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Volume 30 Issue 5
Oct.  2019
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Coronas around Olivine in the Miaowan Olivine Norite, Yangtze Craton, South China

  • Coronitic microstructures have been used to interpret the late-stage solidification history of igneous rocks and to constrain the corresponding chemical and/or physical changes. Coronas with three shells were also recognized in the Miaowan olivine norite, Yangtze Craton, South China. In our study, orthopyroxene intergrowth with vermicular magnetite in the inner shell is in optical continuity with magnetite-free orthopyroxene in the middle shell. In the outer shell of brown amphibole remaining magnetite-free orthopyroxene inclusions sporadically occur. Meanwhile Mg# values of orthopyroxene (76-80) in the inner and middle shells are basically consistent with olivine (78-81). In this paper, we propose a multi-stage genetic model for the formation of coronas in the Miaowan olivine norite. In the first stage, the magnetite-free orthopyroxene shell formed through reaction between primocrystal olivine with the residual Si-rich melt at 990-1 053 ℃ and 6.2-6.5 kbar. In the second stage, the orthopyroxene-magnetite symplectite shell formed when primocrystal olivine reacted with the late-stage residual Fe-rich melt promoted by high oxygen fugacity condition at 927-1 035 ℃ and 6.0-6.5 kbar. In the third stage, the brown amphibole shell formed as the presence of residual hydrous melt and replaced the middle shell at 821-900 ℃ and 5.5-6.0 kbar.
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Coronas around Olivine in the Miaowan Olivine Norite, Yangtze Craton, South China

    Corresponding author: Zhaochong Zhang, zczhang@cugb.edu.cn
  • 1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China
  • 2. College of Desert Control Science and Engineering, Inner Mongolia Agricultural University, Hohhot 010018, China
  • 3. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China
  • 4. School of Earth Sciences, Lanzhou University, Lanzhou 730000, China

Abstract: Coronitic microstructures have been used to interpret the late-stage solidification history of igneous rocks and to constrain the corresponding chemical and/or physical changes. Coronas with three shells were also recognized in the Miaowan olivine norite, Yangtze Craton, South China. In our study, orthopyroxene intergrowth with vermicular magnetite in the inner shell is in optical continuity with magnetite-free orthopyroxene in the middle shell. In the outer shell of brown amphibole remaining magnetite-free orthopyroxene inclusions sporadically occur. Meanwhile Mg# values of orthopyroxene (76-80) in the inner and middle shells are basically consistent with olivine (78-81). In this paper, we propose a multi-stage genetic model for the formation of coronas in the Miaowan olivine norite. In the first stage, the magnetite-free orthopyroxene shell formed through reaction between primocrystal olivine with the residual Si-rich melt at 990-1 053 ℃ and 6.2-6.5 kbar. In the second stage, the orthopyroxene-magnetite symplectite shell formed when primocrystal olivine reacted with the late-stage residual Fe-rich melt promoted by high oxygen fugacity condition at 927-1 035 ℃ and 6.0-6.5 kbar. In the third stage, the brown amphibole shell formed as the presence of residual hydrous melt and replaced the middle shell at 821-900 ℃ and 5.5-6.0 kbar.

0.   INTRODUCTION
1.   GEOLOGICAL SETTING
  • The Huangling dome is located in the northern Yangtze Craton, which is bounded to the west by the Longmenshan orogenic belt, to the north by the Qinling-Dabie orogenic belt and to the southeast by the Jiangnan orogenic belt (Chen C et al., 2018; Zhang and Zheng, 2013; Wu et al., 2012; Ma et al., 2002; Fig. 1a). The Miaowan Ophiolite Complex, defined by Peng et al.(2012, 2010), crops out in the southern Huangling dome (Jiang et al., 2016; Gao et al, 2011; Ma et al., 1997; Fig. 1b). Subsequently, on the basis of new field, geochemical and geochronological evidence, Deng et al. divided the Miaowan ophiolite complex into two distinct lithological suites including an older MORB-type ophiolitic suite and a younger arc-related magmatic suite, recording the evolution of a Proterozoic ocean (Cheng et al., 2018; Wu et al., 2018; Deng et al., 2017).

    The older ophiolite suite, occurs along a WNW strike for 13 km long and with nearly 4 km wide, which has tectonic contact with the Archean TTG (tonalite-trondhjemite-granodiorite) gneisses and Paleoproterozoic metasedimentary rocks and has intrusive contact with the Neoproterozoic Huangling granitoids (Fig. 1b). The suite has a formation age of ~1 115 Ma and consists from base to the top of ultramafic rocks, metagabbro and metadiabase, sheeted dike complex, metabasalt, pillow lava and metasedimentary rocks (Deng et al., 2017; Jiang et al., 2016). All rock units were deformed and foliated strongly as the 942–935 Ma metamorphic deformation imprint (Jiang, 2014).

    The younger magmatic suite, intruding and cutting across the older suite during 1 000–970 Ma and intruded by Neoproterozoic granite-granodiorite in Dengcuncun-Yuanjiaping area, consists of pegmatitic-massive gabbro, massive diabase and locally, massive olivine norite (Deng et al., 2017; Han et al., 2017; Gao et al., 2011; Fig. 1b).

    The olivine norite (08HL09–1, hereafter referred to as the Miaowan olivine norite) including coronas in this study occurs in a small exposure within the younger magmatic suite (Fig. 1b), and generally, intrude and cut across the gabbro and diabase. The sample collected from the olivine norite is not affected by the later metamorphic imprint.

