have been studied and characterized based on the oxide, elemental and XMg compositional values procured from the examination of thin sections of the representative samples under the CAMECA SX-Five Electron Probe Micro Analyzer (EPMA) instrumentation facility available at the Department of Geology, Institute of Science, Banaras Hindu University, Varanasi, India. The standards adopted to obtain the mineral chemistry of the minerals preserved in textural relations during the peak as well as the retrograde stages of metamorphism are as given:
Wavelength dispersive spectrometry, LaB6 filament, TAP, PET, LIF, LPET and LTAP crystals were used at a voltage of 15 kV and current 10nA with a beam diameter of 1 µm for the analysis. Different natural and synthetic standards were used for calibration. The following X-ray lines were used for oxide analysis: Al-Kα, Mg-Kα, Ba-Lα, Cl-Kα, P-Kα, K-Kα, Ca-Kα, Ti-Kα, Cr-Kα, Cu-Kα, Mn-Kα, Si-Kα, Na-Kα, Fe-Kα and F-Kα. Natural mineral standards: fluorite, albite, halite, periclase, peridote, corundum, wollastonite, apatite, pyrite, orthoclase, rutile, chromite, rhodonite, hematite and barite; pure metal standard: Ni and synthetic glass standard YAG supplied by CAMECA-AMETEK were used for routine calibration, X-ray elemental mapping and quantification (Pandey et al. Reference Pandey, Pandit, Arora, Chalapathi Rao and Pant2019; Chakravarti et al. Reference Chakravarti, Singh, Venkatesh, Patel and Sahoo2018).
This section of the study presents the range of essential chemical compositions of the mineral assemblage that is crucial in delineating the existing metamorphic grade with respect to the minerals identified in different textural relations under the petrographic examination. Table 1 represents the summary of the representative EPMA data for different minerals present in the assemblage.
Table 1. Representative microprobe analyzes data of various minerals present in the Mg-Al assemblage
XMg = Mg /(Mg+Fe2+); C = Core; R = Rim; NR = Near Rim; S = Symplectite.
XMg = Mg /(Mg+Fe2+); XAn = Ca /(K+Na+Ca); C = Core; R = Rim; NR = Near Rim; S = Symplectite.
4. a. Sapphirine
The mineral Sapphirine, calculated on a 10-oxygen basis, present in the assemblage is highly aluminous with its Al2O3 contents ranging from 62.36 to 64.59 wt%. High MgO content ranging from 17.22 to 18.34 wt% contributes to the designated sapphirine-bearing Mg-Al granulites. The XMg values of core and rim data have negligible variation from 0.885 to 0.870. The major highlight of this communication being the occurrence of sapphirine within the plagioclase matrix, is discussed in detail in the following sections of this study. The ternary plot of sapphirine shows high alumina content with significant amounts of magnesian and silica (Fig. 5a).
Figure 5. (a) Ternary plot representing the composition of sapphirine; (b) Ternary plot representing Mg-rich compositions of spinel; (c) Ternary plot representing Mg-rich compositions of pyroxene; (d) Ternary plot of biotite in Al-Mg-Fe diagram; (e) Ternary Or–Ab–An plot representing anorthitic contents of feldspar; (f) Quaternary plot of orthoamphibole between Mg/Mg+Fe+2 and Si representing the compositions of gedrite; (g) Quaternary plot of Mg/Mg+Fe+2 vs Si of calcic amphibole showing Tchermakite CaB ≤1.50, (Na+K)A < 0.50; Ti < 0.50 and Paragsite CaB ≥1.50, (Na+K)A > 0.50; Ti > 0.50 compositions.
4. b. Spinel
The spinel present is calculated on a 4-oxygen basis and is basically of the composition showing solid solution between MgAl2O4-FeAl2O4 with its XMg values ranging from 0.655 to 0.663 which is lesser than the coexisting XMg values of hornblende, gedrite, sapphirine, orthopyroxene, biotite and plagioclase. The ternary plot of spinel suggests a considerable solid solution between spinel and hercynite, however, the data plots towards the field of Mg-spinel (Fig. 5b).
4. c. Orthopyroxene
The mineral orthopyroxene, calculated on a 6-oxygen basis, has high MgO contents ranging from 27.12 to 28.94 wt% showing slight XMg compositional variation from 0.788 to 0.813 (from core of two different grains). The compositional data of orthopyroxene is typical of initial concentrations that should be present as the high-grade rock enters the granulite facies of metamorphism and is stable up to UHT metamorphic condition with negligible variation in its structural formula. The ternary plot for orthopyroxene shows a slight solid solution between enstatite and ferrosilite and the concentration of the plotted data reflects the presence of Mg-rich hypersthene in the assemblage (Fig. 5c).
