Tonalite is a coarse-grained, light-colored, plutonic or intrusive rock. Its composition is mainly plagioclase, quartz, and small amounts of biotite, hornblende, and sometimes pyroxenes. Also, it may have a small amount of alkali feldspar.
Gerhard vom Rath was the first to apply the name tonalite in 1864. This rock is named after its type locality, i.e., Tonale Pass, a high pass across the Rhaetian Alps between Lombardy and Trentino in northern Italy.
Here is more about tonalite, including its properties, characteristics, chemical and mineral composition. We will also look at where and how it forms and some of its uses.
Quick overview and properties
- Name: Tonalite
- Rock type: Igneous
- Origin: Intrusive or plutonic
- Texture: Phaneritic or coarse-grained
- Color: Off-white, light to dark gray with some brownish or greenish-hued
- Chemical composition: Felsic to intermediate.
- Silica content: > 60%
- Cooling history: Slow, deep inside the Earth’s crust
- Mohs hardness scale: 6-7
- Density: 2.75 g/cm3 [source]
- Metamorphic form: Tonalite gneiss
- Tectonic settings: Mainly on continental arc with smaller amounts in island arc, mid-ocean ridges.
What does tonalite look like?
Color, texture, and general appearance should help you identify this rock in the field. However, this exercise may prove futile for newbies people.
Usually, tonalite is a massive off-white (leucocratic varieties are nearly white), light gray, or dark gray, with some having a brownish or dark green hue.
It is dominated by felsic minerals, i.e., white to grayish feldspar or colorless quartz, with a few scattered mafic or dark-colored minerals like magnetite, hornblende, or biotite.
The texture of tonalite is coarse-grained with nearly equigranular grains like granites. However, some may show porphyritic texture.
Porphyritic tonalite will have larger crystals (phenocrysts) of plagioclase, quartz, biotite, hornblende, magnetite, and ilmenite set in coarse-grained groundmass. The groundmass will have mainly quartz and plagioclase.
Sometimes, plagioclase phenocrysts may show chemical zoning, including oscillatory (reversal Na/Ca ratio).
Lastly, tonalite’s typical color index M is 10-40, making it a leucocratic to mesocratic rock. Those rocks with M < 10 are considered leucocratic. Leucocratic varieties include trondhjemite and plagiogranites.
Tonalite composition
Tonalite is a felsic rock. Its essential minerals are plagioclase and quartz and smaller amounts of mafic minerals. Here is more on the chemical composition and mineralogy of this rock.
1. Chemical composition
Tonalite chemical composition is mostly felsic (SiO2 > 63 wt. %), some being intermediate. It is relatively high in alkalis (Na2O and K2O) and low in ferromagnesian (iron and magnesium).
Le Maitre 1976 from 97 analyses, found the average percentage weight chemical composition as SiO2: 61.52%, TiO2: 0.73%, Al2O3: 16.48%, Fe2O3: 1.83%, FeO: 3.82%, MnO: 0.08%, MgO:2.80%, CaO:5.42%, Na2O: 3.63%, K2O: 2.07%, P2O5: 0.25%. This makes it an intermediate rock.
On the other hand, data from Nockolds (1954) SiO2: 66.15%, TiO2: 0.62 %, Al2O3: 15.56%, Fe2O3: 1.36%, FeO: 3.42%, MnO: 0.08%, MgO: 1.94 CaO: 4.65 Na2O: 3.90, K2O: 1.42%, P2O5: 0.21%. Such a rock would be of felsic composition.
2. Mineral composition
Tonalite mainly has plagioclase and quartz with minor biotite, hornblende, and pyroxenes. Sometimes, it may be a small amount of alkali feldspar (up to 10% of total felspar).
The common accessory minerals in this rock include apatite, zircon, titanite (sphene), magnetite, ilmenite, and, less often, allanite and garnet.
Usually, tonalite’s plagioclase is sodic (An30–50), i.e., oligoclase or andesine, while the alkali feldspar is orthoclase.
