Zircon U-Pb ages of the Cretaceous gabbroic and granitic rocks from the Kajishima, northwest Shikoku, southwest Japan

Abstract

We carried out zircon U-Pb dating of the Cretaceous gabbroic and granitic rocks exposed in the Kajishima to determine their magmatic ages. The plutonic rocks in the Kajishima are classified into seven lithologies: Hbl-bearing olivine norite, Type-1 Hbl-bearing troctolite, Type-2 Hbl-bearing troctolite, Type-1 pyroxene hornblende gabbronorite, and Type-2 pyroxene hornblende gabbronorite, massive granodiorite, and deformed granodiorite. The new zircon U-Pb ages are: 90.77 ± 0.99 Ma for the Type-2 Hbl-bearing troctolite, 92.35 ± 0.74 Ma for the Type-1 pyroxene hornblende gabbronorite, 91.33 ± 1.03 Ma for the massive granodiorite, and 84.11 ± 1.12 Ma for the deformed granodiorite. The new zircon U-Pb ages suggest multiple episodes of magmatic intrusion in Kajishima during ∼ 92 and 84 Ma. Although two gabbroic rocks and a massive granodiorite show similar zircon U-Pb ages, subsequent granitic magmatism is unraveled in Kajishima, which is contrary to previous geological studies that postulated an evolution of all plutonic rocks from a single gabbroic parent magma.

INTRODUCTION

Recent compiltation of U-Pb zircon crystallization ages from both volcanic and plutonic rocks together with U-Pb detrital zircon age information have suggested that magmatism in arcs is episodic in space and time, and the periods of increased magma production ‘flare-ups’ are common (cf. Paterson and Ducea, 2015 and references therein). In addition, geochronological and isotopic studies have suggested that the magmatic flare-ups are driven by episodic increases in mantle-derived magma into the arc crust, inducing voluminous continental crust formation (e.g., Martinez-Ardila et al., 2019; Attia et al., 2020). Therefore, zircon U-Pb age data of mafic magmatic rocks would provide critical information to unravel the evolution history and continental crust forming processes in magmatic arcs during flare-ups because mantle-derived mafic rocks record mantle activity.

During the Cretaceous magmatic flare-up event along the continental margin of East Asia, various plutonic rocks, including dominant granitic rocks with related mafic rocks have intruded in the inner zone of southwest Japan (Fig. 1a; e.g., Sato et al., 2016; Takatsuka et al., 2018a). These mafic rocks provide important clues to the genesis of granitic magma to drive the growth of the continental crust (e.g., Sisson et al., 1996; Collins, 2002; Nakajima et al., 2004). Although numerous zircon U-Pb age data of the granitic rocks are available in the literature (Herzig et al., 1998; Watanabe et al., 2000; Nakajima et al., 2004; Ishihara and Tani, 2013; Iida et al., 2015; Skrzypek et al., 2016; Sueoka et al., 2016; Miyazaki et al., 2018; Takatsuka et al., 2018a, 2018b; Mateen et al., 2019; Shimooka et al., 2019; Nakae, 2021; Noda et al., 2021; Tsutsumi, 2021), limited number of those from mafic plutonic rocks, which probably reflect mantle activity directly, has been reported (Nakajima et al., 2004).

Figure 1. (a) Distribution of Cretaceous igneous rocks in southwest Japan (after Imaoka et al., 2014). (b) and (c) Location of the study area. (d) Geological map of Kajishima. Numbers refer to zircon U-Pb ages.

Various types of Cretaceous gabbroic rocks associated with granitic dykes are widely exposed in the Kajishima (Yoshimura, 1940a, 1940b; Horiuchi, 1985), which is one of the uninhabited islands in the central area of Seto Inland Sea. (Fig. 1). Although geological and petrological studies have been performed on Kajishima (Yoshimura, 1940a, 1940b; Horiuchi, 1985), the timing of magmatism in Kajishima is not well understood despite several attempts using isochron dating methods (Kagami et al., 1985; Okano et al., 2000). In this paper, we report new geological, lithological and geochronological data of gabbroic rocks and granitic rocks in Kajishima. Based on these new information, we discuss the geochronological relationship between them and consider the wider context of the Cretaceous to Paleogene magmatic activity in southwest Japan.

