Possible manifestations of slab window magmatisms in Cretaceous southwest Japan

Tectonophysics 344 (2002) 1 – 13 www.elsevier.com/locate/tecto

Possible manifestations of slab window magmatisms in Cretaceous southwest Japan Osamu Kinoshita* College of Integrated Arts and Sciences, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai 599-8531, Japan Received 26 January 2001; accepted 22 October 2001

Abstract The slab window system related to ridge subduction is studied to explain the conspicuous activity and the along-arc migration of the magmatisms in the eastern margin of Eurasian continent in the late Mesozoic. The trend of the granite ages is scrutinized with systems approach and analyzed by the statistical method in Cretaceous southwest Japan, a reference field of the Eurasian eastern margin. The magmatic distribution of simultaneous activity was determined to be the V-shape of the slab window, excluding dating errors from the located age data. The slab window magmatism has the most dominant zone of the acidic rocks at or near the centerline of the window (the spreading center of two plates), and also has the subdominant zone of the acidic to intermediate rocks on either side of the window. As the slab window opens due to the plate motions, the upwelling current is adiabatically induced in the asthenosphere to fill the window gap. The partial melt is generated in the upwelling flux and transports heat to the lower crust to make greater granitic magma chambers by heat conduction to, and assimilation with, the crustal matter. The younger and hotter slabs outside the window also play a role in partial slab melting and/or dehydration to the asthenosphere and generate the subdominant granodioritic and adakitic and/or high-magnesian andesitic magmatisms as the preand post-activities of the main granite genesis. The slab window formed by the subducted ridge between the Kula and Pacific plates was the strong heat source of the active magmatisms and migrated at the rate of 2.8 cm/year from southwest to northeast about 8000 km along the continental margin. The growth of the continent associated with the active acidic to intermediate magmatisms including the adakitic and/or high-magnesian andesitic activities is modeled by the slab window system. D 2002 Elsevier Science B.V. All rights reserved. Keywords: slab window; ridge subduction; growth of continent; granite; adakite; high-magnesian andesite

1. Introduction A long chain of batholithic granites and volcanic equivalents from the Mesozoic to the Early Cenozoic extends about 8000 km in the eastern margin of the Eurasian continent, from the southern region of

*

Tel.: +81-72-254-9729; fax: +81-72-254-9927. E-mail address: [email protected] (O. Kinoshita).

Guangxi in South China to the far eastern region of Russia. Southwest Japan is a good reference field for the construction of a magmatic model of the Eurasian eastern margin; it was a constituent of the Eurasian margin before the opening of the Japan Sea in the Miocene (Otofuji et al., 1989) and has experienced minor deformation thus far despite the block rotation at the Japan Sea formation. It also has more abundant granite ages and other geological data of high quality. I first proposed a migration model of magmatism and

0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 0 1 ) 0 0 2 6 2 - 1

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an assumption of ridge subduction induced by the detailed researches of southwest Japan (Kinoshita and Itoˆ, 1986) and applied them to the other regions of the eastern margin of Eurasian continent (Kinoshita and Itoˆ, 1988, 1990). The model and the assumption were thought out in conformity with the trend of the granite ages and other geological data, although they need to be more quantitatively discussed. I constructed, therefore, a model of magmatism based on a regression plane of the granite ages on the sample localities by systems approach and statistical methods (Kinoshita, 1997, 1999). The granite ages were newly selected according to strict criteria (Kinoshita, 1995). This is an improved magmatic model on quantitative analysis, which explains dynamically the migration of magmatism generated by a linear heat source obliquely extending landward and moving along the continental margin. It agrees very well with the suggestion that volcanism seems to have gradually shifted from south to north and from west to east with time in South China (Wang and Mo, 1995). The migrating linear heat source corresponds to the subducted ridge axis and gives strong support to the assumption of the ridge

subduction beneath the eastern margin of the Eurasian continent. In the present paper, I report the slab window spreading and concerned magmatisms in Cretaceous southwest Japan. The adakitic and/or high-magnesian andesitic magmatisms are also discussed according to the slab window model. The center axis of the slab window is the aforementioned linear heat source. I study what the isochronous spatial distribution of active magmatism is like on both sides of the centerline. The manifestation of the slab window magmatisms is shown by the isochronous magmatic distribution, which suggests the subcrustal upwelling flow of the asthenosphere inside the window and the partial melting of the hotter and younger slabs on either side of the window. The granite ages, other geological data and the magmatic model detailed in Kinoshita (1995, 1999) form the basis of this work. It has been supported by many researchers that an orogenic event or the growth of a continent such as the eastern Eurasia magmatism modeled by the genesis of batholithic granitoids could be ascribed to active ridge subduction (e.g., Isozaki and Maruyama,

