岩浆分异过程的证据:锆石微量元素
——以湘南三叠纪王仙岭花岗岩体研究为例

徐慢1),段晓侠1),陈斌2),王志强1),陈彦交1),黎训飞1)

1) 合肥工业大学资源与环境工程学院,合肥,230009; 2) 南方科技大学,广东深圳,518055

内容提要: 本文以锆石微量元素为着眼点,结合野外勘查和锆石U-Pb测年及Nd—Hf同位素分析,精细研究南岭地区三叠纪王仙岭复式岩体的分离结晶过程。岩体主体部分的粗粒电气石白云母花岗岩(237.8±3.5 Ma)、二云母花岗岩(236.6±5.0 Ma)、强蚀变电气石花岗岩(238.5±3.5 Ma)侵位时代一致,同属于晚三叠世岩浆活动的产物;对应的εHf(t)值分别为-11.8~-7.8、-17.8~-7.4、-16.1~-6.3,εNd(t)值分别为-9.9~-11.1、-10.8~-11.1、-9.2~-10.8;相近的εHf(t)和εNd(t)值说明这些岩浆岩来自同一源区的同一期岩浆事件。三叠纪花岗岩中电气石白云母花岗岩锆石Ce/Sm和Yb/Gd值同时增加的变化趋势以及低的P和Y含量指示存在磷灰石等富MREE和P、Y矿物的分离结晶。另一方面,二云母花岗岩相对亏损LREE和Th,指示锆石结晶之前熔体经历了褐帘石/独居石的分离结晶,而较高的P含量进一步确定了是褐帘石而非富P相独居石的分离结晶;其Nb、Ta含量低、Eu负异常更显著,说明受到金红石和斜长石的分离结晶的影响。电气石白云母花岗岩和二云母花岗岩虽然同期同源,但锆石微量元素变化精细的刻画出岩浆分离结晶过程的差别。本研究表明岩浆岩副矿物地球化学分析是揭示岩浆结晶过程的有效手段。

关键词:锆石微量元素;结晶分异;Nd—Hf同位素;U-Pb定年;王仙岭岩体

图1 华南南岭地区中生代花岗岩及矿床分布图(据孙涛等,2003修改)
Fig. 1 Geological map of Mesozoic granite and mineral deposits in Nanling Mountains, south China(modified from Sun Tao et al., 2003&)

华南南岭地区广泛发育大面积中生代花岗质岩石(图1),这些花岗岩与众多大型W、Sn、Nb、Ta和REE矿床具有成因联系,使得这些花岗岩一直备受关注(毛景文等,2007)。南岭地区岩浆岩中,燕山期花岗岩分布最广泛,与之伴生的钨锡矿床规模大、数量多,因而研究程度高(李献华等,2007;邓晋福等,2016;杨文采,2018;张燕等,2019),相比之下印支期岩体分布较少,研究程度相对低。近年来印支期花岗岩也陆续发现伴生钨锡矿,受到越来越多的重视(陈骏等,2008;郭春丽等,2012);湖南王仙岭复式岩体是湘南印支期岩浆活动的代表,其周边发现有毛家垄、人形岭和野鸡窝等钨矿床,定年表明钨成矿作用发生于印支期(蔡明海等, 2006)。南岭地区印支期岩浆岩一般以多期次岩浆活动构成的复式岩体形式产出(郭春丽等,2012),岩石组成复杂多样,各岩石单元之间的关系不明。王仙岭复式岩体发育多期次、多种类型的岩石,包括三叠纪主体、侏罗纪二云母花岗岩岩株或隐伏侵入体、花岗斑岩脉等;其中主体部分又分为中心相和边缘相两部分,而具体的岩性组成前人报道存在争议:① 王仙岭主体由中心相电气石二云母花岗岩和边缘相黑云母二长花岗岩组成(柏道远等,2006;王显彬等,2012,2013;蔡明海等,2013,2016);② 主体为中心相中粗粒和边缘相中细粒电气石黑云母花岗岩(郑佳浩和郭春丽,2012);③ 主体为中心相电气石二云母花岗岩和边缘相电气石白云母花岗岩组合(章荣清等,2010,2011;Zhang Rongqing et al., 2015)。王仙岭岩体主体的不同岩石单元的厘定及演化关系有待进一步研究;本文通过野外勘查、岩相学、锆石U-Pb年代学及Nd—Hf同位素等手段获得岩石组成、期次和源区性质的基础上,以岩石副矿物地球化学为突破口,揭示不同岩石分异演化过程。岩石中的副矿物如锆石、磷灰石等能够有效反映寄主岩石的地球化学性质和岩石成因,尤其是复杂岩浆过程或由于后期蚀变等因素造成全岩地球化学无法准确反映岩浆演化历史时,矿物地球化学能提供解译岩石成因的独特视角(Braund et al., 2014; Duan Xiaoxia et al., 2019)。锆石(ZrSiO4)是花岗质岩石中普遍发育的副矿物,锆石在结晶过程中吸纳REE、U、Th、Ti等多种微量元素,并保存其母岩浆的同位素组成,其U-Pb,Hf,O同位素体系被开发用于确定岩浆结晶时代以及岩浆源区性质(Kemp et al., 2007; Wang Xiaolei et al., 2013),此外其微量元素组成能够提供岩石成因、成岩温度、氧逸度等信息,是有效的研究岩浆岩成因及演化过程的工具。

图2 湘南王仙岭岩体地质图(据湖南地质队,1983修编)
Fig. 2 Geological map of Wangxianling granitoid in southern Hunan(modified from Hunan Geological Survey Team,1983#)
Q—第四系沉积物;C1y—下石炭统岩关组灰岩;C1d—下石炭统大塘组砂岩;D3—上泥盆统佘田桥组灰岩;D2q—中泥盆统棋梓桥组白云质灰岩、白云岩; D2t—中泥盆统跳马涧组砂岩;γ—花岗斑岩;—中细粒二云母花岗岩:—细粒电气石白云母花岗岩:—粗粒电气石白云母花岗岩
Q—Quaternary sediments; C1y—limestone of the Lower Carboniferous Yanguan Formation; C1d—sandstone of the Lower Carboniferous Datang Formation; D3—limestone of the Upper Devonian Shetianqiao Formation; D2q— dolomitic limestone or dolomite of the Middle Devonian Qiziqiao Formation; D2t—sandstone of the Middle Devonian Tiaomajian Formation; tourmaline—muscovite granite;

