Zircon U-Pb Age and Petrochemical Characteristics of Shimian Granite in Western Sichuan: Petrogenesis and Tectonic Significance
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摘要: 扬子西缘新元古代岩浆岩分布广泛, 目前对其成因和构造背景还存在很大争议.对扬子西缘康滇裂谷北段石棉花岗岩体进行了系统的SHRIMP锆石U-Pb年龄、岩石学和元素-Nd同位素地球化学研究, 结果表明该岩体是弱铝质的高钾钙碱性Ⅰ型花岗岩, 形成于818±7 Ma, 是由前存年轻(中元古代末-新元古代初)岛弧地壳物质部分熔融形成的, 并混染了少量古老地壳物质.石棉花岗岩形成于扬子地块西缘由会聚挤压向陆内伸展的转折时期, 其"岛弧地球化学特征"是继承了源岩的地球化学特征的结果, 不代表其形成时的构造环境.Abstract: Neoproterozoic magmatic rocks are widespread in western margin of the Yangtze block, and their genesis and tectonic setting have been an issue in hot debate at the present time. SHRIMP U-Pb zircon age, petrology, geochemical and Nd isotopic data are reported for the Neoproterozoic Shimian granite in the Kangdian rift of western Sichuan. This pluton is of metaluminous high-K calc-alkalic I-type granite and emplaced at 818±7 Ma. Petrology, geochemical and Nd isotopic characters suggest that the pluton was generated by partial melting of pre-existing, young (Late Mesoproterozoic to Early Neoproterozoic) island arc crust, with contamination of old crust materials during magma ascending and emplacement. The Shimian granite is the product of the tectonic transition from compression to introplate extension in western margin of the Yangtze block. Their arc-like geochemical features (such as Nb-Ta depletion) should have been inherited from the protoliths, rather than reflection of their tectonic setting when the pluton formed.
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Key words:
- granite /
- Kangdian rift /
- Neoproterozoic /
- tectonic setting /
- geochemistry /
- geochronology
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0. 引言
扬子地块西缘新元古代岩浆活动非常强烈,广泛分布新元古代岩浆岩,基本上沿康滇裂谷连续分布,包括花岗岩、花岗闪长岩、英云闪长岩、基性-超基性小侵入体和基性岩墙、岩脉等.目前对这些岩浆岩的成因和构造背景存在两种截然不同的观点:一种观点认为扬子地块西缘新元古代早期(>900 Ma)岩浆岩形成于和Rodinia超级大陆聚合有关的岛弧造山运动,而新元古代中期(860~740 Ma)的岩浆岩为板内非造山成因,并与导致Rodinia超级大陆裂解的地幔柱-超级地幔柱活动有关(Li et al., 2002, 2003a, 2003b, 2003c, 2006; Zhu et al., 2004, 2008; Lin et al., 2007; Huang et al., 2008;林广春,2008);持这种观点的研究人员支持华南大陆是连接北美和澳大利亚-南极大陆之间的桥梁和Rodinia超级大陆的“核心”(Li et al., 1995, 2008).另一种观点则认为扬子地块西缘新元古代岩浆岩是与造山运动有关的大陆边缘岩浆弧,俯冲造山运动可能持续到0.74 Ga或更晚(Zhou et al., 2002a, 2002b, 2006a, 2006b, 2007; Zheng, 2004;杜利林等, 2005, 2006, 2007; Zhao and Zhou, 2007a, 2007b; Zhao et al., 2008).一些研究人员认为华南大陆很可能位于Rodinia超级大陆的边缘(Zhou et al., 2006a, 2006b)或根本不属于Rodinia超级大陆的一部分(Zhou et al., 2002b).显然,扬子西缘新元古代岩浆岩的成因和构造背景研究,对认识华南板块在新元古代时期的构造演化及Rodinia超级大陆重建研究都将产生重要的影响.
本文选取扬子西缘康滇裂谷北段的石棉花岗岩体,报道其SHRIMP锆石U-Pb年龄和元素-Nd同位素地球化学组成,并探讨其岩石成因与构造意义.
1. 地质背景
石棉花岗岩体又称大相岭或黄草山岩体,为一较大的花岗质岩基(约794 km2),岩性主要为中粗粒斑状钾长-二长花岗岩,矿物组成以钾长石、石英、斜长石为主,含少量角闪石、黑云母,副矿物有磁铁矿、磷灰石等.钾长石主要为条纹长石,含量50%左右,宽板状或他形粒状;石英含量25%~30%,呈他形粒状,具波状消光;斜长石含量10%左右,自形-半自形板状,主要为奥长石,可见聚片双晶,已发生绢云母化;角闪石含量5%左右,他形粒状,多已发生绿泥石化和绿帘石化;黑云母含量小于5%,呈细小片状定向分布.岩体普遍含自形的钾长石巨晶,最大直径约1~2 cm,呈斑状结构,岩石具弱片麻状构造.石棉花岗岩体侵入东侧的新元古代苏雄组火山岩,并进一步被东西两侧的古生代-中生代地层所覆盖(图 1).岩体内常见同时代的后期侵入的基性岩墙、岩脉(Li et al., 2003c; Lin et al., 2007),尤其是在岩体边缘部分.
