Latite is an intermediate volcanic or extrusive rock. It has a fine-grained texture and is often porphyritic.
This rock has nearly equal plagioclase and alkali feldspar. Also, it has a small amount of mafic minerals and sometimes quartz.
Frederick Leslie Ransome, an American Geologist, named this rock latite after Latium in Central Italy. Initially, it meant a rock between andesite and trachyte.
However, that definition has changed to emphasize a volcanic rock with an approximately equal amount of plagioclase and alkali feldspar. It is the volcanic equivalent of monzonite.
This post will go into detail about latite rock. We will describe it, including its appearance, texture, and colors. Also, you will have its composition and where it occurs.
Description and appearance
Latite is a massive, whitish, pink, gray, yellowish-colored rock. Its texture is fine-grained, usually porphyritic, but not always. Some show rhomb porphyry.
Porphyritic textured rocks have unusually large crystals (phenocrysts) in a fine-grained matrix. It shows two-stage cooling: a slower one that forms phenocrysts and a faster one that makes groundmass.
Usually, porphyritic latite will have phenocrysts of plagioclase (andesine or oligoclase) and sodic feldspar (sometimes sanidine), fine-grained crystalline to slightly glass matrix. Also, biotite phenocrysts may occur.
The groundmass will have plagioclase, alkali feldspar, mafic minerals, and quartz.
Some latite rocks may show Rhomb porphyry. It has grayish-white rhomb-shaped phenocrysts of anorthoclase in a fine-grained matrix.
Lastly, porphyritic varieties with 5-20% quartz are often known as quartz latite porphyry.
Chemical composition
Latite is an intermediate rock with silica 52–63 wt.%. Its chemical composition lies between felsic and mafic rocks.
Data from Le Maitre (1796) gives the % weight chemical composition of latite at SiO2: 62.80%, TiO2: 0.83%. Al2O3: 16.37%, Fe2O3: 3.34%, FeO: 2.27%, MgO: 2.25 %, CaO: 4.27%, Na2O: 3.88%, and K2O: 3.98%.
Lastly, in the TAS classification, latite is chemically considered a variety of trachyandesite.
Latite mineralogy
Latite has mostly nearly equal plagioclase and alkali feldspar amounts. Also, it has small amounts of biotite, pyroxene, and sometimes hornblende or quartz/tridymite.
Common accessory minerals include tridymite, magnetite, ilmenite, zircon, and apatite.
As mentioned earlier, this rock is an intermediate. Therefore, its mineralogy lies between felsic and mafic rocks.
Felsic minerals in this rock are alkali feldspars like sanidine, orthoclase, or anorthoclase, and plagioclase like sanidine. Also, it may have quartz in the groundmass.
On the other hand, clinopyroxene, usually augite, is the most common mafic mineral. However, it may also have aegirine-augite or orthopyroxenes.
Other common mafic minerals are biotite a mica and hornblende, an amphibole.
On the QAPF diagram for volcanic rocks, latite is a rock where plagioclase accounts for 35-65% of total feldspar by volume, and quartz is 0-5% of the QAPF content by volume. It has no foids.
It lies between trachyte and andesite. An increase in alkali feldspar grades it into trachyte and plagioclase to basalt or andesite. Andesite if the plagioclase is sodic or basalt if calcic.
An increase in quartz to up to 20% grades or foids up to 10% of the QAPF volume grades this rock into:
1. Quartz latite
On the QAPF diagram, quartz latite is a rock in which quartz is 5-20% of the QAPF content and plagioclase 35-65% of the total feldspar. Increasing quartz grades this rock to rhyolite.
2. Foid-bearing latite or tristanite
Felspathoids account for up to 10% of QAPF content in foid-bearing latite with plagioclase 35-65% of total feldspars by volume. You can replace the name foid with the dominant foid this rock has. For instance, you can have leucite or nepheline-bearing latite.
How does it form?
Latite forms when intermediate magmas cool quickly on or near the Earth’s surface. The quick cooling doesn’t give time for the growth of large minerals.
Magma originates from fractional crystallization mantle-derived magma or anatexis of lower alkali basaltic or gabbroic rocks. Also, contamination and magma mixing can cause magma to have an intermediate composition.
Tectonic settings and occurrence
This occurs in active continental margins, usually above subduction zones. However, it can occur in oceanic islands and continental crustal extensional zones but in smaller amounts.
This rock usually occurs as lava flows, intrusive sill, or laccoliths.
Some places where latite is found include Bulgaria and the western half of North America. For instance, in South Dakota, USA, it occurs as sills and laccoliths.
References
- Blatt, H., Tracy, R. J., & Owens, B. E. (2006). Petrology: Igneous, sedimentary, and metamorphic (3rd ed.). W.H. Freeman and Company
- Le Maitre, R. W. (Ed.) (2002). Igneous rocks: A classification and glossary of terms (2nd ed.). Cambridge University Press
- 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