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Igneous flow banding, St Peter Suite, South Australia



Roberto Weinberg
Monash University, Australia





Flow Banding

This page is part of a group of pages on the magmatic aspects of the St Peter Suite. Outcrops of the St Peter Suite in Point Brown and Rocky Point have common flow banding features forming biotite schlieren marking the contacts between magma batches, interpreted as related to flow banding.


granite disaggregation granite disaggregation
a) Rythmic vertical flow banding in granite at Point Brown. b) Detail of flow banding in same outcrop as in (a) with gradation from biotite rich on the right to biotite poor on the left.
granite disaggregation granite disaggregation
c) Flow banding: a top layer cuts across and truncates previously formed, curved flow banding below. This is interpreted as an erosional channel. Point Brown. d) Zoom in: gradation in colour index from a sharp margin and a biotite rich base followed upwards by a gradual decrease in biotite content.
granite disaggregation granite disaggregation
e) Irregular flow banding. f) Flow banding folded in between two xenoliths of red coarse granite.
granite disaggregation granite disaggregation
g) Erosional channel in contact between a grey granitic rock and a reddish granitic rock. Note how the biotite schlieren at the base curves to form the basis of individual layers. Rocky Point. h) Biotite schlieren, irregular in shape and locally discontinuous, at the contact between a grey granitic rock (below) and a reddish granite above. The schlieren is associated with a biotite-free layer underneath. Interpretation: flow related sorting and schlieren formation at the contact, between two magmas, subsequently stretched and folded by magmatic flow. Rocky Point.
granite disaggregation granite disaggregation
i) Banding at the contact between a grey granite and a red granite. Banding is broken up into blocks separated by shear zones. j) Detail of the banded contact.
granite disaggregation
k) Close view of the banding at the contact.