Origin of calcite by magma mixing in mingled rocks of the Ghansura Rhyolite Dome, Bathani volcano-sedimentary sequence, eastern India

The GRD, occurring within the BVSs and exposed in the Ghansura village near Bathani, is a small felsic dome or laccolith, which represents a fossilized subvolcanic magma chamber (Gogoi & Chauhan, 2021). The outcrop records good evidence of magma mixing and mingling (Gogoi et al., 2018). The rhyolite dome exhibits crosscutting relationship with the volcano-sedimentary sequence, indicating that the dome was emplaced later (Fig. 2B). Different rock types ranging from mafic to felsic are observed in the rhyolite dome (Figs. 3A and 3B). Apart from the mafic and felsic rocks, four different types of hybridized rocks are also found in the GRD. These rocks include mafic rocks containing felsic clasts, mingled rocks, porphyritic rocks, and non-porphyritic rocks displaying emulsion texture. These hybrid rocks are described in detail by Gogoi and Chauhan (2021).

Figure 3.

This study focuses on the mingled rocks identified within the rhyolite dome (Figs. 3C and 3D). The mingled rocks are characterized by intermingled mafic-felsic zones where the individual mafic and felsic components are clearly identifiable. The contacts between the mafic and felsic components are sharp and display cuspate-lobate pattern (Fig. 3C). Presence of cuspate and lobate contacts at outcrop scale is indicative of the consequence of magma mingling of coeval felsic and mafic magmas (Barbarin & Didier, 1992). Distinct reaction surfaces, which appear to be intermediate in composition, can be seen at the mafic and felsic contacts in the mingled rocks (Fig. 3D).

The rocks under study can be categorized into three distinct groups: (a) mafic endmember, i.e. basalt, (b) felsic endmember, i.e. rhyolite, and (c) hybrid intermediate rocks resulting from the mingling of ‘a’ and ‘b’. A comprehensive overview on petrography of the rock types found in the GRD is given in Gogoi et al. (2018).

3.2.1.

Basalt

The basalts constitute dominantly of pyroxene (4502,000 μm; 45–55 vol%), plagioclase (90–200 μm; 35–40 vol%), and Fe-Ti oxides (50–350 μm; 5–10 vol%). The rocks depict holocrystalline texture. Some of the samples exhibit porphyritic texture with phenocrysts of augite (Fig. 4A). Most of the plagioclase laths are partly or entirely embedded within augite resembling sub-ophitic and ophitic texture, respectively (Fig. 4B). The basalts also consist of vesicles filled with secondary minerals such as calcite and chlorite, representing amygdaloidal texture (Fig. 4C). Pyroxene occurs only in these mafic rocks. The mineral chemical data of the pyroxenes were taken from already published works (Gogoi & Chauhan, 2021; Gogoi & Saikia, 2018). Pyroxene occurring in the mafic endmember of our study was classified as clinopyroxene and plots in the field of augite (Fig. 5).

Figure 4.

Figure 5.

3.2.3.

Mingled rocks

The mingled rocks are identified by intermingled mafic and felsic zones. These rocks are fine to medium grained and consist dominantly of amphibole (80–180 μm; 20–30 vol%), biotite (30–150 μm; 20–25 vol%), quartz (20–40 μm; 10–15 vol%), plagioclase (30–100 μm; 10–15 vol%), K-feldspar (20–40 μm; 5–10 vol%), ilmenite (30–330 μm; 5–10 vol%), calcite (25–500 μm; <5 vol%), and titanite (30–120 μm; <5 vol%). Two distinct microzones can be observed in thin section under the microscope: (a) mafic zone, with size ranging from a few millimetres to 15 mm, consisting essentially of medium-grained amphiboles that are basically secondary phases after clinopyroxene (henceforth referred to as amphibole-rich microzones [ARM]), and (b) felsic zone, which is similar in size to the mafic zones, consisting essentially of fine-grained quartz, K-feldspar, biotite, and plagioclase. These two distinct microzones are in contact with each other. The contact is marked by fine-grained felsic minerals and medium-grained dark green-coloured amphibole grains (Fig. 6A). The extent of the dark green amphibole grains from the contact with felsic zones reaches up to 2,000 μm, while the entire mafic zone extends up to 15 mm indimension.