2.   PETROGRAPHY AND DESCRIPTION OF CORONAS
  • The Miaowan olivine norite is mainly composed of plagioclase (30 vol.%–35 vol.%), amphibole (30 vol.%–35 vol.%), orthopyroxene (15 vol.%–20 vol.%), olivine (10 vol.%–15 vol.%) and magnetite (> 5 vol.%), with minor ilmenite, green spinel, sulfides, apatite and zircon. The plagioclase, olivine and some orthopyroxenes (Opx1), euhedral to subhedral, up to 4 mm in sizes, appear to crystallize earliest from the magma (Fig. 2a).

    Figure 2.  Photomicrographs illustrating the petrographic characteristics of the coronas around olivine in the Miaowan olivine norite. (a) A general view of the coronas around olivine in the olivine norite. (b) Coronas of orthopyroxene-magnetite symplectite and magnetite-free orthopyroxene partly replacing olivine, surrounded by brown amphibole adjacent to plagioclase. The cracks of olivine (lower right) were inherited by the shells of symplectite and magnetite-free orthopyroxene. (c) Coronitic shells partly replacing olivine. There are cracks in olivine grain, in which pale green amphibole and magnetite inclusions develop. (d) The close-up view of coronas from (c) (the red box), exhibiting three distinct coronitic shells, orthopyroxene-magnetite symplectite, magnetite-free orthopyroxene and brown amphibole. (e) Orthopyroxene-magnetite symplectite pseudomorphs completely replacing olivine, showing orthopyroxene-magnetite symplectite, magnetite-free orthopyroxene and brown amphibole shell. Locally, the magnetite-free orthopyroxene shell is absent. (f) Pseudomorphs after olivine, just minor serpentine relics in the position of olivine core. Abbreviations (according to Kretz, 1983): Ol. olivine; Pl. plagioclase; Opx1. early-formed orthopyroxene; Opx2. symplectitic orthopyroxene; Opx3. magnetite-free orthopyroxene; Opx4. orthopyroxene inclusion in olivine; Am1. brown amphibole shell; Amp2. pale green amphibole inclusion in olivine; Mag1. large interstitial magmatic grain; Mag2. symplectitic magnetite; Srp. serpentine; Act. actinolite. Plane-polarized light.

    In the Miaowan olivine norite, three coronitic shells surrounding olivine are observed: (1) the inner shell of orthopyroxene-magnetite symplectite; (2) the middle shell of magnetite-free orthopyroxene; and (3) the outer shell of brown amphibole. Uncommonly, the orthopyroxene-magnetite symplectites around larger magnetite inclusion or plagioclase inclusion occur within the olivine (Figs. 3d and 4c). Additionally, there are early-formed plagioclase and orthopyroxene (Opx1) abutting against the outer shell (Fig. 3f).

    Figure 3.  Photomicrographs illustrating the petrographic characteristics of the coronas around olivine in the Miaowan olivine norite. (a) Three coronitic shells around olivine. Locally, the contacts between olivine and orthopyroxene-magnetite symplectite are convex towards olivine. (b) The close-up view of (a), exhibiting three distinct coronitic shells, orthopyroxene-magnetite symplectite, magnetite-free orthopyroxene and brown amphibole. The boundaries of them are irregular. (c) Symplectites of orthopyroxene with magnetite around olivine radiate from other minerals into olivine, and serpentinization occur along the cracks in olivine. (d) Symplectites of orthopyroxene-magnetite around larger magnetite grained within olivine. Amphibole and orthopyroxene inclusions are also enclosed within olivine, whose cracks are consistent with the olivine. (e) The magnetite-free shell in direct contacting with olivine, the symplectite shell is absent. Locally, the contacts between olivine and magnetite-free orthopyroxene are convex towards olivine. (f) Three coronitic shells around olivine, orthopyroxene-magnetite symplectite, magnetite-free orthopyroxene and brown amphibole. Early-formed plagioclase and orthopyroxene (Opx1) are adjacent to the outer shell. The mineral abbreviations are identical to those of Fig. 2. Plane-polarized light.

    The orthopyroxene-magnetite symplectite shell, which is in direct contact with olivine, varies from < 100 to 500 μm in width (Figs. 2d and 3b), with some complete pseudomorph after primary olivine (Figs. 2e, 2f and 4b). As is shown in Fig. 3a, contacts between olivine and orthopyroxene-magnetite symplectite are convex towards olivine and locally strongly cuspate. Both the symplectitic orthopyroxene and magnetite are fine grained, grading away from the olivine core into slightly larger grained. The symplectitic magnetite is commonly vermicular, locally lamellar or droplet-like, and is 2–5 μm wide and 10–100 μm long. In a few cases, orthopyroxene-magnetite symplectites radiate from other minerals into olivine, and serpentinization occur along the cracks in olivine (Fig. 3c). Using the image-analysis freeware ImageJ on BSE images, the orthopyroxene and magnetite ratio analyzed is not very stable, approximated to 70 : 30 for the average value.

    The magnetite-free orthopyroxene shell, which commonly displays ambiguous contacts with the inner shell of symplectite (Figs. 2b and 3a), varies from 100 to 450 μm in width (Figs. 2d and 3b). Compared with the symplectitic orthopyroxene in the inner shell, the magnetite-free orthopyroxene is larger, up to 150 μm in diameter, which grades outwards into larger grains (Fig. 4a) and in most cases shows a granular fabric (Fig. 4d). Notably, the magnetite-free orthopyroxene and the symplectitic orthopyroxene are in optical continuity.

