1887

oa South African Journal of Geology - Petrographic, Rb-Sr isotope and geochemical characteristics of intrusive granitoids from the Port Edward-Port Shepstone area, Natal

Volume 89 Number 2
  • ISSN : 1012-0750
  • E-ISSN: 1996-8590

 

Abstract

The Port Shepstone-Port Edward area of Natal contains extensive bodies of granitoid intrusive into older, high-grade, supracrustal gneisses. This paper describes the petrographic, Rb-Sr isotopic and geochemical characteristics of four of these granitoids, namely the Glenmore granite, Nicholson's Point granite, Port Edward enderbite and Bomela charnockite.

The Glenmore, Port Edward, and Bomela granitoids contain abundant feldspar megacrysts whilst the Nicholson's Point granite is coarse-grained. Orthopyroxene is common in the enderbite and charnockite and is also developed in certain exposures of the Nicholson's Point granite. Rare fayalite has also been reported from the Bomela charnockite. Chemical variation in these units can be ascribed mainly to changing proportions of the feldspars.

Rb-Sr isotope data indicate that these granitoids were formed in the period 1 011 to 859 Ma. The youngest age, dating the crystallization of the Bomela charnockite, indicates that this portion of the Namaqua-Natal Mobile Belt was active for longer than previously thought.

At least two of the four intrusives (Port Edward and Bomela) have chemical characteristics (e.g. high HFS element concentrations and FeO*/(FeO* + MgO) ratios) which are typical of A-type granitoids. Their mineralogy, chemistry, and Proterozoic age provide convincing evidence that they are members of the charnockite-rapakivi granite association. Furthermore, field relationships and the A-type character of these two units constrains the age of D2 tectonism in southern Natal to between 987 and 859 Ma.

Comparison of the data presented in this paper with that from mineralogically and chemically suitable source rocks on the Kaapvaal Craton and the older supracrustal gneiss succession in southern Natal indicates that it is very unlikely that these four granitoids were derived by reworking of Archaean crustal material or of metasediments like those presently exposed in the region. It is, however, possible that the granitoids were derived from ∼1400 Ma mafic to intermediate material like the granulite xenoliths caught up in kimberlites in Lesotho.

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1986-05-01
2020-09-23

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