闽北邵武地区玄武安山岩的成因和意义

2024-06-03 20:03:05周伏顺林鑫王郅睿邵程波
关键词:侏罗世安山岩玄武

周伏顺 林鑫 王郅睿 邵程波

周伏顺,林鑫,王郅睿,等. 闽北邵武地区玄武安山岩的成因和意义:来自年代学、地球化学及NdHf同位素的约束.吉林大学学报(地球科学版),2024,54(3):840861.doi:10.13278/j.cnki.jjuese.20230345.

Zhou Fushun,Lin Xin,Wang Zhirui,et al. Petrogenesis and Significance of Basaltic Andesite in the Shaowu Area, Northern Fujian: Constraints from Geochronology, Geochemistry and NdHf Isotopes. Journal of Jilin University (Earth Science Edition),2024,54(3):840861.doi:10.13278/j.cnki.jjuese.20230345.

摘要:

东南沿海地区中生代火山岩研究成果颇丰,但有关中侏罗世晚期基性火山岩的报道较少,限制了对该区域中生代构造岩浆活动及大地构造演化的深入理解。本文对江绍断裂带东南侧邵武地区的中生代火山岩开展了系统的岩石学、锆石UPb年代学、锆石LuHf同位素、地球化学和SmNd同位素研究。结果显示:邵武地区玄武安山岩喷发年龄为(161.0±2.0)Ma;同位素地球化学结果显示,这些样品中—晚侏罗世锆石的εHf(t)值介于-14.33~-10.41之间,εNd(t)值较低(-9.2~-8.4),反映富集Nd同位素的特征;岩石地球化学结果表明,该套火山岩具有高w(Al2O3)、w(Na2O)、低w(MgO)、w(TFe2O3)等特征,稀土总量较低,稀土配分曲线为右倾型,且具弱的Eu负异常,大离子亲石元素Rb、Ba和K相对富集,高场强元素Nb、Ta、Ti、P等相对亏损。综合岩石学和地球化学研究结果,本文认为邵武玄武安山岩起源于交代岩石圈地幔的部分熔融,并经历一定结晶的分异作用,其大地构造背景总体为板内环境。结合前人对区域构造岩浆活动的认识,本文认为在中侏罗世晚期,太平洋俯冲板片发生回撤、撕裂,导致幔源岩浆底侵并置换了古老壳源岩石,从而东南沿海地区虽整体处于挤压背景,但仍存在局部拉張环境。

关键词:

中侏罗世;地球化学;伸展背景;玄武安山岩;东南沿海

doi:10.13278/j.cnki.jjuese.20230345

中图分类号:P588.145

文献标志码:A

收稿日期:20231221

作者简介:周伏顺(1997—),男,硕士研究生,主要从事地球化学方面的研究,E-mail:z913909811@163.com

通信作者:林鑫(1987—),男,副教授,主要从事应用地球化学及其与数据科学交叉领域方面的研究,E-mail: xinlin@chd.edu.cn

基金项目:陕西省重点研发计划(2024GHZDXM26);国家重点研发计划(2016YFC0600601);中央高校基金项目(300102271206)

Supported by the Key Research and Development Program of Shaanxi (2024GHZDXM26), the National Key Research and Development Program(2016YFC0600601) and the Central University Fund Project (300102271206)

Petrogenesis and Significance of Basaltic Andesite in the Shaowu Area, Northern Fujian: Constraints from Geochronology, Geochemistry and NdHf Isotopes

Zhou Fushun,Lin Xin,Wang Zhirui,Shao Chengbo

School of Earth Sciences and Resources, Changan University, Xian 710054, China

Abstract:

