Eclogite - Eclogite (English spelling)

Also called eclogite, it is a massive rock generally composed of clinopyroxene and calcium-poor garnet. Based on the compositional characteristics of clinopyroxene and garnet and on the geological occurrence, it can be divided into the following four types:

(1) Associated with metamorphic rocks formed at relatively high temperatures. Clinopyroxene is poor in alkali, while garnet is rich in magnesium. They are found in the metamorphic rock areas of Norway and Scotland.

(2) Associated with metamorphic rocks of moderate temperature conditions. Garnet is rich in magnesium, but clinopyroxene is also slightly rich in alkali. It is sometimes associated with quartz and kyanite. It occurs in metamorphic areas other than (1) in Norway and Scotland.

(3) Clinopyroxene is rich in alkali, while garnet is poor in magnesium. This type of rock is found in metamorphic rocks that formed under relatively low temperatures and high pressures, such as in the glaucophane metamorphic belt on the west coast of the United States.

(4) Clinopyroxene is poor in alkali, while garnet is rich in magnesium. They are found in peridotite bodies, as xenoliths in kimberlites and basalts, and probably formed under the high pressure and temperature conditions in the upper mantle.

Eclogites generally have a chemical composition similar to that of basalt, but instead of basalt, which is composed mainly of pyroxene and plagioclase, eclogites are made up of denser garnet and pyroxene, indicating that they formed under high-pressure conditions deep underground.

[Mitsuo Hashimoto]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

榴輝岩(りゅうきがん)ともいう。一般的には単斜輝石とカルシウムに乏しいざくろ石とからなる塊状の岩石。単斜輝石とざくろ石の組成上の特徴や、地質学的産状からみると、次の4種類に分けられる。

(1)比較的高温で生成した変成岩に伴うもの。単斜輝石はアルカリに乏しく、ざくろ石はマグネシウムに富む。ノルウェーやスコットランドの変成岩地帯に産する。

(2)中程度の温度条件の変成岩に伴うもの。ざくろ石はマグネシウムに富むが、単斜輝石もややアルカリに富む。ときには石英や藍晶石(らんしょうせき)を伴う。ノルウェーやスコットランドの、(1)とは違う変成岩地域に産出する。

(3)単斜輝石はアルカリに富むが、ざくろ石はマグネシウムに乏しい。この種のものはアメリカ西海岸の藍閃(らんせん)変成岩地帯など、比較的低い温度と高い圧力の下で生成した変成岩に伴って産出する。

(4)単斜輝石がアルカリに乏しく、ざくろ石はマグネシウムに富む。橄欖(かんらん)岩体の中、キンバレー岩や玄武岩中の捕獲岩などとしてみいだされ、おそらくマントル上部の高温高圧下で生成したものであろう。

　エクロジャイトは一般に玄武岩に似た化学組成をもつが、玄武岩が輝石と斜長石を主成分とするのに対し、密度の高いざくろ石や輝石からなる。これはエクロジャイトが地下深部の高圧条件の下で生成したことを示す。

［橋本光男］

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