Possible secondary minerals include uralite, iddingsite, and chlorite, which replace pyroxenes, biotite, or hornblende. Also, it may have epidote and sericite from feldspar replacement. However, quartz isn’t susceptible to alteration.
On the QAPF diagram for coarse-grained plutonic rocks, tonalite is a rock with plagioclase making up more than 90% of the total feldspar and quartz 20-60% of the QAPF content by volume.
Tonalite is one of the granitoids, i.e., rocks in which quartz is 20-60% of QAPF content by volume and variable feldspars. Other granitoids are granite, granodiorite, and alkali feldspar granite.
However, tonalite differs from granite since it mainly has sodic plagioclase, while alkali feldspar dominates granite. Also, granites are usually higher in silica and lighter in color than tonalite.
Lastly, tonalite shouldn’t be confused with quartz diorite, which has only 5-20% quartz by volume of the QAPF content.
Naming tonalites
The naming follows the convention of naming other igneous rocks. Therefore, you can have textural, mineralogical, and chemical composition prefixes, among other generic ones.
For instance, biotite or hornblende tonalite indicates they have considerable (> 5% by volume) amounts of biotite or hornblende.
If a rock has more than two considerable minerals, follow the increasing order of abundance and hyphenate. For instance, biotite-hornblende tonalite indicates that biotite < hornblende.
Varieties and related rocks
In granitoids, S-I-A-M or alphabet classification, metaluminous to weakly peraluminous tonalites and granodiorites belong to the I-type of granites.
Some of the varieties and related rocks include:
1. Microtonalite
Microtonalite describes a tonalite with a medium-grain texture. It forms mainly as a subvolcanic rock with a relatively quicker cooling rate.
The faster cooling doesn’t allow mineral grains to grow much like the coarse-grained plutonic counterparts.
2. Trondhjemite
Trondhjemite is a leucocratic (M < 10%) tonalite variety deficient in orthoclase. This rock is named after the type locality Trondheim, Norway.
You can define trondhjemite as a light-colored, coarse-grained intrusive igneous rock that has essentially sodic plagioclase (oligoclase or sometimes andesine), quartz, and minor amounts of mafic minerals – biotite, pyroxenes, and rarely hornblende – and alkali feldspar. Common accessories include allanite, apatite, zircon, titanian, magnetite and ilmenite, etc.
Trondhjemites are part of the TTG (tonalite–trondhjemite–granodiorite) suite. TTG formed about 90% of the juvenile Archean crust (3.8-2.5 Ga) and are the protoliths of many gneiss complexes on the Archaean craton.
However, Phanerozoic TTG does occur in convergent plate margins, such as in Western parts of the United States, notes Okrusch (2020).
Lastly, the Archean TTG rock suite formed likely from partial melting of metabasalt or oceanic subduction crust not similar to today’s subduction zones.
3. Plagiogranites
Plagiogranites are a trondhjemite variety that occurs in ophiolites and oceanic crusts.
They are also leucocratic and have mainly quartz, sodic plagioclase, and minor silicate minerals (> 10 % biotite or hornblende).
4. Charnockitic tonalite
Charnockitic tonalites are orthopyroxene-bearing tonalite varieties. Please don’t confuse them with charnockites.
Charnockites are medium to coarse-grained granoblastic, non-foliated, high-pressure, and temperature metamorphic rocks. They have feldspar, quartz, and orthopyroxene and were initially mistaken for igneous.
These rocks are an endmember of the charnockite series and commonly occur in metamorphic granulite facies.
5. Enderbite
Enderbite is an igneous rock with quartz, feldspar (mostly anti-perthite or sometimes perthite), orthopyroxene, and magnetite.
It is equivalent to orthopyroxenes bearing tonalite and is named after its type locality, Enderby Land, Antarctica.
How is tonalite formed?
Tonalites are plutonic or intrusive igneous rocks formed when magma intrudes through areas of weakness, faults, or cracks and cool slowly deep inside the Earth’s crust. Uplift and erosion may expose this rock to the surface.
The slow cooling allows mineral crystals to grow. Thus, these rocks have larger grains visible to the naked eye.
