[S-MP25] Supercontinents and Crustal Evolution

[SMP25-06] Petrogenesis of mafic dyke swarms from Western Dharwar craton, India: Insights from olivine composition and Sr-Nd isotope geochemistry

*Silpa Ammini Sasidharan¹, Madhusoodhan Satish-Kumar², Krishnan Sajeev³, Toshiro Takahashi² (1.Research Institute for Natural Hazards and Disaster Recovery, Niigata University, 2.Faculty of Science, Niigata University , 3.Centre for Earth Sciences, Indian Institute of Science)

Keywords:Precambrian mafic dyke swarms, Mantle evolution, Olivine mineral chemistry, Dharwar craton, Sr-Nd isotope geochemistry

Understanding the early mantle evolution, including the early differentiation and the nature of distinct geochemical reservoirs, is crucial in understanding Earth’s evolutionary history. The mantle compositions from time to time are preserved in the highly resistant mafic dyke swarms that resulted from the large-scale igneous activities. Olivine is a critical mineral found in the mafic dykes, the potential for understanding the composition of the primitive magmas and the melt evolution of the mantle source [1, 2]. Hence, the olivine dolerite dykes from the Western Dharwar craton of India are considered in the present study to obtain information about the mantle source regions and the crystallization sequences of the magma. The petrographic observations and mineral chemical composition are used to constrain the petrogenetic history, including the conditions of crystallization and the parent melt composition. Dykes of various dimensions with country rock relations are exposed along the craton. The studied dyke samples consist of plagioclase (~2mm), clinopyroxene (1-2mm), olivine (0.5-2mm) and minor orthopyroxenes (0.2-0.5mm). The sample shows the typical poikilitic texture formed by dominant olivine and clinopyroxene grains in a matrix of plagioclase. The olivine was anhedral and subrounded with a few grains of Ti-magnetite present in the core, as well as a thin rim around the resorbed olivine grain associated with the plagioclase matrix. The high whole-rock and olivine core Mg# (>70) suggest that they could represent primitive melt composition. The olivine grains are more or less homogenous with less compositional zoning, indicating that they were derived from the unfractionated melt formed by the direct partial melting of the mantle [3]. The cores and rims do not change drastically in composition and are magnesian as the Fo content has limited variations of Fo_(71-77). This combined with the whole rock trace element patterns with large ion lithophile elements (LILEs) and light rare earth elements (LREEs) depletion, low initial Sr isotopic ratio of around 0.70088 and negative epsilon Nd values can be interpreted as the ancient crust recycled into the mantle source. This also points towards the subduction related crustal recycling and interaction of the mantle source with oceanic crustal componentsin the Archean [4]. The Rb-Sr and Sm-Nd isotopic ratios showing a good isochron relationship with an errorchron age of 2718 ± 20 Ma (MSWD=20) indicative of the depleted mantle following major continental growth activity in the Dharwar craton. The primitive melt composition that can be derived from the olivine mineral chemistry can provide direct information of the mantle source beneath the Dharwar craton during the early Proterozoic.

References: [1] Sun et al. Chemical Geology 405 (2015) 10–18. [2] Gross et al. Meteoritics and Planetary Science 48(5) (2013). 854-871. [3] Sarkar et al. Lithos 406–407 (2021) 106524. [4] Silpa et al. Lithos 388–389 (2021) 106023.