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  地质通报  2018, Vol. 37 Issue (5): 840-852  
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沈淑鑫, 贾东, 曾庆, 谢忱, 资金平, 张勇, 胡晶. 四川龙门山北段唐王寨-仰天窝向斜麻柳村辉绿岩斜锆石U-Pb同位素定年及其地质意义[J]. 地质通报, 2018, 37(5): 840-852.
[复制中文]
Shen S X, Jia D, Zeng Q, Xie C, Zi J P, Zhang Y, Hu J. Baddeleyite U-Pb geochronology of Maliucun diabase from Tangwangzhai-Yangtianwo syncline in the northern segment of Longmen Mountain, Sichuan, and its geological significance[J]. Geological Bulletin of China, 2018, 37(5): 840-852.
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基金项目

国家重大科技专项《前陆冲断带及复杂构造区地质过程、深层结构与储层特征》(编号:2016ZX05003-001)和《含油气盆地深层构造研究》(编号:2017ZX005008-001)

作者简介

沈淑鑫(1991-), 女, 在读硕士生, 构造地质学专业。E-mail:shen3352332@163.com

通讯作者

贾东(1960-), 男, 博士, 教授, 从事构造地质学方面的研究。E-mail:djia@nju.edu.cn

文章历史

收稿日期: 2017-03-20
修订日期: 2017-05-10
四川龙门山北段唐王寨-仰天窝向斜麻柳村辉绿岩斜锆石U-Pb同位素定年及其地质意义
沈淑鑫1 , 贾东1 , 曾庆2 , 谢忱2 , 资金平1 , 张勇1 , 胡晶1     
1. 南京大学地球科学与工程学院, 江苏 南京 210023;
2. 中国石油西南油气田公司勘探开发研究院, 四川 成都 610041
摘要: 为确定龙门山北段唐王寨-仰天窝向斜内发育的NE向辉绿岩脉的侵位时代、构造背景,探讨其是否与峨眉山地幔柱活动相关,对向斜内麻柳村处的辉绿岩样品进行了斜锆石LA-ICP-MS U-Pb同位素测年、主量和微量元素分析。结果表明,辉绿岩斜锆石U-Pb同位素年龄为261.1±1.8Ma;属于高钛型拉斑玄武岩,高铝(13.39%~14.02%)、高铁(13.88%~14.67%)、低镁(4.43%~4.56%),富集大离子亲石元素和轻稀土元素,Nb/La=0.84~0.85、Th/Ta=2.40~2.45、Ta/Hf>0.25,具有与洋岛玄武岩相似的地球化学特征,Nb-Ta略微负异常,暗示其可能源于富集型地幔并受到少量陆壳物质混染,形成于大陆板内环境。与峨眉山玄武岩质火成岩对比发现,两者具有高度一致的形成时代及相似的地球化学特征。据此认为,该区辉绿岩为峨眉山地幔柱活动的产物,峨眉山玄武质岩浆的活动已经波及至龙门山北段及川西北地区,可能影响了该区的生物环境及油气成藏。
关键词: 辉绿岩    斜锆石U-Pb年龄    峨眉山地幔柱    唐王寨-仰天窝向斜    
Baddeleyite U-Pb geochronology of Maliucun diabase from Tangwangzhai-Yangtianwo syncline in the northern segment of Longmen Mountain, Sichuan, and its geological significance
SHEN Shuxin1, JIA Dong1, ZENG Qing2, XIE Chen2, ZI Jinping1, ZHANG Yong1, HU Jing1     
1. School of Earth Science & Engineering, Nanjing University, Nanjing 210023, Jiangsu, China;
2. Exploration and Development Research Institute of PetroChina Southwest Oil & Gasfield Company, Chengdu 610041, Sichuan, China
Abstract: To confirm the emplacement age and tectonic setting of the NE-striking diabase dykes developed in the Tangwangzhai-Yangtianwo syncline of northern Longmen Mountain as well as to reveal whether or not the diabases were associated with the Emeishan mantle plume activity, the authors investigated U-Pb baddeleyite age, major element content and trace element abundances of the diabase samples collected from the Maliucun area within the syncline. The results showed that the diabases yielded a baddeleyite concordant U-Pb age of 261.1±1.8Ma; they belong to the high-Ti (Ti/Y>500) tholeiitic series, and are characterized by high Al (13.39%~14.02%), high Fe (13.88%~14.02%), and low Mg (4.43%~4.56%) values as will as enrichment of the large iron lithophile ele-ments and light rare earth elements; coupled with the data Nb/La (0.84~0.85), Th/Ta (2.40~2.45), and Ta/Hf>0.25, the features of the diabases are similar to the geochemical characteristics of ocean island basalt; the slightly negative anomaly of Nb-Ta indicates that they were probably derived from the enriched mantle with minor contamination by crust materials. A comparison with the Emeishan basaltic rocks shows that they are consistent highly with each other in ages and geochemical characteristics. It is considered that the diabases were formed by Emeishan mantle plume activity and the activity of Emeishan basaltic magma was spread to the northern section of Longmen Mountain and northwestern Sichuan, which had an impact on the biotic environment and hydrocarbon accumulation of this area.
Key words: diabase    baddeleyite U-Pb dating    Emeishan mantle plume    Tangwangzhai-Yangtianwo syncline    