图 1 川西泸定-石棉地区前寒武纪地质略图右上角插图为华南新元古代裂谷系构造简图(Li et al., 1999)Fig. 1. Simplified Precambrian geological map of the Luding-Shimian region, western Sichuan本文对采自岩体的10个样品进行元素地球化学和同位素分析,SHRIMP年龄样品04KD23-1采自石棉县城南约10 km的公路旁.
2. 分析方法
在双目镜下挑选晶型完好、具有代表性的锆石颗粒和标准锆石TEM(TEM的206Pb/238U=0.066 8,相应的年龄为417 Ma,Black et al., 2003) 粘贴在环氧树脂表面,然后抛光并镀金.在SHRIMP同位素分析前,对待测锆石进行了透射光和反射光显微照相以及阴极发光图像分析,以检查锆石的内部结构,从而帮助选定最佳的待测锆石部位和数据解释.锆石的U-Th-Pb同位素分析在地质科学院离子探针中心的SHRIMP-Ⅱ离子探针上进行.
主量元素和微量元素分别在中国科学院广州地球化学研究所的Rigaku RIX 2000型荧光光谱仪(XRF)和Perkin-Elmer Sciex ELAN 6000型电感耦合等离子体-质谱仪(ICP-MS)上分析,XRF主量元素分析方法与Goto and Tatsumi(1994)报道的相似,分析精度优于5%,ICP-MS微量元素分析流程见刘颖等(1996),分析精度优于3%.Sm-Nd的化学分离采用常规的阳离子树脂(REE和其他元素分离)和HDEHP(Sm、Nd分离)方法,Nd同位素组成测试在广州地球化学研究所的Micromass Isoprobe型MC-ICPMS上进行,详细的分析程序参见梁细荣等(2003).143Nd/144Nd比值用146Nd/144Nd=0.721 9校正,测得的143Nd/144Nd比值校正到Shin Etsu JNdi-1标准的0.512 115(Tanaka et al., 2000).Nd同位素分析的全流程本底 < 100 pg.
3. 结果
3.1 SHRIMP锆石U-Pb年龄
对样品04KD23-1(29°06′41″N,102°20′29″E)的17个锆石颗粒进行了17个分析点的U-Pb同位素年龄分析,分析结果列于表 1,锆石U-Pb谐和图解见图 2a.所分析的锆石颗粒大多数是破碎晶,少数为透明的自形晶体,锆石颗粒大小差异明显(图 2b).阴极发光图像显示大多数锆石具有继承核,边部发育完好的环带结构.1号分析点的U-Pb年龄稍微偏年轻,可能有Pb的丢失;4号分析点的U-Pb年龄稍微偏老,可能是因为继承锆石的影响.其余15个分析点在误差范围内有一致的U-Pb同位素组成,其206Pb/238U年龄的加权平均值为818±7 Ma(2σ),代表了石棉花岗岩体的结晶年龄.与石棉岩体已有的年龄结果在误差范围内一致(~800 Ma,Zhao et al., 2008;786±36 Ma,沈渭洲等,2000).
