Page 61 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Part 3. Lavas, syn-volcanic intrusions and

related volcaniclastic deposits

As magma rises to shallow levels prior to an eruption it texture, with glassy, cryptocrystalline or aphanitic
may become saturated in volatiles, as a result of groundmass. Non-explosive autoclastic processes of
decompression, and/or as a result of crystallization of autobrecciation and quench fragmentation generate
anhydrous phases (Sparks, 1978; Burnham, 1983). If significant volumes of fragmented lava as normal by-
volatile contents are very low, or if volatiles are able to products of effusive eruptions, regardless of
escape from the magma, an effusive eruption will occur, composition or setting. The same processes may cause
generating lava flows or domes. Several processes are brecciation of intrusions emplaced into wet sediment.
involved in degassing of magmas: Thus, the autoclastic facies comprise, for lavas,
(1) minor, magmatic-volatile-driven explosive autobreccia and/or hyaloclastite and, for syn-volcanic
eruptions; intrusions, intrusive autobreccia, intrusive hyaloclastite
(2) the steady loss of gas and condensates through and peperite.
fractured, permeable conduit wall rocks, in some cases
associated with shallow hydrothermal systems; Syn-volcanic intrusions and intrusive complexes are an
(3) vesiculation prior to eruption and during outflow; important product of magmatism in subaqueous
(4) formation, passive rise and escape of bubbles from sedimentary basins. Magmas of all compositions can be
the magma in the conduit. emplaced as syn-volcanic intrusions. They can be
coarsely porphyritic or aphyric, with aphanitic,
Vigorous but short-lived explosive activity (1) and gas cryptocrystalline or partly glassy groundmass, and range
loss through permeable conduit walls (2) are very in vesicularity from non-vesicular to partly pumiceous.
common precursors and accompaniments to eruptions of Hanson (1991) described extensive, thick, andesitic and
subaerial intermediate and silicic lavas (Ncwhall and rhyolitic intrusions and associated intrusive breccia in a
Melson, 1983; Taylor et al., 1983; Eichelberger et al., Devonian arc sequence in California. Drill core
1986; Heiken and Wohletz, 1987). Detailed studies of retrieved from the Gulf of California (Einsele, 1986),
textures and H 2O contents in high-viscosity silicic flows the Juan de Fuca Ridge (ODP Leg 139, 1992) and the
and domes (Eichelberger et al., 1986; Fink and Manley, Japan Sea (Thy, 1992) revealed the presence of multiple
1987) and low-viscosity basaltic flows (Mangan et al., basaltic sills in deep marine sediments. The sill complex
1993; Wilmoth and Walker, 1993) suggested that discovered at the Juan de Fuca Ridge is adjacent to a 95
volatiles are lost rapidly but non-explosively by m-thick massive sulfide deposit. At Hellyer, in the
vesiculated lava (3). This mechanism requires that the Mount Read Volcanics, western Tasmania, basaltic
vesicles are interconnected (important in high-viscosity sheets above the massive sulfide deposit were, in many
flows), or else are able to rise and escape (important in cases, emplaced as sills into unconsolidated mudstone
low-viscosity flows). The fourth process is mainly (McPhie and Allen, 1992; Waters and Wallace, 1992).
confined to low-viscosity magmas (mostly basaltic), and Lower in the sequence at Hellyer, and elsewhere in the
can extend to mild fountaining and emission of sprays Mount Read Volcanics, syn-volcanic intrusions of
of fluid magma that coalesce and recombine to form intermediate and silicic composition are common but
coherent lava when deposited on the vent rim (fountain- difficult to recognize and easily mistaken for lava flows.
fed lava flows — Wilson and Head, 1981). In this part, important lithofacies characteristics of lavas
and syn-volcanic intrusions are reviewed, with emphasis
The physical properties of magmas (composition, on related autoclastic deposits, internal structures and
temperature, viscosity, volatile content, phenocryst facies geometry. Lithofacies information is obtained
content) have a major influence on the internal textures, from map-, drill-section- and outcrop-scale
facies geometry and facies associations of lavas and observations. Some textures evident in hand specimens
syn-volcanic intrusions. Other important controls are, of lavas and intrusions are also found in volcaniclastic
for lavas, the circumstances of eruption, such as deposits. Genetic interpretations should be consistent
discharge rate, vent character, substrate gradient and with all the available lithofacies information and not
subaerial versus subaqueous setting (Walker, 1973b; depend solely on textures evident in hand specimens or
Hulme, 1974; Moore et al., 1978; Wilson et al., 1987; thin-sections.
Griffiths and Fink, 1992), and, for intrusions, the host
sediment character (Busby-Spera and White, 1987). In Autobreccia (10)
most cases, both lava flows and syn-volcanic intrusions
include varying proportions and arrangements of Autobrecciation involves the non-explosive
coherent facies and autoclastic facies. The coherent fragmentation of flowing lava. Parts of lava flows that
facies consists of solidified lava or magma and is are cooler, more viscous, or subject to locally higher
characterized primarily by evenly porphyritic or aphyric strain rates than the rest respond to stress in a brittle
fashion. The process commonly affects the outer

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