Abstract:The expulsion of interstitial liquid from crystal mush is closely related to the abundant crystallization of Fe- Ti oxides during the mid- late stage of the solidification of layered intrusions. However, the mechanism behind the expulsion of interstitial liquid at the early stage, prior to the abundant crystallization of Fe- Ti oxides, remains enigmatic. The highly differentiated Bijigou intrusion, located in the northern margin of the Yangtze Block, is the largest layered intrusion with Fe- Ti- (V) mineralization in China at present. It consists of three main zones: the lower zone (LZ), middle zone (MZ), and upper zone (UZ), composed mainly of plagioclase peridotite, gabbro, and diorite, respectively. In this study, we analyzed the crystal size distributions (CSDs) and spatial distribution patterns (SDPs) for plagioclase and clinopyroxene of samples from the plagioclase peridotite unit in the LZ. The objective was to examine the expulsion mechanism of interstitial liquid from the crystal mush during the early stage of solidification of layered intrusions. Our modeling results showed that the fraction of trapped liquid (FTL in these samples ranged from 9% to 13%, suggesting that the interstitial liquid was effectively expelled from the crystal mush. The consistent slopes of the CSD curves for clinopyroxene and plagioclase of samples from the plagioclase peridotite unit mainly vary from 3. 36 to 2. 45 and from 2. 99 to 1. 87, respectively. The intercepts varied from 0. 45 to 3. 35 for clinopyroxene and from 1. 06 to 2. 83 for plagioclase. These values are similar to those observed in mechanical compaction. In addition, the SDP R value (ratio of observed and predicted nearest neighbor distance of the same mineral) and the mineral mode, except for plagioclase/clinopyroxene, are negatively correlated, consistent with the trend of mechanical compaction. Therefore, the expulsion of interstitial liquid during the early stage of solidification in the Bijigou intrusion is controlled by mechanical compaction. On the other hand, we compared the Bijigou intrusion with the Skaergaard, Sept Iles, and Kiglapait layered intrusions, in terms of scale, fraction of trapped liquid, density contrast of interstitial liquid and cumulus crystal, accumulation rate and compaction rate relationship, and thickness of mush layer. Our analysis suggests that compaction plays a critical role in expelling interstitial liquid at the early stage of solidification in layered intrusions, particularly in the absence of magma replenishment and convection. However, the factors controlling the degree of compaction are distinct under different conditions. Furthermore, our comparison indicates that the density contrast of interstitial liquid and cumulus crystals primarily controls the onset of compaction when intrusion sizes are similar, with a higher degree of compaction observed for greater density contrasts. In addition, compaction is more likely to operate in large intrusions than small ones at the early stage of solidification.