Mesozoic-Cenozoic Mafic Magmatism in Western Shandong Province and Its Implication for the Chemical Evolution of the Mantle
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摘要: 镁铁质火成岩作为分布最为广泛的典型幔源岩石, 已成为探索地幔化学性状及示踪岩石圈深部过程的主要研究对象.通过对典型样品元素-同位素组成的系统测定, 并结合前人已有资料, 综合研究了鲁西中生代和新生代镁铁质岩石的地质与地球化学特征.研究结果表明, 中生代镁铁质火成岩总体具有富轻稀土和大离子亲石元素、贫高场强元素、ISr值变化范围大(0.70396~0.71247)、εNd (t) 值显著偏低(-9.20~-21.21) 的地球化学特征, 但该区南部和北部的中生代镁铁质岩石在元素-同位素组成上仍存在一定差别, 主要表现在南部较之北部镁铁质岩石具有更高的稀土总量(ΣREE为325.52×10-6~555.75×10-6)和轻、重稀土比值(LREE/HREE=17.75~25.97), 以及更高的LILE/HFSE比值(如La/Nb=6.37~13.85, Th/Nb=0.52~1.53).南部镁铁质岩石较之北部镁铁质岩石也更富放射成因锶, ISr值分别为0.70844~0.71247和0.70396~0.70598.元素-同位素综合示踪指示鲁西中生代地幔总体具有因岩石圈大规模拆沉作用形成的EMⅠ型富集地幔特征, 但其南部叠加了因深俯冲而进入地幔的扬子陆壳的影响, 因而表现出EMⅠ和EMⅡ组分混合的富集地幔特征.新生代玄武岩具有类似于大洋玄武岩的地球化学特征, 其源区应为亏损的软流圈地幔, 但在部分熔融形成岩浆之前遭受了近期的交代作用.自中生代至新生代, 华北克拉通地幔具有由富集向亏损演变的趋势, 这一化学性状的演变最可能是中生代以来岩石圈大规模拆沉作用, 导致软流圈地幔上涌并对原有岩石圈地幔再改造所致.Abstract: Mafic igneous rock, a widely distributed and typical mantle-derived rock, has become a major research source for revealing possible mechanisms for the chemical evolution of the mantle. Combined geological and geochemical studies have been carried out on the Mesozoic and Cenozoic mafic igneous rocks in western Shandong Province, determining the elemental and isotopic compositions of typical samples. Generally, the Mesozoic mafic igneous rocks have enriched LREE and LILE concentrations, low HFSE abundance, and display highly variable ISr ratios (0.703 96-0.712 47) and distinctly negative εNd (t) values (-9.20 to -21.21). Compared to the north of western Shandong Province, the Mesozoic mafic rocks in the south have higher total REE contents (ΣREE=325.52×10-6-555.75×10-6), higher LREE/HREE ratios (17.75-25.97), and higher LILE/HFSE ratios (e. g., La/Nb=6.37-13.85, Th/Nb=0.52-1.53). They also have more radiogenic Sr isotopic compositions, with ISr values of 0.708 44-0.712 47 and 0.703 96-0.705 98, respectively. Integrated elemental and isotopic tracing suggest that the Mesozoic mantle of western Shandong Province is generally characterized by the EMⅠ component, which was probably formed by large-scale lithospheric delamination, whereas the mantle source in the south part has been superimposed by the influence of the deep subducted Yangtze continental materials, and thus displays the mixed features of EMⅠ and EMⅡ components. The geochemical characteristics of the Cenozoic basalts are similar to those of the oceanic basalts. It is likely that the rocks were derived from a depleted asthenosphere source but underwent metasomatism shortly before partial melting. From the Mesozoic to the Cenozoic, the nature of the mantle beneath the North China craton evolved from an enriched one to a depleted one. This chemical evolution most likely resulted from an asthenosphere upwelling, which was induced by large-scale lithospheric delamination and thus replaced the original lithospheric mantle with a newly created one.
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0. 引言
地幔的化学性状及其随时间的演化是当前固体地球科学领域倍受关注的前沿和热点, “岩石探针” (lithoprobe) 被视为解决这一问题的最有效手段(邓晋福等, 1996).镁铁质火成岩(玄武岩及相关侵入岩) 作为分布最为广泛的典型幔源岩石, 已成为探索地幔化学性状及示踪岩石圈深部过程的主要研究对象.
