- Email: [email protected]
Structures within large volume rhyolite lava flows of the Devonian Comerong Volcanics, southeastern Australia, and the Pleistocene Ngongotaha lava dome, New Zealand
Journal 21"Volcanology and Geothermal Research, 54 ( 1992 ) 33-51
33
Elsevier Science Publishers B.V., Amsterdam
Structures within large volume rhyolite lava flows of the Devonian Comerong Volcanics, southeastern Australia, and the Pleistocene Ngongotaha lava dome, New Zealand Kelsie A. Dadd Department of Applied Geology, University of Technology, Broadway, P.O. Box 123. Sydney, N.S. 14~ 2007, Australia (Received March 3, 1992; revised version accepted May 18, 1992 )
ABSTRACT Dadd, K.A., 1992. Structures within large volume rhyolite lava flows of the Devonian Comerong Volcanics, southeastern Australia, and the Pleistocene Ngongotaha lava dome, New Zealand. J. Volcanol. Geotherm. Res., 54:33-51. Many rhyolitic units within the Late Devonian Comerong Volcanics erupted as lava flows and domes, some up to 18 km long and 350 m thick. The textural and structural characteristics that distinguish the flows and domes as lava rather than rheomorphic ignimbrite include unbroken phenocrysts, zones of autobrecciation, and finely developed flow layering with individual layers continuous for several metres. The flow layering is typically contorted into isoclinal folds with forms suggesting fluidal deformation and is interlayered with and gradational into restricted zones of pumice-rich lapilli tuff and zones of lenticulite breccia. The lenticulite breccia comprises discontinuous, lenticular rhyolite fragments, the long axes of which define a foliation parallel to flow layering. Lenticles in the breccia vary from elongate layers up to 1 m long and several millimetres thick to short fragments less than 10 cm long and several cenlimetres wide. Similar zones of lenticulite breccia consisting of glassy lenticular clasts in a devitrified, spherulitic "'matrix" of cristobalite and albite, exist within the Late Pleistocene Ngongotaha dome near Rotorua, New Zealand. The lenticulite breccia is considered to form by aqueous diffusion and selective devitrification of the rhyolite along anastomosing fluid paths and to be modified by mechanical fracturing of the lava in a zone of high shear stress. Geochemically the rhyolites are high-Si and A-type, with high Zr and Y contents indicating that they formed from hightemperature, relatively anhydrous, F-rich melts. A-type granitoids crop out intermittently along the length of. and adjacent to the volcanic complex and are comagmatic with the rhyolite. The high temperature, low bubble and phenocryst content, and a high eruptive rate of the rhyolite, likely resulted in a low effective viscosity and extensive flow units.
Introduction The Late Devonian Comerong Volcanics (Fig. 1 ) provide an excellent opportunity to study the evolution o f a bimodal volcanic field (Dadd, 1992). The volcanic and sedimentary rocks that constitute the Comerong Volcanics crop out as two thin belts approximately 1 km wide on the eastern and western limbs of the Budawang Synclinorium, a structural subdiviCorrespondence to: K.A. Dadd, Department of Applied Geology, University of Technology, Broadway, P.O. Box 123, Sydney, N.S.W. 2007, Australia.
0377-0273/92/$05.00
sion of the Lachlan Fold Belt in southeastern Australia. Within this structure the sequence is exposed from its base, an unconformity with Ordovician metasedimentary rocks (Powell, 1983), to its top, where it is interbedded with fluvial siliciclastic sedimentary rocks of the overlying Merrimbula Group. Two thick units of rhyolite of enigmatic origin crop out on the eastern limb of the synclinorium (Fig. 2). Measured sections through the rhyolite units are as thick as 350 m, and outcrop can be traced for over 50 km along strike (Dadd, 1989). The lateral extent of single flows is unknown due to the discontinuous
© 1992 Elsevier Science Publishers B.V. All rights reserved.
34
r,.A.D~DD
150
"E
\ , K~'~
South Wales
~_
f
J
SYDNEY/
(..... •
TN
NOWR .( •
)
"
•
STUDY AREA
35.5"S
.
" x ' ~ EDEN
0
5
10
20 km
A ~ y NS ~-~
Permian- Triassic cover LateDevonian Merfimbula Group M i d - Late .Devonian
~/
//
ma basic Inwusives Mid - Late Devonian Comerong Volcanics
Geological boundary; inferred Fault
Axisof synclinorium
~
granitoid
EarlyDevonian Granitoid intrusives Ordovician basement
Fig. 1. Location of the Comerong Volcanics, southeastern Australia, and the area of detailed study. Points labelled 1-3 indicate the position of columns in Fig. 2.
nature of the outcrop, but thickness variations along strike and variations in structure within the flows susgest that the units consist of several overlapping flows, some as long as t 8 kin. Estimating volumes for individual flows or for the package as a whole is difficult, because the east-west dimensions of flow units cannot be measured.
The Comerong rhyolites have the distribution and aspect ratio of an ash flow tuff but show features typical of siticic lava flows (Dadd, 1986). The origin of lava-like silicic units of great areal extent is controversial (e.g., Ekren et al., 1984; Twist and Harmer, 1986; Bonnichsen and Kauffman, 1987; Henry et al., 1988, 1989); such units are interpreted as
RHYOLITELAVAFLOWSOF THE COMERONGVOLCAN1CSAND THE NGONGOTAHALAVADOME
35
v v v v v ,
, s s s ~ . , s J i s . s ~ s l .
upper mafic unit
e ~ s s s .
upper rhyolite lava unit middle mafic unit ' s ~ s s ~
lacustrine and fluvial sedimentary rocks felsic tuff and lapilli tuff lower rhyolitic lava unit , ~ ¢ s ¢ , s ¢ ¢ ¢ , ¢ ¢ ¢ s
lower mafic unit
t'7.7.7.2'
basal sedimentary breccia unit
~EEZY' I
I
covered areas
U
N
f
200 100
metres
0
Fig. 2. Representative stratigraphic columns for the eastern limb of the Budawang Synclinorium. The position of columns is shown in Fig. 1. The upper rhyolite unit in column 3 is illustrated in detail in Fig. 8.
either ash flow tufts that have undergone extreme rheomorphism or hot and unusually lowviscosity lava flows. Rheomorphic ignimbrites are deposited as pyroclastic flows and undergo mass flowage either directly after deposition (Schmincke and Swanson, 1967; Noble, 1968; Walker and Swanson, 1968; Chapin and Lowell, 1979) or in the final stages of deposition (Hoover, 1964; Ragan and Sheridan, 1972; Wolff and Wright, 1981 ) due to their high temperature, low viscosity and deposition on steep slopes. As a result of this secondary flowage, the ignimbrite develops characteristics similar to those of a silicic lava flow and may be virtually indistinguishable. The original pyroclastic textures of a rheomorphic tuff may be preserved within the basal poorly welded to non-welded zone (Ek-
ren et al., 1984). In cases of extreme rheomorphism however, when the temperature of ejecta is high enough to allow it to coalesce and flow, even the basal zone may be modified to a lava-like structure and be indistinguishable from a cohesive flow. Similarly, extensive silicic lava flows may result from extremely high eruption temperatures leading to flows of low viscosity and causing the structure and texture of the lava flow to diverge from that of a high aspect ratio flow. An unequivocal set of parameters to distinguish tufts modified by extreme rheomorphism from extensive lava flows has not yet been developed from modern well-exposed examples; the origin of such rhyolite units in ancient sequences will even more often be ambiguous.
310
Structural and textural features of the extensive Comerong rhyolites that suggest a lavaflow origin include contorted and planar flow layering, unbroken phenocrysts and few (typically < 1%) lithic fragments. Sections of the rhyolite units, however, have a clastic structure defined by elongate lenticular fragments that resemble the flattened pumice clasts of a rheomorphic ignimbrite and cause some confusion in assigning an origin to the rhyolite units. Similar breccia, comprising lenticular glassy rhyolite fragments in a devitrified matrix, is found in the silicic Ngongotaha lava dome near Rotorua, New Zealand. The breccia is interlayered with and grades into zones of flow-layered rhyolite. The lenticular fragments resemble flattened pumice but occur within the lava dome. This lenticulite breccia structure, previously undocumented in silicic lava flows, provides an analogue for the breccia within the Comerong rhyolite. In this paper I describe the large rhyolite units of the Comerong Volcanics, including their meso- and microscale textural and structural characteristics, and suggest two possible origins for the formation of the lenticulite breccia. The rhyolite units are interpreted as several extensive lava flows and smaller flows and domes based on the lack of pyroclastic textures. The bases of flows however, are not well exposed and so their origin is equivocal.
Rhyolite lavas of the Comerong Volcanics: meso- and microstructure The rhyolite lava flows of the Comerong Volcanics exhibit a range of structural types in which variation in field and microstructural characteristics are inferred to be the result of several physical parameters, including the magma viscosity, volatile content and degree of devitrification and crystallization.
Massive structural type The massive structural type consists of sparsely porphyritic rhyolite with approxi-
L.A I) a, D D
mately 5% phenocrysts of quartz, albite, minor alkali feldspar and altered mica in a groundmass with poikilocrystic or "snowflake" texture (Anderson, 1969; Lofgren, 1971). Quartz and feldspar phenocrysts are typically unbroken, and many are well rounded through resorption. The poikilocrystic texture comprises spherical, very fine-grained, optically continuous quartz patches with inclusions of alkali feldspar, surrounded by darker-coloured areas of albite and orthoclase. Poikilocrysts are up to 1 m m in diameter and have a mottled texture, as the included mineral grains are irregular in shape.
Flow-layered structural type This is the most widespread structural type, and many flows consist of a thick sequence of flow-layered rhyolite with scattered brecciated zones. Flow layering is defined by colour (Fig. 3), with lighter layers having larger poikilocrysts than darker layers, and by a parallel preferred parting plane in the outcrop. Also parallel to the flow layering are fine, l mm wide, quartz veinlets and irregular to lenticular areas of fine intergrown needles of feldspar and quartz in a poikilocrystic groundmass. Individual layers range from 1 m m to several centimetres in width, averaging 1-2 mm. Flow layering varies from planar and constant in orientation to open folds or complexly deformed structures, commonly with refolded isoclinal folds suggesting fluidal deformation. The fold axes in some areas show little consistency, and zones of complex folding are sandwiched between zones of planar layering. Individual layers, only millimetres thick, are in places continuous for several metres.
Lenticulite breccia Lenticulite breccia, in which discontinuous, lenticular fragments define a foliation, is always spatially associated with the flow-layered structure. Lenticles vary from elongate layers
RHYOLITE LAVA FLOWS OF THE COMERONG VOLCANICS AND THE NGONGOTAHA LAVA DOME
37
Fig. 3. Rhyolite outcrop in the Comerong Volcanics showing flow banding defined by a colour variation in outcrop. View taken through water.
Fig. 4. Rhyolite outcrop in the Comerong Volcanics showing long thin lenticles intermediate in structure between flow banding and stubby lenticles.
Fig. 5. In situ lenticulite breccia in the Comerong Volcanics with stubby, black lenticles which stand out as prominent features on the outcrop surface.