龙门山褶皱冲断带位于四川盆地西缘,长约500km,大地构造上属于扬子板块西部边缘[1],受晚三叠世羌塘、华北、扬子块体共同作用和中—新生代青藏地区板块碰撞的叠加影响,形成了龙门山冲断带异常复杂的变形构造[2-4]。唐王寨推覆体位于龙门山北段(图 1-a),在多次推覆抬升中,晚古生界剥蚀殆尽,该时期的地质信息也随之缺失。唐王寨推覆体主体唐王寨-仰天窝向斜泥盆系内发育多条辉绿岩脉(图 1-b),然而迄今为止,其岩石地球化学特征、形成时代及构造背景仍缺少研究报道,这可能与辉绿岩精确定年难度较大有关。自晚泥盆世以来,扬子西南-西北缘存在多期不同的岩浆事件,目前有研究报道的事件,分别为扬子西南缘晚二叠世早期的峨眉山大火成岩省事件[5]、扬子西北缘—秦岭地区二叠纪秦岭洋扩张形成并残留的基性侵入岩[6]事件与中生代华南-华北板块碰撞作用发育的岩浆岩事件[7]、扬子西缘-西南缘中生代早期古特提斯洋闭合发育的岩浆岩事件[8-9]与新生代受欧亚碰撞影响发育的岩浆岩事件[10-11]。唐王寨推覆体处于这些事件的交错地带,其辉绿岩脉的形成时代和成因背景难以推断。有学者[12]认为,该岩脉形成于晚二叠世,但尚需精确的年龄数据支撑。由于宝兴以北地区尚未发现峨眉山玄武质岩浆岩,且该地区离峨眉山地幔柱中心的距离大于700km[13],该地区的辉绿岩与晚二叠世峨眉山玄武岩之间是否具有亲缘性,峨眉山基性岩浆活动是否影响到扬子块体西北缘地区等问题值得进一步研究。

图 1 龙门山北段地质构造简图(a)[4]和麻柳村辉绿岩地质图(b) Fig.1 Geological map of northern Longmen Mountain (a) and Maliucun diabases (b)

此外,龙门山北段是中国西部油气勘探的重点区域之一,近期更是在广元剑阁ST3井发现了泥盆系白云岩的油气显示,强烈且频繁,或将开启川西北泥盆系天然气勘探的序幕[14]。对该地区岩浆热事件进行研究也将有助于进一步分析川西北泥盆系的油气成藏史。

