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现代地质 ›› 2018, Vol. 32 ›› Issue (04): 727-738.DOI: 10.19657/j.geoscience.1000-8527.2018.04.09

• 岩石学 • 上一篇    下一篇

云南省凤庆花岗质糜棱岩的岩石地球化学特征及锆石U-Pb年代学

王宏(), 丛峰(), 李再会, 王海林   

  1. 中国地质调查局 成都地质调查中心,四川 成都 610081
  • 收稿日期:2017-05-14 修回日期:2017-11-26 出版日期:2018-08-10 发布日期:2018-09-19
  • 通讯作者: 丛峰
  • 作者简介:丛 峰,男,博士,副研究员, 1982年出生,地质矿产勘查专业,主要从事地球化学研究。Email: 673103623@qq.com
    王 宏,男,工程师,1985年出生,地质矿产勘查专业,主要从事区域地质矿产调查与研究。Email: wangh1213@sina.com.
  • 基金资助:
    中国地质调查局项目“云南1∶5万诗礼等五幅区域地质调查”(12120114060201);“云南1∶5万宁蒗等三幅区域地质调查”(121201010000150016-05)

Zircon U-Pb Geochronology and Geochemistry of Granitic Mylonites in Fengqing, Yunnan Province

WANG Hong(), CONG Feng(), LI Zaihui, WANG Hailin   

  1. Chengdu Center, China Geological Survey, Chengdu,Sichuan 610081, China
  • Received:2017-05-14 Revised:2017-11-26 Online:2018-08-10 Published:2018-09-19
  • Contact: CONG Feng

摘要:

在野外地质调查基础上,对滇西“三江”地区临沧花岗岩基北部凤庆花岗质糜棱岩的年代学、岩石地球化学开展了较系统的研究。利用LA-ICP-MS对凤庆花岗质糜棱岩进行 U-Pb定年,锆石结晶年龄为(223.3±0.50) Ma、(231.9 ±1.8) Ma,指示岩体形成于中三叠世。地球化学特征表现为Na2O/K2O比值低、铝饱和指数(A/CNK值)大于1.5,具过铝质、高钾钙碱性特征;轻稀土富集,重稀土亏损,Eu明显亏损(δEu =0.27~0.58));相容元素 Cr 和 Ni 含量较低,富集Rb、Th、U 、Sm等元素,亏损 Zr、Hf、Ti 等元素。黑云母花岗闪长质糜棱岩相对花岗伟晶质糜棱岩在Ni、Fe、Mg、Mn、Ca和∑REE上均相对富集。凤庆花岗质糜棱岩与临沧花岗岩具有较一致的地球化学特征和岩浆结晶年龄,且两者在野外露头上呈脆韧性剪切变形渐变过渡接触,表明花岗质糜棱岩在构造剪切变形前是临沧花岗岩体的一部分,而非前人认为的元古界大勐龙岩群高级变质岩,是凤庆一带澜沧江两侧临沧花岗岩遭受后期澜沧江断裂带脆韧性剪切变形作用形成的。

关键词: 锆石 U-Pb 定年, 地球化学, 花岗质糜棱岩, 云南省凤庆

Abstract:

Based on field investigations, new data on the geochronology, petrology and geochemistry of the Fengqing granitic mylonites (north of Lincang batholith) located in southern Sanjiang region are presented. LA-ICP-M zircon U-Pb dating of two Fengqing granitic mylonites are yielded Triassic ages of (223.3 ±0.50) Ma, and (231.9 ±1.8) Ma, much younger than the previously conceived Proterozoic age. Geochemical analyses show that the granitic mylonites are peraluminous (A/CNK=1.54-1.75),high-K calc-alkaline, with low Na2O/K2O ratio. The rocks are enriched in light rare earth elements (LREE), and depleted in heavy rare earth elements(HREE), with distinct negative Eu anomalies( Eu/Eu*=0.27-0.58). The rocks contain low Cr and Ni contents, high Rb, Th, U and Sm contents, and low Zr, Hf and Ti contents. The biotite granodioritic mylonite, compared to the younger granopegmatitic mylonite, is more enriched in Ni, Fe, Mg, Ca and REE. This may reflect the products of co-magmatic fractionation. The Fengqing granitic mylonites and Lincang granite are broadly coeval and geochemically similar, and they show a transitional contact of brittle-ductile shearing in the field. This indicated that the granitic mylonite is part of the Lincang granite before shearing in the Lancangjiang fault zone, rather than being the previously-conceived Proterozoic Damenglong high-grade metamorphic rocks.

Key words: zircon U-Pb geochronology, geochemistry, granitic mylonite, Fengqing,Yunnan Province

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