Journal of Petrology Advance Access originally published online on July 27, 2007
Journal of Petrology 2007 48(9):1761-1791; doi:10.1093/petrology/egm037
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Formation of Distinct Granitic Magma Batches by Partial Melting of Hybrid Lower Crust in the Izu Arc Collision Zone, Central Japan
1Geological Institute, Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai, Hodogaya-Ku, Yokohama 240-8501, Japan
2Department of Geology, University of Maryland, College Park, MD 20742-4211, USA
3Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba 305-8567, Japan
4National Institute of Polar Research, 1-9-10 Kaga, Itabashi 173-8515, Japan
5Department of Geoscience, Shimane University, Matsue 690-8504, Japan
RECEIVED SEPTEMBER 9, 2006; ACCEPTED JUNE 25, 2007
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL METHODSGEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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The Miocene Kofu Granitic Complex (KGC) occurs in the Izu Collision Zone where the Izu–Bonin–Mariana (IBM) arc has been colliding with the Honshu arc since the middle Miocene. The KGC includes rocks ranging in compositions from biotite-bearing granite (the Shosenkyo and Mizugaki plutons), and hornblende–biotite-bearing granodiorite, tonalite, quartz-diorite, and granite (the Shiodaira, Sanpo, Hirose and Sasago plutons), to hornblende-bearing tonalite and trondhjemite (the Ashigawa–Tonogi pluton), indicating that it was constructed from multiple intrusions of magma with different bulk chemistry. The Sr-isotopic compositions corrected to sensitive high-resolution ion microprobe (SHRIMP) zircon ages (SrI) suggest that the primary magmas of each pluton were formed by anatexis of mixed lower crustal sources involving both juvenile basalt of the IBM arc and Shimanto sedimentary rocks of the Honshu arc. After the primary magmas had formed, the individual plutons evolved by crystal fractionation processes without significant crustal assimilation or additional mantle contribution. SHRIMP zircon U–Pb ages in the KGC range from 16·8 to 10·6 Ma and overlap the resumption of magmatic activity in the IBM and Honshu arcs at c. 17 Ma and the onset of IBM arc–Honshu arc collision at c. 15 Ma. The age of the granite plutons is closely related to the episodic activity of arc magmatism and distinct granitic magma batches could be formed by lower crustal anatexis induced by intrusion of underplated mantle-derived arc magmas. Based on pressures determined with the Al-in-hornblende geobarometer, the KGC magmas intruded into the middle crust. Thus, the KGC could represent an example of the middle-crust layer indicated throughout the IBM arc by 6·0–6·5 km/s seismic velocities. This granitic middle-crust layer acted buoyantly during the IBM arc–Honshu arc collision, leading to accretion of buoyant IBM arc middle crust to the Honshu arc.
KEY WORDS: arc–arc collision; crustal anatexis; granite; Izu–Bonin–Mariana (IBM) arc; Izu Collision Zone
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL METHODSGEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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The Izu Collision Zone, where the northern Izu–Bonin–Mariana (IBM) arc system has been colliding with the Honshu arc (Fig. 1b), is a key place to study the petrological effect of remelting of arc crust because (1) Neogene granite plutons are widely exhumed by tectonic uplift associated with arc collision (e.g. Sato, 1991
; Kawate & Arima, 1998), (2) the geodynamic process of arc collision has been extensively investigated (e.g. Niitsuma, 1989; Taira et al., 1989; Amano, 1991; Soh et al., 1991; Takahashi & Saito, 1997; Aoike, 1999), and (3) the episodic magmatic activity of the IBM and Honshu arcs is well constrained (e.g. Cambray et al., 1995). Young granites developed in modern arcs provide important clues to understanding geodynamic processes of continental crust formation (see Davidson & Arculus, 2006), in contrast with granites in Archean terranes, where the geodynamic information is modified by polyphase deformation and metamorphism. It has been suggested by previous workers that the tectonic setting of arc–arc collision and arc accretion in the Izu Collision Zone is similar to that of Archean orogenic belts (e.g. Taira et al., 1992). Therefore, understanding the petrological processes of granite formation in the Izu Collision Zone may contribute to understanding ancient orogenic belts, especially those related to arc collisional settings.
