卢氏八宝山花岗斑岩地球化学特征及成因研究

曾令君, 周 栋, 星玉才, 赵太平, 姚军明, 包志伟*



卢氏八宝山花岗斑岩地球化学特征及成因研究

曾令君1,2, 周 栋1,2, 星玉才3, 赵太平1, 姚军明1, 包志伟1*

(1. 中国科学院 广州地球化学研究所 矿物学与成矿学重点实验室, 广东 广州 510640; 2. 中国科学院大学, 北京 100049; 3. 河南省地矿局 第一地质调查院, 河南 洛阳 471023)

河南省卢氏县八宝山岩体位于华北克拉通南缘、东秦岭造山带西段。岩体呈筒状, 可能为古火山机构岩颈相的超浅成侵入体。岩体的形成时代为(146±2) Ma(LA-ICPMS锆石U-Pb年龄)。岩体边缘相为钾长花岗斑岩、中心相为黑云母二长花岗斑岩。岩体出现以铁为主的多金属矿化, 与区内同时代成矿花岗岩有着显著的差别。岩石SiO2含量较高(64.8%～73.5%), ACNK值为0.98 ~ 1.56, 属于准铝质-过铝质钾玄岩系列花岗岩类。轻稀土相对富集(La/Yb)N=2.29～23.8, Eu异常不显著(Eu=0.61 ~ 1.39), Rb、Ba、U和K等元素强烈富集, Sr、Nb、Ti和P等元素显著亏损, 属于高分异的I型花岗岩, 并且钾长花岗斑岩具有相对较高的演化分异程度。岩体具有较低的Nd()初始值(-19.46～-16.4)和较古老的二阶段模式年龄(2.49～2.27 Ga), 与锆石Hf同位素模式年龄一致, 表明该岩体的源区物质以壳源物质为主。斑岩体为碰撞后的拉伸环境下地壳物质部分熔融的产物。根据黑云母地质压力计估算, 八宝山岩体的的侵位深度为1.89～2.55 km, 与该岩体浅成相的地质产状一致, 推测其深部可能存在有利于斑岩Cu-Au-Mo-Pb-Zn的多金属成矿区段。

高分异I型花岗岩; Nd同位素; 斑岩; 八宝山岩体; 华北克拉通南缘

0 引 言

秦岭造山带是中国大陆中央造山带的重要组成部分, 总体上是由华北、秦岭和扬子三板块沿两个主缝合带(商丹和勉略带)经历早古生代、晚古生代晚期和中生代早期三次板块俯冲、碰撞造山作用形成的; 之后又经历了中新生代强烈的陆内造山作用叠加, 是一个典型的大陆复合造山带[1]。秦岭造山带, 特别是华北克拉通南缘中生代花岗质岩浆活动强烈, 与之有关的Mo、W、Pb、Zn和Au等多金属矿产特别发育, 是我国最重要的多金属成矿带之一[2–3]。华北南缘中生代花岗岩时空分布、源区组成、成因演化规律、成矿专属性及其与秦岭造山带演化和华北克拉通中生代岩石圈减薄过程的联系一直是一个研究的热点[4–6]。

河南省卢氏县八宝山岩体位于华北克拉通南缘、东秦岭造山带的西段, 岩体为火山颈相的超浅成斑岩, 目前已发现的矿化类型以铁矿化为主, 并伴有少量的铜、钼和铅锌矿化。该岩体目前已获得了精确的同位素年龄(146±2) Ma (LA-ICPMS锆石U-Pb[7]), 但是该岩体及有关矿床目前尚缺少深入的地质、地球化学研究。为探讨岩体的地质、地球化学特征及成因过程, 本文在详细的野外地质工作基础上, 对八宝山岩体进行了岩石地球化学及同位素地球化学研究, 在此基础上探讨了岩体成因。

1 区域地质背景

秦岭造山带是中国大陆中央造山带的重要组成部分, 它由2个主缝合带(商丹和勉略缝合带)和3个块体(华北、秦岭和扬子)组成[1,8]。华北南缘是秦岭造山带中最重要的中生代构造-岩浆-成矿带, 其范围北至三宝断裂(三门峡-宝丰), 南部以栾川断裂为界(图1)。

卢氏八宝山地区的大地构造属于华北克拉通南缘、东秦岭造山带的西段。华北南缘结晶基底为新太古界太华群中深变质岩系, 盖层为古元古界熊耳群、中元古界官道口群、新元古界栾川群和陶湾群。区内出露的地层主要是中元古代的碳酸盐岩和碎屑岩。

八宝山地区主要发育了一系列近东西向的逆冲断层和挤压破碎带及复式褶皱。并且其褶皱形态自北向南由开阔逐渐变为线状紧密型, 断裂密度相应由小变大。区内成矿岩体均为中生代岩浆作用的产物, 岩性包括闪长岩、钾长花岗斑岩及二长花岗斑岩。岩体出露面积为0.6 ~ 1.3 km2, 呈北北东向的近平行排列, 自西向东依次为: 蒲镇沟闪长岩带、银家沟-夜长坪花岗斑岩带、后瑶峪-八宝山花岗斑岩带、郭家河闪长岩带。工业矿床主要见于银家沟-夜长坪花岗斑岩带和后瑶峪-八宝山花岗斑岩带上, 而中性闪长岩带仅发现零星的矿化。区内矿产较为丰富, 包括业已探明的曲里小型铁锌铜矿床[10]、银家沟大型多金属硫铁矿床[11]、夜长坪大型钨钼矿床[12]、后瑶峪小型铁铅锌矿床[13]及八宝山小型铁铜矿床。

