ERB222 Chap.4 Granites, Rhyolites and Crustal Melting
Granites, Rhyolites and Crustal Melting
Granite needs a crustal source
Melting mantle peridotite yields basalt, yet thick batholiths and huge rhyolite eruptions sit almost entirely on the continents. Week 3 explains them as products of melting inside the continental crust, supported by inherited zircon, xenoliths, migmatites, isotopes and the bulk chemistry of the upper crust.
Because solid crust will not melt by decompression, the heat comes from basaltic magma that ponds at the Moho, underplates the crust and raises the geotherm.
Water and micas set the melt budget
Water-saturated metasediments begin to melt near 650°C, but only small amounts of immobile melt form until hydrous minerals break down.
Muscovite breakdown gives less than 10% melt, while biotite breakdown near 760°C can release a mobile granitic magma through fluid-absent melting, written H = A + M. The quartz, albite and orthoclase system then shows why so many granites share a minimum-melt composition.
Naming and typing a granite
The Weeks 3 and 4 practical asks for an IUGS name first, then a type.
M, I, S and A letters record the source rock and come with distinct mafic minerals, while Buddington's epizonal, mesozonal and catazonal classes record emplacement depth. The alumina saturation index, calculated in molar proportions, separates peraluminous from metaluminous granites and links chemistry to what you see under the microscope.
What this chapter covers
- 01
Evidence for crustal melting
- 02
Heat from basalt underplating
- 03
Fluid-absent melting of micas
- 04
Haplogranite ternary minimum
- 05
Granitoid textures and intrusion types
- 06
Buddington depth zones
- 07
I, S, A and M granite types
- 08
Alumina saturation index
Worked example · free
Classify a hornblende granodiorite with the alumina saturation index
- 1Convert alumina to moles: 14.2 / 101.96 = 0.1393.
- 1Convert the other oxides: CaO 3.4 / 56.08 = 0.0606; Na2O 3.5 / 61.98 = 0.0565; K2O 3.0 / 94.2 = 0.0318.
- 1Sum the denominator: 0.0606 + 0.0565 + 0.0318 = 0.1489.
- 1Divide: A/CNK = 0.1393 / 0.1489 = 0.94; this is at most 1, so the rock is metaluminous, consistent with its hornblende and an I-type source.
Key terms
- Migmatite
- A mixed rock, part igneous and part metamorphic, in which granitic domains of crystallised melt sit beside darker unmelted domains.
- Xenocryst
- A crystal carried into a magma from foreign rock, such as an inherited zircon older than its host granite.
- Fluid-Absent Melting
- Partial melting in which the water comes from the breakdown of a hydrous mineral, summarised as H = A + M.
- Underplating
- The ponding of mantle-derived basaltic magma at the base of the crust, where its heat can melt the crust above.
- Haplogranite
- A simplified experimental granite made of alkali feldspar and quartz, used to model natural granite melting.
- Alumina Saturation Index
- The molar ratio Al2O3/(CaO + Na2O + K2O) that separates peraluminous from metaluminous igneous rocks.
- Pegmatite
- A very coarse-grained granitic rock with crystals from centimetres to metres, crystallised from the last water-rich melt.
- Batholith
- A large, generally discordant body of one or more plutons with an exposed area greater than 100 square kilometres.
Granites, Rhyolites and Crustal Melting FAQ
Why can't decompression melting make granite?
Continental crust is solid in its normal state, and lowering the pressure on it does not produce melt, so some thermal disturbance is required. That heat comes mostly from basaltic magma trapped at the Moho or intruded into the lower crust, which lifts the geotherm until hydrous minerals start to break down.
What temperature does the continental crust start melting at?
With free water present, metasediments begin melting at about 650 degrees Celsius, but only a little immobile melt forms. Substantial, mobile granitic magma needs biotite breakdown near 760 degrees, and a metapelite can reach about 20% melt at 800 degrees, while amphibolite needs close to 950 degrees for similar volumes.
How do you tell an S-type granite from an I-type in thin section?
Look for aluminium-rich phases. Primary muscovite, cordierite, garnet and red biotite with monazite point to an S-type, whereas hornblende, magnetite and sphene point to an I-type. Confirm with the alumina saturation index, which is above 1 for S-types, and with initial strontium isotope ratios when available.
What is the difference between an aplite and a pegmatite?
Both are granitic veins or dykes but differ sharply in grain size. Aplite is light, sugary and fine-grained quartz plus feldspar, while pegmatite has crystals from several centimetres to metres. Pegmatites form from the final, most water-rich melt and often hold tourmaline, beryl, topaz and elements such as lithium and tantalum.
Exam move
Draw the melt-fraction curve from memory with its 650, 760 and 800 degree anchor points and the under-10% and 60% melt levels, then explain each step aloud. Rebuild the I, S, A and M table on a blank page, and time yourself computing molar A/CNK for two analyses until four-decimal working takes under three minutes. Finish by matching every texture in the practical list to a sketch.
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