sulfur isotopic composition of the magnetite-series and ilmenite-series granitoids in japan

Contrib. Mineral. Petrol. 68, 107-115 (1979) Contributions to Mineralogy and Petrology �9 by Springer-Verlag 1979

Sulfur Isotopic Composition of the Magnetite-Series and Ilmenite-Series Granitoids in Japan

Akira Sasaki and Shunso Ishihara

Geological Survey of Japan, Tokyo Branch, 8 Kawada-cho, Shinjuku-ku, Tokyo 162, Japan

Abstract. Sulfur isotopic composition has been mea- sured on 30 granitoids and 11 gabbroids from the Cretaceous and Tertiary granitic terranes of Japan. The two series of granitoids, the magnetite-series and ilmenite-series, defined by Ishihara (1977), show two specific isotope trends. The magnetite-series granitoids all have positive 6 3 4 S (CDT) values from + 1 to + 9 % o, while the ilmenite-series rocks are dominated by negative values between - 1 1 and + 1% o. The trend in the ilmenite-series is consistent with the thesis that the magma has been influenced by light biogenic sulfur from the continental crust. The inferred large scale magma-crust interaction in the ilmenite-series granitoids indicates that the emplacement of this series of magma has been governed by a stoping mechanism.

In contrast, the magnetite-series granitoids have little if any evidence for significant magma-crust interaction, indicating that the intrusion of this series of magma may have been more or less of fissure-filling type. Their trend towards positive c5 3 4S values (average ~ +5% o) argues for the introduction of some heavy sulfur, probably of seawater origin, into the mantle derived sulfur. This is most likely to occur in an arctrench system by the subduction of an oceanic plate which accompanies the sulfate-bearing pelagic sediments.

The isotopic data of gabbroids, mostly between - 1 and + 3 % o, are close to the commonly assumed value for mantle sulfur. Nevertheless, the gabbroids from the magnetite-series granitic terranes tend to have higher 6 34 S value than those from the ilmenite- series belts. It is inferred that the factors controlling the isotope characteristics of the granitoid sulfur have also been operative in these grabbroids at least to some extent.

Introduction

Recent work by one of us (Ishihara, 1977) established that common granitoids are divided into two series

according to their opaque mineral assemblage. The magnetite-series granitoids are characterized by the presence of magnetite (0.1-2 vol%), ilmenite, hema- tite, and pyrite or chalcopyrite, while the ilmenite- series granitoids are practically free of opaque oxide minerals but contain very small amount of ilmenite (less than 0.1 vol%) with the occasional presence of varying amounts of pyrrhotite. The two series of granitoids are thought to have been derived from different source magmas, the former being generated presumably in deeper levels, namely, the upper mantle and/or the lower crust, whereas the latter may be derived from shallower levels of the crust with possible incorporation of carbon-bearing metamorphic and sedimentary rocks.

The isotopic composition of terrestrial sulfur is known to be rather uniform and close to the meteoritic value in mantle materials but more or less fraction- ated in crustal constituents. It is therefore expected that if the above assumption on the two series of granitoids is true their sulfur isotopic data must differ. Such data, if they become available, would provide another useful approach to the problems of granitoid genesis. Thus in this study the ~ 3 ~S values were mea- sured on 30 granitoid samples which were selected to represent the most common, and fresh members of the well documented Cretaceous and Tertiary granitic terranes in Japan. Eleven specimens of gabbroids occurring in the same terranes were also examined. The data are reviewed and discussed in terms of their possible bearings on the magma genesis.

Geological Setting and Samples

Japanese granitoids occur extensively over the four major islands and are relatively young, viz. from early Cretaceous onward, except for the Funatsu granitoids of Jurassic age. Major granitic masses tend to follow geotectonic units controlled by basement geology, and are grouped chronologically into the Cretaceous-Paleogene and the Miocene ones. The former granitoids are further divided

0010-7999/79/0068/0107/$01.80

108

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MAGNETITE SERIES ILMENITE SERIES

