Petrogenesis of wollastonite bearing syenites from Sendraimalai, Samalapatti Complex, Tamil Nadu, India

The Sendraimalai syenite forms part of the Samalapatti Complex within the “Southern Granulite Terrane” (SGT) of Tamil Nadu, India, and is intruded along a major NE–SW trending, crustal-scale shear zone associated with the Dharmapuri rift system. The syenite occurs as a discordant, massive, non-foliated intrusive body within hornblende-epidote gneiss and is spatially associated with ultramafic and carbonatite rocks. Petrographically, the rocks are coarse-grained, leucocratic (C.I. 14–20), and exhibit hypidiomorphic to porphyritic textures. These rocks are dominated by alkali feldspar (68–75 vol%), sodic plagioclase (7–12 vol%), clinopyroxene (8–14 vol%), and minor quartz (1.5–3 vol%). Apatite, grossular, and wollastonite occurs as accessories. The textural relations suggest that garnet and wollastonite have a primary magmatic origin with no evidence of metamorphism. The mineral chemistry data shows that the alkali feldspar has Or 88−93 composition with high BaO concentrations of 5.51 to 10.36 wt% and the plagioclase is albite-rich (Ab 98−99 ). Clinopyroxenes express the Fe-rich diopside (Wo 49−50 En 26−35 Fs 15−23 ) with Mg# of 54.97–73.92. Wollastonite shows SiO₂ of 50.80–51.90 wt% and CaO of 46.81–48.42 wt%. Garnet is dominated by grossular (66.07–78.63%) with subordinate almandine (19.90–32.51%) and low TiO₂ (0.24–0.80 wt%). Whole-rock geochemistry indicates SiO₂ variation (61.41–63.04 wt%), high Al₂O₃ (16.65–18.83 wt%), and high total alkalis with K₂O (7.18–8.41 wt%) exceeding Na₂O (2.25–2.95 wt%). The rocks are metaluminous, alkaline, and shoshonitic in nature. These rocks are enriched in LILE (Rb, Ba, Sr, K) and depleted in HFSE (Nb, Ta, Zr, Ti) and have negative P anomalies in the mantle-normalized diagram, indicating crustal contamination and inheritance of the subduction signature. The chondrite-normalized REE pattern shows enrichment in LREE and a positive Eu anomaly, which suggests plagioclase feldspar accumulation. Petrogenetically, the Sendraimalai syenites were generated by low-degree partial melting of a K-rich, subduction-modified lithospheric mantle source, followed by extensive fractional crystallization of an alkaline basaltic parent magma. The occurrence of wollastonite and grossular-rich garnet suggests magma–carbonate interaction and Ca enrichment within a CO₂-rich magmatic system at crustal depths. These characteristics are consistent with the evolution of subduction-related magmatism toward a more alkaline magmatic affinity during the later stages of crustal evolution.