Nature of hydration in Japanese Paleozoic geosynclinal basalt

EARTH AND PLANETARY SCIENCE LETTERS 15 (1972) 271-285. NORTH-HOLLAND PUBLISHING COMPANY

NATURE OF HYDRATION

IN J A P A N E S E P A L E O Z O I C G E O S Y N C L I N A L

BASALT

Hitoshl HATTORI*, Ryuichi SUGISAKI** and Tsuyoshi TANAKA* Received 17 November 1971 Revised version received 17 February 1972 Further revised version received 5 May 1972

The petrochemistry of geosynclinal basalts of late Paleozoic age in southwest Japan is discussed by examining statistically the result of over 120 new chemical analyses. The rocks are rich in H20 (+), but there is found to be only a slight systematic variation of elements along with increasing content of H 2O (+). Spilitization and/or palagonitization has nothing to do with the alteration, and the effect of reaction with deep sea water is not significant. Two types of geosynclinal basalt occurring in the axial part (Mikabu zone) and on the flanks of the geosyncline, respectively, are clearly discriminated on the basis of petrochemical characteristics. The basalt on the flanks is rich in TiO2, Na20, K20, P2Os, Rb and St, and poor in MgO and CaO, compared with the basalt in the Mikabu zone.

1. Introduction Basaltic rocks, often encountered in eugeosynclinal sedimentary facies, are invariably altered in such a way that prominent hydration is manifested by abundant clayey and hydrous minerals together with frequent obliteration of primary texture. It is practically hard to identify environments under which hydrous minerals such as chlorite, mica, epidote, prehnite, pumpellyite, actinolite, kaolinite and smectites are formed. 'Geosynclinal basalt' [ 1] intercalated in the Paleozoic group of Japan is quite markedly different in petrochemical characteristics from recent mafic volcanics on the Japanese Islands. Geosynclinal basalt has recently received much attention in an attempt to envisage the nature of volcanism. Comparison of chemical characteristics of geosynclinal basalt with those of basaltic rocks in oceanic regions became popular in Japan [ 1 - 9 ] , Australia [10], Canada [ 1 1 - 1 5 ] , Germany [16], Greece [17], Iceland [18], and Switzerland [19]. Particularly noted is the similarity to basalts of oceanic ridges in terms of rubidium and potassium contents [ 1,20,21 ], and rare earth elements

patterns [7,16]. Geosynclinal basalt commonly has high contents of water and carbonate. This seems to be an essential feature of these basalt or allied volcanic rocks. Evidently the high state of hydration often inhibits genetic studies of geosynclinal basalt by petrographic as well as petrochemical means. Chemical reaction of oceanic ridge basalt or pillow lava with deep sea water has been studied for chemical changes, and diverse opinions have been given concerning trends of alteration of such geosynclinal basalt. Difficulties may arise from the fact that the geosynclinal basalt must have experienced a complicated history of hydration in deuteric, hydrothermal, cooling, devitrification, diagenesis, burial and uplift stages. Nevertheless, it is important to determine whether or not the original chemical characters of the parent material have been maintained, i.e. isochemical system. In this paper an attempt is made to establish and explain probable limits of redistribution of some key elements in Japanese Paleozoic geosynclinal basalt.

2. Geology * Geological Survey of Japan, Hisamoto 135, Kawasaki, 213 ** Japan. Department of Earth Sciences, Nagoya University, Chikusa, Nagoya, 464 Japan.

The Japanese Paleozoic group of Silurian to Permian age, consists of eugeosynclinal sedimentary rocks such

272

H. Hattori et al., Hydration in Paleozoic synclinal basalt

as greywacke sandstone, shale, chert and limestone, and contains plenty of 'green' volcanics, occasionally called Schalstein or greenstone. In southwest Japan (fig. 1) the volcanics occur by far the most commonly in early to middle Permian as lava and pyroclastics of basaltic composition, not infrequently with pillow structure. The volcanics are undoubtedly products of submarine volcanism. Holocrystalline rocks(coarse-grained and equigranular) such as microgabbro and dolerite also occur with the volcanics. These rocks are tentatively divided into the following two geologic units: Permian and Mikabu, i.e., geosynclinal basalt from the Permian group and those from the Mikabu zone respectively. The Mikabu unit represents the axial part of the Japanese late Paleozoic geosyncline and the Permian unit represents the flank part [8]. Mikabu: mainly of Permian age, this is found intermittently in an elongated area called the 'Mikabu zone', or well known as the Mikabu green rocks. The Mikabu zone extends along much of the present Japanese Islands in southwest Japan, and each rock body in the zone is typically shown on a geologic map with a dimension from 1 km × 1 km to 3 km × 15 km or much more elongated. The massive and holocrystalline na-

