NEW RECOGNITION OF KANGDING-MIANNING METAMORPHIC COMPLEXES FROM SICHUAN, WESTERN YANGTZE CRATON: EVIDENCE FROM Nd ISOTOPIC COMPOSITION
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摘要: 通过对四川冕宁沙坝及康定—泸定地区的斜长角闪岩-TTG片麻岩的系统Sm-Nd同位素的测定, 在该区首次获得70 6Ma的Sm-Nd等时线年龄.据此等时线年龄及其他样品的Nd亏损地幔模式年龄证明长期以来这套被当作太古宙-古元古代的变质岩系是新元古代大陆裂解与其后紧接着的地幔柱引起的热变质作用的产物.与已有数据相结合, 初步认为扬子克拉通目前所出露的基底没有统一演化历史, 对扬子克拉通的基底有待更深入的研究.
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关键词:
- 基底 /
- Sm-Nd年代学与地球化学 /
- 地幔柱 /
- 新元古代 /
- 扬子克拉通西缘
Abstract: The Kangding Group, outcropping around the northern segment of the western Yangtze craton, has been regarded as a typical Archean high-grade terrain for a long time. The Kangding-Luding and Mianning complexes are representatives of the group. An Sm-Nd isochron of 706 Ma was first obtained through the systematic determination of Sm Nd isotopic composition in the amphibolites-TTG gneisses from the Shaba, Mianning county, Sichuan Province. The isochron and Nd depleted mantle model ages of other samples show that the complexes were metamorphic and anatectic products of Neoproterozoic-mafic-ultra-mafic rocks after breakup of Rodinia by a mantle plume. All these evidences suggest that the present outcropping basements of the Yangtze craton are free of a united evolutionary history. Therefore, further study should be made of the basements of the Yangtze craton.-
Key words:
- basement /
- Sm-Nd geochronology and geochemistry /
- mantle plume /
- Neoproterozoic /
- western Yangtze craton
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图 1 四川康定—泸定、冕宁地区地质略图(据四川区域地质志中所附地质图[1]改编)
1.喜马拉雅构造层; 2.燕山-印支构造层; 3.华里西构造层; 4.加里东构造层; 5.吕梁亚构造层; 6.花岗岩; 7.闪长岩; 8.二长岩; 9.安山岩; 10.玄武岩; 11.深大断裂; 12.取样点
Fig. 1. Sketch geologic map of Kangding-Luding and Mianning region, Sichuan Province
图 2 四川康定—泸定、冕宁地区变质基底中片麻岩的岩石化学分类图解(据O'Connor[14])
Fig. 2. Petrochemical classification of gneisses in metamorphic basements from Kangding-Luding and Mianning region, Sichuan Province
图 3 四川康定—泸定英云闪长质片麻岩的w (143Nd) /w (144Nd)-w (147Sm) /w (144Nd) 同位素组成的分布
Fig. 3. Plot of w (143Nd) /w (144Nd) versus w (147Sm) /w (144Nd) for felsic gneisses from Kangding-Luding area, Sichuan Province
图 4 四川冕宁泽远片麻岩的w (143Nd) /w (144Nd)-w (147Sm) /w (144Nd) 等时线
Fig. 4. Sm-Nd isochron for Zeyuan gneisses from Mianning area, Sichuan Province
图 5 康定—冕宁灰黑色麻粒岩、斜长角闪岩与崆岭群角闪岩类稀土元素分布型式对比
KY04, KH35, KH15, KH29为崆岭群中的斜长角闪岩, KY09, KH27为崆岭群中的闪长岩质-英云闪长岩质-奥长花岗质片麻岩. (数据引自Gao等[17]); 98822-1-2和98922-4-1为灰黑色麻粒岩, 98919-10-2, 98919-11-2, 98921-4-1为斜长角闪岩类. (球粒陨石稀土组成据Boynton (1984) 的资料——据文献[18])
Fig. 5. Comparison of REE profiles between granulites, amphibolites from Kangding-Mianning and amphibolites from Kongling Group, Northern Yangtze craton
图 6 四川康定—泸定、冕宁地区变质基底岩石的初始Nd-Sr同位素协变图
Fig. 6. Covariant plot between initial Nd and Sr isotopic compositions for metamorphic basement in Kangding-Luding and Mianning region, Sichuan Province
表 1 四川康定—泸定—冕宁变质基底的Sm-Nd、Rb-Sr同位素分析结果
Table 1. Compositions of Sm-Nd and Rb-Sr isotopes for the metamorphic basement in the Kangding-Luding and Mianning region, Sichuan Province
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