GEOLOGICAL BACKGROUND

Kajishima is predominantly underlain by the Cretaceous gabbroic rocks with minor granitic rocks occurring as dyke (Yoshimura, 1940a, 1940b; Horiuchi, 1985). Previous works (Yoshimura, 1940a, 1940b; Horiuchi, 1985) have illustrated geological maps of the island and revealed petrological characteristics of gabbroic rocks. They also described the granitic rocks as ‘aplite’ without detailed petrographic observation. Based on field observations, Yoshimura (1940a) has suggested that the mafic rocks and granitic rocks (‘aplite’) are formed from noritic parental magma. Horiuchi (1985) classified gabbroic rocks of Kajishima into five types including 1) orthopyroxene allivalite (troctolite composed of olivine and highly calcic plagioclase), 2) noritic gabbro-A, 3) noritic gabbro-B, 4) olivine gabbro, and 5) hyperite (an old name originally used for a variety of hypersthene-bearing norite) based on detailed field and petrographic observations. Horiuchi (1985) has further suggested that various types of mafic rocks are formed through differentiation in the same magma chamber based on field observations and chemical composition of minerals.

Kagami et al. (1985) reported Sr isotope data for noritic rocks and cortlanditic rocks (pyroxene olivine hornblendite composed of large hornblende crystals enclosing olivine, and pyroxene) of Kajishima, those initial ⁸⁷Sr/⁸⁶Sr ratios range from 0.70741 to 0.70770. Kagami et al. (1987) further investigated whole-rock Rb-Sr and Sm-Nd isotopes of gabbroic rocks and suggested that gabbroic rocks were not derived from a single magma chamber because the data showed ‘multiple isochrons’. Okano et al. (2000) reported contrasting mineral isochron ages of 101.9 ± 3.2 Ma (Rb-Sr isochron age) and 220 ± 46 Ma (Sm-Nd isochron age) for olivine-bearing gabbroic rocks of Kajishima.

LITHOLOGY AND PETROGRAPHY

The geological map of the Kajishima based on the fieldwork and petrographical investigation in this study is shown in Figure 1b. Samples for this study were collected from five lithological varieties of gabbroic rocks and two types of granitic rocks. The texture, grain size, mineral assemblage, and modal proportion of gabbroic rocks and granitic rocks were investigated with a polarizing microscope. The modal proportions were determined for 37 gabbroic rock samples and 7 granitic rock samples by point counting in thin sections. Petrographic features are summarized in Table 1.

Table 1. Summary of petrographic features and modal composition of plutonic rocks from Kajishima

Rock type	Field observations	Textual remark	Grain size
Hbl-bearing olivine norite	Layered	Massive, Porphyritic, Cumulus	Medium-coarse
Type-1 Hbl-bearing troctolite	Layered	Massive, Porphyritic, Cumulus	Coarse
Type-2 Hbl-bearing troctolite	Layered	Massive, Porphyritic, Cumulus	Medium-coarse
Type-1 pyroxene hornblende gabbronorite	Layered	Massive, Porphyritic, Cumulus	Fine-medium
Type-2 pyroxene hornblende gabbronorite	Massive	Massive, Equigranular	Medium
Massive granodiorite	Massive	Massive, Equigranular	Fine-medium
Deformed granodiorite	Deformed	Deformed, Mylonitic	Fine-medium

Rock type	Range of modal composition 1) 2) 3)
	Ol	Opx	Cpx	Hbl	Bt	Pl	Qz	Kfs	Opq
Hbl-bearing olivine norite	10-15	6-17	<7	5-28	<1	41-66	-	-	Tr
Type-1 Hbl-bearing troctolite	13-43	<3	<9	13-42	<1	30-42	-	-	<2
Type-2 Hbl-bearing troctolite	5-11	<1	2-3	18-19	Tr	66-74	-	-	Tr
Type-1 pyroxene hornblende gabbronorite	Tr	<15	1-20	3-47	<1	51-93	-	-	<1
Type-2 pyroxene hornblende gabbronorite	<1	<16	1-11	7-29	<1	61-76	-	-	<3
Massive granodiorite	-	-	-	-	4-12	37-59	31-44	<27	Tr
Deformed granodiorite	-	-	-	-	13-14	30-33	43-44	11-12	Tr

¹⁾Mineral abbreviations after Warr (2021). ²⁾All numbers are vol%. ³⁾Tr, trace.

