Metamorphic and Thermal History of a Fore-Arc Basin: the Fossil Bluff Group, Alexander Island, Antarctica

The Himalia Ridge Formation (Fossil Bluff Group), Alexander Island is a 2·2-km-thick sequence of Upper Jurassic–Lower Cretaceous conglomerates, sandstones and mudstones, derived from an andesitic volcanic arc and deposited in a fore-arc basin. The metamorphic and thermal history of the formation has been determined using authigenic mineral assemblages and vitrinite reflectance measurements. Metamorphic effects include compaction, pore-space reduction, cementation and dissolution and replacement of detrital grains by clay minerals (smectite, illite/smectite, corrensite and kaolinite), calcite, chlorite, laumontite, prehnite, pumpellyite, albite and mica, with less common quartz, haematite, pyrite and epidote. The authigenic mineral assemblages exhibit a depth-dependence, and laumontite and calcite exhibit a strong antipathetic relationship. Detrital organic matter in the argillaceous layers has vitrinite reflectance values (R_o) ranging from 2·3 to 3·7%. This indicates considerable thermal maturation, with a systematic increase in reflectivity with increasing depth. There is good correlation of metamorphic mineral assemblages with chlorite crystallinity and vitrinite reflectance values—all indicating temperatures in the range of 140 ± 20°C at the top of the sequence to 250 ± 10°C at the base of the sequence. The temperatures suggest a geothermal gradient of 36–64°C/km and a most likely gradient of 50°C/km. It is suggested that this higher-than-average gradient for a fore-arc basin resulted either from rifting during basin formation or from a late-stage arc migration event.