表 1 川西石棉花岗岩SHRIMP锆石U-Pb同位素分析结果Table Supplementary Table SHRIMP U-Pb isotopic data for zircons from Shimian granite in western Sichuan点号 Th(μg/g) U(μg/g) Th/U 206Pb*(%) 207Pb/206Pb±1σ 206Pb/238U±1σ 207Pb/235U±1σ 206Pb/238U±1σ(Ma) 207Pb/206Pb±1σ(Ma) 1 173 219 0.81 0.17 0.069 49±0.000 67 0.124 7±0.000 7 1.172±0.014 757±4 872±22 2 69 99 0.72 0.63 0.071 08±0.000 93 0.139 0±0.001 1 1.261±0.036 838±6 801±57 3 57 113 0.52 0.99 0.071 76±0.000 85 0.135 3±0.001 0 1.186±0.037 817±6 727±64 4 211 203 1.07 0.76 0.072 86±0.000 76 0.142 5±0.000 9 1.308±0.027 858±5 824±41 5 160 264 0.63 0.52 0.067 65±0.000 55 0.136 8±0.000 8 1.194±0.019 826±4 719±32 6 92 161 0.59 0.80 0.070 25±0.000 72 0.136 6±0.000 9 1.197±0.029 825±5 727±50 7 92 143 0.67 0.91 0.070 10±0.000 76 0.138 7±0.000 9 1.196±0.034 836±5 693±58 8 147 317 0.48 0.35 0.067 79±0.000 49 0.136 9±0.000 7 1.226±0.017 827±4 772±28 9 69 119 0.60 1.15 0.071 60±0.001 40 0.134 6±0.001 0 1.150±0.042 813±5 673±77 10 77 126 0.63 0.73 0.070 48±0.000 81 0.135 2±0.000 9 1.202±0.029 817±5 756±50 11 174 210 0.85 0.51 0.070 54±0.000 78 0.137 6±0.000 8 1.259±0.023 831±5 817±37 12 58 119 0.50 0.40 0.070 00±0.001 20 0.134 1±0.001 0 1.234±0.034 811±6 829±56 13 51 96 0.55 0.46 0.073 00±0.001 50 0.132 8±0.001 0 1.266±0.037 803±6 904±59 14 143 198 0.75 0.12 0.068 81±0.000 64 0.133 8±0.000 8 1.250±0.017 809±4 863±35 15 54 92 0.60 0.76 0.074 40±0.001 50 0.136 1±0.001 0 1.279±0.041 822±6 873±64 16 115 182 0.66 0.39 0.071 10±0.001 20 0.131 1±0.000 8 1.227±0.028 794±5 865±45 17 148 162 0.94 0.48 0.071 49±0.000 69 0.132 5±0.000 8 1.233±0.021 801±5 854±34 注:206Pb*表示普通206Pb占总206Pb的百分比;采用204Pb校正方法计算年龄. 3.2 主量和微量元素地球化学
石棉花岗岩体10个样品主量和微量元素分析结果见表 2.岩体SiO2含量较高(71.02%~77.83%),富K2O,K2O/Na2O为1.10~2.35,A/CNK= 0.98~1.14,为弱过铝质.岩石样品属于高钾钙碱性系列(图 3a),在TAS分类图解上(Middlemost, 1994),样品落入花岗岩范围(图 3b).在以SiO2含量为横坐标的Harker图解上(图 4),样品的Al2O3、Fe2O3、MgO、TiO2、CaO、P2O5和Sr、Eu、Zr、V均与SiO2呈良好的负相关关系,表明随岩浆演化,可能存在斜长石、角闪石、Fe-Ti氧化物和磷灰石等的分离结晶.
表 2 川西石棉花岗岩主量元素(%)和微量元素(μg/g)分析结果Table Supplementary Table Major (%) and trace element (μg/g) analyses of Shimian granite in western Sichuan样号 04KD20-6 04KD20-10 04KD21-4 04KD23-1 04KD23-4 04KD24-2 98KD71 98KD72 98KD74 98KD86 主量元素(%) SiO2 77.83 77.23 71.19 71.02 71.98 76.28 74.57 76.26 74.69 77.19 TiO2 0.08 0.08 0.35 0.32 0.35 0.11 0.19 0.09 0.11 0.13 Al2O3 11.87 12.26 14.16 13.44 14.18 12.37 13.38 12.47 12.18 11.52 TFe2O3 1.65 1.68 3.58 3.91 3.39 1.62 1.91 1.47 1.59 1.63 MnO 0.03 0.03 0.06 0.06 0.06 0.03 0.04 0.03 0.03 0.02 MgO 0.16 0.16 0.38 0.36 0.36 0.09 0.36 0.01 