1 地质背景

南岭地区位于华南中南部的华夏地块,地理位置为111°~117°E, 23°20′~26°40′N(舒良树,2006),总面积200000 km2,由越城岭、都庞岭、萌渚岭、骑田岭、大庾岭五大山岭组成。南岭构造带发育在华南前震旦纪早古生代强烈褶皱—变质的基底之上,自三叠纪华南—华北陆块和华南—印支陆块拼合以来,经历了中—新生代陆内多期构造—岩浆沉积作用,发育大规模的中生代花岗岩和裂谷盆地。该区基底变质岩的原岩为陆源碎屑岩,这些变沉积岩的沉积时代为新元古代到早中生代。南岭地区岩浆活动非常普遍,发育多时代、大面积的岩浆岩,加里东期、印支期、燕山期均有出露,以燕山期岩体占最大比重(Zhou Xinmin et al., 2006)。加里东期花岗岩主要分布在湘—赣、湘—桂和桂—粤交界地区,大部分属于强过铝质S 型(孙涛等,2003);印支期花岗岩分布较为分散,包括广西栗木花岗岩(锆石U-Pb 定年为214~218 Ma,康志强等,2012);桂东北苗儿山—越城岭复式岩体中云头界白云母花岗岩(锆石U-Pb 年龄约220 Ma,伍静等,2012),王仙岭花岗岩(锆石年龄223.5Ma,章荣清等,2010)。燕山期花岗岩分布最广,其中湘南—桂北地区集中发育与钨锡成矿的花岗岩,形成一条北东向分布的铝质A 型含锡花岗岩岩带,与柿竹园、瑶岗仙、西华山等众多大型钨锡铌钽多金属矿床伴生(毛景文等,2007,2008)。

王仙岭岩体位于湖南省郴州市东南约10 km处,出露面积 19.7 km2,北大南小,呈倒葫芦状,NNE向展布,侵入泥盆系灰岩、砂岩和石炭系灰岩、白云岩(图2)。岩体周边出露地层主要为泥盆系—中三叠统滨、浅海相碳酸盐岩和碎屑岩,以及上三叠统—古近系陆相碎屑岩。区内构造以 NE向张扭性断裂和不同地层岩性界面附近的层间滑动构造为主,并控制了区内锡多金属矿化的空间分布和花岗斑岩脉的产出。岩体内部蚀变作用十分普遍,以云英岩化和硅化为主。在岩体周边围岩中主要有大理岩化、矽卡岩化等蚀变。王仙岭岩体东南侧发育有荷花坪锡多金属矿,蔡明海等(2006)获得早期矽卡岩型矿石辉钼矿Re-Os 等时线年龄为224.0±1.9 Ma, 认为区内早期成矿与王仙岭早期中粗粒二云母花岗岩有关。岩石学研究方面,柏道远等(2006) 、Wei Daofang等(2007) 、章荣清等( 2010,2011) 、郑佳浩和郭春丽( 2012) 、王显彬等(2012)、Zhang Rongqing等(2015)等开展了王仙岭岩体的岩石学、地球化学、年代学以及Hf 同位素示踪研究。

图3 湘南王仙岭岩体样品手标本照片
Fig. 3 Photographs of hand specimens of Wangxianling granitoid in southern Hunan
(a) 粗粒电气石白云母花岗岩手标本;(b) 细粒电气石白云母花岗岩手标本;(c) 粗粒电气石白云母花岗岩和细粒电气石白云母花岗岩过渡;(d) 二云母花岗岩手标本;(e) 蚀变电气石花岗岩手标本;(f) 花岗斑岩手标本
(a) Hand specimen of coarse tourmaline—muscovite granite; (b) hand specimen of fine tourmaline—muscovite granite; (c) one sample with coarse tourmaline—muscovite granite and fine tourmaline—muscovite granite coexisting together; (d) hand specimen of two-mica granite ; (e) hand specimen of altered tourmaline granite; (f) hand specimen of granite porphyry

2 样品及测试方法

2.1 采样及岩相学

根据野外勘查结合岩相学观察,王仙岭岩体主体主要由粗粒电气石白云母花岗岩和细粒电气石白云母花岗岩组成,两者界线不显著,呈过渡接触(图3c,4c)。另有二云母花岗岩、花岗斑岩等小规模岩脉或岩株产出(图2)。

粗粒电气石白云母花岗岩分布于岩体边部,岩石呈灰白色,粗粒似斑状结构(图3a,4a),主要矿物有斜长石(含量约25%)、石英(约30%)、钾长石(约30%),白云母(约10%),电气石含量约5%,呈自形长柱状,约0.5~2 cm,可见球面三角形截面。副矿物为锆石、磷灰石、榍石等;细粒电气石白云母花岗岩分布在岩体中部,呈灰白色或者灰色,细粒花岗结构(图3b,4b),主要矿物有石英(含量约30%)、钾长石(约35%)以及斜长石(约20%)以及白云母(约5%),电气石含量更高,可达10%左右,自形—半自形细长针状,长约0.2~4 mm。副矿物为锆石、磷灰石等;电气石白云母花岗岩局部形成强蚀变电气石花岗岩(图3e,4e),电气石含量显著增加,高达10%~15%,多呈自形长柱状晶体,粒度粗大,约0.5~3 cm长,整体蚀变程度高,长石普遍绢云母化。二云母花岗岩呈岩脉产于岩体东北部分,岩石新鲜面呈灰白色,中粒结构(图3d,4d),主要矿物有钾长石(含量约35%)、斜长石(约30%)以及石英(约25%),次要矿物为、黑云母(~8%),白云母(~2%),少见电气石,副矿物为锆石、磷灰石以及褐帘石等。花岗斑岩呈岩脉分布于岩体东南侧(图3f,4f),新鲜面青灰色,斑状结构,斑晶约占45%,主要为石英(含量约20%)、钾长石(约20%),以及少量黑云母(约5%),其中石英它形粒状,粒度2~3 mm,钾长石半自形板状,粒度2~4 mm。基质约占55%,为石英、斜长石,钾长石和黑云母。