    Figure 4.  Backscattered-electron (BSE) images exhibiting the petrographic characteristics of the coronas around olivine in the Miaowan olivine norite. (a) Three coronitic shells around olivine, orthopyroxene-magnetite symplectite, magnetite-free orthopyroxene and brown amphibole. Cracks in olivine are partly filled with serpentines. (b) Orthopyroxene-magnetite symplectite pseudomorphs completely replacing olivine. (c) A thin film of amphibole rim around plagioclase inclusion within olivine. (d) Orthopyroxene with a granular fabric. The mineral abbreviations are identical to those of Fig. 2.

    The brown amphibole shell is wider than the inner and middle shells (Figs. 2b and 3a). The amphibole, irregular shape, is up to 250 μm in size, in which sporadically sub-grained orthopyroxene inclusions occur.

    At some places the inner and/or middle shells are discontinuous, i.e., the magnetite-free orthopyroxene shell is directly contacted with olivine (Fig. 3e) and/or the brown amphibole shell is in direct contact with the orthopyroxene-magnetite symplectite shell (Fig. 2e).

3.   ANALYTICAL METHODS AND RESULTS
  • Chemical compositions of minerals of the Miaowan olivine norite (08HL09–1) were determined by electron microprobe (JEOL JXA-8100) at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences, Wuhan. The element standards used in the present paper are silicates and oxides. Operating conditions were as follows: 15 kV accelerating voltage, 20 nA beam current and 2–5 μm beam diameter, with counting times of 20 s on peaks and 10 s in the background. The analytical precision is better than 1% for elements of which oxide content exceeds 5 wt.%, while precision is better than 5% for elements of which oxide content is from 1 wt.% to 5 wt.%.

  • The analyses of electron microprobe from the Miaowan olivine norite include early-crystallized minerals such as olivine, large orthopyroxene (Opx1), large interstitial magnetite grain (Mag1); orthopyroxene inclusion within olivine (Opx4), pale green amphibole inclusion within olivine (Am2); and coronitic minerals such as symplectitic orthopyroxene (Opx2) and magnetite (Mag2) in the inner shell, magnetite-free orthopyroxene (Opx3) in the middle shell and brown amphibole (Am1) in the outer shell. The representative results are reported in Tables 14.

    Grain 1 Grain 2 Grain 3
    Rim Mantle Mantle Core Core Mantle Mantle Rim Rim Mantle Core Mantle Rim Rim Rim Mantle Core Mantle Rim Rim
    SiO2 38.27 39.61 39.13 38.97 39.26 39.15 39.28 38.23 38.31 38.39 38.70 37.97 38.26 41.94 38.59 38.66 39.40 39.43 39.26 37.52
    TiO2 0.02 0.03 0.00 0.04 0.01 0.05 0.01 0.02 0.02 0.02 0.03 0.02 0.02 0.28 0.03 0.02 0.03 0.01 0.03 0.16
    Al2O3 0.05 0.02 0.05 0.04 0.02 0.04 0.04 0.04 0.03 0.15 0.00 0.06 0.02 1.14 0.04 0.05 0.03 0.03 0.04 2.64
    Cr2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.09
    FeO* 19.51 18.21 18.40 19.25 18.43 18.19 17.95 19.38 19.92 20.42 20.40 19.52 18.99 30.99 19.76 19.32 18.06 18.50 18.27 35.69
    MnO 0.33 0.34 0.38 0.31 0.34 0.36 0.33 0.38 0.35 0.35 0.38 0.35 0.38 0.27 0.39 0.38 0.35 0.40 0.34 0.26
    MgO 41.64 41.55 41.18 40.89 41.53 41.33 41.69 41.81 40.84 39.59 39.90 41.83 42.11 24.87 41.25 41.40 41.34 41.18 41.50 23.47
    CaO 0.02 0.00 0.00 0.01 0.00 0.02 0.02 0.00 0.01 0.06 0.00 0.02 0.00 0.06 0.01 0.03 0.00 0.00 0.00 0.05
    Na2O 0.01 0.00 0.00 0.03 0.00 0.04 0.01 0.00 0.04 0.01 0.02 0.01 0.01 0.03 0.01 0.01 0.00 0.00 0.00 0.01
    K2O 0.00 0.01 0.00 0.02 0.00 0.00 0.01 0.00 0.00 0.01 0.03 0.01 0.01 0.01 0.02 0.01 0.00 0.01 0.00 0.01
    Total 99.86 99.77 99.14 99.56 99.59 99.18 99.34 99.85 99.52 99.04 99.47 99.79 99.79 99.58 100.09 99.87 99.22 99.56 99.44 99.92
    Oxy 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4
    Si 0.985 1.011 1.007 1.003 1.005 1.006 1.006 0.984 0.991 1.000 1.003 0.979 0.984 1.121 0.992 0.993 1.011 1.010 1.006 1.033
    Ti 0.000 0.001 0.000 0.001 0.000 0.001 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.006 0.001 0.000 0.001 0.000 0.001 0.003
    Al 0.002 0.001 0.002 0.001 0.001 0.001 0.001 0.001 0.001 0.005 0.000 0.002 0.001 0.036 0.001 0.002 0.001 0.001 0.001 0.086
    Cr 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.002
    Fe 0.420 0.389 0.396 0.414 0.395 0.391 0.385 0.417 0.431 0.445 0.442 0.421 0.408 0.693 0.424 0.415 0.387 0.396 0.392 0.822
    Mn 0.007 0.007 0.008 0.007 0.007 0.008 0.007 0.008 0.008 0.008 0.008 0.008 0.008 0.006 0.008 0.008 0.008 0.009 0.007 0.006
    Mg 1.598 1.581 1.580 1.569 1.586 1.583 1.592 1.604 1.575 1.537 1.541 1.608 1.614 0.991 1.580 1.586 1.581 1.573 1.586 0.964
    Ca 0.001 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.002 0.000 0.001 0.000 0.002 0.000 0.001 0.000 0.000 0.000 0.002
    Na 0.001 0.000 0.000 0.001 0.000 0.002 0.001 0.000 0.002 0.000 0.001 0.001 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.001
    K 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000
    Total 3.014 2.989 2.992 2.997 2.994 2.993 2.993 3.015 3.009 2.998 2.998 3.020 3.016 2.856 3.008 3.006 2.988 2.990 2.993 2.920
    Mg# 79 80 80 79 80 80 81 79 79 78 78 79 80 59 79 79 80 80 80 54
    Note: *Total Fe as FeO.