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Recent seismic experiments (Suyehiro et al., 1996
; Kodaira et al., 2007; Takahashi et al., 2007) define the presence of a middle-crustal layer with compressional velocity (Vp) = 6·0–6·5 km/s throughout the IBM arc, interpreted to represent granitic rocks (Suyehiro et al., 1996; Takahashi et al., 1998; Kitamura et al., 2003). Oceanic arcs such as the IBM arc, therefore, are recognized as ‘factories’ where granites are formed either by direct crystallization of differentiated arc magmas (e.g. Haraguchi et al., 2003), or by melting of the crystallization products of earlier arc magmas by later magmas (e.g. Nakajima & Arima, 1998; Tatsumi, 2000). Consequently, exposed granites in the Izu Collision Zone could represent the granitic middle crust exhumed by tectonic uplift during continuing arc–arc collision (Kawate & Arima, 1998). The Kofu Granitic Complex (KGC), which is the largest plutonic complex in the Izu Collision Zone (Fig. 1b), is composed of various types of granite ranging from granite (sensu stricto) to granodiorite, quartz-diorite, tonalite and trondhjemite, which reflect a wide range of petrographic, geochemical and isotopic signatures (Kato, 1968a, 1968b; Sato & Ishihara, 1983; Shimizu, 1986; Takahashi, 1989; Sato, 1991; Saito et al., 2004, 2007). In this paper, we present geochemical and Sr-isotopic compositions, and sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb age data for the KGC. The objectives of this study are (1) to propose a petrogenetic model for the various granites that make up the KGC, and (2) to evaluate whether the KGC could represent an example of the 6·0–6·5 km/s middle crust of the IBM arc. |
GEOLOGICAL BACKGROUND |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL METHODSGEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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The Izu Collision Zone (Taira et al., 1989
; Soh et al., 1991, 1998; Aoike, 1999) is located near the Boso triple junction (Ogawa et al., 1989), where, as a consequence of the northwestward migration of the Philippine Sea Plate, the northern IBM arc system has been colliding with the Honshu arc (Fig. 1a). The collision has been occurring since the middle Miocene (c. 15 Ma) (Niitsuma, 1989; Soh et al., 1991; Takahashi & Saito, 1997), nearly coeval with the beginning of clockwise rotation of the SW Honshu arc associated with the opening of the Japan Sea (17–15 Ma, Otofuji et al., 1994) (Fig. 2). The collision has led to the accretion of IBM arc crust to the Honshu arc, associated with a southward migration of the plate boundary and trench system (Soh et al., 1991, 1998; Taira et al., 1998). Collision-induced deformation resulted in (1) the northward bending of the pre-Neogene accreted terranes and the Median Tectonic Line of the Honshu arc (Kanto Syntaxis, Takahashi & Saito, 1997, Fig. 1a), and (2) the formation and uplift of imbricated thrust-bound segments, which are several tens of kilometers in lateral dimension (Fig. 1b). As a result of the tectonic thickening, the crustal thickness reaches 
40 km beneath the Izu Collision Zone (Asano et al., 1985).
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The northern and central parts of the KGC intruded into the Shimanto Belt, a Cretaceous to Paleogene accretionary complex composed of clastic sedimentary rocks, whereas the southern part was emplaced into the Miocene Nishiyatsushiro Group, a submarine volcanic sequence of the IBM arc (Fig. 1c). Amphibolite-facies metamorphic rocks are developed within the contact aureoles of the KGC (Kato, 1968a
, 1968b; Shimazu et al., 1976; Shimizu, 1986), and anatectic migmatites locally occur (Saito et al., 2007). K–Ar dating studies of the KGC documented ages ranging from 15·7 to 7·4 Ma (Kawano & Ueda, 1966; Shibata et al., 1984; Saito & Kato, 1996; Saito et al., 1997) (Fig. 2). Itoh et al. (1989) reported zircon fission-track ages of 12·5–11·3 Ma from the southern part of the KGC (Fig. 2).
In this paper, we subdivide the KGC into seven plutons on the basis of their petrographic, geochemical, and Sr-isotopic characteristics: the Shosenkyo, Mizugaki, Shiodaira, Sanpo, Hirose, Sasago, and Ashigawa–Tonogi plutons. Following the studies by Shimizu (1986) and Sato (1991), we subdivided the Sanpo, Hirose and Sasago plutons into an external and an internal part (Fig. 1c), corresponding to ilmenite-series and magnetite-series rocks (Ishihara, 1977), respectively. The external parts of these plutons are characterized by a higher 87Sr/86Sr isotopic ratio than the internal part (Sato, 1991). They contain abundant metasedimentary xenoliths derived from the country rocks and were previously interpreted as a marginal facies generated by magma–country-rock interaction (Shimizu, 1986; Takahashi, 1989; Sato, 1991).
The petrographic characteristics of the plutons are summarized in Table 1. The Shosenkyo and Mizugaki plutons consist of biotite-bearing granite. The Shiodaira, Sanpo, Hirose, and Sasago plutons are composed of hornblende–biotite-bearing granodiorites, tonalite, quartz-diorite, and granite, and the Ashigawa–Tonogi pluton consists mainly of hornblende-bearing tonalite. The rocks of the northern part of Mizugaki pluton and the southern part of Ashigawa–Tonogi pluton are characterized by porphyritic textures and/or graphic intergrowth of quartz and feldspars, which are rare in the other plutons. The northern extreme of the Sanpo pluton consists of leucogranite that represents a roof zone of the pluton. These features are indicative of the relatively shallower emplacement depth of the northern and southern extremes of the KGC compared with the central parts.
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Rocks with preferentially oriented plagioclase and/or large poikilitic hornblende crystals occur locally in the Sanpo, Hirose and Sasago plutons. We interpret these as cumulates (see Saito et al., 2007
) (Table 1). Abundant mafic enclaves ranging from 10 to 30 cm in length and consisting of fine-grained plagioclase, hornblende, biotite, magnetite and interstitial quartz, occur in the Sanpo pluton. Mafic enclaves are smaller, ranging from sub-millimeter to tens of centimeters in size, and less abundant in the Shiodaira, Hirose, Sasago and Ashigawa–Tonogi plutons, and rarely observed in the Shosenkyo and Mizugaki plutons (Table 1). |
ANALYTICAL METHODS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL METHODSGEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Mineral analyses were carried out using an automated energy-dispersive electron microprobe (LINK QX2000 system) at the Geological Institute, Yokohama National University, Japan. The operating conditions were 15 kV and 0·155 nA on a cobalt standard. Data were processed using a LINK ZAF-4/FLS correction program. We used standard materials of natural quartz for SiO2, synthetic corundum for Al2O3, synthetic rutile for TiO2, synthetic cosmochlore for Cr2O3, forsterite for MgO and FeO(total) (total iron as FeO), wollastonite for CaO, albite for Na2O, orthoclase for K2O, and apatite for P2O5.
