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首页> 外文期刊>Journal of geophysical research. Solid earth: JGR >Magnetic mineralogy of pyroxenite xenoliths from Hannuoba basalts, northern North China Craton: Implications for magnetism in the continental lower crust
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Magnetic mineralogy of pyroxenite xenoliths from Hannuoba basalts, northern North China Craton: Implications for magnetism in the continental lower crust

机译：华北克拉通北部汉诺巴玄武岩中辉石辉石异岩的磁性矿物学：对大陆下地壳的磁性意义

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Studies of the petrology, mineral chemistry, and rock magnetic properties of nine pyroxenite xenoliths from Hannuoba basalts, northern North China Craton, have been made to determine the magnetization signature of the continental lower crust. These pyroxenites are weakly magnetic with low average susceptibility (χ) and saturation isothermal remanent magnetization (M_(rs)) of 39.59 × 10~(-8)m~3 kg~(-1) and 12.05 × 10~(-3) Am~2 kg~(-1), respectively. The magnetic minerals are mainly magnetite, pyrrhotite, and Fe-rich spinel, which significantly contribute to χ and natural remanent magnetization. Magnetite occurs as interstitial microcrystals together with zeolite aggregates, indicating a secondary origin in a supergene environment. In contrast, pyrrhotite and Fe-rich spinel were formed prior to the xenoliths’ ascent to the surface, as evidenced by their dominant occurrence as tiny inclusions and thin exsolution lamellae in pyroxene. The Fe-rich spinel has ~ 50% mole fraction of Fe_3O_4 and corresponds to the strongest magnetization, and its coexistence with Mg-rich spinel implies a reheating event due to the underplating of basaltic magma. Besides, armalcolite and ilmenite were found in the reaction rims between xenoliths and the basalt, but they contribute little to the whole rock magnetization. However, these pyroxenite xenoliths would be nonmagnetic at in situ depths, as well as peridotite and mafic granulite xenoliths derived from the crust-mantle transition zone (~ 32–42 km). Therefore, we suggest the limiting depth of magnetization at the boundary between weakly magnetic deep-seated (lower crust and upper mantle) xenoliths and strongly magnetic Archean granulite facies rocks (~ 32km) in Hannuoba, northern North China Craton.
机译：对华北克拉通北部汉诺巴玄武岩中的9种辉绿岩异岩进行了岩石学，矿物化学和岩石磁学研究，以确定该大陆下地壳的磁化特征。这些辉石岩具有弱磁性，平均磁化率（χ）低，饱和等温剩余磁化强度（M_（rs））为39.59×10〜（-8）m〜3 kg〜（-1）和12.05×10〜（-3） Am〜2 kg〜（-1）。磁性矿物主要是磁铁矿，黄铁矿和富铁尖晶石，它们对χ和自然剩余磁化强度有显着贡献。磁铁矿作为间隙微晶与沸石聚集体一起出现，表明在超基因环境中是次生起源。相比之下，磁黄铁矿和富铁尖晶石是在异种岩上升到表面之前形成的，这一点可以证明，它们主要存在于辉石中的细小夹杂物和薄的析出薄片。富铁尖晶石具有约50％的Fe_3O_4摩尔分数，并且对应于最强的磁化强度，并且其与富镁尖晶石的共存暗示着由于玄武岩浆岩的底层而发生的再加热事件。此外，在异石与玄武岩之间的反应缘中发现了辉石和钛铁矿，但它们对整个岩石的磁化作用很小。但是，这些辉绿岩异岩在原位深度以及从地幔幔过渡带（〜32-42 km）衍生的橄榄岩和镁铁质粒状辉石异岩都将是非磁性的。因此，我们建议在华北克拉通北部汉诺巴的弱磁性深层（下地壳和上地幔）异岩和强磁性太古宙粒岩相岩石（〜32km）之间的边界处限制磁化深度。

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