Amphibolite is a rock associated with the convergent plate boundaries where heat and pressure cause regional metamorphism of mafic igneous rocks such as basalt and gabbro or from the clay rich sedimentary rocks that can be either marl or greywacke. The metamorphism sometimes also flattens and elongates the mineral grains which produces schistocity in the rock. Because metamorphism creates minerals based entirely upon the chemistry of the protolith, Certain 'dirty marls' and volcanic sediments may actually metamorphose to an amphibolite assemblage.
Deposits containing dolomite and siderite also readily yield amphibolites especially where there has been a certain amount of contact metamorphism by adjacent granitic masses.
Metamorphic rocks composed primarily of amphibole, albite, with subordinate epidote, zoisite, chlorite, quartz, sphene, and accessory leucoxene, ilmenite and magnetite which have a protolith of an igneous rock are known as Orthoamphibolites. Often the easiest way to determine the true nature of an amphibolite is to inspect its field relationships; especially whether it is inter-fingered with other sediments, especially greywackes and other poorly sorted sediments. If the amphibolite appears to transgress apparent protolith bedding surfaces it is an ortho-amphibolite, as this suggests it was a dyke.
Picking a sill and thin metamorphosed lava flows may be more troublesome. Thereafter, whole rock geochemistry will suitably identify ortho- from para-amphibolites. The word metabasalt was thus coined, largely to avoid the confusion between ortho-amphibolites and para-amphibolites.
While not a true metamorphic rock name, as it infers an origin, it is a useful term. Amphibolites define a particular set of temperature and pressure conditions known as the amphibolite facies. However, caution must be applied here before embarking on metamorphic mapping based on amphibolites alone.
Firstly, for an ortho amphibolite to be classed as a metamorphic amphibolite, it must be certain that the amphibole in the rock is a prograde metamorphic product, and not a retrograde metamorphic product. For instance, actinolite amphibole is a common product of retrograde metamorphism of basalts at upper greenschist facies conditions.
Often, this will take on the crystal form and habit of the original protolith assemblage; actinolite pseudomorphically replacing pyroxene is an indication that the amphibolite may not represent a peak metamorphic grade in the amphibolite facies.
Hullvann, Norway. Garnet amphibolite sample with plagioclase white , hornblende balck and garnet red with reaction rim. From James St. Skiddaw 1 Spotted slate 2 Andalusite slate 3 Cordierite slate. Amphibolite The name amphibolite has been used in geological literature for nearly years, since Brongniart Other common minerals include quartz, clinopyroxene, garnet, epidote-group minerals, biotite, titanite and scapolite.
The common varieties are tschermakitic and magnesio- and ferro-hornblende. When pyroxene prevails, the rock should be named hornblende-pyroxene rock or calc-silicate rock, depending on its composition and on the composition of the clinopyroxene. The boundary with the higher grade, granulite-facies metamorphic rocks, is determined by the appearance of orthopyroxene.
Changes in mineralogy depends very much on protolith, however, production of abundant garnet and hornblende are most characteristic. Sodic feldspars are oligoclase rather than the albite that dominates at lower T. Biotite and muscovite are both abundant in pelitic rocks of amphibolite facies. Kyanite and sillimanite are often produced by reaction of muscovite and quartz. Amphibolites are usually either black or dark green, depending on the color of the dominant amphibole.
These rocks are usually coarse-grained enough for the individual mineral grains to be seen with the unaided eye. Garnet is a common mineral in amphibolites.
The sample above contains small reddish garnet crystals. Width of sample 11 cm. A contact between amphibolite on the right and tonalite pegmatite. Dark mineral in the lower left is biotite. Other minerals in tonalite are plagioclase and quartz.
Width of sample 10 cm. Amphibolite is a metamorphosed mafic igneous rock basalt , gabbro although it is usually difficult to determine the protolith because original features are often obliterated. In order to make amphibolite out of basalt, we need heat and pressure to initiate chemical reactions and also need to add water because amphiboles, unlike pyroxenes, are hydrous minerals. According to some sources, impure carbonate rocks rich in clay content may also metamorphose to amphibolitic rock.
Prexifes para- and ortho- were used in the past to denote sedimentary and igneous protoliths, respectively. It is hard to think of an impurity in carbonate rocks where you can end up with amphibole and plagioclase irrespective of the type of metamorphism.
Amphibolite either lacks or has weakly developed foliation. Schistose rocks with a similar composition are hornblende schists. High grade dark-colored hornblende-bearing rock is hornblende gneiss. It is a common rock type in mountain ranges and shield areas. It is a very common product of regional metamorphism which is associated with mountain building episodes such rocks are often lineated or foliated because mountain building is a dynamic event that involves motion.
0コメント