岩石学报  2012, Vol. 28 Issue (12): 3938-3950   PDF    
福建太武山花岗岩体成因:锆石U-Pb年代学与Hf同位素制约
赵姣龙, 邱检生, 李真, 刘亮, 李友连     
南京大学内生金属矿床成矿机制研究国家重点实验室,地球科学与工程学院,南京 210093
摘要: 太武山岩体位于福建东南沿海,为一大致呈北东向延伸的不规则状岩株体,出露面积约40km2。岩体主体岩性为中细粒花岗岩,环岩体北部边缘尚发育有似斑状黑云母花岗岩。锆石LA-ICP-MS U-Pb定年表明,岩体的形成年龄为96.9±1.3Ma (MSWD=1.09, 2σ),属晚白垩世早期岩浆活动产物。化学组成上,该岩体富硅,碱含量中等,弱过铝,铝饱和指数(A/NKC值) 为1.01~1.04,碱铝指数(AKI值) 为0.73~0.92,贫钙、镁、铁,属亚碱弱过铝质花岗岩类。微量和稀土元素组成上,岩体富Cs、Rb、U、Th、Pb和轻稀土,贫Ba、Sr、P、Ti,Rb/Sr比值高,具中到强的铕负异常(Eu/Eu*=0.85~0.04),其Zr、Nb、Ce、Y等高场强元素均较之典型A型花岗岩偏低,锆石饱和温度也较低(726~809℃),综合地质地球化学资料指示该岩体应属高分异的I型花岗岩。太武山花岗岩的锆石εHf(t) 值散布于正值与负值之间(-1.44~2.78),tDM2值偏低(0.98~1.25Ga,平均值为1.06Ga),指示成岩过程中应有显著的亏损地幔组分参与。综合分析表明,岩体的形成首先经历了幔源岩浆与其诱发地壳物质熔融产生的长英质岩浆在地壳深部混合,随后这一混合岩浆又经进一步分异演化的二阶段成岩过程。
关键词: 高分异I型花岗岩     锆石U-Pb定年     Hf同位素组成     岩石成因     福建太武山岩体    
Petrogenesis of the Taiwushan granite pluton in Fujian Province: Constraints from zircon U-Pb ages and Hf isotopes
ZHAO JiaoLong, QIU JianSheng, LI Zhen, LIU Liang, LI YouLian     
State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China
Abstract: The Taiwushan granite pluton, with an outcropped area of about 40km2, is roughly elongated NE-trending as an irregular stock in the coastal region of SE Fujian Province. Lithologically, this pluton consists mainly of medium to fine-grained granites, with minor porphyraceous biotite granites surrounding the north marginal parts. Zircon LA-ICP-MS U-Pb dating yields an age of 96.9±1.3Ma (MSWD=1.09, 2σ), indicating that this pluton was generated in the initial stage of Late Cretaceous. Chemically, the Taiwushan granites are enriched in silicon, and depleted in calcium, magnesium and iron. They also have moderately alkaline contents and show weakly peraluminous signature with A/NKC values of 1.01~1.04 and AKI values of 0.73~0.92, thus can be grouped into subalkaline and weakly peraluminous granitoids. On trace and REE aspects, the granites are enriched in Cs, Rb, U, Th, Pb and LREE, depleted in Ba, Sr, P, Ti, and show high Rb/Sr ratios and moderately to strongly negative europium anomalies (Eu/Eu*=0.85~0.04). They also have lower Zr, Nb, Ce, Y concentrations and lower zircon saturation temperatures (726~809℃) relative to that of the typical A-type granites. Integrated geological and geochemical data suggest that the Taiwushan pluton should be genetically ascribed to highly fractionated I-type granites. Zircon Hf isotopic compositions of the granites are variable, with εHf(t) values ranging from negative to positive (-1.44~2.78). Correspondingly, they show younger two-stage Hf model ages ranging from 0.98Ga to 1.25Ga with a mean value of 1.06Ga, indicating that large amounts of depleted mantle materials had been involved in magma genesis. Based on a synthesis of geology, geochronology, elemental and isotopic geochemistry, we suggest that the Taiwushan granites were most likely generated via a two-stage process including formation of parental magma by mixing of a depleted mantle-derived magma and an induced crustal-melted felsic magma in the deep crust, and then suffered further differentiation during magma ascent.
Key words: Highly fractionated I-type granites     Zircon U-Pb dating     Hf isotopic compositions     Petrogenesis     The Taiwushan pluton, Fujian Province    

