This family comprises the basic members of the calc-alkali series of plutonic rocks; in other words, the gabbro are rocks of granitic habit with less than 52 per cent silica, and composed essentially of a lime-bearing plagioclase, together with one or more ferro-magriesian constituents. Usually more or less iron ore in black metallic-looking grains can be seen, and in some varieties considerable olivine may occur. This latter can sometimes be detected with the lens as yellowish or green grains. The color of gabbros is usually dark, dark gray or greenish to black; very rarely reddish. In some varieties in which diallage is the kind of pyroxene present and the grain is moderately coarse the rock is much lighter in tone and of a medium gray or greenish gray. The same is true in many cases where the rock is more or less altered. The texture is granitoid or granular, sometimes with a porphyritic tendency from the elongation of the feldspars, but true porphyritic texture is very rare in gabbro.
The word gabbro is derived from an Italian place name, and originally signified a serpentine containing diallage. Von Buch first gave the type more precision by defining it as a rock composed essentially of plagioclase feldspar and diallagic augite. The feldspar is a basic plagioclase generally labradorite, but occasionally bytownite or anorthite. Some oligoclase or andesine may be present in subordinate quantity. Orthoclase is absent except in rare instances, when it is associated with quartz in micrographic intergrowth.
Chemically the gabbros are
characterized by high percentages of lime, magnesia, and ferrous oxide,
together with low alkalies; potash is practically absent. There is a natural
passage grading into diorites; and plagioclase-augite rocks with more than
52 per cent, silica may appropriately be termed augite-diorites. Rocks on
the border line are sometimes termed gabbro-diorite or diorite-gabbro.
According to the nature of the ferro-magnesian constituents occurring in
conjunction with the plagioclase, we may distinguish the following types:
Gabbro = labradorite + augite.
Olivine-gabbro = labradorite + augite + olivine.
Norite = labradorite + augite + hypersthene.
Olivine-norite = labradorite + augite + hypersthene + olivine.
Troctolite = labradorite + olivine.
Eucrite = anorthite + augite + olivine.
Allivalite = anorthite + olivine.
Anorthite-rock== anorthite alone.
By a diminution in the proportion of feldspar and corresponding increase in the pyroxenic constituent, the gabbros pass into the pyroxenites; and similarly if the olivine be increased at the expense of the feldspar, they pass into the peridotite. The feldspar of the gabbros occurs in allotriomorphic, or in partially idiomorphic grains. It is twinned on the Carlsbad design, albite and pericline types, often in combination. The well-marked albite twin-lamellation gives high extinction angles in properly oriented thin sections. When zonal structure is observed, the outermost zone is less basic than the interior, as shown by the variations in the extinction angles. Thus labradorite may be bordered by andesine. Though often perfectly fresh, the feldspar is sometimes weathered into an opaque white substance known as saussurite, which can be resolved by microscopic examination into a confused and intimate aggregate of granules and fibers of zoisite, epidote and secondary feldspar (albite), the last-named being of water-clear and glassy habit.
The augite is usually the variety known as malacolite. Besides the usual prismatic cleavage there is a pinacoidal parting, with which is associated a marked metallic or pearly lustre. Under the microscope, in sections oblique to this lamination, the fine striation characteristic of diallage is often apparent. The hypersthene is characterized by its pleochroism; and by a marked coppery luster, due to the reflection of light from numerous platy and rodlike inclusions arranged along parallel planes ("Schiller structure "). The hornblende, when an original constituent, is sometimes a brown, sometimes a green variety. Secondary hornblende (actinolite, smaragdite, uralite), derived from the pyroxene, is also not uncommon, especially in those rocks that have suffered mechanical disturbance. Rocks in which the pyroxene is completely converted into hornblende may be appropriately termed uralitic gabbro. The term epidiorite has been used in this sense, but it is not to be commended. The olivine occurs in idiomorphic grains, which, when fresh, are colorless. They are, however, often stained with limonite, and crowded with rods and granules of magnetite, which may become resorbed when the olivine is altered to serpentine.
Minerals accessory to the gabbros are dark mica, quartz, apatite, ilmenite, rutile and green and brown spinels (pleonaste, picotite, chromite). In structure the gabbros are essentially holocrystalline and granitic, the constituent grains being allotriomorphic, although the feldspars tend to be idiomorphic towards the augite. When this tendency is accentuated the ophitic structure is developed. In size the grains vary considerably, so that both coarse and fine textures occur. A finely granular or granulitic structure, in which the augites tend to show idiomorphic outlines, is developed in the so-called pyroxene-granulite, which is the name given to certain gabbro-like rocks associated with crystalline schist occurring in Sutherland, in Saxony, in Madagascar, and in the United States. These rocks consist essentially of plagioclase feldspar, hornblende, both monoclinic and rhombic pyroxene, and garnet; and are characterized by the freshness of the minerals, as well as by their typical (or "granulitic") structure.
