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

The Nigerian Younger Granite province is one of the world’s classic examples of anorogenic magmatism. It comprises 53 distinct silica-oversaturated syenite-granite ring complexes spread across roughly 7,500 square kilometres within a north-trending belt about 400 kilometres long and 160 kilometres wide. These complexes were emplaced into the Precambrian Nigerian Basement Complex during the Jurassic period, with radiometric ages ranging from approximately 147 to 173 million years. Geologists interpret the province as the product of within-plate magmatism linked to the early opening of the South Atlantic Ocean, meaning the alkaline igneous activity was decoupled from the compressional forces of subduction zones. Individual complexes typically display multiple intrusive phases, from earlier basic and intermediate rocks to later, highly evolved granites and rhyolites, reflecting progressive fractional crystallisation of mantle-derived or hybrid melts.

The Tongolo area, located in Toro Local Government Area about 80 kilometres southeast of Bauchi city, sits within the broader Jos Plateau-Bauchi ring complex province. It has historically been recognised for tin-niobium mineralisation hosted in biotite granites. However, recent artisanal mining activity and reports of new base-metal occurrences prompted field investigations that confirmed the presence of lead, zinc and copper minerals. The area is underlain by three principal lithological units: the Tongolo Younger Granite Complex, composed of biotite granites, porphyritic granites and rhyolites; the Dagga-Allah Porphyry, a polygonal system of dykes cutting across the basement south of the granite complex; and the Palaeoproterozoic migmatite-gneiss basement reworked during the Pan-African Orogeny around 600 million years ago.

To characterise these rocks, the researchers collected seventeen representative samples from fresh outcrops across the roughly 282-square-kilometre study area during two years of fieldwork. After petrographic analysis at the University of Jos, the samples were crushed, pulverised and digested using a multi-acid mixture of hydrofluoric, nitric and perchloric acids to ensure complete dissolution of resistant minerals such as zircon and monazite. The resulting solutions were analysed for major and trace elements on an Agilent 4200 Microwave Plasma-Atomic Emission Spectrometer. Analytical accuracy was verified against certified reference materials, with recoveries for arsenic in the BGS 119 standard reaching 92.96 percent and relative standard deviations for key elements remaining below 3 percent across replicates.

The major-element results paint a clear picture of highly evolved, ferroan magmatism. The granitic rocks contain high silica dioxide, ranging from 68.50 to 75.25 weight percent with a mean of 72.66 weight percent, excluding a single dolerite sample. They also show elevated iron numbers, averaging 0.74, and low magnesium numbers averaging 36.56, a combination that is diagnostic of A-type granitoid affinity. On the Total Alkali-Silica classification diagram, the granites and rhyolites plot predominantly in the granite field, while the dolerite falls in the gabbroic diorite field. Harker variation diagrams reveal systematic negative correlations between silica and aluminium, iron, magnesium, calcium, manganese and phosphorus oxides, consistent with fractional crystallisation as the dominant differentiation process. Notably, the declining phosphorus trend points to early apatite fractionation, a mechanism widely recognised for generating metal-enriched residual melts in ore-forming granitic systems.

The trace-element data are where the story becomes genuinely striking. Compared with average upper continental crust, the Tongolo suite is dramatically enriched in uranium, with concentrations of 96 to 319 parts per million representing enrichment factors of roughly 35 to 118 times crustal values. Lead is enriched 4 to 19 times, zinc 3 to 37 times, and gallium about 4 times crustal abundances, while thorium, barium, vanadium, chromium and nickel are depleted. Primitive mantle-normalised spider diagrams show strong positive anomalies at uranium, lead and gallium alongside pronounced negative anomalies at barium, chromium, vanadium and nickel, a signature consistent with A-type affinity and advanced fractional crystallisation. Europium anomalies calculated for five viable samples range from 0.059 to 0.228, with a mean of 0.111, indicating extensive plagioclase fractionation characteristic of high-temperature, anhydrous A-type magmatic systems.