图4 湘南王仙岭岩体显微照片
Fig. 4 Microscopic photos of the Wangxianling granitoid in southern Hunan
(a) 粗粒电气石白云母花岗岩正交偏光(25+),电气石自形—半自形,含量约5%;(b) 细粒电气石白云母花岗岩正交偏光(50+),电气石细长针状,含量高(~10%);(c) 粗粒和细粒电气石白云母花岗岩过渡,左侧为粗粒,右侧为细粒,电气石含量更高;(d) 二云母花岗岩正交偏光(25+);粒度中等,主要矿物有自形黑云母、白云母、斜长石、钾长石、石英等;少见电气石;(e) 蚀变电气石花岗岩正交偏光(25+):电气石含量高(10%~15%),自形长柱状,粒径约3~10 mm,斜长石强烈绢云母化;(f) 花岗斑岩正交偏光(25+),斑晶主要为石英、钾长石等,斑状结构;Q—石英;Pl—斜长石;Bt—黑云母;Kf—钾长石; Tur—电气石; Ser—绢云母;Fl—萤石;Ms—白云母
(a) Coarse tourmaline—muscovite granite, crossed polars (25+), coarse grained texture, tourmaline content reaches 5% and is euhedral to sub-euhedral; (b) fine tourmaline—muscovite granite, crossed polars (50+), fine grained texture, tourmaline takes a significant proportion of the intrusives(~10%) and exhibits elongate needle shape; (c) transition between coarse tourmaline—muscovite granite and fine tourmaline—muscovite granite with coarse grains on the left and fine grains on the right which shows higher tourmaline content; (d) two-mica granite , crossed polars (25+), medium grain size, and the main minerals are biotite, muscovite, plagioclase, potassium feldspar, quartz, etc. tourmaline is rarely seen; (e) altered tourmaline granite, crossed polars (25+): strong sericitization of plagioclase is pervasive through the rock and the tourmaline content is as high as 10%~15% and shows 3~10 mm elongated prism; (f) granite porphyry crossed polars (25+) the phenocrysts are mainly quartz, potassium feldspar etc., with porphyry structure. Q—quartz; Pl—plagioclase; Bt—biotite; Kf—K-feldspar; Tur—turmaline; Ser—sericite; Fl—fluorite; Ms—muscovite

本文选取王仙岭主体部分粗粒电气石白云母花岗岩WX-58、细粒电气石白云母花岗岩WX-22、二云母花岗岩WX-63、蚀变电气石花岗岩WX-68进行了锆石LA-ICP-MS U-Pb测年分析,由于WX-22样品定年数据不理想,仅对WX-58、63、68进行了微量元素和Hf同位素测试。

2.2 测试方法

图5 湘南王仙岭岩体锆石阴极发光(CL)图像和定年数据
Fig. 5 CL images and dating results of representative zircons of the Wangxianling granitoid in southern Hunan

锆石由河北省地质矿产调查研究所实验室挑选,然后在双目镜下挑选出晶形较好,无明显裂痕和包体的锆石颗粒,并将其粘贴在环氧树脂表面,打磨抛光后露出锆石的表面,对其进行透射光、反射光和阴极发光(CL)图像的采集。锆石U-Pb和微量元素测试在合肥工业大学LA-ICPMS实验室完成。测试仪器为Agilent 7500a 四极杆等离子体质谱仪 (Q-ICPMS)和193 nm的激光剥蚀进样系统,激光剥蚀系统为美国Coherent Inc,激光器为ArF准分子激光器,激光剥蚀束斑直径为30 μm,能量密度范围为1~45 J/cm2,最高重复频率为20 Hz;剥蚀时间80~120 s,剥蚀深度为40~60 μm,每个分析点的气体背景采集时间为15 s,信号采集时间为50 s。以国际标准锆石91500,GJ-1作为标样,美国国家标准技术研究院研制的人工合成硅酸盐玻璃NIST610作为外标,29Si为内标。测试使用的激光脉冲速率为8~10 Hz,激光束脉冲能量0.6~1.3 MJ。具体分析过程及数据精度等详见Griffin等(2004)。数据通过ICPMSDataCal软件计算获得相应的同位素比值,年龄及误差,并采用Andersen (2002)的方法进行必要的普通铅校正。年龄计算采用国际标准程序Isoplot程序出图(Ludwig, 2003)。

锆石Hf同位素测试在南京聚谱检测科技有限公司LA-MC-ICPMS仪器上完成。锆石微量元素和Lu-Hf同位素测试均在U-Pb测试同一颗粒上进行。采用Analyte Excite的193 nm ArF准分子激光剥蚀系统和Nu Plasma II多接收器电感耦合等离子体质谱(MC-ICPMS)。准分子激光发生器产生的深紫外光束经匀化光路聚焦于锆石表面,能量密度为60 J/cm2,束斑直径为50 μm,频率为8 Hz,共剥蚀40 s。测试过程中每隔10颗样品锆石,交替测试2颗标准锆石(GJ、91500、Mud Tank、Penglai),以检验锆石Hf同位素数据质量。Hf同位素数据处理过程:使用n(176Lu)/n(175Lu) = 0.02658 和 n(176Yb)/n(173Yb) = 0.796218扣除176Lu 和176Yb 对 176Hf的同量异位干扰(Vervoort et al., 2004);标准锆石GJ-1和91500的n(176Hf)/n(177Hf)加权平均值分别为0.282004±0.000009(n=10, 2σ)和0.282311±0.000010(n=10, 2σ)与推荐值一致,εHf(t)和 Hf 模式年龄计算使用以下参数:[n(176Lu)/n(177Hf)]CHUR = 0.0332,[n(176Hf)/n(177Hf)]CHUR,0 = 0.282772 (Blichert-Toft and Albarède, 1997),[n(176Lu)/n(177Hf)]DM = 0.0384,[n(176Hf)/n(177Hf)]DM = 0.28325 (Griffin et al., 2000),λLu = 1.867×10-11 a-1 et al., 2004),[n(176Lu)/n(177Hf)]CC = 0.015 (Griffin et al., 2002)。

