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Volume 31 Issue 3
Jul.  2020
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Liangliang Huang, Liyuan Wang, Hong-Rui Fan, Musen Lin, Wenhui Zhang. Late Early-Cretaceous Magma Mixing in the Langqi Island, Fujian Province, China: Evidences from Petrology, Geochemistry and Zircon Geochronology. Journal of Earth Science, 2020, 31(3): 468-480. doi: 10.1007/s12583-019-1022-6
Citation: Liangliang Huang, Liyuan Wang, Hong-Rui Fan, Musen Lin, Wenhui Zhang. Late Early-Cretaceous Magma Mixing in the Langqi Island, Fujian Province, China: Evidences from Petrology, Geochemistry and Zircon Geochronology. Journal of Earth Science, 2020, 31(3): 468-480. doi: 10.1007/s12583-019-1022-6

Late Early-Cretaceous Magma Mixing in the Langqi Island, Fujian Province, China: Evidences from Petrology, Geochemistry and Zircon Geochronology

doi: 10.1007/s12583-019-1022-6
More Information
  • The mafic enclaves from Mesozoic intermediate-acid magmatic rocks, widely developed along Fujian coast, are considered to be the results of large-scale crust-mantle interaction and magma mixing. In this paper, petrography, mineralogy, and geochemistry of granites and mafic microgranular enclaves (MMEs) in Langqi Island are studied to provide new information for tracing crust-mantle interaction. The zircon U-Pb dating results show that the Langqi rocks were formed at ~101 Ma, which are metaluminous, enriched in silica and high-K calc-alkaline I-type granites. The enclaves have a typical magmatic structure, which is characterized by magma mixing between high-temperature basic magma and low-temperature acidic magma through injecting. The enclaves and host granites show a tendency to mixed major and trace elements, displaying a clear-cut contact relationship, which is indicative of coeval magmatism. The genesis of Langqi rocks is related to the extensional setting caused by the subduction of Paleo-Pacific Plate, and they are the results of mixing of subduction-related metasomatized mantle-derived mafic and induced crustal-melted granitic magma originating from partial melting of the crustal material.
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Late Early-Cretaceous Magma Mixing in the Langqi Island, Fujian Province, China: Evidences from Petrology, Geochemistry and Zircon Geochronology

doi: 10.1007/s12583-019-1022-6

Abstract: The mafic enclaves from Mesozoic intermediate-acid magmatic rocks, widely developed along Fujian coast, are considered to be the results of large-scale crust-mantle interaction and magma mixing. In this paper, petrography, mineralogy, and geochemistry of granites and mafic microgranular enclaves (MMEs) in Langqi Island are studied to provide new information for tracing crust-mantle interaction. The zircon U-Pb dating results show that the Langqi rocks were formed at ~101 Ma, which are metaluminous, enriched in silica and high-K calc-alkaline I-type granites. The enclaves have a typical magmatic structure, which is characterized by magma mixing between high-temperature basic magma and low-temperature acidic magma through injecting. The enclaves and host granites show a tendency to mixed major and trace elements, displaying a clear-cut contact relationship, which is indicative of coeval magmatism. The genesis of Langqi rocks is related to the extensional setting caused by the subduction of Paleo-Pacific Plate, and they are the results of mixing of subduction-related metasomatized mantle-derived mafic and induced crustal-melted granitic magma originating from partial melting of the crustal material.

