Magma hybridization and crystallization in coexisting gabbroic and granitic bodies in the mid-crust, Akechi district, central Japan

Abstract

Petrological and geochemical features of gabbros and fine-grained mafic rocks (mafic microgranular enclaves; MMEs) in the Inagawa Granite of the Ryoke Plutonic Complex were investigated to assess the interactions between coexisting mafic and silicic magmas, and the petrogenetic relationships between the MMEs and surrounding gabbros. The MMEs exhibit mingling textures that imply the coexistence of mafic and silicic magmas that did not undergo complete mixing, but the geochemical compositions of the MMEs require substantial hybridization and homogenization. The gabbroic rocks exhibit disequilibrium textures and mineral compositions, such as quartz–hornblende ocellar textures and patchy plagioclase crystals with bimodal anorthite contents. These textures and compositions record an abrupt decrease in crystallization temperature and mechanical mixing between crystallizing gabbroic mush and silicic (granitic) melt. Geochemical variations of the gabbroic rocks can be explained by hybridization and fractional crystallization (HFC) processes between crystallizing gabbroic mush and granitic melt. Extrapolation of the mixing trend to a basaltic composition suggests that the primitive mafic end-member was a low-K basaltic magma. Given that HFC yields magnesian andesite by the addition of a small amount of silicic melt to a primitive mafic end-member, the compositional modification of mafic magmas by magma mixing might be an essential process in the formation of andesitic magma in arc crust.

Introduction

Input of mafic melt into silicic magma chambers triggers eruptions, rejuvenates silicic magma chambers, and keeps large silicic magmatic systems active for millions of years (e.g., Sparks et al. 1977; Pallister et al. 1992; Murphy et al. 1998; Kent et al. 2010; Cooper and Kent 2014; Cashman et al. 2017). Interaction between mafic and silicic melts in a pluton is described as magma mixing, mingling, and/or hybridization. In this paper, based on Ubide et al. (2014), the term “magma mixing” is used in the broad sense of interactions between magmas without implying whether the final product is homogeneous. “Mingling” and “hybridization” refer to the physical dispersion and chemical interactions of magmas, respectively. Our understanding of the physicochemical processes involved during magma mixing is still limited (Perugini and Poli 2012). It is thought that the compositions of the end-member magmas have a strong effect on the mixing outcome, whereas rheologically similar magmas produce a homogeneous hybridized magma as a result of efficient mixing. For magmas with contrasting compositions (e.g., mafic and silicic magmas), hybridization is hindered by the large thermal and rheological differences between the magma end-members (e.g., Sparks and Marshall 1986; Grasset and Albarède 1994; Bateman 1995; Perugini et al. 2008). Geochemically, the mixing of two magmas is considered to generate linear correlations in Harker-type diagrams, with hybrid magmas plotting along a mixing line between the end-members. However, recent studies have shown that chaotic dynamics may enhance efficient hybridization between contrasting magmas and produce non-linear geochemical correlations between the hybrids (Perugini et al. 2006, 2008; De Campos et al. 2011). Mixing features are relatively common in plutonic rocks, and mingling textures between mafic and silicic magmas suggest incomplete mixing (e.g., Waker and Skelhorn, 1966; Didier and Barbarin 1991; Finders and Clemens 1996; De Campos et al. 2008).

From a geological and petrological perspective, previous studies of magma mixing/mingling have focused mainly on processes within a silicic (granitic) magma rather than a mafic magma or body. It has been suggested that intermediate magmas are formed by multiple magmatic processes operating in mafic magmas, including fractional crystallization, assimilation, and mixing with silicic magmas in the crust (Eichelberger 1978, 2006; Rudnick 1995, Reubi and Blundy 2009; Kent et al. 2010). Therefore, a detailed understanding of the chemical modification of mafic magmas and petrological features due to such modification is important for understanding magmatic processes in crustal magma chambers.

The Ryoke belt is distributed along the northern side of the Median Tectonic Line (MTL) of southwest Japan, and extends for 700 km from east to west (Fig. 1a). It consists of the Ryoke Metamorphic Complex and Ryoke Plutonic Complex. The Ryoke Metamorphic Complex consists of Late Cretaceous low-pressure/high-temperature metamorphic rocks. The main protoliths of the Ryoke Metamorphic Complex are sedimentary rocks of a Jurassic accretionary complex. The Ryoke Plutonic Complex consists mainly of granitic rocks, with minor amounts of dioritic and gabbroic rocks. Monazite and zircon U–Pb dating of the Ryoke Metamorphic Complex and Ryoke Plutonic Complex has yielded ages of ca. 102–98 and 95–69 Ma, respectively (Takatsuka et al. 2018, and references therein). It is important to determine the heat source(s) of the high-T metamorphism and associated granitic magmatism to understand the evolution of island arc crust. In general, mafic magmatic processes are key in the transfer of heat and materials from the mantle to crust. In this respect, the possible products of mantle-derived mafic magmas in the Ryoke belt are small bodies of gabbroic rocks (e.g., Noto 1977; Tainosho et al. 1989; Kagami et al. 1995; Kutsukake 2000; Takagi et al. 2010). Some of these gabbroic rocks are closely associated with MMEs in the granitic rocks, which clearly indicate there was coeval mafic and silicic magmatism (e.g., Ishihara et al. 2003; Nakajima et al. 2004; Ishihara and Chappell 2007).

Fig. 1

(a) Simplified tectonic map of Japan and eastern Eurasia. (b) Geological map of the Akechi District (after Yamasaki et al. 2020). The white areas in (b) indicate Neogene and Quaternary strata, rivers, and lakes

In this paper, I present petrological observations and geochemical data for gabbros and MMEs in the Inagawa Granite of the Ryoke Plutonic Complex, and discuss the interactions between coexisting mafic and silicic magmas, petrogenetic relationships between the fine-grained mafic rocks and surrounding gabbros, and magma mixing in a mid-crustal magma chamber.

