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北大巴山平利一带晚三叠世粗面岩和基性岩墙群成因及其地质意义

陈涛, 陈隽璐, 李平, 朱卫红, 郝晨羽, 孙吉明, 王健, 颜玲丽

陈涛,陈隽璐,李平,等. 北大巴山平利一带晚三叠世粗面岩和基性岩墙群成因及其地质意义[J]. 西北地质,2025,58(1):93−105. doi: 10.12401/j.nwg.2023071
引用本文: 陈涛,陈隽璐,李平,等. 北大巴山平利一带晚三叠世粗面岩和基性岩墙群成因及其地质意义[J]. 西北地质,2025,58(1):93−105. doi: 10.12401/j.nwg.2023071
CHEN Tao,CHEN Junlu,LI Ping,et al. Petrogenesis of Late Triassic Trachyte and Basic Dike Swarms in Northern Dabashan and Its Geological Significance[J]. Northwestern Geology,2025,58(1):93−105. doi: 10.12401/j.nwg.2023071
Citation: CHEN Tao,CHEN Junlu,LI Ping,et al. Petrogenesis of Late Triassic Trachyte and Basic Dike Swarms in Northern Dabashan and Its Geological Significance[J]. Northwestern Geology,2025,58(1):93−105. doi: 10.12401/j.nwg.2023071

北大巴山平利一带晚三叠世粗面岩和基性岩墙群成因及其地质意义

基金项目: 中国地质调查局项目(DD20243439、DD20230228、12120114020501、DD20230215),陕西省自然科学基础研究计划(2023-JC-ZD-15、2023-JC-YB-268)和国家自然科学基金项目(40972150)联合资助。
详细信息
    作者简介:

    陈涛(1992−),男,助理工程师,长期从事基础地质研究工作。 E−mail:297041366@qq.com

    通讯作者:

    陈隽璐(1964−),男,研究员,长期从事大地构造研究工作。E−mail:chjl0116@163.com

  • 中图分类号: P581

Petrogenesis of Late Triassic Trachyte and Basic Dike Swarms in Northern Dabashan and Its Geological Significance

  • 摘要:

    北大巴山紫阳−平利−竹溪一带较为广泛地分布有一期基性岩墙与碱性火山岩,这些火山−侵入岩系多被认为形成于早古生代。通过对基性岩和粗面岩的LAICP−MS锆石UPb同位素测年工作发现,其形成时代分别为(219.5±2.2)Ma和(223.9±2.8)Ma,属于晚三叠世岩浆作用的产物。其中,基性岩墙中的辉绿岩显示出贫Si、高Ti的特征,粗面岩显示高Ti、富碱的特征,两类岩石轻、重稀土元素分馏存有一定差异。辉绿岩的Pb−Sr−Nd同位素组成上显示出OIB物质源区组成且具有EMⅡ和上地壳富集物质成分的加入,碱性系列的粗面岩在原始地幔标准化图解上则显示出大隆起的微量元素分布样式,构造环境判别图解均显示出二者与板内岩浆活动具有成因关系。结合区域地质特征,认为北大巴山地区晚三叠世的粗面岩−辉绿岩组合形成于勉−略洋闭合之后持续的板内伸展活动,为南秦岭地区深部地幔岩浆物质演化的地质记录。

    Abstract:

    An amount of basic dikes and alkaline igneous rock occurred in Ziyang−Pingli−Zhuxi regions of the North Daba Mountain, most of these rock series are believed to have been formed in Early Paleozoic magmatic activity, while the zircon U−Pb isotope dating of the basic dike swarms and trachyte in this study show them formed at (219.5 ± 2.2)Ma and (223.9 ± 2.8) Ma, respectively. The rock series can be regarded as a product of Late Triassic magmatism. The diabase from the basic dike swarms shows a characteristic of high Ti and low Si, trachyte shows a feature of high Ti and is rich in alkali, both of which have some differences for the fractionation of REE. The Pb−Sr−Nd isotopic composition of the diabase shows a source region of OIB, with some additions of enrichment material of EmⅡ and upper crust, the trachyte shows an obvious rise inthe Primitive Mantle normalized diagram. Thus, both of them are formed by the intra-plate magmatism. The Late Triassic trachyte−diabase assemblage in North Beidaba Mountain is considered to have been formed by the continuous intraplate extension after the closure of the Mian−Lue Ocean, also is a geological record of mantle derived magma in South Qinling.

  • 石墨是中国24种战略性矿产资源之一,因其具有良好的导电性、导热性、化学稳定性、耐高温性、耐腐蚀性等特性,而成为新兴产业的重要原材料(王登红,2019陈正国等,2021张艳飞等,2022)。石墨矿床按成因可划分为区域变质型、接触变质型和同化混染型3类(彭素霞等,2018)。近年来,中国石墨矿产的找矿勘探不断取得突破成果,石墨资源储量增长迅速(颜玲亚等,2018张艳飞等,2020a)。

