Unveiling the petrogenesis of alkaline silicate rocks at the Sevattur Carbonatite Complex, India

doi:10.21203/rs.3.rs-3452344/v1

This study investigates the evolution of alkaline silicate rocks in the Southeast of the Sevattur Carbonatite Complex, intruding granite gneiss country rock. These rocks include monzodiorite, monzonite, syenite, albitite, and granite, alongside ultramafic enclaves known as pyroxene hornblendite. The present work unveils the intricate fractional crystallization processes within an alkali-rich basaltic magma system supported by mineralogical and geochemical studies. Mafic minerals such as diopside, pargasite, interstitial magnetite, and ilmenite indicate early cumulate textures forming at high temperatures. Amphibole and diopside alongside apatite inclusions suggest the presence of alkalis, fluorine, phosphorus and H₂O in the primary magma. A coherent mineralogical and geochemical continuum from pyroxene hornblendite to monzodiorite, monzonite, and syenite with decreasing temperatures indicates fractional crystallization from a common primary magma. Furthermore, the absence of chilled margins and distinct boundaries dismisses a xenolithic origin for pyroxene hornblendite. The formation of granite, likely occurring during late-magmatic to hydrothermal stages, is characterized by K-feldspar, albite, and quartz, with clinozoisite and chlorite. Albitite occurrences as pegmatitic veins in syenite monzonite with shared geochemical traits with syenite and monzonite, suggesting a late-stage derivation from a common magma. Notably, these rocks lack the characteristic alkali-rich mineral assemblages, dispelling the notion of fenitization.

Alkaline silicate rocks 1

fenitization

fractional crystallization

magmatic

hydrothermal evolution

Elevated concentrations of Na and K characterize alkaline igneous rocks, which usually contain modal alkali feldspars or feldspathoids together with sodic- pyroxenes and -amphiboles (Sørensen, 1974; Streckeisen, 1980; Le Maitre, 2002). A majority of alkaline rocks are spatially and genetically associated with carbonatites (a rock containing > 50 vol.% igneous carbonates; Le Maitre, 2002) and are together known as alkaline rocks and carbonatite complex (ARC) (Woolley & Kjarsgaard, 2008). These rocks are economically significant due to their potential to host ore-grade rare earth elements (REE) (Chakhmouradian & Zaitsev, 2012). They are usually associated with inter-continental rifts, stable intraplate settings, and collisional zones (Goodenough et al., 2021). The REE enrichment in alkaline rocks is essentially governed by low-degree partial melting of the lithospheric mantle (Dostal, 2017). However, alkali-rich rocks can also form by carbonatite-induced metasomatism (fenitization) or alkali metasomatism of silica-saturated or -undersaturated rocks (Le Bas, 2008; Eliot et al., 2018). Such metasomatically formed rocks often show mineralogical similarities to magmatic syenites (Drüppel et al., 2005; Elliott et al., 2018), making distinguishing between them difficult (Elliott et al., 2018).

Several Neoproterozoic alkaline complexes such as Yelagiri, Sevattur, Samalpatti, and Pakkanadu are present along the NE-SW trending Dharmapuri Shear Zone in Tamil Nadu, India (Miyazaki et al., 2000; Mukhopadhyay et al., 2011; Dey et al., 2021a, b; Srivastava et al., 2022). Among these complexes, Sevattur carbonatites are associated with various alkaline silicate rocks such as monzodiorite, monzonite, syenite, albitite, and granite (Dey et al., 2021). There are ongoing debates regarding the genesis of these rocks and their relation with the spatially associated carbonatites. Previous works have reported that the granite gneiss country rock is fenitized, and consequently, it is hard to distinguish between the magmatic and fenitized syenites at Sevattur (Viladkar & Subramanian, 1995). Further studies have demonstrated the spatial association between the syenites and carbonatites is coincidental and genetically unrelated (Miyazaki et al., 2000). However, these studies lack a detailed textural and mineralogical characterization of these saturated alkaline suit of rocks at Sevattur. The primary objective of this contribution is to systematically study mineralogy and major element bulk-rock geochemistry of these associated silicate rocks, both proximal and distal to the carbonatites, and to assess their genetic lineage and the effects of fenitization with respect to the spatially associated carbonatites.

Sevattur alkaline complex (12°25'12" N 78°31'12" E) is an oval-shaped pluton (measuring about 12 X 5 square kilometres) emplaced in granite gneiss country rock (Miyazaki et al., 2000). Silicate rocks occupy a significant portion of the complex. At the same time, carbonatite occurrences are restricted to the NNW part of the complex, mainly divided into calcite-, dolomite-, banded-, and blue carbonatite varieties (Dey et al., 2021a). Pyroxenites are in contact with carbonatites in the SW; and with gabbro and tonalite rocks in the NW part of the complex (Fig. 1).

The silicate rocks present around the Sevattur carbonatites include a variety of syenites together with monzonite and quartz monzonite (Viladkar & Subramanian, 1995; Miyazaki et al., 2000; Ramasamy et al., 2001). Additionally, shonkinites (a variety of nepheline syenites) were reported from the peripheral parts of the complex (Ramasamy, 2018). Geochronological studies of the complex estimate the Rb-Sr whole rock mineral isochron age of the syenites as 755 ± 21 Ma (Miyazaki et al., 2000), while that of the carbonatites is 771 ± 18 Ma (Kumar & Gopalan, 1991). On the other hand, the whole rock Pb/Pb age of carbonatites is estimated as 801 ± 11 Ma (Schleicher et al., 1997).

Based on the field occurrences, we found varieties of saturated to oversaturated silicate rocks towards the south of the carbonatite body, intruding the granite gneiss country rock (Fig. 1). These silicate rocks, primarily based on their mineralogy and whole rock geochemistry (discussed in the later sections), are classified as monzodiorite, monzonite, syenite, albitite, and granite (Fig. 1). Monzodiorite, syenite, and albitite are proximal to the carbonatite. In contrast, monzonite mainly occurs in the southern part of the complex (distal from carbonatites). However, there are some smaller occurrences of scattered monzonite proximal to the carbonatites are also present. Granite is present near the peripheral part of the complex, in contact with the country rock (Fig. 2a). Based on the field observations, monzonite and monzodiorite seem to have similar mafic abundance (Fig. 2b–d), which decreases in syenite, albitite, and granite (Fig. 2e, f). Ultramafic enclaves are frequent in monzonite and monzodiorite, with sharp to serrated contacts (Figs. 2c, d). Megacrystic feldspars (1–2 cm) are rare in monzonite (Fig. 2b), while, their abundances increase in syenite and albitite (Fig. 2e). Albitite is late, crosscuts monzonite and syenite, and some of the syenite occurrences are capped by albitite (Fig. 2e). Similar albitite veins with profuse pyrochlore are found to crosscuts the blue carbonatite at Sevattur (Dey et al., 2021a) (Fig. 2f).