鲁西中、新生代镁铁质岩浆作用广泛发育, 中生代镁铁质火成岩较集中的分布区有2个: 其一位于鲁西地块南部的费县—蒙阴一带, 包括费县方城含辉石岩捕虏体的玄武岩和蒙阴钾玄质火山岩; 另一位于鲁西地块北部的济南—邹平一带, 包括济南辉长质杂岩体、邹平辉长质杂岩体及相关的玄武质火山岩.新生代镁铁质火成岩主要分布在临朐—昌乐一带, 为碱性玄武岩(图 1).前人对上述岩石进行过不同程度的研究, 并据此探讨了源区地幔的性状与演化(陈道公, 1992; 支霞臣等, 1990, 1994; Guo et al., 2001, 2003; Qiu et al., 2002; Zhang et al., 2002, 2004; 裴福萍等, 2004; Zheng et al., 2005), 但以往的工作主要是针对局部地段或局部时段的岩石开展的, 对源区地幔性质及其富集机制的认识存在分歧.本次研究将区内中、新生代镁铁质火成岩置身于一个统一的整体来考虑, 通过对典型样品元素-同位素组成的系统测定, 并结合前人已有资料, 对鲁西乃至整个华北克拉通中、新生代地幔的化学性状和演化及其形成机制进行了全面探讨.
图 1 鲁西中、新生代镁铁质岩石分布1.新生代碱性玄武岩; 2.中生代火山岩; 3.辉长质杂岩体; 4.金伯利岩; 5.碳酸岩; 6.镁铁-超镁铁质煌斑岩; 7.断裂; Ⅰ.华北板块; Ⅱ.扬子板块Fig. 1. Sketch map showing the distribution of the Mesozoic and Cenozoic mafic igneous rocks in western Shandong Province1. 地质概况及岩石学特征
鲁西位于华北板块东南部, 处于沂沭断裂带西侧, 构造上紧邻大别-苏鲁聚合带(图 1).其基底地层主要由太古界泰山群中高级变质岩组成, 基底之上为寒武—奥陶系碳酸盐岩地层, 石炭—二叠纪出现沉积间断.中、新生代火山岩多分布于断陷型火山盆地中, 主要为中心式火山喷发的产物.辉长质杂岩体分布于鲁西地块北部的济南—邹平一带, 呈岩镰或岩墙状产出, 其中以济南杂岩体规模最大, 出露面积达400 km2 (李昶绩, 1992).除中、新生代镁铁质火成岩外, 鲁西地块内还分布有金伯利岩、碳酸岩和镁铁-超镁铁质煌斑岩(图 1).
临朐—昌乐一带新生代玄武岩主要形成于中新世, K-Ar年龄介于10.6~18.9 Ma之间(金隆裕, 1985), 主要岩性为碱性橄榄玄武岩.岩石多呈细粒斑状或显微斑状结构、致密块状或气孔-杏仁构造, 斑晶矿物主要为橄榄石(Fo=80~100)和钛普通辉石, 基质由钛普通辉石、斜长石和少量橄榄石及磁铁矿组成.岩石中普遍含二辉橄榄岩包体及铝质辉石和蓝刚玉等矿物巨晶.
中生代典型玄武岩见于费县方城一带, K-Ar年龄为119~125 Ma (Zhang et al., 2002; 裴福萍等, 2004).按化学成分可分为碱性玄武岩和拉斑玄武岩两类, 其中含有辉石岩捕虏体, 最近还发现有橄榄石捕虏晶(Zhang et al., 2004).但鲁西中生代火山岩的主体为钾玄质火山岩, 主要岩性包括粗面玄武岩、玄武粗安岩及偏中性的粗安岩、安粗岩等.蒙阴盆地钾玄质火山岩的40Ar-39Ar年龄变化于114.8~124.3 Ma之间(Qiu et al., 2002), 这套岩石中未发现深源岩石包体.
济南—邹平一带辉长质杂岩体报道的年龄较多, 但数据较分散(李昶绩, 1992). 谭东娟和林景仟(1994)测得济南辉长质杂岩体的40Ar-39Ar年龄为115 Ma, 这一年龄与邹平盆地辉长质岩体侵入青山组火山岩(K-Ar年龄为128~130 Ma, 山东省地质矿产局, 1991) 的地质事实较为吻合, 因此, 它们应属于燕山晚期岩浆活动的产物.杂岩体的主体岩性为橄榄苏长辉长岩, 组成矿物为基性斜长石、普通辉石、紫苏辉石和少量橄榄石、黑云母等, 其中济南杂岩体分异良好, 可区分为基性程度渐次降低的中心相、过渡相和边缘相, 而邹平辉长质杂岩体的分异程度较差.
