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RbSr whole-rock ages of Precambrian metamorphic rocks in the Kamiaso conglomerate from central Japan
Earth and Planetary Science Letters, 21 (1974) 277-287 © North-ttolland Publishing Company, Amsterdam - Printed in The Netherlands
Rb-Sr
WHOLE-ROCK
AGES OF PRECAMBRIAN
IN THE KAMIASO CONGLOMERATE
METAMORPHIC
FROM CENTRAL
ROCKS
JAPAN
Ken SHIBATA
Geological Surf O, of Japan, Kawasaki (Japan} and Mamoru ADACHI
Department of Earth Sciences, Nagoya University, Nagoya (Japan) Received 30 July 1973 Revised version received 8 November 1973
Whole-rock samples of metamorphic and granitic cobbles and boulders from the Kamiaso conglomerate in central Japan yield well-defined Rb-Sr isochron ages of 1985 + 25 my and 1820 + 40 my. These ages are the oldest yet obtained for rocks in the Japanese Islands, and provide key evidence for the middle Precambrian metamorphic and igneous events in the provenance of these rocks. The age of 1985 my defined by six samples of quartzo-feldspathic gneiss may be the time of emplacement of the original granitic rocks. The 1820 my age indicates the time of extensive regional metamorphism and igneous activity. Precambrian episodes in the provenance of the Kamiaso conglomerate are summarized as follows: (1) 2000 my - formation of granitic rocks, (2) 1800-1600 my highgrade metamorphism accompanied by igneous activity, (3) 1200 1000 my some significant thermal event. Judging from rock types and geochronological data, it can be said that metamorphic rocks in the Kamiaso conglomerate are remarkably similar to those of the Matenrei and Nangnim systems in North Korea. The Precambrian complex from which the metamorphic and granitic rocks were derived, was exposed to the north not far from the present site of the Kamiaso conglomerate in the late Paleozoic time, and it was probably a part of the large Precambrian continent in East Asia.
1. Introduction The Japanese Islands are situated between the Asian continent and the Pacific Ocean, and knowledge of the Japanese basement rocks is fundamental to understand the geologic phenomena in the island arc system. The oldest fossiliferous formations recognized in Japan are Silurian, and there has been much debate whether or not the Japanese Islands have a Precambrian basement. Some metamorphic complexes exposed in limited areas have been thought to be originally Precambrian. Most of the isotopic dating results, however, are younger than 500 my with two exceptionally old 2 o 7 pb/206 Pb ages obtained for detrital zircons: 1493 my from the Hida metamorphic rocks 11], and 1782 my from the Ryoke metamorphic rocks [2], the latter o f which
are metamorphosed equivalents of the Paleozoic sediments. Until recently there was no record of Precambrian rocks in Japan that have not been involved in a younger metamorphism. In 1970, Adachi found an intraformational conglomerate in a Permian formation at Kamiaso, central Japan (Fig. I), and named it the Kamiaso conglomerate [3]. This conglomerate is characterized by having rounded gravels of sillimanite gneiss and orthoquartzite [4]. We have carried out K - A t and R b - S r dating on micas from the metamorphic rocks contained in the Kamiaso conglomerate, and obtained middle Precambrian ages of 1 7 0 0 - 1 5 0 0 my [ 5 - 7 ] . This was the first geochronological evidence o f Precambrian rocks in Japan. Encouraged by these results, we have carried out the R b - S r whole-rock dating on metamorphic and granitic rocks in the Kamiaso conglomerate. We here
278
K. SHIBATAAND M. ADACHI
,3,o*
,;0"
50"
Poleozoic group 0
50
IO0 KM poleocurrent
direction
Fig. 1. Geologic sketch map of central Japan showing location of Kamiaso and distribution of Hida metamorphic complex and Paleozoic group in the Mino terrain. Arrows indicate paleocurrent directions (after Adachi and Mizutani [9] ). present the R b - S r whole-rock analytical data and discuss more detailed geochronology of these rocks in relation to the Precambrian of East Asia.
2. Geology The upper Paleozoic group is widely developed in the inner zone of central Japan. This region is called the Mino Paleozoic terrain, where geosynclinal sediments attaining 10 km in thickness are composed mainly of sandstone, shale and chert with lesser amounts of limestone, basic volcanics and conglomerate. The region is bounded by the Hida metamorphic complex in the north and gradually changes with NE-SW trend into the Ryoke metamorphic belt in the south. In these geosynclinal sedimentary facies, coarse clastic tongues of intraformational conglomerate are sporadically found. The Kamiaso conglomerate is one of them and is intercalated in a turbidite formation of the Permian system in the medial part of the Mino terrain. Stratigraphically, the Kamiaso conglomerate
is considered to be early Permian [4], since this conglomerate is about 1,500 m lower than the limestone horizon of the middle Permian (Parafusulina zone), and contains the limestone gravels of the middle Carboniferous (ProJhsulinella-Fusulinellazone). The Permian system of the Kamiaso area consists of graywacke, shale and bedded chert with local intercalation of limestone, mafic volcanics and conglomerate, forming a westerly plunging syncline with the wavelength of about 10 km [8]. There is no sign of shear folding and contact metamorphism, and sedimentary structures such as sole markings, graded bedding, cross bedding and slump structure are well preserved. Paleocurrent studies [4,9] indicate that almost all clastic materials including the Kamiaso conglomerate came from the north (Fig. 1). The Kamiaso conglomerate is an ill-sorted polymictic intraformational paraconglomerate and is made up of four beds with the thickness of 3~8 m. Rounded to subangular pebble to boulder-sized gravels of sedimentary, igneous and metamorphic rocks are contained in the coarse-grained graywacke matrix. Also contained in the conglomerate are angular fragments of black shale and chert locally derived. Among gravels sedimentary rocks such as graywacke, shale, chert, limestone and marl are abundant, and metamorphic rocks make up as much as 12%. Minor but very important constituents of the Kamiaso conglomerate are well-rounded gravels of orthoquartzite, quartzose sandstone, sillimanite gneiss, garnet gneiss, two-mica granite, syenite, granodiorite, quartz monzonite, quartz porphyry, rhyolite and amygdaloidal andesite. Rounded gravels of limestone containing the middle Carboniferous fusuline fossils and carbonatized arkose sandstone having cobble of sillimanite gneiss are also noteworthy. The high-grade metamorhpic rocks and orthoquartzite are considered to have been derived from the Precambrian continent in the north [4].
3. Sample
description
All the samples of metamorhic and granitic rocks have suffered some chloritization and sericitization probably due to the later hydrothermal alteration and weathering. While biotite and plagioclase are commonly altered, potassium feldspar is generally fresh in
Rb-Sr WHOLE-ROCKAGES OF PRECAMBRIAN METAMORPHICROCKS
279
TABLE 1 Rb-Sr analytical data for whole-rock samples of metamorphic and granitic rocks in the Kamiaso conglomerate Sample No.
Rock type
Rb (ppm)
Sr (ppm)
Quartzo-feldspathic gneiss 1-1 medium-grained garnet -sillimanite-biotite gneiss
265.6 266.2
122.9 122.1
2 -6
294.8
80.4
fine-grained muscovite-sillimanite-biotite gneiss
87Rb/86Sr
6.255 6.312 10.61
87Sr/86Sr
0.8899 0.8895 0.9935
2 -9
coarse-grained garnet-biotite gneiss
270.1
130.3
6.000
0.8674
2 -10
medium-grained sillimanite-biotitegneiss
343.6
111.0
8.964
0.9450
4-32
medium-grained garnet-sillimanite-biotitegneiss 315.9
124.9
7.320
0.9215
4 -36
coarse-grained sillimanite-biotitegneiss
277.9
135.7
5.927
0.8663
4 -41
coarse-grained muscovite-biotite grandioritic gneiss
132.5
267.0
1.437
0.7459
4 -50A
coarse-grained muscovite-biotite granitic gneiss
219.9
170.9
3.725
0.8145
4 -50B
coarse-grained sillimanite-muscovitebiotite granitic gneiss
134.3
190.5
2.041
0.7643
4 -58
coarse-grained muscovite-biotite granodioritic gneiss
156.1
230.5
1.961
0.7276
4 -65
medium-grained sillimanite-muscovite-biotite gneiss
361.7 358.8
115.3 113.0
9.080 9.196
0.9710 0.9712
4-73
coarse-grained sillimanite-biotitegneiss
256.1
165.0
4.494
0.8233
4-76
medium-grained garnet-sillimanite-biotite gneiss
287.5
170.5
4.882
0.8345
1-19
medium-grained sillimanite-muscovite-biotite gneiss
205.0
120.7
4.918
0.8283
4-23
coarse-grained sillimanite-biotitegneiss
166.9
198.8
2.432
0.7708
4 -24B
coarse-grained sillimanite-biotitegneiss
244.2
199.5
3.543
0.7988
Granitic rock G-1
coarse-grained muscovite-biotite granite
244.5
123.3
5.743
0.8629
G-2
coarse-grained syenite
420.2
105.0
G-3
coarse-grained hornblende-biotite granodiorite
84.3
346.7
0.7038
0.7172
G-4
medium-grained quartz monzonite
70.2
287.5
0.7074
0.7167
Pelitic gneiss
all samples. These samples are divided by microscopic examination into: (1) quartzo-feldspathic gneiss, (2) pelitic gneiss, and (3) granitic rock (Table 1). Quartzo-feldspathic and pelitic gneisses were formed under the high-grade metamorphic condition up to the upper amphibolite facies [ 10]. Generally the quartzo-feldspathic gneisses are mas-
11.58
1.017
sive and fine- to coarse-grained, composed chiefly of potassium feldspar, quartz and plagioclase with lesser amounts of biotite, sillimanite, garnet, muscovite and accessories. Sillimanite is the most common index mineral in the quartzo-feldspathic gneiss characterizing the high-grade metamorphism. Potassium feldspars are perthitic orthoclase and microcline with
280 large values of 2Vx(67-87°). Some plagioclases are markedly zoned (normal in character) and antiperthite is occasionally ol:served. All samples of the pelitic gneiss are medium- to coarse-grained sillimanite-muscovite-biotite-oligoclase(An t 6.23)-potassium feldspar-quartz gneiss with minor amounts of tourmaline, cordierite, apatite, zircon, graphite, rutile, sphene and trace sulfides. Sillimanite occurs in close association with flaky biotite displaying chocolate brown to reddish brown Z-axial color. Orthoclase porphyroblasts (up to 30 mm in size) are common in sample 4 24. Samples of four granitic rocks have suffered varying degrees of later alteration; chloritized biotite and sericitized plagioclase are common. Sample G-2, dark gray, coarse-grained syenite, is partly penetrated by a veinlet of quartz-tourmaline-albite. In sample G-3, hornblende is completely replaced by carbonate and chlorite, and biotite is also almost entirely altered to chlorite. Unlike the other three samples, sample G-4 has undergone a stight recrystallization and small grains of quartz are observed at the original grain boundaries.
