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黄金科学技术, 2020, 28(1): 1-11 doi: 10.11872/j.issn.1005-2518.2020.01.128

矿产勘查与资源评价

滇西保山地块黑牛凹金矿区辉绿玢岩地球化学特征、锆石U-Pb年龄及地质意义

罗应,1, 卢映祥2, 刘学龙,1, 薛顺荣2, 王帅帅1, 李振焕1, 张昌振1, 陈建航1

1.昆明理工大学国土资源工程学院,云南 昆明 650093

2.自然资源部三江成矿作用及资源勘查利用重点实验室,云南 昆明 650051

Geochemical Characteristics,Zircon U-Pb Age and Geochronological Significance of Diabase in Heiniu’ao Gold Deposit,Baoshan Block,Western Yunnan

LUO Ying,1, LU Yingxiang2, LIU Xuelong,1, XUE Shunrong2, WANG Shuaishuai1, LI Zhenhuan1, ZHANG Changzhen1, CHEN Jianhang1

1.Faculty of Land Resource Engineering,Kunming University of Science and Technology,Kunming 650093,Yunnan,China

2.Key Laboratory of Sanjiang Mineralization and Resource Exploration and Utilization,Ministry of Land and Resources,Kunming 650051,Yunnan,China

通讯作者: 刘学龙(1983-),男,甘肃张掖人,教授,从事西南三江地区的地质矿产研究工作。xuelongliu@foxmail.com

收稿日期: 2019-07-08   修回日期: 2019-10-08   网络出版日期: 2020-03-06

基金资助: 云南省科学技术奖“杰出贡献奖”项目“西南三江复合成矿作用疑难问题研究”.  2017001
云南省基础研究计划项目“滇西北中甸铜厂沟燕山期斑岩型Mo-Cu矿床热液蚀变分带与成矿流体研究”.  2019FB062
云南省万人计划“青年拔尖人才”专项.  20190028
昆明理工大学重点学科建设经费项目“地质资源与地质工程(省级一流学科)”.  1407839305

Received: 2019-07-08   Revised: 2019-10-08   Online: 2020-03-06

作者简介 About authors

罗应(1992-),男,贵州安顺人,硕士研究生,从事矿床地质研究工作52938746@qq.com , E-mail:52938746@qq.com

摘要

通过对保山地块黑牛凹金矿区内辉绿玢岩进行LA-ICP-MS原位微区锆石U-Pb定年和岩石地球化学特征分析,获得了辉绿玢岩的成岩年龄为(212.0±4.9) Ma,这一时限与核桃坪矿区内辉绿玢岩成岩时限(195.0±5.3) Ma,以及金厂河矿区辉绿玢岩成岩时限(217.3±5.8) Ma具有一致性,均属晚三叠世。黑牛凹辉绿玢岩具有低SiO2、低MgO及高钾特征,属于钾玄系列。微量元素具有富集轻稀土元素(LREE)、大离子亲石元素(LILE)及亏损高场强元素(HFSE)的特征,暗示其源区为岩石圈地幔。岩石中的Cr、Ni元素含量明显比原始玄武岩浆低,表明岩浆可能经历了橄榄石和辉石的结晶分离作用。Nb、Ta元素具明显负异常,暗示在岩浆上升侵位过程中受到了地壳物质的混染。黑牛凹辉绿玢岩形成的构造背景与区域内金厂河辉绿岩、核桃坪辉绿岩一致,均为板内拉张环境,这可能与古特提斯洋闭合导致保山地块因碰撞而发生的内部拉张、伸展活动有关。