Among the Mesozoic volcanic rocks along the southeast coast of China, Middle Jurassic rocks are seldom reported, yet they hold significant importance for understanding Mesozoic magmatic activities and tectonic evolution in this region. The Shaowu basaltic andesite, exposed in the southeast of the Jiangshao fault zone, provides an ideal opportunity for study. This paper systematically presents petrological, zircon UPb geochronological, LuHf isotopic, whole-rock geochemistry and SmNd isotopic analyses of the basaltic andesite, indicating an eruption age of (161.0±2.0) Ma. The isotopic data shows that all samples are characterized by εHf(t) values ranging from -14.33 to -10.41 and low εNd(t) values between -9.2-8.4. The geochemical results shows high w(Al2O3)  and w(Na2O) contents, along with low levels of w(MgO) and w(TFe2O3) etc. The total rare earth elements (REE) content is low, with chondrite-normalized REE patterns showing LREE enrichment and weak Eu negative anomalies. Furthermore, large ion lithophile elements such as Rb, Ba and K are enriched, while the high field strength elements such as Nb, Ta, Ti and P are relatively depleted. The geochemical and petrological characteristics of the basaltic andesite imply that it originated from partial melting of a metasomatic mantle wedge, underwent certain crystallization differentiation, and formed in an intraplate tectonic environment. Based on previous studies on regional tectonic-magmatic activities, it is proposed that Pacific subducted plate retreat and tearing, resulting in encroachment of mantle-derived magma and the replacement of ancient crust-derived rocks, occurred amidst local extension within the broader compressional background of the south-eastern coast of China during the Middle Jurassic.

Key words:

Middle Jurassic; geochemical; extensional setting; basaltic andesite; southeast coast of China

0  引言

玄武安山岩直接导源于上地幔,受地幔源岩成分的制约,其岩浆的形成与全球构造,如裂谷扩张、板块俯冲消减及地幔的深部作用(地幔对流、局部隆升、流体交代)等过程有关。因此,玄武安山岩研究对于反演地幔物质成分、分析构造环境及地球深部动力学均具重大意义[1],是国际火山岩研究的热点[25]。

华南地区以大面积分布的岩浆岩[6]及金属矿床[7]为特色,其内部可划分出多个构造带[8]及成矿带[9],使得该地区成为国内外学者研究的热点地区之一。现有研究表明,这些大规模的岩浆及矿床多与晚中生代的火山作用密切相关,并被普遍解释为是古太平洋板块向欧亚大陆俯冲作用所造成的应力体制转换的产物。然而,前人[1011]对于转换的时限是否为中侏罗世目前还存在争议。目前,对于该时期华南地区的应力状态也相应存在不同的观点,主要包括:由挤压转为伸展体制[10]、挤压体制[11]以及整体挤压体制下的局部伸展[6]。

玄武安山岩作为深部地质过程的记录者,能够为理解造山带构造演化过程提供很好的窗口,也为解决上述争议提供了新的思路。闽北地区侏罗纪火山岩广泛发育,其成因及构造背景的界定对于确定区域构造格架及演化起着至关重要的作用。该地区火山岩以大规模中酸性火山岩为主[1217],而对于玄武安山岩的报道较少。本文以邵武地区采集的玄武安山岩为研究对象,通过对样品进行岩石学、年代学、锆石LuHf同位素、全岩地球化学以及SmNd同位素的研究,确定玄武安山岩的形成时代及其成因,阐明中侏罗世古太平洋板块俯冲作用对闽北地区的影响。

1  地质背景及样品描述

华南板块由扬子地块、华夏地块以及其间的缝合带 — 江南造山带共同组成[1821],南北方向上分别夹持于东南亚块体和秦岭造山带之间,西邻青藏高原,东侧为太平洋板块。自元古宙板块拼合以来,华南板块经历了多阶段的构造岩浆活动,成为研究大陆体制转换的理想区域[2223]。其中,中生代板块间构造体制的变化导致了区域巨量岩浆的侵位(图1a),引发了华南大规模的成矿事件,成为国内外地质学家研究的焦点。

