science

I-type granite

I-type granite is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand I-type granite rather than just read about it. In short: I-type granites are a category of granites originating from igneous sources, first proposed by Chappell and White (1974). They are recognized by a specific set of mineralogical, geochemical, textural, and isotopic characteristics that indicate, for example, magma hybridization in the deep crust.

Key takeaways

I-type granite belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect I-type granite to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of I-type granite from memory before moving on to harder problems.

Reference excerpt

I-type granites are a category of granites originating from igneous sources, first proposed by Chappell and White (1974). They are recognized by a specific set of mineralogical, geochemical, textural, and isotopic characteristics that indicate, for example, magma hybridization in the deep crust. I-type granites are saturated in silica but undersaturated in aluminum; petrographic features are representative of the chemical composition of the initial magma. In contrast S-type granites are derived from partial melting of supracrustal or "sedimentary" source rocks.

Petrographic characteristics

Primary minerals Minerals that crystallized from the silicate melt are considered primary minerals. They are grouped into "Major", "Minor", and "Accessory" minerals based upon their modal percentages in the rock.

Major mineralogy Primary minerals in I-type granites are plagioclase, potassium feldspar, and quartz as in S- and A-type granites. I-type granites have less quartz then their S-type granite color index equivalents. Plagioclase displays zonation and albite twinning. Potassium feldspar can show perthite textures, carlsbad twinning, and, in microcline, tartan twinning. Quartz and potassium feldspar scarcely show granophyric textures.

Minor minerals Biotite is the most common minor mineral in I-type granites. The biotites in I-type granites are greener in general than those in S-type, both in hand sample and in plane polarized light. More mafic composition granites, those with a higher color index, contain more hornblende and biotite. Hornblende is a typical I-type granite mineral which never occurs in S-type granite. Hornblende crystals can be twinned and compositionally zoned.

Accessory minerals Zircon and apatite can occur in both I- and S-type granites, whereas titanite (sphene) and allanite are considered diagnostic accessory minerals for I-type granites. Allanite is typically surrounded by radial fractures, caused by the subsolidus increase in volume of allanite as a result of metamict alteration due to radioactive decay. While apatite inclusions are common, they are not as abundant or large as those in S-Type granites. Primary muscovite can occur in weakly peraluminous fractionated I-type granites. Therefore, the presence of muscovite alone is not diagnostic of S-type granites.

Subsolidus and alteration minerals Minerals that form in the rock as a result of chemical reactions that take place between primary minerals and hydrothermal fluids are classified as subsolidus minerals. They form below the temperature and pressure conditions of the solidus in the absence of a silicate melt. Other alteration minerals may form at surface conditions from interaction of the minerals present in the rock with groundwater and the atmosphere. Alteration of biotites can produce fluorite, chlorite, and iron oxides such as magnetite and ilmenite. Sericitic alteration is seen within feldspars. In more evolved I-Type granites, calcite occurs as a late stage and/or a subsolidus mineral. Fluorite, like calcite, is rare and where observed it is associated with the more evolved I-type granites. It can form as a late stage product of crystallization. It is commonly observed as part of the subsolidus alteration of biotite along with chlorite and opaque oxides. Muscovite occurs as an alteration of feldspars and biotite. Epidote can be found, especially on the edges of allanite.

Color index Color index, or the modal abundance of minerals other than quartz, plagioclase and alkali feldspar (e.g., mafic silicates, oxides, sulfides, phosphates, etc.), can be used to infer the maturity of a granite. Juvenile I-type granites have a higher color index. Amphibole, biotite, sphene, allanite, and oxides are typically more abundant. In contrast, more evolved (i.e. fractionated) I-type granites have a lower color index, and may contain minerals such as muscovite that are indicative of their fractionated nature.

Textures I-type granites can have variable textures. I-type granites, like other granite types, can vary in crystal size from aphanitic to phaneritic; crystal size distributions include porphyritic, seriate, and rarely equigranular textures. Like other granites, phenocrysts in I-type granites are commonly feldspars, but can also be hornblende. Amphibole is a diagnostic feature on the hand sample scale between S-type and I-type granites.

Geochemistry

Major elements I-type granites are rich in silica, calcium and sodium but contain lesser amounts of aluminium and potassium when compared to S-type granites. I-type granites are typically metaluminous to weakly peraluminous. This is expressed mineralogically by the presence of amphibole and accessory minerals such as sphene and allanite in the metaluminous I-type granites. Note that weakly peraluminous fractionated I-type granites may crystallize primary muscovite and rare spessartine-rich garnet.

Trace and rare earth elements The rare earth element diagrams of I-type granite suites tend to be flatter than those of S-type granites, which has been inferred to be caused by the lesser amounts of apatite in I-type granites. I-type granites have lower rubidium/strontium (Rb/Sr) ratios than S-type granites.

Isotopic characteristics Initial strontium isotopic ratios (87Sr/86Sr)i are a good differentiator between I- and S-type granites, with I-type granites having lower initial strontium isotopic ratios than S-type granites.

Interpretation(s)

Source characteristics I-type granites are interpreted to be generated from the melting of igneous rocks. The “I” in I-type in fact stands for igneous. This interpretation was made by Chappell and White in their 1974 paper based on their observations in the Lachlan Fold belt of southeastern Australia.

The I-S line The I-S line is an observed contact between I- and S-type granites in an igneous terrane. This contact is usually clearly defined; one example of this occurring is within the Lachlan fold belt of Australia. The I-S line is interpreted to be the location of a paleo-structure in the subsurface that separated the generation zones of the two different melts.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with I-type granite

Start with the simplest possible case. Write down what I-type granite claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to I-type granite before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about I-type granite ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of I-type granite

In research: I-type granite appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses I-type granite in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: I-type granite is common in secondary-school and first-year university syllabi. It links to neighbouring topics Igneous rocks, so understanding it makes those chapters shorter.
In everyday life: Look for I-type granite outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study I-type granite in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what I-type granite means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain I-type granite out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is I-type granite in simple terms?

Why does I-type granite matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study I-type granite?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on I-type granite.