全岩Nd同位素测试在北京大学造山带与地壳演化教育部重点实验室完成分离纯化,并天津地质调查中心完成上机测试。Sm和Nd 的含量通过同位素稀释剂法获得,元素Sm和Nd的误差小于0.5% 。n(143Nd)/n( 144Nd)的测量值是通过n(146Nd)/n( 144Nd )= 0.7219 进行矫正。样品测试过程中,LRIG Nd 标样给出的测量值n(143 Nd)/n( 144 Nd) =( 0.512111 ± 4) (2σ),以同样的化学流程处理BCR-2 标样得出的测试数值:n( 147 Sm)/n(144 Nd) = 0.1376,n(143 Nd)/n(144 Nd)=0.512624 ± 3 ( 2σ) 。

图6 湘南王仙岭岩体U-Pb谐和图: (a) 二云母花岗岩(WX-63); (b) 粗粒电气石白云母花岗岩(WX-58); (c)和(d) 细粒电气石白云母花岗岩(WX-22); (e) 蚀变电气石花岗岩(WX-68)
Fig. 6 U-Pb concordia digram of zircons from the Wangxianling granitioid in southern Hunan: (a) two-mica granite (WX-63); (b) coarse tourmaline—muscovite granite(WX-58); (c) and (d) fine tourmaline—muscovite granite(WX-22); (e) altered-tourmaline granite(WX-68)

3 数据

3.1 锆石U-Pb测年

王仙岭复式岩体的五个岩浆岩样品锆石U-Pb测试数据列于表1,测试锆石多呈自形—半自形棱柱状,较小,长约50~150 μm,长短轴比主要为1∶1到2∶1。阴极发光(CL)图像显示测试锆石颗粒均发育振荡环带(图5),U含量普遍高,238U=(282~15463)×10-6。部分锆石图像暗,反映U含量非常高,对应的n(206Pb)/n(238U)、n(207Pb)/n(235U)等同位素比值偏低,获得的年龄明显偏年轻,暗示存在放射性蜕变造成的铅损失,这些数据弃用。测点的锆石232Th/238U值在0.01~1.32,结合振荡环带判断测试的锆石均为岩浆锆石(Hoskin and Schaltegger, 2003)。

锆石U-Pb谐和图(图6a)显示二云母花岗岩(WX-63)14个测试数据有3个显示新元古代年龄,集中在939~1087 Ma(n(207Pb)/n(206Pb)年龄),为继承古老锆石;另外11个数据集中在U-Pb谐和线附近,以n(206Pb)/n(238U)计算(误差为2σ)得到的加权平均年龄为236.6±5.0 Ma(n=11,MSWD=1.4),代表二云母花岗岩岩浆结晶年龄。粗粒电气石白云母花岗岩(WX-58)共获得12个有效数据(图6b),集中在U-Pb谐和线附近,n(206Pb)/n(238U)计算的加权平均年龄为237.8±3.5 Ma(n=12,MSWD=0.97),代表其岩浆结晶年龄。细粒电气石白云母花岗岩(WX-22)锆石CL图显示多数锆石为暗黑色,U含量异常高(可达1%以上),本次研究分析了近30颗锆石,大部分数据谐和度低,偏离谐和线很远(图6c),说明存在放射性Pb损失,其中有7个数据集中于U-Pb 谐和线周围(图6d),n(206Pb)/n(238U)年龄集中在213~236 Ma,指示细粒岩体侵位于230 Ma左右。蚀变电气石花岗岩(WX-68)共12个数据点,普通铅略高,经过矫正后数据集中在U-Pb谐和线周围(图6e),n(206Pb)/n(238U)加权平均年龄为238.5±3.5 Ma(n=12,MSWD=0.94),指示其侵位于三叠纪。

3.2 Nd—Hf同位素

全岩Nd同位素和Hf同位素初始值通过加权平均年龄数据计算,数据列于表2和表3,Nd同位素数据显示三种岩石初始Nd同位素组成相近,二云母花岗岩εNd(t)值集中在-10.8~-11.1,电气石白云母花岗岩εNd(t)值-9.9~-11.1;蚀变电气石花岗岩εNd(t)值为-9.2~-10.8。

图7 湘南王仙岭岩体锆石εHf(t)和二阶段模式年龄TDM2 统计直方图
Fig. 7 Histograms of zircon εHf(t) values and two-stage model ages TDM2 (Ma) of the Wangxianling granitoid in southern Hunan

二阶段模式年龄TDM2代表假定平均大陆地壳n(176Lu)/n(177Hf)值为0.015,通过n(176Hf)/n(177Hf)初始值回算到亏损地幔模式曲线的年龄(Griffin et al., 2002)。所有的测试点均显示较低的n(176Lu)/n(177Hf )(0.0009~0.0046),说明锆石结晶后放射积累的Hf极低。除了继承锆石数据,所有的初始Hf同位素εHf(t)值均为负值,粗粒电气石白云母花岗岩具有相对一致的Hf同位素组成,εHf(t)值集中在-11.8~-7.8(均值-9.8),二阶段Hf模式年龄为(TDM2) 为1762 ~2016 Ma。蚀变电气石白云母花岗岩εHf(t)变化范围为-16.1~-6.3(均值-10.2),二阶段Hf模式年龄为(TDM2) 为1670 ~2288 Ma。二云母花岗岩εHf(t)变化范围为-17.8~-7.4(均值-11.6),二阶段Hf模式年龄 (TDM2) 为1738 ~2391 Ma。三者初始Hf同位素和二阶段模式年龄值均相近(图7)。

