G. A. IVANKIN

COCKADE TEXTURES OF PYROXENE–FELDSPAR METASOMATIC FORMATIONS

In 1948 and 1949, in the area of the Sartygoy mine in Khakassia, metasomatic formations were found with a highly original texture, very similar to cockade structures of ores. The unusual textural features of the products of the metasomatic process prompted the author to describe them.

Below is a brief description of the factual material and conclusions concerning the origin of cockade textures in metasomatites.

East of the Sartygoy mine, on the left side of the middle and lower course of the Tibek River, the right tributary of the Uybat River, several small lens-like bodies—remnants of Cambrian limestones—have been recorded, elongated parallel to the southern contact of a large limestone remnant, which extends from WNW to ESE from the Uybat River to the middle course of the Tibek River. The limestone remnants occur among rocks of gabbro-dioritic composition, representing a hybrid belt of the Uybat granite pluton. The gabbro-diorites are cut by numerous dikes composed of granite-aplites and aplites, as well as by dikes of porphyrites, which probably represent the roots of Devonian extrusions.

At the contacts of small limestone remnants with gabbro-diorites, and within the field of development of the latter, skarnization of the rocks is observed; this, it must be supposed, is connected with postmagmatic activity of the granitic intrusion. At the western end of one of the small limestone remnants, measuring \(3 \times 50\) m, along with garnet and pyroxene-scapolite skarns, pyroxene–feldspar metasomatic formations with very interesting textural features were encountered.

The pyroxene–feldspar metasomatic formations of this locality are situated either directly at the contact of limestones with gabbro-diorites or occur among the latter at some distance from the contact.

In terms of mineral species, the pyroxene–feldspar formations are poor. The principal minerals are monoclinic pyroxenes and feldspars. As accessory minerals, sphene is always present, and garnet is sometimes present.

The monoclinic pyroxenes occur as small crystals having a bottle-green or dark-green, almost black color. Under the microscope, the bottle-green pyroxenes are colorless or tinted pale green, show no pleochroism, and are ordinary diopside. The dark-green or nearly black pyroxenes in thin section are deep green and show pleochroism from green to deep green. Sometimes brownish tones are observed in their coloration. Measurements of optical constants \((2V, cNg,\) etc.) by means of the Fedorov stage give grounds for assigning them to aegirine-diopside (aegirine from 0 to 20%; diopside from 80 to 100%).¹ It should be noted that the properties of the pyroxenes vary greatly

in space; their changes are sometimes noticeable within a single schlieren, but in most cases, within the limits of one schlieren the pyroxenes are represented by one and the same variety.

The feldspars are represented by acid and intermediate plagioclases and by alkali feldspars. Their quantitative ratios vary from a sharp predominance of plagioclase to a sharp predominance of alkali feldspar.

Fig. 1

Fig. 2

Of special interest are the textural features of the formations described. Small crystals of pyroxene form aggregates which, in sections, have the appearance of closed rings, loops, and complex patterns (Figs. 1, 2, 3, 4, and 5).

Pyroxene crystals, moderately elongated along the third crystallographic axis, are generally arranged radially in the rings. If

Fig. 3

Fig. 4

two or three pyroxene rings are observed, separated by feldspathic intervals, then such an arrangement of the pyroxene crystals very much resembles radial septa, and the figures as a whole resemble archaeocyathid fossils (Figs. 3 and 4). At the same time, differences in the coloration of the pyroxene in adjacent rings are often noticeable. The spaces between the rings, loops, and inside them are filled with feldspathic material, among which individual crystals of sphene, pyroxene, and, more rarely, garnet are very often observed.

It is important to note that metasomatites with a well-expressed cockade texture occupy a certain central position in the zones of skarnification.

and toward the periphery of these zones they pass into pyroxene–feldspar rocks of an equigranular structure.

Attention is drawn to the quantitative ratios of plagioclase and alkali feldspar within the rings and outside them. Very often it is observed that within the rings and loops the feldspars are represented by plagioclase, and outside them by alkali feldspars (Fig. 5). At least, in all cases in our observations, an increased content of plagioclase within the rings was always noted.

Cases were observed in which a pyroxene ring was breached by alkali feldspars. In such instances the direction of movement of the feldspathic material can be clearly seen: the ring is deformed, and pyroxene grains are carried along by the moving material, forming a “trail” of movement. In Fig. 4 one can see the deformation of a pyroxene ring in an upward direction from below and the rupture of the ring. These facts indicate a somewhat later deposition of alkali feldspar than of pyroxene.

Fig. 5

Fig. 6.

