新疆塔里木板块西部瓦吉里塔格地区二叠纪超镁铁岩的岩石成因与岩浆源区

Petrogenesis and magma source of the ultramafic rocks at Wajilitag region, western Tarim Plate in Xinjiang.

瓦吉里塔格地区位于塔里木板块西部,前人在此区发现了晚二叠世形成的6个爆破角砾岩筒和32条岩脉。爆破角砾岩筒属火山口相和火山管道相,脉岩属次火山相。爆破角砾岩筒由角砾和胶结物组成。角砾的主要类型是纯橄榄岩、橄榄岩、橄榄辉石岩、辉石岩等;胶结物主要是麦美奇岩和苦橄岩。据岩石化学组成和液相线橄榄石的Fo分子确定的原生岩浆的MgO含量分别是18.78％和18.8％。它们的CaO/Al_2O_3平均值为3.9,Al_2O_3/TiO_2平均值为2.6。这些超镁铁岩都富集LREE、大离子亲石元素和Nb、Ta,而重稀土元素贫化,并有Zr、Hf负异常,不同样品间∑REE和(La/Yb)_N的差异反映了橄榄石和单斜辉石堆晶作用的影响,依据实验岩石学研究成果,熔融深度应为450km～650km。副矿物组合异常复杂,其中,刚玉的结晶深度有可能大于575km,它们的ε_(Nd)(t)=+3.3～+5.4,ε_(Sr)(t)=-4.27～-10.78,^(206)Pb/^(204)Pb=18.47～22.78,^(207)Pb/^(204)Pb=15.52～15.90,^(208)Pb/^(204)Pb=38.45～39.62。Nd、Sr、Pb同位素组成与洋岛玄武岩和一些地幔柱组分相同或相近。综合各方面的研究成果可以证明,这些超镁铁岩很有可能源自于核一幔边界之上的D^n层,源区本属于亏损型地幔,但在岩浆生成前不久被富含轻稀土元素和大离子亲石元素的流体交代。

Six diatreme breccia pipes and 32 dikes, which occurred in late Permian epoch, were found by predecessors in the Wajilitag region located in the western Tarim plate. The dikes belong to ultra-hypabyssal phase of subvolcano phase. The diatreme breccia pipes belong to volcanic crater phase and volcanic conduit phase, and consist of breccias and binding material. The breccias are mainly slithers (dunite, peridotite, olivine pyroxenite, pyroxenite, et al.) and crystal debrises (olivine, clinopyroxenes and phlogolite), which are mainly comprised of early cumulative phases. While all binding material are volcanic lava, main petrological types are meimechite and picrite. The MgO content of primary magma determined by petrochemical compositions and Ni abundances is 18.78%; on the other hand, by Fo molecular of liquidus olivine is 18.8%. Their average CaO/Al2O3 is 3.9, and average Al2O3/TiO2 is 2.6. All of ultramafic rocks are enriched in LREE, LILE and NbTa, but HREE are depleted, and show negative Zr and Hf abnormality. The differences of SigmaREE and (La/Yb)(N), between breccias and binding materials reflect cumulation effects of olivines and clinopyroxenes. According to research results of experimental petrology, they should be generated from a high degree partial melting in the majozite stability field with depths ranging from 450km to 650km. Their accessory mineral assemblage shows extreme complication, among them, the crystallization depth of corindon should surpass 575km. epsilon(Nd) (t) of these rocks varies from +3.3 to +5.4, epsilon(Sr) (t) = -4.27 similar to -10.78, Pb-206/Pb-204 = 18.47 similar to 22.78, Pb-207/Pb-204 = 15.52 similar to 15.90, Pb-208/Pb-204 = 38.45 similar to -39.62. Nd, Sr and Pb isotopic compositions are identical to or similar to some ocean island basalts and mantle plume constituent. It is proved by various research results that these ultramafic rocks were possibly derived from D' layer which lay above core-mantle boundary. This source region shows a long time-integrated depletion and therefore, the infarred enrichment event in the mantle sources must have taken place relatively shortly prior to melt segregation.