ture represented by microgabbro and dolerite is charac teristic and occasionally accompanied by pillow lava, pillow breccia, ultramafics and rarely chert and limestone. The lithologic association in the Mikabu zone constitutes the general feature of ophiolite suite. The chemical features observed between two facies of lavas and holocrystalline rocks are illustrated separately in figures (figs. 2A, 2B, 4, 5, 6 and 7). Samples collected in Shikoku are here designated as Mikabu-I (M-I), and rocks from the Kanto Mountains are Mikabu-E (M-E). Petrographically the holocrystalline rocks of M-E are usually composed of clinopyroxene with hourglass extinction, plagioclase and sphene. In the holocrystalline rocks of M-I augite and plagioclase are the main constituents [6], with sphene as a minor component. The petrographic distinction, particularly in relative abundance of sphene, observed between the rocks of the eastern Mikabu zone (M-E) and those of the western Mikabu zone (M-I) may be regarded as a lateral variation of the parental material along the Mikabu zone. In both regions mafic minerals are often altered to chlorite, actinolite, epidote and calcite, and calcic plagioclase to albitic plagioclase and sericite. Leucoxene,

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Fig. 1. Distribution of the upper Paleozoic formations in southwest Japan. Key - 1: unmetamorphosed; 2: covered or metamorphosed; 3: Mikabu zone.

H. Hattori et al., Hydration in Paleozoic synclinal basalt

prehnite, pumpellyite, stilpnomelane, and rarely glaucophane are identified as metamorphic minerals. Permian: basalt of this age is most common in various parts of the Japanese Paleozoic group. Usually, lava and/or pyroclastic sequences, several 10 to 100 m thick, are found to be laterally persistent. Rarely a volcanic complex attains over 1 km thickness. Chert and limestone are commonly associated. Microscopically sphene and clinopyroxene, often with hourglass extinction, are commonly recognized, in rocks that are not intensely altered. Like the Mikabu rocks, mineralogic reconstitution to altered mineral assemblages is quite widespread. Prehnite, pumpellyite, actinolite and stilpnomelane are common. For the sake of convenience the Permian rocks are subdivided into two groups according to tectonic division; inner zone (to the north of the axial part: continental side) and outer zone (to the south: Pacific side). However, there is no distinctive petrographic criterion to subdivide them into these two zones. As stated above, the two types of geosynclinal basalt have undergone mineralogic reconstitution in various degrees; mafic minerals to chlorite, epidote, actinolite, calcite, quartz and leucoxene, and plagioclase to sericite, prehnite, pumpellyite and albite.

EXPLANATION

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3. Chemical analysis All specimens of the geosynclinal basalt treated here were collected in such a way as to avoid weathered parts; individual specimens were usually heavier than 2 kg in weight; they were sawed into slices and chips to eliminate veinlets and vesicles. But it seems entirely impossible to locate fresh samples, i.e., non-hydrated. This degree of hydration (fig. 2A) seems to be diagnostic of any geosynclinal basalt, and stands in contrast to recent unaltered volcanics. Pyroclastics are not considered here so as to avoid complicated processes superposed by introduction of terrestrial material, although the chemical similarity between lava and associated pyroclastics has been discussed by Uchida [22] and Tanaka [2]. The basalt is usually very heterogeneous and frequently contains carbonate minerals along with hydrated minerals, as reported elsewhere [23-25]. Particularly, amygdaloidal basalt with vesicles of various sizes are commonly found in the Japanese Paleozoic

27 3

I

2

3

4

5

6

*l,

H20 (+1

Fig. 2A. Frequency distribution of H20 (+) content in the three geologic units. group, and the vesicles are mostly filled with secondary minerals. Even when any veinlet or pore healed with or without quartz and carbonates or others is not recognized microscopically, it does not always mean that the specimen analyzed might be wholly free from such secondary minerals (fig. 2B). Practically, elimination of all unfavorable portion of heterogeneous geosynclinal basalt is not always attained in spite of much effort. 3.1. Major elements

Since hydrated minerals and carbonates are abundant in geosynclinal basalt, H20 (+) and carbonate contents were carefully determined. A collective analytical method pertinent to the present study was described in a separate paper [26]. 122 samples of lava and ho-

274

H. Hattori et al., Hydration in Paleozoic synclinal basalt

with a Mo-target tube operated at 90 kV and 20 mA. Reproducibility of X-ray intensity is commonly within 1%. Detection limit is placed at 0.5 ppm. An analytical error is expected to be within 15% in the case of more than 10 ppm; however, it may be much larger at the 1 ppm level. Naturally K/Rb ratios in the case of 1 ppm level of Rb may introduce a larger error, thus not always indicating the original values. Tables for chemical data used in this paper will be published elsewhere.

