Magnetic mineralogy of pyroxenite xenoliths from Hannuoba basalts, northern North China Craton

implications for magnetism in the continental lower crust

Zhiyong Li, Jianping Zheng*, Qingli Zeng, Qingsheng Liu, W. L. Griffin

*Corresponding author for this work

Research output: Contribution to journalArticle

5 Citations (Scopus)

Abstract

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 × 10-8m3 kg-1 and 12.05 × 10-3 Am2 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 Fe3O4 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.

Original languageEnglish
Pages (from-to)806-821
Number of pages16
JournalJournal of Geophysical Research: Solid Earth
Volume119
Issue number2
DOIs
Publication statusPublished - 2014

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