Major elements and 12 trace elements (Ba, Co, Cr, Cu, Nb, Ni, Rb, Sr, V, Y, Zn and Zr) were analyzed by X-ray fluorescence (XRF) spectrometry. Samples were crushed manually and ground in an agate ball mill. The sample powder was mixed with Li2B4O7 (weight proportion of 1:5) then fused in a crucible (Pt–Au alloy) at 1200°C. Resulting glass disks were analyzed using XRF (RIGAKU RIX-3000) at the National Institute of Polar Research (NIPR), Japan. The analytical procedure followed the methods described by Motoyoshi & Shiraishi (1995) and Motoyoshi et al. (1996). Additional trace elements [rare earth elements (REE), Li, Be, Hf, Ta, Pb, Th and U] were analyzed using solution inductively coupled plasma-mass spectrometry (ICP-MS) (Thermo Elemental, VG PQ-3) at the Department of Geoscience, Shimane University, Japan. Kimura et al. (1995, 2002) and Roser et al. (2000) described the alkali fusion after acid digestion, standard addition analytical methods, and the precision and accuracy of the analysis.
Sr-isotopic compositions were determined using a thermal ionization sector field mass spectrometer at the Geological Survey of Japan (VG Elemental, Sector 54), following the procedure described by Nakajima et al. (1990). 87Sr/86Sr ratios were normalized to 86Sr/88Sr = 0·1194. Repeated analyses of the NBS987 standard during this study gave 87Sr/86Sr of 0·71025 ± 0·00002 (1
). Initial Sr-isotopic ratio (SrI) was recalculated back to an age determined by the SHRIMP U–Pb dating method (this study). Representative whole-rock compositions and Sr-isotopic compositions are given in Table 2. The full data are available as an electronic appendix, which may be downloaded from the Journal of Petrology website at http://www.petrology.oxfordjournals.org.
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Zircon grains for SHRIMP dating were separated from seven rock samples of the KGC. The U, Th and Pb isotope compositions of zircon were measured with the SHRIMP II at NIPR. The analytical techniques essentially follow those of Williams (1998
). An 30 µm spot was used for analysis. The reference value of U concentration in zircon was obtained by measurement on standard zircon SL13 (238 ppm U; 206Pb/238U age of 572 Ma) (Roddick & van Breemen, 1994). Pb/U ratios were corrected for instrumental fractionation based on measurement of the zircon standard FC1 (1099 Ma; Paces & Miller, 1993). Data reduction and processing were conducted using the computer programs SQUID ver. 1 (Ludwig, 2000) and ISOPLOT ver. 2 (Ludwig, 1999). The SHRIMP analytical results, 204Pb-corrected 206Pb/238U ages, and 207Pb-corrected 206Pb/238U ages are given in Table 3.
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GEOCHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL METHODSGEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Major and trace element data
In terms of normative mineralogy, the Shosenkyo and Mizugaki plutons plot in the granite field of the An–Ab–Or diagram, the Shiodaira, Sanpo, Hirose, and Sasago plutons fall in the tonalite, granodiorite, and monzogranite fields, and the Ashigawa–Tonogi pluton falls in the tonalite and trondhjemite fields (Fig. 3). Selected major elements are plotted against SiO2 (Fig. 4a). The samples from the Shosenkyo and Mizugaki plutons are characterized by relatively high K2O and a narrow range of SiO2 (72–78 wt %), they are also poorer in TiO2, FeO(total), MgO and CaO than the rocks from the other plutons. Unlike the rocks from the other plutons, those from the Shosenkyo and Mizugaki plutons show a decrease in Na2O and K2O with increasing silica. There are two trends for TiO2, Al2O3, CaO, Na2O and K2O in the other plutons. One trend is dominated by the Hirose samples, which show higher K2O and lower CaO contents compared with those for the Shiodaira, Sanpo, Sasago and Ashigawa–Tonogi plutons. Rocks of the Ashigawa–Tonogi pluton show a wide range of major element variation (SiO2 62–76 wt %), and are rich in TiO2 and FeO(total) and poor in Al2O3 and K2O. The TiO2, Al2O3, FeO(total), MgO and CaO contents decrease with increasing SiO2 whereas the Na2O content exhibits the opposite trend. The Shosenkyo and Mizugaki plutons are alkali–calcic to calc-alkalic whereas the other plutons are calcic based on the modified alkali–lime index (MALI: Na2O + K2O – CaO, Frost et al., 2001
). All the samples show an increase in Aluminum Saturation Index [ASI: molecular Al2O3/(CaO + Na2O + K2O)] with increasing silica. Most rocks are metaluminous, but the most silica-rich portions of the Hirose and Sasago plutons and the samples from the Shosenkyo and Mizugaki plutons are peraluminous.