The plutonic rocks collected in this study are mainly classified into gabbroic rocks and granitic rocks (Fig. 2a). The gabbroic rocks are divided into more detailed categories based on color index: anorthosite, leuco-gabbroids, gabbroids, and mela-gabbroids based on color index (Fig. 2b). The modal compositions of gabbroic rocks are plotted on Pl-Ol-Px, Pl-Px-Hbl and the Pl-Opx-Cpx ternary diagrams (Figs. 2c-2e). The Pl-Ol-Px ternary system was used to classify the lithology when the olivine content was greater than 5 vol%, and the Pl-Px-Hbl ternary system was used when the olivine content was 5 vol% or less. In addition, lithologies outside the troctolite composition range are classified as gabbro, gabbronorite, or norite by Pl-Opx-Cpx ternary system. As a result, the gabbroic rocks are classified into five lithologies: 1) Hbl-bearing olivine norite, 2) Type-1 Hbl-bearing troctolite, 3) Type-2 Hbl-bearing troctolite, 4) Type-1 pyroxene hornblende gabbronorite, and 5) Type-2 pyroxene hornblende gabbronorite. The Type-1 Hbl-bearing troctolite are coarse-grained and relatively melanocratic (Colour index = 58-70%). In contrast, the Type-2 Hbl-bearing troctolite are medium- to coarse-grained and relatively leucocratic (Colour index = 26-34%). The Type-1 pyroxene hornblende gabbronorite shows porphyritic and cumulus textures consisting of fine- to medium-grained minerals, whereas the Type-2 pyroxene hornblende gabbronorite, which is texturally distinctive from the Type-1 pyroxene hornblende gabbronorite, shows medium-grained equigranular texture. (Table 1). Gabbroic rocks, except the Type-2 pyroxene hornblende gabbronorite, show a layered structure consisting of plagioclase-rich light-colored layers and mafic mineral-rich dark-colored layers, whereas Type-2 pyroxene hornblende gabbronorite show no layered structures. Anorthosite locally develops within the light-colored layer of Type-1 pyroxene hornblende gabbronorite.

Figure 2. Qz-Kfs-Pl ternary diagram (a) for the plutonic rocks and color index (b), modal Pl-Px-Ol (c), Pl-Px-Hbl (d), and Pl-Opx-Cpx (e) ternary diagrams for the gabbroic rocks from Kajishima. Classification boundaries are after Streckeisen (1976).

Granitic rocks intrude into gabbroic rocks as dykes with 0.3-3 m width. Based on the field observations, granitic rocks are classified into two types; 1) massive granodiorite and 2) deformed granodiorite. The massive granodiorite intrudes into a mass of single type gabbroic rock, whereas the deformed granodiorite intrudes into boundaries between different types of gabbroic rocks. The deformed granodiorite is characterized by occurrences of porphyroclasts of K-feldspar and plagioclase in the matrix comprising subgrains of quartz and plagioclase. The modal compositions of studied granitic rock samples are plotted in the Qz-Kfs-Pl ternary diagrams (Table 1 and Fig. 2a). In the Qz-Kfs-Pl diagram, the massive granodiorite are classified as granodiorite to tonalite. In contrast, the deformed granodiorite are mainly classified as granodiorite although there are some sample that fall in the granite compoisition.