Geology of the study area

The Akechi District is located in the western Mikawa–Tono region, including the prefectural boundary between northeastern Aichi and southeastern Gifu prefectures and the northern part of the Mikawa Plateau (Fig. 1a–b). Granitic rocks of the Ryoke Plutonic Complex cover nearly the entire area of the Akechi District, with minor outcrops of metamorphic rocks of the Ryoke Metamorphic Complex and Neogene strata (Fig. 1b). The Ryoke Metamorphic Complex consists of meta-mudstone, -sandstone, and -siliceous rocks (i.e., meta-chert). The rocks of the Ryoke Metamorphic Complex in the Akechi District generally strike NE–SW to ENE–WSW, and have a metamorphic mineral assemblage of K-feldspar + cordierite, corresponding to the K-feldspar–cordierite zone that is part of the contact aureole of the Ryoke Plutonic Rocks (Miyazaki 2010; Yamasaki et al. 2020). Although the exact age of the contact aureole has not been reported, zircons in metamorphic rocks in the surrounding area show two stages of rim growth that are texturally, chemically and chronologically distinct; CL-bright, low-U inner rims with ca. 97 Ma are overgrown by CL-dark, high-U outer rims with ca. 89 Ma (Takatsuka et al. 2018), suggesting contact metamorphism due to granitic intrusion after the regional metamorphism. The Ryoke Plutonic Rocks in the Akechi District consist of the Late Cretaceous Inagawa Granite, Busetsu Granite, and biotite granite (Fig. 1b). The Inagawa Granite is subdivided into four lithological units: massive, gneissose, weakly gneissose, and foliated melanocratic facies (Yamasaki et al. 2020).

The massive facies of the Inagawa Granite consists mainly of coarse- to medium-grained (hornblende)–biotite monzogranite and granodiorite. The gneissose facies consists mainly of coarse- to medium-grained hornblende–biotite granodiorite and tonalite, which is characterized by an obvious gneissose texture and more common occurrence of K-feldspar megacrysts than in the massive facies. The weakly gneissose facies has gradational contacts with the gneissose and massive facies, and comprises coarse-grained (hornblende)–biotite monzogranite and granodiorite. The foliated melanocratic facies consists mainly of medium-grained, hornblende–biotite granodiorite, with minor amounts of hornblende–biotite monzogranite. This lithology is characterized by abundant mafic inclusions that commonly range in length from 10 to 50 cm. Such mafic inclusions were very rare in the gneissose facies and almost completely absent in the weakly gneissose facies and massive facies, except at the periphery of the gabbro body. The Inagawa Granite intrudes the Ryoke Metamorphic Complex, and is itself intruded by the Busetsu Granite. The Busetsu Granite consists mainly of medium- to fine-grained muscovite–biotite granodiorite. The biotite granite is dominantly medium-grained biotite granodiorite and monzogranite, with minor amounts of quartz monzonite and syenogranite.

In addition to the aforementioned lithologies, several mafic plutonic bodies that are up to 4 × 2 km in size are distributed within the Inagawa Granite (Fig. 1b). These mafic rocks consist of medium- to fine-grained quartz- and biotite-bearing hornblende gabbro. The fine-grained gabbro exhibits magma mixing features, and was coeval with the Inagawa Granite. Detailed mode of occurrence of the coeval magmatism is described in later section.

Sample preparation and analytical methods

Sample collection and preparation

Studied samples were collected from fresh and representative part of outcrops. To ensure the representativeness of the samples, samples of 10–15 cm square were taken according to the grain size of the samples. The sample was split into two pieces using a rock cutter, and a thin section was made from one side, and a sample for whole-rock geochemical analysis was made from the other side.

Analytical methods

Mineral compositions and back-scattered electron images were obtained using a JEOL JXA-8800R electron microprobe analyzer at the GSJ-Lab. A 12 nA beam current, 15 kV accelerating voltage, and 2 μm beam spot size were used. Eleven elements (Si, Ti, Al, Cr, Ni, Fe, Mn, Mg, Ca, Na, and K) were analysed as divalent oxides in wavelength-dispersive X-ray spectrometry modes. The measured X-ray lines are Kα for all elements. X-ray intensities for each peak and each background were integrated over 20 and 10 s, respectively. Silicates and oxides including forsterite (Mg), manganese ferrite (Mn, Fe), jadite (Na, Al, Si), wollastonite (Ca), pyrophanite (Ti), K-feldspar (K), nickel oxide (Ni), and eskolaite (Cr) were used as calibration standards. Matrix corrections were performed using the ZAF quantitative calculation for oxides. Selected data for plagioclase, amphibole, and biotite are listed in Table 1.

Table 1 Selected mineral chemistry of constituent minerals in the gabbros, mafic magmatic enclave, Inagawa Granite in the Akechi District

Powdered rock samples were prepared for X-ray fluorescence (XRF) spectrometry and whole-rock laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analyses. The rock samples were first cut into slabs that were a few millimeters thick, and the surfaces of the slabs were scraped with a diamond disk to remove any contamination from the saw. The slabs were then cleaned with ultrapure water in an ultrasonic bath and dried in an oven for > 24 h. The dried samples were coarsely crushed in a tungsten carbide mortar. Volume of the coarsely crushed samples were reduced by coning and quartering procedure. The reduced samples were then powdered using an agate mill.

Glass beads were prepared using the following methods. About 1.0 g of powdered samples were weighed in a ceramic crucible and ignited in a muffle furnace for 2 h at 900 ˚C. The weight difference between before and after heating was defined as loss on ignition (LOI). Glass beads were prepared by mixing 0.5 g of ignited powdered rock sample and 5.0 g of lithium tetraborate (Li₂B₄O₇: MERK Co. Ltd., Spectromelt A10, #1.10783) flux (10 times the amount of the standard powder sample). The mixture was put into a platinum crucible (95% Pt-5% Au alloy) and two drops of lithium bromide aqueous solution (LiBrH₂O: H₂O = 1:1) were added as exfoliation agent. Fusing and agitation were carried out with an automated high frequency bead sampler (Tokyo Kagaku Co. Ltd. TK-4500); at 120 s prefusion (~ 1070 °C), 180 s fusion (~ 1070 °C), and 180 s agitation. Whole-rock major element compositions were measured with a PANalytical Axios XRF spectrometer at the Geological Survey of Japan (GSJ) Laboratory (GSJ-Lab), Tsukuba, Japan, following the methods of Yamasaki (2014). A Rh anode X-ray tube was used and Kα line was measured for all elements. X-ray intensities at both lower and higher angles against a peak position were used for background corrections, except for Mg and Na. For Mg and Na, one point at a higher angle against a peak position was used to avoid overlapping the slope of the adjacent peak. Tube currents and voltages were 50 mA and 50 kV, respectively. The calibration curves were calculated using software equipped with PANalytical Axios, and the linear functions were adopted for all elements. Matrix corrections were also carried out using software equipped with the instrument, and the de Jongh model was adopted. Since both the standard and unknown samples were ignited before bead preparation, concentration of the calibration standard and the measurement results of the unknown sample were given as anhydrous basis, and Fe content was given as total Fe₂O₃. XRF data quality was monitored by analysis of the USGS geochemical reference materials, G-2 (Gladney and Roelandts 1988) (Table 2).

