1 Introduction

The large Igneous Provinces (LIPs) are generally known for its enormous basaltic magmatism and their economic potential.1,2,–3 The magma generation and diverse magmatic assemblages in the LIP record are intriguing and the magma generation processes involves mantle melting, creation of pathways for the melt transfer and wall rock interactions.2, 4 However, while considering the complex nature of the magmatic systems, metasomatic reactions, compositional variations during ascent, branching and splitting of magma volumes are also needed to be evaluated. Moreover, the formation of intermediate magma chambers in the lower crustal regions and upper mantle discontinuities and other magmatic intrusions associated with LIPs are often accompanied by melt differentiation.5 The LIPs may contain volumetrically minor amount silica saturated peralkaline volcanic rocks with anorogenic genetic characters, widely known as A-type granites. Even though these ‘within plate’ granitic magmatism is volumetrically small, they are important in understanding the evolution of the crust and they give important insights into the origin and diversity of magmas produced during the LIP events.6 Generally, these granitic batholiths exhibit variable compositions and are thought to be formed either by the fractionation of a basaltic magma with or without crustal contamination or by the plume-induced melting of the crust. In some cases, compositionally diverse peraluminous, peralkaline and metaluminous A-type granitic intrusions in one particular region contemporaneous with the same LIP event is rather special because their sources could be different or they owe different petrogenetic histories.7

The Deccan Large Igneous Province (Fig. 1), one of the largest igneous province in the world with an eruptive aerial extent of 106 km,2 predominantly composed of tholeiitic basalts exhibit a wide range of isotopic ratios, major and trace element compositions.8,9,–10 The magmatic imprints of Deccan LIP indicate diverse rock assemblages which might have been produced by various modification processes such as fractional crystallization, crustal contamination, AFC, etc. However, the addition of continental lithospheric magmas into the main plume impulse could also be attributed to the wide compositional spectrum of Deccan LIP.11, 12 The major derivative phases such as alkali basalts, carbonatites, lamprophyres, and syenites were studied by various researchers13,14,15,16,–17 to evaluate the magmatic and metasomatic histories. However, considering the felsic volcanic imprints associated with the Deccan LIP, only the rhyolites have been studied in detail.18, 19 The granites associated with Deccan LIP are minor and the majority of these A-type granites are confined to the Phenai Mata region. In the present study, we discuss the mineralogy and geochemistry of the A type granites and basic units from Phenai Mata Igneous Complex (PMIC) to deduce the petrogenetic history of PMIC.

Figure 1:
figure 1

The generalized geological map of the Deccan Large Igneous Province showing the location of the Phenai Mata Igneous Complex50.

Full size image

2 Field Relations

The Chhotaudepur sub-province of Deccan LIP comprises mainly of two major igneous complexes namely, Ambadungar alkaline igneous complex in the southeast and the PMIC in the western part (Fig. 2). The stratigraphic succession of this area includes Bagh sandstone of Cretaceous age underlying the Deccan trap basaltic flows.20, 21 The Ambadungar complex is mainly characterized by alkaline silicates and carbonatites. The main alkaline rocks of this terrane are nephelinite, tinguaite, etc. The presence of calcite-carbonatite and small plugs of red-brown ankeritic-carbonatite from Ambadungar complex is intriguing.22, 23 The PMIC comprises of gabbro, granophyre, tholeiitic basalt, doleritic dykes and dyke-lets of granite.24, 25 Felsic rocks constitute 1/4th of the Phenai Mata hill which mainly includes monazite, quartz monzonite, granite and granodiorite (Fig. 3a). The magma mixing and mingling processes are conspicuous in many portions of the Phenai Mata hill.26 Majority of the Phenai Mata region is characterized by the presence of xenoliths of mafic material in felsic rock indicating that the felsic magmatism followed was by the mafic magmatism or else contemporaneous eruption of both types of magma might have taken place. Gwalani et al.27 reported basalt granophyre, hornblende dolerite, layered gabbro, gabbro, anorthosite and pulaskite from this area. Kumar24 reported syenite, dolerite, lamprophyre, microgranite, tholeiitic gabbro, basalt and fine-grained gabbro from PMIC. Hari.et al.28 reported alkali feldspar syenites with shoshonitic affinities from this area and proposed that they were derived from a metasomatically enriched mantle in an intracratonic rifting environment. Many fine-grained Mafic magmatic enclaves (MMEs) are found as inclusions in the host granites (Fig. 3b, c). Several syn-plutonic dykes (Fig. 3d) have also been demarcated from the PMIC. Hybrid rock zones where fine and coarse-grained mafic and felsic material being mixed is sporadically seen in the field. Texturally and mineralogically, diverse MMEs include densely porphyritic, lensoidal, ultramafic pegmatoidal, densely porphyritic mesocratic, sparsely porphyritic leucocratic, moderately porphyritic mesocratic, moderately porphyritic leucocratic, and felsic pegmatoidal types are noticeable in the PMIC.26 The synplutonic dykes of PMIC exhibit feature such as necking down and changing trends, irregular, curvy and concave margins, ropy twists, intense back-veining and protuberances/dykelets within the granites.26