2 岩体地质及岩相学特征

八宝山岩体位于河南省卢氏县境内。区内出露的地层主要是中元古界官道口群含硅质条带、条纹的白云岩。岩体侵入中元古界官道口群白云岩中, 与围岩边界平直, 呈岩株状产出。岩体地表出露似一头东尾西的鱼形, 长约2 km, 面积约1.05 km2。在600 m标高以上, 岩体与围岩接触带内倾, 呈喇叭形, 600 m标高以下接触带陡直呈筒状。铁矿体主要产于岩体和围岩的接触带上。

图1 秦岭造山带构造简图及中生代花岗岩体分布图(据文献[9])

1–第三系; 2–中生代碎屑沉积岩; 3–古生代海相碳酸盐岩与陆源碎屑岩; 4–古元古界秦岭群黑云斜长片麻岩和斜长角闪岩; 5–中元古界熊耳群火山岩及中上元古代海相碎屑沉积岩; 6–新太古界太华群片麻岩、麻粒岩和混合岩; 7–加里东期花岗岩; 8–白垩纪火山岩; 9–燕山期花岗岩; 10–燕山期闪长岩; 11–推断断裂; 12–断裂; 13–城市。

岩体可分为两个岩相: 中心相为黑云母二长花岗斑岩、边缘相为钾长花岗斑岩, 两岩相一般呈渐变过渡关系, 局部见指状穿插关系。铁矿化主要发生在岩体与围岩的接触带。岩体及围岩中分布较多不同岩性的岩脉, 如正长斑岩脉、花岗斑岩脉、闪长斑岩脉(图2)。岩体与围岩接触带局部发育有夕卡岩, 但夕卡岩与铁矿化无明显的空间联系。据野外产状及定年结果(LA-ICPMS锆石U-Pb, (146±2) Ma),两种不同的岩性应为同期岩浆作用的产物[7]。

黑云二长花岗斑岩呈灰白色, 斑状结构, 块状构造(图3a)。斑晶主要是斜长石、钾长石、石英及少量黑云母、角闪石(图3b)。石英斑晶占15%, 大小0.75 ~ 1.25 mm, 斑晶呈他形, 边缘有被熔蚀的现象(图3c); 斜长石斑晶占30%, 颗粒较大, 约2 ~ 3.5 mm, 自形程度较好, 主要为中长石, 具有环带构造、常见聚片双晶和卡纳复合双晶, 也有少量更长石, 具有细而密的聚片双晶, 有些斜长石完全被绢云母置换, 呈斜长石的假像; 钾长石约25%, 大小0.6 ~ 2 mm, 钾长石表面较浑浊, 内部和边缘有熔蚀现象, 有些钾长石内部发生了碳酸盐化、绢云母化和泥化(图3d)。黑云母和角闪石含量较少(约5%), 镜下可见角闪石的黑云母化及黑云母绿泥石化现象(图3e)。基质和斑晶成分相同, 主要是钾长石、斜长石和石英, 含量约占30%。副矿物以磁铁矿、榍石和磷灰石为主, 含量很少。

图2 八宝山岩体地质图

据河南省卢氏县地质勘查研究所《1﹕2000河南省卢氏县八宝山铁铜矿区地形地质及工程布置图(2011年)》修改。1–第四系; 2–中元古界官道口群; 3–黑云母二长花岗斑岩; 4–钾长花岗斑岩; 5–花岗斑岩; 6–正长斑岩; 7–闪长斑岩; 8–断层。

图3 八宝山岩体典型岩石的手标本和岩相学特征

(a)黑云母二长花岗斑岩手标本特征; (b)黑云母二长花岗斑岩镜下特征; (c)他形石英, 边缘被熔蚀; (d)钾长石碳酸盐化; (e)黑云母绿泥石化; (f)钾长花岗斑岩手标本特征; (g)钾长花岗斑岩镜下特征; (h)钾长石的卡斯巴双晶, 发生泥化。

钾长花岗斑岩呈肉红色, 斑状结构, 块状构造(图3f)。斑晶主要是钾长石和石英(图3g), 含量55%。石英斑晶25%, 粒度比黑云母二长花岗斑岩中的石英斑晶稍大, 约1 ~ 2.5 mm, 斑晶呈他形, 石英斑晶熔蚀现象明显; 钾长石斑晶30%, 粒度较大2 ~ 5 mm, 钾长石具有卡斯巴双晶(图3h), 有些钾长石发生泥化、绢云母化、碳酸盐化, 并且多数钾长石和石英斑晶的边缘有被熔蚀的现象, 这是浅成斑岩特有的现象[14]。斑晶和基质成分相同, 由钾长石和石英组成, 含量约占45%。副矿物以黄铁矿、锆石和金红石为主, 含量很少。

3 分析方法

本文选取了33件较新鲜的花岗岩样品进行全岩主元素和微量元素、Nd同位素和黑云母电子探针分析。

主元素和微量元素分析在中国科学院广州地球化学研究所同位素地球化学国家重点实验室完成。主元素采用碱熔玻璃片X射线荧光光谱法(XRF)测试, 分析仪器为Rigaku ZSX 100e, 分析精度优于2%。样品处理工作主要包括样品的烧失、称样(0.7 ~ 1 g的样品于一次性痰杯中, 加入8倍量的LiBO4)以及在XRF玻璃熔片震荡电热熔融炉中将样品熔成玻璃片待测。微量元素分析采用酸溶法。用ICP-MS测试, 所用质谱仪为Thermo Plasma-Quad Excell ICP-MS。在质谱分析过程中用Rh元素作为内标来进行内部校正, 纯的元素标样作为外部校正[15]。实验室标样BHVO-2(玄武岩)、GSR-1(花岗岩)、GSR-2(花岗岩)和GSR-3(花岗岩)作为参考标准, 分析精度优于5%, 详细分析流程见刘颖等[16]。

