Pyroclasts volcanic breccia

The Exposure Hill Formation of northern Victoria Land occupies the same stratigraphic position as the Mawson Formation in the Allan and Coombs hills of southern Victoria Land. Both formations are composed of volcanic breccias and related pyroclastic rocks that formed at the start of the eruption of the Kirkpatrick Basalt and the intrusion of sills of the Ferrar Dolerite of the Middle Jurassic Ferrar Group which is the subject of Chapter 12. 

Fig. 12.4 The Allan Hills and Coombs Hills at the edge of the polar plateau in southern Victoria Land expose the Mawson Formation as well as the underlying formations of the Beacon Supergroup and the overlying Kirkpatrick Basalt and Ferrar Dolerite sdUs. The Mawson Formation consists of laharic deposits and volcanic breccias interbedded with layers of tuffaceous sandstone. This formation represents a transition from felsic pyroclastics, that characterize the Triassic Lashly Formation, and the large-scale eruption of tholeiitic baseilt in Gondwana during the Jurassic (Adapted from ElUot et til. 2006)...

Fedele, L., Tarzia, M., Belkin, H. E., De Vivo, B., Lima, A., and Lowenstern, J. B. (2006). Magmatic-hydrothermal fluid interaction and mineralization in alkali-syenite nodules from the Breccia Museo pyroclastic deposit, Naples, Italy. In Volcanism in the Campania Plain Vesuvius, Campi Flegrei and Ignimbrites (B. De Vivo, ed.). Developments in Volcanology, 9, pp. 125-161. Elsevier, Amsterdam, The Netherlands. 

Fig. 8.13 The chemical compositions of volcanic rocks of the Hawkes Porphyry Member of the Gambacorta Formation (Appendix 8.5.14) in the southern Neptune Range vary predictably with increasing silica concentrations. The only exception is sample 294 which is a pyroclastic breccia (clast ) from the basal part of the Elliott Sandstone of the Neptune Group. However, sample 294 does not deviate significantly from the Rb-Sr data array in Fig. 8.12 (Data from D.L. Schmidt communicated to Eastin 1970)...

The phreatomagmatic volcanic eruptions expelled both compressed gases and rock fragments and formed shallow maar craters at the surface surrounded by a tephra ring. The superheated steam that was discharged caused pervasive hydrothermal alteration of the breccia pipes and associated pyroclastics, including the deposition of zeolites in the open spaces of the... 

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