ZIRCON U-Pb DATING

Methodology

Zircon grains for U-Pb geochronology were separated from two gabbroic rocks (KJS-006, KJS-047) and two granitic rocks (KJS-007, KJS-025) samples using high-voltage pulse power fragmentation device (SELFRAG-Lab) at the National Museum of Nature and Science (NMNS) in Japan. The heavy minerals were concentrated by panning and further processed with a hand magnet, and the remaining fractions were purified using heavy liquid (diiodomethane) separation. An adequate amount of zircons (approximately 150 grains) was randomly handpicked from each sample. The backscattered electron and cathodoluminescence (CL) images were obtained using a scanning electron microscope (JEOL JSM-6610) at NMNS to select the sites for analysis. In order to determine the crystallization age of the zircon, the rim domains were selected for analysis. The U-Pb dating was undertaken using laser ablation inductively coupled plasma mass spectrometry system (LA-ICP-MS) at the NMNS in Japan, with the NWR213 laser ablation system (Electro Scientific Industries) in conjunction with an Agilent 7700x quadrupole ICP-MS (Agilent Technologies). Zircon grains from the samples, the zircon standard TEMORA2 (417 Ma; Black et al., 2004) and the glass standard NIST SRM610 were mounted in an epoxy resin and polished until the surface was flattened with the center of the embedded grains exposed. The spot size of the laser was approximately 25 µm. The experimental conditions and the procedures followed for the measurements were based on the methods described in Tsutsumi et al. (2012). A correction for common Pb was corrected based on the measured ²⁰⁷Pb for mean age calculation and measured ²⁰⁸Pb for Tera-Wasserburg concordia diagrams (e.g., Williams, 1998) and the model for common Pb compositions proposed by Stacey and Kramers (1975). The ages presented in this study were calculated using IsoplotR software (Vermeesch, 2018). The uncertainties in the mean ²³⁸U-²⁰⁶Pb ages represent 95% confidence intervals.

Weighted mean zircon U-Pb ages of the samples, together with rock types, bodies and localities, are listed in Table 1. The full LA-ICP-MS U-Pb data and calculated ages of zircon, together with the analytical results of secondary standards (FC1 and OD3), are available on request to the corresponding author.

Results

Representative CL images of zircon grains indicating the analysis position and calculated ²⁰⁶Pb/²³⁸U ages are shown in Figure 3. Analyzed zircon grains in gabbroid rocks exhibit sector zoning, and those in the granitic rocks exhibit concentric oscillatory zoning. Tera-Wasserburg concordia diagrams and age distribution plots for analysis of rim domains are shown in Figure 4. Weighted mean ages of zircons in the four samples collected from the Kajishima are 90.77 ± 0.99 Ma for the Type-2 Hbl-bearing troctolite, 92.35 ± 0.74 Ma for the Type-1 pyroxene hornblende gabbronorite, 91.33 ± 1.03 Ma for the massive granodiorite, and 84.11 ± 1.12 Ma for the deformed granodiorite (Table 2; Fig. 4).

Figure 3. Cathodoluminescence images (CL) of representative zircon grains. The spot analysis locations are indicated by circles. Ages shown are ²³⁸U/²⁰⁶Pb ages with 2σ error determined by LA-ICP-MS.

Figure 4. Tera-Wasserburg U-Pb Concordia diagrams and age distribution plots for zircon samples. The uncertainties in the mean ²³⁸U/²⁰⁶Pb* ages represent 95% confidence intervals. ²⁰⁷Pb* and ²⁰⁶Pb* indicate radiometric ²⁰⁷Pb and ²⁰⁶Pb, respectively.

Table 2. Weighted mean zircon U-Pb age, rock type, body, and locality of the sample

Sample No.	Mean age (Ma)	Rock type	Locality
KJS-047	90.77 ± 0.99	Type-2 Hbl-bearing troctolite	N34°07′28.5′′, E133°09′31.7′′
KJS-006	92.35 ± 0.74	Type-1 pyroxene hornblende gabbronorite	N34°07′15.8′′, E133°09′35.8′′
KJS-007	91.33 ± 1.03	Massive granodiorite	N34°07′15.4′′, E133°09′35.1′′
KJS-025	84.11 ± 1.12	Deformed granodiorite	N34°07′26.1′′, E133°09′27.9′′

DISCUSSION

The new zircon U-Pb ages obtained in this study range from ∼ 92-91 and ∼ 84 Ma. Because the zircon rim domains selected for U-Pb analysis are thought to have formed during the crystallization of magma (Fig. 3), the obtained weighted mean ages can be interpreted as the age of zircon crystallization during the cooling of the magmas. Therefore, our results indicate that there were two episodes of magmatic activities in the Kajishima at ∼ 92-91 and ∼ 84 Ma, although Okano et al. (2000) have reported older mineral isochron ages of 101.9 ± 3.2 Ma (Rb-Sr isochron age) and 220 ± 46 Ma (Sm-Nd isochron age).