Fig. 6. Photomicrograph of elongate lenticles. The lenticles have an axiolitic rim surrounding an intergrowth of coarsegrained quartz and feldspar. The surrounding "matrix" consists of micropoikilitic quartz grains with a flow-banded structure parallel to the elongate lenticles. Under crossed polarizers. Bar scale: 1 ram.
RHYOLITE LAVA FLOWS OF THE COMERONG VOLCAN1CS AND THE NGONGOTAHA LAVA DOME
up to 1 m long and several millimetres thick (Fig. 4 ) to short fragments less than 10 cm long and several centimetres thick (Fig. 5 ). There are two broad groups of lenticles, elongate and stubby. Both groups contain up to 5% euhedral to rounded quartz and feldspar phenocrysts, as in the groundmass. The stubby lenticles are darker and more resistant than the surrounding rhyolite, and are defined by areas of large micropoikilitic quartz grains. Elongate lenticles are commonly recessive features and consist of a rim of axiolitic spherulites with a central region of coarse-grained quartz and feldspar and large spherulites. Many of the lenticles are surrounded by a dark halo (Fig. 6), consisting presumably of material excluded during spherulite crystallization as shown by the experiments of Lofgren ( 1971 ). These lenticles typically are more altered, as indicated by the presence of white mica and chlorite, than the surrounding rhyolite. The coarse-grained intergrowth of quartz and feldspar within the lenticles suggests a decrease in volume during devitrification of the glassy rhyolite, creating a
39
low-pressure area and promoting the growth of vapour-phase minerals at these sites.
Brecciated structural type Brecciation, a c o m m o n feature of all other structural types, ranges from closely spaced fractures where there has been little m o v e m e n t of the clasts to zones of angular rhyolite clasts in a jumbled block arrangement. A c o m m o n structure is a "jigsaw puzzle" arrangement of clasts with rhyolite of a similar colour and texture as the cementing material. This structure may result from the autobrecciation of a partially solidified or devitrified rhyolite lava. The zones of brecciation are both concordant and discordant with layering in the outcrop and vary from microscale zones to areas of several square metres. Contacts between the brecciated and unbrecciated rhyolite vary from sharp to gradational. The breccia cement is typically similar in composition to the clasts but in some cases consists of silica (Fig. 7). Brecciated zones occur throughout the flow
Fig. 7. Rhyolite breccia in the Comerong Volcanics. Angular clasts of rhyolite in a "jigsaw puzzle" arrangement with a siliceous cement. View taken through water.
4f~
units and are not preferentially concentrated at any one level in the flow.
Pumice-rich lapilli tuff The pumice-rich lapilli tuff is associated with the flow-layered structure, and contacts between the two are often irregular. The lapilli tuff has a restricted distribution, occurring in only two adjacent sections. Layers of the tuff have a maximum thickness of approximately 50 m. Clasts within the tuffrange from several millimetres to 15 cm in diameter, averaging 23 cm, are irregular in shape, randomly distributed and constitute up to 50% of the outcrop. The clasts are amygdaloidal and are possibly pumice. A few clasts contain spherulites and were once glassy. Pumice is often associated with rhyolite lava flows (Bristow and Duncan, 1983; Fink, 1983; Fink and Manley, 1987; Heiken and Wohletz, 1987 ), either as precursor pumice eruptions or as frothy vesicular layers within the flow. The individual pumice clasts within the tuff are more typical of a near vent air-fall pumice deposit. Basal section of lava flows The basal part of each lava flow is covered or poorly exposed with a thick cover of lichen and moss. At one place (Fig. 8), the contact is covered for 2.5 m between the top of the underlying basalt unit and the rhyolite. In the basal metre of exposed rhyolite the unit is brecciated and contains vesicular basalt fragments and patches and angular clasts of jasper. These accidental clasts may have been incorporated into the flow from the palaeo-ground surface. One metre above the exposed base, the rhyolite has a poorly developed foliation defined by an alignment of sparse feldspar, laths. A faint, wavy colour layering is visible on some surfaces. The unit grades vertically into flowlayered rhyolite. In the second section, the rhyolite is exposed for 5 m above the contact with a bedded silt-
~ • DAI)I>
stone sequence. The basal section of the rhyolite comprises a 5-m-thick, polymictic breccia with a non-porphyritic rhyolite matrix. Clasts in the breccia include rhyolite, similar to the matrix, as well as siltstone, basalt, and banded porphyritic rhyolite. Clasts are mostly angular except those close in composition to the matrix, which have diffuse edges. They are in a close-packed arrangement with clasts up to 30 cm in width. The next 3 m of outcrop consists of a matrix-supported breccia in which the clasts are almost entirely porphyritic rhyolite and have diffuse contacts with the matrix. The unit grades upward into an intermingling of porphyritic rhyolite and the non-porphyritic rhyolite that formed the matrix to the breccia. Extent of individual lava flows Vertical continuity in mapped sections is good, but the lateral extent of individual flows is difficult to establish due to the uniform texture and phenocryst content of the rhyolite and to breaks in outcrop along strike. No flows can be physically traced between measured sections, which are typically separated by several kilometres of poor outcrop. Abrupt structural and lithological changes occur between some measured sections. Rhyolite flows tend to be steep sided, so many of these changes are likely depositional features. However in some cases, the boundaries between different lithological units correlate with Landsat and air photo lineaments and are probably fault controlled. The chemical composition of the rhyolite, proved unreliable as a tool for distinguishing separate flows. The scatter of elemental abundances, due largely to alteration, now masks any primary differences in flow units. Much of the rhyolite outcrop occupies the same stratigraphic position in adjacent sections (Fig. 2 ) and can be divided into two main eruptive episodes, the last being most voluminous. The upper unit can be correlated for approximately 50 km along strike, 18 km of which is so similar in texture and flow layering ori-
RHYOLITE LAVAFLOWS OF THE COMERONG VOLCANICS AND THE NGONGOTAHA LAVADOME
41
metres
560t -~-'~---~,,,~_
520-~ ~ghz~
480 - o o , = .
BST
Thundereggs up to 15 cm in diameter in top metre. Very open folds.
BST/
Auto-brecciated bands with no systematic orientation; open to isoclinal folds.
BX / Light-coloured lenticles and lithic clasts up to 2 cm; BST [ clasts decrease in abundance upward. LB
440-~ BST 400-
~
Large black lenticles up to 5 x 1 cm. Spherulitic light-coloured lenticles up to 10 x 3 cm; rounded 'pumiceous' fragments. Covered section. Alternating buff and purple bands; very open folds; some lighter coloured bands are spherulitic and lenticular in shape.
JBlack LB BX LB
320- % o ~ o'~
lenticles up to 20 x 2 cm; no grading.
JOccasional
'pumiceous-looking' clasts up to 3 cm.
Black lenticles; av. 3 x 0.5 cm. Light coloured lenticles up to 20 x 1.5 cm; normally graded.
_ *oo=
-
~,
BST/ BX
Fine flow banding; brecciated.
LB
Light coloured lenticles up to 10 x 1 cm; open folds.
BST
Open folds.
LB
Discontinuous, light coloured spherulitic streaks up to 0.5 cm wide in section view; spherulitic surfaces in plan view.
280- ,=,,=, ~ 0'~0
240
-
A streaky, indented lineation and alignment of phenocrysts; planar.
200I
r
a
-
-
77S
BST Colurrmar jointing perpendicular to flow banding.
16o- _---L
A streaky, indented lineation and alignment of phenocrysts; planar. MST
Faint colour banding; folds with wavelengths 5 to 10 cm.
~AAA &AAA A&A~
BST/ BX Indented parting planes; grades from underlying wispy structure.
80 . . . . .
Dark-coloured wisps and streaks. Bands <1 to 3 cm wide. BST
BX Polymictic breccia with rhyolite and vesicular basalt fragments. 0 Fig. 8. S t r a t i g r a p h i c c o l u m n s h o w i n g the u p p e r rhyolite unit on t h e e a s t e r n limb o f t h e B u d a w a n g S y n c l i n o r i u m . T h e c o l u m n illustrates the r e l a t i o n s h i p b e t w e e n c o n t i n u o u s f l o w - b a n d e d rhyolite a n d in situ lenticulite breccia. B S T = b a n d e d structural type; B X = b r e c c i a t e d structural type; LB = lenticulite breccia; M S T = m a s s i v e structural type.
42
K.L I)ADD
entation that it may represent a single, large rhyolite flow. Rhyolite lava flows o f this scale occur elsewhere, for example, the Plateau Rhyolite o f the Yellowstone caldera (Wyoming, U.S.A.; Christiansen and Blank, 1972), where some flows are as much as 300 m thick and extend for 20 km from their source. The origin of some larger silicic volcanic units is a matter of debate due to the similarities of rhyolite lava flows with rheomorphic ignimbrites. In ancient volcanic provinces the distinction between rheomorphic ignimbrites
~-
~
|
~-_._.~
_
~--
_
l _ _ _ _
.,~
-
~
_
--
~
and large volume lavas could be especially unclear due to the effects of devitrification, recrystallization and tectonic disruption of the units (cf. Allen, 1988). The large extent of the Comerong rhyolites and the enigmatic lenticulite breccia they contain are features similar to rheomorphic ignimbrites. R h e o m o r p h i c ignimbrites do occur elsewhere in the Comerong Volcanics and show significant differences to the lava flows in the study area, including recognizable fiamme, abundant lithic clasts (Fig. 9a), lithic-rich horizons and a discontinuous
:-I
. . . . . .
Fig. 9. (a) Foliation developed in rheomorphic ignimbrite in the Comerong Votcanics, showing flattened pumice and lithic ctasts (solid black). Bar scale is 5 cm. (b) Lineation developed on foliation surface of outcrop in (a), defined by elongate, flattened pumice. Note the discontinuous nature of the lineation. Bar scale is 5 cm. (c) Lineation developed on flow-banding surface in a rhyolite lava flow. Note the continuous nature of the lineation and absence of lithic clasts.
RHYOLITE LAVA FLOWS OF THE COMERONG VOLCANICS AND THE NGONGOTAHA LAVA DOME
lineation developed on the foliation plane (Fig. 9b). In contrast, the rhyolite lavas have continuous flow layering, an interlayered and gradational relationship between the lenticulite breccia and flow-layered rhyolite, few lithics and a more continuous lineation developed on the flow layering surface (Fig. 9c). Many rheomorphic ignimbrites in the TransPecos, Texas, U.S.A. (Henry et al., 1988 ) have a poorly welded to non-welded base in which pumice or ash textures are still visible. Above this zone the unit grades into more lava-like features. No ash-rich basal zones have been recognised in the Comerong rhyolites; howe'~er, such bases may be covered or obscured due to devitrification.
