Granite rock, classification and origin

What is Granite?

Granite is a coarse-grained plutonic igneous rock composed of about 25 percent quartz and roughly 65 percent feldspar, mostly potassium and sodium-rich feldspar. Quartz are roughly spherical in shape, are often glassy and clear to light gray in colour. Feldspar crystals are generally white to gray or salmon pink in colour and exhibit a rectangular rather than spherical shape. Other minor constituents of granite are muscovite and some dark silicates, particularly biotite and amphibole. Although the dark components generally make up less than 10 percent of most granites, dark minerals appear more prominent than their percentage indicates. Granite is a holocrystalline and leucocratic rock because it is completely crystalline and a light-coloured rock.

Mineralogy of Granite

Granite is composed of only primary minerals. Among these, feldspars and quartz occur as essential minerals and the common accessory minerals are mafic minerals such as hornblende, biotite, hypersthene, augite, muscovite, magnetite, pyrite and epidote. Of these, hornblende and biotite are particularly common in granites. In case of essential minerals, feldspars account for roughly 65% of granite. In feldspars too, alkali feldspars (orthoclase and microcline) are larger in quantity than plagioclase feldspars. In normal granite, the usual plagioclase feldspar is oligoclase (i.e., 10 to 30% of An and 70 to 90% Ab). Quartz is usually present to the extent of 25 to 30%. The accessory minerals are generally less than 15%.

Classification of Granite

Granite is a type of igneous rock that is primarily composed of quartz, plagioclase and alkali feldspar. There are many different types of granite, and the classification of granite is based on various factors, including its mineral composition, texture, geochemistry and tectonic setting. In short, granite can originate from a variety of processes. Here are a few of the most widely accepted classifications of granite:

1. Mineralogical classification (IUGS classification)
2. Chemical classification (Alumina Saturation, S-I-A-M classification)
3. Tectonic classification (Based on tectonic environment)

1. IUGS classification

The traditional IUGS petrographic classification of granitoids is based upon their modal abundances of quartz, plagioclase and alkali feldspar. The IUGS classification focuses on differences in abundances and compositions of the feldspars and accounts for a wide variety of granitoid. It is inexpensive, simple to use, and truly non-genetic. The advantage of the IUGS classification is that it can be readily applied in the field for most rocks.
Mineralogical classification of granites are classified into four granite domains: tonalite, granodiorite, granite (monzogranite, and syenogranite) and alkali feldspar granites according to IUGS (Figure 1).

A major disadvantage of the IUGS classification scheme is that it is not considering compositional variations except those that affect the feldspar abundances. The classification cannot address the presence or absence of minor constituents, for example muscovite, which may carry significant petrologic implications. In such cases, mafic and felsic granitoids may fall in the same field but are having significantly different chemical compositions.

2. Chemical classification of Granite

Geochemical classifications were developed to counter the shortcomings of IUGS classification. One such classification was presented by Chappell and White in 1974. They recognized two distinct granitoid types in the Lachlan Fold Belt of eastern Australia based on source regions for producing molten granite: igneous and sedimentary protoliths (source rocks). This scheme of classification was later substantiated by Loiselle & Wones in 1979. Later this classification is widely populated as Genetic alphabetical classification or S-I-A-M classification.

2.1 Genetic alphabetical classification

Genetic alphabetical classification includes: I-type granites (I = igneous); S-type granites (S = sedimentary); M-type granites products of mantle melts (M = mantle) and A-type (A = anorogenic) [3]: A = type granites were referred as alkaline or anorogenic granites [4].

I-type granites

I-type granites are derived from igneous protoliths, first proposed by Chappell and White (1974). Itcontains moderate amounts of Al₂O₃ and high amounts of Na₂O. I-type granites are saturated in silica but undersaturated in aluminium. Its petrographic features are representative of the chemical composition of the initial magma.

Major primary minerals in I-type granites are plagioclase, potassium feldspar, and quartz. 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. I-type granites have less quartz than their S-type granite colour index equivalents. Biotite is the most common minor mineral in I-type granites. Hornblende is a typical I-type granite mineral that never occurs in S-type granite. Accessory minerals in I-type granites are zircon, apatite, titanite (sphene), and allanite. 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.

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. I-type granites have lower rubidium/strontium (Rb/Sr) ratios than S-type granites. 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.

S-type granites

S-type granites are a category of granites first proposed in 2001. S-type granitoids, derived from sedimentary protoliths and containing high amounts of Al₂O₃ and relatively low amounts of Na₂O.They are recognized by a specific set of mineralogical, geochemical, textural, and isotopic characteristics. S-type granites are over-saturated in aluminum, with an ASI index greater than 1.1.