The representative samples of all the silicate rocks and their locations are provided in Online Resource 1. Initially, petrographic studies were carried on polished thin sections using optical microscopy to identify potential areas for analysis before proceeding with data collection by electron probe microanalyzer (EPMA). The selection of analysis points was guided by backscattered electron (BSE) images obtained using a Carl Zeiss EVO-18 Scanning Electron Microscope (SEM). The instrument was operated with a 20 µA beam current and a 20 kV Electron High Tension (EHT) setting in High-Definition Backscattered Electron (HDBSD) mode. The mineral compositions were determined parallelly using CAMECA SXFive and CAMECA SXFive Tactics EPMA at the DST-SERB National Facility, Department of Geology (Centre for Advanced Studies), Banaras Hindu University and National Centre for Earth Science Studies (NCESS), India, respectively. The EPMA analyses used wavelength-dispersive spectrometry (WDS) mode with TAP, PET, LIF, LPET, and LTAP crystals. The analytical conditions were kept at accelarating voltage of 15 kV and current of 10–20 nA with beam diameters of 1–10 µm, depending on the grain diameter. Peak counting times were 10 s as well as the background time were 5s for all the elements. Natural silicate mineral and andradite as internal standard was used to verify the positions of crystals (SP1-TAP, SP2-LiF, SP3-LPET, SP4-LTAP, and SP5-PET). After repeated analysis, it was found that the error on major element concentrations is < 1%, whereas the error on trace elements varied between 3–5%. 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, peridot, 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. Routine calibration, acquisition, quantification, and data processing were carried out using SxSAB version 6.1 and SX-Results software of CAMECA (Pandey et al., 2017; Chakravarti et al., 2018). While the EPMA at NCESS was operated with 15 kV accelerating voltage, 20 nA beam current, and 1 µm beam diameter. Respective background intensities were measured on both sides of the peak for half the peak times. Data reduction has been carried out using X-Phi method. Additional operational details of are discussed in Sorcar et al. (2021).

Major oxides were analyzed by X-ray Fluorescence (XRF) at the National Centre for Earth Science Studies (NCESS), Trivandrum, India. Pressed pellets were used for major element analysis. The pellets were prepared by sprinkling finely powdered samples over a boric acid binder filled in aluminium cups and pressing in a 40-ton hydraulic press for 30 seconds. Analyses were performed on a Bruker S8 Tiger wavelength dispersive (WD) XRF instrument. The reproducibility of the instrument was monitored using the international reference materials GSP-2 and JB-3. The accuracy for the reference standards is better than 0.5% for SiO₂ and < 5% for other major oxides.

Pyroxene mineral formulae and mol.% of end-members were calculated following the method of Marks et al. (2008). Amphibole group minerals are classified based on the scheme of Hawthorne et al. (2012). Mineral formulae of magnetite and their end-members were calculated using the End Member Generator (EMG) software (Ferrakutti et al., 2015).

The crystallization temperatures of amphiboles were determined using the Ti in calcium amphibole thermometer [\(T\left(ᵒ\text{C}\right)=\frac{2400}{1.52-{\text{l}\text{o}\text{g}\text{T}\text{i}}^{\text{A}\text{m}\text{p}}}-273;\)where T is temperature and logTi^Amp is the Ti content of amphibole in atoms per formula unit (apfu) expressed in the logarithm to base 10; Liao et al., 2021]. The thermometry is particularly applicable in the present work because of the of equilibrium assemblage of Ti-bearing calcium amphiboles with Ti-rich magnetite and ilmenite, and overall low oxygen fugacity condition. The temperature estimates of the mica group minerals were carried out using Ti-Fe⁺² thermometry [\(T\left(ᵒK\right)=\frac{838}{1.0337-\left(\frac{\text{T}\text{i}}{{\text{F}\text{e}}^{2+}}\right)}\); where T is temperature and Ti and Fe²⁺ content of amphibole in atoms per formula unit (apfu); Luhr et al., 1984].

Monzodiorite, monzonite, syenite, and albitite

The monzodiorite, monzonite, and syenite are characterized by plagioclase, K-feldspar and microcline, along with amphibole, clinopyroxene, and biotite in decreasing order of modal abundance (Figs. 3; 4a). The relative modal abundance of mafic minerals increases from syenite to monzonite and monzodiorite while that of the K-feldspar decreases (Figs. 3; 4a). Syenite and monzonite are characterized by a bimodal size distribution of K-feldspar, wherein megacrystic K-feldspar and microcline (> 1000 µm) are present in a matrix of K-feldspars (200–400 µm), plagioclase, clinopyroxene, and phlogopite (Fig. 3c–f). These megacrystic K-feldspar in monzonite and syenite often show a perthite texture (Fig. 3c–e). The contact between K-feldspar and plagioclase is often marked by low AZ albite (Fig. 3d)

The mafic minerals in monzonite, monzodiorite, and syenite are clinopyroxene (diopside), amphibole (pargasite, hastingsite, and actinolite), and phlogopite (see section 4). Diopside is prismatic, subhedral, occur as discrete grains, and often partially replaced by hastingsite (Fig. 3a, b, f). Pargasite is discrete subhedral-to-anhedral and is often co-crystallizing with hastingsite, while phlogopite outlasts their crystallization (Fig. 3a–c). Actinolite is late in the paragenetic sequence and replaces hastingsite (Fig. 4a inset). Accessory phases in these lithologies include apatite, titanite, magnetite, and ilmenite. Apatite is early, subhedral and restricted as inclusions in amphibole or interstitial to amphibole and clinopyroxene (Fig. 3b, f, g). Titanite is late, and either form along fractures present in amphibole and clinopyroxene (Fig. 3a, f) or rimming magnetite (Fig. 4a). Ilmenite is often replaced by magnetite (Fig. 3f). Epidote and clinozoisite are late and replaces plagioclase.

Irrespective of their association, albitite is characterized by > 95 vol.% of megacrystic (> 1 mm), elongated albite with minor discrete annite (200–400 µm) (Fig. 4b). Magnetite is accessory and frequently replaced by ilmenite along the grain margin. In contrast, albitite veins in carbonatites (dolomitic carbonatite) are characterized by U-rich pyrochlore with an orbicular mantle of Ba-rich K-feldspar together with minor calcite, magnesio-riebeckite, ferri-winchite, baryte, phlogopite, magnetite, pyrite, and thorite (Dey et al., 2021).

Granite

Granite is essentially composed of K-feldspar (~ 40 vol.%), quartz (~ 30 vol.%), and albite (~ 25 vol.%) with accessory magnetite, clinozoisite, and chlorite (Fig. 4c, d). Clinozoisite and chlorite replace albite and K-feldspar, respectively. Anhedral magnetite is scattered within the matrix of K-feldspar, quartz, and albite; and is often replaced by titanite (Fig. 4d). Granite shares contact with the granite gneiss country rock, mainly characterized by plagioclase and quartz with alternate bands of amphiboles (hornblende, pargasite and actinolite) (Fig. 2a).

Ultramafic enclave

The ultramafic enclave is coarse-grained, mainly composed of pargasite (~ 60 vol.%) and diopside (Fig. 4e, f). Thus, according to IUGS nomenclature the ultramafic enclave is pyroxene hornblendite (Streckeisen 1973; Le Maitre 2002). The diopside and pargasite show a cumulate texture with interstitial magnetite, ilmenite, and apatite (Fig. 4e). The magnetite and ilmenite are co-precipitating often sharing grain contacts, while apatite is either restricted as inclusions in the diopside or interstitial to pargasite and diopside (Fig. 4f).

