地球科学  2018, Vol. 43 Issue (4): 1253-1265.   PDF    
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北祁连青山花岗岩体矿物学特征及其对岩石成因的约束
王楠1, 吴才来2, 雷敏2, 陈红杰2, 李名则3     
1. 中国地质科学院矿产资源研究所, 国土资源部成矿作用与资源评价重点实验室, 北京 100037;
2. 中国地质科学院地质研究所, 北京 100037;
3. 四川省地质调查院, 四川成都 610081
摘要:北祁连造山带经历了洋盆的打开到闭合这一完整的Wilson旋回,并发育有大量与之相关的花岗质岩体.加深对花岗岩的研究可以为重建俯冲-增生/碰撞造山格架提供关键线索.对带内西段的青山二长花岗岩体进行了岩相学以及造岩矿物化学成分的电子探针原位分析,厘定了其矿物形成的物理化学条件,进一步约束了其岩石成因和构造背景.研究表明,青山二长花岗岩中钾长石全部为正长石,斜长石为中酸性的中长石和更长石,黑云母属镁质黑云母,角闪石则为镁角闪石亚类.锆石饱和温度平均为750 ℃,黑云母的结晶温度平均为647 ℃,氧逸度为-15,推测岩体固结压力约为1.85×108 Pa,形成深度约6.73 km.矿物化学特征显示,青山二长花岗岩为具Ⅰ型花岗岩特征的低熔线花岗岩,并具有壳幔岩浆混源的特点,可能为含水条件下形成的钙碱性花岗岩.
关键词地质温压计    岩石成因    矿物化学    二长花岗岩    北祁连造山带    岩石学    
Mineralogical Characteristics of Qingshan Granitic Pluton in North Qilian Orogenic Belt and Their Constraints on Petrogenesis
Wang Nan1 , Wu Cailai2 , Lei Min2 , Chen Hongjie2 , Li Mingze3     
1. Institute of Mineral Resources, Chinese Academy of Geological Sciences, Key Laboratory of Metallogeny and Mineral Resourees Assessment of Ministry of Land and Resources, Beijing 100037, China;
2. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
3. Sichuan Institute of Geological Survey, Chengdu 610081, China
Abstract: The North Qilian orogenic belt underwent a complete Wilson Cycle through opening and closing of an ocean basin. Deepening the research of granite within can provide pivotal clues to reconstruct the subduction-accretion/collision orogenic framework. On the basis of systematic petrological and petrographical research, this paper conducts electron microprobe analysis on the main rock-forming minerals, aiming to determine the physicochemical conditions during rock formation and provide further constraints on the petrogenesis and tectonic setting. Results show that Qingshan monzogranite is mainly composed of K-feldspar (orthoclase), plagioclase (andesine-oligoclase), biotite(magnesian biotite), amphibole (magnesiohornblende) and quartz. Zircon saturation thermometer shows that the average temperature of initial magma is 750℃. Meanwhile, mineral chemistry analysis reveals that the average crystallization temperature for biotite is 647℃ and the oxygen fugacity during the rock formation is -15, corresponding to solidification depth of 6.73 km and solidification pressure of 1.85×108 Pa. The data in this paper indicates that Qingshan monzogranite is Ⅰ-type subsolvus granite, with the features of crust-mantle mixed source. Thus, it is proposed that Qingshan monzogranite belongs to calc-alkaline granite and produced under water-bearing condition.
Key Words: geothermo barometer    petrogenesis    mineralogical chemistry    monzogranite    North Qilian orogenic belt    petrology    

北祁连造山带是一个早古生代缝合带,经历了新元古代至古生代期间因Rodinia超大陆裂解导致的洋盆打开、扩张,到洋壳初始俯冲,最后发生大陆碰撞和剥蚀这一完整的Wilson旋回(Song et al., 2009a, 2009b, 2013; Yu et al., 2013).北祁连造山带内出露大量早古生代花岗岩,是上述地质过程的完整记录者,其时代为520~380Ma,前人根据岩相学、锆石U-Pb年代学和地球化学将其分为多个阶段,分别对应北祁连洋的南向俯冲、北向俯冲-洋盆闭合-碰撞、造山后调整和陆内演化过程,但在碰撞阶段的起止时间、碰撞花岗岩的特征等问题上存在较大的争议(吴才来等, 2004, 2006, 2010; Song et al., 2013).