关键词: 辉绿玢岩 ; 锆石U-Pb定年 ; 地球化学特征 ; 晚三叠世 ; 板内拉张环境 ; 滇西

Abstract

The special geotectonic location of Baoshan block makes it undergo a series of Tethys evolutionary activities with frequent regional tectonic-magmatic activities.Granite is developed from Paleozoic to Cenozoic.In addition,a large number of basic intrusive rocks occurring as dikes and dikes are also developed in the region.Previous research and analysis of granite in the region is relatively systematic,but the study and analysis of basic rocks in the region,especially basic dikes commonly developed in typical polymetallic ore areas in the ore concentration area,is relatively weak.Previous studies and analyses only the chronology of diabase and its relationship with mineralization in the Hetaoping lead-zinc polymetallic ore area.The study and analysis of the petrogenesis of basic rocks is almost blank.Therefore,the diabase porphyrite in typical boreholes of Heiniu’ao gold deposit has been systematically analyzed in terms of petrochemical characteristics and zircon U-Pb dating in LA-ICP-MS in situ.The main elements of Heiniu’ao diabase porphyry are characterized by low SiO2,low TiO2,high Al2O3,low MgO,high FeOT and low K2O,belonging to the shoshonite series.Trace and rare earth elements are characterized by enrichment of light rare earth elements (LREE) and large ion lithophile elements (LILE) and depletion of high field strength elements(HFSE),suggesting that their source areas are lithospheric mantle.The content of Cr and Ni in the rocks is obviously lower than that in the original basalt magma,indicating that the magma may have undergone the crystallization separation of olivine and pyroxene.The obvious negative anomalies of Nb and Ta suggest that they were contaminated by crustal materials during the process of magma emplacement.The tectonic environment is consistent with the tectonic background of Jinchanghe diabase and Hetaoping diabase,which are intraplate extensional environments.The diagenetic age of the diabase porphyrite obtained in the mining area is (212.0±4.9) Ma,which is similar to the diagenetic age of the Hetaoping diabase obtained by predecessors (195.0±5.3) Ma,and that of the Jinchanghe diabase (217.3±5.8) Ma are all Late Triassic.It shows that the basic dikes in the area are the products of the Triassic magmatic activities.The closure of the Triassic Paleo-Tethyan Ocean resulted in the collision between Baoshan and Simao blocks,which resulted in extensional activities in Baoshan block.The basalt magma originating from the mantle emplaced upward along the weak tectonic zone to form diabase porphyrite of this time limit,and experienced crystallization differentiation and crustal contamination during the process of magma emplacement.

Keywords: diabase porphyrite ; zircon U-Pb dating ; geochemistry ; Late-Triassic ; intraplate extensional environment ; Western Yunnan

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罗应, 卢映祥, 刘学龙, 薛顺荣, 王帅帅, 李振焕, 张昌振, 陈建航. 滇西保山地块黑牛凹金矿区辉绿玢岩地球化学特征、锆石U-Pb年龄及地质意义[J]. 黄金科学技术, 2020, 28(1): 1-11 doi:10.11872/j.issn.1005-2518.2020.01.128

LUO Ying, LU Yingxiang, LIU Xuelong, XUE Shunrong, WANG Shuaishuai, LI Zhenhuan, ZHANG Changzhen, CHEN Jianhang. Geochemical Characteristics,Zircon U-Pb Age and Geochronological Significance of Diabase in Heiniu’ao Gold Deposit,Baoshan Block,Western Yunnan[J]. Gold Science and Technology, 2020, 28(1): 1-11 doi:10.11872/j.issn.1005-2518.2020.01.128