基性岩墙的常用定年方法有K-Ar或40Ar/39Ar同位素定年、锆石U-Pb同位素定年等[15]。然而,K-Ar或40Ar/39Ar系统在后期地质、构造作用下,容易存在封闭性不足的问题;锆石具有较高的U-Pb体系封闭温度,定年精确,但基性岩中原生锆石很少,且多为捕获锆石,难以获得具统计意义的年龄数据[15]。近年研究发现,斜锆石(ZrO2)不仅具备锆石U-Pb定年的优点,而且U含量更高,初始Pb含量低,Pb不易丢失,定年更精确可靠[15-17]。此外,斜锆石形成于硅不饱和的岩浆环境,常见于基性岩中,如辉长岩[18-19]、辉绿岩[20],其U-Pb同位素年龄更能代表基性岩的形成年龄。因此,斜锆石无疑是基性岩定年的理想矿物。在本次研究的辉长岩样品(MLC-4)中发现了100多颗斜锆石,为获取该区辉绿岩墙的斜锆石U-Pb年龄提供了良好的基础。

本文通过对龙门山北段唐王寨-仰天窝向斜泥盆系内麻柳村地区的辉绿岩进行岩石学、LA-ICPMS斜锆石U-Pb同位素年龄和地球化学特征分析,确定其形成时间,并探讨该地区辉绿岩脉与峨眉山玄武岩的亲缘性,为扬子板块西北缘的岩浆-构造背景提供进一步认识。

1 地质背景与采样

研究区位于四川省广元市剑阁县境内,其大地构造位于扬子西部边缘,龙门山褶皱带北段唐王寨-仰天窝向斜内部,东侧以马角坝断裂带与天井山背斜接壤,西侧以青川断裂与轿子顶推覆体相隔[3-4]。二叠纪时期,区域处于面向古特提斯洋的被动大陆边缘环境,之后,区域各构造皆经历了晚三叠世和晚白垩世—新生代2期挤压构造变形,其中晚三叠世冲断-褶皱作用尤为强烈,发育倒转向斜、紧闭背斜,晚三叠世运动后仰天窝向斜、天井山、矿山梁背斜基本已经形成,仰天窝向斜为沿马角坝断裂带发生巨大滑移量的滑覆体,遭受了强烈剥蚀[3-4]。新生代构造变形较弱,部分改造了早期形成的冲断构造[21-22]

区内地层主要为志留系—泥盆系海相碎屑岩和碳酸盐岩,与西侧轿子顶推覆体古生界浅变质碎屑岩-碳酸盐岩为2套完全不同的地层,而与天井山背斜古生界岩性一致,同属于四川盆地西北缘的沉积构造[23]。区域内发育多条基性岩脉,未发现其他岩浆岩类型。基性岩脉主要岩石类型为辉长岩、辉绿岩,主要侵位于泥盆系,少部分侵入志留系。

研究区共见6条基性岩脉,皆分布于仰天窝向斜内部,呈NE走向,长1~7km,宽约60m,斜穿多个层位,包括上志留统沙帽群(S3sh)黄灰色-紫色页岩-石英细砂岩,下泥盆统平驿铺组(D1p)灰白色中厚层石英砂岩、甘溪组(D1g)黄褐色铁质粉砂岩,中泥盆统养马坝组(D2y)石英砂岩-灰岩互层、观雾山组(D2g)灰白色中厚层白云岩和上泥盆统沙窝子组(D3s)灰色薄层泥质-白云质灰岩、茅坝组(D3m)深灰色中厚层泥质灰岩[23]。本次研究的样品采于剑阁县六合乡麻柳村处采石坑,此处岩脉所侵入围岩为中泥盆统养马坝组石英砂岩,所采样品为坑内灰黑色-灰绿色的辉绿岩-辉长岩(图 2-ab),样品编号为MLC-4和MLC-5。镜下观察(图 2-cd)发现,岩石具辉长-辉绿结构,主要矿物为斜长石和辉石,皆遭受一定程度的绿泥石化,斜长石含量约55%,辉石约35%,暗色含铁矿物(磁铁矿、钛铁矿)约5%,绿泥石3%,黑云母、绢云母2%。

