A Geochemical Study Of Early Cretaceous Mafic Intrusions In The Southeastern North China Block

The southeastern edge of the North China Block (NCB) is close to the Dabie-Sulu orogenic belt that was formed by the Triassic continental collision between the South China Block (SCB) and the NCB. There are widespread Mesozoic igneous rocks in this region. Studying the nature of their magma source and geodynamic setting can provide insights into the complex crust-mantle interaction above the continental subduction zone, and have important bearing on the recycling of subducted continental crustal materials with respect to the evolution of subcontinental lithospheric mantle (SCLM). Therefore, deciphering the petrogenesis of these igneous rocks together with the deep subduction of the SCB beneath the NCB can provide tight constraints on the geodynamics of the NCB destruction and the crust-mantle interaction above continental subduction zone.This PhD thesis deals with Early Cretaceous mafic intrusive rocks from the southeastern edge of the NCB by means of a combined study of petrography, zircon U-Pb geochronology, whole-rock major-trace elements and Sr-Nd isotopes, whole-rock and mineral O isotopes as well as zircon Lu-Hf isotopes. The results are used to constrain their petrogenesis, and to explore the crust-mantle interaction above the continental subduction zone and its modification on the overlying SCLM.This thesis investigated five gabbroic intrusions from the southeastern edge in the NCB, which are the Jinan pluton, the Zouping pluton, the Jinling pluton, the Tietonggou pluton and the Yinan pluton. The Yinan pluton is located close to the Dabie-Sulu orogenic belt, whereas the Jinling, Jinan and Zouping plutons are located far away from the Dabie-Sulu orogenic belt. The Tietonggou pluton is located in between. The samples are composed of gabbro and gabbroic diorite. LA-ICPMS zircon U-Pb dating yield Early Cretaceous ages of119±4to136±2Ma for their emplacement. The gabbroic rocks have low SiO2contents of47.43to54.46wt.%but higher MgO contents of4.10to16.09wt.%with Mg#values of56to77, Cr contents of58.0to1318ppm, Ni contents of14.8to316ppm. They are characterized by the arc-like patterns of trace element distribution such as enrichment of light rare earth elements (LREE) and large ion lithophile elements (LILE), depletion of heavy rare earth elements (HREE) and high field strength elements (HFSE). In addition, they exhibit enrichment of radiogenic isotopes, with high whole-rock initial87Sr/86Sr ratios of0.7042to0.7111and variably low εNd(t) values of-16.3to-5.0as well as variably low zircon εHf(t) values of-23.3to-0.6. They have two-stage whole-rock Nd model ages of1.33to2.24Ga and two-stage zircon Hf model ages of1.21to2.64Ga. They mostly have higher whole-rock and mineral δ18O values than those of the normal mantle.The above geochemical characteristics indicate their derivation from partial melting of enriched mantle sources. In particular, their two-stage whole-rock Nd and zircon Hf model ages are similar to those of UHP metaigneous rocks in the Dabie-Sulu orogenic belt, suggesting involvement of the subducted SCB continental crustal materials in their petrogenesis. The enriched mantle sources would be generated by crustal metasomatism through underplating reaction of the overlying NCB SCLM-wedge peridotite with felsic melts derived from the subducting SCB continental crust during the Triassic continental collision. Such metasomatized mantle would be fertile in major elements and enriched in trace elements and radiogenic isotopes, which are the results of the slab-mantle interaction in continental subduction channel. Because of its fertile and enriched properties, the metasomatized mantle domains were susceptible to partial melting in an extensional tectonic setting, resulting in the Early Cretaceous gabbroic magmatism.It is notable that the whole-rock elements and isotopic compositions of the Early Cretaceous gabbroic rocks from the southeastern edge of the NCB exhibit spatial variations from the southeast (adjacent to the Dabie-Sulu orogenic belt) to the northwest (far from the Dabie-Sulu orogenic belt). There are decreases in the whole-rock K2O/Na2O, Ce/Pb, La/Nb and initial87Sr/86Sr ratios, but increases in whole-rock εNd(t) values and zircon εHf(t) values from the southeast to the northwest. These spatial variations indicate incorporation of the crustal components of deeply subducted SCB into the overlying NCB SCLM-wedge, with the reduced influence of the subducted SCB from the southeast to the northwest. Based on the distances of these rocks to the Dabie-Sulu orogenic belt, the spatial extent of the influence of the subducted SCB on the overlying NCB SCLM is estimated to about200-300km.In summary, the Mesozoic mafic igneous rocks in the southeastern edge of the NCB were derived from partial melting of the metasomatized SCLM sources that were produced by the recycling of the subducted SCB continental crustal materials and its crust-mantle interaction in continental subduction channel. Therefore, they provide petrological and geochemical records of crust-mantle interaction above continental subduction zone.