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Volume 42 Issue 6
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Article Navigation >  Acta Sedimentologica Sinica  > 2024  >  42(6): 1890-1902

LIU LongYu, CUI XiaoZhuang, CHEN FengLin, LONG XunRong, MA ZhiXin, SUN ZhiMing. Geochronology, Petrogenesis, and Tectonic Significance of the Neoproterozoic Diabases in the Jinkouhe Area, Western Sichuan Province[J]. Acta Sedimentologica Sinica, 2024, 42(6): 1890-1902. doi: DOI:10.14027/j.issn.1000-0550.2024.111
Citation: LIU LongYu, CUI XiaoZhuang, CHEN FengLin, LONG XunRong, MA ZhiXin, SUN ZhiMing. Geochronology, Petrogenesis, and Tectonic Significance of the Neoproterozoic Diabases in the Jinkouhe Area, Western Sichuan Province[J]. Acta Sedimentologica Sinica, 2024, 42(6): 1890-1902. doi: DOI:10.14027/j.issn.1000-0550.2024.111
shu

Geochronology, Petrogenesis, and Tectonic Significance of the Neoproterozoic Diabases in the Jinkouhe Area, Western Sichuan Province

doi: DOI:10.14027/j.issn.1000-0550.2024.111
  • 1.

    College of Earth and Planetary Sciences, Chengdu University of Technology, Chengdu 610059, China

  • 2.

    Chengdu Center, China Geological Survey (Geosciences Innovation Center of Southwest China), Chengdu 610218, China

Funds:

China Geological Survey DD20243477

National Natural Science Foundation of China 41872120

  • Received Date: 2024-10-19
  • Accepted Date: 2024-11-07
  • Rev Recd Date: 2024-10-22
    • Available Online: 2024-11-07
  • Publish Date: 2024-12-10
  • Objective An extensive suite of Neoproterozoic diabase dykes is exposed in the Jinkouhe area of western Sichuan province, offering crucial insights into the tectonic evolution of the western Yangtze Craton. These diabases bear key implications for understanding the early tectono-magmatic history of the Yangtze Craton and its relationship with the assembly and fragmentation of the Rodinia supercontinent. Methods Sensitive High-Resolution Ion MicroProbe（SHRIMP） zircon U-Pb geochronology and whole-rock geochemistry analyses were performed on the Jinkouhe diabases. Results SHRIMP zircon U-Pb dating of the diabase samples yields an age of 823±10 Ma （MSWD=1.30, n=10）. The diabases are classified as a tholeiitic basalt series, displaying Nb-Ta depletion and negative Eu anomalies. Their geochemical signatures are indicative of both island arc and intraplate basaltic affinities. Conclusions The Jinkouhe diabases are interpreted to have originated from partial melting of a spinel-garnet lherzolite mantle source, which was metasomatized by subduction-related melts. Combined with regional geological data, these diabases were most likely formed within a post-collisional extensional setting, reflecting tectonic processes associated with the final amalgamation of the Yangtze Craton within the context of the Rodinia supercontinent assembly.
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  • Received:  2024-10-19
  • Revised:  2024-10-22
  • Accepted:  2024-11-07
  • Published:  2024-12-10

Geochronology, Petrogenesis, and Tectonic Significance of the Neoproterozoic Diabases in the Jinkouhe Area, Western Sichuan Province

doi: DOI:10.14027/j.issn.1000-0550.2024.111
  • 1. College of Earth and Planetary Sciences, Chengdu University of Technology, Chengdu 610059, China
  • 2. Chengdu Center, China Geological Survey (Geosciences Innovation Center of Southwest China), Chengdu 610218, China
Funds:

China Geological Survey DD20243477

National Natural Science Foundation of China 41872120

  • Received Date: 2024-10-19
  • Accepted Date: 2024-11-07
  • Rev Recd Date: 2024-10-22
  • Available Online: 2024-11-07
  • Publish Date: 2024-12-10