43
Comparison with modern lava flows: Ngongotaha lava dome and flow, New Zealand The structures developed in the rhyolite units of the Comerong Volcanics are similar to those in the Pleistocene Ngongotaha lava dome, near Rotorua, New Zealand (Fig. 10). The lava dome, situated on the edge of Rotorua Caldera, is approximately 4 km in diameter. It is older than the 50,000 yr B.P. rise of the lake level and younger than 140,000 yr B.P. (B. Houghton, pers. commun., 1988). The flowlayered character and local brecciated areas are excellently exposed in a quarry on the northeastern face of the dome. Flow-layered structures
Controls on the viscosity of the Comerong rhyolites The Comerong rhyolite lavas are predominantly alkalic and high SiO2. Major elements show a broad range of abundances likely due to alteration. Y, Zr and Nb abundances are high with values intermediate to those of peralkalic (Mahood, 1981 ) and calc-alkalic (Ewart et al., 1968; Hildreth, 1979) rhyolite. They are most likely comagmatic with A-type, alkalic granites suggesting that the melt was high temperature (possibly greater than 830 ° C ), anhydrous and had a high F content (Collins et al., 1982; Clemens et al., 1986; Whalen et al., 1987). A high F content leads to a low viscosity (Dingwell et al., 1985 ), particularly in high-silica melts and thus may have been an important control on viscosity in the high-Si Comerong rhyolites. Both an increase in bubble content and an increase in the abundance and size of crystals in a melt lead to an decrease in fluidity (McBirney and Murase, 1984). The Comerong rhyolites have a low crystal content and small crystals. The lack of lithophysal cavities and vesicular horizons in the flows, except for the rare pumice-rich lapilli tuff, suggests the magma may also have had a low bubble content.
Flow layering in the Ngongotaha lava dome is defined by colour variation and by a change from obsidian to light-coloured devitrified rhyolite consisting of cristobalite and albite. Layers are continuous for several metres, are commonly folded and have wispy, discontinuous and interfingering contacts. Elongate lithophysae and layers rich in lithophysae occur parallel to the colour layering. Some lithophysae, however, are wrapped by the foliation, so that they developed late in the formation of the layering while the rhyolite could still behave plastically. Brecciated structures The flow layering in the lava dome is brecciated in several areas. These include a brecciated carapace; internal areas discordant with the flow layering in the dome, in which large blocks of the rhyolite are in a jumbled arrangement, and areas of brecciation, concordant with the trend of layering in the dome, in which fragments are lenticular and lie within the plane parallel to flow layering. In this paper this breccia type is called lenticulite breccia. The first two breccia types are well known in lava flows and domes. In the lenticulite brec-
x.~
44
l).~,[ ~[:,
rr~
L,2
52{I I)O)N r
and alluvial deposits t
/
Lake Rotorua
~ [ Rotoiti Breccia Te Wairoa Breccia ~k Huka Group Haparaagi Rhyolite Mamaku Ignimbrite Haparangi Rhyolite
Ngongotaha
Mokoa Island Geological boundary
f
Fault
"J /
ROTORUA
Volcanic centre and caldera margin
km 0 r "' IIII "
I IIJ
~/
/
5 ]
500 000N Fig. 10. Location of the Ngongotaha lava dome and flow near Rotorua, New Zealand. Simplified geology taken from Thompson (1974).
cia, lenticular to wispy segments of obsidian are contained within a light-coloured, devitrified rhyolite that constitutes a "matrix". The obsidian clasts have both feathered and planar ends (Figs. 11 and 12). They range from massive with sparse spherulites and perlitic cracking to clasts "veined" with devitrified rhyolite and having a wispy texture. The lenticular clasts range from 1 m m to approximately 2 cm thick and are up to 10 cm long in the samples studied. The flow-layered and lenticulite breccia structures occur together within t h e Ngongotaha dome, grading vertically one from the other in a similar style to that developed in the Comerong lava flows.
Microscalefeatures The lenticular, obsidian clasts are typically flow layered, and many exhibit microfolding with a fluidal appearance (Fig. 13a). Phenocrysts make up less than 5% of all structural types in the Ngongotaha lava dome and are typically unbroken. Fragmental phenocrysts are more abundant in areas with a lenticulite breccia structure, occurring in trains concentrated at the boundary between breccia and continuous flow layering (Fig. 13b and c). Devitrification of the rhyolite commenced parallel to layering and in apparently random spherical structures. Although large devitrified
RHYOLITE LAVA FLOWS OF THE COMERONG VOLCANICS AND THE NGONGOTAHA LAVA DOME
45
Discussion
2-1 Fig. 11. A zone of in situ lenticulite breccia in the Ngongotaha Dome, bounded by flow-banded rhyolite and with gradational contacts. The contact between breccia and flow-banded rhyolite is indicated by a dashed line. Individual glassy clasts in the breccia are highlighted in solid black. Bar scale is 5 cm.
patches in some samples have developed independent of, and obscuring, the flow layering, the devitrification more typically enhances the layered and lenticulite breccia structure.
Many clastic structures in rhyolite lava flows and domes result from a continual change in the rheology of the moving mass. As part of the rhyolite devitrifies sufficiently to become brittle it behaves differently from the remaining ductile rhyolite. With continued movement, the brittle component reaches its yield point and fractures to form breccia. At any stage an increase in temperature, possibly caused by thermal feedback within the flow (Nelson, 1981 ), can change the behaviour of the lava mass and the breccia can be re-cemented. At the top and sides of the flow, where cooling is more rapid than the interior, a brecciated carapace develops. Many other breccia structures are produced by: ( 1 ) contact with groundwater and subsequent water movement through the lava; and (2) water expansion on heating, especially that trapped in pores (Heiken and Wohletz, 1987). Within the interior of the moving lava mass during laminar flow, shear produces a folia-
Fig. 12. Black lenticular clasts in the Ngongotaha lenticulite breccia are glassy and contain large light-coloured spherulites. The surrounding white-coloured rhyolite is totally devitrified. The black glassy clasts resemble flattened pumice.
46
~,.a, 1)4I ~i)
Fig. 13. Photomicrographs of lenticulite breccia from the Ngongotaha dome. Bar scales: 1 m m . ( a ) Clasts have both planar, banded structures and folded flow banding (plane-polarized light). (b) and (c). A train of broken phenocrysts occurs along the contact of the lenticulite breccia domain (lower part of photo) and a cohesive flow-banded domain. The phenocrysts in the flow-banded domain are unbroken. (b) Plane-polarized light. (c) Under crossed polarizers.
RHYOLITE LAVA FLOWS OF THE COMERONG VOLCANICS AND THE NGONGOTAHA LAVA DOME
47
Fig. 13. (Continued).
tion. This foliation is typically d e f n e d by colour variation and enhanced by devitrification structures. The formation oflenticulite breccia involves sections of flow layering within this portion of the flow.
Textural changes accompanying devitrification The flow-layered rhyolite and fragments in the lenticulite breccia both show microscale colour layering, folding of the flow layering and a similar percentage of phenocrysts, suggesting they form initially by the same processes. The few spherulites found in the obsidian layers and fragments of the Ngongotaha samples are larger than those in the devitrified "matrix"; thus a completely devitrified lava of similar composition should contain areas or layers of more coarse-grained devitrification textures. Similarly, in the Comerong lavas, the micropoikilitic quartz grains and spherulites in the lenticulite breccia fragments are larger than
those in the surrounding devitrified rhyolite, suggesting by analogy that the fragments remained glassy while the surrounding rhyolite devitrified. After some cooling of the rhyolite mass, the glassy lenticles devitrified at a lower temperature (Lofgren, 1971). The textural differences between the stubby and elongate lenticles in the Comerong rhyolite therefore probably reflect variation in the rate and temperature of devitrification, volume changes accompanying devitrification, and post-devitrification alteration. Features similar to these lenticles are described as early formed, planar lithophysal cavities by Hausback (1987) in a Miocene rhyodacite lava flow in Baja California, Mexico. The cavities were shear planes active in the last stages of laminar flow and were later filled with vapour-phase minerals. The lenticular zones in the Comerong flows, however, contain isolated phenocrysts, a feature inconsistent with a lithophysal origin.
The role of water Lenticulite breccia in the Ngongotaha dome is interlayered with flow-layered rhyolite. These layers may develop different textures due to water and other volatile movement within the lava mass, which would reduce the viscosity and shear strength of the lava. Water may diffuse from a lava flow or dome on cooling and decompression (Heiken and Wohletz, 1987 ). The water would be at higher pressure than the atmosphere and may cause fracturing and explosion of the lava mass (Heiken and Wohletz, 1987). Two cases of diffusion that Heiken and Wohletz ( 1987 ) consider are molecular transport of one material through another, and mixing of lava with groundwater in the vent during extrusion. Apart from the obvious role of water pressure in producing a brecciated structure, the first of these processes may be important in the development of flow-layered and 1. Development of cracks in the rhyolite due to shear during flow. Volatiles begin to move through the rhyolite.
~
[
'~ "~ 1 -~ ~" ~ ~ -~ "~ "~'~" ~ I"~ -~ ~, [--~ "-~ ~, ~
~ ~ I~,~""~-'1 ~
2. With continued volatile movement along interconnecting fractures, the rhyolite selectively devitrifies. 3. The pattern of devitrified zones (volatile movement paths) and undevitrified glassy zones, produces a "pseudobreccia" texture.
Fig. 14. Anastomosing volatile movement paths producing a "pseudobreccia" texture.
lenticulite breccia structure, by migration of magmatic volatiles from the lava to fracture planes, vesicles and lithophysae (Heiken and Wohletz, 1987) accompanied by chemical alteration of the medium. Lofgren ( 1971 ) suggested that a rhyolitic glass devitrifies most rapidly in the presence of alkali-rich solutions and that these solutions might occur locally within a cooling rhyolitic magma. Movement of such fluids in a glassy rhyolite mass, along fractures parallel to the flow layering, may lead to accelerated alteration and devitrification of selected layers and the development of a "pseudobreccia structure" (cf. Allen, 1988) by isolating lenticles of fresh obsidian where fluid paths anastomose (Fig. 14 ).
Models for the formation of lenticulite breccia The lenticulite breccia is interlayered with continuously flow-layered rhyolite within the non-pumiceous section of both large and small rhyolite lava flows and domes. Two possible models for the formation of the lenticulite breccia are outlined. The breccia may have formed solely by one mechanism or by a combination of the two. ( 1 ) Brecciation of flow layering may have formed by drawing out and breaking of plastic glassy layers with low shear strength, due to an increase in the shear stresses at particular lev-
A
B
C
Fig. 15. The stretching and brecciation of flow-banded rhyolite in a plastic state to produce the lenticular segments of the lenticulite breccia structure. (A) Banding is developed due to shear in the moving rhyolite flow. Bands are both glassy and partially devitrified leading to differences in rheological behaviour. (B) Stretching of bands during flow produces a boudinage structure while the rhyolite behaves in a plastic manner.(C) The more glassy bands with a lower yield strength than the devitrified bands break to form lenticular segments.