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Fig. 2B. Frequency diagram showing carbonate content. locrystalline rocks were selected from all analyzed samples with 5% in weight or less calcium and magnesium carbonate contents. This means that the samples of geosynclinal basalt used in this paper contain less than 3% CO 2 at most. CaO combined with carbonates is not included in CaO %; thus it represents a real CaO content in silicates. Among 122 samples, about 70 samples (60%) contain less than 0.5% carbonates equivalent to 0.2% CO 2, and 85 samples (70 %) contain less than 1% carbonates equal to 0.45% CO 2. Similar restriction for calculation of the average lava composition has been attempted by Wilson et al. [23] who place the limit of normative calcite contents at 4%. All chemical data are shown in figures on a carbonate free basis, that is, normalized to 100% silicates without carbonates. The results contain, therefore, both H20 (+) and ( - ) as in the silicate portion. 3.2. M i n o r e l e m e n t s : R b a n d S r Rubidium and strontium were determined by a modified method of Hattori and Shibata [27], using a Philips' Semi-automatic X-ray Spectrograph equipped

The chemical variations observed in parts of a single pillow may give us useful information on probable redistribution of elements in a hydration process. Three pillow lavas were selected within the limits of low carbonate content and wide range of H20 (+) content over 2% from core to rim of a pillow. Brief description of the three pillows (I..19, 1-45 and F-81) are given in the Appendix. Analytical data of the three pillows are listed in table 1. For trend analysis of major elements with respect to H20(+) content, all data are shown in fig. 3 at the same ordinate scale. In the 1-19 pillow, SiO 2 and Na20 are distributed reversely to A1203, total Fe, MgO and CaO. In the 1-45 pillow, as hydration proceeds, A1203, CaO and Fe203/FeO increase, whereas SiO 2 decreases. In the F-81 pillow, considerable amounts of carbonates may mask the general trend, but SiO 2 and Na20 are opposite to total Fe and MgO. SiO 2 content fluctuates widely, particularly in the F-81 pillow. This may be caused by the presence of quartz with which carbonate minerals are associated in fine veinlets at the core and middle parts. Fe203 and FeO behave reciprocally, but total Fe is nearly constant except the rim of F-81 pillow. MgO varies considerably, and this may reflect a large extent of chloritization by which MgO migrates. CaO varies half as much as MgO does. Na20 content is small in all three pillow rims. The introduction of sodium,.i.e., spilitization may not have occurred in these pillows, although the 1-45 and F-81 pillows are rich in Na20. A similar leaching phenomenon of Na20 has been reported in many pillow lavas [28-34]. K20 is not added in any prominent amount, and the range of change is not large. Palagonitization of submarine basalt as described

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Fig. 3. Chemical change in the rim, middle and core parts of three pillows. Square: 1-19; circle: 1-45; triangle: F - 8 1 . Open: rim; dot: middle; solid: core. Table 1 Chemical composition of three pillow lavas (for petrographic description, see Appendix). A Weight %

