Under the microscope, the charnockites exhibit medium to coarse-grained granoblastic texture. The grains have well-defined boundaries and occasionally large grains of orthopyroxene are noticed in some sections ( Figure 2 ). Plagioclase and quartz are the dominant minerals with appreciable amounts of pyroxene. Biotite, magnetite and apatite occur as accessory minerals and the rocks are garnet-free.

Plagioclase occurs are fresh with albite, pericline and occasional carlsbad twinning. It shows characteristic antiperthitic texture with coarse blebs of K-feldspar elongated parallel to the twin lamellae and rarely cutting across them. It contains inclusions of quartz and apatite and shows an alter to sericite. Anhedral grains of quartz show undulate extinction. Orthopyroxene (hypersthene) occurs as sub-idiomorphic grains showing characteristic pleochroism from light green to pale yellow. It encloses hornblende and biotite suggesting a reaction relationship between them ( Figures 2(a)-(f) ).

This relation and the orthopyroxene grains closely mimicking the shape and mineral fabric orientation of the biotite and hornblende indicate the prograde transformation of hornblende and biotite to orthopyroxene. However, some pyroxene grains show hornblende rims around them in which case it appears that hornblende is later than pyroxene suggesting local retrograde metamorphism. The retrogression might probably have resulted due to local fluctuation in physical conditions or during shearing. The modal compositions of the rocks of Marandahalli fall in the charnockite (tonalitic) field of the QAP diagram [4] [41] - [43] ( Figure 3(a) ).

Mineral chemistry plots (EPMA) of various minerals, such as orthopyroxene,

biotite, K-feldspar and plagioclase and their respective SGT charnockitic areas like Marandahalli, Madurai, Coorg and Nagercoil ( Table 1 ). The orthopyroxene of charnockites of Maranadahalli has MgO ranging from 16 - 22 wt.% ( Table 2 ), and falls in the hypersthene composition of (En_55.03 Fs_43.57 Wo_1.4) enstatite-ferrosilite solid solution series [44] ( Figure 3(b) ). It exhibits varying Mg# (0.50 to 0.71), and plot in the igneous pyroxenes field in the XMg Vs Al diagram (Figure 3(c) ) similar to the pyroxenes of charnockites from Coorg [45] , on the contrary the charnockites of the Madurai Block [46] and the Nagercoil [47] that plot in metamorphic pyroxene field ( Figure 3(c) ).

The biotites have abundant K₂O 8 - 10 wt.%, MgO 10 - 16 wt.%, FeO 13 - 20 wt.%, TiO₂ 4 - 6 wt.%, and Al₂O₃ 13 - 15 wt.% ( Table 2 ) respectively. The Fe/(Fe + Mg) ratio in biotite is 0.38 - 0.50 with an average of 0.58 and the Si vs. Fe/Fe + Mg diagram is plotted in the biotite field ( Figure 3(d) ) [48] . These biotites appear to be formed in the last phase of magmatism. The analyzed biotites ( Table 2 ) plot in the primary biotite field in the 10TiO₂-(FeO + MnO)-MgO diagram ( Figure 4(a) ) [49] . For composition, the biotites from the charnockites of Madurai [46] , Nagercoil [47] and Coorg [45] are also shown in Figure 3(d) and Figure 4(a) .

Plagioclase exhibit consisting of anorthite (An_26-33) content along with the plagioclase from charnockits of Madurai [46] and Nagercoil [47] also get plotted in the anorthite field ( Figure 4(b) and Table 3 ). However, the plagioclase from charnockites of Coorg [45] gets plotted in the albite to andesine fields ( Figure 3(f) ). K-feldspar being in the composition of Marandahalli varying in range Or_93-99 and plotted in the orthoclase/microcline field ( Table 3 ; Figure 4(b) ) and other charnockite plutons are also getting in similar fields.

The whole rock composition of Marandahalli charnockites (major, trace and RE elements) are provided in Table 4 & Table 5 ) and are marked by a high SiO₂ concentration (~65 - 69 wt.%) and a reasonably high Al₂O₃ (~12 - 15 wt.%). They exhibit the metaluminous nature in terms of the Alumina Saturation Index (ASI) ( Figure 5(a) ), along with those from Coorg, Madurai and Nagercoil. However, some of the Nagercoil charnockites [47] displaying a slightly peraluminous nature [52] . The Modified Alkali-Lime Indices (MALI) [53] of Marandahalli indicates a

1, 5 [Present work work]; 2, 7 Madurai—Indu et al. 2021; 3 Nagarcoil—Rajesh et al. 2011; 4, 6 Coorg—Dev et al. 2023; nd = not determined; (*) Number of mineral grains.