0.19 0.16 CaO 0.20 0.14 2.00 1.92 2.07 0.60 0.47 0.41 0.82 0.33 K2O 5.03 5.27 4.11 4.50 3.75 5.16 4.20 4.70 6.01 5.52 Na2O 3.00 3.06 3.37 3.22 3.33 2.74 3.83 3.53 2.56 2.52 P2O5 0.01 0.01 0.07 0.07 0.07 0.01 0.05 0.03 0.03 0.04 烧失 0.51 0.41 0.85 0.58 0.75 0.47 0.22 0.46 0.79 0.47 总量 100.36 100.31 100.10 99.40 100.27 99.46 99.22 99.46 99.00 99.52 A/CNK 1.11 1.12 1.04 0.98 1.07 1.11 1.14 1.07 1.00 1.08 (Ga/Al)×104 2.66 2.53 2.64 2.54 2.53 2.45 2.55 2.85 2.19 3.53 微量元素(μg/g) Ga 16.70 16.40 19.90 18.30 19.10 16.20 18.20 19.00 14.30 21.70 Rb 261.00 244.00 170.00 189.00 165.00 229.00 142.00 206.00 204.00 368.00 Sr 15.30 15.20 178.00 117.00 179.00 81.00 64.70 18.20 64.30 7.21 Y 41.20 56.30 49.00 44.80 41.90 29.10 28.30 39.50 33.30 72.20 Zr 85.20 109.00 195.00 240.00 185.00 113.00 179.00 84.80 143.00 177.00 Nb 18.00 14.30 15.90 13.90 15.20 8.61 12.50 13.00 11.70 18.00 Ba 93.60 96.20 608.00 827.00 469.00 274.00 739.00 175.00 976.00 65.70 La 16.70 22.50 54.30 63.60 63.10 36.20 35.10 38.20 35.60 66.10 Ce 36.10 47.10 109.00 128.00 127.00 75.30 72.70 79.60 71.30 131.00 Pr 4.70 6.13 13.70 15.40 15.70 9.01 8.79 9.92 8.80 16.60 Nd 18.00 23.00 48.10 54.90 53.70 30.60 32.30 35.30 31.10 55.80 Sm 4.56 5.81 9.73 9.64 9.77 5.64 5.98 7.18 6.03 11.90 Eu 0.14 0.16 1.23 1.22 1.19 0.63 0.94 0.41 0.56 0.33 Gd 4.96 6.09 8.52 8.11 7.87 4.54 5.33 6.25 5.31 11.50 Tb 1.00 1.23 1.41 1.36 1.25 0.73 0.87 1.05 0.89 2.04 Dy 6.59 8.52 8.72 7.88 7.57 4.55 5.19 6.47 5.88 12.80 Ho 1.50 1.94 1.79 1.58 1.56 1.00 1.03 1.35 1.22 2.63 Er 4.40 5.84 5.25 4.56 4.46 3.15 2.85 4.12 3.69 7.65 Tm 0.76 0.99 0.83 0.68 0.72 0.55 0.47 0.70 0.61 1.24 Yb 5.06 6.51 5.47 4.53 4.59 3.80 3.30 5.15 4.16 7.90 Lu 0.79 1.07 0.84 0.70 0.72 0.62 0.48 0.81 0.64 1.15 Hf 3.89 4.18 6.09 6.68 5.46 4.03 5.55 3.51 4.81 7.07 Ta 2.22 1.58 1.39 1.17 1.28 1.06 1.19 1.80 1.29 2.02 Th 23.20 30.10 22.90 21.50 23.00 27.10 19.30 23.60 22.00 57.80 U 4.72 6.21 4.74 3.56 6.08 5.38 3.94 4.41 4.33 8.04 注:A/CNK=Al2O3/(CaO+Na2O+K2O)(摩尔比);TFe2O3为全铁. 在球粒陨石标准化图解(图 5a)上,样品表现出轻稀土元素略微富集、重稀土元素基本水平的形态,具有明显的Eu负异常.在MORB标准化蛛网图(图 5b)上,样品表现出强烈的Sr、Ba、P、Ti负异常和明显的Nb、Ta负异常,以及Rb、Th正异常.Sr、Eu强烈负异常表明岩浆发生了明显的长石分离结晶,P、Ti负异常可能是磷灰石、钛-铁氧化物分离结晶的结果,Nb、Ta的亏损类似于形成岛弧环境岩石的特征.
图 5 石棉花岗岩样品的稀土元素分布形式(a)和微量元素蛛网图(b)球粒陨石和MORB数据引自Sun and McDonough, 1989Fig. 5. Chondrite-normalized REE diagram (a) and MORB-normalized spidergram (b) for Shimian granite3.3 Nd同位素
石棉花岗岩体6个样品的Nd同位素分析结果列于表 3.样品的147Sm/144Nd=0.106 1~0.153 0,143Nd/144Nd=0.5120 76~0.512 371,具有较高的εNd(t)值(-2.55~+2.38).两阶段的Nd模式年龄T2DM=1.406~1.792 Ga.
