Formation of a laccolith by magma pulses: Evidence from modal and chemical composition of the 500 m long borehole section through the Permo-Carboniferous Landsberg laccolith (Halle Volcanic Complex)

Abstract

The 500 m long section through the upper part of the Permo-Carboniferous Landsberg laccolith (Halle Volcanic Complex) was sampled every 25 meters. The modal proportions between plagioclase and K-feldspar phenocrysts vary in the section and the laccolith may be divided into four parts with different proportions of Pl/Kfs, which, in subvolcanic rocks, should reflect different proportions of these minerals in the magma plumbing system. Chemical composition of whole rock samples is uniform, but the correlations of Si and other elements with depth within all of the four sections suggest that the sections based on modal composition are also reflected in chemical composition of the rock. Also, the uppermost 100 meters of the laccolith has slightly higher contents of Fe, Ti, Zr and Nb compared to those in the rest of the laccolith and this is consistent with it being a separate magma pulse derived from a distinct source. Detailed analyses of chemical variations within each section are consistent with the model that the upper 500 meters of the Landsberg laccolith was formed by three successive pulses with slightly different chemical compositions. The best documented is the uppermost pulse, which was over-accreted on the first pulse. Another pulse was probably emplaced in the middle of the first pulse. The thickness of the pulses was 100‐300 m, which is consistent with previous 2D and 3D emplacement models of the Halle laccoliths. However, the contacts between the pulses based on modal and chemical compositional variations are not always concurrent with the presence of shearing zones, the discrepancy that is not yet well understood. In general, because silica-rich laccoliths are relatively small bodies that cool quickly due to high level of emplacement, they may preserve better evidence for separate magma pulses compared to plutonic batholiths.