Abstract: Objective An extensive suite of Neoproterozoic diabase dykes is exposed in the Jinkouhe area of western Sichuan province, offering crucial insights into the tectonic evolution of the western Yangtze Craton. These diabases bear key implications for understanding the early tectono-magmatic history of the Yangtze Craton and its relationship with the assembly and fragmentation of the Rodinia supercontinent. Methods Sensitive High-Resolution Ion MicroProbe（SHRIMP） zircon U-Pb geochronology and whole-rock geochemistry analyses were performed on the Jinkouhe diabases. Results SHRIMP zircon U-Pb dating of the diabase samples yields an age of 823±10 Ma （MSWD=1.30, n=10）. The diabases are classified as a tholeiitic basalt series, displaying Nb-Ta depletion and negative Eu anomalies. Their geochemical signatures are indicative of both island arc and intraplate basaltic affinities. Conclusions The Jinkouhe diabases are interpreted to have originated from partial melting of a spinel-garnet lherzolite mantle source, which was metasomatized by subduction-related melts. Combined with regional geological data, these diabases were most likely formed within a post-collisional extensional setting, reflecting tectonic processes associated with the final amalgamation of the Yangtze Craton within the context of the Rodinia supercontinent assembly.

LIU LongYu, CUI XiaoZhuang, CHEN FengLin, LONG XunRong, MA ZhiXin, SUN ZhiMing. Geochronology, Petrogenesis, and Tectonic Significance of the Neoproterozoic Diabases in the Jinkouhe Area, Western Sichuan Province[J]. Acta Sedimentologica Sinica, 2024, 42(6): 1890-1902. doi: DOI:10.14027/j.issn.1000-0550.2024.111
Citation: LIU LongYu, CUI XiaoZhuang, CHEN FengLin, LONG XunRong, MA ZhiXin, SUN ZhiMing. Geochronology, Petrogenesis, and Tectonic Significance of the Neoproterozoic Diabases in the Jinkouhe Area, Western Sichuan Province[J]. Acta Sedimentologica Sinica, 2024, 42(6): 1890-1902. doi: DOI:10.14027/j.issn.1000-0550.2024.111
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0.   引言

  • 扬子克拉通是东亚地区最大的前寒武纪克拉通之一,其新元古代及显生宙岩石覆盖严重,仅零散出露少量太古代结晶基底[1]。近年来,越来越多的研究工作揭示目前已确认的太古代基底岩系记录了不同的地壳演化过程,可能代表当时分别独立的微陆块[2⁃3]。然而,它们普遍经历了2.00~1.95 Ga碰撞相关的变质和岩浆作用[3⁃7],伴随大量~1.85 Ga A型花岗岩和基性岩脉的侵位[6],被认为与全球Columbia超大陆的聚合有关。据此,有学者认为扬子克拉通自古元古代晚期以来一直是一个统一的块体[2,8]。然而,最近在扬子克拉通内部识别出数套中元古代晚期的蛇绿岩,如庙湾[9]、石棉[10]和菜子园[11]蛇绿岩,指示其当时很可能存在几个独立的块体[12]。

    川西金口河新元古代辉绿岩发育于石棉蛇绿岩以东约120 km处,恰好位于一些学者推测的石棉—庙湾一线的隐蔽缝合带上（图1）[13⁃14]。迄今为止,前人仅对金口河辉绿岩开展过少量同位素年代学研究工作[15],其岩石成因及构造意义一直未得到有效约束。鉴于此,本文对金口河辉绿岩进行了系统的岩石学、锆石U-Pb年代学和全岩地球化学研究,旨在进一步限定其侵位时代,揭示其岩石成因和构造环境,并讨论其大地构造意义。研究成果有助于更好地理解Rodinia超大陆聚散背景下扬子克拉通的地质演化历史。

    Figure 1.  (a) Precambrian geological map of South China (modified from reference [13]); (b) Precambrian geological map of the western Yangtze Craton (modified from reference [18]); (c) geological map of the Jinkouhe area in the western Sichuan province (modified from references [15, 21⁃22])