Feldspar

Alkali-feldspar and plagioclase are common in all lithologies (except pyroxene hornblendite) and show a broad range of compositions (Table 1). Plagioclase compositions in monzonite and monzodiorite varies between andesine and oligoclase (Ab_67–96An_32–4O_11–0) and gradually become more albite rich (Ab_88–100An_11–0Or_1–0) in syenite, granite, and albitite (Fig. 5a). All alkali feldspar compositions are near end-member K-feldspar (Or _> 90) (Fig. 5a). The albite lamellae in K-feldspar within syenite and monzonite also show near-end-member albite compositions.

Table 1

Representative compositions of feldspar.
	Monzodiorite			Monzonite			Syenite			Granite			Albitite
Wt.%	1	2	3	4	5	6	7	8	9	10	11	12	13	14
SiO₂	61.61	68.72	63.87	59.87	68.31	64.66	69.24	66.91	63.88	63.90	65.42	67.06	64.42	69.09
Al₂O₃	24.02	19.46	18.99	24.46	19.93	19.17	19.58	20.03	18.53	22.27	18.71	20.98	18.01	19.36
CaO	6.03	0.66	0.03	6.68	0.19	0.01	n.d.	0.55	0.05	2.29	n.d.	0.59	n.d.	n.d.
BaO	n.d.	n.d.	1.32	n.d.	n.d.	1.02	n.d.	n.d.	0.41	n.d.	n.d.	n.d.	n.d.	n.d.
Na₂O	7.97	10.99	0.59	7.79	11.37	0.23	11.74	11.54	0.43	10.50	0.47	11.66	n.d.	11.86
K₂O	0.14	0.06	14.67	0.10	0.04	14.98	0.04	0.05	15.46	0.15	15.86	0.08	15.66	0.05
Total	99.77	99.89	99.47	98.89	99.83	100.08	100.61	99.08	98.76	99.11	100.46	100.37	98.09	100.35
Formula calculation based on 8 oxygens
Si	2.739	3.000	2.974	2.695	2.985	2.983	3.002	2.957	2.988	2.842	2.999	2.928	3.020	3.005
Al	1.258	1.001	1.042	1.297	1.026	1.042	1.001	1.043	1.021	1.167	1.011	1.080	0.995	0.992
Ca	0.287	0.031	0.002	0.322	0.009	-	-	0.026	0.003	0.109	-	0.028	-	-
Ba	-	-	0.024	-	-	0.018	-	-	0.008	-	-	-	-	-
Na	0.687	0.931	0.053	0.680	0.963	0.021	0.987	0.989	0.039	0.905	0.042	0.987	-	1.000
K	0.008	0.003	0.871	0.006	0.002	0.882	0.002	0.003	0.923	0.009	0.928	0.004	0.936	0.003
Ab	70	96	6	67	99	2	100	97	4	88	4	97	-	100
Or	1	-	94	1	-	98	-	-	96	1	96	-	100	-
An	29	3	-	32	1	-	-	3	-	11	-	3	-	-
Ab: albite; Or: orthoclase (K-feldspar); An: anorthite (abbreviations after Warr, 2021)
n.d.: not detected

Amphibole

Amphibole group minerals are present in pyroxene hornblendite, monzodiorite, monzonite, and syenite. Compositionally, all are calcium amphiboles (Fig. 5b; Table 2). Pargasite and hastingsite from pyroxene hornblendite and monzodiorite are Mg-rich and Fe-poor (9.1–11.2 wt.% MgO; 13.2–16.4 wt.% FeO; Table 2), compared to in monzonite and syenite (6.7–15.0 wt.% MgO and 15.6–18.7 wt.% FeO; Table 2, comp. 10; Fig. 5c). Additionally, the pargasite and hastingsite are characterized by high Ti, Na and K contents compared to late actinolite. Note that pargasite in pyroxene hornblendite has the highest Ti, Na and K contents (3.1–3.4 wt.% TiO₂; 1.8–2.2 Na₂O; up to 2 wt.% K₂O; Table 2, comp.13, 14) compared to other lithologies. The actinolite have end-member composition with variable Fe and Mg (13.7–18.3 wt.% MgO), low Na, K, Al and Ti contents (Table 2. comp. 6, 11, 14, 15). Hastingsite and pargasite show a gradual Fe enrichment and decreasing Mg and Ti contents from pyroxene hornblendite to monzodiorite, monzonite and syenite (Fig. 5c, d). However, all actinolite have high Mg and low Ti contents compared to pargasite and hastingsite (Fig. 5c, d)