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The Shosenkyo and Mizugaki plutons are rich in Ba, Nb, Rb and Y, and poor in Sr and Zr compared with the other plutons (Fig. 4b). Compared with the Mizugaki pluton, the Shosenkyo pluton is poorer in Ba and Nb, and richer in Rb and Y. The Ashigawa–Tonogi pluton shows a well-defined variation in trace element contents and is depleted in Ba and Rb compared with the other plutons. The Y and Zr contents of the Ashigawa–Tonogi pluton increase with increasing SiO2, in contrast to the Hirose pluton, which shows a decrease of Y and Zr with increasing SiO2 indicating zircon fractionation. The Shiodaira pluton has higher Sr than the Sanpo, Hirose, Sasago and Ashigawa–Tonogi plutons. The Hirose samples show a distinctive trend in Ba, Nb and Rb.
The cumulates in the Sasago, Sanpo, and Hirose plutons are tonalite in composition (Fig. 3). The major and trace element compositions of the cumulates are distinct from those of the host granites (Fig. 4a and b). The cumulates are characterized by low SiO2 (55–58 wt %) and their Al2O3, Na2O, Nb, Ni, Sr and Y contents are scattered on plots against SiO2 (Fig. 4a and b). In the Zr/Y–Sr/Y diagram (Fig. 4b), however, the cumulates exhibit a distinctive trend of low Zr/Y over a wide range of Sr/Y. Enclaves in the Sanpo pluton are mafic to intermediate in composition (57–60 SiO2 wt %) with distinctly higher Na2O (3·0–3·6 wt %) and lower Ni (2·0–6·6 ppm) at a given SiO2 than the main granite trend (Fig. 4a and b).
The Shosenkyo and Mizugaki plutons are characterized by enriched light REE (LREE) (2·3 and 2·6 [La/Yb]N, respectively) and a negative Eu anomaly (0·2 and 0·4 Eu/Eu*, respectively) (Fig. 5). The other plutons also show LREE enrichment ([La/Yb]N; 5·7 for Shiodaira, 2·9–7·8 for Sanpo, 2·7–3·7 for Hirose, 2·6 for Sasago, and 1·7–2·0 for Ashigawa–Tonogi) with or without a negative Eu anomaly (Eu/Eu*; 1· 0 for Shiodaira, 0·6–0·9 for Sanpo, 0·7–0·9 for Hirose, 0·7 for Sasago, and 0·6–0·8 for Ashigawa–Tonogi). The total REE contents increase with increasing SiO2 in the Sanpo and Ashigawa–Tonogi plutons, in contrast to those in the Hirose pluton, which shows a decrease of total REE contents with increasing SiO2 indicating fractionation of zircon and/or apatite. Primordial mantle normalized trace element patterns for the KGC granites (Fig. 6) display enrichment in large ion lithophile elements (LILE; Rb, Ba, Th, U and K), and depletion in high field strength elements (HFSE; Nb, Ta and Ti). These characteristics are diagnostic of rocks formed in subduction-related settings (see Pearce & Parkinson, 1993). KGC granites show a higher abundance of LREE, LILE (Rb, Ba, Th, and K) and Nb compared with those of the Tanzawa plutonic complex (Fig. 1b, Kawate & Arima, 1998) occurring south of the KGC (Figs 5 and 6).
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SHRIMP zircon U–Pb dating
Zircon grains selected for analysis are prismatic euhedral crystals generally exhibiting a pinkish color. Cathodoluminescence (CL) imaging reveals that the zircon grains have well-developed concentric oscillatory zoning (Fig. 7). Most of the grains have a euhedral center, which is interpreted to have crystallized from melt.
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We analyzed 27 zircon grains (one spot in each grain) collected from seven sample locations. The 207Pb-corrected U–Pb age yields a more precise age than the 204Pb-corrected U–Pb age (1
error: 0·1–0·6 compared with 0·1–1·1, Table 3), therefore, the 207Pb-corrected U–Pb ages are used in the following discussion. The SHRIMP zircon U–Pb ages range from 16·8 to 10·3 Ma (Table 3). The majority of the data are concordant on a Tera–Wasserburg diagram (Fig. 8). As all of the analyzed zircon grains show concentric oscillatory zoning (Fig. 7), we consider that the SHRIMP zircon U–Pb ages of the KGC represent the crystallization age of zircon from granitic melts.
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Two grains in each of the Mizugaki and Shiodaira plutons plot on the concordia line (Fig. 8b and c) and yield mean ages of 14·3 ± 0·2 Ma and 11· 4 ± 0·3 Ma. Grains from the Sanpo pluton show relatively scattered plots (Fig. 8d). Five concordant data points (gray symbols in Fig. 8d) yield a mean age of 14·0 ± 0·1 Ma. Two grains from the Hirose pluton plot on the concordia line but two other grains are discordant (Fig. 8e); the concordant grains yield a mean age of 10·6 ± 0·2 Ma. Two grains from each of the Sasago and Ashigawa–Tonogi plutons plot on the concordia line (Fig. 8f and g), yielding mean ages of 12·7 ± 0·3 Ma and 13·2 ± 0·1 Ma.
The U–Pb ages for the Shosenkyo pluton range from 16·8 Ma to 13·4 Ma (Table 3, Fig. 7). In the Tera–Wasserburg diagram, all data plot along the concordia line (Fig. 8a). These ages could suggest a prolonged history of Shosenkyo magma activity at depth over 3 Myr (e.g. Cesare et al., 2003) or significant Pb loss. If Pb loss is the cause of the variability in ages, the data should result in a negative correlation of U concentration with U–Pb age (higher U concentration will produce higher Pb, and Pb loss will result in a younger age), but this is not observed in our data (Table 3). We prefer, therefore, the interpretation that the data represent a real age range.
