Mafic, Large‐Volume, Pyroclastic Density Current Deposits from Phreatomagmatic Eruptions in the Ferrar Large Igneous Province, Antarctica

Abstract

Mafic volcaniclastic deposits are increasingly recognized as important components of continental flood basalt provinces, but the mode of origin of these deposits and the possible environmental influence of the eruptions that formed them remain poorly constrained. The Mawson Formation (Ferrar large igneous province, Jurassic) contains the best‐known examples of large‐scale, mafic, primary pyroclastic deposits from any flood basalt province worldwide. At southern Allan Hills, in contrast with other sites, the Mawson Formation consists predominantly of subhorizontal, laterally continuous, meter‐thick layers that fall into four facies: (1) poorly sorted, coarse lapilli‐tuff or tuff‐breccia beds; (2) tuff or fine lapilli‐tuff beds; (3) block‐rich layers; and (4) accretionary lapilli‐bearing beds. We interpret the first facies as deposits of high‐concentration pyroclastic density currents (PDCs) resulting from the collapse of phreatomagmatic eruption plumes, some from a source probably several kilometers away in Coombs Hills. The finer‐grained layers were probably deposited by dilute PDCs, whereas block‐rich layers originated from local sources predominantly by lateral ejection and ballistic fall. Dilute, moist turbulent currents were also likely responsible for the generation and deposition of large (up to 4.5 cm) rim‐type accretionary lapilli. Emplacement of meters‐thick basaltic PDC deposits over areas of at least 100–200 km2 is suggested, with individual deposit volumes on the order of a cubic kilometer. Prelava pyroclastic eruptions of the Ferrar mafic magmas were explosive, produced significant eruption clouds, and distributed their products over substantial areas.