Table 2

Representative compositions of amphibole.
	Pyroxene hornblendite		Monzodiorite				Monzonite					Syenite
Wt%	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15
SiO₂	39.92	40.49	41.30	43.61	44.41	56.12	40.80	39.62	40.98	41.94	52.08	38.53	38.88	48.65	52.48
TiO₂	3.10	3.43	1.47	1.02	1.12	0.01	0.28	2.92	1.79	2.30	0.08	1.25	0.86	0.20	0.03
Al₂O₃	12.95	12.70	10.45	8.96	9.30	1.03	10.56	11.51	11.08	10.79	2.68	12.15	12.11	4.95	1.76
Cr₂O₃	n.d.	0.03	n.d.	0.06	0.03	0.03	0.03	0.03	0.05	n.d.	n.d.	0.04	0.03	0.02	n.d.
FeO	0.35	0.16	0.51	0.56	0.39	0.56	14.98	0.40	0.81	16.32	13.80	0.43	0.48	0.78	11.21
Fe₂O₃	13.23	13.70	16.22	13.23	16.45	8.68	7.24	15.65	17.57	1.93	1.23	18.70	16.78	14.81	1.19
MnO	0.73	1.23	3.33	5.44	2.47	1.73	1.14	3.19	1.25	0.56	0.30	4.26	6.69	2.29	1.03
MgO	11.03	10.70	9.12	11.20	10.43	18.28	7.87	9.58	8.65	10.39	13.69	6.69	6.79	11.98	14.53
CaO	11.42	11.88	11.62	11.29	10.80	13.05	12.86	11.37	11.22	11.12	11.55	11.33	11.01	11.20	11.18
Na₂O	2.20	1.83	1.73	1.66	1.71	0.31	1.88	2.08	1.92	2.10	0.89	1.67	1.61	1.44	0.88
K₂O	1.96	1.99	1.30	1.09	1.09	0.04	0.74	1.69	1.57	1.70	0.22	2.22	1.92	0.43	0.29
H₂O+	1.92	1.83	1.96	2.00	1.99	2.11	1.66	1.92	1.92	1.97	2.07	1.82	1.84	1.98	2.09
F	0.11	0.27	0.02	0.01	0.01	0.01	0.23	0.01	n.d.	n.d.	n.d.	0.13	0.10	0.08	n.d.
Cl	0.08	0.07	n.d.	n.d.	n.d.	n.d.	0.67	0.16	0.16	n.d.	n.d.	0.16	0.15	0.01	n.d.
O = F,Cl (calc)	0.06	0.13	0.01	0.00	0.01	0.00	0.25	0.04	0.04	0.00	0.00	0.09	0.08	0.04	0.00
Total	98.92	100.09	99.05	100.14	100.18	101.99	100.70	100.08	98.93	101.13	99.79	99.27	99.06	98.78	99.67
Formula based on 23 oxygens
Si	6.058	6.090	6.349	6.536	6.663	7.805	6.248	6.053	6.333	6.295	7.712	6.055	6.092	7.288	7.838
Al	1.942	1.910	1.651	1.464	1.337	0.169	1.752	1.947	1.667	1.705	0.288	1.945	1.908	0.712	0.162
Ti	-	-	-	-	-	0.002	-	-	-	-	-	-	-	-	-
Fe³⁺	-	-	-	-	-	0.024	-	-	-	-	-	-	-	-	-
∑T	8.000	8.000	8.000	8.000	8.000	8.000	8.000	8.000	8.000	8.000	8.000	8.000	8.000	8.000	8.000
Ti	0.354	0.388	0.171	0.115	0.126	-	0.032	0.335	0.208	0.260	0.009	0.148	0.101	0.022	0.004
Al	0.374	0.341	0.243	0.120	0.308	-	0.153	0.125	0.351	0.204	0.172	0.304	0.329	0.162	0.136
Cr	-	0.003	-	0.008	0.003	0.003	0.004	0.004	0.006	-	-	0.005	0.003	0.002	-
Fe³⁺	0.056	0.021	0.388	0.615	0.278	0.159	0.834	0.351	0.137	0.219	0.135	0.485	0.663	0.258	0.128
Mn	0.014	0.008	0.027	-	-	0.041	0.148	-	0.028	-	-	0.014	-	0.025	0.053
Fe²⁺	1.706	1.841	2.082	1.641	1.953	1.007	1.919	2.002	2.279	1.993	1.675	2.476	2.318	1.855	1.349
Mg	2.496	2.398	2.090	2.502	2.332	3.790	1.797	2.182	1.992	2.325	3.009	1.568	1.586	2.676	3.330
∑C	5.000	5.000	5.001	5.001	5.000	5.000	4.887	4.999	5.001	5.001	5.000	5.000	5.000	5.000	5.000
Mn	0.032	0.012	0.040	0.071	0.049	0.026	-	0.052	0.078	0.071	0.037	0.043	0.064	0.074	0.073
Fe²⁺	-	-	-	0.017	0.111	-	-	0.013	-	0.054	-	-	0.007	-	-
Ca	1.857	1.915	1.914	1.814	1.736	1.945	2.000	1.860	1.857	1.788	1.798	1.907	1.848	1.798	1.723
Na	0.111	0.073	0.046	0.099	0.104	0.030	-	0.075	0.064	-	0.164	0.050	0.081	0.128	0.204
∑B	2.000	2.000	2.000	2.001	2.000	2.001	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000
Ca	-	-	-	-	-	-	0.110	-	-	-	-	-	-	-	-
Na	0.535	0.460	0.471	0.382	0.392	0.055	0.558	0.541	0.512	0.523	0.086	0.460	0.407	0.292	0.042
K	0.379	0.383	0.254	0.208	0.209	0.006	0.144	0.330	0.310	0.326	0.041	0.445	0.384	0.082	0.053
∑A	0.914	0.843	0.725	0.590	0.601	0.061	0.812	0.871	0.822	0.849	0.127	0.905	0.791	0.374	0.095
OH	1.928	1.852	1.991	1.997	1.993	1.997	1.712	1.956	1.958	2.000	2.000	1.894	1.908	1.959	2.000
F	0.052	0.129	0.009	0.003	0.007	0.003	0.113	0.003	-	-	-	0.064	0.051	0.038	-
Cl	0.020	0.019	-	-	-	-	0.175	0.041	0.042	-	-	0.042	0.041	0.004	-
∑W	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.001	2.000
Species	Ti-rich Prg	Ti-rich Prg	Mhst	Mhst	Prg	Act	Hst	Ti-rich Mhst	FPrg	Mhst	Act	Hst	Hst	Act	Act
Temp. (° C)	945	970	776	703	719	472	523	930	817	867	400	748	681	482	340
Prg: pargasite, Fprg: ferro-pargasite: Mhst: magnesiohastingsite; Hst: hastingsite, Act: actinolite (abbreviations after Warr, 2021)
^# FeO and Fe₂O₃ determined from the stoichiometry.
* H₂O calculated from (F + Cl + OH = 2).
n.d.: not detected

Clinopyroxene

Clinopyroxene in pyroxene hornblendite, monzodiorite, monzonite, and syenite is diopside with variable hedenbergite contents (Fig. 5e). Similar to amphiboles, there is a gradual Fe-enrichment in syenites (7.9–10.5 wt.% FeO; Table 3) from monzonite, monzodiorite and pyroxene hornblendite (4.5–9.6 wt.% FeO; Table 3; Fig. 5e). Note that all the clinopyroxene has variable Na and low Mn contents (0.7–1.2 wt.% Na₂O; 0.2–0.9 wt.% MnO; Table 3) while clinopyroxene in pyroxenite hornblendite is marked by highest Ti contents (up to 1.0 wt.% TiO_2; Table 3)

Table 3

Representative compositions of clinopyroxene.
	Pyroxene hornblendite		Monzodiorite			Monzonite				Syenite
Wt.%	1	2	3	4	5	6	7	8	9	10	11
SiO₂	49.81	47.85	52.64	51.59	48.87	49.78	51.29	49.42	50.85	49.02	49.30
TiO₂	0.51	1.05	0.25	0.32	0.53	0.24	0.20	0.81	0.58	0.47	0.63
Al₂O₃	3.60	4.68	1.44	1.76	3.68	1.65	1.32	3.14	2.73	3.04	3.34
Fe₂O₃^#	3.31	4.48	4.22	3.35	5.83	7.54	4.66	7.41	1.91	5.35	3.67
FeO^#	6.64	4.70	4.99	4.87	6.34	6.24	9.19	4.05	9.57	7.91	10.51
MnO	0.25	0.37	0.67	0.38	0.56	0.86	0.93	0.50	0.51	0.54	0.69
MgO	11.01	11.44	13.57	13.44	10.32	10.59	10.57	11.50	10.64	9.96	8.78
CaO	22.02	22.71	23.38	22.60	21.85	22.20	21.10	23.23	21.60	21.98	21.69
Na₂O	1.14	0.77	0.72	0.81	1.18	1.15	1.19	1.08	0.99	1.01	1.05
K₂O	n.d.	n.d.	n.d.	0.02	0.04	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
Total	98.29	98.06	101.94	99.16	99.20	100.24	100.44	101.14	99.38	99.30	99.67
Formula calculation based on 6 oxygens and 4 cations
Si	1.899	1.829	1.929	1.934	1.863	1.891	1.942	1.843	1.931	1.878	1.892
Al	0.162	0.211	0.062	0.078	0.165	0.074	0.059	0.138	0.122	0.137	0.151
Ti	0.015	0.030	0.007	0.009	0.015	0.007	0.006	0.023	0.016	0.014	0.018
Fe³⁺	0.095	0.129	0.116	0.094	0.167	0.215	0.133	0.208	0.055	0.154	0.106
Mn	0.008	0.012	0.021	0.012	0.018	0.028	0.030	0.016	0.016	0.018	0.022
Mg	0.626	0.652	0.741	0.751	0.587	0.600	0.597	0.639	0.603	0.569	0.503
Fe²⁺	0.212	0.150	0.153	0.153	0.202	0.198	0.291	0.126	0.304	0.253	0.338
Ca	0.899	0.930	0.918	0.908	0.893	0.903	0.856	0.928	0.879	0.018	0.022
Na	0.084	0.057	0.051	0.059	0.087	0.084	0.087	0.078	0.073	0.075	0.078
K	-	-	-	0.001	0.002	-	-	-	-	-	-
Ae	9	7	6	6	10	10	10	9	6	9	9
Di	67	74	76	77	65	65	58	74	61	62	53
Hd	24	19	18	17	25	25	32	17	33	29	38
^#FeO and Fe₂O₃ estimated from stoichiometry (after Marks et al., 2008).
n.d.: not detected