Liangliang Huang, Liyuan Wang, Hong-Rui Fan, Musen Lin, Wenhui Zhang. Late Early-Cretaceous Magma Mixing in the Langqi Island, Fujian Province, China: Evidences from Petrology, Geochemistry and Zircon Geochronology. Journal of Earth Science, 2020, 31(3): 468-480. doi: 10.1007/s12583-019-1022-6
Citation: Liangliang Huang, Liyuan Wang, Hong-Rui Fan, Musen Lin, Wenhui Zhang. Late Early-Cretaceous Magma Mixing in the Langqi Island, Fujian Province, China: Evidences from Petrology, Geochemistry and Zircon Geochronology. Journal of Earth Science, 2020, 31(3): 468-480. doi: 10.1007/s12583-019-1022-6
  • There have been strong tectonic events and magmatic activities since the Yanshanian Period in South China, and the magmatic rocks in this district are diverse with a wide exposure area (Li et al., 2018; Mao et al., 2014; Guo et al., 2012; Tao et al., 2007; Li, 2000). The Mesozoic magmatism in South China inherited the material composition of Precambrian orogenic crust and lithospheric mantle, and was affected by the Paleo-Pacific Plate subduction and slab roll-back in geodynamic initiation mechanism (Zheng et al., 2013). Mantle-derived magma underplating induced by rollback of the subducted Paleo-Pacific Plate is the main mechanism for the formation of the medium-acid intrusive rocks. The tectonic deformation in different directions and the spatial and temporal distribution characteristics of magmatic rocks indicate that the Mesozoic magmatic rocks in South China are the results of multi-plate convergence and multi-directional extrusion- extension orogeny (Dong et al., 2015; Mao et al., 2014, 2013). The multi-stage basic-intermediate dyke swarms produced in the South China coast since the Early Jurassic may correspond to the multiple lithospheric extension structures. Heat and depleted material provided by the mantle-derived magma underplating in an extensional tectonic regime may play an important role in the genesis of contemporary granites (Hu et al., 2007). The Cretaceous crust-mantle interaction was intense in the Fujian coastal region, and the intermediate-acid volcanic-intrusive rocks were formed through varying degrees of mixing of the mantle-derived magma and subsequently induced crustal felsic magma by underplating (Liu et al., 2014; Yang et al., 2013). The Early Cretaceous magmatism in the coastal areas is characterized by the occurrence of a large number of I-A composite rock body and the emplacement of basic dikes. The mafic microgranular enclaves (MMEs) are common in granites from Fujian coastal areas. They have typical magmatic structure, unbalanced structure and acicular apatite, etc., indicating the development of magma mixing and crust-mantle interaction in this area. Magma mixing is one of the important reasons for the diversity of granites (Yang et al., 2007). The range of magma mixing is very broad, not limited to I-type granites, but also occurs in A-type and S-type granites, which can occur between mafic and felsic magma (Barbarin, 2005), as well as different crustal magma. The MMEs often indicate the existence of magma mingling and the occurrence of material and energy exchange between different endmember magma, which can provide important evidence for the initiation and evolution of calc-alkaline granitoid magma (Barbarin, 2005). In this study, the MMEs and their host granitic rocks in the Langqi area of Fujian, China, were studied through systemic petrography, petrology and geochemistry. The genesis of MMEs, the process of magmatic evolution and the model of crust-mantle magmatic interaction, are discussed, which has significant implications for understanding the mechanism of Mesozoic magma activities and the type of crustal growth in South China.

  • The study area is situated in the Langqi Island of Fuzhou, southeastern coast of Fujian Province, China. It is tectonically located in the east of Changle-Nanʼao fault zone (Fig. 1). The occurrence of a large number of I-A composite granites and basic-intermediate dykes since the Early Cretaceous indicates that the coastal areas of Fujian have already been in an extensional tectonic setting, and the granites are also considered to be related to the lithospheric extension by the roll-back of Paleo- Pacific Plate (Ding et al., 2015; Wang et al., 2013; Hu et al., 2007; Li et al., 1997). The granites exposed in the coastal areas of Fujian show good polar characteristics in the ocean direction, the ages of magmatic rocks are increasingly younger from inland to ocean, and the influence of mantle-derived components on the diagenesis process of granites in the region is continuously strengthened (Li et al., 2014). The Langqi pluton is located in the Mesozoic active fault subduction zone along the coast of Zhejiang, Fujian and Guangdong provinces. The intrusive strata consist of the Upper Jurassic Nanyuan Formation and the Lower Cretaceous Xiaoxi Formation.

    Figure 1.  Sketch geological map of the Langqi region.

  • The Langqi rocks are mainly off-white and reddish granites with medium-coarse grains on the whole, which have block and granitic structure (Fig. 2). The rocks are mainly composed of K-feldspar (35%-40%), quartz (25%-35%), plagioclase (15%-20%), biotite (2%-4%) and hornblende (3%) (Figs. 3a, 3c), in which the accessory minerals are zircon and titanite, etc. The K-feldspar is anhedral granular and scattered interstitial distribution with kaolinized and minor metasomatic plagioclase (Fig. 3d). Plagioclase is usually euhedral, subhedral, and long-prism texture, which generally develops polysynthetic twins and melting corrosion structure, and often develops sericitization in the interior of plagioclase (Fig. 3c). The helminth texture can be observed in locally eroded metasomatism of K-feldspar silkworm (Fig. 3). Hornblende and biotite are widely distributed in other minerals in the form of euhedral and subhedral crystal, and the former is often altered into chlorite or epidote (Fig. 3b).

    Figure 2.  Field photographs of the Langqi pluton. (a), (b) Macroscopic photographs of host-enclave relationships in the Langqi pluton; (c), (d) mafic enclaves varied in shape from ellipsoidal to stilliform with different sizes.

    Figure 3.  Representative photomicrographs (a)-(d) of host granites and enclosed mafic microgranular enclaves (e)-(i). The higher interference color of quartz and plagioclase is due to the thicker thin sections. Qtz. Quartz; Pl. plagioclase; Kfs. alkali feldspar; Amp. amphibole; Bt. biotite; Ap. apatite; Ep. epidote.