4. Experimental procedures Rb-Sr analyses were done on 20 whole-rock sampies of cobbles and boulders of metamorphic and granitic rocks from the Kamiaso conglomerate. In order to obtain a representative whole-rock sample, at least 1 kg of the rock was crushed to > 115 mesh. Rb and Sr concentrations were measured by isotope dilution using ~TRb and 84Sr spikes, and STSr/ 86 Sr ratios were calculated from the spiked runs. 87 Sr/86 Sr ratios were separately determined for some unspiked samples. The mass spectrometric analyses were done by the JEOL-05RB mass spectrometer with 90 °, 30 cm radius analyzer. Triple filaments were used and samples were loaded on the side filaments as RbzSO4 or Sr(NO3)z. Ion currents were detected with an electron multiplier, output of which was amplified by a vibrating reed electrometer, recorded on a digital voltmeter and punched on paper tape. Data reduction was done by a computer. All 87 Sr/86 Sr ratios were normalized to 8 6 Sr/88 Sr ratio = 0.1194. The standard deviation for the average of 10 20 peak ratios is mostly between 0.05-0.12%.
K. SHIBATAAND M. ADACHI Eight analyses of the Elmer and Amend Sr standard range from 0.70816 to 0.70872 with an average of 0.70843 +- 0.00018 (lo). lsochrons were calculated by the least-square method of York [11 ] taking account of uncertainties of + 3% in 8~Rb/86Sr ratios and + 0.15% in~7 Sr/~6Sr ratios, and the errors are at the 95% confidence level. All ages given in this paper were calculated using 87Rb decay constant of 1.47 × 10-1 lyr-1. In addition to the whole-rock analyses, R b - S r analyses of constituent minerals were made for some samples. The plagioclase samples contain a small amount of quartz. K - A r age determinations were made also on biotite, muscovite, plagioclase and potassium feldspar from sample 4-50. The analytical method is essentially the same as that described by Shibata [12]. The constants used for K Ar age calculations are: k~ --4.72 X 10-l°yr-1, Xe = 0.584 N 10 -~° yr-1,4OK/K = 0.0119 atomic %. 5. Results Rb Sr analytical data for whole-rock samples of metamorphic and granitic rocks from the Kamiaso conglomerate are given in Table 1. R b - S r analyses of minerals are given in Table 2, and K - A r ages of minerals for sample 4-50 in Table 3. Rb Sr whole-rock data for 20 samples are plotted on the isochron diagram in Fig. 2. It is remarkable that, although these samples are cobbles and boulders collected from different horizons in the Kamiaso conglomerate, they yield rather well-defined isochrons. Six samples of the quartzo-feldspathic gneiss (4-65, 4-32, 1-1,4-50A, 4-50B, 4-41) give an isochron of 1985 +- 25 my with an initial aTsr/86Sr ratio of 0.7036 -+ 0.0010. This is the oldest age yet determined for any rocks in the Japanese Islands. Excluding four samples (1-19, 4-58, G-3, G-4), the remaining 10 samples define an isochron of 1820 +- 40 my with an initial ratio of 0.7039 -+ 0.0027. Among these 10 samples, two samples (G-l, G-2) are granite and syenite, two (4-23, 4-24B) are pelitic gneisses, and the remaining six are quartzo-feldspathic gneisses. Four samples that do not fall on either of the two isochrons give model ages of 780 my (4-58), 1120 my (G-4), 1170 my (G-3) and 1680 my (1-19) by assuming the initial 87 Sr/86 Sr ratio = 0.705.
281
Rb-Sr WHOLE-ROCK AGES OF PRECAMBRIAN METAMORPHIC ROCKS TABLE 2 Rb-Sr analytical data for minerals from metamorphic rocks in the Kamiaso conglomerate Sample No.
Mineral
1-19
plagioclase
41.4
t 71.3
0.6990
0.7265
2-9
K-feldspar
423.3
189.9
6.454
0.8929
K-feldspar
285.3
418.3
1.975
0.7632
plagioclase
57.2
159.4
1.038
0.7338
638,4
42.2
43.86
1.814
biotite (chloritized)
306.3
81.9
10.83
0.9346
muscovite
430.4
111.6
11.17
0.9830
K-feldspar
367.5
239.0
plagioclase
60.0
K-feldspar
483.8
4-24 4-50
4-73
Rb (ppm)
biotite (fresh)
As shown in Table 4, micas in some o f these samples yielded K - A t and R b - S r ages of 9 5 0 - 1 6 9 0 my. It is evident, therefore, that the R b - S r whole-rock ages o f 1985 my and 1820 m y indicate the times o f older events. Fig. 3 shows an isochron diagram for six wholerock samples, and four potassium feldspar and three plagioclase samples. The combined isochron defines an age of 1970 +- 30 my with an initial ratio of 0.7047 -+ 0.0011, which agrees with that defined by whole-rock samples within the limit o f experimental error. This suggests that the feldspars became closed to Rb and Sr isotopes about 2000 my ago together with whole-rock samples. The quartzo-feldspathic gneiss contains abundant potassium feldspar and subordinate amounts of quartz and plagioclase with lesser amounts o f biotite, muscovite, sillimanite and
Sr (ppm)
87Rb/a6Sr
S7Sr/a6Sr
4.451
0.8339
110.0
1.579
0.7514
171.3
8.176
0.9488
garnet. Based on the petrological study o f the metamorphic rocks in the Kamiaso conglomerate, some o f the quartzo-feldspathic gneiss were considered to be orthogneiss [101. The 1985 my isochron age may be regarded as the time of emplacement o f the original granitic rocks. The R b - S r analytical results of minerals for the quartzo-feldspathic gneiss 4-50 are shown in Table 2 and K - A t age results in Table 3. This is a gray, coarse-grained, large boulder about 60 cm in diameter, composed o f potassium feldspar ( 2 V x = 73 75 °, A = 0.82), quartz, plagioclase (An2s), biotite, muscovite with accessories of zircon, apatite, sphene, futile and iron ore. Biotite is almost entirely altered to chlorite. Plagioclase is also dusted with sericite. The rock is partly very coarse-grained and heterogeneous, having large lenticular clots of biotite
TABLE 3 K-At ages of minerals from quartzo-feldspathic gneiss 4-50 Mineral
K20 (%)
4°Ar rad (10 ..4 ccSTP/g)
Atmospheric 4°Ar(%)
Age (my)
Biotite (fresh) Biotite (fresh) Biotite (chloritized) Muscovite K-feldspar Plagioclase
6.53 6.63 3.29 9.16 11.54 1.26
4.51 4.60 1.53 7.41 4.97 0.649
3.3 6.0 2.8 0.4 5.3 18.3
1410 1410 1050 1570 990 1140
+ 50 ± 50 -+40 ± 50 ± 30 + 40
TABLE 4 K - A r and R b - S r ages of minerals from m e t h a m o r p h i c rocks in the Kamiaso conglomerate Sample No.
Rock type
Mineral
K - A r age (my)
R b - S r age (my)
1-8 1-19
G-B, P S-M-B, P S - B , QF G-B, P S-B, P S-B, P ( S ) - M - B , QF
950** 1440" 1640" 1160" 1290** 1540"* 1160* 1410
1510"* 1470"* 1630"*
2-6 4-21 4-23 4-24 4-50
biotite biotite muscovite Biotite biotite biotite biotite biotite (fresh) biotite (chloritized) muscovite K-feldspar plagioclase
1680** 1490"* 1660" * 1690
1050
1310
1570 990 1140
1610
Abbreviations: G = garnet, B = biotite, S = sillimanite, M = muscovite, P = pelitic gneiss, QF = quartzofeldspathic gneiss. 4°K: h = 4.72 × 10 - 1 ° yr -1, h e = 0.584 × 10 -1° yr ' l , 4 ° K / K = 0.0119 at.%. 87Rb: ~k = 1.47 × 10 -11 yr -1. * [51; ** [7].