4. d. Biotite
The studied Mg-Al granulite containing biotite, calculated on a 22-oxygen basis, has low TiO2 contents ranging from 2.34 to 2.83 wt% corresponding to the retrograde stage of metamorphism. Thus, the main source of potassium for biotite formation is generally an external source, typically through metasomatic fluids or the breakdown of existing K-feldspar. The presence of biotite essentially occurs during the hydration stage of metamorphism and shows symplectitic texture with gedrite and plagioclase. The MgO and Al2O3 contents are considerably on the higher side with their values ranging between 19.98 to 21.69 wt% and 17.15 to 17.76 wt%, respectively. The obtained XMg values for biotite vary slightly between 0.849 and 0.890. From the ternary plot of biotite, the data is concentrated at the boundary of eastonite and biotite (Fig. 5d).
4. e. Plagioclase
Plagioclase present is calculated on a 8-oxygen basis and has been divided into two types based on their anorthitic contents. The plagioclase present during the peak stage of metamorphism in the matrix (surrounding sapphirine) shows lower anorthitic content with their CaO values ranging from 17.56 to 17.85 wt% resulting in XAn values in the range of 0.854 to 0.874. On the other hand, the plagioclase present in the retrograde stage forming symplectites in association with gedrite and biotite has relatively higher anorthitic content with their CaO values ranging from 18.27 to 18.51 wt% resulting in XAn values in the range of 0.895 to 0.902. In the ternary plot of feldspar, it is evident that the plagioclase present in the assemblage is essentially anorthite/bytownite (Fig. 5e).
4. f. Sodic-gedrite
The mineral gedrite (an orthoamphibole) present in the studied assemblage is calculated on a 23-oxygen basis containing a negligible amount of CaO content, unlike the hornblende present in its association. However, it is rich in Al2O3 contents (18.60–20.61 wt%) and even richer in its MgO contents (21.02–21.72 wt%). The gedrite in the studied assemblage is found to be associated with hornblende and sapphirine present in the plagioclase matrix. The XMg value of gedrite varies negligibly from 0.79 to 0.82. The gedrite contains a significant amount of Na2O content ranging between 1.88 wt% and 2.76 wt%.
The higher Na2O contents (>2.5 wt%) of gedrite are observed to be present in the peak metamorphic assemblage of the UHT metamorphism (Tsunogae et al. Reference Tsunogae, Santosh and Shimpo2007), while the lower Na2O contents (<2.0 wt%) are found to be present in the gedrite of the retrograde metamorphic assemblage. The analyzed compositions of the gedrite present in the assemblage are shown in Fig. 6, coupled with its representative composition in silica-deficient rocks of the granulite facies from different localities. Fig. 6a shows that the NaA contents in all the samples of gedrites are found to increase with the increasing values of AlIV Robinson et al. Reference Robinson, Ross and Jaefe1971. The sum of NaA and AlIV linearly increases with decreasing values of Si (Fig. 6b) due to NaAAlIV ↔ Si substitution. The obtained negative correlation between Fe+Mg+Si and Al from Fig. 6c could probably be asserted to Tschermak substitution (Fe,Mg)VISi ↔ AlVIAlIV It is observed that the gedrite is slightly less hydrous than the associated hornblende phase. The quadrilateral plot of orthoamphibole reflects the compositional parameter of the mineral gedrite (Fig. 5f).
Figure 6. Compositional diagrams illustrating chemistry of gedrite through the correlation between compositional data of Na-bearing gedrite in silica-deficient granulite-facies rocks from the present study with the reports from Eastern Ghats (Dasgupta et al. Reference Dasgupta, Sengupta and Ehl1999), In Ouzegane and Boumaza (Reference Ouzegane and Boumaza1996) and Karur Koshimoto et al. (Reference Koshimoto, Tsunogae and Santosh2004) are shown. (a) AlIV vs. NaA diagram, (b) NaA + AlIV vs. Si diagram, (c) Fe+Mg + Si and Al diagram.
4. g. Calcium-amphibole
The hornblende present in the Mg-Al granulite is significantly rich in its CaO content (11.57 – 11.83 wt%), and hence, it is designated as calcium amphibole (Hawthorne et al. Reference Hawthorne, Oberti, Harlow, Maresch, Martin, Schumacher and Welch2012). The high calcic content of hornblende is, therefore, utilized to balance the calcic contents of the plagioclase sustaining equilibrium metamorphic reactions during different stages of metamorphism in the formation of two generations of plagioclase as discussed above. The XMg values of hornblende show higher values with slight variation from 0.833 to 0.934. The quadrilateral plots for amphibole show two varieties of the mineral, viz. Tchermakite and pargasite, based on their calcic contents (Fig. 5g).
5. Geothermobarometric Estimations and P-T Path
There are various methods to estimate the average P-T condition such as THERMOCALC and pseudosection modelling using Perple_X. All of these allow us to determine the extent of metamorphism using various thermodynamic equations.
