geochemistry and mineral chemistry of mafic dykes

309Mafic dykes of the Hiei GraniteJournal of Mineralogical and Petrological Sciences, Volume 105, page 309─319, 2010

doi:10.2465/jmps.090603T. Kutsukake, [email protected] Corresponding author***CRGGLB: Taga, M. (Katata High School, Otsu), Kiji, Y. (Mino-o

High School, Osaka), Nishimura, S. (Board of Education, Kusatsu), Sawada, K. (Fuzoku Junior High School,　Shiga Univ.), Sugii, K. (Kyoto Fire Dept., Kyoto), Takemoto, K. (Ibara Junior High School, Ibara) and Tenpaku, T.

Geochemistry and mineral chemistry of mafic dykes associated with Hiei Granite pluton, southwest Japan

Toshio Kutsukake*, Satoshi Nakano** and The Collaborative Research Group for the Granites Around Lake Biwa (CRGGLB)***

*Laboratory of Geological Sciences, Science Hall, Aichi University, Toyohashi 441-8522, Japan**Department of Natural Science, Faculty of Education, Shiga University, Otsu 520-0862, Japan

Many thin mafic dykes occur within and around the Cretaceous Hiei Granite pluton in southwest Japan. Petro-graphically, these dykes are spessartite, basalt, and dolerite. Petrographical and geochemical characteristics in-dicate their designations as calc-alkaline basalt and basaltic andesite. Their magmas have probably originated in the LILE-rich mantle wedge under the Eurasian continental margin matured arc. These mafic rocks are crystal-lized and have suffered successive hydrothermal alteration under high vapor condition. Among the Cretaceous granites around Lake Biwa, the older ones of ~ 95 Ma, such as the Hiei and Kaizuki-yama plutons, are accom-panied exclusively by these mafic dykes including spessartite; however, the younger (~ 70 Ma) ones are never accompanied by these dykes.

Keywords: Cretaceous, Geochemistry, Hiei Granite, Mafic dyke, Spessartite

INTRODUCTION

In the course of our geological and petrological study of the Hiei Granite pluton (C.R.G.G.L.B., 2008), we exam-ined dykes of both felsic and mafic compositions distrib-uted within and/or around the pluton. The occurrence of lamprophyre has been established by Hiki’s (1917) geo-logical study of the region around Mt. Hiei. Yoshizawa and Ishizaka (1961) analyzed the mode of occurrence and presented a petrography of the lamprophyres of this area. They classified these mafic dykes into the following three types: pyroxene-andesitic lamprophyre, doleritic lampro-phyre, and their hydrothermally altered products. Spessar-tite seems to be included in the first group of their classifi-cation. Most of these dykes do not exhibit the textural characteristics of lamprophyre (Nockolds et al., 1978). Kimura et al. (1998) did not consider these dykes to be any kind of lamprophyres, and they classified all of them as “andesite,” without the support of geochemical data.

However, our study revealed that some of them can be classified as spessartite and others as both basalt and dol-erite. Although the rock classified as basalt has a basaltic composition, it exhibits an andesitic texture.

In this paper, we report the mode of occurrence, pe-trography, geochemistry, and mineral chemistry of these mafic dykes and discuss their petrogenesis and geological situation together with the Cretaceous allied dykes in the western Chubu and Kinki districts in southwest Japan.

GEOLOGICAL OCCURRENCE

The Hiei Granite pluton, 6 km (E-W) × 7 km (N-S) in extent, is one of the Cretaceous granites around Lake Biwa and is distributed over Kyoto and Otsu Cities. The geological map of this pluton is shown in Figure 1b (C.R.G.G.L.B., 2008).

In the eastern part of this pluton, both granite por-phyry and granodiorite porphyry dykes, in parallel ar-rangement, cut through the granite along the N-S trend. Their maximum width is 200 m and 150 m, respectively. Thin mafic dykes, ranging in width from a few tens of centimeters to several meters, are numerous, especially in the equigranular granite, which forms the outer portion of the pluton; however, within the core porphyritic granite, mafic dykes are rare. The biggest mafic dyke (>5 m in

310 T. Kutsukake, S. Nakano and CRGGLB

width) is located within the hornfels south of the granite. The trend of dykes is predominantly in 40°N-60°E, but other trends have also been recognized.