    The MMEs are commonly present in the Langqi pluton which are uneven distributed in space. The enclaves are various in shape, most of which are elliptical or goose-like, with a small number of angular or irregular ones. The relationship between the enclaves and the host rocks are mostly clear-cut contact (Figs. 2b, 2c). The edges of the enclaves are clear and the chilled borders are developed (Fig. 2d), and diameters of the enclaves are generally 3-20 cm. Their sizes are smaller than those of the host rocks, and they have porphyritic-granule and block structures. The MMEs are more uniform in color, significantly deeper than the host rocks, and mostly black (Fig. 2). The color ratio of MMEs is mainly related to the degree of magma mixing, that is, the lower the degree of magma mixing is, the higher the color ratio is. The sizes of the enclaves vary widely, which are generally smaller than the host rocks and mostly fine-grained.

    The MMEs have a relatively high content of dark minerals, and are mostly dioritic and partly gabbro, the main minerals of which include plagioclase (~50%), K-feldspar (~15%), hornblende (~10%), pyroxene (~5%), biotite (~10%) and quartz (~5%). They show different structures such as gabbro, diabasic, porphyritic, granule, and heterogranular textures (Fig. 3). The zoning textures of plagioclase develop and the centers are often ablated (Fig. 3i). The plagioclases can be divided into two types, one is matrix plagioclases, which are long-strip, fine-grained, euhedral, subhedral, long-prismatic, and forming a gabbro-like and diabasic-like texture, the other is medium-grained plagioclase phenocrysts, which are euhedral, and part of which develop zoning textures (Fig. 3i). Some plagioclases encapsulate multi-layer fine-grained dark minerals (Fig. 3h), which show that feldspars are characterized by multi-stage growth.

  • Zircon grains were separated using flotation and electromagnetic methods at the Regional Geological and Mineral Investigation Institute, Langfang, Hebei Province, China. Zircons with good crystal shape and transparency were selected and pasted into epoxy resin sample targets. Reflected, transmitted microscopic photography, and the cathodoluminescence (CL) images of zircons were conducted at the Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. The U-Pb dating and trace element analyses of zircon were simultaneously performed using Agilent 7500 ICP-MS at the same place by using the laser ablation system Geolas2005. Detailed operating process and data reduction are the same as the description by Liu et al (2008). Zircon 91500, glass NIST610 were used as external standards for U-Pb dating, trace element calibration, whereas the content of 29Si as internal standard for trace element calibration, respectively. The composition of major and trace elements were measured in Guangzhou Aussie Analytical Testing Co., Ltd. The major elements were determined by X-ray fluorescence spectroscopy (PAN analytical Axios XRF) method, whereas the trace elements composition was determined by inductively coupled plasma mass spectrometry (PE ElAN6000 ICP-MS).

  • Zircons of samples are euhedral, subhedral and column, mostly grainy and long-column, up to 40-150 μm long, and having length to width ratios between 2 : 1 and 3 : 1. They have smooth surfaces and sharp edges, showing distinct rhythm bands on the CL images (Fig. 4). Analytical results are shown in Table 1. The U and Th concentrations vary widely, with Th/U ratios of > 0.4 that are generally similar to magmatic zircon. Fourteen analyses of 17 spots from sample LQ1 have better harmonicity, with U 30.1 ppm-141 ppm, Th 32.5 ppm-326 ppm and Th/U ratios mostly vary between 1.08 and 2.03 (Table 1). Fourteen analyses of 18 spots from sample LQ5 have better harmonicity, with U 46.3 ppm-153 ppm, Th 59.9 ppm-271 ppm and Th/U ratios mostly vary between 1.31 and 1.97 (Table 1). Chondrite-normalized REE patterns of the Langqi pluton invariably show obvious fractionation of light and heavy REE, Ce positive abnormalities and slightly Eu negative abnormalities. They display a LREE deficiency type with left inclination model (Fig. 5). The above characteristics indicate that these zircons are magmatic zircons, whose U-Pb ages can represent the crystallization ages of the samples. The 206Pb/238U weighted mean ages of the two samples from the Langqi granite pluton are 101.6±1.3 (MSWD=0.58) and 100.1±1.1 Ma (MSWD= 0.93), respectively.

    Figure 4.  Zircon CL images of typical single-crystals and LA-ICP-MS U-Pb concordant age diagram and weighted histogram for Langqi pluton.

