Syenite – Intermediate Lands of Tigrinyaland and Semien

This is the seventh part of the geological description of Tigrinyaland¹ and Semien,² which has been adapted from Messrs. Ferret and Galinier’s work published in 1847.³ In this installment, the intermediate or transitional lands of Tigrinyaland and Semien constituted of syenite rocks will be presented.

Syenite is a massive crystalline rock, composed essentially of orthoclase, amphibole hornblende and hyaline quartz. It’s true that some geologists regard quartz as unnecessary to the composition of syenite, with all the more reason that rocks normally composed of orthoclase and amphibole have been called syenite, quartz being regarded as accidental in these rocks when it is present. Nevertheless, we say that the typical syenite, the Syene rock in Egypt, as well as all Abyssinian⁴ syenites, contain quartz. Consequently, this mineral, far from being incidental in syenite, forms an essential element of it. This is why we retain the name syenite for Abyssinian rocks, and why we would be inclined to regard as another rock a syenite that does not contain quartz as an essential element. In this, we follow the theoretical ideas of Mr. Rivière.⁵

In Abyssinia, as in many other regions of Africa, syenite is a very important rock. The appearance of this rock therefore played a very important role in the geological phenomena of this part of the world. However, even though syenite is very abundant in Abyssinia, it is often masked by other rocks. We even do not see the syenite exposed over a very large area. The main visible deposits of this rock are between the Red Sea and Tarenta,⁶ at Gundet, at Abune-Alef, around Aksum and Mai-Braziyo, at Selekhlekha, around Hadish-Adi, at Tekezé and in the ravines which lead to this valley.

Between the Red sea and Tarenta, the syenite forms a main massif which occupies the west of the surroundings of Aylet and the foot of Tarenta. In the latter place, it is red or greenish pink. It forms a very considerable massif in the middle of the gneiss, the micaschist, etc; that is to say, in the middle of the rocks of the primary terrains and even of the transitional lands that it has crossed. But it is in turn cut, in various places, by diorite and by a lepidolite which is attached to the amphibolic rocks.

Syenite reappears around Gundet, where it forms the base of the soil. To the north, it is limited by basalts, and to the south by granite. At Adi-Keteyo and Gundet, it is crossed and crowned by basalt while further south, towards the Mareb, it pierces the granite. It is cut by diorite and amphibolite. This syenite forms the soil, from the limit of the basalts, at Adi-Keteyo, to beyond Gundet. It therefore constitutes the base of the Shakné valley. In the latter place it is red, while near Gundet it is generally greenish. In all cases, this rock has a granitoid texture.

From the preceeding facts, we may assign the relative age of the syenite, by considering its position with respect to other rocks. Thus it is more modern than granite, gneiss, micaschist, and even the rocks of transitional lands. But it is older than diorite, etc. It is therefore probable that syenite came to light after the formation of the transitional terrains, and that its appearance completed the series of phenomena of this period. These are the reasons that motivated us to connect syenite and porphyry to the transitional lands.

Around Abune-Alef, we find a syenite which is surrounded and crossed by diorite. This syenite is greenish, very feldspathic, and passes into granite by various characters. In this locality, the syenite is apparently little developed, but it is linked, in a southwesterly direction, beneath the other rocks, to the syenite hillocks which occur, on a large scale, to the north of Aksum and around May-Braziyo.

At Mount Sebahat, the syenite is bluish gray, very grainy, very fine. It appears to have been modified by trachytes, and passes into diorite. The same varieties and the same facts occur around Mégaréa-Zambré and Aksum, and in the Shiré plain.

South of Aksum, syenite reappears below the tertiary lands of Shiré. Indeed, we see this rock in the vicinity of Adi-Gahad, and between Adi-Berak and Hadish-Adi, around basalt mountains. The porphyry and pegmatite seem here to be only an extreme variety of syenite. They are, moreover, both crossed by veins of quartz.

