Granodiorite is a coarse-grained, light-colored plutonic rock. It has mainly plagioclase, quartz, lesser alkali feldspar, and minor amounts of mafic minerals like mica and amphibole.
A hand specimen of granodiorite somewhat rock resembles granite, and its composition is between tonalite and granite.
The word granodiorite comes from two rock names. Grano is from the Latin word granum (associated with granite), which means grain, and diorite is from the Greek word διορίζειν (diorízō), meaning to distinguish. It described the coarse-grained appearance of this rock with distinguishable minerals.
George Ferdinand Becker was the first to propose the term granodiorite in 1893 on Sierra Nevada’s maps of the Gold Belt, and Waldemar Lindgren published it. Back then, it referred to any rock with an intermediate composition between diorite and granite.
However, today, granodiorite is defined as a plutonic rock in which plagioclase is 65-90% of total feldspar, and quartz accounts for 20-60% of QAPF content by volume. Also, it has minor mafic minerals.
You should know that granodiorite is among the most abundant rocks in the Earth’s continental crust. Also, the upper continental crust has a crude average composition like this rock.
Unfortunately, many confuse this rock with other plutons, especially granite, diorite, and tonalite. Furthermore, most people commercially call it ‘granite’, creating more confusion.
Discover granodiorite’s properties (texture, color, and hardness), uses, chemical and mineral composition. We will also discuss how it forms, where it is found, and how it differs from look-alikes like granite and diorite.
Quick properties and facts
- Name: Granodiorite
- Rock type: Igneous
- Origin: Plutonic
- Color: Light colored, usually off-white to grayish, with some slightly pinkish to reddish.
- Texture: Coarse-grained or phaneritic
- Silica content: 63%-69%
- Chemical composition: Felsic
- Plutonic equivalent: Dacite
- Density: 2.6-2.7 g/cm3
- Mohs hardness scale: 6-7
- Tectonic environment: Mainly convergent zones (continental arc margins, continental-continental collision, and less volumes in oceanic arc)
What does granodiorite look like?
Color and texture are key to identifying this rock. Granodiorite is usually a massive, off-white, grayish-white, or light-gray rock but can be reddish, pinkish, or other colors. It has a coarse-grained texture (visible to the naked eye) with grains more or less equigranular like granite.
Also, granodiorite has a speckled or salt-and-pepper appearance, like granite. The mafic minerals like hornblende and biotite form fewer darker speckles. In contrast, lighter minerals, i.e., feldspars and quartz, are lighter grains you will see.
However, granodiorite is darker than granite since it contains more mafic minerals and plagioclase content.
Besides coarse-grained and equigranular texture, some may have porphyritic and, less frequently, orbicular or pegmatitic.
Porphyritic granodiorite will have large (up to 6 cm) crystals or phenocrysts of usually pinkish subhedral sodic alkali feldspar. These phenocrysts are embedded in a finer but phaneritic matrix or groundmass of quartz, feldspars, biotite, amphibole, and accessory minerals.
However, according to Márton et al. (2007) study, this rock may have “phenocrysts of plagioclase, which show complex twinning, zoning and pronounced epitactic growth.”
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Although mostly massive, this rock may show modal layering common in gabbro plutons. An example is the Tarçouate Laccolith in Morocco’s Anti-Atlas ranges. It has foliated, homogeneous biotite granodiorite surrounding inner hornblende granodiorite (rich in melanocratic monzodiorite inclusions), showing modal biotite and hornblende variation notes Gill, (2010).
Also, some granodiorite may show comb layering, such as in the Sierra Nevada batholith. Such a rock will have elongated banding in each layer, usually, plagioclase and hornblende, oriented perpendicular to the layering plane. It resembles the teeth of a comb, which become broader or branch as they go into the interior parts of the plutons.
Lastly, according to (Bentor, 1989, p. 202), the color index M varies from <5 (leucogranodiorite, 8%) to 5-30 (mesogranodiorite, 91%) and 30-50 (melagranodiorite, 1%). Thus, we can say that this rock is mainly mesocratic.
Granodiorite composition
What is the mineral and chemical composition of granodiorite?
1. Chemical composition
The chemical composition of granodiorite is felsic, with 63-69% silica. However, some specimens may be intermediate in composition. I
This rock is relatively high in felsic content (alkali oxides and silica) and low in mafic (iron and magnesium oxides) content.
