J. Japan. Assoc. Min. P etr. Econ. Geol. 67, 218-241, 1972

PETROLOGY OF THE MUROTOMISAKI GABBROIC COMPLEX

TOSHIHIKO YAJIMA

Department of Earth Sciences, Faculty of Education, The Saitama University, Urawa

The Murotomisaki differentiated gabbroic complex locates at the extreme point of the Cape Murotomisaki, Shikoku Island. This complex intruded into the Murotohanto Group of Eocene in age. The zonal arrangement of which complex shows the indication of the fractional crystallization with gravitational sinking. Principal rock types of the complex are as follows: chilled marginal dolerite, picrite gabbro, fine-grained hypersthene bearing olivine augite gabbro, medium-grained hypersthene bearing olivine augite gabbro, augite gabbro pegmatite and ferrogabbro. The pegmatitic rocks of the latest stage intruded as small veins. Any grain of pigeonite or inverted pigeonite is not found, whereas a small amount of hypersthene is observable as internal precipitates. Iron concentration in olivine and in augite is conspicuous at the upper part of the complex. Ilmenite is the predomi nant opaque minerals through the course of fractionation and magnetite appears at the latest stage. Fractional crystallization of OLIVINE THOLEIITE to intermediate iron concentration under low vapour pressure and low oxygen partial pressure is studied on these bases.

1. INTRODUCTION Atumi Dolerite (Kushiro, 1964), Semi Doleri There are many petrological contribu te (Kuno, unpublished), Koyama Complex tions to the differentiation of basaltic mag (Yamazaki, 1967), basaltic rocks of Oshima mas such as Skaergaard Intrusion (Wager (Kuno, et al., 1957), Takakusayama (Tiba, and Deer, 1939), Palisade Diabase (Walker, 1966), basaltic rocks of Sidara (Kuno, un 1940), Stillwater Complex (Hess, 1960), published), Sanogawa Gabbro-Diorite Com Black Jack Sill (Wilkinson, 1957, 1958) and plex (Yajima, 1970), etc. Guadalupe Igneous Complex (Best, 1963, For the identification of the rock types 1967). These studies are necessary not only of the differentiated igneous complex, pre for the determination of the process of soli sence or absence of pigeonite is an import dification, but also for the estimation of ant indicator. In the tholeiitic intrusive initial petrochemical condition of the intrud complex as the Palisade Diabase and the ed magma. The Skaergaard Intrusion, the Stillwater Igneous complex, pigeonite is Palisade Diabase and the Balck Jack Sill present, whereas in the alkalic intrusive represent the products of high alumina complex such as the Black Jack Sill, pigeo basalt magma, tholeiite basalt magma and nite is quite lacking. On the other hand, alkali olivine basalt magma, respectively. Kushiro found pigeonite in the Atumi alkali The contributions to the differentiation olivine dolerite and the present writer ob of basaltic magmas in the Japanese Islands served inverted pigeonite in the Sanogawa and in the related islands include those of Gabbro-Diorite Complex, which belongs to Morotu Igneous Complex (Yagi, 1953), calc-alkali rock.

(Manuscript received, March 21, 1972) Petrology of the Murotomisaki gabbroic complex 219

These petrochemical and mineralogical cal data as well as field and microscopic characters suggest that the physical condi observations, the mineralogy and chemistry tions of the magmas are independent from of the igneous complex are described. Pet the chemical nature of the original magma; rochemistry and differentiation process of in other words, a magma of certain chemi the complex will be described and discussed cal composition does not necessarily crystal in the following another papers. lize at a certain definite temperature or pressure. 2. GENERAL GEOLOGY To investigate this problem, it is highly The Murotohanto (Muroto Peninsula), significant to study the gabbroic complex occupying the south-eastern end of Shikoku in which pigeonite does not appear. Since Island, consists mostly of Murotohanto the Murotomisaki complex appears to be Group of Eocene in age (Katto, 1961, 1965). long to this type, studies on this gabbroic (Fig. 1) This group is composed of Oyama complex have been done in detail. misaki formation (3100m in thickness),

A geologic map of the Murotohanto Muroto formation (400m) and Naharigawa Peninsula was prepared by Suzuki (1930), formation (5300m), in the ascending order. who described the rocks of the igneous com They are in contact with each other and plex of Murotomisaki as uniform gabbroic intersected by many faults. Shijiderayama rock. In 1946, when earthquake attacked formation (300m) of Oligocene in age covers the Nankai district, the Cape Murotomisaki Muroto formation with unconformity. showed upheaval of 3 to 4 metres, and in Muroto formation is composed of sand consequence, field observation along the stone-shale alternation, shale, sandstone and continuous exposure of the complex came conglomerate with pillow lava exposed at to be possible. After this upheaval, Yoshi the seashore of Shiina. The clastic rocks of zawa (1953, 1954) investigated the complex Muroto formation show complicated folding in optical and petrochemical respects. In of various kinds of scale. The Muroto these papers, he concluded the Murotomi misaki gabbroic complex intruded into the saki complex to belong to the calc-alkali above clastic rocks, giving thermal effects rock series (hypersthenic rock series defined corresponding up to the grade of pyroxene by Kuno, 1950), on the basis of presence of hornfels facies. The Murotomisaki complex hypersthene and absence of pigeonite, also tilts 80•K southeastwards. But the zonal on the basis of chemical properties in the arrangement of the differentiated products AFM diagram (K2O+Na2O-FeO+Fe2Oa MgO diagram). The present writer studied the same igneous complex and reached a different conclusion. That is, the original magma of the Murotomisaki intrusion belongs to OLI VINE THOLEIITE and has a character of differentiation trend of intermediate iron concentration. Fourteen new chemical ana lyses of constituent minerals are presented in this paper. On the basis of these chemi 220 T. Yajima developed parallel to the structure of the tion of heavy mafic minerals as olivine and entire bodies, and gravitational concentra augite indicates the horizontal intrusion of the magma.

