Petrogenesis of rift-related tephrites, phonolites and trachytes (Central European Volcanic Province, Rhön, FRG): Constraints from Sr, Nd, Pb and O isotopes.

Jung, S., Mezger, K., Hauff, Folkmar, Pack, A. and Hoernes, S. (2013) Petrogenesis of rift-related tephrites, phonolites and trachytes (Central European Volcanic Province, Rhön, FRG): Constraints from Sr, Nd, Pb and O isotopes. Open Access Chemical Geology, 354 . pp. 203-215. DOI 10.1016/j.chemgeo.2013.06.026.

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Supplementary data:

Abstract

Highlights:
• Some differentiated alkaline rocks may evolve by FC or AFC but not both
• Analyses of acid-leached samples necessary to detect unsupported 87Sr
• Crustal contamination hardly detectable in high-Sr lavas but obvious in low-Sr lavas
• Deep crustal contamination confirmed by high-precision Pb double-spike data
• Positively correlated 87Sr/86Sr ratios - δ18O values also indicate crustal contamination
The volcanic rocks of the Rhön area (Central European Volcanic Province, Germany) belong to a moderately alkali basaltic suite that is associated with minor tephriphonolites, phonotephrites, tephrites, phonolites and trachytes. Based on isotope sytematics (87Sr/86Sr: 0.7033-0.7042; 143Nd/144Nd: 0.51279-0.51287; 206Pb/204Pb: 19.1-19.5), the inferred parental magmas formed by variable degrees of partial melting of a common asthenospheric mantle source (EAR: European Asthenospheric Reservoir of Cebria and Wilson, 1995). Tephrites, tephriphonolites, phonotephrites, phonolites and trachytes show depletions and enrichments in some trace elements (Sr, Ba, Nb, Zr, Y) indicating that they were generated by broadly similar differentiation processes that were dominated by fractionation of olivine, clinopyroxene, amphibole, apatite and titaniferous magnetite ± plagioclase ± alkalifeldspar. The fractionated samples seem to have evolved by two distinct processes. One is characterized by pure fractional crystallization indicated by increasing Nb (and other incompatible trace element) concentrations at virtually constant 143Nd/144Nd ~ 0.51280 and 87Sr/86Sr ~ 0.7035. The other process involved an assimilation-fractional crystallization process (AFC) where moderate assimilation to crystallization rates produced evolved magmas characterized by higher Nb concentrations at slightly lower 143Nd/144Nd down to 0.51275. Literature data for some of the evolved rocks show more variable 87Sr/86Sr ranging from 0.7037-0.7089 at constant 143Nd/144Nd ~ 0.51280. These features may result from assimilation of upper crustal rocks by highly differentiated low-Sr (< 100 ppm Sr) lavas. However, based on the displacement of the differentiated rocks from this study towards lower 143Nd/144Nd ratios and modeled AFC processes in 143Nd/144Nd vs. 87Sr/86Sr and 207Pb/204Pb vs. 143Nd/144Nd space assimilation of lower crustal rocks seems more likely. The view that assimilation of lower crustal rocks played a role is confirmed by high-precision double-spike Pb isotope data that reveal higher 207Pb/204Pb ratios (15.62-15.63) in the differentiated rocks than in the primitive basanites (15.58-15.61). This is compatible with incorporation of radiogenic Pb from lower crustal xenoliths (207Pb/204Pb: 15.63-15.69) into the melt. However, 206Pb/204Pb ratios are similar for the differentiated rocks (19.13-19.35) and the primitive basanites (19.12-19.55) implying that assimilation involved an ancient crustal end member with a higher U/Pb ratio than the mantle source of the basanites. In addition, alteration-corrected δ18O values of the differentiated rocks range from c. 5 to 7 ‰ which is the same range as observed in the primitive alkaline rocks. This study confirms previous interpretations that highlighted the role of AFC processes in the evolution of alkaline volcanic rocks in the Rhön area of the Central European Volcanic Province.

Document Type: Article
Additional Information: WOS:000325751200016
Keywords: Igneous petrology; geochemical modeling; radiogenic isotope geochemistry; stable isotope geochemistry; intraplate processes
Research affiliation: OceanRep > The Future Ocean - Cluster of Excellence
OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-MUHS Magmatic and Hydrothermal Systems
Refereed: Yes
Open Access Journal?: No
DOI etc.: 10.1016/j.chemgeo.2013.06.026
ISSN: 0009-2541
Projects: Future Ocean
Date Deposited: 18 Jul 2013 09:00
Last Modified: 07 Dec 2017 13:16
URI: http://oceanrep.geomar.de/id/eprint/21605

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