/ 4-73~ . _ / T " 19 70130 m.y. R." 0.7047,0.0011
4-~.1
/
1.00
---.1.-4_32
/ /
0.95
0.0010/
T = 1985--. 25 m.y. R,- 0.7036±
2-9K /
~ / ~ ' / ~ 'o ,7,
0.90
II
4-50K
oe5
0.15
/i-i
m.,.
4_soA //'4~;,3 -
Ro- 0.7039_ 0.0027
Y • 24~ S
$°1
/ 2
07lb/'~6Sr Fig. 2. R b - S r whole-rock isochrons for m e t a m o r p h i c and granitic rocks in the Kamiaso conglomerate.
4
6
$
|TRb//16Sr Fig. 3. R b - S r isochron diagram for six whole-rock samples, and four potassium feldspar and three plagioclase samples. K = potassium feldspar, P = plagioclase.
I0
Rb-Sr WHOLE-ROCKAGES OF PRECAMBRIAN METAMORPHICROCKS up to 35 mm in size. Sillimanite partially replaced by muscovite is usually present; plagioclase and biotite dominate over potassium feldspar. Two samples were prepared; one from a massive part (4-50A) and the other from a gneissose heterogeneous part (4-50B). Both samples fall, however, on the whole-rock isochron of 1985 my. The mica-whole-rock isochron ages are calculated using the whole-rock sample 4-50B, since this contains more mica than 4-50A. Although the biotite clots in sample 4-50B have also suffered alteration, fresh biotite that survived the chloritization is partly retained. It is of much interest that this fresh biotite gives a R b - S r isochron age of 1690 my, while chloritized biotite 1310 my. This difference in biotite age between 1690 my and 1310 my may be interpreted as their differential chloritization. The Rb Sr isochron age of muscovite is 1610 my. The K - A r biotite ages are 1410 my (fresh) and 1050 my (chloritized), while muscovite age 1570 my; this is very close to the R b - S r age. The potassium feldspar and plagioclase give K - A r ages of 990 my and 1140 my, both are similar to the chloritized biotite age. In marked contrast with the younger K - A r ages, potassium feldspar and plagioclase are on the R b - S r whole-rock isochron of 1985 my, suggesting that they have remained closed systems since the original formation of the rock. The 1820-my whole-rock isochron age seems to be rather ubiquitous throughout the provenance of the metamorphic rocks, because the isochron is well defined by various rock types; quartzo-feldspathic and pelitic gneisses and granitic rocks. This age indicates the time of extensive regional metamorphism associated with igneous activity. Samples G-1 and G-2, both of which lying on the 1820-my isochron, are typical granite and syenite, suggesting that these zocks were emplaced 1820 my ago. It should be noted that all these whole-rock systems appear to have been closed since 1820 my ago, although the rocks yielded mica ages of 1500-1700 my (Table 4). Because the micas in the metamorphic rocks were probably formed or recrystallized under the upper amphibolite facies metamorphic condition, the mica ages of 1500-1700 my may represent the time of this metamorphism. If this is the case, the 1820-my whole-rock age may indicate either the older metamorhpic event separable from the 15001700-my event, or the period of the earlier phase
283
within one and the same prolonged metamorphic event. Among R b - S r and K - A r dates muscovites and some biotites give rather concentrated ages of 16001700 my; therefore, the period of the high-grade regional metamorphism may be interpreted to have occurred between 1800 and 1600 my. The ages younger than 1600 my are then ascribed to its retrogressive metamorphic phase. Some of the biotites give much younger ages of 1000 1200 my (Table 4). All these biotites are considerably chloritized as shown by lower K20 contents, and the lower ages might be due to rather recent weathering or alteration. Alternatively, the 1000-1200-my age may indicate a thermal event that has rejuvenated biotite ages, and this is supported by the evidence that two of the whole-rock samples (G-3, G-4) give R b - S r model ages of about 1150 my. Adachi [10] pointed out, on the basis of the petrological study of the gneisses, that the 1000-1200-my episode may represent some thermal event which led to the varying degrees of chloritization and sericitization accompanied by the formation of secondary opaque minerals such as pyrite. It is concluded that the 1000-1200-my age is not the result of argon loss due to weathering or alteration, but represents some significant thermal event. Based on the isotopic ages and petrographic evidence of metamorphic and granitic rocks in the Kamiaso conglomerate, the Precambrian episodes in the provenance area for these rocks are suggested as follows: (1) emplacement of granitic rocks at about 2000 my; (2) high-grade regional metamorphism up to the upper amphibolite facies associated with emplacement of granite and syenite at 1800-1600 my, by which most of the metamorphic rocks were formed; and (3) some thermal event at 1200-1000 my, by which some of biotite ages were rejuvenated.
6. Discussion Systematic analyses of the paleocurrent direction [9] and coarse clastic materials [8] in the Mino terrain disclosed that almost all of clastics including the metamorphic rocks in the Kamiaso conglomerate, were transported from the north. A detailed petrological study of clastic plagioclase and heavy minerals in Permian graywacke of the central Mino terrain shows
284 that the materials were derived from an extensive metamorhpic terrane dominated by the granodioritic rocks [ 13]. This fact, together with the paleocurrent evidence and isotopic age data, may suggest that an extended Precambrian land mass existed in the north not so far from the present site of the Kamiaso conglomerate. Then question arises whether the Hida metamorphic complex, situated about 70 km north of Kamiaso, is a possible source for the metamorphic rocks in the Kamiaso conglomerate. The Hida metamorphic complex is composed mainly of quartzo-feldspathic gneiss, marble, amphibolite and pelitic gneiss metamorphosed generally in the epidote-amphibolite to the amphibolite facies. Petrological study and isotopic dating of the Hida complex have been done extensively [1, 1 4 - 1 9 ] , but the controversy regarding the age of metamorphism and original rocks is not yet clearly settled. Most of K - A r and R b - S r mineral ages are heavily concentrated at 180 my with minor concentrations at 250 my and 500 my by mostly R b - S r and U - T h - P b ages. The oldest record is the =07pb/=O6pb age of 1493 my for the detrital zircon from the Amo gneiss [ 1]. This age is similar to the mica ages of gneisses in the Kamiaso conglomerate. The R b - S r whole-rock analyses of the Hida complex, though limited in number, indicate that the ages are generally 2 0 0 - 5 0 0 my with fairly low initial a vSr/a~ Sr ratios of 0.7030.708 [18, 19]. These results indicate that the Hida metamorphic complex, at least for the rocks analyzed, is not reworked material of Precambrian crust. In view of the 1500-my age for zircon, it is probable that some of the Hida metamorphic complex contain the material originated back to the Precambrian time. At present it is, however, rather difficult to correlate the Hida complex with metamorphic clasts in the Kamiaso conglomerate. Another evidence for the existence of a Precambrian land mass is a =°VPb/=°6pb age of 1782 my for the detrital zircon in a banded gneiss from the Ryoke metamorphic rocks in the Kinki district of southwest Japan [2]. Since these metamorphic rocks are believed to have been metamorphosed from the Paleozoic sediments, the zircon was probably derived from a Precambrian terrane and laid down in the clastic sediments of the Japanese Paleozoic geosyncline. Aside from the metamorphic rocks having exceptionally old z o 7pb/~ o 6 Pb ages mentioned above,
K. SHIBATAAND M. ADACHI metamorphic and igneous rocks considered to have been derived from the pre-Silurian basement complex are sporadically found either as squeezed-out masses in the tectonic zone or as xenoliths in younger volcanics. The metamorphic and igneous rocks intimately accompanied by Silurian strata in the Kurosegawa tectonic zone of southwest Japan are typical examples, although results of isotopic dating for these rocks concentrate at about 400 my [20, 21 ]. Regarding the metamorphic and igneous rocks with about 400-my age, discussion should be extended to the additional new evidence from northeast Japan that recent K - A r dating of hornblende in the Hikami granitic rocks associated with Siluro-Devonian strata in the Kitakami district also gave the similar age of 354 my [22]. Further accumulation of isotopic age data as well as geologic evidence will give a sharper view on the basement geology of the Japanese Islands. Precambrian formations are widely developed in the eastern part of the Asian continent near the Japanese Islands, especially in North China and Korea (Fig. 4). The ages of basement rocks in Korea are important for discussing the relationship between the provenance of the metamorphic rocks in the Kamiaso conglomerate and the Precambrian rocks in East Asia. There are about 20 Precambrian dates on metamorphic and granitic rocks in Korea, ranging from 8 0 0 2900 my [23]. Among them are included 12 R b - S r whole-rock model ages in South Korea [24]. The whole-rock isochron age could not be obtained because of a wide scatter of points probably due to the o
500 KM
S i n o ;.'1~ o r ~ a fi - s h i e i c] )5~;2~.% ) e,z','Z;f,~,-~'-"
'~', 2 2 0 0 - 2 4 0 0 ~ " ' "
~ . , ~ ' - . a ~ - ~ ; . ~
",x;;~ 3~5c'~" ~:,,-'<.'Z'
MATENR~
.....