Nd同位素分析测试在中国科学院广州地球化学研究所同位素地球化学国家重点实验室的Micromass ISOPROBE型多接收电感耦合等离子体质谱仪(MC-ICPMS)上进行。对Nd同位素分析所需样品采用酸淋滤法进行预处理, 以减少后期蚀变作用对同位素比值的影响。对Sm和Nd的分离和纯化是用石英交换柱用1.7 mL Teflon粉末作为交换介质完成的。实验所用的Nd标样为国际标样Shin-Etsu JNdi-1[17]。143Nd/144Nd比值用143Nd/144Nd=0.72190进行校正。详细的实验流程和分析方法见梁细荣 等[18]和韦刚健等[19]。

电子探针定量分析在南京大学内生金属矿床成矿机制研究国家重点实验室完成, 使用的仪器是JEOL JXA-8800M电子探针。工作条件为: 加速电压15 kv, 加速电流10 nA, 束斑直径小于1 µm, 所有测试数据都进行了ZAF (atomic number effect, absorption effect and fluorescence effect) 修正。使用的标样均为美国国家标准局的矿物标样。详细的实验流程和分析方法见Amli.[20]和Roeder[21]。

4 分析结果

4.1 全岩主元素和微量元素地球化学

八宝山斑岩体岩石主元素和微量元素分析结果见表1。

由于岩石普遍、强烈的蚀变作用(绢云母化、高岭土化和碳酸盐化等), 样品的烧失量较高, 因此在进行地球化学投图时将样品去除烧失量后, 重新换算为100%。钾长花岗斑岩和黑云母二长花岗斑岩都比较富硅, SiO2变化于64.8% ~ 73.5%之间。岩体碱含量较高, 钾长花岗斑岩的Na2O和K2O含量分别为0.24% ~ 1.98%和8.00% ~ 11.7%, K2O+Na2O总量为8.32% ~ 13.0%; 黑云母二长花岗斑岩的Na2O和K2O含量分别为2.02% ~ 3.88%和3.93% ~ 5.76%, K2O+Na2O总量为8%左右。在TAS岩石分类图解上(图4)样品位于碱性系列和亚碱性系列的过渡区。其中, LSB-31样品由于钾长石化较强烈, K2O含量较高, 导致在TAS图解上落入正长岩范围。其余的点主要落入花岗岩和石英二长岩范围, 与岩石的实际矿物组成的定名基本一致。由于岩体富钾, 钾长花岗斑岩(K2O/Na2O=4.97 ~ 24.7)和黑云母二长花岗斑岩(K2O/Na2O=1.01 ~ 2.79)样品落入钾玄岩系列(图5)。

表1 八宝山花岗岩体主元素(%)、微量元素及稀土元素(µg/g)组成分析结果

(续表1)

样号钾长花岗斑岩 LSB-8LSB-10LSB-11LSB-15LSB-19LSB-20LSB-26LSB-31LSB-32LSB-33LSB-39LSB-40LSB-43LSB-44LSB-47LSB-51LSB-67 Hf3.933.844.083.763.823.833.324.393.963.464.7755.354.723.554.394.13 Ti168613261277130210718268161321103112017327469097109261399915 Y18.313.014.21713.310.114.621.312.011.78.0111.813.815.619.314.29.62 Ga19.117.518.117.917.418.415.120.016.616.217.918.718.618.918.117.916.8 1000*Ga/Al2.482.222.072.312.312.372.182.302.142.092.252.372.282.442.42.262.23 La41.27.6723.516.735.54.947.558.7320.227.217.812.324.968.833.949.77.24 Ce74.018.544.138.159.610.517.418.243.247.427.322.445.212868.187.911.9 Pr8.092.75.025.326.011.372.572.315.865.142.622.594.8713.48.19.182.87 Nd26.911.316.420.919.75.5811.19.4920.617.28.249.3116.740.62829.911.7 Sm4.182.262.383.963.091.412.732.243.102.531.451.752.745.454.34.152.45 Eu1.060.620.750.850.670.640.610.700.830.630.650.710.821.050.691.140.63 Gd3.691.842.273.282.71.362.342.512.512.361.291.612.484.63.853.61.93 Tb0.560.340.340.520.410.250.440.520.390.360.190.290.410.60.590.510.32 Dy3.232.152.193.152.381.712.733.652.262.021.311.832.492.953.482.71.85 Ho0.680.490.510.660.510.390.580.820.470.450.30.440.550.630.770.560.39 Er2.161.481.671.951.571.131.642.381.491.391.031.411.531.922.271.681.18 Tm0.330.240.260.320.260.190.270.390.240.230.190.230.240.290.360.280.2 Yb2.411.732.042.231.971.301.722.721.81.61.321.811.772.072.541.961.45 Lu0.360.290.310.340.330.210.260.430.280.250.240.30.290.310.370.310.23 ΣREE16951.710198.413531.051.955.210310863.95710527015719351.3 LREE15543.192.385.912624.441.941.793.810058.149.195.225714318243.7 HREE13.48.589.6212.410.16.579.9813.49.478.695.867.929.7513.414.211.67.55 LREE/HREE11.55.029.596.8712.53.724.193.109.9111.59.916.29.7619.21015.65.79 δEu0.800.900.970.700.701.390.710.900.880.781.411.280.940.620.510.870.86 (La/Yb)N12.23.178.265.3512.92.703.132.298.0312.19.614.8710.123.89.5518.13.59 (La/Sm)N6.362.186.372.727.412.251.782.514.206.947.924.545.878.155.097.721.91 (Gd/Yb)N1.260.880.911.211.130.861.110.761.151.210.80.731.161.831.251.511.1 TZr(℃)797782806799773798756777807796824834839808786825781

(续表1)

