北山地区照壁山南A型花岗岩地球化学特征及构造意义

2016-10-10 09:26王怀涛任文秀赵淇馨刘子锐王庭超赵洪星
西北地质 2016年1期
关键词:蛇绿岩照壁山南

王怀涛,任文秀,赵淇馨,刘子锐,王庭超,赵洪星

(1.兰州大学地质科学与矿产资源学院,甘肃 兰州 730000; 2. 甘肃省地质调查院,甘肃 兰州 730000;3. 甘肃省第三地质勘察院,甘肃 兰州 730000)



北山地区照壁山南A型花岗岩地球化学特征及构造意义

王怀涛1,2,任文秀2,赵淇馨3,刘子锐2,王庭超3,赵洪星2

(1.兰州大学地质科学与矿产资源学院,甘肃 兰州730000; 2. 甘肃省地质调查院,甘肃 兰州730000;3. 甘肃省第三地质勘察院,甘肃 兰州730000)

侵入到红柳河蛇绿岩中的照壁山南黑云母二长花岗岩,其SiO2含量为68.86% ~ 73.27%,具高碱略富K贫Na,(K2O+Na2O=7.03 ~ 7.98,K2O/Na2O=1.16 ~ 1.36),AI含量中等(Al2O3=13.75%~14.1%)、弱过铝质(A/NKC=1.03 ~ 1.07)的特点,属高钾钙碱性系列弱过铝质花岗岩类。岩石具较高的Ga×104/Al(5.22 ~ 6.48)和Y/Nb (2.71 ~ 3.43) 值,在原始地幔标准化图解上显示出K、La、Nd、Zr、Hf、Sm富集, Nb、Ta、Sr、P、Ti强烈亏损,轻、重稀土分异明显,(La/Yb)N=7.19 ~13.15,负Eu异常(δEu=0.82 ~ 0.84),REE配分曲线呈略右倾“海鸥型”型的特点。研究结果表明黑云母二长花岗岩是在早泥盆世后碰撞伸展环境下,早期俯冲的与岛弧有关的物质受到底侵幔源玄武岩浆的加热部分熔融后,而形成的A(A2)花岗岩。红柳河-牛圈子-洗肠井蛇绿岩带西段所代表的古洋壳在晚奥陶世—早泥盆世间闭合(446.4~404.8Ma),之后出现一个以挤压结束伸展开始为特征的动力学演化阶段,其转化时间可能在415~404.8Ma。

红柳河蛇绿岩;后碰撞;地球化学;A型花岗岩;北山

北山造山带是中亚造山带的重要组成部分,地处哈萨克斯坦板块、塔里木板块、西伯利亚板块三大板块交汇部位,隶属于中亚造山带南缘,经历了早古生代至早中生代长期的多阶段、复杂的俯冲-拼贴历史,对认识中亚造山带的构造演化和古亚洲洋的最终闭合具有重要研究意义(贺振宇等,2014;XIAO et al., 2010; SONG et al., 2013a,2013b; TIAN et al., 2014)。在复杂的地质作用下,北山地区自北向南分布4条蛇绿岩带:红石山-百合山-蓬勃山蛇绿岩带、芨芨台子-小黄山蛇绿岩带、红柳河-牛圈子-洗肠井蛇绿岩带、辉铜山-帐房山蛇绿岩带(杨合群等,2010),通过分析这些蛇绿岩带及其相关的岩浆岩,对于认识北山地区的构造演化具有重要的意义。前人对红柳河-牛圈子-洗肠井蛇绿岩带做了大量的研究,多数学者认为红柳河-牛圈子-洗肠井蛇绿岩属于洋壳型蛇绿岩,该蛇绿岩带为早古生代缝合带,分隔塔里木板块和哈萨克斯坦板块(任秉琛等,2001;何世平等,2002;王立社等,2007;杨合群等,2010;武鹏等,2012),也有部分学者认为红柳河-牛圈子-洗肠井蛇绿岩形成于弧后盆地环境(左国朝等,1990;于福生等,2000;郑荣国等,2012)。目前,已发表资料表明北山地区仅红柳河-牛圈子-洗肠井蛇绿岩带与北山早古生代洋盆有关(周国庆等,2000;任秉琛等,2001;于福生等,2006;郭召杰等,2006;张元元等,2008 ;武鹏等,2012;李向民等,2012;侯青叶等,2012)。