00~/~ "~g'"
\
.
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,8o-,soo
........... "'~
215o ~v
-
i~00-1800 2000
AGES IN MY.
Precambrian
Formations
Fig. 4. Distribution of Precambrian formations and their isotopic ages in east Asia.
Rb-Sr WHOLE-ROCKAGES OF PRECAMBRIAN METAMORPHICROCKS disturbance by the extensive Mesozoic plutonism. However, Hurley et al. [24] concluded that some of the basement rocks in South Korea were originally at least as old as 2000 my. A U - P b zircon age of 2150 my for the Yoogoo granite gneiss confirms the middle Precambrian event in South Korea [25]. The Nangnim and Matenrei systems in North Korea yielded K - A r ages of 2040 my [26] and 1740-1700 my [27] respectively. According to the compilation of isotopic age data from the Chinese platform by Yanagi and Yamaguchi [28], there is a remarkable concentration of ages between 1800-1900 my, with minor peaks at 2300-2400 my and about 1400 my, indicating the epochs of metamorphism associated with plutonic activity in the Chinese platform. The Huto system in North China was dated at 1550 -+ 200 my by the common lead method [29], and the upper limit of the Liaoho system in northeast China was determined at 1400 m.y. [29]. The Wutai and Anshan systems in North and northeast China yielded many K - A t ages ranging from 2400 to 1700 my [30]. The 1700-1500-my mica ages on the metamorphic rocks in the Kamiaso conglomerate are similar to those of the Huto, Liaoho and Matenrei systems, whereas the 2000-my whole-rock isochron age on the quartzo-feldspathic gneiss is correlated to ages of the Wutai, Anshan and Nangnim systems. This isochron age is roughly comparable with the zircon age of 2150 my from South Korea [25]. It is also notable that the 1800-1900-my peak in the Chinese platform corresponds to the 1820-my whole-rock age given in this paper. Judged by rock types, metamorphic conditions and geochronological data, it can be said that metamorphic rocks in the Kamiaso conglomerate are remarkably similar to those from the Matenrei and Nangnim systems in North Korea [10]. There remains a possibility that the basement rocks in South Korea have much affinity with the metamorphic rocks in the Kamiaso conglomerate. Well-rounded orthoquartzite pebbles and cobbles in the Kamiaso conglomerate give us another important clue to the discussion on the provenance area. Similar occurrence of orthoquartzite pebbies has been reported in the Paleozoic terrain [31]. Interestingly enough, orthoquartzitic sandstone is commonly developed in the Sinian and Shogen systems which
285
cover unconformably the Huto, Liaoho and Matenrei systems. It is possible to suppose that orthoquartzite clasts in the Kamiaso conglomerate were derived from the Sinian and Shogen systems. The late Precambrian non-metamorphosed Sinian system yielded K-Ar ages of 900-1100 my on glauconite, and its lower limit is estimated to be about 1300 my [30]. The 10001200-my ages recorded on some of the metamorphic and granitic rocks in the Kamiaso conglomerate may indicate a thermal event that took place while the Sinian system was deposited. On the basis of the paleocurrent evidence and the similarity in ages and rock types of some of the Precambrian rocks in East Asia to the metamorphic rocks in the Kamiaso conglomerate, it is supposed that the Precambrian complex, from which the metamorphic clasts were derived, was exposed not so far from the present site of the Kamiaso conglomerate in the late Paleozoic time, and it probably formed a part of the Precambrian continent including the Matenrei and Nangnim systems [5,10] and also the Precambrian formations in South Korea. According to the explosion-seismic observations [32], a thick layer with the velocity of 6 km/sec exists in central Japan reaching a depth of 25 km, whereas the underlying layer with the velocity of 7 km/sec is thin. The low-velocity material constituting the 6-km/sec layer may be referable to the acidic crust composed of granite or gneiss. Presumably, the 6-km/sec layer inferred from the explosion seismology contains the remnants of the Precambrian continent discussed here, which is possibly now deeply hidden under the present Japanese Islands. The existence of the Precambrian continent proposed by us has an important meaning when considering the geologic development of the Japanese Islands, especially the Precambrian geology in Japan and East Asia and the origin of the Japan Sea. The development of marginal seas such as the Japan Sea has recently been much discussed [24,33-36] ; the discussion has been focussed on an interpretation that the Japan Sea area was formed by opening under some tensional condition. The isotopic ages of the metamorphic rocks in the Kamiaso conglomerate, which are comparable with those from the middle Precambrian formations in East Asia, in particular from the basement complex in Korea, support the idea that the Japanese Island were originally a part of the Asian
286 c o n t i n e n t , a n d t h a t t h e y were severed f r o m the cont i n e n t giving rise to the f o r m a t i o n o f t h e J a p a n Sea.
Acknowledgements We are very grateful to Dr. S. Mizutani of Nagoya University for his fruitful discussions and constructive criticisms of the manuscript. Our thanks are also extended to Dr. H. Hattori of the Geological Survey of Japan for his encouraging advices during this investigation. The work was partly financed by Grant-inAid for Scientific Research, Ministry of Education.
References [ 1 ] K. Ishizaka and M. Yamaguchi, U - T h - P b ages of sphene and zircon from the Hida metamorphic terrain, Japan, Earth Planet. Sci. Lett. 6 (1969) 179. [2] K. Ishizaka, U - T h - P b ages of zircon from the Ryoke metamorphic terrain, Kinki District, J. Japan. Assoc. Min. Petrol. Econ. Geol. 62 (1969) 191 (in Japanese with English abstract). [3] M. Adachi, Granite-bearing conglomerate in the Permian system from Kamiaso, Shichiso-mura, Kamo-gun, Gifu Prefecture, central Japan, Abst. Program Ann. Meet. Geol. Soc. Japan (1970) 201 (in Japanese). [4] M. Adachi, Permian intraformational conglomerate at Kamiaso, Gifu Prefecture, central Japan, J. Geol. Soc. Japan 77 (1971) 471. [5] K. Shibata, M. Adachi and S. Mizutani, Precambrian rocks in Permian conglomerate from central Japan, J. Geol. Soc. Japan 77 (1971) 507. [6] K. Shibata, M. Adachi and S. Mizutani, Precambrian geochronology of metamorphic rocks in Permian conglomerate from central Japan, Rep. 24th I.G.C. Sec. 1 (1972) 288. [7] K. Shibata and M. Adachi, R b - S r and K - A r geochronology of metamorphic rocks in the Karniaso conglomerate, central Japan, J. Geol. Soc. Japan 78 (1972) 265. [8] S. Mizutani, Superficial folding of the Paleozoic system of central Japan, J. Earth Sci. Nagoya Univ. 12 (1964) 17. [9] M. Adachi and S. Mizutani, Sole markings and paleocurrent system in the Paleozoic group of the Mino terrain, central Japan, Mem. Geol. Soc. Japan 6 (1971) 39 (in Japanese with English abstract). [ 10] M. Adachi, Pelitic and quartzo-feldspathic gneisses in the Kamiaso conglomerate - a study of Precambrian geology in Japan and East Asia, J. Geol. Soc. Japan 79 (1973) 181.
K. SHIBATA AND M. ADACHI [ 11 ] D. York, Least-squares fitting of a straight line, Can. J. Phys. 44 (1966) 1079. [12] K. Shibata, K - A r age determinations on granitic and metamorphic rocks in Japan, Geol. Survey Japan Rep. 227 (1968). [13] S. Mizutani, Clastic plagioclase in Permian graywacke from the Mugi area, Gifu Prefecture, central Japan, J. Earth Sci. Nagoya Univ. 7 (1959) 108. [14] K. Ishioka, Staurolite and kyanite near Unazuki in the lower Kurobe-gawa area, J. Geol. Soc. Japan 55 (1949) 156 (in Japanese). [15] K. Ishioka and K. Suwa, Metasomatic development of staurolite schist from rhyolite in the Kurobe-gawa area, central Japan, J. Earth Sci. Nagoya Univ. 4 (1956) 123. [16] K. Suwa, Finding of conglomerate schist from the upper Katakai river area, Toyama Prefecture, central Japan, J. Geol. Soc. Japan 72 (1966) 585. [17] S. Sato, Precambrian-Variscan polymetamorphism in the Hida massif, basement of the Japanese Islands, Sci. Rep. Tokyo Univ. Education, Sec. C 10 (1968) 15. [18] K. Shibata, T. Nozawa and R.K. Wanless, R b - S r geochronology of the Hida metamorphic belt, Japan, Can. J. Earth Sci. 7 (1970) 1383. [19] M. Yamaguchi and T. Yanagi, Geochronology of some metamorphic rocks in Japan, Eclogae Geol. Helv. 63 (1970) 371. [20] I. Hayase and K. Ishizaka, R b - S r dating of the rocks in Japan, 1 - South Western Japan, J. Japan. Assoc. Min. Petrol. Econ. Geol. 58 (1967) 201 (in Japanese with English abstract). [21] I. Hayase and S. Nohda, Geochronology of the "oldest rock" of Japan, Geochem. J. 3 (1969)45. [22] K. Shibata, K - A r ages of the Hikami granite and the Usuginu granitic clasts, J. Geol. Soc. Japan 79 (1973) 705 (in Japanese). [23] Geological Survey Korea, Isotope ages and geologic map of Korea (1972). [24] P.M. Hurley, H.W. Fairbairn, W.H. Pinson, Jr. and J.H. Lee, Middle Precambrian and older apparent age values in basement gneisses of South Korea, and relations with southwest Japan, Geol. Soc. Am. Bull. 84 (1973) 2305. [25] H.E. Gaudette and P.M. tturley, U - P b zircon age of Precambrian basement gneiss of South Korea, Geol. Soc. Am. Bul. 84 (1973) 2305. [26] S.S. Zimin et al., Intrusive complex of northeast Korea and southwest Primorye, in: Magmatism and Useful Minerals of Northeast Korea and South Primorye, ed. Ye. Radkevich (Nauka, Moscow, 1966) 7 (in Russian). [27] N.I. Polevaya, V.K. Putintsev and V.D. Sprinston, Absolute age of certain magmatic and metamorphic rocks in North Korea, Soviet Geol. 6 (1961) 119 (in Russian). [28] T. Yanagi and M. Yamaguchi, Ages of some Precambrian metamorphic rocks in North China, Mem. Fac. Sci. Kyushu Univ. Set. D 20 (1970) 177. [29] P. Li et al., Absolute ages of rocks of the Chinese Peoples' Republic, Geochemistry 7 (1960) 682. [30] P. Li, Geochronological data by the K - A t dating method, Sci. Sinica 14 (1965) 1663.