    宁夏卫宁北山–内蒙古阿拉善左旗南部地区属构造活动带与稳定地块的交接部位,沉积环境多样,并经历了复杂的变质变形过程,矿化蚀变广泛发育,为石墨成矿提供了优越的成矿地质条件。目前在两省交界处内蒙古一侧已发现库井沟、阎地拉图、石驼山–新工地、喇嘛敖包、炭井沟等处石墨矿,预估矿物量超1000万吨(梁利东等,2020),显示了该区良好的石墨找矿前景,而靠宁夏一侧则发现有新照壁山和乌车梁等处石墨矿,勘探程度低、探明储量少,亟需加强基础地质研究。笔者在详细的矿产地质调查基础上,以宁夏卫宁北山单梁山地区库井沟石墨矿床为研究对象,结合前人勘探成果,通过全岩主微量元素分析、C同位素分析等手段,研究含矿岩系的地球化学特征,恢复原岩性质,并与其他石墨矿床进行类比,进而探讨石墨矿床成因。研究成果对卫宁北山地区同类型矿床的研究和找矿前景预测具有借鉴意义。

    宁夏卫宁北山地区处于北祁连造山带的东段,向北与阿拉善陆块相接,向东与鄂尔多斯地块相接,是构造活动带与稳定地块的交汇部位(图1a)。区域上主要出露古生界地层,其次为中、新生界地层。早古生界形成了以次深海斜坡相陆源碎屑–泥质沉积为主的浊积岩系,构成本区古生代各类建造的基底层(仲佳鑫等,2012)。其中自石炭纪开始本区接受连续沉积,地层由老至新分别为前黑山组、臭牛沟组、土坡组、太原组、大黄沟组、红泉组和五佛寺组,其中前黑山组、臭牛沟组为一套滨海–泻湖相碎屑岩和碳酸盐岩沉积组合,是卫宁北山地区矿床产出的主要层位。(图1b)。卫宁北山地区岩浆活动不强烈,仅在苦井沟、黄石坡沟、二人山、金场子等地零星发育一些闪长玢岩脉,岩脉一般宽1~6 m,延伸长度变化大,从数十米到数百米不等,走向主要呈近EW向,侵入余上泥盆统和下石炭统中。显微镜下观察发现闪长玢岩蚀变严重,主要为绿泥石化、绢云母化、高岭土化。锆石U-Pb年代学研究表明闪长玢岩的成岩年龄在143~150 Ma左右(刘勇等,2010艾宁等,2011)。

    图  1  研究区大地构造位置图(a)(霍福臣等,1989郭佩等,2017)及区域地质略图(b)(据中卫幅1∶20万区域地质图)
    Figure  1.  (a) Geotectonic location and (b) regional geological maps of Zhongwei

    卫宁北山地区构造类型复杂,主要为一系列走向近EW的略呈弧形弯曲的复式背、向斜,轴向长5~28 km,宽度可达数公里。与石墨矿床密切相关的是单梁山–骆驼山复式背斜。北屋子梁向斜呈紧闭线状分布于矿区北侧,走向近EW向,轴长大于16 km,两翼倾向相向,其中北翼倾向150°~190°,倾角40°~75°,南翼倾向330°~10°,倾角60°~75°。断裂构造多为走滑性质的逆冲断层,在研究区西部主要呈EW向或近EW向,在东部则以NW向、SN向为主。

    库井沟矿区范围内主要出露泥盆纪—石炭纪地层,自下而上分为中宁组、臭牛沟组、靖远组,此外还有少量第四系分布(图2)。中宁组主体色调为紫红色,局部夹灰绿色,主要由砾岩、长石石英砂岩、粉砂岩和泥岩组成,具上粗下细的特征,显示内陆湖泊相碎屑岩沉积环境。臭牛沟组主要岩性为灰白色石英砂岩、灰黑色含石墨碳质板岩、浅灰绿色页岩灰岩及白云质灰岩,是区内晶质石墨矿的含矿层位。靖远组主要分布在矿区北部,岩性以灰白色变质含砾石英砂岩夹粉砂岩、页岩及生物灰岩透镜体为主。第四系主要为风积、冲洪积和残坡积。矿区范围内未见岩浆岩发育。

    图  2  库井沟石墨矿床矿区地质略图(据张春林等,2017
    Figure  2.  Geological map of the Kujinggou graphite deposit

    矿区范围内的构造形迹主要为褶皱和断裂。骆驼山–碱向山背斜位于矿区东南部,两翼地层对称,倾向变化较大,倾角为30°~40°,石炭系臭牛沟组出露于两翼,泥盆统中宁组二段出露于核部,背斜轴向呈NWW—SEE向,对矿区地层分布、构造形态影响明显,背斜卷入的地层在各处出露厚度不一。矿区内主要的断层为F1逆断层,分布于矿区的中部,呈NE向延伸,倾向北西,倾角约为60°,断层延长近300 m。F2逆断层与F3平移断层规模小,未对矿体产生破坏作用。

    矿区内已勘探发现4条主要矿体,分别被命名为Ⅰ号、Ⅱ号、Ⅲ号、Ⅳ号(表1)。矿体呈平行层状分布于变质石英砂岩中,与地层产状基本一致,走向近东西(图3图4A),倾向为350°~10°,倾角为9°~25°。主矿体长约为1500 m,宽约为690 m,矿体厚度为0.83~81.54 m,平均为41.19 m。固定碳品位为2.56%~6.53%,平均为5.53%。赋矿围岩为含石墨碳质板岩,近矿围岩主要是含石墨碳质板岩,实际就是具石墨矿化的碳质板岩,只是未达到工业要求,局部为变质石英砂岩。