Fig. 1. The Cartesian coordinate system X – Y denoting the localities of the granite age samples (circles), the adakites and/or high-Mg andesites (asterisks), and the areas of Iwakuni (I) and Mikawa – Tono (M). The paleotrench is shown in the northern Shimanto belt. MTL = Median Tectonic Line (X-axis); ISTL = Itoigawa – Shizuoka Tectonic Line. Stars: Ka = Karatsu City ( Y-axis passes through this city), Hi = Hiroshima City and Ky = Kyoto City. Ryoke, San’yo and San’in are the magmatic belts, but Ryoke is also the metamorphic belt (low P/T) paired with the Sanbagawa belt (high P/T).

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1991; Kiminami et al., 1993; Kimura, 1997; Maruyama, 1997; Maruyama et al., 1997; Okada, 1999; Iwamori, 2000). It is characterized by episodic and high activity with along-arc migration. Even in an ordinary subduction zone, such a peculiar event occurs in an occasion of change of a plate for another one; it is also a transitional tectonics of subduction from one plate to another.

2. Granite age trend explaining the magmatic migration model Granite age data in southwest Japan are taken at the localities (circles) in Fig. 1 and listed in Table 1, which contains the data sets (X, Y, Z ) of the granite age Z and its locality (X, Y ) (Kinoshita, 1999). The Xand Y-axes are set on the Median Tectonic Line (MTL) and a line normal to the MTL passing through Karatsu

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City (Ka; star mark) in Kyushu, respectively. That is, the localities of all sample rocks with age data Z are shown by the coordinate (X, Y) in southwest Japan. The MTL axis was parallel to the paleotrench in the Cretaceous, which was confirmed in the northern Shimanto belt, a part of the southern zone of the MTL. All granite ages are K – Ar biotite ages because measuring one type of mineral age by only one method is reasonable enough to detect the magmatic trend, and the emplacement ages can be checked by the cooling histories of the granite bodies. The cooling histories have almost the same trend and the age gaps between K – Ar biotite ages and emplacement ages are taken to be 10 Ma all over southwest Japan (compiled in Kinoshita, 1995). The criteria for the age data selection, the properties or sorts of the sample rocks and data sources were shown in Kinoshita (1995). The chronological cross-checks on the granitic trend of the K – Ar biotite age were also performed by the Rb – Sr

Table 1 K – Ar biotite age data of the Ryoke and San’yo granites No.

X

Y

Saga Prefecture 1 14 96 2 22 98 Fukuoka Prefecture 3 14 120 4 18 124 5 27 126 6 28 123 7 30 102 8 35 108 9 66 105 10 72 87 11 84 84 12 84 87 13 86 114 14 90 93 Kumamoto Prefecture 15 5 50 16 33 3 17 48 45 Oita Prefecture 18 48 45 19 95 3 20 135 25 Yamaguchi Prefecture 21 150 89

Z 85 84 91 92 93 82 96 91 96 94 93 93 94 96 95 ± 4.8 99 89 95 ± 4.8 92 ± 4.6 86 ± 4.3 85

No.

X

Y

22 182 63 23 185 63 24 185 68 25 198 54 Hiroshima Prefecture 26 207 75 27 225 88 28 240 55 29 240 57 30 240 65 31 245 81 32 246 93 33 247 117 34 255 54 35 279 105 Okayama Prefecture 36 327 43 37 363 70 38 375 90 39 375 108 40 381 135 41 387 91 Kagawa Prefecture 42 340 20 43 363 20 44 375 34

Z

No.

89 ± 4.4 86 88 87

45 381 13 46 381 43 47 393 18 Hyogo Prefecture 48 405 70 49 443 15 50 450 31 51 459 37 52 486 51 53 486 51 54 486 51 Osaka Prefecture 55 513 69 56 525 35 Kyoto Prefecture 57 546 34 58 560 48 Nara Prefecture 59 546 15 Shiga Prefecture 60 546 71 61 546 78 62 570 66 63 570 72 Mie Prefecture 64 580 40

85 84 80 ± 4.8 89 ± 4.5 86 ± 4.3 78 82 82 82 ± 4.1 89 96 86 80 ± 2.8 80 ± 4.0 65 76 ± 2.0 81 ± 1.8 77 80

X

Y

Z

No.

