SECONDARY WELDING OF SUBMARINE, PUMICE-LITHIC BRECCIA AT MOUNT-CHALMERS, QUEENSLAND, AUSTRALIA

Very thick units of massive pumice and lithic Introduction clast-rich breccia in the Early Permian Berserker beds at Mount Chalmers, Queensl and, are deposits from cold, water-supported, volcaniclastic mass flow s emplaced in a below-wave base submarine setting. Adjacent to syn-vol canic andesitic and rhyolitic sills and dykes, the pumice-lithic brecc ia shows a well-developed eutaxitic texture. The eutaxitic foliation i s parallel to intrusive contacts and extends as far as a few metres aw ay from the contact. At these sites, pumice clasts are strongly flatte ned and tube vesicles within the pumice clasts are compacted and align ed parallel to the direction of flattening. Some lenticular pumice cla sts contain small (less than or equal to 2mm), round, quartz-filled am ygdales and spherulites. Further away from the sills and dykes, the pu mice clasts have randomly oriented, delicate tube vesicle structure an d are blocky or lensoid in shape. Round amygdales were generated by re -vesiculation of the glass and the spherulites indicate devitrificatio n of the glass at relatively high temperatures. The eutaxitic texture is therefore attributed to re-heating and welding compaction of glassy pumice-lithic breccia close to contacts with intrusions. In cases inv olving sills, secondary welding along the contacts formed extensive, c onformable, eutaxitic zones in the pumice-lithic breccia that could be mistaken for primary welding compaction in a hot, primary pyroclastic deposit.