3.3 锆石微量元素

锆石LA-ICPMS原位微量元素测试过程中排除了具有Ca、Sr、Th、Al以及P异常峰值的测试点,因为这些峰值暗示可能存在其他富REE的矿物包裹体干扰,如磷灰石、榍石、独居石、褐帘石等。锆石微量元素数据列于表4。蚀变电气石花岗岩(WX-68)的锆石普遍显示LREE相对富集的特征,研究表明锆石数据中高LREE含量(LaN>1且PrN>10,Cavosie et al., 2006)通常代表锆石受到次生蚀变影响或微小富LREE矿物包裹体的影响,或者他们是热液成因而非岩浆成因的锆石(Rayner et al., 2005; Claiborne et al., 2010)。WX-68样品所有测点均显示LaN>1且PrN>10,这说明不是个别的富LREE包裹体的影响,而是锆石本身是热液蚀变的锆石。其普通Pb含量异常高,同样说明遭受了热液蚀变的强烈影响。其U、Th、Nb、Ta、Hf、Ti等元素含量与电气石白云母花岗岩相近,数据更分散,分布范围更大,但REE、Y、P等元素存在差别(图8)。

表3 湘南王仙岭岩体Nd同位素
Table 3 Nd isotope compositions of Wangxianling granitoid in southern Hunan

样品号岩性年龄(Ma)Sm(μg/g)Nd(μg/g)n(147Sm)/n(144Nd)测值1σn(143Nd)n(144Nd)n(143Nd)n(144Nd) tεNd(t)SiO2(%)WX-12WX-13WX-63WX-64二云母花岗岩236.64.4121.70.122850.0000040.5119680.511778-10.873.80236.65.0324.70.123100.0000060.5119690.511779-10.872.56236.64.4922.10.122810.0000040.5119590.511769-11.072.96236.65.1025.40.121370.0000040.5119550.511767-11.172.74WX-88wx-58WX-60wx-91电气石白云母花岗岩237.82.8513.90.123940.0000050.5119840.511791-10.673.61237.82.7413.60.121790.0000050.5120150.511826-9.974.93237.82.6912.70.128040.0000100.5120160.511817-10.173.85237.82.359.70.146450.0000030.5119890.511761-11.176.06WX-67WX-68WX-69WX-74蚀变电气石花岗岩238.51.8011.30.096290.0000110.5119730.511822-9.963.88238.52.4012.50.116060.0000130.5119610.511780-10.867.78238.53.2117.70.109630.0000160.5120290.511858-9.265.73238.52.5011.10.136150.0000050.5120220.511810-10.266.59

图8 湘南王仙岭岩体微量元素图
Fig. 8 Trace elemental diagrams of the Wangxianling granitoid in southern Hunan

所有样品稀土配分曲线均显示HREE富集,LREE亏损(图9),均具有很高的Hf、Y、Ta、P含量。三叠纪岩浆岩样品中,电气石白云母花岗岩的锆石Hf含量分布在11670×10-6~15636×10-6,均值14146×10-6。Y含量集中于1931×10-6~3024×10-6(均值2391×10-6),Th含量范围461×10-6~2137×10-6(均值939×10-6),Ta含量为8.4×10-6~21.7×10-6(均值13.4×10-6);REE总量为1392×10-6~2184×10-6,其中LREE含量为21.1×10-6~89.1×10-6 ,HREE含量1338×10-6~2095×10-6 ,具有一定的Eu负异常(Eu/Eu*=0.10~0.18)和比较显著的Ce正异常(Ce/Ce*=6.64~68.79)。二云母花岗岩锆石Hf含量范围为13547×10-6~18612×10-6,均值14824×10-6。Y含量(3470×10-6~4698×10-6,均值4134×10-6)明显高于电气石白云母花岗岩,而Th 含量分布在133×10-6~360×10-6(均值248×10-6)显著低于电气石白云母花岗岩,同时,Ta含量(6.20×10-6~17.26×10-6,均值9.72×10-6)也低于电气石白云母花岗岩;二云母花岗岩REE总量为2205×10-6~3225×10-6,稀土配分曲线显示(图9d),相比电气石白云母花岗岩,二云母花岗岩显示LREE含量明显偏低(7.8×10-6~24.2×10-6), Eu负异常更显著(Eu/Eu*=0.03~0.11)而Ce正异常更弱(Ce/Ce*=1.54~7.75)。

4 讨论

4.1 岩浆侵位序列和源区示踪

王仙岭复式岩体岩性划分及侵位时代存在较大争议,关于三叠纪主体部分,柏道远等(2006)认为岩体边部岩性为细中粒斑状黑云母二长花岗岩(人形岭单元),岩体中部主要为中粗粒电气石二云母二长花岗岩(邓家石单元);蔡明海等(2013,2016)和王显彬等(2012)认为印支期岩体主要由早期中粗粒似斑状富含电气石的二云母花岗岩(222.5 ~226.0 Ma)和晚期呈岩枝、岩脉侵入早期岩体的中细粒黑云母花岗岩(212 Ma,Wei Daofang et al., 2007)组成;郑佳浩和郭春丽(2012)认为王仙岭岩体中心相主要为中粗粒电石气黑云母花岗岩,边缘相为细中粒电气石黑云母花岗岩(235.0 Ma);章荣清等(2010, 2011)及Zhang Rongqing 等(2015)认为主体为中心相中粗粒电气石二云母花岗岩和边缘相中粗粒电气石白云母花岗岩。此外,岩体东南侧发育有花岗斑岩脉,年龄集中在142~159 Ma(Wei Daofang et al., 2007; Zhang Rongqing et al., 2015)。另外报道有侏罗纪黑云母二长花岗岩株、隐伏侵入体 (156~157 Ma,郑佳浩和郭春丽,2012;章荣清等,2011)。本文基于野外填图和室内分析厘定了岩体的主要岩石类型和分布(图2),通过锆石U-Pb年代学测试结果厘定了岩浆岩侵位序列:粗粒电气石白云母花岗岩时代为237.8±3.5 Ma,强蚀变电气石花岗岩时代为238.5±3.5 Ma,二云母花岗岩侵位于236.6±5.0 Ma,三者锆石U-Pb年龄误差范围内一致;这些岩浆岩同属于晚三叠世岩浆活动的产物。