青山花岗岩体位于研究程度薄弱的北祁连造山带西段,仅有的少量研究表明其侵位于~440Ma,但未对岩体形成温度进行讨论(王楠等,2017a).加深对青山岩体的研究可以为重建俯冲-增生/碰撞造山格架提供关键线索,但原本薄弱的研究,特别是缺少矿物尺度的研究在一定程度上制约了大家对其岩石成因的认识.对花岗质岩石内部结构构造、矿物组合以及矿物成分的研究可以约束矿物形成的物理化学条件和岩浆演化特征,借此判别花岗岩的成因类型、源区性质和构造环境等(Abdel-Rahman,1994郭耀宇等,2015赛盛勋等,2016王楠等,2017b周云等,2017).因此,本文将对青山岩体的主要造岩矿物开展岩相学、矿物化学方面的分析,讨论其对岩石成因,特别是岩浆形成的温度、压力等物理化学条件方面的约束,为解析北祁连造山带早古生代造山过程中的构造-岩浆活动提供矿物尺度的制约.

1 地质背景与岩石学特征

祁连造山带位于青藏高原北缘(图 1),与之关系密切的主要构造单元自东北至西南可依次分为阿拉善地块、北祁连造山带、祁连地块、柴北缘超高压变质带和柴达木地块.其中,北祁连造山带是一条早古生代缝合带,其整体走向呈NW向,长约1000km,夹持于东北侧的阿拉善地块和西南侧的祁连地块之间,西部为左旋走滑的阿尔金断裂所截,东南部与秦岭造山带相连(Song et al., 2013; Yu et al., 2012, 2013, 2014崔加伟等,2016王楠等, 2016a, 2016b).

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图 1 北祁连造山带构造位置(a)、早古生代花岗岩分布(b)及青山二长花岗岩地质简图(c) Fig. 1 Tectonic position of North Qilian orogenic belt (a), distribution of Early Paleozoic granites (b) and geological sketch of Qingshan monzogranite (c) 图a据许志琴等(1999);图b据Song et al.(2013)

北祁连造山带内发育有新元古代-早古生代蛇绿混杂岩、高压变质岩、岛弧火山岩、花岗质侵入岩、志留纪复理石建造、早泥盆纪磨拉石建造和石炭纪-三叠纪沉积盖层(Song et al., 2013).其中,南蛇绿岩带呈NW-SE向展布,主要岩石类型包括橄榄岩、基性-超基性堆晶岩、枕状玄武质熔岩等,其中玄武岩表现出类N-MORB的特征(史仁灯等,2004侯青叶等,2005; Tseng et al., 2007; Song et al., 2009b).北蛇绿岩带产超基性岩石、堆晶岩、辉绿岩墙枕状熔岩等,上覆玄武质熔岩和沉积岩组成的互层岩系,部分地球化学特征类似于具N-MORB、E-MORB和IAB特征的玄武岩(夏小洪和宋述光,2010; Xia et al., 2012).两条蛇绿岩带中夹有岛弧岩浆带,岛弧岩浆带出露有弧火山杂岩、HP/LT榴辉岩和蓝片岩为主的次级变质带以及钙碱性中酸性岩(苏建平等,2004; Tseng et al., 2009夏小洪和宋述光,2010; Song et al., 2007, 2009a, 2013).出露的大量花岗质岩体岩石组合类型多样,闪长岩、二长岩、英云闪长岩、花岗闪长岩、花岗岩和钾长花岗岩等均有发育.大量年代学证据表明区内花岗岩时代为512~383Ma(吴才来等, 2006, 2010陈育晓,2014; Chen et al., 2014; Yu et al., 2015).

