Origin of gem-quality megacrysts in the Cenozoic alkali basalts from the Muling area, northeastern China

Megacrysts hosted by mantle-derived volcanic rocks provide valuable information on the chemical evolution and ascending history of the parental magmatic systems. However, the origin of the megacrysts has still been far from consensus. Here, we present systematic major-and trace-element data of a variety of gem-quality megacrysts in the Cenozoic basalts from Muling area, northeastern China, with the aim of investigating the origin and for-mational conditions of these megacrysts. The studied megacrysts are composed of clinopyroxene, garnet and orthopyroxene. All megacrysts are optically and compositionally homogeneous. Clinopyroxene megacrysts, with Mg# of 85.3-87.5, contain Al2O3 of 6.64-8.07 wt%, thus can be ascribed as Al-augites. Garnet megacrysts are rich in pyrope component with Mg# of 74.0-76.0 and contain low Cr2O3 less than 0.05 wt%. All orthopyroxene megacrysts are enstatite in composition. According to the color, they can be further divided into two groups (Group 1: greenish brown; Group 2: dark brown) which are apparently different in chemical composition. Group 2 orthopyroxenes have lower Mg# (85.1-86.3) and Cr2O3 (0.09-0.18 wt%) and higher Al2O3 (6.78-7.71 wt%) and TiO2 (0.20-0.24 wt%), while Group 1 orthopyroxenes are characterized by higher Mg# (89.2-89.8) and Cr2O3 (0.55-0.66 wt%) and lower Al2O3 (5.13-5.93 wt%) and TiO2 (0.11-0.15 wt%). Clinopyroxene, garnet and Group 2 orthopyroxene megacrysts can be easily distinguished with equivalent phases in accompanied peridotite xenoliths or peridotite xenoliths in other locations through eastern China by their major-element compositions. They show good correlations between Mg# and major-oxide contents (such as Al2O3 and TiO2), indicating their derivation from melts which had undergone variable extent of fractional crystallization. Mg# and trace-element compositions of their equilibrium melts are incomparable with those of host basalts. We infer clinopyroxene, garnet and Group 2 orthopyroxene megacrysts from Muling are most likely produced by fractional crystallization of basaltic melts that formed earlier than host basalts. Pressure estimations suggest clinopyroxene and garnet megacrysts from Muling were formed at 2.0-2.5 GPa, corresponding to a depth of 66-80 km, near the boundary between lithosphere and asthenosphere based on local lithosphere thickness. Compared with orthopyroxenes in accompanied peridotites, Group 1 orthopyroxene megacrysts have slightly lower Mg# and higher CaO and Al2O3, similar with those produced by basaltic melts-peridotite interaction experiments in which orthopyroxene precipitated with the expense of olivine and clinopyroxene. A revised model (modified from Liu and Ying (2019): Liu, Y.D., Ying, J.F., 2019. Origin of clinopyroxene megacrysts in volcanic rocks from the North China Craton: a comparison study with megacrysts worldwide. International Geology Review 62, 1845-1861.) has been proposed to decipher the formation of variable megacrysts from Muling.