�9 GRANITOIDS 0

�9 GABBROIDS r l

lttMIOCENE PLUTON OFGREEN TUFF BELT

~ STRONGLY - - MA GNE T/TE-SERIES BELT - - WEAKLY 1

- - I L M E N I T E - S E R I E S B E L T

A. Sasaki and S. Ishihara : Sulfur Isotopic Composition of Granitoids

% /~.ES'N - - 7 T

+

sAN'It

jApA N

r

0 ~ , 300 KM

BELT

~A TECTONIC LINE

--~--SO'N

Fig. 1. Distribution of the magnetite-series and ilmenite-series granitic belts and 634S data of granitoids and gabbroids in the Japanese islands. Original 334S values (Tables 1 and 2) are rounded. M T L : Median tectonic line; ISL: Itoigawa- Shizuoka line; SBB." Sanbagawa metamorphic belt; KMB." Kamuikotan metamorphic belt. On the east of ISL is the Fossa Magna area. The dotted line is the eastern edge of the Green Tuff plutonic belt of northeast Japan; that of southwest Japan is not shown

tectonically into the groups of northeast Japan and southwest Ja- pan, which are separated by the Tanakura tectonic line (Fig. 1). Gabbro and diorite are sporadically distributed in the granitic terranes as small xenolithic masses having exposed areas of about 10 km 2. Where radiometric ages are available, the gabbroid and

granitoid show no difference in age (Fukasawa and Onuki, 1972). The Cretaceous-Paleogene granitoids of northeast Japan are

generally massive and yield Cretaceous mineral ages, although some foliated ones in the westernmost part of the Abukuma belt may be older. The granitoids of northeast Japan vary in composition from tonalite to monzogranite and belong mainly to the magnetite series. There appears to be a trend in each belt such that the magnetite content increases slightly toward the oceanic side. Gabbro and diorite are rather dominant.

The largest exposure of the Cretaceous-Paleogene granitoids occurs in the area to the west of the Tanakura tectonic line, which is assigned to the Inner Zone of southwest Japan. The rocks are mostly granodiorite and monzogranite, and gabbroids occur sporadically as isolated masses in and near the granitoids. In the south- ernmost part of the Ryoke belt, the granitoids are generally foliated and the exposure is bounded on the south by the Median tectonic line. Both granitoids and gabbroids belong to the ilmenite-series in the Ryoke belt.

The Ryoke granitoids change gradually northward into massive granitoids of the San-yo belt, but both are chemically similar, if these are compared with granitoids of the San-in belt (Ishihara

and Terashima, 1977). Granitoids of the San-yo belt are monzogranite and some granodiorites with an enrichment of lithophile elements. The majority of these belong to the ilmenite-series but magnetite occurs in the northern half and its amount increases gradually to the north. Thus, granitoids of the San-in belt cannot be separated sharply from those of the San-yo belt and are characterized by abundant magnetite and by a depletion of the lithophile elements. Within the Cretaceous-Paleogene granitic terranes of the Inner Zone, magnetite of both granitoids and gabbroids tends to increase toward the marginal sea side.

Miocene granitoids occur along the Japanese island arc, parallel to the major trench. Granitoids of the Green Tuff belt cut the structures of the older basement. The granitoids are massive having the composition of tonalite and granodiorite in general. Gabbro and diorite are often accompanied by the granitoids. All are strongly magnetite bearing. The other type of Miocene pluton, belonging to the ilmenite series, is present in the oceanic side of the island arc. The granitoid occurrences follow the geotectonic units of the basement (e.g., Shimanto and Hidaka belts). They are massive consisting mostly of granodiorite and monzogranite. Mafic rocks are very rare in these belts. In the Outer belt of southwest Japan, the granitoids contain many xenoliths of pelitic origin, while in the Hidaka belt the interaction with the crustal materials results in migmatites seen along the westernmost zone.

The samples used in this study, 30 granitoids and 11 gabbroids, were all from representative exposures of the above mentioned

A. Sasaki and S. Ishihara : Sulfur Isotopic Composition of Granitoids 109

Table 1. a 343 values of Japanese granitoids

Serial Sample No. Locality Rock Name S(%) 6 3~S Sulfide No. (~

Magnetite-series Belts

Kitakami-Abukuma Belt

1 K-49 2 A-51

San-in Belt

3 RS-88

4 M-16 5 11-116 6 328-12

Green Tuff Belt

? HK-1

8 AN-7 9 TN- 154

10 HD-3 a II CB-84 ~ 12 TI-3 ~

Tono, Iwate Pref. Hornblende-biotite granodiorite 0.005 + 1.2 Pyrrhotite(?) Naraha, Fukushima Pref. Hornblende-biotite granodiorite 0.002 +4.4 Pyrite

Shirakawa, Gifu Pref.