Table 2 Whole rock major element (wt%) and trace element (ppm) compositions of the gabbros, mafic magmatic enclave, Inagawa Granite in the Akechi District and reference materials

Whole-rock trace element compositions were measured with a LA–ICP–MS system at the GSJ-Lab, which consists of a New Wave Research NWR213 LA system coupled to an Agilent 7700 × quadrupole ICP–MS. The high-dilution ratio (sample:flux = 1:10) glass bead for XRF analyses were used for the LA–ICP-MS analyses. Argon (Ar) gas is used to plasma, auxiliary and nebuliser (carrier) gas. Helium (He) gas is used to flush the ablated material out of the laser cell, and is then mixed with Ar gas just before entry into the ICP-MS. Flow rates of the He carrier gas were 0.5 l min⁻¹. The production rate of oxide was monitored by ²⁴⁸ThO/²³²Th and was maintained below 0.5%. A spot size of 100 µm and a laser emission repetition rate of 10 Hz were used as laser settings. 50 sweeps of 33 elements from ⁴⁵Sc to ²³⁸U with dwell times 0.05–0.4 s were replicated three times in the peak hopping and spectrum modes. All signal intensities were corrected with respect to the background signal obtained from the measurement of a gas blank for 40 s prior to initiating the calibration standard and unknown measurements. The internal standard ⁴²Ca was used for all the measurements in this study. The GSJ geochemical reference material JB-2 were used as the external calibration standard material. Data reduction was conducted using MassHunter Workstation software installed with the Agilent 7700x. Calibration lines were calculated with the calibration standard, and a series of data reduction, which involved subtraction of the gas blank intensity and calculation of the concentration after normalization using the internal standard element, was performed with the MassHunter software. The precision and accuracy of the analytical results of the GSJ geochemical reference material was mostly < 30%. Lower limits of detection (DL) were < 0.01 ppm for all elements (Table 2). The details of this method were described by Yamasaki and Yamashita (2016). LA–ICP–MS data quality was monitored by analysis of the GSJ geochemical reference material JA-1 (Imai et al. 1995) (Table 2).

Mode of occurrence and petrography

The gabbroic bodies are mainly distributed in the Higashi-hagihira, Akasawa-gawa, and Tatsuhara areas (Fig. 1b). Although the sizes of the gabbroic bodies vary, the mode of occurrence is similar at all locations. The gabbros occur as lumps or blocks that are a few meters wide along ridges and on hillsides (Fig. 2a). The fine-grained gabbros occur as mafic microgranular enclaves (MMEs) in the massive facies of the Inagawa Granite (Fig. 2b–f). Although the enclaves generally have a spindle shape (several tens of centimeters in length), angular enclaves are also present (Fig. 2b). In some cases, the contacts between the MMEs and host granite are irregular (Fig. 2c–f), without chilled margins (Fig. 2d–e). The irregularly shaped MMEs may be intruded by small veinlets of granite (Fig. 2c and f). These modes of occurrence are consistent with those of MMEs (Didier and Barbarin 1991), which suggest the coexistence of mafic and silicic magmas. In particular, Barbarin and Didier (1991) suggested that crenulate to cuspate contacts (typically shown as Fig. 2f) occurred especially between two liquids that had strong temperature, composition, and rheology contrasts. Whereas numerous spindle-shaped mafic enclaves were existed in the foliated mela-facies, existence of the irregularly shaped MMEs were restricted to the periphery of the gabbro body.

Fig. 2

Field occurrence of gabbros and mafic microgranular enclaves (MMEs) in the study area. (a) Mode of occurrence of the gabbro outcrops. The gabbros occur as lumps or blocks that are a few meters wide along ridges and on hillsides. (b) Angular MMEs of various sizes and colors. The areas surrounded by dashed lines (shown by arrows) are very pale-colored enclaves. (c) Irregularly shaped MMEs. The arrow indicates a small granitic veinlet intruding a MME. (d) Irregularly shaped MME. The arrow indicates the location of the close-up shown in (e). (e) Contact between a MME and host granite. A chilled margin is not observed in either the MME or granite. (f) Irregularly shaped mixture of mafic and granitic magmas

The gabbros are medium- to coarse-grained hornblende gabbro and consist mainly of plagioclase, hornblende, and cummingtonite (Fig. 3a–b), along with minor biotite, quartz, apatite, zircon, and ilmenite, and rare clinopyroxene, orthopyroxene, and K-feldspar. The gabbros have a seriate texture, with a grain size of 0.1–6.0 mm. Plagioclase is euhedral and occurs as phenocrysts (5.0–6.0 mm in length) and relatively small crystals (0.1–1.0 mm in length) (Fig. 3c). The large crystals commonly exhibit prominent dusty zoning and clear rims (Fig. 3c–e). Skeletal and dendritic plagioclase phenocrysts with patchy zoning, including boxy and spongy cellular plagioclase (Hibbard 1995), occur in the gabbros (Fig. 3d–e). Hornblende occurs mainly as interstitial and poikilitic crystals, and exhibits brown to pale brown pleochroism. Distinctive rims with a greenish brown color characterize some hornblende. Corroded clinopyroxene crystals are occasionally observed in the cores of brown hornblende. Cummingtonite is subhedral–euhedral (0.5–1.0 mm in length), nearly colorless, and commonly exhibits polysynthetic twinning. Orthopyroxene is rarely associated with cummingtonite. Ilmenite occurs as interstitial (Fig. 3a) and discrete subhedral crystals (Fig. 3f). Quartz occurs as interstitial crystals and bleb-like inclusions in phenocrystic plagioclase. In some cases, a quartz–hornblende ocellar texture (Hibbard 1981) is observed in the gabbros (Fig. 3g). This texture comprises a relatively large quartz crystal with small hornblende and cummingtonite, and rare biotite crystals in its rim. Euhedral cummingtonite crystals also occur in the quartz crystals (Fig. 3h). K-feldspar occurs as anhedral interstitial crystals. Biotite is subhedral–euhedral (0.1–2.5 mm) and exhibits dark to pale brown pleochroism.