RHYOLITE LAVA FLOWS OF THE COMERONG VOLCANICS AND THE NGONGOTAHA LAVA DOME
els in the rhyolite mass (Fig. 15 ). This process is similar to the development of shear zones, as they are both planar zones of concentrated deformation that help to accommodate a local strain rate the country rock cannot accommodate by bulk deformation (White et al., 1980). The shear zone develops parallel to the major heterogeneity in the rock, that is, the flow layering, which itself is produced by shear during flow of the lava mass. Obsidian zones or layers deform plastically and with a low shear strength during flow until they reach their yield point and fracture. Clasts in the lenticulite breccia have both high- and low-angle terminations relative to flow layering and suggest a range in the ductility & t h e clasts prior to failure (White et al., 1980). The development of isolated zones of lenticulite breccia or shear zones within the rhyolite mass may be due to an increase in water content or water movement in these zones. The presence of a pore fluid can both reduce the strength of a rock (Fyfe et al., 1978) and increase the rate of devitrification, thus creating a heterogeneity and promoting the development of a shear layer. Alternatively, the breccia horizon may develop because of an increased strain rate due to pulsing flow rate or some heterogeneity in the flow of the lava. Features of the lenticulite breccia that support a brecciation model include sharp terminations of clasts (Fig. 13a), rotation of clasts within the breccia zones, the termination of clast fabric at the edges of breccia zones (Fig. 13a), and the concentration of broken phenocrysts within the breccia zones indicating movement. The "trains" of phenocryst fragments at the contact of the breccia zones and cohesive rhyolite (Fig. 13b) suggest that the contact is the site of a high strain rate. (2) The second model for the development of lenticulite breccia is the formation of a "pseudobreccia" by a process of aqueous diffusion and selective devitrification. Clasts in this model are not formed by mechanical fracturing of the lava. Features that support this
49
process, as outlined above and in Figure 14, include the juxtaposition of clasts with similar internal fabrics, and the gradation of devitrification within clasts in the Ngongotaha lenticulite breccia from glassy, through partially devitrified to totally devitrified "matrix". The gradation may reflect the extent of aqueous diffusion and acceleration of devitrification processes in the glass. Following the development of a lenticulite breccia horizon by this second process, the zone could be modified by shear, as it would act as a more heterogeneous layer than the flow foliation and hence concentrate strain. Conclusions The Comerong rhyolites erupted as lava with several large flow units as well as smaller domelike bodies and flows. A typical section through a flow unit consists of continuously flow-layered rhyolite with planar flow layering interlayered with lenticulite breccia and rare pumice-rich lapilli tuff. The layering in a few localities is contorted into fluidal flow folds and cut by auto-brecciated zones. The lenticulite breccia consists of clasts of rhyolitic composition similar to the host rhyolite but differing in devitrification texture and degree of alteration. They may have formed by aqueous diffusion and selective devitrification of the rhyolite with anastomosing fluid paths and have been modified by mechanical fracturing in a zone of high shear stress. Lenticulite breccia and flattened pumice lenticles could be easily confused in devitrified rhyolite, but the Comerong rhyolite lavas lack other features diagnostic of ignimbrites. The similarity of the lenticulite breccia in the Comerong rhyolites with those in the Ngongotaha lava dome indicates a lava-flow origin. The Comerong rhyolites typically have a low crystal content, very few lithic clasts and few lithophysal cavities. Their distinctive A-type chemistry and high temperatures, possibly accompanied by the rapid extrusion of the rhyo-
lite, led to a low effective viscosity of the magma and to large eruptive units. Acknowledgements This research was supported by an Australian Commonwealth Postgraduate Research Award. I would like to thank R.H. Flood, C. Henry, D.A. Swanson and an anonymous reviewer for critical appraisal of the manuscript. The time and effort given by C. Henry and J. Wolff in assistance with many aspects of this paper is greatly appreciated. References Allen, R.L., 1988. False pyroclastic textures in altered silicic lavas, with implications for volcanic-associated mineralization. Econ. Geol., 83: 1424-1446. Anderson, Jr., J.E., 1969. Development of snowflake texture in a welded tuff, Davis Mountain, Texas. Geol. Soc. Am. Bull., 80: 2075-2080. Bonnichsen, B. and Kauffman, D.F., 1987. Physical features of rhyolite lava flows in the Snake River Plain volcanic province, southwestern Idaho. In: J.H. Fink (Editor), The Emplacement of Silicic Domes and Lava Flows. Geol. Soc. Am., Spec. Pap., 212:119-145. Bristow, J.W. and Duncan, A.R., 1983, Rhyolite dome formation and plinian activity in the Bumbeni Complex, Southern Lebombo. Trans. Geol. Soc. S. Afr., 86: 273-279. Chapin, C.E. and Lowell, G.R., 1979. Primary and secondary flow structures in ash flow tufts of the Gribbles Run palaeovalley, central Colorado. Geol. Soc. Am., Spec. Pap., 180:137-154. Christiansen, R.L. and Blank, Jr., H.R., 1972. Volcanic stratigraphy of the Quaternary Rhyolite Plateau in Yellowstone National Park. U.S. Geol. Surv., Prof. Pap. 729-B, 18 pp. Clemens, J.D., Holloway, J.R. and White, A.J.R., 1986. Origin of an A-type granite: Experimental constraints. Am. Mineral., 71: 317-324. Collins, W.J., Beams, S.D., White, A.J.R. and Chappell, B.W., 1982. Nature and origin of A-type granites with particular reference to southeastern Australia. Contrib. Mineral. Petrol., 80: 189-200. Dadd, K.A., 1986. A look at some rhyolite lavas of the Comerong Volcanics, southeastern N.S.W., Australia. Int. Volcanol. Congr., New Zealand, Abstr., p. 143. Dadd, K.A., 1989. The stratigraphy, volcanic evolution and tectonic setting of the Comerong Volcanics, Australia. Ph.D. Thesis, Macquarie University, Sydney, N.S.W. (unpublished).
Dadd, K.A., 1992. The Middle to Late Devonmn EdenComerong-Yalwal Volcanic Zone of southeastern Australia: an ancient analogue of the YellowstoneSnake River Plain region of the U.S.A. In: ~.k. Fergusson and R.A. Glen (Editors), The Palaeozoic Eastern Margin of Gondwanaland: Tectonics of the Lachlan Fold Belt, Southeastern Australia and Related Orolgens. Tectonophysics, 214:277-291. Dingwell, D.B., Scarfe, C.M. and Cronin, D.J., 1985. The effect of fluorine on viscosities in the system NazOA1203-SIO2: Implications for phonolites, trachytes and rhyolites. Am. Mineral., 70: 80-87. Ekren, E.B., Mclntyre, D.H. and Bennett, E.H., 1984. High-temperature, large volume, lavalike ash-flow luffs without calderas in southwestern Idaho. U.S. Geol. Surv., Prof. Pap. 1272, 76 pp. Ewart, A., Taylor, S.R. and Capp, A.C., 1968. Trace and minor element geochemistry of the rhyolitic volcanic rocks, central North Island, New Zealand. Contrib. Mineral. Petrol., 18: 76-104. Fink, J.H., 1983. Structure and emplacement of a rhyolite obsidian flow: Little Glass Mountain, Medicine Lake Highland, northern California. Geol. Soc. Am. Bull., 94: 362-380. Fink, J.H. and Manley, C.R., 1987. Origin of pumiceous and glassy textures in rhyolite flows and domes. In: J.H. Fink (Editor), The Emplacement of Silicic Domes and Lava Flows. Geol. Soc. Am., Spec. Pap., 212: 77-88. Fyfe. W.S., Price, N.S. and Thompson, A.B., 1978. Fluids in the Earth's Crust. Elsevier, Amsterdam, 384 pp. Hausback, B.P.. 1987. An extensive, hot, vapour-charged rhyodacite flow, Baja California, Mexico. In: J.H. Fink (Editor), The Emplacement of Siticic Domes and Lava Flows. Geol. Soc. Am., Spec. Pap., 212:111-118. Heiken, G. and Wohletz, K., 1987. Tephra deposits associated with silicic domes and lava flows. In: J.H. Fink (Editor), The Emplacement of Silicic Domes and Lava Flows. Geol. Soc. Am., Spec. Pap., 212: 55-76. Henry, C.D., Price, J.G., Rubin, J.N., Parker, D.F., Wolff, J.A., Sell, S., Franklin, R. and Barker, D.S., 1988. Widespread, lavalike silicic volcanic rocks of TransPecos Texas. Geology, 16:509-513. Henry, C.D., Price, J.G., Parker, D.F. and Wolff, J.A., 1989. Mid-Tertiary silicic alkalic magmatism of TransPecos Texas: Rheomorphic luffs and extensive silicic lavas. N.M. Bur. Mines Miner. Resour., Mem.~ 46: 231-274. Hildreth, E.W., 1979. The Bishop Tuff: Evidence for the origin of compositional zonation in silicic magma chambers. In: C.E. Chapin and W.E. Elston (Editors), Ash-flow Tuffs. Geol. Soc. Am.. Spec. Pap., 180: 4376. Hoover, D.L., 1964. Flow structures in a welded tuff, Nye County, Nevada. Geol. Soc. Am., Spec. Pap., 76: 83. Lofgren, G.E., 1971. Experimentally produced devitrification textures in natural rhyolite glass. Geol. Soc. Am. Bull.. 82:111-123.
RHYOLITE LAVA FLOWS OF THE COMERONG VOLCANICS AND THE NGONGOTAHA LAVA DOME
Mahood, G.A., 1981. Chemical evolution of a Pleistocene rhyolitic center: Sierra La Primavera, Jalisco, Mexico. Contrib. Mineral. Petrol., 77: 129-149. McBirney, A.R. and Murase, T., 1984. Rheological properties of magmas. Annu. Rev. Earth Planet. Sci., 12: 337-357. Nelson, S.A., 1981. The possible role of thermal feedback in the eruption of siliceous magmas. J. Volcanol. Geotherm. Res., 11: 127-137. Noble, D.C., 1968. Laminar viscous flowage structures in ash flow tuff from Gran Canaria Islands: A discussion. J. Geol., 16: 721-723. Powell, C.McA., 1983. Geology of the N.S.W. south coast. S.G.T.S.G. Field Guide, 1, Geol. Soc. Aust. Ragan, D.M. and Sheridan, M.F., 1972. Compaction of the Bishop Tuff. Geol. Soc. Am. Bull., 83: 95-106. Schmincke, H.U. and Swanson, D.A., 1967. Laminar viscous flowage structures in ash-flow tufts from Gran Canaria, Canary Islands. J. Geol., 75: 641-664. Thompson, B.N., 1974. Geology of the Rotorua Geother-
51
mal District. In: Geothermal Resource Survey, Rotorua Geothermal District. Dep. Sci. Ind. Res. (N.Z.) Geotherm. Rep. No. 6, pp. 10-36. Twist, D. and Harmer, R.E., 1986. Voluminous rheoignimbrites in the Bushveld Complex: field relations and petrogenesis. Int. Volcanol. Congr., New Zealand, Abstr., p.83. Walker, G.W. and Swanson, D.A., 1968. Laminar flowage in a Pliocene soda rhyolite ash-flow tuff, Lake and Harney counties, Ore. U.S. Geol. Surv., Prof. Pap.. 600B: 37-47. Whalen, J.B., Currie, K.L. and Chappell, B.W., 1987. Atype granites: geochemical characteristics, discrimination and petrogenesis. Contrib. Mineral. Petrol., 95: 407-419. White, S.H.. Burrows, S.E., Carreras, J., Shaw, N.D. and Humphreys, F.J., 1980. On mylonites in ductile shear zones. J. Struct. Geol., 2: 175-187. Wolff, J.A. and Wright, J.V., 1981. Rheomorphism or welded tufts. J. Volcanol. Geotherm. Res., 10:13-34.
