The geology of the Okonjeje igneous complex and surrounding Damara sequence metasediments, South West Africa/Namibia

Abstract

No abstract provided. The following is taken from the author's Summary and Conclusions:

Field, hand specimen and thin section studies, together with XRD and chemical analysis (after Simpson, 1954), enabled the writer to classify the rocks into the following broad groups: (l) Damara Sequence metasediments, (2) Karoo Supergroup metasediments, (3) post-Karoo dykes, and (4) tholeiitic suite, alkaline suite, and radial dykes of the Okonjeje Igneous Complex. A late magnetite-hematite quartzite cross cuts the Damara Sequence metasediments, and recent sediments consists of unconsolidated alluvial sand, and scree deposits

Structural analysis of the three subareas by the use of rose diagrams, the equal area Schmidt Net, and contoured diagrams confirmed the presence of: (l) symmetric upright plunging anticline (subarea l), (2) asymmetric upright plunging syncline (subarea 2), (3) symmetric reclined syncline (subarea 3), (4) a dominant 090° quartz vein trend over the whole area. Folding, boudinaging, jointing, shearing, veining, and faulting of the Damara Sequence metasediments was observed

The Damara Sequence metasediments were regionally metamorphosed to the low grade greenschist facies. A later contact (thermal) metamorphic event related to the intrusion of the Okonjeje Complex upgraded the metamorphism to the high temperature pyroxene-hornfels facies, which affected the adjacent Damara Sequence and Karoo Supergroup lithologies

FMA plots by Simpson (1954) indicate alkaline and tholeiitic trends for the two rock suites of the Okonjeje Igneous Complex. Simpson (1954) suggests that the two basic magmas representing the two rock suites originate from a common parent. However, there are two contrasting lines of liquid decent: (l) fairly undersaturated liquid which produced the tholeiitic rocks, and (2) an undersaturated liquid which differentiated to produce the alkaline rocks. The foyaites and camptonites are a product of a highly undersaturated. residual liquid

Simpson (1954) also suggests that cauldron subsidence was responsible for the form of the alkaline intrusion, while cone-sheet intrusions produced the present form of the alkali and tholeiitic gabbros. The inward dip of the beds in the northern and north eastern Karoo-outliers is evidence for cauldron subsidence. Hughes (1982) and Richey (1961) provide evidence for the coexistence of cauldron subsidence and cone sheet intrusion as the mechanism of emplacement of the ring complexes such as Okonjeje

The rock names, general geology, and interpretation of the Okonjeje Igneous Complex are different in some respects and similar in others, to the work done by Simpson (1954). The geology of the surrounding Damara Sequence metasediments, to the writer's best knowledge, has not been mapped in as great a detail previously; although, geologists from the geological survey have mapped the area