Fig. 3

Photomicrographs of the (a–g) gabbros, (h) mafic microgranular enclaves, and (i) host Inagawa Granite. Field of view = 4.5 mm. (a)–(b) and (f)–(h) are in plane-polarized light. (c)–(e) and (i) are in cross-polarized light. Pl = plagioclase; Cpx = clinopyroxene; Hbl = hornblende; Cum = cummingtonite; Ilm = ilmenite; Qz = quartz; Bt = biotite; Zrn = zircon. See text for a detailed explanation

The fine-grained gabbros consist of quartz–biotite-bearing hornblende gabbro. The gabbro is holocrystalline, equigranular, and consists mainly of plagioclase, hornblende, and biotite, with minor quartz, apatite, zircon, and ilmenite. Plagioclase is subhedral and mostly 0.1–0.5 mm in length. Several subhedral phenocrystic crystals (up to 2.0 mm in length) were also observed. These crystals exhibit conspicuous zoning, including dusty zones in crystal mantles, and abundant corroded hornblende inclusions in crystal cores (Fig. 2h). Biotite is subhedral (0.1–0.5 mm in length) and exhibits dark to pale brown pleochroism. Hornblende is subhedral, 0.1–0.5 mm in length, and exhibits greenish brown to pale greenish brown pleochroism. Quartz occurs interstitially and as pool-like aggregates.

The massive facies is distributed around the gabbro body, and is also the host rock of the MMEs in the Higashi-hagihira area. The granite consists of coarse- to medium-grained biotite monzogranite, hornblende-bearing biotite monzogranite, and hornblende-bearing biotite granodiorite. The granite is homogeneous and does not contain inclusions or xenoliths, except at the periphery of the gabbro body. The granite has a seriate texture and grain size of 0.5–5.0 mm. The granite consists mainly of plagioclase, quartz, K-feldspar, biotite, and hornblende, with minor apatite, zircon, allanite, and opaque minerals (Fig. 3i). Plagioclase is euhedral–subhedral (0.1–5.0 mm in length) and zoned. Quartz occurs interstitially to other minerals and commonly exhibits weak undulatory extinction. K-feldspar also occurs interstitially and locally exhibits a perthitic texture. Biotite is subhedral (0.5–4.0 mm in length) and exhibits dark to pale brown pleochroism. Hornblende is subhedral, 0.5–4.5 mm in length, and generally exhibits dark greenish brown to brown pleochroism. The granite contains accessory euhedral allanite (0.1–0.5 mm in length), which exhibits strong reddish to pale brown pleochroism.

Mineral and whole-rock geochemistry

Mineral chemistry

Anorthite contents [An = 100 × Ca/(Ca + Na + K) in moles] of both plagioclase cores and rims in the Inagawa Granite are An_45–27 (Fig. 4). Plagioclase in the MMEs has systematically higher An contents (An_60–39) relative to plagioclase in the granite (Fig. 4). Plagioclase compositions in the gabbroic rocks are bimodal (An_89–22 with a compositional gap at An_68–64; Fig. 4). The low-An plagioclase has compositions similar to those in the MMEs and granite. High- and low-An plagioclase generally corresponds to core and rim compositions of each plagioclase crystal, respectively, but complex distributions of high- and low-An plagioclase can be observed in some single crystals. For example, skeletal and dendritic plagioclase phenocrysts with patchy zoning have irregularly shaped, patchy, high-An areas surrounded by low-An areas, including rims (Fig. 5).

Fig. 4

Histograms showing plagioclase anorthite contents (An = 100 × Ca/[Ca + Na + K] in moles) in the Inagawa Granite, mafic microgranular enclaves (MMEs), and gabbros. The gabbros have a bimodal distribution of An contents. HH area = Higashi-hagihira area; TS area = Tatsuhara area; AK area = Akasawa-gawa area

Fig. 5

Back-scattered electron image and plagioclase anorthite contents for skeletal plagioclase phenocrysts in gabbro (sample #AK126A). Light-colored areas in the back-scattered electron image have high An contents; other areas, including the clear rim, have low An contents. Both high- and low-An areas have bimodal compositions and each compositional group has relatively uniform An contents

Amphiboles in the gabbroic rocks can be chemically classified into two sub-groups based on the International Mineralogical Association classification scheme (Hawthorne et al. 2012): Mg–Fe–Mn and Ca amphiboles. The Mg–Fe–Mn amphiboles are cummingtonite and grunerite. Most of the Ca amphiboles are magnesio-hornblende, with lesser amounts of actinolite, ferro-hornblende, and ferro-actinolite. Amphiboles in the MMEs are magnesio-hornblende. The Ca amphiboles in the gabbroic rocks and MMEs form a single trend characterized by decreasing Si and increasing Ti and Mn, with decreasing Mg/(Mg + Fe) (Fig. 6a–c). The Mg–Fe–Mn amphiboles are characterized by higher Si and lower Ti and Mn contents relative to the Ca amphiboles at a given Mg/(Mg + Fe) ratio. Biotite in the MMEs and gabbroic rocks have the same compositions (Fig. 6d–f), which are different to the biotites in the Inagawa Granite in terms of Fe/(Fe + Mg) ratios. Mn contents of biotite in the Inagawa Granite are higher than those in the gabbroic rocks and MMEs (Fig. 6f).

Fig. 6

Amphibole and biotite compositions in the gabbros, mafic microgranular enclaves (MMEs), and Inagawa Granite. The classification scheme for amphibole is after Hawthorne et al. (2012)

Whole-rock geochemistry

Whole-rock SiO₂ contents of the gabbroic rocks vary from 51.26 to 58.51 wt.% (Table 2; Fig. 7). All samples from the Akasawa-gawa area and one sample from the Tatsuhara area are TiO₂-rich, probably due to the presence of relatively large amounts of ilmenite. Two of these samples have high P₂O₅ contents. The MMEs are similar to the gabbroic rocks in terms of major element compositions (Fig. 7). SiO₂ contents of the Inagawa Granite vary from 68.64 to 71.92 wt.% and are clearly different from those of the gabbroic rocks and MMEs. However, the whole-rock compositions of most of the gabbroic rocks and MMEs likely exhibit a compositional trend that vectors towards the Inagawa Granite (Fig. 7).