表4 湘南王仙岭岩体锆石微量元素数据(×10-6)
Table 4 zircon trace element data of the Wangxianling granitoid in southern Hunan (×10-6)

编号PTiYNbLaCePrNdSmEuGdTbDyHoErWX-58 粗粒电气石白云母花岗岩WX-58-237357.29202621.00.1028.000.192.145.080.6933.913.818167.5309WX-58-249504.28207921.30.0118.690.121.923.720.7625.512.217067.4325WX-58-1910983.10247110.60.0014.970.071.024.380.6336.416.221582.5379WX-58-1412396.72232214.00.3033.431.449.559.110.8142.817.221178.3345WX-58-06134716.50302426.30.6066.020.636.8213.381.6062.922.6282103460WX-58-02139015.56288781.20.7631.600.787.0310.671.1351.222.127398.8440WX-58-229305.88193116.81.3540.360.844.406.250.6834.214.117264.5293WX-63 二云母花岗岩WX-63-0325873.3646988.510.131.930.051.347.670.2762.830.3402151.6673WX-63-0623754.3144038.180.182.970.292.577.950.5062.128.2370140.9628WX-63-0823986.46443010.770.073.300.273.009.630.6172.331.4400142.9615WX-63-1417775.0435158.660.193.600.243.318.730.6061.425.4321114.7491WX-63-2021704.4541437.540.524.510.705.4611.230.6865.528.9368131.2580WX-63-2222755.2742398.140.646.110.886.079.890.6263.028.2369136.2597WX-63-1621614.55347013.310.194.370.100.422.270.4625.816.6264106.4558WX-63-0526532.90417512.650.263.580.311.573.460.4732.019.6308127.1653WX-68 蚀变电气石花岗岩WX-68-0214706.89307321.500.6716.020.734.617.510.8040.618.6253101.0482WX-68-0570310.2214820.781.2878.691.088.7612.371.6449.817.519973.0319WX-68-1113259.57270331.721.7929.971.8112.111.981.1147.919.824190.7421WX-68-1313926.48287222.911.0134.340.655.638.270.8144.919.024895.2443WX-68-15116911.9245020.204.5439.422.6915.711.190.9943.216.721079.9379WX-68-198404.61194614.320.5835.420.564.016.730.8631.312.716264.7305WX-68-20256014.7382959.213.1848.123.9425.123.011.7069.528.4345129.0618WX-68-21236322.8386570.094.4064.104.8230.329.251.9381.631.4368130.1604WX-68-22216411.6380033.902.7130.203.3721.220.311.5166.827.8336126.9599WX-68-2412348.24245930.561.9753.611.9011.113.901.4348.119.122582.5383WX-68-2527906.76268422.936.3939.353.5518.913.001.0642.917.722587.1420WX-68-2923527.11497944.761.3469.670.848.3215.541.3886.435.8445166.9750

编号TmYbLuHfTa204PbcPbThUTh/UδEuδCeCe/SmYb/Gd∑REEt(℃)WX-58 粗粒电气石白云母花岗岩WX-58-2366.96211141451911.800.5814878536320.220.1237.865.5118.291443750WX-58-2475.87331361422214.550.6315546138290.120.1844.835.0228.761570702WX-58-1985.6796152156368.430.8414047235870.130.1168.793.4221.881784675WX-58-1476.0694130146689.440.4815274237270.200.106.643.6716.221649742WX-58-0698.89021641167015.8212.2242213750370.420.1423.594.9314.322184833WX-58-0299.19091631417721.730.13251112660910.180.129.042.9617.762108827WX-58-2262.95881091413011.790.6216785236430.230.119.096.4517.171392730WX-63 二云母花岗岩WX-63-03144.61300244139437.951.4021225154440.050.035.540.2520.693019681WX-63-06134.31225231137987.391.6318823849190.050.052.530.3719.732834702WX-63-08128.511372111354712.77-19335349230.070.053.470.3415.732754738WX-63-14101.9911162142146.203.0314536038910.090.063.580.4114.852205716WX-63-20120.61084201136316.531.6016924144500.050.061.540.4016.552602705WX-63-22124.41143207137636.880.2217724446070.050.061.670.6218.152692720WX-63-16144.214852831861217.261.8736916393650.020.117.751.9257.512891707WX-63-05159.316153021708812.780.9435813396110.010.092.681.0350.493225669WX-68 蚀变电气石花岗岩WX-68-02112.610701961418815.330.1018254245600.120.114.982.1326.362304744WX-68-0567.76251161139812.061.27128154929210.530.1715.356.3612.551571783WX-68-1195.69061661320118.022.0717364744580.150.123.682.5018.942047776WX-68-13101.19541751244118.157.98205100949090.210.1010.154.1521.232131739WX-68-1586.28351581278112.336.9913170833010.210.122.723.5219.351883798WX-68-1971.06801321399512.43-9545723770.190.1513.825.2721.751508708WX-68-20142.313592481466836.457.38255118863810.190.122.862.0919.573045821WX-68-21135.412502311394348.1512.9260127561950.210.113.022.1915.322967870WX-68-22139.212992411453517.054.8518835644510.080.112.101.4919.452915795WX-68-2484.87841441440414.354.1715287335720.240.156.173.8616.321855761WX-68-2596.09231731364421.263.3118369544470.160.122.003.0321.512068743WX-68-29158.914282551249226.474.454593173105460.300.0915.734.4816.543424747

图9 湘南王仙岭岩体锆石REE配分型式图解
Fig. 9 Zircon REE pattern diagram of the Wangxianling granitoid in southern Hunan

图10 湘南王仙岭岩体锆石Hf同位素演化图
Fig. 10 Hf isotope evolution diagram of zircon from the Wangxianling granitoid in southern Hunan