扬子地块内部可划分为同一盖层的不同基底,新元古代以来拼合成为稳定的克拉通[25];华夏地块主要由大面积的新元古代碎屑岩夹火山岩、岩浆岩及碳酸盐岩组成。新元古代末期,扬子和华夏地块发生碰撞拼合,形成江南造山带,沿该造山带南界的江绍断裂带分布了大量冷侵位的蛇绿岩套、蓝片岩

和具岛弧属性的岩浆岩[2627]。加里东期和印支期华南板块经历了两期陆内造山运动,主要体现在扬子地块东半部和整个华夏地块,区域内岩石卷入强烈变质变形过程中[25]。尤其是加里东期造山运动使得前泥盆纪岩石大多变质成为片岩、片麻岩和混合岩,且伴随强烈的岩浆活动,形成变质变形岩石花岗岩复合体系[27]。

邵武地区位于华夏地块中北部,区域出露的地层由老到新依次为:震旦系、下古生界罗峰溪群、侏罗系(图1b)。震旦系为黑云母斜长片麻岩、云母片岩夹斜长角闪岩,变质程度达角闪岩相,且发育强烈的混合岩化作用[2829]。下古生界罗峰溪群为灰绿色变砂岩、千枚岩、片岩,局部夹大理岩。侏罗系通过断层与下古生界分隔,内部岩石单元包括:兜岭群,下部为砂岩夹凝灰岩及灰岩透镜体,上部为晶屑凝灰岩;漳平群,为红绿相间的粉砂岩及细砂岩;梨山群,分为下部的灰黑色砂砾岩、粗砂岩、粉砂岩及上部的砂岩夹灰岩透镜体。研究区南东侧及北西侧分别出露NE走向的政和—大埔断裂带及江绍断裂带,区内断裂构造发育,包括NE向、NW向及EW向等多组,其中近NE向最为发育。区内岩浆岩主要为印支期和燕山期黑云母花岗岩,侵位于震旦系中,其次是玄武安山岩和闪长岩,零星分布于下古生界羅峰溪群中[30]。

本次研究的玄武安山岩出露于邵武市南侧的罗峰溪群千枚岩中,规模较小,呈NE向延伸,长度约300 m,宽度约100 m。由于植被覆盖,玄武安山岩与千枚岩接触界面不易观察,岩石表面风化较强,呈黄褐色,而人工揭露的断面上岩石相对新鲜,呈深灰色,节理发育(图2a),十分破碎。岩石表面可见气孔构造,内部被方解石矿物充填(图2b)。

镜下观察显示,玄武安山岩具间粒间隐结构,块状构造,斑晶体积分数为35%~40%,多为斜长石,自形板状,表面可见弱绢云母化,粒径为0.2~0.5 mm,见少量石英,粒径多为0.1~0.2 mm。局部可见杏仁体,主要由方解石和石英充填,大小为1.0~1.5 mm,体积分数为5%~10%。基质由细粒斜长石和辉石等组成,基质中见半自形粒状的次生绿帘石和次生方解石(图3)。根据岩石结构、构造及矿物成分组成,定名为玄武质安山岩。

2  分析方法

本次工作采集了露头上较新鲜的玄武安山岩样品,采样位置见图1b。选择其中1件样品进行年代学及锆石LuHf同位素分析,4件样品进行全岩主微量及稀土元素、SmNd同位素测试。

锆石单矿物挑选在河北省廊坊市尚艺岩矿检测技术服务有限公司完成。将野外采集的样品粉碎,通过常规重选法选出单矿物,在双目镜下挑选出形态较好的锆石。锆石制靶在南京宏创地质勘查技术服务有限公司完成。首先通过环氧树胶将待测锆石和标准锆石固定在玻璃板上,然后抛光至锆石暴露出