These mafic dykes occur in several modes, which are shown in Figure 2. Most dykes replenish joints of the host granite; therefore, they usually exhibit a bent and/or curved shape. Also an en echelon arrangement of dykelets is seen (Fig. 3e). These modes of occurrence suggest in-trusions after the consolidation of the host granite.

PETROGRAPHY

Mafic dyke rocks are petrographically classified into three types: spessartite, basalt, and dolerite.

Figure 1. (a) Zonal division of Cretaceous to Paleogene granitoids in the western Chubu and Kinki districts, and localities of studied and reference rocks. (b) Geological map of the Hiei Granite plu-ton and sample localities of the analyzed mafic dyke rocks.

Figure 2. Sketches of the mode of occurrence of mafic dykes in the Hiei Granite pluton, modified from Yoshizawa and Ishizaka (1961). (a) A thin mafic dyke cuts across an aplite vein (Ap). (b) Bent mafic dyke along the joints of the host granite. (c) A mafic dyke filling the fractures formed by conjugate joints. (d) Branch-ing of mafic dykes along the joint. (e) En echelon arrangement of mafic dykelets.

311Mafic dykes of the Hiei Granite

SpessartiteThis rock is compact and dark green in color. Acicular amphibole crystals are visible to the naked eye.

Phenocrysts are mainly made up of reddish-brown amphibole and plagioclase (Fig. 3a). The amphiboles are sometimes euhedral prism, but are usually subhedral. Patchy, pale-green amphibole (actinolitic) is developed

within the phenocrystic amphiboles. Actinolite is also de-veloped in a parallel growth along with the brown amphi-bole (Fig. 3b). The plagioclase is mostly pseudmorphic and is replaced by zoisite and sericitic mica. Therefore, it is difficult to estimate the plagioclase composition. Pseu-domorphic clinopyroxene, mostly replaced by actinolitic amphibole, is also present. Occasionally, a mosaic of bio-

Figure 3. Photomicrographs of the Hiei mafic dyke rocks. (a) Spessartite, Ca-amphibole phenocrysts (Hb) are seen. (b) Spessartite, an inter-growth of pargasite (Hb) and actinolite (Act). (c) Basalt, phenocrysts of clinopyroxene (Cpx), plagioclase (Pl), and kaersutite (Hb) are seen. (d) Basalt, kelyphitic corona around the quartz xenocryst (Qtz). (e) Dolerite; phenocrysts of clinopyroxene (Cpx) and plagioclase (Pl) are seen. (f) Dolerite, hydrothermal reaction products of garnet (Grt) and chlorite (Chl) between the clinopyroxene and plagioclase.


tite flakes is observed and is believed to be a product of amphibole alteration. Groundmass is composed mostly of fine amphibole and plagioclase, which is frequently lath-

shaped. The products of hydrothermal alteration are mus-covite, epidote, zoisite, chlorite, titanite, and others.

BasaltThis rock is black and compact; mineral grains, with the exception of quartz xenocrysts, are invisible to the naked eye.

Phenocrystic plagioclase and clinopyroxene are glomeroporphyritic and lie in the fine groundmass (Fig. 3c). Reddish-brown amphibole (kaersutite) also occurs as phenocrysts. The texture of this rock is typical to andesite, and atypical to basalt; however, its chemical composition is basaltic (refer to the geochemical section). The ground-mass consists mainly of plagioclase, brown amphibole (stout prism), granular clinopyroxene, and biotite. Quartz xenocrysts are surrounded by a kelyphitic corona (Fig. 3d), suggesting a reaction between the xenocryst and the host melt.

Phenocrystic plagioclase is euhedral and subhedral and sometimes has a thin sodic rim; its composition is An55−70 (Yoshizawa and Ishizaka, 1961). Groundmass pla-gioclase is An30−50. Clinopyroxene is replaced by actinolite from the periphery.

Secondary minerals are the same as the above-men-tioned spessartite.

