Geochemical Characteristics and Tectonic Significance of Rapakivi Granites in Yingfeng
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摘要: 柴达木北缘鹰峰环斑花岗岩出露于柴达木地块与南祁连地体之间的柴北缘造山带,是我国发现的又一元古宙环斑花岗岩体.初步研究表明,鹰峰环斑花岗岩是具环斑结构和A-型花岗岩特征的典型的元古宙环斑花岗岩体,且属于A1亚型,岩浆组合具双峰式特征.环斑结构主要由几个钾长石斑晶颗粒形成聚斑,中心有一斜长石内核,斑晶表面具不均匀高岭土化,条纹构造明显且有规律,基质由细粒-微粒的石英组成,有明显重结晶及定向构造; 岩石化学组成以高钾为特征,A/ NKC < 1,A/NK > 1,属准铝质; 微量元素组成上富集Ba、U、Th、Ce、Hf、Sm,亏损Sr、Ta、Nb、Zr、Y,Rb/Sr (0.17~0.6)和Rb/Ba (0.03~0.24) 很低,岩石分异演化程度不高; 稀土元素: REE、Ce、Zr含量高,Ga含量高达25×10-6以上,远远高出其他类型花岗岩,但Eu (0.75~4.3) ×10-6轻度亏损,属轻稀土富集型.通过对微量元素和稀土元素的地球化学行为分析,鹰峰环斑花岗岩是发生在板内的一种岩浆作用,是下地壳的麻粒岩受底侵或拆沉作用地幔上涌影响,发生部分熔融,然后经过分异演化形成了碱性的“干”岩浆,并在后碰撞的区域拉伸构造环境下侵位.同时伴随温度的降低,钠质的斜长石从钾长石中出溶,并迁移到钾长石的边沿,形成了具环斑结构的A1型花岗岩.Abstract: Yingfeng rapakivi granite, outcropping in the north of the Qaidam basin orogenic belt between the Qaidam plate and South Qilian, is one of the Properozoic rapakivi granites found in China. This research shows that Yingfeng rapakivi granite is a typical Proterozoic rapakivi granite that has rapakivi texture and features of A-type granite, and belongs to A1 subtype, and the magma assemblage has two-apex characteristics. Rapakivi texture is mainly a polymer composed of several K-feldspar porphyritic crystals, with one plagioclase nuclear in it. The surface of the porphyritic crystal is kaolinized unevenly, its stripped structure is obvious and has orderliness. The matrix is composed of fine-particulate quartz and has obvious recrystalization and orientation textures. Its petrochemistry is characterized by high K, A/NKC < 1, A/NK > 1, belonging to quasi-aluminous rock. It is rich in minor elements Ba, U, Th, Ce, Hf, Sm, depleted in Sr, Ta, Nb, Zr, Y, Rb/Sr ((0.17)-0.6), and its Rb/Ba (0.03-0.24) ratio is very low. The differentiation of the rock is not high. The ∑REE, Ce, Zr contents are high, and the content of Ga can reach over 25×10-6, far higher than that of other granite types. Its Eu ((0.75) ×10-6-4.3×10-6) is depleted slightly, belonging to LREE enrichment type. Through the analysis of the geochemistry of minor elements and REE, Yingfeng rapakivi granite is a kind of magmatism produced in the plate. It is the "dry" magma that is formed by the granulite, which is influenced by the uprush of the mantle of base intrusion or delamination, and then melted and then differentiated, and invaded in the post-collision area at the regional extentional tectonic environment. At the same time, with the decrease of temperature, Na-plagioclase is separated from the K-feldspar and transferred to the brim of the K-feldspar to form the ring porphyritic A1 granite.