    Table 1.  Representative microprobe analyses of olivine in the Miaowan olivine norite (in wt.%)

    Opx1 Opx2 Opx3 Opx4
    1 2 3 4 5 1 2 3 4 5 1 2 3 4 1 2
    SiO2 53.79 54.35 53.81 54.01 53.93 53.96 54.38 54.30 54.01 54.56 53.69 53.40 53.38 53.16 53.40 53.16
    TiO2 0.21 0.12 0.15 0.15 0.16 0.22 0.12 0.12 0.17 0.16 0.11 0.16 0.19 0.14 0.12 0.09
    Al2O3 1.81 1.27 1.90 1.90 1.68 1.52 2.43 2.48 1.80 2.24 2.15 2.31 2.15 2.59 2.43 3.38
    Cr2O3 0.00 0.00 0.00 0.01 0.00 0.00 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.01
    FeO 16.54 16.21 16.30 15.73 15.74 14.98 13.33 12.90 14.05 12.75 15.39 15.27 15.31 14.68 14.67 14.86
    MnO 0.47 0.45 0.40 0.44 0.45 0.38 0.38 0.41 0.42 0.35 0.44 0.39 0.38 0.36 0.35 0.33
    MgO 26.20 26.67 26.12 26.64 26.62 28.40 28.61 29.17 28.50 28.67 27.67 27.53 27.87 28.01 28.16 27.10
    CaO 0.98 0.97 0.94 0.92 0.96 0.39 0.69 0.55 0.55 0.73 0.56 0.65 0.77 0.77 0.51 0.47
    Na2O 0.03 0.00 0.01 0.01 0.00 0.00 0.02 0.02 0.01 0.02 0.01 0.00 0.03 0.01 0.00 0.00
    K2O 0.00 0.01 0.02 0.01 0.00 0.02 0.00 0.02 0.01 0.00 0.00 0.01 0.00 0.00 0.00 0.00
    Total 100.02 100.05 99.66 99.81 99.54 99.88 99.99 99.95 99.50 99.47 100.01 99.72 100.08 99.73 99.64 99.40
    Oxy 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
    Si 1.951 1.966 1.955 1.954 1.957 1.944 1.940 1.935 1.945 1.951 1.935 1.930 1.925 1.918 1.926 1.921
    Ti 0.006 0.003 0.004 0.004 0.004 0.006 0.003 0.003 0.005 0.004 0.003 0.004 0.005 0.004 0.003 0.003
    Al 0.077 0.054 0.082 0.081 0.072 0.065 0.102 0.104 0.076 0.094 0.091 0.098 0.091 0.110 0.103 0.144
    Cr 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
    Fe 0.502 0.490 0.495 0.476 0.478 0.451 0.398 0.384 0.423 0.381 0.464 0.462 0.462 0.443 0.442 0.449
    Mn 0.015 0.014 0.012 0.014 0.014 0.012 0.011 0.012 0.013 0.011 0.013 0.012 0.012 0.011 0.011 0.010
    Mg 1.416 1.438 1.414 1.437 1.440 1.525 1.522 1.550 1.530 1.528 1.487 1.484 1.498 1.506 1.514 1.460
    Ca 0.038 0.038 0.037 0.036 0.037 0.015 0.026 0.021 0.021 0.028 0.022 0.025 0.030 0.030 0.020 0.018
    Na 0.002 0.000 0.001 0.001 0.000 0.000 0.001 0.001 0.000 0.001 0.001 0.000 0.002 0.001 0.000 0.000
    K 0.000 0.001 0.001 0.000 0.000 0.001 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
    Total 4.006 4.004 4.001 4.002 4.003 4.018 4.006 4.011 4.013 3.998 4.016 4.016 4.025 4.023 4.019 4.005
    Mg# 74 75 74 75 75 77 79 80 78 80 76 76 76 77 77 76
    Note: Opx1. Early-formed orthopyroxene; Opx2. symplectitic orthopyroxene; Opx3. magnetite-free orthopyroxene; Opx4. orthopyroxene inclusion in olivine.