87 ± 1.9 74 83

65 580 47 66 610 64 67 610 70 Gifu Prefecture 68 635 112 69 710 51 70 724 51 71 724 52 72 724 53 73 729 40 74 729 56 Aichi Prefecture 75 688 23 76 695 49 77 700 9 78 703 42 79 703 43 Nagano Prefecture 80 738 21 81 775 30 82 775 30 83 775 30 84 775 33 85 800 30 86 800 30

81 87 87 ± 4.3 81 ± 4.0 79 77 73 77 79 78 75 70 ± 2.1 96 ± 1.9 79 71 ± 3.6 70 ± 3.5

X

Y

Z 67 69 ± 3.5 73 ± 3.7 75 70 67 68 69 74 69 75 66 68 ± 2.1 65 63 64 ± 2.0 65 ± 2.0 66 ± 2.0 67 ± 2.1 70 67 70

69 ± 3.5

They are denoted by the Cartesian coordinate system X – Y – Z, in which the locality and the age correspond to (X, Y) and Z, respectively (Fig. 2, top). Data sources and the properties of the sample rocks were listed in Kinoshita (1995). No.: sample number; X, Y: km; Z: Ma.

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whole rock isochron and the Chemical Th – U – total Pb Isochron Method (CHIME) monazite ages (shown later). The three-dimensional Cartesian coordinate system X – Y– Z is taken for statistically processing the age data (top in Fig. 2). Southwest Japan is simplified to be rectangular ( p0 –q0 –r0 – s0 ) on the X –Y plane. The granite age at the locality (Xi, Yi) corresponds to the coordinate Zi vertical to the X – Y plane (i = 1 – 86, although three outliers are excluded on the regression analysis). Thus, the age trend is formulated by a

regression plane ( p– q– r– s) of Z on X and Y, which inclines in both directions along and normal to the MTL, with a very high multiple correlation coefficient of 0.91. The equation of the regression plane is obtained by the least squares method as Z =  0.0358X  0.0276Y + 95.6, where X and Y are in kilometers and Z is in Ma (this treatment was detailed in Kinoshita, 1999). The bottom diagram in Fig. 2 shows the projections of the regression plane and the age data on the planes Z –X and Z –Y in the directions from s to p and from q to p, respectively.

Fig. 2. The regression plane p – q – r – s of the granite ages on the sample localities (top) and the projections with the age plots on the Z – X and Z – Y planes in the directions from s to p and from q to p, respectively (bottom). Ka (star) = Karatsu City; MTL = Median Tectonic Line (corresponding to Fig. 1).

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The Rb – Sr whole rock isochron ages, almost emplacement ages, were compiled and shown in maps covering many localities in southwest Japan (Kagami et al., 1999). For the granitic bodies lacking in the Rb – Sr age and of a negligible Ar loss due to a thermal or alteration overprint, the oldest K –Ar hornblende ages (closure temperature, 500 – 550 °C) were approved instead. The Rb –Sr and K – Ar ages are 101 data in the Ryoke – San’yo magmatic belt. All the 101 age data are listed in the form of the aforementioned data sets (X, Y, Z) and plotted on the three-dimensional Cartesian coordinate system, as shown in the top diagram in Fig. 2, to be treated by regression analysis. The equation of the regression plane for the 101 data is calculated to be Z =  0.0353X  0.0370Y + 105.4 by the least squares method. This agrees very well with the regression equation of the K –Ar biotite age data as shown above, when the gap between the Rb – Sr whole rock isochron and K –Ar biotite age is taken to be 10 Ma. Fig. 3 shows the projections of the Rb – Sr whole rock isochron (solid circles) and the oldest K –Ar hornblende (solid triangles) data by the same method in Fig. 2 (bottom). The K – Ar biotite ages (hollow circles) and its regression line (heavy line) are shifted upward ( + 10 Ma), considering the age gap for the cooling history.

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The two kinds of the solid marks lie relatively scattered. However, the equation of the regression plane derived from the solid mark data is very significant because the ratio of the regression variance to the residual variance is very high (24.1). The CHIME monazite ages of the granitoids, of which the closure temperature (650 – 750 °C) is almost equal to the solidus temperature of the granitic magma, were recently reported (Suzuki and Adachi, 1998) in the Iwakuni area, Yamaguchi Prefecture and in the Mikawa – Tono area, Aichi – Nagano Prefectures (shown in Fig. 1). Each of the two areas has several granitic bodies of various magmatic stages. I think that the most active magmatism is the Gamano granodiorite (95 Ma) in the Iwakuni area and the Inagawa granodiorite (82 Ma) or the Busetsu granite (78 Ma) in the Mikawa – Tono area because their granitic bodies are conspicuously larger than the others in both areas. The magmatism in one area shows a rise, peak and fall in activity, of which the total chronological interval is dependent on the age dispersion due to several stages of magmatisms and the dating errors. In this study, based on the K – Ar biotite ages, the average total interval of ages in southwest Japan is estimated to be about 20 Ma, as shown in Fig. 6.