Figure 2:
figure 2

(Modified after Hari et al.)28

The regional geological map of the Phenai Mata Igneous complex and adjacent regions.

Full size image
Figure 3:
figure 3

Field photographs showing a the panoramic view of the Phenai Mata hill showing the main mafic exposures and gabbroic rocks in the bottom and central portions and the top portion of the hill is mainly composed of felsic rocks. b interaction of felsic and mafic magmas in the top part of the PMIC, c moderately porphyritic mesocratic MME with in the granitic rocks of PMIC, d irregular syn-plutonic mafic dyke in the granitoids of the PMIC.

Full size image

The felsic samples collected from PMIC are categorized as granite, granodiorite, monzonite and quartz monzonite as per the classification of Middlemost 29 (Fig. 4). The mafic volcanic rocks associated with the felsic rocks of the Pheani Mata hill are classified as basaltic andesites on the basis of TAS diagram (not shown). The mineral chemistry of felspars of granites, monzonites, and quartz monzonites are given in Tables 1, 2, and 3 respectively. Mineral chemistry of pyroxenes of monzonite is presented in Table 4. The major and trace element geochemistry of felsic rocks are presented in Table 5 and the major and trace element geochemistry of the mafic rocks are presented in Table 6.

Figure 4:
figure 4

Classification of the felsic rocks from the PMIC as granites, granodiorites, monzonites and quartz-monzonites based on the classification of Middlemost29.

Full size image
Table 1: Mineral chemistry of feldspar from granite samples.
Full size table
Table 2: Mineral chemistry of feldspar from Monzonite.
Full size table
Table 3: Mineral chemistry of feldspar from Quartz monzonite.
Full size table
Table 4: Mineral chemistry of pyroxene from Monzonite.
Full size table
Table 5: The major and trace element geochemistry of monzonites, quartz monzonites, granites and granodiorites from the PMIC.
Full size table
Table 6: Major and trace element geochemistry of the mafic rocks of PMIC.
Full size table

3 Analytical Techniques

3.1 Mineral Chemistry

Chemical compositions of pyroxenes in the monzonites and the plagioclases in the quartz monzonite, granite and monzonite were analyzed using a CAMECA SX100 electron microprobe at the Institute of Mineralogy and Mineral Resources, Technical University of Clausthal, Germany. A beam current of 20 nA acceleration voltage of 20 kV and a beam diameter of 1 micron was used. Both natural and synthetic standards were employed and the details of the analytical procedures are given in Torab and Lehman.30

3.2 Whole Rock Major and Trace Elements

Major and trace element concentrations were analyzed using X-Ray Fluorescence Spectrometer (XRF) and Inductively Coupled Plasma-Mass Spectrometer (ICP-MS) at Wadia Institute of Himalayan Geology, Dehradun. The samples were crushed and pulverized with agate carbide ring grinder. The pressed pellets were used for the major element analysis. The open digestion method was used for the measurement of trace elements and REEs. Rock powders were dissolved in HF and HNO3 in Teflon crucibles and heated over a hot plate for about 48 h. Further, the sample precipitate was used to prepare solutions for the measurement of trace element abundances using an ICPMS. The details of the analytical procedures are given in Singh et al.31

