Petrology and geochemistry of the Government Springs Laccolith, Big Bend National Park, Texas.

Abstract

The Government Spring Laccolith GSL is a silica oversaturated alkali feldspar quartz syenite which intruded the lower Tertiary age Chisos Formation between late Eocene and 25 ma ago. It contains a small volume <1% of mafic inclusions of basaltic composition. The host laccolith consists predominantly of feldspar >80%, quartz <7%, and some clinopyroxene 10% phenocrysts. Modal mineralogy of associated dike samples is broadly identical to the host quartz syenite. The inclusions exhibit a wide range of textures and mineral proportions. Some samples have abundant plagioclase megacrysts 10mm, and clinopyroxene is the dominant mineral in chilled margins. Mafic inclusions form when a relatively small volume of mafic magma is injected into a larger volume of silicic magma. The hotter mafic magma is quenched by the cooler, solidifying silicic magma. In order for this process of magma mingling to dominate over magma mixing, a large compositional, thermal, and viscosity contrast is required, as well as a larger proportion of silicic melt over the mafic melt. Mafic inclusions formed under these conditions have the following unique characteristics chilled rinds, diminishing grain size towards crenulated margins, acicular apatite needles, and rounded shape. All of these features are present in the mafic inclusions in GSL. This thesis documents the field relations, petrography, petrology, and geochemistry of mafic inclusions from the GSL. Evidence presented suggests a magmatic origin for these inclusions.

14 samples from this reference