长白山新生代玄武岩中橄榄岩包体所揭示的岩石圈地幔特征

Characteristics of the lithospheric mantle revealed by peridotite xenoliths from Changbaishan volcanic rocks

长白山地区位于华北克拉通东北部,广泛出露富含地幔橄榄岩包体的新生代玄武岩,为研究岩石圈地幔的性质和演化提供了优越条件。本文对长白山地区天池和龙岗新生代火山岩群中尖晶石相橄榄岩包体进行了岩石学、全岩主微量元素、矿物主量元素、单斜辉石微量元素和Sr-Nd-Hf同位素分析。研究结果表明,尖晶石相橄榄岩包体由二辉橄榄岩和少量的方辉橄榄岩组成,Mg^#值为87.4~91.2,表现出新生饱满的特征,平衡温度为900~1100℃。橄榄石的Mg^#值(%Fo)为85.6~91.3。单斜辉石包括四种类型:(1)轻稀土元素严重亏损型;(2)轻稀土元素亏损型;(3)向右微倾型和(4)"勺型"。单斜辉石表现出Sr同位素(87Sr/86Sr=0.702749~0.707276)整体亏损,部分样品富集的特征,单斜辉石的Nd-Hf同位素呈现出亏损特征(143Nd/144Nd=0.512886~0.51333、εHf=+17.7^+49.8)。长白山地区二辉和方辉橄榄岩分别经历了小于10%和略大于10%的部分熔融作用,并受到富水硅酸盐熔体的地幔交代作用。太平洋板块向西俯冲作用使得软流圈上涌并携带大量壳源物质进入地幔深部,与岩石圈地幔发生橄榄岩-熔体反应,形成了长白山地区不均一的岩石圈地幔,以新增生饱满地幔为主,夹有少量古老难熔岩石圈地幔碎片。

The Changbaishan area is located in the northeastern North China Craton,in which the Cenozoic basalts bearing abundant mantle peridotite xenolith are widely exposed,providing an excellent geological condition for studying the nature and evolution of lithospheric mantle.In this paper,petrology,whole-rock major and trace elements,mineral major elements,in situ clinopyroxene trace element and Sr-Nd-Hf isotope analysis of peridotite xenolith from Tianchi and Longgang Cenozoic basalts were carried out.The results indicate that the peridotite xenoliths are composed of lherzolite and minor harzburgite.They possess Mg^#values of 87.4~91.2 and show the new and fertile characteristics,with estimated equilibrium temperature of 900~1100℃.The Mg^#values(%Fo)of olivine are 85.6~91.3.Four types of clinopyroxenes are identified:(1)seriously depleted LREE;(2)depleted LREE;(3)gently right-dipping and(4)spoon type.Clinopyroxene shows Sr isotopic features(0.702749~0.707276)mainly depleted and partly enriched.Also clinopyroxene shows depleted Nd-Hf isotopic features with 143Nd/144Nd=0.512886~0.51333,εHf=+17.7^+49.8.The peridotites in Changbaishan area experienced partial melting lower and slightly lager than 10%different degrees of metasomatism.The westward subduction of the Pacific plate caused the asthenosphere to upwelling and carry a large amount of crustal material into the deep mantle,and the lithospheric mantle to undergo peridotite-melt reaction.As a result,new and fertile oceanic lithospheric mantle,with minor ancient refractory lithospheric mantle remnants was formed beneath the Changbaishan area in northeastern North China Craton.