2. 中、新生代镁铁质火成岩地球化学对比
表 1列出了鲁西中、新生代镁铁质火成岩代表性岩石样品的主量、微量和稀土元素含量及Sr-Nd同位素组成, 结合前人已有资料, 区内中、新生代镁铁质火成岩在地球化学上具有以下显著差别:
表 1 鲁西中、新生代镁铁质火成岩代表性样品主量和微量元素含量及Sr-Nd同位素组成Table Supplementary Table Major, trace element contents and Sr-Nd isotopic compositions of the typical mafic igneous rocks in western Shandong Province
(1) 新生代玄武岩明显富钛, 其TiO2含量普遍高于2.00%, 平均为2.61% (据52件样品资料统计), 与洋岛玄武岩相近, 而中生代镁铁质岩石的TiO2含量一般均低于1.30% (图 2a). (2) 除济南辉长质杂岩体因分异良好而样品的TFeO有较大的变化范围外, 其余中生代镁铁质岩石的TFeO含量较新生代玄武岩普遍偏低(图 2b), 指示后者有较深的岩浆源区(Lassiter and Depaolo, 2000). (3) 中、新生代镁铁质火成岩均富轻稀土, 但济南辉长质杂岩体具有最低的稀土总量[∑REE= (35.15~127.88) ×10-6]和轻、重稀土比值[LREE/HREE=3.83~8.07, (La/Yb)N=3.41~11.40, 图 2c及图 3b], 这主要是由于该杂岩体含有较多的斜方辉石所致, 因为斜方辉石的稀土含量显著低于单斜辉石, 但又较单斜辉石相对富重稀土(Henderson, 1982).此外, 鲁西地块南部费县—蒙阴一带镁铁质岩石与北部济南—邹平一带镁铁质岩石具有明显不同的稀土元素组成特征, 前者的稀土总量和轻、重稀土比值显著高于后者(图 2c、2d), 说明二者的源区组成或成岩过程存在差异.新生代玄武岩的稀土元素特征总体较接近北部济南—邹平一带的镁铁质岩石. (4) 中、新生代镁铁质岩石Rb、Ba、Th、U等大离子亲石元素均较富集, 但新生代玄武岩的微量元素分布模式相似于洋岛玄武岩, 缺乏高场强元素的亏损(图 4a), 同样指示其来自较深的地幔源区, 而中生代镁铁质岩石均表现出Nb、Ta、Zr、Hf、P、Ti等高场强元素的亏损(图 4b), 具有与俯冲作用有关岩浆岩的典型特征(Kelemen et al., 1990; Stolz et al., 1996). (5) 与缺乏高场强元素亏损的特点相对应, 新生代玄武岩的LILE/Nb比值显著低于中生代镁铁质岩石, 如样品的La/Nb、Ba/Nb、Th/Nb比值分别为0.57~0.89、4.6~32.7和0.052~0.098, 而中生代岩石的相应值分别为1.54~13.85、59.4~1 265.4和0.201~1.529, 其中鲁西地块南部费县—蒙阴一带的中生代镁铁质岩石具有最高的La/Nb、U/Nb和Th/Nb比值(图 2e、2f, 表 2).新生代玄武岩的La/Nb、Ba/Nb比值与洋脊、洋岛玄武岩及原始地幔较接近, 但中生代岩石的相应值(特别是Ba/Nb比值) 明显偏高(表 2, 图 5), 充分说明区内中、新生代源区地幔性状存在差异. (6) 新生代玄武岩的ISr值普遍低于0.704 5, εNd (t) 值多为正值, 与洋脊和洋岛玄武岩相似(图 6), 而中生代镁铁质岩石则具有迥然不同的Sr-Nd同位素组成特征, 其中最明显的差别体现在εNd (t) 值上, 中生代镁铁质岩石的εNd (t) 值均为显著的负值(一般均低于-10).此外, 鲁西地块南部和北部中生代镁铁质岩石的Sr-Nd同位素组成也存在差别, 南部岩石较之北部岩石有明显偏高的ISr值.大别-苏鲁聚合带北侧的中生代幔源岩石, 如北大别镁铁-超镁铁岩(Jahn et al., 1999)、北淮阳富钾火山岩(邱检生等, 2002)、鲁西地块南缘镁铁-超镁铁质煌斑岩(Qiu et al., 2002), 以及胶莱盆地钾玄质火山岩(Fan et al., 2001)和侵入岩(邱检生和王德滋, 1999), 均具有与鲁西地块南部镁铁质岩石类似的Sr-Nd同位素组成(图 6), 指示这些岩石的岩浆源区性状相似.