core

B

MIKABU (1-19) middle rim

C

Permian (I-45) core middle

rim

Permian (F-81) core middle

rim

51.86 1.65 10.59 1.96 9.06

50.92 2.04 10.22 3.96 6.93

48.93 1.88 10.81 1.92 9.63

52.34 0.94 17.33 2.07 5.76

55.13 0.78 16.23 1.16 4.60

56.57 0.81 15.61 1.06 6.00

49.31 1.44 13.97 0.27 6.08

52.97 1.41 14.87 1.63 4.35

46.37 1.83 15.09 4.80 7.80

MnO MgO CaO Na20 K20

0.14 8.10 9.42 2.24 0.41

tr. 9.78 9.30 2.28 0.37

tr. 11.18 10.85 1.58 0.14

0.08 5.65 5.87 3.95 0.04

0.08 4.27 5.37 5.20 0.05

0.10 4.86 4.95 4.00 0.04

0.12 5.30 7.10 4.57 0.1l

0.11 5.05 9.32 5.38 0.15

0.15 8.00 8.53 2.72 0.77

P2Os H20(+) H20(- ) CaCO3 MgCO3

0.16 5.14 0.22 0.03 0.00

0.09 3.07 0.10 0.03 0.00

0.10 2.92 0.15 0.06 0.00

0.11 4.66 0.30 1.07 0.04

0.09 3.60 0.34 3.81 0.17

0.10 2.68 0,25 2.68 0.12

0.16 2.42 0.22 7.68 0.88

0.24 1.78 0.10 3.14 0.35

0.11 4.11 0.11 0.40 0.05

SiO 2 TiO 2 A1203 Fe20 3 FeO

r/SRb/(ppm) Total

1.5 38.8

4.0 28.4

1.3 74.6

0.5 133

0.5 99.1

100.98

99.09

100.15

100.21

100.88

0.5 130 99.83

0.5 203 99.63

0.5 326

15.3 221

100.85

100.84

276

H. Hattori et al., Hydration in Paleozoic synclinal basalt

by Moore [35] is, therefore, not likely in these three pillows. H20 (+) is commonly concentrated in the core, whereas the rim is not always highly hydrated; nor is it oxidized in respect to iron. This supports similar observations reported by Uchida [33] on the Paleozoic pillow lavas in the Kanto Mountains. It is contrary to a commonly observed trend in pillow lavas of younger geologic ages [5,28,34,36]. Internal concentration of H20 (+) may be explained by a similar process observed in a sort of trap rocks in which the outer and more rapidly cooled part is usually little hydrated, as Hart et al. [37] have clearly demonstrated on a single 10 m lava from Iceland. Based on the evidence, it is difficult to conclude that, as hydration proceeds, any element sympathetically increases or decreases from the core to rim in all three pillows. However, there are some conspicuous trends from the middle toward the rim, that is, total Fe and MgO trends are positive; Na20 trend is negative. These trends indicate more drastic changes from the middle to the rim than the core to the middle as exhibited in fig.3 with steeper slope. It is very likely that the rim represents an extraordinary heterogeneous portion of pillow lava, while the change from the core to the middle is markedly small. Distinct change from the core to the middle of all the three pillows is indicated by a negative sign in A1203, FeO and H20(+), and positive sign in Na20. In other terms, A1203 and FeO increase, whereas Na20 decreases. K20 changes very little. Variations in both Na20 and K20 are unexpectedly small. Increase in FeO means that oxidation of iron along with hydration may not have occurred in these pillows. These relations are similar to those found in basalts of the Kanto Mountains [32,33], but are not consistent with relations found in the dredged pillow lava of abyssal tholeiite from the Mid-Atlantic Ridge [38] and the submarine basalt on the flanks of the Hawaiian volcanoes [39,40]. It is to be noted that minor elements and ratios of K/Rb (excluding 1-19) and Ca/Sr exhibit a narrow range of change. Probably chemical redistribution at the core was nearly isochemical except H20 (+), on the contrary that around the rim was in an open system, particularly in respect to Na20. Pillow lava must have been in contact with sea water for a longer duration than common lava flows.

A pillow's surface is supposed to have been in direct contact with sea water and an aggregate of piled pillow lavas is judged to have tremendously larger total surface than that of a common lava flow with the same volume. Consequently element migration during hydration in pillow lava seems to be highest, particularly from the middle to the rim in the geosynclinal basalt. A noteworthy statement by Vallance [31] is to be cited here. "The pillows from the British Isles probably behaved as open systems with their environments during alteration . . . . Nevertheless, it is suggested that in certain cases pillows may be regarded approximately as closed to all components except water." Unless obvious examples of palagonitization or spilitization are demonstrated, no other significant chemical change that those detected in the pillows (table 1) may be expected elsewhere. The authors are of the opinion that the present composition at the core of the pillows and the indicated range of chemical redistribution furnish a sound basis to assess the original pristine contents of the three pillows.

5. Statistical analysis of chemistry of geosynclinal basalt 5.1. Alkalies and CaO

When interest is focussed on the petrochemical nature of geosynclinal basalt, the contents of alkalies are the most important factor to classify the rocks: alkali basalt, high-alumina basalt, high-alkali tholeiite and low-alkali tholeiite. Naturally much attention must be paid to alkalies, because alkalies in submarine basalts are considered to have moved drastically (34,35,37,38,41-45]. In this connection, the probable migration of alkalies in the geosynclinal basalt should be determined as accurately as possible. As already discussed in the foregoing section, the range of change in the three pillows may mark an established limit. Na20 moves as much as 2.5% in, the pillows. CaO and K20 move less than 2% and 0.7% at most, respectively. On the other hand, a few samples in both the Permian and Mikabu rocks have extraordinarily high contents of Na20 or K20 , i.e., about 2% higher than the upper value of the majority of samples (figs.4 and 5). Because of this and of the results of the three-pillow observations,

277

H. Hattori et al., Hydration in Paleozoic synclinal basalt

a migration of Na20 and K20 of at most approximately 2.5% may (though rarely does) occur. As a further check of element migration during hydration, several diagrams are presented illustrating the relationships between H20 (+) and K, Rb, Na20, Ca, St, and Fe203/FeO (figs. 4-7). These diagrams are not essentially identical with trend analysis of hydration process, but merely indicate