中心面,在偏光显微镜下对锆石进行反射光和透射光拍照,确定锆石的包体以及裂纹情况,最后进行阴极发光(CL)照相,以检查锆石的内部结构。

选择晶形较好,无包体、裂纹的锆石颗粒进行锆石UPb测年分析。锆石分析在中国地质科学院矿产资源研究所使用电感耦合等离子体质谱仪(LAMCICPMS)完成,详细的仪器参数和分析流程见文献[31]。仪器的型号和相应的激光剥蚀系统分别为Finnigan Neptune型LAMCICPMS和NewWave UP213 nm。实验过程采用单点剥蚀,直径为30 μm,采样深度为20~40 μm,频率为10 Hz,输出能量约为2.5 J/cm2。实验过程采用He作为载气。锆石年龄以GJ1为外标,元素质量分数以M217为外标。在测试过程中,每隔10个样品点测试

2个GJ1和1个Plesovice标准锆石。最后使用ICPMSDataCal软件完成数据处理(信号选择、漂移校正和年龄计算),通过Isoplot3.0软件计算加权平均年龄。

锆石LuHf同位素测定在北京科荟测试技术有限公司完成,仪器为激光剥蚀多接收器电感耦合等离子体质谱仪(Neptune plus),激光进样系统为RESOlution SE固体激光器。根据锆石照片选择合适区域,利用激光剥蚀系统对锆石进行剥蚀。测点位置为UPb年龄的同一测点或其附近。激光剥蚀的斑束直径为30~38 μm,能量密度为6 J/cm2,频率为6 Hz,激光剥蚀物质以高纯He为载气送入Neptune Plus(MCICPMS),积分时间为27 s。锆石样品GJ1用作分析的参考标准。在分析过程中,GJ1标准的176Hf/177Hf加权平均值为0.282 007±0.000 007(2σ,n=36),与误差范围内的报告值一致。采用179Hf/177Hf = 0.7325对Hf同位素比值进行指数归一化质量歧视校正,采用173Yb/172Yb=1.35274对Yb同位素比值进行指数归一化质量歧视校正。由于锆石中176Lu/177Hf值通常小于0.002,因此锆石中176Hf的同质异位素的干扰主要来自176Yb。在锆石激光剥蚀过程中直接测定Yb信号,用剥蚀过程中βYb的平均值作为Yb的质量歧视校正系数来进行同质异位素176Yb的干扰校正。

岩石样品主、微量及稀土元素测定在北京燕都中实测试技术有限公司完成。将岩石粗碎至厘米级的碎块,选取无蚀变及脉体穿插的新鲜样品用纯化水冲洗干净,烘干并粉碎至200目备用。主量元素测试:首先将粉末样品称量后加入Li2B4O7 (1∶8)助熔剂混合,然后使用融样机加热至1 150 ℃使其在金铂坩埚中熔融成均一玻璃片体,最后使用XRF(Zetium, PANalytical)测试,测试结果保证数据误差小于1%。微量及稀土元素测试:首先将200目粉末样品称量并置放入聚四氟乙烯溶样罐并加入HF+HNO3,然后在干燥箱中将高压消解罐保持在190 ℃温度72 h,最后取出经过赶酸并将溶液定容为稀溶液上机测试。测试使用ICPMS(M90, analytikjena)完成,所测数据根据监控标样GSR2显示误差小于5%,部分挥发性元素及极低质量分数元素的分析误差小于10%。

SmNd同位素测定在核工业北京地质研究院使用ISOPROBET热电离质谱仪进行。详细的化学制备、质谱法和标准样品测定见文献[32]。使用143Nd/144Nd=0.7219校正Nd同位素分析质量,测定过程中标准样品的测量结果如下:基准物质JMC=0.521109±3.000000(143Nd/144Nd)。Sm和Nd的分析空白总量为5×10-11 g。

3  分析结果

3.1  锆石年龄

玄武安山岩中的锆石大多为短柱状,粒径50~150 μm,长宽比为1∶1~1.5∶1。在CL图像中可见其具有清晰的振荡环带(图4),具有岩浆锆石的特征。选取29颗发育环带的锆石进行UPb同位素测试分析,測试结果见表1。结果显示所有分析点的Th/U值均大于0.1,这也反映其锆石为岩浆成因。其中一个测点的