图 2 鲁西中、新生代镁铁质岩石地球化学对比1.新生代玄武岩; 2.方城中生代玄武岩; 3.蒙阴中生代钾玄质火山岩; 4.济南辉长岩; 5.邹平辉长岩; 6.邹平中生代玄武质火山岩.C.新生代玄武岩; MS.鲁西地块南部中生代镁铁质火成岩; MN.鲁西地块北部中生代镁铁质火成岩.图中除本文资料外, 其余资料引自: 新生代玄武岩据金隆裕(1989)、支霞臣(1990)、陈道公(1992)和牛漫兰等(2001); 中生代镁铁质岩石据邱检生等(1997)、Guo et al. (2001)、Zhang et al. (2002)、Guo et al. (2003)和裴福萍等(2004)Fig. 2. Geochemical comparison diagrams of the Mesozoic and Cenozoic mafic igneous rocks in western Shandong Province
图 3 鲁西中、新生代镁铁质岩石稀土元素球粒陨石标准化配分型式(球粒陨石值据Boynton, 1984)Fig. 3. Chondrite-normalized REE distribution patterns of the Mesozoic and Cenozoic mafic igneous rocks in western Shandong Province
图 4 鲁西中、新生代镁铁质岩石微量元素相对于原始地幔标准化蛛网图(原始地幔值据Sun and McDonough, 1989)Fig. 4. Primitive mantle-normalized spidergrams of trace elements for the Mesozoic and Cenozoic mafic igneous rocks in western Shandong Province表 2 鲁西中、新生代镁铁质火成岩不相容元素比值及与各主要地球化学储库的对比Table Supplementary Table Incompatible element ratios of the Mesozoic and Cenozoic mafic igneous rocks in western Shandong Province and comparison with those of the other geochemical reservoirs
图 5 鲁西中、新生代镁铁质岩石La/Nb-Ba/Nb关系(图例及资料来源同图 2)Fig. 5. La/Nb versus Ba/Nb diagram of the Mesozoic and Cenozoic mafic igneous rocks in western Shandong Province
图 6 鲁西中、新生代镁铁质岩石(87Sr/86Sr) i-εNd (t) 关系1.新生代玄武岩; 2.邹平玄武质火山岩; 3.邹平辉长岩; 4.济南辉长岩; 5.方城玄武岩; 6.蒙阴钾玄质火山岩; 7.鲁西地块南缘中生代镁铁-超镁铁质煌斑岩; 8.北淮阳富钾火山岩; 9.北大别镁铁-超镁铁岩; 10.胶莱盆地钾玄质火山-侵入岩; DM.亏损地幔; MORB.洋脊玄武岩; OIB.洋岛玄武岩; PM.原始地幔; EMⅠOIB.具EMⅠ型富集地幔特征的洋岛玄武岩; EMⅠ.Ⅰ型富集地幔; EMⅡ.Ⅱ型富集地幔; 其余代号同图 2.除本文资料外, 新生代玄武岩据陈道公(1992)和支霞臣等(1994); 中生代镁铁质岩石据Guo et al. (2001)、Qiu et al. (2002); Zhang et al. (2002)和Guo et al. (2003); 北大别镁铁-超镁铁岩据Jahn et al. (1999); 北淮阳富钾火山岩据邱检生等(2002); 胶莱盆地钾玄质火山-侵入岩据邱检生和王德滋(1999)、Fan et al. (2001)Fig. 6. (87Sr/86Sr) i versus εNd (t) diagram for the Mesozoic and Cenozoic mafic igneous rocks in western Shandong Province3. 讨论
由于所研究的镁铁质岩石产于大陆区, 其地球化学特征是源区地幔性状的反映, 还是浅部地壳物质混染的结果, 抑或是二者的叠加, 这是首先需要讨论的问题.
3.1 地壳混染
鲁西新生代玄武岩普遍含幔源捕虏体或捕虏晶, 说明岩浆上升速度较快, 在上升过程中与围岩地壳物质的相互作用不强(Glazaer and Farmer, 1992).岩石中斑晶橄榄石的Fo值多大于90, 全岩MgO含量普遍高于8%, 指示它们应属于原始玄武岩浆(McKenzie and Bickle, 1988).此外, 岩石的LILE/HFSE比值(如La/Nb、Ba/Nb、Th/Nb、Th/Ta等)和Sr-Nd同位素组成与大洋玄武岩相似, 上述特征均说明岩浆未遭受显著的地壳物质的混染作用.