The base-metal concentrations themselves are remarkable. Lead was detected in every sample, ranging from 72.7 to 324.2 parts per million, indicating a consistent and widespread enrichment across the study area. Zinc spans a much wider range, from 154.7 to 8,537.9 parts per million, while copper varies from 10.6 to a staggering 20,498.7 parts per million. The extreme upper values for both zinc and copper occur in a single sample of pegmatitic granite-gneiss from Dagga-Allah, which records copper at approximately 2.05 weight percent and zinc at 0.85 weight percent, one to two orders of magnitude above the suite average. Elevated cadmium, reaching 72.8 parts per million in another sample, tracks the zinc enrichment and serves as a geochemical pathfinder for sphalerite-hosted mineralisation. Copper and zinc show a strong positive correlation of 0.77, pointing to a shared paragenetic control, whereas lead correlates only moderately with both, suggesting a broader and possibly earlier stage of lead mobilisation.

The spatial architecture of the mineralisation tells an equally compelling story. Kriging-interpolated anomaly maps for lead, zinc and copper, overlaid with lineaments interpreted from satellite imagery and field structural measurements, reveal that the highest concentrations of all three metals cluster in the southern sector of the study area, precisely where lineament density is greatest. Two dominant lineament orientations, northwest-southeast and northeast-southwest, coincide with structural trends identified in rose diagrams of joints and veins. Copper anomalies sit proximal to intrusive contacts, reflecting magmatic-hydrothermal fluid inputs, whereas lead and zinc anomalies are more widespread and structurally controlled, signifying distal dispersion by hydrothermal fluids migrating along the dominant lineament sets. High copper-to-lead ratios above 5, as seen in the Dagga-Allah sample, are generally associated with proximity to a high-temperature mineralising centre above 300 degrees Celsius, while distal expressions of the same system typically fall below 0.1 as lead becomes dominant.

Perhaps the most consequential insight concerns the origin of the controlling structures. The dominant north-northeast to south-southwest structural fabric mapped across all rock types aligns with regional Pan-African structural fabrics in the northeastern Nigerian basement. This suggests the brittle structures now hosting mineralisation were not generated by the Younger Granite emplacement itself but instead reactivated pre-existing basement-scale weaknesses inherited from the Pan-African orogeny. The implication for exploration is profound: similar structural corridors elsewhere in the Pan-African basement of northeastern Nigeria may represent comparable base-metal targets, independent of whether a Younger Granite intrusion is exposed at the surface.

The study’s authors acknowledge limitations, including an incomplete rare-earth-element dataset limited to five elements, the absence of isotopic or fluid-inclusion constraints, and the qualitative nature of the lineament analysis. Future work will integrate aeromagnetic data to map subsurface structural architecture, apply uranium-lead zircon geochronology to constrain the emplacement age of the Tongolo Complex, and conduct fluid-inclusion microthermometry to determine the temperature, pressure and salinity of the mineralising fluids. For now, the southern Tongolo sector stands as a high-priority exploration target, and the study adds fresh geochemical constraints on how A-type magmatism and magmatic-hydrothermal processes conspire to build base-metal deposits in one of Africa’s most distinctive granite provinces.

Subject of Research: Lithogeochemistry, petrogenesis and base-metal mineralisation of A-type granites in the Tongolo area of the Nigerian Younger Granite province

Article Title: Lithogeochemistry and petrogenesis of A-type granites and associated rocks from the Tongolo area of northeastern Nigeria and implications for lead-zinc-copper mineralisation

Article References: Olorunyomi, A. E., Olobaniyi, S. B., & Odukoya, A. M. (2026). Lithogeochemistry and petrogenesis of A-type granites and associated rocks from the Tongolo area of northeastern Nigeria and implications for lead-zinc-copper mineralisation. Discover Geoscience, 4(1), Article 326. https://doi.org/10.1007/s44288-026-00691-3

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00691-3

Keywords: A-type granite, Nigerian Younger Granite province, Tongolo complex, Pb-Zn-Cu mineralisation, lithogeochemistry, petrogenesis, fractional crystallisation, magmatic-hydrothermal fluids, lineament control, MP-AES, Bauchi State, base-metal exploration