4 Petrography and Mineral Chemistry

The major mineral assemblages of the granite include alkali feldspars, plagioclases and quartz. The accessory minerals include biotite, hornblende, titanite, apatite and zircon (Fig. 5b). Some portions of the granites contain augite to sub calcic augite pyroxene grains. In the feldspar classification diagram (Fig. 6a), all the dominant feldspars from the granites plot in the sanidine field (Or53-83 Ab17-46 An0.07-3). Intergrowth textures like myrmekitic, poikilitic and perthite have also been identified in the granites. Zoning is observed in some plagioclase grains in the granites and granodiorites. Plagioclases observed in the granodiorites are partially sericitized and saussuritized. Micrographic texture (Fig. 5d) is noticeable in monzonite only. Interlocking arrangement of quartz and feldspar are noticeable in most of the samples and it might have been originated by the simultaneous crystallization of quartz and plagioclase from a silicate melt with the presence of a hydrous phase. Chlorite minerals are seen as accessories along with biotite in most of the monzonite samples. The pyroxenes (Fig. 5d) of monzonite is classified as augite (Fig. 6b). The presence of magnetite and titanomagnetite are also noticeable in the monzonite and quartz monzonite. The quartz monzonite samples are characterized by abundant plagioclase phenocrysts (Fig. 5a, c). Most of the plagioclases are sericitized and saussuritized. In the groundmass of quartz monzonite samples, chlorite minerals are found as accessories along with few biotite flakes. Silicate liquid immiscible globules are noted in many sections of quartz monzonite (Fig. 5c). Some xenolith fragments exhibit trachytic texture and which is perceptible in granodiorite (Fig. 5b). The interstitial regions of the granodiorite sample exhibit crystalline nature. Alkali feldspar and plagioclases in the granodiorite samples are generally altered to sericite (Fig. 5b). The abundant granophyric texture observed in some of the samples point towards an anorogenic setting which are related to the continental rift environment.32, 33

Figure 5:
figure 5

a Granophyric texture in the quartz-monzonite show Micrographic groundmass surrounds feldspar phenocryst fragment. b The pyroxene grain exhibit assimilated fragments from the felsic host. c Presence of silicate liquid immiscibility globules in the monzonite sample. d The granophyric texture in the monzonite radiating fringe of feldspar grains and the growth of granophyre. e, f Basalts from the PMIC exhibit flow textures with plagioclases and clinopyroxenes.

Full size image
Figure 6:
figure 6

a Conventional classification diagram of the feldspars from the quartz monzonite, monzonite and granite, b classification diagram of pyroxenes in the monzonite sample.

Full size image

The basaltic andesites of the PMIC mainly composed of pyroxenes and plagioclases as phenocrysts and the groundmass are generally composed of plagioclase, clinopyroxene, glass and opaques (Fig. 5e, f).

5 Whole Rock Geochemistry

The granite samples exhibit higher SiO2 (73.4–75.32 wt%) contents relative to the granodiorite (62.4–66 wt%), monzonite (~ 60 wt%) and quartz monzonite (62.66–66.19 wt%). All the samples exhibit relatively low MgO values (monzonite: 2.87–2.94 wt%; quartz monzonite: 0.13–1.65 wt%; granite: 0.5–1.38 wt% and granodiorite: 1.4–3.28 wt%, respectively) (Table 5). In the variation diagrams (Fig. 7), all major elements show a general decreasing trend with the increase in SiO2 content. When the felsic rocks of PMIC are compared with the associated mafic rocks of PMIC, all the samples except the granites exhibit a linear correlation. Feldspar fractionation is evident from the decreasing Al2O3, CaO and Sr with the SiO2 increase. The Fe–Ti oxide fractionation is evident by the progressive decrease in TiO2 and Fe2O3 with SiO2 increase (Fig. 7). All the felsic samples exhibit enriched REE (Fig. 8) with low concentrations of Co, Sc, Cr (11.14–94.39 ppm), Ni (1.15–9.57), and Eu (0.73–2.65 ppm). The REE chondrite and primitive mantle normalized diagrams of different felsic units are shown in Fig. 8. In the primitive mantle normalized multi-element diagram (Fig. 8), granites exhibit Rb, Ta, Nb, Sr, Hf, Zr, Ti negative anomalies and Th, La, Nd, and Zr positive anomalies, whereas granodiorite, monzonite and quartz monzonite exhibit Ba, Sr, Hf, Zr, and Ti negative anomalies. The granite has the lowest whereas monzonite has the highest REE concentrations. In all the cases, prominent negative Eu anomaly is noticeable indicating plagioclase fractionation.

Figure 7:
figure 7

Variation diagram of SiO2 with other major oxides exhibit definite trend for the mafic and felsic samples except for the granites.

Full size image
Figure 8:
figure 8

Chondrite normalized REE and Primitive mantle normalized multi element diagrams of a, b granite, c, d granodiorite, e, f monzonite and g, h quartz monzonite Normalizing values for the chondrite and Primitive mantle are from Sun and McDonough49.

