冀东马兰庄条带状硅铁建造的变质时代及地质意义

Metamorphic chronology of the BIF in Malanzhuang of eastern Hebei Province and its geological implications

前寒武纪条带状硅铁建造（BIF）是世界,也是中国最重要的铁矿资源类型,无论是储量,还是开采量,均居首位。前寒武纪条带状硅铁建造是地球早期特有的化学沉积建造类型,广泛分布于太古代—早元古代（3.2-1.8 Ga）,记录了地球早期大气和海洋的化学成分、氧化还原状态及演化,是广大地质工作者长期关注的焦点之一。在国外,与BIF有关的古风化壳型富铁矿广泛发育,同时也是国外重点开采的富铁矿类型,但在中国一直没有发现。本文测定了冀东迁安矿区东矿带马兰庄硅铁建造中与硅铁建造互层产出的花岗质岩石的锆石SHRIMP U-Pb年龄,确定了BIF遭受混合岩化的时代,并根据中国BIF的形成年龄和变质时代对造成中国缺失与BIF有关的古风化壳型富铁矿这一问题进行了探讨。在中国,硅铁建造形成不久可能即被深埋,遭受区域变质作用和混合岩化作用,但没有在地表长时间遭受风化剥蚀,这可能是冀东地区,乃至中国缺失与BIF有关的古风化壳型富铁矿的重要原因之一。

The Precambrian Banded Iron Formations（BIF） are the most important iron resource both in China and abroad,whether in the exploratory reserves or in the quantity of the exploitation.BIF were also the special geochemical sediments in the early earth history which recorded the composition and the redoxomorphic stage of the contemporary atmosphere and the ocean as well as their evolution.The paleo-weathering crust of BIF,which is the most enriched iron ore,is widely distributed in the world.However,this type of BIF has never been found in China.Why is the paleo-weathering crust of BIF absent in China？ This is the main focus of this paper.The authors investigated the metamorphic chronology of the BIF in Malanzhuang by means of SHRIPMP II analysis of the zircons from the granitic rocks which occur in the interlayer of the BIF.Based on a comparative study of the time of the BIF formation,the authors have concluded that the BIF in China were deeply buried immediately after their formation,and subsequently they experienced regional metamorphism and migmatization.The BIF in China experienced no weathering for a long time at the earth＇s surface,which is the reason why the paleo-weathering crust type of enriched iron ore cannot be found in China.