Fig. 7

Whole-rock major element–SiO₂ diagrams for the Inagawa Granite, mafic microgranular enclaves (MMEs), and gabbros. Gray arrows show the inferred compositional trends between the mafic rocks and granites

Normal-type mid-ocean ridge basalt (N-MORB)-normalized multi-element patterns of the gabbroic rocks slope gently up to the left, with negative Nb–Ta and small positive Pb anomalies (Fig. 8a). Chondrite-normalized rare earth element (REE) patterns show slight enrichment in light REEs (LREEs; La/Yb_N = 3.44–9.85; Fig. 8b). The trace element patterns of the MMEs completely overlap those of the gabbroic rocks (Fig. 8c, d). N-MORB-normalized multi-element patterns of the Inagawa Granite are similar to those of the gabbroic rocks. The Inagawa Granite exhibits marked negative Ti anomalies and enrichments in Ba, Th, U, La, and Ce, as compared with the gabbroic rocks (Fig. 8e). The Inagawa Granite exhibits two types of chondrite-normalized REE patterns. One is similar (La/Yb_N = 4.01–9.60) to the gabbroic rocks, and the other is LREE-enriched (La/Yb_N = 10.63–20.20; Fig. 8f). Both types of patterns are characterized by negative Eu anomalies.

Fig. 8

Whole-rock trace element patterns for the Inagawa Granite, mafic microgranular enclaves (MMEs), and gabbros. (a), (c) and (e) N-MORB-normalized trace element patterns. (b), (d) and (f) C1 chondrite-normalized REE patterns. Chondrite and N-MORB values are from Sun and McDonough (1989). Symbols are as in Fig. 7. Gray areas in (c)–(f) correspond to those in (a) and (b)

Discussion

Interpretation of the textures and plagioclase compositions

MMEs in the Inagawa Granite exhibit a gradation in color from dark to light gray within a single MME (Fig. 2e), and typical mingling textures. In addition, the colors of MMEs in a single outcrop are commonly variable (Fig. 2b). These modes of occurrence suggest that variable hybridization has occurred between the mafic and silicic (granitic) magmas. The common dusty-zoned plagioclase and unusual mineral assemblages, such as the quartz–hornblende ocellar texture and large amount of biotite in the pyroxene-bearing gabbros, are likely disequilibrium mineral assemblages in the gabbroic rocks and cannot be explained by normal fractional crystallization processes. Plagioclase in the gabbroic rocks exhibits conspicuous chemical zoning and patchy high-An areas (Fig. 5). The chemical compositions of the relatively low-An areas are the same as those of the clear plagioclase rims (Fig. 5). The most important feature of the plagioclase compositions is the clear compositional gap between the low- and high-An plagioclase within a single crystal (i.e., the bimodal An contents; Fig. 4). In contrast, plagioclase in the Inagawa Granite and MMEs is characterized by unimodal compositions.

Plagioclase compositions of the gabbroic rocks are concentrated between An_70–80 (cores) and An_36–60 (rims) (Fig. 4). This rim composition is consistent with the plagioclase compositions in the MMEs. Given the close association with the gabbroic bodies, the mafic melt that formed the MMEs was likely the parental melt that crystallized the plagioclase rims in the gabbroic rocks. The texture of the gabbroic rocks is inconsistent with melt intrusion after solidification, but records the infiltration of late-stage, possibly co-genetic, melt or in situ differentiation of interstitial melt in a crystal mush. However, the latter should have produced continuous compositional zoning in plagioclase. Thus, the plagioclase compositional gap is inferred to reflect an abrupt change in crystallization conditions that resulted in disequilibrium.

Dusty zoning and patchy high-An/low-An regions in plagioclase crystals are typical petrographic features of magma mixing (e.g., Hibbard 1981). Highly elongate interstitial biotite and apatite in gabbroic rocks is also characteristic of magma mixing (i.e., bladed biotite and acicular apatite; Hibbard 1991, 1995). The quartz–hornblende ocellar texture (Hibbard 1981) comprises relatively large quartz crystals surrounded by small hornblende and cummingtonite crystals (Fig. 3g). Given that the corroded clinopyroxenes in hornblende cores are interpreted as the product of interactions between clinopyroxene and melt, the cummingtonite was probably a product of interaction between orthopyroxene and melt. The ocellar texture and plagioclase compositions can be explained by mixing between the parental magma of the gabbros and a relatively low-temperature silicic melt that formed the Inagawa Granite.

The plagioclase compositional gap can be explained by thermal and compositional re-equilibration caused by mixing between a large amount of silicic magma and a relatively small amount of mafic magma. The phase diagram for the anorthite–albite binary system (Fig. 9a) shows that the thermal conditions (T₁) control the crystallizing plagioclase composition (M) for a range of melt compositions (C_M). Mixing between a large amount of silicic melt and a relatively small amount of mafic melt increases the temperature of the silicic melt from T₂ to T₃, and decreases the temperature of the mafic melt from T₁ to T₃. The composition of the hybrid melt is within the range of C_M. A sudden decrease in the magma temperature from T₁ to T₃, and subsequent rapid thermal relaxation, yields a plagioclase compositional gap (M to H; Fig. 9a). This process cannot be achieved by a rapid change in physical conditions and subsequent slow cooling caused by upwelling of mafic magma through a dike, and requires uniform thermal conditions and re-equilibration after the abrupt change of magma temperature. Given that thermal diffusivity is > 10⁵ times higher than chemical diffusivity in a magma (e.g., Watson 1994; Eriksson et al. 2003), thermal relaxation is followed by chemical homogenization.

