Experimental studies of melt-peridotite reactions at 1-2 GPa and 1250-1400°C and their implications for transforming the nature of lithospheric mantle and for high-Mg signatures in adakitic rocks

Abstract

Experiments of the melt-peridotite reaction at pressures of 1 and 2 GPa and temperatures of 1250–1400°C have been carried out to understand the nature of the peridotite xenoliths in the Mesozoic high-Mg diorites and basalts of the North China Craton, and further to elucidate the processes in which the Mesozoic lithospheric mantle in this region was transformed. We used Fuxin alkali basalt, Feixian alkali basalt, and Xu-Huai hornblende-garnet pyroxenite as starting materials for the reacting melts, and lherzolite xenoliths and synthesized harzburgite as starting materials for the lithospheric mantle. The experimental results indicate that: (1) the reactions between basaltic melts and lherzolite and harzburgite at 1–2 GPa and 1300–1400°C tended to dissolve pyroxene and precipitate low-Mg# olivine (Mg#=83.6-89.3), forming sequences of dunite-lherzolite (D-L) and duniteharzburgite (D-H), respectively; (2) reactions between hornblende-garnet pyroxenite and lherzolite at 1 GPa and 1250°C formed a D-H sequence, whereas reactions at 2 GPa and 1350°C formed orthopyroxenite layers and lherzolite; and (3) the reaction between a partial melt of hornblende-garnet pyroxenite and harzburgite resulted in a layer of orthopyroxenite at the boundary of the pyroxenite and harzburgite. The reacted melts have higher MgO abundances than the starting melts, demonstrating that the melt-peridotite reactions are responsible for the high-Mg# signatures of andesites or adakitic rocks. Our experimental results support the proposition that the abundant peridotite and pyroxenite xenoliths in western Shandong and the southern Taihang Mountains might have experienced multiple modifications in reaction to a variety of melts. We suggest that melt-peridotite reactions played important roles in transforming the nature of the Mesozoic lithospheric mantle in the region of the North China Craton.