PHREATOMAGMATIC FORMATION OF POLYPHASE CLASTIC DIKES, MOERAKI PENINSULA, SOUTH ISLAND, NEW ZEALAND

Polyphase clastic dikes exhibiting a range of textures, including cross-lamination and peperitic basalt-mudstone contacts, are exposed in the remnants of an Eocene shallow submarine to emergent volcano on the North Otago Coast, New Zealand. Among numerous basalt dikes intruding basalt lapilli tuff of the Moeraki volcanics are several ~30 cm thick dikes that grade, within the same fracture, into baked, clast-bearing mudstones across a peperitic region of spalled off basalt fragments and baked, vesiculated mudstone. Multiple sets of trough cross-laminations are present within the baked mudstone portions of these dikes, as are wall-parallel planar-laminations. The baked mudstones grade into mudstone breccias showing several episodes of brecciation; pre-brecciation sedimentary structures are preserved in larger mudstone fragments within the dikes. Structureless gray mudstone fills the fractures farthest from the basalt. It is proposed that the phreatomagmatic interaction of basalt with mud and mudstones immediately underlying the Moeraki volcano produced the polyphase clastic dikes exposed at Moeraki. Repeated injections of basalt and/or pulsating phreatomagmatic interaction of magma with wet sediments produced episodic upward flows of fluidized sediments. These pulses entrained volcanic material and fragments of earlier dike injections. Material that remained closest to the basalt was contact metamorphosed to porcelainite. The larger mudstone fragments in breccias were formed through a combination of mechanical fracturing and fragmentation due to heating and expansion of pore fluids within lithified mudstone. Iterative numerical modeling confirms the feasibility of the latter method of rock fragmentation.