样号钾长花岗斑岩黑云母二长花岗斑岩 LSB-68LSB-75LSB-76LSB-78LSB-79LSB-80LSB-87LSB-92LSB-93LSB-94LSB-12LSB-13LSB-46LSB-48LSB-7-1LSB-7-16 SiO268.572.971.470.669.57270.771.469.670.564.867.865.671.363.267.3 TiO20.200.220.250.210.200.200.350.230.220.130.540.390.480.140.450.25 Al2O314.114.614.314.113.514.514.615.214.113.515.914.315.913.715.613.9 Fe2O3T1.951.141.712.743.330.761.430.552.532.094.144.444.652.354.431.13 MnO00000.080.010.880.010.020.040.050.030.070.010.060.03 MgO0.120.120.110.110.160.10.660.100.100.451.631.131.400.641.280.54 CaO0.070.110.080.080.270.190.650.210.200.793.32.083.070.933.231.99 Na2O0.530.560.640.630.580.630.450.610.640.533.432.593.872.763.882.02 K2O9.468.399.589.079.3310.19.9110.310.39.604.075.393.935.654.305.76 P2O50.010.010.030.010.120.10.150.10.100.040.280.200.280.030.280.13 LOI4.61.491.441.842.290.882.610.721.501.791.421.250.332.171.675.61 Total99.699.699.699.599.699.699.499.699.599.599.799.799.899.898.999.6 K2O/Na2O17.614.714.914.315.91621.916.915.918.01.182.071.012.041.102.79 ANK1.261.461.251.301.221.21.271.241.151.191.581.411.491.281.421.46 ACNK1.251.431.231.281.171.161.151.201.111.050.991.020.981.110.931.06 K2O+Na2O9.998.9610.29.709.9110.810.310.91110.17.507.997.818.428.187.78 D.I.89.692.093.091.390.694.792.894.592.893.488.289.389.591.987.286.3 Rb110204240150249261333350269264128124112153124196 Ba908232737382049270237133397410639002629149120492258223328001865 Th21.815.813.825.220.418.417.825.519.720.416.014.516.723.616.118.6 U4.737.944.426.048.114.313.827.356.523.023.082.382.297.13.713.87 Nb23.830.142.420.132.228.527.940.131.237.02318.621.035.923.024.5 Sr13411810821713511099.617710290.5646462643136635150 Zr13912815115113214713116515412819216720691.2180100 Ta1.702.703.572.012.882.542.392.712.153.721.803.151.681.731.802.20 Hf4.253.703.994.093.683.943.614.514.13.714.754.285.073.044.93.3 Ti613126615301942125812122216166312597503051224826468130.401.10 Y15.326.919.114.418.112.619.521.415.216.215.69.921714.716.910.7 Ga1619.520.417.01816.717.522.517.816.020.318.119.315.718.214.8

(续表1)

样号钾长花岗斑岩黑云母二长花岗斑岩 LSB-68LSB-75LSB-76LSB-78LSB-79LSB-80LSB-87LSB-92LSB-93LSB-94LSB-12LSB-13LSB-46LSB-48LSB-7-1LSB-7-16 1000*Ga/Al2.142.512.662.262.512.132.262.782.372.232.402.402.282.162.192.00 La20.613.315.048.625.78.5315.420.86.7817.147.618.962.839.753.218.9 Ce38.830.638.482.151.524.834.848.120.534.485.936.411063.792.835.4 Pr4.674.415.508.836.224.224.816.793.524.459.834.37126.35103.93 Nd16.618.321.429.622.216.919.726.51616.534.316.641.119.432.412.8 Sm3.193.613.844.423.753.483.995.043.73.265.312.685.882.685.302.17 Eu0.610.870.841.060.980.671.101.150.780.761.431.061.690.611.270.56 Gd2.803.293.223.83.382.793.844.212.792.854.402.324.962.643.641.59 Tb0.470.650.540.530.550.440.610.690.490.480.600.330.690.380.530.25 Dy2.764.313.192.753.212.53.743.982.912.883.331.853.572.302.911.52 Ho0.591.030.710.590.710.490.770.850.610.620.660.370.720.550.540.31 Er1.743.092.221.642.091.412.182.381.751.761.871.091.901.601.651.04 Tm0.270.450.350.260.340.230.340.370.280.280.270.150.310.250.270.20 Yb2.023.162.421.822.381.662.572.561.931.931.731.122.101.811.731.35 Lu0.330.490.370.300.40.270.360.400.290.290.270.180.310.260.280.23 ΣREE95.687.798.118612368.594.312362.487.719787.524814220680.2 LREE84.671.28517411158.679.910851.376.618480.123313219473.7 HREE11.016.513.011.7139.8214.415.411.011.113.17.4614.59.8411.56.49 LREE/HREE7.684.316.5114.98.545.975.537.014.626.8814.010.71613.416.811.3 δEu0.610.760.710.770.830.640.840.740.710.740.871.260.930.700.840.88 (La/Yb)N7.313.014.4219.17.753.674.295.832.516.3419.712.021.415.622.110.0 (La/Sm)N4.162.372.527.094.411.582.482.661.183.385.784.546.909.576.481.74 (Gd/Yb)N1.140.861.091.721.171.381.231.361.191.212.091.711.951.205.620.97 TZr(℃)803796799811780791798803788796785783790745769761

注：Fe2O3T为全铁; ACNK=Al2O3/(CaO+Na2O+K2O), 分子数比; ANK= Al2O3/( Na2O+K2O), 分子数比; Mg#=Mg2+/(Mg2++Fe2+)×100;Eu=2EuN/(SmN+GdN); (La/Yb)N为球粒陨石标准化值, 标准化值引自文献[22]; 锆石饱和温度Zr=12900/[2.95+0.85+ln(496000/rmet)],=(Na+K+2Ca)/(Al×Si),rmelt为全岩锆含量。

