济南辉长岩岩浆演化过程:来自熔体包裹体的证据

Magmatic Processes Involved in Formation of the Jinan Gabbros: Evidence from Melt Inclusions

本研究首次报道了早白垩世济南辉长岩中橄榄石斑晶捕获的熔体包裹体的研究结果。济南辉长岩中橄榄石的Fo（60.374.6）,Mn（25003500μg/g）,Ni（701349μg/g）,Fe/Mn比值（61.283.5）,与源区母岩为纯的橄榄岩形成的熔体结晶出的橄榄石性质不同,可能与源区存在辉石岩的贡献有关。橄榄石中熔体包裹体主量元素具有较大的变化范围。熔体包裹体成分的标准矿物计算（CIPW）表明,MgO〉10%的熔体包裹体为含有霞石和橄榄石标准矿物分子的硅不饱和熔体,Mg O〈10%时为含石英标准矿物分子的硅饱和熔体。橄榄石中包裹有辉石和斜长石,说明岩浆演化过程应该处于开放环境。熔体包裹体的（208Pb/206Pb）i和（207Pb/206Pb）i与MgO具有良好的负相关关系,与SiO2具有良好的正相关关系,以及熔体包裹体具有较高的SiO2特征表明岩浆演化过程中可能有下地壳长英质组分的加入。熔体包裹体的Pb同位素落在EMI附近并向EMII延伸,其源区可能有EMI和EMII的贡献,熔体包裹体的主量元素成分说明其源区母岩可能有橄榄岩和辉石岩的贡献。

We present the first analyses of major elements and Pb isotope compositions of olivine-hosted melt inclusions from the Jinan gabbros in the Early Cretaceous. The olivines in the Jinan gabbros display a relatively large compositional variation： Fo values vary from 60.3 to 74.6; Mn contents range from 2500-3500 μg/g; Ni contents show a range of 70-1349 μg/g and Fe/Mn ratios range from 61.2 to 83.5. Different from the olivine crystallized from partial melts of pure peridotite, the compositional variation of the olivine in the Jinan gabbros is consistent with the contribution of pyroxenite other than peridotite in the source. The melt inclusions hosted by the olivine crystals show a significant compositional variation. CIPW norm calculations indicate that melt inclusions with Mg O〉10% contain normative nepheline（Ne） and olivine（Ol）, while melt inclusions with Mg O〈10% contain normative quartz（Q）. The occurrence of clinopyroxene and plagioclase hosted in olivines suggests that the magmatic processes involved the formation of gabbro should be occurred in an open-system. The negative correlation between（208Pb/206Pb）i,（207Pb/206Pb）iand MgO, and positive correlation between（208Pb/206Pb）i,（207Pb/206Pb）i and SiO2, and high SiO2 content of melt inclusions, indicate that felsic component from the lower crust may be involved in the magmatic processes. The Pb isotopic compositions of melt inclusions are close to EMI and extend towards an EMII-type isotopic composition, suggestive of the involvement of the EMI and EMII components during the formation of the Jinan gabbros. The CaO contents and major element compositions of melt inclusions are consistent with partial melting of the mixing of peridotite- and pyroxenite- sources.