"Flindersyty" z Gór Flindersa, unikalne znalezisko odległych pokryw impaktytowych w Australii

Marek Żbik, Victor A. Gostin

Abstrakt


FINDERSITES FROM FLINDERS RANGES, UNIQUE DISCOVERY OF A DISTAL EJECTA BLANKET IN AUSTRALIA

Summary
An extensive layer of a distal ejecta blanket occurs in the 600 million years old Precambrian Bunyeroo Formation in the Flinders Ranges, South Australia. The impact location creating this ejecta blanket is about 300 km west of the Flinders Ranges, at Lake Acraman, Gawler Ranges. Preliminary reports of impact related mineralogy, geochemistry and microstructure of this ejecta blanket are presented in the paper. Samples, collected from Bunyeroo Gorge, are composed of coarse and sandy ejecta sub layers set in a shale host rock. The mineralogy of the coarse layer is determined by the composition of the impacted dacite clasts. The clay fraction of this layer consists of vermiculite and kaolinite, formed from the alteration and weathering of glassy components. The dark red impact dacite clast, containing feldspar phenocrysts surrounded by a felsic matrix, had sunk into the mudstone host rock. According to this study this layer was deposited by vertical fall of ejecta through the water column shortly after impact. The sandstone sublayer is highly porous and more persistent compared to the coarse debris sublayer, and represents finer impact debris that took longer to settle through the atmosphere and water column. The clay fraction separated from samples indicated illite, vermiculite and clinochlorite. Both sublayers contain numerous grains displaying impact-producted features. These include several quartz and zircon grains displaying one or two sets of decorated PDF (planer deformation features). Albite incrusted spherules and shard-like clasts are present in both layers and in the intervening mudstone layer. They consist of secondary minerals which probably replaced primary glass of microtectite origin. In the coarse ejecta clasts, numerous quartz and examined zircon crystals show decorated multiple planar deformation features. Plagioclase feldspar phenocrysts display planar deformations, undulatory extinction and partial isotropisation. Irregular pink dark turbid patches showing wavy extinction are sites of de vitrified melt along grain boundaries. All these features demonstrate that the studied samples were altered by shock metamorphism whose magnitude could be estimated as moderately shocked, with shock pressures between 10 and 30 GPa. Electron microscope images show the fine grained ejecta sandstone sublayer with a typical well developed granular structure and a high intergranular porosity. Highly weathered sand size grains of feldspathic clasts are recognizable with difficulty because they are covered by an intergrowth of diagenetic feldspar and clays. These clays show a high porosity with an ultrafine cellular honeycomb structure developed on the diagenetic albite crystals. Perhaps due to their high specific surface values, these clay minerals have adsorbed an excess of various elements that show on the X-ray fluorescent analyses. Trace and minor element abundances in the above described layers, confirm the similarities between the impact ejecta sublayers, and significant differences between these and the host mudstone. Moreover there is evidence of the enrichment of impact ejecta layers in er and Ni, especially in the clay fraction. Because these elements are not present in the target rocks in such amounts, and because they follow the Ir anomaly, it is intended to investigate the type of impactor from studying the elemental ratios in the clay fraction of ejecta layers. Former impact glasses that were probably contained by impactor elements retained some elemental abundances of the impactor even after transformation of the glasses into clay minerals. The uniqueness of the coarse impact dacites in the Bunyeroo ejecta which, for the first time have been found far away from the impact site, suggest that they be given a special name of "Flindersites" from the mountain range where they occur. Furthermore, the similarity in origin between Flindersites and meteorites from the Moon and Mars provides a special opportunity to study links between them.