a late cretaceous-paleogene cauldron cluster: the nôhi rhyolite, central japan

Bull Volcanol (1991) 53:132-146

V01(fi 610gy �9 Springer-Verlag 1991

A Late Cretaceous-Paleogene cauldron cluster: the N6hi Rhyolite, central Japan

Yoshimitsu Koido

Department of Earth Science, Faculty of Education, Gifu University, Yanagido 1-1, Gifu 501-11, Japan

Received October 2, 1989/Accepted October 1990

Abstract. The N6hi Rhyolite now occupies an area of about 5000 km 2 in central Japan. It is the largest cluster of volcanic piles of Late Cretaceous to Paleogene age on the Inner Side of SW Japan. It unconformably over- lies Paleozoic to Jurassic rocks and is intruded by Pal- eogene granitic intrusions in its southern and northern parts. The N6hi Rhyolite consists chiefly of rhyolitic to rhyodacitic welded-tuff sheets with subordinate amounts of volcaniclastic sedimentary rocks. Each welded-tuff sheet is usually 200 to 700 m thick, extends laterally 20 to 60 km, and is composed of nearly homogeneous densely welded tuff. Six volcanic sequences (I- VI) are recognized in the southern and central parts of the N6hi Rhyolite. Each sequence generally comprises successive accumulations of welded-tuff sheets and volcaniclastic sedimentary layers underlying the sheets. Sequence I, III and V volcanism was derived from zoned magma bodies and was accompanied by granodiorite porphyries. Sequence II, IV and probably VI volcanism was derived from homogeneous magma bodies and was not accompanied by granodiorite porphyries. The estimated total volume is of the order of 300- 700 km 3 for Sequences I, II and IV, 1600 km 3 for Se- quence III, and 50 km 3 or less for Sequences V and VI. The volume of Sequence III is one of the largest vol- umes of ashflow turfs so far recognized. One or more polygonal cauldrons, each 15-40 km across and 400- 1000m deep, formed during each of Sequence I through IV. These cauldrons were generally formed by two successive depressions. The first depression in each sequence is of the Motojuku-type, characterized by collapse prior to the principal eruption. The second depression was formed during the principal eruption. The cauldron of Sequence V has an unusual shape and was probably formed during or after the eruption. The development of a cauldron in Sequence VI is uncertain. The N6hi Rhyolite is elongate in a NW direction and contains a cluster of seven or eight cauldrons. These cauldrons formed in a tension field trending N and NE, and their volcanism migrated northward. The main regional tectonic framework controlling the cauldrons resulted from uplifting of the basement, which is ascribed

to the ascent of a large volume of magma in Late Cre- taceous to early Paleogene time.

Introduction

Cretaceous to Paleogene volcanic rocks are extensively distributed on the Inner Side of SW Japan, together with a great amount of granitic rocks (Fig. 1). This volcanism is characterized by voluminous felsic pyroclastic-flow deposits, especially welded tuffs.

The N6hi Rhyolite is a large-scale product of this volcanism in central Japan (Fig. 2). The N6hi Rhyolite extends from the northern margin of the Ry6ke Terrane to the Hida Terrane through the Mino Terrane and Hida Marginal Belt, which are the principal geotectonic units in central Japan. It is about 120 km long, about 40 km wide on average, and is elongate in a NW direction. The basement of the N6hi Rhyolite includes the Jurassic Mino Terrane, the Jurassic to Lower Creta- ceous Tetori Group, and Paleozoic igneous and metamorphic rocks in the Hida Marginal Belt and Hida Ter- rane. The N6hi Rhyolite is intruded by Paleogene granitic rocks in its southern and northern parts, and is covered by Miocene sedimentary rocks and Quaternary volcanic rocks. The Atera Fault running through the midst of the N6hi Rhyolite in a NW trend is a left- lateral active fault with a displacement of about 5- 8 km.

The rocks of the N6hi Rhyolite had long been called "quartz porphyries" because of their hypabyssal appearance. Kawada et al. (1961) first recognized that the N6hi Rhyolite consisted chiefly of rhyolitic to rhyodacitic welded tufts with subordinate volcaniclastic ma- terial and minor amounts of lava. Hypabyssal rocks also occur, such as granodiorite porphyries and granite porphyries that are co-magmatic with the volcanic rocks (Yamada et al. 1971). The volcanostratigraphy is established for its southern and central parts (Yamada et al. 1971, 1976, 1985; Kawada 1971; C.R.G.N.R. 1973, 1976; Koido 1974, 1989; Shikano 1976; Yamada 1977,

133

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Fig. 1. Cretaceous to Paleogene igneous rocks on the Inner Side of SW Japan and the location of the N6hi Rhyolite (Tanaka and Nozawa 1977). 1-6, principal geotectonic units on the Inner Side of SW Japan Fig. 2. Geological sketch map around the N6hi Rhyolite (simplified from Yamada et al. 1982)

1989; Koido and Sat6 1979; Kawada et al. 1988; Ya- mada and Kobayashi 1988) and for its northern part (C.R.G.N.R. 1979; Kawada 1982). The N6hi Rhyolite has been classified on the basis of these studies into five (I-V) or six (Ia, Ib, II-V) volcanic sequences in its southern and central parts (Yamada et al. 1971; C.R.G.N.R. 1976).

This paper discusses the N6hi Rhyolite as a cluster of cauldrons, most of which have formed by two col- lapses prior to and during the principal eruption. The regional tectonic framework controlling the cauldrons is also described.