    表  1  库井沟矿区晶质石墨矿矿体特征一览表
    Table  1.  Characteristics of orebodies in the Kujinggou graphite deposit
    矿体
    编号
    矿石
    类型
    赋矿标
    高(m)
    埋藏深
    度(m)
    规模(m)厚度厚度变化
    系数(%)
    矿体
    形态
    产状(°)C 品位(%)品位变化
    系数(%)
    长度斜深最小-
    最大平均
    倾向倾角最小-
    最大平均
    晶质
    (鳞片)
    状石墨
    12551340442103110~7212.19~53.0119.7368.23似层状00~143.04~8.974.8752.88
    120113251022200136~10802.00~123.2037.7862.05似层状00~203.03~7.524.3448.49
    11801248159400150~3222.00~50.4825.2366.21透镜状02~144.11~7.525.5061.35
    1270132056400100~35843.92~85.1757.8424.49透镜状04~103.86~5.244.5450.31
    下载: 导出CSV 
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    图  3  库井沟石墨矿床A-A′纵剖面图(据张春林等,2017
    Figure  3.  Vertical section A-A′ showing the distribution and morphology of Kujinggou graphite deposit
    图  4  库井沟石墨矿床矿石特征
    A.矿体的野外露头;B、C.块状矿石;D、E.单偏光下石墨呈鳞片变晶结构,弱定向构造;F.电子显微镜下鳞片状石墨;Gr.晶质石墨
    Figure  4.  Photographs showing mineral associations and textures of Kujinggou graphite ores

    石墨矿石镜下具鳞片变晶结构和变余泥质结构,矿石构造主要包括浸染状构造、块状构造及板状构造。矿石中的有用矿物主要为晶质石墨;脉石矿物主要是碳质和黏土矿物,以及微量黄铁矿、白铁矿、方解石和绢云母。通过109件粒度分析样品的镜下观察,矿石中的石墨主要呈不规则鳞片状单晶或呈块状聚晶集合体产出,石墨晶体大小不等,50目(粒径大于0.287 mm)以上和100目(粒径为0.147 mm)以下的比例相当,绝大部分石墨晶体粒径处于这两个区间内。

    本研究样品采自库井沟石墨矿床Ⅰ号、Ⅱ号、Ⅲ号、Ⅳ号矿体的地表和岩心,岩性均为含石墨碳质板岩,所有样品均较为新鲜,石墨矿物未发生氧化作用。

    主微量元素分析测试由宁夏回族自治区地质矿产中心实验室完成,采用熔片法X-射线荧光光谱法(XRF)分析主量元素,测试结果的相对误差小于2%;采用等离子质谱法(ICP-MS)分析微量元素和稀土元素,测试结果的相对误差低于5%~10%。

    石墨矿石样品先破碎至200目,在双目镜下挑选出纯净的石墨颗粒送至中地大环境地质研究院,采用MAT系列稳定同位素气体质谱仪分析测试碳同位素组成,具体分析方法见Du 等 (2021),分析误差在0.1%以内。

    石墨矿石因遭受不同程度的变质作用和风化作用,其主量元素含量呈现较大的变化范围(表2),SiO2含量为44.22%~70.98%,平均为54.55%;Al2O3含量为8.56%~20.60%,平均为17.13%;K2O含量为1.63%~3.68%,平均为2.80%;Na2O含量为0.32%~3.26%,平均为1.31%;K2O+Na2O含量为2.06%~6.15%,K2O/Na2O值为0.89~5.44,碱度指数(A/NK)为2.6~5.2,铝过饱和指数(A/CNK)为2.0~4.6。石墨矿石总体特征为富Si、Al,碱质含量低且K2O含量大于Na2O含量。