长期复杂的构造演化使北山地区的花岗岩类侵位具有多期性和复杂性,侵位时间可能从前寒武纪一直延续到中生代燕山期。在红柳河地区分布着大量的花岗岩体,主要发育晚加里东期和早华力西期的花岗岩体(李伍平等,2001a,2001b;赵泽辉等,2007;王涛等,2008),这些与造山碰撞有关的花岗岩体对于探讨区域构造演化具有重要的意义。在限定板块缝合带的形成时代的上限时,直接侵入在缝合带的蛇绿岩或蛇绿混杂岩中的岩体构造意义最为明确,而侵入到红柳河蛇绿岩中的黑云母二长花岗岩具有钉合岩体的大地构造意义,其形成年代限定红柳河蛇绿岩的侵位的上限(韩宝福等,2010)。因此,笔者选取侵入红柳河蛇绿岩中的照壁山南黑云母二长花岗岩进行岩石地球化学分析,结合其形成时代,探讨了其成因及构造意义,以期对中亚造山带南缘板块构造演化的深入研究提供可靠的依据。

1 地质背景

北山地区位于甘新蒙交界部位(图1),是东天山造山带的东延部,呈东西向延伸,向东被巴丹吉林沙漠掩盖,向北延伸至蒙古增生造山带,南到阿尔金断裂,长约520km,宽约250km(刘雪亚等,1995;XIAO et al., 2010)。北山造山带由一系列的岛弧、蛇绿混杂岩带和微陆块等构造单元组成,北山造山带的北部是由多个洋内岛弧拼贴形成的复杂增生系统,在古生代晚期拼贴形成公婆泉复合岛弧(贺振宇等,2014;XIAO et al., 2010; SONG et al., 2013a, 2013b),而南部主要涉及了古老微陆块之间的俯冲-碰撞作用(LIU et al., 2011; QU et al., 2011)。

1.第四系;2.二叠纪砂岩;3.早古生代火山碎屑岩;4.枕状玄武岩;5.元古宇大理岩;6.前寒武系片岩片麻岩;7.超镁铁质岩;8.中粗粒堆晶辉长岩;9.细粒堆晶辉长岩;10.片麻状花岗岩;11.二长花岗岩;12.黑云母二长花岗岩;13.闪长岩;14.韧性剪切带;15.采样位置及编号; Ⅰ.准噶尔-哈萨克斯坦板块;Ⅱ.塔里木板块;A.研究区图1 (a)研究区构造背景及(b)红柳河蛇绿岩地区的地质简图 ( 据张元元等,2008)Fig. 1 (a)Sketch geological map of Hongliuhe area and (b)its tectonic setting (After ZHANG Y Y, et al., 2008)

研究区位于红柳河-牛圈子-洗肠井蛇绿岩带西段红柳河地区,处于东天山东段,塔里木板块与哈萨克斯坦板块交接部位的东南侧,属于塔里木板块北部活动大陆边缘及中天山隆起带与南天山弧后盆地褶皱带的交汇部位(图1a)。红柳河蛇绿岩出露在甘新交界处红柳河车站北东方向照壁山南北两侧,基本呈东西展布,向东被第四系冲洪积物覆盖,西侧与二叠纪砂岩角度不整合接触,南以红柳河—前进工区韧性剪切带与前寒武系片岩、片麻岩,志留纪片麻状花岗岩为界,北侧受红柳河隐伏断裂控制并被第四系洪积物覆盖,出露总面积约80km2。红柳河蛇绿岩套中的各种岩石组分出露齐全,各组分之间多为构造接触,变质橄榄岩、堆晶超镁铁岩、堆晶辉长岩、枕状玄武岩、碧玉岩等均有出露,在照壁山一带被蓟县纪大理岩、片岩推覆其上。照壁山南黑云母二长花岗岩位于蛇绿岩的东南部,长约5km,宽约150~650m,呈北西西—南东东向的长条状,以岩枝状侵入红柳河蛇绿混杂岩中的早古生代火山碎屑岩和细粒辉长岩中(图1b)。

