Anatomy of a laccolith complex - geometry and texture of porphyritic rhyolites in the Permocarboniferous Halle Volcanic Complex (Germany)

Quantitative petrographic investigations of felsic phenocrysts (K-feldspar, plagioclase, quartz; modal abundance mostly from 20 to 30%) of voluminous rhyolitic laccoliths of the Permocarboniferous Halle Volcanic Complex in Germany yielded well defined, straight crystal size distributions (characteristic lengths 3.4 to 36 nun) and spatial distribution patterns with R-values from 1.34 to 0.78 (random distribution, no touching frameworks). Complicated phenocryst shapes seem to contradict simple growth histories. Phenocryst populations formed on a time-scale of up to a couple of 1,000 years prior to emplacement. Simple models for filling and cooling of laccoliths suggest time-scales from several hundred to 20,000 years. Laccoliths intruded by distinct magma batches and are more numerous than previously recognised. Dimensions of laccoliths follow a selfsimilar trend of magmatic intrusions. Different phenocryst populations led to a contrast in density (similar to 1%) and viscosity and, thus, level of emplacement of laccoliths. They developed between an upper crustal magma chamber and final emplacement. Contacts of laccoliths appear frequently brecciated with some peperitic textures and, in places, show intercalation of magma and host sediment under conditions of ductile deformation on a cm-scale.