Geochemical And Magnetic Study Of Cenozoic Kamafugite In Lixian Areas,Western Qinling

Kamafugite is an igneous rock rarely exposed in the world.Cenozoic kamafugites reported in this paper occur in West Qinling,Gansu Province,China.Kamafugitic magmas are usually derived from a deep mantle source and always have mantle xenoliths.In addition,many kamafugite suites are associated with coeval carbonatites.Therefore,kamafugite,mantle xenoliths in them and related carbonatites are very important to understand the deep matter of the earth.West Qinling is located in the western part of the Qinling orogenic belt and bounded between the southern margin of the North China Craton and the Northern margin of the Yangtze Craton.It is also a part of the Northeasternmost margin of the Tibet Plateau.The Haoti,Baihe and Goujia areas of the Lixian,Gansu Province,are located near the Tianshui-Lixian strike-slip fault belt.Cenozoic ultrapotassic magmatic rocks are extensively exposed in these areas.In order to constrain the petrogenesis and magma source of these Cenozoic ultrapotassic rocks,as well as the paleolatitude variation of the Lixian area,petrography,whole-rock geochemical,Sr-Nd-Pb isotopic data,rock magnetic and paleomagnetic data are presented in this dissertation.Our studies provide some new conclusions as follows:Kamafugites in Lixian Country of West Qinling are commonly considered to erupted in the Miocene(7.1-22 Ma).These rocks have high total alkaline contents(K2O+Na2O,up to 6.16 wt.%).Different from alkaline basalt,the kamafugite have lower SiO2 and Al2O3,and high CaO,MgO,TiO2 and total alkaline contents(K2O + Na2O).Their major elements display no clear linear relationships with MgO contents,indicating no obvious magmatic fractional crystallization.In addition,they are enriched in LREEs,Nb,and Ta,and show relatively flat HREE patterns with no obvious negative Eu and Sr anomalies.They have OIB-like(but more enriched)REE patterns with high(La/Yb)N ratios(32?50).Extensive mantle-derived xenoliths from these kamafugites and their uniform MgO,Ni,Cr contents and initial 87Sr/86Sr ratios with high Mg#(Mean>73)rule out significant crustal contamination.Their low initial 87Sr/86Sr ratios(0.7042?0.7051)and depleted ?Nd(t)(+3.2?+3.8)suggest an origin from relatively depleted lithospheric mantle metasomatized by carbonatitic melts.The magma source is probably composed of spinel-and garnet-mantle peridotites.Recent 40Ar-39Ar analyses on biotite and hornblende from the kamafugite rocks of the Lixian Country yielded Miocene eruption age.Kamafugites samples collected from Haoti,Baihe and Goujia area were also analyzed for rocks magnetic study.Rock magnetic experiments(isothermal remnant magnetisation acquisition curves and inverse direction demagnetisation curves measurements,three axis isothermal remnant magnetisation systematic thermal demagnetization analysis and magnetic hystersis loop curves)of each sample indicated that the magnetic carriers are mainly of magnetite,except few samples are magnetic pyrite.Almost all the samples have one component.Paleomagnetic results show that the West Qinling Orogenic Belt in the Miocene was close to that of the modern geographical position.In addition,palaeolatitude is close to the current latitude,and no obvious northward movement,suggesting that the West Qinling Orogenic Belt began to stabilize at least before the time when the kamafugite rocks formed.The Western Qinling Orogenic Belt is likely to be affected by the uplift of the Tibet Plateau,and a clockwise rotation has occurred.