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Large Scale Rheoignimbrite Eruptions and Their Sources: An Example from the Springbok Quartz Latites of the Goboboseb Region, Namibia


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Author Ewart A and Milner SC


Four quartz latite rheoignimbrite units
(units I to IV) mapped in the Goboboseb Mountains
have been traced to their eruptive source, the
Messum intrusive complex; this is a complex
multistage ring structure (18 km diameter) with
well defined gabbroic, monzonitic, and syenitic
intrusive phases, and a central ring zone
containing mixed rhyolitic to latitic
agglomerates. Three of the four quartz latite
units are correlated with the upper and lower (2
units) Springbok quartz latites outcropping some
90-130 km north of McIsum. This implies minimum
volumes of 300-500 km for wits I and III (Lower
Springbok) and 2200-2600 km for unit IV (Upper
Springbok). This latter unit is still ti 300 m
thick at its outcrop limit, some 130 km from
Messum. Each of the four units represents single
cooling units. Lithologically, each can be
divided into three zones: A basal, dark grey to
black flinty or glassy zone, often with highly
attenuated fiamme (ressembling flow banding); a
central massive and strongly recrystallised zone;
and an upper variable zone which exhibit
breccias, contorted flow banding, and
amygdaloidal to pumiceous clasts. Their overall
characteristics tend to be intermediate between
ash flows and lava flows. The rheoignimbrites
are interpreted as the oproducts of dense, high
temperature (950°-1100 C) ash flows which
underwent rheomorphism and lava-like flow prior
to their final cooling.
These large scale eruptions are related to
the initial rifting of Africa and S. America,
possiby in response to a major hot spot.