Journal Article

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

A. MOCK, D. A. JERRAM and C. BREITKREUZ

in Journal of Petrology

Volume 44, issue 5, pages 833-849
Published in print May 2003 | ISSN: 0022-3530
Published online May 2003 | e-ISSN: 1460-2415 | DOI: http://dx.doi.org/10.1093/petrology/44.5.833
Using Quantitative Textural Analysis to Understand the Emplacement of Shallow-Level Rhyolitic Laccoliths—a Case Study from the Halle Volcanic Complex, Germany

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In qualitatively homogeneous magmatic bodies, quantitative textural analysis—such as crystal size distribution, modal abundance, and spatial distribution pattern analyses—allows their internal heterogeneity to be measured and interpreted. In this study, these methods are applied to samples from a 300 m drill core through one of the porphyritic rhyolitic laccoliths (Petersberg unit) of the ∼300 Ma Halle Volcanic Complex, Germany.Qualitatively, the geochemically homogeneous Petersberg unit does not show much textural variation. Quantitatively, however, the crystal size distributions of the three most common phenocryst phases (orthoclase, plagioclase and quartz) suggest continuous crystal growth during magma ascent and emplacement, but different growth histories of the phenocryst phases throughout the genesis of the laccolith. In situ cooling did not affect the phenocryst population. Size distributions of the phenocrysts vary on a centimetre to decimetre scale, but are similar on the scale of the laccolith. The modal abundance of the phenocryst phases is very similar throughout the drill core. Quantification of the spatial distribution of phenocrysts, however, reveals a trend for clustering towards the interior or upper part of the laccolith, which is attributed to flow and shear processes during emplacement and discontinuities in the interior relating to the intrusion of different magma pulses. Circular statistics of the orientation of long axes of crystals reveal a weak alignment of the orthoclase and plagioclase phenocrysts on the sample scale as a result of flow in the magma in spite of little acicularity. In general, laccoliths can be fed by several pulses of magma without major cooling between batches.

Keywords: crystal size distribution (CSD); Halle Volcanic Complex (HVC); laccoliths; porphyritic rhyolites; spatial distribution patterns (SDP)

Journal Article.  7224 words.  Illustrated.

Subjects: Petrology

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