大兴安岭根河地区早白垩世火山岩深部源区与构造-岩浆演化：Sr-Nd-Pb-Hf同位素地球化学制约

Characteristics of deep sources and tectonic- magmatic evolution of the early Cretaceous volcanics in Genhe area,Da-Hinggan Mountains:Constraints of Sr-Nd-Pb-Hf isotopic geochemistries.

北大兴安岭根河地区发育大面积晚中生代火山岩,是大兴安岭中生代火山岩的重要组成部分。本文通过年代学(火山岩基质^(40)Ar/^(39)Ar年龄和SHRIMP锆石U-Pb年龄)测定,发现该区火山岩年龄为140～100Ma,属早白垩世火山岩。通过根河西岸旱白垩世火山岩剖面的岩石学和地球化学研究,认为根河早白垩世发生了三次大的火山喷发活动。火山岩剖面下部和上部主要为碱性的中基性-基性火山岩,剖面中部主要为一套碱性-亚碱性过渡性质的中酸性、酸性火山岩组合。该剖面下部和上部的基性、中基性火山岩具有相似的地球化学特征,显示高Al_2O_3、K_2O和全碱含量,较高的MgO和Mg~#值;LREE富集、LREE和HREE分馏较强、Eu异常不明显,富集LILE而亏损HFSE元素,尤其强烈亏损Nb和Ta元素;(^(87)Sr/^(86)Sr)_i-ε_(Nd)(t)和ε_(Nd)(t)-ε_(Hf)(t)图解表现出地幔源区特征。而剖面中部的中酸性火山岩与剖面上和下部的基性火山岩具有相似的地球化学特征,不同的是MgO含量和Mg~#较低,Ti、Sr和Ba元素亏损明显,负Eu异常明显,以及初始(^(87)Sr/^(86)Sr)_i值较高。地球化学特征表明,基性和中基性火山岩均反映陆内伸展环境和陆缘弧火山岩的一些特征,这种特性可能与该区火山岩浆源于受古生代俯冲洋壳流体交代的地幔楔有关。火山岩浆演化以分离结晶作用为主要方式,上升过程基本未受地壳物质混染。结合最新资料,我们认为根河地区早白垩世火山岩形成于古亚洲洋向滨太平洋过渡阶段,是蒙古-鄂霍茨克海造山后伸展和西太平洋板块俯冲共同作用的产物。

Late Mesozoic volcanic rocks section in Genhe area,northeast of Inner-Mongolia,is the important part of Da-Hinggan Mountains Mesozoic volcanic rocks.They are mainly composed of trachy-basalt,basaltic trachy-andesite,trachy-andesite,trachyte and rhyolite.Through geochronological study,including ^(40)Ar-^(39)Ar and SHRIMP zircon U-Pb dating,our study group attained ages of Genbe volcanic section,which have the range from 115Ma to 140Ma.This result indicates that these volcanic rocks were formed in three gyrations of volcanic explosion in early Cretaceous.The major elements of the samples show that geochemical characteristics of rocks in the lower and upper volcanic section are similar.Both of them have higher Al_2O_3,K_2O and total alkaline contents,and relatively high in MgO and Mg~#.The trace elements demonstrate total LREE enrichment with insignificant Eu anomalies(δEu=0.83～1.00)and intense fractionation between LREE and HREE.The primitive mantle-normalized spidergrams arc characterized by LILE enrichment and HFSE depletion,especially for a significantly negative Nb-Ta anomaly.Isotopic geochemistries show that values ofε_(Nd)(t)range from 1.5 to 3.6,and(^(87)Sr/^(86)Sr)_i from 0.704571 to 0.705907.The geochemical characteristics of trachytes,in middle volcanic section,are similar with basic and intermediate-basic volcanics of lower and upper volcanic section.However,MgO content and Mg~# is lower,and Ti shows intense depletion.Rhyolites of middle rocks section have higher SiO_2 and more lower MgO and Mg~# values.Trace elements show the characteristics of significant Eu anomalies(δEu=0.16～0.85),intense depletion of Sr and Ba,and high (^(87)Sr/^(86)Sr)_i values.According to geochemistries of basic and intermediate-basic volcanics,it is inferred that these rocks formed in an intracontinental extension setting.On the other hand,some characteristics of arc volcanies can be recognised in these rocks,which probably relate with dynamics and geochemical processes of subduction among plates in ancient Asian ocean territory.The origination of volcanic magma is speculated from a metasomatized mantle wedge,which had been modified by paleozoic subducted ocean slab. Fractional crystallization is the most important evolutionary type during magma evolvement.The early Cretaceous volcanics of Genhe area were formed in the transition stage from ancient Asia to paleo-pacifie structural domain,which were caused by coupling effect of postorogenic extension Mongol-Okhotsk zone and subduction of west Pacific slab.