Abstract:

Ultramafic xenoliths are abundant in Late Mesozoic high-Mg pyroxene-diorites at Laiwu, western Shandong Province; they are composed of dominant dunite (>80%) with minor harzburgite and olivine-pyroxenite. We determined the abundance of siderophile elements (PGEs, Cr, Co, and Ni) and chalcophile elements (Cu and S) of representative xenolith samples, and calculated oxidation states based on mineral chemistry. The harzburgite shows evidence of variable metasomatism by a Si-rich melt, and contains high concentrations of Cr, Co, Ni and IPGE with high ratios of IPGE/PPGE in bulk rock composition, most likely representing the residue of mantle after high degree of partial melting. Dunite, composed of chromite and high-Mg olivine, contains high Cr, Co and Ni, but low IPGE with low ratios of IPGE/PPGE in bulk rock composition, suggesting it is likely the cumulate of a mafic melt. Some olivine grains in dunite show variable, locally high Mg (up to Fo=94). We attribute these high values to the interaction of the olivine with abundant chromite in the cumulate, and that the parental melt was likely boninitic. Both harzburgite and dunite indicate high log f_O2 values, ranging from FMQ+1.4 to +2.4. The values contrast with f_O2 below that of the graphite-CO₂ buffer for xenoliths in Early Paleozoic diamondiferous kimberlite pipes in the area. The data indicate a sharp increase in f_O2 during Mesozoic time, likely caused by subduction of the Tethyan oceanic plate before collision with the Yangtze continent below the eastern margin of the North China craton.

Key words: Late Mesozoic, subduction zone, platinum group elements and oxidation state, high-Mg pyroxene diorite, dunite, subcontinental lithospheric mantle, western Shandong, lithology