Kamafugitic diatremes: their textures and field relationships with examples from the Goias alkaline province, Brazil

摘要

Kamafugitic rocks intruded the Precambrian basement and Phanerozoic sediments at the northeast border of the Parana basin as part of the Late Cretaceous Goias alkaline province (GAP). Plutonic complexes dominate the north of the province, whereas lavas and pyroclastic rocks prevail in the south. The central GAP is characterized by kamafugitic diatremes, which may crop out continuously for up to 850 m and consist of a central breccia body, surrounded and overlain by lava flows and crosscut by dykes. The breccias contain some special spheroidal juvenile fragments—namely, accretionary and armored lapilli, frozen droplets, spinning droplets, and wrapped fragments—whose textural and mineralogical aspects are described in detail. Irregularly shaped tuff pockets that occur within the breccias contain textures and structures similar to those of subaerial surge deposits and formed in confined, high gas to solid + liquid ratio domains in the conduit. Diatreme emplacement affected the country rock through thermal metamorphism, development of columnar jointing, and formation of pepcrite-like mixtures. There is no evidence of phreatomagmatic activity in the diatremes, and CO_2, rather than H_2O, seems to have been the major volatile component of the kamafugitic magmas. This finding implies that features such as accretionary lapilli and peperites are not exclusively associated with H_2O-dominated processes.