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

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40 Ar-39 Ar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,206 Pb-238 U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-238 U年龄介于124.9~133.0Ma,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。

Constraints of Hydrothermal Zircon U-Pb Ages on the Metallogenic Epoch of the Xishimen Skarn Iron Deposit,Sourthern Hebei Province

Xishimen iron deposit is one of the typical skarn iron deposits in northern China. The new 40Ar/39Ar mineralization ages of phlogopite in Xishimen iron deposit are 133. lil. 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/238 U concordia age of 135.6±1.5 Ma, which represents the crystallization age of the Wu＇ an com- plex. Zircons of skarn dikes are brown, transparent to translucent. In contrast to magmatic zir- cons, hydrothermal zircons have low Th, U, total REE contents （Th=222.37×10^-6-1 541.11×10^-6, U=218. 44×10^10-6 --989.17X10-6） 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 206 pb/238 U 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 hydro- thermal zircons, showing that 129 Ma is the chief metallogenie period in Xishimen area.