The petrology and significance of a stratiform mafic segregation pegmatite in a Karoo-aged dolerite sheet

University of Durban-Westville, Department of Geology, Durban, South Africa

A sub-horizontal stratiform mafic segregation pegmatite, of the order of 30 cm thick, occurs within 25 m of the top contact of a Karoo-aged dolerite sheet at Pietermaritzburg, South Africa. The host rock is an orthopyroxene-rich dolerite containing 5-8% interstitial granophyre. The pegmatite is similar to the host dolerite in many respect that the granophyre content is higher, and acicular augite is the only pyroxene. At its upper contact, a laterally continuous sub-horizontal joint forms a sharp boundary to the pegmatite, whilst the lower contact is gradational over a few millimetres. Low MgO and chalcophiles, and elevated incompatible element contents, are the main geochemical attributes of the pegmatite. The footwall of the stratiform pegmatite is characterized by slightly elevated incompatible element contents up to 0.74 m from the contact, and small, irregular pods of pegmatite occur within 0.5 m of the main pegmatite layer. In the hanging-wall dolerite, incompatible element enrichment is confined to within 0.3 m of pegmatite. Inter-element plots of the incompatible elements (T1O ₂ , Zr, and Y) and compatible elements (MgO, Cr, and Ni) produce linear regression lines with correlation coefficients of 0.98 or better, indicating the consanguinity of the hanging-wall and footwall dolerites and the enclosed pegmatite, with no evidence for assimilation of country rock material. The pegmatite represents an estimated 25 to 35% fractionation of the dolerite, which accords with the results of comparable studies on similar bodies of rock. In common with other documented cases, the pegmatite is situated within the upper 50% of the intrusion. Although segregation may have taken place within a single intrusion, the sharp, jointed upper contact suggests that the pegmatite could have accumulated at the interface between a new influx of magma and the overlying, partially crystalline, products of an earlier influx within a multiple intrusion. Pegmatites in small intrusions of the type described here are considered to be incipient forms of the more complexly evolved pegmatites found in large layered intrusions.

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