冀南西石门矽卡岩型铁矿床成矿年代:热液锆石U-Pb年龄的证据
Constraints of Hydrothermal Zircon U-Pb Ages on the Metallogenic Epoch of the Xishimen Skarn Iron Deposit, Sourthern Hebei Province
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摘要: 西石门铁矿床是华北地区最为典型的矽卡岩型铁矿床之一,最新获得金云母40Ar-39Ar成矿年龄为(133.1±1.3) Ma、(137.1±1.5) Ma,略老于其成矿母岩(武安杂岩体)形成的年龄(126～136 Ma),这是矛盾的.笔者对角闪正长岩和矽卡岩脉中的锆石进行了LA-ICP-MS U-Pb同位素年代学分析.结果表明,角闪正长岩中锆石呈浅黄色-无色、透明、棱角状、不规则状,振荡环带结构不发育,部分锆石中含棱角状继承锆石内核,206Pb-238U年龄加权平均值为(135.6±1.5) Ma,代表了武安杂岩体的结晶年龄.矽卡岩脉中锆石呈棕色,含有少量细小的包裹体,呈透明-半透明,相对于岩浆锆石具有低的Th、U、REE含量及Th/U值(Th=(222.37～1541.11)×10-6,U=(218.44～989.17)×10-6,Th/U=0.90～1.56),并具有强烈的Ce正异常(δCe=4.46～196.22)和Eu负异常(δEu=0.59～0.80),206Pb-238U年龄介于124.9～133.0 Ma,加权平均值为(129.4±2.6) Ma,代表了热液锆石的形成年龄,说明129 Ma是该西石门地区主要成矿期.Abstract: Xishimen iron deposit is one of the typical skarn iron deposits in northern China. The new 40Ar/<sup>39Ar mineralization ages of phlogopite in Xishimen iron deposit are 133.1±1.3 Ma and 137.1±1.5 Ma, slightly older than the ages (126-136 Ma) of mineralization parent rocks (Wu'an complex), which is unreasonable. This paper deals with the analyses of geochronology of the metallogenic event on the basis of LA-ICP-MS zircons U-Pb dating for hornblende syenite and skarn dikes developed in this area. The zircons of hornblende syenite are light yellow-colorless, transparent, angular and irregular with weak oscillatory zonings, and a few zircons have irregular inherited zircon core. LA-ICP-MS U-Pb analyses of these zircons yields a weighted mean 206Pb/238U concordia age of 135.6±1.5 Ma, which represents the crystallization age of the Wu'an complex. Zircons of skarn dikes are brown, transparent to translucent. In contrast to magmatic zircons, hydrothermal zircons have low Th, U, total REE contents (Th=222.37×10-6-1 541.11×10-6, U=218.44×10-6-989.17×10-6) and Th/U value (Th/U=0.90-1.56). They display a strong positive Ce anomaly(δCe=4.46-196.22) and negative Eu anomaly(δEu=0.59-0.80). Zircons from skarn dikes yielded 206Pb/238U ages ranging from 124.9 Ma to 133.0 Ma with a weighted mean age of 129.4±2.6 Ma, which is interpreted as the age of crystallization of hydrothermal zircons, showing that 129 Ma is the chief metallogenic period in Xishimen area.
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Key words:
- Xishimen iron deposit /
- skarn dikes /
- hydrothermal zircons /
- mineralization age
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