33
Elsevier Science Publishers B.V., Amsterdam
Structures within large volume rhyolite lava flows of the Devonian Comerong Volcanics, southeastern Australia, and the Pleistocene Ngongotaha lava dome, New Zealand Kelsie A. Dadd Department of Applied Geology, University of Technology, Broadway, P.O. Box 123. Sydney, N.S. 14~ 2007, Australia (Received March 3, 1992; revised version accepted May 18, 1992 )
ABSTRACT Dadd, K.A., 1992. Structures within large volume rhyolite lava flows of the Devonian Comerong Volcanics, southeastern Australia, and the Pleistocene Ngongotaha lava dome, New Zealand. J. Volcanol. Geotherm. Res., 54:33-51. Many rhyolitic units within the Late Devonian Comerong Volcanics erupted as lava flows and domes, some up to 18 km long and 350 m thick. The textural and structural characteristics that distinguish the flows and domes as lava rather than rheomorphic ignimbrite include unbroken phenocrysts, zones of autobrecciation, and finely developed flow layering with individual layers continuous for several metres. The flow layering is typically contorted into isoclinal folds with forms suggesting fluidal deformation and is interlayered with and gradational into restricted zones of pumice-rich lapilli tuff and zones of lenticulite breccia. The lenticulite breccia comprises discontinuous, lenticular rhyolite fragments, the long axes of which define a foliation parallel to flow layering. Lenticles in the breccia vary from elongate layers up to 1 m long and several millimetres thick to short fragments less than 10 cm long and several cenlimetres wide. Similar zones of lenticulite breccia consisting of glassy lenticular clasts in a devitrified, spherulitic "'matrix" of cristobalite and albite, exist within the Late Pleistocene Ngongotaha dome near Rotorua, New Zealand. The lenticulite breccia is considered to form by aqueous diffusion and selective devitrification of the rhyolite along anastomosing fluid paths and to be modified by mechanical fracturing of the lava in a zone of high shear stress. Geochemically the rhyolites are high-Si and A-type, with high Zr and Y contents indicating that they formed from hightemperature, relatively anhydrous, F-rich melts. A-type granitoids crop out intermittently along the length of. and adjacent to the volcanic complex and are comagmatic with the rhyolite. The high temperature, low bubble and phenocryst content, and a high eruptive rate of the rhyolite, likely resulted in a low effective viscosity and extensive flow units.
Introduction The Late Devonian Comerong Volcanics (Fig. 1 ) provide an excellent opportunity to study the evolution o f a bimodal volcanic field (Dadd, 1992). The volcanic and sedimentary rocks that constitute the Comerong Volcanics crop out as two thin belts approximately 1 km wide on the eastern and western limbs of the Budawang Synclinorium, a structural subdiviCorrespondence to: K.A. Dadd, Department of Applied Geology, University of Technology, Broadway, P.O. Box 123, Sydney, N.S.W. 2007, Australia.
0377-0273/92/$05.00
sion of the Lachlan Fold Belt in southeastern Australia. Within this structure the sequence is exposed from its base, an unconformity with Ordovician metasedimentary rocks (Powell, 1983), to its top, where it is interbedded with fluvial siliciclastic sedimentary rocks of the overlying Merrimbula Group. Two thick units of rhyolite of enigmatic origin crop out on the eastern limb of the synclinorium (Fig. 2). Measured sections through the rhyolite units are as thick as 350 m, and outcrop can be traced for over 50 km along strike (Dadd, 1989). The lateral extent of single flows is unknown due to the discontinuous
© 1992 Elsevier Science Publishers B.V. All rights reserved.
34
r,.A.D~DD
150
"E
\ , K~'~
South Wales
~_
f
J
SYDNEY/
(..... •
TN
NOWR .( •
)
"
•
STUDY AREA
35.5"S
.
" x ' ~ EDEN
0
5
10
20 km
A ~ y NS ~-~
Permian- Triassic cover LateDevonian Merfimbula Group M i d - Late .Devonian
~/
//
ma basic Inwusives Mid - Late Devonian Comerong Volcanics
Geological boundary; inferred Fault
Axisof synclinorium
~
granitoid
EarlyDevonian Granitoid intrusives Ordovician basement
Fig. 1. Location of the Comerong Volcanics, southeastern Australia, and the area of detailed study. Points labelled 1-3 indicate the position of columns in Fig. 2.
nature of the outcrop, but thickness variations along strike and variations in structure within the flows susgest that the units consist of several overlapping flows, some as long as t 8 kin. Estimating volumes for individual flows or for the package as a whole is difficult, because the east-west dimensions of flow units cannot be measured.
The Comerong rhyolites have the distribution and aspect ratio of an ash flow tuff but show features typical of siticic lava flows (Dadd, 1986). The origin of lava-like silicic units of great areal extent is controversial (e.g., Ekren et al., 1984; Twist and Harmer, 1986; Bonnichsen and Kauffman, 1987; Henry et al., 1988, 1989); such units are interpreted as
RHYOLITELAVAFLOWSOF THE COMERONGVOLCAN1CSAND THE NGONGOTAHALAVADOME
35
v v v v v ,
, s s s ~ . , s J i s . s ~ s l .
upper mafic unit
e ~ s s s .
upper rhyolite lava unit middle mafic unit ' s ~ s s ~
lacustrine and fluvial sedimentary rocks felsic tuff and lapilli tuff lower rhyolitic lava unit , ~ ¢ s ¢ , s ¢ ¢ ¢ , ¢ ¢ ¢ s
lower mafic unit
t'7.7.7.2'
basal sedimentary breccia unit
~EEZY' I
I
covered areas
U
N
f
200 100
metres
0
Fig. 2. Representative stratigraphic columns for the eastern limb of the Budawang Synclinorium. The position of columns is shown in Fig. 1. The upper rhyolite unit in column 3 is illustrated in detail in Fig. 8.
either ash flow tufts that have undergone extreme rheomorphism or hot and unusually lowviscosity lava flows. Rheomorphic ignimbrites are deposited as pyroclastic flows and undergo mass flowage either directly after deposition (Schmincke and Swanson, 1967; Noble, 1968; Walker and Swanson, 1968; Chapin and Lowell, 1979) or in the final stages of deposition (Hoover, 1964; Ragan and Sheridan, 1972; Wolff and Wright, 1981 ) due to their high temperature, low viscosity and deposition on steep slopes. As a result of this secondary flowage, the ignimbrite develops characteristics similar to those of a silicic lava flow and may be virtually indistinguishable. The original pyroclastic textures of a rheomorphic tuff may be preserved within the basal poorly welded to non-welded zone (Ek-
ren et al., 1984). In cases of extreme rheomorphism however, when the temperature of ejecta is high enough to allow it to coalesce and flow, even the basal zone may be modified to a lava-like structure and be indistinguishable from a cohesive flow. Similarly, extensive silicic lava flows may result from extremely high eruption temperatures leading to flows of low viscosity and causing the structure and texture of the lava flow to diverge from that of a high aspect ratio flow. An unequivocal set of parameters to distinguish tufts modified by extreme rheomorphism from extensive lava flows has not yet been developed from modern well-exposed examples; the origin of such rhyolite units in ancient sequences will even more often be ambiguous.
310
Structural and textural features of the extensive Comerong rhyolites that suggest a lavaflow origin include contorted and planar flow layering, unbroken phenocrysts and few (typically < 1%) lithic fragments. Sections of the rhyolite units, however, have a clastic structure defined by elongate lenticular fragments that resemble the flattened pumice clasts of a rheomorphic ignimbrite and cause some confusion in assigning an origin to the rhyolite units. Similar breccia, comprising lenticular glassy rhyolite fragments in a devitrified matrix, is found in the silicic Ngongotaha lava dome near Rotorua, New Zealand. The breccia is interlayered with and grades into zones of flow-layered rhyolite. The lenticular fragments resemble flattened pumice but occur within the lava dome. This lenticulite breccia structure, previously undocumented in silicic lava flows, provides an analogue for the breccia within the Comerong rhyolite. In this paper I describe the large rhyolite units of the Comerong Volcanics, including their meso- and microscale textural and structural characteristics, and suggest two possible origins for the formation of the lenticulite breccia. The rhyolite units are interpreted as several extensive lava flows and smaller flows and domes based on the lack of pyroclastic textures. The bases of flows however, are not well exposed and so their origin is equivocal.
Rhyolite lavas of the Comerong Volcanics: meso- and microstructure The rhyolite lava flows of the Comerong Volcanics exhibit a range of structural types in which variation in field and microstructural characteristics are inferred to be the result of several physical parameters, including the magma viscosity, volatile content and degree of devitrification and crystallization.
Massive structural type The massive structural type consists of sparsely porphyritic rhyolite with approxi-
L.A I) a, D D
mately 5% phenocrysts of quartz, albite, minor alkali feldspar and altered mica in a groundmass with poikilocrystic or "snowflake" texture (Anderson, 1969; Lofgren, 1971). Quartz and feldspar phenocrysts are typically unbroken, and many are well rounded through resorption. The poikilocrystic texture comprises spherical, very fine-grained, optically continuous quartz patches with inclusions of alkali feldspar, surrounded by darker-coloured areas of albite and orthoclase. Poikilocrysts are up to 1 m m in diameter and have a mottled texture, as the included mineral grains are irregular in shape.
Flow-layered structural type This is the most widespread structural type, and many flows consist of a thick sequence of flow-layered rhyolite with scattered brecciated zones. Flow layering is defined by colour (Fig. 3), with lighter layers having larger poikilocrysts than darker layers, and by a parallel preferred parting plane in the outcrop. Also parallel to the flow layering are fine, l mm wide, quartz veinlets and irregular to lenticular areas of fine intergrown needles of feldspar and quartz in a poikilocrystic groundmass. Individual layers range from 1 m m to several centimetres in width, averaging 1-2 mm. Flow layering varies from planar and constant in orientation to open folds or complexly deformed structures, commonly with refolded isoclinal folds suggesting fluidal deformation. The fold axes in some areas show little consistency, and zones of complex folding are sandwiched between zones of planar layering. Individual layers, only millimetres thick, are in places continuous for several metres.
Lenticulite breccia Lenticulite breccia, in which discontinuous, lenticular fragments define a foliation, is always spatially associated with the flow-layered structure. Lenticles vary from elongate layers
RHYOLITE LAVA FLOWS OF THE COMERONG VOLCANICS AND THE NGONGOTAHA LAVA DOME
37
Fig. 3. Rhyolite outcrop in the Comerong Volcanics showing flow banding defined by a colour variation in outcrop. View taken through water.
Fig. 4. Rhyolite outcrop in the Comerong Volcanics showing long thin lenticles intermediate in structure between flow banding and stubby lenticles.
Fig. 5. In situ lenticulite breccia in the Comerong Volcanics with stubby, black lenticles which stand out as prominent features on the outcrop surface.
Fig. 6. Photomicrograph of elongate lenticles. The lenticles have an axiolitic rim surrounding an intergrowth of coarsegrained quartz and feldspar. The surrounding "matrix" consists of micropoikilitic quartz grains with a flow-banded structure parallel to the elongate lenticles. Under crossed polarizers. Bar scale: 1 ram.
