大兴安岭南部钠闪石流纹岩的岩石成因:年代学和地球化学证据

摘要

本文报道了在大兴安岭南部白音高老组新发现的钠闪石流纹岩的锆石U-Pb定年结果和岩石地球化学资料,以便揭示富碱钠闪石流纹岩的成因与区域构造演化.钠闪石流纹岩中锆石为自形-半自形的长柱状、短柱状,具有明显岩浆成因的震荡生长环带,结合其Th/U比值(1.2～1.9),暗示其为岩浆成因.锆石LA-ICP-MS U-Pb定年结果表明,其206Pb/238U年龄介于134 ～ 149Ma之间,其加权平均年龄为141Ma,表明钠闪石流纹岩形成于早白垩世早期.锆石εHf(t)=+9.07～+12.08,二阶段Hf模式年龄介于415 ～616Ma之间.岩石地球化学资料表明,钠闪石流纹岩以富硅、碱、铁质和贫钙、镁质为特征,在地球化学上,具有典型的A型流纹岩特征.该类岩石不仅具有高的稀土元素总量(REE=307×10-6～ 1395×10-6)、显著的Ce正异常(Ce/Ce* =6.52 ～ 18.6)和显著的Eu负异常(Eu/Eu*=0.007 ～0.009),而且表现出高场强元素(如Zr、Hf、Nb、Ta)的显著富集、大离子亲石元素(如Ba、Sr)的强烈亏损和很高的Ga/A1比值(104×Ga/Al =4.88～6.41).上述资料表明,钠闪石流纹岩的原始岩浆应是古俯冲蚀变洋壳部分熔融的产物,并形成于蒙古-鄂霍茨克缝合带闭合后的岩石圈伸展构造环境.%This paper reports the zircon U-Pb age and geochemical data of riebeckite rhyolites in Baiyingaolao Formation in the southern Da Hinggan Mts. , with the aim of revealing the petrogenesis of riebeckite rhyolites and the regional tectonic evolution. Zircons from the riebeckite rhyolite are euhedral-subhedral in shape, and display typical oscillatory zoning on CL images, together with their high Th/U ratios (1.2 ~1.9) , suggesting their magmatic origin. LA-ICP-MS zircon U-Pb dating results indicate that 206Pb/238U ages of 24 analytical spots range from 134Ma to 149Ma, yielding a weighted mean age of 141 ± 1Ma, implying that the riebeckite rhyolite formed in the early Early Cretaceous. Their εHf(t) values range from + 9. 07 to + 12. 08 , and their Hf two-stage model ages vary from 415Ma to 616Ma. The riebeckite rhyolites have SiO2 = 74. 3% ~ 76. 4% , FeOT = 2. 97% ~ 5. 94% , ( Na2O + K2O) = 7. 07% ~ 8. 51% , CaO = 0. 12% ~ 0. 84% , and MgO = 0. 03% ~ 0. 09% , similar chemically to typical A-type rhyolite. Additionally, the riebeckite rhyolites have high total REE contents ( 307 ×10-6 ~ 1395 × 10-6 ) and Ga/Al ratios ( 104 × Ga/Al = 4. 88 ~ 6. 41) , strong positive Ce anomalies (Ce/Ce* =6.52 ~ 18.6), and strong negative Eu anomalies ( Eu/Eu * =0.007~0.009), and display enrichment in HFSEs (e. g. , Zr, Hf, Nb, Ta) and strong depletion in LILEs (e. g. , Ba, Sr). Taken together, we conclude that the riebeckite rhyolites could be derived from partial melting of a fossil altered oceanic crust during an extensional setting after the closure of the Mongol-Okhotsk Ocean.