Phlogopite

The mica group minerals in Sevattur silicate rocks belong to phlogopite-annite solid solution series with high Al and variable Ti contents (14.2–16.9 wt.% Al₂O₃; 1.1–4.5 wt.% TiO₂; Table 4; Fig. 5f). In monzodiorite, monzonite, and syenite they are intermediate to Mg-rich (X_Fe: 0.4–0.6) in composition, whereas in albitite and some in monzonite are relatively Fe-rich (X_Fe~ 0.7; Table 4; Fig. 5f).

Table 4

Representative compositions of mica group minerals.
	Monzodiorite			Monzonite				Syenite			Albitite
Wt.%	1	2	3	4	5	6	7	8	9	10	11	12
SiO₂	37.11	36.85	36.76	35.56	34.97	36.13	38.43	39.67	36.76	36.52	32.39	33.25
TiO₂	2.24	3.69	3.76	4.52	4.51	4.10	2.23	0.79	2.43	0.94	2.55	3.74
Al₂O₃	15.66	14.19	14.41	15.10	15.00	14.75	15.11	14.90	13.57	14.02	16.87	16.93
Cr₂O₃	0.02	0.03	0.03	0.08	0.05	0.02	n.d.	n.d.	n.d.	n.d.	0.10	0.15
MgO	11.70	10.93	11.19	7.64	6.88	13.05	14.32	15.21	13.65	11.77	5.43	5.37
MnO	-	-	-	-	-	-	-	0.75	0.73	1.19	-	-
FeO	19.12	19.81	19.78	23.51	23.83	17.54	16.21	16.24	18.88	22.28	28.64	26.63
CaO	n.d.	n.d.	n.d.	0.15	0.13	0.15	n.d.	n.d.	n.d.	n.d.	0.04	0.07
Na₂O	0.05	0.14	0.12	n.d.	n.d.	0.06	0.06	0.08	0.06	0.04	n.d.	n.d.
K₂O	9.11	9.04	9.09	9.13	9.07	9.07	8.89	8.57	9.69	9.31	8.92	9.18
F	0.50	0.01	n.d.	0.33	0.29	0.09	0.03	1.00	1.20	0.98	0.19	0.22
Cl	0.07	-	-	0.15	0.16	0.13	-	0.07	0.10	0.10	0.01	0.01
O = F,Cl	0.23	0.00	0.00	0.17	0.16	0.06	0.01	0.44	0.53	0.44	0.08	0.09
H₂O*	3.65	3.83	3.86	3.59	3.54	3.83	3.98	3.57	3.32	3.40	3.62	3.61
Total	99.73	98.94	99.42	100.18	98.93	99.25	99.64	100.41	99.85	100.12	99.09	99.52
Formula calculation based on 7 (T + M) cations
Si	2.845	2.879	2.854	2.817	2.819	2.777	2.888	2.931	2.817	2.811	2.616	2.681
Al	1.155	1.121	1.146	1.183	1.181	1.223	1.112	1.069	1.183	1.189	1.384	1.319
ΣT	4.000	4.000	4.000	4.000	4.000	4.000	4.000	4.000	4.000	4.000	4.000	4.000
Al	0.261	0.186	0.172	0.227	0.244	0.113	0.226	0.229	0.043	0.083	0.222	0.290
Mg	1.337	1.273	1.295	0.902	0.827	1.495	1.604	1.676	1.559	1.351	0.654	0.646
Mn	-	-	-	-	-	-	-	0.047	0.048	0.078	-	-
Fe²⁺	1.226	1.295	1.284	1.557	1.606	1.127	1.019	1.004	1.210	1.434	1.935	1.796
Ti	0.129	0.217	0.220	0.270	0.274	0.237	0.126	0.044	0.140	0.054	0.155	0.227
Cr	0.001	0.002	0.002	0.005	0.003	0.001	-	-	-	-	0.007	0.009
ΣM	3.000	3.000	3.000	3.000	3.000	3.000	3.000	3.000	3.000	3.000	3.000	3.000
Na	0.007	0.022	0.018	-	-	0.009	0.009	0.011	0.008	0.006	-	-
K	0.891	0.901	0.900	0.923	0.932	0.889	0.852	0.807	0.947	0.914	0.919	0.944
Ca	-	-	-	0.013	0.012	0.012	-	-	-	-	0.004	0.006
ΣI	0.899	0.922	0.917	0.935	0.944	0.910	0.862	0.819	0.955	0.920	0.923	0.950
F	0.122	0.002	-	0.082	0.073	0.021	0.006	0.234	0.291	0.240	0.049	0.056
Cl	0.010	-	-	0.020	0.022	0.017	-	0.009	0.013	0.013	0.002	0.001
OH	1.868	1.998	2.000	1.898	1.905	1.963	1.994	1.758	1.696	1.747	1.949	1.943
O	10.132	10.211	10.192	10.268	10.285	10.144	10.114	10.034	10.048	9.962	10.041	10.195
Xfe	0.478	0.504	0.498	0.633	0.660	0.430	0.388	0.375	0.437	0.515	0.747	0.735
Species	Phl	Phl	Phl	Ann	Ann	Phl	Phl	Phl	Phl	Phl	Ann	Ann
Phl: phlogopite; Ann: annite (abbreviations after Warr, 2021)
* H₂O calculated from stoichiometry.
n.d.: not detected

Other minerals

Other minerals in these lithologies include magnetite, ilmenite, titanite, apatite, and epidote group minerals, while chlorite group minerals are restricted to granite (Table 5; Online Resource 2).