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Key words:
- rapakivi granite /
- north of Qaidam basin /
- geochemistry /
- tectonic setting
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典型元古代环斑花岗岩均产于稳定的地盾, 指示特定的非造山构造环境及超大陆裂解事件, 在芬兰、瑞典、乌克兰等国外以及我国北京密云等地的前寒武地层中相继发现, 近年来受到国内外学者广泛关注和深入研究, 并对环斑花岗岩和环斑结构进行了定义(宋彪, 1992; Rämö and Haapala, 1995; Rämö et al., 1995; 郁建华等, 1996; Rämö and Haapala, 1996; Haapala and Rämö, 1999; Väino and Flodén, 1999).
狭义的环斑结构指的是像芬兰南部维堡环斑花岗岩体中的那种结构, 其特征是卵形的钾长石巨晶具有奥长石-中长石外壳, 也有一些卵球形的钾长石巨晶不具外壳; 钾长石和石英均有2期, 自形成早期的石英是高温的.广义的环斑结构则泛指钾长石巨晶具有斜长石外壳, 钾长石巨晶可以是卵球的, 也可以是自形的, 斜长石外壳可从钠长石(An6) 到中长石(An36).
我国学者卢欣祥等(1999)通过多年研究发现, 在秦岭-昆仑造山带, 沿古缝合带分布着相当数量的环斑或环斑状花岗岩体, 主要为与造山作用有关的印支期环斑花岗岩(卢欣祥等, 1996, 1999).近期, 柴达木北缘鹰峰环斑花岗岩的研究对于环斑花岗岩及秦岭造山带演化等具有重要意义(肖庆辉等, 2003).本文主要对鹰峰环斑花岗岩的环斑结构及地球化学特征进行了初步研究, 并探讨了其形成的构造环境.
1. 岩体地质特征及其时代
鹰峰环斑花岗岩位于柴达木盆地北缘, 柴北缘断裂带的北侧, 也就是柴达木地块与南祁连地体之间的柴北缘构造带, 形成时代为(1 772±33) Ma (肖庆辉等, 2003).鹰峰岩体出露面积约20多km2, 为复式杂岩体, 主要由环斑花岗岩及辉长岩脉组成, 二者构成典型的双峰式岩套.
2. 环斑结构特征
球斑-环斑长石主要为钾长石, 大小1~5 cm不等, 一般为2~3 cm, 含量达60%以上(实际面统计结果).钾长石斑晶边缘有一圈1~1.5 mm的斜长石环边, 形成较典型的环斑结构, 基质由细粒-微粒的石英组成, 受后期构造较明显, 石英有明显的重结晶及定向构造.
钾长石斑晶: 由几个颗粒(成分同为钾长石) 形成聚斑, 中心有一斜长石的内核(图 1a), 斑晶表面具不均匀的高岭土化, 条纹构造明显, 条纹分布且有规律.条纹主要为钠长石, 可见聚片双晶, 干涉色I级黄(薄片稍厚), 格子双晶发育.不同颗粒间有一圈厚度为0.01 mm左右的交代边.钾长石可见包裹有斜长石颗粒和石英颗粒(图 1b, 1c).局部可见钾长石和石英组成的文象交生结构, 钾长石受构造影响, 内部见有较多裂隙, 裂隙中充填有后期(或同期) 进入的钠长石.钠长石沿裂隙穿过2~3个颗粒.
钾长石边缘具斜长石环边(图 1d, 1e), 聚片双晶发育, 绢云母化、高岭土化较强(图 1e).
基质为细粒-微粒结构, 较大斜长石、石英颗粒基质无变形, 较小的石英、角闪石变质强, 具重结晶、定向排列(图 1f), 多平行于斑晶颗粒, 可能为钾长石斑晶在生长过程中对基质造成挤压而形成.
3. 环斑花岗岩地球化学特征
3.1 岩石化学
鹰峰环斑花岗岩以高钾为特征, w (K2O) 达5×10-2以上, w (Na2O) 为3.9×10-2, w (CaO) 为1.8×10-2, w (TiO2) 为0.88×10-2, w (K2O)/w (Na2O) > 1.3, w (K2O)/w (Na2O+K2O) > 0.57, 而w (SiO2) 含量并不高, 平均为67%, w (A)/w (CNK) 平均为0.94, w (A)/w (NK) 约为1.20, w (A)/w (NK)-w (A)/w (CNK) 图解表明属准铝质(图 2).表 1反映出, 本区环斑花岗岩岩石化学特征总体上与A型花岗岩特征相似.