2 岩石特征

黑云母二长花岗岩呈浅灰色,具细粒花岗结构,粒径0.1~2mm,块状构造,主要矿物为斜长石(40%±)、钾长石(20%±)、石英(25%±)、黑云母(10%±)、角闪石(5%±)及微量的金属矿物、磷灰石、锆石。长石晶体呈宽板状和短柱状,少量为他形粒状,斜长石发育卡式和双晶纹较细密的聚片双晶,具微弱的钠黝帘石化;钾长石发育卡式和格子双晶,具明显的环带构造和条纹构造,包裹细粒斜长石,并见2种长石形成的蠕英结构;石英呈他形粒状,波状消光;黑云母片呈鳞片体,具深褐-淡黄多色性,具轻微的绿泥石和帘石化;角闪石呈短柱状,横断面呈多边形或近六边形,蓝绿-浅黄绿多色性,个别晶体具微次闪石化。

3 分析方法

照壁山南黑云母二长花岗岩的样品采集位置见图1。样品的岩石地球化学分析由国土资源部中南矿产资源监督检测中心完成。主量元素测定采用AXIOS X射线荧光光谱仪分析,分析误差<5%,分析结果见表1。微量元素采用X seroies Ⅱ等离子体质谱仪分析,分析误差< 10%。析结果见表2、表3。

4 岩石地球化学特征

4.1主量元素特征

照壁山南黑云母二长花岗岩岩石化学成分见表1,照壁山南黑云母二长花岗岩SiO2含量为68.86% ~ 73.27%,K2O+Na2O含量较高,平均为7.37%,除一个样品(K2O/Na2O=0.89)外,其余样品略富K贫Na(K2O为3.32% ~ 4.6%,Na2O为3.29% ~ 3.71%,K2O/Na2O为1.16 ~ 1.36),Al含量中等(13.75% ~ 14.1%),贫CaO(1.71% ~ 2.3%)和MgO(0.39% ~ 0.81%),低Ti(TiO2为0.29% ~ 0.58%),低TFeO(1.95% ~ 4.16%),具弱过铝质(A/CNK=1.03 ~ 1.07,A/NK=1.31~1.47)地球化学特征。在SiO2-K2O投影图上落入高钾钙碱性系列(图2),在铝饱和指数判别图解中显示弱过铝质花岗岩特征(图3)。

图2 岩石w(SiO2)-w(K2O)图解Fig. 2 w(SiO2)-w(K2O) diagram for granitoids

图3 岩石 A/CNK-A/NK 图解Fig.3 A/CNK-A/NK diagram for granitoids

4.2微量元素特征

照壁山南黑云母二长花岗岩微量、稀土元素分析结果见表2、表3。岩石中HFSE元素含量高,元素组合(Zr+Nb+Ce+Y)为410.3×10-6~ 470.9×10-6,明显大于A型花岗岩下限值350×10-6,岩石具有较高的Ga×104/Al值(5.22 ~ 6.48),大于 A 型花岗岩的下限值(2.6),同时Y/Nb值(2.71 ~ 3.43)较高,大于 A 型花岗岩的下限值(1.2)(WHALEN et al.,1987)。在原始地慢标准化的微量元素蛛网图上(图4),岩体微量元素标准化曲线均呈轻微右倾的锯齿状,总体表现为K、La、Nd、Zr、Hf、Sm富集, Nb、Ta、Sr、P、Ti强烈亏损。稀土元素总含量不高,∑REE为266.6×10-6~ 309.3×10-6;轻、重稀土分异明显(LR/HR=7.59 ~ 12.55,平均值为10.31,(La/Yb)N为7.19 ~ 13.95);弱至中等Eu负异常,δEu为0.82 ~ 0.84,说明岩石经历了一定的斜长石分离结晶作用;稀土元素球粒陨石标准化配分型式为向右倾斜且左陡右平,Eu处“V”型谷的曲线(图5)

