Magma mixing in Neoarchean granite from Nalgonda region, Eastern Dharwar Craton, India: Morphological, mineralogical and geochemical evidences

Abstract

We present field, petrographic, mineralogical and whole-rock geochemical data for part of the Neoarchean granite and associated microgranular enclaves (MEs) occur in the Nalgonda region, NE part of Eastern Dharwar Craton (EDC), and demonstrate the end-member magma mixing processes in the petrogenesis of the host granite. Extensive occurrence of ME and uni-directional flow band structures (N-S trend) in all the studied outcrops exposed over about 20 km depict that intensive magma mixing-mingling occurred at magma chamber scale. MEs are the portions of intermediate mafic magma that had interacted at two stages with acidic host granite. Mixing at the initial stage promoted efficient thorough mixing which resulted in rapakivi texture and mesocratic enclaves. Wispy filament structures around these enclaves indicate that mafic magma globules were mechanically diluted in the host granite magma by chaotic advection. These filamental magmas were further linearly diluted along with convection-related flow of the host magma. Smaller mafic globules also got linearly stretched along with this flow. Upon solidification of the host magma, these wispy mafic filaments were preserved as flow structures. The second stage of mafic magma mixing did not promote thorough mixing due to the large viscosity contrast with the host granite magma and preserved their physical entity. These enclaves are melanocratic with sharp boundary and devoid of gradational contact and filament structures around it. They characteristically show network of granitic vein injection which often show crenulation folding. These evidences indicate that during the second-stage interaction, the enclave magma was rigid to plastic nature with respect to the host granite magma. Coherent linear Harker variation trend of CaO, MgO, TiO 2 , K 2 O/Na 2 O, V, Y and Sc indicates that mixing has promoted to develop chemical gradient between ME and host granite magmas. Mechanical dilution by chaotic advection must have enhanced the chemical diffusion of both magmas. Smooth decreasing Harker trend of Y content in both the magmas indicates that theyhad undergone certain degree of fractional crystallisation. In contrast, elements like Rb, Ba and Sr with large value of diffusion coefficient (D) show scattered behaviour in element-element plots suggesting that diffusive fractionation was active during chaotic advection mixing. It is concluded that heterogeneity in compositional variation of Nalgonda granite can be attributed to difference in degree of mechanical dilution of mafic magma, fractional crystallisation and diffusive fractionation. The geochemical evidences indicate the role of subduction in the evolution of these rocks.

Injection of enriched lithospheric mantle magmas explains the formation of microgranular enclaves in the Rio Jacaré Batholith, Borborema Province, Brazil

“…These features indicate that mafic magmas were injected during two different crystallization stages in the RJB magmatic chamber. According to the model of Fernandez and Barbarin (1991), MEs with globular shapes were formed when mafic magma was injected into the felsic magmatic chamber of the RJB, with up to 30% crystallized, and disaggregated by convective movements; this also agrees with the interpretations of other authors about the genesis of globular MEs (e.g., Vernon et al 1988, Castro et al 1991, Liu et al 2013, Shukla and Mohan 2019. Since MEs with such features are well distributed throughout the RJB, we believe that the input of mafic magma in this stage was important.…”

“…According to Janoušek et al (2004), the compositional variation of MEs can be considered the mixing of a mantle-derived mafic magma with crustal magmas. Some authors (e.g., Didier 1991, Shukla andMohan 2019) consider some features that are also found in the RJB MEs (e.g., variation in the color of the MEs, presence of multiple MEs, and diffuse contacts), a reflection of different degrees of homogenization and the role of mixing between the parental mafic magma and the felsic host.…”