3. PETROGRAPHY 1. General profile of the complex The Murotomisaki complex is exposed at the extreme point of the Cape Muroto (Murotomisaki) as inclined sheet with maxi mum thickness of 230 metres. It tapers northeastwards. The country rocks of this intrusive sheet include shale-sandstone al ternation, slate, arkose sandstone and con glomerate. All the contacts between the complex and surrounding rocks are knife sharp and the marginal rocks of the complex exhibit the characteristic feature of rapid cooling of the intruded magma. Distribu tion of different rock types in the complex Fig. 2. Perspective view of the Murotomisaki show the zonal arrangement parallel to the complex. contact plane, and almost continuous out

Fig. 3. Geologic map of the Murotomisaki complex . Petrology of the Murotomisaki gabbroic complex 221

in about 20m from the contact are very much hardened like hornfels, and composed essentially of plagioclase, quartz and alkali feldspar with small amount of chlorite and sericite. There are many reddish brown lenses reaching 20cm in size, concentrated with jaroisite, pyrite, hematite and magne tite. Near the contact, pseudomorphs of hypersthene and cordierite are observed. At some parts of the contact, the country rocks of arkose sandstone origin Fig. 4. Schematic profile of the Murotomisaki exhibits granitic fabric under the micro complex. scope, and at the northeastern end of Bisha crops from lower to upper contact are ob goiwa, some portion of country rocks intrud served. ed into the chilled margin of the complex The principal rock types of the com as small dikes of 20cm in width (Fig. 5). plex from the lower to the upper part are A quartz-feldspathic white vein parallel to as follows: chilled marginal dolerite, basaltic the contact boundary is observed at the dike (branch of chilled marginal dolerite), chilled marginal part of the Kobishagoiwa. picrite gabbro, fine-grained hypersthene This vein may reach 10 metres in extension bearing olivine augite gabbro, medium and 30 centimetres in width. Chemical grained hypersthene bearing olivine augite composition of this vein is given in Table 1. gabbro, augite gabbro pegmatite and ferro This rock is composed essentially of norma gabbro. The term "picrite gabbro" is used tive quartz and feldspar (An19.4Ab67.8Or12.8) in this paper for the fine-grained hypers with small amount of normative corundum. thene bearing olivine augite gabbro, in Since this composition is quite different from which modal olivine occupies nearly 30%. those of ordinary other differentiation pro "Fine-grained hypersthene bearing olivine ducts of this complex, this rock is thought augite gabbro" is for rocks without any to have been formed by the mechanical significant variation of modal minerals in mobilization and partial melting of the handspecimen, while "medium-grained hy country rock by the heat supplied from the persthene bearing olivine augite gabbro" is intruded magma. for rocks with remarkable variation as com positional bandings. The former is charac teristic to the lower part of the sheet, where as the latter to the middle part. "Ferro gabbro" is used for rocks with olivine and augite, both of which are enriched in iron. Plagioclase with considerable amounts of ilmenite and magnetite are also character istic to this rock type. 2. Country rocks The country rocks of the complex with Fig. 5. A sketch of quartz-feldspathic vein. 222 T. Yajima

Table 1. Chemical analysis and norm value rocks showed inclination of mobilization. of quartz-feldspathic vein from upper There are many small ptygmatically marginal part of the gabbroic com folded veins in the chilled marginal parts plex. within the narrow belt of about 50cm from

the contact. Width of these veins reaches

1 cm. These folded veins are deduced to

have been formed by the deformation of

the chilled marginal parts which enclosed

the country rocks as xenolith at the time of

magmatic intrusion.

Both of the lower and upper chilled

margins are traversed at numerous localities

by small white veins composed mostly of

quartz, calcite and prehnite. They intruded along joint planes or small faults which dist

ribute almost vertically to the contact

planes. The width of them reaches about

5cm. These veins are probably the pro

ducts of later hydrothermal stage after the

solidification of the main part of the

complex.

3. Chilled marginal dolerite

The marginal part of the complex is

composed of fine-grained chilled marginal

dolerite, the width of which attains 2m.

The marginal part shows fine-grained basal

tic texture, however, many extremely thin

Fig. 6. A sektch of blocks of chilled marginal white veins intersect one another, forming dolerite in the country rock. networks. The principal constituents are

Many small blocks of chilled marginal plagioclase (An65•}) and ilmenite, with small dolerite are enclosed in the country rock at amount of magnetite. The mafic minerals some parts of the contact as shown in Fig. were altered into chlorite or actinolite. Ex 6. The diametre of these blocks attains to cept this narrow basaltic band in the chilled 20cm. These blocks are deduced to have marginal rocks, alteration of mafic minerals been formed from the chilled marginal dole is not remarkable, so that the principal rite by the action of mobilized country rock. constituents of the chilled margin are oli The melting point of the country rocks is vine, augite, plagioclase and ilmenite with estimated to be lower than that of dolerite, small amount of magnetite and hypersthene. because chemical composition of the country Olivine within this chilled marginal part is rock corresponds to granitic igneous rocks. partly altered in some grains. Alteration

Then, it is deduced that there was a certain products are in some cases grayish green stage when marginal part of the intruded to straw yellow nontronite-like mineral with gabbroic magma was solidified while country an extremely low birefringence. In this Petrology of the Murotomisaki gabbroic complex 223

pies the southeastern part of the complex (lower part of the sheet) for 20m in thick ness. This part shows pale greenish appea rance. The main constituents of this picrite gabbro are olivine, augite, plagioclase and ilmenite with small amounts of hypersthene and chlorite. Because of the intervenig sheared zone between the chilled margin and the picrite gabbro. The relation between them is not observed in this portion, but continuous, gradational relation is observed at the Maruyama differentiated gabbroic complex, which locates at the seashore 6 km northeast of Murotomisaki. The same relation is, therefore, expected in this com plex, After the investigation about variation of modal abundance of olivine in both sides of the intervening sheared zone, a rock with

