Subaqueous rhyolite block lavas in the Miocene Ushikiri Formation, Shimane Peninsula, SW Japan

Journal of Volcanology and Geothermal Research, 46 (1991) 241-253

241

Elsevier Science Publishers B.V., Amsterdam

Subaqueous rhyolite block lavas in the Miocene Ushikiri Formation, Shimane Peninsula, SW Japan Kazuhiko Kano a, Keiji Takeuchi a, Takahiro Yamamoto a and Hideo Hoshizumi b a Geology Department, Geological Survey of Japan, 1-3, Higashi 1-chome, Tsukuba, lbaraki 305, Japan b Kyushu Center, Geological Survey of Japan, 1-28, Shiobaru 2-chome, Minami-ku, Fukuoka 815, Japan (Received January 11, 1990; accepted in revised form November 11, 1990)

ABSTRACT Kano, K., Takeuchi, K., Yamamoto, T. and Hoshizumi, H., 1991. Subaqueous rhyolite block lavas in the Miocene Ushikiri Formation, Shimane Peninsula, SW Japan. J. Volcanol. Geotherm. Res., 46: 241-253. A rhyolite mass of the Miocene Ushikiri Formation in the western part of the Shimane Peninsula, SW Japan, is a small subaqueous edifice about 600 m high and 4 km wide, formed at water depths between 200 and 1000 m. It consists mainly of three relatively flat, lava-flow units 50-300 m in maximum thickness, each of which includes lobes and their polyhedral fragments. The lava lobes are poorly to well vesiculated, glassy to microcrystalline and flow-banded and -folded. Compared with mafic pillows, they are large, having thick, quenched and brecciated, glassy crusts because of their high viscosity, surface tension and thermal conductivity. Their surfaces disintegrate into polyhedral fragments and grade into massive volcanic breccia. The massive volcanic breccia composed of the lobe fragments is poorly sorted and covered with stratified volcanic breccia of the same rock type. The rhyolite lavas commonly bifurcate in a manner similar to mafic pillow lavas. However, they are highly silicic with 1-5 vol.% phenocrysts and have elongated vesicles and flow-folds, implying that they were visco-plastic during flowage. Their surface features are similar to those of subaerial block lava. With respect to rheological and morphological features, they are subaqueous equivalents of block lava.

Introduction

Silicic magmas are normally more viscous and have higher thermal conductivities than intermediate to mafic magmas (Shaw, 1963; Friedman et al., 1963; Murase and McBirney, 1973; Fisher and Schmincke, 1984, pp. 51-54 and references therein). This makes it likely for silicic lavas to predominantly form lava domes and piles of their fragments, especially in subaqueous environments (Pichler, 1965) where they are more rapidly chilled than on land. However, several cases are known where subaqueous silicic lavas are fluid enough to form sheet flows (Cas, 1978; de Rosen-Spence et al., 1980) and actively bi0377-0273/91/$03.50

© 1991 - Elsevier Science Publishers B.V.

furcate (Snyder and Fraser, 1963; Bevins and Roach, 1979; de Rosen-Spence et al., 1980; Furnes et al., 1980; Yamagishi and Dimroth, 1985; Yamagishi, 1987). This paper describes additional occurrences of such fluidal, subaqueous silicic lavas. The subaqueous rhyolite lavas described in this paper bifurcate in a manner similar to pillow lava, but are probably subaqueous equivalents of block lava and may be called subaqueous block lava. They are highly silicic with a visco-plastic nature. Their surfaces disintegrate into polyhedral fragments and are covered with their own fragments, similar to subaerial block lava. Classification of subaqueous lava is difficult

242

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K. K A N O

Geologic setting

because of its diverse morphology and behavior. Nevertheless, subaqueous lava can be simply classified in terms of surface features, similarly to the classification of subaerial lava. In fact, there are subaqueous equivalents of pahoehoe lava having pahoehoe surfaces, i.e. pillow lava (Jones, 1986) and abyssal pahoehoe lava (Lonsdale, 1977). Subaqueous pahoehoe lava (Ballard et al., 1979) and subaqueous block lava are probably end members of subaqueous lavas. This classification may have predictive values for the morphology and behavior of subaqueous lava as the surface features should reflect the physical properties of lavas. In this paper, lobes are defined as the unbrecciated or variably brecciated elongate masses that stack to form a part of lava together with their fragments, thus include those lobes, pods and tongues defined by Dimroth et al. (1979).