206Pb/238U年龄

为234.4 Ma,结合其CL测点位置认为该数值代表了样品中锆石的混合年龄。其余锆石分析年龄可以分为两组,在锆石年龄谐和图上,7个年龄数据点的206Pb/238U年龄为458.0~443.2 Ma,其加权平均年龄为(449.6±5.3) Ma (n=7, MSWD=0.49)(图4a);其余21个数据点206Pb/238U年龄为168.1~151.4 Ma,其加权平均年龄为(161.0±2.0) Ma (n=21, MSWD=2.5)(图4b)。

3.2  锆石LuHf同位素

玄武安山岩中锆石LuHf同位素分析结果见图5及表2。样品FJ115中的12颗锆石加权平均

年龄为161 Ma,其(176Hf/177Hf)i值和εHf(t)值分别为0.282 28~0.282 39和-14.33~-10.41,对应于Hf地壳TDMC为2 096~1 847 Ma。

4颗锆石的

加权平均年龄为450 Ma,其(176Hf/177Hf)i值为0.282 22~0.282 25,εHf(t)值为-10.08~-8.98,对应于2 040~1 977 Ma的TDMC。

3.3  主、微量及稀土元素

玄武安山岩的主量元素分析结果见表3。由于采集的样品发生了轻微蚀变,因此在进行元素分析时,尽量不采用对蚀变作用比较敏感的Rb、Ba、K、Si等活动性元素,而利用不活泼的Ti、Zr、Nb、Y、Ta、Hf及稀土元素来判别岩石类型[3334]。w(Th)w(Co)和Zr×0.0001/

TiO2Nb/Y判别图(图6)[3536]中,采集的样品大多落入安山岩或玄武安山岩的范围,综合样品岩相学中较高的石英体积分数、长石的宽板状形态以及测年样品中挑选出的锆石形态,认为样品应属于钙碱性玄武安山岩。

玄武安山岩样品的SiO2质量分数为50.80%~52.48%,MgO质量分数为5.40%~6.59%,TFe2O3质量分数为8.04%~8.54%,TiO2质量分数为0.92%~1.02%,Al2O3质量分数为14.50%~15.64%,Mg#值为55~59。主量元素分析中产生了较高的烧失量(6.70%~7.36%),因此,需要对后期蚀变作用是否对活动性元素产生影响进行判别[37]。活动性元素SiO2与

相对不活动的元素Ce之间明显负相关,

而K2O、Rb、Th与其明显

正相关(图7),这些元素在岩石后期蚀变过程中受到的影响较小,其质量分数的变化是由岩浆作用产生的。另外,在稀土元素及微量元素图解(图8)中,4个样品呈现出大致相同的变化规律,也指出玄武安山岩样品的化学成分没有受到后期蚀变作用的明显改变。

玄武安山岩样品稀土和微量元素质量分数分析结果见表4。稀土元素总量w(ΣREE)较低,为82.33×10-6~92.40×10-6。在球粒陨石标准化稀土元素配分型式图上(图8a),样品总体富集轻稀土,呈现出轻稀土右倾而重稀土近平坦的样式,轻重稀土之间分馏较弱(LREE/HREE=7.38~7.71,(La/Yb)N=7.69~8.21),表现为弱的负Eu异常,δEu为0.76~0.84。样品微量元素组成特征见图8b,整体富集大离子亲石元素Rb、Ba和K,相对亏损高场强元素Nb、Ta、Ti、P。

3.4  SmNd同位素

玄武安山岩SmNd同位素组成相对均匀(表5),147Sm/144Nd为0.131 3~0.135 0,143Nd/144Nd为0.512 104~0.512 139,低于原始地幔现代值0.512 638[39]。采用样品的形成年龄(161 Ma)进行计算,样品的εNd(t)=-9.2~-8.4,这些特征表明在岩石形成过程中有大量地壳物质加入。在计算岩石Nd模式年龄时,SmNd同位素的分馏效应会产生明显的影响,以致其模式年龄结果不合理,为了减少此类影响,本文采用两阶段模式计算岩石Nd同位素模式年龄,用以近似代表样品源岩的地壳存留年龄。Nd单阶段模式年龄(TDM)为2 020~1 890 Ma,与锆石中Hf的模式年龄大体一致。Nd模式年龄值大多分布在古元古代,这也进一步表明岩浆形成过程中有新元古代地壳物质的参与。