图 2 麻柳村辉绿岩野外照片(a)、手标本照片(b)及岩石薄片透射光显微镜照片(c—单偏光,d—正交偏光) Fig.2 The field photograph (a), the specimen photograph (b) and the photomicrographs (c, d) of the Maliucun diabase
2 斜锆石U-Pb同位素定年 2.1 分析方法

样品MLC-4经破碎、重液、磁选等流程,挑选出100多颗斜锆石。将挑选出的斜锆石制靶,并进行透射光、反射光和阴极发光(CL)图像分析(图 3),获取斜锆石的形态、内部结构、环带发育情况,帮助选定最佳的斜锆石分析位置。

图 3 麻柳村地区辉长岩(MLC-4)代表性斜锆石阴极发光(CL)图像 Fig.3 CL images of representative baddeleyite from the diabase sample (MLC-4) in Maliucun

斜锆石U-Pb同位素定年分析采用LA-ICPMS测试方法[16],在南京大学内生金属矿床成矿机制研究国家重点实验室完成测试。激光剥蚀系统为美国New Ware公司的UP-213 Laser Ablation Sys⁃ tem固体激光器,激光束斑直径为32μm。根据前人研究,对同一年龄的斜锆石和锆石,都以标准锆石作为测试分析标样进行LA-ICP-MS U-Pb同位素定年分析,所得的年龄值在误差范围内一致[24]。结合实验室条件,本次选用测试分析标样为GJ-1锆石(608.5±1.5Ma)和Mud Tank锆石(732±5Ma)[25]。单点分析的同位素比值及年龄误差为1σ。各测点的U-Pb年龄计算采用Glitter 4.4.1软件,测试原理、数据处理和铅校正方法详见Griffin等[26]。数据处理采用Isoplot/Ex v.4.15软件[27]

2.2 测试结果

CL图像(图 3)显示,样品MLC-4中的斜锆石种类单一,形态一致,颜色较暗,晶形呈长柱形-针形,自形,晶棱锋利,振荡环带宽,为典型的岩浆成因斜锆石。本次共完成了19个测点的斜锆石年龄测定(图 3),获得19组LA-ICP-MS斜锆石U-Pb年龄测试结果(表 1)。

表 1 麻柳村辉长岩斜锆石U-Th-Pb年龄测试结果 Table 1 LA-ICP-MS U-Th-Pb dating results of baddeleyites from the diabase in Maliucun

从斜锆石U-Pb年龄分布图(图 4)可以看出,测点的年龄数据分布呈现单峰。在谐和图(图 4-a)上,数据点基本分布在谐和线上,可知斜锆石的U-Pb体系封闭性良好,无明显U、Pb的加入和丢失,其206Pb/238U表面年龄值能真实反映岩浆结晶年龄[16, 28]。19个分析点的谐和年龄[29]为261.1± 1.8Ma(MSWD=0.0096),206Pb/238U表面年龄加权平均值为261.3±3.8Ma(MSWD=0.22)(图 4-b),显示很好的一致性。

图 4 麻柳村地区辉长岩(MLC-4)斜锆石U-Pb年龄谐和图(a)和206Pb/238U表面年龄加权平均值计算图(b) Fig.4 Baddeleyite U-Pb concordia plot (a) and weighted mean 206Pb/238U age plot (b) for the diabase in Maliucun
3 岩石地球化学特征 3.1 分析方法

本次共选取2块新鲜的麻柳村辉绿岩样品进行地球化学元素分析,测试工作在澳实分析检测(广州)有限公司完成。主量元素测定采用MEXRF26d化验分析法,即用偏硼酸锂熔融,X荧光光谱分析,共测试14种氧化物含量及烧失量,检测范围为0.01%~100%,RSD=0.1%~1%。微量元素测定采用ME-ICP61方法,用四酸消解,等离子光谱分析检测。稀土、稀有元素测定采用ME-MS81化验分析法,用硼酸锂熔融,等离子质谱仪定量分析。