Permian

f

•

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correlation with H20 (+). Provided that a mass of samples possesses a mean value with small deviation, any regular distribution pattern of plots may bring forth something significant. On this assumption, the following analysis and interpretation were made. It seems possible that K and Rb are very mobile in a hydration process; as a result the K/Rb ratio does not remain constant. Hart [42] reported two-fold

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11. Hattori et al., Hydration in Paleozoic synclinal basalt

enrichment in K and five-fold enrichment in Rb in the altered margin of submarine basalts; and later Hart and Nalwalk [43] likewise disclosed an apparent decrease of K/Rb in many altered submarine basalts. On the other hand, there are a few investigations concerning metamorphic processes. Kamp [25] demonstrated an apparent decreasing trend of both Rb and K20 in the basaltic tuff of the Green Beds of the Scottish Dalradian Series with increasing metamorphic grade up to the amphibolite facies. This means that the higher the dehydration the poorer the rocks are in both elements. The geosynclinal basalt used in this study has undergone low grade metamorphism of up to approximately the pumpellyite-actinolite facies. The basalt may not have experienced such a prominent decreasing process of both elements along with progressive metamorphism as described in the Scottish Dalradian Series. Hart [42] noted an increasing trend of K/Rb with decreasing K in some submarine tholeiftes which have undergone low grade metamorphism. He considered that the higher K/Rb ratio was caused by preferential exclusion of Rb from metamorphic minerals. The average K content of the metabasalts is reported to be only 500 ppm; therefore, K/Rb ratios as high as 1800-3500 appear to be produced only in case of extremely low Rb content. Probably, these high ratios are introduced by some undetermined process in the case of less than several ppm of Rb, and in some cases by analytical error as already stated in an earlier section. Sea water alteration apparently acts always to lower the K/Rb ratio [42]. The high K/Rb ratios in many geosynclinal basalts in this study are significant even if subjected to sea water alteration, and appear to be an inherited primary feature even though involved in complicated alteration processes. In fig. 5, no trend is recognized, but only the lavas in the outer zone of Permian show a negative trend. The negative trend means that Na20 is leached with increasing hydration. In fig. 6, the trend is positive in the holocrystalline rocks of M-l, but faintly negative in the other Mikabu rocks. This means that only in the holocrystalline rocks of M-I CaO was enriched with increasing hydration. On the contrary, the lavas of M-I as well as lava and holocrystalline rocks of M-E show a slight decrease of CaO with hydration as is recognized in sea water alteration [43,46]. Referring to the loss and gain of Sr during sea water alteration as reported by Hart [42] and Hart and Nalwalk [43], the

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above fact supports that sea water alteration has never been widespread in the hoiocrystalline rocks of M-I, but it may have influenced some of the other Mikabu rocks. Above all, several lavas in M-E and the outer zone of Permian may have been affected slightly, if any, by sea water alteration, as judged from the correlation of CaO, K20 and Na20 with H20(+ ). But this assumed trend of alteration has beyond doubt never reached the level advocated by several authors as discussed above. 5.2. Fe203/FeO No apparent pattern is recognized in fig. 7, while many samples lie nearly within ratios of less than 0.5. This is not much higher than a generally accepted value of 0.3 for the unaltered fresh submarine basalts [11,38,43].

H. Hattori et al., Hydration in Paleozoic synclinal basalt

279

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5.3. Average c o m p o s i t i o n Before calculating averages, the following check was made on frequency diagrams of elements. Both the rocks of the inner and outer zones of Permian are similarly stacked in the frequency column. The holocrystalline rocks of M-I tend to have lower TiO2, total Fe, total Fe/MgO, Na20, Rb and Sr, and higher CaO than the associated lavas. But such difference is not recognized between the lavas and holocrystalline rocks of M-E. As these differences are much smaller than those found between the rocks of Permian and Mikabu, both lavas and holocrystalline rocks were collectively treated. The holocrystalline rocks of M-I contain higher

H20 (+) than the lavas (fig. 2A). This may be interpreted to be a phenomenon in trap rocks, as already explained in the earlier section on the pillow lavas. Carbonates are not abundant in the Mikabu rocks, but are quite abundant in the Permian lavas. By statistical testing of the averages and standard deviations (table 2), the three geologic units are definitely discriminated by TiO 2, CaO and P205. The Permian rocks are markedly different from M-I and M-E in contents of TiO2, A1203, MgO, CaO, Na20 , K20, P205, Rb and Sr, thereby making it significant to discriminate the Permia.n and Mikabu rocks on the basis of their petrochemical characteristics.

