岩石学报  2016, Vol. 32 Issue (9): 2793-2806   PDF    
大兴安岭中段晚三叠世四分组效应花岗岩的厘定及其地质意义
李世超1, 李永飞2, 王兴安3, 朱凯1     
1. 吉林大学地球科学学院, 长春 130061 ;
2. 中国地质调查局沈阳地质调查中心, 沈阳 110034 ;
3. 东北师范大学地理科学学院, 长春 130024
摘要: 布敦化岩体位于中亚造山带东段大兴安岭地区的科尔沁右翼中旗,该岩体由中细粒二云母花岗岩和斑状中细粒白云母花岗岩组成。LA-MC-ICP-MS锆石U-Pb同位素年龄测试显示布敦化岩体形成于晚三叠世早期。地球化学分析表明,该岩体为弱过铝质-过铝质、高钾钙碱性系列,主量元素表现出高硅(73.73%~78.33%)、富碱(Na2O+K2O=7.07%~9.06%)、低钙(CaO=0.01%~0.87%)、贫镁(MgO=0.12%~0.52%)的特征;稀土元素具有明显的Eu负异常和M型四分组效应配分型式。岩石学和地球化学特征显示布敦化岩体为高分异I型花岗岩。Lu-Hf同位素研究显示其εHft)值为7.0~12.4,二阶段Hf模式年龄(tDM2)为812~473 Ma。结合邻区晚三叠世地质资料可知,布敦化岩体是在古亚洲洋闭合造山后伸展的构造背景下产生的,是晚元古代-早古生代地壳物质熔融的产物。布敦化地区的四分组效应花岗岩的稀土元素总量高、岩浆侵位后经历了强烈的分异演化,暗示布敦化地区可能具有较好的稀有金属矿的找矿前景。
关键词: 布敦化     四分组效应     稀有金属矿     晚三叠世     大兴安岭    
Delineation of the Late Triassic granitic pluton from the middle part of Greater Xing'an Mountains showing tetrad REE patterns and its geological implications
LI ShiChao1, LI YongFei2, WANG XingAn3, ZHU Kai1     
1. College of Earth Sciences, Jilin University, Changchun 130061, China ;
2. Shenyang Center of Geological Survey, CGS, Shenyang 110034, China ;
3. College of geographical Sciences, Northeast Normal University, Changchun 130024, China
Abstract: The Budunhua pluton is located in the Horqin Right Wing Middle Banner, eastern part of the Central Asia Orogenic Belt. The pluton is mainly composed of medium-fine grained two-mica granites and porphyritic fine-grained muscovite granites. The LA-MC-ICP-MS U-Pb dating of the zircons indicate that the Budunhua pluton was emplaced in the Late Triassic. The rocks have high SiO2 (73.73%~78.33%) and (Na2O+K2O) (7.07%~9.06%) contents, poor CaO (0.01%~0.87%) and MgO (0.12%~0.52%) contents, A/CNK=1.02~1.23, belong to high-K calcalkaline series. The petrogeochemistry of rare earth element is characterized by strong depletion of Eu, and showing an M-type tetrad pattern. These characteristics indicate that the Budunhua pluton belongs to highly fractionated I-type granites. Zircon εHf(t) values for the Budunhua pluton range from 7.0 to 12.4, two-stage model ages (tDM2) ranging from 812 Ma to 473 Ma. Combining with regional geologic data, the primary magma of Budunhua pluton was derived from partial melting of juvenile crustal material from Neoproterozoic to Early Paleozoic, and formed in the extensional tectonic environment after the closure of the Paleo-Asian Ocean. The tetrad effect granite in Budunhua area with high ΣREE and evolution occurred after the intrusion of the primary magma, suggesting that the granite have prospects of REE mineralization.
Key words: Budunhua     Tetrad effect     Rare metal minerals     Late Triassic     Greater Xing'an Mountains    
1 引言

