The Classification of Igneous Rocks Into Suites
Igneous rocks are classified on several different criteria, including color, mineral composition, and chemistry. The classification based on igneous suites is a chemical classification. There are four major suites summarized in the table below.
~A Summary of The Suites~
- Komatiite - An igneous suite distinguished by the presence of ultramafic lavas, including "noncumulate" rocks ranging in composition from peridotite to Basalt or andesite, and cumulate rocks ranging from peridotite to mafic Gabbros. The lavas commonly exhibit spinifex texture. All rocks of the series have low Ti and Fe/Fe+Mg, and high Mg, Ni, Cr. Ultramafic komatiites are restricted to the Archean and are associated with rifting centers.
- Tholeiite - a silica-oversaturated Basalt, characterized by the presence of low-calcium Pyroxenes (orthopyroxene and/or pigeonite) in addition to clinopyroxene and calcic plagioclase. In contrast to the calcalkaline suite, these are iron enriched rocks. Tholeiites are characteristic of rifting sites and have shown up as enormous volcanic outpourings of flood Basalts (to 9 km thick and 1 200,000 km2) of predominantly Quartz-tholeiitic lava.
- Calcalkaline - a series of igneous rocks in which the weight percentage of CaO = K20+Na20, and the weight percent of silica is between 56-61% (over-saturated). Includes many intermediate rocks (monzodiorite, Diorite, Quartz Monzonite, and Quartz Diorite, as well as many, but not all, Granites and Rhyolites). Calcalkaline rocks differ from Basaltic rocks by lacking a strong iron-enrichment. Calcalkaline rocks are one of three series of volcanic rocks generated along subduction zones and emplaced in volcanic arcs. Toward the trench they are replaced by tholeiitic rocks and away from the trench by alkaline rocks.
- Alkalic - rock suites in which the total of alkali oxides (Na20+K20) exceeds that of Ca0 at a silica content less than 51% (undersaturated), of which alkali Basalts are the most important, but also including Syenites, nephaline Syenites, and phonolites (silica undersaturated, alkali feldspar and feldspathoid-rich analogues of Granites and Rhyolites). Alkaline suites occur in a wide variety of tectonic situations, including rifting centers (bimodal associations), subduction zones (both continental margin and island arc), oceanic islands (including hot spots such as Hawaii; alkali Basalts), strike-slip (transform), and within Proterozoic (but not Archean) stable Cratons independent of orogenic activity. They appear to be late fractionation stages in the evolution of magmas.
~A Glossary of Igneous Terms~
Anorthosite - A plutonic rock in which plagioclase makes up at least 90% of the total feldspar, mafic minerals comprise less than 10% of the rock and Quartz is less than 5% of the felsic minerals. Most Anorthosites are Precambrian and most appear to be formed by fractionation of mafic magmas at rifting centers, although Anorthosites are in fact more complex than this.
Amygdule - A gas cavity or vesicle in an igneous rock, which is filled with such secondary minerals as Calcite, Quartz, Epidote, chalcedony, or a zeolite.
Bimodal Association - An association of contrasting igneous rock suites produced at continental rifting sites consisting of tholeiitic and alkaline (alkali Basalts and Syenites, among others) rocks.
Clinopyroxene (Cpx) - A group name for (monoclinic) iron rich Pyroxenes. Clinopyroxenes are less common than orthopyroxenes and include minerals including diopside, hedenbergite, clinoenstatite & augite.
Consanguinity - The genetic relationship that exists between igneous rocks that are presumably derived from the same parent magma. Such rocks are closely associated in space and time and commonly have similar geologic occurrences and chemical and mineralogic characteristics.
Comagmatic - Said of igneous rocks having a common set of chemical and mineralogic features, and thus regarded as having been derived from a common parent magma. Also, said of the region in which such rocks occur.
Cumulate - The part of an igneous rock formed by the Accumulation of crystals that settle out from a magma by the action of gravity.
Dacite - A fine-grained extrusive rock with the same general composition as andesite but having a less calcic plagioclase and more Quartz; according to many, it is the extrusive equivalent of granodiorite.
Feldspathoids - A group of minerals similar to feldspars but under-saturated in silica and tending to form from silica undersaturated solutions rich in alkalis (Na and K), such as alkali magmas. Includes nephaline, leucite, sodalite.
Kimberlites - A porphyritic alkalic peridotite containing abundant phenocrysts of Olivine (commonly serpentinized or carbonatized) and phlogopite (commonly chloritized) in a fine-grained groundmass of Calcite and second-generation Olivine and phlogopite (with assessory minerals). Kimberlites occur in stable continental crust, generally far from plate Boundaries and are hot spot related.
Mode/Modal - The actual volume percent mineral composition of an igneous rock used as a basis of igneous rock classification. Significant modal phases are Quartz, alkali feldsdpars, plagioclase, feldspathoids and mafic minerals.
Norm/Normative constituents - A strategy for comparing chemical rock analyses where composition is expressed as weight percentages of oxides (e.g. Si02, K20, Ca0, etc.). Because a large number of oxides is possible among different rocks, and wide variations in abundances is typical, a theoretical composition is calculated for each rock (the norm consisting of normative constituents [compounds having the chemical formulas of simple end members of the major rock-forming mineral groups]). The norm is calculated by "crystallizing" the minerals out in order, assuming that each normative constituent will form fully and, by leaving less of its oxides in the melt, control which normative constituents will crystallize out later.
Ophiolite - An assemblage of mafic and ultramafic igneous rocks several kilometers thick beginning on the bottom as sheared ultramafics, then progressing to ultramafic cumulates, then Gabbro cumulates, then unlayered Gabbro, then a sheeted dike complex, and then finally pillow lavas interbedded with sedimentary rocks deposited in deep marine water. The suite comprises the oceanic lithosphere.
Orthopyroxene (Opx) - A group name for Mg-rich Pyroxenes crystallizing in the orthorhombic system and usually containing no calcium and little or no aluminum. Includes such mineral as enstatite, bronzite, hypersthene, and orthoferrosilite which are common in peridotites, Gabbros, pyroxenites, norites, and Basalts.
Province - Geographic areas where in a specific span of geologic time magmatic rocks of a restricted range of chemistry were produced.
Silica saturation - a measure of the amount of Si02 available in a magma or rock. Silica under-saturation is when Si02 is low enough that not only is there not enough to form Quartz, there is insufficient amounts to form other minerals such as feldspars, leading to feldspathoid crystallization. Silica saturation is when enough Si02 exists for Quartz to crystallize out. Silica over-saturation is when Si02 exists in enough quantity that the excess leads to unusual mineral associations not formed when silia is just saturated, such as fayalite (Fe rich Olivine) and Quartz existing simultaneously.
Spilites - An altered Basalt, characteristically amygdaloidal or vesicular, in which the feldspar has been albitized and is typically accompanied by Chlorite, Calcite, Epidote, chalcedony, prehnite, or other low-temperature hydrous crystallization products characteristic of greenstone. Spilites often occur as submarine lava flows and exhibit pillow structure.
Spinifex texture - Interpenetrating lacy elongate Olivine crystals in komatiite, commonly considered to have been formed by quenching.
- (a) A set of apparently comagmatic igneous rocks. (b) A collection of rock specimens from a single area, generally representing related igneous rocks. (c) A collection of rock specimens of a single kind, e.g. Granite
s from all over the world.
Contributed by Lynn Fichter