In the Miocene Ushikiri Formation of the western part of the Shimane Peninsula, SW Japan, rhyolite volcanic rocks overlie subaqueous andesite volcanic rocks and are overlain by a sandstone-mudstone turbidite sequence (Fig. 1). The andesite volcanic rocks in the basal part of the Ushikiri Formation form a composite mass of water-chilled and brecciated lavas, dikes and sills, and minor pillow lavas (Kano et al., 1989). Benthic foraminiferal fossils in the turbidite sequence of the Ushikiri Formation and in the mudstone and shale of the conformably underlying Josoji Formation are of outer neritic to inner bathyal paleo-environments (Nomura, 1986). This implies that the andesite and rhyolite volcanic rocks of the Ushikiri Formation emplaced at water depths between 200 and 1000 m.

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The subaqueous rhyolite volcanic rocks are divided into three units composed of lavas and their fragments, with pumice lapilli tufts between Units 2 and 3 and in the western periphery (Fig. 1). These three units were not mapped in the southeastern part (Fig. 1) because of poor exposure, but are probably 50-300 m in maximum thickness. The units form an edifice about 4 km wide and 600 m high, being flanked by a pumice lapilli tuff (Fig. 2). The summit is relatively flat and the flanks dip 30-40 °. No large feeder dikes nor vents have been found in the mapped area. However, individual rhyolite units increase in thickness toward the summit region, and sulfide ore veins of several meters thick occur in the summit region. Unknown vents are presumably present in or around the summit region. Occurrence of the lavas

Unit 2

This unit can be best observed at localities A and B (Fig. 1). At these localities, a poorly sorted, non-stratified massive volcanic breccia is overlain and underlain by stratified volcanic breccia (Figs. 3 and 4). The thickness is 6-10 m at locality A and 20 m at locality B (Fig. 4), increasing eastward. The massive volcanic breccia (Figs. 4 and 5A) is composed mostly of polyhedral fragments ranging in size from several centimeters to several tens of centimeters, of poorly vesiculated, glassy to microcrystalline rhyolite (vesicle volume = 1-5%) and pumiceous rhy-

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Fig. 3. Geologic columns of unit 2 at localities A and B.

244

K. KANOETAL.

Fig. 4. Occurrence of unit 2 at locality B. A view from west to east. Massive volcanic breccia (M) encloses lobes (1 and 2) and is overlain by stratified volcanic breccia (S). Lobes 1 and 2 are elongated in an E - W direction. Scale bar is about 5 m long.

olite (vesicle volume = 10-30%). The breccia also includes lobes (Fig. 6) and their blocks (Fig. 5B). The major constituent fragments have folded flow bands, and are closely to loosely packed and have the interstices filled with comminuted fragments of the same rock. Lobes are elliptical or somewhat irregularly shaped, 1-15 m in diameter, and petrographically and chemically similar to the fragments of host volcanic breccia. They have a rim of dense, glassy rhyolite 20 cm to 2 m thick, and their cores are flow-banded, more or less vesiculated, glassy to microcrystalline rhyolite (Fig. 6A). Columnar joints are common in the lobe margins and are spaced 5-20 cm normal to the lobe surface. Vesicles up to 3-4 cm in diameter are quite common in the periph-

ery of core, sometimes making the periphery pumiceous, and are commonly lined with or filled with chalcedony and quartz. They are often elongated parallel to the 0.5-15-cmthick flow bands. Some lobes are disintegrated in situ with the glassy rhyolite rims brecciated into variably sized angular fragments and the interstices filled with shattered pumiceous rhyolite of the cores (Fig. 6B). Brecciation of the glassy rhyolite rims probably resulted from rapid in situ thermal contraction, differential movement of the lobe and simultaneous intrusion of inner pumiceous rhyolite into the brecciated part. Well preserved lobes have sharp boundaries with the host volcanic breccia, whereas

SUBAQUEOUSRHYOLITEBLOCKLAVASIN THE MIOCENEUSHIKIRIFORMATION,JAPAN

245

Fig. 5. Volcanic breccias of unit 2 at locality B. A. Massive volcanic breccia. Lens cap diameter is 6 cm. B. Large discoid block (L) in the massive volcanic breccia (M) lies parallel to the overlying volcanic breccia beds (S). H a m m e r indicated by an arrow is 30 cm long. C. Volcanic breccia bed 3 m thick.