Fig. 3. The chronological cross-check between the K – Ar biotite age (hollow circles and heavy line) and the Rb – Sr whole rock isochron age (solid circles), including the oldest K – Ar hornblende age (solid triangles). The K – Ar biotite ages and the regression line are shifted upward ( + 10 Ma) due to compensation for the cooling age gap.

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In the coordinates (X, Y) of the localities in this model, the Iwakuni and Mikawa – Tono areas are taken to be (200, 60) and (700, 15) as the middle points of the areas, respectively. Thus, the emplacement ages of the granitoids in the Iwakuni and Mikawa –Tono areas are calculated to be 97 and 80 Ma, respectively, by the model equation considering the cooling age gap of 10 Ma as shown above. These calculated emplacement ages agree very well with the CHIME ages of the most active magmatisms of the Gamano and the Inagawa or Busetsu bodies. The total age interval of the magmatic stages is possible for the Mikawa –Tono area (about 27 Ma). However, the age distribution in the Iwakuni area may be unsymmetric in the most active age of the Gamano granodiorite. The time span of the fall in the Iwakuni area is about 10 Ma but the span of the rise is not recognized in Suzuki and Adachi (1998), although older ages than the Gamano body, the Kita-oshima granitoid and Habu granodiorite, are reported in Okudaira et al. (1993) and Yuhura et al. (1999), respectively. I conclude that the two area data by the CHIME method agree with this model, although more CHIME ages may be necessary for this study by systems approach. According to the chronological discussion and the cross-checks of the K – Ar biotite ages by the Rb – Sr whole rock isochron ages, and the CHIME monazite ages on the magmatic trend in southwest Japan, the trend of the K – Ar biotite ages shifted on the older side by 10 Ma, was concluded to be the trend of the emplacement ages of the granitic bodies; it becomes younger along and across the magmatic arc and the average age gap between the K – Ar biotite ages and the emplacement ages is about 10 Ma in southwest Japan. Once the regression plane of the K – Ar biotite ages on the localities in southwest Japan is determined (Fig. 2) and the chronological cross-checks by plural datings verify the regression plane, the various characteristics of the magmatic activity can easily be obtained. For example, the along- or across-arc trend of the ages and the isochronous line of the magmatic activity are taken out by cutting the regression plane with the plane of Y = constant, X = constant and Z = constant, respectively. The former two are shown in Fig. 2 (bottom) and the last is shown by the line m – n in Fig. 2 (top). The migration rates of magmatism along and across the arc are calculated to be 2.8 and 3.6 cm/year from the slope of the regression line in Fig. 2 (bottom), respectively.

The simultaneous magmatic manifestation on the crustal surface in southwest Japan may have widened across the isochronous line, which suggested a slab window opening under the crust. Such an expansion of the magmatism was shown by an average width of about 140 km at the center of the Ryoke – San’yo magmatic belt, which was taken from the age data distribution by excluding the dating error (Fig. 4 in Kinoshita, 1999). The slab window, however, may actually be in the shape of divergence (V-shape), which is induced from the migration vectors of the Kula and Pacific plates and the ridge axis. Thorkelson (1996) scrutinized the principles of slab window formation for the various types of the ridge subduction systems. The entire shape, instead of the average width as the manifestation of the slab window, is studied in the next section according to the migration model of magmatism and the way of thinking in Kinoshita (1999), and referring to the construction of a slab window determined by plate motion in Thorkelson (1996).

3. Slab window formation related to ridge subduction The migration vectors of the Kula and Pacific plates and the ridge axis were taken in the eastern margin of southwest Japan during the Late Cretaceous (Kinoshita, 1999). The motions and the configurations of the plates and the ridge were determined based on the magmatic age trends in southwest Japan, that is, the result from the land data. The velocity vectors of the plate and ridge motions induced from these results were examined by the researches of Engebretson et al. (1985) and Nakanishi et al. (1992), in which they discussed the plate and ridge motions based mainly on the magnetic anomaly in the Pacific Ocean. The situations of the plates and the ridge induced are compatible with the Late Cretaceous environments, especially on the plate tectonics in the northwestern Pacific (discussed in detail in Kinoshita, 1999). The migration vectors are indicated by the arrows K – K0 (Kula plate), P – P0 (Pacific plate) and R – R0 (ridge axis) in Fig. 4, although the direction of the trench axis should be reconstructed to be N35°E (Otofuji et al., 1989) in the Cretaceous. The triple junction J including the points K, P and R is detailed in the inset (J) in the left-hand side of the figure. The