Fig. 7. A sketch of basalt dike (A) and its the maximum modal abundance of olivine is schematic field relation (B). surmised to be present at the central part of the dissected sheared zone. Tabular pla marginal rocks, ophitic texture is characteri gioclase crystals are arranged parallel to the stic. The opaque minerals are largely lower contact plane. ilmenite with small amounts of pyrrhotite 5. Fine-grained hypersthene-bearing olivine and magnetite. augite gabbro At the northeastern margin of this com The picrite gabbro shows a decrease of plex there are dikes of basalt branching olivine content apart from the marginal from the chilled marginal dolerite (Fig. 7). part, and the rock changes gradually to fine These dikes are extremely hard, possibly grained hypersthene bearing olivine augite because of a sort of recrystallization by the gabbro. This rock contains about 20 volume heat sustained in the country rock after the % of olivine, and plagioclase crystals are consolidation of main part of the complex. arranged parallel to the boundary of the Chlorite, actinolite, epidote, plagioclase and complex. Augite forms many large clots of ilmenite are the principal constituents of about 4cm in diameter. In these clots a these dikes. The chemical composition of large number of olivine and plagioclase the dike rock suggests that it was formed grains are involved. A very small amounts from a later differentiated magma intruded of hypersthene is observed in the interstices into this area. The schematic field relation as rim of olivine or in the altered part of to the chilled marginal dolerite is illustrated olivine. in Fig. 8.

4. Picrite gabbro 6. Medium grained hypersthene-bearing oli

Picrite gabbro containing magnesian vine augite gabbro olivine (Fa27•}) more than 30 volume % occu The continuous gradation of rock types 224 T. Yajima

from fine-grained olivine augite gabbro to of the compositional bandings and wavy medium-grained hypersthene-bearing olivine pegmatitic veins suggest that the banding augite gabbro is observed. This medium represents the squeezed residual liquid form ed in situ. grained gabbro is characterized by the pre sence of small compositional bandings and 8. Wavy pegmatitic vein wavy pegmatitic pockets, and especially by At the lower part of the medium the gabbro pegmatite veins. Olivine and grained hypersthene-bearing olivine augite pyroxene from this rock type are a little gabbro, many small pegmatitic veins of more enriched in iron component compared wavy structure are observed (Fig. 9, 10). with those from the marginal part, while It's wave length is from 0.3 to 1.0m. Petrographical and mineralogical characters plagioclase is a little more sodic (An59•}) 7. Compositional banding in the above rock of this wavy pegmatitic veins are complete type ly the same as those of olivine augite gabbro

Compositional banding is characteristic pegmatite. But the veins are found exclu sively in the lower part of part of the to the melanocratic part of the medium medium grained gabbro, and closely associa grained hypersthene bearing olivine augite ted with the compositional bandings. gabbro, being accompanied by the small wavy pegmatitic veins. Mineral composi Judging from the several sections of

tions are the same as those of surrounding these structures in the field, those wavy veinlets would have a disk like form. The medium grained gabbro, however, volume upper part is composed essentially of plagio ratio of mfic minerals to felsic minerals is clase, while the lower part is principally quite different. Separation of these two mi composed of olivine and augite. They were neral groups is conspicuous. Width of the

compositional banding becomes gradually formed as closed system of residual liquid in the medium grained-gabbro. This system broader upwards, and at the end of broad

est banding, there appear small wavy peg is considered to be a product of later stage of solidifiction of the magma, because there matitic veins as shown in Fig. 8. The com are many independent and isolated wavy positional banding shows wavy curvature, veins apart only several tens centimetres wave length of which is less than 1 to 2 m.

The direction is almost parallel to the ex from the neighbouring one. 9. Olivine augite gabbro pegmatite vein tension of the intruded body. Arrangements This rock occupies the central part of the differentiated complex as veins, the width of which is from 0.2 to 2.0m, or as pockets of several metres across. These veins run in NE-SW direction without ex ception, that is, they are parallel to the contact plane of the intrusive complex. The maximum length of these veins may reach 20m. In most cases, it is within 10m. Their three dimensional perspective views Fig. 8. Transitional field relations from com observed at the Murotomisaki complex indi positional bandings into wavy pegmati tic veinlets. cate that they have a form of a centre-thick Petrology of the Murotomisaki gabbroic complex 225

disk formed by condensation of residual liquid. Foliation of plagioclase is characteristic in the upper part of the vein for 5 to 10cm. (Fig.10-2), showing parallel direction to the extension of the vein. Plagioclase from this narrow foliated part shows weak zoning while mafic minerals therefrom exhibit mostly of alteration by hydrothermal reaction. Lar ge plagioclase crystals reaching 5 cm in length are found to be arranged vertically to the elongation of the vein (Fig. 10-3). Mafic minerals as olivine, augite and ilmenite are concentrated in the lower part of the vein with a little amount of sodic plagio clase (Fig. 10-4, 5). Sometimes augite in cludes small grains of olivine and sometimes the latter occur as isolated grains. 10. Augite gabbyo pegmatite At the upper horizon of the complex, gabbro pegmatite consisting mostly of au gite, plagioclase, ilmenite and magnetite is present. The most conspicuous petrological character in this rock type is the absence of olivine. Olivine or orthopyroxene is utterly absent in this augite gabbro pegmatite ex cept at the lowest portion. The average length of augite or plagioclase is 3 cm, while the maximum length of augite attains to 20 cm, and that of plagioclase 6 cm. Variation of grain size in this rock is very prevailing, although the variation of chemical composi tion is not so distinct. In this gabbro peg Fig. 9. A schematic illustration of process of formation of wavy pegmatitic veinlets. matite zone, there are a great number of At the locally low pressure parts of the intruded regularly arranged melanocratic patches magma, which is mushy state with suspending crystals, residual liquid concentrates as small containing brownish-green hornblende and lense in parallel to the direction of the intruded magma. (1). The intermediate plagioclase cyrstal chlorite. About 10% of clinopyroxene there lizes at first, floating in the heavier liquid and in was altered into fibrous actinolitic horn showing foliation of feldspars. (2). Intermediate to sodic plagioclase crystallizes in secondary blende. Compositions of augite and plagio place, which shows vertical elongation to the lense clase from these patches are almost identi (3). Thirdly, augite begins to precipitate, with small amount of sodic plagioclase (4). Iron ore cal with those of the surrounding parts. concentrates at the lowest part of the wavy According to the study on this rock by veinlet, which indicates the concentration of ferriferous liquid in this portion (5). fission-track method, uranium is ascertained 226 T. Yajima to be more concentrated in the patches than ginal part of the Murotomisaki complex.

the surrounding rocks. The main constituents are fibrous actinilitic

At several lowest part of this augite

gabbro pegmatite, this rock contacts with ferro gabbro with a continuous gradation

relation, while at another point, this rock

contacts with hypersthene bearing olivine

augite gabbro pegmatite with a sharp bound

ary. At the upper part, this rock contacts

with fine-grained hypersthene bearing oli

vine augite gabbro with a continuous grada

tion at the interval of several metres.

