Genesis of Neoproterozoic Amphibolite in Diancangshan, West Yunnan: Evidence from Zircon U-Pb Age and Whole-Rock Geochemistry
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摘要: 目前关于扬子地块西南缘新元古代岩浆作用的研究仍显薄弱，制约了对华南板块新元古代构造演化的完整认识.对滇西点苍山地区出露的变基性岩（斜长角闪岩）进行了详细的锆石U-Pb年·代学和全岩地球化学分析.根据Nb/La比值，将斜长角闪岩样品分为2组，第1组样品的Nb/La比值为0.75~0.76，均大于0.5，类似富铌玄武岩；第2组样品的Nb/La比值为0.20~0.21，强烈亏损Nb、Ta，类似岛弧玄武岩.第1组样品的SiO2含量为52.4%~52.6%，MgO含量为4.1%~4.2%，FeOt含量为8.7%~8.8%，Mg#为53；第2组样品相对第1组具有较低的SiO2含量（47.4%~47.5%）和较高的MgO含量（5.4%~5.5%）、FeOt含量（9.8%~9.9%）、Mg#（57）.样品的轻、重稀土元素分馏程度较强，第1组样品的（La/Yb）cn值为3.0~3.2，（Gd/Yb）cn为1.42~1.51，具有轻微的Eu负异常（δEu=0.75~0.76）；第2组样品具有相对较高的（La/Yb）cn（14.0~14.3）和（Gd/Yb）cn（1.85~1.93）比值，Eu异常不明显（δEu=0.99~1.13）.LA-ICP-MS锆石U-Pb定年结果表明，2个代表性的斜长角闪岩样品给出了764±6 Ma和779±9 Ma的形成年龄，暗示点苍山地区存在新元古代时期基性岩浆活动.综合研究认为斜长角闪岩的形成与新元古代俯冲体制下的弧后拉张作用有关.Abstract: The current research on Neoproterozoic magmatism on the southwestern Yangtze Block is still weak, which restricts the complete understanding of the Neoproterozoic tectonic evolution of the South China Plate. Detailed zircon U-Pb geochronological and whole-rock geochemical analyses of amphibolite in the Diancangshan area in western Yunnan were carried out in this study. The amphibolite samples can be divided into two groups based on their Nb/La ratios. Group 1 has Nb/La ratios of 0.75-0.76, greater than 0.5, resembling those of Nb-enriched basalt, while Group 2 has Nb/La ratios of 0.20-0.21, and shows obvious negative Nb and Ta anomalies, and thus are similar to those of arc basalt. The group 1 samples have SiO2 contents of 52.4%-52.6%, MgO of 4.1%-4.2%, FeOt of 8.7%-8.8%, and Mg# of 53. The group 2 samples are characterized by lower SiO2 contents (47.4%-47.5%), and higher MgO (5.4%-5.5%), FeOt (9.8%-9.9%) contents, and Mg# (57). Both of the two group samples show slightly high degree of fractionation. The group 1 samples have (La/Yb)cn ratios of 3.0-3.2, and (Gd/Yb)cn ratios of 1.42-1.51, and show negative Eu anomalies (δEu=0.75-0.76). The group 2 samples have (La/Yb)cn ratios of 14.0-14.3, and (Gd/Yb)cn ratios of 1.85-1.93, and show unobvious Eu anomalies (δEu=0.99-1.13). LA-ICP-MS zircon U-Pb dating results show that two representative amphibolite samples yield ages of 764±6 Ma and 779±9 Ma, suggesting that there is mafic magmatic activity in the Diancangshan area during the Neoproterozoic. Integrated with regional data, the amphibolite was likely to be produced in a back-arc basin which was caused by oceanic crustal subduction along the southwestern Yangtze Block in the Neoproterozoic.
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Key words:
- zircon U-Pb dating /
- amphibolite /
- back-arc basin /
- Neoproterozoic /
- Diancangshan /
- geochronology /
- geochemistry
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图 1 滇西点苍山-哀牢山构造带及其邻区地质简图
Fig. 1. Simplified geological map showing the Diancangshan-Ailaoshan tectonic belt and surrounding areas
图 3 点苍山-哀牢山构造带元古界地层柱状图
Fig. 3. Simplified stratigraphic column of Proterozoic stratum in the Diancangshan-Ailaoshan tectonic belt
图 4 斜长角闪岩的野外照片(a、b)和显微照片(c、d)
Pl. 斜长石；Hbl. 角闪石；Bt. 黑云母；Qtz. 石英
Fig. 4. Field photos (a, b) and photomicrographs (c, d) of the representative amphibolite
图 5 点苍山地区斜长角闪岩代表性锆石CL图像
Fig. 5. CL images of representative zircons of the amphibolites in the Diancangshan area
图 6 点苍山地区斜长角闪岩锆石U-Pb年龄谐和图及其加权平均年龄
Fig. 6. Zircon U-Pb concordia diagrams and weighted mean ages of the amphibolites in the Diancangshan area
图 7 点苍山地区斜长角闪岩Nb/Y-Zr/TiO2 (a)和MgO-Nb/La (b) 图解(底图据Sajona et al., 1994)
Fig. 7. Plots of Nb/Y vs. Zr/TiO2 (a) and MgO vs. Nb/La (b) for the amphibolites in the Diancangshan area (after Sajona et al., 1994)
图 8 点苍山地区斜长角闪岩球粒陨石标准化稀土元素配分模式(a)和原始地幔标准化微量元素蛛网图(b)
球粒陨石和原始地幔标准化值据Sun and McDonough（1989），云开富Nb玄武岩据Zhang et al.（2012）
Fig. 8. Chondrite-normalized REE pattern (a) and primitive mantle-normalized trace element spidergram (b) of the amphibolites in the Diancangshan area
图 9 点苍山地区斜长角闪岩Hf/3-Th-Ta (a)和Y/15-La/10-Nb/8 (b)图解
底图据Campbell and Griffiths（1990）. A.N-MORB；B.E-MORB-板内拉斑玄武岩；C.板内碱性玄武岩；D.钙碱性弧火山岩.1区为火山弧玄武岩，2区为大陆玄武岩，3区为大洋玄武岩；1A.钙碱性玄武岩，1B.板内火山弧拉斑玄武岩，1C.板内碱性火山弧玄武岩，2A.大陆玄武岩，2B.弧后盆地大陆玄武岩，3A.大陆裂谷碱性玄武岩，3B、3C.富集型洋脊玄武岩，3D.正常洋脊玄武岩
Fig. 9. Plots of Hf/3-Th-Ta (a) and Y/15-La/10-Nb/8 (b) for the amphibolites in the Diancangshan area
图 10 点苍山地区斜长角闪岩Ba/Yb-Nb/Yb (a)及Ba/Nb-Th/Yb (b)图解(据Pearce et al., 2005)
Fig. 10. Plots of Ba/Yb vs. Nb/Yb (a) and Ba/Nb vs. Th/Yb (b) for the amphibolites in the Diancangshan area (after Pearce et al., 2005)
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