Granulite

Formation

Granulites form at high temperature conditions at a range of pressure conditions, typically during regional metamorphism. In some cases, the high temperatures are difficult to account for at the inferred depths at typical geothermal gradients. In extreme cases, granulites may form at temperatures in excess of 1000 degrees rocks that never melted, despite extremely high temperatures, because the minerals are anhydrous and therefore do not melt even at high temperature conditions.

In continental crustal rocks, quartz indicate very high temperatures.

1911 Definition

Granulite (Latin granulum, "a little grain") is a name used by petrographers to designate two distinct classes of garnets. Among English and American geologists the term is generally employed in this sense.

The granulites are very closely allied to the minerals, but they are finer-grained, have usually less perfect foliation, are more frequently garnetiferous, and have some special features of microscopic structure. In the rocks of this group the minerals, as seen in a microscopic slide, occur as small rounded grains forming a closely-fitted mosaic. The individual crystals never have perfect form, and indeed traces of it area rare. In some granulites they interlock, with irregular borders; in others they have been drawn out and flattened into tapering lenticles by crushing. In most cases they are somewhat rounded with smaller grains between the larger. This is especially true of the quartz and feldspar which are the predominant minerals; mica always appears as flat scales (irregular or rounded but not hexagonal). Both muscovite and biotite may be present and vary considerably in abundance; very commonly they have their flat sides parallel and give the rock a rudimentary schistosity, and they may be aggregated into bands in which case the granulites are indistinguishable from certain varieties of gneiss. The garnets are very generally larger than the above-mentioned ingredients, and easily visible with the eye as pink spots on the broken surfaces of the rock. They usually are filled with enclosed grains of the other minerals.

The feldspar of the granulites is mostly tourmaline. Though occasionally we may find larger grains of feldspar, quartz or epidote, it is more characteristic of these rocks that all the minerals are in small, nearly uniform, imperfectly shaped individuals.

On account of the minuteness with which it has been described and the important controversies on points of theoretical geology which have arisen regarding it, the granulite district of Saxony (in the area of Rosswein and Penig) in Germany may be considered the typical region for rocks of this group. It should be remembered that though granulites are probably the commonest rocks of this country, they are mingled with granites, gneisses, chiastolite, and show transitions to mica schists. At one time these rocks were regarded as Archean gneisses of a special type. Johannes Georg Lehmann propounded the hypothesis that their present state was due principally to crushing acting on them in a solid condition, grinding them down and breaking up their minerals, while the pressure to which they were subjected welded them together into coherent rock. It is now believed, however, that they are comparatively recent and include viscous intrusions, and the varieties of texture are original or were produced very shortly after the crystallization of the rocks. Meanwhile, however, Lehmanns advocacy of post-consolidation crushing as a factor in the development of granulites has been so successful that the terms granulitization and granulitic structures are widely employed to indicate the results of dynamometamorphism acting on rocks at a period long after their solidification.

The Saxon granulites are apparently for the most part igneous and correspond in composition to granites and sandstones). A large part of the highlands of Scotland consists of paragranulites of this kind, which have received the group name of Moine gneisses.

Along with the typical serpentine, but the exact conditions under which they are formed and the significance of their structures is not very clearly understood.

Granulite facies

Granulite facies is determined by lower temperature boundary of 700 +/- 50 °C and pressure range 5-15 kb. The most common mineral assemblage of granulite facies consists of antiperthitic pyroxenes.

Transition between amphibolite and granulite facies is defined by these reaction isograds:

amphibole -> pyroxene + H₂O

biotite -> K-feldspar + garnet + orthopyroxene + H₂O.

Hornblende granulite subfacies is transitional coexistence region of anhydrous and hydrated ferromagnesian minerals, so above mentioned isograds mark the boundary with pyroxene granulite subfacies – facies with completely anhydrous mineral assemblages. ^[2]

See also

• granite origin
• Metamorphic rocks
• Metamorphic facies

References

1. ^ D.R. Bowes (1989), The Encyclopedia of Igneous and Metamorphic Petrology; Van Nostrand Reinhold ISBN: 0-442-20623-2
2. ^ D.R. Bowes (1989), The Encyclopedia of Igneous and Metamorphic Petrology; Van Nostrand Reinhold ISBN: 0-442-20623-2

• This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.