280

H. Hattori et aL, Hydration in Paleozoic synclinal basalt

Table 2 Average and standard deviation of the Permian and Mikabu rocks (M-I and M-E). n= numbers of samples. Rb and Sr were determined for 57 samples of Permian, 29 of M-I and 14 of M-E, respectively. Average values of K/Rb were calculated using 47, 7 and 6 samples, respectively, excluding samples containing less than 0.5 ppm Rb.

Fe203 FeO

o

Permian 1.2 o o

®

® ®

0.8

Unit

Permian

M-I

M-E

Weight %

n

70

32

20

SiO 2

g a

49.64 (3.56)

48.42 (1.70)

46.90 (2.79)

~ a

1.97 (0.67)

1.02 (0.43)

1.46 (0.36)

A120 3

K o

15.36 (1.76)

13.23 (2.50)

13.71 (2.41)

Total Fe

x o

11.51 (2.52)

9.94 (2.34)

12.05 (1.06)

MgO

~ a

6.78 (2.44)

9.08 (2.39)

10.41 (3.68)

CaO

~ o

7.19 (2.36)

12.62 (2.36)

10.09 (1.58)

~ a

3.37 (1.08)

2.15 (0.93)

2.37 (1.08)

K20

~ a

0.69 (0.74)

0.15 (0.23)

0.33 (0.52)

P205

~

0.23

0.07

0.11

tr

(0.14)

(0.00)

(0.10)

H20 (+)

~ a

2.94 (1.14)

3.43 (0.79)

2.94 (1.43)

Total Fe/MgO

~

1.18 (0.47)

1.31 (0.46)

1.8 (4.8)

2.4 (3.1)

TiO 2

Na20

o

2.08 (1.46)

®

0.4

® ~b e • o~ o ,tl O ® ~ O0

e

~,

o~°~

o~,

®

I

I ~

~

°o o

a) ®

0

ii ~

o o

~o

o

0 I

1

iil

1.2 Mikabu

0.8

a

0.4

•

a

o

o/~ iO

A • •

0

~A

~AA% A

A[~

o

A

/P

i[~ P

I

I

•I

• i

1

2

3

4

5

6

H20

( -t--)

*10

Fig. 7. F e 2 O a / F e O - H 2 0 (+) diagram. Arrow indicates value higher than the scale.

6 . Discussion Rb (ppm)

~ o

10.2 (10.9)

K/Rb

~ o

596 (293)

930 (700)

426 (104)

Sr (ppm)

~ o

241 (129)

106 (95)

156 (132)

Ca/Sr

x o

291 (227)

1760 (1600)

1240 (1230)

T h e h y d r a t e d state in g e o s y n c l i n a l b a s a l t is b e y o n d d o u b t the result o f c o m p l i c a t e d processes p r o b a b l y involving or after its e f f u s i o n o n t o s e d i m e n t s . M a n y a l t e r a t i o n t r e n d s were disclosed m a i n l y in c o m p a r i n g c h e m i c a l d a t a o b t a i n e d in the rim t o t h e core o f a p r e s e n t - d a y p i l l o w lava or in t h e o u t e r a l t e r e d m a r g i n a n d less a l t e r e d i n t e r i o r o f o n e t h o l e i i t e b l o c k d r e d g e d

H. Hattori et al., Hydration in .Paleozoic synclinal basalt

from the deep ocean floor. Provided that an argument as to whether the alteration is produced by sea water weathering after the magma has cooled or produced by deuteric alteration during the magmatic stage is disregarded, generally the effect of alteration on basalts on the ocean floor may be summarized as follows: (Common features) An increase in H 2 0 : [11,34,35,38,40] ; Fe203: [11,34,38,41] ; K20: [34,35,40,43,45]. A decrease in CaO: [34,35,38,43,47] ; Na20: [31, 34,35,41,45]. (Suggested trend) An increase in TiO2: [35,38,40] ; Total Fe: [41]; Rb,Cs : [43]; P205 • [38,40]. A decrease in MgO • [41]; SiO2: [34,41,43]. Depletion of Na20 in the rim described in table 1 for the three pillows is consistent with the above-mentioned features commonly observed in sea water alteration. By statistical analysis, the negative correlation of CaO or Na20 with H20 (+) is detected in some lavas in M-E and in those of the outer zone of Permian. CaO and Na20 in some of the Paleozoic geosynclinal basalt seem to have behaved in the same manner as those observed in the present ocean floor. On the contrary, the low Fe203/FeO in all the geosynclinal basalts is not consistent with the common features as above. The opportunity for significant sea water reaction particularly with such a holocrystalline rock as M-I might be slim as judged from the variation of CaO and Ca/St. Some of the present geosynclinal basalt probably underwent dehydration under burial condition to crystallize less hydrated minerals. After an alteration of clinopyroxene by chlorite or actinolite plus an ironoxide mineral, or by carbonates plus quartz and epidote, the rocks were subjected to succeeding burial metamorphism, and actinolite, pumpellyite or glaucophane were formed. The presence of higher H 2 0 (+), for example, more than 4% in amount does not always furnish grounds for regarding them as a mere progressive hydration, but partly as dehydration of much more hydrated rocks under rather higher temperature and pressure conditions. On the other hand, there is no reliable information on the probable extent of chemical redistribution detected exclusively in the metamorphic process. Refer-