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Fig. 4. Slab window formation. The triple junction J is detailed in the inset (J). The velocity vectors of the Kula and Pacific plates and the ridges axis are KK0, PP0 and RR0, respectively, although the direction of the trench axis should be reconstructed to be N35°E in the Cretaceous.

points K and P are points on the trench axis as well as of the Kula and Pacific plate edges, respectively. Point R is an intersecting point of the trench axis and the ridge axis. While points K, P and R move to K0, P0 and R0 for 1 year, respectively, the junction J moves to J0 by 2.8 cm along the trench. Referring to Thorkelson (1996), the slab window margins are formed into shapes J0 –K0 and J0 – P0 for 1 year. The slab window geometry is shown by the inner zone of the V-shape between two heavy lines which extend beyond points K0 and P0 from junction J0. Fig. 5 is a manifestation on the crustal surface of the slab window magmatism. The upper part of the

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MTL is the magmatic zone of the Ryoke and San’yo belts in southwest Japan (see also Fig. 1). The plotted hollow and solid circles and triangles show the location distribution of isochronous magmatic activity, which is taken by the following method. In the three-dimensional Cartesian coordinate system in Fig. 2 (top), all the age data are projected on a horizontal plane setting at a certain level (for example, at the level of an average age of 80 Ma in southwest Japan) in the direction from p to q for the upper data of the horizontal plane or from q to p for the lower ones along the regression plane. That is to say, each location of the magmatic activities in the Ryoke – San’yo belt is plotted with respect to the base line, the centerline of the slab window concerned in the activity. The age data projected on the horizontal plane make the location distribution of the simultaneous magmatic activities as shown by the plotted data in Fig. 5. The hollow and solid circles and the hollow and solid triangles are the age data in the magmatic areas X = 0 –200, 201 –400, 401– 600 and 601– 800 km along the MTL axis, respectively. Four symbols mix almost uniformly; the slab window opening is not different between each area along the MTL axis. The plotted data distribute normally in the area about 560 km wide along the MTL, which is divided into 20 sections in a parallel direction with the centerline CL. Such a normal location distribution is shown by a histogram in Fig. 6 together with the normal age distribution. The normal location distribution corresponds to the normal age distribution around the

Fig. 5. Magmatic distribution of the simultaneous activities. The hollow and solid circles and the hollow and solid triangles are the age data in the magmatic areas, X = 0 – 200, 201 – 400, 401 – 600 and 601 – 800 km along the MTL axis, respectively.

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Fig. 6. Histogram of the location or age data distribution across the regression plane. The location or age data distribution corresponds to the horizontal or vertical distribution of the data across the regression plane in Fig. 2 (bottom), respectively.

regression plane; the former width of each section is the latter one multiplied by the migration rate of the magmatism along the arc (discussed later in connection with Fig. 6). The 110- and 220-km graduations on the MTL axis in Fig. 5, show once and twice the standard deviation of the magmatic location distribution, respectively. The half width of the data expansion is actually about 2.3 times the standard deviation. The shape of the heavy lines with the internal angle of 47° is taken from the motions of the Kula and Pacific plates and the ridge axis as aforementioned (Fig. 4). The size of the slab window is decided next by the age error analysis and checked by the geological data. The age data in Z space of both the upper and the lower sides of the regression plane are crowded near or on the regression plane, whereas they become sparse with distance from the plane as shown by the projection on the Z – Y or Z – X plane (Fig. 2). The age distribution based on the regression plane is represented by the histogram in Fig. 6, modified from Fig. 3 in Kinoshita (1999). The latter histogram had little inaccuracy on account of being taken by the graphical method, although it had the same standard deviation (rn = 3.9 Ma) as in Fig. 6. Referring to the discussion on the data dispersion by Kinoshita (1999), the time distribution Z of the raw age data, the data distribution P solely due to the time expansion of the magmatic activity and the age error distribution, e, are all normal with the variances rn 2 , r2 andra 2 , respectively, because of the theorem of reproductivity. That is, de-