Strontium isotopic compositions
The initial Sr-isotope ratio (SrI) at the SHRIMP zircon U–Pb age was calculated for each pluton of the KGC (Table 2, Fig. 9). The SrI for the Shosenkyo pluton was calculated at 16·8 Ma. The calculated SrI values are 0·7056–0·7057 for the Shosenkyo, 0·7054–0·7062 for the Mizugaki, 0·7038–0·7039 for the Shiodaira, 0·7041–0·7044 for the Sanpo, 0·7040–0·7051 for the Hirose, 0·7041–0·7048 for the Sasago, and 0·7037–0·7042 for the Ashigawa–Tonogi plutons (Fig. 9). The two mafic enclaves in the Sanpo pluton have SrI values of 0·7041, which are within the range of host granite (Table 2).
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The external parts of the Sanpo and Hirose plutons tend to have slightly higher SrI than the internal parts of the plutons (Fig. 9). This observation is consistent with a previous interpretation that the external parts of the plutons represents a marginal facies generated by interaction with old crustal materials characterized by high Sr-isotopic composition (Shimanto sedimentary rocks, Table 2) during emplacement (Shimizu, 1986
; Takahashi, 1989; Sato, 1991). One sample from the external part of the Hirose pluton (HRS-05b, SiO2 55 wt %) has lower SrI (0·7040) than other samples (0·7050 SrI), consistent with the interpretation that it formed as early crystallized cumulate that was not involved in significant magma–country-rock interaction (Saito et al., 2007). In summary, excluding the data from the external parts of plutons, each pluton has a nearly uniform SrI value (Table 1), suggesting that it formed from a distinct single primary magma. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL METHODSGEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Origin of the Kofu granite magmas
Various origins have been proposed for metaluminous to weakly peraluminous intermediate to silicic magmas similar to the KGC. These are: (1) crystal differentiation of mantle-derived basaltic and/or boninitic magma (e.g. Meijer, 1983
; Shirey & Hanson, 1984; Haraguchi et al., 2003); (2) partial melting of hydrous basaltic lower crust (e.g. Takahashi, 1986; Kay & Kay, 1988; Kawate & Arima 1998; Hansen et al., 2002; Sisson et al., 2005); (3) partial melting of andesitic middle arc crust (e.g. Tamura & Tatsumi, 2002; Shukuno et al., 2006); (4) partial melting of mixed sources (mantle-derived basaltic rocks and older crustally recycled materials) (e.g. Skjerlie & Patinõ-Douce, 1995); (5) progressive mixing of mantle-derived magma with crust-derived felsic magma (e.g. Tatsumi & Takahashi, 2006); (6) assimilation of crustal rocks into differentiating basaltic magmas (e.g. DePaolo, 1981); (7) a mixing, assimilation, storage, and hybridization (MASH) process, where mantle-derived magma interacts with deep crustal materials (Hildreth & Moorbath, 1988). In addition, partial melting of subducting basaltic oceanic crust has been proposed for low-K intermediate–silicic magmas of tonalite and trondhjemite affinities (e.g. Defant & Drummond, 1990), but this process is unlikely as the REE abundance of the KGC plutons is not consistent with garnet involvement in their genesis (Fig. 5).
The Shosenkyo, Mizugaki, Shiodaira, and Ashigawa–Tonogi plutons have SiO2 contents >60 wt % and lack any chemical trends suggesting derivation from basaltic magma. The relatively mafic samples (SiO2 <58 wt %) in the Sasago, Sanpo, and Hirose plutons show cumulate textures; this feature contradicts a derivation from mantle-derived basaltic magma. Sato (1991) suggested that the petrological diversities occurring within the KGC resulted from various degrees of assimilation of crustal rocks into the mantle-derived magmas. Because each intrusion has a nearly uniform SrI value, however, the whole-rock compositional variation in each pluton is unlikely to be a result of assimilation of crustal rocks into differentiating basaltic magmas or progressive mixing of mantle-derived magma with crust-derived felsic magmas. In addition, the mafic enclaves are Ni-poor and therefore cannot be derived from a basaltic source (Fig. 4b), suggesting little contribution of mantle-derived magma into granite magma. For these reasons, we suggest that crystal differentiation of mantle-derived magmas (e.g. Meijer, 1983; Shirey & Hanson, 1984; Haraguchi et al., 2003), progressive mixing of mantle-derived magma with crust-derived felsic magma (e.g. Tatsumi & Takahashi, 2006), assimilation of crustal rocks into differentiating basaltic magmas (e.g. DePaolo, 1981), or a MASH process (e.g. Hildreth & Moorbath, 1988) are unlikely to produce the KGC magmas. The data suggest that each of the seven distinct intrusions of the KGC was derived from a different intermediate or silicic parental magma.
Parental magmas and fractional crystallization
Saito et al. (2004) suggested a crystal fractionation model for the Ashigawa–Tonogi pluton, which involves removal of hornblende, plagioclase, and magnetite from an intermediate parental magma composition (SiO2 62 wt %). We have modeled crystal fractionation for the other six plutons of the KGC, employing a least-squares mass-balance calculation for the major elements. The most SiO2-depleted sample from each pluton without a cumulate texture was chosen as a starting composition, assuming this to be closest to a parental magma composition. The inferred parental magma is silicic (SiO2 72 wt %) for the Mizugaki and Shosenkyo plutons, and intermediate (SiO2 61–64 wt %) for the Sanpo, Shiodaira, Sasago, and Hirose plutons. The samples from the external part of the Sanpo, Hirose and Sasago plutons are excluded from the modeling because their Sr-isotope compositions (Fig. 9) suggest wall-rock interaction as previously suggested by Shimizu (1986), Takahashi (1989) and Sato (1991).
