(a) Discuss the petrogenesis of carbonatite rocks. (15 marks) (b) What is metasomatic replacement ? What is the role of the…

(a) Petrogenesis of carbonatites (15 marks)

Definition and classification. Carbonatites are igneous rocks with more than 50% primary carbonate minerals and less than 20% SiO₂. They are classed by the dominant carbonate as calciocarbonatite (sövite, coarse; alvikite, fine), magnesiocarbonatite (beforsite, dolomite), ferrocarbonatite and natrocarbonatite. The last is known only from the active volcano Oldoinyo Lengai, Tanzania.

Setting and association. They occur in stable continental cratons and rifts, often as ring complexes and plugs with alkaline silicate rocks (nephelinite, ijolite, melilitite), and are surrounded by a halo of alkali metasomatism called fenitisation. They are rich in Nb, REE, P, Sr, Ba and F.

Models of origin.

• Primary mantle melt: a very small degree of partial melting of carbonated peridotite at depths of more than about 80–100 km produces carbonatite liquid directly.
• Liquid immiscibility: a carbonated silicate melt, such as nephelinite or melilitite, separates at crustal levels into a silicate liquid and a carbonate liquid.
• Fractional crystallisation: a carbonated alkaline silicate magma crystallises olivine, pyroxene and melilite until a residual carbonate liquid is left.

Evidence. Sr, Nd and C–O isotopes are mantle-like, with δ¹³C about −5‰, which supports a mantle source. Experiments support both immiscibility and low-degree mantle melting. Most workers accept that no single model fits all carbonatites. Indian examples are Amba Dongar (Gujarat, related to the Deccan), Sung Valley (Meghalaya), Sevattur and Hogenakal (Tamil Nadu) and Newania (Rajasthan).

(b) Metasomatic replacement and the role of host rock (15 marks)

Metasomatic replacement. It is the process in which hydrothermal or other fluids dissolve the minerals of a rock and simultaneously deposit new ones, volume for volume, with a change in bulk chemistry but with the original texture and structure preserved. Lindgren's rule states that the change occurs without change in volume.

Criteria of recognition.

• Pseudomorphs and relict minerals.
• Preservation of bedding, fossils and other structures across the new mineral.
• Irregular, sharp replacement fronts and ore that cuts across the bedding.
• No open-space filling features such as comb structure or vugs.

Role of host-rock composition.

• Reactive carbonate rocks (limestone, dolomite) neutralise acid fluids, so replacement ore bodies of Pb–Zn–Cu and skarns (garnet–pyroxene–wollastonite) form there, along contacts with granite.
• Reducing hosts, such as carbonaceous shale, graphite or pyrite-bearing rocks, precipitate metals such as gold, uranium and copper, because they reduce the metal-bearing fluid.
• Inert, chemically resistant hosts (quartzite, massive granite) resist replacement, so mineralisation stays in veins along fractures.
• Feldspar-rich rocks alter to sericite and clay, giving disseminated ores.

The host thus controls the site, the mineral and the shape of the deposit.

(c) Evolution of the Deccan Volcanic Province (20 marks)

Extent, volume and age. The Deccan Traps cover about 500,000 km² today, though the original area was over 1.5 million km², with a volume of about 1 million km³ or more, and a thickness of over 2 km in the Western Ghats. Most of the lava erupted in about one million years around 66 Ma, close to the Cretaceous–Palaeogene boundary (chron 29R).

Origin. India passed over the Réunion mantle plume as it drifted north after leaving Gondwana. The plume head melted and the crust was rifted, with the Seychelles separating from India about 62 Ma. Fissure eruptions along the Narmada–Tapi rift, the west coast fault and dyke swarms fed the lava.

Stratigraphy and petrology. The Deccan Basalt Group is divided into the Kalsubai, Lonavala, Wai and Salsette subgroups, with formations such as Khandala, Bushe, Poladpur, Ambenali and Mahabaleshwar. The lavas are mainly tholeiitic basalts, with minor picrite, alkaline rocks and rhyolite. Flows are compound pahoehoe and aa, laid in horizontal sheets 10–50 m thick, separated by red boles and by intertrappean beds carrying freshwater fossils.

Sub-provinces. Western Ghats, Malwa, Mandla, Kachchh and Saurashtra, and Narmada–Tapi.

Consequences. Volcanic CO₂ and SO₂ affected climate, and the eruptions are widely discussed as one factor in the end-Cretaceous extinction, alongside the Chicxulub impact. The lava has weathered to black cotton soil (regur) and laterite, and formed the plateaux and scarp of the Western Ghats. Later erosion and the Deccan aquifers are of economic importance.