The metamorphosed rocks of the Pretoria series and the associated Intrusive diabases in the Enzelsberg Area, Marico District

Abstract

The area is underlain mainly by arenaceous and argillaceous sediments, limestone and andesitic lava and tuff of the Pretoria Series of the Transvaal System. These sediments have been metamorphosed by the Bushveld Complex, and minerals such as andalusite, cordierite, biotite, garnet, hornblende, anthophyllite, grunerite, cummingtonite, diopside, calcite, scapolite, potassium feldspar and plagioclase are developed. Eight rocks have been chemically analysed and can, after Niggli (1934), be divided into four groups, viz. the calcium-rich alkaline silicate rocks, the calcaluminosilicate rocks, the femic silicate rocks and the aluminosilicate rocks. The four analyses falling in the last group were plotted on a C/fm against mg variation diagram. The garnetbearing hornfelses fall in the field with a higher C/fm value, and the cordierite-bearing hornfels in the field with a higher mg value. The andalusite hornfels falls in the field occupied by garnet-bearing hornfels, although it contains no garnet. lt is postulated as being a lower metamorphic grade equivalent of the garnet hornfels. Almandine as well as cordierite form at the expense of andalusite, and partly of biotite. All mineral assemblages plot on ACF and AKF diagrams typical of the hornblende-hornfels facies. The widespread occurrence of such metamorphic minerals as clinozoisite, chlorite, muscovite, hornblende and talc point to pronounced hydrous conditions during metamorphism. The different assemblages of metamorphic minerals suggest that the deposition of the Timeball Hill and Daspoort Stages took place in a uniform epicontinental environment, whereas the greater part of the Magaliesberg Stage was deposited under unstable epicontinental and deeper marine conditions, resulting in sedimentary facies changes along strike. As seen from the scarcity of quartzite and the high tourmaline content of the hornfelses, the sediments in the east were deposited in deeper water than those further to the west. Twelve major sedimentary cycles, consisting mainly of alternating layers of shale and sandstone, are distinguishable, starting with the shales of the Timeball Hill Stage and ending with the uppermost sandstone of the Magaliesberg Stage, with carbonaceous shale being deposited during cycle 6 and limestone during cycle 8. The depths of deposition of the eight chemically analysed rock specimens are compared, by using linear variation diagrams of the values TiO2 and SiO2/ Al2O3 against t. The ferruginous quartzite and the underlying andalusite-muscovite hornfels are apparently genetically interrelated, and the iron as well as the aluminium in these rocks are probably of volcanic origin, being deposited under suitable Eh and pH conditions. The carbon from the graphitic slate is most probably of organic origin. The Ongeluk lava is represented by a dark green andesite, in many places vesicular. From 20 to 22 individual lava flows, with an average thickness of 15 metres per flow, were found. These can further be grouped together into four periods, or units of volcanic action, on the basis of the variation in plagioclase composition of the lava. The andesite forms a member of the calc-alkali series and may have formed by the contamination of basaltic magma by sialic materials, by the fractional crystallization of basaltic or tholeiitic magma, or by the partial or total melting of the lower crust. The hypabyssal rocks comprise unaltered, noritic, Maruleng-type dykes and sills close to the Bushveld Complex, and altered hornblende-bearing diabases generalfy found further away from the Comptex.