新疆麻扎尔塔格地区铁富集型高镁岩浆的源区

Source of the Fe-riched-type High-Mg Magma in Mazhartag Region,Xinjiang

麻扎尔塔格地区位于塔里木板块西缘 ,巴楚县东部。该区有密集的二叠纪脉岩群产出 ,侵入于志留系、泥盆系、石炭系和下二叠统 ,主要岩石类型是辉绿岩、辉绿玢岩、暗色橄榄辉长岩、煌斑岩等。其中 ,暗色橄榄辉长岩的 Fe2 O3+Fe O=14 .4 0 %～ 16 .88% ,Mg O=17.2 1%～ 18.5 9% ,属铁富集型高镁岩浆岩。它们的 Mg#=6 7～ 6 8,Fe Ot/Mg O <1,Ni=4 6 9× 10 - 6 ～ 6 35× 10 - 6 ,大体上代表了原生岩浆。而辉绿玢岩和辉绿岩的 Mg#、Fe Ot/Mg O和 Ni丰度则证明它们属适度演化的岩浆。岩相学与岩石化学证明 ,橄榄石和单斜辉石的分离结晶作用是岩浆演化的主要机制。各种脉岩均具有板内拉张环境下玄武岩的稀土元素和微量元素地球化学特征。这些岩石具有相同的 Nd、Sr、Pb同位素组成 ,1 43Nd/1 44 Nd=0 .5 12 5 0 8～ 0 .5 12 786 ,87Sr/86 Sr=0 .70 4 2 4 6～ 0 .70 6 4 4 4 ,2 0 6 Pb/2 0 4 Pb=18.17～ 19.2 4 ,2 0 7Pb/2 0 4 Pb=15 .4 7～ 15 .71,2 0 8Pb/2 0 4 Pb=38.6 3～ 39.32。由此证明 ,这些脉岩具有原始地幔的 Nd、Sr、Pb同位素组成 ,它们的岩浆源区应该位于下地幔。

A group of Permain vein rocks, which intrude into the stratum of Sliurian system, Devonian system, Carboniferous system and lower Permain series, occur in Mazhartag region, located in the west margin of the Tarimu plate, eastern Bachu county. Their main rock types have diabases, diabase-prophyrites, chromocratic olivine gabbros and lamprophyres. The chromocratic olivine gabbros, with Fe 2O 3+FeO=14.40%～16 88%, MgO=17 21%～18 59%, Mg#=67～68, FeO t/MgO<1 and Ni=469×10 -6～635×10 -6, belong to the Fe-riched-type high-Mg magma and are more or less representative of the primary magma, while Mg#, FeO t/MgO and Ni abundance of the diabases and diabase-prophyrites show these rocks belong to the moderately evolved magma. It is proved by the petrography and petrochemistry that the fractional crystallization of the olivines and clinopyzoxenes is the main mechanism of the magmatic evolution. All varial vein rocks have the characteristics of the rare earth element and trace element geochemical characteristics of basalts formed under the extension settings of the intra-plate. All rocks,with 143Nd/ 144Nd= 0 512508～0 512786, 87Sr/ 86Sr=0 704246～0 706444, 206Pb/ 204Pb= 18 17～19 24, 207Pb/ 204Pb=15 47～15 71 and 208Pb/ 204Pb=38 63～39 32, have the identical Nd, Sr and Pb isotopic compositions. Therefor, it can be proved that these vein rocks have Nd, Sr and Pb isotopic compositions of the primitive mantle and their magmatic source maybe lay in the lower mantle.