5 讨论

5.1岩石成因

照壁山南黑云母二长花岗岩主量元素具高Si、低Mg、低Ti,富碱、贫Ka,略富K贫Na、弱铝质的特点,微量元素蛛网图上K、La、Nd、Zr、Hf、Sm富集, Nb、Ta、Sr、P、Ti强烈亏损, 及 Ga×104/Al(5.22~6.48)和Y/Nb(2.71~3.43)均大于A型花岗岩的下限,均显示与后碰撞或造山后构造环境形成的A型花岗岩特征类似(WHALEN et al., 1987;Eby, 1992; 谭绿贵,2008)。在Whalen et al., 1987年的A型花岗岩判别图解中,照壁山南黑云母二长花岗岩均投到 “A型花岗岩”区(图6)。

图4 照壁山南黑云母二长花岗岩原始地幔标准化的微量元素蛛网图(原始地幔标准值据SUN S S et al., 1989)Fig.4 PM-normalized trace element spider diagram of the biotite monzogranite in the south of Zhaobishan (PM-normalized values from SUN S S et al., 1989)

图5 照壁山南黑云母二长花岗岩稀土元素配分图(球粒陨石标准值据SUN S S et al., 1989)Fig. 5 Chondrite normalized REE distribution patterns of the biotite monzogranite in the south of Zhaobishan (Chondrite-normalized values from SUN S S et al., 1989)

表1 照壁山南黑云母二长花岗岩主量元素地球化学分析结果表(%)

表2 照壁山南黑云母二长花岗岩微量元素地球化学分析结果表(10-6)

表3 照壁山南黑云母二长花岗岩稀土元素地球化学分析结果表(10-6)

Eby根据地球化学特征将A型花岗岩分为A1型和A2型,并认为A1型花岗岩是与洋岛岩浆来源相同的地幔分异产物,且侵位于大陆裂谷或板内的构造环境,A2型花岗岩来源于大陆地壳或板下地壳,且与陆-陆碰撞或岛弧岩浆作用有关(Eby, 1992),照壁山南黑云母二长花岗岩在Y-Nb-Ce和Y/Nb-Rb/Nb判别图中均投影于A2区(图7)。

图6 A型花岗岩判别图解(据WHALENET et al., 1987)Fig. 6 Discrimination diagrams of A-type granites (After WHALENET et al., 1987)

图7 A型花岗岩亚类判别图解(据EBY, 1992)Fig. 7 Discrimination diagrams for the subdivision of A-type granites (After EBY, 1992)

前人对A型花岗岩的成因提出了很多模式:①幔源玄武质岩浆高度结晶分异(TURNER et al.,1992; BEYTH et al., 1994; HAN et al.,1997; MUSHKIN et al., 2003)。②壳源物质和幔源物质混合(邱检生等,1999; MINGRAM et al., 2000; YANG et al., 2006; KONOPELKO et al.,2007)。③特殊源岩的部分熔融。例如,麻粒岩相岩石、英云闪长岩-花岗闪长岩、紫苏花岗岩、新生玄武质地壳(COLLINS et al., 1982; CLEMENS et al., 1986; WHALEN et al., 1987; KING et al., 1997;CREASER et al., 1991; WU et al.,2002)。④上地壳钙碱性岩石低压熔融(PATINO Duce, 1997)。

该岩体的Nb/Ta值为3.48~15.4,平均值为8.58,低于幔源岩石(17.5±2,HOFMANN, 1988; Green, 1995),而较接近陆壳岩石(~11,TAYLOR et al, 1985; GREEN, 1995);Nd/Th值为1.13~4.36,平均为2.63,低于幔源岩石(>15,BEA et al., 2001),而接近壳源岩石(≈3,BEA et al., 2001) ;而亏损的麻粒岩相残留岩作为源岩很难产生本区富Si、富碱的A型花岗岩(CREASER et al., 1991);该岩体Nb负异常也显示其壳源的特点,Rb/Sr值(0.55~1.2,平均值为0.88)大于陆壳平均Rb/Sr值(0.24)(TAYLOR et al., 1986),同样说明其地壳来源;因此,由幔源玄武质岩浆高度结晶分异形成的可能性较小,而可能具壳源特征。

