北山石板泉一带石炭纪双峰式火山岩地球化学特征及构造意义
2016-07-27王晓伟杨春霞王启航王玉玺李鹏兵
王晓伟,杨春霞,王启航,王玉玺,李鹏兵
(1.甘肃省地质调查院,甘肃 兰州 730000;2.甘肃省地质矿产勘查开发局,甘肃 兰州 730000;3.甘肃省地矿局第三地质矿产勘查院,甘肃 兰州 730050)
北山石板泉一带石炭纪双峰式火山岩地球化学特征及构造意义
王晓伟1,2,杨春霞3,王启航3,王玉玺1,李鹏兵3
(1.甘肃省地质调查院,甘肃 兰州730000;2.甘肃省地质矿产勘查开发局,甘肃 兰州730000;3.甘肃省地矿局第三地质矿产勘查院,甘肃 兰州730050)
摘要:北山造山带南缘石板泉一带的石炭纪火山岩(红柳园组)在时空上构成独特的双峰式火山岩组合,具有较短的Daly间断及大量酸性火山岩夹少量中基性火山岩为主要特征,其中安山岩具有富钠贫钾、高Al低Mg的过铝质高钾钙碱性系列特征,并含有相当数量亏损地幔物质,指示了安山岩源区可能源于亏损地幔,显示板内安山岩的特征;流纹岩属于低Ti流纹岩类,指示岩石可能是基性岩浆底侵作用下地壳物质发生部分熔融,源区存在有斜长石残留,形成于大陆裂谷环境,其动力学体制可能与陆内伸展拉张作用有关,表明从晚古生代形成的裂谷在早石炭世晚期-晚早石炭世早期发展到鼎盛时期,晚石炭世由伸展拉张到逐步进入碰撞挤压这个地球动力学环境的重大转折时期,二叠世末进入碰撞造山演化阶段。
关键词:双峰式火山岩;地球化学;构造环境;北山
双峰式火山岩的成分间断通常被称为Daly间断(DALY R A,1925),这种成分间断实际上可以发生在任一段SiO2含量区间内,既可以是通常认为的流纹岩-玄武岩组合,也可以是中性岩-玄武岩组合或流纹岩-中性岩组合(HILDRETH.W,1981;尹得功,2015;朱志新,2012)。故一组时空上紧密伴生的、SiO2含量集中分布在两个区间并且存在一定成分间断的火山岩系就构成一套双峰式火山岩组合,双峰式火山岩可以产生在不同的构造背景中,如洋岛(GEIST. D,1995)、大陆拉张减薄环境(DUNCAN. A .R,1984; GARLAND. F,1995; PIN. C,1993)、洋内岛弧(B ROUXEL. M ,1987)、弧后盆地(HOCH STAEDTER. A. G,1990a,1990b)、成熟岛弧(FREY. F. A ,1984; PIN. C,1997)以及造山后拉张环境(COULON. C,1986)等。探讨双峰式火山岩的形成机制和产出环境,不仅对恢复造山带大地构造格局有意义,而且对于探查与双峰式火山岩有关的矿产资源也是有启发的(王焰,2000;钱青,1999)。笔者拟对北山南带石板泉一带石炭纪红柳园组双峰式火山岩进行岩石学和岩石地球化学的综合研究,试图完善其形成机制,探讨产出环境及意义。
1地质背景及岩石学特征
北山造山带位于中亚造山带的南缘,地处哈萨克斯坦、准噶尔和塔里木板块的交接部位,地质构造复杂,经历了多期次、多阶段的板块裂解—俯冲—碰撞—拼合的复杂地质演化过程,具多旋回复合造山的特点,体现在前造山阶段的多岛(块)洋、俯冲造山阶段的沟-弧-盆-裂谷体系及其软碰撞,主造山期多旋回俯冲碰撞、斜向敛合和构造迁移,以及造山后期造山带侧向挤出和构造逃逸、伸展断陷等复杂的构造演化历史(龚全胜,2002;张新虎,2005,2007,2008;杨合群,2006;王金荣,2001;聂凤军,2002;左国朝1990;RAPP. R.P,1997)。研究区位于北山造山带南带之南缘红柳园-帐房山构造岩浆岩带中白山堂一带(图1),石炭纪红柳园组可见安山岩与流纹岩、流纹质凝灰岩呈互层(局部可见玄武岩和流纹岩互层)产出,但整体上以爆发相(火山碎屑岩类)→溢流相(安山岩到流纹岩)→爆发-沉积相(凝灰岩、凝灰质砂岩夹生物碎屑灰岩)韵律构成一个大的晚古生代火山喷发旋回。安山岩主要具斑状结构、玻晶交织结构,块状构造。斑晶主要为斜长石、石英及暗色矿物;基质占岩石的90%以上;流纹岩具斑状结构,基质具霏细结构、显文象结构,块状构造。斑晶主要为更长石和石英,基质主要为长英质(70%~80%),部分流纹岩中可见斜长石晶屑及少量磷灰石、金属矿物。
2样品采集与测试
样品采集于石板泉南侧的安山岩和流纹岩。选择新鲜无蚀变、隐晶质结构、经镜下薄片鉴定和严格筛选的样品进行化学分析。样品委托甘肃省中心实验室测定,主量元素除FeO和LOI采用标准湿化学分析外,其余的利用XRF分析,测试仪器为帕拉科生产的AXIOS型X-荧光光谱仪,精度一般优于5%;稀土、微量元素采用美国ThermoFisher公司生产的XII Series型ICP-MS分析,数据质量用国家一级标准物质进行质量监控GB/T 14506-1993,分析结果采用DZ130-1991进行数据检查,测试精度介于5%~10%。