noting in the form Z  N(l,rn2), P  N(l,r2) and e  N(0,ra2), where l is the average value of the ages and is dependent on X and Y, the age data structure is recognized as Z = P + e (linear normal regression model). The relation between the variances is rn2 = r2 + ra2 on the ground of the regression analysis. If u is the along-arc velocity of the magmatism, (urn)2= (ur)2+(ura)2 shows the relation in the magmatic location distribution. The age distribution on either side of the average age l(X, Y) multiplied by the along-arc magmatic velocity (u = 28 km/Ma) makes the magmatic location distribution on either side of the centerline (CL) in Fig. 5. Concerning the graduations of the abscissa in Fig. 6, the top (km) is for the magmatic location distribution (Fig. 5) and the bottom (Ma) is for the age distribution (Fig. 2). The standard deviation around the location distribution is about 110 km (urn), corresponding to 3.9 Ma (rn) around the age distribution. The linear normal regression model is agreeable to the magmatic location distribution as well as the magmatic age distribution. The locations of the plotted data in Fig. 5 have some uncertainties, which relate to the dating errors of the samples because the granite age, as aforementioned, is the function of the sample locality on the basis of the migration model of magmatism. That is, the locations of magmatic activities are represented by the expected values (mean values) with error bars on either side of the plotted data. The uncertainties should be deservedly excluded by estimating the variance ra2 appropriate to the error distribution e. The 28 age samples with the error values in Table 1 are checked again by retracing the papers of the data sources and applying the error equation in Cox and Dalrymple (1967). The 13 age data with error values are selected as the data well grounded by the error equation and their average error is 3.41% of the granite age. This agrees with the value 3– 4% of the K –Ar biotite dating error, which has been suggested by K. Shibata’s personal communication (1999). The average error age is calculated to be 2.7 Ma (ra) by multiplying the error rate 3.41% into the average magmatic age 80 Ma in southwest Japan. The standard deviation r, solely due to the time expansion of the magmatic activity, is consequently taken to be 2.8 Ma by using the aforementioned relation between the variances rn2, r2 and ra2. Then, the areal expansion ur of the magmatism along the arc is about 80 km.

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The half width of the magmatic areal distribution is about 185 km (2.3ur). The V-shape slab window drawn by the heavy lines is put on the centerline (CL) of the actual magmatic distribution containing the 83 plotted location data, as shown in Fig. 5. The outlines of the plotted data distribution shown by the broken lines are very similar to the slab window margins, that is, the data distribution forms the upper part of the V-shape, which widens landward (upward in the figure). The distance between the MTL to the paleotrench axis results in about 50 km after evaluating the rear (left) width of the slab window, about 185 km at the northern margin of San’yo belt. The location of the paleotrench in the Cretaceous has been confirmed as being in the northern Shimanto belt shown in Fig. 1. The present distance from the MTL is about 50 km throughout southwest Japan, although the amount of the crustal contraction in the southern part of the MTL cannot be precisely evaluated after the magmatism. The slab window area is not symmetric in the centerline (CL). There is every probability that the magmatic areas outside the front (right in the figure) and rear (left) window margins are about 70 km wide and are not widely existent, respectively, when the location error has been compensated for and the magmatic distribu-

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tion contracts from 110 to 80 km per standard deviation. The magmatism may more easily occur in the front (fertile crust or uppermost mantle) than the rear (relatively depleted) of the slab window. However, the slab window magmatism has the most active zone at or near the spreading center (CL), and also has the subdominant zones on both sides of the window. The tendency of such a magmatic distribution is recognized as a center peak and both side ‘‘horns’’ of the frequency even from the histogram in Fig. 6. Fig. 7 shows the magmatic active zone (inner area between the broken lines) due to the slab window spreading and heating in southwest Japan at 92 Ma. The most dominant magmatism occurs near both the centerline of the window and the boundary between the Ryoke and the San’yo belts (Fig. 5), where the lower crust is heated most by the upwelling current of the hot asthenosphere (Fig. 8). The spacious acidic bodies like the Hiroshima batholith located in the southern region of Hiroshima Prefecture have been emplaced in this active area. The subdominant magmatisms, which are relatively dioritic stocks, occur in the front and rear areas of the window margins as well as in almost all areas of the Ryoke – San’yo magmatic belt. Such dioritic activities are chronologically recognized as the pre- and post-magmatisms of the main acidic activity.

Fig. 7. Slab window magmatism in Southwest Japan at 92 Ma. The Japan Islands should be reconstructed in the Cretaceous; the direction of the MTL axis is restored to be N35°E. The section A – A is shown in connection with Fig. 8.

