新疆博格达东缘色皮口地区晚石炭世裂谷火山岩地球化学、锆石U-Pb年代学及Hf同位素研究

Geochemistry,zircon U-Pb age and Hf isotopes of Late Carboniferous rift volcanic in the Sepikou region,eastern Bogda,Xinjiang

新疆色皮口地区位于博格达造山带东段北部,区域内的上石炭统柳树沟组火山岩为玄武岩-玄武安山岩、角斑岩-石英角斑岩和流纹岩,组成双峰式火山岩建造。岩石SiO 2含量为48.07%~77.62%,赖特碱度率（AR）为1.35~4.7,Na2O＋K2O含量为3.74%~9.02%,K2O/Na2O值为0.04~1.04,为低钾高钠钙碱性-碱性岩石。玄武岩、玄武安山岩TiO 2=0.86%~1.7%,较高的Al、低Mg,以及低K2O/TiO 2和K2O/P2O5比值（分别为0.13~1.81、0.36~6.00）,反映了在岩浆演化过程中发生了不明显的分离结晶作用。玄武岩、玄武安山岩、角斑岩不相容元素K、Rb、Th、Ba强富集,高场强元素Nb、Ta、Zr、Hf无富集,Ti亏损不明显,玄武岩（Th/Nb）N值为1.36~6.55,Nb/La值为0.29~0.44,具有较低的Nb/Zr比值（0.03~0.05）。由玄武岩到石英角斑岩,稀土元素组成略右倾平行曲线簇,倾斜度（轻重稀土分异度）略增大,铕负异常趋于明显（δEu=0.81~1.17）。流纹岩不相容元素K、Rb、Th、Ba富集,高场强元素Nb、Ta、Zr、Hf富集,流纹岩稀土总量增高,轻重稀土分异增大,明显铕负异常（δEu=0.27~0.50）,显示后期较强的岩浆分异作用。石英角斑岩的锆石LA-ICP-MS U-Pb定年结果为314.9±1.2Ma（n=16,MSWD=0.4,Th/U比值在0.56~1.21之间）,这表明石英角斑岩形成时代为晚石炭世。石英角斑岩中锆石的176Hf/177Hf比值均分布在0.282897~0.283097之间,并具有较高的正εHf值（11~18）,平均值为14,Hf的模式年龄tDM2介于180~628Ma。所有锆石的176Hf/177Hf比值和εHf值位于亏损地幔演化线与下地壳之间,并靠近亏损地幔演化线。上述特点反映晚石炭世火山岩形成于板内裂谷环境,下部玄武岩与角斑岩-石英角斑岩具有同源特征,暗示岩浆源区来源于亏损地幔,并受地壳混染。

The Upper Carboniferous Liushugou Formation bimodal volcaric rocks, consisting of basalt, basaltic andesite, keratophyre-quartz keratophyre and rhyolite, are widely distributed in Sepikou ea which is located in the eastern margin of Bogda Mountain, Xinjiang. The rocks in the studied area have low-K （K20 =0. 17% 4. 07% ） and high-Na （Na20 =3.09% 6. 47% ） ; SiO2 contents between 48.07% and 77.62% ; AR exponent between 1.35 and 4.7 ; Na20 ＋ K20 contents between 3.74% and 9. 02% ; the low K20/Na20 ratios （0. 04 1.04） ; these data shows that it belongs to calc-alkalic alkalic rocks. The basahs, basaltic andesites have slight high-A1, low-Mg, low K20/TiO2 and K20/P205 radios （the values are 0. 13 1.81, 0. 36 -6. 00）, indicating that fractional crystallization is not clear. The basalt, basaltic andesite, keratophyre are characterized by high abundance K, Rb, Th, Ba; and depletion in HFSE （Nb, Ta, Zr, Hf）, and slightly depletion in Ti. The basalts are characterized by （Th/Nb）N = 1.36 6. 55 ratios, and Nb/La = 0. 29 0.44 ratios, and low Nb/Zr = 0. 03 0. 05 ratios, Chondrite-normalized REE patterns for basalts- keratophyre is slight-leaning parallel curve clusters, together with slight enrichment in LREE contents and low HREE/LREE ratios, the keratophyre exhibits slightly negative Eu anomalies （SEu = 0. 81 1.17 ）. The rhyolite is characterized by high abundance K, Rb, Th, Ba, and high abundance in HFSE （Nb, Ta, Zr, Hf）, high rhyolite Y.REE contents, low HREE/LREE ratios, They also exhibit that the fractionated REE patterns with prominent negative Eu anomalies （ 8Eu = O. 27 0. 50 ）, suggesting stronger crystallization differentiation of magma. In situ U-Pb ages and Hf isotopes of quartz keratophyre were analyzed on the basis of micro area studies upon zircon. Zircon U-Pb ages of the quartz keratophyre are 314. 9 ＋ 1.2Ma （n = 16, MSWD =0. 4）, respectively with their Th/U ratio of 0. 56 1.21. The quartz keratophyre zircons, with their 176Hf/177Hf ratios varying in 0. 282897 0. 283097, have high positive .f（ 11 18, averaged at 14） and depleted mantle model ages between 180Ma and 628Ma, which are almost equal to their U-Pb ages. In the 176Hf/177Hf-6Fif discrimination diagrams, these zircons are potted on the depleted mantle evolution line. Above characteristics showed that Late Carboniferous volcanic formed in continental rift. Basalt and quartz keratophyre have the same characteristics. Petrology, geochemistry, Hf, Nd isotopic characteristics indicate that they are originated from depleted mantle of contamination by crustal materials. The Late Carboniferous rhyolite formed from both fractional crystallization of basaltic magma of contamination by crustal materials, and the partial melting of crust.