Fig. 1a,b: Landscapes showing a) on the left, a series of basalt flows with, in the foreground, fields developed on basalt-derived soils, and b) a distant view of the mountain of Amba Fekada, the east face of which features a towering rock wall of columnar trachyte. The samples illustrated here were collected as part of a geological mapping program conducted in 2005, in support of a major archaeological survey conducted by staff and students organized by Simon Fraser University in British Columbia, Canada. The 100 km2 field area lies north and west of the town of Adigrat, accessible from the highway running from Adigrat to the Eritrean border near Zalanbesa.
"Rock of the Month # 241, posted for July 2021" ---
Trachyte is a variety of volcanic rock, which is not especially common (as compared with, say, andesite and basalt), yet is widely distributed. Trachytes are rich in the feldspar mineral group. A related term is trachytic texture. Both are derived from the Greek for "rough", via the French. Let's look at some definitions before further studying these rocks.
Trachyte "is a volcanic rock consisting essentially of alkali feldspar", and technically defined by mineralogy in QAPF modal terms, or chemically on a TAS (total alkali-silica) plot (Le Maitre et al., 1989, pp.124, 23, 28, 14). Trachyte is essentially the volcanic equivalent of syenite, and may shade into either silica-oversaturated or undersaturated variants (either into quartz trachyte, or towards feldspathoidal trachyte and phonolite). The typical chemistry would include circa 58-66 wt.% SiO2 and 9-13 wt.% K2O + Na2O, straddling the boundary of intermediate and felsic igneous rocks. Trachyte may occur as subvolcanic intrusions, as flows, and as ash fall deposits (e.g., in Sudan: Paulick and Franz, 1997). An example of a volcano with trachyte and related rhyolite and trachyandesite flows is the Shamsan Caldera, part of the Aden volcano in Yemen (Cox et al., 1979, pp.276-279). The chemistry of the Shamsan Pliocene-Pleistocene lavas (and the phenocrysts entrained in the melts) changed with time, and define a reverse differentiation sequence in which the higher (younger) flows moved to compositions of reduced silica and increased Fe, Mg, Ca and P.
Trachytic texture refers to a rock fabric defined by subparallel alignment of phenocrysts of feldspars in a volcanic rock. This can be interpreted to be a flow texture, in which the movement of the magma squeezes crystals of tabular to prismatic habit into a common direction. Confusingly, the resultant rock need not be a trachyte, as defined by mineralogy and chemistry (see above). Thus this texcture is often seen in basaltic rocks, as in the Cambrian Mount Read Volcanics of western Tasmania (McPhie et al., 1993, pp.36, 49, 140). Associated crystals such as pyroxenes may also be entrained in the resultant directive fabric.
Fig. 2a,b: Sample ETH007, small fragments showing a surface feature of limonitic (?) Liesegang rings. At right: a sanidine K-feldspar microphenocryst in trachytic groundmass (long-axis field of view 1.7 mm, nominal magnification 50X, in crossed-polarized transmitted light). The rock is a sanidine-phyric trachyte, colour index 3%, with accessory primary magnetite and riebeckite.
Local Geology, and this month's specimens
These and other trachyte samples were collected during fieldwork based in Adigrat (Wilson and Pavlish, 2005; Wilson et al., 2014). 107 samples were collected, and polished thin sections prepared from 31 of these (Wilson, 2006). The field area includes the town of Fatsi to the west (on the Adigrat-Asmara highway), and the peak of Amba Fekada, directly north of Fatsi. There is a Precambrian basement exposed low in the northeast of the area: slates and schists, marble and an extensive felsic intrusive suite. The basement is unconformably overlain by non-fossiliferous Paleozoic to Jurassic clastic sediments of the Adigrat Group: sandstone, siltstone, ironstone, quartzite and local chert, conglomerate, shale, grit and orthoquartzite, a sedimentary suite with very minor felsic volcanism. The cap to the steep-walled sandstone canyons, previously identified as a paleosol layer, is evidently more complex, including peperite and/or rheomorphic breccia. The overlying Cenozoic suite includes extensive Oligocene flood basalt flows associated with the Rift Valley to the east, as well as trachytic plugs and satellite bodies, and gabbroic minor intrusives.
