Conditions of formation of the birushin monzonite intrusion, Sikhote Alin

The Birushin monzonite intrusion constitutes the core of a subvolcanic massif composed of crystalloclastic trachyandesites and pyroxene dioritic porphyries. The monzonites are saturated in xenoliths, which form a continuous series from micrograined pyroxene hornfelses and pyroxene microdiorites to medium-grained monzodiorites and monzonites. Xenoliths of different composition and texture occur together within limited areas, thus providing the impression of their transportation from different sources. At the same time, some xenoliths are multiphase and consist of three or more zones of different composition. The micrograined melanocratic xenoliths are characterized by trachytoid alignment of plagioclase and amphibole grains. Numerous measurements have revealed a persistent orientation of trachytic textures. Based on structural, chemical, and mineral data, it has been concluded that the core monzonites were formed by magmatic replacement in situ of the host crystalloclastic trachyandesites and dioritic porphyries.