表 3 川西石棉花岗岩Sm-Nd同位素分析结果Table Supplementary Table Sm-Nd isotopic data for Shimian granite in western Sichuan样品号 Sm(μg/g) Nd(μg/g) 147Sm/144Nd 143Nd/144Nd±2σ εNd(t) fSm/Nd TDM(Ga) T2DM(Ga) 04KD20-6 5.81 23.0 0.153 0 0.512 371±0.000 008 -0.63 -0.222 1.955 1.642 04KD21-4 9.73 48.1 0.122 3 0.512 108±0.000 009 -2.55 -0.378 1.736 1.792 04KD23-1 9.64 54.9 0.106 1 0.512 076±0.000 011 -1.48 -0.460 1.521 1.708 04KD24-2 5.64 30.6 0.111 4 0.512 086±0.000 011 -1.84 -0.434 1.585 1.736 98KD71 5.98 32.3 0.111 9 0.512 306±0.000 009 +2.38 -0.431 1.268 1.406 98KD86 11.9 55.8 0.128 9 0.512 254±0.000 010 -0.38 -0.346 1.606 1.621 注:t=0.818 Ga;Sm、Nd含量采用ICP-MS测试结果;TDM=1/λSm×ln{[(143Nd/144Nd)sample-0.513 15]/[(147Sm/144Nd)sample-0.213 7]+1};T2DM=TDM-(TDM-T)×(fCC-fS)/(fCC-fDM);其中:fS、fCC和fDM分别为样品、平均大陆地壳和亏损地幔的fSm/Nd值,fSm/Nd=(147Sm/144Nd)sample/(147Sm/144Nd)CHUR-1,(147Sm/144Nd)CHUR=0.196 7;fCC=-0.4;fDM=0.085 92. 4. 讨论
4.1 岩石成因
本文研究的石棉花岗岩缺乏同时期共生的辉长岩,因此不可能是幔源玄武质岩浆分异的产物(M型);大多数样品的A/CNK不超过1.1,为弱过铝质成分,而高SiO2的S型花岗岩通常A/CNK超过1.1,是强过铝质成分;样品具相对低的Zr、Nb、Y、Ga和低的10 000×Ga/Al比值,也排除了A型花岗岩的可能(Whalen et al., 1987).笔者注意到岩石含有少量角闪石,这是Ⅰ型花岗岩的特征矿物,同时样品的P2O5含量很低(0.01%~0.07%),在Harker图解上(图 4)P2O5与SiO2呈负相关关系,这是高分异Ⅰ型花岗岩的特征(Chappell, 1999; Broska et al., 2004),部分高SiO2样品为过铝质,具有高的全碱含量,表明岩浆经历了较强的斜长石、角闪石和黑云母的分离结晶.综上所述,石棉花岗岩应为弱铝质的高分异Ⅰ型花岗岩,其源岩应为中—基性火成岩.
石棉花岗岩样品普遍具有较高的εNd(t)值(-2.55~+2.38),但与在时间-空间上密切共生的玄武质岩(苏雄玄武岩,Li et al., 2002a;冷碛辉长岩,李献华等,2002b;康定基性岩墙、岩脉,Lin et al., 2007)相比明显偏低,这排除了新底侵玄武质岩石部分熔融的可能.石棉花岗岩很可能是由前存的初生地壳物质部分熔融形成的,这与在扬子西缘,新元古代初生岛弧地壳的普遍存在相一致(李献华等,2002b; Li et al., 2003b).扬子块体西-西北缘很可能在中元古代晚期-新元古代早期存在一个俯冲带,正是这个俯冲的洋壳导致了扬子块体西缘初生岛弧地壳物质的增生(李献华等,2002b; Li et al., 2003b).岩石相平衡熔融实验研究也表明,对于偏铝的高钾钙碱性Ⅰ型花岗岩,最适合的产生机制是钙碱性-高钾钙碱性的安山质源岩局部熔融(Roberts and Clements, 1993).我们还注意到样品的εNd(t)值变化范围较大(-2.55~+2.38),以及较老的Nd模式年龄(1.4~1.8 Ga),这可能是岩浆混染了古老地壳物质的结果(薛怀民等,2006).通过以上研究,笔者认为石棉花岗岩应是中元古代晚期-新元古代早期扬子块体西缘的俯冲洋壳产生的初生岛弧地壳物质部分熔融形成的,岩浆上升侵位过程中混染了部分古老地壳物质.
在Nb—Y和Rb—Y+Nb构造判别图上(图 6,Pearce et al., 1984),石棉花岗岩样品投影于火山弧、同碰撞和板内环境的交界处,没有明确的指示意义.虽然石棉花岗岩具有明显的Nb-Ta亏损等类似于岛弧环境的地球化学特征,但这些特征主要受源区组成和岩浆结晶演化过程等因素的制约,构造环境往往是第二位的控制因素(Frost et al., 2001).花岗岩的地球化学判别往往给出的是其源岩的构造环境而不是花岗岩本身的形成环境(吴福元等,2007;张旗等,2007).这些花岗质岩具有诸如Nb-Ta亏损等类似于岛弧环境的地球化学特征,应该是继承了源岩(中元古代晚期-新元古代早期的初生岛弧地壳)的地球化学特征,并不代表其形成时的构造环境.值得注意的是,石棉花岗岩富K2O(3.75%~6.01%),贫CaO(0.14%~2.07%),并含有钾长石斑晶,与Barbarin花岗岩分类中KCG相似(Barbarin, 1999).高钾和含钾长石斑晶钙碱性花岗岩(KCG)可以出现在不同的构造环境中,实际上指示的是一种构造体制的变化而不是一个特定的构造环境,KCG的出现可能指示了大陆会聚向离散的转折(Barbarin, 1999).本文研究的石棉花岗岩体应是扬子地块西缘由会聚向离散的转折时期形成的.
