(a) What are migmatites? Describe the important types of migmatites and the processes of their formation. (15 marks) (b) Draw a…

(a) Migmatites

Migmatites are common in high-grade metamorphic belts and are mixed rocks produced when a metamorphic rock is partially melted and the melt remains partly segregated. The light felsic melt is leucosome; the dark mafic residue is melanosome; together they form neosome, while the unmelted parent metamorphic rock is paleosome. The important types are metatexites, coherent rocks in which leucosome forms veins, bands or layers within paleosome, and diatexites or nebulitic migmatites, in which high melt fraction gives a granitic texture and paleosome occurs as enclaves. Mehnert's older morphological types, still widely used, include agmatic (breccia-like), stromatic (layered), ophthalmic (augen), folded, dictyonitic (net-veined), schlieric and nebulitic migmatites. Formation involves partial melting of metapelites, paragneisses or amphibolites at high temperature and pressure, metamorphic differentiation by melt segregation, and metasomatism or fluid-assisted melting that changes residual composition. The leucosome is commonly granitic to tonalitic, while the melanosome may contain biotite, garnet, amphibole or mafic restite. Indian examples include the Banded Gneissic Complex (Bhilwara) of Rajasthan, the Peninsular Gneiss of the Dharwar craton, the Chotanagpur Gneissic Complex and the Southern Granulite Terrain, where migmatites record crustal anatexis and granite genesis. They thus grade from low-melt metatexites to high-melt diatexites, preserving the transition from metamorphism to igneous melt.

(b) Albite–anorthite diagram

The dry 1 atm albite–anorthite diagram is a continuous solid-solution diagram. Sketch: x-axis composition from Ab100 (An0) on the left to An100 on the right; Ab20-An80 is plotted at 80% An, close to the An end. y-axis temperature: Ab100 melts at 1118°C and An100 at 1553°C. The upper curve is the liquidus, the lower curve the solidus, and there is no eutectic or minimum: the liquidus and solidus meet only at the pure end-members.

