大兴安岭温布其地区白音高老组火山岩的形成机制

Formation mechanism of Baiyingaolao volcanic rocks in Wenbuqi area of Da Hinggan mountains

大兴安岭温布其地区白音高老组火山岩主要由流纹质岩屑晶屑凝灰岩和流纹岩等一系列酸性火山岩构成。LA-ICP-MS锆石U-Pb测年结果显示,研究区流纹岩形成于(131±1.5)Ma,为早白垩世;岩石地球化学研究表明,样品具有高硅SiO_2(72.43%~78.64%)、富铝Al2O3(11.48%~14.24%)、富钾K_2O(4.09%~5.05%)和低镁MgO(0.14%~0.34%)的特点;轻重稀土分馏较明显[(La/Yb)_N=8.22~13.68],Eu负异常明显(δEu=0.39~0.90);微量元素相对富集大离子亲石元素Rb、Ba、K,亏损高场强元素Nb、P、Ti。^(176)Hf/^(177)Hf比值为0.282 830~0.282 983,εHf(t)为6.02~10.19。研究发现,温布其地区白音高老组火山岩岩浆来源于新元古代亏损地幔增生的年轻基性地壳的部分熔融,形成于伸展的构造环境。

The volcanic rocks in Baiyingaolao Formation of Wenbuqi area of Da Hinggan mountains are composed of a series felsic volcanic rocks such as rhyolite and rhyolitic lithic crystal tuff. LA-ICP-MS zircon U-Pb dating results show the rhyolite formed in the Early Cretaceous,aged in（131 ± 1. 5） Ma. The geochemical study suggests that the rhyolite is characterized by high SiO_2（72. 43% ~ 78. 64%）,Al_2O_3（11. 48% ~ 14. 24%）,and K_2O（4. 09% ~ 5. 05%）,and low MgO（0. 14% ~ 0. 34%） in contents. Samples are significant in fractionation of HREE and LREE [（ La/Yb）_N= 8. 22 ~ 13. 68],with negative Eu anomalies（ δEu = 0. 39 ~ 0. 90）. The trace element are characterized evidently by enrichment of Rb,Ba,K,depletion of Nb,P,Ti. The ratios of^（176）Hf/^（177） Hf vary in 0. 282 830 ~ 0. 282 983 and the corresponding εHf（t） values are positive ranging from 6. 02 to 10. 19.Above characteristics indicate that the volcanic rocks derived from the basic lower crust accreted from depleted mantle during Neoproterozoic. Tectonic setting is related to the extensional setting.