Table 5

Representative compositions of magnetite.
	Pyroxene hornblendite		Monzodiorite			Monzonite				Syenite	Granite
Wt.%	1	2	3	4	5	6	7	8	9	10	11
TiO₂	1.06	1.67	0.01	0.01	0.15	0.15	0.42	0.15	0.20	0.15	n.d.
Cr₂O₃	0.22	0.23	0.07	0.13	0.21	0.22	0.23	0.06	0.20	0.28	n.d.
Fe₂O₃^#	66.6	65.4	69.17	69.21	67.48	69.2	68.01	67.76	67.67	67.93	67.69
FeO^#	31.66	31.87	30.97	31.22	30.42	30.59	30.74	30.72	30.16	30.53	31.09
MnO	0.33	0.65	0.25	0.10	0.27	0.65	0.15	0.09	0.16	0.23	0.02
NiO	0.07	0.46	0.01	n.d.	0.04	0.10	0.61	0.03	0.60	0.21	n.d.
Total	99.99	100.26	100.47	100.67	98.57	100.72	99.94	98.8	98.99	99.33	98.8
Formula calculation based on 4 oxygens and 3 cations
Ti	0.031	0.048	-	-	0.004	0.004	0.012	0.004	0.006	0.004	-
Cr	0.007	0.007	0.002	0.004	0.006	0.007	0.007	0.002	0.006	0.009	-
Fe³⁺	1.93	1.889	1.995	1.992	1.985	1.985	1.97	1.988	1.982	1.983	1.987
Fe²⁺	1.019	1.023	0.993	0.999	0.994	0.981	0.989	1.001	0.982	0.990	1.014
Mn	0.01	0.021	0.008	0.003	0.009	0.021	0.005	0.003	0.005	0.008	0.001
Ni	0.002	0.0142	-	-	0.001	0.003	0.018	0.001	0.019	0.007	-
^# FeO and Fe₂O₃ calculated from the stoichiometry
n.d.: not detected

Magnetite from monzonite, monzodiorite and granite are of end-member composition, whereas in pyroxene hornblendite, they are Ti-rich (up to 1.7 wt.% TiO₂; Table 5). Monzonite is also characterized by ilmenite-pyrophanite solid solution with variable Mn contents, ranging from Mn-rich ilmenite (2.5–10 wt.% MnO) to pyrophanite (39 wt.% MnO) (Online Resource 2). Epidote group minerals in monzonite are epidote, while those in monzodiorite and granite are clinozoisite. Titanite and fluorapatite across all lithologies are of end-member compositions.

Whole-rock major element oxide data were used to investigate the compositional variations among the silicate lithologies at Sevattur (Table 6). The pyroxene hornblendite shows extreme depletion in alkalis and silica and, thus, are difficult to classify based on the whole rock data (Fig. 6). In the TAS diagram, all other lithologies rocks appear as alkaline and classify as monzodiorite, monzonite, syenite, and granite (Fig. 6), this compositional range is similar to the earlier reported syenites and high silica rocks from Sevattur (Miyazaki et al., 2000). However, based on their aluminium saturation index (ASI; Frost & Frost; 2008), the albitite is peraluminous, while monzodiorite, monzonite, syenite and granite are metaluminous (Fig. 7a). The monzodiorite appears to be alkali-calcic in the modified alkali lime index (MALI; Frost & Frost; 2008) plot, in contrast monzonite, syenite, albitite and granite are alkaline in nature (Fig. 7b). Moreover, all these lithologies show adherence to alkali basalt clan (Fig. 7c).

There is a gradual increase in alkalis (Na₂O and K₂O) and SiO₂ from pyroxene hornblendite to monzodiorite and monzonite to syenite, albitite, and granite (Figs. 6; 8a, b). The highest Na contents characterize albitite, while some syenites have higher K contents (Fig. 8a, b). Pyroxene hornblendites show the highest MgO, CaO, FeO, and TiO₂ compared to other lithologies (Fig. 8). It is interesting to note that the composition of syenites and the high silica rocks of Miyazaki et al. (2000) show a similar variation in oxides as found in the present work (Figs. 6; 8).

Table 6

Whole rock compositions of various alkaline silicate rocks present at Sevattur.
	Pyroxene hornblendite	Monzodiorite		Monzonite						Syenite					Albitite	Granite
Wt%	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16
SiO₂	36.71	50.11	53.17	58.95	57.88	58.81	58.57	59.13	56.53	62.95	63.33	63.18	64.01	64.21	66.23	69.47
TiO₂	3.01	0.97	0.92	0.67	0.64	0.75	0.72	0.70	0.66	0.32	0.36	0.40	0.59	0.55	0.08	0.19
Al₂O₃	6.95	10.97	15.96	17.59	17.12	17.82	17.61	18.02	18.16	15.97	16.76	17.65	17.64	17.86	19.13	16.44
MnO	0.34	0.23	0.12	0.13	0.11	0.14	0.13	0.12	0.14	0.09	0.09	0.07	0.05	0.07	0.03	0.03
Fe₂O₃	19.98	13.14	8.41	6.23	5.10	6.11	6.23	5.90	5.32	3.58	3.80	3.75	4.23	4.23	2.34	1.67
CaO	17.67	10.34	6.70	4.42	4.21	4.17	5.12	4.42	5.39	4.00	2.82	1.36	2.95	2.60	0.13	1.42
MgO	9.39	6.89	4.86	1.15	0.95	1.02	1.14	1.06	1.66	1.13	1.00	0.61	1.67	1.84	0.05	0.37
Na₂O	0.93	1.30	3.46	4.34	4.22	4.39	4.39	4.41	4.58	5.52	1.68	2.89	4.37	4.54	8.74	5.28
K₂O	0.67	5.51	3.35	5.83	5.56	5.80	5.51	5.81	4.35	3.74	10.96	9.81	5.71	5.69	3.22	4.42
P₂O₅	2.29	1.00	0.76	0.04	0.11	0.01	0.04	n.d.	n.d.	0.19	0.12	0.01	0.20	0.20	n.d.	0.05
Total	97.94	100.46	97.71	99.35	95.90	99.02	99.46	99.57	96.79	97.49	100.92	99.73	101.42	101.79	99.95	99.34
A.I.	0.230	0.621	0.427	0.578	0.571	0.572	0.562	0.567	0.492	0.580	0.754	0.720	0.571	0.573	0.625	0.590
A.I.: Alkalinity index (Na₂O + K₂O/Al₂O₃)

Ultramafic enclaves: Primary cumulates or xenoliths?

Plutonic rocks need not necessarily represent their primary melt composition as they are subjected to several degrees of fractional crystallization (Litvinovsky et al., 2002 and references therein). Such process may result in the formation of an early cumulates as blobs or globules of mafic rocks within differentiated felsic rocks (Vernon, 1984). Several studies have shown that the origin of ultramafic enclaves has three major hypotheses: (a) fragments of early cumulate minerals derived from a mafic (basaltic) magma; (b) refractory, residual assemblage from a felsic (granitic) magma and; (c) formed due to incomplete mixing or mingling between mafic and felsic magmas (Noyes et al., 1983; Vernon, 1984; Chappell et al., 1987; Feeley et al., 2008). These cognate enclaves are different from xenoliths, which are carried as fragments of pre-existing rocks and are incorporated in magmas during ascent or emplacement (Vernon, 1984).