保山地块属SIBMASU(滇缅泰马)地体的一部分[1,2],处于西藏—云南—泰国—马来西亚中间板块中段,是滇西特提斯的重要组成部分[3,4,5,6]。由于独特的大地构造位置,该区域经历了古—新特提斯的演化。区域内岩浆活动频繁,从古生代至新生代均发育有岩浆岩。前人对保山地块内各时代的花岗岩体进行了系统的年代学和岩石地球化学研究分析。早古生代以平河花岗岩基为代表,锆石U-Pb年龄为480~486 Ma,属于S型花岗岩[7,8,9];晚古生代以二叠纪木厂花岗岩体为代表,锆石U-Pb年龄约为266 Ma,属于A型花岗岩[10];中生代主要有三叠世耿马岩体和蚌东岩体,耿马岩体锆石U-Pb年龄为(231.9±2.8) Ma,属于S型花岗岩[11],蚌东岩体锆石U-Pb年龄为(228.3±1.7) Ma[12];早白垩世志本山岩体锆石U-Pb年龄为(126.7±1.6) Ma[13];晚白垩世主要有蚌渺岩体和槽间岩体,蚌渺岩体锆石U-Pb年龄为(85.4±4.2) Ma,具有Ⅰ型花岗岩的特征[14],漕间岩体锆石U-Pb年龄为(68.8±1.2) Ma,属于过铝质S型花岗岩[15,16];新生代以双脉地体隐伏花岗岩体为代表,锆石U-Pb年龄为(36.27±0.48)~(35.78±0.49) Ma,属于S型花岗岩[17]

此外,保山地块内发育有大量呈岩墙和岩脉产出的基性侵入岩体,区内多金属矿区内也普遍发育基性侵入岩,然而对区域内基性岩的研究相对匮乏。韩艳伟[18]对核桃坪矿区内的辉绿玢岩进行了SHRIMP锆石U-Pb定年,获得辉绿玢岩成岩时代为(195.0+5.3) Ma。王建荣等[19]对核桃坪矿区内的辉绿玢岩系统的岩石学及岩相学特征进行了首次研究,认为矿区辉绿玢岩(基性侵入岩)作为成矿的物理化学障碍,可作为找矿标志。此外,区域上金厂河铅锌多金属矿亦发现分布有大量基性侵入岩(辉绿岩和辉绿玢岩),对地质找矿具有重要的指导意义。区域内其余多金属矿区内也发育有辉绿玢岩体,但目前缺乏系统的分析研究。因此,在以往研究基础上,对保山地块黑牛凹金矿区内的辉绿玢岩进行岩石地球化学及年代学特征分析,以期探讨区内辉绿玢岩的成岩时代、源区特征以及形成的构造背景。

1 矿区地质特征与岩石学特征

黑牛凹金矿床属于滇西保山Fe-Cu-Pb-Zn-Au多金属成矿带的重要组成部分。矿区位于保山市陇阳区瓦窑镇,其大地构造位置处于“三江”造山带“蜂腰”状弧形构造南侧散开端的末端部分,保山陆块与兰坪—思茅盆地相接且靠近保山陆块一侧[20]

矿区内出露地层主要有上寒武统核桃坪组(Є3h)、上寒武统沙河厂组(Є3sh)和少量第四系(Q4)(图1)。上寒武统核桃坪组划分为上、中、下3段,其中核桃坪组上段零星分布于矿区南东侧,岩性主要为钙质板岩夹粉砂质板岩,局部夹大理岩化(泥质)灰岩,地层厚度为几米至几十米,矽卡岩发育,局部见有辉绿玢岩侵入,并发育有小规模铅锌矿体;核桃坪组中段分布于矿区中部,岩性主要为粉砂质板岩夹钙质板岩,局部夹泥质灰岩透镜体,黄铁矿化和硅化较强烈,矿体主要赋存于该层与核桃坪组下段的接触带中;核桃坪组下段岩性主要为钙质板岩夹粉砂质板岩,局部夹泥质灰岩透镜体,破碎带中硅化、黄铁矿化和磁黄铁矿化较强烈,阳起石矽卡岩发育,为矿区内重要的赋矿层位。上寒武统沙河厂组下段(Є3sh1)主要分布于矿区东部,岩性主要为大理岩化灰岩、钙质板岩和粉砂质板岩;层间破碎带发育,并伴有辉绿岩侵入;矿化强烈,局部有矽卡岩产出,并伴有铅锌铁金矿体产出,为矿区内重要的赋矿层位。第四系洪坡积物(Q4)仅少量分布于矿区河谷及山坡凹地等低洼地。

图1

图1   黑牛凹金矿区地质简图(据杨怀[21]修改)

