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.

Introduction

Fiamme and eutaxitic texture are deformation textures that are unique to volcanic facies. They commonly occur in welded pyroclastic deposits and, as a result, are generally considered to be welding textures (e.g. Fritsch and Reiss, 1868, Zamboninin, 1919, Mansfield and Ross, 1935 cited in Smith, 1960a). Fiamme, described as flame-like or elongate lenses, are interpreted to result from the plastic deformation, flattening and sintering together of hot glassy clasts in a pyroclastic flow deposit during welding (Smith, 1960a). The bedding-parallel alignment of flattened, elongate fiamme and glass shards defines the eutaxitic texture (Fritsch and Reiss, 1868, Mansfield and Ross, 1935 cited in Smith, 1960a, Ross and Smith, 1960).

Fiamme and eutaxitic texture are characteristic of, but are not restricted to, welded pyroclastic flow deposits (e.g. ignimbrites; Smith, 1960b, Ross and Smith, 1960), welded pyroclastic fall deposits (e.g. Sparks and Wright, 1979), welded autobreccia (e.g. Sparks et al., 1993) and pyroclastic deposits that have undergone secondary reheating and welding as a result of contact with hot lavas or intrusions (e.g. Ross and Smith, 1960, Christiansen and Lipman, 1966, Schmincke, 1967, McPhie and Hunns, 1995). Similar fiamme and eutaxitic texture also occur in non-welded altered pumice-rich rocks (e.g. Fiske, 1969, Allen, 1988, Branney and Sparks, 1990, Gifkins et al., 2004) and felsic lavas (e.g. Pichler, 1981, Allen, 1988, Gifkins and McPhie, 1996).

The terms “fiamme” and “eutaxitic texture” are used herein to describe the rock texture or appearance and not to imply any particular origin. Fiamme (Fig. 1) are flame-like, glassy or devitrified lenses, which define a pre-tectonic foliation (cf. McPhie et al., 1993). Eutaxitic texture (Fig. 1) is the pre-tectonic foliation defined by the parallel alignment of fiamme (cf. Ross and Smith, 1960). Eutaxitic texture typically imparts a blotchy or streaky appearance to the rock due to the colour contrast between the darker fiamme and paler matrix.

Discriminating between genuine and apparent welding textures is difficult in ancient volcanic successions. The correct identification of welded pyroclastic deposits can influence interpretations of the depositional environment and eruption style. Welding textures imply high temperatures during or shortly after deposition and are therefore more likely to occur in subaerial deposits. In contrast, apparent welding textures can form at any time in the history of the deposit, from syn-deposition to syn-regional deformation, and can occur in cold deposits in both submarine and subaerial environments.

Apparent welding textures may be the result of: (1) diagenetic alteration and compaction of pumice clasts (e.g. Allen, 1990, Allen and Cas, 1990, Branney and Sparks, 1990, Gifkins and Allen, 2001, Gifkins et al., 2004); (2) syn-depositional alignment of lenticular pumice clasts (e.g. Ross and Smith, 1960, Fiske, 1969); (3) tectonic flattening and attenuation of phyllosilicate-altered clasts (e.g. McPhie et al., 1993); (4) diagenetic alteration and compaction of vesicular flow bands in lava (e.g. Gifkins and McPhie, 1996); (5) devitrification and hydrothermal alteration of variably crystalline versus glassy flow-banded lava (e.g. Allen, 1988); and (6) viscous shear accompanying vesiculation and autobrecciation (e.g. Pichler, 1981 cited in Branney and Sparks, 1990).

Apparent welding textures can be useful indicators of the post-depositional textural evolution of the facies. For example, the selective mechanical compaction of some pumice clasts in a pumice breccia during diagenesis suggests that these pumice clasts were softer and more easily deformed than other clasts. They may have previously been hydrated or altered to soft clay minerals, or else vesicles in the uncompacted pumice clasts may have been filled with competent minerals, such as zeolites, that resisted compaction.

The aim of this paper is to describe examples of apparent welding textures in altered pumice breccias from two ancient submarine volcanic successions: the Cambrian Mount Read Volcanics (MRV) in Australia, and the Miocene Green Tuff Belt (GTB) in Japan. We use the term “pumice breccia” for volcaniclastic facies composed of angular pumice fragments coarser than 2 mm (cf. Fisher, 1960). These examples were chosen because they were previously interpreted as welded ignimbrites based on the presence of fiamme, eutaxitic texture and a high proportion of juvenile pyroclasts. The origins of the apparent welding textures will be considered and their characteristics compared with those of welded ignimbrites. Criteria are proposed to help distinguish between genuine and apparent welding textures in pumice-rich rocks.