Fig. 9

Schematic phase diagrams for the (a) anorthite–albite and (b) forsterite–enstatite–silica systems. Ab = albite; An = anorthite; Fo = forsterite; En = enstatite; Qz = quartz (silica). C_M, C_F, and C_H are hypothetical compositions of mafic, felsic, and hybrid melts, respectively. F, M, and H are hypothetical compositions of felsic, mafic and hybrid melts in equilibrium with the mineral(s), respectively. Gray arrows in (a) and (b) denote the temperature and compositional changes due to mixing between mafic and silicic magmas. The black arrow in (b) is the crystallization path after mixing. The blue area in (a) shows the compositional range of hypothetical melts (C_M) that crystallize An_M plagioclase at a temperature T₁. See the text for further discussion

For crystallization of the mafic minerals, the compositional change is an important factor, in addition to the temperature change. Assuming the melt is silica-saturated or -undersaturated and coexisting with An_70–90 plagioclase (C_M in Fig. 9b), mixing with hypothetical silicic melt that has a composition C_F in Fig. 9b yields a silica-oversaturated melt C_H. The melt will crystallize quartz at H₁, and finally crystallize quartz and orthopyroxene at the eutectic point H₂ (Fig. 9b). Although an orthopyroxene–quartz assemblage can be produced by a silica-saturated melt at the eutectic point H₂, the ocellar texture indicates quartz crystallized after orthopyroxene (Fig. 3g), suggesting the presence of silica-oversaturated domains. Interstitial quartz in the gabbroic rocks (e.g., Fig. 3b) can also be explained by the same mechanism. Abundant hornblende crystals associated with cummingtonite can also be explained by the reaction between the mafic and silicic melts by “hydration crystallization” (Beard et al. 2005). The hydration crystallization reactions involving amphibole are shown as hydrous (felsic) melt + pyroxene + Fe-Ti oxides ± calcic plagioclase = amphibole + quartz ± sodic plagioclase (Beard et al., 2005).

The quartz ocellar texture had been explained by reaction between hybrid or mafic melt and an existing quartz crystal from the granitic magma that has been incorporated into the mafic magma during mingling (e.g., Hibbard 1991). However, when quartz crystals are incorporated into a high-temperature basaltic magma body, the composition of the magma is assumed to remain unchanged from point C_M (Fig. 9b), and the silica component will be completely consumed. The hydration crystallization also involved melts, not quartz xenocrysts (Beard et al. 2005). The xenocrystic origin of the ocellar texture may be possible in case of intermediate composition and/or relatively small amount of mafic magma, such as MMEs within the granite, but in the gabbroic mush, mixing with the liquid state is more reasonably explainable.

Whole-rock geochemical variations

The mafic–silicic magma hybridization inferred from the outcrop and microscopic observations should be recorded by the whole-rock geochemical compositions. However, it is important to assess whether the whole-rock compositions of the gabbros are actually melt compositions. In general, the behavior of phases that are rich in specific elements, such as Fe–Ti oxides or apatite, can be used to identify cumulates. Cumulates that formed before the crystallization of these phases have very low TiO₂ and P₂O₅ contents relative to the melt (i.e., magma) composition. The TiO₂ and P₂O₅ contents of the studied gabbroic rocks, except for samples from the Akasawa-gawa area and one sample from the Tatsuhara area, are 0.6–1.3 and 0.1–0.4 wt.%, respectively. These values are comparable with those of andesitic rocks in the northeastern Japan arc (e.g., TiO₂ = 0.6–1.2 wt.% and P₂O₅ = 0.1–0.3 wt.%; Hanyu et al. 2007). Whole-rock REE patterns can also be used to evaluate the effects of mineral accumulation. Plagioclase has relatively steep LREE-enriched patterns with marked positive Eu anomalies, whereas clinopyroxene and amphibole have slightly LREE-depleted and rather flat middle (M)REE–heavy (H)REE patterns. Thus, the accumulation of plagioclase results in slightly LREE-enriched patterns and positive Eu anomalies. Whereas the magnitude of positive Eu anomaly can be decrease with increasing modal abundances with clinopyroxene and amphibole, subtle negative Eu anomalies in those minerals cannot compensate for the positive Eu anomaly of plagioclase in cumulus rocks (e.g., Godard et al. 2009). The studied gabbroic rocks are slightly LREE-enriched and have no Eu anomalies, and these patterns are the same as in the MMEs, indicating their compositions reflect those of melts (Fig. 8b, d). As such, the gabbroic rocks, apart from the ilmenite-rich lithologies, could have melt compositions.

Harker diagrams are commonly used to examine the whole-rock geochemistry of granitic rocks, but chemical interactions between two end-members with large differences in SiO₂ contents and differentiation trends of silicic (granitic) magmas result in similar elemental trends; and, consequently, Harker diagrams cannot distinguish these processes. In contrast, whole-rock FeO*/MgO ratios can be used to examine the differentiation of gabbroic rocks during the period before the saturation of Fe–Ti oxides, because those ratios reflect FeO*/MgO ratio of constituent mafic minerals that increase with differentiation of parental magma. Given that the An contents of plagioclase decrease with differentiation, a correlation between plagioclase An contents and whole-rock FeO*/MgO ratios would be expected for differentiation of the gabbroic rocks, when there has been no compositional modification by magma mixing. The data exhibit a slight decrease in the An contents of the high-An plagioclase (i.e., cores) with increasing whole-rock FeO*/MgO ratios, although the trends are different for each area, suggesting there was not a common parental magma (Fig. 10a). In addition, the low-An suite shows nearly constant An contents (i.e., similar to rim values) for a range of whole-rock FeO*/MgO ratios (Fig. 10a). Both the high- and low-An plagioclase exhibit no systematic trends with increasing whole-rock SiO₂ contents, and if SiO₂ is the indicators of differentiation, then meaning of the SiO₂ content of samples from the Akasawa-gawa area are largely inconsistent with whole-rock FeO*/MgO (Fig. 10b). These observations suggest that the whole-rock FeO*/MgO ratios and SiO₂ contents cannot be used as indicators of magmatic differentiation.