1.   地质背景与样品描述

  • 扬子克拉通北与华北克拉通以秦岭—大别—苏鲁造山带为界,西南与印支地块以哀牢山—红河断裂带为界,西北与松潘—甘孜地体以龙门山断裂为界,东南以江南造山带与华夏地块为界[1]。由于被新元古代和显生宙沉积盖层覆盖严重,扬子克拉通前新元古代地质记录仅零星出露（图1a）[13,16⁃18]。古元古代晚期至中元古代地层在扬子克拉通西南缘出露较为齐全,包括经历了绿片岩相至低角闪岩相变质作用的大红山群、河口群和东川群以及仅经历了低绿片岩相变质作用的昆阳群、会理群和峨边群[1,19]。区域上,这些地层普遍被中—新元古代花岗岩、辉绿岩、辉长岩以及少量闪长岩所侵入,彼此之间呈断层接触或完全分离（图1b）[1,18⁃19]。

    峨边群主要分布于川西金口河地区,为一套以碎屑沉积岩为主,夹少量碳酸盐岩及酸性—基性的火山岩、火山碎屑岩的地层,其上为苏雄组、开建桥组、列古六组或震旦系不整合覆盖（图1c）[15,20-22]。传统上,峨边群被认为与川西会理—会东地区会理群和滇中地区昆阳群相当,均为中元古代晚期地层[1,18]。熊国庆等[23]报道的峨边群上部凝灰岩锆石U-Pb年龄为779±16 Ma,但其与侵入至其中的辉绿岩年龄813±8 Ma[15]和花岗岩年龄860±4 Ma[21]明显冲突。峨边群底部变玄武岩的锆石U-Pb年龄为1 019±5 Ma[22],但最近获得的变质细粒岩屑砂岩最年轻一组碎屑锆石的年龄峰值为~910 Ma[20]。因此,尽管峨边群的沉积时代目前仍无定论,但可大致限定在1 020~860 Ma。

    在金口河地区,大量辉绿岩脉侵入峨边群中部枷担桥组的灰白色硅化白云岩（图2a,b）,临近辉绿岩脉的硅化白云岩明显破碎,且沿岩脉侵入方向发生明显拖曳变形,可见较为明显的冷凝边。这些辉绿岩脉大多呈NE—SW或E—W向展布,宽0.5~2.5 m不等,一般长30~50 m,表面已明显风化,多呈灰黄色,新鲜面呈灰黑色（图2a,b）。这些辉绿岩脉显示块状构造、辉绿结构,主要组成矿物为斜长石（55%）和辉石（35%）,少量石英（3%）、磁铁矿和钛铁矿（3%）。斜长石自形程度较高,呈规则的长条状或板状,部分镜下可见其定向排列,其格架中充填细粒辉石,辉石呈半自形—他形粒状,部分晶体粗大,包含细小的自形斜长石（图2c,d）。石英呈微细粒状,磁铁矿和钛铁矿多呈细粒状,少数为条状,不均匀分布。

    Figure 2.  Typical field photos (a, b) and micrographs (c, d) of the diabases in the Jinkouhe area, western Sichuan province

2.   分析方法

  • 辉绿岩样品的锆石分选工作在廊坊岩拓地质服务有限公司完成。锆石制靶及阴极发光（CL）照相在北京离子探针中心完成。样品锆石U-Pb同位素定年在北京离子探针中心利用SHRIMPⅡ分析完成。岩石样品经破碎、淘洗、重液分离和电磁分离几道工序后,在双目镜下挑选晶型完好、具有代表性的锆石颗粒和标准锆石TEMORA一起粘在树脂台上,打磨抛光,制成样靶。离子探针测试之前,在电子探针上进行阴极发光（CL）照相,以确定锆石的内部结构和成因。然后将样品靶清洗、镀金后,在SHRIMP Ⅱ离子探针上根据实验室规范程序进行测定。一次离子为4 nA,测试孔径约为32 μm。数据是以5次扫描为一组,通过Zr、Pb、U和Th同位素种类的测定,每测定3或4个分析点之后测定1块标准锆石。标样为M257（U含量为840×10-6）[24]和TEM（年龄为417 Ma）[25],分别用于U含量和年龄校正。标准锆石TEM与未知样品比例为1∶2~1∶3,以检验仪器状态的稳定性,实现对未知样品测定数据的同位素分馏校正。样品锆石采用206Pb/238U年龄,每个分析点数据均为5次扫描的加权平均值,年龄误差为1σ绝对误差,同位素比值为1σ相对误差,详细分析流程见文献[26⁃27]。样品的全岩主、微量元素分析在武汉上谱分析科技有限责任公司进行。其中,主量元素含量分析利用X射线荧光光谱仪（XRF）完成,测试相对标准偏差（RSD）小于2%；微量元素含量分析利用电感耦合等离子体质谱仪（ICP-MS）完成。