    Table 2.  Representative microprobe analyses of orthopyroxene in the Miaowan olivine norite (in wt.%)

    Am1 Am2
    1 2 3 4 5 6 1 2 3 4
    SiO2 42.97 43.50 42.62 42.91 42.61 42.66 42.96 42.82 43.15 43.51
    TiO2 2.60 2.16 2.68 2.46 2.39 2.08 1.42 1.53 1.39 1.19
    Al2O3 10.87 11.43 11.12 11.57 11.20 11.36 12.79 12.63 12.48 12.47
    Cr2O3 0.00 0.00 0.00 0.00 0.00 0.26 0.02 0.01 0.01 0.03
    FeO 12.04 10.89 12.37 12.00 12.00 11.11 9.64 9.75 9.89 9.81
    MnO 0.16 0.20 0.16 0.16 0.16 0.14 0.15 0.15 0.12 0.17
    MgO 14.15 15.14 14.33 14.45 14.98 15.29 15.83 15.86 15.91 15.90
    CaO 11.49 11.45 11.31 11.45 11.75 10.41 11.84 11.97 11.76 11.84
    Na2O 2.63 2.63 2.77 2.68 2.64 2.79 2.82 2.85 2.77 2.84
    K2O 0.52 0.44 0.52 0.53 0.50 1.38 0.42 0.51 0.49 0.40
    Total 97.42 97.82 97.87 98.21 98.23 97.46 97.91 98.06 97.98 98.16
    O 23 23 23 23 23 23 23 23 23 23
    Si 6.317 6.304 6.236 6.239 6.185 6.243 6.182 6.166 6.205 6.244
    Al 1.683 1.696 1.764 1.761 1.815 1.757 1.818 1.834 1.795 1.756
    Al 0.200 0.255 0.154 0.221 0.102 0.202 0.351 0.309 0.321 0.354
    Ti 0.288 0.235 0.295 0.269 0.260 0.229 0.154 0.166 0.150 0.129
    Fe3+ 0.265 0.388 0.381 0.378 0.540 0.409 0.479 0.467 0.509 0.471
    Cr 0.000 0.000 0.000 0.000 0.000 0.030 0.003 0.001 0.002 0.003
    Mg 3.101 3.271 3.125 3.132 3.242 3.335 3.396 3.404 3.412 3.401
    Fe2+ 1.216 0.931 1.132 1.081 0.917 0.950 0.681 0.707 0.680 0.706
    Mn 0.019 0.024 0.020 0.020 0.019 0.018 0.019 0.018 0.015 0.021
    Ca 1.809 1.777 1.773 1.783 1.827 1.632 1.826 1.847 1.812 1.820
    Na 0.748 0.738 0.784 0.755 0.744 0.791 0.787 0.795 0.773 0.789
    K 0.097 0.081 0.097 0.098 0.092 0.257 0.077 0.094 0.090 0.073
    SUM 15.744 15.700 15.761 15.738 15.744 15.852 15.772 15.807 15.764 15.767
    Mg# 72 78 73 74 78 78 83 83 83 83
    Note: Am1. Brown amphibole shell; Am2. pale green amphibole inclusion in olivine.

    Table 3.  Representative microprobe analyses of amphibole in the Miaowan olivine norite (in wt.%)

    Mag1 Mag2
    1 2 3 4 5 6 1 2 3 4 5 6
    SiO2 0.91 0.17 2.17 0.18 0.41 0.16 0.22 0.61 0.24 0.34 0.29 0.20
    TiO2 2.17 5.79 2.35 2.79 2.54 2.00 0.33 0.17 0.26 0.15 0.16 0.25
    Al2O3 1.14 3.07 0.96 1.76 1.24 0.97 0.26 0.37 0.41 0.45 0.37 0.37
    Cr2O3 0.07 0.37 0.04 0.02 0.03 0.16 0.02 0.01 0.10 0.34 0.08 0.11
    FeO 87.26 82.35 85.83 87.99 88.24 88.83 92.25 91.76 91.85 92.06 91.88 91.98
    MnO 0.20 0.78 0.32 0.28 0.29 0.31 0.00 0.00 0.01 0.02 0.00 0.01
    MgO 1.28 0.64 1.43 0.29 0.52 0.67 0.06 0.46 0.16 0.21 0.24 0.23
    CaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02
    Na2O 0.00 0.08 0.02 0.00 0.01 0.05 0.06 0.00 0.01 0.05 0.03 0.00
    K2O 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    Total 93.03 93.27 93.13 93.31 93.27 93.15 93.21 93.38 93.04 93.63 93.04 93.16
    O 4 4 4 4 4 4 4 4 4 4 4 4
    Si 0.034 0.006 0.082 0.007 0.016 0.006 0.009 0.023 0.009 0.013 0.011 0.008
    Ti 0.062 0.165 0.067 0.080 0.073 0.057 0.009 0.005 0.008 0.004 0.005 0.007
    Al 0.051 0.137 0.043 0.079 0.056 0.043 0.012 0.017 0.018 0.020 0.017 0.017
    Cr 0.002 0.011 0.001 0.001 0.001 0.005 0.001 0.000 0.003 0.010 0.003 0.003
    Fe3+ 1.754 1.515 1.659 1.746 1.767 1.829 1.956 1.927 1.945 1.939 1.952 1.951
    Fe2+ 1.017 1.097 1.055 1.062 1.049 1.008 1.006 1.002 1.006 0.997 0.998 1.000
    Mn 0.007 0.025 0.010 0.009 0.009 0.010 0.000 0.000 0.000 0.001 0.000 0.000
    Mg 0.072 0.036 0.081 0.016 0.029 0.038 0.004 0.026 0.009 0.012 0.013 0.013
    Ca 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.001
    Na 0.000 0.006 0.001 0.000 0.001 0.004 0.004 0.000 0.001 0.004 0.002 0.000
    K 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
    SUM 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000
    Note: Mag1. Large interstitial magmatic grain; Mag2. symplectitic magnetite.