图4 八宝山岩体岩石TAS图解

图5 八宝山花岗斑岩K2O-SiO2图解(据文献[23], 样品符号同图4)

尽管两类岩石在主元素的含量上有一定差异, 但其铝含量均较高(Al2O3=13.5% ~ 16.3%), ACNK (Al2O3/(CaO+Na2O+K2O), 分子数比)=0.98～1.56, 在A/NK-A/CNK图解(图6)中, 为准铝质-过铝质花岗岩。因此, 八宝山岩体为准铝质-过铝质的钾玄质岩浆岩。

从图7可以看到, 八宝山花岗斑岩的主元素呈连续演化的特征, TiO2、Al2O3、Fe2O3T、MgO、CaO和P2O5含量均随SiO2含量的增加而降低。中心相的黑云母二长花岗斑岩到边缘相的钾长花岗斑岩SiO2含量依次增加, 除了K2O, 其他氧化物含量逐渐减少, 暗示钾长花岗斑岩和黑云母二长花岗斑岩存在结晶分异的演化关系, 且钾长花岗斑岩具有相对较高的演化程度。

图6 八宝山花岗斑岩A/NK-A/CNK图解(据文献[24], 样品符号同图4)

八宝山花岗岩体的稀土元素总量较低,SREE含量变化范围在51.3~ 271 µg/g之间。在球粒陨石标准化分布模式图上呈向右倾斜的平滑曲线(图8a), LREE/HREE值在3.72 ~ 19.2之间, (La/Yb)N=2.29 ~ 23.8。黑云母二长花岗斑岩((La/Yb)N=10.0~22.1)比钾长花岗斑岩((La/Yb)N=2.29 ~ 12.2, 除了LSB-44, (La/Yb)N=23.8)的轻重稀土分馏明显。(La/Sm)N值为1.18 ~ 9.57, (Gd/Yb)N值为0.76 ~ 2.09, 表明轻稀土分馏程度较重稀土显著。岩石Eu异常不显著(Eu=0.61 ~ 1.39)。其中钾长花岗斑岩的Eu负异常(Eu=0.61 ~ 0.97, 除了LSB-20、LSB-39以及LSB-40)比黑云母二长花岗斑岩(Eu=0.70 ~ 1.26)明显。两个不同岩性斑岩的稀土元素组成的差异可能与早期独居石等富轻稀土矿物及斜长石的分离结晶作用有关,晚期岩相相对亏损轻稀土元素及Eu。

在微量元素原始地幔标准化图解上(图8b), 表现为Rb、Ba、U和K等大离子亲石元素的强烈富集、高场强元素Zr、Hf和Ta无明显异常, Sr、Nb、Ti和P等元素显著亏损。与黑云母二长花岗斑岩相比, 钾长花岗斑岩具有明显高的Rb、Ba、Th、U和K等大离子亲石元素含量, 更高的Rb/Sr比值和更强烈的Ti和P亏损(图7和图8b), 可能与岩浆分异过程中斜长石和铁钛氧化物等矿物的结晶分异有关。

4.2 全岩Nd同位素组成

选取10个样品进行Nd同位素分析, 结果见表2。八宝山岩体的143Nd/144Nd初始比值介于0.511560~0.511730之间;Nd()=-19.2～-16.4, 均为负值。在计算Nd模式年龄时, 对于Sm/Nd值在-0.5～-0.3之间的样品, 用单阶段模式计算DM年龄是合适的, 但对于和大陆地壳Sm/Nd比相差较大的样品, 即Sm/Nd>-0.3或<-0.5, 用单阶段模式计算DM年龄就会产生较大的偏差[27]。在这种情况下, 采用两阶段Nd模式年龄计算方法能有助于得到较合理的DM年龄值[28–29]。八宝山岩体的Sm/Nd值变化于-0.24～-0.57之间, 大部分样品的Sm/Nd<-0.5, 与太古宙大陆地壳Sm/Nd平均值(-0.4)[30]相比偏差较大, 为了最大限度减少因地壳演化阶段内Sm-Nd分馏对Nd模式年龄计算值产生的影响, 本文采用两阶段Nd模式年龄计算方法。DM2年龄介于2.49～2.27 Ga之间, 老的Nd两阶段模式年龄和负的Nd()值指示八宝山花岗岩体主要来源于古老的地壳物质。

图7 主元素和微量元素对SiO2变化图解(据文献[25]和[26], 样品符号同图4)

图8 八宝山花岗斑岩球粒陨石标准化REE分布模式图及原始地幔标准化微量元素蛛网图

球粒陨石和原始地幔标准化值据文献[22]。

表2 八宝山花岗岩体全岩Nd同位素分析测试结果

4.3 黑云母电子探针分析结果

黑云母二长花岗斑岩中的黑云母具有富镁贫铁的特征, 其FeO和MgO的质量分数分别为11.5%～13.3%和13.8%～18.6%, Al2O3的质量分数为13.3%～14.6%。在Mg2+-(Fe2++Mn2+)-(Fe3++Al3++Ti4+)三角图中(图9), 所有黑云母的成分投点均在镁质黑云母区域内, 表明本区黑云母的成分类型为镁质黑云母。

5 讨 论

5.1 成因类型

八宝山花岗斑岩的暗色矿物主要是角闪石和黑云母, 岩体均具有高的SiO2含量(64.8% ~ 73.5%)和全碱含量(K2O+Na2O=8.32% ~ 13.0%), 岩石较高的分异指数、LREE/HREE及Rb/Sr比值的变化范围等均表明它们经历了高程度的结晶分异。

