Abstract:
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. The
εHf 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.