According to data from Le Maitre (1976) from 885 analyzed samples, the average percentage weight composition of granodiorite is SiO2: 66.09%, TiO2: 0.54%, Al2O3: 15.73%, Fe2O3: 1.38%, FeO: 2.73%, MnO: 0.08%, MgO: 1.74%, CaO: 3.83%, Na2O: 3.75%, K2O: 2.73%, and P2O5: 0.18%
2. Mineral composition
Granodiorite’s mineral composition is mainly plagioclase, quartz, lesser alkali feldspar, minor biotite, and hornblende. Sometimes, it may have muscovite, epidote, and pyroxenes.
Accessory minerals include apatite, Fe-Ti oxides (magnetite and ilmenite), ulvöspinel, allanite, zircon, titanite, sulfide minerals, and, less often, tourmaline and garnet.
Plagioclase is calcium-rich and sodium-rich An0 – An50, usually oligoclase or sometimes andesine, while alkali feldspar is orthoclase and less often microcline. Plagioclase often forms twinned crystals that may be entirely enclosed by orthoclase, i.e., poikilitic. Sometimes, sodic plagioclase and potassium feldspar may show a perthitic texture.
Mafic minerals are mainly biotite, hornblende, and pyroxene – augite (clinopyroxene) and enstatite (orthopyroxene). Biotite will have well-formed hexagonal and hornblende needlelike crystals.
Best (2013) states that plagioclase and mafic minerals usually form euhedral to subhedral crystals. In contrast, alkali feldspar and quartz tend to be anhedral, probably from interstitial melt crystallization. This texture is often known as hypidiomorphic-granular, i.e., it has some euhedral, subhedral, and anhedral crystals.
On the QAPF (Quartz, Alkali Feldspar, Plagioclase, and Feldspathoids) diagram for coarse-grained plutonic rocks, granodiorite is defined as a rock in which quartz is 20-60% QAPF content by volume, and plagioclase being 65-90% of total feldspar. It doesn’t have feldspathoids, though.
With the increase in plagioclase, it grades into tonalite, while with an increase in alkali feldspar, you have granite.
Naming this rock
When naming granodioritic rocks, you can use textural, chemical, mineral, and other qualifying prefixes. For instance, you can have biotite or hornblende granodiorite if it has considerable (> 5% vol.) biotite or hornblende.
Name according to increasing order of abundance in the case of two minerals with a hyphen between the abundant minerals. For instance, biotite-hornblende granodiorite implies that both biotite and hornblende occur in considerable amounts. However, hornblende > biotite.
How does granodiorite differ from granite?
Both these two rocks form slow cooling of silicic magma in plutons (magma chambers) deep in the Earth’s crust. Their texture is phaneritic with visible grains or crystals that are more or less equigranular.
Their main difference is granodiorite has more calcic and sodic plagioclase than alkali feldspar and higher mafic minerals, making it darker. In contrast, granite has more potassium feldspar (orthoclase) than plagioclase and less of the mafic minerals, making it lighter in color.
On a hand sample, granodiorite will generally appear darker with darker speckles and a few pinkish or reddish ones due to low alkali feldspar, while granite will be lighter, with fewer darker speckles and more pink or reddish minerals. However, this may not be true for leucocractic varieties.
Basanite
Basanite is a fine-grained, dark-colored volcanic or extrusive igneous rock. This mafic rock has mainly calcic plagioclase, nepheline (with or without other foids), augite, olivine, and other minor minerals.
However, the use of the name basanite varies. For instance, basanite is an informal name for any rock with an intermediate composition between diorite and granite. More than often, it refers to granodiorite that intruded the Late Cretaceous in the Banat in Serbia and Hungary.
In Australia, basanite refers to rock whose composition is intermediate between quartz monzonite and quartz diorite, found in Mount Gulaga in New South Wales.
How is granodiorite formed?
Granodiorite forms when silica-rich magma intrudes and cools slowly inside the Earth’s crust. The slow cooling allows form mineral crystals to grow large, i.e., visible with the naked eye.
Later, the plutons of which some are later exposed by erosion or uplift.
The silicic magma that forms this rock originates from the fractional crystallization of mantle-derived mafic magma or anatexis of lower subcrustal rocks. Magma mixing and crustal contamination may also play a role in magma generation.
Lastly, granodiorites are in the S-type (sedimentary) and I-type (igneous) granites in the alphabet or S-I-A-M classification.
Where is granodiorite found?