对于I型、S型花岗岩的划分, 铝饱和指数ACNK=1.1仅适用于未经强烈结晶分异的花岗岩[33], 对于分异程度较高的八宝山花岗斑岩并不适合。实验研究表明, P2O5在弱过铝质和强过铝质岩浆中随SiO2增加变化趋势不同, 这种性质成功地用于区分I型和S型花岗岩类[34–38]。本文的数据显示, 八宝山花岗岩主要为准铝质到过铝质花岗岩, 在图7中P2O5与SiO2含量呈负相关关系, 与I型花岗岩演化趋势一致。

A型花岗岩的重要特征是形成温度高、无水、铝质、富集HFSE和非造山成因, 尤其是高温、无水特点, 是A型花岗岩区别于高分异花岗岩的重要参数[39]。而八宝山花岗斑岩具有一系列不同于A型花岗岩的特点: (1)八宝山岩体的FeOT/MgO比值主要介于2～8, 有别于A型花岗岩显著富铁的特征(FeOT/MgO>10[40]); (2)岩体的Zr、Nb、Ce和Y等元素的含量均较低, Zr+Nb+Ce+Y = 192～340 μg/g,小于350 μg/g[40]。在区分A型花岗岩与分异的I型花岗岩的有关判别图解上(图10), 样品几乎全部落入高分异花岗岩区域, 并且在Ce-SiO2判别图解中(图7), 投点均落在I型花岗岩区域; (3) 虽然八宝山岩体的岩浆温度很高(锆石饱和温度[41]变化于745～839 ℃, 见表1), 但由于其含有角闪石和黑云母等含水矿物, 与A型花岗岩的定义不符。综上分析, 八宝山岩体应属于高分异I型花岗岩。

图9 黑云母的Mg2+-(Fe3++Al3++Ti4+)-(Fe2++Mn2+)图解(底图据文献[31], Fe2+和Fe3+的计算据文献[32])

个别黑云母二长花岗斑岩和钾长花岗斑岩样品具有高Sr低Y的微量元素组成, 显示埃达克质岩类似的地球化学特征[42–43], 反映花岗岩源区可能存在石榴子石残留相。事实上华北南缘不少150～130 Ma的花岗岩具有不同程度的类似埃达克岩的组成特点[44–45], 表明该时间段内华北南缘由扬子向华北克拉通之下俯冲所形成的加厚下地壳尚未拆沉。

5.2 物质来源

八宝山岩体具有高的SiO2(64.8% ~ 73.5%), 相对低的MgO含量(0.10% ~ 1.40%)和Mg#(8.78 ~ 49.3), 显示出地壳来源的特征。全岩Nd同位素分析结果显示岩体具有较低的Nd()(－19.46～－16.4)和较古老的模式年龄(NdDM2=2.49～2.27 Ga)。

图10 八宝山花岗斑岩成因类型判别图

A–A型花岗岩; FG–分异M+I+S型花岗岩; OGT–非分异M+I+S型花岗岩。据文献[40], 样品符号同图4。

根据壳源岩石的Nd同位素研究, 华北块体壳源岩石Nd同位素模式年龄主要分布于3.6～1.8 Ga范围内, 并且主要集中于3.6～3.3 Ga、2.8～2.6 Ga和2.2～2.0 Ga区段内, 华北陆块主体形成于太古宙, 并以2.8～2.6 Ga时期为最强的陆壳增生期, 其次, 古元古代也是较主要的地壳增生期[46]。扬子块体壳源岩石Nd同位素模式年龄主要分布于2.4～1.0 Ga范围内, 在3.4～2.5 Ga区间内也有少量分布, 这表明扬子块体陆壳主体形成于1.0 Ga前的元古宙[47–49], 太古宙古陆核较少[47]。即扬子地壳的年龄相对年轻。

由于八宝山岩体产于华北克拉通南缘, 其岩浆源区可能为: (1) 华北地块南缘的中-下地壳岩石(即太华群和/或熊耳群)[50]; (2)俯冲于华北克拉通南缘地壳之下的扬子陆壳及南秦岭陆壳[51]。

八宝山花岗斑岩的两阶段Nd模式年龄主要介于2.49～2.27 Ga之间。2.49～2.27 Ga的模式年龄不同于华北克拉通主体形成于太古宙并以2.8～2.6 Ga为强烈的地壳增长期, 而与扬子板块古元古代和中元古代为主的地壳生长期相符合。此外, 八宝山岩体的Nd()值也与Jahn.[52]归纳出来的华北克拉通古老下地壳的Nd(=130 Ma)值(-44～-32)相差很多。因此, 八宝山花岗岩体的岩浆源区更可能为扬子克拉通的地壳物质。

综上所述, 可以推断八宝山岩体的源区可能是俯冲的扬子克拉通及南秦岭陆壳与太华群和熊耳群的混合, 但主要来源于南秦岭和扬子陆块的部分熔融, 这与Hf同位素分析结果一致[7]。

5.3 岩石成因

八宝山岩体的锆石定年结果表明, 钾长花岗斑岩和黑云母二长花岗斑岩是同一期岩浆作用的产物, 只是经历了不同程度的结晶分异[7]。从黑云母二长花岗斑岩到钾长花岗斑岩, 岩石的矿物组成中石英和碱性长石含量逐渐升高, 斜长石的含量逐渐减少, 铁镁矿物含量明显下降; 随着SiO2含量的升高, TiO2、Al2O3、Fe2O3T、MgO、CaO和P2O5逐渐降低, 而全碱含量逐渐升高, Nb/Ta和La/Y没有什么变化(图7)。显示了黑云母二长花岗斑岩和钾长花岗斑岩之间的同源岩浆分异演化的趋势。