Pioneering work (e.g., Merla and Minucci, 1938) long ago provided the framework of regional maps and detailed descriptions of the local rock types, stratigraphy and structure. The local geology can be traced upwards from canyon beds to plateau level to mesas and craggy peaks. Above Precambrian and Paleozoic basement is a sequence of Adigrat sandstone overlain by Antalo limestone, basalt, sedimentary interbeds and trachyte. Basalts cover the Adigrat sandstone in a region from Adigrat westwards to Enticcio (ibid., p.128). Merla and Minucci also described the regional trachytic rocks (ibid., pp.147-150).
Sample ETH007 was collected in situ from the base of the vertical, columnar-jointed east face of Amba Fekada, west of the church of Fekada Maryam. The rock is close-jointed, joint spacing 20-60 cm on some faces, or as little as 2-5 cm. The pale rock type was initially given the field term "felsite", but soon was found to be trachyte. The pale bluish-grey rock is weakly but consistently magnetic, with Mn and Fe oxides on weathered faces, and cut by orangey oxidation bands (Liesegang rings?). Under the microscope, some 85% of the rock is a trachytic groundmass of small, tabular K-feldspar crystals, each <0.2 mm in length. Clear crystals of the K-feldspar sanidine occur as tabular phenocrysts approaching 2 mm in length (12%). These feldspar phenocrysts are stocky and tabular in form, simple-twinned, but without evident exsolution features. Primary oxides (magnetite) are present, as well as secondary goethite (2%). Small prisms of dark blue sodic amphibole (riebeckite) occur between the oriented matrix feldspars (1%). The magnetite imparts a weak magnetism, which in places will just barely attract a pen magnet (magnetic susceptibility on two small pieces averages 1.37x10-3 SI units).
Prior (1898) described the alkaline amphibole riebeckite from trachytic rocks in then-Abyssinia. The mineral had been first described a decade earlier in granitic rocks in Socotra, and then in the Scottish isles of Ailsa Craig (curling stones!) and Skye, in Colorado and Texas, and elsewhere. The Abyssinian rocks had been collected by W.T. Blanford in 1868. Some of these rocks were dubbed paisanite, a K-feldspar microgranite composed of anorthoclase **, quartz and lesser riebeckite. The samples from the region, parts of modern Eritrea and Tigrai, included basalt and trachyte, from localities around Senafe, where a plain is dotted with high volcanic hills. Localities included: the high point, Akub Teriki (Arabi Teliki); Kishyat, which exhibits a columnar jointed north face 300 feet (91 m) or more high (similar hills dot the country to the south); and Fokada (Amba Fekada), almost 30 miles (48 km) to the south. Paisanite has been described in the region at a site referred to by Giovanni Merla and Enzo Minucci as Monte Toquile (Merla and Minucci, 1938, p.133).
** Anorthoclase is a triclinic feldspar of composition intermediate between the potassic, monoclinic sanidines and the K-poor sodic plagiclase members albite and oligoclase. My identification of phenocrysts in the two samples as sanidine is not supported by detailed crystallographic or mineral chemical data, so these too might in principle be anorthoclase, as determined so long ago by Prior! Phenocrysts in strongly peralkaline trachyte in the Azores include sanidine, aegirine-augite, fayalite, alkali amphibole, aenigmatite, pyrite and magnetite. Associated melt inclusions in fayalite and glass in cognate xenoliths record evidence of extreme fractional crystallization with high levels of volatiles such as F and Cl (Mungall and Martin, 1993).