    表  2  库井沟石墨矿床矿石主量元素测试结果(%)
    Table  2.  Major element compositions of ore in Kujinggou graphite deposit (%)
    样品号 位置(m)SiO2CaOMgOAl2O3TFe2O3K2ONa2OP2O5TiO2V2O5固定碳
    ZK704-DH133.6055.090.431.5219.636.833.490.860.2560.430.0182.57
    ZK806-DH227.9053.420.441.8320.057.082.841.090.2140.390.0182.27
    ZK808-DH1136.4051.250.611.8317.857.622.893.260.2980.390.0192.39
    ZK2302-DH263.5053.070.421.5918.617.012.912.750.3350.450.0172.41
    ZK1506-DH1162.9055.730.391.4820.607.503.041.500.1970.350.0212.25
    ZK1508-DH1109.7755.170.361.6320.155.763.280.890.1770.490.0202.56
    ZK008-DH1114.0055.820.651.8219.556.732.990.990.2100.400.0172.48
    ZK1504-DH1127.4054.280.331.5420.206.772.861.140.2700.330.0172.58
    ZK706-DH168.5055.610.291.3020.555.043.680.930.2130.520.0172.51
    ZK2001-DH1106.7754.340.391.8020.197.073.031.090.2420.360.0192.57
    ZK2306-DH186.2054.000.811.6417.617.522.602.740.2400.410.0172.22
    ZK1104-DH170.6070.981.991.3210.763.991.870.480.0720.440.0130.51
    ZK1105-DH183.1055.941.382.3019.976.992.811.250.1780.720.0301.82
    XL XT DH-136.0050.431.601.2714.402.292.740.880.310.580.1910.72
    XL XT DH-255.7063.330.330.3310.661.741.740.320.370.420.1210.65
    XL XT DH-349.1053.420.880.578.567.371.631.590.250.400.0811.52
    XL XT DH-438.0044.220.170.4919.824.973.361.090.130.730.0494.81
    XL XT DH-553.1045.710.240.519.1812.712.690.840.080.290.1112.06
    下载: 导出CSV 
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    库井沟石墨矿床微量元素和稀土元素测试结果如表3表4所示。石墨矿石的微量元素含量总体较稳定,大离子亲石元素Rb、Sr、Ba的含量具有较宽的变化范围,分别为73.8×10−6~204.3×10−6、72.9×10−6~264.2×10−6和264.4×10−6~740.7×10−6,Rb(139.7×10−6)、Sr(194.7×10−6)及Ba(527.7×10−6)的平均值均小于后太古宙澳大利亚平均页岩(PAAS; Taylor et al., 1985)中Rb、Sr、Ba的含量。Sr、Nb、Ti等元素亏损明显,Zr略微富集(图5)。Rb/Sr值为0.25~1.4,Sr/Ba值为0.17~1.16。V/Cr值为0.17~1.16,Ni/Co值为0.50~1.12。

    表  3  库井沟石墨矿床含矿岩石微量元素分析结果(10−6
    Table  3.  Trace elements compositions of graphite ore of the Kujinggou graphite deposit (10−6)
    元素 ZK305-1ZK305-2ZK1104-1ZK1104-2ZK1104-3ZK1104-4ZK1105-1ZK1105-2ZK1105-3ZK307-1ZK307-2ZK306-1ZK306-2
    Rb184.573.8110.0204.3154.8168.1153.794.5127.2113.8159.5162.0110.8
    Sr146.172.987.7432.3264.2221.9580.1206.487.486.8136.5121.087.5
    Ba682.1264.4399.9740.7608.2615.6546342.4489.7430.1628.4671.3441.6
    Nb20.2/17.420.821.922.820.8/17.216.420.920.415.6
    Zr179.3165.6208.5198.8238.6238225.8202.5274.6274.1226.4233.6200.5
    V72.221.332.388.077.486.195.744.932.631.576.086.036.0
    Cr81.933.759.584.082.792.998.350.951.950.882.381.745.2
    Co18.39.714.98.712.211.41716.711.11221.117.713.9
    Ni40.818.222.610.534.91742.325.917.919.640.739.521.8
    Y37.216.521.336.135.734.234.617.92019.229.332.717.7
    La63/50.484.876.965.666.7/54.450.863.663.9/
    P780.3528.9786841.91407.6706.11111.3711549.3467.8860800.7542.8
    Ti4910278041005640562057105380380043204280523053203660
    Rb/Sr1.261.011.250.470.590.760.260.461.461.311.171.341.27
    Sr/Ba0.210.280.220.580.430.361.060.600.180.200.220.180.20
    Ni/Co2.231.881.521.212.861.492.491.551.611.631.932.231.57
    V/Cr1.131.581.840.951.071.081.031.131.591.611.080.951.26
    下载: 导出CSV 
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    图  5  库井沟石墨矿石与围岩微量元素蛛网图
    原始地幔值据Sun 等(1989);上地壳值据Rudnick等(2003);PAAS(澳大利亚后太古宙平均页岩)据Taylor 等(1985)
    Figure  5.  Primitive mantle-normalized trace element patterns of ore and host rock in Kujinggou graphite deposit
    表  4  库井沟石墨矿床含矿岩石系稀土元素分析结果(10−6
    Table  4.  Rare earth elements compositions of graphite ore of the Kujinggou graphite deposit (10−6
    样品号 LaCePrNdSmCdEuTbDyHoErTmYbLuY
    XL XT DH-130.532.59.6930.26.075.501.561.073.631.002.390.472.470.5418.2
    XL XT DH-232.213.18.9427.3264.25.621.391.002.700.591.340.261.260.2810.5
    XL XT DH-331.531.810.635.121.96.411.641.234.451.212.820.552.520.5524.2
    XL XT DH-411.110.23.089.27238.61.380.390.301.360.431.110.241.170.268.15
    XL XT DH-515.316.78.6826.21.073.681.050.772.900.791.810.351.570.3415.3
    下载: 导出CSV 
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    石墨矿石的∑REE为101×10−6~137×10−6,平均为117×10−6;LREE/HREE值为5.96~7.33,平均为6.63;δCe值为0.18~0.45,平均为0.32;δEu值为0.73~0.81,平均为0.76。围岩的稀土元素总量∑REE为42×10−6,明显低于石墨矿;LREE/HREE值为5.71,δCe值为0.41,δEu值为0.75。在球粒陨石标准化稀土元素配分曲线上,石墨矿石和围岩样品均表现为左高右低的形态(图6),表明矿石、围岩样品中轻、重稀土元素分异程度较高,轻稀土元素明显富集。石墨矿石和围岩样品都具有δEu和δCe负异常。