3.2 主量元素

样品的主量元素分析数据见表 2。麻柳村辉绿岩中SiO2含量为47.90%~49.04%,在SiO2-(K2O+ Na2O)图解(图 5)中,样品点落入辉绿岩范畴[30];全碱含量(K2O+Na2O)为4.89%~5.18%,Al2O3含量为13.39%~14.02%,与洋岛拉斑玄武岩的平均Al2O3含量(13.45%)接近。在AFM图解[31]图 6-a)中落入拉斑玄武岩区。此外,样品Fe2O3含量高(13.88%~ 14.67%),MgO含量较低(4.43%~4.56%),TiO2含量高(4.35%~4.39%),属于高钛玄武质岩石(TiO2>3%,Ti/Y>500)[37-38]

表 2 麻柳村辉长岩主量、微量和稀土元素含量 Table 2 Major, trace and rare earth element for the diabase in Maliucun
图 5 麻柳村辉绿岩硅碱图[30] Fig.5 SiO2 versus (Na2O+K2O) diagram for the diabase in Maliucun
图 6 麻柳村辉绿岩及部分峨眉山玄武岩AFM(a)[31]和Ti-Zr-Y(b)[41]判别图 Fig.6 AFM (a) and Ti-Zr-Y (b) discrimination diagrams for the diabase in Maliucun and some of the Emeishan basalt
3.3 微量元素

辉绿岩的微量元素分析数据见表 2。辉绿岩样品的稀土元素总量较高,达293.1×10-6~315.1×10-6。在球粒陨石标准化稀土元素配分图(图 7-a)上,表现为轻稀土元素(LREE)富集的右倾型分布样式,(La/ Yb)N值为14.32~14.75,轻稀土元素富集特征明显,未见明显的Eu异常,与洋岛玄武岩(OIB)配分曲线[39]近平行,与富集型洋脊玄武岩(E-MORB)平缓的配分曲线区别明显,样品点均落入峨眉山高钛玄武岩及基性岩墙区域[36, 40]

图 7 不同岩石类型稀土元素球粒陨石标准化配分曲线图(a)和不相容元素原始地幔标准化蛛网图(b)[39] Fig.7 Chondrite-normalized REE patterns (a) and primitive mantle-normalized incompatible element patterns (b) for different rock types

在原始地幔标准化微量元素蛛网图(图 7-b)中,样品呈现与洋岛玄武岩配分曲线[39]相似的趋势,总体上大离子亲石元素(LILE)富集,如Rb、Ba、Th等,高场强元素(HFSE)略微亏损,如Hf、Nb、Ta,而Sr元素则出现较明显的负异常。样品点同样落入峨眉山高钛玄武岩及基性岩墙区域[35-36]。在Ti-Zr-Y判别图解[41]图 6-b)中,麻柳村辉绿岩样品点落入板内玄武岩区域,同其他地区峨眉山玄武岩的研究结果[32-37]一致。

4 讨论 4.1 辉绿岩的形成年代及构造环境

唐王寨-仰天窝向斜内麻柳村辉绿岩体的年代学研究较匮乏。梁斌[12]在研究唐王寨-仰天窝伸展构造时,对该区的辉绿岩脉进行了K-Ar同位素定年,分别获得330.42 ± 5.24Ma和200Ma的年龄数据。由于该文中缺少具体的年龄数据分析,且同期辉绿岩年龄相差太大,使该地区辉绿岩体的确切形成时代仍存在疑问。

本文在麻柳村辉绿岩脉样品中挑选出斜锆石,其CL图像显示,斜锆石多为自形,针状-柱状,振荡条带宽,揭示其为岩浆结晶成因。通过LA-ICPMS U-Pb定年,测得样品的U-Pb年龄为261.1± 1.8Ma。相对于锆石,斜锆石具有更高的U-Pb封闭温度和U初始含量及更低的Pb初始含量。有学者通过碱性超基性岩体中斜锆石和锆石U-Pb定年结果对比,发现斜锆石相比共存的锆石,具有更高的定年精密度,更适合定年[42]。为此,笔者认为,麻柳村辉绿岩的斜锆石U-Pb年龄具有较高的可信度,即该区辉绿岩脉形成于中晚二叠世。