11. Ferrogabbyo

At the central part of the differentiated

complex is exposed ferrogabbro bordering to

the lowest part of pegmatitic vein. This

ferrogabbro consists of olivine (Fa50), augite,

sodic plagioclase (An46•}) and ilmenite with a considerable amount of magnetite also with

small amount of pyrrhotite and chalcopy

rite. The ferrogabbro is the most iron

enriched rock in this complex. Reddish

brown colour due to the oxidation of iron

ores is characteristic in the field appearance

of this rock.

12. Upper fine-grained olivine augite gab

bro

The uppermost part of the augite gab

bro pegmatite contacts with fine-grained

olivine augite gabbro with a sharp boundary.

Olivine from this part is relatively

magnesian (Fa3o•}).

13. Actinolitic hornblende gabbyo

This rock has a porphyritic texture

with phenocrysts of actinolitic hornblende, Fig. 10. A schematic illustration of differentia as observed at the northeastern upper mar tion process of the intruded magma.

Fig. 11. A branch of country rocks which intruded into the chilled marginal part of the complex. Fig. 12. Quartz-feldspathic vein in the chilled marginal part of the complex. Fig. 13. Ptygmatically folded small veins in the chilled marginal part of the complex. Fig. 14. Contact aureole of the complex. Fig. 15. Basaltic dike branching from the chilled marginal part of the complex. Fig. 16. Compositional bandings in medium-grained gabbro. Fig. 17. Wavy pegmatitic veinlets. Fig. 18. Ditto. Petrology of the Murotomisaki gabbroic complex 227 223 T. Yajima Petrology of the Murotomisaki gabbroic complex 229 hornblende, plagioclase and opaque mine hydrothermal reaction. rals. Sometimes core of augite is observed 4. MINERALOGY within the hornblende crystals, suggesting it to be an alteration product of augite by 1. Mineralogical characteristics

Fig. 27. Mineral parageneses of rocks of the Murotomisaki complex.

Fig. 28. Modal abundance of constituting minerals.

Fig. 19. A wavy pegmatitic veinlet. Fig. 20. Swarm of compositional bandings and wavy veinlets. Fig. 21. Small fragment of picritic gabbro in the medium-grained hypersthene bearing olivine augite gabbro. Fig. 22. Swarm of olivine augite gabbro pegmatite veins. Fig.23. Lower part of a olivine augite gabbro pegmatite. Fig.24. Olivine augite gabbro pegmatite. Fig. 25. Close-up photo of augite gabbro pegmatite. Fig. 26. Olivine augite gabbro pegmatite. 230 T. Yajima

The principal minerals of the complex Table 2. Lattice constants (130 plane) and are olivine, augite, plagioclase (labradorite estimated compositions of olivines to andesine), ilmenite and magnetite with small amounts of orthopyroxene (bronzite to hypersthene) and hornblendes (brown common hornblende and green common horn blende). In some places, actinolitic horn blende is one of the principal constituents.

A very small amount of biotite is present 1. Picrite gabbro (T.Y. 670422071) in the chilled margin and in the ferrogabbro 2. Olivine augite gabbro pegmatite (T. Y. 67042310) which is a product of the latest stage. 3. Olivine augite gabbro pegmatite (T. Y. The accessory minerals are serpentine, 670422072) chlorite, actinolite, prehnite, calcite, apa 4. Ferrogabbro (T. Y. 67042101) A-fraction 5. Ditto. B-fraction tite, pyrrhotite and pyrite. The mineral 6. Ditto. C-fraction paragenesis and modal abundance of con stituting minerals of the complex are sum are shown in Table 2. The later value is marized in Fig. 27 and in Fig. 28. an exceptional case and generally Fo55 is As far as exmained, pigeonite is not the limit composition of olivine of the found and orthopyroxene is rare and not so latest stage product. The compositional petrologically significant if present as stated variation in a single handspecimen is very later. Plagioclases are fairly variable in conspicuous especially in ferrogabbro. As compositions ranging from An70 to An30. stated in the chapter of petrography, there Quartz seems to be absent even in the is an expected compositional difference product of the latest stage, but it is found between those of independent grains and along joint planes of some gabbros or it is those of inclusions in augite. found as veinlets probably of country rock Olivines are short prismatic in Corm, origin. and less than 2mm in grain size. Gene One of the most characteristic features rally, olivines are unaltered. But along the of the mineral paragenesis is the disap lower marginal part of the complex they pearance of olivine in augite gabbro are partially replaced by the aggregates of pegmatite and reappearance in ferrogabbro light green chlorite and serpentine under of the latest stage product. Another the microscope, sometimes leaving the characteristic is the appearance of consider pseudomorph. Very locally a green to able amount of magnetite in gabbro pegma yellow nontronite-like mineral replaces them tites formed at the latest stage of soldi in the lower marginal gabbro, whereas red fication. dish brown iddingsite is a characteristic 2. Olivine alteration product of olivine in some parts The compositions of olivines were of ferrogabbro. estimated by the X-ray powder method of Besides the above described alteration measuring d130 value (Yoder and Sahama, products, occasional presence of orthopy 1957). The results including the most roxene is worth mentioning. Orthopyro magnesian (Fo73)and the most ferroan (Fo48) xene is found sometimes in the pseuod Petrology of the Murotomisaki gabbroic complex 231 morph of olivine in olivine-bearing gabbro, 3. Clinopyyoxene but not found in the gabbro free from In this complex, clinopyroxene is olivine. exclusively augite ((+) 2V about 45•K), and