4  讨论

4.1  火山岩时代

1∶20万顺层幅区调报告中未将本次工作中涉及的玄武安山岩圈出,但在其周边地区零星圈出了多个安山玢岩露头,它们呈岩脉产出于罗峰溪群内,时代定为燕山早期[30]。本次地质考察通过野外和镜下特征认为该地区岩石具有明显的杏仁构造,且SiO2质量分数变化范围为50.80%~52.48% (<

53%),该套岩石属于典型的玄武安山岩。本文选取新鲜样品的LAICPMS 锆石UPb 定年结果表明,玄武安山岩内部包含了(449.6±5.3) Ma和(161.0±2.0) Ma两组年龄。但玄武安山岩产于罗峰溪群内部,表明其喷发时代应该晚于地层,罗峰溪群内部的化

石表明其时代为晚奥陶世(440 Ma)[40]。

因此,玄武安山

岩中(449.6±5.3)  Ma的年龄不能代表其喷发时代。华南地区在早古生代经历了强烈的构造热事件,在华夏地块尤为突出,华夏地块的大多数岩石发生变质变形改造及混合岩化,同时也引发了大面积的岩浆活动[41]。玄武安山岩中(449.6±5.3) Ma的年龄与华南板块加里东期发育的岩浆事件一致,代表了玄武安山岩形成过程中的岩浆锆石捕虏晶。結合上述讨论,(161.0±2.0) Ma的年龄应为邵武地区玄武安山岩的喷发时代,为晚侏罗世。

侏罗纪是华南板块重要的构造岩浆活化期,该时期岩浆岩广泛出露,包括中酸性及基性岩浆岩[42]。其中大量的A型花岗岩、双峰火山岩、板内拉斑玄武岩和板内碱性玄武岩分布于南岭构造带[43]。而东南沿海地区则发育大量的中酸性岩浆岩,如邢光福等[6]报道了福安地区安山岩的年龄为(162.3±3.7) Ma,张伟等[13,44]认为浙江松阳毛弄组火山岩的时代为180~153 Ma。武夷山北缘侏罗纪岩浆活动也有大量的报道和记录,如孟祥金等[4546]认为冷水坑地区的火山岩时代为160~146 Ma,Wang等[47]获得了闽西北地区I型花岗岩的时代为161 Ma。本文年代学结果及上述数据表明,华南东部晚侏罗世大规模岩浆作用形成的火山侵入岩在闽北邵武地区也有体现,由于晚侏罗世—早白垩世处于华南构造域转换的特殊时期,因此,邵武地区玄武安山岩的厘定对于进一步阐明华南大地构造演化具有很好的指示意义。

4.2  岩石成因

玄武安山岩中相对低MgO、TFe2O3质量分数,低Mg#值(标准地幔的Mg#值为68~75[48]),贫不相容元素 Ni、Co、V,富Al2O3 、Na2O、Sr等,均显示出演化岩浆的特点。微量元素蛛网图(图8b)中Sr、P、Ti元素的亏损可能与斜长石、磷灰石和FeTi氧化物的分离结晶有关。稀土元素配分曲线图(图8a)中,样品中弱的Eu负异常(δEu=0.76~0.84)也暗示了可能存在角闪石或斜长石的分离结晶。w(Sm)w(Rb)图(图9a)中可见样品主要分布于含角闪石的分离结晶趋势线附近。根据 100(Fe+Mg+Mn)/Ti100Si/Ti图解(图9b),邵武地区玄武安山岩样品数据分布于单斜辉石分离结晶线附近,同样说明岩浆演化过程中发生了以单斜辉石为主的分离结晶作用。结合上述特征,笔者认为玄武安山岩岩浆演化过程中发生了一定的结晶分异作用。