Figure 6.

An important feature observed during petrographic study of these rocks is that amphiboles occurring at the boundary of the ARM is optically different than the amphiboles occurring at the interior. The amphiboles present in the interior are pale green to colourless, whereas exterior amphiboles show a dark green colour (Fig. 6A). The mafic zones containing dark green amphiboles also consist of minerals like biotite, calcite, and ilmenite (Figs. 6B–D and 7). However, such minerals are not observed in the mafic zones containing pale green amphiboles.

Figure 7.

Mineral chemical analyses of four mineral phases are presented with a view to understand the formation of calcite in the mingled rocks of GRD. The minerals include amphibole, biotite, calcite, and ilmenite. Detailed descriptions of each of the mineral phases are given below.

5.1.1.

Amphibole

Amphibole compositions were determined from the ARM in the mingled rocks. The ARM consist of pale green interior and dark green exterior zones. A total of 16 point analyses were obtained from amphibole grains occurring in the interior and exterior of the ARM (Table 1). The amphibole mineral formulae were determined on the basis of 23 oxygen atoms. The classification of amphibole was done following the method described in Leake et al. (1997). The interior, pale green amphiboles of the ARM are classified as actinolite, while the exterior, dark green amphiboles plot in the fields of magnesio-hornblende and ferro-hornblende (Fig. 8).

Figure 8.

5.1.2.

Biotite

A total of eight point analyses were obtained from biotite crystals occurring in the hornblende-rich mafic zones or dark green exterior regions of the ARM. EPMA analyses of biotite are provided in Table 2. The biotite mineral formulae were determined on the basis of 22 oxygen atoms. According to the classification of Tischendorf et al. (1997), brown-coloured biotite grains associated with hornblende are classified as Fe-biotite (Fig. 9).

Figure 9.

5.1.3.

Ilmenite

A total of 8 point analyses were obtained from ilmenite crystals occurring in the hornblende-rich mafic zones. EPMA analyses of ilmenite are provided in Table 3. The ilmenite mineral formulae were determined on the basis of three oxygen atoms. According to the TiO₂-FeO-Fe₂O₃ classification diagram, the compositional analyses plot near the ideal composition of ilmenite (Fig. 10).

Figure 10.

5.1.4.

Calcite

A total of 12 point analyses were obtained from calcite crystals occurring in the hornblende-biotite-rich zones. EPMA analyses of calcite are provided in Table 4. The locations of spot analyses for calcite are shown in a back-scattered electron (BSE) image (Fig. 11). According to the CaCO₃-MgCO₃-(Fe + Mn)CO₃ classification diagram, the compositional analyses plot near the ideal composition of calcite (Fig. 12).

Figure 11.

Figure 12.

There is no experimental evidence for crystallization of actinolite from melt (Stephens, 2001). Actinolite is usually produced by solid-state reaction from earlier crystallized clinopyroxene crystals. In a magma mixing scenario, when clinopyroxene-bearing mafic magmas come in contact with felsic magmas, diffusion of H ions and volatile species from the felsic to the mafic endmember promotes the replacement of clinopyroxene by actinolite in the mafic phase (Castro & Stephens, 1992). The equation related to this replacement can be written as: 1 4Ca0.5Mg0.75Fe0.75Si2O6+2H→Ca2Mg3Fe2Si8O22(OH)2+Fe(clinopyroxene)(actinolite) \[\begin{matrix} 4\text{C}{{\text{a}}_{0.5}}\text{M}{{\text{g}}_{0.75}}\text{F}{{\text{e}}_{0.75}}\text{S}{{\text{i}}_{2}}{{\text{O}}_{6}}+2\text{H}\to & \text{C}{{\text{a}}_{2}}\text{M}{{\text{g}}_{3}}\text{F}{{\text{e}}_{2}}\text{S}{{\text{i}}_{8}}{{\text{O}}_{22}}{{\left( \text{OH} \right)}_{2}}+\text{Fe} \\ \left( \text{clinopyroxene} \right) & \left( \text{actinolite} \right) \\ \end{matrix}\]

In our scenario, the mafic rocks contain coarser clinopyroxene (augite) crystals (Figs. 4A and 4B), while the mingled rocks are characterized by ARM whose inner portions are dominated by actinolite (Figs. 6A and 6B). The occurrence of actinolite-dominated zones in the mingled rocks suggest that interaction between the mafic and felsic magmas led to the transformation of earlier formed clinopyroxene crystals to actinolite in the mafic phase.