5. a. P-T estimation using THERMOCALC program
Average P-T condition calculations are based on the method of Powell and Holland (Reference Powell and Holland1994) and the updated version of their internally consistent thermodynamic dataset (Holland and Powell Reference Holland and Powell1998) using the THERMOCALC program version 3.21. Mafic granulite containing the assemblage of sapphirine, orthopyroxene, spinel, hornblende and anorthite all of which were stable during peak metamorphism. The end member phases used in the THERMOCALC composition were spr4, spr7, spinel, hercynite, phlogopite, annite, enstatite, mg-tschermak ferrosilite, anorthite, eastonite quartz and H2O. Several independent reactions can be written for the selected end-member phases. All possible independent equilibria used for the P–T estimate of the peak assemblage of mafic granulite are given in Table 2. Samples J-3 and J-6 give precise and synchronous results using the THERMOCALC program, suggesting a near thermal peak condition of granulite facies metamorphism, nearly 925 OC and 8 kbar (Table 2). Gedrite–biotite symplectites were used for the P-T estimation of symplectite formation. The phases involved include spr4, spr7, enstatite, ferrosilite, mg-tschermak, enstatite, phlogopite, annite, eastonite, quartz and H2O. The intersection of three independent reactions in P-T space (Table 2) indicates symplectite formation conditions during decompression and cooling at P-T condition of 6.6 kbar and 896 OC (Table 2).
Table 2. Pressure-temperature results using THERMOCALC program
5. b. P-T estimation using phase equilibria modelling
5. b.1. Peak stage of metamorphism
The constrained isochemical phase equilibrium model in Fig. 7a depicts the possible mineral assemblages across the P-T space (from 4 to 9 kbar and 800 to 1200 °C) for the XRF bulk composition of the studied Mg-Al rich granulites Na2O – 3.26, MgO – 19.38, Al2O3 – 20.26, SiO2 – 40.30, K2O – 0.06, CaO – 4.33, TiO2 – 1.26, FeO – 8.47, O2 – 0.70, LOI – 1.97 in wt%. Pseudosection modelling is used to understand the P-T history of evolution, especially focusing on the deduced ITD path coupled with the assemblages corresponding to the peak and retrograde stages of metamorphism. The utilized bulk composition XRF data was obtained by analyzing the representative sample from the Central Discovery Centre, Banaras Hindu University, Varanasi, India.
Figure 7. (a) P-T pseudosection model of the studied mafic granulite (in NCKFMASHTO system) showing the stability fields of different equilibrium phase assemblages using bulk-rock composition (in wt%) Na2O – 3.26, MgO – 19.38, Al2O3 – 20.26, SiO2 – 40.36, K2O – 0.06, CaO – 4.33, TiO2 – 1.26, FeO – 8.47, O2 – 0.70 and LOI – 1.97; (b) The P-T pseudosection model showing the intersection of plotted XMg isopleth contours denoting the peak stage of metamorphism; (c) P-T pseudosection model illustrating the growth and consumption (yellow arrows) of major mineral assemblages with changing pressure and temperature conditions; (d) T-X (H2O) pseudosection of the studied granulite calculated at 6 kbar; (e) Pseudosection model illustrating T-X diagram which shows sequential removal of sapphirine and spinel phases.
The trajectory derived from the prograde and peak mineral assemblages indicates that the maximum pressure (P) was reached before the maximum temperature (T) along its path for the sapphirine-bearing granulite. The proposed phase equilibrium model is created using the command-line-based Perple_X software (version 6.9.1), which utilizes Gibbs energy minimizations (Connolly Reference Connolly1990, Reference Connolly2005, Reference Connolly2009; Connolly and Petrini Reference Connolly and Petrini2002) and employs a varied range of thermodynamic datasets that are internally consistent Holland and Powell (Reference Holland and Powell1998 and Reference Holland and Powell2011). The proposed P-T stability fields are approximated in the range of 4 to 8 kbar and 800 to 1200 °C for the NCKFMASHTO Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3) system.