Nodagawa, Kyoto Pref. Minari, Shimane Pref. Kumoki, Shimane Pref.

Jozankei, Hokkaido

Ani mine, Akita Pref. Tanigawadake, Niigata Pref. Kadohara, Fukui Pref. Chichibu Mine, Saitama Pref. Tsushima, Nagasaki Pref.

llmenite-series Belts

San-yo Belt

13 JG-1 Sori, Tochigi Pref. 14 10-125 Otani mine, Kyoto Pref. 15 H-126 b Hobenzan, Yamaguchi Pref. 16 H-129 b Hobenzan, Yamaguchi Pref.

Ryoke Belt

17 NI-6 Nezugaseki, Niigata Pref. 18 10614 Tsukuba, Ibaraki Pref. 19 TO-329 Aji, Kagawa Pref. 20 MY-6 Kikuma, Ehime Pref.

Hornblende-biotite quartz 0.006 + 8.3 Pyrite monzodiorite

Hornblende-biotite granodiorite 0.003 +4.8 Pyrite Hornblende-biotite granodionite 0.003 +3.9 Not visible Biotite monzogranite 0.037 +6.3 Pyrite

Hornblende-biotite granodiorite 0.003 + 6.0 porphyry

Hornblende-biotite granodiorite 0.037 + 9.1 Hornblende-biotite granodiorite 0.007 + 5.7 Hornblende-biotite granodiorite 0.006 +4.7 Hornblende-biotite granodiorite 0,077 + 0.5 Biotite monzogranite 0,086 +2.5

Biotite granodiorite 0.002 4. l Biotite granodiorite 0.041 - 4.6 Hornblende-biotite granodiorite 0.005 +2.2 Biotite monzogranite, aplitic 0.007 + 1.3

Biotite monzogranite, stressed 0.035 +0,6 Biotite monzogranite, stressed 0.005 - 5.0 Biotite granodiorite, stressed 0.004 - 3.5 Hornblende-biotite granodiorite, 0.003 2.1

stressed

Not visible

Pyrite Not visible Chalcopyrite Pyrite Pyrite,

pyrrhotite

Pyrrhotite Pyrrhotite Chalcopyrite Pyrrhotite

Pyrrhotite Not visible Not visible Not visible

Hidaka belt

21 HK-27 Ichinohashi, Hokkaido Pyroxene-hornblende-biotite 0.053 -7 .6 Pyrrhotite tonalite

22 HK-41 Nissho, Hokkaido Pyroxene-hornblende-biotite 0.11 5.6 Pyrrhotite tonalite

23 HK-44 Nissho, Hokkaido Biotite granodiorite 0.032 - 5.8 Pyrrhotite 24 HK-94-2 Sarugawa, Hokkaido Biotite granodiorite 0.05 10.7 Pyrrhotite 25 HK-94-3 Sarugawa, Hokkaido Biotite granodiorite 0.13 - 10.9 Pyrrhotite

Outer Belt of Southwest Japan

26 S-303 Omogokei, Ehime Pref. Hornblende-biotite granodiorite 0.006 -7.5 Pyrrhotite 27 TS-38 Takatsukiyama, Ehime Pref. Biotite granodiorite 0.086 - 10.0 Pyrrhotite 28 OK-92 Okueyama, Miyazaki Pref. Biotite granodiorite 0.19 4.2 Pyrrhotite 29 OK-98 Okueyama, Miyazaki Pref. Biotite monzogranite aplitic 0.17 -4 .8 Pyrrhotite 30 031601 Shibisan, Kagoshima Pref. Biotite monzogranite 0.11 5.0 Pyrrhotite

Miocene granitoids intruded into non-Green Tuff members near the Green Tuff belt b Located in the San-yo belt, but this pluton consists of nearly equal amount of the magnetite series (e.g., H-126) and the ilmenite series (H-129) Samples Nos. 6 and 30 are provided by T. Imaoka and T. Sato, respectively