``svg <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 640 376" width="640" height="376"> <title>Albite–anorthite phase diagram and crystallisation of An80 melt</title> <style>text{font-family:Arial,Helvetica,sans-serif;font-size:13px;fill:#1f2937}.t{font-size:15px;font-weight:bold}.b{font-weight:bold}.s{font-size:12px;fill:#4b5563}.c{text-anchor:middle}.e{text-anchor:end}.i{font-style:italic}</style> <rect width="640" height="376" fill="#fff"/> <text x="320" y="20" class="t c">Albite–anorthite diagram (dry, 1 atm): crystallisation of Ab20–An80 melt</text> <rect x="66" y="68" width="390" height="250" fill="#fff" stroke="#1f2937" stroke-width="1.4"/> <line x1="62" y1="295" x2="66" y2="295" stroke="#1f2937"/><text x="58" y="299" class="e s">1100</text> <line x1="62" y1="250" x2="66" y2="250" stroke="#1f2937"/><text x="58" y="254" class="e s">1200</text> <line x1="62" y1="204" x2="66" y2="204" stroke="#1f2937"/><text x="58" y="208" class="e s">1300</text> <line x1="62" y1="159" x2="66" y2="159" stroke="#1f2937"/><text x="58" y="163" class="e s">1400</text> <line x1="62" y1="113" x2="66" y2="113" stroke="#1f2937"/><text x="58" y="117" class="e s">1500</text> <line x1="62" y1="68" x2="66" y2="68" stroke="#1f2937"/><text x="58" y="72" class="e s">1600</text> <line x1="66" y1="318" x2="66" y2="322" stroke="#1f2937"/><text x="66" y="336" class="c s">0</text> <line x1="144" y1="318" x2="144" y2="322" stroke="#1f2937"/><text x="144" y="336" class="c s">20</text> <line x1="222" y1="318" x2="222" y2="322" stroke="#1f2937"/><text x="222" y="336" class="c s">40</text> <line x1="300" y1="318" x2="300" y2="322" stroke="#1f2937"/><text x="300" y="336" class="c s">60</text> <line x1="378" y1="318" x2="378" y2="322" stroke="#1f2937"/><text x="378" y="336" class="c s">80</text> <line x1="456" y1="318" x2="456" y2="322" stroke="#1f2937"/><text x="456" y="336" class="c s">100</text> <text x="261" y="354" class="c">composition, wt % An (Ab at 0, An at 100)</text> <text transform="translate(20,193) rotate(-90)" class="c">temperature (°C)</text> <path d="M66.0,287.1 L85.5,267.5 L105.0,248.0 L124.5,228.5 L144.0,208.9 L163.5,196.4 L183.0,183.9 L202.5,171.4 L222.0,158.9 L241.5,151.0 L261.0,143.0 L280.5,135.0 L300.0,127.1 L319.5,121.4 L339.0,115.7 L358.5,110.0 L378.0,104.4 L397.5,100.6 L417.0,96.9 L436.5,93.1 L456.0,89.4 L456.0,89.4 L436.5,103.3 L417.0,117.3 L397.5,131.3 L378.0,145.3 L358.5,155.5 L339.0,165.7 L319.5,176.0 L300.0,186.2 L280.5,196.4 L261.0,206.6 L241.5,216.9 L222.0,227.1 L202.5,235.0 L183.0,243.0 L163.5,251.0 L144.0,258.9 L124.5,266.0 L105.0,273.0 L85.5,280.0 L66.0,287.1 Z" fill="#fde8e8" stroke="none"/> <path d="M66.0,287.1 L85.5,267.5 L105.0,248.0 L124.5,228.5 L144.0,208.9 L163.5,196.4 L183.0,183.9 L202.5,171.4 L222.0,158.9 L241.5,151.0 L261.0,143.0 L280.5,135.0 L300.0,127.1 L319.5,121.4 L339.0,115.7 L358.5,110.0 L378.0,104.4 L397.5,100.6 L417.0,96.9 L436.5,93.1 L456.0,89.4" fill="none" stroke="#b03030" stroke-width="2.2"/><path d="M66.0,287.1 L85.5,280.0 L105.0,273.0 L124.5,266.0 L144.0,258.9 L163.5,251.0 L183.0,243.0 L202.5,235.0 L222.0,227.1 L241.5,216.9 L261.0,206.6 L280.5,196.4 L300.0,186.2 L319.5,176.0 L339.0,165.7 L358.5,155.5 L378.0,145.3 L397.5,131.3 L417.0,117.3 L436.5,103.3 L456.0,89.4" fill="none" stroke="#1d4e78" stroke-width="2.2"/> <text x="152" y="118" class="b">liquid (melt)</text> <text x="175" y="229" class="b" style="fill:#7f1d1d">melt + plagioclase</text> <text x="261" y="282" class="b" style="fill:#1d4e78">plagioclase solid solution</text> <text x="113" y="186" class="s" style="fill:#b03030">liquidus</text><text x="308" y="200" class="s" style="fill:#1d4e78">solidus</text> <text x="70" y="281" class="s">1118 °C</text><text x="452" y="105" class="e s">1553 °C</text> <line x1="378" y1="73" x2="378" y2="159" stroke="#5b3a80" stroke-width="1.6" stroke-dasharray="5 4"/> <text x="378" y="69" class="c s" style="fill:#5b3a80">bulk An80</text> <line x1="378" y1="104" x2="435" y2="104" stroke="#374151" stroke-width="1.2"/> <circle cx="378" cy="104" r="3.5" fill="#b03030"/><circle cx="435" cy="104" r="3.5" fill="#1d4e78"/> <text x="372" y="108" class="e s b">1</text> <line x1="300" y1="127" x2="403" y2="127" stroke="#374151" stroke-width="1.2"/> <circle cx="300" cy="127" r="3.5" fill="#b03030"/><circle cx="403" cy="127" r="3.5" fill="#1d4e78"/> <text x="294" y="131" class="e s b">2</text> <line x1="255" y1="145" x2="378" y2="145" stroke="#374151" stroke-width="1.2"/> <circle cx="255" cy="145" r="3.5" fill="#b03030"/><circle cx="378" cy="145" r="3.5" fill="#1d4e78"/> <text x="249" y="149" class="e s b">3</text> <text x="470" y="66" class="b">Tie-lines (lever rule)</text> <text x="470" y="82" class="s">1 1520 °C: first crystals</text> <text x="470" y="98" class="s"> ≈ An95, melt An80</text> <text x="470" y="114" class="s">2 1470 °C: melt ≈ An60,</text> <text x="470" y="130" class="s"> solid ≈ An87; solid</text> <text x="470" y="146" class="s"> fraction ≈ 0.75</text> <text x="470" y="162" class="s">3 1430 °C: last melt</text> <text x="470" y="178" class="s"> ≈ An49; solid = An80</text> <text x="470" y="210" class="b">Zoning</text> <text x="470" y="226" class="s">Equilibrium: crystals</text> <text x="470" y="242" class="s">re-equilibrate to An80.</text> <text x="470" y="258" class="s">Fractional: calcic core,</text> <text x="470" y="274" class="s">sodic rim (normal zoning).</text> <text x="470" y="290" class="s">Reverse zoning: input of</text> <text x="470" y="306" class="s">new calcic melt.</text> <text x="320" y="366" class="c s">Melt path follows the liquidus toward Ab; crystal path follows the solidus. No eutectic.</text> </svg> `` For an initial melt Ab20-An80, crystallization begins at the liquidus with plagioclase richer in anorthite than the melt, about An90–An95. On cooling, the crystal follows the solidus toward lower An while the residual melt follows the liquidus toward higher Ab. Under equilibrium, crystals re-equilibrate and the final plagioclase approaches the bulk Ab20-An80 composition. Under fractional crystallization, removed crystals make the residual melt move faster toward albite, producing more strongly zoned crystals. The lever rule, fraction solid = (C0-CL)/(CS-CL), is applied using a tie-line between liquidus and solidus at any temperature. Normal zoning, An-rich cores and Ab-rich rims, records continuous cooling and melt evolution; reverse zoning, Ab-rich cores and An-rich rims, implies late addition of anorthite-rich melt, assimilation, volatile effects, or crystal reaction. If diffusion is slow, zoning is preserved; if fast, it is erased.