花岗岩作为大陆地壳的重要组成部分,蕴含有壳幔演化及其相互作用过程的丰富信息,加之其与构造演化及成矿作用密切相关,因而历来是备受关注的重要研究对象(Chappell and White, 1974, 2001; Pitcher, 1982; 王德滋和周新民,2002; 吴福元等, 2007; 周新民, 2007)。我国闽浙沿海地区广泛发育晚中生代花岗岩,前人对该区花岗岩进行过不同程度的研究,这些研究为揭示岩浆的成因演化及探讨大地构造背景与动力学过程提供了重要信息(Martin et al., 1994Chen et al., 2004; 洪大卫等,1987周珣若和吴克隆,1994王德滋和周新民,2002),目前已普遍认识到幔源岩浆底侵及壳幔相互作用是制约区内花岗岩成因的主要机制(Zhou and Li, 2000Qiu et al., 2004; Li et al., 2012a),但对幔源组分参与花岗岩成岩过程的方式尚存在分歧,此外,由于区内花岗岩普遍经历了高程度的分异演化,对岩体成因类型的归属也存在不同认识,如区内花岗岩普遍发育晶洞构造,以往多认为这类晶洞碱长花岗岩都可归为A型花岗岩(洪大卫等,1987蒋叙良,1991),特别是随着铝质A型花岗岩(aluminous A-type granite) 概念的提出,人们普遍认为不含碱性铁镁矿物的

晶洞碱长花岗岩均可归属到铝质A型花岗岩范围,但实际上晶洞构造的出现只是反映岩体定位深度较浅,并经历较高程度的分异演化,并不能作为A型花岗岩所特有的鉴别标志,即晶洞碱长花岗岩的成因类型既可以是A型,也可以是其它类型(邱检生等,2008)。由于花岗岩成因类型的判定直接涉及到对壳幔深部作用过程与区域动力地质背景及其演化的正确理解,因此,这一方面的研究亟待加强。本文以福建东南部的太武山岩体为对象,在系统的全岩地球化学研究基础上,阐明了岩体的成因类型归属,并结合锆石Hf同位素组成,探讨了幔源组分参与成岩的方式及可能的成岩过程。

1 地质概况及岩相学特征

福建省以政和-大埔断裂和长乐-南澳断裂为界,大致可划分为3个主要构造带,自西向东依次为早古生代褶皱带、燕山期岩浆岩带和平潭-东山变质带(图 1a)。早古生代褶皱带指政和-大埔断裂以西的闽西地区,该区域内出露有华夏地块的主要前寒武纪变质基底岩石,如麻源群和马面山群等,此外,还广泛发育加里东期花岗岩(张爱梅等,2010Li et al., 2012b)。燕山期岩浆岩带出露于政和-大埔断裂和长乐-南澳断裂之间的广大区域,长约500km,宽约100km,主要由晚侏罗世-白垩纪的花岗岩和火山岩组成。平潭-东山变质带分布于长乐-南澳断裂的东南侧,主要由晚中生代的区域变质岩与动力变质岩、花岗岩和火山岩以及少量中新生代的基性-超基性岩石组成,花岗质岩石在变质带中多呈NE向平行于长乐-南澳断裂带分布,它们多呈岩株或岩基状侵入于周围的变质岩系中。最新的定年资料显示,该变质带存在~187Ma、150~140Ma、133~130Ma和108~100Ma四期岩浆作用和变质作用(Liu et al., 2012)。

图 1 福建省基本构造格局(a,据马丽芳,2002修改) 及太武山花岗岩体地质略图(b,据福建省地质局,1977修改) 1-黑云母花岗岩;2-中细粒花岗岩;3-佛昙组玄武岩;4-第四系;5-动力变质花岗岩;6-断裂(①长乐-南澳断裂;②政和-大浦断裂);7-采样点 Fig. 1 Diagram showing the tectonic framework of Fujian province (a, modified after Ma, 2002) and sketch geological map of the Taiwushan granitic pluton 1-biotite granite; 2-intermediate-fine grained granite; 3-basalt of the Fotan Formation; 4-Quaternary; 5-dynamic metamorphic granite; 6-faults (①Changle-Nan'ao fault; ②Zhenghe-Dapu fault); 7-sample localities

① 福建省地质局. 1977.厦门幅1:20万地质图(内部资料)