    图  6  库井沟石墨矿石与围岩稀土元素配分模式
    原始地幔值据Sun 等(1989);上地壳值据Rudnick等(2003);PAAS(澳大利亚后太古宙平均页岩)据Taylor 等(1985)
    Figure  6.  Chondrite-normalized REE patterns of ore and host rock in Kujinggou graphite deposit

    矿石中石墨样品的δ13C值变化很小,5个样品的δ13C值为−24.3‰~−24.0‰(表5)。

    表  5  库井沟石墨矿床矿石碳同位素组成测试结果
    Table  5.  Carbon isotope compositions of graphite ores in the Kujinggou deposit
    序号样品岩性δ13C(‰)
    1含石墨碳质板岩−24.0
    2含石墨碳质板岩−24.3
    3含石墨碳质板岩−24.3
    4含石墨碳质板岩−24.3
    5含石墨碳质板岩−24.2
    下载: 导出CSV 
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    前人对宁夏卫宁北山石炭纪地层进行过沉积环境研究,认为该区石炭纪—早二叠世早期地层形成于基底坳陷下沉、盆地范围不断扩大的构造背景,臭牛沟组岩性以陆源碎屑岩和碳酸盐岩为主(李红霞等,2016中国区域地质志·宁夏志,2018)。本次研究的石墨矿体产出在碳质板岩中,明显保留沉积层序。石墨矿石和围岩样品显示出与典型变质沉积岩相似的主要元素地球化学特征,表现为低Si、低碱、烧矢量大。石墨矿石和围岩样品的微量元素测试数据表明Rb、Ba等大离子亲石元素较富集,Zr略微富集,Sr、Nb、Ti亏损明显(图5),显示出近海陆源碎屑物的特征。矿石、围岩中轻、重稀土元素分异明显,且二者都具有δEu和δCe负异常,与上地壳和PAAS的稀土元素配分模式相似,但是稀土元素总量低于上地壳和PAAS(图5)。在La/Yb-REE图解(图7)中,样品点全部落入沉积岩-钙质泥岩区域。

    图  7  石墨矿石La/Yb-∑REE图解(底图据Allegre et al., 1978
    Figure  7.  La/Yb-∑REE diagram for the graphite ores

    Ba、Sr含量可以较好地指示水体盐度变化(杨季华等,2020)。Sr与Ba化学性质相似,但是在溶液中Sr比Ba迁移能力强,淡水与海水相混合时,Ba易形成BaSO4沉淀,因此Sr/Ba值可以用于区分海相和陆相沉积环境,当Sr/Ba<1,表明沉积岩为陆相沉积;当Sr/Ba>1,表明沉积岩形成于海相环境(段威等,2020)。研究区石墨矿石的Sr/Ba值除一个值为1.06外,其余全部小于1,反映其原岩形成于陆相环境。在Sr-Ba图解(图8)中,全部样品落在半咸水区和咸水区,并且大部分数据点围绕现代三角洲半咸水黏土区范围边缘分布。变价元素(如V、Cr、Co、Ni)的溶解度随氧化还原条件发生改变为改变,因此,这些元素的含量可以反演古海洋的氧化还原条件,如Ni/Co>7、V/Cr>4.5时,均指示缺氧还原环境(程仕俊等,2021)。库井沟石墨矿床石墨矿石的Ni/Co值为1.2~2.2,V/Cr值为0.3~1.1,指示石墨矿石的原岩形成于相对氧化的沉积环境。Al2O3/(Al2O3+Fe2O3)值m可以指示沉积岩形成的大地构造环境,当m为0.6~0.9时指示大陆边缘环境,m为0.4~0.7时指示远洋深海环境,m为0.1~0.4时指示洋脊海岭环境。库井沟石墨矿床的Al2O3/(Al2O3+Fe2O3)值为0.42~0.86,平均为0.73,接近于大陆边缘环境下Al2O3/(Al2O3+Fe2O3)值(0.6~0.9)。

    图  8  石墨矿石Ba-Sr图解(底图据王仁民等,1986)
    Ⅰ.现代三角洲半咸水黏土区;Ⅱ.太平洋远洋沉积物区;Ⅲ.俄罗斯台地不同年代海相碳酸盐岩区;Ⅳ.现代高咸水沉积物区
    Figure  8.  Ba-Sr diagram for the graphite ores

    因此,推测矿区矿体原岩沉积于大陆边缘的滨浅海,沉积微环境相对氧化,沉积时水体盐度较低,并混合有不均匀的咸水–半咸水。

    石墨C同位素被广泛用于约束石墨物质来源。形成石墨的碳主要来自:①有机物;②碳酸盐岩;③地幔碳。其中有机物的C同位素值介于−17‰~−40‰,集中于−26‰~−28‰范围内(Schidlowski, 1987, 2001; Hoefs, 2009);典型海洋碳酸盐的碳同位素值位于−2‰~+4‰范围内(Sharp, 2007);地幔碳的碳同位素值约为−7‰(Hahn-Weinheimer et al., 1981; Weis et al., 1981)(图9)。陈衍景等(2000)认为生物成因的有机碳和化学成因的无机碳都是华北石墨矿床中的物质来源,其对应不同的矿床成因类型。朱建江等(2021)认为,由富12C的有机物形成的石墨具有轻的C同位素组成特征,由富13C的碳酸盐或地幔碳形成的石墨具有重的C同位素组成特征,当富12C和富13C的两种流体混合时,形成的石墨的C同位素组成介于二者之间。

