ZIRCON U-Pb DATING AND GEOCHEMISTRY OF THE FANGJIAGOU PLUTON IN ZHALANTUN AREA, INNER MONGOLIA
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摘要: 方家沟地区二长花岗岩中锆石颗粒的晶体内部结构清晰，振荡生长环带发育和较高的Th/U比值（0.99~3.10），反映了岩浆成因特征.LA-ICP-MS锆石U-Pb定年结果为（152.9±2.2）Ma.岩石SiO2含量73.06%~76.25%，里特曼指数（σ）2.51~2.63，具有高钾钙碱性特征，呈过铝质特点.ΣLREE/ΣHREE比值和（La/Yb）N比值分别为6.21~10.43和4.98~12.18，为轻稀土富集、重稀土亏损型.δEu值0.52~0.72，为中弱亏损.以上特征表明方家沟岩体为过铝质高分异I型花岗岩.其成因可能是由于造山后的拉张环境使得地壳减薄，促使软流圈的物质上涌和幔源岩浆的底侵作用，导致地壳的温度升高，减压熔融形成二长花岗岩.Abstract: The Fangjiagou monzogranite pluton shows a typical magmatic origin with its clear crystal interior structures of zircon, developed oscillatory growth zonation and high Th/U ratio of 0.99 -3.10. The LA-ICP-MS zircon U-Pb dating is (152.9±2.2) Ma. The rock is peraluminous, belonging to high-K calc-alkali series, with SiO2 content of 73.06%-76.25% and Rittman index (σ) of 2.51-2.63. The ratios of ΣLREE/ΣHREE and (La/Yb)N are respectively 6.21-10.43 and 4.98-12.18, with enrichment in LREEs and depletion in HREEs. The δEu values range from 0.52 to 0.72. The features above indicate that the Fangjiagou pluton belongs to peraluminous highly fractionated I-type granite, which may be formed due to the crust melting by temperature rising and pressure reducing in post-orogenic extensional environment, with crust thinning, materials from asthenosphere upwelling and mantle-derived magma underplating.
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Key words:
- Fangjiagou pluton /
- monzogranite /
- zircon U-Pb dating /
- geochemistry /
- Inner Mongolia /
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