太武山岩体位于福建东南沿海龙海太武山一带,构造上处于平潭-东山变质带内,紧邻长乐-南澳深大断裂带产出(图 1a),为一大致呈NE向延伸的不规则状岩株体,出露面积约40km2。岩体北部侵入于片理化或片麻理化的动力变质花岗岩中,南部为第三系佛昙组玄武岩覆盖(图 1b)。岩体主体岩性为浅肉红色中细粒花岗岩,环岩体北部边缘尚发育有灰白色-浅肉红色似斑状黑云母花岗岩(图 1b),其中可见少量变质岩捕虏体。主体相花岗岩呈中细粒花岗结构,块状构造,组成矿物主要为石英(25%~30%)、钾长石(50%~55%,主要为微纹长石)、斜长石(20%~25%,An=24~29) 和黑云母(2%~3%)(图 2a, b),副矿物有锆石、磷灰石、榍石、磁铁矿等。边缘相具似斑状结构,主要矿物组成为石英(20%~25%)、钾长石(45%~50%)、斜长石(25%~30%,An=25~35) 和黑云母(5%~8%),副矿物组成与主体花岗岩类似,其中钾长石常有钠长石净边(图 2c),斜长石可见环带结构(图 2d)。岩体各相带岩性较均一,未见有暗色镁铁质微粒包体发育。

图 2 太武山花岗岩体典型样品的岩相学显微照片 (a、b) 为主体相;(c、d) 为边缘相.Q-石英;Per-微纹长石;Pl-斜长石;Bt-黑云母.均正交偏光下 Fig. 2 Microphotographs of representative rock samples from the Taiwushan granite pluton (a, b)-samples from the main phase; (c, d)-samples from the marginal phase. Q-quartz; Per-microperthite; Pl-plagioclase; Bt-biotite. All pictures were taken under crossed nicol
2 样品与分析方法

选取太武山岩体主体岩性中细粒花岗岩(ZP-3) 的典型样品进行锆石U-Pb同位素定年和Hf同位素组成分析,具体采样位置示于图 1,采样经纬度见表 1。在严格避免污染的条件下,对全岩样品进行破碎、淘洗和磁选分离出锆石精样,然后在双目镜下挑选出表面平整光洁,具不同长宽比、不同柱面特征和颜色的锆石颗粒。将所挑选锆石排列于双面胶上,置于模具内,注入环氧树脂胶结,待其固结后抛光至锆石颗粒核部出露,制成样品靶以待测试。在分析之前先对锆石样品进行阴极发光(CL) 内部结构照相,以作为分析时选点的依据。

表 1 太武山花岗岩体LA-ICP-MS锆石U-Pb定年结果 Table 1 LA-ICP-MS zircon U-Pb dating results of the Taiwushan granite pluton

锆石的CL照相、U-Pb年龄和Lu-Hf同位素组成测定均在西北大学大陆动力学国家重点实验室完成。CL照相采用安装有Mono CL3+型(Gatan, USA) 阴极荧光探头的扫描电镜(Quanta 400 FEG, USA) 完成,锆石U-Pb测年和Lu-Hf同位素组成分析采用的激光剥蚀系统为德国Microlas公司生产的GeoLas200M,其激光发生器为Lambda Physik公司生产的ComPex102准分子激光器(193nm ArF Excimer),采用He作为剥蚀物质的载气。U-Pb年龄测定采用的等离子体质谱为Agilent公司生产的7500a ICP-MS,激光剥蚀束斑直径为30μm,采用国际标准锆石91500做外标进行同位素质量分馏校正。样品的同位素比值及元素含量计算采用Glitter (ver. 4.0) 软件,并使用嵌入Excel的ComPbCorr#3_15G程序(Andersen,2002) 来进行普通铅校正,年龄谐和图用Isoplot程序(ver. 2.49,Ludwig,2001) 绘制。Lu-Hf同位素分析所用仪器为英国Nu Instruments公司生产的Nu Plasma HR多接收电感耦合等离子体质谱仪(MC-ICP-MS),激光束斑直径为44μm,剥蚀频率为10Hz,脉冲能量为80mJ,剥蚀时间为50s,实验过程中获得锆石91500、GJ-1和MON-1三个外标的176Hf/177Hf值分别为0.282309±0.000012(n=24, 2σ)、0.282022±0.000016(n=21, 2σ) 和0.282738±0.000008(n=34, 2σ),详细分析方法见Yuan et al.(2008)

全岩地球化学分析样品先经岩相学观察,选出新鲜岩石样品,然后细碎至200目以上。ZP-3样品的主量元素在南京大学现代分析中心利用ARL9800XP+型X射线荧光光谱仪测定,使用Li2B4O7和LiBO2(67:33) 混合熔剂及加拿大Glaisse高温自动燃气熔样机制样,测试条件为:X射线工作电压40kV,电流60mA,分析精度优于5%;其余样品的主量元素由南京大学内生金属矿床成矿机制研究国家重点实验室采用Thermo9900xp型X射线荧光光谱仪测定,测试电压为50kV,电流为50mA,每个元素扫描时间20s。相对于标准样品的测定值,相对误差在元素丰度>1.0%时为±1%,元素丰度 < 1%时为±10%。所有样品的微量和稀土元素均由南京大学内生金属矿床成矿机制研究国家重点实验室采用Finnigan Element Ⅱ型电感耦合等离子体质谱(ICP-MS) 测定,相对标准偏差优于5%,详细测试方法和流程见高剑峰等(2003)

