沂沭断裂带中南段钾质火山岩的元素地球化学与Sr-Nd-Hf同位素组成及其对岩石成因的制约

Petrogenesis of Potassic Volcanic Rocks in the Middle-South Parts of the Yishu Deep Fault Zone:Constraints from Elemental Geochemistry and Sr-Nd-Hf Isotopes

本文以沂沭断裂带中南段莒南-大店一带的钾质火山岩为对象,系统研究了它们的岩石学、元素地球化学和Sr-Nd-Hf同位素组成特征,据此探讨了源区地幔富集机制及岩石成因。莒南钾质火山岩主要为一套安粗质火山-潜火山岩组合,大店多发育粗面质火山岩,二者的锆石LA-ICP-MS U-Pb年龄基本一致,均约为118Ma。在化学组成上,这套火山岩均偏碱性,里特曼指数(σ)介于4.10~5.07,且富钾(K_2O=4.21%~5.77%),在SiO_2-K_2O和K_2O-Na_2O关系图上,均投影在钾玄岩系区。安粗质和粗面质火山岩均富大离子亲石元素(Cs,Rb,Th,U,Pb)和轻稀土元素,贫高场强元素(Nb,Ta,P,Ti),但粗面质火山岩具较显著的Ba、Eu负异常,且轻重稀土比值较低。二类火山岩均具有富集且较均一的Sr-Nd同位素组成(I_(Sr)=0.7086~0.7091,ε_(Nd)(t)=-16.9^-18.2),锆石ε_(Hf)(t)值均为显著的负值(安粗质岩:-22.3^-24.3,粗面岩:-19.7^-23.6)。元素和同位素组成的综合分析表明,区内钾质火山岩的源区地幔极可能遭受了因扬子板块深俯冲和华北克拉通岩石圈拆沉等多次富集事件的影响。至早白垩世,随着郯庐断裂引张的加剧,诱发了软流圈地幔上涌,同时在断裂减压作用的影响下,促使了这一经叠次改造的富集地幔的部分熔融,由此产生的岩浆经进一步分异演化,导致了区内钾质火山岩的形成。

In this paper, we report the petrological, major and trace element geochemistry and Sr-Nd-Hf isot data for the potassic volcanic rocks around the Junan-Dadian region in the middle-south parts of the Yi deep fault zone, with aims to constrain the enrichment mechanism of the mantle source and to elucidate ope shu petrogenesis. Potassic volcanic rocks outcropped around Junan are a set of latitic volcanic-subvolcanic rock association, and those in Dadian consist mainly of trachyte. They have consistent zircon LA-ICP-MS U-Pb ages of about 118Ma. Chemically, they show higher K2Oq-Na20 contents with Rittmann＇s indices ranging from 4.10 to 5.07, and enriched in potassium （K20=4.21~5.77~）. On theSiO2-K2OandK20-Na20 plots, these volcanic rocks all fall into the field of shoshonite series. Both the latitic and trachytic volcanic rocks are enriched in LILE （e. g. , Cs, Rb, Th, U, Pb） and LREE, and depleted in HFSE （e. g. , Nb, Ta, Ti, P）, but the trachytes show distinct negative anomalies of Ba and Eu in the primitive mantle- normalized trace element spidergrams, and have lower LREE/HREE ratios. The volcanic rocks have homogeneous and enriched Sr-Nd isotopic compositions with Isr ratios of 0. 7086 ~ 0. 7091 and eNd （t） values of --16. 9-18. 2, and show distinct negative zircon era（t） values （specially -22.3-24.3 for the latitic volcanic rocks, and -19.7-23.6 for the trachytes）. Based on a synthesis of the elemental and isotopic geochemistry, we suggest that the mantle source of the potassic volcanic rocks might have underwent multiple enrichment events induced by source contamination of continental crustal materials both from deep subduction of the Yangtze Craton and lithospheric delamination of the North China Craton. During Early Cretaceous, the continued extension of Tanlu Fault induced the upwelling of asthenospheric mantle and led decompression partial melting of these multiple modified enriched mantle. The generated magmas subsequently suffered further differentiation, thus finally resulted in the formation of these potassie volcanic rocks.