中亚造山带东段——中国东北地区显生宙花岗岩广泛发育,是该区地壳的重要组成部分,不同时代、不同类型的花岗岩为我们研究中亚造山带东段的地壳组成、生长以及演化过程提供了丰富的研究素材(吴福元等, 1999; 孙德有等, 2004a, b; Wu et al., 2011)。近年来大量的高精度同位素年龄资料表明该区花岗岩的形成时代可分为前中生代和晚中生代两个主要阶段,二者以三叠纪为界,被理解为分别对应古亚洲洋构造域和太平洋构造域的演化阶段(Wu et al., 2011; Xu et al., 2013; 许文良, 2013),并归纳了古亚洲洋于晚古生代自西向东剪刀式闭合(Miao et al., 2007, 2008; Zhang et al., 2008; 孙德有等, 2004a; 王玉净和樊志勇, 1997; 李锦轶等, 2007)和太平洋从侏罗纪开始自东向西俯冲(朱光等, 2004)的演化模式。介于以上两个构造域之间的三叠纪的构造属性则尚有争论。

布敦化岩体位于内蒙古自治区兴安盟科尔沁右翼中旗(HRWMB)西部,地处大兴安岭隆起与松辽盆地之间的大兴安岭中南段东坡,分布面积数百平方千米,由于缺乏精确同位素数据,布敦化岩体成岩年龄一直被认为是晚侏罗世末-早白垩世初(吉林省地质局, 1974),然而高精度锆石U-Pb同位素测试显示,其形成时代为晚三叠世。本文通过对布敦化岩体进一步的野外地质调查、岩石学、地球化学以及Lu-Hf同位素研究,为中亚造山带东段三叠纪的地质演化提供新的证据。

①吉林省地质局. 1974. 1:20万科尔沁右翼中旗幅区域地质调查报告

2 区域地质概况和岩体特征

布敦化岩体位于世界上最大的显生宙造山带——中亚造山带的东段,夹持于贺根山-黑河断裂带和西拉木伦河断裂带之间,岩体侵入到晚古生界地层和二叠纪岩体之中,被晚中生代火山岩所覆盖(图 1)。该区前中生代地质演化与古亚洲洋的开合息息相关,中生代以来岩浆活动频繁,与之相关的岩浆热液型金属矿产广泛分布于该区(毛景文等, 2005)。


图 1 布敦化岩体所在位置(a)及地质简图(b) Fig. 1 Location (a) and simplified geological map (b) of the Budunhua pluton

布敦化岩体的岩性较为单一,主体岩性为中细粒二云母花岗岩(图 2a)和斑状中细粒白云母花岗岩(图 2b)。中细粒二云母花岗岩为中细粒花岗结构,块状构造,矿物组成主要为石英,粒度1~3 mm,含量30%左右;正长石,粒度1~3 mm,含量30%~35%,个别发生高岭土化而表面浑浊;斜长石,粒度1~4 mm,含量30%左右;黑云母,多褐铁矿化,含量不足5%;白云母,含量5%左右,分布不均匀,个别样品中未见(图 2c)。斑状白云母花岗岩具似斑状结构,块状构造。斑晶为正长石,粒度4~6 mm,含量5%~10%。基质中细粒结构,矿物组成主要为石英,粒度2 mm左右,含量25%~30%左右;正长石,1~2 mm,含量30%~35%左右;斜长石,粒度1~2 mm,含量25%~30%左右;白云母,含量5%左右,分布不均匀(图 2d)。


图 2 布敦化岩体野外(a、b)及显微镜下照片(c、d) (a、c)中细粒二云母花岗岩;(b、d)斑状中细粒白云母花岗岩 Fig. 2 Macro-characteristics (a, b) and microphotographs (c, d) for Budunhua pluton (a, c) two-mica granite; (b, d) biotite granite
3 分析方法 3.1 主量和微量元素分析