3 分析结果 3.1 锆石U-Pb年龄

表 1中列出了太武山岩体锆石U-Pb年龄测定结果,代表性被测锆石颗粒的阴极发光(CL) 图像及测定点位和相应的206Pb/238U视年龄示于图 3图 4为年龄谐和图。

图 3 太武山花岗岩体代表性被测锆石的阴极发光图像、LA-ICP-MS分析点位及206Pb/238U视年龄 Fig. 3 CL images, localities of the points for LA-ICP-MS measurements and the 206Pb/238U apparent ages of representative detected zircons from the Taiwushan granite pluton

图 4 太武山花岗岩体锆石U-Pb谐和图 Fig. 4 U-Pb concordia diagram for zircons of the Taiwushan granite pluton

太武山岩体被测锆石呈淡棕-棕黄色,透明-半透明,柱状或长柱状,长径大者可达200~300μm,长宽比2:1~3:1。CL图像显示清晰的岩浆振荡环带(图 3),Th/U比值高(=0.59~1.52,表 1),表明被测锆石为典型的岩浆结晶锆石(Corfu et al., 2003Wu and Zheng, 2004)。本次共获得12个有效锆石点测试数据,在206Pb/238U-207Pb/235U谐和图上,所测点均投影在谐和线上或位于谐和线附近(图 4),指示被测锆石未遭受明显的后期热事件影响。这12个锆石点的206Pb/238U年龄分布于93~100Ma之间,经计算获得的206Pb/238U年龄统计权重平均值为96.9±1.3Ma (MSWD=1.09, 2σ),代表该岩体的形成年龄。这一年龄与福建北东沿海高分异I型花岗岩的年龄(=91.5~96.1Ma,邱检生等,2008) 相似,说明它们均为晚白垩世早期岩浆活动的产物。

3.2 主量元素

表 2列出了太武山岩体的主量元素分析结果及经计算所得的相关参数。由表中数据可看出,太武山岩体主体岩性--中细粒花岗岩在主量元素组成上具有以下特征:①富硅,SiO2含量为73.52%~77.36%,分异指数高(D.I=94.6~96.3),说明岩体经历了高程度的分异作用。②碱含量中等偏高(K2O+Na2O=8.40%~9.21%),富钾(K2O=4.40%~5.39%,K2O/Na2O=1.04~1.41),碱铝指数(AKI值) 变化于0.87~0.92,在SiO2-(K2O+Na2O) 关系图上,样品点落在亚碱性系列区(图 5a)。③弱过铝,A/NKC值变化于1.01~1.04,在A/NKC-A/NK图解上,样品点均落入亚碱弱过铝区(图 5b)。样品的CIPW标准矿物计算结果中均出现刚玉分子,但含量都在1%以下(表 2),与S型花岗岩强过铝(A/NKC>1.1,CIPW标准矿物计算结果中刚玉分子含量大于1%,Chappell and White, 2001) 的特点明显不同。④低铁、镁、钙、钛,贫磷,P2O5含量基本均低于0.10%,也不同于S型花岗岩,后者常具较高的P2O5含量(>0.20%, Chappell,1999)。

表 2 太武山花岗岩体代表性样品主量元素含量(wt%)、CIPW标准矿物及主要岩石化学参数 Table 2 Major element contents (wt%), CIPW-normative minerals and predominant petrochemical parameters of the representative samples from the Taiwushan pluton

①福建省地质局. 1977.泉州幅、厦门幅1:20万区域地质调查报告(内部资料)

图 5 太武山花岗岩体SiO2-(K2O+Na2O) 岩石分类图(a,底图据Middlemost, 1994) 及A/NKC-A/NK关系图解(b, 底图据Maniar and Piccoli, 1989) Fig. 5 SiO2-(K2O+Na2O) diagram (a, after Middlemost, 1994) and A/NKC-A/NK plot (b, after Maniar and Piccoli, 1989) of the Taiwushan granite pluton