Magma that forms tonalites and related rocks like granodiorites and granites originate from 1) partially melting subducted oceanic crust or mantle wedge, 2) metabasalt/metagabbro anatexis, or 3) fractional crystallization.
However, the larger tonalite volume in batholith belts results from the fusion of basalt or gabbro magma ponding and crystallizing at the crust’s base (underplating).
Where is tonalite found?
Tonalites occur in batholiths, dikes, sills, stocks, and other igneous intrusions or plutons, including laccoliths and laccoliths. This rock is part of the TTG, an important component of the juvenile crust and Archean Cratons.
Depending on the tectonic setting, tonalite usually occurs in plutonic rocks like granodiorite, granite, diorite, monzogranite, synenogranite, anorthosite, quartz monzodiorite, gabbro, and ultramafic rocks like peridotite.
Some places where tonalite is found are in batholiths along the North and Western American orogenic belts. These batholiths are associated with subducted oceanic crust lying on continent margins. Examples include the Peninsular Range, Sierra Nevada, Coastal Range, Idaho batholith (western side), and Coastal Batholiths of Peru. Here, it occurs with granodiorite and other granitic rocks with less gabbro.
Smaller volumes of tonalite and granodiorite occur in island arcs. However, these arcs are dominated by intermediate, ultramafic (peridotite), mafic (gabbro) plutons, and extrusive rocks like basalt and andesite. Examples include the Tobago plutonic complex, Aleutian, Kurile, Izu– Bonin – Mariana (IBM) arcs, the Philippines, and the Solomon Islands.
Also, tonalite occurs in mid-ocean ridges and is associated with basaltic pillow lava, sheet flows, sheeted dikes, and gabbro.
More places this rock occurs include Great Tonalite Sill (Alaska USA), Adamello batholith (Italy), Tertiary Bergell intrusion (Central Alps), Chingezi and Mashaba (Zimbabwe), and in the Caledonian belt of Scandinavia.
Lastly, tonalite occurs in small volumes in ophiolites like Dongwanzi ophiolite (China), Boil Mountain ophiolite (Maine, USA), Semail ophiolite (Oman), Canyon Mountain ophiolite (California, USA), and Char ophiolitic belt (Kazakhstan).
What is tonalite used for?
Tonalite is a hard and durable rock, making it a valuable aggregate and dimensional stone for various construction and architectural uses. It is also ideal for landscaping, riprap, marine walls, monuments, headstones, etc.
As a crushed stone, it makes highway subbases, asphalt, concrete, and concrete subbases. Also, you can use it on unpaved walkways, driveways, etc.
On the other hand, in the dimensional stone industry, it may be cut and used in masonry, paving, making retaining walls, etc.
Cut and polished tonalite may make slabs, stair treads, and windowsills, among other architectural stones.
Lastly, some tonalites host copper-gold porphyry with economic value. Examples of gold-bearing ones are in Batu Hijau.
References
- Gill, R. (2010). Igneous rocks and processes: A practical guide (1st ed.). Wiley-Blackwell.
- Best, M. G. (2013). Igneous and metamorphic petrology (2nd ed.). Blackwell Publishers.
- Le Maitre, R. W. (Ed.) (2002). Igneous rocks: A classification and glossary of terms (2nd ed.). Cambridge University Press
- Winter, J. D. (2014). Principles of igneous and metamorphic petrology. Pearson Education.
- Haldar, S. K., & Tisľjar, J. (2014). Introduction to Minerology and petrology (1st ed.). Elsevier.
- Okrusch, M., & Frimmel, H. (2020). Mineralogy: An introduction to minerals, rocks, and mineral deposits (1st ed.). Springer.
- Nockolds, S. R. (1954). Average chemical compositions of some Igneous Rocks. Geological Society of America Bulletin, 65(10), 1007. https://doi.org/10.1130/0016-7606(1954)65[1007:accosi]2.0.co;2
- Le Maitre, R. W. (1976). The chemical variability of some common igneous rocks. Journal of Petrology, 17(4), 589–598. https://doi.org/10.1093/petrology/17.4.589