青山岩体侵入于早古生代火山-沉积地层中,主体呈东西向展布.岩体北侧为奥陶系地层,西侧和东侧为寒武纪火山岩,南侧主要为新生代沉积物.围岩在与岩体的接触带附近受接触热变质作用,自围岩向岩体依次为变质凝灰岩、变质火山熔岩和变质中粒石英砂岩.岩石有轻微风化,风化面为灰黑色,新鲜面为肉红色至灰白色,块状、中粒,镜下可见典型花岗结构,主要造岩矿物为钾长石(30%~35%)+斜长石(30%)+石英(25%)+黑云母(0~5%)+角闪石(0~5%),另有榍石、磁铁矿和锆石等副矿物(图 2).其中,钾长石自形-半自形,可见简单双晶和格子双晶,轻微高岭土化,呈星点状分布;斜长石自形板状,同样可见简单双晶、聚片双晶,有绢云母化,但程度较低;石英他形充填,暗示形成较晚,可见受构造作用影响的波状消光,此外常呈滴状交代钾长石;黑云母呈片状,可见一组平行解理,多色性弱,干涉色较高;角闪石粒度稍小,可见简单双晶,横切面对称消光,可见56°解理,纵切面斜消光,有聚集现象,内部局部被熔蚀或有磁铁矿等矿物的析出现象.

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图 2 青山二长花岗岩野外(a~b)及显微特征(c~d,正交偏光;e~f,单偏光) Fig. 2 Outcrops (a-b), crossed polarising filters (c-d) photomicrographs and plane polarized light (e-f) of Qingshan monzogranite Kfs.钾长石;Pl.斜长石;Qz.石英;Bt.黑云母;Amp.角闪石;缩写据Whitney and Evans(2010)
2 分析方法

电子探针X射线显微分析仪(EPMA),是一种现代成分分析仪器,可以获得矿物微米量级微区内的化学成分,因此为研究样品的成分分析提供了有效的分析方法.本文在详细的岩相学观察基础上,圈定新鲜的钾长石、斜长石、黑云母和角闪石用于电子探针测试.探针片喷碳和电子探针矿物化学成分分析在中国地质科学院矿产资源研究所国土资源部成矿作用与资源评价重点实验室完成.仪器型号为JEOLXA-8100,加速电压15kV,电流为20nA,束斑直径5μm,标样为天然或合成的矿物和氧化物,主要氧化物的分析误差约为1%.

3 矿物地球化学特征 3.1 钾长石

青山二长花岗岩中钾长石的电子探针测试结果见表 1.结果显示,青山二长花岗岩钾长石成分有一定的变化,K2O含量为13%~16.51%,Na2O含量为0.26%~2.34%,CaO含量较低,小于0.1%;同时FeOT、MgO、MnO、TiO2和P2O5等含量均较低,表明相关元素的类质同象现象较少,钾长石形成温度偏低.经过计算,除测点104-Q2-3的Or值偏低为78.42外,其余钾长石的Or值为87.36~97.67,各钾长石的Ab值为2.72~21.46,An值为0~0.45,全部为正长石(图 3),与北祁连柴达诺黑云母二长花岗岩,小柳沟二长花岗岩和花岗闪长岩内钾长石矿物特征相似(Chen et al., 2014赵辛敏等,2014).成分具有一定程度变化表明钾长石形成过程中岩浆成分和物理化学条件有所波动.