Table 3. Chemical analyses and atomic numbers of clinopyroxene (Analyst: H. Haramura)

1. Augite from picrite gabbro, lower marginal part of the complex, (1779, T.Y. 670422071a) 2. Augite from small augite gabbro pegmatite vein, lower part of the complex (1773, T. Y. 67042101) 3. Augite from augite gabbro pegmatite, uppermost part of the complex, (1782, T. Y. 67042203) 4. Augite from ferrogabbro (1790, T. Y. 67042101') 5. Augite from large vein of olivine augite gabbro pegmatite, central part of the complex (B., T. Y. 670422072) 6. Augite from small vein of olivine augite gabbro pegmatite, central part of the complex (A., T. Y. 67042310) 7. Augite from leucocratic part of large vein of olivine augite gabbro pegmatite (1826, T. Y. 670422073) 232 T. Yajima

Table 4. Optical properties of clinopyroxenes

Sample number correspondes to that of Table 3., except number 8, which is chemically unanalyzed augite from chilled marginal dolerite (T. Y. 68051903).

the other gabbros of tholeiite or calc-alkali

type. Wollastonite molecules of these

augites are about 42 mole %, and they are

on a trend almost parallel to that of augites

from the Skaergaard intrusion. Augite

from the picrite gabbro has a higher MgO

and a little lower CaO contents compared

with those from the differentiated products Fig. 29. Fractional trend of pyroxene (augite) of later stage. Variation of Fe2+/Mg2+ with composition of coexisting olivine (broken tie lines). ratio is restricted within a narrow range. 4. Orthopyroxene pigeonite or inverted pigeonite is not found Orthopyroxene of this complex is rather as far as examined. The augite exhibits subordinate in modal abundance, i.e., up to idiomorphic outline in the marginal 0.1 modal %. In the chilled marginal hypersthene-bearing olivine augite gabbro, dolerite, isolated grains are found under the whereas irregular outline in augite gabbro microscope in the neighbourhood of olivine, pegmatite. exhibiting reaction relation between them. In fine and medium-grained gabbros, However, in most cases, it is found as fibrous separate augite grains show simultaneous small grains near the central parts of extinction frequently in thin section, but altered olivine grains. The pleochroism do not always suggest the three-dimensional of the orthopyroxene from inner part of the continuity of the grains. Augites from complex is extremely weak from pale pink gabbro pegmatites are not so rarely idiomor to colourless, and less remarkable than phic. The maximum length is 20cm. In that of orthopyroxene from the other spite of such a large size, zoning is not parts. The range of (+) 2V of orthopy observed optically. Pleochroism is remark roxene is from 77•K to 53•K, corresponding able from pale bluish green to pale yellowish to En80Fs20-Enb3Fs42 after Hess (1952). green, especially in the augite from the Orthopyroxenes are exclusively found in gabbro pegmatite of the latest formation. the rocks containing olivine and especially Seven chemical analyses of augites from in the altered parts along leucocratic this complex are given in Table 3. They veinlets less than 1mm width. From this contain low Na2+K2O (0.4-0.6 wt%), microscopical feature, orthopyroxene of this

relatively low Al2O3 (0.5-4.0 wt%) and low complex may well be regarded as of internal

TiO2 (0.7-1.4 wt%), like the augites from precipitation origin; that is, they may have Petrology of the Murotomisaki gabbroic complex 233

been formed by the reaction of olivine and a Table 5. Chemical analyses of plagioclase (Analyst; H. Harmaura) solution of deuteric origin. This solution might have been under the effect of country sedimentary rocks, so that the solution was slightly silica rich, being accompanied by the reaction mentioned above. Thus orthopy

roxene in this complex is regarded as

product of secondary and not as product of

primary crystallization stage at which most

part of the intruded magma was solidified. 5. Plagioclase

Plagioclase of chilled marginal dolerite

shows generally a weak zoning from An65

to An55, but such distinct zoning is rather

exceptional. The most calcic plagioclase

(An80•}) is found in basaltic dike, which forms a branch of chilled marginal dolerite, while Atomic Number ( 0=32) the most sodic one (An35•}) is found in

ferrogabbro, which is a product of the later

stage. Chemical composition of plaigoclase

from picrite gabbro is about An71, whereas

normative plagioclase composition of the

chilled marginal dolerite is about An64.

Accordingly, at the earliest stage of solidifica

tion, plagioclase of An71, was crystallized

from the magma, however, normative

plagioclase of the rock was about An64. In this complex, chemically analyzed values

are nearly equal to those normative com

position values. This coincidence of modal compositions with normative ones may be 1. Plagioclase from picrite gabbro, lower mar due to the low content of Al2O3 in clino ginal part of the complex (1784, T. Y. pyroxene; most of Al2O3 in the rock allotted 670422071) 2. Plagioclase from augite gabbro pegmatite, to normative plagioclase is estimated to be uppermost part of the complex (1825, T. Y. used up to form modal plagioclase. 67042203) Alteration product of plagioclase is 3. Plagioclase from ferrogabbro (1783, T. Y. 67042101) quite lacking except several cases at some 4. Plagioclase from augite gabbro pegmatite parts of augite gabbro pegmatite. Five vein (1824, T. Y. 67042101)

plagioclases were chemically analyzed, the 5. Plagioclase from augite gabbro pegmatite result of which are shown in Table 5. (1827, T. Y. 670422073)

There is not so much outstanding regularity in the variation of normative orthoclase plagioclase components (Fig. 30). Plagio components compared with normative clase of picrite gabbro represents highest 234 T. Yajima

Table 6. Lattice constants of magnetite

1. Augite gabbro pegmatite (T.Y. 67042203) 2. Olivine augite gabbro pegmatite (T.Y. 68042310) 3. Olivine augite gabbro pegmatite (T.Y. 670422072)