110 A. Sasaki and S. Ishihara: Sulfur Isotopic Composition of Granitoids

Table 2. 634S values of Japanese gabbroids occurring in granitic terranes

Serial Sample No. Locality Rock Name S(%) 634S Sulfide No. (~

Magnetite-series Belts

Kitakami-Abukuma Belt

31 K-330 32 K-45 33 A-233

San-in Belt

34 HH-238A

35 11-120

Green Tuff Belt

36 TA-7

Ilmenite-series Belts

Ryoke Belt

37 115 38 RG-10

39 MN-4 40 MY-10

Hidaka Belt 41 HK-64

Himekami, Iwate Pref. Kurihashi, Iwate Pref. Kuroishiyama, Fukushima Pref.

Hino, Tottori Pref.

Zakka, Shimane Pref.

Tanzawa, Kanagawa Pref.

Tsukuba, Ibaraki Pref. Dando, Aichi Pref.

Nabari, Mie Pref. Mategata, Ehime Pref.

Tottabetsu, Hokkaido

Pyroxene-biotite monzodiorite 0.008 + 1.0 Amphibole gabbro 0.036 +2.3 Hornblende-biotite gabbro 0.18 + 2.2

Not visible Chalcopyrite Pyrite, pyrrho- tire

Pyroxene-hornblende gabbro 0.057 +2.7 Pyrite, pyrrhotite

Amphibole quartz gabbro 0.10 + 4.3 Pyrite

Amphibole gabbro 0.063 + 6.7 Pyrite

Amphibole gabbro 0.035 +2.5 Pyrrhotite Hornblende-biotite gabbro 0.086 -0.9 Pyrrhotite,

chalcopyrite Amphibole gabbro 0.10 - 4.0 Pyrrhotite Amphibole gabbro 0.093 +0.5 Pyrrhotite

Pyroxene-hornblende gabbro 0.090 - I. 1 Pyrrhotite

Sample No. 34 is donated by H. Hattori

Cretaceous and Tertiary granitic terranes. Wherever possible the sampling sites were chosen carefully in non-mineralized areas so as to avoid any contamination by sulfur of doubtful origin.

Analytical Methods

Sulfur was extracted as H2S from powdered rock samples by reac- tion with tin(II)-strong phosphoric acid ("Kiba reagent") at 280_+ 10~ in a nitrogen gas flow (Sasaki et al., 1976). Details of the procedure will be published elsewhere but, except for a few minor modifications, it is very similar to that described by Kiba et al. (1955). With this technique, sulfur in rocks, both as sulfide and sulfate, is easily reduced to H2S. The evolved HzS gas is then collected as AgzS for weighing and is finally converted to SO2 according to the method of Robinson and Kusakabe (1975). The isotopic analysis is made with a McKinney type mass spec- trometer (90 ~ sector, 20 cm radius) and the result is expressed in 6 34S0/o ~ value relative to the Cation Diablo troilite sulfur. Uncer- tainty of the reported value is within _+0.2O/oo.

Sulfur content of the sample is estimated from the amount treated and the recovered Ag2S, thus being semi-quantitative. A small fraction of the Ag2S produced commonly adheres to the glass tubing and results in a value lower than that obtained by the more exact technique of Arikawa et al. (1972). Comparative examination employing a few reference samples, however, indicates that the error in the chemical yield may not exceed 20%.

Results

T o t a l su l fur c o n t e n t s a n d ~ 3 4 S ( C D T ) va lues o f the

g r a n i t o i d s a n d g a b b r o i d s s tud ied are g iven in T a b l e s 1 a n d 2 a n d p l o t t e d in F i g u r e s 1 a n d 2. M a j o r sulf ide

species, i den t i f i ed u n d e r the re f l ec t ing m i c r o s c o p e at

m a g n i f i c a t i o n x 100, a re a lso l i s ted in the tables .

O t h e r f o r m s o f sulfur , such as su l fa te m i n e r a l s a n d

l a t t i c e - b o u n d SO42 in c o m m o n r o c k - f o r m i n g m i n -

erals, m a y be expec t ed in lesser a m o u n t bu t a re n o t

i den t i f i ed by o r d i n a r y obse rva t i on .