3.   分析结果

    3.1.   锆石U⁃Pb年代学

  • 金口河辉绿岩样品中锆石主要呈自形—半自形的棱柱或短柱状,少部分锆石呈板片状。锆石长轴为80~150 μm不等,长宽比为1∶1~2∶1。大部分锆石具有宽缓的震荡环带,少部分具有较为均匀的内部结构,表明为基性岩浆成因（图3）。

    Figure 3.  Zircon U⁃Pb age concordia (a) and weighted average (b) plots of diabase samples from the Jinkouhe area, western Sichuan province

    对辉绿岩样品JKHD2的12颗锆石进行了12个点位的U-Pb定年分析（表1）。其中#1表现出明显年轻的不谐和年龄,可能为后期热事件所致；#10具有明显较老的年龄,可能为捕获锆石。剩余10个分析点的Th含量介于31~348 μg/g,U含量介于66~480 μg/g,Th/U比值介于0.46~1.22,206U/238Pb年龄介于838~798 Ma,加权平均年龄为823.5±9.6 Ma（MSWD=1.3,n=10）,与其谐和年龄825.1±8.5 Ma（MSWD=1.1,n=10）完全一致,解释为辉绿岩样品JKHD2的结晶年龄。

    样品号206Pbc/%U/×10-6Th/×10-6232Th/238U206Pb/×10-6206Pb/238U 年龄/Ma207Pb/206Pb年龄/Ma207Pb*/206Pb*误差±%207Pb*/235U误差±%206Pb*/238U误差±%
    JKHD2-010.69372800.2213.1257.7±3.828±1600.046 66.70.2626.90.040 81.5
    JKHD2-020.093492500.7440.6819±10815±380.066 31.81.2372.30.135 41.3
    JKHD2-030.572321520.6827.8838±11798±620.065 72.91.2583.30.138 71.4
    JKHD2-040.442651170.4631.2824±11677±680.062 13.21.1673.50.136 41.4
    JKHD2-050.104803480.7556.5828±10837±300.067 01.51.2652.00.137 01.3
    JKHD2-060.463282050.6438.4820±11803±680.065 93.31.2323.50.135 61.4
    JKHD2-070.623102350.7937.2838±11667±700.061 83.31.1823.60.138 71.4
    JKHD2-080.2297930.9911.0798±14953±730.070 93.61.2884.00.131 81.8
    JKHD2-091.0666310.487.9833±22786±1500.065 47.11.2437.60.138 02.8
    JKHD2-100.341341621.2516.9880±14828±610.066 72.91.3453.40.146 21.7
    JKHD2-110.4972841.228.6838±15863±950.067 84.61.2985.00.138 82.0
    JKHD2-120.092471730.7328.1803±11803±460.065 92.21.2052.60.132 61.5

    Table 1.  Sensitive High Resolution Ion MicroProbe(SHRIMP) zircon U⁃Th⁃Pb isotope analyses of diabase samples from the Jinkouhe area, western Sichuan province

    • 3.2.   全岩地球化学

  • 对8件金口河辉绿岩样品进行全岩主、微量地球化学测试,分析结果列于表2。由于辉绿岩样品较高的烧矢量（LOI=2.97%～7.44%）,因此表2中已将其主量元素含量在去除烧矢量后回算为100%。金口河辉绿岩样品具有相对较低的SiO2含量（47.48%~51.67%）以及相对较高的TiO2含量（2.14%~3.38%）和Fe2O3t含量（12.74%~17.77%）。此外,其Al2O3含量介于14.28%~15.64%,MgO含量介于5.71%~9.59%。在Zr/TiO2-Nb/Y岩性判别图解中,几乎所有样品落入亚碱性玄武岩区域（图4a）[28]；在FeOt/MgO-SiO2图解中,所有样品均落入拉斑玄武岩区域（图4b）[28]。