R b - S r WHOLE-ROCK AGES OF PRECAMBRIAN METAMORPHIC ROCKS [31] Res. Group Tanba Belt, The Paleozoic system in the Tanba belt, part 2, Earth Sci. 25 (1971) 211 (in Japanese with English abstract). [32] H. Aoki, T. Tada, Y. Sasaki, T, Ooida, I. Muramatsu, H. Shimamura and I. Furuya, Crustal structure in the profile across central Japan as derived from explosion seismic observation, J. Phys. Earth 20 (1972) 197. [33] V.V. Beloussov and E.M. Ruditch, Island arcs in the development of the earth's structure (especially in the region of Japan and the Sea of Okhotsk), J. Geol. 69 (1961) 647.
287
[34] D.E. Karig, Origin and development of marginal basins in the western Pacific, J. Geophys. Res. 76 (1971) 2542. [35 T. Matsuda and S. Uyeda, On the Pacific-type orogeny and its model - extension of the paired belts concept and possible origin of marginal seas, Tectonophysics 11 (1971)5. [36] G.H. Packham and D.A. Falvey, An hypothesis for the formation of marginal seas in the Western Pacific, Tectonophysics 11 (1971) 79.
Rb-Sr
WHOLE-ROCK
AGES OF PRECAMBRIAN
IN THE KAMIASO CONGLOMERATE
METAMORPHIC
FROM CENTRAL
ROCKS
JAPAN
Ken SHIBATA
Geological Surf O, of Japan, Kawasaki (Japan} and Mamoru ADACHI
Department of Earth Sciences, Nagoya University, Nagoya (Japan) Received 30 July 1973 Revised version received 8 November 1973
Whole-rock samples of metamorphic and granitic cobbles and boulders from the Kamiaso conglomerate in central Japan yield well-defined Rb-Sr isochron ages of 1985 + 25 my and 1820 + 40 my. These ages are the oldest yet obtained for rocks in the Japanese Islands, and provide key evidence for the middle Precambrian metamorphic and igneous events in the provenance of these rocks. The age of 1985 my defined by six samples of quartzo-feldspathic gneiss may be the time of emplacement of the original granitic rocks. The 1820 my age indicates the time of extensive regional metamorphism and igneous activity. Precambrian episodes in the provenance of the Kamiaso conglomerate are summarized as follows: (1) 2000 my - formation of granitic rocks, (2) 1800-1600 my highgrade metamorphism accompanied by igneous activity, (3) 1200 1000 my some significant thermal event. Judging from rock types and geochronological data, it can be said that metamorphic rocks in the Kamiaso conglomerate are remarkably similar to those of the Matenrei and Nangnim systems in North Korea. The Precambrian complex from which the metamorphic and granitic rocks were derived, was exposed to the north not far from the present site of the Kamiaso conglomerate in the late Paleozoic time, and it was probably a part of the large Precambrian continent in East Asia.
1. Introduction The Japanese Islands are situated between the Asian continent and the Pacific Ocean, and knowledge of the Japanese basement rocks is fundamental to understand the geologic phenomena in the island arc system. The oldest fossiliferous formations recognized in Japan are Silurian, and there has been much debate whether or not the Japanese Islands have a Precambrian basement. Some metamorphic complexes exposed in limited areas have been thought to be originally Precambrian. Most of the isotopic dating results, however, are younger than 500 my with two exceptionally old 2 o 7 pb/206 Pb ages obtained for detrital zircons: 1493 my from the Hida metamorphic rocks 11], and 1782 my from the Ryoke metamorphic rocks [2], the latter o f which
are metamorphosed equivalents of the Paleozoic sediments. Until recently there was no record of Precambrian rocks in Japan that have not been involved in a younger metamorphism. In 1970, Adachi found an intraformational conglomerate in a Permian formation at Kamiaso, central Japan (Fig. I), and named it the Kamiaso conglomerate [3]. This conglomerate is characterized by having rounded gravels of sillimanite gneiss and orthoquartzite [4]. We have carried out K - A t and R b - S r dating on micas from the metamorphic rocks contained in the Kamiaso conglomerate, and obtained middle Precambrian ages of 1 7 0 0 - 1 5 0 0 my [ 5 - 7 ] . This was the first geochronological evidence o f Precambrian rocks in Japan. Encouraged by these results, we have carried out the R b - S r whole-rock dating on metamorphic and granitic rocks in the Kamiaso conglomerate. We here
278
K. SHIBATAAND M. ADACHI
,3,o*
,;0"
50"
Poleozoic group 0
50
IO0 KM poleocurrent
direction
Fig. 1. Geologic sketch map of central Japan showing location of Kamiaso and distribution of Hida metamorphic complex and Paleozoic group in the Mino terrain. Arrows indicate paleocurrent directions (after Adachi and Mizutani [9] ). present the R b - S r whole-rock analytical data and discuss more detailed geochronology of these rocks in relation to the Precambrian of East Asia.
2. Geology The upper Paleozoic group is widely developed in the inner zone of central Japan. This region is called the Mino Paleozoic terrain, where geosynclinal sediments attaining 10 km in thickness are composed mainly of sandstone, shale and chert with lesser amounts of limestone, basic volcanics and conglomerate. The region is bounded by the Hida metamorphic complex in the north and gradually changes with NE-SW trend into the Ryoke metamorphic belt in the south. In these geosynclinal sedimentary facies, coarse clastic tongues of intraformational conglomerate are sporadically found. The Kamiaso conglomerate is one of them and is intercalated in a turbidite formation of the Permian system in the medial part of the Mino terrain. Stratigraphically, the Kamiaso conglomerate
is considered to be early Permian [4], since this conglomerate is about 1,500 m lower than the limestone horizon of the middle Permian (Parafusulina zone), and contains the limestone gravels of the middle Carboniferous (ProJhsulinella-Fusulinellazone). The Permian system of the Kamiaso area consists of graywacke, shale and bedded chert with local intercalation of limestone, mafic volcanics and conglomerate, forming a westerly plunging syncline with the wavelength of about 10 km [8]. There is no sign of shear folding and contact metamorphism, and sedimentary structures such as sole markings, graded bedding, cross bedding and slump structure are well preserved. Paleocurrent studies [4,9] indicate that almost all clastic materials including the Kamiaso conglomerate came from the north (Fig. 1). The Kamiaso conglomerate is an ill-sorted polymictic intraformational paraconglomerate and is made up of four beds with the thickness of 3~8 m. Rounded to subangular pebble to boulder-sized gravels of sedimentary, igneous and metamorphic rocks are contained in the coarse-grained graywacke matrix. Also contained in the conglomerate are angular fragments of black shale and chert locally derived. Among gravels sedimentary rocks such as graywacke, shale, chert, limestone and marl are abundant, and metamorphic rocks make up as much as 12%. Minor but very important constituents of the Kamiaso conglomerate are well-rounded gravels of orthoquartzite, quartzose sandstone, sillimanite gneiss, garnet gneiss, two-mica granite, syenite, granodiorite, quartz monzonite, quartz porphyry, rhyolite and amygdaloidal andesite. Rounded gravels of limestone containing the middle Carboniferous fusuline fossils and carbonatized arkose sandstone having cobble of sillimanite gneiss are also noteworthy. The high-grade metamorhpic rocks and orthoquartzite are considered to have been derived from the Precambrian continent in the north [4].