The hp04ver.dat thermodynamic data file of Holland and Powell (Reference Holland and Powell1998) was chosen as default for constraining the P-T phase equilibria diagram using the recent solution models of governing phases present in the petrographic assemblage. These models include melt (HP) for melt phases (Holland and Powell Reference Holland and Powell1996), Sp(WPC) for spinel (White et al. Reference White, Powell and Clarke2002), Sa(WP) for sapphirine (Wheller and Powell Reference Wheller and Powell2014), Ilm (WPH) for ilmenite (White et al. Reference White, Powell, Holland and Worley2000), Bio(TCC) for biotite (Tajcmanova et al. Reference Tajčmanová, Konopásek and Connolly2007), oAmph(DP) for gedrite (Diener et al. 2011), cAmph(DP) for hornblende (Diener et al. 2011), Opx(HP) for orthopyroxene (Holland and Powell Reference Holland and Powell1996) and feldspar for plagioclase (Fuhrman and Lindsley Reference Fuhrman and Lindsley1988). The proposed pseudosection model therefore depicts the stability fields of different equilibrium mineral assemblages possible at suitable P-T conditions. The related pure phases and their end members were incorporated in the ‘phase list’ of the Perple_X program (version 6.9.1) based on petrographic study and their respective confirmation through EPM analysis. The constrained equilibrium phase diagram is further overlapped with isopleth contours (XMg = Mg/Mg+Fe values) of the existing dominant mineral phases, such as XMg spinel, XMg sapphirine, XMg hornblende and XMg orthopyroxene.
The peak metamorphic assemblage as deduced from the XMg of the plotted isopleth contours (XMg spinel, XMg hornblende, XMg sapphirine and XMg orthopyroxene) and their intersection in the stability field containing the related assemblage as observed in the petrographic study, suggests the stability field of the assemblage feldspar-melt-spinel-sapphirine-ilmenite-hornblende-orthopyroxene, corresponding to a pressure of about 5.5 kbar and a temperature of about 995 °C (Fig. 7b).
The modal proportions of minerals present in the assemblage change in the P-T space in the constrained mode diagram of the pseudosection model (Fig. 7c). The mode diagram is related to the stability fields of the peak assemblage in such a way that the growth and consumption of minerals with respect to varying P-T conditions can be understood and verified. The modal proportion of sapphirine starts to appear from the peak zonal assemblage and increases further at higher P-T conditions. For hornblende, a significant modal proportion is visible at higher pressures and grows effectively towards higher P-T values. The modal proportions of spinel increase significantly with rising temperatures, while the modal proportions for orthopyroxene increase steeply with rising pressure values.
5.b.2. Peak T-X (H2O) pseudosection modelling
Fig. 7d represents the constrained T-X (H2O) (fixed at 6 kbar) pseudosection model, which reveals the H2O concentration present during the peak stage of UHT metamorphism in the assemblage feldspar, melt, spinel, sapphirine, ilmenite, hornblende and orthopyroxene. Here, hornblende (a clino-amphibole) is the only hydrous phase that accounts for the H2O content in the peak metamorphic assemblage. From the isomode plot of the constrained pseudosection model (Fig. 7c), the modal proportion of hornblende falling in the peak stability field of the studied assemblage corresponds to the value 73.12, which is in sound correlation with the hornblende isomode plotted in the T-X (H2O) diagram (Fig. 7d) corresponding to modal proportion value of 72.90. Now, at the derived peak thermal condition 995 °C in the pseudosection diagram (Fig. 7b), the correlated hornblende T-X (H2O) isomodes (Fig. 7d) in the representative peak assemblage at 995 °C reveal a very low value of XH2O corresponding to 0.100. The low H2O value present during the peak metamorphic stage further complements the attainment of UHT metamorphism of the studied assemblage.
5.b.3. Retrograde stage of metamorphism
The analyzed bulk-composition XRF data was further recalibrated to account for the retrograde assemblage from the textural observations in the same representative rock sample. This recalibration of the XRF data was done to make it feasible to plot the retrograde assemblage around the favourable P-T condition revealed by conventional methods of geothermobarometric estimation. Another factor that accounts for the recalibration is that using the initially obtained bulk-composition XRF data, the retrograde P-T stability field containing the observed textural relation in the petrographic records could not be plotted unless the sapphirine and spinel data were subtracted to form retrograde bulk composition as shown in the approximated T-Xsapphirine diagram (Fig. 7e) where the composition of sapphirine and spinel is sequentially removed till they disappear from the stability field of the retrograde assemblage.
After recalibrating the bulk-composition data, suitable phase equilibria modelling has been done to reveal the stability field of the retrograde stage of metamorphism as shown in Fig 8a. The observed phases in the stability field of the retrograde assemblage are feldspar, ilmenite, biotite, gedrite, hornblende, rutile and H2O. Through the intersection of XMg isopleth contours of the dominant retrograde phases (biotite and gedrite), it can be inferred that the retrograde assemblage was stable around 775 °C at 2.2 kbar (Fig. 8b). The modal proportions of these dominant retrograde phases (biotite and gedrite), it is observed that gedrite and biotite are consumed as the temperature rises (Fig. 8c).
Figure 8. (a) P-T pseudosection model representing the retrograde assemblage of metamorphism among the different stability field of the equilibrium phase assemblages using recalibrated bulk-rock composition of the mafic granulite from Rampur Domain of the Eastern Ghats Province; (b) The P-T pseudosection model showing the intersection of plotted XMg isopleth contours during retrograde stage of metamorphism; (c) P-T pseudosection model illustrating the growth of biotite and consumption of ortho-amphibole (shown with yellow arrows) with changing pressure and temperature conditions; (d) T-X (H2O) pseudosection of the studied granulite calculated at 2 kbar.