Fig. 10

Plot of average plagioclase An content and ranges of An versus whole-rock FeO*/MgO and SiO₂ for the gabbros. Symbols are as in Fig. 7. Filled and open symbols denote high- and low-An (i.e., core and rim compositions) plagioclase, respectively. The high-An core compositions exhibit at least two discrete decreasing trends with increasing FeO*/MgO, and no clear trend with increasing SiO₂. The low-An rim compositions exhibit no systematic trends and have nearly constant An contents regardless of whole-rock FeO*/MgO and SiO₂

As such, correlations between whole-rock FeO*/MgO ratios and major element contents in terms of magma-mixing were examined. The estimation of end-members are the most important issue for the evaluation. A silicic end-member (sample #AK036) was chosen, which is a sample from a homogeneous outcrop of the Inagawa Granite that is relatively close to an outcrop exhibiting magma mixing. However, all the studied gabbroic rocks exhibit some mixing (based on petrographic observations), and do not represent pre-mixing melt compositions. Therefore, a sample that experienced the least mixing (sample #AK130) was used as the mafic end-member. The results of a simple mixing calculation between the two end-members is shown in Fig. 11. Calculated results of the differentiation of the mafic end-member using rhyolite-MELTS (Gualda et al. 2012; Ghiorso and Gualda 2016) are also shown in Fig. 11. The parameters for the calculations were as follows: the pressure was assumed to be the lower limit for the biotite zone (2.9–3.7 kbar) in the Ryoke Metamorphic Complex (Miyazaki 2010); the magma water content was assumed to be 5 wt.%, which is slightly lower than the saturation point at the assumed pressure, based on the maximum water content of a basaltic magma (e.g., ~ 5 wt.% at 2 kbar and ~ 6 wt.% at 3 kbar; Newman and Lowenstern 2002); and 5 ˚C steps of fractional crystallization at a redox state of FMQ–1.

Fig. 11

Results of mixing calculations between the gabbros and Inagawa Granite, along with fractional crystallization modeling of mafic magma using rhyolite-MELTS, plotted on major element–FeO*/MgO diagrams. Gray lines with a black cross indicate the results of a mixing calculation. Numbers denote the percentage of the granite component at 10% intervals. Green x's indicate the fractional crystallization results obtained using rhyolite-MELTS (Gualda et al. 2012; Ghiorso and Gualda 2016). Other symbols are as in Fig. 7. A detailed explanation and the calculation method are given in the text

The MnO, CaO, Na₂O, and K₂O contents of the samples from the Higashi-hagihira area can be explained by mixing between mafic and silicic magmas. However, the SiO₂, TiO₂, and FeO* contents of these samples do not agree with the mixing trend. Large variations of these elements with little change in FeO*/MgO ratios cannot be explained by fractional crystallization, and thus can be qualitatively explained by magma mixing. The MMEs plot on the mixing trend for all elements. The samples from the Akasawa-gawa area do not plot on the mixing trend for the samples from the Higashi-hagihira area, but show similar elemental behavior, such as large variations in Al₂O₃, FeO*, MnO, CaO, Na₂O, and K₂O contents with little change in FeO*/MgO ratios. In contrast, the gabbroic rocks from the Tatsuhara area appear to be plot on a fractional crystallization trend (Fig. 11). The overall whole-rock geochemistry indicates that some hybridization has occurred, although the MMEs have mingling textures and record incomplete mixing. In general, granitic contamination of a MME initially results from the rapid infiltration of granitic melt (e.g., Nédélec and Bpuchez 2015). The irregularly shaped quartz pools and quartz–hornblende ocellar textures indicate the intrusion of small veinlets or globules of silicic (granitic) melt into the gabbroic mush. In addition, diffusion processes across the surface contact of immiscible liquids (such as the contact between MMEs and granitic magma) are also an important hybridization mechanism (e.g., Debon 1991; Perugini et al. 2008; Morgavi et al. 2013). Therefore, actual hybridization processes that produce mixing lines in geochemical plots are the result of two phenomena. In contrast, the high TiO₂ and P₂O₅ contents (not shown) of some gabbroic rocks cannot be explained by either mixing or fractionation, and probably reflect ilmenite and apatite accumulation. In the case of the gabbroic magmas in the Akasawa-gawa and Tatsuhara areas, ilmenite and apatite had probably already started to crystallize. Therefore, the TiO₂ and P₂O₅ contents of the hybridized magmas were controlled by mechanical transfer of these minerals between the mafic and silicic magmas. Mechanical transport properties of minerals depend largely on the magma rheology, and trace element transfer can be controlled by such phases that are enriched in specific elements (e.g., zircon).

Irrespective of the complexity of elemental behavior, as a first-order interpretation, the main whole-rock geochemical features can be explained by mixing between mafic magmas and silicic magmas of the Inagawa Granite. The mafic end-member was already partially hybridized. Thus, the actual mafic end-member must have been more primitive in composition. The granitic magmas had a relatively wide range of compositions (Fig. 11), and the silicic end-member may not have been the selected composition. In reality, mixing and hybridization can occur between both mafic and silicic compositions that have experienced variable degrees of differentiation. If the mixing trend is extrapolated to a basaltic composition, the mafic end-member (Fig. 12) is similar to a primitive, low-K basaltic magma from the northeastern Japan arc (e.g., Ryozen Formation; SiO₂ = 48.51–51.38 wt.%, FeO* = 8.24–9.71 wt.%, MnO = 0.15–0.18 wt.%, and K₂O = 0.13–0.59 wt.%; Shuto et al. 2013). Mixing between end-member compositions that were variably differentiated can explain the geochemical variations of the studied gabbroic rocks (Fig. 12). This is a hybridization and fractional crystallization (HFC) process, similar to the assimilation and fractional crystallization (AFC) concept (DePaolo 1981).

Fig. 12

Schematic illustration of the relationship between the fractional crystallization (FC) paths for the mafic and silicic (granitic) magmas, and mixing between both magmas due to hybridization and fractional crystallization (HFC) processes. Gray arrows are FC paths for the mafic and silicic (granitic) magmas, and the gray dashed curves are the hybridization paths between the mafic and silicic (granitic) magmas. The dashed gray arrows are schematic HFC paths of hybrid melt. Gray and pink fields in the upper left panel (SiO₂) denote compositional range of basaltic andesite and andesite, respectively. The HFC process can produce a andesite composition. The open star is the inferred composition of the parental mafic melt. Other symbols are as in Fig. 11. See the text for details

Trace element behavior during magma mixing is more complicated. Although the gabbros broadly preserve melt compositions, the petrographic observations suggest that some crystals had already formed at the time of mixing. Therefore, mechanical transfer of such minerals would control the extent of hybridization, as well as the TiO₂ and P₂O₅ contents. A simple mixing calculation was undertaken between the granite (AK036) and gabbro (AK130) samples that have the most different compositions of the analyzed samples. The results demonstrate that < 30% hybridization of granitic magma can reproduce the incompatible element contents (Rb–Gd) in the gabbros and MMEs (Fig. 13). In contrast, relatively compatible elements within the REEs, such as the HREEs (Tb–Lu), show reversal content between the granite and gabbros, and this cannot be reproduced by addition of silicic (granitic) magma to mafic (gabbroic) magma. This may be due to the strong partitioning of these elements into amphibole in andesitic magmas (e.g., Kd_Yb = 1.1–1.8; Luhr and Carmichael 1980; Fujimaki et al. 1984).

