Abstract: The Cenozoic volcanic rocks, mainly composed of alkaline basalts, are widely distributed across Eastern China. These volcanic rocks contain abundant deep-derived rock xenoliths and high-pressure megacrysts, providing good opportunities for understanding the composition and processes experienced by deep lithosphere beneath Eastern China. However, compared to the well-studied rock xenoliths, people's understanding of the formational mechanism and conditions of the accompanied megacrysts is relatively weak. These megacrysts include a variety of minerals such as pyroxene, garnet, amphibole, zircon, ilmenite, corundum. More importantly, they are characterized by large size and high clarity, and can be used as high-quality colored gemstone resources. Therefore, this study conducted in-situ major- and trace-element analysis on pyroxene and garnet megacrysts hosted by the Cenozoic alkali basalt from Muling (Heilongjiang Province), Huadian (Jilin Province) and Mingxi (Fujian Province) are as with the aim of revealing the origin of pyroxene and garnet megacrysts from the Cenozoic volcanic rocks in Eastern China, and further constraining the geological processes undergone by the deep lithosphere.The volcanic rocks in Muling and Mingxi areas contain pyroxene and garnet megacrystals, but only pyroxene megacrystals are found in Huadian area. The shape of these megacrysts is commonly irregular, making it difficult to identify the original crystal form and related face characteristics. Among them, the pyroxene megacrysts are large in size, with diameters mostly ranging from 1-4 cm. Garnet megacrystals are relatively small, with diameters mostly less than 3 cm. The pyroxene megacrystals are black under naked eyes and brown light green under transmitted light, while the garnet megacrysts are light pink to wine red in colour.According to major-element analytical results, the megacrysts are chemically homogeneous, without systematic differences of element contents between core and rim. The pyroxene megacrysts from Muling and Huadian include clinopyroxene and orthopyroxene, while only clinopyroxene is found from Mingxi. The clinopyroxene from the three regions have high Al₂O₃ content (6.55-10.30 wt%), which can be ascribed to Al-augite. The major-oxide contents of clinopyroxenes are significantly different from those of clinopyroxenes in mantle rocks (such as peridotite), indicating that they are not fragments of mantle rock fragments captured during the upwelling of host magma. It is worth noting that the major-element content (such as TiO₂ and Al₂O₃) of clinopyroxene megacryst increases with the decreasing of Mg#, while Cr₂O₃ decreases with the decreasing of Mg#. The variation of CaO content is limited. These features indicate that the formation of clinopyroxene megacrysts is mainly controlled by magma fractional crystallization. The orthopyroxene megacrysts in Muling and Huadian are enstatite in composition, and their major-oxide contents are also significantly different from those of orthopyroxenes in mantle rocks, indicating that they are not products of mantle rock fragmentation. The orthopyroxene megacrysts in Muling can be further divided into two groups. The first group is light green with a brownish tone, while the second group is brown. These two groups of orthopyroxene also exhibit significant differences in chemical composition. The first group of clinopyroxene megacryst has high Mg# (89.2-89.8), Cr₂O₃ (0.55-0.66 wt%), and low Al₂O₃ content (5.13-5.93 wt%). The second group of clinopyroxene megacryst has lower Mg# (85.1-86.3), Cr₂O₃ (0.09-0.18 wt%), and higher Al₂O₃ content (6.78-7.71 wt%). Similar to the orthopyroxene in Huadian, the major-element content (such as TiO₂ and Al₂O₃) of the first group of orthopyroxene in Muling is not correlated with Mg#, implying that the orthopyroxene megacrysts in Muling and Huadian are not products of fractional crystallization of a melt. Their higher Cr₂O₃ and Mg# suggest that these orthopyroxene megacrysts may be products of melt-peridotite reaction. The content of TiO₂ and Al₂O₃ in the second group of orthopyroxene megacrysts in Muling increases with the decreasing of Mg#, reflecting that the formation of orthopyroxene of this group is mainly controlled by fractional crystallization. The garnet megacrysts in Muling and Mingxi areas are mainly pyrope in composition, but the Mg# of Muling garnet is significantly higher than that of Mingxi garnet (74.0-76.0 vs. 45.0-61.8). The Cr₂O₃ content of garnet in these two regions is very low (< 0.18 wt%), which is significantly different from that of garnet in mantle rocks. The content of garnet TiO₂ and Al₂O₃ in the two areas also show an increasing trend with the decrease of Mg#, indicating that they are also products of fractional crystallization of a melt rather than fragments of mantle rocks.Estimated results show that there are significant differences in the REE contents between the equilibrium melts of pyroxene and garnet megacrysts and their host basalts from the three areas. Moreover, the Mg# of the equilibrium melt of pyroxene megacrysts is completely different from that of their host basalts. This indicates that the pyroxene and garnet megacrysts in the studied areas are not products of fractional crystallization of their host magma, but rather formed from crystallization processes of earlier basaltic melts, and were subsequently captured during the upwelling of their host magma. It is worth noting that there are obvious differences in the REE contents of equilibrium melts between pyroxene and garnet megacrysts from the same area (such as Muling), or the REE contents of equilibrium melts between clinopyroxene and clinopyroxene from the same area (such as Huadian) is also apparently different. This indicates that the megacrysts may not derive from the common melt. The calculation results of clinopyroxene thermobarometers show that the crystallization pressures of clinopyroxene megacrysts in Muling, Huadian, and Mingxi are 2.0-2.2 GPa, 1.4-1.8 GPa, and 2.0-2.5 GPa, respectively, corresponding to depths of 66-68 km, 46-58 km, and 73-84 km. Based on regional geophysical data, the clinopyroxene from Muling and Mingxi probably formed near the boundary between the lithosphere and asthenosphere (LAB). The clinopyroxene in Huadian area is formed at a shallower location (lithospheric mantle). The pressure results mentioned above are also roughly comparable to the results obtained by empirical formula of Na content of clinopyroxene. The empirical formula estimations based on Ca content of garnet shows that the crystallization pressure of garnet megacrysts from Muling and Mingxi are 2.5 GPa and 1.2-1.9 GPa, respectively, roughly comparable to 77-80 km and 37-57 km depths. However, due to the fact that the megacrysts from the three regions all occur as single crystals, it is hard to provide constraints on the temperature and pressure conditions for the formation of orthopyroxene megacrysts.Overall, the origin of pyroxene and garnet megacrysts in the Cenozoic volcanic rocks through eastern China, represented by Muling, Huadian and Mingxi areas, is complex. The studied megacrysts were mainly formed by fractional crystallization processes, with a minor contribution of melt peridotiteinteractions. Although the formational depths of different types of megacrysts vary slightly, it can be seen that LAB is the main location for the formation of these gem-quality megacrysts. Deep-derived melts with different stages stayed near LAB (like a melt pool) and underwent slow crystallization processes (probably accompanied by melt-peridotite reactions) under relatively stable conditions, forming gem-quality pyroxene and garnet megacrysts. Correspondingly, the deep lithosphere beneath eastern China also inevitably experienced variable degrees of modifications by these deep-derived melts, such as accumulation and melt-peridotite reaction.