5.b.4. Retrograde T-X (H2O) pseudosection modelling
Fig. 8d represents the constrained T-X (H2O) (fixed at 2 kbar) pseudosection model, which reveals the H2O concentration present during the retrograde stage of metamorphism in the assemblage feldspar, ilmenite, biotite, gedrite, hornblende, rutile and H2O. Here, the entire stability field of the retrograde assemblage falls within a narrow range in the P-T space as encircled by the dashed lines. However, irrespective of the stability field being narrow, the retrograde stage holds ample amount of H2O content with its XH2O values ranging between 0.95 and 1.00, that is the assemblage is almost saturated with H2O contents to its possible limit due to the presence of high hydrous biotite and amphibole phases along with the remaining amount of H2O itself that is required to saturate the assemblage at 775 °C and 2 kbar of temperature and pressure. From the inferences drawn with the T-X (H2O) diagram of the retrograde stage, it is evident that the H2O phase played a significant role in altering the textural records of mineral assemblage that once experienced UHT metamorphic condition. Although the exact source of H2O could not be precisely predicted at this stage, it could be concluded that an external water source must have existed in forming the excess amount of biotite and amphibole that has led to a noticeable drop in the temperature from their initial thermal maximum.
5.c. P-T path of evolution
The P-T trajectory traversed by the granulites of the investigated terrane is shown in Fig.9. Here, the path traversed during the exhumation of the studied UHT granulite suggests a decompression path as a result of which significant drop in both the temperature and pressure are observed from 995 °C to 775 °C of temperature and 5.5 to 2 kbars of pressure. It can be inferred that the P-T path followed is in a clockwise direction hence attributing to thrusting and crustal thickening processes during their exhumation and subsequent uplift.
Figure 9. The obtained P-T path suggesting decompression path of the studied granulites resulting in slow rate of exhumation.
6. Interpretation of monazite EPMA (U-Th-Pb) chemical ages
Monazite, known for its high thermal stability, is widely employed as a dependable method for dating, especially in the examination of the intricate geological evolution of metamorphic rocks (Crowley and Ghent Reference Crowley and Ghent1999). It aids in determining the timing of metamorphic events and structural deformations experienced by rocks within a specific geographic region (Williams et al. Reference Williams, Jercinovic and Terry1999). A total of 37 texturally controlled data points (Table 3) from two representative granulite samples (sample no J-3, sample no. J-6) of the studied terrane were subjected to chemical dating using electron microprobe geochronological technique.
Table 3. U-Th-Pb monazite chemical data of the representative granulite from Junagarh sector, EGB
** Same data point number for second phase of analysis of new points.
The monoclinic crystal system of monazite permits a limited degree of substitution with minerals possessing similar structures, such as cheralite where Th4+ + Ca2+ can substitute for 2REE3+ (Rose Reference Rose1980). In the area under investigation, the monazite mineral examined contains a significant amount of substituted calcium, indicating the potential for cheralite substitution. Interestingly, the considerable presence of silicon dioxide, which signifies substitution with another common isostructural mineral, huttonite where Th4+ + Si4+ can substitute for LREE3+ + P5+ (Pabst and Hutton, 1951), is partially substituted in most of the monazite grains imparting brighter zones (Hokada and Motoyoshi Reference Hokada and Motoyoshi2006) in the Back Scattered Electron (BSE) images (Fig.10).
Figure 10. BSE Image showing the points from where the data has been taken for EPM chemical dating of the samples J-3 and J-6 of Junagarh, Kalahandi, Odisha.
The dating of the monazite mineral was conducted by analyzing the prominent mineral grains utilizing the EPMA CAMECA_SX-Five instrument facility at the Department of Geology, Institute of Science, Banaras Hindu University. Before conducting the analysis, two representative samples were prepared by applying a thin layer of carbon 20 nm) using the LEICA-EM ACE 200 carbon coating instrument. The analysis was carried out using an electron gun with a LaB6 source, operating at an operational voltage of 15 kV and a supplied current of 200nA to generate an electron beam. The analysis followed the analytical protocol for U-Th-Pb monazite dating as described by Pandey et al. ( Reference Pandey, Pandit, Arora, Chalapathi Rao and Pant 2019 ). The chemical ages were synthesized using Geo-standards derived from the Alok_Std of Progress Granite from Antarctica. This granite has been estimated to be between 470 and 553 million years old by Carson et al. ( Reference Carson, Fanning and Wilson 1996 ). The standard run checks in Table 3 revealed an age of 477 million years for this standard granite as shown in Table 3.