    图  9  库井沟与不同地区石墨矿床及含碳物质的C同位素值特征对比(数据引自刘敬党等,2017
    Figure  9.  Carbon isotope compositions of graphite and carboniferous from Kujinggou and other regions

    库井沟石墨矿床的C同位素值变化很小,集中在−24.3‰~−24.0‰,靠近有机物C同位素的主要变化区间,暗示库井沟矿床中碳质来源主要为有机物,并掺杂部分无机碳。考虑到矿区内臭牛沟组为一套滨浅海相陆源碎屑岩–碳酸盐岩沉积组合,矿区外围也主要由奥陶系、志留系陆源碎屑岩和碳酸盐岩组成,地层中的碳酸盐岩在发生硅酸盐化蚀变过程中释放出的CO2可以参与到石墨结晶作用,从而作为无机碳的来源。区域上金场子金矿床碳酸盐矿物的C同位素组成特征也表明了沉积碳酸盐是必须的流体CO2来源之一(海连富等,2021)。

    岩石中的石墨可以由有机质通过变质作用生成,也可以在一定的温压条件下从碳过饱和的C-H-O流体中结晶析出(Luque et al., 1998; Zhu et al., 2020张艳飞等,2020b)。库井沟石墨矿床赋存于下石炭统臭牛沟组的浅变质岩中,矿体空间展布严格受地层产状制约,主要呈层状、似层状和透镜体状产于碳质板岩中,与围岩产状近于一致,矿石仍显示出一定的原生沉积特征。赋矿地层臭牛沟组形成于海进作用过程(与地质历史上气候变暖有关),富含动植物化石(宁夏区域地质志,2018),形成碳质的初始富集。区域构造运动为有机碳质成分的石墨化提供了合适的地质环境条件。印支期造山作用活化了先成的断裂构造,并使区域地层进一步挤压变形,原岩中的有机质在区域变质作用过程中发生脱氧、脱氢的还原反应形成碳氢化合物,此外碳酸盐岩发生硅酸盐化蚀变释放CO2,两者易于结合发生氧化还原反应生成石墨。库井沟石墨矿床中鳞片状石墨在围岩中均匀分布,矿石中固定碳含量变化稳定,表明石墨晶体形成后没有发生大范围迁移,而是在进变质作用过程中,在原地重结晶形成粒度更大的鳞片状石墨晶体或集合体。由于区域变质程度不高,臭泥沟组的变质变形程度较弱,导致释放的CO2少,石墨结晶时间短,因此石墨的13C值更靠近有机物的13C值主要变化区间。

    (1)库井沟石墨矿床中矿石类型以含石墨碳质板岩为主,石墨矿石的主量元素特征为低Si、低碱、烧矢量大,指示原岩为一套滨浅海相陆源碎屑岩–碳酸盐岩沉积组合。原岩沉积时的沉积微环境相对氧化,沉积时水体介质性质主要为盐度较低、混合不均匀的咸水–半咸水。

    (2)库井沟石墨矿床的碳同位素值变化很小,集中在−24.3‰~−24.0‰,碳质来源主要为有机碳。

    (3)库井沟矿床赋存于石炭系臭牛沟组变质岩系中,属典型的区域变质型石墨矿床,沉积作用形成碳质的初始富集,区域变质作用造成晶质石墨的富集。

  • 图  1   北大巴山地区地质图及采样位置图(据徐学义等,2014

    Figure  1.   Geological map of north Daba mountain and sampling location

    图  2   平利地区辉绿岩(a)和粗面岩(b)显微照片(正交偏光)

    Kfs. 钾长石;Cpx. 单斜辉石;Hbl. 角闪石;Chl. 绿泥石;Ep. 绿帘石

    Figure  2.   Micrographs of (a) diabase and (b) trachyte in Pingli (cross–polar light)

    图  3   平利地区粗面岩(a)和辉绿岩(b)锆石CL图像

    Figure  3.   CL images of zircon for (a) trachyte and (b) diabase in Pingli

    图  4   锆石U–Pb谐和图及加权平均年龄图

    a、b. 粗面岩;c、d. 辉绿岩

    Figure  4.   Zircon U–Pb concordance diagram and weighted average age diagram

    图  5   平利地区粗面岩和辉绿岩岩石类型判别图

    a. Nb/Y–Zr/TiO2图(Winchester,1977);b. SiO2–(K2O+Na2O)图(Miyashiro, 1974

    Figure  5.   Rock types of trachyte and diabase in Pingli

    图  6   稀土元素球粒陨石标准化图(a)和微量元素原始地幔标准化图(b)

    标准化数据自Taylor 等(1985)Sun等(1989)

    Figure  6.   (a) Chondrite–normalized REE distribution patternss and (b) primitive mantle–normalized trace elements spider diagram