    Spot Th U Th/U 207Pb/206Pb 207Pb/235U 206Pb/238U 208Pb/232Th 207Pb/235U 206Pb/238U
    (ppm) Ratio ±1σ Ratio ±1σ Ratio ±1σ Ratio ±1σ Age(Ma) ±1σ Age(Ma) ±1σ
    LQ1:Mean=101.6±1.3 Ma, n=14, MSWD=0.58
    LQ1-01 79.8 61.1 1.30 0.0550 0.0064 0.1092 0.0094 0.0161 0.0004 0.0051 0.0002 105 8.6 103 2.5
    LQ1-02 75.6 57.3 1.32 0.0526 0.0059 0.1133 0.0101 0.0162 0.0004 0.0051 0.0002 109 9.3 103 2.5
    LQ1-03 132 85.1 1.55 0.0516 0.0050 0.1127 0.0099 0.0161 0.0004 0.0051 0.0002 108 9.0 103 2.3
    LQ1-05 178 102 1.74 0.0464 0.0039 0.0965 0.0068 0.0158 0.0004 0.0049 0.0002 93.6 6.3 101 2.3
    LQ1-06 107 55.0 1.95 0.0585 0.0081 0.1174 0.0109 0.0160 0.0005 0.0049 0.0002 113 9.9 102 3.0
    LQ1-07 32.5 30.1 1.08 0.0667 0.0162 0.1267 0.0214 0.0172 0.0007 0.0056 0.0003 121 19.3 110 4.7
    LQ1-08 175 106 1.65 0.0523 0.0044 0.1113 0.0081 0.0158 0.0003 0.0051 0.0002 107 7.4 101 2.2
    LQ1-09 80.9 67.2 1.20 0.0553 0.0055 0.1183 0.0078 0.0164 0.0005 0.0049 0.0002 114 7.1 105 3.0
    LQ1-10 159 78.1 2.03 0.0514 0.0045 0.1077 0.0069 0.0161 0.0004 0.0052 0.0002 104 6.3 103 2.5
    LQ1-11 90.9 67.4 1.35 0.0543 0.0052 0.1094 0.0092 0.0155 0.0005 0.0050 0.0002 105 8.4 99.3 3.2
    LQ1-12 326 141 2.31 0.0474 0.0041 0.1023 0.0081 0.0158 0.0003 0.0048 0.0001 98.9 7.4 101 1.8
    LQ1-15 111 74.9 1.49 0.0532 0.0055 0.1103 0.0104 0.0156 0.0004 0.0052 0.0002 106 9.5 99.8 2.3
    LQ1-16 183 136 1.34 0.0519 0.0038 0.1112 0.0073 0.0157 0.0003 0.0050 0.0001 107 6.7 101 1.7
    LQ1-17 127 82.4 1.54 0.0531 0.0059 0.1061 0.0096 0.0156 0.0004 0.0050 0.0002 102 8.8 99.7 2.7
    LQ5:Mean=100.1±1.1 Ma, n=14, MSWD=0.94
    LQ5-02 271 153 1.77 0.0502 0.0036 0.1079 0.0073 0.0155 0.0003 0.0047 0.0001 104 6.7 99.2 1.7
    LQ5-03 122 78.3 1.56 0.0506 0.0051 0.1078 0.0095 0.0159 0.0004 0.0048 0.0002 104 8.7 102 2.4
    LQ5-05 143 92.4 1.55 0.0502 0.0049 0.1052 0.0093 0.0155 0.0003 0.0052 0.0002 102 8.5 99.5 2.0
    LQ5-06 217 121 1.80 0.0526 0.0046 0.1147 0.0083 0.0160 0.0003 0.0053 0.0002 110 7.6 102 2.0
    LQ5-08 109 73.1 1.50 0.0495 0.0046 0.1062 0.0083 0.0159 0.0004 0.0049 0.0002 102 7.6 102 2.5
    LQ5-09 78.6 60.2 1.31 0.0555 0.0061 0.1127 0.0085 0.0159 0.0004 0.0053 0.0002 108 7.8 102 2.7
    LQ5-10 176 95.2 1.84 0.0501 0.0052 0.1034 0.0092 0.0155 0.0003 0.0051 0.0001 99.9 8.5 99.0 2.1
    LQ5-11 59.9 43.6 1.37 0.0560 0.0077 0.1190 0.0118 0.0167 0.0005 0.0055 0.0002 114 10.7 107 3.2
    LQ5-13 134 88.1 1.52 0.0463 0.0050 0.0943 0.0090 0.0153 0.0003 0.0049 0.0002 91.5 8.3 97.6 2.0
    LQ5-14 123 79.8 1.55 0.0509 0.0057 0.1014 0.0097 0.0152 0.0003 0.0047 0.0002 98.1 9.0 97.0 2.2
    LQ5-15 142 109 1.31 0.0521 0.0048 0.1096 0.0086 0.0156 0.0003 0.0051 0.0002 106 7.8 100 2.1
    LQ5-16 107 70.3 1.53 0.0529 0.0050 0.1122 0.0086 0.0160 0.0004 0.0052 0.0002 108 7.8 102 2.2
    LQ5-17 248 158 1.57 0.0531 0.0033 0.1090 0.0058 0.0155 0.0003 0.0050 0.0001 105 5.3 99.1 2.0
    LQ5-18 91.2 46.3 1.97 0.0511 0.0076 0.1060 0.0124 0.0156 0.0005 0.0048 0.0002 102 11.4 99.9 3.0

    Table 1.  Zircon LA-ICP-MS U-Pb isotope data for the Langqi granites

    Figure 5.  Chondrite-normalized REE distribution pattern of zircons for Langqi granitic batholith.

