Submarine Eruptive Processes in the Brook St Terrane at Colac Bay, Southland, New Zealand

Abstract

A sequence of volcanic and volcaniclastic rocks of the Brook Street Terrane crops out along the Southland coast from Howells Point, Riverton, into Colac Bay. This sequence provides an along-strike exposure, revealing lateral variations in these submarine deposits of a Permian oceanic island arc; such sequences are rarely studied, and allow examination of an arc environment within a thin slice of time, in a manner which is impossible in the more common vertical sequences. Lithofacies analysis and mapping, and geochemical analyses were carried out. The sequence includes pillow lavas, dikes, hydroclastic breccias, tuffs, lapilli tuffs and argillites. All is hydrothermally altered and metamorphosed to prehnite-pumpellyite facies, so major and trace element analyses were conducted on the freshest augite phenocrysts wherein the magmatic signal is likely to be preserved. Geochemical data indicate variable primitive island arc to MORB-like magma sources. Two distinct groups of signatures exist, one with typical island arc characteristics, the other more closely resembling back-arc (MORB-like) magma. The distribution of these suggests two separate small volcanic centres, perhaps tapping different mantle domains in a source region transitional between the arc and back-arc. The size and proximity of the two may indicate they are satellite vents of a larger arc volcano. The sedimentary structures and facies relations of tuffs and lapilli tuffs suggest formation in association with pillow lava as hyaloclastite, and redeposition by subaqueous mass flows. The pillow lavas observed have peperitic contacts with the volcaniclastic rocks, indicating that they intruded into these deposits while they were unconsolidated. These relationships indicate that two generations of pillow lava formed in the sequence, the first associated with production of hyaloclastite which was redeposited in fans and channels on the volcano flanks, and the second intruding into these deposits