Granitoids

Sul fu r c o n t e n t o f the magne t i t e - s e r i e s g r an i t o id s var ies f r o m 0.002 to 0 .09%. T h e i r 6 34S va lue r anges

f r o m + 0 . 5 to +9 .1~ g iv ing an a r i t h m e t i c m e a n o f + 4 . 8 % o . T h e r e is n o a p p a r e n t c o r r e l a t i o n b e t w e e n

the i so top ic d a t a a n d the sulfur c o n c e n t r a t i o n data .

Su l fu r c o n c e n t r a t i o n o f the i lmeni te - se r i es g r a n i t o i d s r anges f r o m 0.002 to 0 .19% S. T h e i r 634S va lues a re

d is t inc t ly l ower t h a n t hose o f t he m a g n e t i t e series,

v a r y i n g b e t w e e n - 2 a n d - 1 1 % o.

A. Sasaki and S. Ishihara: Sulfur Isotopic Composition of Granitoids 111

g

-10]

10

MAGNETITE SERtES ILMENITE SERIES �9 GRANITOIDS 0

�9 GABBROIDS E]

�9 B I �9

�9 Q I o o o

r~

o

o 0

0

[3

o o o~ o o

0 �9

0 0 0

s'{ ppmi ,00 1000

Fig. 2. S (ppm) vs. c53~S (~ plot of Japanese granitoids and gabbroids

Some granitoids of this series show clear evidence for the interaction with pelitic rocks from wall or roof-pendant, and they tend to reveal relatively high concentrations of sulfur and low 634S values. The Takatsukiyama pluton in western Shikoku, for example, contains abundant xenoliths from the surrounding Shimanto Supergroup. A low (~34S value, -10%o, observed for this granitic body (No. 27 in Table 1) is thus most likely due to the assimilation by the magma of sulfur from sedimentary rocks. Average ~r for the Shimanto Supergroup in this area is estimated to be - 12~ (Table 3). Similar observation can be made for the data of the Hidaka belt in Hokkaido. Slightly contaminated tonalite and

granodiorite masses of this belt yield 6 348 values around -6~ (Nos. 22 and 23), while granodiorites alternating with gneisses in the migmatite zone have more negative values around -11~ 0 (Nos. 24 and 25). Excluding the above three samples (Nos. 22, 23, and 27), the mean 634S of the ilmenite-series granitoids is computed as -4.6O/oo.

The Omogokei granodiorite in Shikoku (No. 26) intrudes mafic igneous rocks, yet its sulfur is found to be clearly enriched in 32S, indicating that the negative 63~S trend of the ilmenite-series granitoid may have already existed in the ascending magma.

Gabbroids

The sulfur concentration of gabbroids has a narrow range of variation compared with that in the granitoids. With an exception of sample No. 31 which contains 0.008% S (Table 2), ten other samples examined contain 0.04 to 0.18% S. There appears little if any difference between the sulfur contents of gabbroids occurring in the two series of granitic terranes.

The range of ~34S values is also narrower than that of the granitoids. However, there is a distinction between the data of the two series and 634S varies from + 1.0 to +6.7% o in the magnetite-series samples, in contrast to the variation from -4.0 to +2 .5%o in the ilmenite-series. The trend that the ilmenite-series samples have more 32S is in accord with the observation for the granitoids.

There is a regional difference in the isotopic data of the magnetite-series gabbroids. The Kitakami- Abukuma belt yields ~ 3~S values ranging from + 1.0 to +2.3% o with an average of +1.8, while the San- in and the Green Tuff belt have +2.7 to +6.7% o and an average of +4.6. The latter is, therefore, isotopically similar to the magnetite-series granitoids.

Table 3. ~34S values of roof sedimentary rocks in some granitic terranes of Japan

Serial No. Sample No. Rock Name S(%) ~34S (~ Sulfide

Sarugawa Area, Hidaka Belt, Hokkaido

42 HK-88 Pyroxene-biotite hybrid 0.14 - 8 . 3 24 HK-94-2 Biotite granodiorite 0.05 - 10.7 25 HK-94-3 Biotite granodiorite 0.13 - 10.9 43 HK-94-1 Cordierite-biotite gneiss 0.46 - 12.3 44 SHK-51 Fine sandstone hornfels 0.083 -13 .6 45 HK-46 Shale hornfels 0.47 - 16.9