In the present work, the pyroxene hornblendite (ultramafic enclave) primarily comprises diopside and pargasite, showing a typical cumulate texture with interstitial magnetite and ilmenite. The amphibole thermometry indicates a high crystallization temperature for this assemblage (~ 970–873°C). Additionally, the presence of amphibole and absence of plagioclase probably indicate high water content (4–6 wt.%) of the primary magma at crustal depths (Anderson, 1980; Barker and Eggler; 1983; Litvinovsky et al., 2002). Furthermore, Na and K in diopside and pargasite (Tables 2; 3) containing apatite inclusions (Fig. 4e, f) reflect the magma's early alkali, fluorine and phosphorus saturation. A similar inference has been drawn for the origin of cumulate rocks (pargasite + clinopyroxene + apatite) at Oshurkovo Complex, Russia (Litvinovsky et al., 2002).

At Sevattur, the pyroxene hornblendite is exclusively present in monzonite and monzodiorite, having a similar assemblage of pargasite + diopside + magnetite + ilmenite (See sections 3 and 4). Similar petrographic character together with coherent geochemical variations are essential for identifying co-magmatic rocks (Mukhopadhyay et al., 2011). Compositionally, the pargasite, diopside and magnetite in the pyroxene hornblendite are relatively Mg-Ti-rich compared to monzodiorite, monzonite, and syenite (Fig. 5b–e). Such mineralogical parity combined with decreasing modal proportion of mafic minerals from pyroxene hornblendite to monzodiorite, monzonite, and syenite is a characteristic of fractional crystallizations from a common primary magma. This is also well complemented by field observations against a xenolithic origin of pyroxene hornblendite due to the absence of chilled margins, sharp boundaries and intermixing with their host rocks (Fig. 2c, d).

Petrogenesis of the monzodiorite, monzonite, syenite, albitite and granite

Fractional crystallization of a basaltic magma leads early precipitation of mafic minerals such as olivine, clinopyroxene and Ca-feldspars, and the magma becomes gradually depleted in Mg and Ca. This led to a concomitant enrichment of Na, K, Fe, Al, Si, and H₂O in the evolving magma, which in turn stabilizes amphibole, biotite, K-feldspar, and quartz (Bowen, 1922; 28; Nandedkar et al., 2014; Marxer et al., 2022). The textural dispositions of diopside and pargasite across lithologies at Sevattur, suggest that these phases were co-crystallizing together with interstitial magnetite and ilmenite and represents the earliest magmatic assemblage, while hastingsite formed late, replacing diopside (Figs. 3b, f; 9; 10). The diopside show an evolutionary trend of increasing hedenbergite content from pyroxene hornblendite → monzodiorite and monzonite → syenite (Fig. 5e). As discussed previously, Ti-rich pargasite in pyroxene hornblendite has the highest crystallization temperature (873–970°C) followed by pargasite and hastingsite (660–938°C; Table 7) in monzodiorite, monzonite and syenite. Based on the texture and composition, mica group minerals formed late in the paragenetic sequence, complemented by their lower crystallization temperature (630–754°C) (Table 7) compared to hastingsite and pargasite. A Fe²⁺ enrichment at the expense of Mg, similar to that observed in diopside and amphiboles, is also prevalent for mica group minerals with the highest Fe contents in albitite (Fig. 5f). Such a Fe enrichment trend is expected in micas from silicate rocks and corroborates a magmatic differentiation process (Brod et al., 2001; Giebel et al., 2019). Absence of ternary feldspars in all lithologies impedes the determination of temperatures from feldspar thermometry (Fig. 5a). However, in monzonite some feldspars show minor orthoclase component, constraining its crystallization temperatures (~ 700°C; Fig. 5a). Similar temperatures of feldspars with as phlogopite suggest an increasing K/Na ratio of the magma. In accordance with the evolution of clinopyroxene, amphibole and mica, there is a decrease in their bulk-rock MgO, FeO, TiO₂ and CaO contents in the sequence of ultramafic enclaves → monzodiorite and monzonite → syenite (Fig. 8).

Table 7

Amphibole and mica group minerals thermometry in pyroxene hornblendite, monzodiorite, monzonite, syenite and albitite.
Rock type	Amphibole nomenclature	Amphibole temperature range (°C)	Mica group mineral nomenclature	Mica group mineral temperature range (°C)
Pyroxene hornblendite	Ti-rich pargasite	970–873	-	-
Monzodiorite	Magnesiohastingsite	804–703	Phlogopite	698 − 629
	Pargasite	795–719
	Actinolite	472
Monzonite	Magnesiohastingsite	938–660	Phlogopite	754 − 649
	Pargasite	876–684	Annite	700 − 697
	Actinolite	400	Annite	700 − 697
Syenite	Hastingsite	748–681	Phlogopite	640 − 570
Syenite	Actinolite	340–540	Phlogopite	640 − 570
Albitite	-	-	Annite	651 − 600
^Calculated from: \(T\left(ᵒ\text{C}\right)=\frac{2400}{1.52-{\text{l}\text{o}\text{g}\text{T}\text{i}}^{\text{A}\text{m}\text{p}}}-273),\)where T is temperature and logTi^Amp is the Ti content of amphibole in atoms per formula unit (apfu*) expressed in the logarithm to base 10 (Liao et al., 2021);
^#Calculated from: \(T\left(ᵒK\right)=\frac{838}{1.0337-\left(\frac{\text{T}\text{i}}{{\text{F}\text{e}}^{2+}}\right)}\); where T is temperature and Ti and Fe²⁺ content of amphibole in atoms per formula unit (apfu) (Luhr et al., 1984).

Additionally, the early incorporation of alkalis in pargasite and hastingsite increases the Ca content of the magma resulting in the crystallization of Ca-rich plagioclase in monzodiorite and monzonite (Litvinovsky et al., 2002). The plagioclase compositions gradually vary from andesine and oligoclase in monzodiorite and monzonite to more albite-rich in syenite and granite (Fig. 5a). This is reflected by the negative correlation of CaO and the positive correlation of Na₂O relative to SiO₂ (Fig. 8a, c). Furthermore, the positive correlation of K₂O and Al₂O₃ relative to SiO₂ can be attributed to increasing modal abundance of K-feldspars in monzodiorite, monzonite, syenite and granite (Figs. 7; 8b, d).

Actinolite in monzonite, monzodiorite and syenite replaces hastingsite (Fig. 4a) and is formed at hydrothermal conditions with low crystallization temperatures (340–540°C; Table 7) compared to phlogopite, pargasite and actinolite. Epidote and clinozoisite also forms at hydrothermal stages in monzonite and monzodiorite with increasing H₂O and fO₂ contents in the magma derived fluids (Figs. 9; 10).

The granite is characterized mainly by end-member feldspars (K-feldspar and albite) and quartz with abundant chlorite and epidote. Additionally, it lacks any high temperature mafic minerals as found in pyroxene hornblendite, monzodiorite and monzonite. Such abundance of low temperature chlorite (~ 188°C; Online Resource 2) and epidote indicates late-magmatic to hydrothermal origin for granite (Thomas et al., 2021) (Figs. 9).