    Table 4.  Representative microprobe analyses of magnetite in the Miaowan olivine norite (in wt.%)

    The olivine grains are generally chemically homogeneous from core to rim with Mg# value [Mg#=100×Mg/(Mg+Fe2+)] ranging from 78 to 81 (Table 1). In a few cases, however, the Mg# value is dramatically decreased (54–59) where some analytical spots abut against clusters of vermicular magnetite.

    The Mg# value of early-formed orthopyroxene (Opx1) is from 74 to 75 (Table 2). The Mg# value of symplectitic orthopyroxene (Opx2) ranges from 77 to 80. The magnetite-free orthopyroxene (Opx3) displays Mg# value of 76–77, consistent with that of the orthopyroxene inclusions (Opx4) in olivine (Mg#=76–77).

    The amphiboles in the Miaowan olivine norite belong to pargasite or magnesiohastingsite in the classification of Leake et al. (1997). The Mg# value of brown amphibole (Am1) in the outer shell is from 72 to 78, and the value of pale green amphibole inclusions (Am2) in olivine is 83 (Table 3). Additionally, the Am2 is characterized by lower contents in TiO2 and FeO than the Am1, i.e., 1.19 wt.%–1.53 wt.% vs. 2.08 wt.%–2.68 wt.%, 9.64 wt.%–9.89 wt.% vs. 10.89 wt.%–12.37 wt.%, respectively.

    The vermicular magnetite (Mag2) shows TiO2 contents ranging from 0.15 wt.% to 0.33 wt.%, lower than that of the large interstitial magmatic grain (Mag1), which ranges from 2 wt.% to 5.79 wt.% (Table 4).

4.   DISCUSSION
  • The temperature and pressure estimates may help to explain the formation of coronas. Using single-pyroxene thermometer and geobarometer proposed by Mercier (1976), we have got the thermobarmetric information acquired from the compositions of the coronitic orthopyroxene. The calculated results show that the temperature and pressure for symplectitic orthopyroxene (Opx2) and magnetite-free orthopyroxene (Opx3) is similar, i.e., 927–1 035 ℃ at 6.0–6.5 kbar and 990–1 053 ℃ at 6.2–6.5 kbar respectively. It suggests that the inner shell of orthopyroxene-magnetite symplectite occurred simultaneously or slightly later than the formation of the middle shell of magnetite-free orthopyroxene.

    Ti geothermometer can be applied to magmatic and subsolidus amphiboles (Otten, 1984; Helz, 1973). Given the formation temperature of amphiboles in the Miaowan olivine norite below 970 ℃, the equation from Otten (1984), i.e., T(℃)= 1 024×(Ti/23O)+545 (T < 970 ℃), is more suitable for our samples. In this case, the calculated temperature of brown amphibole in the outer shell (Am1) is 821–900 ℃.

    A geobarometer on Al in amphibole in the Miaowan olivine norite according to Hammarstrom and Zen (1986) gives pressure 5.5–6.0 kbar for brown amphibole in the outer shell (Am1).

    The estimated P-T conditions for the outer shell suggest that the formation of brown amphibole was later than the inner and middle shells.

  • Although the metamorphic origin of the coronas is irrefutably proposed by several petrologists (e.g., Grant, 1988; Mongkoltip and Ashworth, 1983; Zeck et al., 1982; England, 1974), the rocks of olivine norite discussed here are not affected by the metamorphic deformation imprint and the samples are unaltered. Thus, we infer that the formation of coronas around olivine is probably related to magmatic process.

    Based on detailed petrographic observations and chemical compositions of minerals, there is no significant distinction between coronas from the Miaowan olivine norite (LMUO) and those from layered mafic and ultramafic intrusions (LMU). Thus we try to use the previous theories to account for the origins of coronas in this paper.

    In common, a single-stage origin of the coronas was proposed in some instances by most researchers (Grant 1988; Mongkoltip and Ashworth 1983; Gardner and Robins 1974), i.e., the individual shells formed simultaneously. The textural features and geochemical data of coronas in the Miaowan olivine norite, however, indicate a different mechanism (elaborated below) during formation of the three coronitic shells. Thus, we infer that a multi-stage origin of coronas in the Miaowan olivine norite seems to be plausible.

  • In the Miaowan olivine norite, where the orthopyroxene-magnetite symplectite shell is absent, the magnetite-free orthopyroxene shell in direct contact with olivine occurs (Fig. 3e). Thus, the orthopyroxene is considered as a reaction rim (Xie et al., 2017). The formation of orthopyroxene reaction rim around olivine is commonly attributed to elevated silica activity in the magmatic melt during the fractional crystallization (Holness et al., 2011; Turner and Stüwe, 1992). The reaction is expressed as: olivine+SiO2(l)=orthopyroxene (where l is liquid), suggesting that this reaction occurs in the presence of melt. This inference is also suitable for interpreting the formation of magnetite-free orthopyroxene of the coronas around olivine discussed here in the light of the following evidence: (1) the magnetite-free orthopyroxene in the middle shell and the symplectitic orthopyroxene in the inner shell were in optical continuity; (2) the cracks of olivine were also inherited by the shell of magnetite-free orthopyroxene (Fig. 2b); and (3) the magnetite-free orthopyroxenes grew inward into the core of olivine (Fig. 3e). Therefore, we infer that the middle shell of magnetite-free orthopyroxene formed through the reaction of the early-formed olivine with the Si-rich melt during the fractional crystallization at the time of earliest stage among the formation of three coronitic shells.

