Page 72 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Subaqueous syn-volcanic sills and dykes Syn-volcanic dykes are, in most cases, actually feeders
to lava flows and domes, to piles of in situ and
In ancient volcanic terranes, it is useful to make a resedimented hyaloclastite (Fig. 20), or to syn-volcanic
distinction between sills and dykes that are essentially sills. Contacts of syn-volcanic dykes show the effects of
coeval with the enclosing sequence and those that quenching in the near-surface, water-saturated
significantly post-date the enclosing sequence. environment, and of interaction with the poorly
Especially in subaqueous settings, a significant consolidated host sequence. Yamagishi (1987, 1991)
proportion of the magmatism may in fact comprise syn- has recognized different types of feeder dykes
volcanic, high-level intrusions rather than surface lava distinguished by the character of their margins and of
flows. Magma movement towards the surface is associated hyaloclastite. The margins of apophyseal-
influenced by its density and hydrostatic pressure type feeder dykes consist of bulbous or finger-like
compared with the density of the enclosing rocks (or protrusions which extend into the host and break up into
sediments) and the lithostatic pressure they exert. concentric pillow fragments (13.3). The margins of
Magma that is denser than the host is more likely to massive-type feeder dykes are very closely jointed and
remain subsurface and be intruded as sills than erupted grade outward to angular fragment breccia and peperite
(McBirney, 1963; Walker, 1989a). This condition is (13.7).
commonly met in subaqueous settings where rising
magma encounters unconsolidated sediments that have Volcaniclastic deposits associated with silicic
accumulated to substantial thicknesses. Host sequences
to syn-volcanic sills and dykes consist of ambient basin lava dome eruptions in shallow water (18)
sediments, volcanic deposits and other co-genetic sills Silicic lava dome eruptions at vents in shallow water
and dykes. typically involve complex combinations of
phreatomagmatic, explosive magmatic and effusive
Identification of syn-volcanic or syn-sedimentary sills activity, complicated further by syn-eruptive mass-flow
usually depends on the upper contact relationships, resedimentation and reworking. Volcaniclastic deposits
especially evidence indicating that the host sequence formed in this setting can be mixtures of quench-
was unconsolidated or poorly consolidated when fragmented juvenile clasts and pyroclasts, and show
intruded (Fig. 30). Important features include the evidence of heat retention in spite of subaqueous
presence of tongues or lobes projecting upward from the deposition (e.g. Tamura et al., 1991). Sustained eruptive
sill into the host, the development of peperite along the activity may lead to subaerial emergence of the volcanic
upper margin, induration of the host, destruction or pile, followed by cycles of growth and collapse of the
disturbance of bedding in sediments at the contact, and edifice. In many cases, only a remnant of volcanic
the presence of contacts that are locally discordant to structure survives, and significant amounts of the
adjacent bedding (Kokelaar, 1982; Branney and volcanic products are reworked, eroded and
Suthren, 1988; McPhie, 1993). A chilled margin may resedimented, contributing Volcaniclastic debris to
occur along upper surfaces of both sills and surface lava neighboring deeper water settings. The final record of
flows and is not diagnostic of the mode of emplacement. dome-related, shallow-water volcanism is thus expected
Along the lower contacts of sills, textures can develop to include a wide range of volcanic facies and marked
that are similar to those along the upper contact or along variations in facies geometry, especially in proximal
the bases of lava flows. sequences.

Fig. 29 Character and arrangement of volcanic facies that develop in association with the emplacement of partly
extrusive cryptodomes (top). (1), (2), (3) mark the sites of sections depicted in the graphic logs (right).

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