1.第四纪洪坡积物;2.沙河厂组下段一层:灰色大理岩化灰岩;3.核桃坪组中段三层:灰绿色粉砂质板岩;4.核桃坪组中段二层:浅灰色泥质灰岩;5.核桃坪组中段一层:灰绿色粉砂质板岩;6.核桃坪组下段:灰绿色粉砂质板岩;7.辉绿玢岩;8.灰岩透镜体;9.断层及编号;10.构造蚀变带;11.金矿体;12.采样位置

Fig.1   Simplified geological map of Heiniu’ao gold deposit(modified after Yang[21]


矿区内构造发育,主要为近SN、NE和NW向3组张扭性断裂,这些断裂具有多期次活动特征,其中SN向断裂的形成时代早于NE、NW向断裂。不同方向的断裂之间相互切错,断裂带附近侵入有不同规模的辉绿玢岩,且构造破碎带发育,构成了良好的导矿与容矿构造。矿区内主要分布有5条断裂,分别为F2、F3、F4、F5、F12和F13断裂,其中与矿体关系密切的断裂有F3、F4和F12,是主要的导矿与容矿构造。

F3断裂分布于矿区中部核桃坪组一段与核桃坪组二段的接触带部位,为主要的导矿和容矿构造,北部走向为近SN向,南部走向变为NE向,走向总体约为20°,倾向85°~131°,倾角54°~87°,局部向西陡倾。破碎带发育,沿破碎带发育有细脉状、团块状和胶状黄铁矿化;细脉状硅化局部呈石英脉,矿体主要分布于破碎带中。

F4断裂分布于矿区东部核桃坪组二段的层间破碎带内,走向为SN向,倾向近东向,倾角为20°~45°。断裂带发育有褐铁矿化和硅化,矿体分布于该断裂带中。

F12断裂分布于矿区西侧核桃坪组中,其南北两端被后续地层掩盖,出露长度约为300 m。探槽揭露显示其倾向为近西向,倾角约为76°,断层性质为逆断层,东侧发育有辉绿玢岩,岩石局部破碎,并发育褐铁矿化。在矿化强烈地段可见金矿化发育。

矿区内岩浆岩零星分布于矿区中部、南部和南西部的辉绿玢岩岩脉,整体呈近SN向展布。该岩脉与寒武系呈斜交侵入接触,且在接触带局部发育金属矿化。蚀变带局部发育有辉绿玢岩(图2),暗示着岩体与金矿体之间存在联系,且在手标本上可见辉绿玢岩中发育有零星黄铁矿化[图3(a)]。镜下观察岩石具有斑状结构,斑晶主要为斜长石(50%)和辉石(30%),且斜长石已发生蚀变,部分蚀变为绿帘石和钠长石,其中有少量辉石蚀变为角闪石[图3(b)]。

图2

图2   黑牛凹金矿区18线地质剖面图

1.第四系残坡积物;2.上寒武统核桃坪组上段钙质板岩;3.辉绿玢岩;4.蚀变带;5.金矿体;6.探槽;7钻孔

Fig.2   Geological profile map of No.18 line in Heiniu’ao gold deposit


图3

图3   黑牛凹金矿区辉绿玢岩手标本(a)与显微照片(b)

Py-黄铁矿;Pl-斜长石;Aug-辉石;Ab-角闪石;Ep-绿帘石;Hb-角闪石

Fig.3   Hand specimen (a) and micrographs (b) of diabase in Heiniu’ao gold deposit


2 样品采集与分析方法

从矿区典型钻孔ZK3801中采集4件辉绿玢岩样品进行岩石地球化学和年代学研究,分别采自孔深38.6 m、38.7 m和38.9 m处,钻孔位置见图1。用于全岩主量和微量元素分析的岩石粉末样品由廊坊市地源矿物测试分选技术服务有限公司进行处理,具体处理方法如下:(1)将样品磨碎至1 mm以下;(2)用稀HCl和去离子水进行超声浸泡和清洗;(3)将样品烘干,用不锈钢体把样品粉碎至75 μm以下。主量和微量元素分析工作在核工业北京地质研究院分析中心完成,主量元素采用飞利浦PW240 X射线荧光谱仪测定,微量元素采用高温高压消解,并采用ELEMENT等离子质谱分析仪进行分析。主量元素的分析精度高于5%,微量元素的分析精度高于10%。

