Geochemistry, geochronology and their significances of Haizi bimodal intrusions in Kangdian fault-uplift zone, southwestern margin of Yangtze platform
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摘要: 海孜辉绿岩-花岗斑岩双峰式侵入岩体位于扬子地台西南缘康滇断隆带中的武定-元江裂陷槽内,本文对海孜辉绿岩和花岗斑岩主要进行了LA-ICP-MS锆石U-Pb定年和地球化学研究。花岗斑岩的定年结果为1764±18Ma,辉绿岩的定年结果为1765±5.4Ma。岩相学及地球化学测定数据显示海孜地区辉绿岩具有细粒和粗粒两种结构,与花岗斑岩之间存在明显的组分间断,形成双峰态,其中花岗斑岩为钙碱性(SiO2=70.93%~73.04%,σ=1.28),辉绿岩属过碱性(SiO2=42.46%~48.26%,σ=15.3),Hf同位素特征表明两者可能均与幔源母岩浆相关。而海孜双峰式分布特征显示大陆裂谷拉张环境的构造背景,暗示海孜岩体是1.7Ga时昆阳裂谷裂解时期产物,与全球Columbia超大陆的裂解事件相呼应。作为扬子地台西南缘首次发现的1.7Ga双峰式侵入岩,海孜双峰式侵入岩的发现具有重要的地质意义。
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关键词:
- 海孜 /
- 1.7Ga双峰式侵入岩 /
- 地球化学及年代学 /
- 地质意义 /
- 扬子地台西南缘
Abstract: The Haizi diabase-granite porphyry bimodal intrusions are located in the Wuding-Yuanjiang Aulacogen, southwestern Yangtze platform. According to field investigation, the diabase can be divided into fine-grained and coarse-grained two types. The granite porphyry had directly contact relation with the fine-grained diabase without transitional intermediate rock. In this article, petrographic investigation analysis, geochemical and LA-ICP-MS zircon U-Pb dating have been conducted. The results showed that the zircon 207Pb/206Pb weighted average age is 1764±18Ma for the granite porphyry and is 1765±5.4Ma for the diabase. The data also produced a geochemical discontinuity between diabase(SiO2=42.46%~48.26%, σ=15.3) and granite porphyry (SiO2=70.93%~73.04%, σ=1.28). The geochemical investigations revealed that the Haizi diabase belonged to high-potassium alkaline basalt series and the granite porphyry to low-potassium rhyolite series. Both of them are enriched in high field strength elements and depleted in large-ion lithophile elements. They represent a bimodal distribution, which is the characteristics of a continental rift. This meant that crustal remelting and accretion of new crust occurred in this region in the 1.7Ga period. The age of 1764±18Ma for the granite porphyry and 1765±5.4Ma for the diabase are probably relict of the Kunyang rift break, which can serve as a petrological record of Columbia supercontinent break. This is significantly first time to recognize the petrological record for the continental breakup at southwestern margin of the Yangtze platform. -
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