  • Four mafic enclaves and four host rocks were selected for geochemical analysis through measuring the rock body profile and filed mapping, and the analytical results are shown in Table 2. The granites have a high SiO2 content, varying from 71.82 wt.% to 72.59 wt.% with an average of 72.39 wt.%. The SiO2 content of the MMEs is between 47.16 wt.% and 60.16 wt.%, with an average of 55.47 wt.%. The host rocks are relatively high in total alkalis content, with Na2O+K2O ranging from 8.37 wt.% to 8.72 wt.%, which are within the range of granite and belong to high potassium calc-alkaline rocks (Figs. 6a, 6c). They have medium Al2O3 content ranging from 13.52 wt.% to 13.98 wt.% and A/CNK values of 0.95 to 1.00, showing metaluminous and weak- peraluminous properties (Fig. 6b). The mafic enclaves have higher incompetent element Sr and lower Ba than the host granites (Fig. 6d), indicating that the dioritic enclaves should be a fully modified product of the mantle-derived magma. They are relatively high in total REE (214.1 ppm-225.7 ppm) and present right-leaning partition curves of significant fractionation of light and heavy REE (LREE/HREE=9.17-9.90), and moderate Eu negative anomalies (Eu/Eu*=0.65-0.71) in the chondrite-normalized REE patterns (Fig. 7a). The host rocks show enrichment in large ion lithophile elements (LILE) (Rb, Th, U, and K) and strong depletion in high field strength elements (HFSE) (Nb, Ta, and Ti) in the primitive mantle-normalized trace element diagrams (Fig. 7b).

    Sample LQ1 LQ2 LQ3 LQ4 BT-1 BT-2 BT-3 BT-4
    Lithology G G G G MMEs MMEs MMEs MMEs
    Major element (wt.%)
    SiO2 72.41 72.59 72.74 71.82 56.24 47.16 60.16 58.32
    TiO2 0.29 0.28 0.26 0.28 0.93 1.44 0.71 0.80
    Al2O3 13.88 13.98 13.94 13.52 18.19 17.32 17.29 17.62
    FeOT 1.96 1.95 2.08 2.20 7.06 8.78 5.46 6.08
    MnO 0.09 0.10 0.10 0.26 0.14 0.20 0.17 0.31
    MgO 0.39 0.40 0.36 0.46 3.44 4.90 2.58 2.91
    CaO 1.10 1.04 1.03 1.44 7.66 12.60 5.62 5.89
    Na2O 4.62 4.64 4.62 4.39 4.66 2.00 5.01 4.88
    K2O 4.07 4.03 4.10 3.98 0.96 0.20 1.42 1.41
    P2O5 0.07 0.07 0.06 0.06 0.38 0.31 0.28 0.31
    LOI 0.41 0.43 0.55 0.44 0.50 4.13 0.84 1.01
    Total 99.29 99.51 99.84 98.85 100.16 99.04 99.54 99.54
    A/CNK 0.99 1.01 1.00 0.96 0.80 0.66 0.86 0.87
    A/NK 1.16 1.17 1.16 1.17 2.09 4.94 1.77 1.84
    Mg# 28.3 28.9 25.5 29.3 49.1 52.5 48.3 48.7
    Trace element (ppm)
    La 48.6 50.4 47.9 48.6 29.6 12.5 33.8 30.5
    Ce 95.4 97.5 92.2 93.9 54.6 27.7 64.3 61.3
    Pr 10.5 10.6 10.05 10.25 6.32 3.66 7.27 7.28
    Nd 37.7 37.6 35.7 36.6 24.5 15.8 28 27.8
    Sm 7.31 7.13 6.67 6.6 4.61 4.07 5.3 5.02
    Eu 1.55 1.4 1.44 1.4 1.71 1.51 1.68 1.77
    Gd 6.65 6 5.7 5.68 3.98 4.42 4.77 4.73
    Tb 0.95 0.88 0.86 0.8 0.57 0.63 0.66 0.63
    Dy 5.60 5.60 5.44 5.20 3.2 3.71 3.88 4.01
    Ho 1.14 1.06 1.06 1.02 0.68 0.76 0.80 0.81
    Er 3.25 3.19 2.98 2.98 1.88 2.01 2.23 2.35
    Tm 0.50 0.50 0.49 0.47 0.29 0.27 0.35 0.36
    Yb 3.31 3.28 3.1 3.29 1.99 1.63 2.45 2.43
    Lu 0.53 0.54 0.51 0.49 0.32 0.25 0.40 0.41
    Y 32.1 30.9 28.9 28.9 17.8 19.1 21.5 22.4
    ΣREE 222.99 225.68 214.1 217.28 134.25 78.92 155.89 149.4
    LaN/YbN 10.53 11.02 11.08 10.60 10.67 5.50 9.90 9.00
    δEu 0.68 0.65 0.71 0.70 1.22 1.09 1.02 1.11
    Ba 1 235 1 185 1 270 1 180 484 157 556 301
    Cr 20 20 20 20 60 230 50 50
    Ni 5.5 7.2 6.6 8.7 13.4 35.8 27.9 25.6
    Cs 1.81 1.49 1.68 1.58 1.27 0.97 2.02 3.61
    Ga 17.5 17.7 17.7 17.1 19.8 18.4 19 19.7
    Hf 6.4 6.2 5.9 6.1 7.1 2.6 7.4 7.5
    Nb 14.6 14.9 13.6 14 5.7 5.8 8.2 8
    Rb 128 131.5 121 127.5 27.3 2.7 46.4 63.8
    Sn 2 3 2 2 1 1 2 2
    Sr 137 124 120.5 129 788 526 557 448
    Ta 1.1 1.9 1 1.1 0.4 0.3 0.7 0.6
    Th 13.95 14.3 13.35 14.25 5.97 0.96 8.23 6.62
    U 3.2 3.24 2.24 3.7 1.54 0.22 2.88 2.65
    V 15 14 14 15 137 197 95 112
    Zr 220 209 205 208 376 98 365 395
    G. Granite; MMEs. mafic microgranular enclaves.