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The slab window at 92 Ma is drawn on the presentday location of southwest Japan because it can possibly be corresponded to the localities of various geological data. Southwest Japan was subjected to negligible internal deformation due to crustal stability for all the block rotation in the Miocene. Therefore, the direction of the MTL or the paleotrench axis can be reconstructed to be N35°E in the Cretaceous in the condition of almost retaining the present island shape. If so, the direction of the ridge axis is taken to be almost west or a little to the south, which concords very much with the direction of the magnetic anomalies of the chron M0 (124 Ma) near the Japan or Kuril trench. In Kinoshita and Itoˆ (1990), southwest Japan was restored during the pre-Tertiary age based on the trends of the Mesozoic igneous rock ages. That is, the deformation of southwest Japan is so minimal as to certify the expressiveness of the magmatic trend even after rotating to restore the Japan Islands. Maruyama et al. (1997) showed the plate tectonic synthesis from 750 Ma to the present. The paleographic map of the eastern margin of the Eurasian continent, including Japan Islands at 90 Ma, is drawn therein with the Kula and Pacific plates and the ridge in the northwest Pacific Ocean. The reconstruction diagrams of Japan Islands in Kinoshita and Itoˆ (1990) and Maruyama et al. (1997) indicate that the entire drawing in Fig. 7 solely rotated the MTL axis to be N35°E can be explainable for the geological data shown implicitly as well as explicitly in Fig. 1. The V-shape slab window widening landward from the TTR triple junction migrated along the paleotrench from southwest to northeast at the rate of 2.8 cm/year. The spreading of the slab window caused the upwelling flow of the hot asthenosphere to transport heat to the lower crust and stimulated the magmatism to more vigorous activity in the eastern margin of the Eurasian continent. The migration of the subducted ridge also caused the migration of other geological phenomena such as the Ryoke and Sanbagawa metamorphisms (Fig. 1) and the formation of the sedimentary basins (Kinoshita and Itoˆ, 1986; Kinoshita, 1995). Fig. 8 is a cross-sectional illustration at the section A – A in the middle of the Ryoke –San’yo magmatic belt (Fig. 7), which shows the upwelling of the hot asthenosphere and the magmatic underplating caused by the slab window motion. As the slab window opens, the upwelling current is adiabatically induced in the asthenosphere. If the velocity of the upwelling

Fig. 8. Cross-sectional illustration of the slab window magmatism at the section A – A in Fig. 7. The hot asthenosphere upwelling caused by the window opening motion promotes the crust melting to form the larger granitic magma chambers. Hotter and younger slab margins (hatching parts and even out of the hatching parts) play roles of partial melting themselves and dehydration to generate the adakitic and/or high-Mg andesitic magmatisms.

flow becomes faster than a few centimeters/year, the flow generates the partial melt of the asthenosphere in the flux (White and McKenzie, 1989). The melting parts come up and form larger magma chambers of the basalt or gabbro in the lower crust or the uppermost mantle (the dark shading in order from the lower/ smaller to the upper/larger in the figure). The lower crust is subjected to anatexis by heat conduction from the mafic magma and causes successively partial melt to generate the greater granitic magma chambers (the cross symbols in Fig. 8). The acidic magmatism heated by the basic magma chambers occurs dominantly in the center of the slab window. The younger and hotter slabs (the hatching of the subducted slab) have the ability of magmatism up to this point, where they play roles of partial melting themselves and/or dehydration to the mantle. The subdominant granodioritic and adakitic and/or highmagnesian andesitic magmatisms (mentioned later) are observed on the crustal surface overriding upon the younger slab. In the Ryoke –San’yo magmatic belt in southwest Japan, many basic enclaves are observed in the

O. Kinoshita / Tectonophysics 344 (2002) 1–13

granitic bodies, where the basic and acidic magmas are mixed or mingled with each other (Yoshikura and Yamamoto, 1995). None of them, however, have yet been ascertained petrologically to be the primitive basic rocks from the N-MORB magmatism, which have come up through the slab window gap. Thorkelson (1996) discussed in detail the slab window magmatisms in the various cases of the systems. He suggested that the chemistry of the magmatic products could vary from alkalic to tholeiitic and calc –alkalic, consistent with the variety of igneous processes of the slab windows including mantle flows, heating of the wedge mantle, mixing between sub- and supra-slab mantles and partial melting of or dehydration from the slab margins. The basic rocks in southwest Japan are all very small outcrops, because the present denudation level is not so deep to observe entirely the mantlederived basic rocks. A large volume of them may be overlaid by the greater granites and volcanics. One of the important constraints for the magmatism is the heat source of the high thermal gradient, which has episodic activity and migration along the arc. I suggest that the effective heat source in this case has been the convectional heat of the upwelling mantle by the slab window opening and the partial melting and/or dehydration due to the younger and hotter slabs, which is reasonable for the episodic and migrating magma geneses not only of the granites but also of the high-magnesian andesitic and/or adakitic rocks.

