Rare Wollastonite-Bearing Rocks in India Reveal a Hidden Subduction Legacy

Deep in the Southern Granulite Terrane of Tamil Nadu, India, a small and unassuming body of rock is rewriting what geologists thought they knew about how alkaline magmas evolve beneath ancient continents. The Sendraimalai syenite, part of the Samalapatti Complex, has long been overlooked amid the better-studied alkaline plutons of northern Tamil Nadu. Now, a team of Indian geologists has subjected this coarse-grained, pale-colored intrusion to a full battery of petrographic, mineral-chemical, and whole-rock geochemical analysis, and the results are striking: the rock contains wollastonite and garnet that appear to have crystallized directly from magma, a phenomenon so rare that only a handful of localities worldwide preserve it.

Wollastonite, a calcium silicate mineral, is normally the signature of contact metamorphism. It forms when limestone or other carbonate rocks are baked by intruding magma or buried in skarn environments, driving off carbon dioxide in classic decarbonation reactions. Finding it as an apparently primary igneous mineral, intergrown with clinopyroxene and alkali feldspar and lacking any reaction rims, replacement textures, or retrograde alteration, is what makes the Sendraimalai occurrence remarkable. The researchers argue that the wollastonite grew in textural equilibrium with its host minerals, a relationship that points to direct crystallization from a calcium-rich, carbon dioxide-bearing silicate melt rather than post-emplacement transformation.

The rock itself is a leucocratic syenite with a color index of 14 to 20, coarse grains exceeding six millimeters, and hypidiomorphic to porphyritic textures. Under the microscope, tabular alkali feldspar megacrysts dominate, making up 68 to 75 percent of the rock and displaying Carlsbad and perthitic twinning. Sodic plagioclase contributes 7 to 12 percent, clinopyroxene 8 to 14 percent, and quartz a mere 1.5 to 3 percent. Apatite, grossular garnet, and wollastonite occur as accessory phases, while biotite and amphibole are conspicuously absent. On the standard QAP classification diagram, the assemblage plots squarely in the syenite field, but the mineral chemistry tells a far more exotic story.

Electron microprobe analyses conducted at the Geological Survey of India in Hyderabad revealed that the alkali feldspars are extraordinarily potassic, with orthoclase contents between 88 and 93 percent, and astonishingly rich in barium, containing 5.51 to 10.36 weight percent BaO. Barium substitutes readily for potassium in the feldspar lattice, and concentrations this high are rarely seen outside highly evolved alkaline systems. The plagioclase, by contrast, is nearly pure albite, with Ab values of 98 to 99. The clinopyroxenes are iron-rich diopsides with magnesium numbers between 55 and 74, while the garnets are dominated by the grossular component at 66 to 79 percent, accompanied by subordinate almandine and very low titanium. Wollastonite itself shows limited compositional variation, with silica near 51 percent and lime near 47 percent.

Whole-rock geochemistry reinforces the picture of an unusual magma. The syenites contain 61.41 to 63.04 weight percent silica, high alumina of 16.65 to 18.83 percent, and total alkalis dominated by potassium: K2O ranges from 7.18 to 8.41 percent, always exceeding sodium oxide at 2.25 to 2.95 percent. The rocks are metaluminous, alkaline, and shoshonitic, a combination that places them firmly in the potassium-enriched lineage of continental magmatism. Trace element abundances are equally dramatic, with barium reaching nearly 47,500 parts per million and strontium around 7,800 parts per million, values that speak to extreme enrichment of large-ion lithophile elements during the magma’s evolution.

These geochemical fingerprints carry tectonic meaning. The rocks are enriched in rubidium, barium, strontium, and potassium but depleted in niobium, tantalum, zircon, and titanium, with negative phosphorus anomalies on mantle-normalized diagrams. That pattern, the authors argue, indicates both crustal contamination and an inherited subduction signature, a chemical memory of fluids and melts released by an ancient descending slab. Chondrite-normalized rare earth element patterns show strong light rare earth enrichment and a pronounced positive europium anomaly, which the researchers attribute to accumulation of plagioclase feldspar under relatively reducing conditions, where divalent europium was preferentially incorporated into the feldspar structure.