Geology of the Nfihi Rhyolite

Stratigraphic units

Individual flow units or cooling units (Smith 1960) are difficult to determine for welded tufts in the N6hi Rhyolite. Hence, a welded-tuff sheet defined by Smith (1960) is used as the primary stratigraphic unit. Each sheet is usually 200 to 700 m thick, and extends laterally 20 to 60 kin. It consists of nearly homogeneous densely welded tuff, but it has some variation in modal and chemical compositions. Contacts between succes-

134

sive sheets are generally determined by a discontinuous change in general lithofacies. Less welded materials or thin airfall tuffs are recognized neither within each sheet nor at the contact between successive sheets.

Layers of volcaniclastic sediments are intercalated with welded-tuff sheets at several stratigraphic hori- zons. They are composed mainly of various kinds of non-welded turfs, tuffaceous sandstones, mudstones and conglomerates, and are 30 to 400 m in thickness. Most of them are lacustrine deposits. Conglomerates contain pebbles of the underlying welded-tuff sheets and basement rocks in a tuffaceous sandy matrix.

Stratigraphy and volcanic sequence

The N6hi Rhyolite consists of numerous welded- tuff sheets, several interlayered volcaniclastic sediments

and lavas (Table 1). A volcanic sequence is defined here as the successive accumulat ion of welded-tuff sheets and volcaniclastic sedimentary layers underlying the sheet. The older volcaniclastic sedimentary layers of each volcanic sequence unconformably cover the underlying welded-tuff sheet. Six volcanic sequences ( I - VI) are recognized in the southern and central parts of the N6hi Rhyolite (see Table 1).

Petrography

Each welded-tuff sheet shows similar petrographic features, and is composed of abundant crystal fragments, devitrified matrix, essential fragments and lithic fragments.

Crystal fragments, commonly smaller than 3 m m and rarely up to 5-6 m m in diameter, are of quartz,

Table 1. Stratigraphic succession in the southern and central parts of the N6hi Rhyolite (after Yamada et al. 1971, 1985; C.R.G.N.R. 1973, 1976; Yamada 1977, 1989; Koido and Sat6 1979; partly modified)

Volcanic Stratigraphic unit [thickness, composition] (Fission-track age, Ma) Sequence (ca. age) Southern part Southern and central parts Central part

Granite porphyry (57.0 + 2.3)

VI-3 W.T.S. [150 m, Rh] VI VI-2 W.T.S. [120-180 m, Rh] (58.2_+3.2) (58 Ma) VI-1 W.T.S. [150-200 m, Rh]

Granodiorite porphyry (63.1 _+ 2.6)

V-4 W.T.S. [300 m, Rd] V V-3 W.T.S. [500 m, Rh-Rd] (66.0 _+ 3.8)

V-2 V.S.L. [50-150 m, --] (63 Ma) V-1 V.S.L. [30 m, --]

IV-4 W.T.S. [350-500 m, Rh] IV IV-3 W.T.S. [250-500 m, Rh]

IV-2 W.T.S. [400-700 m, Rh] (64.5 + 2.5) (65 Ma) IV-1 V.S.L. [50-400 m, --]

Granodiorite porphyry ( - Qdp) (68.8 + 2.7, 72.9 _ 3.1)

III

(70 Ma)

1II-5 W.T.S. [200-400 m, Rd] III-4 W.T.S. [200-300 m, Rh] Ili-3 W.T.S. [700-1000 m, Rd] (67.0_+ 1.5, 72.9+3.1)

III-2S W.T.S. [350 m, Rh] (71.7+4.1) III-2C W.T.S. [300+rn, Rh] III-1S V.S.L. [150 m, Rh] III-1C V.S.L. [?, Rh]

II-4 W.T.S. [500 m, Rh] (76.3 + 4.0, 76.3 + 1.5) II I1-3 V.S.L. [50 m, --] (75 Ma) II-2 W.T.S. [50+m, ?] (72.4_+3.8)

II-1 V.S.L. [50 m, --]

Granodiorite porphyry

I

(80 Ma?)

I-7 Lava [100 m, Da] I-6 W.T.S. [500 m, Rd] 1-5 W.T.S. [500 m, Rh] I-4 V.S.L. [50 m, --] I-3 W.T.S. [150 m, Rh] I-2 W.T.S. [100 m, R.h] I-1 V.S.L. [200 m, --]

W.T.S., Welded-Tuff Sheet; V.S.L., volcaniclastic sedimentary layer; Qdp, quartz diorite porphyry; Rh, rhyolitic; Rd, rhyodacitic; Da, dacitic

plagioclase, alkali feldspar, biotite, hornblende, some- times ortho- and clino-pyroxenes, and rarely olivine, with accessory ilmenite, allanite and zircon. The amounts of these crystals are mostly 30-60 vol.% exclusive of lithic fragments. Modal proportions of these crystals vary between sheets in the same volcanic sequence and have some variation within each sheet (Fig. 3). Plagioclase is mostly calcic oligoclase and andesine (or labradorite) and generally shows normal zoning. Plagioclase is strongly zoned in some rhyodacitic welded tufts. Plagioclase is partly altered to calcite, sericite and epidote. Most alkali feldspars are orthoclase; sanidine is rarely found in very fresh rocks. Orthoclase has perthitic and cryptoperthitic structures with optical angles of about ( - ) 40 ~ ~ It is often turbid and are partly altered to calcite. Mafic minerals are commonly from 2 to 5 vol.%. They are characterized by Fe-rich species, namely biotite rich in annite-siderophyllite components, ferroedenite-ferrohornblende, ferroaugite, ferrohypersthene and ferrohortonolite (Kawada 1971; Harayama 1979). Such minerals are frequently found in the equivalent volcanic and plutonic rocks in the sur- rounding areas and are interpreted as primary magmatic constituents. Most mafic minerals are altered to chlorite, calcite, sericite, magnetite and others.

