Lithostratigraphy of Miocene–Recent, alkaline volcanic fields in the Antarctic Peninsula and eastern Ellsworth Land

Miocene–Recent alkaline volcanic rocks form numerous outcrops scattered widely throughout the Antarctic Peninsula and eastern Ellsworth Land. They occur mainly as short-lived (typically 1–2 million years) monogenetic volcanic fields but include a large outcrop area in northern Antarctic Peninsula which includes several substantial polygenetic shield volcanoes that were erupted over a 10 million year period (the James Ross Island Volcanic Group (JRIVG)). As a whole, the outcrops are of considerable importance for our understanding of the kinematic, petrological and palaeoenvironmental evolution of the region during the late Cenozoic. Until now, there has been no formal stratigraphical framework for the volcanism. Knowledge of the polygenetic JRIVG is still relatively poor, whereas a unifying lithostratigraphy is now possible for the monogenetic volcanic fields. For the latter, two new volcanic groups and twelve formations are defined, together with descriptions of the type sections. The volcanic fields (both polygenetic and monogenetic) vary in area from c. 1 to 4500 km2, and aeromagnetic data suggest that one may exceed 7 000 km2. The rocks are divisible into two contrasting petrological ‘series’, comprising basanites–phonotephrites and alkali basalts–tholeiites. The JRIVG is dominated by alkali basalts–tholeiites but also contains rare basanites, and phonotephrite–tephriphonolite compositions occur in minor pegmatitic segregations in sills. By contrast, in the monogenetic volcanic fields, basanites–phonotephrites generally form the older outcrops (mainly 15–5.4 Ma) and alkali basalts–tholeiites the younger outcrops (4(?)–<1 Ma). Throughout the region, erupted volumes of alkali basalts–tholeiites were an order of magnitude greater, at least, than those of basanite–phonotephrite compositions. Interpretation of the lithofacies indicates varied Miocene–Recent palaeoenvironments, including eruption and deposition in a marine setting, and beneath Alpine valley glaciers and ice sheets. Former ice sheets several hundred metres thick, and fluctuating ice surface elevations, which were generally higher during the eruptive periods than at present, can also be demonstrated.