RHYOLITE LAVA FLOWS OF THE COMERONG VOLCAN1CS AND THE NGONGOTAHA LAVA DOME
up to 1 m long and several millimetres thick (Fig. 4 ) to short fragments less than 10 cm long and several centimetres thick (Fig. 5 ). There are two broad groups of lenticles, elongate and stubby. Both groups contain up to 5% euhedral to rounded quartz and feldspar phenocrysts, as in the groundmass. The stubby lenticles are darker and more resistant than the surrounding rhyolite, and are defined by areas of large micropoikilitic quartz grains. Elongate lenticles are commonly recessive features and consist of a rim of axiolitic spherulites with a central region of coarse-grained quartz and feldspar and large spherulites. Many of the lenticles are surrounded by a dark halo (Fig. 6), consisting presumably of material excluded during spherulite crystallization as shown by the experiments of Lofgren ( 1971 ). These lenticles typically are more altered, as indicated by the presence of white mica and chlorite, than the surrounding rhyolite. The coarse-grained intergrowth of quartz and feldspar within the lenticles suggests a decrease in volume during devitrification of the glassy rhyolite, creating a
39
low-pressure area and promoting the growth of vapour-phase minerals at these sites.
Brecciated structural type Brecciation, a c o m m o n feature of all other structural types, ranges from closely spaced fractures where there has been little m o v e m e n t of the clasts to zones of angular rhyolite clasts in a jumbled block arrangement. A c o m m o n structure is a "jigsaw puzzle" arrangement of clasts with rhyolite of a similar colour and texture as the cementing material. This structure may result from the autobrecciation of a partially solidified or devitrified rhyolite lava. The zones of brecciation are both concordant and discordant with layering in the outcrop and vary from microscale zones to areas of several square metres. Contacts between the brecciated and unbrecciated rhyolite vary from sharp to gradational. The breccia cement is typically similar in composition to the clasts but in some cases consists of silica (Fig. 7). Brecciated zones occur throughout the flow
Fig. 7. Rhyolite breccia in the Comerong Volcanics. Angular clasts of rhyolite in a "jigsaw puzzle" arrangement with a siliceous cement. View taken through water.
4f~
units and are not preferentially concentrated at any one level in the flow.
Pumice-rich lapilli tuff The pumice-rich lapilli tuff is associated with the flow-layered structure, and contacts between the two are often irregular. The lapilli tuff has a restricted distribution, occurring in only two adjacent sections. Layers of the tuff have a maximum thickness of approximately 50 m. Clasts within the tuffrange from several millimetres to 15 cm in diameter, averaging 23 cm, are irregular in shape, randomly distributed and constitute up to 50% of the outcrop. The clasts are amygdaloidal and are possibly pumice. A few clasts contain spherulites and were once glassy. Pumice is often associated with rhyolite lava flows (Bristow and Duncan, 1983; Fink, 1983; Fink and Manley, 1987; Heiken and Wohletz, 1987 ), either as precursor pumice eruptions or as frothy vesicular layers within the flow. The individual pumice clasts within the tuff are more typical of a near vent air-fall pumice deposit. Basal section of lava flows The basal part of each lava flow is covered or poorly exposed with a thick cover of lichen and moss. At one place (Fig. 8), the contact is covered for 2.5 m between the top of the underlying basalt unit and the rhyolite. In the basal metre of exposed rhyolite the unit is brecciated and contains vesicular basalt fragments and patches and angular clasts of jasper. These accidental clasts may have been incorporated into the flow from the palaeo-ground surface. One metre above the exposed base, the rhyolite has a poorly developed foliation defined by an alignment of sparse feldspar, laths. A faint, wavy colour layering is visible on some surfaces. The unit grades vertically into flowlayered rhyolite. In the second section, the rhyolite is exposed for 5 m above the contact with a bedded silt-
~ • DAI)I>
stone sequence. The basal section of the rhyolite comprises a 5-m-thick, polymictic breccia with a non-porphyritic rhyolite matrix. Clasts in the breccia include rhyolite, similar to the matrix, as well as siltstone, basalt, and banded porphyritic rhyolite. Clasts are mostly angular except those close in composition to the matrix, which have diffuse edges. They are in a close-packed arrangement with clasts up to 30 cm in width. The next 3 m of outcrop consists of a matrix-supported breccia in which the clasts are almost entirely porphyritic rhyolite and have diffuse contacts with the matrix. The unit grades upward into an intermingling of porphyritic rhyolite and the non-porphyritic rhyolite that formed the matrix to the breccia. Extent of individual lava flows Vertical continuity in mapped sections is good, but the lateral extent of individual flows is difficult to establish due to the uniform texture and phenocryst content of the rhyolite and to breaks in outcrop along strike. No flows can be physically traced between measured sections, which are typically separated by several kilometres of poor outcrop. Abrupt structural and lithological changes occur between some measured sections. Rhyolite flows tend to be steep sided, so many of these changes are likely depositional features. However in some cases, the boundaries between different lithological units correlate with Landsat and air photo lineaments and are probably fault controlled. The chemical composition of the rhyolite, proved unreliable as a tool for distinguishing separate flows. The scatter of elemental abundances, due largely to alteration, now masks any primary differences in flow units. Much of the rhyolite outcrop occupies the same stratigraphic position in adjacent sections (Fig. 2 ) and can be divided into two main eruptive episodes, the last being most voluminous. The upper unit can be correlated for approximately 50 km along strike, 18 km of which is so similar in texture and flow layering ori-
RHYOLITE LAVAFLOWS OF THE COMERONG VOLCANICS AND THE NGONGOTAHA LAVADOME
41
metres
560t -~-'~---~,,,~_
520-~ ~ghz~
480 - o o , = .
BST
Thundereggs up to 15 cm in diameter in top metre. Very open folds.
BST/
Auto-brecciated bands with no systematic orientation; open to isoclinal folds.
BX / Light-coloured lenticles and lithic clasts up to 2 cm; BST [ clasts decrease in abundance upward. LB
440-~ BST 400-
~
Large black lenticles up to 5 x 1 cm. Spherulitic light-coloured lenticles up to 10 x 3 cm; rounded 'pumiceous' fragments. Covered section. Alternating buff and purple bands; very open folds; some lighter coloured bands are spherulitic and lenticular in shape.
JBlack LB BX LB
320- % o ~ o'~
lenticles up to 20 x 2 cm; no grading.
JOccasional
'pumiceous-looking' clasts up to 3 cm.
Black lenticles; av. 3 x 0.5 cm. Light coloured lenticles up to 20 x 1.5 cm; normally graded.
_ *oo=
-
~,
BST/ BX
Fine flow banding; brecciated.
LB
Light coloured lenticles up to 10 x 1 cm; open folds.
BST
Open folds.
LB
Discontinuous, light coloured spherulitic streaks up to 0.5 cm wide in section view; spherulitic surfaces in plan view.
280- ,=,,=, ~ 0'~0
240
-
A streaky, indented lineation and alignment of phenocrysts; planar.
200I
r
a
-
-
77S
BST Colurrmar jointing perpendicular to flow banding.
16o- _---L
A streaky, indented lineation and alignment of phenocrysts; planar. MST
Faint colour banding; folds with wavelengths 5 to 10 cm.
~AAA &AAA A&A~
BST/ BX Indented parting planes; grades from underlying wispy structure.
80 . . . . .
Dark-coloured wisps and streaks. Bands <1 to 3 cm wide. BST
BX Polymictic breccia with rhyolite and vesicular basalt fragments. 0 Fig. 8. S t r a t i g r a p h i c c o l u m n s h o w i n g the u p p e r rhyolite unit on t h e e a s t e r n limb o f t h e B u d a w a n g S y n c l i n o r i u m . T h e c o l u m n illustrates the r e l a t i o n s h i p b e t w e e n c o n t i n u o u s f l o w - b a n d e d rhyolite a n d in situ lenticulite breccia. B S T = b a n d e d structural type; B X = b r e c c i a t e d structural type; LB = lenticulite breccia; M S T = m a s s i v e structural type.
42
K.L I)ADD
entation that it may represent a single, large rhyolite flow. Rhyolite lava flows o f this scale occur elsewhere, for example, the Plateau Rhyolite o f the Yellowstone caldera (Wyoming, U.S.A.; Christiansen and Blank, 1972), where some flows are as much as 300 m thick and extend for 20 km from their source. The origin of some larger silicic volcanic units is a matter of debate due to the similarities of rhyolite lava flows with rheomorphic ignimbrites. In ancient volcanic provinces the distinction between rheomorphic ignimbrites
~-
~
|
~-_._.~
_
~--
_
l _ _ _ _
.,~
-
~
_
--
~
and large volume lavas could be especially unclear due to the effects of devitrification, recrystallization and tectonic disruption of the units (cf. Allen, 1988). The large extent of the Comerong rhyolites and the enigmatic lenticulite breccia they contain are features similar to rheomorphic ignimbrites. R h e o m o r p h i c ignimbrites do occur elsewhere in the Comerong Volcanics and show significant differences to the lava flows in the study area, including recognizable fiamme, abundant lithic clasts (Fig. 9a), lithic-rich horizons and a discontinuous
:-I
. . . . . .
Fig. 9. (a) Foliation developed in rheomorphic ignimbrite in the Comerong Votcanics, showing flattened pumice and lithic ctasts (solid black). Bar scale is 5 cm. (b) Lineation developed on foliation surface of outcrop in (a), defined by elongate, flattened pumice. Note the discontinuous nature of the lineation. Bar scale is 5 cm. (c) Lineation developed on flow-banding surface in a rhyolite lava flow. Note the continuous nature of the lineation and absence of lithic clasts.
RHYOLITE LAVA FLOWS OF THE COMERONG VOLCANICS AND THE NGONGOTAHA LAVA DOME
lineation developed on the foliation plane (Fig. 9b). In contrast, the rhyolite lavas have continuous flow layering, an interlayered and gradational relationship between the lenticulite breccia and flow-layered rhyolite, few lithics and a more continuous lineation developed on the flow layering surface (Fig. 9c). Many rheomorphic ignimbrites in the TransPecos, Texas, U.S.A. (Henry et al., 1988 ) have a poorly welded to non-welded base in which pumice or ash textures are still visible. Above this zone the unit grades into more lava-like features. No ash-rich basal zones have been recognised in the Comerong rhyolites; howe'~er, such bases may be covered or obscured due to devitrification.
