Chemistry Reference
In-Depth Information
Occurrences of ultramafic rocks are:
bodies of cumulative origin associated with basic-ultrabasic complexes,
in particular, as more or less stratiform levels in Bushveld type layered
complexes;
usually small ultramafic intrusions, as the Pyrenean lherzolites;
inclusions in alkali basalts (basanite, nephelinite) and kimberlites; it
may be either cumulates, or preserved fragments of the mantle, or relicts
of a partial melting of the mantle;
large bodies, made mostly of harzburgites (and minor lherzolites), in
Alpine type ophiolites.
The upper mantle is composed of lherzolite: olivine, orthopyroxene,
clinopyroxene (diopside-endiopside) and a minor aluminous phase, spinel
or, at higher pressure/depth, garnet.
In summary, most ultramafic rocks are:
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either cumulates formed from basic magmas;
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or fragments of the mantle emplaced by tectonics or inclusions in vol-
canic lava or kimberlites.
Stony meteorites (stones), chondrites and achondrites, which represent
90% of the meteorites, are composed of olivine, pyroxene and ore (less than
10%, as iron-nickel alloys and sulfides):
iron-poor chondrites (L chondrites) with olivine and hypersthene;
iron-rich chondrites (H chondrites) with olivine and bronzite;
calcium poor achondrites with pigeonite or hypersthene;
calcium-rich achondrites with diopside or augite.
Metamorphic rocks
Granulite facies
(and that of pyroxene hornfels) are defined by the paragen-
esis orthopyroxene
calcic plagioclase. This paragenesis appears in the basic
rocks. In the orthogneisses with a composition of granitoids, orthopyroxene
associated with potassium feldspar is formed in the place of the biotite of
common granites. Orthopyroxene is rare in aluminous paragneisses: associ-
ation garnet
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K-feldspar replace aluminium-rich biotite (see chapter 3.2.1d
on micas). Clinopyroxene may be present in basic rocks of the granulite and
pyroxene hornfels facies. It is rare in basic rocks of the amphibolite facies,
where it is commonly replaced by hornblende.
Aluminous diopside
also occurs in calcic-magnesian, aluminous, silica-
undersaturated (Tschermak molecule is under-saturated in silica compared
to anorthite) rocks of high temperature (pyroxene hornfels facies, granulite
facies and high temperature amphibolite facies).
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