281

ring to the observation that the chemical variation from the core to the middle of the three pillows is markedly small even accompanied by the textural change and relative abundance of metamorphic minerals, i.e., pumpellyite and/or actinolite, it is suggested that the metamorphic process has nothing to do with metasomatism. As a first approximation, however, we may be allowed to assume that the determinative factor having controlled alteration processes in geosynclinal basalt was sea water reaction among various possible agents, and that all the observed trends of alteration in the Paleozoic geosynclinal basalt are simply compared with the prominent sea water alteration. Then the nature of hydration in the basalt suggests only slight sea water alteration, but the majority of the basalt may have undergone different alterations from sea water alteration. As already pointed out by Hart and Nalwalk [43], the nature of hydration related to low grade metamorphism in the Mid-Atlantic Ridge, 22°N Latitude [48], in the Ordovician basaltic lavas, Australia [24], and in the Mesozoic to Cenozoic andesitic lava flows, central Chile [49] may be different from that of sea water alteration. The Permian rocks and Mikabu rocks (M-I and M-E) are discriminated by TiO2, A1203, MgO,CaO, Na20, K20*, P205 , Rb and Sr* with the highest probability at the 99% confidence (table 2). Similarly the three units, i.e., Permian, M-I and M-E are clearly discriminated by TiO2, CaO and P205 . The Permian rocks rich in carbonates (fig. 2B) suggest a relatively high CO 2 activity. Since visible veinlets have been removed prior to chemical analysis, some carbonates (mainly CaCO3) are probably altered products of clinopyroxene or plagioclase. About 70% samples of the Permian rocks contain less than 1% carbonates equivalent to 0.55% CaO. This size of widening of o value does not affect the highly significant differences in CaO. *Significant difference at the 95% confidence. Two samples of Permian and two of M-E include extraordinarily high K20 content as seen in fig.4. The abundant presence of sericite was detected by X-ray diffraction analysis on the clay fraction extracted from the four samples. Petrographically, the sericite is secondary products disseminated on plagioclase, suggesting introduction of K20. If these four samples are omitted for calculation of average composition, standard deviation becomes small enough to discriminate the three geologic units with the highest probability at the 99% confidence. A similar result is obtained in the case of Sr, when two samples of M-E (fig.6) are disregarded.

2H2

H. ltattori et aL. Hydration in Paleozoic synclinal basalt

Let us compare o values of TiO 2, CaO and P205 with the range of change of corresponding elements in the three pillows. The total amount of increase or decrease from the core to the rim never exceeds one o value in any case. This indicates that the above-mentioned highly significant differences among the three geologic units are quite clear and are established with confidence. Suggested ranges of total change of other major elements in the three pillows may also limit the maximum extent of redistribution of corresponding elements in the geosynclinal basalt. When the ranges are defined only from the core to the middle, the redistribution of almost all major elements in the pillows are fairly within the range of half to one o value of individual elements. In fact, sandstone and shale associated with these basalts in the Paleozoic group are scarcely metasomatized, and this implies that an appreciable material transfer did not take place. Conclusively, average chemical compositions of the rocks of the three geologic units are considered to have been preserved in a relatively closed system even through a much complicated geologic history, thereby indicating that the average compositions could be very close to those of the original material. The mode of emplacement of basaltic rocks in other geosynclines has recently been explained in terms of plate-tectonics by such a mechanism that the basaltic rocks are formed in the mid-oceanic ridge, and then brought into and incorporated in the trench or geosyncline on a moving plate [50,51]. The general features of hydration of the late Paleozoic geosynclinal basalt in the Japanese Islands are not entirely compatible with alteration by present-day sea water which is demonstrated by the systematic chemical trends in the major elements with distance from oceanic ridge-spreading centers [44]. This leads to the conclusion that the geosynclinal basalt was exposed to or immersed in deep sea water for only a short time, and was rapidly covered with sedimentary sequences. Significant differences in the average chemical compositions of the three geologic units, particularly between the axial part (Mikabu green rocks, both M-I and M-E) and the flank parts are not explained by the moving plate hypothesis mentioned above. The areal distribution of the basalt type in the Japanese Islands is geotectonically interpreted elsewhere [8,52]. The hydrated state in sites of basaltic magma generation and the presence of anomalous upper mantle around the Mid-Atlantic Ridge is suggested to be of