Major primary minerals in S-type granites are alkali-feldspar and plagioclase feldspar and quartz. Thus, S-type granites are silica over-saturated (containing quartz), and do not contain feldspathoids. Minor minerals include cordierite, muscovite, garnet, and sillimanite. Biotite compositions from S-type granites are more aluminous than those of I-type granites consistent with the higher ASI index of S-type granites. Accessory minerals commonly observed in S-type granites include zircon, apatite, tourmaline, monazite and xenotime. Monazite is considered a diagnostic accessory mineral of S-type granites, whereas allanite is diagnostic of I-type granites. Oxide minerals in S-type granites will more commonly be ilmenite rather than magnetite.

Major element characteristics of S-type granites include lower levels of sodium and calcium, and elevated levels of silica and aluminum. S-type granites contain more magnesium than iron. With respect to aluminium, S-type granites are always peraluminous, or have a total alkali (+calcium) to aluminium ratio of greater than one. S-type granites contain elevated levels of potassium, rubidium and lead, and are depleted in strontium.

M-type granite

The term M-type granite is introduced by White in 1979. M-type or mantle-derived granite was proposed later, to cover those granites which were clearly sourced from crystallized mafic magmas, generally sourced from the mantle. These are rare because it is difficult to turn basalt into granite via fractional crystallization.

A-type Granite

The term A-type granite was proposed by Loiselle and Wones in 1979. The ‘A’ stands for Anorogenic or Anhydrous, as these granites are characterized by low water content and a lack of orogenic or transitional tectonic fabric. The A-type granites dominantly form within continental intraplate rifting or uplifting or at regional post-orogeny uplift or collapse. Their formation could be either anorogenic, meaning far from any orogeny, or after orogeny is completed.

A-type granites are generally peralkaline in composition. Minerals like the sodic amphiboles – riebeckite and arfvedsonite, and the sodic pyroxene – aegerine, are commonly found in these rocks. Chemical characteristics of A-type granites include high silica, alkalis, zirconium, niobium, gallium, yttrium and cerium. In addition, they tend to be relatively Fe-rich and thus fayalitic olivine sometimes occurs. The source for A-type granites could be dry granulite left over from the loss of wet magma during orogenies.

2.2 Granite classification based on alumina saturation index

Alumina saturation index (ASI) was introduced by Shand in 1943. Alumina saturation classes are based on the molar proportions of Al₂O₃/(CaO+Na₂O+K₂O) (“A/CNK”).

Granites can be subdivided on the basis of their chemistry into peralkaline, metaluminous, and peraluminous on the basis of the ratio Al₂O₃/(Na₂O + K₂O + CaO). The related terminology is:

Peraluminous

If Al₂O₃ > (Na₂O + K₂O + CaO) or Al₂O₃/(Na₂O + K₂O + CaO) > 1.1; Rock will probably have muscovite and may have garnet; will be corundum-normative.

Metaluminous

If Al₂O₃ < (Na₂O + K₂O + CaO) but Al₂O₃ > (Na₂O + K₂O) or Al₂O₃/(Na₂O + K₂O + CaO) > 1.0; rock may have hornblende.

Peralkaline

If Al₂O₃ < (Na₂O + K₂O) or Al₂O₃/(Na₂O + K₂O + CaO) << 1.0; rock will probably have a lot of K-feldspar in the norm, probably a feldspathoid or very little, if any quartz.

3. Tectonic classification of Granite

Granites formed in a variety of tectonic settings around the world, either at plate margins or intraplate. They are subdivided according to their tectonic environments into four main groups—(i) ocean ridge granites (ORG), (ii) volcanic arc granites (VAG), (iii) within plate granites (WPG) and (iv) collision granites (COLG) using trace element Y-Nb, Yb-Ta, Rb-(Y + Nb) and Rb— (Yb + Ta) diagrams.

Origin of Granite

Granite is an igneous rock that is formed through the slow crystallization of magma below the Earth’s surface. The exact origin of granite is still a subject of debate among geologists, but the prevailing theory is that it is formed through the partial melting of the Earth’s continental crust.
The continental crust is composed of a variety of rocks, including sedimentary, metamorphic, and igneous rocks. When these rocks are subjected to high temperatures and pressures, they can partially melt and become the source of the magma that forms granite.

The partial melting of the crust is thought to occur at the boundary between the lower crust and the upper mantle, which is known as the Moho discontinuity. The heat required for this melting is generated by the radioactive decay of certain elements, such as uranium, thorium, and potassium, which are present in the crust.

As the magma rises towards the surface, it cools and crystallizes, forming granite. The slow cooling of the magma allows for the formation of large, visible crystals of minerals such as quartz, feldspar, and mica, which give granite its distinctive texture and appearance.

References

• Chappell, B. W. & White, A. J. R. (1974). Two contrasting granite types. Pacific Geology 8, 173–174.
• Loiselle, M. C. & Wones, D. S. (1979). Characteristics and origin of anorogenic granites. Geological Society of America, Abstracts with Programs 11, 468
• Shand, S. J. (1943). The Eruptive Rocks, 2nd edn. New York: John Wiley, 444 pp.
• White, A. J. R. (1979). Sources of granite magmas. Geological Society of America, Abstracts with Programs 11, 539.