    Table 2.  Major and trace elements for the Langqi granites and mafic microgranular enclaves

    Figure 6.  Chemical classifications of host quartz diorites and mafic microgranular enclaves from the Langqi pluton. (a) TAS diagram; (b) A/CNK vs. A/NK diagram; (c) K2O vs. SiO2 diagram; (d) Sr vs. Ba diagram.

    Figure 7.  Chondrite-normalized rare earth element distribution patterns (a) and primitive mantle-normalized trace element diagrams (b) for the Langqi granites and mafic microgranular enclaves.

    The element characteristics of the MMEs from Langqi pluton are significantly different from those of granites, which are as follows. (1) The MMEs are relatively poor in silicon (47.16 wt.%-60.16 wt.%), alkali (2.20 wt.%-6.43 wt.%) and rich in aluminum (17.29 wt.%-18.18 wt.%), one of which is in the range of gabbro and belongs to low potassium series rocks, and the others are in the range of diorite and belong to calc-alkaline rocks (Figs. 6a, 6c). The A/CNK values of enclaves range from 0.65 to 0.86, which are metaluminous that are lower than those of host rocks (Fig. 6b). The K2O/Na2O values (0.86-0.90) of the host rocks are lower than the crust-derived granites of South China with an average of 1.61 (Mao et al., 2014). The K2O/Na2O values of the MMEs are from 0.1 to 0.28, obviously lower than the host granites, showing the characteristics of relatively low potassium. (2) The MMEs are higher in Fe2O3T (5.46 wt.%-8.78 wt.%), MgO (2.58 wt.%-4.90 wt.%), CaO (5.62 wt.%-12.6 wt.%), Cr (50 ppm-230 ppm), and Mg# (48.3-52.5), and higher in P2O5 than that in the host granites, which may be related to volatile- rich minerals such as apatite, biotite and amphibole, suggesting that they are water-rich magma. (3) The MMEs and the host rocks generally show similar REE distribution curves, and they are relatively enriched in LREE, but relatively depleted in the HREE. Except the gabbro enclaves (BT-2) with weak fractionation of light and heavy REE, the fractionation degree of the other three samples is similar (LREE/HREE=8.5-9.4) with slight Eu positive anomalies (δEu=1.02-1.22).

  • The genesis of the MMEs in granites can be divided into four major models: (1) residues, the MMEs are the melting residues during crustal anatexis responsible for the generation of granitoid magmas (Chappell et al., 1987); (2) capture or contamination of basic rocks (Maas et al., 1997); (3) cognate enclaves, the MMEs are the accumulation of mafic minerals formed by crystal-liquid phase separation of the homologous parent magma of granites (Shellnutt et al., 2010; Dodge and Kistler, 1990); (4) magma mixing, representing remnants of a mafic component added to granitic magma (Niu et al., 2018; Barbarin, 2005). Previous studies show that the major and trace element contents of enclaves and host rocks are linearly changed and silicate glasses are visible in enclaves (Dorais and Spencer, 2014). However, these characteristics are not consistent with Langqi mafic enclaves in this study. Homologous accumulation mode is contradicted against the irregular random distribution of the enclaves and field features of clear-cut contact. Moreover, the low content of dark minerals (biotite, hornblende), and the porphyritic and aplitic structure do not support the homologous accumulation model either. The surrounding rock xenoliths are usually distinguished from the host rocks by typical metamorphic structures and older ages of crystallization, which are inconsistent with the magmatic structure of the mafic enclaves and the coeval ages with the granites.