图 2 鹰峰环斑花岗岩w (A)/w (NK)-w (A)/w (CNK) 图解Fig. 2. w (A)/w (NK)-w (A)/w (CNK) diagram for rapakivi granites in Yingfeng表 1 鹰峰环斑花岗岩主元素组成Table Supplementary Table Major element compositions of rapakivi granites in Yingfeng
3.2 微量元素和稀土元素
稀土总量较高, ∑REE达(565~585) ×10-6, Ce含量达210×10-6以上, Zr含量达265×10-6以上, 岩石Ga含量高达25×10-6以上, 10 000*Ga/Al高达3.4以上, 远远高出其他类型花岗岩, 属轻稀土富集型, 但Eu (0.75~4.3) ×10-6轻度亏损(表 2).稀土分布曲线并非呈特征的“海鸥”型, 在图 3 (Pearce, 1983) 上表现为较陡的右倾型, 轻、重稀土分异明显.
表 2 鹰峰环斑花岗岩稀土元素、微量元素组成Table Supplementary Table Rare earth and trace element compositions of rapakivi granites in Yingfeng
图 3 鹰峰环斑花岗岩w (Na2O)-w (K2O) 图解Fig. 3. w (Na2O)-w (K2O) diagram for rapakivi granites in Yingfeng微量元素富集Ba、U、Th、Ce、Hf、Sm, 亏损Sr、Ta、Nb、Zr、Y, 且不含F, 在微量元素蛛网图(图 4b) (Taylor and McLennan, 1985) 上呈明显的Ba、Rb、Th、Ce、Hf、Sm峰和Sr、Ta、Nb、Zr、Y谷, 显示了准铝质A型花岗岩的一般特征.w (Rb)/w (Sr) (0.17~0.6)、w (Rb)/w (Ba) (0.03~0.24) 很低, 反映了岩石的分异演化程度不高.
图 4 鹰峰环斑花岗岩REE及微量元素分布模式Fig. 4. REE and trace elements distribution patterns for rapakivi granites in Yingfeng3.3 鹰峰环斑花岗岩具A型特征
分别对w (Na2O)-w (K2O) (4a) (Collins et al., 1982)、w (Rb)/w (Nb)-w (Y)/w (Nb) (图 5) (Whalen, 1987; Eyb, 1992)和w (La)-w (La)/w (Sm) (图 6) (Trenil, 1978) 进行图解, 结果是: w (Na2O)-w (K2O) 全部落入A型花岗岩区; 鹰峰环斑花岗岩w (La)/w (Sm)-w (La) 呈水平线, 具A型特征(I型花岗岩为倾斜线) 主要以分离结晶作用为主; w (Rb)/w (Nb)-w (Y)/w (Nb) 中样品全部落入A1亚区.上述表明本区花岗岩具A型特征, 且属于A1亚型.
图 5 鹰峰环斑花岗岩w (Rb)/w (Nb)-w (Y)/w (Nb) 图Fig. 5. w (Rb)/w (Nb)-w (Y)/w (Nb) diagram for rapakivi granites in Yingfeng
图 6 鹰峰环斑花岗岩w (La)/w (Sm)-w (La) 图Fig. 6. w (La)/w (Sm)-w (La) diagrams for rapakivi granites in Yingfeng另外Whalen (1987)认为w (Ga)/w (Al) 比值高是A型花岗岩的典型特征, 故鹰峰环斑花岗岩具典型A型花岗岩特征.
