| Popis: |
Sand-sized spheroids in the 3.4 billion-year old Onverwacht Group, South Africa, previously interpreted as silicified marine carbonate ooids, were constructed of loose volcanic dust and ash and closely resemble accretionary lapilli found in younger volcanic sequences. They commonly show concentric internal structuring, including a nucleus, a relatively coarse-grained detritus layer made up of recrystallized vitric and crystal grains, and an outer rind of recrystallized fine-grained ash. The outer rind was rigid when the grains were deposited whereas the detritus layer was soft and plastic. Graded lap Jill beds lacking current structures represent air-fall lapilli and ash which accumulated rapidly under shallow water to subaerial conditions. The tops of many units were reworked by curr nts to form flat- and cross-laminated layers. Accretionary lapilli are known from many younger volcanic sequences, but seldom represent more than trace accumulations and are rare as water-deposited grains. Their apparent abundance in Archean terranes in both South Africa and Australia may suggest that the geologic and hydrologic setting of certain Archean volcanic complexes favored the formation and preservation of accretionary lapilli. It may reflect, however, an abnormally wet early atmosphere or the relatively higher rate of outgassing of volatiles through early Precambrian volcanoes. |