表 1 青山二长花岗岩钾长石化学成分(%) Table 1 Chemical compositions of K-feldspars from Qingshan monzogranite
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图 3 青山二长花岗岩长石端元组分图解 Fig. 3 End-member discrimination diagram for feldspars from Qingshan monzogranite Smith(1974)
3.2 斜长石

斜长石的测试结果见表 2,其中K2O含量较低,为0.22%~0.44%,Na2O含量为6.29%~8.36%,测点104-Q2-4的CaO含量较低为1.88%,其余斜长石的CaO为5.14%~8.85%;与钾长石相似,FeOT、MgO、MnO、TiO2和P2O5等含量偏低,表明相关元素的类质同象现象较少.经计算,斜长石的Or为1.29~2.48,Ab为55.53~86.65,An为10.97~43.18.在长石分类图中(图 3),各斜长石落入了中酸性的中长石和更长石区域内.An牌号明显大于柴达诺黑云母二长花岗岩(An=0.65~10.64)、小柳沟二长花岗岩(An=0.13~9.82)和花岗闪长岩(An=20.26~21.17)、神木头石英二长岩(An=13~18)和屈木山花岗闪长岩(An=12~24),但与雷公山英云闪长岩(An=33~42)相似(Tseng et al., 2009; Chen et al., 2014赵辛敏等,2014; Chen et al., 2016).斜长石的SiO2-An关系图显示二者具有负相关关系,且相关程度较高(R2=0.9719),符合岩浆结晶过程(图 4).

表 2 青山二长花岗岩斜长石化学成分(%) Table 2 Chemical compositions of plagioclases from Qingshan monzogranite
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图 4 青山二长花岗岩斜长石SiO2-An相关图 Fig. 4 Diagram of chemical variation of SiO2 versus An in plagioclases from Qingshan monzogranite
3.3 黑云母

黑云母是花岗质岩石中最丰富的镁铁质矿物之一,用阴离子法以氧原子数为22计算出的黑云母阳离子数见表 3.青山二长花岗岩中黑云母成分均匀,其中SiO2含量变化于35.62%~37.68%之间,TiO2含量相对较高,变化于4.03%~4.76%之间,FeOT含量高于MgO,二者含量分别介于17.21%~19.22%和12.36%~13.22%.此外,多数CaO低于检测限,表明黑云母应为原生岩浆成因,不受或较低程度地受到大气流体循环或岩浆期后初生变质引起的绿泥石化、绢云母化或碳酸盐化的影响(Kumar and Pathak, 2010).根据黑云母Mg-(AlVI+Fe3++Ti)-(Fe2++Mn)分类图(图 5a),青山二长花岗岩内黑云母为镁质黑云母,同时靠近铁质区域,可能为二者之间的过渡类型.化学成分明显不同于小柳沟二长花岗岩,但与小柳沟花岗闪长岩内黑云母表现出了一定程度的相似性(赵辛敏等,2014).另外,在Fe3+-Fe2+-Mg图解中,黑云母落在了Ni-NiO和Fe2O3-Fe3O4缓冲剂线之间(图 5b).

表 3 青山二长花岗岩黑云母化学成分(%) Table 3 Chemical compositions of biotites from Qingshan monzogranite
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图 5 青山二长花岗岩黑云母分类图(a)和黑云母组成与氧缓冲对相关图(b) Fig. 5 Classification diagram for biobite (a) and correlative diagram between biotite composition and oxygen buffer-reagents (b) from Qingshan monzogranite 图a据Foster(1960);图b据Wones and Eugster(1965)
3.4 角闪石

角闪石的分析结果显示其成分均匀(表 4),FeOT、MgO和CaO含量分别为13.20%~14.34%、13.67%~15.20%和12.11%~12.69%.电子探针数据计算以23个氧原子和16个阳离子为基准.角闪石CaB=1.89~1.97,(Na+K)A=0.223~0.435,Si=7.19~7.47,Mg/(Mg+Fe2+)=0.64~0.7,在Si-Mg/(Mg+Fe2+)图中落入普通角闪石中的镁角闪石亚类区域内(图 6),明显不同于屈木山花岗闪长岩的浅闪石(Chen et al., 2016).