Fig. 30. Contents of orthoclase component in plagioclase (An-Ab-Or diagram). 6. Ilmenite and magnetite Solid circle: chemically analyzed, Opaque minerals of this complex were Open circle: normative composition. studied by reflection microscope, and X-ray diffraction patterns were used for determination of their lattice constants (Table 6). The most predominant opaque minerals are iron oxides, ilmenite and mag netite. They are found as independent grains, but not as pararell intergrowth of exsolution products. Ilmenite is found in all rock species without exception, while magnetite is observed only in the gabbro pegmatite of later stage. Therefore, in the early half stage of differentiation of the Fig. 31. Variation of 2Į(131)-2Į (131) against magma, ilmenite was the only predominant composition of plagioclase. opaque mineral, whereas magnetite ap value of orthoclase component. According peared in the later pegmatitic stage. This to the results of X-ray powder pattern characteristic mineral parageneses throw studies, the values of 2Į (131)-2Į (131) of light upon the intermediate iron concentra plagioclases represent high form at the tion in this complex. This character may early stage, while low form at the latter have been due to intermediate Poz in the stage. For this determination the figure magma intruded into this region. proposed by Smith and Yoder (1955) is used. Pyrrhotite and chalcopyrites are found

(Fig. 31) as subordinate opaque minerals in fer rogabbro.

Fig. 32. Fine-grained olivine augite gabbro. Fig. 33. Medium-grained olivine augite gabbro. Fig. 34. Olivine augite gabbro pegmatite. Fig. 35. Augite gabbro pegmatite. Fig. 36. Ferrogabbro. Fig. 37. Photomicrograph of quartz-feldspathic vein (crossed nicols). Fig. 38. Photomicograph of chilled marginal dolerite (open nicol). Fig. 39. Photomicrograph of chilled marginal dolerite (open nicol.) Petrology of the Murotomisaki gabbroic complex 235 236 T. Yajima Petrology of the Murotomisaki gabbroic complex 237

5. DISCUSSIONS gabbro. The compositional gap is also It is one of the most characteristic found in augites, that is , augites from mineralogical features of this complex that ferrogabbro are a little more enriched in ferrosilite molecule than that of augite olivine disappears at augite gabbro peg matite stage and it reappears at the gabbro pegmatite, which is a little more enriched in ferrosilite molecule than that of stage of ferrogabbro which is possibly a medium-grained gabbro. product of the latest differentiation stage. In ferrogabbro, two different modes of The mineral parageneses are shown in Fig. occurences of olivine are recognized; in one 27. case olivine is included in augite grains, and At the upper part of the Murotomisaki in another case, olivine is present as isolated intrusive sheet is present augite gabbro grains. The presence of two different modes pegmatite without olivine. This rock is in of occurences and of compositional range contact with medium-grained hypersthene in olivines in ferrogabbro suggests that bearing olivine augite gabbro. Ferrogabbro olivines crystallized at least in two different including considerable amounts of olivine, stages; one is early pegmatitic stage and augite, ilmenite and magnetite, is present another is later pegmatitic stage. In the as small pockets in augite gabbro pegmatite former stage, olivine precipitated down to or along the lower boundary between augite the lower parts of the molten body, while gabbro pegmatite and medium-grained in the latter, olivine crystallized in situ and hypersthene-bearing olivine augite gabbro. did not precipitate. Magnetite crystallized The most iron rich olivine in medium also at this stage. However, there was a grained gabbro is about Fa40 in composition, stage between the above-mentioned two while the most magnesian olivine in fer stages, when olivine was not formed as rogabbro is about Fa42. Hence, there is proved by the formation of olivine-free augite no significant range gap between them, gabbro pegmatite. suggesting the presence of continuous These petrographical and mineralogical succession in compositional variation of characteristics suggest that some parts of olivine. ferrogabbro were formed after the soli On the contrary, composition of dification of main parts of augite gabbro plagioclase of ferrogabbro is a little more pegmatite and other parts of ferrogabbro sodic than that of augite gabbro pegmatite, were formed contemporaneously. In other while the latter is a little more sodic than words, the ferrogabbro proves that it in that of medium-grained gabbro. Then, cludes the products of successive differentia there is a short range gap in plagioclase com tion and of contemporaneous formation and positions of ferrogabbro and medium-grained there was a stage when olivine was not

Fig. 40. Photomicrograph of picrite gabbro (open nicol). Fig. 41. Photomicrograph of fine-grained olivine augite gabbro (crossed nicols). Fig. 42. Photomicrograph of medium-grained olivine augite gabbro (crossed nicols). Fig. 43. Photomicrograph of augite gabbro pegmatite (open nicol). Fig. 44. Ditto (crossed nicols). Fig. 45. Photomicrograph of ferrogabbro (open nicol). Fig. 46. Photomicrograph of ferrogabbro, olivines in augite (crossed nicols). Fig. 47. Photomicrograph of hypersthene in medium-grained olivine augite gabbro (open nicol). 238 T. Yajima crystallized within the successive differentia 5. Wo-Fo-FeO-SiO2system (Osborn, 1964, tion process. based on Bowen, 1914,Schairer and Bowen, The disappearance and reappearance 1942; Osborn and Muan, 1960; Bowen and of olivine are possibly explained in the Schairer, 1935; Bowen, Schairer and Posna light of the system forsterite-diopside-silica jak, 1933) system by Kushiro and Schairer (1963) as 6. Wo-Fo-Fe2O3-SiO2system (Osborn, will be discussed hereafter. 1964; Muan and Osborn, 1956) The main constitutents of this complex 7. Wo-MgO-FeO(Fe2O3)-SiO2system (Pres are olivine, augite, plagioclase, ilmenite and nall, 1966) magnetite as described in the preceding 8. MgO-FeO-Fe2O3-CaA12Si2O8-SiO2system chapters. Since, it is necessary for the in (Roeder and Osborn, 1966). terpretation of differentiation of the magma The last two systems, 7 and 8, cover to refer the synthetic systems including the all ranges of the above other systems. following components: The process of the crystallization, there 1. Olivine (Forsterite-Fayalite) fore, will be reasonably followedin the Wo 2. Augite (Diopside-Hedenbergite MgO-FeO(Fe2O3)-SiO2system, by - con Enstatite-Ferrosilite)- sidering the effect of Anorthite-Albite con 3. Plagioclase (Anorthite-Albite) tent in this system. 4. Ilmenite, Magnetite. Principal mineral parageneses of this Orthoclase component in plagioclase is complex are summarized as follows; negligible in this case (2.1-0.8 mole %). A. Intrusion of magma and start of cryst TiO2 content of augite is very small (1.43 allization of olivine, followedby cryst -0.48 wt%), and crystallization of ilmenite allization of plagioclase, is only one principal factor of regulating B. Start of crystallization of augite, TiO2 content in the residual liquid, and in C. Disappearanceof olivine and appearance consequence, precipitation of ilmenite took of magnetite, place. Crystallization of magnetite has a D. Local crystallization of Fe-olivine. much more important role in the course of The course of differentiation of magma differentiation, governing the chemical is schematicallyshown in Fig. 48. Chemical compositions of augite and olivine. There composition of the intruded magma locates fore, it is necessary to refer the natural and at a point in the olivine volume in this synthetic systems including oxidation tetrahedron (A). After the precipitation of reduction of iron. The synthetic systems a certain amount of olivine, magma changes chemically close to the magma of this com its compositiontowards B, which is on the plex are as follows; extension of a line connecting point Fo and 1. MgO-FeO-SiO2 system (Bowen and point A. At point B, the magma reaches Schairer, 1935) the boundary surface (cotectic surface) 2. Diop-An-Ab system (Bowen, 1915; between olivine volume and diopsidic Yoder, 1965) pyroxene volume (Ca-pyroxene volume), 3. Diop-Fo-SiO2 system (Bowen, 1914; and starts to precipitate augite, forming Kushiro and Schairer, 1963) olivine augite gabbro. At point C, pre 4. Hed-Fa-SiO2 system (Bowen, Shairer cipitation of magnetite begins in this and Posnajak, 1933) tetrahedron. On the contectic line X-Y, Petrology of the Murotomisaki gabbroic complex 239

