Monzonite

Monzonite, the group-name of a type of rocks which have acquired it from their most celebrated occurrence, that of Monzoni in Tirol. The rocks are of granitic appearance, usually rather dark grey in colour and fine to moderately coarse grained. The special characteristic which distinguishes them from granites and ordinary syenites is the presence of plagioclase and orthoclase felspars in nearly equal amounts. Labradorite, andesine and oligoclase are present, usually in well-shaped crystals, often zoned; orthoclase forms large irregular plates in which the other minerals are embedded. There is rarely any considerable amount of quartz, though in a few of these rocks this mineral occurs (the quartz-monzonites). Other features are the abundance of augite, pale green or brownish green, and of large bronze-coloured plates of biotite which are of quite irregular shapes and full of enclosures. Hypersthene or bronzite is less common, but dark brown and green hornblende are sometimes abundant. Olivine also may be present; when the rock contains this in notable quantity it may be called an olivine monzonite. Numerous large prisms of apatite often characterize micro-sections of monzonites, and zircon, iron ores and pyrites are frequent accessory minerals.

The monzonites of Tirol show a great variability in appearance, structure, and the relative proportions of their minerals. They tend to pass into rocks which have been called diabases and gabbros, and near the margins of the outcrop fades very rich in pyroxene (pyroxenites) occur. Many authors believe that this variety of types is associated with the fact that the monzonites occupy a middle place as regards their chemical composition between the acid and the basic igneous rocks, and that such a magma is naturally somewhat unstable, and likely to split up or differentiate into partial magmas of more siliceous and less siliceous character. The monzonites in fact approach rather closely to the calculated mean composition of the outer portion of the earth's crust and from a molten magma of this nature it is natural to suppose that all kinds of igneous rocks have been derived.

Rocks of monzonitic facies occur also in Norway, where they have been described as akerites. They contain quartz, orthoclase and plagioclase, augite and dark brown biotite; hornblende and hypersthene also may be present. Some of them have porphyritic rather than granitic texture, especially near the margins of the laccolites. From a study of these and other occurrences Brogger proposed to define the monzonites as orthoclase-plagioclase rocks in which the two chief classes of felspar occur in nearly equal quantities (as distinguished from the orthoclase rocks or granites and syenites and the plagioclase rocks or diorites and gabbros).

At Yogo Peak and Beaver Creek in Montana, U.S.A., there are masses of granitoid rock which bear a close resemblance to the monzonites of Tirol. Two main types occur: (a) yogoite, which differs little from monzonite, and (b) shonkinite, which is a more basic rock richer in plagioclase and augite; this rock contains olivine and in places passes into dark pyroxenites. In shonkinite also a little nepheline may be present. In several places in the west of Scotland (Argyllshire) intrusive bosses are known which consist of an olivine-bearing rock closely related to monzonite. It has been called kentallenite because it is quarried at Kentallen in Argyllshire. Large crystals of pale green augite and irregular plates of biotite which enclose idiomorphic plagioclase felspar are conspicuous in micro-sections of this rock, and the abundance of olivine is rather greater than is usual in the monzonites; it is associated with diorites of lamprophyric character and dark pyroxenites and peridotites.

The following analyses show the chemical peculiarities of the principal rocks of the monzonite group: S10 2 Al 2 0 3 Fe 2 0 3 FeO MgO CaO K 2 0 Na20 Monzonite, Monzoni. 54.20 1 5.73 3.6 7 5.4 0 3.4 0 8.5 o 4.4 2 3.07 Yogoite, Yogo Peak. 54.4 2 14.28 3.32 4.13 6'12 7.7 2 4' 22 3.44 Kentallenite,Argyllshire 52.09 1193 1.84 7.11 12.48 7.84 3.01 2.04 (J. S. F.)