1.长城纪古硐井群;2.蓟县纪平头山组;3.石炭纪红柳园组安山岩;4. 石炭纪红柳园组流纹岩;5. 石炭纪红柳园组流纹斑岩;6.白垩纪新民堡群;7. 第三纪冲洪积层;8.花岗闪长岩;9.二长花岗岩;10.白山堂铜矿;11采样位置;12研究区图1 研究区区域地质略图Fig.1 Regional Geological Sketch of Study Area
3地球化学特征
3.1主元素地球化学特征
红柳园组火山岩(表1)SiO2介于62.97%~64.79%和73.50%~76.07%,具较短的Daly成分间断,在时空上可以构成双峰式火山岩组合。该Daly成分间断和美国-加拿大的喀斯卡特Mazama、Medicine、墨西哥的Ceborco及日本Mashu等火山地区组成的双峰式火山岩十分相似(王焰,2001)。在TAS图解上样品分别落入安山岩和流纹岩域内(图2),其中安山岩具富Na、贫K、高Al低Mg特征,Mg#(31~37)远远小于典型的MORB的Mg#值(60)(RAPP. R.P,1997),σ=1.86~2.88,TiO2>0.7%,暗示岩浆来源和上地幔有关,K2O/TiO2=2.5~3.7,K2O/P2O5=7.3~12.8,比值较低且变化范围较小反映了在岩浆演化过程中地壳混染不强烈(郭峰,2005)。流纹岩具高Si、富Na、贫K、低Al、Ca特征,σ=1.22~2.68,低Ti(TiO2<0.7%)和P,应归属于低Ti流纹岩类(RILEY.T.R,2001)。 K2O/TiO2=7.7~29.9,K2O/P2O5=42.4~93.3,明显高于与其伴生的安山岩, 暗示流纹岩可能是大陆地壳厚度较薄的下地壳中基性物质部分熔融的产物(CONDIE. K.C,1986;郭锋,2001)。在SiO2-K2O 图解上流纹岩为钙碱性-高钾钙碱性系列,而安山岩为高钾钙碱性系列(图3),在火山岩的铝饱和指数图解中(图4)安山岩为过铝质,而流纹岩具有过铝-偏铝质特点。
B.玄武岩(含CIPW霞石标准矿物分子为碱性玄武岩);O1.玄武安山岩;O2.安山岩;O3.英安岩;R.流纹岩;S1.粗面玄武岩;S2.玄武粗安岩;S3.粗面安山岩;T.粗面岩和粗面英安岩;Pc.苦橄岩;U1.碧玄岩和碱玄岩;U2.响岩质碱玄岩;U3.碱玄质响岩;Ph.响岩;F.似长石岩图2 火山岩TAS图解(据M. J. LE BAS,等.1986)Fig.2 TAS diagrams(After M. J. LE BAS,et al.1986)
表1 常量元素(%)、微量元素(10-6)、稀土元素(10-6)表
注:测试单位:甘肃省中心实验室。
Ⅰ.拉斑系列;Ⅱ.钙碱性系列;Ⅲ.高钾钙碱性系列;Ⅳ.橄榄玄粗岩系列图3 火山岩SiO2-K2O图解(据PECCERILLO et al.,1976)Fig.3 SiO2-K2O diagrams(After PECCERILLO et al.,1976)
3.2稀土元素和微量元素地球化学特征
石炭纪红柳园组火山岩微量元素表明,安山岩中Ba、Ta含量较高且变化较小,Ni和Cr的含量较低且变化较大,指示原始岩浆发生过橄榄石的分离结晶明显,Nb和Ti的含量明显偏低,表明本区岩浆可能受到过陆壳物质的较弱的混染,或者表明幔源岩浆在壳内次生岩浆房中曾发生过辉石和钛氧化物的分离结晶(邱家骧,1991;李昌年,1992)。Zr/Nb为26.5~42.7,Zr/Y为7.02~9.75,均大于4,显示板内安山岩的特征。流纹岩中RbN/YbN=4.42~14.98,属于强不相容元素弱富集型。Ba、Zr较高,Sr、P、Ti、Cr、Ni含量较低。Zr含量多数介于125~450,Hf含量大于5,Yb含量为3~12,这些特征与裂谷特征相一致。在原始地幔标准化的蛛网图(图5)上,曲线呈轻微右斜的锯齿状,总体上表现为Rb、Ba、Th、K、La、Ce等不相容元素相对富集,高场强元素Nb、Ta、P、Ti、Sr等具有明显的负异常。
稀土元素含量相差不大,稀土总量、轻稀土含量以及重稀土含量安山岩均明显高于流纹岩,LREE/HREE为1.98~2.04,(La/Yb)N=4.63~4.86,表明轻重稀土分馏不明显;流纹岩LREE/HREE为1.87~5.19,(La/Yb)N=4.13~17.48,表明轻重稀土具有一定程度的分馏。Sm/Nd为0.18~0.23,均小于0.33,表明该地区火山岩为轻稀土弱富集型,但流纹岩富集较安山岩更明显。球粒陨石标准化配分图(图6)显示,明显富集LREE,亏损HREE,具弱Eu负异常(δEu=0.46~0.86,介于0.4~1),结合低Sr及高Yb和Y表明,在岩浆作用过程发生过斜长石和角闪石分结离晶作用或源岩在部分熔融工程中存在有斜长石的残留(HUGH R.,2000)。
图4 火山岩的铝饱和指数图解Fig.4 A/NK-A/CNK diagrams