3. Sample
description
All the samples of metamorhic and granitic rocks have suffered some chloritization and sericitization probably due to the later hydrothermal alteration and weathering. While biotite and plagioclase are commonly altered, potassium feldspar is generally fresh in
Rb-Sr WHOLE-ROCKAGES OF PRECAMBRIAN METAMORPHICROCKS
279
TABLE 1 Rb-Sr analytical data for whole-rock samples of metamorphic and granitic rocks in the Kamiaso conglomerate Sample No.
Rock type
Rb (ppm)
Sr (ppm)
Quartzo-feldspathic gneiss 1-1 medium-grained garnet -sillimanite-biotite gneiss
265.6 266.2
122.9 122.1
2 -6
294.8
80.4
fine-grained muscovite-sillimanite-biotite gneiss
87Rb/86Sr
6.255 6.312 10.61
87Sr/86Sr
0.8899 0.8895 0.9935
2 -9
coarse-grained garnet-biotite gneiss
270.1
130.3
6.000
0.8674
2 -10
medium-grained sillimanite-biotitegneiss
343.6
111.0
8.964
0.9450
4-32
medium-grained garnet-sillimanite-biotitegneiss 315.9
124.9
7.320
0.9215
4 -36
coarse-grained sillimanite-biotitegneiss
277.9
135.7
5.927
0.8663
4 -41
coarse-grained muscovite-biotite grandioritic gneiss
132.5
267.0
1.437
0.7459
4 -50A
coarse-grained muscovite-biotite granitic gneiss
219.9
170.9
3.725
0.8145
4 -50B
coarse-grained sillimanite-muscovitebiotite granitic gneiss
134.3
190.5
2.041
0.7643
4 -58
coarse-grained muscovite-biotite granodioritic gneiss
156.1
230.5
1.961
0.7276
4 -65
medium-grained sillimanite-muscovite-biotite gneiss
361.7 358.8
115.3 113.0
9.080 9.196
0.9710 0.9712
4-73
coarse-grained sillimanite-biotitegneiss
256.1
165.0
4.494
0.8233
4-76
medium-grained garnet-sillimanite-biotite gneiss
287.5
170.5
4.882
0.8345
1-19
medium-grained sillimanite-muscovite-biotite gneiss
205.0
120.7
4.918
0.8283
4-23
coarse-grained sillimanite-biotitegneiss
166.9
198.8
2.432
0.7708
4 -24B
coarse-grained sillimanite-biotitegneiss
244.2
199.5
3.543
0.7988
Granitic rock G-1
coarse-grained muscovite-biotite granite
244.5
123.3
5.743
0.8629
G-2
coarse-grained syenite
420.2
105.0
G-3
coarse-grained hornblende-biotite granodiorite
84.3
346.7
0.7038
0.7172
G-4
medium-grained quartz monzonite
70.2
287.5
0.7074
0.7167
Pelitic gneiss
all samples. These samples are divided by microscopic examination into: (1) quartzo-feldspathic gneiss, (2) pelitic gneiss, and (3) granitic rock (Table 1). Quartzo-feldspathic and pelitic gneisses were formed under the high-grade metamorphic condition up to the upper amphibolite facies [ 10]. Generally the quartzo-feldspathic gneisses are mas-
11.58
1.017
sive and fine- to coarse-grained, composed chiefly of potassium feldspar, quartz and plagioclase with lesser amounts of biotite, sillimanite, garnet, muscovite and accessories. Sillimanite is the most common index mineral in the quartzo-feldspathic gneiss characterizing the high-grade metamorphism. Potassium feldspars are perthitic orthoclase and microcline with
280 large values of 2Vx(67-87°). Some plagioclases are markedly zoned (normal in character) and antiperthite is occasionally ol:served. All samples of the pelitic gneiss are medium- to coarse-grained sillimanite-muscovite-biotite-oligoclase(An t 6.23)-potassium feldspar-quartz gneiss with minor amounts of tourmaline, cordierite, apatite, zircon, graphite, rutile, sphene and trace sulfides. Sillimanite occurs in close association with flaky biotite displaying chocolate brown to reddish brown Z-axial color. Orthoclase porphyroblasts (up to 30 mm in size) are common in sample 4 24. Samples of four granitic rocks have suffered varying degrees of later alteration; chloritized biotite and sericitized plagioclase are common. Sample G-2, dark gray, coarse-grained syenite, is partly penetrated by a veinlet of quartz-tourmaline-albite. In sample G-3, hornblende is completely replaced by carbonate and chlorite, and biotite is also almost entirely altered to chlorite. Unlike the other three samples, sample G-4 has undergone a stight recrystallization and small grains of quartz are observed at the original grain boundaries.
4. Experimental procedures Rb-Sr analyses were done on 20 whole-rock sampies of cobbles and boulders of metamorphic and granitic rocks from the Kamiaso conglomerate. In order to obtain a representative whole-rock sample, at least 1 kg of the rock was crushed to > 115 mesh. Rb and Sr concentrations were measured by isotope dilution using ~TRb and 84Sr spikes, and STSr/ 86 Sr ratios were calculated from the spiked runs. 87 Sr/86 Sr ratios were separately determined for some unspiked samples. The mass spectrometric analyses were done by the JEOL-05RB mass spectrometer with 90 °, 30 cm radius analyzer. Triple filaments were used and samples were loaded on the side filaments as RbzSO4 or Sr(NO3)z. Ion currents were detected with an electron multiplier, output of which was amplified by a vibrating reed electrometer, recorded on a digital voltmeter and punched on paper tape. Data reduction was done by a computer. All 87 Sr/86 Sr ratios were normalized to 8 6 Sr/88 Sr ratio = 0.1194. The standard deviation for the average of 10 20 peak ratios is mostly between 0.05-0.12%.
K. SHIBATAAND M. ADACHI Eight analyses of the Elmer and Amend Sr standard range from 0.70816 to 0.70872 with an average of 0.70843 +- 0.00018 (lo). lsochrons were calculated by the least-square method of York [11 ] taking account of uncertainties of + 3% in 8~Rb/86Sr ratios and + 0.15% in~7 Sr/~6Sr ratios, and the errors are at the 95% confidence level. All ages given in this paper were calculated using 87Rb decay constant of 1.47 × 10-1 lyr-1. In addition to the whole-rock analyses, R b - S r analyses of constituent minerals were made for some samples. The plagioclase samples contain a small amount of quartz. K - A r age determinations were made also on biotite, muscovite, plagioclase and potassium feldspar from sample 4-50. The analytical method is essentially the same as that described by Shibata [12]. The constants used for K Ar age calculations are: k~ --4.72 X 10-l°yr-1, Xe = 0.584 N 10 -~° yr-1,4OK/K = 0.0119 atomic %. 5. Results Rb Sr analytical data for whole-rock samples of metamorphic and granitic rocks from the Kamiaso conglomerate are given in Table 1. R b - S r analyses of minerals are given in Table 2, and K - A r ages of minerals for sample 4-50 in Table 3. Rb Sr whole-rock data for 20 samples are plotted on the isochron diagram in Fig. 2. It is remarkable that, although these samples are cobbles and boulders collected from different horizons in the Kamiaso conglomerate, they yield rather well-defined isochrons. Six samples of the quartzo-feldspathic gneiss (4-65, 4-32, 1-1,4-50A, 4-50B, 4-41) give an isochron of 1985 +- 25 my with an initial aTsr/86Sr ratio of 0.7036 -+ 0.0010. This is the oldest age yet determined for any rocks in the Japanese Islands. Excluding four samples (1-19, 4-58, G-3, G-4), the remaining 10 samples define an isochron of 1820 +- 40 my with an initial ratio of 0.7039 -+ 0.0027. Among these 10 samples, two samples (G-l, G-2) are granite and syenite, two (4-23, 4-24B) are pelitic gneisses, and the remaining six are quartzo-feldspathic gneisses. Four samples that do not fall on either of the two isochrons give model ages of 780 my (4-58), 1120 my (G-4), 1170 my (G-3) and 1680 my (1-19) by assuming the initial 87 Sr/86 Sr ratio = 0.705.
281
Rb-Sr WHOLE-ROCK AGES OF PRECAMBRIAN METAMORPHIC ROCKS TABLE 2 Rb-Sr analytical data for minerals from metamorphic rocks in the Kamiaso conglomerate Sample No.
