The noble gas isotopic constrains on the petrogenesis of mafic-ultramafic intrusions in the western end of the Pobei complex, Xinjiang, northwestern China

The large Pobei Permian layered mafic-ultramafic complex in the northeastern margin of the Tarim craton, western China is composed of a gabbro intrusion and more than 20 ultramafic intrusions. The isotopic compositions of noble gases in the Poyi ( No. 1) ultramafic intrusion revealed information of mantle plume. In present study the abundance and isotopic compositions of noble gases were analyzed for fresh minerals in mafic-ultramafic rock samples from No. 1 , 4 , 10 and 14 ultramafic intrusions in the western end of Pobei complex. The results showed that olivine ( O1) , pyroxene ( Px) and plagioclase ( P1) magmatic minerals in the Pobei (#)1 , 4, 10 and 14 ultramafic intrusions were in low values of He-3/He-4 ratios ( 0. 26 similar to 2. 79Ra ) . The Ne-21/(22) Ne and Ne-20/(22) Ne were plotted and distributed between the Ne mass fraction line ( MFL) and L-K line, the Ar-40/(36) Ar ratios ranged from 295 to 598. These isotopic compositions of noble gases indicated a mixture of different portion of mantle ( plume) , crust and air during the Pobei magmatic processes with different petrogenesis. The mantle plume was identified in the Poyi ultramafic intrusion, and more crustal fluid was added into lithospheric mantle in other three ultramafic intrusions. The magmatic source area of Pobei ultramafic intrusion was superimposed by deep mantle plume on lithospheric mantle that was reformed by subducted fluids.