Field, petrographic and geochemical characteristics of Sullya alkaline complex in the Cauvery Shear Zone (CSZ), southern India: Implications for petrogenesis

Abstract

Significant, but volumetrically smaller, unmetamorphosed and largely undeformed alkaline magmatic suites have been reported from the Southern Granulite Terrain in southern India. These Neoproterozoic alkaline magmatic rocks occur as lenses, dykes and plugs that are mostly within, or proximal to, major shear zones or transcrustal faults. In this contribution, field, petrographic and whole-rock geochemical data of Sullya syenites and associated mafic granulites from the Mercara Shear Zone (MSZ), which separates low-grade (greenschist to upper amphibolite facies) Dharwar Craton and high-grade (granulite facies) Southern Granulite Terrain is presented. The isolated body of the Sullya syenite, similar to other alkaline plutons of the Southern Granulite Terrain, shows an intrusive relationship with the host hornblende-biotite gneisses and mafic granulites. The Sullya syenites lack macroscopic foliations and unlike, other plutons, they are not associated with carbonatites and ultrapotassic granites. Potash feldspar and plagioclase dominates the felsic phases in the Sullya syenite and there is negligible quartz. The studied syenites show evidence of melt supported deformation, but show no evidence of recrystallization. Geochemically, they most resemble the Angadimogar syenites (situated 3 km west of the Sullya syenites) with similar major oxide and trace element concentrations. The petrogenetic studies of the Sullya syenite have indicated that they were generated by mixing of two different sources derived from the partial melting of metasomatized continental mantle lithosphere and lower crustal mafic granulites. This melt source could have been emplaced in a rift-related tectonic setting. The emplacement is considered to be controlled by shears.

Appendix: Analytical methods

Major oxides were analyzed by X-ray fluorescence (XRF) technique at National Centre for Earth Science Studies (NCESS), Trivandrum, India. Pressed pellets were used respectively for major element analysis. The pellets were prepared by sprinkling finely powdered sample over boric acid binder filled in aluminium cups and pressing in a 40-ton hydraulic press for 30 s. Analyses were performed on a Bruker S4 Pioneer wavelength dispersive (WD) XRF instrument. The detection limit of major element was ~0.01% and analytical precision is always better than 1%. Precision for trace elements is estimated to be better than 5% on the basis of repeated analysis of reference rock standards (Ravindra Kumar and Sreejith 2016; Sorcar et al. 2019). The precision and accuracy of calibration curves and data reliability is available at http://cess.res.in/groups/crustal-processes-crpgroup/laborataries/xrf-lab-2.

Trace element concentration analyses were performed at Department of Earth Sciences in Indian Institute of Technology Kanpur. Approximately 0.25 g of sampler powder was initially digested in pre-cleaned teflon beakers at 130 ± 5°C using a 5 mL mixture of concentrated HF (2 parts), concentrated HCl (1 part) and concentrated HNO₃ (1 part) for 48 hrs. The acid was then slowly evaporated at 80 ± 5°C, and again the samples were re-dissolved in 4 mL of Aqua Regia acid (3 mL of concentrated HNO₃ + 1 mL of concentrated HCl). Aqua Regia was fluxed for 24 hrs. Further, the samples were dried and re-dissolved in 5% HNO₃. The acid digestion steps were only repeated when digestion was incomplete. Trace element concentrations were determined at ~200 ppm total dissolved solid solutions. Three procedural blanks, reference material SBC-1 (shale) and AGV-2 (andesite) rock standard from US Geological Survey (USGS) were also digested following the same procedures. The blanks were analyzed to quantify the total procedural blank, whereas AGV-2 was analyzed as an unknown to assess the data quality. SBC-1 was diluted to seven appropriate concentrations to construct the calibration curve, and trace element concentrations were determined based on the SBC-1 calibration curve. Since rock-matrix matched reference materials were unavailable, all the samples and standards were spiked by ~5 ppb In solution and In was used as an internal standard. The instrument was run both in standard and He kinetic energy discrimination mode to optimize the separation of measured isotopes from interfering polyatomic interferences. The final concentrations were blank – corrected using the average procedural blank concentrations and matrix effect was corrected by In normalization. Average blank corrections were less than 10% for most of the elements. The measured trace element concentration of AGV-2 agrees well with the USGS certified values.