Origin Of High-Cr Stratiform Chromitite In The Fangmayu Alaskan-Type Ultramafic Intrusion, North China Craton

Y The study of chromite from chromitite-bearing layered intrusions can provide significant insights into their petrogenetic origin and tectonic setting. The Fangmayu Alaskan-type ultramafic intrusion in the North China Craton contains layered and massive chromitite associated with a serpentinized ultramafic suite. This study presents zircon and apatite geochronology, chromite mineral chemistry, platinum-group element (PGE) and ReOs isotopic data for the purpose of constraining the origin, evolution, and composition of the Fangmayu chmmitite parental melts. Zircon U-Pb data from the chromitites define various age populations, with oldest age component (>2.49 Ga) interpreted to represent xenocrystic grains incorporated from crystalline basement. A dominant age component (similar to 2.42 Ga) may represent the age of magma emplacement. Ages in the range of 2375-2068 Ma likely represent variable partial radiogenic Pb-loss during subsequent thermal events. Zircon ages in the range of 1.96-1.90 Ga record metamorphic events, potentially related to the collision between the Eastern Block and Western Block in the North China Craton. Ages of 1.85-1.81 Ga likely represent zircon growth during retrograde metamorphic process in a post-collisional setting; near contemporaneous apatite (similar to 1.8 Ga) tracks cooling through 500 degrees C. High F-o contents in olivine (Fo(90_92)) and high Cr# in chromite (72-77) suggest a low Al2O3 ultramafic parental magma formed by a high degree of partial melting. However, total concentrations of PGEs are abnormally low, and the Re-Os system indicates multiple disturbances during later metamorphic events. Experimental data are used to recover the compositions of parental melts to the chromitite, the calculations yield parental melts with 10-11 wt% Al2O3 and 0.7-1.1 wt% TiO2, which define the arc and IAB affinity. The high Cr#s, low Al2O3 (8-10 wt%), and high TiO2 (0.58-0.96 wt%) contents of chromite, combined with calculated parental melt compositions, suggest that the parental magma of Fangmayu stratiform chromitites was a high-Ti island-arc basalt origin and formed within supra-subduction zone setting.