Using Quantitative Textural Analysis to Understand the Emplacement of Shallow-Level Rhyolitic Laccoliths--a Case Study from the Halle Volcanic Complex, Germany

Abstract

RECEIVED FEBRUARY 1, 2002; ACCEPTED NOVEMBER 8, 2002In qualitatively homogeneous magmatic bodies, quantitativetextural analysis—such as crystal size distribution, modalabundance, and spatial distribution pattern analyses—allowstheir internal heterogeneity to be measured and interpreted. Inthis study, these methods are applied to samples from a 300mdrill core through one of the porphyritic rhyolitic laccoliths(Petersberg unit) of the ˘300Ma Halle Volcanic Complex,Germany.Qualitatively,thegeochemicallyhomogeneousPetersbergunit does not show much textural variation. Quantitatively,however, the crystal size distributions of the three most commonphenocryst phases (orthoclase, plagioclase and quartz) suggestcontinuouscrystalgrowthduringmagmaascentandemplacement,but different growth histories of the phenocryst phases throughoutthe genesis of the laccolith. In situ cooling did not affect thephenocryst population. Size distributions of the phenocrysts varyon a centimetre to decimetre scale, but are similar on the scale ofthe laccolith. The modal abundance of the phenocryst phases isvery similar throughout the drill core. Quantification of thespatial distribution of phenocrysts, however, reveals a trend forclusteringtowardstheinteriororupperpartofthelaccolith,whichis attributed to flow and shear processes during emplacement anddiscontinuities in the interior relating to the intrusion of differentmagmapulses.Circularstatisticsoftheorientationoflongaxesofcrystals reveala weakalignment oftheorthoclaseand plagioclasephenocrystsonthesamplescaleasaresultofflowinthemagmainspite of little acicularity. In general, laccoliths can be fedby several pulses of magma without major cooling betweenbatches.