Field occurrences suggest albitite occur as pegmatite veins and crosscuts monzonite, syenite, and carbonatite (Figs. 2e, f). Albitite crosscutting the carbonatites is exclusively characterized by pyrochlore, magnesioriebeckite, and calcite (Dey et al., 2021a). The absence of such assemblage in albitite crosscutting monzonite and syenite suggests that the albitite, which crosscuts the dolomite carbonatite, may be contaminated (Fig. 10). The only mafic minerals present in these albitite is annite which bears compositional resemblance to some micas in monzonite and syenite forming at low temperatures (600–651°C; Table 7) attributing their hydrothermal origin (Figs. 5f, 9; 10). Furthermore, the end-member compositions of albite and K-feldspar, coupled with shared geochemical traits between albitite and syenite (Figs. 5a, 6, 7), and the pegmatitic occurrences of albitite, all point towards its origin during a later, evolved stage emanating from the syenite.

The formation of similar monzodiorite-monzonite-syenite suite is reported from many global occurrences such as Oshurkovo plutonic sheeted complex, Transbaikalia, Russia (Litvinovsky et al., 2002), Yamato mountains, East Antarctica (Zhao et al., 1995), Amran Massif, Southern Israel (Mushkin et al., 2003), Mongolian-Transbaikalian Belt (MTB), Russia (Litvinovsky et al., 2015) and Nechalacho layered suite, Canada (Möller and Williams-Jones, 2016) is related to fractional crystallization of a basaltic magma. Previous studies on Sevattur syenite and granites show similar Sr and Nd isotope ratios, inferring a common mantle source with minimal crustal contamination (Miyazaki et al., 2000). The Sevattur silicate rocks in the present work also show a geochemical similarity to Yelagiri syenites with an alkaline basalt affinity (Figs. 6; 7c). Considering the mineralogical and geochemical characteristics presented in this work, we infer that monzodiorite, monzonite, syenite, and granite are co-magmatic and related through fractional crystallization of a parental alkali-rich basaltic magma (Fig. 7c).

Fenitization or hydrothermal mineralization?

Previous works have shown that monzodiorite and monzonite [Fig. 6; similar to the syenites of Miyazaki et al. (2000)] are younger (~ 755 Ma) than the most evolved ankerite carbonatites (~ 801 Ma) at Sevattur; implying that the syenites and albitite cannot be fenitized by carbonatite. However, previous works have proposed the formation of syenites through the fenitization of the granite gneiss country rocks (Viladkar and Subramanian, 1995), but they lack detailed mineralogical evidence supporting fenitization.

Generally, fenitization is marked by an increase in whole rock alkali and a decrease in silica contents (Brögger, 1921). On the contrary, at Sevattur, the only alkali-bearing phases contributing to high alkalis in granite and syenite are the K-feldspar, albite and a few phlogopites, evidenced by the concomitant increase of Al₂O₃, K₂O and Na₂O with SiO₂ in the whole rock during magma evolution (Fig. 9). Fenitized rocks are usually rich in alkalis, complemented by the presence of alkali-rich clinopyroxene (aegirine) and amphibole (richterite, arfvedsonite, and riebeckite), along with phlogopite and alkali feldspars (Elliott et al., 2018). Thus, the absence of alkali-rich pyroxenes and amphiboles in the studied rocks suggests the absence of fenitization in the formation of the studied silicate rocks

The texture and mineralogy of the Sevattur silicate rocks suggests that these are genetically related. This is well complemented by their whole-rock compositions. The magmatic to hydrothermal evolution and corresponding mineral paragenesis of pyroxenite hornblendite, monzodiorite, monzonite, syenite, albitite and granite is modelled (Fig. 10). The following conclusions are drawn from our discussions above:

A coherent mineralogical evolution and the whole-rock compositions suggest that the silicate rocks at Sevattur are co-magmatic and related by fractional crystallization of an alkali-rich basaltic parental magma.

The ultramafic enclaves of hornblende pyroxenite in monzodiorite and monzonite represents the early cumulates derived from a primitive magma with high alkali, fluorine and phosphorus and water content.

The granite and albitite represent a shallower, late-fractionates of the same magmas that generated monzodiorite-monzonite-syenite suite with relatively high H₂O and oxidizing conditions.

The studied lithologies are late-intrusives to the carbonatites and the effect of fenitization is ruled out. The silicate rock formations found at Sevattur have their own distinct evolutionary history. When these rocks came into contact with carbonatites, they incorporated certain materials from the carbonatites. This is evident in the presence of albitite, which contains pyrochlore and is located in close proximity to the carbonatites.

Acknowledgements

S.D.S, M.D, S.B and A.C thank the Director of IISER Tirupati for providing financial support through an Institute Travel Grant that enabled them to carry out the fieldwork. The Ministry of Education, Government of India, is acknowledged by S.B and M.D for providing financial assistance through the Institute Fellowship programme. This work is significant component of S.D.S Master’s thesis work and a part of M.D’s PhD work. The EPMA work at Department of Geology, Banaras Hindu University, was supported by two major research projects granted to N.V.C.R. by the Department of Science and Technology Science and Engineering Research Board (DST-SERB), New Delhi (SR/S4/ES-599/2011 dated 27.5.2013 and IR/S4/ESF-18/2011 dated 12.11.2013). NS and K.B.J thanks Secretary, MoES and the Director, NCESS, Thiruvananthapuram for providing unconditional support to carry out the EMP and XRF work. We also thank Marco Brenna for comments and suggestions on the final draft of the manuscript.

The authors assert that there are no discernible financial conflicts or personal relationships that could be perceived as influencing the research presented in this paper.

Author contributions

Data curation, Methodology, Writing - original draft preparation, Writing – review and editing: Swastik Dilip Shinde,Monojit Dey, Sourav Bhattacharjee;Supervision, Methodology, Data curation, Formal analysis, Writing – review and editing: Rohit Pandey, Nittala Venkata Chalapathi Rao; Formal analysis and Writing–review and editing: Sneha Mukherjee, Nilanjana Sorcar, and Kumar Batuk Joshi; Supervision, Methodology, Resources and Writing – review: Supratim Pal; Conceptualization, Methodology, Funding acquisition, Investigation, Project administration, Resources, Supervision, Writing - review and editing: Aniket Chakrabarty

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No competing interests reported.

Unveiling the petrogenesis of alkaline silicate rocks at the Sevattur Carbonatite Complex, India

Status:

Version 1

Abstract

Figures

Introduction

Regional geology and field settings

Analytical methods and sample methods

Petrography

Monzodiorite, monzonite, syenite, and albitite

Granite

Ultramafic enclave

Mineral compositions

Feldspar

Amphibole

Clinopyroxene

Phlogopite

Other minerals

Whole-rock geochemistry

Discussion

Ultramafic enclaves: Primary cumulates or xenoliths?

Petrogenesis of the monzodiorite, monzonite, syenite, albitite and granite

Fenitization or hydrothermal mineralization?

Conclusions

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1