(c) Granites and Deccan

(i) This alphabet classification (Chappell and White, 1974, for I and S; later M and A) is based on source. I-type granites are igneous-source, usually metaluminous, with hornblende, biotite, quartz and plagioclase, and little excess aluminium; they form by melting of mafic to intermediate crust or mantle-derived material, typical of Cordilleran arcs such as the Ladakh batholith; they belong to the magnetite series, carry titanite and have an aluminium saturation index below 1.1, and may contain mafic enclaves. S-type granites are sedimentary-source, peraluminous, with muscovite, garnet, cordierite or K-feldspar, and form by anatexis of metasediments, as in Himalayan leucogranites (e.g., Badrinath, Manaslu); they belong to the ilmenite series, have an aluminium saturation index above 1.1 and high initial 87Sr/86Sr, and may contain metasedimentary xenoliths and restite. M-type granites are mantle-derived, oceanic, low in K, with calcic plagioclase, hornblende and pyroxene, and form from partial melting of mantle or ultramafic crust. M-type may be associated with ophiolites. A-type granites are anorogenic, alkaline, often high in incompatible elements, with K-feldspar, Na-plagioclase, biotite, hornblende or pyroxene, and form in rift or post-collisional extensional settings (Loiselle and Wones, 1979); they carry iron-rich mafic minerals such as annite, fayalite or sodic amphiboles (riebeckite, arfvedsonite), have high Ga/Al, Zr and REE, and are exemplified in India by the Neoproterozoic Malani granites (Siwana, Jalor) of Rajasthan. (ii) The Deccan Traps are Cretaceous–Palaeogene continental flood basalts, with the main eruptive phase at about 66.3–65.5 Ma, linked to the Réunion mantle plume as India drifted over it. Covering about 500,000 km² today and reaching about 2 km in thickness in the Western Ghats, the province is among the largest on Earth. They are tholeiitic, layered in formations such as Jawhar, Igatpuri, Poladpur and Ambenali (Kalsubai, Lonavala and Wai subgroups), with simple and compound pahoehoe and aa flows, red boles, intertrappean sediments, dyke swarms (Narmada–Tapi, Konkan) and minor picrites and alkaline–carbonatite complexes (Amba Dongar, Mundwara), and their massive volcanism is linked to environmental stress and mass-extinction effects at the K–Pg boundary. CO2 and SO2 emissions are linked to climate change and ocean acidification. The basalts weather to black cotton soil (regur) and host aquifers in vesicular and fractured zones.