锆石颗粒分选由廊坊市地源矿物测试分选技术服务有限公司完成。首先在双目镜下挑选晶形较好、裂隙较少的锆石颗粒,粘贴在环氧树脂表面制成品靶,打磨品靶,使锆石主体部位充分暴露,并进行抛光;然后对锆石进行透反射光显微照相和阴极发光(CL)图像分析,根据透反射光显微照相和阴极发光(CL)图像挑选出没有包体和裂隙的锆石和微区用于圈定激光剥蚀区域。

锆石U-Pb同位素定年在中国地质大学(武汉)地质过程与矿产资源国家重点实验室(GPMR)利用LA-ICP-MS完成。激光剥蚀系统为GeoLasHd,等离子体质谱仪为Agilent7900。激光能量为80 mJ,频率为5 Hz,激光束斑直径为32 µm。激光剥蚀过程中采用氦气作为载气、氩气为补偿气以调节灵敏度,二者在进入ICP之前通过一个T型接头混合。在等离子体中心气流(Ar+He)中加入了少量氮气,以提高仪器灵敏度、降低检出限和改善分析精密度[22]。另外,激光剥蚀系统配置了一个信号平滑装置,即使激光脉冲频率低至1 Hz,采用该装置后也能获得光滑的分析信号[23]。每个时间分辨分析数据包括20~30 s的空白信号和50 s的样品信号。数据分析处理(样品和空白信号的选择、仪器灵敏度漂移校正、元素含量及U-Th-Pb同位素比值以及年龄的计算)采用软件ICPMSDataCal 12.0[24,25]完成。锆石谐和年龄图的绘制使用Isoplot3.0软件完成[26]。所获得同位素比值和年龄误差均在1 δ程度,锆石采用206Pb/238U年龄。

3 分析结果

3.1 地球化学特征

黑牛凹金矿区内4件辉绿玢岩样品的主量和微量元素分析结果如表1所示。辉绿玢岩的SiO2含量为48.92%~49.06%,TiO2含量在1.42%~1.47%之间;Al2O3具有较高含量,在15.59%~15.93%之间变化;MgO含量低,为4.70%~4.97%,Mg#介于42.71~43.88之间;CaO含量变化范围为6.49%~7.21%;FeOT含量较高,在11.24%~11.46%之间;Na2O含量为1.85%~2.10%;K2O含量为2.31%~2.75%,K2O含量大于Na2O含量,具有高钾特征;P2O5含量为4.85%~5.55%。将4件辉绿玢岩样品主量元素扣除烧矢量投点于TAS图解[图4(a)]上,4件样品均投到碱性辉长岩区域,在SiO2-K2O图解[图4(b)]中,全部样品均投影在钾玄岩岩石系列区域。

表1   黑牛凹金矿区辉绿玢岩主量和微量元素特征

Table 1  Characteristics of major elements and trace elements of diabase in Heiniu’ao gold deposit

