Geology and geochemistry of the Paresis igneous complex, South West Africa

Abstract

Abstract by author:

Basalt flows - intercalated with rhyolites of cycles I and II - and associated gabbroic dykes, are correlated with the regional Stormbergvolcanism and are not directly linked to central igneous activity

Dykes of syenite, bostonite, and microgranite are shown to have been intruded, in that order, under high magmatic pressure along conical fractures. The arcuate form of the syenite dyke of the Central Massif is ascribed to asymmetric collapse of the comendite caldera-block. The different focal depths of the syenite and bostonite conical dykes - about 17 Km. and 6 Km., respectively, are taken as evidence of a compositional stratification in the magma reservoir. Last in the intrusive sequence was a composite stock of foyaite, phonolite, and tinguaite, located on the final ring-fault. The felspathoidal rocks are interpreted as rheomorphic fenites

Petrographic studies indicate that the rhyolitic felspar-pheno-crysts were derived from crystallizing microgranite in the upper levels of the reservoir. Rooks associated with the cauldrons have been deuterically altered to varying degrees. By comparisons with related rocks outside the subsidence area, it is shown that, apart from an increase in H2O, CO2, and the Fe2O3/FeO ratios in the altered rocks, changes in bulk composition were negligible

The Paresis suite has marked alkaline affinities and is chemically comparable with rocks in numerous igneous provinces of world-wide distribution. Basalt-dyke specimen 61a is considered a close approximation to the parent-magma of the Paresis mafic rocks and has analogues at Messum and Okonjeje. This accords with the interpretation that these rocks are representatives of the regional Stormberg igneous activity

Available evidence indicates that neither the rhyolites nor the granite plutons of the central oonplexes in Damaraland are differentiates of the regional basaltic magma. They are confined to the axial region of the Cambrian Damara orogen and their emplacement is correlated with the movement of a crustal segment during the Jurassic Period. It is suggested that granitic magma was produced by remelting of the basement granite - heat for which was generated by shearing associated with the block-faulting

Thirty-eight new analyses are presented

Differentiation of the Paresis central magma is shown to have followed two main trends: the succession of rhyolite cycles is characterized by an increase in the "mafic index" (M = Fe x 1OO/Mg + Fe), whereas in the dyke rocks variation of the "felsic index" (F = (Na + K) x 100/Ca + Na + K) is more prominent. The rhyolites fall into the low-temperature area of the system Q-Or-Ab and are interpreted as successive residual liquids of the crystallizing magma

Discontinuities between variation curves of the basic rooks and of the central felsic suite of Paresis, as well as the absence of intermediate compositions, corroborate structural evidence for their separate origin

The distribution of trace-elements, and their relationships to major constituents during the course of rnagmatic differentiation, is examined in detail. Significant differences between their behaviour in the rhyolites and dyke rocks are noted. It is suggested that, in crystalline phases geometric considerations play a leading role in controlling the entry of a minor element into a lattice-site, whereas its energy properties may be more important in determining its concentration in the coexisting liquids. Marked systematic variations of certain major/minor element ratios confirm the rhyolites as products of extreme magmatic fractionation

Details of mineralogical and analytical methods used are given in the appendix