Interactions between dioritic and granodioritic magmas in mingling zones: plagioclase record of mixing, mingling and subsolidus interactions in the Gęsiniec Intrusion, NE Bohemian Massif, SW Poland

Abstract

Dioritic and granodioritic rocks coexist in the Gesiniec Intrusion in SW Poland showing typical relationships in many mafic–felsic mingling zones worldwide, such as dioritic syn-putonic dykes and microgranular enclaves within granodioritic host. Plagioclase zonation from granodioritic rocks suggests late stage mixing probably with dioritic magma, whereas no magma mixing is recorded in plagioclase from dioritic rocks. The diorites seem to show effects of interaction with evolved, leucocratic melts derived from granodiorite, not with the granodioritic melt itself. We conclude that the diorites’ compositions were modified after their emplacement within the granodioritic host, when the diorites were essentially solidified and injection of evolved melt from granodiorite did not involve marked modification of plagioclase composition. Compositional zoning patterns of plagioclase in diorites can be modeled by closed system fractional crystallization interrupted by resorption induced probably by decompression. Granodioritic plagioclase seems to be affected by the same resorption event. Plagioclase that crystallized in dioritic magma before the resorption does not record interaction between dioritic and granodioritic magmas, suggesting that both magmas evolved separately.