246

K. KANO ET AL.

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disintegrated lobes grade into the breccia. These lobes are elongated mainly in an E W direction (Fig. 4). Large blocks of flow-banded, flow-folded and poorly vesiculated glassy to microcrystalline rhyolite, 1-5 m in long diameter, occur in the massive volcanic breccia (Fig. 5B). They are petrologically similar to the cores of lobes, and are probably the remains of disin-

tegrated lobes. Discoid blocks of lobes in the upper part of massive volcanic breccia often occur parallel to the overlying volcanic breccia beds (Fig. 5B). These large blocks probably slid down from disintegrated lobes to their present positions. The volcanic breccias underlying and overlying the massive volcanic breccia are stratified in beds of 10 cm to 6 m thick (Figs. 3,

247

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4 and 5C). The major constituent fragments (more than 60% in volume) are several centimeters to a few tens of centimeters in diameter, angular to subangular, and appear to be petrographically similar to the fragments of the massive volcanic breccia. Subrounded to rounded microcrystalline rhyolite blocks, possibly derived from Unit 1 rhyolite, also occur in the breccia. Individual volcanic breccia beds are massive or normally graded, and are overlain by thin layers of sand-sized debris. The underlying and overlying stratified volcanic breccias have a gradational top and a sharp base, respectively.

sharp contact; (4) stratified and foreset bedded volcanic breccia which overlies the massive volcanic breccia with a sharp base. The massive bodies are 1-2 m thick and 110 m wide, and tabular or elliptical (Fig. 9A). They appear to extend from SE to NW (Fig. 7) and are more or less fractured at their

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Unit 3 overlies a pumice lapilli tuff and underlies a sandstone-mudstone turbidite sequence (Fig. 1). This unit can be best observed at locality C (Figs. 7 and 8). At this locality, it includes four rhyolite lithofacies (Fig. 9): (1) unbrecciated or partially brecciated massive body which is microcrystalline and locally flow-banded with 1-2 vol.% vesicles of 1-2 mm in diameter and elongated parallel to the flow bands; (2) in situ volcanic breccia which grades from and encloses the massive body; (3) poorly sorted, non-stratified massive volcanic breccia which surrounds the in situ volcanic breccia with a gradational to

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248

K. KANO ET AL.

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Fig. 9. Lithofacies organization of Unit 3 at locality C. A. Unbrecciated mass (U) grades into in situ volcanic breccia (I) and is covered with massive volcanic breccia (M). Hammer is 30 cm long. B. Schematic facies organization.

margin. The marginal fractures, being filled with the comminuted fragments, are mainly normal or parallel to the outlines of massive bodies, and polygonally run with widths of 5-80 cm. Blocks separated by the fractures are often connected with adjacent blocks by narrow necks, and are variably rounded but with a rough surface (Fig. 10A). Interconnected, rounded blocks was misinterpreted as pillows by Kano et al. (1989) because

of their appearance similar to pillows. They resemble pseudo-pillows which are blocks rounded through water-chilling and brecciation (Mimura et al., 1975; Watanabe and Katsui, 1976; Yamagishi, 1987). However, they are different from known pseudo-pillows in terms that they are interconnected similarly to pillow lobes. These blocks become smaller as their comminuted fragments increase outward, but still can be fitted together to form in

SUBAOUEOUS RHYOLITEBLOCK LAVASIN THE MIOCENEUSHIKIRI FORMATION,JAPAN

Fig. 10. Occurrence of volcanic breccias in unit 3 at locality C. A. Interconnected blocks in in situ volcanic breccia. Lens cap d i a m e t e r is 6 cm. B. Massive volcanic breccia. C. Stratified volcanic breccia. Scale bar is about 30 cm long.