    The intertwined relationship between the host rocks and the mafic enclaves suggests that they may be formed at the same time, and the petrology and mineralogy of the mafic enclaves show that they were originated from magma. The Langqi microgranular enclaves are unevenly distributed in the granites, mainly elliptical, minor angular or irregular (Fig. 2), indicating that they are mainly mafic enclaves adding to the host magma under plastic conditions. The clear-cut contact relationship between the enclaves and the host rocks means that the temperature between them is significantly different (Bora et al., 2013). The complex banding and erosion phenomenons of plagioclase are also strong evidences for mixing genesis of enclave magma (Figs. 3i, 9c, 9d). The plagioclase nucleuses develop sieve textures and contain dark minerals (Figs. 9c, 9d), which may be due to injection of high-temperature mafic magma into colder granite magma and corrosion occurring through material exchange and heating. The main oxide contents of the enclaves and host rocks also support the cause of magma mixing. They are distributed along the hybridization trend in the MgO-FeOT diagram (Fig. 8b; Zorpi et al., 1989), and show good curves and linear correlation in the CaO/Al2O3 vs. K2O/MgO diagrams (Fig. 8c) and Al2O3/CaO vs. Na2O/CaO diagrams respectively, which accords with the trend of magma mixing.

    Figure 8.  Element correlation diagrams of mafic mircogranular enclaves and host rocks from the Langqi pluton. (a) After Tsuchiya et al. (2005); (b) after Zorpi et al. (1989).

    Figure 9.  Mafic enclaves with ophitic-gabbro texture and center alteration of the plagioclase.

    Langqi mafic enclaves have lower SiO2, higher MgO contents and higher values of Mg# than the host rocks, suggesting that they maybe come from the partial melting mantle or the mafic lower crust. Although the contents of Cr and Ni in enclaves are lower than those in the primitive mantle magma, part of the enclaves samples are within the range of the mantle- derived magma (Tsuchiya et al., 2005; Fig. 8a), indicating that the enclaves should be the product of magma mixing.

    The MMEs are approximately ophitic and diabasic textures, which mainly consist of basic plagioclase, hornblende phenocrysts and a small amount of pyroxene matrix without biotite. The basic plagioclase is needle-like filled between the granitic pyroxene, and its composition is close to the gabbro (Fig. 9a, 9b). There are many gradual relationships between different structural enclaves, such as diabasic-like and gabbro- like texture→fine grained texture→porphyritic-like texture→ heterogranular texture→porphyritic-like texture host rocks. The content of dark minerals is gradually decreasing, and the content of felsic phenocrysts is gradually increasing. The lithology changes from intermediate-basic to intermediate-acid, the content of biotite is gradually increasing, and the content of apatite varies from high to low. Different textures of enclaves and host rocks are very likely to form a magma mixing series: the MME with diabasic-like and gabbro-like texture may represent the basic endmembers of the mixed magma (Figs. 9a, 9b), while the host magma represents the acidic endmembers. Nb and Ta are high field strength elements with similar ionic radius and similar geochemical properties. The Nb/Ta ratio is usually applied to investigate the genesis of magma. The Nb/Ta ratios of the enclave samples are from 11.7 to 19.3, and the maximum Nb/Ta ratio of enclave sample (BT-2) is 19.3, which is significantly higher than that of basaltic oceanic crust (average values of 17). These features also suggest that the MMEs may represent the basic endmember of magma mixing.

    In the enclave (BT-2), most of the basic plagioclase has undergone severe alteration with rare quartz phenocrysts (Fig. 9), indicating that there were no large-scale materials exchange between the mafic magma and the acidic magma at this stage. The ages of the Langqi pluton in this study are close to those of the Jiaomei biotite granodiorite and the mafic enclaves (Yang et al., 2013; the age of the host rock is 106.4±1.8 Ma; the enclave is 105.6±1.0 and 106.5±1.0 Ma, respectively) in Zhangzhou area, Fujian, China, which were formed under the same regional geological setting. Both the enclaves and the host biotite granodiorite have positive Hf isotopic compositions (the values of the host rocks are 2.2 to 3.7; the values of enclaves are 0.9 to 5.5), indicating that Jiaomei biotite granodiorites were sourced from partial melting of juvenile crust.

    The subduction of the Pacific Plate is accompanied by large-scale magmatism. The magma usually comes from the partial melting of the subduction oceanic crust and the overlying mantle wedge, and partial melting of the crust due to the dehydration of the subducted oceanic crust. The Nb/Ta ratio of the Langqi dioritic enclaves are 11.7 to 19.3 (average 14.6) close to the primitive mantle (17.5±2.0, Jochum et al., 1989) suggesting that these enclaves may document the input from lithospheric mantle.

    The magmatic source of the enclaves maybe derived from the mantle wedge that has experienced extensive fluid metasomatism in the subduction zone. the enclaves were inferred as products by different percentage mixing of subduction related metasomatized mantle-derived mafic and felsic magmas originating from partial melting of the crustal material.

