ANTIPERTHITE IN A MAFIC XENOLITH FOR A VOLCANIC BRECCIA IN NORTHERN BAVARIA

Mafic xenoliths in a Tertiary pipe breccia near Schweinshaupten (northem Bavaria) occasionally contain lamellar antiperthite. Larger grains of K-feldspar are perthitic. Usually three sets of K-feldspar lamellae are present in each plagioclase grain. The lamellar shape in one of these sets, near (512) of the plagioclase host, is better developed than that of the other two. The compositions of the plagioclase host and the Or-rich lamellae are An 62 and Or 87, respectively. The K-feldspar of the lamellae is disordered (t1 = 0.57). The plagioclase is also disordered (t1o near 0.50, no ''e'' reflections). Near the K-rich feldspar the composition of the plagioclase changes from An 62 towards An 85. The lamellae and their anorthite-rich rims were formed by unmixing, with long-range migration of the alkalies and Ba (millimetres) and short-range migration of Ca, Si, and Al (micrometres).