The results from conducting the established procedures for systematic EPMA dating indicate a clear range of ages falling within the Neoproterozoic-Mesoproterozoic period. Specifically, the ages obtained range from 790 million years to 1104 million years, as detailed in Table 3. About 25 of the 37 points that were analyzed in the representative samples (J-3 and J-6) produced an age greater than 900 Ma. Likewise, 12 of the 37 total points that were examined produced an age greater than 900 Ma. These geochronological findings fall within acceptable margins allowing for the calculation of average ages for distinct thermal events affecting the analyzed assemblage with minimal error. The BGC-ISOPLOT_3.75 software is utilized to generate the weighted average distribution (Fig. 11a and Fig. 11b), linearized probability (Fig. 11c and Fig. 11d) and probability density (Fig. 11e and Fig. 11f) plots using an unmixing of data technique implemented through the BGC-ISOPLOT_3.75 program (Ludwig, Reference Ludwig2003). These visualizations aid in narrowing down the likely age of the metamorphic event, while providing a graphical representation of all calculated ages, considering their associated error margins. The plotted linearized probability plot supplements the analyzed data as all the age data with their error margins for the younger event and almost all the age data with their error margins for the older event lie on the probability conformity line of 95%.
Figure 11. (a, c, e) Representative graphs of weighted average, linearized probability plot and probability density plot of the distributions yielding the average age of 846 Ma corresponding to the later and relatively less extreme metamorphic event in the study area, obtained using BGC-ISOPLOT_3.75 program; (b, d, f) Representative graphs of weighted average, linearized probability plot and probability density plot of the distributions yielding the older average age of 959 Ma corresponding to the extreme thermal metamorphic event in the study area, obtained using BGC-ISOPLOT_3.75 program.
From the obtained monazite chemical ages, two different monazite ages have been identified in the studied granulite. The obtained ages for the two sets of monazite chemical ages are represented in Fig. 11. The representative ages for the different set of ages suggest the metamorphic events at around 846 ±74 Ma and 959 ±48 Ma. The pair of ages having a difference of about 113 Ma could be attributed to two different thermal activities during the Neoproterozoic-Mesoproterozoic periods as evidenced by the zoning produced in the analyzed monazite grains (Fig. 10). The observed concentric and sector zoning present in the monazite may have possibly resulted due the derived thermal events in the investigated terrane. The older thermal event corresponding to thermal activity must have been the probable episode for the attainment of the extreme thermal condition of UHT metamorphism. The validation for the older extreme thermal event comes from the textural implications signifying the older dates from the monazite being associated with the peak assemblage while, the later, less extreme thermal activity is observed along the rims of the monazite present in the matrix (Fig. 10). Another important aspect that caused the revealed thermal maxima for the older metamorphic event may be due to the reported emplacement of the repeated influx of anorthosite magmatism (Krause et al. Reference Krause, Dobmeier, Raith and Mezger2001; Dobmeier, Reference Dobmeier2006; Rao et al. Reference Rao, Santosh and Zhang2014) in and around the study area for more than ∼100 Ma of time span.
7. Discussion and conclusions
In this section of the present study, the inferred results from petrographic observations, mineral chemistry, pseudosection modelling and geochronological constraints have been correlated and discussed in detail to sustain a geodynamic evolution of the investigated UHT granulite from the EGB.
7. a. Occurrence of sapphirine within plagioclase
The important findings from the studied segment of Rampur domain (North-Western domain of EGB are summarized in this section which includes the occurrence of rare mineral assemblage viz., sodic-gedrite, calcic-amphibole, biotite and spinel along with sapphirine within plagioclase matrix. The significant thing about the observed assemblage is the existence of sapphirine within plagioclase matrix which is being reported for the first time from the Indian sub-continent and is the second report worldwide only after the report of Raith et al. (Reference Raith, Rakotondrazafy and Sengupta2008), where they reported a unique association of corundum-, spinel- and sapphirine-bearing anorthitic to phlogopitic rock and designated it as ‘Sakenites’. The name ‘Sakenite’ seems to have originated from the type locality of Sakena River NW of Ihosy southern Madagascar region where it was discovered. Another peculiar similarity (or disparity you can say) in the studied granulite from Sakenites is the absence of corundum and the presence of gedrite and orthopyroxene, other than which the textural records of sapphirine within plagioclase are almost similar in the compared high-grade rocks. However, P-T estimation of both the rocks differ in in their temperature ranges where the granulite in the present study suggests UHT metamorphism, while the Sakenites have only attained the granulite facies of metamorphism. However, the pressure ranges of both the studied UHT granulite and Sakenite are relatable at a moderate pressure of 6 to 7 kbars.
