大兴安岭中段晚中生代安山岩年代学和地球化学特征及成因分析

Geochronology and Geochemical characteristics of Late Mesozoic andesites in the Central Da-Hinggan Mountains,and its genesis.

本文通过大兴安岭中段安山岩基质^(40)Ar/^(39)Ar 和 SHRIMP 锆石 U-Pb 年代学的测定,表明安山岩形成于122～125Ma和114～115Ma 两个时期,即为早白垩世的两期喷发。主量元素分析表明,安山岩 SiO_2含量为59.18%～60.89%,Al_2O_3含量为15.19%～17.23%,全碱(K_2O+Na_2O)介于6.16%～7.26%之间,MgO 含量为1.55%～3.77%和 Mg~#值为33～54;微量元素分析表明,安山岩轻稀土元素(LREE)富集、轻重稀土元素(HREE)分馏较强、略具负 Eu 异常(δEu=0.74～0.92)、富集大离子亲石元素(LILE),而亏损高场强元素(HFSE),尤其强烈亏损 Nb 和 Ta。Sr-Nd-Pb 同位素组成表明,安山岩(^(87)Sr/^(86)Sr)_i 变化范围为0.70454～0.70483;ε_(Nd)(t)变化范围为0.97～3.17;初始^(206)Pb/^(204)Pb、^(207)Pb/^(204)Pb和^(208)Pb/^(204)Pb 组成变化范围分别为18.17～18.28、15.50～15.60和38.02～38.29。岩石学和地球化学综合研究表明,大兴安岭早白垩世安山岩的岩性主要为钙碱性系列-钾玄岩系列,安山质岩浆来源于富集 LILE 和水的岩石圈地幔。结合前人资料及本文研究成果,初步认为研究区安山岩形成于板内伸展环境,在岩石圈伸展和减薄背景下,软流圈地幔的上涌和地温梯度增高导致上覆岩石圈地幔中的低熔组分(水和 LILE 富集的交代地幔)发生部分熔融而形成。

^40Ar/^39Ar and zircon SHRIMP U-Pb dating indicate that andesites in the Central Da-Hinggan Mountains were mainly formed in two periods： 122 - 125Ma and 114 - 115Ma. That means the andesites were formed in two volcanic explosions in Early Cretaceous. Geochemical results indicate that the SiO2 are in 59.18% - 60.89%, Al2O3 and K2O ＋ Na2O rang from 15. 19% to 17.23% and 6. 16% to 7. 26%, and Mg^# changes from 33 to 54. The trace elements analysis demonstrate the total LREE enrichment with insignificantly negative Eu anomalies （δEu = 0. 74 - 0. 92 ） and intense fractionation between LREE and HREE. The primitive mantle-normalized spider diagrams are characterized by LILE enrichment and HFSE depletion, especially a significantly negative Nb-Ta anomalies. Isotopic geochemistries show that values of εNd （t） range from 0. 97 to 3.17, and （ ^87 Sr/^86 Sr）i from 0. 70454 to 0. 70483, respectively. The ratios of ^206pb/^204Pb, ^207pb/^204pb and ^208pb/^204Pb vary from 18.17 to 18.28, 15.50 to 15.60, and 38.02 to 38.29, respectively. Based on our petrologic and geochemical study, the Early Cretaceous andesites in the Central Da-Hinggan Mountains are mainly calc-Alkaline, high K calc-Alkaline and shoshonitic, the andesitic magma originated from LILE- and fluid-rich mantle. Combining with previous studies, we suggest that the andesites were formed in the interplate-extending settings, and their formations were due to partially melting of the LILE- and fluid-rich lithospheric mantle, which was caused by upwelling of asthenospheric mantle and rising of ground temperature under extensional and weaken tectonic setting in northeastern China.