Granodiorite rock occurs mainly in batholiths and stocks. However, it can also form sills, dikes, laccoliths, lopoliths, and other intrusions.
This rock is most common in active continental margins and continent-continent collision orogenic zones. However, a few emplacements may occur in island arcs.
Usually, it occurs with other granitoids, i.e., tonalite, granite, and related rocks like quartz-diorite, quartz monzodiorite, or sometimes even with mafic plutons like gabbro.
Going to specific locations, some of the North American Cordilleran batholiths with granodiorite include Sierra Nevada, Peninsular, Coastal, Idaho, and Boulder. Besides cordilleras, it is found in Mount Logan (Canada), the Adirondack Mountains (NY, USA), and the Appalachians (USA).
On the South American cordilleran, it occurs Coastal Batholith of Peru, the Patagonian Batholith, and the Cordillera Blanca Batholith.
Other places with this rock include the Gangdese Batholith Belt (Tibet), the Lachlan Fold Belt, especially Comma and Warburton (Australia), and Massif Central (France).
Also, it occurs in Trans-Himalayan granitoid belts (Trans-Himalayan and Karakorum batholiths), Caledonian Fold Belt (UK), Variscan granite intrusions (central Europe), Batholiths in Mikashevichi Igneous Belt (Russia), and Tokwe Terrane (Zimbabwe)
Lastly, a very small amount occurs in oceanic arcs like New Papua Guinea, Aleutian Island, and New Britain.
What is granodiorite used for?
Granodiorite is a tough, hard, and durable rock. It is sold in the decorative stone industry as ‘commercial granite’ with other rocks like gabbro, granite, diorite, gneiss, syenite, granite, and anorthosite.
Some of the uses of granodiorite include making aggregate (gravel) for construction, including roads and railway ballast, subbases, etc. Also, it makes dimension stones for construction, façades, memorial monuments, curbing, paving (pavers or cobblestones)
If cut and polished, it can make kitchen countertops, slabs, stair treads, windowsills, fireplace surround, floor and wall tiles, etc.
Also, you can use this stone for landscaping, riprap for soil erosion control, or gravel for unpaved walkways, patios, and driveways.
More uses include making statues, headstones, carvings, spheres, pillars, etc.
That is not all. Porphyritic varieties may contain significant deposits of ores, especially copper-porphyry, and associated ores like molybdenum, gold, and silver. Also, some, like Boulder batholith, have copper veins.
Lastly, Ancient Egypt used granodiorite to make steles like the Rosetta stone. Also, it was quarried in Egypt (Mons Claudianus) and shipped to Rome between the 1st and 3rd AD to make structures like the Pantheon columns and Hadrian’s Villa.
Frequently asked questions
Granodiorite is a felsic rock. It has mostly felsic minerals (feldspars, quartz, and sometimes a small muscovite mica) with a small amount of mafic minerals. Its composition lies between tonalite and granite and has about 63-69 wt. % silica.
The Plymouth Rock is a glacial erratic granodiorite boulder upon which traditions claim that pilgrims fathers first stepped after disembarking from the Mayflower ship in 1620.
Yes. Some reddish or pink granodiorite exists. However, the relatively low alkali content makes such rock less common.
Granodiorite is a felsic rock with more quartz (20-60% QAPF content by volume) and lower mafic minerals, making its color lighter than diorite. In contrast, diorite is an intermediate rock with less quartz (< 5% of QAPF content by volume) and more mafic minerals, making it darker.
References
- Best, M. G. (2013). Igneous and metamorphic petrology (2nd ed.). Blackwell Publishers.
- Gill, R. (2010). Igneous rocks and processes: A practical guide (1st ed.). Wiley-Blackwell.
- Kemp, A. I. S. (2005). Granite. In Selley, R. C., Morrison, C. L. R., & Plimer, I. R. (Eds.). Encyclopedia of geology (Vols. 1-5, pp. 234-247). Elsevier Academic.
- Le Maitre, R. W. (Ed.) (2002). Igneous rocks: A classification and glossary of terms (2nd ed.). Cambridge University Press.
- Klein, C., & Philpotts, A. R. (2017). Earth materials: Introduction to mineralogy and petrology (2nd ed.). Cambridge University Press.
- Bentor, Y. K. (1989). Granodiorite. In Bowes, D. R. (ed.). The encyclopedia of igneous and metamorphic petrology (pp. 201-203). New York: Van Nostrand Reinhold.