Characteristics and Tectonic Affinity of Lajishan Paleo-Mantle in Qilian Orogenic Belt: A Geochemical Study of Basalts
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摘要: 拉脊山火山岩带位于祁连造山带中部, 通过对该区早古生代基性火山岩系统的地球化学研究, 揭示该区早古生代地幔的性质及其地幔域的构造归属.研究表明, 拉脊山基性火山岩可以分为两类: Ⅰ类为大陆板内碱性玄武岩, 其稀土元素组成模式为轻稀土富集型, 并具有明显的Nb、Ta负异常, 而Zr、Hf无明显的负异常; Ⅱ类为与地幔柱活动有关的拉斑玄武岩, 具有洋岛玄武岩(OIB) 特征.稀土元素组成模式同样表现为轻稀土富集型, 但其富集程度比Ⅰ类基性火山岩的富集程度弱, 无Nb、Ta、Zr和Hf负异常.基性火山岩的Sr、Nd、Pb同位素组成特征显示, 基性火山岩的地幔源区具有亏损地幔(DM) 和第二类富集地幔(EMⅡ) 混合的特点, 而第二类富集地幔端元(EMⅡ) 占主导地位, 亏损地幔(DM) 物质混入的程度较低; 并具有Dupal异常的同位素特征.通过与华北南缘、北秦岭和扬子北缘西段地幔的Pb同位素组成相比, 表明拉脊山造山带古地幔与北秦岭、南秦岭西段和扬子北缘西段地幔的Pb同位素组成相似.进而表明拉脊山造山带古地幔属于扬子型富放射性成因铅地幔, 而非华北型贫放射性成因铅地幔Abstract: The Early Paleozoic Lajishan volcanic rock belt is located in the middle part of the Qilian orogenic belt. In order to reveal the mantle nature and tectonic affinity of the Lajishan belt, this paper presents a study of the major-trace elements and Pb-Sr-Nd isotopes of the Lajishan basalts. Based on major and trace elemental data, the Lajishan basalts can be divided into two groups: continental alkaline basalts (Group Ⅰ) and tholeiites basalts (Group Ⅱ). Group Ⅰ displays LREE enrichment, distinct Nb and Ta negative anomalies and negligible Zr and Hf anomalies. Group Ⅱ is characterized by typical OIB basalts with LREE enrichment and no Nb, Ta, Zr and Hf anomalies. The formation of Group Ⅱ is related to mantle plume. Sr-Nd-Pb isotopic compositions of basalts indicate that both Group Ⅰ and Group Ⅱ were derived from a mixing source between DM and EMⅡ mantle end-members, and have Dupal Pb isotope anomaly. A regional comparison shows that the high radiogenic Pb isotopic composition of the Lajishan basalts is similar to that of the Yangtze block mantle, and is distinctly from that of the North China mantle. This implies that the Lajishan paleo-mantle has an affinity to the Yangtze mantle instead of a part of the south margin of the North China block.
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Key words:
- basalt /
- geochemistry /
- continental rift /
- Lajishan /
- Qilian orogenic belt
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图 1 拉脊山造山带地质简图(邱家骧等, 1997)
1.前寒武系; 2.下奥陶统; 3.中寒武统; 4.上寒武统; 5.下白垩统; 6.中、上奥陶统; 7.中酸性侵入岩; 8.基性、超基性侵入岩; 9.断层; 图中黑色实心圆圈为示意的采样位置点.Pm109、139、140、107为采样剖面
Fig. 1. Sketch geological map of Lajishan orogenic belt
图 2 拉脊山基性火山岩w (Na2O+K2O) -w (SiO2) (TAS) 图解(Le Maitre et al., 1989)
实心方块代表Ⅰ类基性火山岩; 五角星代表Ⅱ类基性火山岩; 实心圆圈曲线和三角形曲线为碱性玄武岩系列和拉斑玄武岩系列的分界线(Kuno, 1968; Irvine and Baragar, 1971)
Fig. 2. w (Na2O+K2O) -w (SiO2) (TAS) diagram of basalts in Lajishan orogenic belt
图 3 拉脊山基性火山岩球粒陨石标准化稀土元素组成
球粒陨石REE组成参考McDonough and Sun, 1995
Fig. 3. Chondrite-normalized REE patterns of basalts from Lajishan orogenic belt
图 4 拉脊山基性火山岩原始地幔标准化微量元素蛛网图解
原始地幔微量元素组成参考McDonough and Sun, 1995
Fig. 4. Primary mantle-normalized trace element patterns of basalts from Lajishan orogenic belt
图 5 拉脊山基性火山岩Th-Hf-Ta (a)、2Nb-Zr/4-Y (b)、Ti/100-Zr-3Y (c) 构造环境判别图解
a图据Wood (1980);A.N-MORB; B.E-MORB和板内玄武岩; C.板内碱性玄武岩; D.岛弧玄武岩; 实心方块代表Ⅰ类基性火山岩; 五角星代表Ⅱ类基性火山岩; b图据Meschede (1986);AⅠ.板内碱性玄武岩; AⅡ.板内碱性玄武岩, 板内拉斑玄武岩; B.E-MORB; C.板内拉斑玄武岩, 岛弧玄武岩; D.N-MORB; c图据Pearce and Cann (1973);A.岛弧拉斑玄武岩; B.MORB; C.钙碱性玄武岩; D.板内玄武岩; 实心方块代表Ⅰ类基性火山岩; 五角星代表Ⅱ类基性火山岩
Fig. 5. Th-Hf-Ta (a), 2Nb-Zr/4-Y (b) and Ti/100-Zr-3Y (c), discrimination diagrams of basalts in Lajishan orogenic belt
图 6 拉脊山基性火山岩Sr, Nd, Pb同位素判别图解(Rollison, 1993)
DM.亏损地幔; EMI.第一类富集地幔; EMⅡ.第二类富集地幔; HIMU.高U/Pb比值地幔; BSE.全硅质地幔; PREMA.流行地幔; 图中黑色虚线表示基性火山岩现代值的落点范围; 实心方块代表Ⅰ类基性火山岩; 五角星代表Ⅱ类基性火山岩
Fig. 6. Sr, Nd, Pb isotopic diagram of basalts in Lajishan orogenic belt
图 7 拉脊山与周缘不同地质单元内玄武岩Pb同位素对比
资料来源于胡云鹏等(1989)、解广轰等(1989)、李英和任崔锁(1990)、王寿琼(1993)、张旗等(1995)、张本仁等(2003)、夏林圻等(2004)
Fig. 7. Comparison of Pb isotopic compositions of basalts between Lajishan and different geological units in adjacent area
表 1 拉脊山Ⅰ类基性火山岩常量元素和微量元素组成
Table 1. Geochemical composition of basalts (Group Ⅰ) from Lasjishan orogenic belt
表 2 拉脊山Ⅱ类基性火山岩常量元素、微量元素组成
Table 2. Geochemical composition of basalts (Group Ⅱ) from Lasjishan orogenic belt
表 3 拉脊山基性火山岩Sr、Nd、Pb同位素组成
Table 3. Sr, Nd, Pb isotope compositions of basalts in Lajishan orogenic belt
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