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

(a) Crystallisation of a liquid of composition An₅₀ in the albite–anorthite system at 1 atm

Albite (melting point 1118 °C) and anorthite (1553 °C) form a complete solid-solution series, so the diagram (Bowen, 1913) has a lens-shaped loop between the liquidus above and the solidus below, with no eutectic. The temperatures and compositions below are approximate values from the loop.

Cooling path.

(i) 1500 °C is above the liquidus, so the system is entirely liquid of An₅₀.

(ii) Liquidus at about 1450 °C. The first crystals are much richer in anorthite than the liquid, about An₈₀ to An₈₅.

(iii) Below the liquidus, at each temperature the liquid and solid compositions are given by the ends of a horizontal tie line through the temperature. The solid takes the anorthite-rich end, and the liquid moves towards albite along the liquidus. The solid re-equilibrates with the liquid by continuous reaction, moving along the solidus.

At 1400 °C: liquid about An₃₃, solid about An₇₃, and by the lever rule the solid fraction is (50 − 33)/(73 − 33), about 43%. At 1350 °C: liquid An₂₁, solid An₆₄, solid about 67%. At 1300 °C: liquid An₁₃, solid An₅₄, solid about 91%.

(iv) Solidus at about 1280 °C. The last drop of liquid, about An₁₀, disappears and the solid is An₅₀, equal to the starting composition.

The solid changes from An₈₀ to An₅₀, and the liquid from An₅₀ towards An₁₀, by continuous reaction. There is no eutectic halt and only the start and end of crystallisation show a change in cooling rate.

Fractional crystallisation. If early crystals are removed or armoured, as in zoned plagioclase, the liquid evolves to albite-rich values, and the crystals are zoned from An-rich cores to Ab-rich rims (normal zoning), because the solid cannot re-equilibrate.

(b) Definition of granite and petrogenesis of a calc-alkaline peraluminous granite

Definition. Granite is a felsic, coarse-grained plutonic rock. In the IUGS QAPF classification it has quartz 20–60% of the light minerals (Q + A + P) and plagioclase 10–65% of the total feldspar. It includes syenogranite and monzogranite, and the total of dark minerals is low (typically below 20–35%). Its silica is above about 65–70 wt%.

Peraluminous. A/CNK = Al₂O₃/(CaO + Na₂O + K₂O) in molecular proportions exceeds 1 (above 1.1 for S-type). Aluminous minerals occur: muscovite, biotite, cordierite, garnet, andalusite or tourmaline.

Petrogenesis.

(i) Source. Partial melting of aluminous crustal rock, such as metapelite and greywacke, which gives S-type melts. Melting takes place by dehydration of muscovite and biotite, or water-fluxed melting, at 650–850 °C at amphibolite to granulite conditions.

(ii) Heat source. Crustal thickening in collision zones, decompression, radioactive heating, or mantle-derived basalt underplating.

(iii) Evolution. The melt separates from the residue (restite), rises as diapirs or along shear zones, and crystallises. Fractionation of plagioclase and biotite, assimilation, and some mixing with mantle-derived magma give the calc-alkaline trend.

The Himalayan leucogranites, such as the Gangotri and Badrinath granites, arose from melting of the Higher Himalayan Crystalline metasediments.

(c) Assumptions for plotting quartz-bearing metabasic rocks on an ACF diagram

The ACF diagram (Eskola) plots the bulk composition of a metamorphic rock in an equilibrium assemblage as a triangle of three components: A = Al₂O₃ + Fe₂O₃ − (Na₂O + K₂O), C = CaO − 3.3 P₂O₅, and F = FeO + MgO + MnO, normalised to 100. The assumptions are:

(i) Quartz is present in excess, so SiO₂ is not plotted, and the rock is projected from quartz.

(ii) Water (and CO₂) is abundant and mobile, so H₂O too is a component excluded from the triangle.

(iii) Na₂O and K₂O are taken up in feldspar (albite and K-feldspar), which consumes Al₂O₃ in proportion, hence the subtraction from A. The diagram is projected from these feldspars.

(iv) Apatite takes up CaO and P₂O₅ in the ratio 3.3:1, so that part of CaO is removed from C.

(v) Fe and Mg substitute for each other completely, as do Mn, so FeO + MgO + MnO is one component, F. Fe₂O₃ is added to Al₂O₃, as Fe³⁺ replaces Al.

(vi) Equilibrium assemblage at one set of P–T, with no more phases than the three components allow (the mineralogical phase rule).

A basaltic composition plots towards the C–F side, inside the triangle, since its Al₂O₃ is moderate, and gives hornblende + plagioclase in amphibolite facies.