图5 原始地幔标准化微量元素蛛网图(标准值据SUN et al.,1989)Fig.5 Preliminary mantle-normalized trace element patterns (Normalizing values from SUN et al., 1989)
4讨论
地质证据表明,双峰式火山岩可以出现在不同的构造背景中, 故双峰式火山岩并不只是大陆裂谷环境的产物,只要是岩浆作用的地质条件许可均可出现在不同的构造环境中(王焰,2000;钱青,1999)。形成于陆壳基底上的岩浆弧,其岩浆活动总是以安山岩类的大量出现为特征(郭锋,2001;许红英,2011)。研究区处于北山白山堂一带,双峰式火山岩以安山岩和流纹岩为主,但该区Daly成分间断比较小,而且以大量流纹岩为主要特征。地球化学特征指示安山岩中含有相当数量亏损地幔物质,指示了安山岩源区可能为亏损地幔(图7),形成于板内环境,岩浆早期明显发生过橄榄石、辉石、斜长石的分离结晶作用,Th、Nb、Ta相对负异常及LREE和LILE适度富集可能是与地幔岩浆上侵引起下地壳物质部分熔融有关。流纹岩中Nb和Ti的含量明显偏低,表明本区岩浆可能受到过陆壳物质的一定混染,或者表明幔源岩浆在壳内次生岩浆房中曾发生过辉石和钛氧化物的分离结晶。通过对岩石学、岩石地球化学特征研究表明,研究区可能是在裂谷发育初期,地壳拉薄引起软流圈地幔上隆降压形成玄武岩浆,在局部喷发地表形成玄武岩,但大部分在地表相对较厚的地壳内聚集成一定规模的岩浆房,通过强烈的结晶分异作用和同化混染作用,演化成中性岩浆,随着地壳进一步拉伸减薄喷出地表,形成安山岩。随后由于底侵作用的影响,还可能存在一定规模的热的地幔底劈体进入下地壳促使大规模的重熔作用,相应形成了流纹岩,在局部形成了玄武岩-流纹岩和安山岩-流纹岩分布比例的双峰式火山岩组合,其动力学体制可能与陆内伸展拉张作用有关。
图6 流纹岩稀土元素球粒陨石标准化配分图解(标准化值据SUN et al.,1989)Fig.6 Plots of chondrite-normalized REE patterns for the rhyolite(Normalizing values from SUN et al., 1989)
区域岩石-构造研究表明,红柳园裂谷岩浆作用始于泥盆纪,石炭纪进入大规模裂谷发育期,局部形成典型的双峰式火山岩建造。晚石炭世末—早二叠世,辉绿岩及大量中酸性侵入岩岩石学,地球化学表明,中酸性侵入体定位是紧随内陆造山带的碰撞,是代表动力学环境由拉张变为挤压的转折区(赵玮,2014;杨春霞,2015),二叠纪晚期红柳园大陆裂谷闭合进入碰撞造山演化阶段(龚全胜,2002;张新虎,2007),三叠纪形成大型逆冲推覆,在区域上形成了一系列相互平行的北西向逆冲断裂,之后石板泉一带进入后造山伸展环境(朱江,2015)。总体上,本区双峰式火山岩表现出的共同特点是富碱、富钠,Ti、P、Fe和Mg略偏低的特点,和北山其他地区石炭纪火山岩形成的大陆裂谷基本一致(许红英,2011;刘明强,1999;杨合群,2006)。进一步证实了北山南带在石炭纪时期处于大陆裂谷演化过程的观点,和博格达造山带内石炭纪大陆裂谷双峰式火山岩明显不同(王金荣,2010)。其为深入理解北山南带地区晚古生代构造格局及板块构造体系提供了地质依据。
图7 火山岩δEu-Sr图解Fig.7 δEu-Sr diagrams
5结论
(1)北山南带红柳园裂谷在石板泉一带开始于晚泥盆世,闭合于二叠纪晚期,三叠纪末期石板泉一带进入后造山伸展垮塌演化阶段。
(2)石板泉一带石炭纪双峰式火山岩形成于大陆裂谷环境,但和典型的双峰式火山岩明显不同,以安山岩和流纹岩为主,Daly成分间断比较小,而且以含大量流纹岩为主要特征。安山岩源区可能为亏损地幔,是幔源玄武岩岩浆分异作用的产物,流纹岩可能是地幔底劈体进入下地壳促使大规模的重熔,斜长石为源区主要的残留相,揭示该区双峰式火山岩可能为异源的。
(3)石板泉一带在石炭纪处于红柳园裂谷发展的鼎盛时期,其动力学体制可能与陆内伸展拉张作用有关,但双峰式火山岩中的安山岩/流纹岩的相对比例约为1∶6,明显不同于典型的异源双峰式火山岩的分布规律,其可能与该区独特的构造岩浆作用有关。
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收稿日期:2015-06-29;修回日期: 2016-03-03
基金项目:甘肃国土资源厅“石板泉一带1∶5万矿产远景调查”(2010254)
作者简介:王晓伟 (1976-),男,汉,甘肃省岷县人,高级工程师,硕士研究生,构造地质学专业,主要从事区域地质调查研究。E-mail: wangxw07@lzu.edu.cn
中图分类号:P534.45; P59
文献标志码:A
文章编号:1009-6248(2016)02-0025-09
