Understanding mafic-felsic magma interactions in a subvolcanic magma chamber using rapakivi feldspar: A case study from the Bathani volcano-sedimentary sequence, eastern India

Abstract

The Ghansura Rhyolite Dome of the Bathani volcano-sedimentary sequence in eastern India originated from a subvolcanic felsic magma chamber that was intruded by volatile-rich basaltic magma during its evolution leading to the formation of a porphyritic andesite. The porphyritic andesite consists of rapakivi feldspars, which are characterized by phenocrysts of alkali feldspar mantled by plagioclase rims. Results presented in this work suggest that intimate mixing of the mafic and felsic magmas produced a homogeneous hybrid magma of intermediate composition. The mixing of the hot volatile-rich mafic magma with the relatively colder felsic magma halted undercooling in the subvolcanic felsic system and produced a hybrid magma rich in volatiles. Under such conditions, selective crystals in the hybrid magma underwent textural coarsening or Ostwald ripening. Rapid crystallization of anhydrous phases, like feldspars, increased the melt water content in the hybrid magma. Eventually, volatile saturation in the hybrid magma was reached that led to the sudden release of volatiles. The sudden release of volatiles or devolatilization event led to resorption of alkali feldspar phenocrysts and stabilizing plagioclase, some of which precipitated around the resorbed phenocrysts to produce rapakivi feldspars.

Introduction

Mixing between mafic and felsic magmas disrupts the physical and chemical equilibrium of the interacting magmas and may lead to apparent disequilibrium conditions of crystallization. Under such disequilibrium conditions, the nucleation and growth of crystals are significantly affected leading to the formation of textural features like mantled minerals among others (Hibbard, 1981). Such mantled minerals are often exhibited by the feldspar group of minerals (Baxter and Feely, 2002; Vernon, 2016; Gogoi et al., 2018a, 2018b). Feldspars often record compositional changes during mafic-felsic magma interactions and may preserve changes in crystallization environment during their growth (Anderson, 1984; Pietranik and Koepke, 2009). The compositional and morphological changes in growth of igneous feldspars are usually determined with the help of their optical properties observed under the polarizing microscope. Analytical techniques like the electron microprobe can offer immense support to trace the compositional changes in mantled minerals (Ginibre et al., 2002).

A common mantled texture produced as a result of magma mixing is the rapakivi texture. This particular texture is characterized by ovoid phenocrysts of alkali feldspar that are entirely mantled by plagioclase, commonly oligoclase (Sederholm, 1891; Muller, 2007; Vernon, 2016). Mantling of alkali feldspar by plagioclase occurs in both volcanic and plutonic environments. Rapakivi feldspars from these two different environments are characterized by morphological variation in plagioclase overgrowth. In the plutonic environment the morphology of the plagioclase overgrowth is non-cellular in nature, while in case of volcanic environment plagioclase overgrowth displays dendritic morphology. The preservation of dendritic plagioclase in rapakivi feldspar is clearly associated with marked undercooling in the crystallizing magma (Hibbard, 1981; Muncill and Lasaga, 1987).

Textural study of rapakivi feldspar may improve our understanding of magmatic processes and provide important clues to the evolution of certain magmatic systems (Nekvasil, 1991; Slaby and Gotze, 2004; Calzia and Ramo, 2005). Efforts have been going on to infer the genesis of rapakivi texture since the earliest work of Sederholm (1891), yet there is a lack of consensus about the rapakivi-forming processes. Nevertheless, such efforts have put together significant petrographic, analytical and experimental information on the rapakivi texture database that must be taken into consideration before proposing any hypothetical model of rapakivi genesis. The origin of rapakivi feldspars is still a matter of active debate and a number of mechanisms have been presented to explain the formation of mantled feldspars. One model explains the origin of rapakivi texture by mixing of an alkali feldspar-rich felsic magma and a mafic magma, which increases the temperature of the felsic magma and changes the magma composition. This causes resorption of alkali feldspar phenocrysts and crystallization of plagioclase surrounding them (Bladh, 1980; Hibbard, 1981; Slaby and Gotze, 2004; Mondal et al., 2017). Another model explains its origin by sub-isothermal decompression (Stewart, 1959; Elders, 1968; Abbott, 1978; Nekvasil, 1991; Eklund and Shebanov, 1999). According to this model, decompression of a crystal-saturated felsic magma shifts the two-feldspar stability field, reducing the alkali feldspar stability field and promoting plagioclase stability (Whitney, 1975). This causes previously precipitated alkali feldspar to undergo resorption and crystallize plagioclase around them. A few studies have attributed the origin of this texture to an abrupt decrease in pressure caused by a sudden loss of volatiles during the late stages of crystallization (Cherry and Trembath, 1978; Calzia and Ramo, 2005). Dempster (1994) proposed a subsolvus model in which high-temperature exsolution of plagioclase from ternary feldspars and its subsequent redistribution around the margins of phenocrysts in a fluorine-rich environment promoted the growth of rapakivi feldspars. Furthermore, Mondal et al. (2017) proposed that subsolidus fluid-induced dissolution of alkali feldspar phenocrysts followed by pseudomorphic replacement by oligoclase and albite may lead to the formation of rapakivi feldspars. Thus, a number of mechanisms may be responsible for the origin of rapakivi feldspars. Here, we try to investigate the formation mechanism of rapakivi feldspars preserved in the porphyritic andesitic rocks of the Ghansura Rhyolite Dome (GRD) in eastern India. The objective of this work is not to propose a new mechanism of formation of rapakivi texture. Instead, we have integrated the results of earlier studies with our own observations to understand the processes responsible for the development of rapakivi feldspar in our studied rocks. Nevertheless, our approach may add new insights into the already existing models of rapakivi texture genesis.