Piecing the evidence together, the team proposes that the Sendraimalai syenites were generated by low-degree partial melting of a potassium-rich, subduction-modified lithospheric mantle source, followed by extensive fractional crystallization of an alkaline basaltic parent magma. The potassic character is unlikely to reflect crustal contamination, because even the earliest-crystallized minerals preserve strong potassium signatures, and fractional crystallization alone cannot explain it since no systematic potassium depletion is observed across the pluton. Instead, the signature appears to be source-inherited, locked into the mantle during an earlier episode of subduction-related metasomatism. The exceptionally barium-rich feldspars point to a barium-metasomatized mantle melted at low degrees, with further barium enrichment during differentiation and possible late-stage fluid interaction.

The calcium-rich mineralogy adds another layer of complexity. The coexistence of diopside, wollastonite, and grossular garnet suggests crystallization within the CaO–MgO–Al2O3–SiO2–CO2–H2O system under unusually high calcium activity. The researchers outline plausible reactions, including the conversion of calcite and silica to wollastonite with carbon dioxide release, and reactions linking clinopyroxene, clinozoisite, and wollastonite to garnet, calcite, and water. Yet they are careful to note that no carbonate minerals, xenoliths, calc-silicate enclaves, or carbonate veins have been found in the outcrops, so these reactions remain inferential models. The calcium enrichment may reflect interaction between the evolving syenitic melt and carbonate material at crustal depths, or it may record derivation from a carbonate-metasomatized mantle source that stabilized wollastonite-bearing assemblages during late-stage crystallization.

Comparisons with neighboring complexes sharpen the interpretation. The Yelagiri Alkaline Complex, also shoshonitic and metaluminous, evolved through open-system magma mixing, with roughly 60 percent shoshonitic mafic magma blending into the resident syenitic melt, as recorded by oscillatory feldspar zoning and abundant mafic enclaves. The Ihouhaouene Syenite-Carbonatite Complex in Algeria, by contrast, evolved through silicate-carbonate liquid immiscibility and crystal mush hybridization. Sendraimalai shares magmatic wollastonite and grossular-rich garnet with Ihouhaouene and silica-saturated normative mineralogy with both, but its clinopyroxenes plot nearer the forearc peridotite field, suggesting a mantle source more thoroughly metasomatized by subduction. Yelagiri lacks wollastonite entirely, likely because its calcium was consumed early by diopside, amphibole, and plagioclase before wollastonite saturation could be reached.

Crystallization temperatures estimated from normative feldspar systematics fall between 800 and 900 degrees Celsius, and rubidium-strontium systematics suggest emplacement beneath continental crust thicker than 30 kilometers. High lanthanum-to-ytterbium ratios of 36.95 to 56.56, combined with relatively constant lanthanum-to-samarium and samarium-to-ytterbium ratios, indicate that melting occurred within the garnet stability field at considerable mantle depths. Taken together, the findings portray Sendraimalai as the product of a volatile-rich, subduction-modified mantle source that generated alkaline basaltic melts, which then fractionated extensively under thick continental crust, with late-stage calcium enrichment producing one of the rarest mineral assemblages in igneous petrology. For a body of rock barely studied until now, the Sendraimalai syenite offers an unusually complete window into how ancient subduction signatures linger in the mantle and resurface, billions of years later, in the chemistry of the continents.

Subject of Research: Petrogenesis of wollastonite-bearing alkaline syenites in the Southern Granulite Terrane, India

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

Article References: Amarendhar, S., Lingaswamy, V., Sreenu, K., & Prasad, K. R. (2026). Petrogenesis of wollastonite bearing syenites from Sendraimalai, Samalapatti Complex, Tamil Nadu, India. Discover Geoscience, 4(1), Article 302. https://doi.org/10.1007/s44288-026-00668-2

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00668-2

Keywords: syenite, wollastonite, grossular garnet, alkaline magmatism, subduction, Southern Granulite Terrane, Tamil Nadu, petrogenesis, mineral chemistry, geochemistry, lithospheric mantle, fractional crystallization