During magma mixing, further hybridization between the mafic and felsic magmas leads to the conversion of actinolite to hornblende. With further hybridization, sluggishly diffusing cations, like Al, migrate from the felsic to the mafic zones and react with the newly formed actinolite crystals to produce hornblende (Castro & Stephens, 1992). The equation related to this replacement can be written as: 2 Ca2Mg3Fe2Si8O22(OH)2+2Al→Ca2Mg3Fe2Si6Al2O22(OH)2+2Si(actinolite)(hornblende) \[\begin{matrix} \text{C}{{\text{a}}_{2}}\text{M}{{\text{g}}_{3}}\text{F}{{\text{e}}_{2}}\text{S}{{\text{i}}_{8}}{{\text{O}}_{22}}{{\left( \text{OH} \right)}_{2}}+2\text{Al}\to & \text{C}{{\text{a}}_{2}}\text{M}{{\text{g}}_{3}}\text{F}{{\text{e}}_{2}}\text{S}{{\text{i}}_{6}}\text{A}{{\text{l}}_{\text{2}}}{{\text{O}}_{22}}{{\left( \text{OH} \right)}_{2}}+\text{2Si} \\ \left( \text{actinolite} \right) & \left( \text{hornblende} \right) \\ \end{matrix}\]

In our scenario, the ARM are in contact with the fine-grained felsic zones. Amphiboles constituting the interior portions of the mafic zones are actinolite in composition, while those occurring in the exterior regions in contact with the felsic zones are hornblende (Fig. 6A). The preservation of hornblende in the outer regions of the mafic zones indicates reaction of already formed actinolite crystals with sluggishly diffusing Al cations from the adjacent Al-rich felsic zones.

With further hybridization, more sluggish cations, like K, start to diffuse from the felsic to the mafic zones. Diffusing K cations react with the newly formed hornblende crystals to produce biotite in the mafic zones (Bateman et al., 1992; Lavaure & Sawyer, 2011; Tate et al., 1997; Ubide et al., 2014). Consequently, CaO is released during the replacement reaction. There is also a surplus of FeO and SiO₂ released in the replacement reaction, likely hosted by ilmenite and quartz found in the hornblende-rich mafic zones. The equation related to this replacement can be written as: 3 Ca2Mg3Fe2Si8O22(OH)2+0.5K2O+0.5Al2O3→KMg3AlSi3O10(OH)2+2CaO+2FeO+5SiO2(amphibole)(biotite) \[\begin{matrix} \text{C}{{\text{a}}_{2}}\text{M}{{\text{g}}_{3}}\text{F}{{\text{e}}_{2}}\text{S}{{\text{i}}_{8}}{{\text{O}}_{22}}{{\left( \text{OH} \right)}_{2}}+0.5{{\text{K}}_{2}}\text{O}+0.5\text{A}{{\text{l}}_{2}}{{\text{O}}_{\text{3}}}\to & \text{KM}{{\text{g}}_{3}}\text{AlS}{{\text{i}}_{3}}{{\text{O}}_{10}}{{\left( \text{OH} \right)}_{2}}+2\text{CaO}+2\text{FeO}+5\text{Si}{{\text{O}}_{2}} \\ \left( \text{amphibole} \right) & \left( \text{biotite} \right) \\ \end{matrix}\]

In our scenario, biotite occurs with hornblende in the amphibole-rich mafic zones (Figs. 6 and 7). This suggests that hornblende in the mafic zones interacted with diffusing K cations from the adjacent felsic zones to produce biotite. The absence of biotite in the actinolite-dominated areas suggests that actinolite has to be converted to hornblende before forming biotite during magma mixing. Thus, the replacement reactions outlined so far all occurs sequentially (Castro & Stephens, 1992; Lavaure & Sawyer, 2011; Tate et al., 1997; Ubide et al., 2014).