为了具有代表性,本文选取不同地点的新鲜、无蚀变的样品进行主微量元素分析,采样位置见图 1。分析测试在国土资源部沈阳地矿所实验室完成。主量元素分析采用X射线荧光光谱仪(PW4400)测定,测试精度优于2%,微量元素分析利用X-Series型电感耦合等离子体质谱仪(ICP-MS)完成测定,测试精度优于5%。

3.2 锆石U-Pb定年和Hf同位素分析 3.2.1 锆石LA-ICP-MS U-Pb定年

样品的粉碎、锆石挑选由河北省廊坊市区域地质调查研究所完成。先将样品粉碎至80~100目之后经过淘洗和电磁方法进行分离,并在双目镜下将具有较好晶形且无明显包裹体和裂痕的锆石挑选出来。锆石的制靶、透射光、反射光和阴极发光(CL)图像的采集以及U-Pb同位素测试在西北大学大陆动力学实验室完成,方法如下:应用193nm的ComPex 102型ArF准分子激光器和Agilent 7500a型ICP-MS仪器,实验中采用He气作为剥蚀物质的载气,激光频率为6Hz,激光强度为50 mJ,激光斑束为30 μm,使用人工合成硅酸盐玻璃标准物质NIST 610进行仪器仪器状态调整参考,使用国际标准锆石91500作为同位素组成的外标,用Andersen的方法(Andersen and Griffin, 2004)进行同位素比值校正,以扣除普通Pb的影响。锆石测年中Pb同位素比值、U-Pb表面年龄和微量元素含量运用ICPMS DataCal程序(Liu et al., 2010)进行处理。年龄计算及成图采用Isoplot(Ludwig, 2001),测试数据、加权平均年龄的误差均为1σ。锆石年龄较为年轻( < 1000 Ma)因此采用206Pb/238U年龄值。

3.2.2 锆石Hf同位素分析

在锆石U-Pb定年的基础上,选择协和度较好的年龄点,分析点与U-Pb定年分析点为同一位置。锆石Hf同位素测试是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Neptune多接收等离子质谱和Newwave UP213紫外激光剥蚀系统(LA-MC-ICP-MS)上进行的,实验过程中采用He作为剥蚀物质载气,根据锆石大小,剥蚀直径采用55 μm或40 μm,测定时使用锆石国际标样GJ1和Plesovice作为参考物质。相关仪器运行条件及详细分析流程见侯可军等(2007)。分析过程中锆石标准GJ1的176Hf/177Hf测试加权平均值为0.282007784±0.0000116639(2σ, n=18),与文献报道(侯可军等, 2007; Morel et al., 2008)值在误差范围内完全一致。εHf(t)计算采用的176Lu衰变常数为1.865×10-11a-1(Scherer et al., 2001),球粒陨石现今的176Hf/177Hf=0.282772,176Lu/177Hf=0.0332。亏损地幔Hf模式年龄(tDM1)计算采用的现今亏损地幔176Hf/177Hf=0.28325和176Lu/177Hf=0.0384进行计算;二阶段Hf模式年龄(tDM2)采用平均大陆壳176Lu/177Hf=0.015进行计算(Griffin et al., 2000)。

4 分析结果 4.1 主量和微量元素 4.1.1 主量元素

布敦化岩体的SiO2含量较高为66.62%~78.63%(表 1),在TAS图解(图 3a)中样品投影在花岗岩区间中,Al2O3含量为11.00%~16.43%,K2O含量为3.92%~5.76%,Na2O含量为2.64%~4.87%,CaO含量为0.01%~0.87%,全碱(Na2O+K2O)为7.07%~9.06%,K2O/Na2O为0.83~2.18,A/CNK值为1.02~1.26,为弱过铝质-过铝质岩石。由SiO2-K2O图解(图 3b)中,花岗岩主要为高钾钙碱性系列(图 3b)。



表 1 布敦化花岗岩的主量元(wt%)、稀土和微量元素(×10-6) Table 1 Major (wt%) and (×10-6) trance elements compositions for granites of Budunhua area

