Abstract:Jiaobei area is located in the south part of Paleoproterozoic Jiaodong-Liaoning-Jilin tectonic and magmatic belt. In the past years, early Precambrian geological events have been studied, but the study on mafic-ultramafic rocks in the Paleoproterozoic period is very lack. In this paper, through analysis on geochemitry of mafic-ultramafic rocks, zircon U-Pb dating and Hf isotope analysis, the origin, ore-forming age and the significance of tectonic and magmatic evolution have been discussed. It is regarded that mafic-ultramafic rocks are partial aluminous rock with the characteristics of low silicon and poor potassium. The La/Yb differentiation is weak, and there is no obvious Eu anomaly. The age of zircon is ~2.1 Ga, and the forming age is Palaeoproterozoic, which is consistent with regional Proterozoic tectonic thermal events. At the same time, two generations of inherited zircon ages of gabbro are ~2.4Ga and ~2.7 Ga, and metamorphic ages of 1.8~1.9 Ga have been obtained. The ε Hf (t) value and TDM2 of magmatic zircon of a metamorphic gabbro sample are -11.1~-2.1 and 2859~3408 Ma (10 data points) respectively; while the ε Hf (t) value and TDM2 of an ultra-basic rock sample are 4.6~5.3 and 2355~2398 Ma (2 data points) respectively. It is showed that basic rocks are derived from the mantle, but are strongly influenced by the materials of the continental crust. The metamorphic ultrabasic is mainly composed of Paleoproterozoic depleted mantle or its new crust components. The degree of magma melting is high. It is generally reflected that the area changed from the early extrusion mechanism to the Paleoproterozoic extension mechanism.
