鲁西韩旺BIF铁矿的形成时代——锆石SHRIMPU-Pb定年证据

Formation Age of the Hanwang BIF Deposit in Western Shandong:Evidence from Zircon SHRIMP U-Pb Dating

鲁西地区太古宙表壳岩包括新太古代早期和晚期两期表壳岩,早期表壳岩主要由变质玄武岩-科马提岩组成,晚期表壳岩主要由变质火山岩-碎屑沉积岩和BIF(Banded Iron Formations)组成。韩旺铁矿位于鲁西的西北部,铁矿区内存在大量变质玄武岩-科马提岩,早期认为该BIF形成于新太古代早期,而新的研究认为鲁西地区BIF都形成于新太古代晚期。本文对韩旺铁矿区内与BIF互层的黑云变粒岩和侵入其中的片麻状花岗闪长岩开展锆石SHRIMP U-Pb定年,获得年龄分别为(2529±7)Ma和(2534±11)Ma。黑云变粒岩和片麻状花岗闪长岩的TREE、(La/Yb)N、Eu/Eu*分别为76×10^(–6)、19.8、0.84和82.7×10^(–6)、17.3、1.14,它们的岩浆锆石的εHf值、单阶段Hf同位素模式年龄分别为5.5~9.46、2.5~2.6 Ga和6.3~9.4、2.48~2.60 Ga。研究支持了鲁西地区BIF形成于新太古代晚期的认识。表壳岩形成、变质变形和花岗闪长岩侵入发生在一个很短的时间范围内。研究还表明,黑云变粒岩的原岩为英安质火山岩,很可能形成于新生玄武质岩浆的强烈结晶分异作用,花岗闪长岩形成于新生玄武质岩石部分熔融,形成过程中有陆壳物质的加入。

Archean supracrustal rocks in western Shandong include two stages of supracrustal rocks during the Early Neoarchean and the Late Neoarchean.Early supracrustal rocks are mainly composed of metamorphic basalt-Komatiite,whereas late supracrustal rocks are mainly composed of metamorphic volcanic rocks-clastic sedimentary rocks and BIF(Banded Iron Formations).The Hanwang iron deposit is located in the northwest of western Shandong.There are a large quantities of meta-basalt-Komatiite in the iron ore area.It is believed that the BIF was formed during the Early Neoarchean,but a recent study suggests that the BIF in western Shandong area was formed during the Late Neoarchean.In this study,the zircon SHRIMP U-Pb dating of the biotite granulite interbedded with BIF and the gneissic granodiorite intruded in the Hanwang iron ore area was carried out,and the ages were(2529±7)Ma and(2534±11)Ma,respectively.The TREE,(La/Yb)N,and Eu/Eu*of biotite granulite and gneissic granodiorite are 76×10^(–6),19.8,and 0.84 and 82.7×10^(–6),17.3,and 1.14,respectively.TheεHf values and single-stage Hf isotopic model ages of their magmatic zircons are 5.5–9.46 Ga and 2.5–2.6 Ga,and 6.3–9.4 Ga and 2.48–2.60 Ga,respectively.This study supports the recognition that BIF in western Shandong was formed during the Late Neoarchean.The formation of supracrustal rocks,metamorphic deformation,and granodiorite intrusion occurred in a very short-time range.The study also shows that the protolith of the biotite granulite is dacitic volcanic rock,which was likely to be formed by the strong crystallization differentiation of the newborn basaltic magma.The granodiorite was formed by the partial melting of the newborn basaltic rock,and the continental crust material was added during the formation process.