对区内中生代镁铁质岩石而言, 方城玄武岩含有辉石岩捕虏体和橄榄石捕虏晶, 也指示其岩浆上升速度快, 不具备与陆壳物质发生强烈相互作用的地质前提.蒙阴钾玄质火山岩尽管未发现幔源包体, 但岩石的K2O/P2O5、K2O/TiO2比值相对稳定, 全岩Sr、Nd同位素组成与从全岩中分离出的单斜辉石单矿物十分接近(Qiu et al., 2002), 其元素-同位素组成特征基本可与方城玄武岩相对比.济南辉长质杂岩低硅高镁, Mg#值普遍大于0.65, 与原始玄武岩浆相似.邹平一带镁铁质火山-侵入杂岩尽管分异演化程度略高, 但其微量元素分布模式及Sr-Nd同位素组成与济南杂岩体类似.此外, 同一产地样品的Sr同位素组成相对均一, ISr值不随相应样品SiO2含量和Mg#指数的变化而发生显著改变(图 7a), 上述特征均不支持区内镁铁质岩石成岩过程中存在强烈的浅部地壳物质混染作用, 其他学者的研究也得出类似的结论(Guo et al., 2001, 2003; Zhang et al., 2002).因此, 我们可以有效地利用全岩地球化学特征反演源区地幔性状.
3.2 源区示踪
由于岩石的LILE/HFSE比值及Sr-Nd同位素组成一般不受部分熔融和分离结晶过程的影响, 因此是示踪源区化学性状的良好指示剂.由图 6可看出, 新生代玄武岩的Sr、Nd同位素组成落在大洋玄武岩范围内, 靠近亏损地幔(DM) 端元, 指示鲁西地幔在新生代具亏损特征, 或具软流圈地幔的属性, 但岩石富轻稀土和大离子亲石元素的特征又与同位素示踪所得出的源区信息不吻合, 这一元素和同位素组成的解耦现象被认为是源区地幔在部分熔融形成岩浆之前遭受过近期交代作用所致, 因为交代作用发生的时间距今较近, 使得放射成因同位素的积累有限, 因而保留了亏损地幔的同位素印记(支霞臣, 1990).
中生代镁铁质岩石的Sr、Nd同位素组成明显有别于原始或亏损地幔, 在(87Sr/86Sr) i-εNd (t) 关系图上, 均显示出富集的岩石圈地幔的属性, 其中鲁西地块北部济南—邹平一带镁铁质岩石的投影点靠近EMⅠ端元, 而地块南部费县—蒙阴一带镁铁质岩石的投影点则指示地幔源区中含有EMⅡ组分(图 6).利用不相容元素比值对岩浆源区性状反演也得出类似信息, 如中生代镁铁质岩石的La/Nb、Ba/Nb、Rb/Nb和Th/Nb比值较之原始或亏损地幔显著偏高(表 2, 图 5), 北部济南—邹平一带镁铁质岩石的Ba/Nb、Ba/La和Ba/Th比值又明显高于南部费县—蒙阴一带的镁铁质岩石(表 2, 图 7b), 这一特征也指示北部镁铁质岩石的岩浆源区主要为EMⅠ组分, 而南部费县—蒙阴一带镁铁质岩石的岩浆源区则含有EMⅡ组分, 表现出EMⅠ和EMⅡ组分混合的富集地幔特征, 因为与EMⅡ组分相比, EMⅠ组分有更高的Ba/Nb、Ba/La和Ba/Th比值(Weaver, 1991; Tatsumoto et al., 1992). 范蔚茗和郭锋(2005)进一步认为华北内部EMⅠ型富集地幔的主要岩性为弥散状金云母相橄榄岩.综上所述, 鲁西地幔在中生代总体具有富集特征, 但北部和南部富集地幔的端元组分存在差异.