DoleriteThis rock is compact and black. Under the microscope, it exhibits a typical doleritic texture (Fig. 3e). Plagioclase, augite, and brown amphibole phenocrysts are observed. Both plagioclase and augite are glomeroporphyritic. Groundmass consists of plagioclase, ophitic augite (al-tered to amphibole in part), and brown and pale-green amphiboles. Hydrothermal alteration is intense; the pla-gioclase undergoes sericitization and the phenocrystic au-gite changes to chlorite, tiny garnet aggregates, and ti-tanite (Fig. 3f). The garnet shows anomalous birefrin - gence, interference color, and belongs to the grossular-andradite series. Ameba-like translucent chromite is also seen.

GEOCHEMISTRY

Four analyses of the Hiei mafic dykes carried out with the purpose of detecting major and trace elements including REEs are presented in Table 1. The analyses were carried out at Activation Laboratories, Ltd., Ontario, Canada, by the ICP-MS, INAA methods in conjunction with titration using KMnO4 (FeO).

Major elementsSiO2 content is less than 50% for basalt and dolerite, whereas it is higher than 50% for spessartite. Na2O + K2O content is less than 3.7% for basalt and dolerite, whereas it is higher than 4.5% for spessartite. Mg# = 53.3-65.5 for these rocks.

These compositions have been plotted in the IUGS

Analyses were carried out at Activation Laboratories Ltd., On-tario, Canada.

Table 1. Chemical analyses of the Hiei mafic dyke rocks


classification diagram of volcanic rock-types (Le Maitre, 1989), shown in Figure 4. Related rocks of the Cretaceous age, occurring around Lake Biwa, such as the spessartites of the Kasuga-mura area (where the Kaizuki-yama Gran-ite is distributed) and the basalts of the Inuyama and Yoro areas (their distribution is shown in Fig. 1a), are also plot-ted in the same diagram for comparison. Both Hiei doler-ite and basalt fall in the basalt field, whereas one spessar-tite falls in the basaltic andesite field and another spessartite in the basaltic trachyandesite field, similar to the spessartites of the Kasuga-mura area (Shiraki et al., 2002). The SiO2 content of the Hiei spessartites is slightly lower than that of Kasuga-mura spessartites.

Trace elementsCr content in two samples of the spessartite is 18.3 ppm (E14a) and 90.6 ppm (AB-4). The Cr content of the for-mer sample is lower than the chromite-bearing spessar-tites (65.6 ppm and 106.8 ppm) in Kasuga-mura (Suzuki and Shiraki, 1980); however, the latter sample has Cr con-tent similar to that of the chromite-bearing spessartites in Kasuga-mura. These values are far lower than those (233-409 ppm) of the basalts from the Inuyama and Yoro areas (Shiraki et al., 2002). Among the rocks from the Hiei mafic dykes, the Sr content of the spessartites is higher than that of the basalts and dolerites. The Sr con-tent of the Kasuga-mura spessartites is similar to that of the Hiei spessartites, whereas the Sr content of the In-uyama basalts is nearly half of the above-mentioned val-ues (Shiraki et al., 2002). Other characteristic trace ele-ment concentrations in the Hiei spessartites are Zr and Rb, which are lower and higher, respectively, compared with their concentrations in the Kasuga-mura spessartites (Shiraki et al., 2002).

N-MORB-normalized geochemical patterns of these dykes are shown in Figure 5, together with minette from Wasatch Plateau, Utah, U.S.A. (Tingey et al., 1991), JB-1 (Kitamatsuura basalt), a Japanese geochemical standard rock (Imai et al., 1995), and Aleutian and Andean basalts (Kelemen et al., 2005) for comparison. As minette repre-sents a typical alkaline lamprophyre, it was chosen to compare the spessartite such as that from the Hiei, which

Figure 4. Plots of the analyses of maf-ic dyke rocks in Na2O + K2O ver-sus. SiO2 IUGS classification diagram of the volcanic rocks. Normalized to 100% and devoid of volatility.