表 4 青山二长花岗岩角闪石化学成分(%) Table 4 Chemical compositions of amphibole from Qingshan monzogranite
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图 6 青山二长花岗岩角闪石分类 Fig. 6 Classification for amphiboles from Qingshan monzogranite Leake et al.(1997)
4 讨论 4.1 岩浆形成的物理化学条件

角闪石全铝压力计和角闪石-斜长石温压计是估算花岗岩形成压力的常用工具,但使用范围较窄.其中,前者要求角闪石Fe#介于0.4~0.65,同时Fe3+/(Fe3++Fe2+)≥0.25(Johnson and Rutherford, 1989; Schmidt, 1992; Anderson and Smith, 1995),后者计算温度的前提是先通过角闪石压力计获得压力值(Holland and Blundy, 1994).青山二长花岗岩并不适用于角闪石温度压力计算.Uchida et al.(2007)提出,黑云母中的全铝含量与花岗岩的固结压力存在良好的正相关性,但仅适用于压力小于2.0×108Pa的条件;此外,该压力计是通过其他方法获得的压力来标定黑云母全铝与压力之间的关系,且未经实验岩石学标定,因此可靠性存在较大疑问(赛盛勋等,2016).而角闪石-黑云母矿物对所得温度往往低于花岗岩固相线温度,可能是由于岩浆冷凝过程中角闪石与黑云母发生镁铁交换所致(Henry et al., 2005).总之,种种因素导致上述温压计无法适用于青山二长花岗岩.

锆石具有较高的封闭温度,也是花岗质岩浆体系中较早结晶的副矿物,锆石饱和温度可近似代表花岗质岩石近液相线的温度,可用于估算初始岩浆温度来限定岩体形成温度的上限(刘春花等,2013).Watson and Harrison(1983)通过高温研究了锆石的饱和行为,在此基础上提出了锆石溶解度模型:

$ \begin{array}{*{20}{l}} {{\rm{ln}}D_{{\rm{Zr}}}^{{\rm{zircon/melt}}} = \left\{ { - 3.8 - \left[ {0.85\left( {M - 1} \right)} \right]} \right\} + }\\ {12900/T,} \end{array} $ (1)

式中:DZrzircon/melt是化学计量锆石中Zr与熔体中的Zr的浓度比,T为温度(K).公式(2)定义了在地壳深熔过程中,锆石的饱和行为是岩浆化学成分和温度的函数,实验的温度范围是750~1020℃.令:

$ {\rm{Si + Al + Fe + Mg + Ca + Na + K + P = 1}}, $ (2)

则全岩岩石化学参数M为(Na+K+2×Ca)/(Al×Si)的阳离子含量比值.若假设不作锆石矿物的Zr和Hf校正,纯锆石中含Zr=497657×10-6,可用岩石中的Zr含量近似代表熔体Zr含量.因此Miller et al.(2003)将计算出锆石饱和温度的公式修正为:

$ \begin{array}{l} {T_{{\rm{Zr}}}} = 12900/[2.95 + 0.85M + {\rm{ln}}(496000/\\ {\rm{Z}}{{\rm{r}}_{{\rm{melt}}}})]. \end{array} $ (3)

全岩岩石化学成分见王楠(2016),通过计算,青山二长花岗岩的锆石饱和温度为732~764℃,平均温度为750℃.