Fig. 3a,b: Two views of an area of sample ETH007, showing the alkali amphibole riebeckite, large numbers of small (<0.05 mm) crystals aligned within curving planes of host feldspars. Each photomicrograph shows a long-axis field of view of 1.7 mm, nominal magnification 50X, in plane-polarized and (right) crossed-polarized transmitted light. The amphibole has a deep blue to blue-green body colour, which obscures the very weak birefringence in crossed polars. The host fabric curves around "obstacles" such as larger quartz or sanidine grains. Riebeckite is typically found in sodic granitoids and trachytes, often with aegirine, a sodic pyroxene.
Sample ETH060 was collected in situ near Ona Adi, northwest of Fekada, from a sill or small plug, outcropping as a small knoll of massive, friable, somewhat vuggy trachyte, which displays hexagonal jointing. This sample exemplifies a distinct trachyte which is lightweight, pale buff and somewhat vuggy. The matrix feldspar is once again about 85% of the rock by volume. Small (approaching 0.5 mm) equant, rounded quartz phenocrysts (8%) are scattered through the rock along with minor (sanidine and plagioclase) phenocrysts (1%). Ragged miarolytic cavities are swathed by the trachytic fabric. Oxides are abundant: mostly limonite / goethite / hematite (6%). The rock is not very magnetic: magnetic susceptibility on the main sample averages 0.25x10-3 SI units. The rock is a miarolytic quartz trachyte with appreciable interstitial oxides, in a strong, swirling directive fabric.
Fig. 4a,b,c: Sample ETH060, a relatively coarse, vesicular trachyte. The two photomicrographs (long-axis field of view 1.7 mm, nominal magnification 50X, in crossed-polarized transmitted light) display trachytic texture once again, with traces of interstitial iron oxides. The feldspar laths wrap around one of many vesicles (the margins of which may be decorated by small, late crystals of quartz). Technical note: the polarizer and analyser are slightly offset in the microscope, thus the rounded vesicles do not appear jet-black, as they properly should.
Further notes on Trachytes:
The generally intermediate magma series that give rise to trachytes seem to be found in localities displaying fairly complex geology: magma mixing, with enclaves of one melt in another; complementary suites of volcanic and subvolcanic rocks (dykes, sills, cone sheets); and associated plutonic rocks of the syenite clan. There is a predominance of younger examples in the literature.
The Kerguelen archipelago is composed of thick piles of Tertiary flood basalts intruded by transitional to alkaline igneous centres. During the evolution of the volcanic complex, trachytic pyroclastic flows and falls were then emitted from younger caldera volcanoes, and trachyte ignimbritic flows infilled valleys, while related pumice falls were widespread (Bonin et al., 2004). Other Cenozoic occurrences include the trachyte and phonolite plugs associated with basaltic lavas in Cameroon (Deruelle et al., 1987) and trachyte lava flows and domes in northwest British Columbia, Canada (Edwards and Russell, 1994). Gough Island, which is located on 34 Ma crust on the African plate some 550 km east of the mid-Atlantic ridge, displays lavas that range from picrite basalt to sodalite-bearing aegirine-augite trachytes (Le Roex, 1985).
In immediate economic terms, the relatively soft and fissile trachytes of the area can be worked into rough stone slabs for artisanal construction purposes, as was seen on our stay in 2005. The directed feldspars no doubt help a skilled worker to split the rock along planes of weakness.