The nature of Fe-Ti oxides in granitoids may give us a good idea about the environmental parameters related to their formation, i.e. whether these oxide minerals formed under oxidized or reduced environmental conditions. Two categories of granitoids have been classified on the basis of the abundance of Fe-Ti oxides: magnetite-bearing magnetite-series and magnetite-free ilmenite-series (Ishihara, 1977). The magnetite-series granitoids are formed under oxidized environmental conditions, whereas the ilmenite-series rocks are generated under reduced environmental conditions (Ishihara, 1977; Nagamine & Araki, 2020). Nagamine and Araki (2020) have proposed that ilmenite-series granitoids would be produced from reduced magmas that have higher concentrations of C-bearing gas species like carbon dioxide, methane, and ethane. Therefore, the presence of ilmenite in the mingled rocks (Fig. 6) suggests that these rocks were formed under reducing conditions with elevated levels of C-bearing gases.

The transfer of heat and volatiles from the mafic to the felsic endmember, during magma mixing and mingling, triggers physico-chemical alterations in both magmas. Crystallisation and degassing induced by undercooling in the mafic endmember raises the bulk viscosity and possibly lowers density (Caricchi et al., 2007; Cashman & Blundy, 2000). Conversely, the felsic endmember undergoes partial melting triggered by superheating (Pistone et al., 2017). This can provide a temporal phase during which the bulk viscosities of mafic and felsic magmas become closer, thus enabling mingling and mixing before further increase in the viscosity of mafic magma due to continued crystallisation (Jarvis et al., 2021). The volatile phases present in mafic magmas are primarily constituted by water and carbon dioxide, but significant concentrations of sulphur and halogen gases, and minor concentrations of volatile metals are also found (Gerlach, 1980; Symonds et al., 1994). These volatile species are primarily contributed by the mantle, but assimilation of crustal rocks and fluids may also act as important contributors (Mason et al., 2017). In general, basaltic magma sourced from the mantle consists of 0.1–4 wt% water and <2 wt% carbon dioxide (Edmonds & Woods, 2018 and references therein). The concentrations of these primary volatile species may reach several wt% in arc magmas. The BVSs represents a back-arc tectonic setting (Gogoi, 2022). The preservation of abundant vesicles in the mafic rocks of our study area (Figs. 3A and 4C) indicates that the mafic magmas were rich in the primary volatile phases. The crystallisation-driven degassing of mafic magma, caused by heat and water loss to the felsic magma, triggers the exsolution of volatile phases, promoting magma mingling (Cashman & Blundy, 2000; Jarvis et al., 2021; Pistone et al., 2017; Wiesmaier et al., 2015). Thus, when the volatile-rich mafic magma interacted with the felsic magma, the mingled system must have had high amount of carbon dioxide. The minerals clinopyroxene and hornblende are Ca-rich phases; therefore, their replacement by Ca-poor phase, i.e. biotite, results in the formation of Ca-bearing accessory phases (Ubide et al., 2014). The available carbon dioxide in the mingled system could have interacted with the CaO liberated during hornblende-biotite transformation (Eq. 3) to produce calcite in the mingled rocks of our study area. The equation related to this reaction can be written as: 4 CaO+CO2→CaCO3(Calcite) \[\begin{matrix} \text{CaO}+\text{C}{{\text{O}}_{2}}\to \text{CaC}{{\text{O}}_{3}} \\ \left( \text{Calcite} \right) \\ \end{matrix}\]

We propose that the calcites in our current study have a secondary origin rather than primary, based on their inhomogeneous distribution in the mingled rocks. We rule out the possibility of the origin of calcite being hydrothermal or from assimilation, as this would result in uniform distribution of the calcites throughout the mingled rocks. The calcites are confined exclusively in the hornblende-biotite zones of the mafic zones and are absent elsewhere in the rock specimens. The only plausible explanation for the absence of calcites from the actinolite zone and their occurrence in the hornblende-biotite zone is diffusion of ions from the felsic zone to the mafic zone, resulting in the sequential formation of new minerals: actinolite, hornblende, biotite, and calcite. This clearly indicates that the calcites resulted from specific replacement reactions and are of secondary origin. Therefore, in our case we dismiss other formation mechanisms and propose for the first time that magma mixing is also responsible for the formation of calcite in hybrid igneous rocks.