大兴安岭南端红山子盆地流纹岩的成因:元素和Sr-Nd-Pb同位素制约

Petrogenesis of Rhyolite from Hongshanzi Basin in Southern Greater Xing'an Range: Elements and Sr-Nd-Pb Isotope Constraints

红山子晚侏罗世早期火山盆地位于西拉木伦河—长春缝合带以南、康宝—围场—赤峰—开原断裂带以北的辽源地块,其中赋存铀钼矿床的流纹岩为肉红色流纹岩和灰白色流纹岩。为了查明流纹岩的物质来源和形成的构造背景,本文分析了流纹岩的主量元素、微量元素、稀土元素和Sr-Nd-Pb同位素组成。分析结果显示:流纹岩具有高SiO_2、K_2O和Fe_2O_3/FeO值,低Al_2O_3、CaO和MgO的特征,肉红色流纹岩比灰白色流纹岩的SiO_2含量略低、(K_2O+Na_2O)含量高和Fe_2O_3/FeO值略高;肉红色流纹岩和灰白色流纹岩均有较高的稀土含量,明显富集轻稀土,显著亏损铕;高场强元素Nb、Ta、Zr、Hf、Ce、Y、Ga和大离子亲石元素Rb、Th、U的含量较高,但大离子亲石元素Ba、Sr的含量低且变化较大,具有A1型流纹岩、低Sr-Ba流纹岩和板内拉张构造流纹岩的微量元素特征;具有较小的(87Sr/86Sr)i、较高的εNd(t)、较年轻的TDM 2和较低的(206Pb/204Pb)i、(^(207)Pb/^(204)Pb)i、(^(208)Pb/^(204)Pb)i,指示流纹岩是源于EMⅠ富集地幔的中元古代年轻下地壳部分熔融的产物,并在岩浆过程中经历了结晶分异。肉红色流纹岩和灰白色流纹岩的差异可能与年轻下地壳物质的成分差异有关。

The Hongshanzi early Late Jurassic volcanic basin is located in Keshenketeng Banner,Inner Mongolia, the southern tip of the Greater Xing＇an Range. Its tectonic posi＇tion is within the Liaoyuan block between the Xilamulun-Changchun suture zone and the Kangbao-Weichang-Chifeng- Kaiyuan fault belt. The fresh red and grey rhyolites, in Hongshanzi basin, are the wall rocks of hydrothermal-type U - Mo deposits. Both have the features of rich in SiO2, K2O and high ratio of Fe2O3/FeO, with lack of Al2O3, CaO and MgO contents, the fresh red rhyolite is higher in（K2O＋Na2O） contents and Fe2Oa/FeO ratio, while lower in SiO2. Both fresh red and grey rhyolites, are relatively high the total amount of REE, showing rich LREE obviously and right REE patterns, with negative Eu anomalies. The high contents of HSFE（Nb, Ta, Zr, Hf, Ce, Y, Ga） and LIRE（Rb, Th, U） , low contents of the LIRE Ba and Sr, show the trace elements feature of A1 type rhyolite, low Ba - Sr type rhyolite and the extensional tectonic setting. The rhyolites have smaller （87 Sr/S6 Sr）i, higher eNd（t）, younger TDM2,1ower （206Pb/204pb）i, （207Pb/204pb）i, （208Pb/204pb）i. Obviously, both fresh red and grey rhyolites were formed by the partial melting of Mid-Mesoproterozoic young lower crust materials originated from the EM I enriched mantle under the extensional tectonic setting within the plates, and had been accompanied by. the fractional crystallization during the magmatic process. Their differences may be related to the different composition of young lower crustal materials.