    样品号JKHD1-H02JKHD2-H01JKHD2-H02JKHD2-H03JKHD2-H04JKHD3-H01JKHD3-H02JKHD3-H05
    地化指标
    SiO251.6747.5447.9647.4848.1850.5150.9450.45
    Al2O315.2914.6114.5014.2814.6415.4515.5715.64
    Fe2O3t12.7417.4817.7717.3616.7115.4215.1114.41
    CaO6.977.596.627.867.445.114.606.02
    MgO7.775.776.155.935.719.369.599.30
    K2O1.171.431.401.411.640.430.460.45
    Na2O1.691.551.461.531.540.970.900.94
    TiO22.143.293.363.363.382.172.252.22
    P2O50.360.520.530.540.520.340.350.35
    MnO0.190.230.230.240.230.240.230.22
    LOI2.977.126.447.446.236.426.477.14
    FeOt11.4715.7315.9915.6215.0413.8713.5912.97
    La17.5025.9025.2026.4026.4015.5014.9014.20
    Ce40.1060.6059.3061.3062.2037.2035.6034.30
    Pr5.728.278.178.448.535.255.114.87
    Nd25.7037.5037.5038.7038.8025.7024.5024.00
    Sm6.358.848.668.899.176.366.415.95
    Eu1.792.602.572.652.581.821.791.70
    Gd8.0610.109.7810.3010.207.517.627.31
    Tb1.191.571.561.591.621.251.221.14
    Dy7.299.059.089.379.507.707.587.09
    Ho1.471.861.881.921.881.521.511.48
    Er4.125.305.235.355.464.364.374.20
    Tm0.650.830.860.850.870.700.720.66
    Yb4.085.115.145.165.464.514.354.24
    Lu0.630.750.770.790.780.660.640.61
    Y39.1048.1047.5047.2050.0040.3038.6037.50
    Sr214225210217239113111122
    Ba6 2222 1591 4961 9102 266132275177
    V249186184180185194196184
    Sc43.4036.3035.5035.4036.3038.1037.9035.90
    Th2.192.622.712.712.741.531.471.39
    U0.420.470.750.480.480.30.280.25
    Co44.2043.7045.8043.5045.9056.6054.2050.40
    Ni48.9024.3030.8024.1024.80176.00165.00130.00
    Rb51.3054.9054.8054.7063.3029.5029.6026.30
    Nb12.1018.5018.7018.2019.2010.109.799.65
    Cr587125120116117400419386
    Ta0.590.910.930.900.910.520.490.46
    Pb3.995.034.834.736.101.682.582.20
    Zn122152149148153133136118
    Ga20.9024.4024.3023.8025.2020.4020.0019.30
    Zr183289289282295181174170
    Hf4.707.107.347.087.214.764.754.45
    Ge1.961.951.941.911.941.901.781.74
    Cs5.697.808.578.187.436.306.205.45
    Ti12 81719 69820 16220 15620 28413 01213 46913 302
    Eu/Eu*0.760.840.850.850.820.810.780.79
    Ce/Ce*0.981.021.011.011.021.011.001.01
    Mg#58.0335.1439.4535.6735.9672.5878.4670.00
    (La/Yb)N3.083.643.523.673.472.472.462.40
    (Gd/Yb)N1.631.631.571.651.551.381.451.43
    Ti/V51.47105.91109.58111.98109.6467.0868.7272.29
    LREE97.16143.71141.40146.38147.6891.8388.3185.02
    HREE27.4934.5734.3035.3335.7728.2128.0126.73
    REE124.65178.28175.70181.71183.45120.04116.32111.75
    LREE/HREE3.534.164.124.144.133.263.153.18
    注：JKHD1(103°08′35″ E, 29°20′24″ N)；JKHD2(103°07′25″ E, 29°17′25″ N)；JKHD3(103°08′23″ E, 29°21′13″ N)；主量元素单位wt.%；微量元素单位μg/g；稀土元素单位μg/g。

    Table 2.  Geochemical results of diabase samples from the Jinkouhe area, western Sichuan province

    Figure 4.  (a) Zr/TiO2⁃Nb/Y and (b) FeOt/MgO⁃SiO2 diagrams of the Jinkouhe diabase samples (based on reference [28])

