syenite - display specimen of a very dark melasyenite from Mountain Pass in the Mescal Range, California
Syenite is an uncommon coarse-grained intrusive igneous rock with the same general composition as granite, but with less than 5% quartz. The feldspar, dark pink to purple in this syenite, is microcline. Hornblende, aegirine, phlogopite and biotite occur in varying amounts, giving the outcrop a maroon cast and making the syenite darker than usual. Melasyenite, from the Greek melas for black, refers to the dark shade of this syenite.
This syenite occurs in the Mescal Range in California, at Mountain Pass, where two composite syenite-shonkinite bodies are associated with the Mountain Pass carbonatite complex that contains the ore that is being mined for rare earth elements by MP Mine Operations. Melasyenite and shonkinite are both unusual igneous rocks.
Syenite forms with the low degree of melting that occurs in a granitic parent rock in a subduction zone or in an area of thick continental crust. A low degree of melting releases potassium into the melt to produce orthoclase/microcline, with slower cooling producing microcline. A greater degree of melting would cause the release of calcium and sodium, the feldspar would become plagioclase, and a granite would form.
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