249

250

situ volcanic breccia. Constituting brecciated parts of lobes, the in situ volcanic breccias of 2-4 m thick stack vertically (Fig. 8) and are extensively developed laterally (Fig. 7). The in situ volcanic breccia which predominantly includes interconnected blocks ts further transitional to a massive volcanic breccia (Fig. 10B) composed of smaller and more predominantly isolated blocks and their comminuted fragments (volume of block = 5070%). The volcanic breccia in the upper part of this unit is stratified at intervals of 30 cm to 1 m (Fig. 10C). Each bed is a poorly sorted massive mixture of variable amounts of blocks and finer-grained fragments of poorly vesiculated microcrystalline rhyolite. Discussion

Units 2 and 3 are constituted of three lithofacies; lobe, massive volcanic breccia and stratified volcanic breccia. Lobes show in situ brecciation, and grade into or are covered with massive volcanic breccias. Therefore, they, together with surrounding massive volcanic breccias, form variably brecciated and fingered lavas. Stratified volcanic breccias, which overlie and underlie the lavas, are composed of the fragments of the same rock type as the brecciated lavas. The breccias are debris flows and/or turbidites as indicated by their internal structures; they were reworked. As discussed in the following sections, the rhyolite lavas of units 2 and 3 are probably subaqueous equivalents of subaerial block lava with respect to the rheologica! and morphological features. However, they were more intensely brecciated and more repeatedly bifurcated than subaerial block lava.

Rheology of the lavas As discussed by Fink (1983) and Yamagishi (1987), and according to the viscosity measurements by Shaw (1963), Friedman et

K. KANOET AL. TABLE 1 XRF analyses of a dense, glassy rhyolite (1) from locality A, Unit 2 and a microcrystalline rhyolite (2) from locality C, Unit 3 (analyst: K. Takeuchi)

SiO2 TiO2 A1203 Fe203* MnO MgO CaO Na20 K20 P205 Ig.L. Total

(1)

(2)

77.33 0.25 9.10 0.95 0.01 0.03 0.75 3.56 1.18 0.04 7.30 100.50

75.13 0.33 12.40 2.79 0.04 0.07 1.75 4.62 0.77 0.09 1.97 100.05

*Total Fe as Fe203.

al. (1963) and Murase and McBirney (1973), it is generally believed that rhyolite magmas behave as a Newtonian fluid above the liquidus temperatures, but in a plastic manner like a Bingham fluid below the liquidus temperatures. Rhyolite lavas of units 2 and 3 are glassy to microcrystalline, highly silicic (Table 1), and include 1-5 vol.% phenocrysts of plagioclase and lesser amounts of quartz, pyroxene (hypersthene and/or augite) and opaque minerals. These rhyolites are therefore presumed to have behaved mainly in a plastic manner until they were quenched in contact with water. Compared with mafic lavas, the aspect ratios of units 2 and 3 are high; they are 50-200 m thick and 4 km long in maximum (Fig. 2). those flow folds and vesicles elongated parallel to flow bands, as typically observed in unit 2 lava, are the features resulting from shear in a viscous flow such as subaerial block lava (Macdonald, 1972, pp. 93-95).

Morphological features of the lavas We interpret the lobes of units 2 and 3 as the fingers of master lavas. Our observation is not sufficient to support this interpretation. However, several cases are known where

