关键词: 鲁西, 新太古代, 花岗岩类, 地球化学, U-Pb年代学, Lu-Hf同位素, 同碰撞

Abstract: Western Luxi is the only area in the world where the Early Neoarchean TTG and greenstone belts are completely preserved, which are typical for study the evolution types of Archean magma, the crust-mantle interaction, and the Archean tectonic model. Based on field geological surveys, in this paper, the geochemical characteristics and formation background of the Neoarchean granites and monzonites in western Shandong s are discussed by means of trace element geochemistry, isotope geochemistry, and isotope chronology. The U-Pb ages of Neoarchean granites and monzonite granites in western Luxi are mainly 2 537 Ma and 2 566 Ma. The granite (TA1802) εHf (t) value is -1.4-2.9(average 0.65), and the two-stage model age is about 2.9 Ga; The monzonite granite (TA1812) εHf (t) value is -0.4-2.7( average 1.31), the age of two-stage model is 3 073-2 886 Ma(average 2.8 Ga), the monzonite granite (TA1817) εHf (t) is 0.3-4.7, the average is 3.35, the age of the second-stage model is 3 032-2 762 Ma, the average is about 2.8 Ga. On the εHf(t)-t diagram, the age evolution lines of the Neoarchean granites and monzonites in Luxi fall on the 2.9-2.8 Ga crustal evolution line, and are roughly the same as that of the second-stage model, which indicates that the Luxi Neoarchean granites and monzonites originated from the re-melting of the ancient crust at 2.9-2.8 Ga. The Neoarchean granites and monzonites in western Shandong are characterized by high w(SiO2), w(Al2O3), and w(Na2O), and they are all quasi-aluminum rocks. In the standardized distribution pattern of rare earth element chondrites, they all show light rare earth element (LREE) enrichment, heavy rare earth element (HREE) depletion, and medium-heavy rare earth element fractionation. There are two samples (TA1801-1 and TA1824) showing Ta enrichment and the rest samples showing large ion lithophile element enrichment, such as K, Rb, Ba and Th, and Nb, Ta, and Ti depletion. The monzonites also show the enrichment of large ion lithophile element such as K, Rb, Ba and Th, the depletion of Nb, Ta, Ti, and the partial molten residual minerals include garnet, rutile, and a small amount of plagioclase and amphibole. Based on the above geochemical characteristics, combined with the regional geological characteristics, the tectonic background of the Neoarchean granites and monzonites in western Shandong is syn-collision, which is the reason of the effective accretion of continental crust.

Key words: western Shandong Province, Neoarchean, granites, geochemistry, U-Pb chronology, Lu-Hf isotopes, syn-collision