The matrix is made of dense glass shards, which are completely devitrified into compact cryptocrystalline

Quartz

135

aggregates of salic minerals with traces of welded texture.

Essential fragments have a lenticular to ovoid shape and show a eutaxitic structure. Most of them are smaller than 5 cm but some are up to 30 cm in length. They carry approximately the same assemblage of phenocrysts with nearly the same proportions as crystal fragments of enclosing welded tufts. Such fragments are, therefore, interpreted to be cognate with the enclosing welded tufts. Their groundmasses are more coarsely crystallized into aggregates of salic minerals than the devitrified matrix of the enclosing rocks, and often show spherulitic or axiolitic structure without traces of vesicular texture.

Lithic fragments are common in most of the welded tufts and are locally up to 20-30 vol.%. They are very small (less than lmm) to large (more than 10cm across). Accidental fragments are principally of the basement rocks such as sandstone, chert or shale constituting the Mino Terrane, and rarely of granitic rocks. Other accessory fragments are of welded tufts and pyroclastic rocks.

Intrusive rocks

Stocks and dikes intrude the adjacent basement rocks as well as the N6hi Rhyolite. They are composed of

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Fig. 3. Modal composit ion of quartz, alkali feldspar and plagioclase of the welded-tuff sheets (W.T.S.) in Sequence III of the

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N6hi Rhyolite (after Yamada et al. 1971; C.R.G.N.R. 1976; Koido and Sat6 1979)

136

granodiorite porphyry, granite porphyry, and subordinate quartz diorite porphyry. Quartz diorite porphyry occurs only as the core of a granodiorite porphyry stock.

Granodiorite porphyries carry abundant euhedral phenocrysts of plagioclase, alkali feldspar, quartz, biotite and hornblende in a microgranitic or micrographic groundmass. Most marie minerals and some plagioclase and alkali feldspar are altered to chlorite, calcite, sericite and other minerals. The granodiorite porphyries occur as stocks, usually 5 to 10 km in length, within the areas dominated by welded-tuff sheets of Se- quences I and III. One small stock intruded the welded-tuff sheet of Sequence V. The granodiorite porphyries probably intruded into the vent areas for these sheets, because unusually abundant large lithic fragments occur within the sheets in the probable vent areas and small-scale breccia dikes are distributed around them (C.R.G.N.R. 1976; Yamada and Kobayashi 1988).

Granite porphyries contain fewer marie minerals than granodiorite porphyries. They usually occur as dikes or dike swarms several tens or hundreds of meters wide along the margins of, or whithin, the N6hi Rhyo- lite. Zircon fission-track ages suggest that the N6hi Rhyolite was intruded by granite porphyries immedi- ately after Sequence VI volcanism along or near the rims of several cauldrons.

Age of the N6hi Rhyolite

No appropriate minerals occur in either the welded-tuff sheets or the intrusive rocks for determining K-Ar ages, owing to alteration. Hence, fission-track dating of zir- cons was used for age estimations (Harayama and Su- zuki 1984; Yamada et al. 1985; Koido and Danbara 1986; Yamada and Kobayashi 1988). Precise determi- nation of the age for individual sheets or intrusive rocks is impossible, however. Taking into consideration both the stratigraphic succession and the Rb-Sr whole-rock isochron ages of several sheets (Okamoto et al. 1975; Shirahase 1982), I estimate the approximate age of each of the volcanic sequences as follows (see Table 1): Sequence I: ca. 80 Ma Sequence II: 75Ma Sequence III: 70 Ma Sequence IV: 65 Ma Sequence V: 63 Ma Sequence VI: 58 Ma

The age of Sequence I is not well determined, because the rocks have to a great extent been thermally metamorphosed by successive granite intrusions. The intervals between other sequences suggest that the age of Sequence I is ca. 80 Ma. On this basis, the N6hi Rhyolite is inferred to have formed during a long period of about 20 m.y., from late Cretaceous to early Pal- eogene.

Development of cauldrons in the N~hi Rhyolite

Many of the primary features of the N6hi Rhyolite are poorly preserved owing to later granitic intrusions and

recent block faulting. Thus, the distribution, variation in thickness and original structure of individual welded-tuff sheets are difficult to determine precisely. Their vent areas also are difficult to identify. In spite of these difficulties, however, the spatial relationship of welded-tuff sheets and volcaniclastic sedimentary layers in each volcanic sequence can be roughly defined.

The main welded-tuff sheets in Sequence I-V and the volcaniclastic sedimentary layer in Sequence IV were deposited within steep-walled depressions. Their thicknesses change abruptly from several hundred meters to several meters at the margins of the depressions. These depressions are interpreted to have been formed by collapse prior to or during accumulation of the main units in each sequence. Granodiorite porphyries or granite porphyries intruded into the probable vent areas within the depression and locally along the margins of the depressions. These depressions probably formed as cauldrons as defined by Smith and Bailey (1968).

The outlines of the cauldrons in each volcanic sequence are defined below, according to published studies (Yamada et al. 1971, 1985; Kawada 1971; C.R.G.N.R. 1973, 1976; Koido 1974, 1989; Yamada 1977, 1989; Koido and Sat6 1979; Yamada and Ko- bayashi 1988).

Sequence I

Rocks of Sequence I occur in the southern margin of the N6hi Rhyolite and are widely intruded by granitic rocks. They are divided into two volcaniclastic sedimentary layers, four welded-tuff sheets and a dacitic lava flow (Table 1). The I-5 and I-6 Welded-Tuff Sheets are the main units in Sequence I.