麻柳村辉绿岩属于高钛拉斑玄武岩系列,富Ti(TiO2=4.35% ~4.39%),高Fe(Fe2O3=13.88% ~ 14.67%)、Al(Al2O3=13.39%~14.02%),低Mg(MgO= 4.43%~4.56%)。Ti-Zr-Y判别图(图 6-b)表明,麻柳村辉绿岩形成于板内环境,属于板内玄武岩系。麻柳村辉绿岩明显富集不相容元素,其中强不相容元素Th/Ta值为2.40~2.54,明显低于上地壳的Th/Ta值(约10)[43],而接近原始地幔的Th/Ta值(约2.3)[44],暗示麻柳村辉绿岩源自于富集地幔[45]。此外,不相容元素Rb/Sr和Nb/Sm值较高,并具有同洋岛玄武岩相似的右倾型稀土元素配分模式,(La/Yb)N>1(14.32~14.75),同样表明其源区为富集地幔[46-47]。但相对洋岛玄武岩,麻柳村辉绿岩具有轻微的Nb-Ta负异常,揭示了板内伸展的应力体制,指示可能受到陆壳物质的混染,表现出地幔柱起源的P型大陆溢流玄武岩(P型CFB)的特征[47-48]。此外,研究区和邻区绵延数十千米的多条辉绿岩脉组成的镁铁质岩墙群也反映了当时扬子西北缘的岩石圈伸展背景。广元一带上二叠统大隆组放射虫硅质岩的沉积相和深水海绵骨针、假提罗菊石等深水化石的存在,也是该区伸展应力环境的表现[12, 23]

综上所述,麻柳村辉绿岩墙形成于中晚二叠世,属于板内高钛型拉斑玄武质侵入岩,具有P型大陆溢流玄武岩的地球化学特征,反映其源于富集型地幔并受到陆壳物质混染,结合该区域发育的深水相沉积,中晚二叠世扬子西北缘地区处于拉张的构造环境。该区域系列岩墙群的发育很可能与同期扬子西南缘的峨眉山大火成岩省事件有关。

4.2 麻柳村辉绿岩与峨眉山玄武岩的亲缘性及其地质意义

峨眉山玄武岩为分布于扬子板块西缘川滇贵地区晚二叠世早期的玄武岩(图 8),一般以龙门山-小菁河断裂带为其西北界,西南界大致为金沙江-哀牢山-红河断裂带,向东至贵阳以东的都匀—瓮安一线,面积为2.5 × 105~3.0 × 105km2;体积达5 × 105km3,西厚东薄[37, 47, 73]。峨眉山玄武岩是中国迄今唯一被国际学术界认可的大火成岩省(LIPs),目前普遍认为其形成与地幔柱活动有关[13, 37, 40, 73-80]。有学者研究认为,四川宝兴地区大面积分布的大石包组玄武岩同属于峨眉山玄武岩(图 8),并将峨眉山大火成岩省的范围向北扩至宝兴地区[32, 35]。本次研究的辉绿岩体位于宝兴峨眉山玄武岩区北北东向约160km处。

图 8 峨眉山大火成岩省及其岩浆岩年龄分布[9, 49-72] Fig.8 Sketch map showing the age and distribution of the Emeishan large igneous province

