Petrogenesis And Magma Source Of Wengen A Mafic-ultramafic Intrusion, Inner Mongolia, China

Wengen A mafic-ultramafic intrusion locates in western section of the northern margin of the North China Plate. Petrology, mineralogy, major, trace element and Sr-Nd isotopes are presented to reveal lithofacies, magmatic character, magma source character and formation mechanism of mafic-ultramafic instusion.Wengen A mafic-ultramafic intrusion is composed of troctolite, gabbronorite, anorthosite, olivine-gabbro, norite and troctolite The crystallization sequence of the intrusion is Ol→Ol-Pl→Ol-Pyx-Pl→Pyx-Pl and Hbl. According to major element, the whole rock is low MgO and K2O+Na2O. It can be found from the relationship with the major oxides in the MgO diagram. MgO and major oxide contents in a linear relationship, which, SiO2, Al2O3, CaO, total bases and a ne14gative correlation between MgO, FeOT positive correlation with MgO. Most samples have similar REE patterns. The rocks are enriched LREE in relative to HREE, and exhibit strong positiveδEu anomaly (+1.23to +2.29). Rocks are depleted in high field strength elements (Th, Nb, Ta, Zr) and relatively enriched in large ion lithophile elements (U, Ba, Sr, Pb). These characters are similar with the curve of lower crustal. Sr isotopic compositions of samples were systematically analyzed and fall in a range of 0.7029 to 0.7083 for (87Sr/86Sr)i. The variations in Sr isotopes are close to EMI.The zircon SHRIMP age of anorthosite is 285±2 Ma. It belongs to early Permian.Using a Mg-Fe distribution coefficient (Kd=(Fe/Mg)Ol/(Fe/Mg)magma) of 0.30±0.03 (Roeder and Emslie, 1970), we obtain parental magma compositions of Wengen layed intrusion. The MgO of it is 7.0%. The parental magma is higher magnesium tholeiite. The rising magma significantly underwent lower crustal contamination.The formation and evolution of magma can be summarized as: basaltic magma originated from lower crust - upper mantle, should occur in North China platform product of underplating, instead of Paleo-Asian Ocean subduction, the residual part of oceanic crust fraction melting process.Which can be shown that the early Permian of North China platform underplating already exists, based on material non-mixed oceanic crust.We can be further speculated that Wulatehouqi region of the Paleo- Asian Ocean subduction process has ended after 285Ma. Magma rising process because of the density relationships formed deep magma chamber, place of olivine and plagioclase fractional crystallization. Evolution of magma rose to high magma chamber occurred pyroxene, hornblende, etc. fractional crystallization, so the gabbro formed.