ABSTRACT *

The Middle to Upper Miocene James Ross Island Volcanic Group is composed predominantly of alkalic olivine-basa1t lavas, palagonite-breccias and subordinate palagonite-tuffs. These volcanic rocks, which form magnificent cliff exposures, have been systematically mapped and collected at James Ross and Vega Islands, Tabarin Peninsula, and at most of the smaller islands of the James Ross Island group. The sub-horizontal lavas and steeply dipping palagonite-breccias beneath them (136 kb) are genetically related, and are separated from each other by level contacts. Lavas and palagonite-breccias occur either as "deltaic" structures around the margins of James Ross Island, where they probably resulted from aqueous brecciation of large lava streams flowing from a major volcanic centre, or as individual lava/ palagonite-breccia volcanoes, which were produced by the subaqueous initiation of central eruptions. Ash cones and the cores of lava/palagonite-breccia volcanoes are composed of massive or thinly bedded fine-grained palagonite-tuff. Although well-preserved sedimentary structures confirm the subaqueous origin of some other palagonite-tuffs, the distinction between those formed by the aqueous brecciation of molten basalt and those formed by true pyroclastic activity is not always clear. Overlap structures, some of which were caused by relative sea-level changes, occur in some palagonite-breccias and against the palagonite-tuffs which form ash cones. Dykes, rare sills, volcanic plugs and a laccolith comprise the associated intrusive rocks. Although it is absent from the dykes, primary interstitial analcite is ubiquitous in the olivine-dolerite plugs and the laccolith.

Large-scale faulting has not been observed displacing the rocks of this volcanic group, except in the Tabarin Peninsula area, where faults with throws of up to 1,000 ft. (305 m.) have been inferred. The termination of some rock units by overlap structures accounts for most of the stratigraphic discontinuities.

Four grades of secondary zeolitization (involving chabazite, phillipsite and anal cite) have been recognised in the extrusive rocks and dykes. There is a correlation between the zeolite grade and the degree of palagonitization of the constituent hyalobasalt fragments in the palagonite-tuffs and breccias, and in these rocks zeolites are generally more abundant than in the olivine-basalt lavas. The zeolites which occur in the plugs and the laccolith (analcite, thomsonite, chabazite, natrolite and gismondite) have a pyrogenetic origin related to the final stages of olivine-dolerite crystallisation.

This volcanic group has alkalic basalt geochemical characteristics, which are more closely related to alkalic basalts from orogenic environments than to those from oceanic ones. Little differentiation has been observed in the extrusive rocks, although peralkaline rocks may exist as plugs, or flows of limited extent, high on James Ross Island beneath the ice cap of Mount Haddington. However, crystallisation differentiation has resulted in dolerite-pegmatite schlieren and late-stage peralkaline veins in the analcite-bearing olivine-dolerite laccolith of Palisade Nunatak. The course of differentiation in this intrusion is comparable with those of alkalic basalts generally but it is most characteristic of differentiation in a single intrusive mass. Although little differentiation has occurred in the dykes, they were evidently committed to a geochemical trend of initial absolute iron enrichment.

Five phases of eruption and lava/palagonite-breccia deposition have been recognised in the James Ross Island area. Although the first phase certainly post-dates the formation of a major volcanic centre situated near the middle of James Ross Island, the superposition of one lava/palagonite-breccia unit on another indicates that there was a gradual submergence of the area during the formation of the volcanic group. Because the upper 2,000 if. (610 m.) of James Ross Island are obscured by ice, the total extent of this submergence is unknown. However, lava/palagonite-breccia horizons occur at intervals up to nearly 3,000 if. (914 m.) above sea-level, suggesting that at least this amount of submergence took place. Since the close of vulcanicity, re-elevation of the landmass has returned this area to its present position. Raised beaches and benches around the margins of James Ross and Vega Islands testify to minor sea-level fluctuations in the recent past.

* Since 1975, when this monograph was eventually published, further work reveals much of the JRIV Group to be younger than indicated here, and having an intra-glacial provenance.