The cauldron in Sequence I was formed by two successive depressions (Fig. 4). The first depression devel- oped in the eastern part of the cauldron. The I-1 Vol- caniclastic Sedimentary Layer contains the oldest deposits in the first depression. It is composed chiefly of thick tuffaceous sedimentary rocks. The I-2 and I-3 Welded-Tuff Sheets have similar extents to that of the I-1 Volcaniclastic Sedimentary Layer. These welded- tuff sheets are interpreted to have filled the depression, but its shape is not clearly shown owing to the extensive granitic intrusions. The proposed depression is about 500m deep, about 10km wide, and extends about 20 km in an ENE direction.

The second depression formed a gigantic cauldron. It is about 500 m deep, about 15 km wide, and extends about 35 km in an ENE direction. The I-5 and I-6 Welded-Tuff Sheets were deposited in the eastern and western parts of the cauldron, respectively. The I-5 Welded-Tuff Sheet conformably covers the 1-4 Volcani- elastic Sedimentary Layer, which is thin and composed mainly of volcanic breccias. The 1-6 Welded-Tuff Sheet grades into and covers the I-5 Welded-Tuff Sheet. The I-6 Welded-Tuff Sheet abuts the vertical wall of the basement along the western margin of the cauldron and contains megablocks derived from the basement walls

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of the cauldron. The second depression formed during the accumulation of the 1-5 and 1-6 Welded-Tuff Sheets. Granodiorite porphyry stocks in the northeastern part of the cauldron intrude through the probable vent areas of the 1-5 Welded-Tuff Sheet. Those in the western part of the cauldron intrude through the probable vent areas of the 1-6 Welded-Tuff Sheet, and a small vent of this sheet is identified in the western part of the cauldron.

The granitic rocks around the Sequence I cauldron intrude the younger rocks of Sequences II and III and are distributed more widely than the Sequence I cauldron, so that they have no direct petrogenetic relations to the Sequence I rocks.

Sequence H

The Sequence II rocks are distributed in the western part of the Nrhi Rhyolite (Fig. 4). They are divided into two volcaniclastic sedimentary layers and two welded- tuff sheets (Table 1). The 1I-4 Welded-Tuff Sheet is the main sheet of this sequence.

The II-1 Volcaniclastic Sedimentary Layer is composed mainly of conglomerate and accompanied by some intercalated stratified tuffs. Conglomerate contains pebbles of the 1-6 Welded-Tuff Sheet, so that erosion proceeded before the accumulation of this layer. The II-1 Volcaniclastic Sedimentary Layer is the lowest deposit in the depression. It is uncertain if the depression corresponds to the proposed cauldron in this sequence, owing to the restricted exposure of the lowest unit. The cauldron was buried mainly by the 1I-4 Welded-Tuff Sheet, but its shape is not clearly shown, because the 1I-4 Welded-Tuff Sheet is widely covered by rocks of Sequences III and IV. The proposed cauldron is about 500 m deep, about 15 km wide and extends about 35 km in a NW direction. The 11-4 Welded- Tuff Sheet is more than 200 m thick outside the cauldron. Presumably, the proposed cauldron was formed by the second depression in the same way as was the Sequence I cauldron, and the first depression corresponds to that formed prior to the accumulation of the lowest unit. Granodiorite porphyry does not intrude Se- quence II rocks, but granite porphyry occurs as ring dikes along the southern rim of the cauldron.

Sequence III

The III-1S Volcaniclastic Sedimentary Layer in the southern cauldron unconformably covers the Sequence II rocks. It is composed mainly of conglomerate, which is interbedded with stratified tuffs in its lower part. Conglomerate contains pebbles of the 11-4 Welded-Tuff Sheet and Sequence I rocks. The Ill-IS Volcaniclastic Sedimentary Layer was deposited in the depression formed prior to its accumulation. The III-1C Volcani- clastic Sedimentary Layer in the central cauldron is composed chiefly of non-welded rhyolitic tuff breccia and volcanic breccia with pumice fragments, and is commonly accompanied by stratified turfs. It contains megablocks of the basement, which are injected by lapilli-tuff dikes. The III-1C Volcaniclastic Sedimentary Layer presumably resulted from a phreatic eruption and was deposited within or around its vent area within the depression.

The 11I-3 Welded-Tuff Sheet, the greatest welded- tuff sheet in the N6hi Rhyolite, grades into and covers the II-2S and II-2C Welded-Tuff Sheets in each cauldron. It is about 700-1000 m thick within both cauldrons and pinches out abruptly outside the cauldrons. The cauldrons formed prior to the accumulation of the upper part of the 111-3 Welded-Tuff Sheet, but it is uncertain if they correspond to the first depression. Most likely they formed as a second depression in the same way as did the Sequence I cauldron. Granodiorite porphyry stocks intrude through the probable vent areas of the III-2C, III-2S and Ili-3 Welded-Tuff Sheets.

The 111-4 and 111-5 Welded-Tuff Sheets covered flat areas where the cauldrons had been completely buried by the 111-3 Welded-Tuff Sheet, and consequently they show no abrupt change in thickness along the margins of the cauldrons. The Ill-4 Welded-Tuff Sheet contains no accessory fragments of the 111-3 Welded-Tuff Sheet, and it gradually thickens towards the northern side of the southern cauldron. The 111-5 Welded-Tuff Sheet also gradually thickens towards the northwestern side of the southern cauldron. It abouts the basement along the western margins of its exposed areas, where it is interstratified with poorly sorted breccias. The breccias are monolithologic, with microbreccias of the same materials as the matrix, and they were derived from the depression walls formed by collapse. From these facts, it is interpreted that these upper sheets probably erupted outside the above-mentioned cauldrons, and a third depression formed during the accumulation of these sheets, but its dimensions are uncertain.

