鲁东青山组中性火山岩的地球化学特征:岩石成因和地质意义  

Geochemistry of Qingshan Formation volcanic rocks from Jiaolai Basin,eastern Shandong Province: Petrogenesis and geological significance

作　　者：闫峻[1] 陈江峰[2] 

机构地区：[1]合肥工业大学资源与环境工程学院,安徽合肥230009 [2]中国科学技术大学地球与空间科学学院,安徽合肥230026

出　　处：《地球化学》2007年第1期1-10,共10页Geochimica

基　　金：国家自然科学基金(40472031);中国科学院知识创新工程重要方向项目(KZCX2-107)

摘　　要：山东郯庐断裂东部胶莱盆地下白垩统青山组中性火山岩SiO_2含量在61.14%～64.43%之间,在TAS上位于亚碱性系列岩石区域,多数属于粗面英安岩,少数为粗面安山岩。全碱(K_2O+Na_2O)含量为7.41%～8.86%。岩石富集大离子亲石元素Ba、Th和LREE、Pb,亏损高场强元素Nb和Ti,具有富集的Sr、Nd同位素组成和低的放射成因Pb。初始^(87)Sr/^(86)Sr(t)比值为0.70698～0.70874,εNd(t)值为-13.78～-16.44。^(206)Pb/^(204)Pb(t)、^(207)Pb/^(204)Pb(t)和^(208)Pb/^(204)Pb(t)比值分别为16.585～16.719、15.400～15.411和36.724～37.011。岩石的地球化学特征表明其起源于深俯冲的扬子地壳,可能为下地壳的部分熔融,并且有少量的来自软流圈地幔物质的加入,在岩浆房中经历了单斜辉石和少量斜长石的结晶分异。胶莱盆地110Ma时期,同位素富集性质的岩石圈地幔在火山岩中没有反映,暗示了可能已经减薄或置换为同位素亏损的岩石圈地幔,但深俯冲的扬子地块下地壳在该地区岩石圈之下依然存在,并对火山活动有物质贡献。The Late Cretaceous Qingshan Formation intermediate volcanic rocks in Jiaolai Basin located in east Tancheng-Lujiang fault are silicon-saturation with SiO2 of 61.14% ～ 64.43%. These rocks belong to recta-alkaline series and most of them are trachydacite with a few show trachyandesite affinity. The total alkaline contents are 7.41%～8.86%. They are characterized by Ba, Th, LREE and Pb enrichment and Nb and Ti depletion, with moderate initial ^87Sr/^86Sr（t） ratios （0. 706 98 ～0. 708 74）, high negative εNd（t） values （ - 13.78 ～ - 16.44） and low radiogenic Pb isotopic ratios [^206Pb/^204Pb（t） = 16. 585～ 16. 719, ^207Pb/^204Pb（t） = 15.400 ～ 15.407, ^208Pb/^204Pb（t） =36. 724～37. 011]. The geochemical characteristics of these rocks indicate that the primitive magma was dominantly derived from partial melting of deep-subducted crust, probably lower crust of the Yangtze Block, together with a small quantity of melt from the isotopic depleted asthenosphere and underwent crystallized fractionation of clinopyroxene and small plagioclase in magma chamber. At 110 Ma, the isotopic enriched lithospheric mantle under Jiaolai Basin was not geochemically reflected in the volcanic rocks, which implies that the enriched lithospheric mantle might have thinned or transformed into the isotopic depleted lithospheric mantle. However, the deep-subducted lower crust of the Yangtze Block still existed under the lithosphere and contributed to the Late Mesozoic volcanic rocks.

关 键 词：火山岩 地球化学 扬子下地壳 青山组 岩石成因论 胶莱盆地 

分 类 号：P581[天文地球—岩石学] P597[天文地球—地质学]