prime importance [53]. Isotopic data from the Tertiary intrusion complexes, western Scotland, indicate that very large hydrothermal convection systems involving heated meteoric waters were established in that area at the time of igneous intrusion [54]. From these investigations, it is inferred that considerable amounts of water permeated deep enough through sediments to mix with magma and the ensuing ascent of the hydrous magma thus formed became the original magma of the geosynclinat basalt. In order to make the genesis of the geosynclinal basalt magma clearer, more detailed surveys of geology and geochemistry must be made.

Acknowledgements The authors are greatly indebted to Prof. S. Mizutani, Nagoya University, who offered constructive criticism and advice on this paper. The authors are also grateful to Dr. H. Isomi, the Geological Survey of Japan, Prof. A. Masuda, Tokyo Science University, and Prof. S. Oana, Nagoya University, who generously extended helpful comments.

Appendix: Description of pillow lavas (I-19): Locafity, 18 km west of Komatsujima city, Tokushima Pref. Largest axis, 30 cm. Rim, surface to 3 cm deep, middle, 5 8 cm, and core 8-11 cm. Rim: Glassy groundmass contains a small amount of clinopyroxene phenocryst with hourglass extinction, measuring 0.05 mm in diameter. Irregular-shaped aggregates of micro-spherulite are also sporadically found. Minute accicular actinolite is widespread both in the groundmass and around vesicles which are filled with chlorite. Middle: The same as in the core, but no quartz veinlet. Core: Groundmass composed of micro-lath of plagioclase and minute mafic minerals (probably sphene and clinopyroxene) is partly altered to actinolite. Vesicles are mostly altered to chlorite. Quartz veinlet with a small amount of chlorite is rarely seen. (I-45): Locality, 35 km west of Anan city, Tokushima Pref. Largest axis, 30 cm. Rim, surface to 3 cm deep, middle, 3 - 6 cm, and core 7 10 cm.

H. ttattori et aL, Hydration in Paleozoic synclinal basalt

Rim: The same as in the middle, and no textural change at all. Vesicles are exclusively filled with pumpellyite with strong pleochroism. Middle: Mossaic aggregates of quartz, measuring up to 2 m m across exceed phenocryst plagioclase in amount. Rarely, clinopyroxene is detected in epidote and calcite grains. In the groundmass, lath-shaped plagioclase is p r e d o m i n a n t with a small a m o u n t of quartz. Vesicles are filled with pumpellyite. Core: Phenocryst clinopyroxene is altered to epidote. Plagioclase and mosaic aggregates of quartz measure 1 m m across. Occasionally calcite entirely replaces some phenocryst. In the groundmass pilotaxitic texture characterized by lath-shaped plagioclase, mafic minerals and a small a m o u n t of quartz is distinct as in the middle. Actinolite, pumpellyite, calcite and chlorite are widespread in the groundmass. Mosaic aggregates of quartz (ambiguous origin) are scattered evenly in the three parts of this pillow. (F-81): Locality, Fujihashi-mura, Ibi-gun, Gifu Pref. Longest axis, 40 cm. Rim, outer surface to 2 cm deep, middle, 3 - 6 cm, and core, 8 - 1 0 cm. Rim: Very fine-grained, tachylitic groundmass of brownish material. Plagioclase phenocrysts are scattered sporadically, and are "altered to quartz and clayey mineral with a small a m o u n t of pumpellyite. Variole grows larger inwards, measuring 1 to 3 mm. Network veinlet composed of quartz and pumpellyite is clearly cut by veinlet of chlorite and quartz. Middle: Fan-shaped feathery aggregates of least crystallized material are predominant. Characteristic r o u n d e d variole is n o t recognized. Calcite is scattered in the aggregate. Plagioclase phenocrysts are altered to clinozoisite + quartz + clayey material. Two types of veinlet are thicker than in the rim. Core: G r o u n d m a s s of feathery aggregate is rather coarse-grained. Pyroxene phenocryst measuring 5 m m across is altered to chlorite. Plagioclase phenocryst to calcite with small a m o u n t s of quartz and pumpellyite. Lenticular vesicle, measuring 2 m m at its longest in this pillow, is altered to calcite a c c o m p a n y i n g with pumpellyite and quartz. Calcite is widespread in b o t h veinlet and groundmass. Calcite veinlet is at its thickest in this core part.

283

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H. Hattori et aL, Hydration in Paleozoic synclinal basalt

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