Full size image

The mafic rocks of Phenai Mata exhibit low SiO2 (56.8–57.1 wt%) and high Fe2O3(10.7–11.3 wt%), Al2O3 (14.1–15.6 wt%) and TiO2 (1.1–1.57 wt%) contents (Table 6). In the chondrite normalized REE diagram, mafic samples exhibit LREE enrichment relative to the HREE (Fig. 9a). In the primitive mantle normalized multi-element diagram, the samples exhibit Pr, Zr and Ti negative anomalies and La, Nb, Ta, Nd and Gd positive anomalies (Fig. 9b). When the mafic samples of PMIC were compared with the basalts from the chhotaudepur province, and found that the REE and primitive mantle normalized multi-element patterns are consistent with the reported data34.

Figure 9:
figure 9

a Chondrite normalized REE and b primitive mantle normalized multi element diagram of basalts of PMIC. Basalts from the Chhotaudepur province34 are also given for comparison. Normalizing values for the chondrite and Primitive mantle are from Sun and McDonough49.

Full size image

6 Discussions

In the 1000*Ga/Al vs. Ce and 1000*Ga/Al vs. Zn classification diagrams32 (Fig. 10) all the felsic samples from the PMIC plot in the A-type granites field. Further, when the felsic samples were plotted in the granitoid discrimination diagrams proposed by Pearce et al.35 and Whalen et al.32, all the samples fall on WPG (within plate granitoid) field (Fig. 11). Generally, A-type granitoids will be enriched in LREE (La/Yb = 12.3–37.3, except sample P-5b with 65.1) and will have relatively flat to somewhat depleted HREE (Ce/Yb = 2.2–5.5) with prominent negative Eu anomalies in the chondrite normalized REE patterns (Fig. 8). The prominent negative Eu anomalies and depleted HREE in the studied samples substantiate the anorogenic nature of the felsic units of PMIC. Loiselle and Wones36 opined that A-type granitoid contain high K2O/Na2O, Fe/(Fe + Mg), K2O and incompatible trace element concentrations (including REE, Zr, Nb and Ta). Whalen et al.32 proposed that A-type granitoid contain higher abundance of Al, Mg, Ca, V, Fe, K + Na, Zn, Ga, Nb, Y, Zr and REE (except Eu). In the present study, the K2O/Na2O ratios of the monzonite and quartz monzonite samples exhibit ≥ 1 values, whereas the granites and granodiorites exhibit < 1 values. Except granites (Zr: 45.9–72.8 ppm; Nb: 16.7–24 ppm) all other samples (Zr: 116.6–1395 ppm; Nb: 56.1–111 ppm) exhibit relatively higher incompatible element concentrations.

Figure 10:
figure 10

The a 1000*Ga/Al vs. Ce and b 1000*Ga/Al vs. Zn classification diagrams32 shows all the felsic samples from the PMIC plot with in the field of A-type granites.

Full size image
Figure 11:
figure 11

a Y (ppm) vs. Nb (ppm) and Nb + Y (ppm) vs. Rb (ppm) tectonic discrimination diagrams indicates a with in plate granite character for the PMIC felsic rocks (after Pearce et al.)35.

Full size image

The exact mode and mechanisms of formation of the anorogenic granitic intrusions are still not clear. Loiselle and Wones36 suggested that A-type granitoid was formed by the fractionation of the mantle-derived alkali basalts without crustal interaction. Subsequently, Collins et al.37 proposed that high temperature, vapor absent melting of residual felsic granulite source may generate an anhydrous melt containing halides and higher amount of highly charged cations such as Zr and REE that are abundant in A-type granitoid. Anderson38 suggested that partial melting of quartz diorite, tonalite and granodiorite also exhibit a compositional character similar to the A-type granitoids. The crustal component that had undergone a previous partial melting episode along with mantle plume impingement/diapirism in an extensional regime may also contribute to the formation of anorogenic magmatism.39 According to Frost and frost,40 the combination of partial melting of crustal rocks and differentiation of basaltic magma, and assimilation of crustal components into the ascending basaltic magma can produce A-type granitoids.