Based on petrographic observations (Table 1), the following minerals were used in the modeling: plagioclase + K-feldspar + biotite for the Mizugaki and Shosenkyo plutons; plagioclase + hornblende + biotite + magnetite for the Sanpo, Shiodaira, and Hirose plutons; plagioclase + hornblende + magnetite + quartz for the Sasago pluton. Mineral compositions used for the modeling are listed in Table 4. The weight proportions of crystal phases and liquid calculated in the fractional crystallization modeling are given in Table 5 and Fig. 10. The sum of squared residuals of the calculation is low in all the modeling (< 0·2) and the calculated melt compositions compare well with the observed rock compositions of each pluton (Table 5, Fig. 11).
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To evaluate the crystal fractionation model, Rb, Ba and Sr abundances in the fractionated melts were calculated using the Rayleigh fractionation model assuming that the whole-rock composition represents the melt composition. The crystal–melt elemental partition coefficients used in the present modeling are from the data compilation by Rollinson (1993
). Because the assumed parental magmas for the Shosenkyo and the Mizugaki plutons are silicic and those for the other plutons are intermediate, different elemental partition coefficients for biotite–melt and plagioclase–melt were used, depending on the melt composition (Table 6). The calculations resulted in good agreement with the observed compositional variation in all the plutons (Table 5, Fig. 11).
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In summary, the SrI data suggest that each pluton was derived from a single parental magma. The observed chemical variations can be explained by crystal fractionation; mixing of either felsic crust-derived magma or mafic mantle-derived magma to produce the range of silica contents in the KGC magmas is disallowed by the isotopic compositions.
Anatexis of hybrid lower crust sources
Lower crustal anatexis has been proposed for the tonalitic parental magma of the Tanzawa plutonic complex (Kawate & Arima, 1998) and experiments show that melts with compositions similar to those of the Tanzawa plutonic complex can be formed in this way (Nakajima & Arima, 1998). Similarly, Saito et al. (2004) suggested that the parental magma of the Ashigawa–Tonogi pluton could be derived by partial melting of hydrous basaltic lower crust with a low-K basaltic composition.
The Tanzawa plutonic complex, which was intruded into the volcaniclastic rocks of the IBM arc, is characterized by lower SrI (0·7034–0·7037 at 7 Ma) similar to the Sr-isotopic composition of volcanic rocks occurring in the northern IBM arc (Kawate, 1996; Kawate et al., 2000). In contrast, the KGC, which has significantly higher SrI, was emplaced in both the volcaniclastic rocks of the IBM arc and the Shimanto sedimentary rocks of the Honshu arc.
The isotopic evidence supports the concept that the KGC granites are generated by anatexis of hybrid lower crustal sources of the Izu Collision Zone. Here we suggest that intra-crustal melting of hybrid lower crustal sources—a mixture of juvenile basaltic material of the IBM arc and mature sedimentary rock (Shimanto Belt) of the Honshu arc—could account for the observed geochemical and Sr-isotopic characteristics of the KGC.
The Shimanto Belt in the Izu Collision Zone is the eastern extension of the pre-Neogene Shimanto accretionary complex of the SW Honshu arc (Takahashi & Saito, 1997; Taira et al., 1998; Takahashi, 2006). A recent wide-angle seismic survey conducted along the Nankai Trough and Shikoku area in the SW Honshu arc revealed the subduction-related structure (Nakanishi et al., 2002). It consists of three distinct parts, the subducting oceanic crust, the sedimentary wedge of the Neogene–Quaternary accretionary prism beneath the continental slope, and the thick older crustal block of the Cretaceous–Tertiary accretionary prism (Shimanto Belt) immediately below the sedimentary wedge. Because the Izu Collision Zone lacks the Neogene–Quaternary accretionary complex, we consider the Shimanto Belt in the area of the KGC to be the main constituent of the fore-arc crust of the Honshu arc at the time of IBM and Honshu arc collision.
The weight proportion of the Honshu arc component present in the source region can be estimated, assuming that the average SrI of the IBM arc basalt (Taylor & Nesbitt, 1998) and of the Shimanto sedimentary rocks represent the IBM arc component and the Honshu arc component, respectively (Fig. 12, Table 7). The result is 45 wt % of the Honshu arc component for the Shosenkyo, 40 wt % for the Mizugaki, 20 wt % for the Hirose, 15 wt % for the Sasago and Sanpo, 10 wt % for the Ashigawa–Tonogi and 5 wt % for the Shiodaira plutons.
