Classification


 
Introduction to Igneous Classification
 
All igneous rock classifications are based on two main criteria: 1) mineral content of the rock, and 2) texture (e.g. grain size). A complete classification must include both components. Here we examine three classifications, beginning with the simplest (but also the most inaccurate).
 
~Color/Texture Igneous Classification~
Minerals at the top of Bowen's Reaction Seriestend to be dark in color (e.g., pyroxene, amphibole) and minerals at the bottom tend to be light in color (e.g.na plagioclase, quartz). The temperature at which a mineral forms dictates its color. Magma compositions tend to segregate out: Mafic Magmas at the top of Bowen's Reaction Series, Intermediate Magmas in the middle, and Felsic at the bottom. The result is that mafic magmas produce dark colored rocks made of dark minerals (basalt) intermediate magmas produce intermediate colored rocks (diorite) and felsic magmas produce light colored rocks (e.g.granite).
   
As a first approximation, a classification based on color and texture is acceptable, however, for a complete classifications, it is inadequate and one must apply additional classification techniques.
 
 
~Modal Igneous Classification~
A modal classification classifies igneous rocks on the relative abundance of five minerals they may contain:
  1. quartz
  2. Alkali feldspars (orthoclase, but including albite [sodium plagioclase] if anorthite [calcium plagioclase] content does not exceed 5%),
  3. Plagioclase,
  4. Feldspathoids (silica poor minerals; no samples available) 
  5. Mafic minerals (such as pyroxene and amphibole). 
The classification is based on arbitrarily defined boundaries between classes on a Ternary Diagram. In general the modal names correspond with names derived from a color/texture classification. The name of the rock is based on mineral content rather than color. Such a classification works readily in a key.
The classification is also commonly displayed as a "Mineral Percent Abundance" chart.
Several crucial observations and decisions must be made to classify rocks using this method:
  1. Determine the percentage of quartz in the rock. 
  2. Determine if the rock is dominated by feldspars (See Key).
  3. Determine if the rock is about a 50/50 mixture of mafics and feldspar (See Key).
  4. Determine if the rock is mostly mafics (See Key).
A mineral/texture classification captures the essential differences among igneous rocks. Minerals must individual minerals must be identified and their abundances estimated. As a result, this next key is more difficult to use.
~Suites~
The normative rock classification groups together igneous rocks we normally think of as unrelated, such as Basalt, Diorite, and Granite. For this reason normative rock classification is more challenging than color/texture or composition/texture system. These rocks do have different mineral assemblages, but may be very similar in their chemistry, reflecting an origin from a common parent magma via FractionationA normative classification works well when igneous rocks are examined in terms of plate tectonic processes.
    
The normative classification arranges igneous rocks into suites, each suite characterized by a particular chemistry. The four major suites are summarized in a Table along with descriptions of each. 
 
Suite Chemistry
Suites are characterized by three chemical signatures: silica saturation, iron enrichment, and the alkali index, each discussed below and summarized in the table link.
    
Silica saturation is a measure of the amount of SiO2 available in a magma or rock. Silica under- saturation is when SiO2 is low enough that quartz nor feldspar can form. The result is silica poor feldspathoid minerals, such as nephaline and sodalite. In contrast, over-saturation is when enough SiO2 exists for Quartz to crystallize out. If SiO2 is high enough it is possible to have a Basalt with Quartz, an uncommon association (Go To Table Link.)
    
The alkali index measures the amount of Ca (calcium) from the top of Bowen's Reaction Series (BRS) relative to the amount of Na+K (sodium+potassium) from the bottom of BRS. Alkali indexes greater than 1 indicate high Ca content typical of the top of BRS. Indexes less than 1 indicate low Ca and high Na+K, typical of the bottom of BRS. (Go To Table Link.)
    
Since in the fractionation process elements low in the reaction series are "sweated" out first we expect the first fractionated melts to be higher in Na+K than the unmelted residue. Since the tholeiitic, calcalkaline, and alkaline suites have alkali indexes (>1) (1) (<1), they form a fractionation Sequence (see below).
    
Iron enrichment declines steadily with fractionation. This is a measure of the decrease in the importance of ferromagnesium minerals down the reaction series. Iron is low in the Komatiite suite because the ultramafic components Mg, Ni, and Cr are so high. (Go To Table Link.)

Suite Fractionation
Igneous rock evolution can occur both within and among the suites.
    
Within-suite evolution occurs when, for example, a calcalkaline suite evolves from a Diorite to a Granite, or a komatiite suite evolves from a peridotite to a Basalt to an andesite.
    
Among-suite evolution occurs in volcanic arcs, and other places, when the first igneous activity begins as silica over-saturated with alkali indexes >1, and evolves to silica under-saturated with alkali indexes <1. That is, tholeiitic, followed by calcalkaline, and finally alkaline suites. Cross Section.
    
Another evolutionary process occurs when one fractionated igneous rock is re-fractionated at a later time. This would occur, for example, if a fractionated Diorite magma emplaced and solidified into a batholith. If this batholith is later heated, a second, more felsic, fractional product (Granite) could be sweated out of it, leaving behind a more mafic residue. Also, a rock of one suite may re-fractionate to a melt with the characteristics of another suite. 

Suite Tectonic Association
One important feature of the suites is their association with particular tectonic regimes (Go To Table). This knowledge is valuable in understanding and reconstructing ancient tectonic events when most of the evidence is destroyed or otherwise unavailable. By analyzing the chemistry of the rocks we can reconstruct the processes by which they formed.
    
The Cross Section shows typical tectonic conditions under which each suite forms. A divergent plate boundary (rift) is on the right and a subduction on the left. The large arrows rising on the right, pointing horizontally across the middle, and descending into the subduction zone on the left marks the path the igneous rocks take. From step to step the evolutionary processes described above occur in sequence.
    
Fractionation takes place in two primary tectonic regimes. The first is at Rifting Centers. Silica over- saturated parent rocks (komatiites in the Archean, other ultramafics since then) rise to the surface and fractionally melt. The melt is tholeiitic and rises to the surface to form the pillow Basalts and sheeted dikes of ocean crust. The unmelted residue is usually silica under saturated ultramafics which stay in the mantle as layer 4 in the ophiolite suite. (Note that the ophiolite "suite" is NOT a suite in the same sense as calcalkaline, etc. suites).
    
The second fractionation takes place at Convergent Boundaries. The tholeiitic oceanic crust moves away from the rifting center until it is subducted. It heats up during subduction and fractionally melts. Typically the first melts erupting closest to the trench are still tholeiitic, but in time the melts evolve to the calcalkaline suite, which build most of the volcanic arc. Later, melts become alkaline. These may come from the secondary fractionation of a melt or rock of the calcalkaline suite.
    
The residue of the fractionation occurring along the subduction zone is ultramafic (peridotite) and continues to descend into the mantle, where it is permanently stored.

Note that the sequence of fractionation is a one way path. This is because the original parent rock begins as silica over-saturated, but at each fractionation silica and elements low in the reaction series are sweated off. At the end the only thing remaining is the most sterile, ultramafic residue, rich in Mg, Cr, and Ni, and poor in silica and alkali elements. There is nothing remaining to fractionate off.
 

Contributed by Lynn Fichter 

Tuesday, October 07, 2014
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