图 3 布敦化岩体TAS图解(a, 据Middlemost, 1994)和SiO2-K2O图(b, 据Maniar and Piccoli, 1989) Fig. 3 TAS diagram (a, after Middlemost, 1994) and SiO2 vs. K2O diagram (b, Maniar and Piccoli, 1989) for Budunhua granite
4.1.2 微量元素

由布敦化岩体的球粒陨石标准化稀土元素配分图解(图 4a)可知,该期花岗岩具有相对平坦的稀土配分型式和强烈的Eu负异常(δEu=0.06~0.39)。其稀土元素含量较高(∑REE=83×10-6~312×10-6),轻重稀土元素基本无分异,(La/Yb)N值为0.50~4.86,LREE/HREE比值为0.22~2.85。在原始地幔标准化微量元素蛛网图(图 4b)中,该期花岗岩相对富集高场强元素U、Pb,强烈亏损大离子亲石元素Ba、Sr和高场强元素Ti、Nb(表 1)。


图 4 布敦化岩体球粒陨石标准化稀土元素配分图解(a, 标准化值据Boynton, 1984)和原始地幔标准化微量元素蛛网图(b, 标准化值据Sun and McDonough, 1989) Fig. 4 Chondrite-normalized REE patterns (a, normalization values after Boynton, 1984) and primitive mantle-normalized trace element spidergrams (b, normalization values after Sun and McDonough, 1989) of the Budunhua granite
4.2 锆石U-Pb年龄和Hf同位素特征 4.2.1 测年结果

布敦化岩体中细粒二云母花岗岩的LA-ICP-MS U-Pb同位素定年结果见表 2,锆石阴极发光(CL)图像见图 5。样品D1019-2-2和D1019-7-2中的锆石多呈自形-半自形,个别发生机械破碎呈不规则状。锆石普遍呈浅灰-暗灰色,内部见有清晰的岩浆振荡环带,个别锆石颜色较深内部结构不清晰,但仍可识别出岩浆振荡环带,其Th/U值分别为0.14~0.69(平均0.32)和0.25~0.62(平均0.40),表明锆石为岩浆成因(Koschek, 1993)。锆石U-Pb协和图显示,大部分数据都投影在协和线上(图 6),其中样品D1019-2-2的23颗锆石206Pb/238U年龄为226~230 Ma(1σ),加权平均年龄为228±0.3 Ma(MSWD=0.052);样品D1019-7-2的12颗锆石206Pb/238U年龄为234~239 Ma(1σ),加权平均年龄为236±0.4 Ma(MSWD=2.6)。综上所述,可将布敦化岩体的侵位时代置于晚三叠世。



表 2 锆石U-Pb同位素LA-ICP-MS测试结果 Table 2 Results of LA-MC-ICP-MS zircon U-Pb age dating for granites of Budunhua area

图 5 布敦化花岗岩的锆石CL图像 Fig. 5 CL images of selected zircons from Budunhua pluton

图 6 布敦化岩体锆石LA-ICP-MS U-Pb年龄协和图 Fig. 6 Zircon U-Pb concordia diagrams from Budunhua pluton
4.2.2 Hf同位素特征

对2件样品(D1019-2-2、D1019-7-2)中的年龄测试结果较好且具有代表性的26颗锆石进行了Hf同位素分析(表 3),样品分析点176Lu/177Hf的比值的均值为0.002,表明由176Lu衰变而形成放射性成因的Hf极少,因此可用176Hf/177Hf的值代表锆石形成时的176Hf/177Hf的值(吴福元等, 2007)。样品中的26个分析点的年龄集中(239~226 Ma),其176Hf/177Hf为0.282833~0.282988,εHf(t)值分布范围为+7.03~+12.4,对应的亏损地幔Hf模式年龄tDM变化范围为617~392 Ma,二阶段Hf模式年龄tDM2为812~473 Ma(表 3图 7),表明其源岩可能是新元古代-早古生代新增生的地壳物质。