4. 构造环境分析
微量元素和稀土元素的地球化学行为, 对于研究花岗岩形成的构造环境具有重要意义. (1) w (K2O)-w (Na2O) (图 3)、w (Rb)/w (Nb)-w (Y)/w (Nb) (图 5)、w (La)/w (Sm)-w (La) (图 6) 图解和高Ga/Al比值表明本区花岗岩具典型A型花岗岩特征, 且属于A1亚型.指示其岩浆来源深, 碱度大, 岩体小, 多与碱性侵入岩、火山岩共生, 环状杂岩体, 大陆或大洋板块内地壳隆起或裂谷拉张环境产物(Trenil, 1978; Collins et al., 1982; Whalen, 1987; Eyb, 1992). (2) 由w (Ce)/w (Gd)-w (Ce)/w (Yb)和w (Gd)/w (Yb)-w (La)/w (Yb) 协变关系(图 7) (Pearce et al., 1984) 表明, 本区的红色环斑花岗岩和灰色环斑花岗岩具有较好的相关性, 说明它们在成因环境上互有联系. (3) w (Nb)-w (Y) (图 8a)和w (Rb)-w (Y+Nb) (图 8b) (Pearce et al., 1984) 图解表明, 本区样品均落入板内花岗岩区或两者的过渡区, 反映构造环境从火山弧向近似板内的方向演化, 即从较活动的挤压环境, 向温度的拉张环境演化.
图 7 鹰峰环斑花岗岩w (Ce)/w (Gd)-w (Ce)/w (Yb) (a), w (Gd)/w (Yb)-w (La)/w (Yb) (b) 协变图Fig. 7. w (Ce)/w (Gd)-w (Ce)/w (Yb) (a), w (Gd)/w (Yb)-w (La)/w (Yb) (b) covariance diagram for rapakivi granites in Yingfeng
图 8 鹰峰环斑花岗岩w (Nb)-w (Y) (a)和w (Rb)-w (Y+Nb) (b)Fig. 8. w (Nb)-w (Y) (a) and w (Rb)-w (Y+Nb) (b) diagram for rapakivi granites in Yingfeng5. 结论与讨论
(1) 鹰峰环斑花岗岩出露于柴达木地块与南祁连地体之间的柴北缘造山带. (2) 鹰峰环斑花岗岩的年龄为(1 772±33) Ma. (3) 结构特征: 钾长石斑晶由几个颗粒(成分同为钾长石) 形成聚斑, 中心有一斜长石内核, 斑晶表面具不均匀的高岭土化, 条纹构造明显, 条纹分布且有规律.基质由细粒-微粒的石英组成, 受后期构造较明显, 石英有明显的重结晶及定向构造. (4) 地球化学特征: 岩石化学组成以高钾为特征, A/CNK < 1, A/NK > 1, 属准铝质; 微量元素组成上富集Ba、U、Th、Ce、Hf、Sm, 亏损Sr、Ta、Nb、Zr、Y, Rb/Sr (0.17~0.6)和Rb/Ba (0.03~0.24) 很低, 岩石分异演化程度不高; 稀土元素: ∑REE、Ce、Zr含量高, Ga含量高达25×10-6以上, 远远高出其他类型花岗岩, 但Eu含量(0.75~4.3) ×10-6轻度亏损, 属轻稀土富集型. (5) 环斑花型花岗岩, 应是一种“干岩浆”, 通常产于挤压造山作用以后的区域拉伸构造环境, 往往是岩石去根作用的产物.通过对其地球化学特征综合分析表明, 鹰峰环斑花岗岩是发生在板内的一种岩浆作用, 是下地壳的麻粒岩受底侵或拆沉作用地幔上涌影响发生部分熔融, 然后经过分异演化形成了碱性的“干”岩浆, 并在后碰撞的拉伸构造环境下侵位(卢欣祥, 2000).同时伴随温度的降低, 钠质的斜长石从钾长石中出溶, 并迁移到钾长石的边沿, 形成了具环斑结构的A1型花岗岩.
