Alkaline Rocks and Carbonatites of the World

Setup during HiTech AlkCarb: an online database of alkaline rock and carbonatite occurrences

Songwe

stripes

Occurrence number: 
163-00-035
Country: 
Tanzania
Location: 
Longitude: 33.27, Latitude: -8.85
Carbonatite: 
Yes

The Songwe Scarp is a topographical feature aligned northwest-southeast and reaching nearly 1000 m in height marking the boundary between the Mbeya Range to the northeast and the Songwe Valley, which is part of the Rukwe Trough, to the southwest. The Mbeya Range consists predominantly of quartzo-feldsphathic schists and gneisses which strike northwest-southeast, a direction that the rift faults tend to follow. Carbonatite outcrops along the base of the scarp for some 19 km, is broadly conformable with the schists, although in detail cross-cutting, and varies between about 6 and 30 m thick. In many places complex systems of veins cut the schists. Schists near to the carbonatite are feldspathized and immediately adjacent rocks commonly brecciated; both rock types have a strong red colour and are commonly veined by carbonate. Areas of feldspathization also occur along the scarps away from the carbonatite, and up to 10 km to the northeast, the schists frequently being transformed to K-feldspar rock, although the primary structures and textures are generally preserved. In contact with the carbonatite on its southwestern side, and veining it, is a fragmentary rock which forms a dyke-like body up to 9 m thick. The carbonatite is a fine-grained rock composed predominantly of what is probably ankerite but with a variable content of fragmentary material derived from the country rocks. Quartz, altered feldspar and altered schist fragments commonly form 20-30% of the rock. Accessory minerals include apatite, pyrite, magnetite, barite, galena and rutile. Partial analyses of carbonatite are given by Brown (1964), but including values for Nb2O5 (0.01-0.06%), Th and U. van Straaten (1989) gives analyses of magnesiocarbonatite and ferrocarbonatite, together with chondrite normalized plots for REE. Chemical analyses of the feldsphathized rocks are also given and range up to 15% K2O. An analysis of the fragmentary rock lying along the southwestern margin of the carbonatite indicates 88% SiO2.

Age: 
A K-Ar determination on K-feldspar from a fragment in a feldspathic breccia plug gave 100±10 Ma (Miller and Brown, 1963).
References: 

BROWN, P.E. 1964. The Songwe scarp carbonatite and associated feldspathization in the Mbeya Range, Tanganyika. Quarterly Journal of the Geological Society of London, 120: 223-40.MILLER, J.A. and BROWN, P.E. 1963. The age of some carbonatite igneous activity in south-west Tanganyika. Geological Magazine, 100: 276-9.VAN STRAATEN, P. 1989. Nature and structural relationships of carbonatites from southwest and west Tanzania. In K. Bell (ed.) Carbonatites: genesis and evolution. 177-99. Unwin Hyman, London.

Map: 
Fig. 3_314 Songwe (after Brown, 1964, Fig. 1). The positions of the Musensi (No. 36) and Panda Hill (No. 38) occurrences are indicated.
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith