Compositional variation in apatite, phlogopite and other accessory minerals of the ultramafic Delitzsch complex, Germany: implication for cooling history of carbonatites

Abstract

A representative suite of samples from the ultramafic lamprophyre–carbonatite (UML–CR) complex of Delitzsch, Germany, shows significant variations in mineral composition and geothermobarometry. Petrographically distinct carbonatites of intrusive dolomitic and subvolcanic dolomitic and subvolcanic calcitic types clearly show marked compositional differences in such minerals as apatite, phlogopite and pyrochlore. Increasing concentrations are established for S, Cl, Si in apatite, and Ti, Al, Cr, Ba, Na in phlogopite during the crystallization history, whereas decreasing trends are shown for the REEs, Sr, Na, F in apatite, and Mg and F in phlogopite. Pyrochlore from beforsites and alvikites on average are enriched with W, Ta, Si, Ti, Zr, Th, U, Fe, Sr, Ba, and Pb relative to the plutonic dolomite carbonatite. Apatite–phlogopite geothermometry combined with hydrogen fluoride barometry shows that carbonatites were formed from melts produced during several magmatic events. At least three crystallization trends (A–C) at different levels of HF fugacity are recognized. Intrusive, coarse-grained, dolomite carbonatites were in chemical equilibrium with phlogopite glimmerites. They show flat cooling trends in a plot of log( f HF/ f H 2O ) vs. 1/ T, whereas the subvolcanic dolomite carbonatites (beforsite) possess a steeper cooling trend. Apatite and phlogopite formed throughout the crystallization history of the complex, from the early stage of the plutonic dolomite carbonatite through the late stage beforsite dykes and, finally, fine-grained calcite carbonatite veins (alvikite). These events cover temperatures from 1200°C to about 600°C. The chemical relationship between carbonatites and ultramafic lamprophyres remain obscure.