图 7 鲁西中、新生代镁铁质岩石ISr-Mg#、SiO2关系(a)和La/Nb-Ba/La关系(b)Fig. 7. ISr versus Mg# and SiO2 diagrams (a) and La/Nb versus Ba/La diagram (b) for the Mesozoic and Cenozoic mafic igneous rocks in western Shandong Province3.3 鲁西地幔化学性状的演化及其形成机制
由前述元素-同位素的综合示踪可看出, 自中生代至新生代, 鲁西地幔具有由富集向亏损或由岩石圈地幔向软流圈地幔演变的趋势.大量研究资料表明, 这一演变趋势在整个华北板块具有普遍性(Fan et al., 2000; Zhou et al., 2002).因此, 正确理解鲁西中、新生代地幔化学性状的演变, 涉及到对整个华北板块富集地幔形成机制及岩石圈大规模减薄过程的认识.
华北富集地幔是早已有之, 还是至中生代才形成, 这是首先要探讨的问题. Guo et al. (2001)认为华北富集地幔在太古代时已存在, 并对中生代镁铁质岩石的形成具有重要制约作用.然而自太古代至早元古代, 华北板块幔源岩石的εNd (t) 值基本均为正值(李曙光和张宗清, 1990; 张宗清, 1998), 主要集中在+2~+3范围内(洪大卫等, 2003), 说明这一时段华北地幔应处于亏损状态.早古生代蒙阴金伯利岩εNd (t) 值变化于-4.78~+2.76 (图 8a), 平均为-1.10, 其中镁铝榴石斑晶的εNd (t) =-1.21~-4.78, 平均为-3.01 (路凤香和郑建平, 1996), 说明此时华北地幔已开始自亏损向弱富集状态转变.至中生代, 华北幔源岩石普遍具有显著偏低的εNd (t) 值(表 3), 其中鲁西地块中生代镁铁质岩石的εNd (t) 值介于-9.20~-21.21, 峰值变化于-12~-18之间(图 8b), 说明华北地幔大规模的富集应发生在中生代.
图 8 鲁西不同时代幔源岩石εNd (t) 值频率分布古生代金伯利岩据路凤香和郑建平(1996);中、新生代岩石同图 6Fig. 8. Histogram of εNd (t) values for mantle-derived rocks of different ages in western Shandong Province表 3 华北地块东部中生代镁铁质岩石εNd (t) 值Table Supplementary Table εNd (t) values of the Mesozoic mafic igneous rocks in east part of the North China block
对华北中生代大规模富集地幔的形成机制目前存在着不同的认识, 概括起来主要有2种观点: 一种观点认为华北中生代地幔的富集主要与扬子-华北板块碰撞及它们之间的大陆深俯冲作用有关, 深俯冲的扬子大陆地壳同软流圈地幔相互作用形成富集地幔(Zhou et al., 2002); 另一种观点认为华北中生代地幔的富集主要与岩石圈大规模拆沉作用有关, 太古代古老地壳物质被拆沉而重循环进入地幔, 导致地幔成分发生改变形成富集地幔(洪大卫等, 2003; Gao et al., 2004).
由于深俯冲的扬子大陆地壳的影响难以波及到华北腹地和北缘, 而中生代镁铁质岩石具显著偏低εNd (t) 值的特征在整个华北板块又具普遍性, 因此, 我们认为岩石圈大规模拆沉作用应该是导致华北中生代地幔富集的主控因素.众多学者也从不同角度探讨了华北中生代大规模拆沉作用形成的可能机制, 如张旗等(2001)指出燕山中晚期中国东部(主要为华北地块) 可能是一个高原, 其下地壳在分熔出埃达克质岩石后, 残余物转变成密度大的榴辉岩, 同时加厚的岩石圈地幔比软流圈地幔冷, 导致其密度也大, 因此, 岩石圈地幔和下地壳可以发生拆沉作用而进入软流圈地幔, 并促使软流圈地幔上涌. 洪大卫等(2003)认为华北中生代岩石圈大规模拆沉作用主要是由Pangea超大陆的裂解所致.最近, Gao et al. (2004)报道了华北北部侏罗纪火山岩起源于榴辉岩相下地壳拆沉进入上地幔后发生部分熔融的直接地球化学证据, 并认为发生于中国东部中—新生代克拉通内部的大陆岩石圈减薄及强烈的构造-岩浆作用, 均与岩石圈下部的拆沉作用和软流圈物质上涌作用有关(Gao et al., 1998, 2003).因此, 中生代华北岩石圈大规模拆沉作用已得到多数学者的认同.
