松辽盆地南部早白垩世火山岩的地球化学特征及其成因

对松辽盆地南部早白垩世火石岭组和营城组火山岩进行的岩石地球化学研究表明,火石岭组火山岩主要以玄武质粗面安山岩和粗安岩为主,存在少量粗面英安岩,其Si O2=53.8%~68.6wt%、K2O+Na2O=7.70%~9.59%、Mg#=26.72~33.89,以富集轻稀土和大离子亲石元素(LILEs,如Rb、Ba、Th、U等),亏损重稀土和高场强元素(HFSEs,如Nb、Ta、Ti、P等)为特征,这些特征与俯冲带火山岩类似。营城组火山岩以流纹岩为主,存在少量的玄武质粗面安山岩、粗面安山岩和英安岩,基性端元具有低硅(Si O2=49.3%~56.6%)、低碱(K2O+Na2O=4.57%~6.60%)、富镁(Mg#=33.49~59.51)的特征,主要属于钙碱性系列,暗示其原始岩浆主要来源于地幔;酸性端元的Si O2=65.4%~74.9%,K2O+Na2O=3.23%~7.75%,低镁(Mg#=21.76~39.18)。营城组火山岩稀土配分型式呈右倾型,富集轻稀土和大离子亲石元素,亏损重稀土和高场强元素,与俯冲环境火山岩相似。其中,营城组流纹岩地球化学特征类似A型花岗岩,暗示形成于伸展环境。结合区域研究成果,认为松辽盆地南部早白垩世火山岩的形成与古太平洋板块向欧亚大陆之下的俯冲作用有关。火石岭组火山岩形成于活动大陆边缘环境,营城组火山岩的形成与岩石圈拆沉作用引发的伸展背景有关。

Geochemical characteristics and petrogenesis of Early Cretaceous volcanic rocks in southern Songliao Basin

The geochemistry of Early Cretaceous volcanic rocks from Huoshiling and Yingcheng Formations in southern Songliao Basin shows that the volcanic rocks from Huoshiling Formation consist of basaltic andesites，tra- chy- andesites，and minor trachy- dacites，in which SiO 2 contents of 53. 8% ～68. 6%，total alkali (K 2 O + Na 2 O) of 7. 70% ～9. 59% and Mg # of 26. 72 ～33. 89， with the characteristics of enrichment in light rare elements (LＲEEs) and large ion lithophile elements (LILEs; Ｒb， Ba， Th， U ，etc)， and depletion in heavy rare elements (HＲEEs) and high field strength elements (HFSEs; Nb，Ta，Ti，P，etc)． The above characteristics are similar to those of the volcanic rocks formed in subduction zone． Volcanic rocks in Yingcheng Formation mainly compose of rhyolites with minor basaltic trachy- andesites，trachy- andesites and dacites，which are characterized by low silicon (SiO 2 = 49. 3% ～56. 6%)，low alkali (K 2 O + Na 2 O = 4. 57% ～ 6. 60%) and high Mg # (Mg # = 33. 49 ～ 59. 51)． The rocks mainly belong to the calc- alkaline series，indicating the mantle materials involved in their primary magma． Felsic volcanic rocks of Yingcheng Formation are characterized by SiO 2 contents of 65. 4% ～ 74. 9%，total alkali content (K 2 O + Na 2 O) of 3. 23% ～7. 75% and Mg # of 21. 76 ～39. 18． Chondrite- normalized ＲEE patterns of vol- canic rocks from Yingcheng Formation are right- inclined in type，enriched in light rare elements (LＲEEs) and large ion lithophile elements，and depleted in heavy rare elements (HＲEEs) and high field strength elements，sili- mar to the volcanic rocks formed in subduction zone． The geochemical characters of rhyolites from Yingcheng For- mation are similar to those of the A- type granite，implying an extensional environment for forming． Combined with the regional geological research，the authors suggest that the Early Cretaceous volcanic rocks in southern Songliao Basin mainly formed under a subduction setting related with the westward- movement of the Paleo- Pacific plate un- derneath the Eurasian Continent． The volcanic rocks from Huoshiling Formation formed under an active continental margin， while those from Yingcheng Formation formed under an extensional setting， related with the delamination of the lithosphere．