样品编号岩性主量元素及含量/%
SiO2TiO2Al2O3Fe2O3TMgOCaONa2OK2OMnOP2O5LOITotal
HNA7-02-1辉绿玢岩49.061.4315.5911.454.967.191.882.730.180.164.8599.48
HNA7-02-2辉绿玢岩48.971.4215.6611.464.937.211.852.750.180.174.8599.45
HNA7-09辉绿玢岩48.951.4715.9311.244.706.492.102.700.170.165.5599.46
HNA7-10辉绿玢岩48.921.4515.8011.334.976.992.052.310.180.165.3099.46
JCH辉绿岩48.431.7114.8811.786.599.632.980.080.300.203.38100.21
HTP辉绿岩48.621.3414.2211.635.749.151.772.990.480.143.3899.46
样品编号岩性微量元素及含量/(×10-6)
RbBaThUTaNbSrZrHfCrCuPb
HNA7-02-1辉绿玢岩112.06973.410.601.0512.338968.12.2374.51306.48
HNA7-02-2辉绿玢岩109.06733.280.630.9211.737966.32.0675.01286.07
HNA7-09辉绿玢岩117.08853.141.170.9911.933159.91.8965.01247.26
HNA7-10辉绿玢岩87.305293.390.580.8912.033860.11.9871.71045.50
JCH辉绿岩1.34293.540.681.6028.439749.11.75153.0334.00
HTP辉绿岩90.055103.360.620.7110.9342237.351.88176.0129.3522.16
样品编号岩性微量元素及含量/(×10-6)
ZnNiCoLaCePrNdSmEuGdTbDy
HNA7-02-1辉绿玢岩105.045.441.016.833.14.4319.14.741.384.751.026.04
HNA7-02-2辉绿玢岩100.044.839.716.032.34.2118.54.471.324.620.995.89
HNA7-09辉绿玢岩100.038.936.915.631.54.1317.94.421.264.440.955.79
HNA7-10辉绿玢岩93.841.738.816.532.94.3218.74.561.154.580.945.88
JCH辉绿岩124.058.238.619.738.14.8017.54.411.524.250.874.78
HTP辉绿岩205.867.646.915.132.84.0918.94.921.445.560.935.92
样品编号岩性微量元素及含量/(×10-6)

LREE/

HREE

(La/Yb)NδEu
HoErTmYbLuYΣREELREEHREE
HNA7-02-1辉绿玢岩1.222.950.583.220.4032.599.7379.5520.183.943.740.88
HNA7-02-2辉绿玢岩1.192.910.563.090.3930.496.4476.819.643.913.710.88
HNA7-09辉绿玢岩1.162.840.543.020.3830.593.9374.8119.123.913.710.86
HNA7-10辉绿玢岩1.212.930.553.020.3831.997.6278.1319.494.013.920.76
JCH辉绿岩0.952.670.412.250.2925.1102.4585.9916.465.226.281.06
HTP辉绿岩1.113.070.402.350.3229.496.8977.2319.663.930.84

注:主量和微量元素测试工作在核工业北京地质研究院分析中心完成,2017

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图4

图4   黑牛凹金矿区辉绿玢岩TAS图解(a)及SiO2-K2O图解(b)

Fig.4   TAS diagram (a) and SiO2-K2O diagram (b) of diabase in Heiniu’ao gold deposit


黑牛凹辉绿玢岩的岩石地球化学特征与保山区域内典型铅锌多金属矿区(金厂河、核桃坪)内辉绿岩具有一致性,主量元素除金厂河辉绿岩K2O含量较低之外,其余主量元素差异不大。在稀土元素配分模式图[图5(a)]中,黑牛凹辉绿玢岩与其余2个矿区的辉绿岩具有一致性,微量元素原始地幔标准化蛛网图[图5(b)]中除金厂河辉绿岩具有Rb、Ba、K亏损(可能是金厂河辉绿岩蚀变导致的)之外,其余微量元素特征具有一致性。

图5

图5   黑牛凹金矿区辉绿玢岩的稀土元素配分模式图(a)和原始地幔标准化微量元素蛛网图(b)(底图据Sun等[29]

Fig.5   Chondrite-normalized rare earth distribution pattern diagram (a) and primitive mantle-normalized trace element spider diagram(b) for diabase in Heiniu’ao gold deposit(base map modified after Sun,et al[29]