Fig. 13

CI chondrite- and N-MORB-normalized trace element compositions of the gabbros, mafic microgranular enclaves (MMEs), and Inagawa Granite, along with mixing calculation results between the gabbros and Inagawa Granite. Chondrite and N-MORB values are from Sun and McDonough (1989). Numbers denote the percentage of the granite component. See the text for details

Implications for magma hybridization in the mid-crust

For MMEs, it is generally assumed that mingling, rather than hybridization, dominates the interaction between mafic and silicic melts (e.g., Vernon 1984, 1990; Sparks and Marshall 1986; Bateman 1995). In fact, the rheological contrast between basaltic (recharge) and granitic (resident) melts limits the mixing during replenishment of a silicic magma chamber (Jellinek and Kerr 1999; Jellinek et al. 1999). However, numerous petrological and geochemical studies, particularly from an isotopic perspective, have suggested that hybridization is a common magmatic process in the continental and arc crust (e.g., Eichelberger et al. 2006; Reubi and Blundy 2009; Kent et al. 2010; Clemens and Bezuidenhout 2014). In general, the mixing efficiency is strongly affected by physical properties and, in particular, the rheological properties of the end-members (e.g., Ubide et al. 2014, and references therein). Barbarin and Didier (1992) proposed that various types of enclaves are produced by the injection of mafic magma into granitic magma at different stages of crystallization of the latter. De Campos et al. (2011) noted that the geochemical trends produced by experimentally induced chaotic mixing between mafic and silicic melts depart significantly from linear trends between the two end-members on Harker diagrams. In addition, uphill chemical diffusion allows the concentrations of some elements to reach levels exceeding those of either end-member (De Campos et al. 2011). The variable mobility of different elements in a mixing system has been described previously (e.g., Perugini et al. 2008; Morgavi et al. 2013), and is known as “differential (selective) diffusion” (Debon 1991). Nevertheless, De Campos et al. (2011) noted that most of the compositions resulting from mixing still produce strongly linear trends.

The compositional variations and petrological features of the studied gabbros and MMEs suggest that the compositions of even the gabbros have been modified by mechanical entrapment of melt globules with or without diffusion processes. Compositional modification of silicic magma and the production of andesitic magma is commonly thought to result from magma mixing in a crustal magma chamber (e.g., Clynne et al. 1999; Tepley et al. 1999; Suzuki and Nakada 2007; Kent et al. 2010). However, compositional modification of mafic magma due to magma mixing–hybridization is also important in generating a diversity of mafic igneous rocks that cannot be produced by fractionation. Such modification of basaltic magma by the addition of silicic magmas is likely to be a common process in generating andesitic magmas. In fact, there have been many reports of basaltic–andesitic magmas that have mixed with silicic magma (e.g., Eichelberger 1975; Sakuyama 1979, 1981; Bloomfield and Arculus 1989). Importantly, mixing calculations between the studied mafic and silicic magmas in this study clearly demonstrate that magnesian andesite can be produced by the addition of a small amount of silicic melt into primitive low-K basaltic magma (Figs. 12–13). Such an origin for primitive andesite by magma mixing has been reported elsewhere (Streck et al. 2007; Beier et al. 2017). Therefore, at least in some cases, compositional modification of mafic magma by magma mixing–hybridization likely plays an important role for the formation of andesitic magma in continental and arc crust.

Since the early 1990s, research into the origins of granites has transitioned from geochemical approaches to understanding the underlying physical processes. Important insights from these studies include that most granitic plutons in the upper continental crust appear to have been emplaced as tabular intrusions fed from depth by small magma batches that ascend rapidly (within 100,000 yr; e.g., Petford et al. 2000). As such, melt generation and transport to the site of final emplacement is rapid, and thus lithological variability in granitic plutons is presumably caused mainly by crustal processes (e.g., Petford et al. 2000; Glazner et al. 2004; Miller et al. 2011). This study demonstrates that such processes generate compositional variations in both granitic and mafic magmas at the final emplacement level in continental and arc crust.

Conclusions

The gabbroic rocks associated with the Inagawa Granite in the Akechi district, Japan, exhibit disequilibrium textures and mineral compositions, such as quartz–hornblende ocellar textures and patchy plagioclase crystals with bimodal An contents. These textures and compositions suggest that an abrupt decrease in crystallization temperature and mechanical mixing between crystallizing gabbroic mush and silicic (granitic) melt occurred. Similarly, MMEs in the Inagawa Granite exhibit mingling textures that indicate the coexistence of mafic and silicic magmas and incomplete mixing, although the geochemical compositions of the MMEs require substantial hybridization and homogenization. The geochemical variations of the gabbroic rocks can be reasonably explained by HFC processes between crystallizing gabbroic mush and granitic melt. Whole-rock trace element compositions of the gabbroic rocks can be broadly reproduced by < 30% hybridization with granitic magma, in terms of incompatible elements (Rb–Gd). However the model calculations do not work for some other elements, such as the HREEs (Tb–Lu), which suggests that these elements were strongly partitioned into crystallizing amphibole in the gabbroic magma. Extrapolation of the mixing trend to a basaltic composition suggests that the primitive composition of the mafic end-member is similar to a primitive, low-K basaltic magma in the northeastern Japan arc. Given that the HFC process yielded magnesian andesite by the addition of a small amount of silicic melt into a primitive mafic end-member, compositional modification of mafic magma caused by magma mixing–hybridization likely has an important role in andesitic magma formation in continental and arc crust.