Geochemische und Isotopen-geochemische Untersuchungen an tertiären Vulkaniten der Hocheifel - Ein Beitrag zur Identifizierung der Mantelquellen von Rift-bezogenen Vulkaniten
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Philipps-Universität Marburg
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Abstract
Major, and trace element and radiogenic (Nd, Sr, Pb) isotope
data are presented for an alkaline suite (nephelinites and basanites)
from the Hocheifel area (Germany) that belongs to the Central European
Volcanic Province (CEVP). Based on variations between compatible and
incompatible trace elements and the Mg# (Mg#: Mg/Mg+Fe) two main suites can be
distinguished.
One suite has affinities to near-primary alkaline
volcanic rocks with high MgO, Ni and Cr and low SiO2 concentrations of < 45
wt % whereas the other suite has lower Mg# < 0.65, mostly lower Ni and Cr
concentrations and higher SiO2 concentrations (> 45 wt %).
However, the most primitive samples from this suite still have Ni (250 ppm)
and Cr (450 ppm) concentrations typical for undifferentiated mantle-derived
volcanic rocks. Rare Earth Element concentrations are high with 100-300 x
chondrite for LREE and c. 10 x chondrite for HREE. The inferred mantle source
for the alkaline rocks is asthenospheric metasomatized garnet peridotite in
which negative anomalies of Rb and K in primitive mantle-normalized diagrams
and Rb/K vs. K, K/La and Ti/La covariations suggest that amphibole was the
major OH-bearing mineral phase. Incompatible trace element ratios
(La/Nb, Ba/Nb, Zr/Nb, Rb/Nb, K/Nb) indicate a common HIMU source for the
primitive rocks whereas elevated trace element ratios of some samples indicate
involvement of an enriched mantle source (EM) and/or crustal contamination.
Trace element modeling indicates that at least three different suites occur
in the Hocheifel area with estimated degrees of partial melting ranging
from 2% to 8 % for the most primitive alkaline rocks.
Constraints on the source composition are in agreement with previously
published trace element concentrations obtained on mantle-derived peridotite
xenoliths.
Furthermore, trace element modeling indicate the presence of garnet and
spinel together with significant amounts of amphibole and aptite suggesting a
depth for melt generation of c. 60-75 km (20-25 kbar).
Initial Nd (e Nd: +7 - +4), Sr (87Sr/86Sr: 0.7031-0.7035) and
Pb (206Pb/204Pb: 19.3-19.8, 207Pb/204Pb: 15.54-15.63; 208Pb/204Pb: 39.0-39.6)
isotope analyses for the most primitive samples lie between plume compositions
and enriched mantle compositions and are similar to those measured in
Ocean Island Basalts (OIB) and the Central European Volcanic Province
elsewhere. These isotope compositions reflect the existence of a depleted
endmember (depleted mantle, DM or high-m, HIMU) and an enriched
endmember (enriched mantle, EM 1). More evolved samples show distinct isotope
compositions in which the Nd isotope compositions become less radiogenic
(e Nd: -1) and Sr isotope compositions become more
radiogenic (87Sr/86Sr: 0.7046). 207Pb/204Pb isotope ratios for these samples
are either less radiogenic (207Pb/204Pb: 15.25) or more
radiogenic (207Pb/204Pb: 15.68) reflecting AFC processes involving at least
two different lower crustal reservoirs during stagnation of the basalts within
the lower crust. Miocene intraplate basaltic volcanism in the Hocheifel
area occurred probably as a result of minor plume activity coupled with
lithospheric extension and some lower lithospheric thermo-mechanical erosion
by the underlying plume head. The basalts show a spatial variation in their
chemical composition; the early-stage alkali basalts have chemical signatures
more compatible with a lower lithospheric source whereas the late-stage alkali
basalts probably originate from asthenospheric souces. This model, which
involves small-scale plume impact followed by continental extension and
asthenosphere-lithosphere interaction together with minor crustal
contamination, should also be applicable to other intra-continental
rift-related areas.
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Dates
Created: 2003Issued: 2003-08-08Updated: 2011-08-10
Faculty
Fachbereich Geowissenschaften
Publisher
Philipps-Universität Marburg
Language
ger
Data types
DoctoralThesis
Keywords
MantelquellePlumeEifelGeochemistryAlkaliolivinbasaltIsotopes
DFG-subjects
IsotopengeochemieGeochemieHocheifel
DDC-Numbers
550
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Jung, Caroline (124950809): Geochemische und Isotopen-geochemische Untersuchungen an tertiären Vulkaniten der Hocheifel - Ein Beitrag zur Identifizierung der Mantelquellen von Rift-bezogenen Vulkaniten. : Philipps-Universität Marburg 2003-08-08. DOI: https://doi.org/10.17192/z2003.0146.
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This item has been published with the following license: In Copyright