CHEMICAL ALTERATION IN ZONED ALKALI FELDSPAR XENOCRYSTS IN THE SILVER MINES QUARTZ BASALT DIKE, ST. FRANCOIS MOUNTAINS, SOUTHEASTERN MISSOURI

The “Silver Mines Quartz Basalt”, a peculiar mafic dike exposed in the St. Francois Mountains of southeastern Missouri, contains numerous xenocrysts of quartz and strongly zoned alkali feldspar. We used energy dispersive x-ray fluorescence (XRF) in a scanning electron microscope to determine chemical variation within the feldspar xenocrysts and their reaction rims within the aphanitic matrix.

Many of the larger feldspar xenocrysts have cores consisting of both white and pink regions with at least two concentric zones surrounding the core material, an inner reddish-brown zone and an outer yellow zone. Each of these four distinct regions shows evidence of extensive chemical alteration. The white regions within the cores have K:Na:Ca ratios that plot geochemically within the range of naturally occurring feldspars at approximately Or₆₅Ab₃₀An₀₅. However, these regions have up to 7% “missing mass” in the XRF analysis, which we interpret to be H₂O in phyllosilicate alteration products. The pink regions within the cores have a significantly higher concentration of calcium and a moderately higher concentration of both sodium and aluminum. The K:Na:Ca ratio in these pink areas plots well outside the stability field of any single naturally occurring feldspar, approximately half way between the compositional stability fields of the alkali feldspars and the plagioclases. We interpret this as evidence for extensive ion exchange during alteration of the xenocrysts by the mafic magma.

The two concentric zones surrounding each core also have K:Na:Ca ratios well outside the stability field for naturally occurring feldspars, indicating extensive alteration from their original feldspar compositions. The reddish-brown zone has a significantly higher concentration of calcium and a moderately higher concentration of both iron and magnesium relative to the white core material. The outer yellow zone has a significantly higher concentration of both calcium and iron relative to the white core material. Outside the outermost yellow zone of each xenocryst lies a distinct fine-grained reaction rim within the matrix. The average composition of this reaction rim plots geochemically very close to half-way between the average composition of the white xenocryst core material and the basaltic-andesite bulk composition of the matrix.