Alkali feldspar sometimes forms aggregates resembling pairs of brushes, which are arranged symmetrically with respect to a certain central plane. Along the central plane there are open cracks. The feldspar crystals are oriented perpendicular to the cracks. There is no doubt that such aggregate forms could have formed from a comparatively mobile medium—liquid (solution) or gaseous. The formation of the “brushes” apparently took place at the final stage of formation of the pyroxene–feldspar metasomatites. With subsequent manifestation of metasomatism, the pyroxene–feldspar metasomatites are transformed into pyroxene–scapolite ones (Fig. 6). The replacement of plagioclase by scapolite is in general very characteristic of metasomatites formed at the expense of gabbro-diorites and leucocratic diorites: at first the plagioclase grains are only partly replaced by scapolite; then scapolite pseudomorphs after plagioclase are formed; and, finally, with further alteration, several scapolite grains coalesce and its crystals form with a more or less pronounced crystallographic outline. The replacement of hornblende by pyroxene occurs somewhat earlier than the replacement of plagioclases by scapolite. In the peripheral parts of the skarnification zones one can see how hornblende is partly or completely replaced by pyroxene, while no development of scapolite is observed. Thus, pyroxene–feldspar rocks are formed first. The arrangement of pyroxene grains is preserved during the replacement of feldspar by scapolite and during the formation of crystals of the latter. Therefore, scapolite crystals always include pyroxene grains.

All the previously listed mineral associations are sometimes cut by veins of calcite. The presence of calcite has also been established inside the rings among the feldspars.

In addition, brushes of prehnite crystals have been noted along jointing cracks in gabbro-diorites.

At another point of the same area, distinctive banded textures of pyroxene–feldspar metasomatites were observed: bands composed of diopside alternate with bands composed of albite–oligoclase (Fig. 7). The pyroxene crystals are oriented perpendicular to the extent of the bands and have a semi-perfect crystallographic outline. The grains of albite–oligoclase composing the feldspar bands are xenomorphic with respect to diopside and surround its crystals. Sometimes large grains of albite–oligoclase (up to 1 cm in diameter) include, almost completely, comparatively large crystals of diopside. Evidently, the formation of albite–oligoclase generally took place somewhat later than the formation of the diopside crystals. It should be noted that, on one side, the pyroxene bands have a macroscopically even, straight—

Fig. 7

linear (under the microscope, finely serrated) boundary with the feldspar bands, whereas on the other side the boundary is coarsely serrated. This is explained by the fact that one (lower) part of the pyroxene band is composed of small (0.1 mm–1 mm) crystals, whereas the other (upper) part is composed of comparatively large diopside crystals moderately elongated along the third crystallographic axis.

In one outcrop, located on the left bank of the Bei River, above the settlement of Maganat, a zone of skarnification was noted. The development of skarns here proceeded by metasomatism to which white diorites were subjected. In the area, pyroxene and pyroxene–garnet skarns predominate. But it is interesting to note the presence of rounded inclusions of skarns in the white diorites. The white diorites include small xenoliths, predominantly of amphibole composition. During metasomatic processes, these rounded xenoliths are replaced first of all, with diopside (sometimes aegirine–diopside) surrounding the xenoliths in the form of rings. Its crystals are arranged perpendicular to the surface of the xenoliths. The common hornblende composing the xenoliths is replaced by actinolite. Replacement of xenoliths by garnet–pyroxene skarns can often be observed; in this case the outer parts consist of an alternation of several zones, now predominantly of garnet, now predominantly of pyroxene composition. The skarn “nodes” are surrounded by feldspar streams or are cut by veinlets of the same (alkaline) feldspar. These observations also indicate the later formation in the zones of skarnification of alkaline feldspar than of pyroxenes, and sometimes of garnets as well.

The foregoing may be summarized as follows:

1. Pyroxene–feldspar metasomatites of cockade texture are among the earlier products of metasomatism in comparison with pyroxene–scapolite skarns.

2. There are gradual transitions from gabbro-diorites unaffected by metasomatism to pyroxene–feldspar metasomatites.

3. The formation of alkali feldspars, as well as albite-oligoclase, took place somewhat later than the formation of pyroxenes (and sometimes also of garnet-pyroxene skarns).

4. The presence of several rings of pyroxene, separated by feldspathic interspaces, indicates the periodicity of the process.

On the basis of the factual material described, the mechanism of formation of the cockade textures of metasomatites may be represented as follows. Along certain zones the gabbro-diorites were subjected to crushing. The action of metasomatic agents in the first stage was manifested in the transformation of the gabbro-diorites: the sharp corners of the fragments were intensely destroyed—the fragments became rounded; hornblende was partly replaced by pyroxene in place, and partly its material passed into solution. In the second stage the rounded fragments became encrusted with brushes of pyroxene crystals; at the same time the deposition of feldspar (in particular albite-oligoclase) followed the formation of pyroxene. The formation of alkali feldspars constituted the third stage of metasomatism. In the later stages of mineralization there occurred the formation of pyroxene-scapolite and garnet-pyroxene skarns, as well as veinlets of calcite and prehnite.

1. Unfortunately, it was not possible to carry out chemical analyses of the pyroxenes. ↩