与主体相岩石相比,边缘相似斑状黑云母花岗岩的硅、碱含量均相对偏低(SiO2=69.60%~74.31%,K2O+Na2O=7.76%~8.24%,AKI=0.73~0.81),K2O/Na2O比值也略低(0.95~1.09),但均具有弱过铝(A/NKC=1.02~1.04) 和贫P2O5(0.04%~0.11%) 特征。在SiO2对主要氧化物的变异图解中,整个岩体的Fe2O3T、MgO、Al2O3、TiO2、CaO、P2O5与SiO2均呈不同程度的负相关(图 6),这些成分变异特点与镁铁质矿物、斜长石、Ti-Fe氧化物及磷灰石分离结晶所引起的成分演变趋势一致,说明分离结晶作用对太武山岩体岩浆成分的演化具有重要的制约作用,即边缘相为早期演化程度相对较低的岩浆侵位结晶形成,而岩体主体岩性则经历了进一步的分异演化。

图 6 太武山花岗岩体主量元素组成变异图解 图例同图 5 Fig. 6 Major element variation diagrams for the Taiwushan granite pluton Symbols are the same as those in Fig. 5
3.3 稀土及微量元素

表 3列出了太武山岩体代表性样品的稀土和微量元素分析结果。由表中数据可看出,岩体主体岩性--中细粒花岗岩的稀土总量中等偏低,∑REE=71.7×10-6~131.0×10-6;富轻稀土,LREE/HREE=2.22~10.91,(La/Yb)N=1.87~9.66,其中轻稀土较重稀土分馏更为明显,(La/Sm)N及(Gd/Yb)N值分别为1.59~4.96和0.82~1.23,稀土配分型式呈明显的右倾斜型(图 7a),不同于典型S型花岗岩常表现出的“海鸥型”稀土配分型式(徐克勤等,1989)。边缘相似斑状黑云母花岗岩的稀土总量略高(136.2×10-6),轻、重稀土的分馏程度均更明显(LREE/HREE=12.22,(La/Yb)N=12.28),且从边缘相到主体相,铕负异常显著增强(二者的Eu/Eu*值分别为0.85和0.04~0.62)。上述稀土元素的变异趋势指示在太武山岩体岩浆演化过程中伴随有富轻稀土矿物(如磷灰石) 和斜长石的分离结晶作用,这与主量元素演变趋势所指示的矿物分离结晶作用特点一致。

表 3 太武山花岗岩体微量及稀土元素含量(×10-6) Table 3 Trace and rare earth element concentrations (×10-6) of the Taiwushan granite pluton

图 7 太武山花岗岩体稀土元素球粒陨石标准化配分曲线(a, 标准化值据Boynton, 1984) 及微量元素原始地幔标准化蛛网图(b, 标准化值据McDonough and Sun, 1995) Fig. 7 Chondrite-normolized REE distribution patterns (a, normalization values after Boynton, 1984) and primitive mantle-normalized spidergrams (b, normalization values after McDonough and Sun, 1995) of the Taiwushan granite pluton

在原始地幔标准化蛛网图(图 7b) 上,太武山岩体明显富集Cs、Rb、U、Th、Pb,贫Ba、Sr、P、Ti。边缘相岩石的Rb/Sr和Rb/Ba比值分别为0.30和0.11,而岩体主体岩性中细粒花岗岩的Rb/Sr和Rb/Ba比值显著偏高,其值分别为1.63~44.67和0.24~13.16,且主体相岩石Ba、Sr、P、Ti等元素的亏损明显增强(图 7b),也指示自边缘相至主体相,岩浆经历了显著的分异演化。

3.4 Hf同位素组成

对已测锆石U-Pb年龄的样品进行了原位锆石Hf同位素组成测定,表 4中列出了测定结果及计算的相关参数。由表中数据可看出,被测锆石的176Hf/177Hf比值变化于0.282673~0.282794,εHf(t) 为-1.44~2.78,散布于正值与负值之间(图 8a)。在t-εHf(t) 关系图上,样品点均落在东华夏地块基底演化域之上(图 9),且位于球粒陨石均一储库(CHUR) 附近,其二阶段Hf模式年龄(tDM2) 变化于0.98~1.25Ga (图 8b),平均为1.06Ga,较之华夏地块基底年龄(主要为1.80~2.20Ga,陈江峰等,1999) 显著偏低,指示成岩过程中应有显著的亏损地幔组分参与,相邻的漳浦复式岩体及福建北东沿海的高分异I型花岗岩均具有类似的锆石Hf同位素组成特征(邱检生等, 2008, 2012, 图 9)。

表 4 太武山花岗岩体锆石Hf同位素分析结果 Table 4 Zircon Hf isotopic compositions of the Taiwushan granite pluton

图 8 太武山花岗岩体锆石εHf(t) 值(a) 和二阶段Hf模式年龄(tDM2) 频率分布直方图(b) Fig. 8 Histograms of zircon εHf(t) values (a) and two-stage Hf model ages (tDM2) (b) for the Taiwushan granite pluton

