alkaline

alkaline earth metal

[al-kuh-lahyn-urth, -lin-]

Any of the six chemical elements in the second leftmost group of the periodic table (beryllium, magnesium, calcium, strontium, barium, and radium). Their name harks back to medieval alchemy. Their atoms have two electrons in the outermost shell, so they react readily, form numerous compounds, and are never found free in nature.

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Calc-alkaline and calc-alkalic are equivalent terms applied to groups of igneous rocks that commonly occur together and that have compositions related by the characteristic chemical trends specified below. Understanding the genesis of these rocks is important, because they make up a major part of the crust of the continents.

Calc-alkaline series

The diverse rock types in the calc-alkaline series include volcanic types such as basalt, andesite, dacite, rhyolite, and also their coarser-grained intrusive equivalents (gabbro, diorite, granodiorite, and granite). They do not include silica-undersaturated, alkalic, or peralkaline rocks.

Geochemical characterization

Characteristic chemical trends relate compositions of rocks as arranged in a series from basalt to rhyolite (intrusive equivalents from gabbro to granite). Analyses document that the chemical constituent silica (SiO2) in compositions of rocks in this series typically ranges from almost 50 weight percent (basalt and gabbro) to over 70 percent (rhyolite and granite). Peacock named the series in 1931 as part of a proposal to distinguish rock series based on concentrations of other chemical constituents plotted against silica: his proposal is rarely applied rigorously today, but his scheme still provides insight. Barker (1983, page 89) has explained Peacock's scheme as follows: "On a variation diagram in which weight percentages of other oxide components are plotted against silica, CaO generally declines with increasing SiO2 and is eventually exceeded by the sum of Na2O and K2O ... The silica content at which CaO equals (Na2O + K2O) is estimated by interpolation and is defined as the alkali-lime index ... Rock suites in which the total of alkali oxides exceed that of CaO at a silica content less than 51 wt % are called alkalic ... Those with an alkali-lime index between 56 and 61 are calcalkalic ..."

Yet other criteria have been used to separate rocks of the calc-alkaline and tholeiitic series. For the calc-alkaline rock series, iron decreases as silica increases. In contrast, for the tholeiitic series, iron increases as silica increases, silica-rich rocks are less common, and potassium concentrations at similar silica values are lower.

Calc-alkaline magmas are typically hydrous, and also typically are more oxidized, with higher oxygen fugacities.

Geologic context

Calc-alkaline rocks typically are found above subduction zones, commonly in volcanic arcs, and particularly on those arcs on continental crust.

Petrologic origin

Rocks in the series are thought to be genetically related by fractional crystallization and to be at least partly derived from magmas of basalt or andesite composition formed in the Earth's mantle. Trends in composition can be explained by a variety of processes. Many explanations focus on water content and oxidation states of the magmas. Proposed mechanisms of formation begin with partial melting of subducted material and of mantle peridotite altered by water and melts derived from subducted material. Mechanisms by which the calc-alkaline magmas then evolve may include fractional crystallization, assimilation of continental crust, and mixing with partial melts of continental crust.

References

  • Daniel S. Barker, Igneous Rocks, Prentice Hall, Inc. Englewood Cliffs, NJ, 417 pages (1983)

See also

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