Figure 5. N-MORB normalized trace-element patterns of the Hiei mafic dyke rocks, minette from Wasatch Plateau, Utah, U.S.A. (Tingey et al., 1991), and a basalt (JB-1) of the Geochemical Standard Rocks of GSJ (Imai et al., 1995). Andean and Aleutian arc basalts (Kelemen et al., 2005). N-MORB data from Sun and McDonough (1989).


is a calc-alkaline lamprophyre. The Aleutian and Andean basalts represent the young arc and matured arc basalts, respectively. All of the Hiei mafic dykes show similar pat-terns. Among the reference rocks, with the exception of Rb content, the pattern most similar to these is that of the Andean basalt. Minette has higher values for most of the normalized elements except for Nb, whereas the Aleutian basalt has lower values except for the Sr content, than the Hiei rocks.

The normalized Rb values of the Hiei dykes are greater than 100, which is higher than other basalts. Mi-nette is fairly higher than the Hiei spessartite with regards to LIL element content. The concentrations of HFS (high-

field strength) elements such as Zr and Ti are fairly con-

sistent among these mafic rocks except for the Aleutian basalt and are concentrated around an N-MORB-normal-ized value of 1.

In the Ti-Zr-Sr discrimination diagram for basalts (Pearce and Cann, 1973), they are exclusively plotted in the field of calc-alkaline basalt very close to the boundary of island-arc tholeiite (Fig. 6). These trace element char-acteristics indicate that they belong to the calc-alkaline basalt and basaltic andesite, as inferred from the petro-graphical characteristics such as the presence of spessar-tite, a typical calc-alkaline lamprophyre, and of the Ca-amphibole phenocrysts in the basalt.

REEChondrite-normalized REE patterns of the mafic dykes and other reference rocks are shown in Figure 7. All of the Hiei dykes show a similar pattern, which are quite similar to that of the Andean basalt and different from the other reference rocks. LREE content of the Hiei rocks is lower than that of JB-1, whereas HREE content is almost the same for all of these rocks. Minette has substantially higher LREE content than the Hiei dykes. The Aleutian basalt is fairly lower than other rocks with regard to all REEs.

MINERAL CHEMISTRY

Major ferromagnesian minerals were analyzed by the electron-probe microanalyzer (JXA8800M) at the Depart-ment of Natural Science, Shiga University. The analytical procedure is the same as that used in the previous study (Nakano et al., 2002).

ClinopyroxeneBasalt and dolerite contain clinopyroxene. Most of them are drastically altered, and a few unaltered crystals were analyzed by EPMA. The results from these analyses are presented in Table 2. They are plotted in the pyroxene quadrilateral (Morimoto, 1988) (Fig. 8) and fall in both the diopside and augite fields. Al2O3 content is rather low in these clinopyroxenes; in the plot of Mg# versus Al2O3 (Fig. 6 of Shiraki et al., 2002), clinopyroxenes in the ba-salt fall within the HMA (high-magnesian andesite) field. The clinopyroxenes in the Yoro basalts have higher (>3.0 wt%) Al2O3 content (Shiraki et al., 2002) than that of the Hiei basalts.

Calcic amphibolesThe main mafic minerals in the spessartite are calcic am-phiboles of both magmatic and secondary origins. Further, in other rock-types, various calcic amphiboles occur as products of hydrothermal alteration. An electron-probe

Figure 6. Plots of the analyses of mafic dyke rocks from Hiei and other areas in the Ti-Zr-Sr discrimination diagram for basaltic rocks (Pearce and Cann, 1971). Symbols are the same as in Fig-ure 4.

Figure 7. Chondrite-normalized REE patterns of the Hiei mafic dyke rocks, minette from Wasatch Plateau, Utah, U.S.A. (Tingey et al., 1991), and basalt (JB-1) of the Standard Geochemical Rocks of GSJ (Imai et al., 1995). Andean and Aleutian arc ba-salts (Kelemen et al., 2005); Chondrite (carbonaceous chondrite II)-normalizing values are derived from Sun and McDonough (1989)


scan indicates that the Ca-amphiboles in these rocks dif-fer in chemical composition grain by grain within a single crystal. Their designations are variable and are plotted in the IMA classification diagram (Leake et al., 1997) shown in Figure 9.

Representative analyses are shown in Table 3. Fe+3 is estimated by the calculation scheme of ΣCa = 15 on the basis of 23(O) (Stout, 1972).