图 6 川西石棉花岗岩构造判别图(Pearce et al., 1984)(a)Nb-Y判别图;(b)Rb-(Y+Nb)判别图Fig. 6. Tectonic discrimination diagrams for Shimian granite, western Sichuan4.2 构造意义
我们对近年来获得的扬子地块西缘新元古代其他相关岩浆岩的研究结果进行简要的回顾和综述,总结扬子地块西缘新元古代岩浆活动的演化序列如下:(1)1.0~0.9 Ga的造山岩浆活动,如1 028±9 Ma天宝山组酸性火山岩(耿元生等,2007)、938±30 Ma石棉镁铁-超镁铁质岩(Shen et al., 2002;朱维光等,2004)、937±106 Ma石棉大水沟斜长角闪岩(Xu et al., 1998)等;(2)0.86~0.85 Ga小规模板内岩浆活动,如857±13 Ma盐边关刀山岩体(Li et al., 2003b)、864±26 Ma丹巴格宗岩体(Zhou et al., 2002a;Li et al., 2003b)等;(3) 构造环境由会聚造山向大规模陆内伸展转变,如本文研究的818±7 Ma石棉花岗岩及~795 Ma瓦斯沟花岗岩(Zhou et al., 2002b;林广春,2008)等;(4)与大规模非造山裂谷作用有关的790~760 Ma基性岩墙群(Zhu et al., 2008)、780~760 Ma基性岩墙群(Lin et al., 2007)、780 Ma的A型花岗岩(Huang et al., 2008)及770~755 Ma瓦斯沟花岗闪长岩(Li et al., 2003c;林广春,2008)等.可见,扬子地块西缘新元古代岩浆活动是由约1.0~0.9 Ga造山带的岩浆活动和0.86~0.74 Ga非造山的岩浆活动组成.造山运动的结束代表华南Rodinia超大陆最终形成,非造山运动的开始代表华南Rodinia超大陆裂解的最初时间,这两种不同的岩浆活动代表华南新元古代的重要构造事件的转折.而818±7 Ma的高钾和含钾长石斑晶钙碱性的石棉花岗岩体侵位,标志着扬子地块西缘构造环境由会聚挤压向大规模的陆内伸展转变.
虽然石棉花岗岩具有岛弧岩浆的一些元素地球化学特征,但区域地质特征和进一步的岩石地球化学分析表明,它应该是扬子地块西缘由会聚向离散的构造环境下形成的,类似于岛弧环境的地球化学特征,应该是继承了源岩-初生岛弧地壳的特征,不代表其形成时的构造环境.笔者认为地幔柱模式/超级地幔柱能够更好地解释川西康滇裂谷新元古代岩浆岩的成因,这对于限定扬子西缘在新元古代时期的构造属性以及确定华南板块在Rodinia超级大陆重建中的位置具有重要意义.本文的研究倾向于支持华南位于澳大利亚和Laurentia大陆之间的Rodinia超级大陆重建模式(Li et al., 1995, 2008).
5. 结论
最新的SHRIMP锆石U-Pb年龄表明,川西石棉花岗岩体侵位结晶发生在818±7 Ma.石棉花岗岩是弱铝质高钾钙碱性的高分异Ⅰ型花岗岩,是中元古代末-新元古代初扬子块体西缘的俯冲洋壳产生的初生岛弧地壳部分熔融形成的,岩浆上升侵位过程中混染了部分古老地壳物质.石棉花岗岩形成于扬子地块西缘由会聚向离散的转折时期,其“岛弧地球化学特征”是继承了源岩(初生岛弧地壳)的地球化学特征的结果,不代表其形成时的构造环境.