SUBAQUEOUS RHYOLITE BI.OCK LAVAS IN THE MIOCENE USHIKIRI FORMATION. JAPAN

subaqueous rhyolite lobes bifurcated directly from the vent (Yamagishi and Dimroth, 1985; Yamagishi, 1987) or bifurcated from a large sheet flow of several kilometers long and several hundred meters thick (de Rosen-Spence et al., 1980), and were stacked together with their fragments. The occurrences of the lobes and their fragments are very similar to our case. Lava lobes in units 2 and 3 may be morphologically comparable to mafic pillows described by Jones (1968), Moore et al. (1971, 1973), Moore (1975), Ballard and Moore (1977) and Yamagishi (1985). However, compared with mafic pillows, the rhyolite lobes are large in size, are not closely packed, have very thick, quenched and more or less brecciated glassy crusts, and are covered with their own fragments, as predicted by Dimroth et al. (1979) and described by de Rosen-Spence et al. (1980), Yamagishi and Dimroth (1985) and Yamagishi (1987). Rhyolite magma is highly viscous and has a large surface tension and a large thermal conductivity (Murase and McBirney, 1973), thereby rhyolite lobes can grow large but are rapidly quenched and easily brecciated. With respect to their surface features, the subaqueous rhyolite lava lobes that are covered with their own polyhedral fragments are similar to subaerial block lava (Macdonald, 1972, pp. 93-95). The fragmentation of the lobes probably occurred mainly through thermal contraction and autobrecciation. Given the inferred water depths of lava emplacement, this may have occurred also through secondary explosions induced by volatile exsolution or water-lava interaction. Subaerial silicic lavas locally explode during flowage, as volatile gas concentrates in near-surface domains of the interior (Fink and Manley, 1989). Vapor generated at water-magma interface has a potential to cause explosions if it is enclosed by magma at low ambient pressures (Kokelaar, 1982; Wohletz, 1986; Busby-Spera and White, 1987). In addition, violent vapor explosions

251

produce abundant ash-sized clasts (Wohletz, 1983). However, no explosion craters, diatremes nor volcaniclastic cones were found in units 2 and 3, and block- and lapilli-sized fragments are dominant in massive volcanic breccias. Secondary explosions were probably of minor importance in our case, even if they occurred.

Emplacement of subaqueous block lava The preceding discussion allows us to interpret the fingered subaqueous rhyolite lavas of units 2 and 3 and those described by de Rosen-Spence et al. (1980), Yamagishi and Dimroth (1985), Yamagishi (1987) as subaqueous equivalents of block lava. The subaqueous block lavas, however, actively bifurcated, because they were quenched and brecciated upon contact with water. As lava extrudes in water, it is chilled in contact with water and brecciated due to thermal contraction. While this process proceeds inward, the lava advances being autobrecciated, and carrying and cascading the waterchilled fragments, similar to subaerial block lava. During flowage, the plastic lava extrudes lobes into and through the cover of their own fragments when the inner pressure exceeds the yield strength of the quenched crust. This should occur the most when it comes across steeper slopes or large obstacles and the inner pressure is suddenly raised according to the difference of gravity potential. Repeated bifurcations enlarge the surface area of lava and make the lava cool rapidly and brecciate further, before it is frozen. The resulting deposit includes waterchilled lava fragments and variably brecciated lava lobes and lava sheets covered with the fragments. Stratified volcanic breccia is then deposited from sediment gravity flows probably generated on the unstable slopes of the lavas. Reworking of lava fragments may be also induced by vapor- or pyroclastic explosions on lava piles at shallow water depths

252

(Cas et al., 1990), though we have no evidence in units 2 and 3. In our model, a subaqueous lava flow is longer and its aspect ratio is smaller if its viscosity is lower and its effusion rate is higher. Those rhyolite lavas described by Yamagishi and Dimroth (1985) and Yamagishi (1987) were likely higher in viscosity and lower in effusion rate than those described by de RosenSpence et al. (1980), thus formed no large sheet flows as shown in their emplacement model. No large lava-sheet flows have been yet found in our case. The aspect ratios of units 2 and 3 are, however, as low as those of the rhyolites described by de Rosen-Spence et al. (1980). Compared with the rhyolite lavas described by Yamagishi and Dimroth (1985) and Yamagishi (1987), the rhyolite lavas of units 2 and 3 were more fluidal or their effusion rates were higher, presumably. Conclusions (1) A rhyolite mass in the Miocene Ushikiri Formation is a relatively flat, small subaqueous edifice composed mainly of three lavaflow units. (2) The rhyolite lavas are possibly 50-300 m thick and 4 km long in maximum and include lobes and their fragments, being enclosed by reworked volcanic breccia. (3) They erupted probably at water depths between 200 and 1000 m, and plastically flowed down forming water-chilled polyhedral fragments through thermal contraction and differential movement of themselves and extruding lobes around the flow front. They are subaqueous equivalents of block lava. Acknowledgements K. Ono, N. Isshiki, K. Mimura, H. Kamata and C. Finn are thanked for their critical comments on this paper. Instructive reviews by C.J. Busby-Spera and an anonymous reviewer are much appreciated.

K. K A N O ET AL.

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