表 3 布敦化岩体的Lu-Hf同位素分析数据 Table 3 LA-MC-ICP-MS Lu-Hf isotope data of zircon for granites of Budunhua area

图 7 布敦化岩体锆石εHf(t)-207Pb/206Pb年龄图解(a)和Hf同位素二阶段模式年龄图谱(b) 地壳演化趋势线中176Lu/177Hf为0.011(after Wedepohl, 1995).锆石εHf(t)的范围参考自Yang et al. (2006) Fig. 7 Plot of εHf(t) vs. 207Pb/206Pb age of the analyzed zircons (a) and two-stage zircon-Hf model age spectra (b) of the samples from the Budunhua granite The crustal evolution path assuming a crustal 176Lu/177Hf ratio of 0.011 (after Wedepohl, 1995). The zircon εHf(t) fields of magmatic rocks are after Yang et al. (2006)
5 讨论 5.1 成因类型

布敦化岩体石英含量高、不具有碱性暗色矿物,主量元素SiO2含量(介于73.73%~78.63%之间,均值75.75%)、全碱(K2O+Na2O)含量(介于7.06%~9.06%之间,均值8.20%)均较高、微量元素富Rb、Th、U、Pb,贫Ba、Sr、P、Ti,具有显著的Eu负异常(δEu均值0.17),这些特征表明岩浆经历了高程度的分异演化。高场强元素Zr、Nb、Y的含量低,Ga/Al比值小于2.6(Whalen et al., 1987),锆石饱和温度(表 1TZr)均值为779 ℃与I型花岗岩的平均值781 ℃相似(King et al., 1997)。以上信息均显示出布敦化岩体为高分异的I或S型而非A型花岗岩(图 8)。


图 8 A型花岗岩判别图解(底图据Whalen et al., 1987) Fig. 8 Discrimination diagrams for A-type granitoids (after Whalen et al., 1987)

尽管布敦化岩体在岩相学上含有白云母和黑云母,但白云母并非是判别S型花岗岩的有效标志(Miller, 1985; 吴福元等, 2007)。在地球化学方面,布敦化岩体的A/CNK值为1.02~1.26,为过铝质岩石,但大量斜长石、普通角闪石和黑云母的分馏作用也可以导致其成分向过铝质转变(Zen, 1986),形成富铝矿物。而钙、钠含量过低也可能使得A/CNK值过高(White and Chappell, 1988)。布敦化岩体的Na2O含量的均值高达3.56%,这通常也被认为是I型花岗岩的重要标志(Chappell and White, 1974)。另外,I型和S型花岗岩中P2O5随着SiO2含量变化的变化趋势二者皆然不同(Chappell, 1999)。其中I型花岗岩中,P2O5含量随SiO2含量增加而降低,而S型花岗岩则相反或保持不变。同时,I型花岗岩的Th和Y含量高,且与Rb之间呈正相关演化关系,而S花岗岩的Th和Y含量低,与Rb之间呈负相关演化或没有变化。由布敦化花岗岩的相关图解(图 8)可见,布敦化花岗岩应该属于高分异的I型花岗岩。

布敦化岩体的稀土元素总量较高,轻重稀土元素则无分异,具有平坦的稀土配分型式和十分强烈的负Eu异常(δEu值0.06~0.39平均0.17),这与一般的花岗岩较强的轻重稀土分异和右倾的稀土配分曲线具有较大的差别,是以Nd或Pm、Gd、Ho或Er为分界点,四个元素为一组分为La-Nd、Pm-Gd、Gd-Ho、Er-Lu四组,每一组形成一个平滑的凸起或凹陷的M型配分型式,具有显著的四分组效应稀土配分型式。这种稀土元素配分型式在正常花岗岩中十分少见,往往在经历了强烈的热液蚀变的高分异I型花岗岩中出现(Jahn et al., 2001; Monecke et al., 2002; 王兴安, 2014)。在判别四分组效应关键元素比值特征图(图 9)中,布敦化岩体的具有典型的四分组效应花岗岩特征,其K/Rb值在受四分组效应后明显低于正常花岗岩,相反,其K/Ba值则高于正常花岗岩一个数量级;其Nb/Ta值和Zr/Hf值则明显的低于正常岩浆岩的范围。综上,布敦化岩体为M型四分组效应花岗岩(Fidelis and Siekierski, 1966; Peppard et al., 1969; Masuda and Ikeuchi, 1979; Masuda et al., 1987; Masuda and Akagi, 1989; Kawabe et al., 1991),是受到热液流体交代作用的高分异I型花岗岩。