玄武安山岩的SmNd同位素组成特征显示,它们具有较小的εNd(t)值,从原始地幔标准化微量元素蛛网图(图8b)中可见,样品显示出 Nb、Ta强烈亏损,Pb 正异常和 Ti 弱负异常,造成这种特征的原因可能为:1)源区富集,即岩浆源区存在有因俯冲进入地幔的地壳物质组分;2)玄武安山岩在形成过程中受到了强烈的地壳物质混染。物质组成方面,Nb/Ta值在岩浆形成之后基本上能够保持不变,对于反映岩浆源区有很好的指示意义。地壳物质相对于地幔熔体具有较低的Nb、Ta质量分数,较低的Nb/Ta值(地壳中Nb/Ta=8.33~13.33,地幔中Nb/Ta=18.27[49])和较高的Th质量分数;相比于原始岩浆,地壳混染后的岩浆具有较高的Th/Nb(>5)和Th/Ta(>10)值[48],而样品中Nb/Ta值为14.98~23.89,Th/Nb和Th/Ta值分别为0.30~0.41和5.40~8.15。据上述特征,认为邵武玄武安山岩岩浆并未表现出明显地壳混染的特征。另一方面,其他研究表明,高场强元素中Zr/Hf值对于岩浆源区的区分具有很好的指示意义,如地幔中Zr/Hf值约为50,而地壳中Zr/Hf值约为36[50]。样品中Zr/Hf值为38.85~40.19,指示邵武玄武安山岩的形成过程又涉及到一定壳源物质加入。岩浆上升过程中,无论是上地壳还是下地壳的混染,在造成岩浆 Pb 正异常的同时,均能够导致混染岩浆中 Ti 质量分数降低从而引起 Ti 的负异常[51],样品中具有明显的Pb 正异常和Ti负异常,表明有地壳物质的加入。稀土元素配分曲线图(图8a)中,样品曲线与OIB曲线样式相似,但其微量元素中却表现为Nb、Ta负异常及Pb正异常等,显示出富集型地幔的特征。另外,玄武安山岩内部锆石εHf(t)值为很小的负值,表明其源区主要来自古老的地壳物质。样品εHf(t)和εNd(t)均具有很小的负值,分别为-14.33~-8.98和-9.2~-8.4,这在东南沿海地区的镁铁质火成岩是很少见的[52]。相反地,这种情况在长英质岩石中较常见,如Zhao等[53]获得了南岭地区晚侏罗世桃山花岗质岩石较低的εHf(t)(-14.3~-4.8)和εNd(t)(-11.2~-8.8)值,并根据地球化学特征认为它们源于古老变沉积岩的部分熔融。Li等[54]得出晚侏罗世佛冈岩基的εHf(t)和εNd(t)值分别为(-11.5~-3.1)、(-12.2~-4.3),属于古元古代中基性岩的部分熔融产物。刘高峰等[55]报道了遂川晚侏罗世花岗岩εHf(t)值为(-19.5~-8.5),代表了深部热驱动下古老地壳熔融的产物。类似的岩石在紫金山、沩山等其他地区也均有报道[5658]。华夏地块东部古老基底的εHf(t)和εNd(t)值分别在<-10和-16~-8。因此,结合本次研究得出的玄武安山岩εHf(t)和εNd(t)较小的负值,本文认为邵武玄武安山岩源区在发生部分熔融之前就已经发生了富集,幔源岩浆不断底侵到中、下地壳,破坏并置换了原有的老地壳,两者在源区发生相互作用,并通过部分熔融产生了母岩浆,同时也将古老地壳和地幔的部分地球化学信息传递到母岩中,随后通过演化形成了玄武安山岩。在岩浆演化和发展过程中并未经历明显的地壳混染。