上述表明, 鹰峰环斑花岗岩是具环斑结构和A1型花岗岩特征的典型的元古宙环斑花岗岩体, 岩浆组合具双峰式特征.同时, 鹰峰环斑花岗岩产出于柴达木地块与南祁连地体之间的柴北缘造山带, 可以视为中元古代全球超大陆裂解事件的标志(Xiao et al., 2003).
本文在撰写过程中得到张克信、朱云海等老师的热情帮助和支持, 在此一并致谢! -
图 2 鹰峰环斑花岗岩w (A)/w (NK)-w (A)/w (CNK) 图解
Fig. 2. w (A)/w (NK)-w (A)/w (CNK) diagram for rapakivi granites in Yingfeng
图 3 鹰峰环斑花岗岩w (Na2O)-w (K2O) 图解
Fig. 3. w (Na2O)-w (K2O) diagram for rapakivi granites in Yingfeng
图 4 鹰峰环斑花岗岩REE及微量元素分布模式
Fig. 4. REE and trace elements distribution patterns for rapakivi granites in Yingfeng
图 5 鹰峰环斑花岗岩w (Rb)/w (Nb)-w (Y)/w (Nb) 图
Fig. 5. w (Rb)/w (Nb)-w (Y)/w (Nb) diagram for rapakivi granites in Yingfeng
图 6 鹰峰环斑花岗岩w (La)/w (Sm)-w (La) 图
Fig. 6. w (La)/w (Sm)-w (La) diagrams for rapakivi granites in Yingfeng
图 7 鹰峰环斑花岗岩w (Ce)/w (Gd)-w (Ce)/w (Yb) (a), w (Gd)/w (Yb)-w (La)/w (Yb) (b) 协变图
Fig. 7. w (Ce)/w (Gd)-w (Ce)/w (Yb) (a), w (Gd)/w (Yb)-w (La)/w (Yb) (b) covariance diagram for rapakivi granites in Yingfeng
图 8 鹰峰环斑花岗岩w (Nb)-w (Y) (a)和w (Rb)-w (Y+Nb) (b)
Fig. 8. w (Nb)-w (Y) (a) and w (Rb)-w (Y+Nb) (b) diagram for rapakivi granites in Yingfeng
表 1 鹰峰环斑花岗岩主元素组成
Table 1. Major element compositions of rapakivi granites in Yingfeng
表 2 鹰峰环斑花岗岩稀土元素、微量元素组成
Table 2. Rare earth and trace element compositions of rapakivi granites in Yingfeng
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Collins, W.J., Beams, S.D., White, A.J. R., et al., 1982. Nature and origin of A-type granites with particular reference to southeastern Australia. Contrib. Miner. Petro., 80: 189-200. doi: 10.1007/BF00374895 Eyb, G.N., 1992. Chemical subdivision of the A2type granitoids: Petrogenetic and tectonic implications. Geology, 20 (7): 641-644. doi: 10.1130/0091-7613(1992)020<0641:CSOTAT>2.3.CO;2 Haapala, I., Ramö, O.T., 1999. Rapakivi granite and related rocks: An introduction. Precambrian Research, 95(12): 1-7. Lu, X.X., 2000. Granitoid tectonic map of Qinling belt. Xi'an Cartographic Publishing Home, Xi'an(in Chinese). Lu, X.X., Dong, Y., Chang, Q.L., et al., 1996. Indosinian Shahewan rapakivi granite in Qinling and its dynamic significance. Science in China(Series D), 26(3): 244248(in Chinese). Lu, X.X., Wei, X.D., Xiao, Q.H., et al., 1999. Geochronological studies of rapakivi granites in Qinling and its geological implications. Geological Journal of China Universities, 5(4): 372-377(in Chinese with English abstract). Pearce, J.A., Harris, N.B.W., Tindle, A.G., 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. J. Petrol., 25(4): 956-983. doi: 10.1093/petrology/25.4.956 Pearce, J.A., 1983. Role of the sub-continental lithosphere in magma genesis at active continental margins. In: Hawkesworth, C. J., Norry, M. J., eds., Continental basalts and mental xenoliths. Shiva, Nantwich, 230249. Rämö, O.T., Haapala, I., 1995. One hundred years of rapakivi granite. Mineralogy and Petrology, 52: 129-146. doi: 10.1007/BF01163243 Rämö, O.T., Haapala, I., Vaasjoki, M., et al., 1995. 