Acid and basic igneous rock associations west of Helmeringhausen, South West Africa

Abstract

Abstract by author:

Post-Kheis igneous rocks west of Helmeringhausen are believed to belong to a single cycle of prolonged igneous activity which can be divided into three major phases:-

First phase: Extrusion of acid volcanics of the Nagatis Formation, and emplacement of granite

The distribution of Nagatis exposures near the centre of the area could indicate a lateral migration of the focus of igneous activity in a southeasterly direction. Two distinct periods of Nagatis volcanism were separated by a brief interlude of erosion, sedimentation and minor basic volcanicity. During the first period rhyolite, ignimbrite and localised agglomerate were extruded from a series of fissures, whilst the second period saw the emplacement of quartz porphyry, porphyritic rhyolite and ignimbritic material. Following the latter period of extrusion, a large portion of the porphyry subsided; an arcuate fracture bounding the subsided mass was intruded by granite-porphyry. Exactly when the emplacement of this Okarus (Oka) granite-porphyry took place, is not certain, but there is a strong possibility that emplacement of the two porphyry-types was separated by a short space of time only, and that these rocks originate from a common melt

Subsidence of Nagatis quartz porphyry coincided with the emplacement of red granite which, during its ascent, weakened the support beneath roof rocks, passively displaced, invaded and reconstituted much of the overlying Nagatis. It is postulated that most of the red granites exposed today were intruded during the first phase of igneous activity

Cessation of the Nagatis volcanicity was followed by an interval of quiescence, of unknown duration, during which the existing landsurface has eroded and the main sedimentary succession of the Sinclair Formation was deposited in large basins. The subsidences could have been related to a variety of factors, particularly to block-faulting, extrusion of felsic material from near-surface reservoirs, and migration of granitic magma to epizonal levels

Second phase: Extrusion of basic lavas of the Sinclair Formation. Outpouring of a considerable volume of felspar-phyric lava is believed to have accompanied block-faulting, which could in part have been attributable to an increasing weight of accumulating sediments

Third phase: Final intrusion of granite. This could have been the direct consequence of heat transfer from the upper mantle to the sialic crust during the Sinclair volcanic episode. Some of the granite intruded into the northwest zone of structural weakness along which granites of the first phase of igneous activity had been emplaced. Granite magma also utilised major fractures that had begun to develop in the Sinclair Formation, formed by further crustal subsidences north of the main granite mass. Quartz porphyry dyke swarms in the northeastern part of the area are probably directly related to the intrusion of GR granite. Linear dykes of post-Sinclair granite-porphyry that follow meridional fractures in the western parts of the area may represent the closing stages of igneous activity west of Helmeringhausen

Large depressions in the north provided basins for accumulation of Auborus sediments, which was followed, after extensive planation of the region, by deposition of the Nama System

The major structural features west of Helmeringhausen follow a general northwest strike, and it is apparent that the emplacement of the major igneous bodies has been controlled by a line of structural weakness conforming to the regional tectonic axis of the Kheis. From the structure of the three lithologic units there is no evidence of orogenesis; folding of strata is localised near the periphery of post-Sinclair granite northwest of the Helmeringhausen

Field, petrographic and geochemical data suggest that the acid racks in the area are consanguineous. In many respects their emplacement history is comparable with that of the younger granite province of Nigeria where extrusion of acid volcanics has preceded granite intrusions

The red granites cover an area of 900 square kilometres. The main exposure extends diagonally across the area over a distance of 80 kilometres and represents a composite batholith, which, as indicated by radiometric dating, consists of both pre- and post-Sinclair intrusive phases. The field-relationship of this red granite mass to the Sinclair Formation is, however, uncertain due to absence of exposures. It has therefore become necessary to regard the composite granite batholith as a geographical entity, to which in this study has been assigned the term Tumuab (GT) granite. Textural variations that characterise the GT granite mass are attributed to differences in the cooling history of several intrusive phases, reconstitution of pre-existing acidic rocks and to low-grade regional metamorphism. From field observations the age-relationship between GT granite and a pink gneissic granite of the Tiras Mountains is obscure. Recent radiometric dating, however, indicates that in age the gneiss is contemporaneous with the older GT granites of which it could represent a marginal facies that has been metamorphosed and sheared by crustal deformation. The possibility of an affiliation of Tiras gneiss with GT granite is supported by geochemical data

Red granite which is chemically and petrographically similar to that of the GT mass, but which cuts the Sinclair Formation as well-defined plutons, is collectively termed Rooikam (GR) granite. This granite belongs to the third phase of the igneous cycle in the area and is in parts considerably contaminated by Sinclair basic lava which it intrudes

A pre-Sinclair/post-Nagatis heterogeneous granite, termed Kotzesrus (GK) granite south of Helmeringhausen, is not considered as belonging directly to the suite of acid igneous rocks, though it could represent a sub-facies of GT granite. GK granite, which is cut by GT, is believed to be the product of mobilised grey granodiorite which has been fluxed and impregnated by emanations from a nearby granitic magma

Whereas acid rocks in the area are represented by plutonic, hypa-byssal and volcanic textural types, basic rocks occur only as dykes and extensive flows. Sinclair basic lavas attain an estimated thickness of 2500 metres, cover 300 square kilometres within the area under discussion and are thought to have flowed from a series of tensional fractures. The lava sheets dip north and have been considerably faulted. Stepfaults with downthrow to south, strike northwest, parallel to the GT mass and to the strike of the succession. Sinclair lavas are characteristically felspar-phyric, and andesitic to basaltic in composition. Anomalous high values of K2O and Sr are not readily accounted for. The lavas belong to the Barby Series of the Sinclair Formation and rest conformably on sediments and a thin succession of felsic material

Post-Kheis igneous rocks west of Helmeringhausen fall into two distinct compositional groups: acid and basic. Hybrid and contaminated rocks are rare and confined to localities where granitic magma has intruded consolidated basic lava flows. The paucity of intermediate rock-types that could have bean produced either by mixing of acid and basic magmas, or by differentiation processes, is significant, considering the recurrence of basic intrusives and extrusives in a terrain, of acid rocks. The voluminous outpouring of Sinclair basic lava took place during an episode when acid igneous rocks, belonging to one suite, were emplaced. The separate occurrence of acid and basic rocks points to two sources of derivation. This would be in accordance with the modern concept that acidic magma has evolved by partial fusion of sialic crustal material, whilst basic magma has been derived by complete melting of a layer of basaltic composition at the base of the crust, or by partial melting of mafic material in the upper mantle. Lack of evidence for mixing of acid and basic magmas can be attributed to the time-intervals separating their respective emplacements. The presence of granitic magma in the crust would, however, be expected to have led to some intermingling of acid and basic magmas during the Sinclair igneous episode. A study of composite dykes in the area has shown that acid and basic magmas can coexist simultaneously, in at least small quantities, and that mixing of these contrasting melts is inhibited by their respective temperature differences

Thirty chemical analyses representing the main post-Kheis/pre-Nama igneous rocks are provided. Major element data indicate a close similarity between Nagatis effusives, acid porphyries, GT and GR granites, all of which are of magmatic origin and which are intermediate in composition between normal calc-alkaline and alkaline acid rocks. K/Rb ratios of the acid volcanics are higher than those of the red granites, suggesting an enrichment of rubidium in the magma after extrusion of Nagatis effusives had taken place. During the prolonged period of plutonism, however, the degree of differentiation in the granitic magma was slight as shown by the similar K/Rb values of GT and GR granites respectively. From the study of both major and minor elements there is no evidence for a genetic relationship between basic lavas of the Sinclair Formation and the suite of acid rocks west of Helmeringhausen