(a) Describe the changes in crystallized solid composition in albite-anorthite system at 1 atm pressure during cooling of a…

Phase equilibria in the albite–anorthite system explain plagioclase zonation in felsic magmas, while ACF projections simplify metamorphic equilibria in quartz-saturated mafic rocks.

(a) At 1 atm, albite (An₀) and anorthite (An₁₀₀) form a continuous high-temperature solid solution. A liquid of An₅₀ cooled from 1500 °C is first entirely liquid. On reaching its liquidus, about 1450 °C, the first plagioclase is more anorthite-rich than the liquid, say An₆₅–₇₀, because anorthite has the higher melting point. With further cooling the solid composition moves continuously along the solidus toward albite, while the residual liquid becomes progressively more albite-rich. The lever rule is applied on each isothermal tie line: the solid fraction equals the distance from the liquid composition to the bulk composition divided by the total tie-line length. At 1400 °C, if the coexisting liquid is about An₄₅ and the solid about An₇₀, the bulk An₅₀ lies between them; the solid fraction is (50−45)/(70−45)=20%. At 1300 °C, take liquid An₁₅ and solid An₆₅; then the solid fraction is (50−15)/(65−15)=35/50=70%. Near the solidus, about 1200 °C, the solid approaches An₅₀ and the vanishing liquid is strongly albite-rich, e.g. An₅; at completion the solid fraction is effectively 100%. Thus the crystallized solid changes from An-rich to An₅₀, the liquid from An₅₀ to Ab-rich, and the system shows continuous solid-solution behaviour without a eutectic arrest; only the onset and completion of crystallization mark thermal breaks.

(b) In the IUGS classification, granite is a granitoid with Q+A+P >90% of total, quartz 20–60 vol% of Q+A+P and alkali feldspar/(alkali feldspar+plagioclase) = 35–90; this includes monzogranite where plagioclase may exceed alkali feldspar, and syenogranite where alkali feldspar dominates. It is distinguished from granodiorite by a higher alkali feldspar/plagioclase ratio and from syenite by sufficient quartz. It is felsic, usually SiO₂ >70 wt%, with quartz, feldspars and Fe–Mg minerals. A peraluminous granite has A/CNK = Al₂O₃/(CaO+Na₂O+K₂O) >1, is corundum-normative, and commonly contains muscovite, garnet or tourmaline. A calc-alkaline peraluminous granite plots in the calc-alkaline field on SiO₂–K₂O or FeO*/MgO diagrams, indicating subduction-related crustal melting rather than peralkaline alkaline affinity. Its petrogenesis is commonly S-type: partial melting of hydrated metapelitic or metasedimentary crust at amphibolite to granulite facies, followed by fractional crystallisation of plagioclase and Fe–Mg minerals, crustal assimilation, and possible mixing with mantle-derived basaltic or andesitic melts. Indian examples include calc-alkaline granites of the Malani Igneous Complex, Rajasthan, peraluminous granites of the Bundelkhand craton, and Himalayan leucogranites such as Lepcha Group rocks.

(c) The ACF diagram uses A = Al₂O₃, C = CaO and F = FeO+MgO. For quartz-bearing metabasites, the bulk composition is assumed to be SiO₂-saturated, so quartz is treated as an excess phase and the composition is projected from the quartz apex of the relevant four-component A-C-F-Q (AFMQ-type) system onto the ACF plane; it is not projected from AFM, which is a different ternary in which Ca is omitted. Na₂O and K₂O are ignored or not treated as separate components, and MnO is neglected. Fe is taken as FeO, with Fe³⁺ ignored or reduced, and Fe and Mg are combined as F. Because basaltic rocks contain appreciable Al₂O₃ as well as CaO and Fe–Mg oxides, their ACF positions lie inside the triangle, not near the C–F join; a position near C–F would imply very low Al₂O₃. The diagram is therefore useful for interpreting amphibolite- and granulite-facies assemblages such as plagioclase, amphibole, garnet and pyroxene with quartz, but it cannot represent Na/K feldspar chemistry, Fe³⁺/Fe²⁺ variations, Mn, or volatile components.

Thus, phase diagrams provide a common framework: the plagioclase binary tracks igneous crystallisation, while ACF projection summarises metamorphic equilibria in quartz-saturated mafic rocks.