A Contribution to the Petrography of Benguella, Based on a Rock Collection Made by Professor J
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( 537 ) XIV.—A Contribution to the Petrography of Benguella, based on a Rock Collection made by Professor J. W. Gregory. By G. W. Tyrrell, A.R.C.Sc, F.G.S., Lecturer in Geology, Glasgow University. Communicated by Professor J. W. GREGORY, F.R.S. (With One Plate.) (MS. received June 2, 1915. Read June 28, 1915. Issued separately August 11, 1916.) Very little is known of the petrography of the Portuguese West African colony of Angola, and of its three provinces that of Benguella is probably least well known in this respect. Granites, gneisses, schists, limestones, and red sandstones have been recognised by the earlier observers of the geology of the region, and detailed descrip- tions of rocks from Angola, and especially of its northern province, Loanda, have been given by BERG* and HOLMES.t From Benguella itself J. P. GOMES\ has described r the collection of R. P. LECONTE, w hich contained granites, amphibolite, adinole with epidote veins, various schists, quartzites, diabase-porphyrite, and basic eruptives. Senhor J. P. BE NASCIAMENTO§ in 1912 recorded granite, diorite, basalt, and quartzite on the Benguella plateau. During his recent journey in Benguella, Professor GREGORY collected about a hundred and fifty specimens of igneous rocks or their metamorphic derivatives, and these offer a most interesting and varied study. This paper is concerned only with their petrographic character ; for their geological relations the reader is referred to the preceding paper by Professor GREGORY. For descriptive purposes the classification given below has been found to be most natural and convenient. The order in which the rocks are arranged also closely follows their sequence in time :— I. Basement gneisses and schists. II. Charnockite series. III. A series of hornfelsed porphyries and other hornfels. IV. Granites, granodiorites, and associated rocks. V. Rhyolite (dellenite). VI. An alkaline series, including nepheline-sodalite-syenite and other syenites, shonkinite, solvsbergite, and ouachitite. VII. Basic intrusions. I am much indebted to A. SCOTT, M.A., B.Sc, for two excellent chemical analyses of rhyolite and shonkinite respectively. * BERG, G., "Gesteine von Angola, Sao Thome, und St Helena," Tscher, min. u. petr. Mitth., xxii (1903), pp. 357-362. t HOLMES, A., " Contribution to the Petrology of North-western Angola," Oeol. Mag., dec. vi, vol. ii, pp. 228-232 ; 267-272 ; 322-328 ; 366-370, 1915. I GOMES, J. P., " Echantillons de roches recueillis entre Benguella et Catoco," Comtn. Dir. Trab. Geol. Portugal, vol. iii, fasc. 11, pp. 239-243, 1898. § NASCIAMENTO, J. P. DE, and MATTOS, A. A. DE, A Golonisagao de Angola, Lisbon, 163 pp., 1912. TRANS. ROY. SOC. EDIK, VOL. LI, PART III (NO. 14). 78 538 MR G. W. TYRRELL. I. THE GNEISSES AND SCHISTS. The gneisses are predominantly quartzo-felspathic varieties with biotite as the principal ferro-magnesian constituent. They are medium- to coarse-grained, pink or grey, well-foliated rocks, with the biotite aggregated into thick clots or folia, or less often evenly spangling the rock. Hornblendic gneisses occur, but are much less common than the biotite-bearing varieties. In thin section the biotite-gneisses are coarse-grained aggregates of quartz and potash-felspar with folia of biotite, and show marked dynamo-metamorphic effects. The larger quartz crystals are broken up into a comparatively coarse granular mass of angular fragments; but the felspars have only suffered a fine peripheral granula- tion, the products of which, mingled with quartz, form a kind of groundmass in which the larger, more or less rounded, "eyed" crystals of felspar are set. The felspar is prevailingly microcline. Orthoclase is subordinate, and a little oligoclase is occasionally present. The felspars show the effects of crushing in the irregularity, curvature, and fracture of their twin striations. The biotite is of the common yellow variety, and occurs in large clots or folia consisting of small flakes felted together with minute grains of quartz, epidote, and occasionally a little muscovite. Felspar is absent from these aggregates. The above description refers particularly to a rock from the Bui River, S. of Ochilesa (185),* which may be taken as typical of the predominant biotite-gneisses. Other rocks of the- same type occurring at Bailundo (152) are very rich in " gitter" microcline. Rocks from the Lengwe Gorge (105, 106, 109) are decidedly richer in soda-lime felspars (oligoclase or albite-oligoclase), and show a development of minute, colourless, euhedral crystals within the felspars. These appear to be of two or three different kinds. Some are clearly small zircons, but others are referred to clinozoisite and epidote. In some types (e.g. (156), W. branch of Lovule River), the foliation is not so definite as in the above-described rocks. The texture is evenly granular and the biotite is uniformly distributed. The granulitic texture becomes more pronounced in a rock from the Benguella Railway, 55 kilometres, B. of San Pedro (114). In this specimen the biotite occurs as thin leaves which appear as narrow bands of biotite flakes in thin section, and produce a bacillar type of foliation. This rock has some resemblance to the Moine gneisses of the Scottish Highlands. Biotite becomes very scarce and the cataclastic texture very prominent, in a pink, granulitic, quartzo- felspathic gneiss from Kambengi (195). The rocks described above are all orthogneisses derived by dynamo-metamorphism from ordinary biotite-granites. These rocks grade into biotite-schists, e.g. at 55 kilo- metres, Benguella Railway (115). The texture becomes more even and finer-grained, the amount of quartz and biotite increases, whilst that of felspar diminishes, in com- parison with the gneisses described above. Another mode of transition is into a * The numbers refer to trie registration numbers of the rocks in Professor GREGORY'S collection. A CONTRIBUTION TO THE PETROGRAPHY OP BENGUELLA. 539 group of gneisses with very pronounced cataclastic textures, showing all gradations to mylonite. In the first stage the pseudo-porphyritic " eyed "..felspar becomes highly rounded, and the quartz is broken into coarse-grained aggregates of angular frag- ments. Both minerals are set in a minutely crystalline paste of crushed quartz and felspar (190, N. of Saccanjimba). In succeeding stages all the quartz is reduced to mylonite, and the crushed groundmass is thus increased at the expense of the intact crystals of felspar. The latter are much reduced in size, show angular outlines, and are traversed by lines of mylonised material (173, W. of Ochilesa). The ferro- magnesian constituents, such as biotite, have been crushed out of recognisable existence, and appear now as aggregates of chlorite and limonite. A few large crystals of anthophyllite, now in process of alteration to talc, have been formed in the Ochilesa rock. The three hornblendic gneisses in the collection differ considerably from those described above. They occur towards the lower end of the Lengwe Gorge (1056), N. of the Saccanjimba Mission (189), and in the river W. of Kambengi (197). In thin section they consist of a coarse foliated aggregate of hornblende and plagioclase felspar, with subordinate quartz, orthoclase, and biotite. The ferro-magnesian minerals bulk more largely in these rocks than in the biotite-gneisses. The horn- blende is a strongly pleochroic variety, of strong absorption, and with colour extremes of bluish-green and yellowish-brown. Small felts of biotite and epidote cluster with the larger hornblende crystals. The felspars mostly belong to andesine, and contain colourless needles similar to those described above in the biotite-gneisses of the Lengwe Gorge. Cataclastic structures are also frequent. These rocks are typical diorite-gneisses. Only one basic hornblende rock occurs in the collection. This comes from the river W. of Kambengi (195). It is a fine-grained, dark-green, non-foliated rock, con- sisting, in thin section, of an evenly granular mass of hornblende crystals of the usual bluish-green colour. The small and sparse interspaces are filled with quartz and decomposed felspathic material. The rock is a granulitic amphibolite. II. ROCKS OF THE CHARNOCKITE SERIES. These are dark-greenish or greyish fine-grained rocks, in which none of the minerals, save an occasional flake of biotite, are macroscopically identifiable. The outstanding microscopical characters of these rocks are their thoroughly granulitic texture, their pyroxenic composition, and their ideal freshness. The mineral con- stituents, occurring in varying proportions throughout the series, are a pale-green monoclinic pyroxene, hypersthene, plagioclase, orthoclase, quartz, hornblende, biotite, and magnetite. This assemblage is identical with that of the charnockite series of Southern India.* The invariable presence of hypersthene, the granulitic texture, • HOLLAND, "The Charnockite Series," Mem. Geol. Surv. India, vol. xxviii, part ii, p. 124, 1900. 540 MR G. W. TYRRELL. and the almost exact correspondence of petrographic types, make the identity of the two series even more certain. Rocks referable to three of the four divisions into which HOLLAND divides the charnockite series are to be found in this collection, as well as a rock which may be described as a charnockite-porphyry. A rock comparable to charnockite in the narrow sense is found to the W. of Ochilesa (177). It consists mainly of quartz, oligoclase, orthoclase, with subordinate biotite, hypersthene, and monoclinic pyroxene. Both quartz and orthoclase