Mineral
1-19
plagioclase
41.4
t 71.3
0.6990
0.7265
2-9
K-feldspar
423.3
189.9
6.454
0.8929
K-feldspar
285.3
418.3
1.975
0.7632
plagioclase
57.2
159.4
1.038
0.7338
638,4
42.2
43.86
1.814
biotite (chloritized)
306.3
81.9
10.83
0.9346
muscovite
430.4
111.6
11.17
0.9830
K-feldspar
367.5
239.0
plagioclase
60.0
K-feldspar
483.8
4-24 4-50
4-73
Rb (ppm)
biotite (fresh)
As shown in Table 4, micas in some o f these samples yielded K - A t and R b - S r ages of 9 5 0 - 1 6 9 0 my. It is evident, therefore, that the R b - S r whole-rock ages o f 1985 my and 1820 m y indicate the times o f older events. Fig. 3 shows an isochron diagram for six wholerock samples, and four potassium feldspar and three plagioclase samples. The combined isochron defines an age of 1970 +- 30 my with an initial ratio of 0.7047 -+ 0.0011, which agrees with that defined by whole-rock samples within the limit o f experimental error. This suggests that the feldspars became closed to Rb and Sr isotopes about 2000 my ago together with whole-rock samples. The quartzo-feldspathic gneiss contains abundant potassium feldspar and subordinate amounts of quartz and plagioclase with lesser amounts o f biotite, muscovite, sillimanite and
Sr (ppm)
87Rb/a6Sr
S7Sr/a6Sr
4.451
0.8339
110.0
1.579
0.7514
171.3
8.176
0.9488
garnet. Based on the petrological study o f the metamorphic rocks in the Kamiaso conglomerate, some o f the quartzo-feldspathic gneiss were considered to be orthogneiss [101. The 1985 my isochron age may be regarded as the time of emplacement o f the original granitic rocks. The R b - S r analytical results of minerals for the quartzo-feldspathic gneiss 4-50 are shown in Table 2 and K - A t age results in Table 3. This is a gray, coarse-grained, large boulder about 60 cm in diameter, composed o f potassium feldspar ( 2 V x = 73 75 °, A = 0.82), quartz, plagioclase (An2s), biotite, muscovite with accessories of zircon, apatite, sphene, futile and iron ore. Biotite is almost entirely altered to chlorite. Plagioclase is also dusted with sericite. The rock is partly very coarse-grained and heterogeneous, having large lenticular clots of biotite
TABLE 3 K-At ages of minerals from quartzo-feldspathic gneiss 4-50 Mineral
K20 (%)
4°Ar rad (10 ..4 ccSTP/g)
Atmospheric 4°Ar(%)
Age (my)
Biotite (fresh) Biotite (fresh) Biotite (chloritized) Muscovite K-feldspar Plagioclase
6.53 6.63 3.29 9.16 11.54 1.26
4.51 4.60 1.53 7.41 4.97 0.649
3.3 6.0 2.8 0.4 5.3 18.3
1410 1410 1050 1570 990 1140
+ 50 ± 50 -+40 ± 50 ± 30 + 40
TABLE 4 K - A r and R b - S r ages of minerals from m e t h a m o r p h i c rocks in the Kamiaso conglomerate Sample No.
Rock type
Mineral
K - A r age (my)
R b - S r age (my)
1-8 1-19
G-B, P S-M-B, P S - B , QF G-B, P S-B, P S-B, P ( S ) - M - B , QF
950** 1440" 1640" 1160" 1290** 1540"* 1160* 1410
1510"* 1470"* 1630"*
2-6 4-21 4-23 4-24 4-50
biotite biotite muscovite Biotite biotite biotite biotite biotite (fresh) biotite (chloritized) muscovite K-feldspar plagioclase
1680** 1490"* 1660" * 1690
1050
1310
1570 990 1140
1610
Abbreviations: G = garnet, B = biotite, S = sillimanite, M = muscovite, P = pelitic gneiss, QF = quartzofeldspathic gneiss. 4°K: h = 4.72 × 10 - 1 ° yr -1, h e = 0.584 × 10 -1° yr ' l , 4 ° K / K = 0.0119 at.%. 87Rb: ~k = 1.47 × 10 -11 yr -1. * [51; ** [7].
/ 4-73~ . _ / T " 19 70130 m.y. R." 0.7047,0.0011
4-~.1
/
1.00
---.1.-4_32
/ /
0.95
0.0010/
T = 1985--. 25 m.y. R,- 0.7036±
2-9K /
~ / ~ ' / ~ 'o ,7,
0.90
II
4-50K
oe5
0.15
/i-i
m.,.
4_soA //'4~;,3 -
Ro- 0.7039_ 0.0027
Y • 24~ S
$°1
/ 2
07lb/'~6Sr Fig. 2. R b - S r whole-rock isochrons for m e t a m o r p h i c and granitic rocks in the Kamiaso conglomerate.
4
6
$
|TRb//16Sr Fig. 3. R b - S r isochron diagram for six whole-rock samples, and four potassium feldspar and three plagioclase samples. K = potassium feldspar, P = plagioclase.
I0
Rb-Sr WHOLE-ROCKAGES OF PRECAMBRIAN METAMORPHICROCKS up to 35 mm in size. Sillimanite partially replaced by muscovite is usually present; plagioclase and biotite dominate over potassium feldspar. Two samples were prepared; one from a massive part (4-50A) and the other from a gneissose heterogeneous part (4-50B). Both samples fall, however, on the whole-rock isochron of 1985 my. The mica-whole-rock isochron ages are calculated using the whole-rock sample 4-50B, since this contains more mica than 4-50A. Although the biotite clots in sample 4-50B have also suffered alteration, fresh biotite that survived the chloritization is partly retained. It is of much interest that this fresh biotite gives a R b - S r isochron age of 1690 my, while chloritized biotite 1310 my. This difference in biotite age between 1690 my and 1310 my may be interpreted as their differential chloritization. The Rb Sr isochron age of muscovite is 1610 my. The K - A r biotite ages are 1410 my (fresh) and 1050 my (chloritized), while muscovite age 1570 my; this is very close to the R b - S r age. The potassium feldspar and plagioclase give K - A r ages of 990 my and 1140 my, both are similar to the chloritized biotite age. In marked contrast with the younger K - A r ages, potassium feldspar and plagioclase are on the R b - S r whole-rock isochron of 1985 my, suggesting that they have remained closed systems since the original formation of the rock. The 1820-my whole-rock isochron age seems to be rather ubiquitous throughout the provenance of the metamorphic rocks, because the isochron is well defined by various rock types; quartzo-feldspathic and pelitic gneisses and granitic rocks. This age indicates the time of extensive regional metamorphism associated with igneous activity. Samples G-1 and G-2, both of which lying on the 1820-my isochron, are typical granite and syenite, suggesting that these zocks were emplaced 1820 my ago. It should be noted that all these whole-rock systems appear to have been closed since 1820 my ago, although the rocks yielded mica ages of 1500-1700 my (Table 4). Because the micas in the metamorphic rocks were probably formed or recrystallized under the upper amphibolite facies metamorphic condition, the mica ages of 1500-1700 my may represent the time of this metamorphism. If this is the case, the 1820-my whole-rock age may indicate either the older metamorhpic event separable from the 15001700-my event, or the period of the earlier phase
283
within one and the same prolonged metamorphic event. Among R b - S r and K - A r dates muscovites and some biotites give rather concentrated ages of 16001700 my; therefore, the period of the high-grade regional metamorphism may be interpreted to have occurred between 1800 and 1600 my. The ages younger than 1600 my are then ascribed to its retrogressive metamorphic phase. Some of the biotites give much younger ages of 1000 1200 my (Table 4). All these biotites are considerably chloritized as shown by lower K20 contents, and the lower ages might be due to rather recent weathering or alteration. Alternatively, the 1000-1200-my age may indicate a thermal event that has rejuvenated biotite ages, and this is supported by the evidence that two of the whole-rock samples (G-3, G-4) give R b - S r model ages of about 1150 my. Adachi [10] pointed out, on the basis of the petrological study of the gneisses, that the 1000-1200-my episode may represent some thermal event which led to the varying degrees of chloritization and sericitization accompanied by the formation of secondary opaque minerals such as pyrite. It is concluded that the 1000-1200-my age is not the result of argon loss due to weathering or alteration, but represents some significant thermal event. Based on the isotopic ages and petrographic evidence of metamorphic and granitic rocks in the Kamiaso conglomerate, the Precambrian episodes in the provenance area for these rocks are suggested as follows: (1) emplacement of granitic rocks at about 2000 my; (2) high-grade regional metamorphism up to the upper amphibolite facies associated with emplacement of granite and syenite at 1800-1600 my, by which most of the metamorphic rocks were formed; and (3) some thermal event at 1200-1000 my, by which some of biotite ages were rejuvenated.