Geochemical Characteristics of Carboniferous Bimodal Volcanic Rocks in Shibanquan, Beishan and Its Tectonic Significance
WANG Xiaowei1,2, YANG Chunxia3, WANG Qihang3,WANG Yuxi1,LI Pengbing3
(1.Geological Survey of Gansu Province, Lanzhou 730000, Gansu, China; 2.Gansu Provinvial Bureau of Geology and Mineral Exploration and Development, Lanzhou 730000, Gansu, China; 3.The 3th Gansu Provincial Bureau of Geology and Mineral Exploration and Development, Lanzhou 730050, Gansu, China)
Abstract:Located in the southern margin of Beishan orogenic belt, the carboniferous volcanic rocks(Hongliuyuan Group) have constituted a unique bimodal volcanic rock combination in time and space, which are mainly characterized byshorter Daly discontinuous and a large number of acidic volcanic rock, with a small amount of intermediate and basic volcanic rocks. In which, the andesites are rich in sodium and poor in potassium, with high Al values and low Mg contents, belonging to the peraluminous high potassium calc-alkaline series, containing substantial amount of depleted mantle materials, and indicating that these andesites may be derived from depleted mantle, with geochemical characteristics of intraplate andesite. The rhyolites belong to the low Ti type, indicating that they may be formed by partial melting of lower crust materials under theinfluence from the underplating of basic magma, with some plagioclaseresidued inmagmatic source.And, these rhyolites were formed in a continental rift environment, its dynamics system may be associated with intracontinental stretching tension effect, suggesting that the late Paleozoic rift were developed to the peak stage in the late period of Early Carboniferous and/or the early period of Late Carboniferous. Thus, the Late Carboniferous serves as the major turning point for the the geodynamic environment gradually changing from the extension and tension to the collision and compression, and this studying area was steped into the ollision orogenic evolution stage at the end of Permian.
Keywords:bimodal volcanic rocks; geochemistry; tectonic environment; Beishan