Shimanto Supergroup, Outer Belt of Southwest Japan

46 Composite Sandstone 0.12 - 1.4 47 Composite Siltstone 0.44 - 13.2 48 Composite Claystone 0.16 - 21.5

Pyrrhotite, chalcopyritd Pyrrhotite Pyrrhotite Pyrrhotite Pyrrhotite Pyrrhotite

112 A. Sasaki and S. Ishit)ara: Sulfur Isotopic Composition of Granitoids

One of the ilmenite-series gabbroid (No. 39, Ta- ble 2), whose 634S value is -4.0~ has probably been modified due to the intrusion of granitoid, as indicated by decomposition of original ferromagne- sian silicates. The other samples which are apparently fresh give an average of +0.3%o.

Discussion

Sulfur of the Magnetite-Series Granitoids

On the basis of the isotopic studies of mafic and ultramafic rocks and associated ores, it has generally been accepted that the sulfur of upper mantle or lower crust material is isotopically close to meteoritic sulfur (Thode et al., 196I ; Shima et al., 1963; Smitheringale and Jensen, 1963) or, possibly slightly enriched in 34S up to about 1~ (Schneider, 1970; Sasaki, 1973). Apart from the detailed features, which will be discussed later, the majority of our data of Japanese gabbroids (Table 2) are fairly close to this estimated mantle value. However, the whole rock sulfur of the magnetite-series granitoids with an average isotopic value of about +5%o seems unlikely to have been derived from the same sulfur that is found in common mafic and ultramafic rocks.

If the sulfur with original ~348 of +1 to +20/00 changes its isotopic composition to +5% o with no incorporation of external sulfur, there must have been preferential loss of the lighter isotope from the system. The only possibility in this regard would be isotopic partitioning among different sulfur-bearing species in the system which is followed by preferential removal of less-oxidized species enriched in 32S. However, a brief calculation of the mass balance indicates that this is improbable. Available data of the equilibrium isotope fractionation (Sakai, 1968) shows that fractionation between any pair of sulfur species at the tem- perature of magmatic processes is very unlikely to exceed 5%0 in 6 34S. It then follows that the amount of outgoing sulfur required to shift the isotopic composition of the system from +1 to +5% o becomes unreasonably large. It is also rather difficult to pro- pose any mechanism that allows the reduced sulfur species to have been selectively removed from the magma when the major form of sulfur fixed in the rock is sulfide.

Thus, the only probable way to yield the 3r enriched magma seems to be the introduction of some external 34S into the host materials from which the granitic magma is generated. If cS34S values of the magnetite-series granitoids and gabbroids from different belts are compared (Tables 1 and 2), there are slight differences. The average value for the Kitakami and Abukuma belts is +2.2%0, while that for the

San-in belt and Green Tuff belt is around +5%0. This difference can be considered meaningful, because there is a clear difference on their tectonic histories, which may affect the process of magma genesis.

Magmatism of the Green Tuff belt is a representative example of that triggered by subduction of a major plate (Ishihara, 1974). Granitoids of the San-in belt may have had a similar tectonic setting during Cretaceous times. On the other hand, there is no evidence for the Kitakami-Abukuma belt that the magmatism was related to subducting plate motion. Instead, it is most probable that the magmatism was triggered by lateral shearing along a deeply developed fracture system in the oceanic plate (Ishihara, 1978). Thus, the contribution of the then current ocean-floor materials may be worth considering.

Involvement of ocean floor sediments and seawater in island arc magmatism has been discussed by many authors. Tatsumoto (1969) considers, on the basis of lead isotope studies, a significant contribution of pelagic sediments into the Quaternary volcanism of the Green Tuff belt. The oxygen isotope results, however, do not appear to be consistent with this interpretation (Matsuhisa, 1977; in press).