The relationship between the studied UHT granulites of the Rampur domain from the EGB and the Sakenites from Madagascar is being established only to understand the nature of their evolution and characteristic patterns between their similar mineral assemblage. In terms of paleo-reconstruction of the Eastern Gondwana (Collins and Pisarevsky Reference Collins and Pisarevsky2005, and Singh et al. Reference Singh, Prakash, Kharya and Sachan2023) during the Meso-Neoproterozoic times, it is evident that the present study area of the EGB and the type locality of Sakenites in Madagascar lie along opposite boundaries of the Indian peninsula. As such, it may be not easy to correlate their metamorphic evolution in terms of synchronous tectonic activities but none the less, this correlation leads to the understanding that both the compared granulites could have probably evolved from the Archean anorthosites during their regional shortening (Lacroix Reference Lacroix1941, Rakotondrazafy Raith Reference Rakotondrazafy and Raith1997, Krause 2001, Dobmeier Reference Dobmeier2006).
7. b. Inferences from petrographic study and phase equilibria modelling
Unlike the usual encounter of UHT granulite consisting sapphirine within the cordierite matrix or its association with orthopyroxene and garnet (Prakash et al. Reference Prakash, Yadav, Tewari, Frimmel, Koglin, Sachan and Yadav2018, Reference Prakash, Vishal, Naik, Yadav, Rai, Tewari and Pattnaik2019 and Singh et al. Reference Singh, Prakash, Singh, Srivastava, Yadav, Singh and Kumar2022, Reference Singh, Prakash, Kharya and Sachan2023), the present study reports the occurrence of sapphirine within plagioclase matrix in the gedrite and hornblende dominated UHT assemblage. So far in this study, we have related the studied granulites with the magmatism and emplacement of anorthosites. Now, we shall see the correlation brought up from the petrography and pseudosection modelling of the UHT granulite from the investigated terrane.
While constraining the pseudosection model, it was found that both the P-T conditions of the peak and retrograde stages of metamorphism were not been able to plot in the constrained wide range of P-T space using the obtained XRF bulk composition. Such incompatibility arises due to the major transition in the dominant phase in the overall phase assemblages. One such noticeable phase is the sodic-gedrite which is dominantly present in the peak (>2.5 wt%) and post-peak (<2 wt %) stages of assemblage, undergoing significant variation in the P-T space. The characteristic composition of sodic-gedrite present in the assemblage is detailed above under the mineral chemistry section of this communication. Here, we shall discuss further implications of sodic-gedrite in the observed UHT assemblage. The high-Na contents in gedrite as reported in this study are reported from hydrothermal veins in granite and meta-dolerite from non-UHT terrains (Otten Reference Otten1984 and Damman Reference Damman1989). Considering this aspect along with the correlated anorthosites, a probable igneous protolith is suggested that underwent extreme thermal metamorphism. A similar high-Na gedrite is reported from the Napier Complex of Eastern Antarctica (Harley Reference Harley1985), where he inferred the sodic-gedrite as a product of later shearing. Therefore, the well-established connection between the Greater Indian Landmass and Eastern Antarctica is supplemented by this work through the examination of a hitherto unknown UHT locality comprising rare mineral assemblages of Na-rich gedrite in association with sapphirine within plagioclase matrix.
The thermodynamic calculations of gedrite, estimated by Ouzegane and Boumaza (Reference Ouzegane and Boumaza1996) suggesting 850–900 °C at 7–9 kbar, can be considered as a high-grade phase in the studied Mg-Al rich UHT granulite. In the present case, the approximated P-T condition for the Na-rich gedrite-bearing UHT assemblage corresponds to about 995 °C at 5.5–6 kbar for the peak metamorphic stage and 775 °C at 2.2 kbar for the retrograde stage of metamorphism. The significant drop in both pressure and temperature showing the decompression path, which signifies a slow rate of unroofing, of the clockwise P-T trajectory that may be the result of crustal shortening due to suturing and thrusting processes associated with collisional tectonic activities. As the decompression proceeds (due slow rate of exhumation), only the element of heat accumulated due to stress is lost in the documented process of crustal thickening resulting from clockwise P-T path while the other heat-inducing factors such as an abundance of radioactive elements and magmatic underplating still prevail and were also responsible for the UHT conditions in the peak stage of metamorphism (Brown Reference Brown2003, Harley, Reference Harley2008, and Clark et al. Reference Clark, Fitzsimons, Healy and Harley2011).
During the retrograde stage, the formation of symplectites of mainly gedrite along with biotite and plagioclase dominates the observed textual relations as discussed earlier under the ‘textural relations and interpretation’ section. The dominant symplectites of gedrite and biotite in the petrographic phase relation potentially provide a wealth of knowledge in decoding the metamorphic history of the studied metamorphic assemblage. The observed vermicular intergrowth of biotite and gedrite with plagioclase represents the symplectite stage where the derived high-grade phase of sodic-gedrite is broken down while the biotite and plagioclase intergrow together to form the matrix (Keevil et al. Reference Keevil, Namur and Holness2020). Such textural vermicular intergrowths representing symplectites are typical of late-stage hydration and cooling processes and holds the scope of further study through 40Ar-39Ar dating to define their cooling history (McDonald et al. 2017) which may be tectonically controlled attributing to rapid upliftment of the terrane.