图 9 太武山花岗岩体t-εHf(t) 关系图及其与福建沿海相关岩体对比 长桥、程溪和湖西资料据邱检生等(2012); 南镇、大层山、三沙和大京资料据邱检生等(2008); 东华夏地块地壳基底演化域据Xu et al. (2007) Fig. 9 t-εHf(t) diagram of the Taiwushan granite pluton and comparison with related intrusions in the coastal area of Fujian province Zircon Hf isotopic compositions for granitic plutons of Changqiao, Chengxi and Huxi after Qiu et al. (2012), and those for Nanzhen, Dacengshan, Sansha and Dajing after Qiu et al. (2008). The evolutionary area of crustal basement in eastern Cathaysia Block is after Xu et al. (2007)
4 讨论 4.1 岩体成因类型归属

花岗岩成因类型的判定是花岗岩研究的最重要基础问题,自20世纪70年代以来,以花岗岩物质来源为基础的分类方案受到普遍推崇,先后提出过多种方案,其中最具影响的首推Chappell and White (1974)提出的I型与S型分类方案,他们认为I型花岗岩是由未经地表风化作用的火成岩部分熔融的产物,而S型花岗岩则是由经历过地表风化作用的沉积物质部分熔融形成。Pitcher (1982)认为自然界中有少数花岗岩可能是地幔岩浆直接分异的产物,称为M型花岗岩。Loiselle and Wones (1979)提出了A型花岗岩概念,尽管这一花岗岩类型并非从物质来源角度提出的,但因其具有明确的指示拉张构造背景意义,因而也受到广泛重视,众多学者先后对这类花岗岩的岩石学特征及其识别标志(Whalen et al., 1987; Sylvester, 1989; Eby, 1990; Frost and Frost, 2011)、岩石学亚类的进一步划分(Eby, 1992; 洪大卫等,1995),以及岩石成因与构造意义等(Collins et al., 1982; Clemens et al., 1986; Creaser et al., 1991; Qiu et al., 2004; Sears et al., 2005; Bonin, 2007; Wong et al., 2009; Li et al., 2012; Yang et al., 2012; 洪大卫等, 1987; 邱检生等, 1999, 2000蒋少涌等, 2008) 进行了广泛深入的研究。

由于自然界中M型花岗岩极少,因此对I、S和A型花岗岩的判定备受关注,不同学者先后从不同角度提出过多种判别准则(Chappell and White, 1974, 2001; Whalen et al., 1987; Chappell, 1999Frost and Frost, 2011; 洪大卫等,1995),如从矿物组成上,角闪石、堇青石和碱性暗色矿物的出现被认为是判定I型、S型和A型花岗岩最有效的矿物学标志(吴福元等,2007),此外,一系列地球化学图解也广泛应用于上述花岗岩成因类型判别中(Whalen et al., 1987; Sylvester, 1989; Eby, 1990, 1992; Frost et al., 2001)。但对于经历高程度分异演化的花岗岩,由于其矿物组成和化学成分都趋近于低共结的花岗岩,使得利用已有的一些标志难以进行有效判定,为此,对这类花岗岩往往需要结合岩石学、矿物学和地球化学等多种证据进行综合判别(Chappell and White, 1992; Chappell, 1999; Wu et al., 2004; Li et al., 2007; 吴福元等,2007)。

就太武山岩体而言,其花岗岩具弱过铝特性,A/NKC值均小于1.1,CIPW标准矿物计算结果中刚玉分子含量都在1%以下,岩石矿物组合中未见白云母、堇青石和石榴石等富铝矿物出现,不同于S型花岗岩的强过铝特征。Chappell (1999)认为分异的S型花岗岩A/NKC值也可能低于1.1,因此对高分异花岗岩成因类型判别还需结合其他证据。研究表明,在强过铝的S型花岗质岩浆中,磷灰石的高溶解度使得其主要呈不饱和状态,因而P2O5含量随着分异演化作用的进行而升高(Wolf and London, 1994),同时会伴随着微量元素Th、Y含量相应的降低。太武山花岗岩体的P2O5含量普遍较低,基本均低于0.10%,且具有随分异作用增强而降低的变异趋势(图 6),同时Th、Y与Rb之间具有一定的正消长演化关系,这些特征与S型花岗岩的演化特点明显不同,因此可排除岩体属于S型花岗岩的可能,其成因类型或为A型花岗岩或为高分异的I型花岗岩。