    金口河辉绿岩样品的稀土元素总量介于111.75~183.45 μg/g。在球粒陨石标准化稀土元素配分图中（图5a）[29],所有样品的稀土元素配分曲线均显示出右倾的特征,（La/Yb）N比值介于2.40~3.67,（Gd/Yb）N比值介于1.38~1.65。所有样品均显示出一定程度的Eu负异常（Eu/Eu*=0.76~0.85）,表明其在岩浆演化过程中经历了一定程度的斜长石分离结晶作用。在原始地幔标准化蛛网图中（图5b）[29],这些样品表现出一定程度的Nb-Ta亏损。

    Figure 5.  Chondrite⁃normalized rare earth element (REE) distribution patterns (a) and primitive mantle⁃normalized spidergrams (b) of the Jinkouhe diabase samples (standard values are based on reference [29])

4.   讨论

    4.1.   形成时代

  • 选择1件金口河辉绿岩样品进行了SHRIMP锆石U-Pb年代学分析。CL图像显示,样品的锆石颗粒形态较为均一,内部多发育宽缓的振荡环带,少量可见均匀的内部结构,表明其为基性岩浆成因。其Th/U比值介于0.46~1.22,均高于0.1,也指示其为岩浆成因。测得的锆石U-Pb年龄的谐和度高,且较为一致,谐和年龄为825.1±8.5 Ma（MSWD=1.1,n=10）,加权平均年龄为823.5±9.6 Ma（MSWD=1.3,n=10）（图3）,与前人报道的SHRIMP锆石U-Pb年龄813.4±8.2 Ma在误差范围内基本一致[15]。因此,金口河辉绿岩的侵位时代可以限定为~820 Ma。

    • 4.2.   岩石成因

    4.2.1.   蚀变作用的影响

  • 辉绿岩样品具有较高的烧失量（2.97%~7.44%）,表明可能受到了一定程度的后期蚀变作用影响。因此,在使用地球化学数据讨论其岩石成因之前,有必要讨论蚀变作用的影响。锆（Zr）作为最不易迁移的元素之一,其与特定微量元素的双变量图被广泛应用于评估这些元素在后期岩浆过程中的活性[30]。本文分析结果表明,高场强元素（HFSEs；如Nb、Ta、Th和Hf）、稀土元素（REEs；如Nd、Yb）、相容元素（如Cr和Ni）、大部分的大离子亲石元素（LILEs；如Mg、Ca、Pb和Ba）以及Y都与样品中的Zr具有良好的相关性。因此,这些元素在后期岩浆过程中相对不易迁移,可以用于后续的岩石成因的解释和讨论。

    • 4.2.2.   地壳混染与分离结晶

  • 地幔来源的岩浆在演化过程中可能受到地壳物质的混染作用[31],由于地壳中Nb-Ta-Ti等元素的剧烈亏损,一定程度的地壳混染作用往往会导致岩石具有岛弧岩浆岩的地球化学特征[32⁃33]。金口河辉绿岩表现出轻微的Nb-Ta亏损,表明其可能受到了一定程度的地壳混染作用。但是,这些样品的La/Nb比值（1.35~1.53）和Th/Nb比值（0.14~0.18）较低,显著低于大陆地壳的比值（La/Nb=2.20,Th/Nb=0.440）,并接近原始地幔的比值（La/Nb=0.94,Th/Nb=0.117）[34]。此外,少量的地壳混染通常会产生正的Zr-Hf异常[32],但在金口河辉绿岩样品中未观察到这些特征（图5）。因此,金口河辉绿岩在岩浆上升侵位过程中受到的地壳混染作用可以忽略不计。

    玄武质岩石的初始岩浆通常具有较高Cr（>1 000 μg/g）、Ni（>400 μg/g）含量和Mg#值（Mg#是岩石中镁铁之间的比例,即Mg2+和Fe(t)2+的比值,大于73）[35⁃36]。金口河辉绿岩样品则具有相对较低的Cr（<600 μg/g）、Ni（>200 μg/g）以及较低的Mg#值（平均值为53）,表明其在岩浆演化过程中经历了一定程度的铁镁质矿物（如单斜辉石和橄榄石）的分离结晶作用。如Harker图解所示,MgO与Cr和Ni均表现较强的正相关性（图6a,b）,指示橄榄石和单斜辉石分离结晶；MgO与CaO和CaO/Al2O3表现负相关性（图6c,d）,同样指示单斜辉石分离结晶；Fe2O3和TiO2与MgO呈负相关（图6e,f）,表明Fe-Ti氧化物的分离结晶。此外,金口河辉绿岩样品还表现出明显的Eu负异常（Eu/Eu*=0.76~0.85）,表明在岩浆上涌和演化过程中发生了斜长石分离结晶。

