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Magnetic mineralogy of pyroxenite xenoliths from Hannuoba basalts, northern North China Craton: Implications for magnetism in the continental lower crust
Author(s) -
Li Zhiyong,
Zheng Jianping,
Zeng Qingli,
Liu Qingsheng,
Griffin W. L.
Publication year - 2014
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1002/2013jb010599
Subject(s) - geology , geochemistry , granulite , xenolith , basalt , mafic , craton , continental crust , crust , facies , geomorphology , paleontology , structural basin , tectonics
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 3 O 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 (~ 32 km) in Hannuoba, northern North China Craton.