1 700 Ma Shachang complex, Northeast China—Proterozoic rapakivi granite not associated with Paleoproterzoic orogenic crust. Geology, 23(9): 815-818. doi: 10.1130/0091-7613(1995)023<0815:MSCNCP>2.3.CO;2 Rämö, O. T., Haapala, I., 1996. Rapakivi granite magmatism: A global review with emphasis on petrogenesis. In: Demaiffe, D., ed., Petrology and geochemistry of magmatic suites of rock in the continental and oceanic crusts. A volume dedicated to Professor Jean Michot. Universite Libre de Bruxelles: Royal Museum for Central Africa(Tervuren), 177-200. Song, B., 1992. Isotope geochronology, REE characteristics and genesis of Miyun rapakivi granites. Bulletin of the Institute of Geology Chinese Academy ofGeological Sciences, 25: 137-156(in Chinese with English abstract). Taylor, S. R., McLennan, S. M., 1985. The continental crust: Its composition and evolution. Blackwell, Oxford. Trenil, M., 1978. Trace element in igneous. In: Allegre, ed. Petrology. [s.l.]: [s.n.], 1-12. Väino, P., Flodén, T., 1999. Rapakivi-granite-anorthosite magmatism—A way of thinning and stabilisation of the Svecofennian crust, Baltic Sea basin. Tectonophysics, 305(1-3): 75-92. doi: 10.1016/S0040-1951(99)00019-0 Whalen, J.B., 1987. A2 type granites: Geochemical characteristics, discrimination and petrogenesis. Contrib. Miner. Petro., 95: 407-419. doi: 10.1007/BF00402202 Xiao, Q.H., Lu, X.X., Wang, F., et al., 2003. Geochronological studies of rapakivi granites in North of Qaidam basin and its geological implications. Science in China (Series D), 33(12): 1193-1200(in Chinese). Yu, J.H., Fu, H.Q., Zhang, F.L., et al., 1996. Anorogenic rapakivi granites and related rocks in northern of the North China craton. China Science and Technology Press, Beijing, 44-61. 卢欣祥, 2000. 秦岭花岗岩大地构造图及说明书. 西安: 西安地图出版社. 卢欣祥, 董有, 常秋玲, 等, 1996. 秦岭印支期沙河湾奥长环斑花岗岩及其动力学意义. 中国科学(D辑), 26(3): 244-248. https://www.cnki.com.cn/Article/CJFDTOTAL-JDXK199603007.htm 卢欣祥, 尉向东, 肖庆辉, 等, 1999. 秦岭环斑花岗岩的年代学研究及其意义. 高校地质学报, 5(4): 372-377. https://www.cnki.com.cn/Article/CJFDTOTAL-GXDX199904001.htm 宋彪, 1992. 密云环斑花岗岩的同位素地质年代学、稀土地球化学特征及成因讨论. 中国地质科学院地质研究所所刊, 25: 137-156. https://cpfd.cnki.com.cn/Article/CPFDTOTAL-ZGDJ199300002015.htm 肖庆辉, 卢欣祥, 王菲, 等, 2003. 柴达木北缘环斑花岗岩的时代及地质意义. 中国科学(D辑), 33(12): 1193-1200. https://www.cnki.com.cn/Article/CJFDTOTAL-JDXK200312007.htm 郁建华, 傅会芹, 张凤兰, 等, 1996. 华北地台北部非造山带环斑花岗岩及有关岩石. 北京: 中国科学技术出版社, 44-61. -
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