EARLY STAGES OF FERRAR LARGE IGNEOUS PROVINCE (LIP) EMPLACEMENT: PHREATOMAGMATISM IN A RIFT SETTING

The Ferrar LIP (late Early Jurassic in age) extends as a linear belt for more than 3,000 km across the center of Antarctica. All flood basalt sequences are underlain by basaltic pyroclastic rocks, mainly of phreatomagmatic origin, which include abundant tuff breccia and lapilli tuff, and minor tuff. Coarse deposits contain significant but variable proportions of accidental clasts of basaltic rocks and underlying Permo-Triassic Beacon strata. In some localities the pyroclastic rocks form extra-vent stratigraphic sequences up to 200 m thick, whereas in others the rocks appear to fill caldera-like structures and have thicknesses up to 370+ m. Subaerial volcanoes are represented by remnants of tuff and tuff rings. Calderas are suggested by steep contacts with Beacon strata. At Allan Hills, for example, caldera collapse is demonstrated by Beacon country rock characterized by a zone of phreatic disaggregation, followed inward by a zone of megaclasts formed of stratigraphically higher Beacon strata, and then by tuff breccia with laterally and vertically increasing proportions of pyroclasts. This caldera (at least 10 km long) is filled by weakly stratified tuff breccia and lapilli tuff, both cut by intrusive bodies of tuff breccia and lapilli tuff, and by basalt diatremes. In the Queen Alexandra Range, underlying Lower Jurassic silicic volcaniclastic strata, which include interbedded arkosic sandstones with angular K-feldspar grains, are deformed by monoclinal structures one of which has a basaltic tuff-breccia body in the hinge line; penecontemporaneous vertical tectonism is inferred. Monoclinal structures also displace overlying basalt lavas, indicating continued tectonism during Ferrar time. The paleovocanological setting is interpreted to be an active rift with abundant groundwater which promoted the widespread occurrence and thickness of phreatomagmatic deposits. The switch to flood basalt eruption reflects either increased rates of magma supply or reduced availability of water. Phreatomagmatism also preceded eruption of Karoo central area basalts in South Africa.