6. Discussion Systematic analyses of the paleocurrent direction [9] and coarse clastic materials [8] in the Mino terrain disclosed that almost all of clastics including the metamorphic rocks in the Kamiaso conglomerate, were transported from the north. A detailed petrological study of clastic plagioclase and heavy minerals in Permian graywacke of the central Mino terrain shows
284 that the materials were derived from an extensive metamorhpic terrane dominated by the granodioritic rocks [ 13]. This fact, together with the paleocurrent evidence and isotopic age data, may suggest that an extended Precambrian land mass existed in the north not so far from the present site of the Kamiaso conglomerate. Then question arises whether the Hida metamorphic complex, situated about 70 km north of Kamiaso, is a possible source for the metamorphic rocks in the Kamiaso conglomerate. The Hida metamorphic complex is composed mainly of quartzo-feldspathic gneiss, marble, amphibolite and pelitic gneiss metamorphosed generally in the epidote-amphibolite to the amphibolite facies. Petrological study and isotopic dating of the Hida complex have been done extensively [1, 1 4 - 1 9 ] , but the controversy regarding the age of metamorphism and original rocks is not yet clearly settled. Most of K - A r and R b - S r mineral ages are heavily concentrated at 180 my with minor concentrations at 250 my and 500 my by mostly R b - S r and U - T h - P b ages. The oldest record is the =07pb/=O6pb age of 1493 my for the detrital zircon from the Amo gneiss [ 1]. This age is similar to the mica ages of gneisses in the Kamiaso conglomerate. The R b - S r whole-rock analyses of the Hida complex, though limited in number, indicate that the ages are generally 2 0 0 - 5 0 0 my with fairly low initial a vSr/a~ Sr ratios of 0.7030.708 [18, 19]. These results indicate that the Hida metamorphic complex, at least for the rocks analyzed, is not reworked material of Precambrian crust. In view of the 1500-my age for zircon, it is probable that some of the Hida metamorphic complex contain the material originated back to the Precambrian time. At present it is, however, rather difficult to correlate the Hida complex with metamorphic clasts in the Kamiaso conglomerate. Another evidence for the existence of a Precambrian land mass is a =°VPb/=°6pb age of 1782 my for the detrital zircon in a banded gneiss from the Ryoke metamorphic rocks in the Kinki district of southwest Japan [2]. Since these metamorphic rocks are believed to have been metamorphosed from the Paleozoic sediments, the zircon was probably derived from a Precambrian terrane and laid down in the clastic sediments of the Japanese Paleozoic geosyncline. Aside from the metamorphic rocks having exceptionally old z o 7pb/~ o 6 Pb ages mentioned above,
K. SHIBATAAND M. ADACHI metamorphic and igneous rocks considered to have been derived from the pre-Silurian basement complex are sporadically found either as squeezed-out masses in the tectonic zone or as xenoliths in younger volcanics. The metamorphic and igneous rocks intimately accompanied by Silurian strata in the Kurosegawa tectonic zone of southwest Japan are typical examples, although results of isotopic dating for these rocks concentrate at about 400 my [20, 21 ]. Regarding the metamorphic and igneous rocks with about 400-my age, discussion should be extended to the additional new evidence from northeast Japan that recent K - A r dating of hornblende in the Hikami granitic rocks associated with Siluro-Devonian strata in the Kitakami district also gave the similar age of 354 my [22]. Further accumulation of isotopic age data as well as geologic evidence will give a sharper view on the basement geology of the Japanese Islands. Precambrian formations are widely developed in the eastern part of the Asian continent near the Japanese Islands, especially in North China and Korea (Fig. 4). The ages of basement rocks in Korea are important for discussing the relationship between the provenance of the metamorphic rocks in the Kamiaso conglomerate and the Precambrian rocks in East Asia. There are about 20 Precambrian dates on metamorphic and granitic rocks in Korea, ranging from 8 0 0 2900 my [23]. Among them are included 12 R b - S r whole-rock model ages in South Korea [24]. The whole-rock isochron age could not be obtained because of a wide scatter of points probably due to the o
500 KM
S i n o ;.'1~ o r ~ a fi - s h i e i c] )5~;2~.% ) e,z','Z;f,~,-~'-"
'~', 2 2 0 0 - 2 4 0 0 ~ " ' "
~ . , ~ ' - . a ~ - ~ ; . ~
",x;;~ 3~5c'~" ~:,,-'<.'Z'
MATENR~
.....
00~/~ "~g'"
\
.
/
,8o-,soo
........... "'~
215o ~v
-
i~00-1800 2000
AGES IN MY.
Precambrian
Formations
Fig. 4. Distribution of Precambrian formations and their isotopic ages in east Asia.
Rb-Sr WHOLE-ROCKAGES OF PRECAMBRIAN METAMORPHICROCKS disturbance by the extensive Mesozoic plutonism. However, Hurley et al. [24] concluded that some of the basement rocks in South Korea were originally at least as old as 2000 my. A U - P b zircon age of 2150 my for the Yoogoo granite gneiss confirms the middle Precambrian event in South Korea [25]. The Nangnim and Matenrei systems in North Korea yielded K - A r ages of 2040 my [26] and 1740-1700 my [27] respectively. According to the compilation of isotopic age data from the Chinese platform by Yanagi and Yamaguchi [28], there is a remarkable concentration of ages between 1800-1900 my, with minor peaks at 2300-2400 my and about 1400 my, indicating the epochs of metamorphism associated with plutonic activity in the Chinese platform. The Huto system in North China was dated at 1550 -+ 200 my by the common lead method [29], and the upper limit of the Liaoho system in northeast China was determined at 1400 m.y. [29]. The Wutai and Anshan systems in North and northeast China yielded many K - A t ages ranging from 2400 to 1700 my [30]. The 1700-1500-my mica ages on the metamorphic rocks in the Kamiaso conglomerate are similar to those of the Huto, Liaoho and Matenrei systems, whereas the 2000-my whole-rock isochron age on the quartzo-feldspathic gneiss is correlated to ages of the Wutai, Anshan and Nangnim systems. This isochron age is roughly comparable with the zircon age of 2150 my from South Korea [25]. It is also notable that the 1800-1900-my peak in the Chinese platform corresponds to the 1820-my whole-rock age given in this paper. Judged by rock types, metamorphic conditions and geochronological data, it can be said that metamorphic rocks in the Kamiaso conglomerate are remarkably similar to those from the Matenrei and Nangnim systems in North Korea [10]. There remains a possibility that the basement rocks in South Korea have much affinity with the metamorphic rocks in the Kamiaso conglomerate. Well-rounded orthoquartzite pebbles and cobbles in the Kamiaso conglomerate give us another important clue to the discussion on the provenance area. Similar occurrence of orthoquartzite pebbies has been reported in the Paleozoic terrain [31]. Interestingly enough, orthoquartzitic sandstone is commonly developed in the Sinian and Shogen systems which
285
cover unconformably the Huto, Liaoho and Matenrei systems. It is possible to suppose that orthoquartzite clasts in the Kamiaso conglomerate were derived from the Sinian and Shogen systems. The late Precambrian non-metamorphosed Sinian system yielded K-Ar ages of 900-1100 my on glauconite, and its lower limit is estimated to be about 1300 my [30]. The 10001200-my ages recorded on some of the metamorphic and granitic rocks in the Kamiaso conglomerate may indicate a thermal event that took place while the Sinian system was deposited. On the basis of the paleocurrent evidence and the similarity in ages and rock types of some of the Precambrian rocks in East Asia to the metamorphic rocks in the Kamiaso conglomerate, it is supposed that the Precambrian complex, from which the metamorphic clasts were derived, was exposed not so far from the present site of the Kamiaso conglomerate in the late Paleozoic time, and it probably formed a part of the Precambrian continent including the Matenrei and Nangnim systems [5,10] and also the Precambrian formations in South Korea. According to the explosion-seismic observations [32], a thick layer with the velocity of 6 km/sec exists in central Japan reaching a depth of 25 km, whereas the underlying layer with the velocity of 7 km/sec is thin. The low-velocity material constituting the 6-km/sec layer may be referable to the acidic crust composed of granite or gneiss. Presumably, the 6-km/sec layer inferred from the explosion seismology contains the remnants of the Precambrian continent discussed here, which is possibly now deeply hidden under the present Japanese Islands. The existence of the Precambrian continent proposed by us has an important meaning when considering the geologic development of the Japanese Islands, especially the Precambrian geology in Japan and East Asia and the origin of the Japan Sea. The development of marginal seas such as the Japan Sea has recently been much discussed [24,33-36] ; the discussion has been focussed on an interpretation that the Japan Sea area was formed by opening under some tensional condition. The isotopic ages of the metamorphic rocks in the Kamiaso conglomerate, which are comparable with those from the middle Precambrian formations in East Asia, in particular from the basement complex in Korea, support the idea that the Japanese Island were originally a part of the Asian
286 c o n t i n e n t , a n d t h a t t h e y were severed f r o m the cont i n e n t giving rise to the f o r m a t i o n o f t h e J a p a n Sea.
Acknowledgements We are very grateful to Dr. S. Mizutani of Nagoya University for his fruitful discussions and constructive criticisms of the manuscript. Our thanks are also extended to Dr. H. Hattori of the Geological Survey of Japan for his encouraging advices during this investigation. The work was partly financed by Grant-inAid for Scientific Research, Ministry of Education.
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R b - S r WHOLE-ROCK AGES OF PRECAMBRIAN METAMORPHIC ROCKS [31] Res. Group Tanba Belt, The Paleozoic system in the Tanba belt, part 2, Earth Sci. 25 (1971) 211 (in Japanese with English abstract). [32] H. Aoki, T. Tada, Y. Sasaki, T, Ooida, I. Muramatsu, H. Shimamura and I. Furuya, Crustal structure in the profile across central Japan as derived from explosion seismic observation, J. Phys. Earth 20 (1972) 197. [33] V.V. Beloussov and E.M. Ruditch, Island arcs in the development of the earth's structure (especially in the region of Japan and the Sea of Okhotsk), J. Geol. 69 (1961) 647.
287
[34] D.E. Karig, Origin and development of marginal basins in the western Pacific, J. Geophys. Res. 76 (1971) 2542. [35 T. Matsuda and S. Uyeda, On the Pacific-type orogeny and its model - extension of the paired belts concept and possible origin of marginal seas, Tectonophysics 11 (1971)5. [36] G.H. Packham and D.A. Falvey, An hypothesis for the formation of marginal seas in the Western Pacific, Tectonophysics 11 (1971) 79.