峨眉山玄武岩主体喷发于约260Ma(图 8),持续喷发时间小于3Ma[33, 40, 80-82]。本文获得的麻柳村辉绿岩斜锆石U-Pb年龄为261.1±1.8Ma,与距离较近的宝兴大石包组玄武岩年龄(263±2Ma)[35]及其他典型的峨眉山玄武岩年龄在误差范围内一致[40, 80]。此外,麻柳村辉绿岩为拉斑板内玄武岩,与峨眉山玄武岩一样皆具有与洋岛玄武岩(OIB)类似的微量元素分布模式,Ti/Y>500、Ta/Hf>0.25、Nb/La=0.84~ 0.85,在AFM和Ti-Zr-Y判别图解(图 6),以及稀土元素、不相容元素蛛网图(图 7)中,皆落入峨眉山玄武岩区域。麻柳村辉绿岩无论是在侵入时间还是在岩石地球化学特征上,均与高钛型峨眉山玄武岩一致,这种时间和成因上的密切联系,表明麻柳村辉绿岩应是峨眉山玄武质岩浆活动的部分产物。

随着峨眉山玄武岩研究数据的增多,其分布范围被进一步拓宽,如向东南延伸至桂北地区[83],向北至四川宝兴地区[32, 35]。本文研究区的位置比宝兴玄武岩分布区偏北约160km,是目前发现位置最北的峨眉山玄武质岩浆活动的产物。值得一提的是,研究区位于唐王寨推覆体内,而唐王寨推覆体是在晚三叠世受扬子、华北和羌塘块体的作用自北西方向推覆而来的,推覆距离约为30km[84-85],说明峨眉山玄武质岩浆的活动范围已延伸至唐王寨推覆体现位置更西北的区域。此外,广元矿山梁、车家坝等地区皆有侵入于栖霞组-茅口组的辉绿岩体零星出露[23],这些辉绿岩体很可能是峨眉山玄武质岩浆活动的产物,由此推测峨眉山玄武质岩浆活动可能已经影响到川西北大片区域。峨眉山地幔柱的规模和波及范围比之前估测的更大,其产生的影响也应重新认识。

Zhou等[86]、He等[87]和Xu等[88]认为,发生于约258Ma的End Guadalupian生物灭绝事件与峨眉山玄武岩的喷发有关,但该观点曾因峨眉山玄武岩规模较小等原因而遭到质疑[73, 89]。峨眉山玄武岩的实际规模更大,这将加强峨眉山大火成岩省事件与同期End Guadalupian生物灭绝事件的因果关联。

此外,在油气勘探领域,岩浆活动对油气成藏的影响越来越受重视。有研究表明,岩浆活动在提高有机质的热演化程度、改善储层的孔渗条件等方面都有重要的作用[90-94]。四川盆地热史演化研究表明,峨眉山地幔柱对四川盆地中二叠统及下伏烃源岩的热演化具有重要的影响,可能控制了四川盆地下古生界油气成藏的格局[95]。川西北地区发育寒武系、泥盆系、二叠系等多套烃源岩,以及可作优质储层的白云化碳酸盐岩。然而,由于该区域受后期构造改造强烈,地质条件较复杂,油气成藏及白云化机理的研究也相对困难。本文对该区域岩浆活动的时代及构造背景的研究,为川西北地区烃源岩热演化史及油气成藏条件提供了重要的资料,而该期热事件对区域油气成藏史的具体影响则有待进一步研究。

5 结论

(1)麻柳村辉绿岩的斜锆石U- Pb年龄为261.1±1.8Ma,表明其形成于中晚二叠世,与峨眉山大火成岩省的主体喷发期基本一致。

(2)麻柳村辉绿岩为高钛型拉斑玄武岩,高铝、铁,低镁,富集大离子亲石元素,Rb/Sr和Nb/Sm值较高,具有与洋岛玄武岩相似的地球化学特征,其Nb-Ta略负异常,暗示其源于富集型地幔并受到少量陆壳物质混染,形成于大陆板内环境。

(3)麻柳村辉绿岩在形成时间、地球化学特征上皆与峨眉山玄武岩一致,表明麻柳村辉绿岩应是峨眉山玄武质岩浆活动的产物,峨眉山玄武质岩浆活动已经波及至川西北地区。

致谢: 锆石U-Pb定年测试和论文修改得到南京大学王孝磊老师的大力协助,谨致谢忱。

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