致谢: SHRIMP锆石U-Pb年代学测试得到中国地质科学院北京离子探针中心宋彪研究员、陶华工程师的指导和帮助;刘颖、涂湘林和梁细荣对主量、微量元素和Nd同位素测试给予了细心的指导;野外考察得到了李武显的大力支持和帮助;专家的评审意见对本文的改进起到了很大的作用,在此表示衷心感谢. -
图 1 川西泸定-石棉地区前寒武纪地质略图
右上角插图为华南新元古代裂谷系构造简图(Li et al., 1999)
Fig. 1. Simplified Precambrian geological map of the Luding-Shimian region, western Sichuan
图 3 石棉花岗岩的(a)K2O-SiO2图解(Peccerillo and Taylor, 1976)和(b)(K2O+Na2O)-SiO2岩石分类图解(Middlemost, 1994)
Fig. 3. Plots of (a) K2O-SiO2 and (b) (K2O+Na2O)-SiO2 for classification of Shimian granite, western Sichuan
图 5 石棉花岗岩样品的稀土元素分布形式(a)和微量元素蛛网图(b)
球粒陨石和MORB数据引自Sun and McDonough, 1989
Fig. 5. Chondrite-normalized REE diagram (a) and MORB-normalized spidergram (b) for Shimian granite
图 6 川西石棉花岗岩构造判别图(Pearce et al., 1984)
(a)Nb-Y判别图;(b)Rb-(Y+Nb)判别图
Fig. 6. Tectonic discrimination diagrams for Shimian granite, western Sichuan
表 1 川西石棉花岗岩SHRIMP锆石U-Pb同位素分析结果
Table 1. SHRIMP U-Pb isotopic data for zircons from Shimian granite in western Sichuan
点号 Th(μg/g) U(μg/g) Th/U 206Pb*(%) 207Pb/206Pb±1σ 206Pb/238U±1σ 207Pb/235U±1σ 206Pb/238U±1σ(Ma) 207Pb/206Pb±1σ(Ma) 1 173 219 0.81 0.17 0.069 49±0.000 67 0.124 7±0.000 7 1.172±0.014 757±4 872±22 2 69 99 0.72 0.63 0.071 08±0.000 93 0.139 0±0.001 1 1.261±0.036 838±6 801±57 3 57 113 0.52 0.99 0.071 76±0.000 85 0.135 3±0.001 0 1.186±0.037 817±6 727±64 4 211 203 1.07 0.76 0.072 86±0.000 76 0.142 5±0.000 9 1.308±0.027 858±5 824±41 5 160 264 0.63 0.52 0.067 65±0.000 55 0.136 8±0.000 8 1.194±0.019 826±4 719±32 6 92 161 0.59 0.80 0.070 25±0.000 72 0.136 6±0.000 9 1.197±0.029 825±5 727±50 7 92 143 0.67 0.91 0.070 10±0.000 76 0.138 7±0.000 9 1.196±0.034 836±5 693±58 8 147 317 0.48 0.35 0.067 79±0.000 49 0.136 9±0.000 7 1.226±0.017 827±4 772±28 9 69 119 0.60 1.15 0.071 60±0.001 40 0.134 6±0.001 0 1.150±0.042 813±5 673±77 10 77 126 0.63 0.73 0.070 48±0.000 81 0.135 2±0.000 9 1.202±0.029 817±5 756±50 11 174 210 0.85 0.51 0.070 54±0.000 78 0.137 6±0.000 8 1.259±0.023 831±5 817±37 12 58 119 0.50 0.40 0.070 00±0.001 20 0.134 1±0.001 0 1.234±0.034 811±6 829±56 13 51 96 0.55 0.46 0.073 00±0.001 50 0.132 8±0.001 0 1.266±0.037 803±6 904±59 14 143 198 0.75 0.12 0.068 81±0.000 64 0.133 8±0.000 8 1.250±0.017 809±4 863±35 15 54 92 0.60 0.76 0.074 40±0.001 50 0.136 1±0.001 0 1.279±0.041 822±6 873±64 16 115 182 0.66 0.39 0.071 10±0.001 20 0.131 1±0.000 8 1.227±0.028 794±5 865±45 17 148 162 0.94 0.48 0.071 49±0.000 69 0.132 5±0.000 8 1.233±0.021 801±5 854±34 注:206Pb*表示普通206Pb占总206Pb的百分比;采用204Pb校正方法计算年龄. 表 2 川西石棉花岗岩主量元素(%)和微量元素(μg/g)分析结果
Table 2. Major (%) and trace element (μg/g) analyses of Shimian granite in western Sichuan