1, 4 [Present work work]; 2 Coorg—Dev et al. 2023; 3, 5 Madurai—Indu et al. 2021; nd = not determined; (*) Number of mineral grains.

distinct calcic signature, while the Nagercoil charnockites [47] show calcic to calc-alkalic nature ( Figure 5(b) ). Even based on their Fe number [53] , the Marandahalli charnockites along with the charnockite of other plutons [45] - [47] [54] depicts in magnesian rich nature ( Figure 5(c) ). In the TAS diagram, the charnockite samples show granodiorite to granite composition and display a distinctive sub-alkali trend ( Figure 5(d) ). The result in the samples plotting in high Sr + Ba granite field ( Figure 6 ) on the Sr-Rb-Ba ternary diagram [55] [56] is resulted of the low Rb and high Sr and Ba of the charnockites from the study area. Furthermore, the high Sr + Ba content of these rocks may be a result of mafic and felsic melt mixing and mingling from the lower crust and enriched mantle [57] .

The charnockites of Marandahalli are derived from (meta-igneous) amphibolite source indicated by the low ratios of (Na₂O + K₂O)/(FeO^t + MgO + TiO₂) and (Na₂O + K₂O + FeO^t + MgO + TiO₂) [58] ( Figure 7(a) ), which is also reflected in the Al₂O₃/(FeO + MgO + TiO₂) vs. (Al₂O₃ + FeO + MgO + TiO₂) plot ( Figure 7(b) ) and when the charnockites from the Marandahalli along with charnockite rocks of other areas of SGT. The Mg# vs. SiO₂ binary plot of charnockites ( Figure 7(c) ) suggests the origin of charnockites be related to the melting of pre-existing continental crust with dehydration within the mantle. Even, the ^+veEu anomalies strengthen the observation that these rocks may have been formed by the re-melting of (metamorphic rocks) TTGs, supra-crustals or amphibolites. The lithological variations in the continental crust (ultramafic to felsic) to much diversified charnockites could be a consequence of re-melting of crust and possibly the interaction with mantle may be promoted by the diversity in charnockites aided by mantle dynamics and plate tectonics [59] . Against this scenario, given the limitations, it is thus surmised that the protoliths of Marandahalli formations have been initiated by the melting of amphibolite, followed by intra-crustal re-melting of protoliths of Marandahalli within the plagioclase stability and removal of potassic melt. Thus, the primary source of protolith is mantle melt, with a little amount of crustal contribution.

Mafic granulites are found in the charnockites as enclaves, and even the geochemical data is suggestive of an amphibolitic source ( Figure 7(d) ) for the charnockites parent magma. The usual Archean TTG nature of these rocks is revealed together with high Sr/Y ratios ( Figure 7(d) ), perhaps TTGs underwent charnockite transformation. However, the Kollaimalai and Pachaimalai [54] , the Nagercoil charnockite [47] and Coorg charnockites [45] exhibit a spread over the Archaen TTG to arc magma fields. The overall petrological and geochemical similarity of the Marandahalli charnockites and the Archean TTG suites suggests that these rocks were derived from amphibolite. The Marandahalli charnockites exhibit lower Sr/Y values in contrast with melts derived from garnet-bearing ecologitic sources (that contain high Sr/Y and low Y values) due to the partitioning of Y into residual garnet [60] and are broadly similar to melts yielded by garnet free amphibolitic sources. It can be stated that enrichment of amphibole and plagioclase in the residue, along with an LILE-enrichment is noticed in the Marandahalli charnockites.

*Standard analysis.

*Standard analysis.

The Ca-Fe-Mg-pyroxene phase relations at 500˚C - 1200˚C and 1 bar to 15 kbar, for estimating geo-thermometry of Marandahalli charnockites by utilizing orthopyroxene assemblage [50] . The resulting curve in the pyroxene quadrilateral ( Figure 3(b) ) yields temperatures ranging from 1000˚C - 1100˚C and pointing to the peak metamorphism or the crystallisation stage of pyroxene. The crystallization temperatures of charnockites can be deduced from binary plots like SiO₂ vs. TiO₂ and SiO₂ vs. P₂O₅. The charnockites plots between the isotherms of 700 ˚C - 800˚C (isotherms of Coorg charnockites are marginally higher) on SiO₂ vs. TiO₂ plot ( Figure 8(a) ) and temperature between 750˚C - 850˚C in the SiO₂ vs. P₂O₅ plot ( Figure 8(b) ). Table 4 demonstrates a lower TiO₂ content in the charnockites of Marandahalli mostly confined in an average 0.30 wt.% (ranges: 0.01 - 0.6 wt.%). Figure 8(a) and b show the Fe-Ti [63] and apatite saturation temperatures [64] for the Marandahalli charnockites stand at 7.5 kbar. Charnockitic rocks and the consistent decrease in temperature with increasing SiO₂ contents could be a result of fractionating charnockite intrusion [65] .