在稀土元素分布模式和微量元素蛛网图上(图8), 两者具有一定的相似性及演化关系, 据此推断两者可能是由同一母岩浆分异形成的。这与两种岩石的锆石Hf同位素和全岩的Nd同位素分析结果也相当一致[7]。例如, 黑云母二长花岗斑岩和钾长花岗斑岩轻、重稀土分馏和Eu负异常程度的差别显示独居石和斜长石等结晶分异的影响, 这与主元素的分析结果相一致。八宝山花岗斑岩具有富硅、明显亏损Nb、Sr、P、Ti和Eu等地球化学特征, 指示其母岩浆经历了显著的分离结晶演化。Nb、Ti元素的亏损指示了富Ti矿物相的分离, 而P的强烈亏损表明发生了磷灰石的分离结晶, Sr和Eu的亏损与斜长石和钾长石的分离结晶作用有关[34]。与黑云母二长花岗斑岩相比, 钾长花岗斑岩相对富集Rb、Ba、Th、U和K等大离子亲石元素, 显示了强烈分异的地球化学特征, 表明两类花岗岩之间存在一定的分异演化关系。钾长花岗斑岩的Rb/Sr和Ba/Sr比值明显高于黑云母二长花岗斑岩(图7), 这也表明钾长花岗斑岩具有更高程度的分异-演化。

在Ba-Sr对数图解上(图11a), 钾长花岗斑岩和黑云母二长花岗斑岩都落在钾长石和斜长石的结晶演化线上, 这些数据的变化趋势也进一步证明了岩体形成过程中主要经历了钾长石和斜长石的分异。相对于主要矿物来说, 副矿物的结晶分异对稀土元素含量变化影响较大。在(La/Yb)N-La变异图解上(图11b), 从岩体样品点的分布来看, 八宝山花岗斑岩存在独居石和褐帘石的结晶分异, 它们可能对稀土元素的含量有一定影响。根据岩体的岩石地球化学特征可以看出钾长花岗斑岩的演化程度更高。

5.4 构造意义

前人的研究指出, 从晚侏罗世到早白垩世, 秦岭造山带处于挤压到伸展的转换阶段, 并且大规模发育钾长花岗岩和富碱中酸性火山岩[1–2,5]。在Pearce.[58–59]提出的花岗岩构造背景判别图上(图12), 钾长花岗斑岩和黑云母二长花岗斑岩都属于碰撞后花岗岩范围。我们对华北南缘其他大型斑岩矿床岩体地球化学数据做了总结(如金堆城、南泥湖和东沟等), 发现八宝山岩体的形成构造背景与华北南缘的其他同时代岩体的形成构造背景一致(图12)。

八宝山岩体岩石地球化学、锆石U-Pb定年和Hf、Nd同位素组成研究所获得的结论, 也与秦岭造山带的形成和演化的研究结果相吻合。秦岭造山带是以新元古代到中生代初期的多板块、多类型碰撞造山为主体, 并遭受中新生代强烈陆内造山作用叠加改造的一个复合型造山带[1]。其形成和演化主要经历了3个不同构造演化阶段: (1)晚太古代-古元古代造山带前寒武纪结晶基底的形成演化; (2)新元古代-中三叠世, 以现代板块构造体制为基本特征的板块构造演化; (3)中生代以来的陆内造山作用与构造演化[1]。原秦岭造山带的华北板块、扬子板块和秦岭微板块在三叠纪中、晚期完成了全面碰撞造山后, 形成的强烈挤压缩短增厚的岩石圈根, 在中生代新的壳幔动力学系统中, 发生强烈壳幔物质交换、伸展减薄, 造成显著的岩石圈去根作用[1,8], 伴随大量岩浆侵位发育了与陆内构造-岩浆活动有关的大规模热液矿床的成矿作用。

图11 八宝山花岗斑岩造岩矿物(a)和副矿物(b)分离结晶判别(图解底图据文献[53], 样品符号同图4)

PlAn15–斜长石(An=15); PlAn50–斜长石(An=52); Ms–白云母; Bt–黑云母; Kfs–钾长石; Grt–石榴子石; Amp–角闪石; Allan–褐帘石; Mon–独居石; Ap–磷灰石; Zr–锆石;Sph-榍石。磷灰石的分配系数据文献[54]; 锆石分配系数据文献[55]; 褐帘石分配系数据文献[56]; 独居石分配系数据文献[57]。

图12 八宝山花岗岩体的构造环境判别图(底图据文献[58–59], 数据来自文献[2,60–62], 样品符号同图4)

ORG–大洋中脊花岗岩; WPG–板内花岗岩; VAG–火山弧花岗岩; Syn-COLG–同碰撞花岗岩; post-COLG–后碰撞花岗岩。阴影区是金堆城、汤家坪、南泥湖和东沟岩体。

八宝山岩体的形成年龄正对应于秦岭造山带在晚侏罗世-早白垩世的挤压向伸展的转变阶段[63–65]。八宝山岩体的形成机制可概括为: 在晚侏罗世-早白垩世的挤压向伸展转换时期, 至少在八宝山斑岩形成时华北南缘加厚地壳尚未拆沉, 而这种加厚的下地壳可能主要源于俯冲于华北南缘陆壳之下的扬子及南秦岭地壳, 俯冲陆壳在上涌地幔热源影响下, 发生部分熔融形成的酸性岩浆上升侵位, 从而形成八宝山岩体碰撞后花岗岩。

5.5 岩体形成深度与找矿前景

对黑云母二长花岗斑岩中的黑云母进行了电子探针分析, 分析结果见表3。利用黑云母中TAl的含量估算八宝山岩体形成的压力以及深度[66](表3)。黑云母结晶的压力为52 ~ 57 MPa, 对应的侵位深度为1.89 ~ 2.55 km, 表明该岩体形成于较浅的环境, 这与岩相学显示的特征相符。而斑岩成矿系统的深度一般在1 ~ 6 km, 最大成矿深度达9 km。因此岩体被剥蚀程度较低, 可以推断八宝山斑岩成矿系统未受到构造抬升的破坏, 深部成矿和找矿潜力较大。