Sulfur isotopic composition of ocean-floor material may be variable. Unfractionated fresh magmatic sulfur in the abyssal basalt may have 6 34S of mantle type (Kanehira et al., 1973). A large c~34S spread (from -24 to +23O/oo) has been reported for epigenetic pyrites in some ocean-floor basalts but their average value may be close to 0~ (Field et al., 1976). The contribution of such sulfur of basaltic origin would not therefore bring about any 34S enrichment in the magmas concerned. On the other hand, pelagic sediments may contain isotopically heavy sulfur as trapped seawater or certain sulfate minerals. In fact, Dymond et al. (1973) reported that the sulfur in metalliferous sediments from the southeast Pacific Ocean has 6 34S close to the seawater sulfate values, ranging from +18 to +21~

There is also a possibility that large amounts of calcium sulfate are deposited from the down-circulat- ing seawater in mid-oceanic ridge hydrothermal sys- tems. If such heavy sulfur is brought down with the subducting plate to the site of magma generation, the magma produced may have varying degrees of 34S enrichment as compared with mantle derived sulfur. The contribution of oceanic sulfur to magma generation may be particularly important in an island arc under which the subduction rate is high.

Sulfur of the Ilmenite-Series Granitoids

We have seen that, in some ilmenite-series granitoids with high sulfur concentration, there exists clear evi-


dence that the magma has been contaminated with shallow crustal materials. However, other specimens which have little visible evidence of crustal contamination are also specified with negative c53~S trend. The observation thus appears to be favorable for, or at least not incompatible with, a crustal origin for the ilmenite-series magma.

The crustal material, especially that of the continental margin area may contain varying amounts of sedimentary rock sulfur which is most likely enriched in 32S as compared with igneous sulfur. The isotopic results of pelitic and psammitic rocks from the Shi- manto Supergroup and the southern part of the Hi- daka belt mentioned before (Table 3) are good exam- ples. Paleozoic sediments of the Chichibu geosyncline, the backbone geologic unit of the Japanese island arc, also show evidence for such isotopically light sulfur (Endo et al., 1973). It would therefore be probable that if the magma generation involves crustal materials under the Japanese island the sulfur in the magma produced would be more or less enriched in 328.

The sulfur isotopic evidence, however, cannot serve as proof for the crustal origin of the ilmenite- series granitic magma. Since the evidence of interaction between magma and crustal material at shallow depth is so clear in some ilmenite-series granitoids, it would not be unexpected that the same process also occurs at deeper levels to the ascending "magnetite-series" magma, converting it to the "ilmenite series".

In contrast, there are very few indications of large scale magma-crust interaction in the magnetite- series granitoids. This probably means that the emplacement of these rocks has been of more or less fracture-filling type, while that of the ilmenite series has been governed by a stoping mechanism.

Sulfur of Gabbroids

As already mentioned, gabbroids occurring in the magnetite-series granitic terranes have the 6 3~S trends parallel to those ofgranitoids. Thus, the samples from the Kitakami-Abukuma belts whose magmatism is likely to be of non-plate type give an average aa~s of + 1.8~ while the gabbroids related presumably to the plate-type magmatism of the Green Tuff belt and San-in belt yield a value of +4.6%o. The interpretation applied to the magnetite-series granitoid also appears most acceptable for these data.

It is commonly accepted that upper mantle material would be the sole source for gabbroic magmas. Any interpretation of the c~3r data of gabbroids which assumes two distinct source materials, there-

fore, appears improbable and crustal contamination would be the case for the gabbroids in the ilmenite- series terranes. As compared to granitoids, isotopic difference between the two series of rocks is much less in gabbroids; a small amount of crustal contamination would be able to cause such an isotopic shift.

Even assuming the original isotopic composition of a given gabbroid magma to be + 4.6% 0, the average c534S for the ilmenite-series gabbroids, +0.3~ can be obtained if the system incorporates some twenty percent of sulfur from an external source whose isotopic composition is -15%o. In any case the sulfur concentration level of the magma would not be changed much by such contamination. The observation that little if any difference exists between the sulfur content data of gabbroids from the two series is consistent with this view.

Data From Other Granitic Terranes

Systematic studies of the sulfur isotopic composition of granitoids are limited. Shima et al. (1963) obtained c~ 34S data of sulfide separates from some Precambrian intrusive granitoids in Canada, and suggested the possibility of discriminating two types, namely, granitoids of primary igneous origin with isotopic values which are more or less meteoritic and those of reworked sedimentary or crustal material origin which showed remarkably heavy value up to + 30%o. However, this was not endorsed by Siewers' (1974) work in which nearly one hundred European granitoids were examined for whole rock c~ 34S. His results range only from - 4 to +9~ giving an average of +2.5o/o0.