Ilmenite is present in the samples under study and biotite falls within the requisite compositional ranges for Ti (0.47 - 0.65) and XMg (0.50 - 0.67), the Ti in biotite thermometer is used in this particular study. The Ti in biotite thermometer and pressure values have an estimated uncertainty of (P = 400 - 600 MPa) used in their calibration [66] . The metamorphic temperatures obtained by Ti thermometry for all charnockite samples are ~740˚C - 800˚C ( Figure 8(c) ). The ternary feldspar geo-thermometer diagram is using plagioclase composition on An-Ab-Or [67] less than 700˚C is shown for the rocks under study show ( Figure 4(b) ), that may correspond to a late retrograde temperature of magnesian charnockite.

The systematics of trace elements are used to understand the tectonic evolution of the charnockite rocks of Marandahalli, which display a significantly higher La/Nb ratio, and as made evident in the Nb versus La/Nb variation diagram ( Figure 9(a) ) [68] characterize the charnockites to be subduction-related magma plots. Even, La/Nb vs. Th/Nb ( Figure 9(b) ) and La/Nb vs. U/Nb ( Figure 9(c) ) demonstrate the charnockites have subduction-related magma source, and that these rocks exhibit substantially higher LILE/HFSE elemental ratios ( Figure 9 ) [68] . The tectonic descrimnation diagrams i.e. Zr vs. Nb/Zr diagram ( Figure 9(d) ) [69] , also hint a subduction field setting. Charnockites are plotted in the volcanic arc granite plot (VAG) [70] . Immobile elemental systematics also support the subduction-derived magmatism in the volcanic arc granites (VAG) field in the tectonic discrimination diagram ( Figure 10(a) ).

The Cr (38 - 124 ppm) and Ni (7 - 19 ppm) contents in charnockites are extremely varied but relate to SiO₂. The plots like Ni and V vs. Rb and Ni vs. Ba plots can be effectively used to illustrate the corresponding importance of fractional crystallisation (FC) and partial melting (PM). While, partial melting produces sub-horizontal patterns in differentiated magmas, fractional crystallisation produces almost vertical trends [71] [72] . It is observed that the PM process dominants the other processes, Ni vs. Rb ( Figure 10(b) ) as reflected by preventing one from favouring either mechanism. However, on V vs. Rb ( Figure 10(c) ) and Ni vs. Ba plots ( Figure 10(d) ), the Marandahalli follow flat coherent trends suggesting the role of partial melting in their petrogenitic evolution.

Charnockites from the study area marked by the enrichment of large ion lithophile elements (LILE). The multi-element spider diagrams [73] ( Figure 11(a) ) display prominent troughs of Nb, P, and Ti indicating mineral phases like magnetite, apatite and sphene are retained in the subduction source, and peaks at Ba and Pb indicate that they go into melt phase. The positive anomalies Th and Sr indicate they have entered into feldspars. The normalised REE chondrite patterns display [74] enrichment of the LREE and depletion of HREE in these rocks indicate a low degree of partial melting in the source, a feature that is common to the charnockites/granites that formed in the continental arc setting; and the subtle positive Eu anomaly is due to the incorporation of plagioclase (fractionation/separation) in the rock ( Figure 11(b) ).

Though the charnockites have an igneous parentage [37] [75] , however, [76] advocate a sedimentary parentage to these rocks. They suggested a sedimentary (psammite and pelite-dominated greywacke-type) genesis for the charnockites using major element discrimination (ASI > 1.1; negative values of discriminant factor (DF) and oxygen isotope data [77] . However, they may also form by the melting of biotite-bearing metaluminous felsic rocks [78] or even by water-excess melting of mafic rocks [79] . Further, igneous enclaves present within Marandahalli charnockite strongly hints that more mafic magmas and/or other igneous sources may have been involved in the origin of igneous types of magma [80] [81] ( Figure 11(c) & Figure 11(d) ).