Whalen et al.(1987) 提出了一系列以Ga/Al (×104) 值为基础的判别图解,用于判别A型花岗岩和其它类型的花岗岩,认为A型花岗岩以Ga/Al (×104)>2.6区别于其它类型花岗岩。从表 3中数据可以看出,太武山岩体Ga/Al (×104) 变化于1.97~3.02,显然用这一指标很难判定其成因类型是属于I型还是属于A型花岗岩,但太武山岩体具有一系列明显不同于A型花岗岩的化学组成特征,如:(1) 岩体的碱铝指数(AKI值) 变化于0.87~0.92,低于A型花岗岩的平均值(=0.95, Whalen et al., 1987)。(2) Zr、Nb、Ce、Yb等高场强元素含量较低,Zr+Nb+Ce+Y变化于132×10-6~308×10-6,低于A型花岗岩下限值(350×10-6, Whalen et al., 1987)。在(Zr+Nb+Ce+Y) vs. (K2O+Na2O) /CaO判别图解中,岩体主体岩性均落入分异的I型花岗岩区(图 10)。(3) 尽管对于A型花岗岩的成因仍存在着广泛的争议,但普遍认为其形成于高温环境(Clemens et al., 1986; 吴福元等,2007)。根据Watson and Harrison (1983)提出的锆石饱和温度计,计算出太武山岩体的锆石饱和温度为726~809℃,与福建北东沿海高分异I型花岗岩的形成温度(730~779℃,邱检生等,2008) 相似,而明显低于闽浙沿海魁歧(817~885℃,据Qiu et al., 2004资料计算)、太姥山(816~850℃,作者未刊资料) 和瑶坑(873~921℃,肖娥等,2007) 等典型A型花岗岩。综上所述,我们认为太武山岩体应属高分异的I型花岗岩。

图 10 太武山花岗岩体(Zr+Nb+Ce+Y)-(K2O+Na2O)/CaO关系图(底图据Whalen et al., 1987) OGT-I、S和M型花岗岩区;FG-分异的I型花岗岩区;A-A型花岗岩区.图例同图 5 Fig. 10 (Zr+Nb+Ce+Y) vs. (K2O+Na2O)/CaO diagram of the Taiwushan granite pluton (after Whalen et al.Whalen et al., 1987) OGT-Field for I-, S-and M-type granitoids; FG-field for fractionated I-type granitoids; A-field for A-type granitoids. Symbols are the same as those in Fig. 5
4.2 成岩过程

近年来,幔源组分在花岗成岩过程中的重要性与普遍性已越来越多地得到证实,它不仅为成岩提供物源,更重要的是为地壳物质熔融产生花岗质岩浆提供热源(Bergantz, 1989; Petford et al., 2000; Zhou and Li, 2000; Annen and Sparks, 2002, 王德滋和周新民,2002; 周新民, 2007)。一般认为幔源组份可通过两种方式参与成岩,其一为幔源组分诱发地壳物质部分熔融产生长英质岩浆,并与其发生混合直接参与花岗岩的形成(Griffin et al., 2002; Belousova et al., 2006; Kemp et al., 2007; Yang et al., 2007; Li et al., 2012a; Zhao et al., 2010, 2012);另一种为幔源组分通过底侵方式形成初生地壳,然后在后期热事件的影响下,这种初生地壳再发生部分熔融形成花岗岩(Pitcher et al., 1985; Jahn et al., 2000; Wu et al., 2006; Zheng et al., 2007)。就太武山岩体而言,其在空间上位于漳浦复式花岗岩体东缘,侵位时间与漳浦复式岩体中的湖西花岗闪长岩相近(为96Ma,邱检生等,2012),指示两者深部可能连为一体,共同构成一复式大岩基。漳浦复式花岗岩体的野外地质和Nd-Hf同位素组成特征均指示其最可能为壳幔岩浆混合成因,结合太武山岩体锆石εHf(t) 值散布于正值与负值之间的特点,我们认为幔源组分也应是通过与其诱发地壳物质熔融产生的长英质岩浆混合的方式参与成岩。