Fig. 49. Phase diagram of forsterite-diopside silica system, partly enlarged.

Fig. 48. Mg2SiO4-iron oxide CaSiO9 SiO2 tetra hedron, partly enlarged. at each differentiation stage. Precipitating clinopyroxene of c from liquid C, the parallel crystallization of forsterite, diopsi residual liquid changes its composition to de and magnetite (also with plagioclase and D, where olivine does not crystallize. Pre ilmenite in the natural system) continues, cipitating clinopyroxene of d, e, from liquid however, fractional crystallization of for D,E, the residual liquid reaches to cotectic sterite in large amounts result poverty of line and follows reappearance of olivine. this component in the residual magma, following disappearance of olivine (C-D-E). 6. SUMMARY In this case, there is a possibility of re The Murotomisaki differentiated gab apearance of olivine after subtraction of broic complex locates at the extreme point fairly amounts of diopsidic pyroxene (augite) of the Cape Murotomisaki, Shikoku Island. from the magma at point F. For this This complex intruded into the Muroto course of crystallization, reaction of olivine hanto Group of Eocene in age. The zonal with clinopyroxene is necessary. arrangement of this complex shows the Diop-Fo-En-SiO2 phase diagram cor indication of the fractional crystallization responds to a projection of this process on with gravitational sinking in situ. Prin the Wo-MgO-SiO2plane of that tetrahedron cipal rock types of the complex are as (Fig. 49). In the Diop-Fo-En-SiO2 system, follows: chilled marginal dolerite, picrite following course of crystallization is con gabbro, fine-grained hypersthene bearing cluded. After the precipitation of forsterite olivine augite gabbro, medium-grained from original magma A, the residual liquid hypersthene bearing olivine augite gabbro, changes its composition to B. At point B, augite gabbro pegmatite and ferrogabbro. crystallization of diopsidic pyroxene begins. The pegmatitic rocks of the latest stage Brokened lines represent the direction of situate at the upper part of the complex variation of residual liquid and b, c, d, e as small veins. Any grain of pigeonite or represent the composition of clinopyroxene inverted pigeonite is not found, whereas a 240 T. Yajima

small amount of hypersthene is recognized - 217. Hess, H.H. (1960), Stillwater igneous complex, as internal precipitates. Iron concentra Montana. Geol. Soc. Amer. Mem. 80, 284pp. tion in olivine and augite is conspicuous Katto, J., et al. (1961), Geology and mineral re at the upper part of the complex. Olivine sources of Kochi Prefecture (in Japanese). Govern. Kochi Pref. is absent in the augite gabbro pegmatite, (1965), A note•\•\•\•\ on some concretions from whereas present in ferrogabbro which con the Muroto formation (Eocene) of Kochi Pre tains a considerable amount of magnetite. fecture, Shikoku, Japan. Kochi Univ. Res. Rept. 14, Natur. Sci. 1. 2. The absence of olivine in the augite gabbro Kuno, H. (1950), Petrology of Hakone Volcano and pegmatite is explained by the experimental the adjacent area, Japan. Bull. Geol. Soc. results of the forsterite-diopside-silica system Amer. 61, 957-1020.•\•\•\•\ at 1 atm. Because of the precipitation of Yamasaki, K., Iida, C., and Nagashima, K., (1957), Differentiation of Hawaiian mag magnetite at the middle stage of solidifica mas. Japan. Jour. Geology and Geography, tion, the concentration of iron in the 28, 179-218. residual liquid was not so conspicuous com •\•\•\•\(1965), Fractionation trends of basaltic magmas in lava flows. Jour. Petr. 6, 302 pared with the Skaergaard intrusion, leading 321.•\•\•\•\ - to the formation of a rock type with in (1968), Differentiation of basaltic magmas termediate degree of iron concentration (Basalts, vol. 2. edited by H.H. Hess,) John during the process of fractionation. Petro Wiley, New York, 623-688. Kushiro, I. (1964 a), The system diopside-forsteri chemical study about this complex will be te-enstatite at 20 kilobars. Geophys. Labr. published in another papers. Year Book 63, 102-108.