全岩Nd同位素和锆石 Hf 同位素分析显示王仙岭岩体所有样品εNd(t)值和εHf(t)值均为负值,且Hf投点分布在地壳演化线之上(图10),指示岩浆源区主要是壳源。三叠纪岩浆岩的电气石白云母花岗岩、二云母花岗岩、蚀变电气石花岗岩的εHf(t)值分别为-11.8~-7.8、-17.8~-7.4、-16.1~-6.3,εNd(t)值分别为-9.9~-11.1、-10.8~-11.1、-9.2~-10.8; 三者具有相近的εHf(t)值和εNd(t)值,二阶段Hf 模式年龄(TDM2) 分别为1762~2016 Ma、 1738~2391 Ma、1670~2288 Ma,TDM2年龄分布图中相互重叠,结合三者锆石U-Pb年龄相近,说明这些岩浆岩来自同一源区的同一期岩浆事件。Hf二阶段模式年龄暗示来自于古元古代古老地壳物质的重熔。前人研究也一致认为南岭地区三叠纪花岗岩(220~240 Ma)由古元古代的变质基底部分熔融而来(周新民等,2007)。华南早中生代构造框架研究表明,印支地块与华南地块在243~258 Ma 期间曾经发生碰撞,南岭地区晚三叠世属于伸展减薄时期,构造环境应属后碰撞环境(周新民等,2007)。王仙岭岩体是碰撞被加厚地壳发生垮塌减薄,应力释放条件下,变质基底发生部分熔融的产物。

4.2 锆石对岩浆分离结晶的指示

锆石微量元素的变化能够反映岩浆分异过程以及分异过程中副矿物的结晶顺序等信息(Bruand et al., 2014)。矿物从熔体中结晶时其化学成分主要受控于两方面:①熔体—矿物间元素的分配系数,例如锆石中Hf、U、HREE等元素分配系数远大于1,在熔体—锆石之间分配时强烈倾向进入锆石,导致锆石普遍富集这些元素。②该矿物结晶时刻的熔体的元素丰度,除了岩浆原始成分差别外,还需要考虑早期结晶分离矿物对残留熔体的影响,例如富Ta的金红石从岩浆中晶出后会导致残留熔体相对亏损Ta,其后从该亏损Ta的熔体中结晶的锆石也会继承低Ta的特征。

图11 湘南王仙岭岩体Nd同位素与SiO2相关图
Fig. 11 Nd isotope and SiO2 diagram of the Wangxianling granitoid in southern Hunan

图 12 湘南王仙岭岩体锆石元素对比指示
Fig. 12 Trace element comparsion and implication diagrams of zircon from the Wangxianling granitoid in southern Hunan
Ap—磷灰石;Ttn—榍石;Hb—角闪石;Rt—金红石;图a 据Lee et al., 2017修改
Ap—apatite; Ttn—titanite; Hb—hornblende; Rt—rutile; fig. a was modified from Lee et al., 2017

电气石白云母花岗岩、二云母花岗岩同属于同一期三叠纪岩浆作用,Nd—Hf同位素特征表明其源区一致,岩浆成分相近,而且Nd同位素组成不随SiO2变化而变化(图11)表明未受到同化混染过程影响,因此两者锆石微量元素的差异主要由不同的分异过程造成,可以用于讨论矿物分离结晶过程。岩浆演化过程中早期不同矿物的分离结晶会对锆石成分有所影响,如果锆石Ce/Sm和Yb/Gd值随着演化进行逐渐升高,可能是磷灰石、榍石、角闪石等富MREE的矿物分离结晶会造成残余熔体相对亏损MREE(Sm,Gd)进而导致Ce/Sm和Yb/Gd值同时增高(Grimes et al., 2015;Lee et al., 2017)。具体而言,不同矿物由于REE分配系数不同,在Ce/Sm—Yb/Gd相关图上表现出不同的变化趋势,例如磷灰石除了富集MREE外,本身相对富集LREE(Prowatke and klemme, 2006),其分离结晶会大幅改变Ce/Sm值,而对Yb/Gd值影响较小,在Ce/Sm—Yb/Gd相关图(12a)中表现为斜率大的变化趋势;而角闪石本身富HREE(Hanchar and Westrenen, 2007),对Yb/Gd值影响较大,因而在相关图(12a)上表现为斜率较缓的变化趋势。本文样品中电气石白云母花岗岩Ce/Sm和Yb/Gd值显示随演化增加的趋势,投图显示其主要落在磷灰石和榍石演化线之间,说明存在这两种矿物的分离结晶的影响。但是二云母花岗岩具有异常低Ce/Sm值,而Yb/Gd值变化不大,说明其主要受到富LREE的矿物分离影响,同时二云母花岗岩REE配分曲线显示具有更低的LREE含量(图9d),Ce的正异常不显著,同样暗示其LREE相对亏损。Hf—Th相关图显示二云母花岗岩相对亏损Th元素(图12b),结合低含量LREE的特征,暗示该岩浆结晶锆石之前经历了富LREE和Th矿物的分离结晶。岩浆体系中最主要的赋存LREE和Th的矿物是褐帘石和独居石,他们早期结晶分离会显著降低残余熔体中LREE和Th含量(Klimm et al., 2008; Stepanov et al., 2012),继而造成后续结晶的锆石相对贫LREE和Th,进一步根据二云母花岗岩P含量高的特征,可以排除富P矿物相独居石((LREE)PO4)分异的可能性,即褐帘石的分离结晶造成其低LREE,高P,低Th的特征(图12d)。电气石白云母花岗岩P和Y含量低,且两者呈正相关(图12c),指示其受到富P和Y的矿物相磷灰石分离结晶的影响,与前述Ce/Sm—Yb/Gd判定结果一致。此外,二云母花岗岩Nb和Ta 含量低,呈正相关(图12e),推测为金红石的分离结晶产生的影响。Hf—δEu相关图(图12f)显示二云母花岗岩具有更低的δEu值,Eu负异常更显著,暗示存在斜长石的分离结晶。综上,锆石微量元素表明二云母花岗岩经历了褐帘石、金红石、斜长石的早期分离结晶,而电气石白云母花岗岩主要经历了磷灰石、榍石的分离结晶。两者虽然同期同源,但是岩浆分异结晶过程存在明显差别。前人全岩地球化学未诠释具体的分离结晶过程信息,本研究补充了岩浆过程的精细约束,同时通过本案例研究说明锆石微量元素是研究岩浆演化过程中矿物结晶信息的有效手段。