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and subsequent interaction between the magma and host rocks in the wedge mantle because the granitoid is relatively rich in CaO and MgO contents. In the adjacent areas of northern Kyoto, the activities of the high-magnesian andesites contemporary with the adakites were also observed at eight localities and dated at three of them (Kimura and Kiji, 1993). Fig. 9 shows the magmatic age trend (stars) of the adakites and/or high-magnesian andesites which are located at the asterisks in southwest Japan in Fig. 1. The hollow circles and the regression line (Fig. 9) are the trend of the granite emplacement age, which are shifted on the older side by 10 Ma than in Fig. 2 (bottom). A few data of the star marks may be a little scattered and plotted separately. However, all stars plotted on either side of the granite regression line are within the age limits of 20 Ma from the regression line. They clearly show the trend of along-arc younging with the same slope as the regression line of the granites. Defant and Drummond (1990) suggested that the oceanic crust younger than 25 Ma is hot enough to initiate melting of the slab and the adakitic magmas are derived by partial melting of the subducted younger slab. The adakitic and/or high-magnesian andesitic activities, as the preand post-magmatisms of the dominant acidic activities were caused by the hot slab parts of the Kula and Pacific plates, which were even in the outer parts of the hatching zone in Fig. 8, not to mention the hatching zone.

4. Adakitic and/or high-Mg andesitic magmatisms The magmatic activities of the adakites and/or the high-magnesian andesites occurred in Cretaceous southwest Japan as shown by the asterisks in Fig. 1, in which the emplacement or extrusion ages of the magmatisms were reported (Seki, 1981; Imaoka et al., 1993; Kimura and Kiji, 1993; Matsumoto et al., 1994; Yuhara and Kagami, 1998; Kamei et al., 1999; Sawada and Saito, 1999; Senou, 1999). In northern Kyoto, Kinki district, Kiji et al. (2000) studied the small but many adakitic stocks (named Tamba granitoid and of about 100 Ma emplacement), which have higher Sr/Y and lower Y contents than the surrounding granitoids of the San’yo belt. They considered that the magma of the adakitic Tamba granitoid was generated by melting the hotter and younger slab

Fig. 9. The age trend of the adakitic and/or high-Mg andesitic magmatisms (stars) associated with the granitic trend and regression line (circles and heavy line). Stars plotted on either side of the granite trend show the trend of along-arc younging with the same slope as the granite regression line.

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5. Conclusions

Acknowledgements

The study on the genesis of the batholithic granites is very significant in connection with the growth of the continent. In the eastern Eurasian continental margin, one of the big growths of the continent occurred episodically and migrated along the arc in the late Mesozoic. The slab window model related to ridge subduction is explainable for the conspicuously active heat source migrating along the continental margin. Time and space distribution of the magmatic activities was studied in detail. The trend of the granite ages is a key to the dynamic system of the magmatism. In southwest Japan, a reference field in the Eurasian eastern margin, the slab window model was constructed to make the system clear concerning not only the dominant activity of the granitic magmatism at the center of the slab window, but also the subdominant granodioritic, adakitic and/or the high-magnesian andesitic magmatisms on both sides of the window. The slab window forms itself into a V-shape, widening in an inland direction from the paleotrench, which was derived from the Kula and Pacific plate motion and inspected by the magmatic distribution of the simultaneous activity. As the slab window opens, it promotes the adiabatic upwelling of the hot asthenosphere to fill the window gap. The upwelling current generates the partial melt in the flow to make the magma chambers of the basalt or gabbro in the lower crust. The heat conduction from the basic magma chambers to the lower crust and the heat transportation into the chambers by the upwelling flow play the effective roles to make the larger granitic magma chambers in the center of the slab window. The slab melting due to younger and hotter slabs causes the pre- and post-magmatisms of the main acidic activity, although it is not so strong but in relatively wider regions of the subducted younger slab margins ( < 20 Ma) of the Kula and Pacific plates. The chronological method is suitable for understanding the dynamics and migration of the magmatic activities. I think that the slab window model in this study chronologically concords very well with the crustal surface data of the eastern margin and is petrologically suggestive of the subcrustal magmatisms and mantle dynamics. A subsequent research for petrology is now in the course of obtaining good results, the next paper will be concerned with this.

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