7. c. Terrane evolution through P-T pseudosection and EMPA U-Th-total Pb ages
Through the examination of the findings mentioned above, this communication is focused towards addressing the mode of evolution, from the obtained metamorphic ages of the UHT granulites from the studied portion of the EGB. The undertaken area of investigation, in particular, is well-suited to understand the tectono-metamorphic evolution of the EGB as it is an integral part of the EGP of the EGB. The lack of geoscientific reports on the Rampur domain of the EGP and the unique mineral assemblage present in the studied terrane serves as a major significance to study this discrete section and correlate its evolutionary P-T path with the reports documented in the surrounding domains and provinces, for a better understanding of the complex metamorphic terrane, of the EGB as a whole. The geographic positioning of the studied terrane plays a key factor as it lies in proximity to the EGBSZ towards its west along which suturing and thrusting processes are most prominent. The yielded results from the phase equilibria modelling show uniform decompression path with respect to P-T conditions unlike the ITD path reported from various UHT localities of the eastern ghats (Bhattacharya Reference Bhattacharya1996, Mohan et al. Reference Mohan, Tripath and Motoyoshi1997, Prakash et al. Reference Prakash, Vishal, Naik, Yadav, Rai, Tewari and Pattnaik2019, Singh et al. Reference Singh, Prakash, Singh, Srivastava, Yadav, Singh and Kumar2022, Reference Singh, Prakash, Kharya and Sachan2023). The difference in the P-T path of evolution may probably be due to a faster rate of exhumation followed by tectonic upliftment processes in the studied terrane.
In addition to the above factors, the geochronological study has postulated two episodes of thermal event for the analyzed UHT granulites, the older event of around 959 Ma corresponds to the extreme thermal conditions of UHT metamorphism, as deciphered through textural constraints, during the Rodinian assembly (Fig. 12a) of the Grenvillian orogeny (Ghose et al. Reference Ghosh, Bose, Das, Dasgupta, Yamamoto, Hayasaka and Mukhopadhyay2016, Bose and Dasgupta Reference Bose and Dasgupta2018, Nasipuri et al. Reference Nasipuri, Corfu and Bhattacharya2018). The later event at around 846 Ma signifies the second metamorphic episode that was probably influenced by the initiation of Rodinia breakup (Fig. 12b) and records a comparatively lesser extent of thermal maxima than the older event. The derived metamorphic events have been compared with the other domains and provinces of the EGB to get an integrated overview of the major thermal events across the EGB (Fig. 13).
Figure 12. (a) Diagram showing the possible reconstruction of the palaeocontinent constituting India–East Antarctica–Australia during the assembly of Rodinia at ca. 959 Ma modified after (Harley et al. Reference Harley, Fitzsimons and Zhao2013; Bose et al. Reference Bose, Das, Kimura, Hidaka, Dasgupta, Ghosh and Mukhopadhyay2016; Li et al. Reference Li, Bogdanova, Collins, Davidson, De Waele, Ernst and Vernikovsky2008; Flowerdew et al. Reference Flowerdew, Tyrrell, Boger, Fitzsimons, Harley, Mikhalsky and Vaughan2013) and b) showing the initiation of succeeding break up at ca. 846 Ma.
Figure 13. Metamorphic episodes, magmatic history and deformational events of different provinces and domains of EGB modified after Bose et al. (Reference Bose, Banerjee, Jain, Dasgupta and Bajpai2020).
This study, therefore, helps to correlate and evolve a basic understanding regarding the tectono-metamorphic evolution of a high-grade terrane from the NW flank of EGB, constituting a unique mineral assemblage which has been characterized for the first time from the Indian subcontinent.
Availability of data and materials
All the data related to this work are included in the manuscript, in the form of tables and figures. There is no other dataset for access.
Acknowledgements
This research work has been made possible through RJP-PDF to Saurabh Singh (R/SRICC/RJP-PDF/23-24/8720), DST-SERB project to Divya Prakash (CRG/2022/001124) and PMRF Fellowship to Rajeev Kumar Pandey (0102562). The authors also thank the Head, Department of Geology, Banaras Hindu University for providing necessary infrastructural facilities. The authors sincerely thank the anonymous reviewers for their insightful comments, which have significantly improved the manuscript.
Financial support
This work was supported by funds utilized from DST-SERB project availed to Divya Prakash (CRG/2022/001124).
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