5 结论

(1)王仙岭复式岩体主体由粗粒和细粒电气石白云母花岗岩组成,另有二云母花岗岩、花岗斑岩等呈小规模岩脉或岩珠产出。锆石U-Pb测年获得粗粒电气石白云母花岗岩时代为237.8±3.5 Ma,强蚀变电气石花岗岩时代为238.5±3.5 Ma;二云母花岗岩侵位于236.6±5.0 Ma,同属于晚三叠世岩浆活动的产物。

(2)印支期岩浆岩电气石白云母花岗岩、二云母花岗岩、蚀变电气石花岗岩的εHf(t)值分别为-11.8~-7.8、-17.8~-7.4、-16.1~-6.3,εNd(t)值分别为-9.9~-11.1、-10.8~-11.1、-9.2~-10.8;三者具有相近的εHf(t)值和εNd(t)值,且侵位时代一致,暗示这些岩浆岩来自同一源区的同一期岩浆事件。

(3)锆石P、Y、Ta、Nb、Th、REE等微量元素显示二云母花岗岩岩浆结晶锆石之前经历了褐帘石、金红石和斜长石的分离结晶,而电气石白云母花岗岩锆石受到榍石和磷灰石等矿物分离结晶的影响。两者虽然同期同源,但是岩浆分异演化过程存在差别。研究表明锆石微量元素的变化能够精细的反映岩浆矿物结晶和分异演化历史等信息。

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Trac.elements i.zircon: constrai.o.magmati.differentiation——.cas.stud.o.Triassi.Wangxianlin.granitoi.i.souther.Huna.Province

XU Man1), DUAN Xiaoxia1), CHEN Bin2), WANG Zhiqiang1), CHEN Yanjiao1), LI Xunfei1)

1) School of Resources and Environmental Engineering, Hefei University of Technology, Hefei, 230009; 2) Southern University of Science and Technology, Shenzhen, Guangdong, 518055

Objectives:This study targets on trace element in zircon as a breach to elaborate the petrogenesis and differentiation features of the Wangxianling granitoid in southern Hunan Province, Nanling Mountains, on the basis of field work and zircon U-Pb geochronology and Hf isotopes.

Methods:The microscope observation of rock sample was conducted based on field work with subsequent zircon U-Pb dating, trace element measurement and Nd—Hf isotopes analysis.

Results:The zircon U-Pb dating yielded the age of 237.8±3.5 Ma for coarse grained tourmaline—muscovite granite. The two-mica granite emplaced at 236.6±5 Ma and altered tourmaline granite age was constrained at 238.5±3.5 Ma. And theirεHf(t) values are confined as -11.8~-7.8、-17.8~-7.4、-16.1~-6.3, and εNd(t) values of -9.9~-11.1、-10.8~-11.1、-9.2~-10.8 respectively. Zircon trace element of the tourmaline—muscovite granite shows the REE reaches 1392×10-6~2184×10-6 with negative Eu anomaly(Eu/Eu*=0.10~0.18)and significant positive Ce anomaly(Ce/Ce*=6.64~68.79). Besides, it contain high P and Y contents ranging from 735×10-6~1390×10-6 and 1931×10-6~3024×10-6 respectively. The Ta and Th contents are confined between 8.4×10-6~21.7×10-6 and 461×10-6~2137×10-6 respectively. In comparison, two mica granite show lower LREE and more prominent negative Eu anomaly(Eu/Eu*=0.03~0.11) and weaker Ce anomaly(Ce/Ce*=1.54~7.75). The Y content is higher (3470×10-6~4698×10-6) and P content (1777×10-6~2653×10-6) are higher and Ta(6.20×10-6~17.26×10-6) and Th (133×10-6~360×10-6) contents are lower.

Conclusions:The Wangxianling granitoid is compounded with Late Triassic intrusions, namely the tourmaline—muscovite granite, the two-mica granite and altered tourmaline granite. TheirεHf(t) and εNd(t) values overlap with each other and they are product of a same magmatic event. Zircon trace element shows the increase of Ce/Sm—Yb/Gb ratios and low Y and P content of the tourmaline—muscovite granite which suggests there is MREE and Y-bearing mineral such as apatite fractionation. On the other hand, two-mica granite was depleted of LREE and Th relative to tourmaline—muscovite granite which indicates its melt experienced LREE and Th-enriched minerals such as allanite and monazite saturation and the high P content further imply that it’s allanite instead of P-enriched monazite. The low Nb, Ta content and more prominent negative Eu anomaly indicate rutile and feldspar fractionation. The tourmaline—muscovite granite and two-mica granite are contemporary and originated from the same source region, yet zircon trace element tells the distinct crystallization processes. This study suggests accessory mineral chemistry could effectively reveal differentiation and crystallization sequences of its host magma.

Keywords:zircon; trace element; fractional crystallization; Nd—Hf isotope; U-Pb dating; Wangxianling granitoid; southern Hunan

注:本文为国家自然科学基金重点资助项目(编号:41530206)和中国地质调查项目(编号:DD20160123)的成果。

收稿日期:2019-10-11;改回日期:2020-01-20;责任编辑:刘志强。Doi:10.16509/j.georeview.2020.03.010

作者简介:徐慢,男,1992年生,硕士研究生,岩石学专业,Email:18556506330@163.com。

通讯作者:段晓侠,女,1987年生,博士,讲师,主要从事岩浆热液矿床研究与教学工作,Email:duanxiaoxia@hfut.edu.cn。

Acknowledgements:This research was supported by National Science Foundation of China (Grant No. 41530206) and China Geological Survey Program (No. DD20160123)

Fris.author:XU Man, male, born in 1992, graduate student of petrology and geochemistry; Email: 18556506330@163.com

Correspondin.author:DUAN Xiaoxia, female, born in 1988, work on magmatic rock petrogenesis and the magmatic—hydrothermal mineralization; Email: Duanxiaoxia@hfut.edu.cn

Manuscript received on: 2019-10-11; Acceptedon: 2020-01-20; Editedby:LIU Zhiqiang

Doi: 10.16509/j.georeview.2020.03.010