花岗岩的微量元素组成明显受其成岩的构造环境制约,Rb-( Y + N b) 微量元素构造判别图解表明 ,照壁山南花岗岩具火山弧花岗岩的特征,按 Pearce新圈出的后碰撞花岗岩区域 ,正好落在后碰撞区域范围内(图8)。另外,岩体Y/Nb为2.71 ~ 3.4,大于1.2,同样说明了本区花岗岩为后碰撞型花岗岩(EBY, 1990, 1992)。

VAG.火山弧花岗岩; COLG.同碰撞花岗岩; WPG.板内花岗岩; ORG.洋中脊花岗岩;syn-COLG.同碰撞花岗岩;post-COLG.后碰撞花岗岩图8 花岗岩Rb-(Y+Nb)图(据PEARCE et al., 1984)Fig.8 Rb-(Y+Nb)Trace element discrimination of tectonic setting for granitoids (After PEARCE et al., 1984)

事实上,以红柳河蛇绿岩为代表的古洋盆伸展扩张阶段发生在 512Ma前,而洋盆闭合至少在405Ma闭合(郭召杰等,2006),之后出现一个以挤压结束伸展开始为特征的动力学演化阶段。因此,综合分析,认为研究区在后碰撞环境下发生了大规模的岩石圈伸展,并伴随软流圈地幔上涌、玄武岩浆上侵,早期俯冲的与岛弧有关的物质受到底侵幔源玄武岩浆的加热部分熔融后形成的。这与典型A2型花岗岩的形成模式(EBY, 1992)比较一致。

5.2构造意义

红柳河地区侵入岩出露面积较大,侵入时代主要为元古宙、早古生代、晚古生代—早中生代。胡霭琴等测得东天山天湖东黑云母二长花岗岩的SHRIMP U-Pb年龄为(467±10)Ma,具岛弧花岗岩的地球化学特征(胡霭琴等,2007),李伍平等测得红柳河北和前进工区花岗岩岩体的锆石U-Pb分别为(441.4 ±1.6)Ma、(440.9±1.3)Ma,并认为是洋壳在俯冲过程中脱水引起上地幔发生部分熔融形成玄武岩浆,上侵玄武岩浆和流体带去高温并绝大部分底侵于下地壳,诱发前寒武纪变质岩发生部分熔融形成中酸性岩浆侵入地壳后形成的(李伍平等,2001b);照壁山南黑云母二长花岗岩的锆石U-Pb年龄为(404.8±5.2)Ma(张元元等,2008),笔者研究结果显示其为A(A2)型花岗岩,其岩石地球化学、构造特征及区域地质背景显示,该岩体形成于后碰撞环境,该时期红柳河地区已经由同造山挤压环境向转化为造山或造山晚期伸展环境,而后碰撞花岗岩类的形成一定晚于碰撞事件,也晚于蛇绿岩的构造侵位(韩宝福,2007)。因此,红柳河地区古洋壳可能在441.4~404.8Ma闭合,该区由同造山挤压环境向后造山或造山晚期伸展环境的转化时间可能为404.8Ma。而牛圈子蛇绿岩中辉长岩的锆石U-Pb年龄为(446.5±4)Ma(武鹏等,2012),在其南侧形成于后碰撞环境的双峰山A2型花岗岩的锆石U-Pb年龄为(415±3)Ma(李舢等,2009),说明该区古洋壳在446Ma仍存在,在(415±3)Ma前已经闭合。因此,牛圈子地区古洋壳闭合时间也应为446.5~415Ma。据此初步得出结论:红柳河-牛圈子-洗肠井蛇绿岩带西段所代表的古洋壳在晚奥陶世—早泥盆世间闭合(446.5~404.8Ma),之后出现一个以挤压结束伸展开始为特征的动力学演化阶段,其转化时间可能为415~404.8 Ma,而其东段的构造演化过程需要作进一步的研究工作,这对理清北山地区中古生代板块构造演化具有十分重要的意义。