在八宝山岩体周围分布着一些燕山期的中酸性小斑岩体, 均呈北北东向有规律地平行排列。这些岩体的展布具有明显的方向性、等距性和分带性规律。特别是银家沟岩体在岩性和成岩成矿年龄与八宝山岩体比较接近(表4)。除了蒲阵沟岩体岩性为中酸性外, 其余岩体的岩性均为酸性, 并且矿体与钾长花岗斑岩关系最为密切。通过与卢氏地区不同岩体成矿特征的对比, 我们推测这些岩体属于相似的成矿体系。并且通过钻探和野外观察发现八宝山主要是在岩体内部和围岩的接触带中发现有铁铜矿体, 可能在岩体的外围会有铅锌矿体存在。

表3 黑云母矿物的电子探针分析结果

注：(kbar)=3.03×TAl-6.53(±0.33)[66]; 上地壳静岩压力梯度=27.5 MPa/km;TAl指在O=22基础上的黑云母中铝阳离子总数。

表4 卢氏地区岩体、矿体概况

八宝山岩体为含黑云母和角闪石的富水岩浆体系, 黑云母组成以较高的Mg/Fe比值、低铝、贫钙、K/Na>>30(108 ~ 475)与全球典型的含矿斑岩特征一致[14]; 锆石Ce4+/Ce3+的比值也很高(286 ~ 786)[7], 这种高氧化性的富水岩浆有利于斑岩铜钼(金)矿床的形成, 所以其深部或边缘应存在有利的成矿部位, 如较大的岩枝、岩体外缘的突变部位。

6 结 论

(1)八宝山岩体主要由中心相的黑云母二长花岗斑岩和边缘相的钾长花岗斑岩组成, 属于准铝质-过铝质, 钾玄岩系列的高分异I型花岗岩类。两者具有同源岩浆演化分异的趋势, 并且钾长花岗斑岩演化分异程度更高。

(2)全岩Nd同位素分析结果表明, 八宝山岩体的源区可能主要来源于南秦岭和扬子陆块的部分熔融, 但在岩浆形成过程中可能混入了少量太华群和熊耳群的物质。

(3)根据黑云母地质压力计估算, 八宝山岩体的的侵位深度为1.89 ~ 2.55 km, 表明该岩体形成侵位较浅, 剥蚀程度较小, 斑岩成矿体系可能得到很好的保存, 在其深部可能存在有利于斑岩Cu-Au-Mo-Pb-Zn的多金属成矿区段。

野外工作得到了卢氏县北方矿业有限公司的大力支持; 主元素和微量元素分析、Nd同位素分析得到了中国科学院广州地球化学研究所同位素地球化学国家重点实验室的支持; 黑云母电子探针分析得到了南京大学内生金属矿床成矿机制研究国家重点实验室的支持, 在此一并表示感谢。同时作者要特别感谢两位审稿专家和责任编辑提出的建设性的修改意见和宝贵的评论。

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Geochemistry and petrogenesis of the Babaoshan granite porphyry in Lushi county, Henan Province

ZENG Ling-jun1,2, ZHOU Dong1,2, XING Yu-cai3, ZHAO Tai-ping1, YAO Jun-ming1and BAO Zhi-wei1*

1. Key Laboratory for Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China; 2. University of Chinese Academy of Sciences, Beijing 100049, China; 3. No.1 Institute of Geological Survey, Geological Bureau of Henan Province, Luoyang 471023, China

The Babaoshan granite porphyries in Lushi county, Henan province, is tectonically located at the southern margin of North China Craton, in the western part of the East Qinling orogen. The granite porphyries occur as circular column which might have been the neck of volcanic edifice. Formed at (146±2) Ma (LA-ICPMS zircon U-Pb), the intrusion is unique for the associated iron predominated ore deposit with accompanying Cu-Mo-Pb-Zn mineralization, which makes it a meaningful target for metallogenetic investigation in the East Qinling area. The Babaoshan granite intrusion is a zoned granitic stock consisting of syenogranite porphyry in the outer zone and biotite monzogranite porphyry in the inner part. The granite porphyries have high SiO2(64.8%－73.5%) contents, with ACNK values varying between 0.98－1.56. It can be classified as metaluminous- peraluminous granite. The rocks are relatively enriched in LILE such as Rb, Ba, U, K and LREE ((La/Yb)N= 2.29－23.8), depleted in Sr, Nb, Ti, and P, with insignificant Eu anomalies. The geochemical characteristics of the Babaoshan granite porphyries suggest a highly fractionated I-type origin, while the syenogranite porphyry is more fractionated. The lowNd() (-19.46－-16.4) and rather old model ages (NdDM2= 2.46 Ga to 2.27 Ga) are consistent with Hf isotopic compositions. It demonstrates that the rocks might have been derived from partial melting of crustal material. It possibly derived from partial melting of the lower crust under extensional tectonic regime following the collision condition. Biotite geobarometer calculations show that the Babaoshan granite porphyries may form at depth of 1.89 km to 2.55 km, which is consistent with the sub-volcanic occurrence. We deduce that there might be an underlying porphyry Cu-Mo-Pb-Zn polymetallic mineralization section at depths.

highly fractionated I-type granite; Nd isotope; porphyry; Babaoshan granite; southern margin of the North China Craton

P581; P597; P595

A

0379-1726(2013)03-0242-20

2012-11-14;

2013-01-14;

2013-02-05

国家重点基础研究发展计划项目(2012CB416602)

曾令君(1989–), 女, 硕士研究生, 矿床学专业。E-mail: zenglingjun0213@163.com

BAO Zhi-wei, E-mail: baozw@gig.ac.cn, Tel: +86-20-85290105