Five welded-tuff sheets and two volcaniclastic sedimentary layers formed in this sequence (Table 1), together with two gigantic cauldrons (Fig. 5). One cauldron is about 30 x 20 km across and more than 700 m deep in the southern part of the N6hi Rhyolite, and the other is about 40x25 km across and more than 800-1000 m deep in its central part. The depth is estimated from the thickness of the welded-tuff sheets in the cauldrons, because the cauldron floor is not exposed.

Both cauldrons were filled with the volcanic sedimentary layer and the overlying two welded-tuff sheets.

Sequence IV

The Sequence IV rocks are divided into three welded- tuff sheets and one volcaniclastic sedimentary layer (Table 1). The IV-1 Volcaniclastic Sedimentary Layer unconformably covers the III-5 Welded-Tuff Sheet. The cauldrons in this sequence are recognized in the central and southern parts of the N6hi Rhyolite (Fig. 6), but are smaller than those in Sequence III. Grano- diorite porphyry is not recognized in Sequence IV.

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own

140

Two cauldrons occur along the southwestern and northeastern margins of the Sequence III central cauldron. Both cauldrons have polygonal outlines and were buried by the IV-1 Volcaniclastic Sedimentary Layer and IV-2 Welded-Tuff Sheet. These units in the southwestern cauldron abut the steep wall of the basement, along which poorly sorted monolithologic breccias are distributed in the base of the IV-1 Volcaniclastic Sedi- mentary Layer. The southwestern cauldron was apparently formed by collapse prior to the accumulation of the lowest unit. The IV-2 Welded-Tuff Sheet buried the cauldron, overflowed both cauldrons, and widely covered the area. Granite porphyries occur as ring dikes along the rims of both cauldrons. The northeastern cauldron is about 20x 10km across and more than 500 m deep, but the dimensions of the southwestern cauldron are uncertain.

The southern cauldron in this sequence formed along the northern margin of the Sequence III southern cauldron. It has a polygonal outline and is about 15 x 10 km across and about 300-400 m deep. It was completely buried by the IV-I Volcaniclastic Sedimen- tary Layer. This layer is composed mainly of non- welded ruffs and tuffaceous sedimentary rocks. It is about 400 m thick within the cauldron and pinches out abruptly outside the cauldron. The IV-2 Welded-Tuff Sheet flowed across flat terrain with no abrupt change in thickness. Whether this sheet erupted within the southern cauldron is unknown. This cauldron is recognized only as the steep-walled depression, formed prior to the accumulation of the lowest unit. It is not interpreted to be a product of regional extension without direct relation to volcanism, because the IV-1 Volcani- clastic Sedimentary Layer is accompanied by pyroclastic-flow deposits. The source area of the Ill-3 Welded- Tuff Sheet is interpreted to be in the northeastern side of the southern cauldron, judging from the size and volume variations of enclosed lithic fragments. The upper units in this sequence probably erupted and accumulated within the second depression, which had formed in the same way as did the second depression in Se- quences I, II and III, but its dimensions are uncertain.

Sequences V and II1

The V-3 Welded-Tuff Sheet, the main sheet in Sequence V, is distributed along the eastern margin of the Nrhi Rhyolite in a NW direction (Fig. 7). This sheet probably occupies a cauldron with an unusual shape. The V-1 Volcaniclastic Sedimentary Layer unconformably covers the IV-4 Welded-Tuff Sheet and is composed of tuffaceous sedimentary rocks. It was deposited in a depression formed by regional subsidence, because the V- 1 Volcaniclastic Sedimentary Layer is thin but extensive and lacks any abrupt change in thickness. In contrast, the thickness of the V-3 Welded-Tuff Sheet changes abruptly at its contact with be basement. The Sequence VI volcanic rocks directly overlie the basement, with no intervening V-3 Welded-Tuff Sheet, possibly because of

erosion. The cauldron collapse in this sequence probably occurred during or after the eruption of the V-3 Welded-Tuff Sheet. Small granodiorite porphyry stocks and granite porphyry dikes intrude along the rim of the cauldron.

The volcanic rocks in Sequence VI are locally distributed along the eastern margin of the N6hi Rhyolite (Fig. 7). They are composed of three welded-tuff sheets that cover both the underlying volcanic rocks and the basement, extending in a NW direction. The development of a cauldron in Sequence VI is uncertain.

Chemical compositions

Essential fragments for chemical analysis cannot be ob- tained from each welded-tuff sheet. The bulk chemical compositions of the welded turfs exclusive of lithic fragments are therefore used. The bulk compositions are similar to those of enclosed essential fragments that were analyzed, and are correlative with the modal compositions. Figure 8 shows the SiO2 content and differentiation index (Thorton and Tuttle 1960) for each of the welded-tuff sheets, intrusive rocks and lava. The SiO2 contents of the welded tuffs are around 68-77 wt.% (rhyolite to rhyodacite), and those of the lava are 65-67 wt.% (dacite). SiO2 contents are 67-72 wt.% for granodiorite porphyries, 74-77 wt.% for granite porphyries, and 66 wt.% for quartz diorite porphyry.