幔源组分通过壳幔岩浆混合的方式参与太武山岩体的形成也得到以下证据的支持:(1) 华夏地块基底年龄主要为早、中元古代(沈渭洲等,1993陈江峰等,1999),Xu et al. (2007) 通过河流碎屑锆石研究,进一步指出东华夏地块基底主体形成于早元古代(1.85~1.87Ga,2.10~2.40Ga),因此,太武山岩体(基底归属于东华夏地块) 偏年轻的Hf模式年龄(0.98~1.25Ga) 并不对应于区域的地壳生长事件,而更可能是底侵的玄武质岩浆与其诱发地壳熔融产生的长英质岩浆相混合所致。(2) 已有的闽浙沿海晚中生代花岗岩的εHf(t) 值普遍具有较大的变化范围(图 9),如福建北东沿海高分异I型花岗岩的εHf(t) 值介于-11.6~4.5(邱检生等,2008),福建东南部漳浦复式岩体中长桥、程溪和湖西三单元岩石的εHf(t) 值分别为-8.3~+3.0、+1.7~+10.2和-2.5~+3.5(邱检生等,2012)。众多的研究表明,锆石U-Pb年龄均一,而其εHf(t) 值散布范围大的特点指示其寄主岩应经历不同来源岩浆的混合过程(Griffin et al. 2002Belousova et al., 2006; Yang et al., 2007; Zhao et al., 2010, 2012)。(3) 闽浙沿海晚中生代岩浆作用过程中的壳幔岩浆混合作用已有大量文献记述(Dong et al., 1998; Xu et al., 1999周金城等,1994王德滋和谢磊,2008董传万等,2009; 刘亮等,2011),充分说明壳幔岩浆混合作用在区内燕山晚期岩浆活动中的普遍性。尽管太武山岩体中缺乏镁铁质包体等直接指示壳幔岩浆混合作用的证据,但研究表明,当在地壳较深部位,由于处于较高的温度、压力环境,由幔源基性岩浆底侵诱发地壳物质部分熔融形成的长英质岩浆尚未开始结晶或结晶程度较低,基本还处于一种均匀状态,这时幔源岩浆的注入,既有良好的混合环境,也有充分的混合时间,二者可以发生完全的混合,产生均一的岩浆,形成钙碱性花岗岩类岩石(Fernandez and Barbarin, 1991)。

太武山岩体富Si,贫Ca、Mg、Fe,亏损Ba、Sr、P、Ti和Eu,这些地球化学组成特征说明在地壳深部形成的壳幔混源岩浆在随后的演化过程中又经历了进一步的分离结晶作用。由边缘相到主体相,暗色矿物黑云母含量减少,岩石的SiO2含量及Rb/Sr、Rb/Ba比值升高,CaO、MgO和Fe2O3T含量显著降低,Ba、Sr、P、Ti、Eu的亏损也愈加强烈,这些变异趋势也指示二相带岩石应为同一原始岩浆经结晶分异演化形成。根据造岩矿物中上述微量元素分配系数的大小(Arth, 1976; Hanson, 1978; Green and Pearson, 1986), Ba、Sr、Eu的亏损指示成岩过程中发生了斜长石的分离结晶,而P和Ti的亏损则分别与磷灰石及含钛矿物(如钛铁矿、榍石等) 的分离结晶有关(Raith, 1995; Wu et al.,2003)。自边缘相至主体相,岩石的轻稀土含量和轻重稀土比值均降低,这一演化趋势极可能与成岩过程中富轻稀土矿物(如磷灰石、褐帘石、独居石等) 的分离结晶有关,由于岩浆体系中褐帘石和独居石的分离会引起Th/U比值的明显下降(Bea et al., 1994; Ewart and Griffin, 1994),而太武山岩体自边缘相至主体相,岩石的Th/U比值(分别为4.64和2.68~5.71,表 3) 并未明显降低,因此,上述稀土元素成分变异应是磷灰石的分离结晶所致,这与自边缘相至主体相,岩石的P2O5含量显著降低的特点相吻合。综上所述,太武山岩体的形成应经历了二阶段的成岩过程,即幔源岩浆与其诱发地壳物质熔融形成的长英质岩浆首先在地壳深部混合形成壳幔混源岩浆,随后这一壳幔混合岩浆又经进一步的分离结晶作用并最终固结成岩。

5 结论

(1) 太武山岩体主体岩性为中细粒花岗岩,其锆石U-Pb年龄为96.9±1.3Ma,属晚白垩世早期岩浆活动产物。

(2) 太武山岩体各相带岩石均具有富硅、亚碱、弱过铝特征,岩石的Rb/Sr、Rb/Ba比值高,并富Cs、Rb、Th、U、Pb,贫Ba、Sr、P、Ti、Eu。从边缘相到主体相,岩石分异演化程度明显增高。岩体的AKI值、Zr+Nb+Ce+Y含量及锆石饱和温度较之A型花岗岩均显著偏低。其P2O5随SiO2增加而降低,Th、Y与Rb之间则表现出一定的正消长演化关系,综合地质地球化学资料指示,太武山岩体应属高分异的I型花岗岩。

(3) 太武山岩体的锆石εHf(t) 值散布于正值与负值之间(-1.44~2.78),二阶段Hf模式年龄(0.98~1.25Ga,平均为1.06Ga) 低于东华夏地块基底年龄,指示成岩过程中应有显著的亏损地幔组分参与。综合分析表明,该岩体的形成首先经历了幔源岩浆与其诱发地壳物质熔融产生的长英质岩浆在地壳深部混合,随后这一混合岩浆又经进一步分异演化的二阶段成岩过程。

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