图 9 四分组效应的关键元素比值特征图(据Wu et al., 2004) 其中TE1, 3值据Irber(1999)计算;○-四分组效应花岗岩(据Wu et al., 2004);□-非四分组效应花岗岩(据Jahn et al., 2001; Wu et al., 2004);●-布敦化岩体 Fig. 9 Variation of key elemental ratios as a function of the tetrad effect (after Wu et al., 2004) TE1, 3 of Irber, 1999; ○-tetrad effect granite (Wu et al., 2004); □-normal granite (Jahn et al., 2001; Wu et al., 2004); ●-Budunhua granite
5.2 构造环境

花岗岩的地球化学性质取决于源岩的矿物组成和化学性质、熔融时的物理化学条件和其后的岩浆演化过程,在某种程度上可以通过大量总结特定构造背景下产生的花岗岩的地球化学性质来进行反演。但四分组效应花岗岩往往是受到热液流体交代作用而产生的(Fidelis and Siekierski, 1966; Kawabe et al., 1991; 赵振华等, 1999),其地球化学性质和行为已经改变,多数经验型构造环境判别图解已经失效。因此我们只能通过同时代的其他地质证据推断其形成时代的构造背景。

近年来,中晚三叠世花岗岩在中亚造山带东段陆续被发现, 其中松嫩-张广才岭-小兴安岭地区的清水岩体(孙德有等, 2004b),毛家屯岩体、密林岩体(Wu et al., 2002),以及红石砬子岩体、大王折子岩体(黑龙江省地质矿产局, 1993)均为A型花岗岩; 兴安地块上的查干岩体(葛文春等, 2005; 周漪等, 2011),河中岩体、扎兰屯岩体、蘑菇气岩体、大石寨岩体(李世超, 2012),塔尔气岩体、查巴奇岩体、库提河岩体多以I型花岗岩为主(Yang et al., 2016)。尽管这些花岗岩的岩石类型不同,但均产生于造山后伸展的环境。

布敦化岩体在大地构造位置上处于中亚造山带东段、大兴安岭的中南段,在二叠纪末-三叠纪初期,古亚洲洋在研究区南部的西拉木伦河-长春-延吉一线经历了由西向东的剪刀式闭合(Chen et al., 2000; Miao et al., 2007, 2008; Wu et al., 2007; 孙德有等, 2004a; 王玉净和樊志勇, 1997; 李锦轶等, 2007; Li et al., 2015)。在三叠纪中-晚期研究区进入了造山后伸展的阶段,从时间上看,与古亚洲洋闭合的主造山时期相差20~30Myr;从岩性上看研究区及相邻地区晚三叠世花岗岩主要为高钾钙碱性系列花岗岩,岩石类型多以正长花岗岩、二长花岗岩为主,岩石成因类型均为I型和A型,是造山后常见的后碰撞花岗岩的典型岩石系列与类型,花岗岩类型的多样化也是后碰撞花岗岩的一个重要特征,这也恰恰反映了造山后花岗岩的来源及成因的复杂性。另外,后碰撞花岗岩作为造山期汇聚作用结束、区域上挤压应力卸载的重要标志,由于造山后阶段地壳伸展、走滑、旋转、差异沉降等所产生或复活的深大断裂,均可为岩浆提供了上升通道,所以其产出应该是区域上广泛分布的,很少受到原有构造格局的影响。研究区及邻区晚三叠世花岗岩的分布均摆脱了晚古生代构造格局的限制,广泛分布于兴安地块、松嫩地块。因此,我们认为布敦化岩体及其邻区的晚三叠世花岗岩是在古亚洲洋西拉木伦河-长春缝合带闭合造山后伸展的构造背景下产生的,此阶段由于应力的释放,加厚的岩石圈开始伸展垮塌,减压导致地幔上升,引发强烈的壳幔作用使新元古代-早古生代新增生的地壳物质熔融,形成长英质岩浆活动(Liégeois, 1998)。