尽管华北中生代地幔普遍表现出因大规模拆沉作用而形成的EMⅠ型富集地幔的特征, 但其南缘幔源岩石明显叠加有因深俯冲而进入地幔的扬子陆壳的印记, 突出表现在南缘幔源岩石的ISr值明显偏高.以鲁西地块为例, 其北部济南—邹平一带镁铁质岩石的ISr值变化于0.703 96~0.705 98, 而南部费县—蒙阴一带镁铁质岩石则变化于0.708 44~0.712 47 (图 9), 北大别及胶莱盆地中生代镁铁质岩石的ISr值均偏高(Jahn et al., 1999; Fan et al., 2001).通过对大别造山带北缘磨子潭—晓天一带富钾火山岩Nd、Sr同位素组成的混合模拟计算, 表明其岩浆源区也存在有因俯冲而进入地幔的扬子古老基底——崆岭群的混染和交代作用(邱检生等, 2002).因此, 华北中生代岩石圈地幔的富集总体受控于岩石圈的大规模拆沉作用, 但其南缘叠加了因深俯冲而进入地幔的扬子陆壳的影响.正是由于深俯冲的陆壳物质对因岩石圈拆沉而形成的EM Ⅰ型富集地幔的再改造, 使得地幔中放射性Sr同位素再次富集, 因此, 华北南缘富集地幔应为EM Ⅰ和EM Ⅱ的混合类型, 这也是导致鲁西地块北部和南部中生代镁铁质岩石地球化学特征存在差异的关键所在.
图 9 鲁西中生代镁铁质火成岩ISr频率分布(资料来源同图 7a)Fig. 9. Histogram of ISr values for the Mesozoic mafic igneous rocks in the north and south parts of western Shandong Province至新生代, 由于岩石圈的拆沉及引张作用的加剧, 促使软流圈地幔上涌, 并对原有岩石圈地幔改造或置换而使岩石圈发生大规模的减薄(郑建平等, 2000, 2003; Zheng et al., 2005).鲁西地块新生代玄武岩的εNd (t) 值普遍大于+3, 峰值集中在+4~+6 (图 8c), 这一特征也佐证了其起源于软流圈地幔, 因为来源于岩石圈地幔玄武岩的εNd (t) 值一般小于+3 (Livaceari and Perry, 1993). 因此, 由岩石圈大规模拆沉导致软流圈地幔上涌并对原有岩石圈地幔再改造, 应是鲁西乃至整个华北板块自中生代至新生代地幔化学性状由富集向亏损演变的主要原因.
4. 结论
(1) 鲁西地块中生代镁铁质火成岩总体具有富轻稀土和大离子亲石元素、贫高场强元素、ISr值变化范围大、εNd (t) 值显著偏低的地球化学特征.但南部和北部镁铁质岩石在微量和同位素组成上仍存在一定差别, 主要表现在南部较之北部镁铁质岩石具有更高的稀土总量和轻、重稀土比值, 以及更高的LILE/HFSE比值和ISr值.元素-同位素综合示踪指示鲁西地块中生代地幔总体具有富集特征, 其中北侧富集地幔应为EMⅠ型, 而南侧则为EMⅠ和EMⅡ的混合类型. (2) 新生代玄武岩具有类似于大洋玄武岩的地球化学特征, 其源区应为亏损的软流圈地幔, 但在部分熔融形成岩浆之前遭受了近期的交代作用. (3) 自中生代至新生代, 鲁西乃至整个华北克拉通地幔具有由富集向亏损演变的趋势, 这一化学性状的演变主要是中生代以来岩石圈大规模拆沉作用, 导致软流圈地幔上涌并对原有岩石圈地幔再改造所致.