    Figure 6.  Harker diagrams of the Jinkouhe diabase samples

    • 4.2.3.   幔源岩浆属性

  • 研究表明,铁镁质岩浆一般来源于地幔橄榄岩的部分熔融。金口河辉绿岩具有亏损高场强元素（如Nb,Ta,Ti）,并富集轻稀土元素（如La,Ce,Pr,Nd）和大离子亲石元素（如Ba,Rb,Sr,Pb）的地球化学特征,在排除显著地壳混染情况下,表明其岩浆源区可能被俯冲板片的熔体或流体交代富集。在Th/Yb-Nb/Yb图解中,显示出金口河辉绿岩的地幔源中加入了2%~3%的俯冲带成分（图7a）[37]。同样,较低的Nb/La比值（0.65~0.74）表明它们可能来源于一个混合的地幔源区,包含软流圈和岩石圈成分（图7b）[38]。由板片熔体改造的地幔源区很可能表现出比由流体改造的地幔更低的Th/Zr比值[39]。辉绿岩样品均表现出较低的Th/Zr比值和较高的Nb/Zr比值（图7c）[40],表明Nb的富集与俯冲作用产生的熔体有关,而非流体。因此,金口河辉绿岩的岩浆源区受到了俯冲熔体交代富集作用。

    Figure 7.  Plots of elements and elemental ratios of the Jinkouhe diabase samples

    稀土元素Sm和Yb被普遍用于限定幔源岩浆起源[41]。根据不同类型地幔源区的部分熔融模型,这些辉绿岩分布于尖晶石—石榴石二辉橄榄岩和尖晶石二辉橄榄岩熔融曲线之间,5%~10%水平（图7d）[42]。这表明软流圈地幔上升到相对较浅的深度,即小于85 km,导致减压熔融,其中地幔中的石榴石向尖晶石的过渡可能发生[43]。因此,金口河辉绿岩可能起源于浅层尖晶石—石榴石二辉橄榄岩的部分熔融,其原始岩浆遭受了俯冲熔体的交代。

    • 4.3.   构造环境

  • 区域上,广泛发育同时期的花岗岩,与金口河辉绿岩构成了双峰式岩浆组合,属于伸展环境中的典型产物。伸展环境可能由弧后伸展[44⁃45]、造山后伸展[46⁃47]或大陆裂谷[48⁃49]引起。在弧后环境中,玄武岩通常显示出从N-MORB到岛弧或钙碱性玄武岩的地球化学特征过渡[50],可能是由于在地幔楔中的俯冲板块相关流体交代作用导致的[51]；而在造山后伸展环境中的玄武岩通常属于钙碱性和拉斑玄武岩系列,并由于来自先前交代作用的岩石圈地幔的部分熔融而表现出明显的Nb-Ta负异常[47,52]。相比而言,在大陆裂谷中,玄武岩由于未受到俯冲物质或交代作用的影响,通常没有明显的Nb-Ta-Ti亏损。如上所述,金口河辉绿岩样品具有拉斑玄武岩的地球化学特征,并表现出较为明显的Nb-Ta亏损（图5b）,这些特征均与造山后伸展环境的玄武岩类似。