6 结论

(1)照壁山南黑云母二长花岗岩为A(A2)型花岗岩,其成因可能为后碰撞背景下,研究区发生了大规模的岩石圈伸展,并伴随软流圈地幔上涌、玄武岩浆上侵,早期俯冲的与岛弧有关的物质受到底侵幔源玄武岩浆的加热部分熔融后,而形成A(A2)型花岗岩。

(2)红柳河-牛圈子-洗肠井蛇绿岩带西段所代表的古洋壳在晚奥陶世—早泥盆世间闭合(446.4~404.8Ma),之后出现一个以挤压结束伸展开始为特征的动力学演化阶段,其转化时间可能为415~404.8 Ma。

致谢:本文得到兰州大学地质科学与矿产资源学院王金荣教授的指导和帮助,同时两位审稿专家对本文提出许多宝贵的改进意见, 在此向他们表示衷心感谢!

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宁夏黏土利用史话:烧制瓷器(2)

经过几千年的历史演变, 古人逐步认识不同原料和提高烧制温度,完成了陶到瓷的发展过程: 陶—釉陶—原始瓷器—成熟瓷器。大约在公元前16世纪的商代中期,古人在烧制白陶、印纹硬陶及器表施釉技术的基础上,烧制温度达1 200℃以上的条件下,创造出了原始瓷器。学术界通过长期的研究, 把商代中期出现的一种带有青绿色、青黄色釉, 胎料和烧成温度(1 250℃)不同于一般陶器的器皿划为瓷的范畴, 定名为原始瓷。后来再逐渐发展,出现白瓷、彩瓷(李辉柄,1978)。宁夏境内考古发现多处西夏(公元1038~1227)瓷器窑址:灵武磁窑堡窑、回民巷窑、石沟驿窑,贺兰插旗口窑、红瓮井窑、皇城台子窑,银川西郊缸瓷井窑,石嘴山窑,中卫老窑沟窑,海原天都山窑等。其中,以灵武窑规模最大,延续时间也最长。这些地区瓷土矿藏多为高岭石泥岩或高岭石泥岩夹矸。

经考古研究,灵武磁窑堡窑始烧于西夏中期,在元代仍在烧造,明清衰落。按用途可分为日常生活用器、文房用具、娱乐用具、雕塑艺术品以及建筑用瓷等。按釉色划分, 则可分为白釉、 黑釉、 茶叶末釉、 青釉、 酱釉、姜黄釉瓷等。以白瓷和剔刻花瓷器最具特色。白瓷是西夏瓷器中数量较多、质量较上乘的产品,其次为黑瓷,这可能与西夏王国崇尚白色、喜爱黑色有关(宋燕等,2010)。但由于当地瓷土含杂质较多, 致使瓷器胎体不够洁白, 多呈灰色、灰白色或米黄色。为此, 该窑白瓷在施透明釉以前均需先施一层洁白的瓷土(俗称“化妆土”),以遮盖坯体。瓷器上的装饰题材主要是花卉、花草纹、弦纹, 也有人物纹、动物纹等, 多以划、剔划花技法予以体现(吕成龙,2006)。

在灵武磁窑堡窑遗址附近有丰富的煤矿和瓷土矿。从窑址东坡附近矿洞中收集的一种黏土进行化学分析和x射线衍射分析,其主要矿物组成为高岭土、白云母、石英及长石等, 矿多分布在煤系露头地方, 化学成分Al2O3含量24.44%,Fe2O3含量1.46%,TiO2含量1.00%, 说明此种黏土亦属于高岭石质的泥岩。烧成瓷器后的瓷胎矿物主要为莫来石、石英、方石英,化学成分Al2O3含量24.94%,Fe2O3含量1.16%,TiO2含量1.31%,烧成温度1 260℃。白釉为“石灰釉” , 即属CaO-Al2O3-SiO2系统范围,推知是石灰石和黏土配合而成(李国祯等,1991)。