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图 1 鲁西中、新生代镁铁质岩石分布
1.新生代碱性玄武岩; 2.中生代火山岩; 3.辉长质杂岩体; 4.金伯利岩; 5.碳酸岩; 6.镁铁-超镁铁质煌斑岩; 7.断裂; Ⅰ.华北板块; Ⅱ.扬子板块
Fig. 1. Sketch map showing the distribution of the Mesozoic and Cenozoic mafic igneous rocks in western Shandong Province
图 2 鲁西中、新生代镁铁质岩石地球化学对比
1.新生代玄武岩; 2.方城中生代玄武岩; 3.蒙阴中生代钾玄质火山岩; 4.济南辉长岩; 5.邹平辉长岩; 6.邹平中生代玄武质火山岩.C.新生代玄武岩; MS.鲁西地块南部中生代镁铁质火成岩; MN.鲁西地块北部中生代镁铁质火成岩.图中除本文资料外, 其余资料引自: 新生代玄武岩据金隆裕(1989)、支霞臣(1990)、陈道公(1992)和牛漫兰等(2001); 中生代镁铁质岩石据邱检生等(1997)、Guo et al. (2001)、Zhang et al. (2002)、Guo et al. (2003)和裴福萍等(2004)
Fig. 2. Geochemical comparison diagrams of the Mesozoic and Cenozoic mafic igneous rocks in western Shandong Province
图 3 鲁西中、新生代镁铁质岩石稀土元素球粒陨石标准化配分型式(球粒陨石值据Boynton, 1984)
Fig. 3. Chondrite-normalized REE distribution patterns of the Mesozoic and Cenozoic mafic igneous rocks in western Shandong Province
图 4 鲁西中、新生代镁铁质岩石微量元素相对于原始地幔标准化蛛网图(原始地幔值据Sun and McDonough, 1989)
Fig. 4. Primitive mantle-normalized spidergrams of trace elements for the Mesozoic and Cenozoic mafic igneous rocks in western Shandong Province
图 5 鲁西中、新生代镁铁质岩石La/Nb-Ba/Nb关系(图例及资料来源同图 2)
Fig. 5. La/Nb versus Ba/Nb diagram of the Mesozoic and Cenozoic mafic igneous rocks in western Shandong Province
图 6 鲁西中、新生代镁铁质岩石(87Sr/86Sr) i-εNd (t) 关系
1.新生代玄武岩; 2.邹平玄武质火山岩; 3.邹平辉长岩; 4.济南辉长岩; 5.方城玄武岩; 6.蒙阴钾玄质火山岩; 7.鲁西地块南缘中生代镁铁-超镁铁质煌斑岩; 8.北淮阳富钾火山岩; 9.北大别镁铁-超镁铁岩; 10.胶莱盆地钾玄质火山-侵入岩; DM.亏损地幔; MORB.洋脊玄武岩; OIB.洋岛玄武岩; PM.原始地幔; EMⅠOIB.具EMⅠ型富集地幔特征的洋岛玄武岩; EMⅠ.Ⅰ型富集地幔; EMⅡ.Ⅱ型富集地幔; 其余代号同图 2.除本文资料外, 新生代玄武岩据陈道公(1992)和支霞臣等(1994); 中生代镁铁质岩石据Guo et al. (2001)、Qiu et al. (2002); Zhang et al. (2002)和Guo et al. (2003); 北大别镁铁-超镁铁岩据Jahn et al. (1999); 北淮阳富钾火山岩据邱检生等(2002); 胶莱盆地钾玄质火山-侵入岩据邱检生和王德滋(1999)、Fan et al. (2001)
Fig. 6. (87Sr/86Sr) i versus εNd (t) diagram for the Mesozoic and Cenozoic mafic igneous rocks in western Shandong Province
图 7 鲁西中、新生代镁铁质岩石ISr-Mg#、SiO2关系(a)和La/Nb-Ba/La关系(b)
图 7a除本文资料外, 其余资料引自Guo et al. (2001)、Zhang et al. (2002)和Guo et al. (2003); 图 7b资料来源同图 2
Fig. 7. ISr versus Mg# and SiO2 diagrams (a) and La/Nb versus Ba/La diagram (b) for the Mesozoic and Cenozoic mafic igneous rocks in western Shandong Province
图 8 鲁西不同时代幔源岩石εNd (t) 值频率分布
古生代金伯利岩据路凤香和郑建平(1996);中、新生代岩石同图 6
Fig. 8. Histogram of εNd (t) values for mantle-derived rocks of different ages in western Shandong Province
图 9 鲁西中生代镁铁质火成岩ISr频率分布(资料来源同图 7a)
Fig. 9. Histogram of ISr values for the Mesozoic mafic igneous rocks in the north and south parts of western Shandong Province
表 1 鲁西中、新生代镁铁质火成岩代表性样品主量和微量元素含量及Sr-Nd同位素组成
Table 1. Major, trace element contents and Sr-Nd isotopic compositions of the typical mafic igneous rocks in western Shandong Province
表 2 鲁西中、新生代镁铁质火成岩不相容元素比值及与各主要地球化学储库的对比
Table 2. Incompatible element ratios of the Mesozoic and Cenozoic mafic igneous rocks in western Shandong Province and comparison with those of the other geochemical reservoirs
表 3 华北地块东部中生代镁铁质岩石εNd (t) 值
Table 3. εNd (t) values of the Mesozoic mafic igneous rocks in east part of the North China block
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