样号 04KD20-6 04KD20-10 04KD21-4 04KD23-1 04KD23-4 04KD24-2 98KD71 98KD72 98KD74 98KD86 主量元素(%) SiO2 77.83 77.23 71.19 71.02 71.98 76.28 74.57 76.26 74.69 77.19 TiO2 0.08 0.08 0.35 0.32 0.35 0.11 0.19 0.09 0.11 0.13 Al2O3 11.87 12.26 14.16 13.44 14.18 12.37 13.38 12.47 12.18 11.52 TFe2O3 1.65 1.68 3.58 3.91 3.39 1.62 1.91 1.47 1.59 1.63 MnO 0.03 0.03 0.06 0.06 0.06 0.03 0.04 0.03 0.03 0.02 MgO 0.16 0.16 0.38 0.36 0.36 0.09 0.36 0.01 0.19 0.16 CaO 0.20 0.14 2.00 1.92 2.07 0.60 0.47 0.41 0.82 0.33 K2O 5.03 5.27 4.11 4.50 3.75 5.16 4.20 4.70 6.01 5.52 Na2O 3.00 3.06 3.37 3.22 3.33 2.74 3.83 3.53 2.56 2.52 P2O5 0.01 0.01 0.07 0.07 0.07 0.01 0.05 0.03 0.03 0.04 烧失 0.51 0.41 0.85 0.58 0.75 0.47 0.22 0.46 0.79 0.47 总量 100.36 100.31 100.10 99.40 100.27 99.46 99.22 99.46 99.00 99.52 A/CNK 1.11 1.12 1.04 0.98 1.07 1.11 1.14 1.07 1.00 1.08 (Ga/Al)×104 2.66 2.53 2.64 2.54 2.53 2.45 2.55 2.85 2.19 3.53 微量元素(μg/g) Ga 16.70 16.40 19.90 18.30 19.10 16.20 18.20 19.00 14.30 21.70 Rb 261.00 244.00 170.00 189.00 165.00 229.00 142.00 206.00 204.00 368.00 Sr 15.30 15.20 178.00 117.00 179.00 81.00 64.70 18.20 64.30 7.21 Y 41.20 56.30 49.00 44.80 41.90 29.10 28.30 39.50 33.30 72.20 Zr 85.20 109.00 195.00 240.00 185.00 113.00 179.00 84.80 143.00 177.00 Nb 18.00 14.30 15.90 13.90 15.20 8.61 12.50 13.00 11.70 18.00 Ba 93.60 96.20 608.00 827.00 469.00 274.00 739.00 175.00 976.00 65.70 La 16.70 22.50 54.30 63.60 63.10 36.20 35.10 38.20 35.60 66.10 Ce 36.10 47.10 109.00 128.00 127.00 75.30 72.70 79.60 71.30 131.00 Pr 4.70 6.13 13.70 15.40 15.70 9.01 8.79 9.92 8.80 16.60 Nd 18.00 23.00 48.10 54.90 53.70 30.60 32.30 35.30 31.10 55.80 Sm 4.56 5.81 9.73 9.64 9.77 5.64 5.98 7.18 6.03 11.90 Eu 0.14 0.16 1.23 1.22 1.19 0.63 0.94 0.41 0.56 0.33 Gd 4.96 6.09 8.52 8.11 7.87 4.54 5.33 6.25 5.31 11.50 Tb 1.00 1.23 1.41 1.36 1.25 0.73 0.87 1.05 0.89 2.04 Dy 6.59 8.52 8.72 7.88 7.57 4.55 5.19 6.47 5.88 12.80 Ho 1.50 1.94 1.79 1.58 1.56 1.00 1.03 1.35 1.22 2.63 Er 4.40 5.84 5.25 4.56 4.46 3.15 2.85 4.12 3.69 7.65 Tm 0.76 0.99 0.83 0.68 0.72 0.55 0.47 0.70 0.61 1.24 Yb 5.06 6.51 5.47 4.53 4.59 3.80 3.30 5.15 4.16 7.90 Lu 0.79 1.07 0.84 0.70 0.72 0.62 0.48 0.81 0.64 1.15 Hf 3.89 4.18 6.09 6.68 5.46 4.03 5.55 3.51 4.81 7.07 Ta 2.22 1.58 1.39 1.17 1.28 1.06 1.19 1.80 1.29 2.02 Th 23.20 30.10 22.90 21.50 23.00 27.10 19.30 23.60 22.00 57.80 U 4.72 6.21 4.74 3.56 6.08 5.38 3.94 4.41 4.33 8.04 注:A/CNK=Al2O3/(CaO+Na2O+K2O)(摩尔比);TFe2O3为全铁. 表 3 川西石棉花岗岩Sm-Nd同位素分析结果
Table 3. Sm-Nd isotopic data for Shimian granite in western Sichuan
样品号 Sm(μg/g) Nd(μg/g) 147Sm/144Nd 143Nd/144Nd±2σ εNd(t) fSm/Nd TDM(Ga) T2DM(Ga) 04KD20-6 5.81 23.0 0.153 0 0.512 371±0.000 008 -0.63 -0.222 1.955 1.642 04KD21-4 9.73 48.1 0.122 3 0.512 108±0.000 009 -2.55 -0.378 1.736 1.792 04KD23-1 9.64 54.9 0.106 1 0.512 076±0.000 011 -1.48 -0.460 1.521 1.708 04KD24-2 5.64 30.6 0.111 4 0.512 086±0.000 011 -1.84 -0.434 1.585 1.736 98KD71 5.98 32.3 0.111 9 0.512 306±0.000 009 +2.38 -0.431 1.268 1.406 98KD86 11.9 55.8 0.128 9 0.512 254±0.000 010 -0.38 -0.346 1.606 1.621 注:t=0.818 Ga;Sm、Nd含量采用ICP-MS测试结果;TDM=1/λSm×ln{[(143Nd/144Nd)sample-0.513 15]/[(147Sm/144Nd)sample-0.213 7]+1};T2DM=TDM-(TDM-T)×(fCC-fS)/(fCC-fDM);其中:fS、fCC和fDM分别为样品、平均大陆地壳和亏损地幔的fSm/Nd值,fSm/Nd=(147Sm/144Nd)sample/(147Sm/144Nd)CHUR-1,(147Sm/144Nd)CHUR=0.196 7;fCC=-0.4;fDM=0.085 92. -
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