    金口河辉绿岩形成于造山后伸展环境还得到了以下证据的支持：（1）相较于在地质过程中较为活跃的大离子亲石元素,高场强元素（如Nb、Ta、Zr、Hf等）在岩浆岩的生成和演化中相对稳定,因此常用于推断岩浆岩的成因环境[53⁃55]。在Hf/3-Th-Ta图解中（图8a）[56],样品大部分落在岛弧钙碱性玄武岩区；在Nb*2-Zr-Y图解中（图8b）[53],所有样品落入板内拉斑和火山弧玄武岩区域；而在Zr-TiO2和Zr-Zr/Y图解中,所有样品都位于板内区域（图8c,d）[37,57⁃58]。这些特征表明,金口河辉绿岩同时具有岛弧岩浆岩和板内玄武岩的地球化学特征。（2）金口河地区侵入枷担桥组中的~860 Ma花岗岩,具有高钾钙碱性、富铝S型花岗岩的地球化学特征,可能形成于与碰撞相关的构造环境[22]。（3）最新的碎屑锆石研究表明,新元古代早期的峨边群枷担桥组在沉积时期接受了同沉积岩浆活动的输入,同时有相当比例来自相邻板块较老物源的加入,可能沉积于碰撞相关的构造背景[20]。

    Figure 8.  Geochemical discrimination diagrams of the diabases samples from the Jinkouhe area

    • 4.4.   大地构造意义

  • 最近,在扬子克拉通内部识别出两套近东西向展布的中元古代晚期蛇绿混杂岩,~1.12 Ga庙湾蛇绿岩[9,59]和~1.07 Ga石棉蛇绿岩[10]。根据二者具有相似的岩石组合、形成时代和地球化学特征,有学者推测当时可能存在一个横贯目前扬子克拉通内部的大洋盆地,将其分割为南、北两个部分（图1）[10,13⁃14]。其他地质事实也支持这一认识：（1）前新元古代岩石分布不均,目前已确认的太古代岩石多出露于扬子北部,而南部以出露大量古—中元古代岩石为特征[1,13⁃14]；（2）扬子北缘与东南缘细粒沉积岩的Nd同位素组成,表明中元古代时期二者具有截然不同的物源,指示当时扬子内部可能为一个陆内裂陷或洋盆[60]；（3）重力异常和航磁异常显示四川盆地沿重庆—华蓥一线可分为两个微地块[61],可能代表扬子内部隐蔽缝合带的中间部位。此外,在川西会理地区还识别出一套近东西向展布的中元古代菜子园蛇绿岩[11],其两侧在中元古代晚期经历了截然不同的构造演化过程[62-63],指示扬子克拉通中元古代时期很可能分为多个微陆块。

    金口河~820 Ma辉绿岩脉发育位置独特,分布于石棉蛇绿混杂岩以东约120 km处,恰好位于推测的扬子克拉通内部的隐蔽缝合带上[13⁃14]（图1）。~1.07 Ga石棉蛇绿岩显示SSZ型蛇绿岩的地球化学特征,被认为形成于洋壳沿扬子北缘向南俯冲导致的石棉—庙湾弧后盆地环境[10]。如前所述,该地区910~860 Ma枷但桥组碎屑物源显示碰撞相关构造背景的特征[20],而侵入其中的S型花岗岩则可能形成于碰撞相关构造环境[22]。可以对比的是,庙湾地区小溪口沉积岩形成时代为900~850 Ma,显示弧前盆地的物源特征,而侵入其中的~850 Ma淡色岩脉具有碰撞后花岗岩体的同位素组成[13]。可以看出,如果石棉—庙湾洋盆存在,其闭合时代可以大致限定为910~850 Ma。考虑到金口河辉绿岩兼具岛弧岩浆岩和板内玄武岩的地球化学特征（图8）,推测其形成很可能与石棉—庙湾洋盆的闭合有关。因此,金口河辉绿岩脉可能是与扬子克拉通新元古代早期拼合形成有关的后碰撞岩浆作用的产物,响应于全球Rodinia超大陆的聚合。

5.   结论

  • （1） 川西金口河地区辉绿岩样品的SHRIMP锆石U-Pb定年结果为823.5±9.6 Ma,限定其侵位时代为~820 Ma。

    （2） 金口河辉绿岩具有拉斑玄武岩的地球化学特征,并显示出一定程度的Nb-Ta亏损和Eu负异常,可能起源于浅层尖晶石—石榴石二辉橄榄岩的部分熔融,其原始岩浆遭受了俯冲熔体的交代。

    （3） 金口河辉绿岩同时具有岛弧岩浆岩和板内玄武岩的地球化学特征,为俯冲流体交代富集的岩石圈地幔在伸展环境下部分熔融的产物,其形成很可能与中元古代石棉—庙湾洋盆的闭合有关。

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