Magmatic Ca-amphiboles of spessartite consist of pargasite and edenite (Fig. 9); kaersutite and pargasite in basalt. Secondary Ca-amphiboles are mainly actinolite.

BiotiteBiotite is contained in the groundmass of the spessartite as tiny flakes. Two representative analyses are presented in Table 4. The compositions are shown in Figure 10. A biotite in the spessartite (I in Table 4) is low in K2O con-tent, suggesting partial chloritization.

EpidoteEpidote occurs as a secondary mineral in every type of rock. It is anhedral granular. An analysis of dolerite is given in III in Table 4. It contains 28 mol% of pistacite.

ChloriteChlorite occurs as a secondary mineral after pyroxene and Ca-amphiboles. It is pale yellowish green in color and shows anomalous interference color. It has scaly flakes. An analysis of the basalt is given in IV in Table 4. It is es-sentially a solid-solution between clinochlore and cham-osite.

TitaniteTitanite occurs in every type of rock as an alteration prod-uct. It is usually an irregular-shaped grain. Several analy-ses show variable contents of TiO2 and SiO2. An analysis is presented in V in Table 4.

GarnetAs an alteration product of clinopyroxene together with chlorite in dolerite, tiny garnet crystals occur in clusters. They show anomalous birefringence and interference col-or, indicating that they belong to the hydrogrossular-an-dradite series. An analysis (VI in Table 4,) supports this inference.

DISCUSSION

Genetical considerationChromite-bearing spessartites, which are associated with the Kaizuki-yama Granite pluton in the Kasuga-mura area, Gifu Prefecture (Sugii and Sawada, 1999), are re-garded to be of mantle origin (Suzuki and Shiraki, 1980). These spessartites bear high-magnesian andesitic charac-teristics; along with the Yoro and Inuyama basalts of Cre-taceous age, they are probably derived from the LILE-

rich upper mantle (Shiraki et al., 2002). The Hiei spessa-

Table 2. Chemical compositions and structural formu-lae of clinopyroxenes

Figure 8. Plots of clinopyroxenes of the Hiei mafic dyke rocks into the Di-Hd-En-Fs quadrilateral (Morimoto, 1988).


rtites also have petrographical and geochemical characteristics similar to those of the Kasuga-mura; therefore, their magmas should have a similar origin.

The Cenozoic high-magnesian andesites in south-west Japan can be classified into the following two types: the sanukitoids bearing high Rb/Sr ratios and the bajaite-adakite series bearing low Rb/Sr ratios (Shiraki et al., 2002). The Hiei mafic dyke rocks as well as the Kasuga-mura spessartites have high Rb/Sr ratios. In this regard, they have an affinity to the sanukitoids. Sanukitoid series rocks are usually rich in Rb, derived from the sediments constituting the upper part of the subducted slab (Shimoda et al., 1998), and also due to the reaction between the melts of sediments and the mantle (Shimoda et al., 2003).

The Hiei mafic dyke rocks have high LILE except Sr, and they are geochemically similar to Andean basalt. These geochemical characteristics are common to the continental margin matured arc basalts and are different

from the young arc tholeiites. The Cretaceous magmatic activities in the Inner Zone of southwest Japan, which led to the formation of these dyke rocks together with the vast granitoids, took place at the Eurasian continental margin (Kutsukake, 1993). Therefore, the tectonic setting is con-sistent with the petrological and geochemical natures of these rocks.

Crystallization and hydrothermal alterationLamprophyres are regarded to be polygenic rocks crystal-lized under volatile conditions (Mitchell, 1994) and char-acterized by abundant phenocrysts of ferromagnesian minerals. Lamprophyres are usually altered (Nockolds et al., 1978). In the Hiei mafic dykes as well, magmatic min-erals are completely or partially altered to secondary ones. Further, they are replaced by chlorite, epidote, titanite, and others. In spessartite, the primary Ca-amphiboles are replaced by actinolite in the same crystallographic orien-

ΣCa=15 indicates the number of cations normalized to 15 except Na and K on the basis of 23(O).

