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Granodiorite thin section

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About Granodiorite

A Granodiorite is a medium- to light-colored coarse-grained rock containing essential quartz (Q), plagioclase feldspar (oligoclase) (P), and alkali feldspar (A), in amounts between 10 to 35% of the total feldspar, and lesser amounts of mafic minerals (M), commonly hornblende or biotite, or both. Whereas granodiorites have 20-30% quartz, diorite has only accessory quartz, if any. The Granodiorite is the plutonic (intrusive) igneous rock equivalent of dacite.

Dacite-Granodiorite family comprises rocks that have color, texture, and mineralogical composition similar to those from rhyolitegranite family, distinguishing them by the fact that plagioclase (P) > K-feldspars (A). Frequently, quartz appears in a smaller quantity, while the ferromagnesian (M) constituents increase considerably.

Type: Plutonic (intrusive)

Equivalent plutonic rock: Dacite


Granodiorite under the microscope

Texture: Granodiorites are holocrystalline, phaneritic, hypidiomorphic-granular rocks, composed of quartz (Q), plagioclase (P), alkali feldspar (A) and mafic minerals (M) (mainly hornblende and biotite).

Rock-forming (essential) minerals: Anhedral quartz (Q) crystals, sometimes with undulating extinction and frequently with liquid, gaseous and solid inclusions (apatite, rutile, zircon microcrystals); in some granodiorites, they form graphic intergrowths (with orthoclase) or myrmekitic (with plagioclase).

Euhedral to subhedral crystals of Plagioclase (P) are easily identified by polysynthetic twins; sometimes with zonation structure. As a rule, the amount of An (anorthite) in plagioclase increases as the amount of amphibole and/or pyroxene increases.

A small proportion of alkali-feldspar (A) may be present. The orthoclase seldom forms phenocrysts and is perthitic more often than in quartz diorites, but less so than in granites and syenites, and it may be accompanied or proxied by microcline.

The mafic (M) (or ferromagnesian) minerals are represented by amphiboles (usually hornblende) and biotite, ± clinopyroxenes (diopside, augite), ± orthopyroxenes. The amphibole and biotite are the most common mafic minerals in the granodiorite rocks and are present as euhdral to subhedral crystals.

Accessory (non-essential) minerals (Ma): zircon, titanite (sphene), apatite, monazite, rutile, iron oxides, etc.

Granodiorite varieties

Granogabbro – granodiorite with basic plagioclase, labradorite type; transition rock between granodiorite and gabbro.

References

  • Best, M. G., & Christiansen, E. H. (2001). Igneous petrology. Blackwell Science.
  • Buzgar N. (2009) – Petrologie magmatică. Ed. Tehnopres, Iaşi, 180 p. (in Romanian)
  • Deer, W. A., Howie, R. A., & Zussman, J. (2013). An introduction to the rock-forming minerals (pp. 498). Mineralogical Society of Great Britain and Ireland, London.
  • Le Bas, M., Maitre, R. L., Streckeisen, A., Zanettin, B., & IUGS Subcommission on the Systematics of Igneous Rocks. (1986). A chemical classification of volcanic rocks based on the total alkali-silica diagram. Journal of petrology, 27(3), 745-750.
  • Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., & Bateman, P. (Eds.). (2005). Igneous rocks: a classification and glossary of terms: recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. Cambridge University Press.
  • mindat.org – The Mineral Database