4.3  大地构造意义

大量研究表明,在中—晚侏罗世,华南地区发生了挤压到板内伸展的构造体制转变[11,54,5960]。但对于具体构造转换的时间仍存在争议。Shi等[10]认为,随着俯冲角度的变化,构造背景从火山弧转变为中侏罗世(165~150 Ma)的弧后伸展,这对应着区域构造应力场从挤压向伸展的转变。余心起等[61]通过沉积盆地研究,指出中国东南部早—中侏罗世普遍为拉张裂陷沉积环境,在泛滥平原环境中有裂陷型盆地共生。而其他学者[6,10,1217,45,47,62110]认为伸展作用在晚侏罗世才开始,中侏罗世为挤压环境(表6)。Su等[111]提出,晚侏罗世发生了有限的地幔上升流,暗示了从挤压到伸展的过渡阶段。Li等[112]指出,中—晚侏罗世太平洋板块持续北西向俯冲造成了大面积的挤压作用,在南岭地区发生了板片撕裂,形成了近EW向的裂谷带。舒良树[27]也表示,在中侏罗世(190~160 Ma)期间东南沿海地区遭受了强烈挤压,此时南岭地区为拉张环境,主要发育双峰式火山岩、层状基性杂岩体、A型花岗岩等。

邵武地区玄武安山岩具有高Zr/Y、Th/Hf、Ta/Hf值,在构造环境判别图中,主要位于板内玄武岩区和大陆拉张带(初始裂谷)玄武岩区(图10),指示出玄武安山岩是在中侏罗世晚期(161.0 Ma)板块内部拉张环境下形成的。根据华南板块内火成岩地球化学的差异性及变化规律,古太平洋板块俯冲过程中,中生代经历了板片的回撤过程[113117],而不均一的板片后撤能够导致板片撕裂[113,118121],这就引发了明显的伸展作用,并导致软流圈物质上涌、底侵、置换华夏板块古老地壳物质,从而形成富集源区。

结合前人研究成果,本文支持研究区在中侏罗世晚期属于拉张环境的观点,即区域伸展作用在中侏罗世晚期已经有所体现。但另一方面,构造地质学[11]、沉积学[26]、岩石学[13,122]等证据显示出中侏罗世东南沿海发育强烈的挤压应力场,结合前人数据及本次研究结果,认为该地区属于整体挤压背景下的局部伸展环境。邢光福等[6]指出中—晚侏罗世华南处于挤压还是拉张环境这一争议的原因在于,火山岩主要沿张性断裂上升侵位或喷发,但这种张性断裂有可能是区域整体挤压背景下局部存在拉张;基于金华地区花岗岩类的地球化学分析,高万里[74]指出由于古太平洋板块俯冲的后退,华南板块经历了从整体挤压和局部伸展背景(中侏罗世)到大规模伸展背景(晚侏罗世)的转变。因此,本文认为邵武地区火山作用发生于板内伸展,表明中侏罗世晚期在东南沿海的俯冲的背景下仍局部存在伸展环境,这源于板块回撤引发的拉伸,同时也诱发了俯冲带深部地幔物质与地壳的混合、置换等作用。

5  结论

1)LAICPMS 锆石 UPb 同位素定年结果表明,邵武地区玄武安山岩形成时代为中侏罗世晚期((161.0±2.0) Ma),指示出闽北地区存在中侏罗世晚期的一期岩浆作用。

2)邵武地区玄武安山岩为钙碱性玄武岩,岩石富集轻稀土及大离子亲石元素Rb、Ba 和K,高场强元素Nb、Ta、Ti和P相对亏损。地球化学特征指示其母岩浆应是由拉张环境下的富集型岩石圈地幔部分熔融产生,经历一定结晶分异作用,其地壳混染作用不明显。

3)邵武地区玄武安山岩指示中侏罗世晚期东南沿海地区俯冲背景下,板片发生回撤导致局部拉张。

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