Metamorphic and Thermal History of a Fore-Arc Basin: the Fossil Bluff Group, Alexander Island, Antarctica |
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Authors: | MILLER S; MACDONALD D I M |
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Institution: | 1 BRITISH ANTARCTIC SURVEY, NERC, HIGH CROSS, MADINGLEY ROAD, CAMBRIDGE CB3 0ET, UK
2 PRESENT ADDRESS: NATIONAL MUSEUMS OF SCOTLAND, CHAMBERS STREET, EDINBURGH EH1 1JF, UK
3 PRESENT ADDRESS: DEPARTMENT OF GEOLOGY & PETROLEUM GEOLOGY, UNIVERSITY OF ABERDEEN, MESTON BUILDING, ABERDEEN AB24 3UE, UK |
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Abstract: | The Himalia Ridge Formation (Fossil Bluff Group), AlexanderIsland is a 2·2-km-thick sequence of Upper JurassicLowerCretaceous conglomerates, sandstones and mudstones, derivedfrom an andesitic volcanic arc and deposited in a fore-arc basin.The metamorphic and thermal history of the formation has beendetermined using authigenic mineral assemblages and vitrinitereflectance measurements. Metamorphic effects include compaction,pore-space reduction, cementation and dissolution and replacementof 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 exhibita depth-dependence, and laumontite and calcite exhibit a strongantipathetic relationship. Detrital organic matter in the argillaceouslayers has vitrinite reflectance values (Ro) ranging from 2·3to 3·7%. This indicates considerable thermal maturation,with a systematic increase in reflectivity with increasing depth.There is good correlation of metamorphic mineral assemblageswith chlorite crystallinity and vitrinite reflectance valuesallindicating temperatures in the range of 140 ± 20°Cat the top of the sequence to 250 ± 10°C at the baseof the sequence. The temperatures suggest a geothermal gradientof 3664°C/km and a most likely gradient of 50°C/km.It is suggested that this higher-than-average gradient for afore-arc basin resulted either from rifting during basin formationor from a late-stage arc migration event. KEY WORDS: Antarctica; diagenesis; fore-arc basin; low-temperature metamorphism; vitrinite reflectance |
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Keywords: | : Antarctica diagenesis fore-arc basin low-temperature metamorphism vitrinite reflectance |
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