Our results indicate that dominant magmatic activity in the Kajishima occurred in ∼ 92-91 Ma (Type-2 Hbl-bearing troctolite, Type-1 pyroxene hornblende gabbronorite, and massive granodiorite). The similar ages of gabbroic rocks and massive granodiorite (∼ 92-91 Ma) are partly consistent with Yoshimura (1940a) argument that all lithologies of plutonic rocks in Kajishima have been formed by differentiation of a single noritic parent magma. However, the deformed granodiorite is relatively younger age (∼ 84 Ma), suggesting at least two magmatic activities of ∼ 92-91 and ∼ 84 Ma in Kajishima.

The new zircon U-Pb ages, together with the wide range of modal compositions in the gabbroic rocks and granitic rocks presented here, collectively suggest multiple episodes of gabbroic and granitic magma intrusions in the Kajishima area during ∼ 92 and ∼ 84 Ma. Figure 5 shows the latitudinal variation of previously published zircon U-Pb ages of Cretaceous-Paleogene mafic and granitic rocks from the inner zone of southwest Japan. The zircon U-Pb ages suggest that plutonic magmatism lasted from ∼ 106 to ∼ 33 Ma, and those of mafic rocks (95-64 Ma) show a marked northward younging trend (cf. Iida et al., 2015). The ages of gabbroic rocks of Kajishima (∼ 92-91 Ma) are also plotted along the younging trend, indicating that they represent early magmatic activities among the mafic rocks of the inner zone of southwest Japan. Because the mafic rocks were probably produced from hydrous mafic magma originating from the upper mantle (e.g., Okano et al., 2000), future petrogenetic study on the gabbroic rocks in Kajishima would help to unravel the early stage of mantle activity which probably triggered the Cretaceous flare-up in southwest Japan.

Figure 5. Latitudinal variation of previously published zircon U-Pb ages of Cretaceous-Paleogene mafic rocks and granitic rocks from the inner zone of southwest Japan. Published zircon U-Pb ages from Herzig et al. (1998), Watanabe et al. (2000), Nakajima et al. (2004), Ishihara and Tani (2013), Iida et al. (2015), Skrzypek et al. (2016), Sueoka et al. (2016), Miyazaki et al. (2018), Takatsuka et al. (2018a), Takatsuka et al. (2018b), Mateen et al. (2019), Shimooka et al. (2019), Nakae (2021), Noda et al. (2021), and Tsutsumi (2021).

SUMMARY

1. (1)    The gabbroic rocks in the Kajishima have variable modal compositions ranging from Hbl-bearing troctolite and Hbl-bearing olivine norite to pyroxene hornblende gabbronorite. The granitic rocks are classified as massive granodiorite and deformed granodiorite based on field characteristics, with massive lithologies ranging from tonalite to granodiorite and deformed lithologies ranging from granodiorite to granite.
2. (2)    The zircon U-Pb ages obtained in this study range from ∼ 92 and 84 Ma. The similar ages of 92-91 Ma for the gabbroic rocks and massive granodiorite are consistent with the previously proposed formation process, in which all gabbroic rocks in Kajishima have been formed by the differentiation from a single gabbroic parental magma. The new zircon U-Pb ages, including the younger age of deformed granodiorite (84 Ma), suggest multiple episodes of magmatic intrusion in Kajishima.

ACKNOWLEDGMENTS

We thank Y. Suda for editorial handling. We also thank T. Okudaira and an anonymous reviewer for their thoughtful and constructive reviews. We are grateful to T. Murakami and F. Fujimoto for their kind help with field survey of Kajishima. We are also grateful to Y. Kusaba and Y. Tsutsumi for their support with zircon U-Pb analysis. This study was partially supported by JSPS KAKENHI Grant Numbers JP21J22080 to K.S. and JP22H01323 to S.S.

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