Petromagnetic Study On The Continental Lower Crust, Northern North China Craton

The research on the composition and nature of the continental lower crust is crucial to understanding the formation and evolution of the lithosphere. Petromagnetic studies focus on the characteristics of the magnetic minerals, magnetization level and their relations to the deep geological processes. Granulite facies terrain rocks have mostly are concerned on by previous studies while the investigations of the lower crustal and upper mantle xenoliths are rare. In the northern North China Craton, widespread Archean granulites and abundant crustal and mantle xenoliths in the Cenozoic basalts provide unique access to the knowledge of the magnetization signature of the continental lower crust. In this thesis, we studied the magnetic properties and mineralogy of the peridotite, pyroxenite and mafic granulite xenoliths from Hannuoba and Yangyuan Cenozoic basalts and compared them with adjacent terrain granulites for the purpose of making clear magnetic minerals and the magnetization distribution in the continental lower crust and upper mantle.The peridotite xenoliths are dominated by fresh spinel lherzolites and have the highest mean density of3.256g cm-3. The pyroxenite xenoliths present black and green in color, and are mostly websterite with spinel, garnet or olivine occasionally present in some samples. They have a mean density of3.27g cm-3, which is higher than that of the mafic granulite xenoliths as3.123g cm"3. The average density of mafic granulites is2.984g cm-3while the felsic granulites have the lowest mean value of2.676g cm-3. Biotite, amphibole and garnet are present or absent in different granulite samples, and granulites have obviously more content of opaque minerals than all xenoliths.The peridotites have the lowest susceptibility (k) of516×10SI, induced magnetization (Ji) of0.013Am-1and natural remanent magnetization (NRM) of0.023Am1. The pyroxenites are mostly magnetic among all the xenoliths with k, Ji and NRM of1517×10-6SI,0.067Am-1and0.15Am-1, respectively. The mafic granulite xenoliths have k, Ji and NRM of960x10-6SI,0.042Am and0.06Am, which are remarkably lower than that of mafic granulites as64x10-3SI,2.82Am-1and0.96Am-1and Felsic granulites, which have middle values of11×10-3 SI,0.48Am-1and1.47Am-1.The xenoliths have the ratio of saturation remanent magnetization and susceptibility (Mrs/k) of2.89-48.32×103Am-1and remanence coercivity (Bcr) of21.54-112.15mT, while that of all the granulites are0.86～17.56×103Am-1and1.08～18.26mT. This indicates that the magnetite is the dominant magnetic mineral in the granulites, however, only plays part contribution to magnetization in the xenoliths. The Day plot (the ratio of Mrs and saturation magnetization vs. the ratio of Bcr and coercivity) implies that the magnetic minerals are multi-domain grains in the granulites while are pseudo-single-domain grains in the xenoliths. Temperature-dependent susceptibility and thermal NRM demagnetization analyses of representative xenoliths reveal that magnetite, pyrrhotite, hematite and Fe-rich spinel are the main magnetic minerals contributing to susceptibility and carrying remanent magnetization.Of all the xenoliths, opaque minerals are mainly present as follows:1. Ilmenite, rutile and armalcolite in the melting area of the silicates. They are probably crystallized from the melt under high temperature and pressure, and underwent slightly retrograde metasomatism.2. Magnetite in the interstice of silicates with zeolite. They are likely the oxidized products of cooling melt in the near-surface environment.3. Hematite or maghematite. They are likely resulted from the oxidation of prior Fe-Ti oxides in the basalts or on the surface.4. Pyrrhotite inclusion in the pyroxene. They were likely trapped grains during the crystallization or precipitates during the cooling of the pyroxene.5. Fe-rich spinel. It occurs as intergrowth with Mg-rich spinel either in the interstice of silicates or as exsolved lamella in the pyroxene sharing the same direction. The Fe-rich spinel is probably induced by the metasomatism of Mg-rich spinel and Fe2+-bearing silicates or melts in the form of solid-solid or solid-liquid reaction.The equilibrium temperatures of the pyroxenite xenoliths are876-916℃, and the equilibrium pressures are<1.5Gpa. This is consistent with the published data of the pyroxenite and mafic granulite xenoliths and the seismic velocity structure of the substrate layer of the lower curst (～33-42km), which is interpreted as the result of the continuous underplating of basaltic magmas from the late Paleozoic with the peak at the Mesozoic. The granulite terrains representing the old lower crust lie in the present upper lower crust (～24-33km). Therefore, we propose that bilevel magnetic lower crust are preserved in the northern North China Craton.