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The involvement of variable proportions of the Honshu arc components (sedimentary component) in the sources is indicated by the degree of alkalinity (MALI: modified alkali–lime index) of the parental magma compositions (Fig. 13). Experimental melts of the IBM arc basalt are strongly calcic (Nakajima & Arima, 1998
), whereas those derived from a mixture of basaltic and sedimentary materials (50% high-Al olivine tholeiite +50% metasedimentary gneiss or 50% hornblende-gabbro +50% metasedimentary gneiss, Patinõ-Douce, 1995; Castro, 2001) are calc-alkalic, suggesting that anatexis of the mixed Shimanto sedimentary and IBM basalt source produces melts with relatively high MALI values compared with those from IBM arc basalt (Fig. 13). The Shosenkyo and Mizugaki plutons with the highest MALI values among the KGC plutons have the highest SrI (0·7058), which is consistent with a significant contribution of the Honshu arc component to the source region (40–45 wt %, Fig. 12). In contrast, the Ashigawa–Tonogi pluton has low SrI (0·7040) (10 wt % involvement of Honshu arc component in its source region, Fig. 12) and a low MALI value similar to the experimental melt compositions of the IBM arc basalt (Nakajima & Arima, 1998). The Hirose pluton has the highest MALI and SrI (0·7047) values among the calcic plutons, consistent with moderate involvement of the Honshu arc component in the source region (20 wt %, Fig. 12).
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Tectonic implications
Variations in emplacement ages and depths of the KGC
Sample locations for the SHRIMP zircon U–Pb (this study), K–Ar hornblende and biotite (Kawano & Ueda, 1966
; Shibata et al., 1984; Saito & Kato, 1996; Saito et al., 1997) and zircon fission-track dating (Itoh et al., 1989) are shown in Fig. 14. The age variation in the KGC is examined by projecting the available age data onto the axis of the Kanto Syntaxis (Aoike, 2003; the dotted line in Fig. 14), which is interpreted to represent the axis of the northern end of the IBM arc underneath the Izu Collision Zone. With the exception of the Shosenkyo pluton, the ages clearly exhibit a spatial variation along the axis, with ages older to the north and south from the center of the KGC (Fig. 15a). The NNW–SSE-trending age variation throughout the KGC probably reflects different timing of magma emplacement.
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We applied the Al-in-hornblende geobarometer of Anderson & Smith (1995
) to rocks from the Sanpo, Hirose, and Sasago plutons, which consist of quartz + plagioclase + K-feldspar + hornblende + biotite + ilmenite + magnetite ± sphene. Hornblende crystals show systematic chemical zoning, ranging from edenite in the cores through hornblende to actinolitic hornblende at the rims. In the studied rock samples, K-feldspar and quartz occur as interstitial phases; therefore, rim compositions of hornblende in contact with quartz were used for the pressure estimate.
The Al-in-hornblende geobarometer is sensitive to temperature; therefore, the equilibration temperature of coexisting hornblende and plagioclase was estimated using the hornblende–plagioclase thermometer of Holland & Blundy (1994). Plagioclase in these samples is chemically zoned, with lower An contents near the rims (average An20). Although rim compositions of hornblende and plagioclase (An20) yield a wide range of temperatures (640–720°C), hornblende compositions that resulted in 700 ± 5°C (close to the solidus temperature in the system Qtz–An20–Or–H2O at 2–3 kbar, Johannes & Holtz, 1996) were used for the pressure determinations. The data yield pressure conditions ranging from 2·9 to 1· 9 kbar (Fig. 15b, Table 8), which are consistent with previous estimates based on metamorphic assemblages occurring within the contact aureoles (3 kbar, Saito et al., 2007).
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The results of the Al-in-hornblende geobarometer suggest a systematic NNW–SSE-trending pressure variation for the Sanpo, Hirose, and Sasago plutons with relatively higher pressure conditions at the central part of the KGC (Fig. 15b). The pressure variations are consistent with field and petrographic observations such as porphyritic textures observed in the northern part of the Mizugaki pluton and the southern part of Ashigawa–Tonogi pluton, and the presence of a leucocratic roof zone at the northern end of the Sanpo pluton.
The pressure variations of the KGC (Fig. 15b) imply the exhumation from greater depth of the central part of the KGC (9 km) compared with the northern and southern extremes (6 km). The KGC is distributed in a tectonic segment bounded by the Butsuzo Tectonic Line and the Tonoki–Aikawa Tectonic Line (Fig. 1b), suggesting a possible tectonic control on the exhumation of the KGC. The collision of the IBM arc against the Honshu arc could result in the formation and uplift of imbricated thrust-bound segments in the Izu Collision Zone, associated with the sequential southward migration of the plate boundary (Soh et al., 1991, 1998; Taira et al., 1998; Fig. 16). The systematic NNW–SSE-trending pressure variations recorded in the KGC probably reflect the uplift and erosion history within the tectonic segment during the IBM–Honshu arc collision (Fig. 16).
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Implications for the 6·0–6·5 km/s middle-crust layer of IBM arc
A wide-angle seismic experiment across the northern part of IBM arc (32°15'N) (Suyehiro et al., 1996
) identified the presence of a relatively thick middle-crustal layer (up to 8 km thick) with compressional velocity (Vp) = 6·0–6·5 km/s, underlain by a lower crust layer with Vp = 6·8–7· 6 km/s. Similar layered crustal structures have also been reported across the southern Bonin and Mariana arc (Takahashi et al., 2007) and along the northern part of the IBM arc (Kodaira et al., 2007). The presence of the Vp = 6·0–6·5 km/s mid-crust layer throughout the IBM arc is significant, as these velocities correspond to those measured for intermediate granitic rocks (Rudnick & Fountain, 1995; Kitamura et al., 2003). The KGC plutons emplaced at a middle-crustal level (2–3 kbar, Fig. 15b; Saito et al., 2007), therefore, could represent an example of the middle-crustal layer reported throughout the IBM arc.