    图  7   平利地区辉绿岩Pb–Sr–Nd物质组成特征图

    Figure  7.   Pb–Sr–Nd composition of diabase in Pingli

    图  8   平利地区辉绿岩和粗面岩构造环境判别图 (据Pearce, 1983

    Figure  8.   Discriminant diagram of tectonic settingfor diabase and trachyte in Pingli

    表  1   平利地区粗面岩和辉绿岩锆石U–Pb同位素物质成分组成表

    Table  1   Zircon LA–ICP–MS U–Pb analytical data for the trachyteand diabase in Pingli

    样品比值年龄(Ma)组成 (10–6U/Th
    207Pb/206Pb±%207Pb/235U±%206Pb/238U±%208Pb/232Th±%207Pb/206Pb±1σ207Pb/235U±1σ206Pb/238U±1σ208Pb/232Th±1σ204Pb206Pb207Pb208Pb232Th238U
    粗面岩(08-66Tw),n=16
    10.050690.001270.24980.006390.035720.000490.00780.00018227342265226315743.4323613.674.67224.6917337.71
    20.051420.001460.246920.007070.034870.000490.005870.00017260402246221311833.5716215.224.86276.3311874.30
    30.051330.001440.261410.007360.037090.000520.010040.00021256392366235320247.3618312.0211.8460.7313552.94
    40.0540.001390.261660.006790.035340.000490.009020.00021371342365224318143.5119713.796.02275.5314615.30
    50.048930.00130.242220.006460.036150.000510.009140.0002114437220522931844<2.1417010.164.97213.8112335.77
    60.054070.00140.26380.006850.035680.00050.011030.00026374342386226322256.621489.854.76174.1611056.35
    70.050880.001550.23790.007190.034240.00050.00960.000252354321762173193583.823092.1892.7231.6910044.33
    80.048390.00130.239920.006420.036430.000520.011350.00027118372185231322856.1721812.847.07263.1414985.69
    90.048090.00140.234010.006750.03580.000520.010650.00029104412146227321464.17208125.32219.6915226.93
    100.048230.00130.230910.006170.035260.00050.008870.00024111372115223317853.9926316.497.03312.2519316.18
    110.049030.001320.229490.006080.034540.00050.00490.0001614936210521939934.311368.885.13443.4610222.31
    120.05180.001350.242330.00620.034630.00050.011810.00031277332205219323763.4622517.799.56307.6817155.58
    130.05630.001510.250470.006560.032960.000480.010750.00029464332275209321663.7620816.9810.2420.1916663.96
    140.051150.002820.252540.013360.035810.000560.011270.0001724812822911227322737.6710311.699.1186.457313.92
    150.050290.001550.223220.006620.033130.000510.01720.0005920841205521033451215.224813.1111.3326.1618385.64
    160.055810.001750.276520.008290.037070.000590.001540.00042445392487235431821.217826.2516.2251.7711874.72
    辉长岩(08-72Tw),n=16
    10.054950.002280.267360.011030.035280.000530.012340.00031410652419224324866.6447.136.78320.28350.41.09
    20.05520.001510.503070.013910.066080.000910.020020.000514203741494126401106.1324815.045.86159.88956.55.98
    30.051050.001550.249550.007610.035450.00050.005860.0002243442266225311843.6478.64.453.23296.43596.92.01
    40.052020.001870.250590.009010.034930.000510.0120.0003286552277221324162.3243.72.5043.46164.02337.92.06
    50.051410.001580.244660.007550.034510.000490.011160.00026259452226219322453.0967.13.874.91246.6520.52.11
    60.054340.001640.259080.007870.034570.000490.010980.00024385432346219322155.581166.8221.41070.3879.80.82
    70.048620.00150.226840.007050.033830.000480.010750.00026130472086214321653.0397.85.7410.1531.6794.81.50
    80.049870.002820.238870.0130.034740.000520.010970.0001318913021711220322133.9796.66.398.71417.06754.41.81
    90.051610.001840.241860.008630.033990.000510.011770.00032268542207215323764.6164.13.937.89353.71496.91.40
    100.05130.002160.242420.010170.034280.000550.011490.00036254672208217323174.7854.93.275.75276.96432.41.56
    110.048610.00450.233740.021320.034870.000560.011050.0002812921021318221322268.5413115.5822.8673.9959.71.42
    120.049710.001890.244410.009290.035660.000550.00970.0003118160222822631956<1.8166.83.756.52372.82506.51.36
    130.055140.002560.258820.011940.034050.000570.011030.000364187323410216422272.9438.42.94.25189.61256.81.35
    140.054660.002040.262820.009840.034880.000550.011190.00037398562378221322574.211228.676.81296.95912.43.07
    150.052360.002120.250410.010120.034690.000560.011650.00039301632278220323482.8152.93.295.29244.19412.81.69
    160.051280.002310.236950.010610.033520.000560.011290.00041253722169213322783.1434.82.0833.2147.24261.91.78
    下载: 导出CSV

    表  2   平利地区粗面岩和辉绿岩主量元素(%)和微量元素(10−6)地球化学数据表

    Table  2   Major elements (%) and trace elements (10−6) compositions for the trachyte and diabase in Pingli