In the Mezanegambassa ravine (Shiré plain), the syenite is pink, changing to pegmatite and porphyry. In the Medera ravine, we find a very altered syenitic quartz porphyry. At Mai-Tenkat, the syenite is reddish porphyry or a grayish syenitic quartz porphyry, often weathered. In the Merakat ravine, it is a true garnetiferous pink quartz porphyry, and often amphibolic. However, it is a dependent of syenite.

Around Selekhlekha, the syenite is still in the same position with respect to the layers of tertiary lands and basalts. In the Firfira ravine, the same fact occurs on a large scale. The syenite of this ravine is so decomposed towards its upper part, that pedestrians were able to make their way through it. It is, on the contrary, very hard and very steep at the bottom of the ravine. It is then greenish gray, cut by veins of crystallized quartz 4 or 5 centimeters thick, and presents porphyry and pegmatite on certain points. This pegmatite, which often contains garnet and zircon, probably results from an accident of the syenite. It can also be that the quartz veins come from a departure made in the melted syenite mass during its appearance. However, it is more probable that they result from cracks produced by dynamic phenomena due to the appearance of diorites, basalts, etc., which were subsequently filled by washing and siliceous infiltration.

The syenite of the Firfira ravine is often micaceous and granitoid. At other times, we find an amphibolic and slightly talcose porphyry which passes into syenite, or which, to speak more precisely, is only an extreme variety of it. Finally, we notice in the Firfira ravine, blocks of syenite which are more than 2000 cubic meters in size, and which are entirely detached from the main mass.

In the Mai-Tenkat ravine, near Adi-Qesti, we still see syenite forming the base of the soil. It is, as we said, a red syenite passing to porphyry, which is covered by the tertiary deposits. Syenite is even found in several other localities, notably on the banks of the Tekezé, where it is more or less crossed by diorites.

We must mention here, for the record, the quartz eurite, which crosses the phyllites around Digsa. This eurite is composed of albite. It contains some imperfect crystals of albite and quartz. Altered, it becomes white and dendritic. But in its normal state, it is generally reddish. Around Digsa, this rock crosses the phyllites which it had modified to the point of making them pass into talcschist, making their texture more crystalline. The eurite in question here is therefore more modern than the phyllites. However, it is not certain that it came to light immediately after the formation of these rocks. It is probable, on the contrary, that it appeared after or during the formation of the terrains of the graywackic group of intermediate lands.

In the Tekezé valley, we also find amphibolic and talcose porphyry that grades into syenite. This rock is sometimes pinkish-gray, then schistoid, due to lamellae of amphibole, which, by its alteration, produces a talcose material.

Reference Notes

1. Tigrinyaland was a collective name of the Midri-Bahri (modern-day state of Eritrea) and Tigray (the northernmost region of modern-day Ethiopia). The term employed for Tigrinyaland by Messrs. Ferret and Galinier in their book is “Tigré”, which had been the designation used by the Amhara rulers of Abyssinia to refer both to the Tigrinya people and the Tigrinyaland. ↩︎
2. Semien was historically the frontier province of the Tigrinya with the Amhara. However, since the reign of Emperor Susenyos, the province of Semien had been governed by members of the Amhara royalty and nobility. Following the death of Dejazmatch Sabagadis in 1831, Semien under its Amhara ruler Dejazmatch Wubbe turned from the frontier province of the Tigrinyaland to the power-center of the Tigrinyaland. This continued until the rise of Emperor Tewodros in 1855. ↩︎
3. Ferret, Pierre Victor Adolphe et Galinier, Joseph Germain (1847) Description Géologique du Tigré et du Samen. Voyage en Abyssinie dans les provinces du tigre, du samen et de L’amhara. Tome troisième. Paulin: Paris. ↩︎
4. Abyssinia is a reference to the region which encompasses modern-day Ethiopia, Eritrea, Djibouti, parts of Sudan and parts of Somalia. ↩︎
5. Rivière, Alphonse (1839) Eléments de Géologie pure et appliquée ou Résumé d’un cours de géologie descriptive, spéculative, industrielle et comparative. Méquignon-Rivière: Paris. ↩︎
6. The Tarenta mountain chain forms the eastern escarpement of modern-day eritrea, and looks down on the Eritrean eastern lowlands which are close to the Red Sea. ↩︎