43
Comparison with modern lava flows: Ngongotaha lava dome and flow, New Zealand The structures developed in the rhyolite units of the Comerong Volcanics are similar to those in the Pleistocene Ngongotaha lava dome, near Rotorua, New Zealand (Fig. 10). The lava dome, situated on the edge of Rotorua Caldera, is approximately 4 km in diameter. It is older than the 50,000 yr B.P. rise of the lake level and younger than 140,000 yr B.P. (B. Houghton, pers. commun., 1988). The flowlayered character and local brecciated areas are excellently exposed in a quarry on the northeastern face of the dome. Flow-layered structures
Controls on the viscosity of the Comerong rhyolites The Comerong rhyolite lavas are predominantly alkalic and high SiO2. Major elements show a broad range of abundances likely due to alteration. Y, Zr and Nb abundances are high with values intermediate to those of peralkalic (Mahood, 1981 ) and calc-alkalic (Ewart et al., 1968; Hildreth, 1979) rhyolite. They are most likely comagmatic with A-type, alkalic granites suggesting that the melt was high temperature (possibly greater than 830 ° C ), anhydrous and had a high F content (Collins et al., 1982; Clemens et al., 1986; Whalen et al., 1987). A high F content leads to a low viscosity (Dingwell et al., 1985 ), particularly in high-silica melts and thus may have been an important control on viscosity in the high-Si Comerong rhyolites. Both an increase in bubble content and an increase in the abundance and size of crystals in a melt lead to an decrease in fluidity (McBirney and Murase, 1984). The Comerong rhyolites have a low crystal content and small crystals. The lack of lithophysal cavities and vesicular horizons in the flows, except for the rare pumice-rich lapilli tuff, suggests the magma may also have had a low bubble content.
Flow layering in the Ngongotaha lava dome is defined by colour variation and by a change from obsidian to light-coloured devitrified rhyolite consisting of cristobalite and albite. Layers are continuous for several metres, are commonly folded and have wispy, discontinuous and interfingering contacts. Elongate lithophysae and layers rich in lithophysae occur parallel to the colour layering. Some lithophysae, however, are wrapped by the foliation, so that they developed late in the formation of the layering while the rhyolite could still behave plastically. Brecciated structures The flow layering in the lava dome is brecciated in several areas. These include a brecciated carapace; internal areas discordant with the flow layering in the dome, in which large blocks of the rhyolite are in a jumbled arrangement, and areas of brecciation, concordant with the trend of layering in the dome, in which fragments are lenticular and lie within the plane parallel to flow layering. In this paper this breccia type is called lenticulite breccia. The first two breccia types are well known in lava flows and domes. In the lenticulite brec-
x.~
44
l).~,[ ~[:,
rr~
L,2
52{I I)O)N r
and alluvial deposits t
/
Lake Rotorua
~ [ Rotoiti Breccia Te Wairoa Breccia ~k Huka Group Haparaagi Rhyolite Mamaku Ignimbrite Haparangi Rhyolite
Ngongotaha
Mokoa Island Geological boundary
f
Fault
"J /
ROTORUA
Volcanic centre and caldera margin
km 0 r "' IIII "
I IIJ
~/
/
5 ]
500 000N Fig. 10. Location of the Ngongotaha lava dome and flow near Rotorua, New Zealand. Simplified geology taken from Thompson (1974).
cia, lenticular to wispy segments of obsidian are contained within a light-coloured, devitrified rhyolite that constitutes a "matrix". The obsidian clasts have both feathered and planar ends (Figs. 11 and 12). They range from massive with sparse spherulites and perlitic cracking to clasts "veined" with devitrified rhyolite and having a wispy texture. The lenticular clasts range from 1 m m to approximately 2 cm thick and are up to 10 cm long in the samples studied. The flow-layered and lenticulite breccia structures occur together within t h e Ngongotaha dome, grading vertically one from the other in a similar style to that developed in the Comerong lava flows.
Microscalefeatures The lenticular, obsidian clasts are typically flow layered, and many exhibit microfolding with a fluidal appearance (Fig. 13a). Phenocrysts make up less than 5% of all structural types in the Ngongotaha lava dome and are typically unbroken. Fragmental phenocrysts are more abundant in areas with a lenticulite breccia structure, occurring in trains concentrated at the boundary between breccia and continuous flow layering (Fig. 13b and c). Devitrification of the rhyolite commenced parallel to layering and in apparently random spherical structures. Although large devitrified
RHYOLITE LAVA FLOWS OF THE COMERONG VOLCANICS AND THE NGONGOTAHA LAVA DOME
45
Discussion
2-1 Fig. 11. A zone of in situ lenticulite breccia in the Ngongotaha Dome, bounded by flow-banded rhyolite and with gradational contacts. The contact between breccia and flow-banded rhyolite is indicated by a dashed line. Individual glassy clasts in the breccia are highlighted in solid black. Bar scale is 5 cm.
patches in some samples have developed independent of, and obscuring, the flow layering, the devitrification more typically enhances the layered and lenticulite breccia structure.
Many clastic structures in rhyolite lava flows and domes result from a continual change in the rheology of the moving mass. As part of the rhyolite devitrifies sufficiently to become brittle it behaves differently from the remaining ductile rhyolite. With continued movement, the brittle component reaches its yield point and fractures to form breccia. At any stage an increase in temperature, possibly caused by thermal feedback within the flow (Nelson, 1981 ), can change the behaviour of the lava mass and the breccia can be re-cemented. At the top and sides of the flow, where cooling is more rapid than the interior, a brecciated carapace develops. Many other breccia structures are produced by: ( 1 ) contact with groundwater and subsequent water movement through the lava; and (2) water expansion on heating, especially that trapped in pores (Heiken and Wohletz, 1987). Within the interior of the moving lava mass during laminar flow, shear produces a folia-
Fig. 12. Black lenticular clasts in the Ngongotaha lenticulite breccia are glassy and contain large light-coloured spherulites. The surrounding white-coloured rhyolite is totally devitrified. The black glassy clasts resemble flattened pumice.
46
~,.a, 1)4I ~i)
Fig. 13. Photomicrographs of lenticulite breccia from the Ngongotaha dome. Bar scales: 1 m m . ( a ) Clasts have both planar, banded structures and folded flow banding (plane-polarized light). (b) and (c). A train of broken phenocrysts occurs along the contact of the lenticulite breccia domain (lower part of photo) and a cohesive flow-banded domain. The phenocrysts in the flow-banded domain are unbroken. (b) Plane-polarized light. (c) Under crossed polarizers.
RHYOLITE LAVA FLOWS OF THE COMERONG VOLCANICS AND THE NGONGOTAHA LAVA DOME
47
Fig. 13. (Continued).
tion. This foliation is typically d e f n e d by colour variation and enhanced by devitrification structures. The formation oflenticulite breccia involves sections of flow layering within this portion of the flow.
Textural changes accompanying devitrification The flow-layered rhyolite and fragments in the lenticulite breccia both show microscale colour layering, folding of the flow layering and a similar percentage of phenocrysts, suggesting they form initially by the same processes. The few spherulites found in the obsidian layers and fragments of the Ngongotaha samples are larger than those in the devitrified "matrix"; thus a completely devitrified lava of similar composition should contain areas or layers of more coarse-grained devitrification textures. Similarly, in the Comerong lavas, the micropoikilitic quartz grains and spherulites in the lenticulite breccia fragments are larger than
those in the surrounding devitrified rhyolite, suggesting by analogy that the fragments remained glassy while the surrounding rhyolite devitrified. After some cooling of the rhyolite mass, the glassy lenticles devitrified at a lower temperature (Lofgren, 1971). The textural differences between the stubby and elongate lenticles in the Comerong rhyolite therefore probably reflect variation in the rate and temperature of devitrification, volume changes accompanying devitrification, and post-devitrification alteration. Features similar to these lenticles are described as early formed, planar lithophysal cavities by Hausback (1987) in a Miocene rhyodacite lava flow in Baja California, Mexico. The cavities were shear planes active in the last stages of laminar flow and were later filled with vapour-phase minerals. The lenticular zones in the Comerong flows, however, contain isolated phenocrysts, a feature inconsistent with a lithophysal origin.
The role of water Lenticulite breccia in the Ngongotaha dome is interlayered with flow-layered rhyolite. These layers may develop different textures due to water and other volatile movement within the lava mass, which would reduce the viscosity and shear strength of the lava. Water may diffuse from a lava flow or dome on cooling and decompression (Heiken and Wohletz, 1987 ). The water would be at higher pressure than the atmosphere and may cause fracturing and explosion of the lava mass (Heiken and Wohletz, 1987). Two cases of diffusion that Heiken and Wohletz ( 1987 ) consider are molecular transport of one material through another, and mixing of lava with groundwater in the vent during extrusion. Apart from the obvious role of water pressure in producing a brecciated structure, the first of these processes may be important in the development of flow-layered and 1. Development of cracks in the rhyolite due to shear during flow. Volatiles begin to move through the rhyolite.
~
[
'~ "~ 1 -~ ~" ~ ~ -~ "~ "~'~" ~ I"~ -~ ~, [--~ "-~ ~, ~
~ ~ I~,~""~-'1 ~
2. With continued volatile movement along interconnecting fractures, the rhyolite selectively devitrifies. 3. The pattern of devitrified zones (volatile movement paths) and undevitrified glassy zones, produces a "pseudobreccia" texture.
Fig. 14. Anastomosing volatile movement paths producing a "pseudobreccia" texture.
lenticulite breccia structure, by migration of magmatic volatiles from the lava to fracture planes, vesicles and lithophysae (Heiken and Wohletz, 1987) accompanied by chemical alteration of the medium. Lofgren ( 1971 ) suggested that a rhyolitic glass devitrifies most rapidly in the presence of alkali-rich solutions and that these solutions might occur locally within a cooling rhyolitic magma. Movement of such fluids in a glassy rhyolite mass, along fractures parallel to the flow layering, may lead to accelerated alteration and devitrification of selected layers and the development of a "pseudobreccia structure" (cf. Allen, 1988) by isolating lenticles of fresh obsidian where fluid paths anastomose (Fig. 14 ).
Models for the formation of lenticulite breccia The lenticulite breccia is interlayered with continuously flow-layered rhyolite within the non-pumiceous section of both large and small rhyolite lava flows and domes. Two possible models for the formation of the lenticulite breccia are outlined. The breccia may have formed solely by one mechanism or by a combination of the two. ( 1 ) Brecciation of flow layering may have formed by drawing out and breaking of plastic glassy layers with low shear strength, due to an increase in the shear stresses at particular lev-
A
B
C
Fig. 15. The stretching and brecciation of flow-banded rhyolite in a plastic state to produce the lenticular segments of the lenticulite breccia structure. (A) Banding is developed due to shear in the moving rhyolite flow. Bands are both glassy and partially devitrified leading to differences in rheological behaviour. (B) Stretching of bands during flow produces a boudinage structure while the rhyolite behaves in a plastic manner.(C) The more glassy bands with a lower yield strength than the devitrified bands break to form lenticular segments.