Henry et al.(2005)认为岩体中黑云母Ti-Mg/(Mg+Fe)温度计可以合理地反映黑云母结晶的物理化学条件(图 7),其公式为:

$ \begin{array}{l} T = \left\{ {[{\rm{ln}}\left( {{\rm{Ti}}} \right) + 2.3594 + 1.7283 \times {\rm{Mg}}/\left( {{\rm{Mg + }}} \right.} \right.\\ {\left. {{{\left. {{\rm{Fe}}} \right)}^3}] \times {{10}^9}/4.6482} \right\}^{0.333}}, \end{array} $ (4)
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图 7 青山二长花岗岩黑云母结晶温度Ti-Mg/(Mg+Fe)图 Fig. 7 Temperature isotherms calculated from the surface-fit equation on a Ti versus Mg/(Mg+Fe) diagram for biotites from Qingshan monzogranite Henry et al.(2005)

式中:限定条件为Mg/(Mg+Fe)=0.275~1.000,Ti=0.04~0.60,T=400~800℃为准确的校正范围.经计算,青山二长花岗岩黑云母的结晶温度介于628~656℃,平均为647℃(表 3).

Q-Ab-Or和Q-Ab-An图解计算的岩石压力往往只是粗略估计,不够精细,为获得更为精确的岩体形成压力,本文将根据温度-水逸度-压力之间的关系来进行计算.在借助黑云母lgfH2O-103/K稳定图解时(Wones, 1981),用锆元素饱和温度计获得的平均温度750℃来近似代表岩石结晶温度(图 8a).在Mg-(AlVI+Fe3++Ti)-(Fe2++Mn)分类图中黑云母全部落入镁质黑云母区域内,根据相关图解,青山二长花岗岩形成的水逸度约为1.15×108Pa,将已知的水逸度和温度投影在花岗岩t-P-f图解中(饶纪龙,1979),求得压力约为1.85×108Pa(图 8b).地壳平均密度约为2750kg/m3(桑隆康和马昌前,2012),据此估算形成深度约6.73km.

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图 8 青山二长花岗岩黑云母lgfH2O-103/K稳定图解(a)和花岗岩温度-压力-水逸度图解(b) Fig. 8 lgfH2O-103/K stability diagram for biotite (a) and temperature-pressure-water fugacity diagram (b) from Qingshan monzogranite 图a据Wones(1981);图b据饶纪龙(1979)

黑云母的Fe3+/Fe2+比值较低,为0.11~0.22,并且落在了Ni-NiO和Fe2O3-Fe3O4缓冲剂线之间,显示了中氧逸度的特征.岩体中存在磁铁矿、石英和榍石,因此可根据Wones(1989)提出的氧逸度公式:

$ \begin{array}{l} {\rm{log}}f{{\rm{O}}_2} = - 30930/(T + 273) + 14.98 + \\ 0.142 \times (P - 1)/(T + 273). \end{array} $ (5)

根据前文计算的压力(1.85×108Pa)和平均温度(750℃)所得的氧逸度为-15.

4.2 岩石成因

黑云母的晶体结构可以容纳长英质熔体中多种元素,承载了寄主岩浆的性质、岩石成因和构造环境等重要信息(郭耀宇等,2015).研究表明,Ⅰ型、S型和A型花岗岩中的黑云母分别相对富集镁、铝和铁(Abdel-Rahman, 1994).青山二长花岗岩相对富集镁、铁元素,同时在分类图解中投入到了镁质黑云母区域内,表现出Ⅰ型花岗岩的特点.Whalen and Chappell(1988)提出可以根据黑云母中Al大小划分Ⅰ型和S型花岗岩,认为Ⅰ型花岗岩中黑云母中Al较低,小于0.224,而S型花岗岩中Al较高(0.353~0.561).青山二长花岗岩黑云母中Al多数低于检测限,仅有的数据亦低于0.224,显示出Ⅰ型花岗岩特征.此外,Ⅰ型花岗岩中的黑云母往往具有更高的MF值(>0.5),本文中黑云母MF为0.54~0.56,同样指示为Ⅰ型花岗岩的特征(谢应雯和张玉良,1995).需要指出的是,角闪石和堇青石是Ⅰ型和S型花岗岩的标志性矿物(Chappell and White, 1974; Miller, 1985),显微镜下可观察到角闪石,是证明青山二长花岗岩为Ⅰ型花岗岩的重要证据.