3.2 锆石U-Pb年龄

本研究对黑牛凹金矿区辉绿玢岩样品(HNA7-06)中的14颗锆石进行了LA-CP-MS锆石原位U-Pb定年,每颗锆石选取一个测试点。最终获得具有一致谐和曲线的点有5个,分析数据见表2。锆石阴极发光图像(CL)(图6)显示,锆石自形程度较好,多呈长柱状,长轴为45~100 μm,具有较大的长宽比值(1.0~2.5),样品U含量为324.24×10-6~805.00×10-6,Th含量为63.90×10-6~693.92×10-6,Th/U比值为0.08~0.93,平均比值为0.58,除HNA7-06-13一个点Th/U比值较小(0.08)之外,其余点的Th/U比值均大于0.1。从锆石阴极发光来看,锆石发育振荡环带,显示基性岩浆锆石特征[27,28]。5个分析点落在一致曲线上及其附近,具有良好的谐和性,锆石206Pb/238U年龄为(209.22±2.46)~(217.68±4.23) Ma,加权平均年龄为(212.3±4.9) Ma(n=5,MSWD=1.5)(图7),表明黑牛凹金矿区辉绿玢岩为晚三叠世岩浆活动的产物。

表2   黑牛凹金矿区辉绿玢岩LA-ICP-MS锆石U-Pb年龄

Table 2  LA-ICP-MS zircon U-Pb age of diabase porphyrite in Heiniu’ao gold deposit

分析点元素含量/(×10-6Th/U比值年龄
总PbThU207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ
HNA7-06-0361.46368.65397.440.930.05620.00300.26520.01400.03430.0007461.16115.73238.8711.27217.684.23
HNA7-06-0445.00252.52324.240.780.05280.00370.24240.01440.03430.0007320.43162.02220.4011.81217.614.55
HNA7-06-08119.74693.92759.570.910.05350.00270.24970.01220.03400.0006350.06114.80226.359.90215.733.82
HNA7-06-1128.19145.00805.000.180.04820.00210.22230.00990.03300.0004200.0868.46203.798.22209.222.46
HNA7-06-1327.4763.90770 .000.080.04900.00250.22330.01090.03300.0005146.38113.87204.659.09209.532.88

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图6

图6   黑牛凹金矿区辉绿玢岩锆石阴极发光(CL)图像和U-Pb同位素分析点位

Fig.6   Cathodoluminescence(CL) images of zircon with U-Pb analyzed from the diabase in Heiniu’ao gold deposit


图7

图7   黑牛凹辉绿玢岩锆石206Pb/238U-207Pb/235U谐和图

Fig.7   Concordia plot of zircon 206Pb/238U-207Pb/235U of diabase in Heiniu’ao gold deposit


4 讨论

通过对黑牛凹矿区内辉绿玢岩进行LA-ICP-MS锆石U-Pb年代学分析,获得其形成年龄为(212.0±4.9) Ma,这与韩艳伟[18]获得的核桃坪矿区辉绿岩锆石SHRIMP U-Pb(195.0±5.3)Ma和金厂河矿区辉绿岩LA-ICP-MS U-Pb年龄(217.3±5.8) Ma(未发表数据)具有一致性,均为晚三叠世岩浆活动的产物。