  • According to previous researchers, the magmatic-origin model of orthopyroxene-magnetite symplectite including: (1) direct crystallization from the magma; (2) solid-state diffusion between olivine and other early formed minerals; and (3) reaction between the early-formed olivine and an interstitial late-stage magmatic liquid phase (Xie et al., 2017; de Hass et al., 2002; Baltatzis and Skounakis, 1990; Ambler and Ashley, 1977; Gardner and Robins, 1974). However, the following petrographic observations and geochemical characteristics suggest that the orthopyroxene-magnetite symplectite in the inner shell of the coronas in the Miaowan olivine norite formed by consumption of olivine grains.

    (1) the components and occurrences of symplectitic magnetite (Mag2) are very different from large interstitial magmatic grains (Mag1); (2) symplectitic orthopyroxene and magnetite regularly surround olivine rather than crystallizing randomly; (3) the cracks of olivine, locally, were inherited by the shell of symplectite (Figs. 2b and 2c); (4) the contacts between olivine and orthopyroxene-magnetite symplectites are convex towards olivine and locally strongly cuspate (Figs. 3a and 4a), indicating that formation of symplectites is at the expense of olivine (de Haas et al., 2002); and (5) except for a few analytical spots, the Mg# value of olivine (78–81) is in common with the symplectitic orthopyroxene (77–80). Thus, we infer that the orthopyroxene-magnetite symplectites were formed by part or entire replacement of the early-crystallized olivine core.

    Some petrologists proposed oxidation modal for the genesis of orthopyroxene-magnetite symplectites (Kendrick and Jamieson, 2016; Efimov and Malitch, 2012; van Lamoen, 1979; Haselton and Nash, 1975; Goode, 1974). They argued that the products of orthopyroxene-magnetite symplectite originated from sub-solidus oxidation of olivine through the following reaction: 3Mg2SiO4+3Fe2SiO4+O2→2Fe3O4+6MgSiO3, in some instances, the incorporation of fluid may occur.

    The compositions of olivine, orthopyroxene and magnetite are determined on the basis of microprobe analysis and corres-ponding densities are assumed of 3.63, 3.44 and 5.20 g/cm3, respectively. Combined with the average ratio of 70 : 30 for the products of orthopyroxene and magnetite, the average Fo of 80 for olivine and En of 80 for symplectitic orthopyroxene, the percentage by weight of gains and losses of MgO, FeO and SiO2 can be calculated in the reaction during symplectite formation. The results are shown in Table 5.

    Reactant Product Mass gain/loss
    Volume 1 0.7 0.3
    Density 3.63 3.44 5.20
    Formula Fo80→En80+Mag
    Mass 3.63 2.41 1.56
    MgO 1.52 0.72 0 Loss 52.4 wt.%
    FeO* 0.68 0.33 1.45 Gain 159.9 wt.%
    SiO2 1.43 1.36 0 Loss 4.7 wt.%
    Note: *Total Fe as FeO.

    Table 5.  Mass balance calculation for symplectites in the Miaowan olivine norite

    It's easy to find that simple oxidation of olivine cannot explain the mass imbalance of reactants and products for Fe, Mg and Si. The mechanism involves introduction of amounts of Fe from the surrounding environment and expulsion of Mg and Si. Moreover, several workers (Efimov and Malitch, 2012; Zeck et al., 1982; Muir and Tilley, 1957) demonstrated that if the reactants are composed of olivine and oxygen, the newly formed orthopyroxene would be poorer in Fe than the reactant olivine. Thus, we can infer that the components of Mg and Si are lost from olivine and Fe is added during the reaction between olivine with a fluid phase.

    Assuming that the formation of orthopyroxene-magnetite symplectites resulted from solid-state diffusion reaction between olivine and other early formed minerals, the symplectites should be confined to the contact between olivine and a particular mineral (Xie et al., 2017; Ambler and Ashley, 1977). However, the occurrence of olivine and all other phases intergrowth has been observed (e.g., olivine-plagioclase). Thus, it is unlikely that the formation of orthopyroxene-magnetite symplectites was resulted from solid-state diffusion reaction.

    During the formation of magnetite-free orthopyroxene shell, the consumption of silicate melt reacting with the outmost of olivine (described above) leaves behind the Fe-rich liquid, which is in disequilibrium with the core of olivine. On the other hand, following the crystallization of olivine, plagioclase and orthopyroxene, the oxygen fugacity of the system increases gradually with the fractional crystallization of the magma (Zhong et al., 2018; Zhang et al., 2014). Therefore, it is suggested that formation of orthopyroxene-magnetite symplectite shell resulted from the reaction of olivine with a residual Fe-rich melt. The reaction can be given as follows: 10(Mg1.6Fe0.4)SiO4+0.6FeO+0.5O2=4(Mg1.6Fe0.4)Si2O6+Fe3O4+9.6MgO+2SiO2, where Mg2+ and Si4+ are taken to the melt.

