Geochemistry of Karroo dolerite sills in the Calvinia district, Western Cape Province, South Africa

✍ Scribed by Anton P. Roex; David L. Reid

• Publisher: Springer
• Year: 1978
• Tongue: English
• Weight: 907 KB
• Volume: 66
• Category: Article
• ISSN: 0010-7999
• DOI: 10.1007/bf00403419

✦ Synopsis

Two Karroo dolerite sills display chemical and mineralogical variation compatible with cumulus enrichment. The Blaauwkrans sill is an olivine tholeiite and contains a central zone slightly enriched in olivine, plagioclase and clinopyroxene. The thicker Hangnest sill is a quartz tholeiite and shows evidence of crystal settling and has a lower zone enriched in cumulus orthopyroxene and plagioclase.

The two sills differ quite markedly in their trace element compositions, with the Hangnest magma enriched by a factor of two in LIL elements (Rb, Ba, Nb, Zr, Y) relative to the Blaauwkrans magma. The Hangnest magma contained extremely low Ni contents (3-5ppm), whereas the Blaauwkrans magma contained higher but more "normal" Ni (100-ll0ppm). Such contrasting trace element compositions preclude any simple genetic relationship between the two Karroo magmas but they may be related either through a common parent or are derivatives from separate parental magmas. * South African Contribution No. 24 to the International Geodynamics Project