Sequences I to VI are classified into two types of vertical variation for both SiO2 content and D.I. One has a systematic variation (Sequences I, III and V), and the other no variation (Sequences I, IV and VI). The SiO2 content of welded-tuff sheets decreases upward from 77 to 72-74 wt.% for Sequence I and from 77 to 68-72 wt.% for Sequence III. Two cycles of such variation are recognized for Sequence III. Furthermore, the SiO2 content of the later intrusive rocks in both Se- quences I and III is 66-72 tw.%. The dacite lava in Se- quence I is more mafic (65-67 wt.%) than the associated intrusive rocks. The SiO2 content of the V-3 Welded- Tuff Sheet in Sequence V changes upsection from 76 to 72 wt.%. The SiO2 content of associated granodiorite porphyries in Sequence V is 72 wt.%. Such systematic variations in SiO2 content are reflected in the upsection variation of D.I. from 93 to 68-73 for Sequences I and III and from 86 to 79 for Sequence V. This pattern toward less silicic compositions with time resembles that observed within single ashflow sheets derived from zoned felsic magma bodies (e.g. Lipman et al. 1966; Bailey et al. 1976; Smith 1979; Hildreth 1981). The gradients extending over the grouped sheets for Sequences I and III can not apply just to those of single ashflow sheets. Each sheet in Sequences I and III, however, shows no significant pauses between eruptions. There- fore, both the volcanic rocks and associated granodiorite porphyries in these sequences are interpreted to have been derived from zoned magma bodies.

The welded tuffs in Sequences II, IV and VI are petrographically and chemically rhyolite and do not show such systematic variation, although the SiO2 con-

141

VOLCANICWELDEDTUFFSHEET(W. INTRUSIVE STAGE T.S.) and LAVA ROCK

G r a n i t e p o r p h y r y

( VI-3 We[ded Tuff Sheet ) VI VI -2 Welded Tuff Sheet

(VI-1 WeLded Tuff Sheet )

V

( V - 4 Welded Tuff Sheet)

V - 3 Welded Tuff Sheet

Granod io r i t e porphyry

IV

IV - 4 Welded Tuff Sheet

( IV -3 Welded Tuff Sheet)

IV- 2 Welded Tuff Sheet

III-5 Welded Tuff Sheet

III - 4 Welded Tuff Sheet

I I I - 3 WeLded Tuff Sheet IIl

II I I - 4 Welded Tuff Sheet

(I I - 2 Welded Tuff Sheet)

Quartz diorite .,porphyry

G ranod io r i t e porphyry

I - 7 Lava

I - 6 Welded Tuff Sheet

[ I - 5 WeIdedTuf f Sheet

I - 3 Welded Tuff Sheet

( I - I Welded Tuff Sheet)

Granod io r i te po rphy ry

S i02 (wt*/o)

i I o

D.I .

I I

64 66

% �9 .%11 ~ 1 7 6 i I T I v I

I i 0 i I ,

% ~

I i i i i i r i

e o �9 �9

r I

~ ". . � 9 ,%... i I i I i i I

. . . . . . . - -

i I i I i I I

; %

I I i I I . . . .

ol i , I

i i ,

68 70 72 74 76 70 80 90 100

Fig. 8. Vertical variations in SiO2 contents and differentiation in- dices of the welded-tuff-sheets, lava and intrusive rocks in the N6hi Rhyolite. Analyses on anhydrous basis. ( ) , no data availa-

ble. Straight lines are simply generalized trend lines (data from Kawada 1971, 1982; Yamada et al. 1971, 1985; C.R.G.N.R. 1973; Koido 1974; Okamoto et al. 1975; present author)

tent and D.I. range from 72 to 76 wt.% and from 80 to 92, respectively. These welded tuffs were apparently derived from individual magma bodies with nearly homogeneous composition. There are no data on the vertical variation in minor elements for these sheets.

Volume

The distribution and thickness variation of individual welded-tuff sheets cannot be precisely determined. The total volume of the sheets for each volcanic sequence

can be roughly estimated, mainly by using the volume of each cauldron (Fig. 9). The estimated volume for each sequence does not indicate that of the corresponding magma body, because some magma was widely dis- persed in airfall tephra and also because the volume of granodiorite porphyry concealed under the roof of the volcanic rocks is unknown. Therefore, the values shown in Fig. 9 are generally minimum values of the volume of the corresponding magma body.

The volcanic activity in the N6hi Rhyolite increased abruptly after Sequence II, reached a maximum in Se-

142

Km 3

1,500-

1,000

500- ~oo

oV//S I II

1.600 77-77

/ / / 1 / / / / / / / .. ,. / / / i / / / / / / / /

/ / / / / / / /

U/A

�9 - . / / / A

".'.'." / / / A / . / . / . / / / / A / , / . / . / ~ / / / /

/ / / I

III IV Sequence

5o 5

V V[

Fig. 9. Estimated original total volume of the welded-tuff sheets in each volcanic sequence in the N6hi Rhyolite

quence III, and systematically decreased thereafter. The estimated volume in Sequence III is nearly equal to that of the largest magnitude of ashflow magmatism (Smith 1979).

Discuss ion

Cauldron collapse prior to and during eruption

Volcaniclastic sedimentary layers and thick welded-tuff sheets successively accumulated within the cauldrons in Sequences I-V. The cauldrons were formed by two or three successive depressions. The first depression was filled by lacustrine units in Sequences I-IV. An abrupt change in thickness of the units is recognized at the margins of the polygonal depressions in Sequence IV. The depression shape and the conditions at its margins in Sequences I-III are uncertain. The first depression was formed by collapse prior to accumulation of the lowest sedimentary unit, perhaps as a premonitory depression to the principal eruption. The depression is not interpreted to be a regional extensional tectonic feature without direct relation to volcanism, because the lowest units contain some volcanic products. Such a depression is not recognized in the Sequence V cauldron.