RHYOLITE LAVA FLOWS OF THE COMERONG VOLCANICS AND THE NGONGOTAHA LAVA DOME
els in the rhyolite mass (Fig. 15 ). This process is similar to the development of shear zones, as they are both planar zones of concentrated deformation that help to accommodate a local strain rate the country rock cannot accommodate by bulk deformation (White et al., 1980). The shear zone develops parallel to the major heterogeneity in the rock, that is, the flow layering, which itself is produced by shear during flow of the lava mass. Obsidian zones or layers deform plastically and with a low shear strength during flow until they reach their yield point and fracture. Clasts in the lenticulite breccia have both high- and low-angle terminations relative to flow layering and suggest a range in the ductility & t h e clasts prior to failure (White et al., 1980). The development of isolated zones of lenticulite breccia or shear zones within the rhyolite mass may be due to an increase in water content or water movement in these zones. The presence of a pore fluid can both reduce the strength of a rock (Fyfe et al., 1978) and increase the rate of devitrification, thus creating a heterogeneity and promoting the development of a shear layer. Alternatively, the breccia horizon may develop because of an increased strain rate due to pulsing flow rate or some heterogeneity in the flow of the lava. Features of the lenticulite breccia that support a brecciation model include sharp terminations of clasts (Fig. 13a), rotation of clasts within the breccia zones, the termination of clast fabric at the edges of breccia zones (Fig. 13a), and the concentration of broken phenocrysts within the breccia zones indicating movement. The "trains" of phenocryst fragments at the contact of the breccia zones and cohesive rhyolite (Fig. 13b) suggest that the contact is the site of a high strain rate. (2) The second model for the development of lenticulite breccia is the formation of a "pseudobreccia" by a process of aqueous diffusion and selective devitrification. Clasts in this model are not formed by mechanical fracturing of the lava. Features that support this
49
process, as outlined above and in Figure 14, include the juxtaposition of clasts with similar internal fabrics, and the gradation of devitrification within clasts in the Ngongotaha lenticulite breccia from glassy, through partially devitrified to totally devitrified "matrix". The gradation may reflect the extent of aqueous diffusion and acceleration of devitrification processes in the glass. Following the development of a lenticulite breccia horizon by this second process, the zone could be modified by shear, as it would act as a more heterogeneous layer than the flow foliation and hence concentrate strain. Conclusions The Comerong rhyolites erupted as lava with several large flow units as well as smaller domelike bodies and flows. A typical section through a flow unit consists of continuously flow-layered rhyolite with planar flow layering interlayered with lenticulite breccia and rare pumice-rich lapilli tuff. The layering in a few localities is contorted into fluidal flow folds and cut by auto-brecciated zones. The lenticulite breccia consists of clasts of rhyolitic composition similar to the host rhyolite but differing in devitrification texture and degree of alteration. They may have formed by aqueous diffusion and selective devitrification of the rhyolite with anastomosing fluid paths and have been modified by mechanical fracturing in a zone of high shear stress. Lenticulite breccia and flattened pumice lenticles could be easily confused in devitrified rhyolite, but the Comerong rhyolite lavas lack other features diagnostic of ignimbrites. The similarity of the lenticulite breccia in the Comerong rhyolites with those in the Ngongotaha lava dome indicates a lava-flow origin. The Comerong rhyolites typically have a low crystal content, very few lithic clasts and few lithophysal cavities. Their distinctive A-type chemistry and high temperatures, possibly accompanied by the rapid extrusion of the rhyo-
lite, led to a low effective viscosity of the magma and to large eruptive units. Acknowledgements This research was supported by an Australian Commonwealth Postgraduate Research Award. I would like to thank R.H. Flood, C. Henry, D.A. Swanson and an anonymous reviewer for critical appraisal of the manuscript. The time and effort given by C. Henry and J. Wolff in assistance with many aspects of this paper is greatly appreciated. References Allen, R.L., 1988. False pyroclastic textures in altered silicic lavas, with implications for volcanic-associated mineralization. Econ. Geol., 83: 1424-1446. Anderson, Jr., J.E., 1969. Development of snowflake texture in a welded tuff, Davis Mountain, Texas. Geol. Soc. Am. Bull., 80: 2075-2080. Bonnichsen, B. and Kauffman, D.F., 1987. Physical features of rhyolite lava flows in the Snake River Plain volcanic province, southwestern Idaho. In: J.H. Fink (Editor), The Emplacement of Silicic Domes and Lava Flows. Geol. Soc. Am., Spec. Pap., 212:119-145. Bristow, J.W. and Duncan, A.R., 1983, Rhyolite dome formation and plinian activity in the Bumbeni Complex, Southern Lebombo. Trans. Geol. Soc. S. Afr., 86: 273-279. Chapin, C.E. and Lowell, G.R., 1979. Primary and secondary flow structures in ash flow tufts of the Gribbles Run palaeovalley, central Colorado. Geol. Soc. Am., Spec. Pap., 180:137-154. Christiansen, R.L. and Blank, Jr., H.R., 1972. Volcanic stratigraphy of the Quaternary Rhyolite Plateau in Yellowstone National Park. U.S. Geol. Surv., Prof. Pap. 729-B, 18 pp. Clemens, J.D., Holloway, J.R. and White, A.J.R., 1986. Origin of an A-type granite: Experimental constraints. Am. Mineral., 71: 317-324. Collins, W.J., Beams, S.D., White, A.J.R. and Chappell, B.W., 1982. Nature and origin of A-type granites with particular reference to southeastern Australia. Contrib. Mineral. Petrol., 80: 189-200. Dadd, K.A., 1986. A look at some rhyolite lavas of the Comerong Volcanics, southeastern N.S.W., Australia. Int. Volcanol. Congr., New Zealand, Abstr., p. 143. Dadd, K.A., 1989. The stratigraphy, volcanic evolution and tectonic setting of the Comerong Volcanics, Australia. Ph.D. Thesis, Macquarie University, Sydney, N.S.W. (unpublished).
Dadd, K.A., 1992. The Middle to Late Devonmn EdenComerong-Yalwal Volcanic Zone of southeastern Australia: an ancient analogue of the YellowstoneSnake River Plain region of the U.S.A. In: ~.k. Fergusson and R.A. Glen (Editors), The Palaeozoic Eastern Margin of Gondwanaland: Tectonics of the Lachlan Fold Belt, Southeastern Australia and Related Orolgens. Tectonophysics, 214:277-291. Dingwell, D.B., Scarfe, C.M. and Cronin, D.J., 1985. The effect of fluorine on viscosities in the system NazOA1203-SIO2: Implications for phonolites, trachytes and rhyolites. Am. Mineral., 70: 80-87. Ekren, E.B., Mclntyre, D.H. and Bennett, E.H., 1984. High-temperature, large volume, lavalike ash-flow luffs without calderas in southwestern Idaho. U.S. Geol. Surv., Prof. Pap. 1272, 76 pp. Ewart, A., Taylor, S.R. and Capp, A.C., 1968. Trace and minor element geochemistry of the rhyolitic volcanic rocks, central North Island, New Zealand. Contrib. Mineral. Petrol., 18: 76-104. Fink, J.H., 1983. Structure and emplacement of a rhyolite obsidian flow: Little Glass Mountain, Medicine Lake Highland, northern California. Geol. Soc. Am. Bull., 94: 362-380. Fink, J.H. and Manley, C.R., 1987. Origin of pumiceous and glassy textures in rhyolite flows and domes. In: J.H. Fink (Editor), The Emplacement of Silicic Domes and Lava Flows. Geol. Soc. Am., Spec. Pap., 212: 77-88. Fyfe. W.S., Price, N.S. and Thompson, A.B., 1978. Fluids in the Earth's Crust. Elsevier, Amsterdam, 384 pp. Hausback, B.P.. 1987. An extensive, hot, vapour-charged rhyodacite flow, Baja California, Mexico. In: J.H. Fink (Editor), The Emplacement of Siticic Domes and Lava Flows. Geol. Soc. Am., Spec. Pap., 212:111-118. Heiken, G. and Wohletz, K., 1987. Tephra deposits associated with silicic domes and lava flows. In: J.H. Fink (Editor), The Emplacement of Silicic Domes and Lava Flows. Geol. Soc. Am., Spec. Pap., 212: 55-76. Henry, C.D., Price, J.G., Rubin, J.N., Parker, D.F., Wolff, J.A., Sell, S., Franklin, R. and Barker, D.S., 1988. Widespread, lavalike silicic volcanic rocks of TransPecos Texas. Geology, 16:509-513. Henry, C.D., Price, J.G., Parker, D.F. and Wolff, J.A., 1989. Mid-Tertiary silicic alkalic magmatism of TransPecos Texas: Rheomorphic luffs and extensive silicic lavas. N.M. Bur. Mines Miner. Resour., Mem.~ 46: 231-274. Hildreth, E.W., 1979. The Bishop Tuff: Evidence for the origin of compositional zonation in silicic magma chambers. In: C.E. Chapin and W.E. Elston (Editors), Ash-flow Tuffs. Geol. Soc. Am.. Spec. Pap., 180: 4376. Hoover, D.L., 1964. Flow structures in a welded tuff, Nye County, Nevada. Geol. Soc. Am., Spec. Pap., 76: 83. Lofgren, G.E., 1971. Experimentally produced devitrification textures in natural rhyolite glass. Geol. Soc. Am. Bull.. 82:111-123.
RHYOLITE LAVA FLOWS OF THE COMERONG VOLCANICS AND THE NGONGOTAHA LAVA DOME
Mahood, G.A., 1981. Chemical evolution of a Pleistocene rhyolitic center: Sierra La Primavera, Jalisco, Mexico. Contrib. Mineral. Petrol., 77: 129-149. McBirney, A.R. and Murase, T., 1984. Rheological properties of magmas. Annu. Rev. Earth Planet. Sci., 12: 337-357. Nelson, S.A., 1981. The possible role of thermal feedback in the eruption of siliceous magmas. J. Volcanol. Geotherm. Res., 11: 127-137. Noble, D.C., 1968. Laminar viscous flowage structures in ash flow tuff from Gran Canaria Islands: A discussion. J. Geol., 16: 721-723. Powell, C.McA., 1983. Geology of the N.S.W. south coast. S.G.T.S.G. Field Guide, 1, Geol. Soc. Aust. Ragan, D.M. and Sheridan, M.F., 1972. Compaction of the Bishop Tuff. Geol. Soc. Am. Bull., 83: 95-106. Schmincke, H.U. and Swanson, D.A., 1967. Laminar viscous flowage structures in ash-flow tufts from Gran Canaria, Canary Islands. J. Geol., 75: 641-664. Thompson, B.N., 1974. Geology of the Rotorua Geother-
51
mal District. In: Geothermal Resource Survey, Rotorua Geothermal District. Dep. Sci. Ind. Res. (N.Z.) Geotherm. Rep. No. 6, pp. 10-36. Twist, D. and Harmer, R.E., 1986. Voluminous rheoignimbrites in the Bushveld Complex: field relations and petrogenesis. Int. Volcanol. Congr., New Zealand, Abstr., p.83. Walker, G.W. and Swanson, D.A., 1968. Laminar flowage in a Pliocene soda rhyolite ash-flow tuff, Lake and Harney counties, Ore. U.S. Geol. Surv., Prof. Pap.. 600B: 37-47. Whalen, J.B., Currie, K.L. and Chappell, B.W., 1987. Atype granites: geochemical characteristics, discrimination and petrogenesis. Contrib. Mineral. Petrol., 95: 407-419. White, S.H.. Burrows, S.E., Carreras, J., Shaw, N.D. and Humphreys, F.J., 1980. On mylonites in ductile shear zones. J. Struct. Geol., 2: 175-187. Wolff, J.A. and Wright, J.V., 1981. Rheomorphism or welded tufts. J. Volcanol. Geotherm. Res., 10:13-34.