(1964 b), Petrology•\•\•\•\ of the Atumi doler ACKNOWLEDGEMENTS ite, Japan. Jour. Fac. Sci., Univ. Tokyo, Sec. 11. 15, 2, 135-202. Main part of this work was carried out and Schairer,•\•\•\•\ J. F. (1963), New data on at the Geological Institute, University of the system diopside-forsterite-silica. Geophys. Labr. Year Book 62, 95-103. Tokyo, under the guidance of late Professor Muan, A., and Osborn, E. F. (1956), Phase equili Hisashi Kuno, to whom I am greatly bria at liquidus temperatures in system MgO thankful for valuable suggestions. FeO-Fe2Oa SiO2. Jour. Amer. Ceram. Soc. 121

The author is also indebted to Drs. S. 140. - Osborn, E. F. (1959), Role of oxigen pressure in the Aramaki, I. Kushiro and A. Kato, who crystallization and differentiation of basalt ma have critically reviewed the manuscript. gma. Amer. Jour. Sci. 257., 609-647. The writer also wishes to express his (1962), Reaction series •\•\•\•\ for subalkaline igneous rocks based on different oxigen pres thanks to Mr. H. Haramura for chemical sure conditions. Amer. Min. 47., 211-226. analyses of rocks and minerals. Presnall, D. G. (1966), The join forsterite-diospide iron oxide and its bearing on crystallization of

REFERENCES basalt and ultramafic magmas. Amer. Jour. Sci. 264., 428-480. Best, M. G. (1963), Petrology of the Guadalupe ig Roeder, P. L. and Osbom, E. F. (1966), Experiment

neous complex, southwestern Sierra Nevada al data for the system MgO-FeO-Fe2Oe CaAl2 Foothill, California. Jour. Petr. 4, 223-259.•\•\•\•\ Si2O8-SiO2 and their petrologic implication. (1967), Compositions and crystallization Amer. Jour. Sci. 264., 753-809. of mafic minerals in the Guadalupe igneous Smith, M. A. (1955), The powder patterns and lat complex, California. Amer. Min. 52, 436-474. tice parameters of plagioclase feldspars, I. The

Bowen, N. L. (1914), The ternary system diopside soda-rich plagioclases. Miner. Mag. 31, 47-68. forsterite-silica. Amer. Jour. Sci. 38, 207-264.•\•\•\•\ Tiba, T. (1966), Petrology of the alkaline rocks of

and Schairer, J. F. (1935), The system the Takakusayama district, Japan. Sci. Rep. MgO-FeO-SiO2. Amer. Jour. Sci. 29, 151 Tohoku Univ. Ser. III, 9, 3., 541-610. Petrology of the Murotomisaki gabbroic complex 241

Wager, L. R. and Deer, W. A. (1939), The petrology 3., 199-230. of the Skaergaard intrusion, Kangerdlugssuaq, Yamazaki, T. (1967), Petrology of the Koyama calc -alkaline intrusive complex East Greenland. Medd. om Grfnland, 105, 4., , Yamaguchi Pre 1-352. fecture, Japan. Sci. Rep. Tohoku Univ. Ser. Walker, F. (1940), Differentiation of the Palisade III, 10, 1., 99-150. diabase, New Jersey. Geol. Soc. Amer. Bull. Yoder, H.S. Jr. and Sahama, Th. G. (1957), Olivine 51., 1059-1105. X-ray determinative curve. Amer. Miner. 42., Wilkinson, J. F. G. (1957), The clinopyroxenes of a 475.•\•\•\•\ differentiated teschenite sill near Gunnedah, New and Tilley, C. E. (1962), Origin of basalt, South Wales. Geol. Mag., 94., 123-134.•\•\•\•\ an experimental study of natural and synthe

(1958), The petrology of a differentiated tic rock systems. Jour. Petr. 3., 342-532. teschenite sill near Gunnedah, New South Yoshizawa, H. (1954), On the gabbro of the Cape Wales. Amer. Jour. Sci. 256, 1-39. Muroto, Shikoku Island, Japan. Part I. Mem. Yagi, K. (1953), Petrochemical studies on the Coll. Sci. Univ. Kyoto, Ser. B. 20, 4., 271 alkalic rocks of the Morotu district, Sakhalin. 284.•\•\•\•\ -

Geol. Soc. Amer. Bull. 64., 769-810. (1954), Ditto. Part II. Ditto. 21, 2, 193 Yajima, T. (1970), The Sanogawa gabbro-diorite 212. - complex. Sci. Rep. Saitama Univ. Ser. B. 5,

室戸岬はんれい岩体の岩石学的研究

矢 島 敏 彦

室戸 岬 はん れ い 岩 体 は始新 世 頃 の窒 戸 層 群 を貫 く,厚 さ最高230mの 層 状 分 化 岩 体 で あ っ て,重 力 場 にお け る結 晶分 化 作 用 の た め 粗,岩 体 下 部 よ り上部 にかけ て,広 範 囲 に わ た る岩相 の 変化 が み ら れ る。 即 ち,下 部 よ り,周 縁 部 粗 粒 玄 武 岩(急 冷 相),ピ ク ライ ト質ハ ン レ イ岩,含 紫 蘇 輝 石-か ん らん石-普 通 輝 石 ハ ン レ イ岩, 普 通 輝 石 ハ ン レ イ岩 質 ペ グ マ タ イ ト,鉄 質 ハ ン レ イ岩等 で あ る。 か ん らん石 、 普 通輝 石 に は漸 進 的 な鉄 分 の増 加 が認 め られ る。 不 透 明 鉱物 は 主 と して チ タ ン鉄 鉱 で あ って,磁 鉄 鉱 は 最末 期 粗 のみ 形成 され る。 これ ら各種 岩 石,鉱 物 の考 察 か ら,か ん らん石 ソ レ イ ア イ ト質マ グ マ の低 水 蒸 気 圧,低 酸 素 圧 下 で の 晶 出 分化 に よる,中 程 度 の鉄 分 濃 集 の分 化 経 路 が 研 究 され た。 特 に,か ん らん石 と普通輝 石 との 間 の反 応 関係 の 存 在 が推 論 され た。