根据丁孝石(1988)对黑云母中MgO含量对黑云母的分类,典型的壳源黑云母中MgO<6%,典型幔源黑云母的MgO>15%.据此判断,青山二长花岗岩(黑云母含量为12.4%~13.2%)具有壳幔混源的特点.同时,黑云母的MgO-FeOT/(FeOT+MgO)判别图解(图 9a)和角闪石的Al2O3-TiO2图解(图 9b)判别图解也支持上述结论,野外偶见的微粒闪长质包体也证明了这一点.

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图 9 青山二长花岗岩黑云母MgO-FeOT/(FeOT+MgO)图解(a)和角闪石Al2O3-TiO2图解(b) Fig. 9 MgO versus FeOT/(FeOT+MgO) diagram for biotite (a) and Al2O3 versus TiO2 diagram for amphibole (b) from Qingshan monzogranite 图a据周作侠(1986);图b据姜常义和安三元(1984)
4.3 构造环境

桑隆康和马昌前(2012)在解释超熔线花岗岩与低熔线花岗岩时提出,超熔线花岗岩形成于伸展构造环境,在低水压条件下当温度低于熔线时,富钠和富钾两个长石才会同时出熔并形成条纹长石,一般仅由碱性长石和石英两种矿物组成,缺乏单粒钠质斜长石;而低熔线花岗岩一般是在含水条件下陆壳较深层位结晶形成的,钠长石、钾长石则会同时结晶出来,并以钾长石和石英颗粒的不连续出现为特征,并且造山花岗岩多数属于低熔线型.在野外调查和实验室显微镜观察过程中,均发现青山二长花岗岩出现钠质斜长石颗粒,并且可见钾长石和石英的不连续出现.因此,应属于低熔线花岗岩,可能是在含水条件下,与造山事件有关的环境中形成.

Abdel-Rahman(1994)总结了不同构造环境下岩浆岩黑云母的主量元素数据,并提出了构造环境判别图解(图 10).由图可以看出,青山二长花岗岩黑云母在多个判别图中均落入了造山带钙碱性花岗岩区域内.吴才来等(2004, 2006, 2010)结合岩相学、锆石U-Pb年代学和地球化学将这些北祁连造山带花岗质岩石分为了4期共5个阶段,分别对应北祁连洋的南向俯冲、北向俯冲-洋盆闭合-碰撞、造山后调整和陆内演化过程.其中,中-晚奥陶世(465~440Ma)为北祁连洋向北俯冲及随后的同碰撞阶段,志留纪(435~421Ma)为晚造山阶段.而Song et al.(2013)Chen et al.(2014)则认为同碰撞花岗岩时代为440~420Ma.青山二长花岗岩的时代为440~438Ma(王楠等,2017a),结合区域地质背景,很可能为北祁连洋闭合后两侧块体发生同碰撞-后碰撞造山的产物,与上述造山带钙碱性花岗岩特征一致.

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图 10 青山二长花岗岩黑云母构造判别图解 Fig. 10 Tectonic discrimination diagrams for biotites from Qingshan monzogranite Abdel-Rahman(1994)
5 结论

(1) 青山二长花岗岩中钾长石全部为正长石,斜长石为中酸性的中长石和更长石,黑云母属镁质黑云母,角闪石则为镁角闪石亚类.

(2) 青山二长花岗岩的锆石饱和温度平均为750℃,黑云母的结晶温度介于628~656℃,平均为647℃,氧逸度为-15,岩体固结压力约为1.85×108Pa,形成深度约6.73km.

(3) 黑云母和角闪石的矿物化学表明青山二长花岗岩为具Ⅰ型花岗岩特征的低熔线花岗岩,并具有壳幔岩浆混源的特点,可能为含水条件下,于碰撞造山事件中形成的钙碱性花岗岩.

致谢 两位匿名审稿专家和编委会对本文提出了宝贵意见和建议,在此表示衷心感谢!

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