    Based on the discussion above, it is convincingly inferred that the orthopyroxene-magnetite symplectites developed when early-formed olivine reacted with the residual Fe-rich melt to produce vermicular magnetite and orthopyroxene. In this process, the oxygen fugacity of melt played a significant role in the formation of vermicular magnetite. The reaction can be summed up: Ol+Fe-rich melt+O2→Opx+Mag+Mg-rich melt. Alternatively, according to the P-T conditions estimated above, the inner shell formed simultaneously with the middle shell, supported by the ambiguous contacts between the two shells.

  • The brown amphibole shell is the thickest shell of the coronas. As is shown in Figs. 2e and 3b, the brown amphibole shell grows inward into the magnetite-free orthopyroxene shell and even is in direct contact with the orthopyroxene-magnetite symplectite (Fig. 2e, lower left). Additionally, the residual orthopyroxene inclusions (Opx3) occurring sporadically in the brown amphibole shell can also be observed. These criteria indicate that brown amphibole post-dates the formation of the magnetite-free orthopyroxene and replaces the latter, and at places the magnetite-free orthopyroxene is absent suggesting an entire replacement by brown amphibole.

    The brown amphibole, characterized by TiO2 contents of 2.08 wt.% to 2.68 wt.%, Al2O3 contents of 10.87 wt.% to 11.57 wt.%, and Mg# values of 72 to 78 (Table 3), very similar to those reported by Ambler and Ashley (1977), is in agreement with a typical of late magmatic origin. Moreover, they plotted into the primary origin district according to Keeditse and Rajesh (2016). In addition, high Sc and V concentrations in the brown amphibole are consistent with an igneous origin (Sc=46.2 ppm–68.8 ppm, V=133 ppm–352 ppm; unpublished data) (Polat et al., 2012; Meurer and Claeson, 2002). As demonstrated by Otten (1984), the presence of magmatic amphibole indicates that the magma must have relatively high water content. Thus, we infer that the outer shell of brown amphibole formed from the residual hydrous melt during the late-stage magmatic crystallization and replaced the middle shell of magnetite-free orthopyroxene.

  • Summarily, we conclude that the coronas around olivine were formed by a three-stage process as shown in Fig. 5. Before this, the early-formed minerals such as olivine, plagioclase and orthopyroxene (Opx1) occur, with the presence of interstitial Si-rich melt, which referred as the stage zero (Fig. 5a).

    Figure 5.  Genetic model for formation processes of three shells around olivine in the Miaowan olivine norite. (a) Early-formed olivine, plagioclase, orthopyroxene and the interstitial Si-rich melt. (b) Formation of magnetite-free orthopyroxene shell in the first stage. (c) Formation of orthopyroxene-magnetite symplectite shell in the second stage. (d) Formation of brown amphibole shell in the third stage. The mineral abbreviations are identical to those of Fig. 2.

    In the first stage, the magnetite-free orthopyroxene shell formed resulting from the reaction of the early-formed olivine with the Si-rich melt during the fractional crystallization (Fig. 5b). The reaction is expressed as: olivine+SiO2(l)= orthopyroxene.

    In the second stage, simultaneous coprecipitation of orthopyroxene and magnetite surrounding olivine occurred and the inner shell formed when early-formed olivine was reacting with the late-stage residual Fe-rich melt promoted by oxidation (at the condition of high fO2). In this process, the oxygen fugacity of melt played a significant role in the formation of vermicular magnetite (Fig. 5c).

    In the third stage, the formation of brown amphibole shell was probably attributed to replacement of the magnetite-free orthopyroxene when the residual hydrous melt occurred at the late stage of magma evolution (Fig. 5d). The fact that the magnetite-free orthopyroxene shell is locally absent may indicate entire replacement of orthopyroxene by brown amphibole.

5.   CONCLUSIONS
  • Petrological investigations and crystal chemical data of the coronas around olivine in the Miaowan olivine norite from the southern Huangling dome, Yangtze Craton, South China lead to the following conclusions.

    The temperature and pressure during the formation of coronas in the Miaowan olivine norite were estimated 990–1 053 ℃ at 6.2–6.5 kbar for the middle shell, 927–1 035 ℃ at 6.0–6.5 kbar for the inner shell, and 821–900 ℃ at 5.5–6.0 kbar for the outer shell, generally indicating the formation sequences of three coronitic shells.

    The coronas around olivine are considered to form through a three-stage process and the oxygen fugacity of melt is a key factor in the formation of vermicular magnetite. The middle magnetite-free orthopyroxene shell formed by the reaction between the early-formed olivine with elevated silica activity melt generated during the fractional crystallization of magma. The inner orthopyroxene-magnetite symplectite shell formed resulting from simultaneous coprecipitation of orthopyroxene and magnetite when early-formed olivine was reacting with the residual Fe-rich melt under the condition of high oxygen fugacity. The outer brown amphibole shell formed by replacement of the magnetite-free orthopyroxene when the residual hydrous melt occurred during the late stage of magmatism.

ACKNOWLEDGMENTS
  • We thank Jianpei Lu for his help for observing thin sections. Shu Zheng, Yang Sun and Shiming Wang are appreciated for their assistance in the microprobe lab, Zhenbing She, Bin Liu, Fuhao Xiong, Zhiguo Cheng for their helpful advices on the interpretation of the coronas. We thank two anonymous reviewers for their constructive and valuable reviews that improved the manuscript. This work is financially supported by the National Key Research and the Development Program of China (No. 2016YFC0600502), the National Natural Science Foundation of China (No. 41502046) and the Geological Survey Project of China (No. DD20160030). The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1012-8.

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