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首页> 外文期刊>Contributions to Mineralogy and Petrology >Amphibole in alkaline basalts from intraplate settings: implications for the petrogenesis of alkaline lavas from the metasomatised lithospheric mantle
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Amphibole in alkaline basalts from intraplate settings: implications for the petrogenesis of alkaline lavas from the metasomatised lithospheric mantle

机译：板内环境中碱性玄武岩中的闪石：对交代岩石圈地幔中碱性熔岩成岩的影响

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摘要

Kaersutite to pargasite phenocrysts from Tertiary alkali basalts (Rhon, Central European Province, Germany) yield new high-precision ~(40)Ar/~(39)Ar ages of 24.0-24.1 Ma. Major and trace element compositions demonstrate that these high-TiO_2 (4-7 wt%) amphiboles are in equilibrium with their host rock. Chemically, these amphibole phenocrysts resemble amphibole from mag-matic veins in upper mantle rocks but differ from disseminated amphibole from peridotite. Most amphiboles have similar isotope characteristics to their alkaline basaltic host rocks (~(87)Sr/~(86)Sr_(24) = 0.7035-0.736, εNd_(24) = +3.8-+4.0, ~(206)Pb/~(204)Pb_(24) = 19.21-19.37, ~(207)Pb/~(204)Pb_(24) = 15.58-15.62, ~(208)Pb/~(204)Pb_(24) = 38.95-39.16), but two samples show contrasting isotopic compositions (εNd_(24) = -4.0 and -2.9; ~(206)Pb/~(204)Pb_(24) = 17.08 and 18.11; ~(207)Pb/~(204)Pb_(24) =15.51 snf 15.58; ~(208)Pb/~(204)Pb_(24)=37.41 and 37.99), indicating involvement of an ancient crust-derived component during melting. The O isotopic composition of the amphibole phenocrysts ranges from 5.4 to 7.5 %o, reflecting O isotope heterogeneity of the upper mantle sources. The contrasting isotopic composition of amphibole and host rock pairs furthermore indicates that phenocrysts record the early stages of the volcanic history of the Rhon volcanic field on a regional scale and at a different depth within the lithospheric mantle. Temperature and pressure estimates range from 1,010 and 1,080 °C and 0.7 and 1.0 GPa and are compatible with the experimental results on the stability of amphibole in alkaline rocks derived from the upper mantle. Geochemistry of major and trace elements and isotopic compositions shows that igneous amphibole from alkali basalts may preserve isotope heterogeneities suggesting that they sample heterogeneous upper mantle lithologies on a small scale.
机译：从叔碱玄武岩（罗恩，德国中欧省，德国）到Kaersutite到辉石辉石的晶状体产生了新的高精度〜（40）Ar /〜（39）Ar年龄24.0-24.1 Ma。主要和微量元素组成表明，这些高TiO_2（4-7 wt％）的闪石与其母岩处于平衡状态。在化学上，这些角闪石的隐晶岩类似于上地幔岩中岩浆脉的角闪石，但不同于橄榄岩中散布的角闪石。大多数闪石具有与其碱性玄武质宿主岩相似的同位素特征（〜（87）Sr /〜（86）Sr_（24）= 0.7035-0.736，εNd_（24）= + 3.8- + 4.0，〜（206）Pb /〜 （204）Pb_（24）= 19.21-19.37，〜（207）Pb /〜（204）Pb_（24）= 15.58-15.62，〜（208）Pb /〜（204）Pb_（24）= 38.95-39.16） ，但两个样品显示出相反的同位素组成（εNd_（24）= -4.0和-2.9;〜（206）Pb /〜（204）Pb_（24）= 17.08和18.11;〜（207）Pb /〜（204）Pb_ （24）= 15.51 snf 15.58;〜（208）Pb /〜（204）Pb_（24）= 37.41和37.99），表明在熔化过程中涉及到了古老的地壳成分。角闪石表晶的O同位素组成为5.4％至7.5％o，反映了上地幔源的O同位素异质性。角闪石和宿主岩对的同位素组成也相反，这表明斑母在岩石圈地幔内区域范围和不同深度记录了罗恩火山场的火山历史的早期阶段。温度和压力估计范围为1,010至1,080°C，0.7和1.0 GPa，并且与闪石在上地幔衍生的碱性岩石中的稳定性实验结果相符。主要元素和微量元素以及同位素组成的地球化学表明，来自碱性玄武岩的火成闪石可能保留了同位素异质性，这表明它们在小范围内对上地幔岩性进行了采样。

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