黑牛凹辉绿玢岩在球粒陨石标准化稀土元素配分曲线和原始地幔标准化微量元素蛛网图上,均显示与洋岛玄武岩(OIB)具有相似特征,表明黑牛凹辉绿玢岩的岩石源区可能为地幔源区。具有相似地球化学特征的不相容元素在岩浆演化过程中,其含量变化具有同步性。元素间含量的比值不易随分离结晶和部分熔融而改变,如Th/Ta和Zr/Hf,这类元素的比值通常可以反映岩石的源区特征。黑牛凹辉绿玢岩Th/Ta比值为3.18~3.80,平均比值为3.44,略大于原始地幔Th/Ta比值(2.3)而小于大陆地壳的Th/Ta比值(10左右)[29];其Zr/Hf比值为30.35~32.18,平均比值为31.19,大于地壳Zr/Hf比值(11)而接近于原始地幔Zr/Hf比值(36.73)[30,31]。Th/Ta和Zr/Hf比值显示黑牛凹金矿区辉绿玢岩具有地幔源区性质,并受到部分地壳物质的混染。基性岩的岩浆一般源自岩石圈地幔或软流圈地幔[32],不同起源的基性岩其微量元素之间存在一定的差异性。源于岩石圈地幔的岩石具有相对富集轻稀土元素(LREE)和大离子亲石元素(LILE)、相对亏损高场强元素(HFSE)的特征;源于软流圈地幔的岩石则具有相对富集大离子亲石元素(LILE)和高场强元素(HFSE)[29]的特征。从微量元素特征来看,黑牛凹辉绿玢岩具有相对富集轻稀土元素(LREE)和大离子亲石元素(LILE)、相对亏损高场强元素(HFSE)的特征,这暗示着黑牛凹辉绿玢岩的源区为岩石圈地幔。另外,其Cr含量(65×10-6~75×10-6)和Ni含量(38.9×10-6~45.4×10-6)比原始玄武岩浆Cr含量(300×10-6~500×10-6)和Ni含量(300×10-6~400×10-6)低很多[33],具低镁特征一致,说明岩浆可能经历了橄榄石和辉石的结晶分离作用,微量元素蛛网图上Nb和Ta元素具有明显负异常,说明岩浆在上升侵位过程中受到了地壳物质的混染。

基性岩是深部玄武质岩浆从源区快速侵位到地表形成的产物[34,35]。由于岩浆产出环境导致其形成构造背景的多样性。从微量元素特征来看,黑牛凹辉绿玢岩具有富集轻稀土元素(LREE)、富集大离子亲石元素(LILE)及亏损高场强元素(HFSE)的特征,这与板内裂谷玄武岩特征一致[36]。结合前人对保山地块构造演化的研究,中三叠世古特提斯洋闭合导致保山地块与思茅地块主体发生碰撞[37],使保山地块内部产生拉张伸展活动[11,36]。结合黑牛凹辉绿玢岩的野外地质特征,辉绿玢岩均产于断裂带及其附近。将辉绿玢岩样品投点于玄武岩FeOT-MgO-Al2O3构造判别图解[图8(a)]上,黑牛凹辉绿玢岩与金厂河辉绿岩、核桃坪辉绿岩均落在大陆环境区域,这与区域构造演化相符合。从Ta/Hf-Th/Hf图解[图8(b)]来看,辉绿玢岩样品与其余2个矿区辉绿岩均投点于陆内裂谷碱性玄武岩范围。综合上述特征,认为黑牛凹辉绿玢岩、金厂河辉绿岩和核桃坪辉绿岩均形成于板内拉张环境,岩石成因与古特提斯洋闭合而导致保山地块发生碰撞引起的造山作用有关。

图8

图8   玄武岩构造环境判别FeOT-MgO-Al2O3图解(a)(底图据Pearce等[38])和玄武岩构造环境判别Th/Hf-Ta/Hf图解(b)(底图据汪云亮等[39]

Fig.8   Discrimination of basalt tectonic environment FeOT-MgO-Al2O3 diagram (a)(base map modified after Pearce, et al[38]) and Th/Hf-Ta/Hf diagram for discrimination of basalt tectonic environment(b)(base map modified after Wang,et al[39]


5 结论

(1)黑牛凹金矿区辉绿玢岩的成岩年龄为(212.3±4.9) Ma,属于保山地块内晚三叠世构造—岩浆活动的产物。

(2)黑牛凹金矿区辉绿玢岩相对富集轻稀土元素(LREE)和大离子亲石元素(LILE),相对亏损高场强元素(HFSE),Nb和Ta元素相对原始地幔显示亏损,表明岩石源区可能为岩石圈地幔,且在岩浆侵位过程中受到地壳物质的混染。

(3)黑牛凹辉绿玢岩形成于板内拉张环境,与金厂河辉绿岩、核桃坪辉绿岩的产出环境一致,均为区域拉张、伸展活动的产物,很可能与保山地块、思茅地块碰撞造山事件有关。

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