煤系高岭土是宁夏重要的瓷土资源,储量丰富,质量好,品位高,但有机质含量高。石嘴山矿务局科研所(1991)利用闲置的瓷用隧道窑进行了煅烧煤系高岭土的研究。成功地将温度控制在1 000~ 1 050℃,煅烧后的高岭土( 1250目)主要技术指标: 粒度≤ 10μm达到95.3% ,白度达到92%~ 93.8% ,莫来石含量为4%~5%。为解决煤矸石的综合利用起到积极的推进作用(葛庆龙等,1999)。

目前,宁夏有20多家生产瓷器的企业,大多分布在石嘴山市和中卫市辖区。在石嘴山市,始建于1943年的光华瓷厂,解放后演变为宁夏石嘴山市瓷器厂,继承“光华”牌商标,主要生产日用瓷、出口瓷、民族用瓷和炻瓷(介于陶与瓷之间,质地致密坚硬,与瓷相似),兼搞工业建筑与艺术陈设等瓷器加工,共140个品种, 600多个花色。产品行销全国18 个省、市、自治区,并出口欧美、东南亚、中东等10余个国家和港澳地区等。而在中卫市,近年大力建设常乐陶瓷园,逐步由建筑陶瓷、日用陶瓷向工业用瓷、卫生陶瓷以及艺术陶瓷方向拓展,倾力打造宁夏“塞上瓷都”。

(西安地质调查中心杨合群;宁夏地质调查院毛自力,艾宁,武丹)

Geochemical Characteristics and Tectonic Significance of A-type Granite in the South Margin of Zhaobishan, Beishan Area

WANG Huaitao1,2, REN Wenxiu2, ZHAO Qixin3, LIU Zirui2, WANG Tingchao3, ZHAO Hongxing2

(1.School of Earth Sciences, Lanzhou University, Lanzhou 730000, Gansu,China;2. Geology Survey of Gansu Province, Lanzhou 73000, Gansu,China; 3. Third Institute Geology and Mineral Exploration of Gansu Province Bureau of Geology and Mineral Rosources, Lanzhou 73000, Gansu,China)

The biotite monzogranite, located in the south margin of Zhaobishan, was intruded in Hongliuhe ophiolite, which are characterized by high-K and calc-alkaline composition with SiO268.86%~73.27%, high alkali (K2O+Na2O=7.03~7.98), coupled with midium aluminum (Al2O3=13.75%~14.1%) and weakly peraluminous (A/NKC=1.03~1.07). These rocks have high ratios of Ga×104/Al(5.22 ~ 6.48)and Y/Nb (2.71 ~ 3.43) in trace elements, which also show evidently loss in Nb, Ta, Sr, P, Ti as well as the enrichments of K, La, Nd, Zr, Hf and Sm. They are also rich in LREE with (La/Yb)N=7.19 ~ 13.15, and negative Eu anomalies(δEu=0.82 ~ 0.84). The results indicate that this biotite monzogranite belongs to A(A2) granite, which was formed by partial melting of young oceanic crust and island arc after had being used to be heated by basaltic magma in the extensional environment of early Devonian. Represented by the west part of Hongliuhe- Niujuanzi-Xichangjing ophiolitic belts, the paleo-ocean was closed between the late Ordovician and early Devonian(446.4~404.8Ma). After that, there was a dynamical evolution which was characterized by the end of the squeeze and the beginning of stretch, with the conversion time may be around 415~404.8 Ma.

Hongliuhe ophiolite; post-collisional; geochemistry; A-type granite; Bashan

2015-05-29;

2015-07-27

中国地质调查局甘肃北山玉石山地区铁金铜镍矿产远景调查项目(1212011120556)

王怀涛(1985-),男,工程师,在读博士,研究方向岩石圈演化与成矿作用。E-mail:wanghtao08@lzu.cn

P588.1

A

1009-6248(2016)01-0039-11

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