Fe–Ti oxides in the lower crustal xenoliths recording significant magmatic events in the crust–mantle transition zone (North China)

English Abstract:

Ilmenite and rutile are found to be common accessory minerals in the lower crustal xenoliths from the Cenozoic Hannuoba basalts, northern North China Craton. Detailed petrography, major chemical composition and trace elements of these Fe–Ti oxides in four pyroxenite xenoliths are studied. The Fe–Ti oxides occur mainly in the interstice of silicates or as inclusions in pyroxenes, and three types are representative: (1) single rounded homogeneous grain; (2) intergrowth of rutile and ilmenite; (3) slender needle-like grains. The ilmenite is especially rich in MgO (6.36~13.1 %) and poor in Fe³⁺ (Fe³⁺/ΣFe, 0~0.063), which is clearly different from the Mg-poor titanomagnetite (MgO, 2.9~3.31 %) in host basalts. The ilmenite on the rims of rutile grains indicates the reaction of rutile and accessional Fe²⁺, which possibly came from interstitial melts or melting silicates. This can be evidenced by the needle-like ilmenite penetrating into the partly-melted pyroxenes. These crustal xenoliths have two-pyroxene equilibrium temperatures of 834~884°C, and ziconium-in-rutile temperature of ~825°C (at 1.2 GPa), which suggest that the xenoliths were derived from the crust–mantle transition zone (~24–33 km), and experienced later magmatic reheating events. Hence, the petrological and geochemical characteristics of the Fe–Ti oxides in the pyroxenite xenoliths record significant geological processes in the continental lower crust beneath Hannuoba, and give insights into the tectonic evolution of the North China Craton.