The second or the third depression in each cauldron is probably more extensive than the first depression, and was buried by the main welded-tuff sheets in each sequence. These sheets show no abrupt change in thickness along the margins of the first depression, and flowed across a low-relief surface. They abut the steep walls of the Second or the third depression, along which they are interstratified with collapse materials in Se- quences I and III. This depression was formed by collapse during the principal eruption. Its shape is poly-

gonal rather than subrounded, but the geometry of the fracture systems bordering the cauldron collapse are unknown owing to lack of deep erosion. The main depression in the Sequence V cauldron also was formed by collapse during or after the principal eruption.

Lipman (1984) summarized a general caldera cycle associated with voluminous ashflow eruptions, on the basis of the greater thickness of the intracaldera sheet and the existence of caldera-collapse breccias in many ashflow fields in western North America, and emphasized that large calderas collapsed concurrently with eruption. The timing of the first collapse events associated with the N6hi Rhyolite cauldrons was different from that suggested by Lipman (1984).

The experimental study on cauldron formation by Komuro et al. (1984) suggests that an ascending magma body domes the ground surface, and as a result radial and concentric fractures are simultaneously produced on the top of the dome. Komuro et al. (1984) conclude that collapse occurs along these fractures and that a polygonal cauldron bounded with normal faults is formed prior to the principal eruption. Komuro (1987) desig- nates the cauldron produced by an ascending magma body as the Motojuku-type cauldron. The Motojuku- type cauldron is often observed in the Miocene volcanic field in Japan (e.g. Fujita 1972) and is quantitatively demonstrated also on the Quaternary caldera which has long been accepted to be the Crater Lake- type (Yahata 1989). The concept of the Motojuku-type cauldon is not always controversial within Japan, one of the reasons for which is thought that the commonly expressed concept on the caldera formation - caldera subsidence after the eruption - is difficult to be sub- stantiated quantitatively on the Quaternary large calderas. Komuro (1987) shows experimentally that a ring fracture is formed by reduced magma pressure or by evacuation of the magma body after the principal eruption.

I believe that the first depression of Sequences I-IV in the N6hi Rhyolite were formed by an ascending magma body prior to the principal eruption and hence is a Motojuku-type cauldron. The second and third depressions in each sequence were formed by reduced magma pressure during the principal eruption. Figure 10 presents the development of two cauldrons according to this model during the Sequence III volcanism, which is the most voluminous volcanic sequence in the N6hi Rhyolite.

Regional tectonic framework of the N6hi Rhyolite mass

The locations of cauldrons migrated northward from Sequence I to II and the subsequent III-VI. The cauldron in Sequence I is elongate in an ENE direction, but the cauldron in Sequence II on the northern side of the Sequence I cauldron is elongate in a NW direction (Fig. 4). The two cauldrons in Sequence III are situated in a NW direction, parallel to that of the N6hi Rhyolite (Fig. 5). The Sequence V cauldron and Sequence VI rocks have NW trends (Fig. 7). The cauldrons in the

A

/ \ /

143

B

C

Tuff breccia & Votcaniclastie sediments

D Rhyolit ic welded-tuff sheet

F Rhyodacitic wetded-tuff sheet

Fig. 10A-F. Cross sections showing the development of Sequence III volcanism. Width of figure is about 100 km, and its vertical scale is emphasized. A cauldron collapse occurred with the ascent of a magma body, resulting in the depressions. The dimensions of the depressions are uncertain; B volcaniclastic sedimentary layers accumulated in the depressions. They were formed by the collapse of cauldron wall, the inflow of volcanic materials from the sur- rounding area and the first phreatic eruptions; C rhyolitic pyroclastic flows erupted from the upper part of the magma body, and relatively minor collapse occurred concurrent with their eruption;

D pyroclastic flows buried the depression and formed rhyolitic welded-tuff sheets; E rhyodacitic pyroclastic flows erupted from the lower part of the magma body, and major cauldron collapse about 20-40 km across and about 700-1000 m deep occurred concurrent with their eruption. The pyroclastic flows completely buried the depressions and formed thick rhyodacitic welded-tuff sheets; F granodiorite porphyries, which represent the deeper part of the magma body, intruded through the vent areas of the sheets and along the fractureS bounding the depressions

144

13

36~ ~

35ON+ " ,.

~ Naegi-Agematsu Granite

~ Naeg i-Agernatsu Granite concealed under the N. R.

~ Inagawa Granite

~ Inagawa Granite concealed under the N.R.

20 Km I I

N6hi Rhyolite(N.R.)

-~] Other granitic rocks in the Ry6ke Terrane

~ / ~ Rybke metamorphic rocks and basic rocks

~ ~ Rocks constituting the Mino Terrone(basement of t he N .R . )

Fig. 11. Elongation of granitic bodies around the N6hi Rhyolite (compiled from Ry6ke Research Group 1972; Yamada et al. 1974). The Inagawa Granite and the southern part of the Naegi-Agematsu Granite in and around the Sequence I cauldron are elongate in a ENE direction. The northern part of the Naegi-Agematsu Granite, interpreted to be concealed under the cauldrons of Sequences II-V, is elongated toward the NW

N6hi Rhyolite were apparently controlled by the regional tectonic framework of ENE and NW directions.

Different directions of elongation in Sequences I and II-V also characterize granitic intrusions (Fig. 11). Those in and around the Sequence I cauldron are arranged in a ENE direction, and the granitic intrusions concealed under the Sequences II-V cauldrons are elongate in a NW direction. These directions resemble those of the cauldron themselves. The granitic intrusions were formed successively after the formation of the cauldrons, so that the regional stress pattern is con- sidered to have existed during the periods of both the N6hi Rhyolite volcanism and granitic intrusions, at least from ca. 80 Ma to 58 Ma.