When the samples of the present study are plotted in different A-type granite discrimination diagrams (Fig. 12), the granites plot in the A2 type granite region whereas the granodiorites, monzonites and quartz monzonites plot in the A1 type granite field. The A2 type granites are widely considered as the product of crustal melting and the genesis A1 type is generally attributed to the fractionation of a basaltic parental melt.41 The association of A1 and A2 type granites along with mafic rocks clearly point towards a complex magmatic history of the PMIC. Greenberg42 suggested that evolved anorogenic magmatism and primitive anorogenic magmatism are the two main processes operated for the formation of these type of rocks. The rocks from Phenai Mata complex are having characteristics of evolved anorogenic magmatic rocks, as well as primitive magmatic rocks.

Figure 12:
figure 12

a Nb-Y-3*Ga, b Y–Nb–Ce and Y/Nb vs. Rb/Na discrimination diagrams for A-type granites of PMIC (From Eby)45.

Full size image

In the variation diagrams with SiO2 (Fig. 7) the monzonites, quartz monzonites and granodiorites exhibit well-defined trends with the mafic rocks of PMIC indicating that these rocks may be cogenetic in nature. However, the granites of PMIC do not exhibit any geochemical trends with the associated felsic and mafic units. Furthermore, granites exhibit prominent Nb–Ta anomalies in the primitive mantle normalized diagram (Fig. 8), whereas other felsic units do not have negative Nb and Ta anomalies pointing that the granites in this area have a different genesis than the other felsic rock units. However, the A-type granitoids appear to represent crustal growth and/or crustal differentiation during tectonic events generally unrelated to the subduction processes.41

The field evidence from the PMIC indicates mixing and mingling of at least two contrasting magma compositions in the genesis of these anorogenic intrusions. The A-type granite discrimination diagrams (Fig. 12) clearly demarcates the granites as the A2 type and the other felsic rocks as A1 type. It is clear that multiple magmatic processes which are contemporaneous with the plume event in the DLIP were attributed to the formation of the A-type granites of PMIC. Bonin41 presented evidence for the generation of A1 type granites from the mantle derived mafic and intermediate magmas. In the generation of the A2 type granites significant amount of crustal components might have been involved.43,44,45 Genetic models proposed for the generation of A2 type granites include the crustal melting, partial melting of lower crust felsic granulites which were depleted in the course of earlier melting episodes,35 melting of meta-igneous,46 melting of alkali-metasomatized compositions,47 and involvement of mixed OIB-crust sources.44, 45

For the crustal melting process, pounded and underplated basaltic magmas in the DLIP might have supplied the necessary heat for the melting of the upper crust.48 The continuous/episodic interaction of plume head with the crustal components might have resulted in the partial melting of the granitic basement to form the A2 type granites in the PMIC. At the same time, the fractional crystallization of the basaltic magma would have ultimately resulted in the generation of A1 type granites. It is also possible that the magmas which were stalled at the base of the crust or coalesced into the local magma chambers would have resulted in the crystal fractionation before the injection into the shallow crust.50 The MMEs, syn-plutonic dykes and hybrid zones which are exposed in the PMIC indicates both A1 and A2 type granites and the basaltic magmas were mixed and mingled at shallow crustal levels before their emplacement.

7 Conclusions

  1. 1.

    The presence of mafic enclaves, syn plutonic dykes, interaction of mafic and felsic magmas at the PMIC point towards a complex magmatic environment for their genesis.

  2. 2.

    All the field and elemental signatures of the felsic rocks from the PMIC exhibit a close proximity to the A-type granites.

  3. 3.

    In the primitive mantle normalized multi-element diagram, granites exhibit negative anomalies at Rb, Ta, Nb, Sr, Hf, Zr, Ti and positive anomalies at Th, La, Nd, and Zr, whereas granodiorite, Monzonite and quartz monzonite exhibit negative anomalies at Ba, Sr, Hf, Zr, and Ti.

  4. 4.

    The geochemical characters of monzonites, quartz monzonites and granodiorites from the PMIC are similar to that of A1 type granites, which is generally considered as a product of fractional crystallization, whereas the granites from the PMIC which exhibit A2 type geochemical characters may be derived by the melting of the pre-existing crust or by the incorporation of the crustal components during the ascent of the magma.

  5. 5.

    The contrasting magmatic signatures of the felsic rocks from the PMIC indicates that the A1 type rocks were derived as a result of fractional crystallisation of basaltic magma, whereas the A2 type might have been derived by the plume-induced crustal melting or by a complex process involving both fractional crystallisation and crustal melting.

  6. 6.

    The field and geochemical evidence from the PMIC indicate that the contrasting felsic magmas and mafic magmas were mixed and mingled at crustal or shallow crustal level to form complex field relationship.