  • The Langqi granite pluton contains only a small amount of hornblende and biotite, without aluminum-rich minerals traditionally as classification criteria of S-type granite such as cordierite, tourmaline, muscovite and garnet. The A/CNK values of the mafic enclaves and granodiorites from the samples in the study are less than 1.1 with relatively low content of P2O5 (0.06 wt.%-0.07 wt.%), and there is a clear negative correlation between P2O5 and SiO2 (Fig. 10a), which is consistent with the evolution trend of I-type granite. The trace elements of mafic enclaves (BT2) are significantly different from other enclaves and host granites. It was inferred as products by different percentage mixing of mantle-derived mafic and crust-derived felsic magmas, indicating that they have different magmatic processes. The mafic dioritic enclaves may be formed by the crystallization of transitional magma, which are formed by the evolution of basic magma or mixed with acidic magma. The significantly different REE contents of the host granites and mafic enclaves (lower HREE contents) also prove that the host granites and the diorite enclaves were from different source regions.

    Figure 10.  Representative binary variation plots for the granites and enclosed MMEs in Langqi pluton (the igneous rocks ages of 119 to 96 Ma of the Shishi region are from Ding et al., 2015, and the data of Zhangpu are from Qiu et al., 2012).

    Their MgO and TiO2 contents decrease with the increase of SiO2 (Figs. 10b, 10c), suggesting that the differentiation of mafic minerals occurred during diagenesis. The contents of Al2O3, CaO, and Sr in quartz diorite are significantly negatively correlated with SiO2 (Figs. 10d-10f), as well as slight Eu negative anomalies, indicating that the crystallization differentiation of the plagioclase is not obvious.

    Studies have shown that I-type granites can be produced by basal crystallization differentiation in the Earthʼs crust (Castro, 2013; Zhang et al., 2008), or formed by the partial melting of crustal rocks with or without the addition of mantle sources (Niu, 2015; Xu et al., 2014; Sisson et al., 2005). Granites with high SiO2 (71.82 wt.%-72.74 wt.%) and low Cr (20 ppm), Ni (10.20 ppm-30.3 ppm) indicate that the parents magma are mainly from the crust. In recent years, through the systematic petrological and geochemical studies on Fujian coast, it is found that a large number of Cretaceous granites show a wide range of Hf isotopic compositions, whereas the εHf(t) values are generally positive (Ding et al., 2015; Yang et al., 2013; Qiu et al., 2012). These Hf isotopic characteristics reveal the contribution of the mantle source to the genesis of the granite, and suggest that it should be the result of partial melting of juvenile crust induced by underplating of mantle-derived magma into the lower crust. The wide distribution of εHf(t) values indicates that the host rocks may be affected by the mixing of magma in different source areas. The Nd isotopic compositions are relatively uniform with high εHf(t) values, and low second-stage Nd model ages (TDM2). The Nd-Hf isotopic compositions indicate that the mantle-derived components should participate in the diagenesis process.

    The dynamic mechanism of the Mesozoic magma in South China is caused by the roll-back of the subducted Paleo-Pacific Plate, which has been widely accepted. Fujian coastal area has been controlled by the subduction tectonic system of the Paleo- Pacific Plate since the Cretaceous, where there are many stages of lithospheric and crustal extension events, and strong mantle- crust interaction such as asthenospheric material upwelling that were caused by roll-back of subducted Paleo-Pacific Plate (Liu et al., 2017; Li Z et al., 2014; Cui et al., 2013; Li J H et al., 2013; Yang et al., 2012). The magmatic rocks show obvious polar characteristics: the ages of magma are increasingly younger from inland to ocean, and the Hf and Nd isotopes change from enrichment to depletion, indicating the influence of upwelling mantle material caused by the subduction of the Paleo-Pacific Plate during the Early and Late Cretaceous on the granite diagenesis process in the area is continuously strengthened. The development of coeval I-type and A-type composite granites and medium-basic dykes also confirms that South China has been in an extensional tectonic setting since the Early Cretaceous. The extension of the crust is beneficial to the underplating of mantle-derived basic magma, providing heat source and partial material for the generation of intermediate- acid magmatism, and promoting the partial melting of the crustal material to produce acid granitic magma, which mixed with the mafic magma.

  • (1) The zircon U-Pb age of the host granites in the Langqi area, Fuzhou, is about 101 Ma. The enclaves and host granites are formed at the same time, which are the results of magma mixing of basic magma produced by the partial melting of the lithospheric mantle and the crust-derived granitic magma.

    (2) The host rocks of the Langqi pluton are metaluminous I-type granites with high-silicon, which is related to the extensional setting caused by the subduction and roll-back of the Paleo-Pacific Plate. The mantle-derived magma underplating induced by subbducted Paleo-Pacific Plate rollback and subsequent break-off is the main mechanism for the formation of intermediate-acid intrusive rocks.

  • We thanks Jiangtao Yang and Pingguang Chen for participation in the fieldworks. This work was supported by the National Natural Science Foundation of China (No. 41873012) and the Fujian Provincial Department of Education (No. JT180063) and the Fujian Provincial Department of Science and Technology (No. 2018J01472). The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1022-6.

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