In a global framework, intermediate (trachytic) magmas may play a central role in the development of a class of gold deposits associated with alkaline rocks. One example of epithermal Au-Ag mineralization in the northern Black Hills of South Dakota is associated with subvertical fracture systems in metasediments and Laramide-age monzonitic and trachytic sills. Pyrite, arsenian pyrite, marcasite and gold occur in sulphidic breccias (Giebink and Paterson, 1986). However, arguably the best example in the world is the storied gold mining camp of Cripple Creek in Colorado, U.S.A. In this area, the Cresson mine alone produced about 50 tonnes of gold in 1903-1935. Local Tertiary igneous activity began about 34 Ma, and Au mineralization is dated at about 29-28 Ma. Local intrusions have pronounced syenitic affinity: syenite, trachyte, phonolite and alkalic basalt. The Cresson deposit is hosted in a diatreme-like breccia pipe, the "blow-out". Gold occurs in cavities from a few mm to >10 m in size, located between breccia fragments (the lining of one cavity yielded >1.8 T of gold!). The hydrothermal alteration associated with Au is rather weak, extending only a few cm from cavity walls: pyrite, silica, chloritization of biotite, plus minor sericite, V mica (roscoelite), adularia, dolomite and fluorite. The telluride calaverite is the main Au mineral in Cresson ore, though various other telluride species occur (Saunders, 1988; see also Thompson et al., 1985; and view a small sample from the Cresson mine).
Fig. 5: Sample ETH007, showing small riebeckite prisms in feldspathic groundmass. Photomicrograph, long-axis field of view 0.45 mm, nominal magnification 200X, in plane-polarized transmitted light.
On the map, Tigrai forms the northern cap to the stocky mass of the large country of Ethiopia, sharing a border with the breakaway nation of Eritrea to the north, along the Red Sea coast. Ethiopia, formerly known as Abyssinia, has a long and proud history. It is the largest country in the Horn of Africa, and in the past century and a half has twice been colonized and then - with or without outside help - liberated (Italian forces occupied the country in the 1890s and 1930s). Remnants of the brief colonial periods include two positive influences: some very robust highway engineering, and Italian facets in the popular national cuisine! In addition, each colonial incursion resulted in a detailed scientific literature (mostly in Italian), suggestive of the work of the "savants" that accompanied Napoleon's forces in their brief invasion of Egypt at the turn of the 19th century.
Tigrai, with its capital at Mekelle, lies in the highlands west of the East African rift valley. There is a regional language (Tigrinya), which, like Somali in the east, is one of the major tongues out of dozens spoken in the country, albeit by less people than the two most commonly spoken languages of Amharic (central Ethiopia, the historic language of government) and Oromo (in the southwest). The history of the region has been tortuous, as described by Kaplan (2003). Looking north from higher parts of the field area, jagged rock peaks in Eritrea formed at impressive rampart on the far horizon. The north end of the field area was set by pragmatic as well as archaeological considerations: the occurrence of a scarred orange blanket of trenches and minefields left over from the wars between Ethiopia and Eritrea in the late 20th century. In the last year, and still in force at the time of writing, there is conflict, displacement of population, and a confusion of human rights abuses throughout Tigray, where pre-existing independence movements have resurfaced and led to intense fighting with the central government in Addis Ababa. It is a tragedy, of a kind all too common in human history, that the Tigrayan population, the farmers, craftsmen and traders living their lives in this austere but beautiful land, are again forced to shelter or flee from heedless clashing armies and opportunists. We hope that this crisis will be resolved rapidly, and that peaceful coexistence and development will return to the region.
Bonin,B, Ethien,R, Gerbe,MC, Cottin,JY, Geraud,G, Gagnevin,D, Giret,A, Michon,G and Moine,B (2004) The Neogene to Recent Rallier-du-Baty nested ring complex, Kerguelen archipelago (TAAF, Indian Ocean): stratigraphy revisited, implications for cauldron subsidence mechanisms. In `Physical Geology of High-Level Magmatic Systems' (Breitkreuz,C and Petford,N editors), Geol.Soc. Spec.Publ. 234, 253pp., 125-149.
Cox,KG, Bell,JD and Pankhurst,RJ (1979) The Interpretation of Igneous Rocks. George Allen and Unwin, 450pp.
