Magma chamber dynamics beneath the Rockeskyller Kopf volcanic centre, West Eifel: A combined field and experimental study
Zusammenfassung der Projektergebnisse
The eruptive history of the Rockeskyller Kopf volcanic centre (West Eifel) has been deduced from field relations. The complex is ~1.5 km across and comprises of a series of partially superimposed structures. The volcanic deposits are composed mostly of pyroclastic deposits, although the final eruptive phase is expressed mostly by several short lava flows. Logging and sampling of about 200 m of tephra stratigraphy has enabled us to reconstruct the eruption history, revealing 7 eruptive phases separated by intervening erosive surfaces. The style of the eruptive events was variable. Several of the early events produced maar-type deposits. Other events exhibit shifts between strombolian and phreatomagmatic eruptive styles. The final eruptive phase is mostly characterised by degassed lava flows that currently define the topographic highs in the area. The initial expression of the last event is a basal welded ashflow tuff that contains abundant carbonised plant remains and moulds. This tuff overlays a paleosol that developed upon deposits of the fifth eruptive phase and indicates a significant depositional hiatus probably on the order of several thousand years. Country rock fragments occur in the deposits of all eruptive phases and are especially abundant in the early maar-type deposits. Lower crustal xenoliths (plagioclase and quartz) are found in the deposits of eruptive phases 1 and 5. Peridotite xenoliths have been found in the phase 1 and phase 3 deposits. They are generally ≤ 5 cm, but can also be much larger. In the phase 1 units, there is a change in character of the peridotite xenoliths. Below an ash marker horizon only wehrlites have been found, where as wehrlites, harzburgites and "composite" peridotite xenoliths with veins of clinopyroxene and biotite occur above this horizon. Discrete clinopyroxenite xenoliths with ± biotite are restricted to the phase 1 and phase 3 deposits, although small mm-size clots of clinopyroxene ± biotite occur in the lavas of most eruptive phases. Megacrysts of clinopyroxene and biotite are found in the deposits of eruptive phases 1 through 5 both as loose crystals in the tephra, as well as forming the cores of small lava bombs. The lavas are leucite and nepheline normative and leucite is a common matrix mineral along with clinopyroxene and magnetite. Major and trace element contents are indicative of an origin by partial melting in the upper mantle in the presence of garnet. Significant fractionation trends are absent, suggesting a repeated supply of primitive magma from the mantle during the lifetime of the volcanic complex (this is supported by the geochemical data). The most notable changes are in fluctuating K2O contents, which can either be related to variable biotite and leucite contents or actual changes in magma composition. The most chemically evolved lavas were erupted during phase 4. A further part of this project involved the experimental investigation of mineral reactions directly applicable to observed textures in the lavas. So far, textures developed within peridotite xenoliths due to interaction with mafic SiO2-undersaturated melts have been investigated. It is apparent that many reaction textures can develop very rapidly. Important progress has been made towards the goal of understanding and modelling magma chamber dynamics (including residence times) during the lifespan of the Rockeskyller Kopf volcanic centre. The degree of scatter in the whole-rock geochemical data indicates that further detailed studies of phenocryst and megacryst chemistry will be necessary to help decipher the evolution of magmatic system. This is the subject of a new research project that will investigate the chemical changes recorded in megacrysts and phenocrysts in the different eruptive phases. This will include detailed measurements of reaction zone thicknesses and diffusion profiles in minerals in order to constrain contact times in the Rockeskyller Kopf magmatic system.
Projektbezogene Publikationen (Auswahl)
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(2005) Stratigraphy, geochemistry, petrography and temporal evolution of the final stages of eruption at Rockeskyllerkopf, West Eifel volcanic field, Germany. Atlantic Universities Geological Conference in St Johns, Newfoundland
Trenholm N
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(2006) The origin of reaction textures in mantle peridotite xenoliths from Sal Island, Cape Verde: experimental and geochemical constraints, GAC-MAC Abstracts v. 31, GAC-MAC Montreal 2006
Shaw CSJ & Dingwell DB
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(2006) The origin of sieve textured minerals in mantle xenoliths. 11th International Conference on Experimental Mineralogy, Petrology and Geochemistry. Bristol, UK. 11-13 September 2006
Shaw CSJ & Dingwell, DB
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(2006) Volcanic Development of the Rockeskyller Kopf Center, West Eifel, Germany. EOS, Transactions American Geophysical Union, 87 (36), Joint Assembly Supplement Abstract V41A-08
Woodland AB, Shaw CSJ, Trenholm ND
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(2006) Volcanology of the upper cone of the Rockeskyller Kopf Volcano, West Eifel volcanic field, Germany. Atlantic Geoscience Society Annual Meeting, Wolfville Nova Scotia, Canada
Trenholm, ND, Shaw, CSJ, Woodland, AB
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(2007) A Combined Experimental and Field Approach to Volcanology: Examples from the Quaternary West Eifel Volcanic Field, Germany. Invited University of New Brunswick SEG-CIM Student Chapter Workshop "A Review of Physical Volcanology: A Metallogenic Perspective" February 2, 2007
Shaw CSJ