5.3 成矿性讨论

花岗岩稀土元素四分组效应是稀土元素地球化学研究领域的前沿问题,研究表明高程度演化的花岗质熔体与富含F、Cl等挥发分的流体相互作用是花岗岩稀土元素四分组效应形成的重要因素,这种流体-熔体间的相互作用使得熔体“整体”具有M型稀土元素四分组效应,并导致流体-熔体共存体系中稀有元素的极端富集,这使得造岩矿物和副矿物均继承了这一特点(赵振华等, 1999)。因此,稀土元素四分组效应花岗岩是寻找超大型稀有金属矿床的重要线索,与之相关的矿产多为稀土矿(丘志力等, 2014)、锡多金属(邓希光等, 2005)、锑矿(彭建堂等, 2005),也有铀矿(张展适等, 2005)和钨矿(Liu et al., 2007)。

一般认为基岩中稀土丰度大于150×10-6即可形成稀土矿化, 更有江西上犹岩体、湖南益将岩体稀土丰度低至120×10-6成矿(苏晓云等, 2014)、沃溪40×10-6成矿(彭建堂等, 2005)的实例,而布敦化岩体的平均稀土总量均值为232×10-6(表 1),具有更好的成矿潜力。杨武斌等(2011)通过对巴尔哲地区四分组效应花岗岩801岩体和802岩体的研究发现,岩浆侵位后的分异演化决定了稀土元素的成矿作用,分异演化使得岩浆-热液过渡阶段熔体中富集足够多的稀土成矿元素,使之能够形成独立的稀土矿物,并最终形成矿床。Eu-Sr、Rb/Sr-1/Sr协变图解显示,布敦化花岗岩样品的投影点基本呈线性排列,指示岩浆在侵位后经历了强烈的分异演化作用,并表现出受长石结晶作用控制的趋势(图 10)。因此我们认为布敦化地区的四分组效应花岗岩无论从稀土元素的含量还是从成矿理论基础上看均具有一定的找矿前景,但成矿规模和找矿方向尚需进一步勘查工作的开展。


图 10 布敦化花岗岩元素协变图 Fig. 10 Variation diagrams for Budunhua granite
6 结论

(1) LA-MC-ICP-MS锆石U-Pb同位素年龄测试结果表明,布敦化岩体形成于228~236 Ma的晚三叠世早期。

(2) 该岩体为准铝质-过铝质、高钾钙碱性系列,主量元素高硅、富碱、低钙、贫镁;稀土配分型式平坦及强烈Eu负异常,具有明显的M型四分组效应配分型式。岩石学和地球化学特征显示布敦化岩体为高分异I型花岗岩。

(3) 结合Lu-Hf同位素测试及邻区晚三叠世地质资料可知,布敦化岩体是古亚洲洋闭合造山后伸展的构造背景下由新元古代-早古生代新增生地壳物质熔融的产物。

(4) 稀土元素四分组效应花岗岩是寻找稀有金属矿床的重要线索,布敦化地区的四分组效应花岗岩稀土元素的含量高、岩浆侵位后经历了强烈的分异演化,指示了该岩体具有一定的找矿前景。

值此杨振升教授八十五华诞之际,谨以此文献给先生,祝先生幸福安康。杨先生治学严谨、为人师表,衷心感谢先生的谆谆教诲和无私帮助。

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