Miarolitic cavities are much less frequent than in granite and syenite. Orbicular gabbro has been found in California. A fluidal or banded structure which is produced by drawn-out layers of varying composition and which simulates a gneissoid structure has been described from several localities, from the Hebrides, California and Minnesota. Pegmatites are also occasionally found in gabbros; they consist of the usual minerals of the rock. In South Norway the pneumatolytic processes attending the intrusion of gabbros have formed much scapolite and other minerals in the gabbro at its border and in dikes in the contact zone; of these minerals apatite is the most prominent and occurs sometimes in large masses. Complementary dikes, etc., occur in gabbro masses but are not perhaps so notable a feature as in the foregoing groups.
Occurrence: Gabbros are widely distributed and common rocks. They are found as large stocks and batholiths and in dikes in the older rock complexes, similarly to granite. They are also found as stocks and necks of old volcanoes cutting the stratified beds of the younger mountain regions. In these they may also be found as thick intrusive sheets. Gabbros have been held to occur also as forming the central portion of thick extrusive sheets, as in the Hebrides, in Sweden and in the Lake Superior region. If this is the case it is due to the low freezing point of the magma, its liquidity and ready crystallization. In the United States, gabbros are found in many places in New England, as in the White Mountains. They are found in the Adirondacks and at Cortlandt on the Hudson River in New York State, in Maryland, etc. They occur in the Lake Superior region and elsewhere in Minnesota and in various places in the Rocky Mountains and in California. They are extensively distributed in Europe, in southern England, in northern Scotland, especially on the islands of the Hebrides, in Norway and Sweden and in Germany. They are in fact almost as widely known as granites though they do not form, as a rule, such large rock masses.
Alteration of Gabbro: It is common to find that where massive bodies of gabbro occur in the older rock complexes and in folded mountain ranges that they are surrounded by a mantle of hornblende schist into which the gabbro gradually passes, by transitional phases. The origin of this is the pressure, shearing and other metamorphic agencies brought about by the orogenic processes, as mentioned under metamorphism, hornblende, diorite, etc., which have acted upon the pyroxene of the gabbro converting it into hornblende and producing the schistose structure. It may happen through pressure and shearing that a schistose or, perhaps better, a gneissoid structure may be induced in the gabbro without change of the pyroxene to hornblende and we would have in this case a gabbro-gneiss or gabbro-schist produced, but generally the change to hornblende occurs. If olivine is present it also forms amphibole. Very often garnet appears as a new mineral resulting from the process. While the change to hornblende is usually accompanied by the assumption of a more or less pronounced gneissoid or schistose structure, this is not always the case; the rock sometimes retains a massive granular character and, if its constituent feldspar and hornblende can be recognized, it would be classed as a diorite, as mentioned under that rock. In another mode of alteration of gabbros the feldspar is changed into a substance called saussurite, which was formerly thought to be a distinct mineral, but which the microscope has shown to be a mixture of albite, zoisite and other minerals. The feldspar, or rather that which replaces it, has no cleavage and is waxy looking. The pyroxene is changed to hornblende, which tends to have a bright to grass green color and is the variety called smaragdite. Other minerals are also formed, but megascopically the waxy looking saussurite and green hornblende predominate. This may take place without formation of schistose structure and it seems probable that in this case the alteration is due more to the chemical and less to the dynamic agencies of metamorphism. Such rocks have been called saussurite-gabbro. In the process of weathering through the agencies of the atmosphere, gabbros give rise to clay soils deeply colored by the oxides of iron and mingled with fragments of still undecomposed minerals.
Iron and Other Ore Deposits: There are frequently found in large gabbro intrusions masses of iron ore, sometimes consisting of magnetite, but generally of ilmenite or mixtures of the two. Usually these are more or less mingled with the minerals of the gabbro itself, especially pyroxene and olivine. The character of the occurrences, their lack of definite form, and the manner in which they gradually shade into the normal gabbro, show that they are only a phase of the rock in which the iron ore. They are usually scattered through it in small grains, is here locally concentrated in great abundance. Such ore deposits are sometimes found at the border of the intrusion, though often scattered in masses through it or at the center. They are known in many places, in the Adirondacks, in northern Minnesota, in Canada, Norway, Sweden and elsewhere. If titaniferous iron ores could be successfully smelted, such deposits would undoubtedly be in many cases of great value. In other cases sulfide ores are developed in gaboro rocks in a similar manner. This is especially true of the sulphide of iron called pyrrhotite, which is often nickel bearing and hence of great value as a source of this useful metal. In some places these deposits are accompanied by valuable amounts of copper ore in the form of chalcopyrite, copper-iron-pyrites, and it has been remarked that as the percentage of copper rises that of nickel falls. Such deposits in gabbros, or in rocks derived from them, are known and have been worked in Norway and Sweden, in Lancaster County, Penn., and at Sudbury, Ontario. The origin of this kind of ore deposit in an igneous rock occurs from magmatic segregation.
Uses of Gabbro: The gabbros are well suited for constructional work and architecture, but as a rule have not been extensively used, probably very largely on account of their dark color. In Sweden they have received considerable attention for monumental and other uses. In the United States they have been used for building in the Lake Superior region, as at Duluth, and quarries of them at Keeseville in the Adirondacks and in Vergennes, Vermont, have been worked. They take a high polish, are sufficiently durable and much easier to work than granite.
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