    样品粗面岩辉绿岩
    08-65h08-67h08-68-1h08-68-2h08-69h08-70h08-71h08-72h08-75h
    SiO261.1663.9466.1866.7850.4450.4749.1949.9348.4
    Al2O318.2316.9215.7915.5413.3312.6713.5613.6812.53
    Fe2O31.010.952.301.553.353.373.312.612.96
    FeO2.563.261.462.069.759.9110.089.6210.56
    CaO1.470.4250.480.568.179.658.757.649
    MgO1.610.9250.640.615.945.466.226.966.66
    K2O4.356.024.194.240.480.460.740.770.72
    Na2O6.665.456.836.312.792.062.643.082.08
    TiO21.090.90.840.812.012.132.031.692.78
    P2O50.20.090.090.080.220.230.220.170.32
    MnO0.20.250.250.260.20.20.210.190.19
    LOLI0.890.490.30.552.311.912.22.552.53
    Total99.4399.61599.3599.353.43.013.323.623.71
    TFeO3.464.103.513.4412.7312.9113.0311.9413.19
    MgO#41.9125.9022.0521.5646.0343.5546.3550.3246.89
    Ritman6.686.285.244.68 1.440.851.852.141.45
    La16422321322213.113.712.910.316.8
    Ce30241539341229.230.328.722.938.3
    Pr32.744.442.144.04.074.243.963.155.42
    Nd11615314414918.819.618.614.826.1
    Sm19.826.924.725.45.335.605.324.237.02
    Eu4.574.564.194.221.491.541.471.251.95
    Gd13.619.518.218.75.495.785.424.507.08
    Tb2.213.383.123.170.981.040.970.781.22
    Dy11.7018.617.218.36.386.606.155.077.15
    Ho2.133.523.273.341.251.321.261.011.34
    Er5.529.278.458.963.023.293.172.503.23
    Tm0.801.411.281.360.470.490.460.370.47
    Yb5.008.988.519.053.063.233.002.472.92
    Lu0.711.261.181.270.450.450.440.350.42
    Y58.197.196.996.533.935.532.927.334.8
    Li15.241.445.369.617.413.919.719.525.1
    Sc2.036.395.765.9438.938.237.836.433.0
    V70.834.831.331.7334336324293286
    Cr2.072.551.942.2750.335.154.189.9194
    Co18.022.847.449.351.053.552.952.257.1
    Ni0.280.630.280.2848.736.649.268.6105
    下载: 导出CSV
    续表2
    样品粗面岩辉绿岩
    08-65h08-67h08-68-1h08-68-2h08-69h08-70h08-71h08-72h08-75h
    Cu5.344.794.724.58126121124118159
    Zn172238221164119114117104146
    Ga34.045.042.641.917.418.819.116.720.1
    Rb1251451381353.273.618.217.9219.20
    Sr40818146.566.7268308402221302
    Zr811157314731593146153143114200
    Nb22934032333610.811.210.78.4419.9
    Cs0.611.280.070.260.210.210.360.140.25
    Ba56449642.780.8278198258276232
    Hf17.035.732.735.33.904.183.923.225.23
    Ta13.722.019.019.60.750.710.730.531.28
    Pb10.517.918.920.34.612.762.622.436.35
    Th19.833.230.733.41.581.641.581.231.70
    U5.408.988.058.550.430.420.410.320.48
    REE680.74932.78882.20920.77 93.0997.1891.8273.68119.42
    LREE/HREE15.4913.3413.6113.56 3.413.383.403.324.00
    (La/Sm)N5.355.365.575.65 1.591.581.571.571.55
    (La/Yb)N23.5417.8217.9617.60 3.073.043.092.994.13
    (Gd/Yb)N2.251.801.771.71 1.481.481.491.512.01
    δEu0.850.610.610.59 0.840.830.840.880.85
    下载: 导出CSV

    表  3   平利辉绿岩Sr–Nd 同位素组成

    Table  3   Sr–Nd composition for diabase in Pingli

    样品87Rb/86Sr87Sr/86Sr±2σIsr147Sm/144Nd143Nd/144Nd±2σεNd(tTDM(Ga)
    08-70h0.0340.705259110.705150.1730.51260540.032.02
    08-71h0.0590.70650060.706320.1740.5126013−0.092.10
    08-72h0.1040.7062160.705890.1740.51261730.232.04
    08-75h0.1840.70552440.704950.1640.51269432.021.39
    下载: 导出CSV

    表  4   平利辉绿岩Pb 同位素组成

    Table  4   Pb composition for diabase in Pingli

    样品206Pb/204Pb±2σ207Pb/204Pb±2σ208Pb/204Pb±2σ238U/204Pb232Th/204Pb206Pb/204Pb)t207Pb/204Pb)t208Pb/204Pb)t
    08-70h18.0351615.5011438.455490.1542.4418.03015.48237.991
    08-71h18.0151715.5081538.396440.1643.0818.00915.48837.926
    08-72h18.1451715.5241338.560380.1336.1618.14015.50838.165
    08-75h17.8912415.5272138.052490.0819.1217.88915.51737.843
    下载: 导出CSV
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  • 收稿日期:  2023-02-07
  • 修回日期:  2023-08-15
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  • 刊出日期:  2025-02-19

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