The orientation of the regional tectonic framework controlling both the N6hi Rhyolite volcanism and granitic intrusions indicates a tension field trending both N and NE. The tension field is recognized also in the equivalent volcanic fields on the Inner Side of SW Ja- pan (Tainosho et al. 1985; Iizumi et al. 1985), and it

was characteristic of Late Cretaceous to early Paleo- gene age on the Inner Side of SW Japan. These tectonic fields have different directions in each district and apparently resulted from extensional stress due to basement uplift rather than only by a regional horizontal tensile stress. I presume that basement uplift was caused by emplacement of a large volume of magma, from which the N6hi Rhyolite volcanism and granitic intrusions were derived. Igneous activity occurred extensively and continually on the Inner Side of WS Ja- pan during this period (Fig. 1), and the tension field has been preserved for a long time. The N6hi Rhyolite volcanism formed the tension field, and the cauldron cluster was so arranged.

Conclusions

1. The southern and central parts of the N6hi Rhyolite are divided into six volcanic sequences (I-VI). Se- quence I, III and V volcanism was derived from zoned

145

m a g m a bod i e s , and Sequence I I , IV and , p r o b a b l y , VI v o l c a n i s m was fed f rom h o m o g e n e o u s m a g m a b o d - ies. 2. One or m o r e p o l y g o n a l cau ld rons , a b o u t 15-40 k m across a n d a b o u t 400-1000 m deep , were f o r m e d in Se- quences I - IV . These c a u l d r o n s were gene ra l ly f o r m e d by two or th ree success ive depress ions . The first dep re s - s ion in each c a u l d r o n is cha r ac t e r i z ed by c o l l a p s e p r i o r to the p r i n c i p a l e r u p t i o n and is c o n s i d e r e d as an exam- p le o f a M o t o j u k u - t y p e cau ld ron . The s econd a n d th i rd d e p r e s s i o n s were f o r m e d by co l l apse du r ing the p r inc i - pa l e rup t ion . 3. The N 6 h i Rhyo l i t e is a c a u l d r o n c lus ter c o n t r o l l e d by the r eg iona l t ec ton ic f r a m e w o r k in E N E a n d N W direc- t ions. This t ec ton ic f r a m e w o r k is a t ens ion f ie ld re- su l t ed f rom up l i f t ing o f the basemen t . Up l i f t i ng is as- c r ibed to a scen t o f a la rge v o l u m e o f m a g m a in Late C r e t a c e o u s to P a l e o g e n e age.

Acknowledgements. I wish to express my sincere thanks to Dr. N. Yamada of the Geological Survey of Japan and the staff members of the Collaborative Research Group for the N6hi Rhyolite for constructive discussions at the various stages of this work. I am also grateful to Prof. K. Suwa of Nagoya University and Prof. Y. Katsui of Hokkaido University for their critical reading of the

manuscript, and to Dr. Y. Shibasaki of Government Industrial Research Institute, Nagoya for the use of XRF. Critical reviews of the manuscript by P. W. Lipman and D. A. Swanson are gratefully acknowledged.

References

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Appendix

The Japanese usage of welded-tuff sheet, volcaniclastic sedimentary layer (V.S.L.) and lava used in this paper is shown as follows

This paper Japanese usage

VI-3 Welded-Tuff Sheet VI-2 Welded-Tuff Sheet VI-1 Welded-Tuff Sheet

1748 m Peak Welded-Tuff Sheet Suzukasawa Welded-Tuff Sheet Senzawa Welded-Tuff Sheet

V-4 Welded-Tuff Sheet V-3 Welded-Tuff Sheet V-2 V.S.L. V-I V.S.L.

Sotobayama Welded-Tuff Sheet Setogawa Welded-Tuff Sheet Kurosawa Breccia Formation Shirakawa Formation

IV-4 Welded-Tuff Sheet IV-3 Welded-Tuff Sheet IV-2 Welded-Tuff Sheet IV-1 V.S,L.

Uguigawa Welded-Tuff Sheet Mayumit6ge Welded-Tuff Sheet Takadaru Welded-Tuff Sheet Atera Formation

III-5 Welded-Tuff Sheet III-4 Welded-Tuff Sheet III-3 Welded-Tuff Sheet III-2S Welded-Tuff Sheet III-2C Welded-Tuff Sheet III-IS V.S.L. III-1C V.S.L.

Yfimoriyama Welded-Tuff Sheet Akaishi Welded-Tuff Sheet Higashimata Welded-Tuff Sheet Kirikoshit6ge Welded-Tuff Sheet S6shima Welded-Tuff Sheet Futatsumoriyama Conglomerate Formation Osaka Pyroclastic Rocks

II-4 Welded-Tuff Sheet II-3 V.S.L. II-2 Welded-Tuff Sheet II-1 V.S.L.

Ak6 Welded-Tuff Sheet Volcanic Breccia Kumuro Welded-Tuff Sheet Kasuri Conglomerate Formation

I-7 Lava I-6 Welded-Tuff Sheet I-5 Welded-Tuff Sheet I-4 V.S.L. I-3 Welded-Tuff Sheet I-2 Welded-Tuff Sheet I-1 V.S.L.

Kasagiyama Lava Agigawa Welded-Tuff Sheet Enasan Welded-Tuff Sheet Volcanic Breccia Ichinosawa Welded-Tuff Sheet Kuroisawa Welded-Tuff Sheet Tozawa Formation

146

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a late cretaceous-paleogene cauldron cluster: the nôhi rhyolite, central japan

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