The northeastern Honshu and the northern IBM arcs experienced minimal volcanic activity during the early Miocene (Taylor, 1992; Arculus et al., 1995; Cambray et al., 1995), coeval with the opening of the Shikoku Basin at c. 25 Ma (Kobayashi et al., 1995; Okino et al., 1999). The cessation of the Shikoku Basin spreading at c. 16 Ma broadly coincided with the resumption of intensive magmatism in the Honshu and IBM arcs at c. 17 Ma (Fig. 2) (Stern et al., 2003). At this time, the southwestern part of the Honshu arc drifted southward, associated with the opening of the Japan Sea Basin (17–15 Ma) (Otofuji et al., 1994) and the collision of the Honshu arc with the IBM arc initiated at c. 15 Ma (Takahashi & Saito, 1997) (Fig. 16a and b). The SHRIMP zircon U–Pb ages (16·8–10·6 Ma) obtained from the KGC overlap with the middle Miocene active magmatic stage of the IBM arc (17–10 Ma) and the onset of the IBM and Honshu arc collision (c. 15 Ma) (Fig. 2). The middle Miocene magmatic arc activity probably led to extensive mantle-derived basaltic underplating, resulting in substantial lower crustal anatexis and the formation of a granitic middle-crust layer represented by the KGC plutons (Fig. 16b and c). Our data suggest that variable proportions of the Honshu arc components were probably involved with the IBM arc component in the anatectic source regions for the KGC magmas.
After the middle Miocene, magmatic activity in the northern IBM arc was episodic, with pulses at 17–14 Ma, 11–10 Ma, and 5 Ma–present (Cambray et al., 1995) (Fig. 2). The first phase of magmatic arc activity led to the formation of the earlier KGC plutons (17–13 Ma, Shosenkyo, Mizugaki, Sanpo, Ashigawa–Tonogi and Sasago plutons, Fig. 16b), and the later plutons (11–10 Ma, Shiodaira and Hirose plutons) could be related to a second phase of activity (Fig. 16c). K–Ar and Ar–Ar dating of biotite and hornblende from the Tanzawa plutonic complex yields ages between 7 and 4 Ma (Saito et al., 1991; Saito, 1993). These ages suggest that the formation of the Tanzawa magma was probably related to the late Miocene reactivation of the arc activity (Fig. 2). The lower crustal anatexis forming the individual granitic plutons with different ages in the Izu Collision Zone, therefore, could be induced by episodic intrusions of underplated mantle-derived arc magmas. The resultant granitic middle-crustal layer was buoyant during the IBM and Honshu arc collision, leading to the accretion of the buoyant IBM arc middle crust to the Honshu arc and consequent tectonic uplift and exhumation of the granitic plutons (Fig. 16d and e).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL METHODSGEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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The KGC is a composite granitic complex formed by multiple intrusions of intermediate to felsic magmas emplaced within a short period of time. It includes seven plutons and each pluton has distinct petrographic, geochemical and Sr-isotopic signatures. The KGC plutons are each derived from a discrete intermediate to felsic parental magma generated by anatexis of the lower crust of the Izu Collision Zone. It is inferred that the lower crust beneath this collision zone has a hybrid nature, consisting of a mixture of two colliding arc components: the IBM arc and the Shimanto fore-arc crust of the Honshu arc. The SrI and MALI values in the KGC plutons reflect the proportion of the sedimentary material (Honshu arc component) involved in the anatectic regions composed mainly of IBM arc basalt.
SHRIMP zircon U–Pb ages for the KGC plutons range between 16·8 and 10·6 Ma. The range in ages suggests that lower crustal anatexis formed the distinct magma batches at different times as a result of episodic intrusions of underplated arc magmas. The KGC magmas were emplaced in the middle crust (2–3 kbar), and probably represent the 6·0–6·5 km/s middle-crust layer reported throughout the IBM arc. During the IBM and Honshu arc collision, the KGC plutons acted buoyantly, leading to the accretion of IBM arc middle crust to the Honshu arc. Continuous collision of the IBM arc against the Honshu arc led to subsequent uplift and exhumation of the granitic middle-crust layer in the Izu Collision Zone. The systematic pressure variation recorded in the KGC probably reflects its uplift and erosion history during the IBM–Honshu arc collision.
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SUPPLEMENTARY DATA |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL METHODSGEOCHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Supplementary data for this paper are available at Journal of Petrology online.
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ACKNOWLEDGEMENTS |
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We thank T. A. Vogel, C. Miller, and K. Nicolaysen for their thoughtful and constructive reviews of an earlier version of this manuscript. Editorial comments by B. R. Frost greatly improved the clarity of the manuscript. The authors also wish to thank M. Ishikawa, Y. Kaneko, S. Igarashi, S. Kawate, K. Aoike, K. Mannen, M. Takahashi, K. Kitamura, H. Kamiyama, K. Tani, T. Kanamaru, J. W. Kokonyangi, F. Korhonen and M. Brown for valuable discussion and comments. We are grateful to K. Shiraishi, Y. Motoyoshi, T. Hokada, T. Usuki, K. Seno and H. Kamioka for their analytical support. Special thanks go to M. Yamamura for his help during fieldwork at an early stage of this study. This study was partly supported by the JSPS Grant-in-Aid for Scientific Research (12440147, 13373005) to M.A. and a Sasakawa Scientific Research Grant from the Japan Science Society (14-305) to S.S.
*Corresponding author. Telephone: +81-45-339-3354. Fax: +81-45-339-3264. E-mail: arima{at}server2.edhs.ynu.ac.jp
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