More recently Coleman (1977) reported a prelimi- nary isotopic study on the whole rock sulfur from the granitic rocks of the Permian New England batholith of southeast Australia. His data comprising the I ("igneous") and S ("sedimentary") type granitoids, defined by Chappell and White (1974), involve a somewhat similar approach to ours and give -3.6 to +5.0~ for the I type and -9 .4 to +7.6O/oo for the S type, although the detailed data are not shown. It is clear that the S type of Chappel and White belongs to the ilmenite-series by Ishihara. However, from the opaque mineral description of these specimens given by O'Neil et al. (1977) only half of the I type specimens seem to be the magnetite- series granitoids. If the I type specimens with negative c534S values are found to belong to the ilmenite-series, their data of the I type may turn out to be similar to our results on the magnetite-series. The wide variation in the S type data with both the negative and positive ~534S is distinct from the trend of our ilmenite-series, indicating a different situation in the Aus- tralian samples.

114 A. Sasaki and S. Ishihara: Sulfur Isotopic Composition of Granitoids

Sulfides in the sedimentary rock are commonly enriched in 32S. Although this is true for most Phane- rozoic samples, there also exist data indicating remarkably heavy sedimentary sulfides up to + 34~ particularly from Precambrian strata (Kulp et al., 1956; Thode et al., 1962; Tugarinov and Grinenko, 1965; Chukhrov et al., 1970). In addition, in the continental crust, evaporite sulfate with high 6 34S should not be neglected. I f crustal materials containing such isotopically heavy sulfur were involved in the formation of ilmenite-series granitoids, their rock sulfur would have varying degrees of 3"S enrichment. This is the basic idea of the McMaster group in their early work (Shima et al., 1963).

In an island arc environment like the Japanese islands, the basement sedimentary pile is thought to be composed of common clastic sediments with little if any sulfate beds. The sulfur in these rocks is therefore expected to be mostly bound in biogenic sedimentary sulfides. In addition, their age is generally young- er than late Paleozoic, the time in which seawater is most depleted in 3~S among Phanerozoic seas. Al- though it is possible, particularly in western Japan, that some Precambrian basement may exist, the assumption of negative 6 34S trend for the average Japanese crust may well be justified. It is therefore probable that the established remarkable isotopic distinction between the magnetite-series and ilmenite-series granitoids in Japan owes much to this unique geologic background. Ilmenite-series granitoids in other areas of the world may or may not have the sulfur isotopic trend comparable to the Japanese samples.

Conclusions

1. Sulfur isotope data of Japanese granitoids fall into two distinct groups which can be correlated respectively with the magnetite-series and the ilmenite-series granitoids defined by Ishihara (1977). The magnetite- series rocks all have positive 6 34S (CDT) values, while the ilmenite-series ones are dominated by negative values.

2. The heavy value of the magnetite-series, about + 5 % o on the average, is difficult to explain, if the sulfur has been simply derived f rom the upper mantle or lower crust material. Incorporat ion of heavy sulfur of probable seawater origin into the mantle derived sulfur is the most likely explanation of this 34S enriched isotopic trend. There is evidence that magmatism related to a subducting plate tends to give higher ~ 34S values, implying this to be the mechanism for the introduction of heavy sulfur.

3. Incorporat ion of crustal sulfur enriched in 32S clearly explains the isotopic trend of the ilmenite-

series granitoids. The sulfur inflow seems to have occurred not only at relatively shallow depths near the site of magma emplacement but also at much deeper levels in the crust where the magma might have been generated. Large scale magma-crust interaction in the ilmenite-series granitoid indicates that the emplacement of magma has been governed by a stoping mechanism. Lack of any such evidence in the magnetite-series may be interpreted as the result of more or less fracture-filling type of intrusion in this series of magma.

4. The possibility that the dramatic distinction between the sulfur isotopic data of the two series of granitoids is a mere peculiarity of the Japanese island arc cannot be excluded since the underlying sedimentary crust, of relatively young age, is thought to be enriched in 32S. There is evidence that in continental regions the crustal sulfur may have remarkably heavy isotopic values. The ilmenite-series granitoids from such areas do not necessarily have negative ~ a4S values.

References

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Received July 20, 1978/Accepted October 25, 1978

sulfur isotopic composition of the magnetite-series and ilmenite-series granitoids in japan

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