Geochemical Characteristics And Significances Of Komatiites In Western Shandong Province

Komatiites are products of decompression melting of mantle which is very hot and they are almost exclusively restricted to the Archean.The Mengyin komatiites are located in the western Shandong Province of eastern block of North China craton.Komatiites have high-MgO?22.92-33.91 wt.%?,but low-Al₂O₃?3.83—7.22 wt.%?,TiO₂?0.12-0.22 wt.%?,CaO?4.83-8.15 wt.%?and Zr(5.6-8.1×10^-6),?REE(3.29-5.74×10^-6)contents,which indicate that they derived from a more refractory mantle source without residual garnet.According to the traditional classification of komatiites,our samples,which possess near-chondritic Al₂O₃/TiO₂?26.8-32.7?and CaO/Al₂O₃?1.1-1.3?ratios with flat HREE??Gd/Yb?_N=0.86-0.88?,belong to the Al-undepleted komatiites?AUK?,similar to Munro,Gorgona and Vietnam komatiites.Zircons in samples are divided into recrystallized and xenocryst grains whose ages distribution suggest a possible 2518-2561 Ma formation age range of komatiites.Based on previous regional research results and characteristics of zircons in samples,komatiitic magma may erupt at²520 Ma,the late Neoarchean.Most zircons have?_Hf?t?values between CHUR and DM line,suggesting that the late Neoarchean was an important period when tectono-thermal events occurred widely in western Shandong Province and mainly involved juvenile crustal additions.Results of olivine EPMA indicate that komatiites captured some olivine xenocrysts.On the basis of relationship between olivine anhydrous liquidus temperature and MgO contents of melts,we obtain a probable eruption temperature of komatiitic magma,¹560??1 atm?,implying whose origin was related to mantle plume.Breakoff of subducting slab might result in plume,changing the tectonic regime in western Shandong Province during the Late Neoarchean period.alphaMELTS simulation results show that slightly depleted garnet peridotite can produce Al-undepleted-komatiitic melts by 10-15%partial melting.