Apparent welding textures in altered pumice-rich rocks

Abstract

Fiamme and eutaxitic texture are common in ancient non-welded, pumice breccias. The fiamme are phyllosilicate-rich lenses that define a bedding-parallel foliation resembling eutaxitic texture in welded ignimbrites. Pumice breccias in the Cambrian Mount Read Volcanics (Australia) are composed of tube pumice clasts, bubble-wall shards, plagioclase crystal fragments and volcanic lithic clasts, and contain dark green fiamme and stylolites. These sericite or chlorite+sericite fiamme are aligned roughly parallel to regional bedding, and are enclosed in pale pink feldspar+quartz-altered domains of uncompacted, randomly oriented, tube pumice clasts and bubble-wall shards. Similarly, pumice breccias in the Miocene Green Tuff Belt in the Hokuroku Basin (Japan) comprise tube pumice clasts, fiamme, bubble-wall shards, crystal fragments and non-vesicular volcanic clasts. The dark green, wispy smectite or chlorite fiamme are aligned parallel to bedding, and are enclosed in domains of variably clay-, zeolite- and sericite-altered, uncompacted pumice clasts and shards. The fiamme in these examples vary in length (0.5 mm to 1 m), length to height ratio (3:1 to 40:1), shape (e.g. flame-like, feathery, branching, bow tie, wedge), and internal texture (porphyritic, massive, vesicular or stylolitic). The fiamme define an anastomosing bedding-parallel, pre-tectonic eutaxitic texture. Delicate feathery terminations and locally preserved vesicles indicate that the fiamme are altered compacted pumice clasts. The presence of bedding-parallel stylolites and solution seams, undeformed shards and pumice clasts adjacent to the fiamme, and the overprinting of fiamme by the regional cleavage suggest that the fiamme formed during diagenesis, prior to tectonic deformation, and that compaction did not involve welding. Fiamme and eutaxitic texture in these examples resulted from the alteration, dissolution and mechanical compaction of cold tube pumice clasts during diagenesis and lithification.