Crystallization of quartz dioritic magmas at 2 and 1 kbar: experimental results

Crystallization experiments were performed on quartz diorite (similar to 55 wt.% SiO2, 3.1-8.4 wt.% MgO) from the GA (TM) siniec Intrusion (Bohemian Massif, SW Poland) at 1-2 kbar, 750-850A degrees C, various mole fractions of water and with fO(2) buffered by the NNO buffer. The two natural quartz diorites (leucocratic poikilitic quartz diorite - 'LPD' and melanocratic quartz diorite - 'MD') differ in whole rock and mineral composition with MD being richer in MgO and poorer in CaO than LPD, probably due to accumulation of mafic minerals or melt removal in MD. LPD represents melt composition and is used to reconstruct crystallization conditions in the GA (TM) siniec Intrusion. The crystallization history of LPD magma, deduced from experimental and natural mineral compositions, includes a higher pressure stage probably followed by emplacement at similar to 2 kbar of partly crystallized magma at temperatures similar to 850-800A degrees C and quick cooling. The mineral assemblage present in LPD requires water contents in the magma of at least 5 wt% and oxygen fugacity below that controlled by the NNO buffer. The compositions of mafic minerals in the MD composition were equilibrated at temperatures below 775A degrees C and at subsolidus conditions. The equilibration was probably due to the reaction between water-rich, oxidizing residual melt and the cumulatic-restitic mineral assemblage. MD is characterized by occurrence of the euhedral cummingtonite and increasing anorthite content in the rims of plagioclase. A similar reaction was reproduced experimentally in both LPD and MD compositions indicating that cummingtonite may be a late magmatic phase in quartz dioritic systems, crystallizing very close to solidus and only from water saturated magma.