Deruelle,B, Ezangono,J, Lissom,J, Loule,J-P, Ngnotue,N, Ngounouno,I, Nkoumbou,C, Nono,A and Simo,E (1987) Mio-Pliocene basaltic lava flows and phonolitic and trachytic plugs north and east of Ngaoundere (Adamawa, Cameroon). In `Current Research in African Earth Sciences' (Matheis,G and Schandlmeier,H editors), A.A.Balkema, Rotterdam, 486pp., 261-264.
Edwards,BR and Russell,JK (1994) Preliminary stratigraphy of Hoodoo Mountain volcanic centre, northwestern British Columbia. In `Current Research: Cordillera and Pacific Margin', GSC Paper 1994-A, 243pp., 69-76.
Giebink,BG and Paterson,CJ (1986) Stratigraphic controls on sediment-hosted epithermal gold mineralization: evidence from the Annie Creek Mine in the Cambrian Deadwood Formation, Black Hills, South Dakota. GSA Abs.w.Progs. 18 no.6, 613, 99th Annual Meeting, San Antonio.
Kaplan,RD (2003) Surrender or Starve: Travels in Ethiopia, Sudan, Somalia, and Eritrea. Vintage Books (Random House, Inc.), New York, 222pp., 1988 work republished with a new epilogue.
Le Maitre,RW, Bateman,P, Dudek,A, Keller,J, Lameyre,J, Le Bas,MJ, Sabine,PA, Schmid,R, Sorensen,H, Streckeisen,A, Woolley,AR and Zanettin,B (1989) A Classification of Igneous Rocks and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. Blackwell Scientific Publications Ltd, Oxford, 193pp.
Le Roex,AP (1985) Geochemistry, mineralogy and magmatic evolution of the basaltic and trachytic lavas from Gough Island, South Atlantic. J.Petrol. 26, 149-186.
McPhie,J, Doyle,M and Allen,R (1993) Volcanic Textures: a Guide to the Interpretation of Textures in Volcanic Rocks. Centre for Ore Deposit and Exploration Studies, University of Tasmania, 198pp.
Merla,G and Minucci,E (1938) Missione Geologica nel Tigrai, volume primo. Reale Accademia d'Italia, Rome, 363pp. plus 2 map sheets at 1:250,000 scale (in Ital.).
Mungall,JE and Martin,RF (1993) Extreme fractional crystallization of trachytic magma, Terceira, Azores. GAC/MAC Abstracts, 74, Edmonton.
Paulick,H and Franz,G (1997) The color of pumice: case study on a trachytic fall deposit, Meidob volcanic field, Sudan. Bull.Volc. 59, 171-185.
Prior,GT (1898) Riebeckite in trachytic rocks from Abyssinia. Mineral.Mag. 12, 92-95.
Saunders,JA (1988) Textural and geochemical characteristics of gold mineralization from Cresson mine, Cripple Creek district, Colorado, U.S.A. TIMM B97, 36-39.
Thompson,TB, Trippel,AD and Dwelley,PC (1985) Mineralized veins and breccias of the Cripple Creek district, Colorado. Econ.Geol. 80, 1669-1688.
Wilson,GC (2006) Petrographic study of rock types from the Gulo Makeda area, eastern Tigrai province, northern Ethiopia. TGSL Report 2005-12, for Simon Fraser University, Vancouver, vi+74pp.
Wilson,GC and Pavlish,LA (2005) Field geology of Gulo Makeda area, eastern Tigrai province, northern Ethiopia. TGSL / U. of T. field report, 12pp. and 10 photographs.
Wilson,GC, Batiuk,SD and Pavlish,LA (2014) The geology of Gulo Makeda. Manuscript for Simon Fraser University monograph on Mezber area, Ethiopia, 26pp., map and legend.
Graham Wilson, posted 05-08,10 July 2021
See the Rock of the Month Thematic Index
or, visit the Turnstone "Rock of the Month" Chronological Archives!
See also other "rocks of the month" from Ethiopia:
Peperite from Tigrai
Amazonite from Sidamo