Table 3. Representative analyses and structural formulae of the Ca-amphiboles


tation (Fig. 3b), suggesting a successive hydrothermal al-teration soon after magmatic crystallization. Also a wide variation in the chemistry of the ferromagnesian minerals indicates a lack of equilibrium during crystallization, as well as local differences in the conditions of hydrothermal alteration. These mineralogical features of the spessartites and their related rocks of the Hiei area suggest a rapid crystallization and hydrothermal alteration under high va-por pressure during the intrusion. The magmas enriched in water usually cause an upward “drilling” by efficient concerted hydraulic and thermoclastic stress, leading to the fracturing of roof rocks (Best and Christiansen, 2001). Then, they incorporate the fragments of the host rock. Quartz xenocrysts with a kelyphitic rim in the Hiei mafic dykes could have been derived from the fragments of the host granite.

Geological bearing of these Cretaceous mafic dykesLamprophyres commonly occur as dykes in the tonalite-granodiorite plutons of the continental arcs. They consti-tute a trilogy together with the host granitoid and mafic

enclaves (Best and Christansen, 2001). The inner zone of southwest Japan, before the Miocene opening of the Japan Sea, constitutes a segment of the Cretaceous-Paleogene magmatic arc of the Eurasian continental margin (Kutsu-kake, 1993). The Hiei Granite pluton is one of the prod-ucts of this arc magmatism. In the tonalites and granodio-rites of the Ryoke Belt, which is located ~ 50 km to the south of the Hiei Granite pluton (Fig. 1a), there occur nu-merous synplutonic dykes of tholeiitic composition and also mafic enclaves (Kutsukake, 2002). The Hiei Granite is quite similar to the Older Ryoke granitoids with regard to the petrological, geochemical, and geochronological characteristics (Kutsukake et al., 2005). However, within granites of the San’yo-Naegi Belt (Fig. 1a), both the maf-ic dykes and mafic enclaves are rare, which can be attrib-uted to compositional constraints, i.e., granite sense strict. Thus, the occurrence of lamprophyres is quite sporadic; hitherto, the spessartites have only been reported in the Hiei and Kaizuki-yama Granites around Lake Biwa (Su-zuki and Shiraki, 1980; C.R.G.G.L.B., 2008). According to Sugii and Sawada (1999), the Kaizuki-yama Granite

Figure 9. Plots of Ca-amphiboles of Hiei mafic dyke rocks into the IMA classification diagrams for Ca-am-phiboles (Leake et al., 1997).


pluton has many geological, petrographical, and geo-chemical similarities with the Hiei Granite pluton (C.R.G.G.L.B., 2008). Both the plutons yield almost the same age of 95 Ma (Sawada and Itaya, 1993), whereas, the younger (ca. 70 Ma) Cretaceous granite plutons

around Lake Biwa are not accompanied by the mafic dykes, and have more evolved characteristics than the Hiei and Kaizuki-yama Granites (C.R.G.G.L.B., 2008). Cretaceous basaltic dykes, which are not associated with the granite, occur in some places such as the Yoro and Inu-yama areas in the western Chubu and Kinki districts (Fig. 1a), and they have geochemical characteristics similar to those of the Kasuga-mura areas (Shiraki et al., 2002). These mafic dykes yield K-Ar ages of ca. 90 and 84 Ma (Kimura and Kiji, 1993), which is less than the Hiei and Kaizuki-yama granites. The Kasuga-mura spessartites yield an age of ca. 83 Ma (personal communication from Prof. K. Suzuki of Nagoya University). Therefore, the magmatic activities of these mafic rocks took place later than the host granites.

ACKNOWLEDGEMENTS

We are grateful to Aichi University and the Fujiwara Fund for the Advancement of Natural History Study for bearing the cost of the chemical analyses of rocks. We would also

Table 4. Chemical compositions and structural formulae of the “secondary” minerals

Figure 10. Composition of biotite in the Hiei mafic dyke rocks.


want to thank the two reviewers for their valuable com-ments, which helped us in improving the style and content of this paper.

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Manuscript received June 3, 2009Manuscript accepted March 24, 2010Published online July 22, 2010Manuscript handled by Takao Hirajima

geochemistry and mineral chemistry of mafic dykes

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