JOURNAL OF PETROLOGY VOLUME 53 NUMBER11 PAGES 2381^2409 2012 doi:10.1093/petrology/egs047 Petrogenesis of Tertiary Alkaline Magmas in the Siebengebirge, Germany S. JUNG1*, K. VIETEN2,R.L.ROMER3, K. MEZGER4,S.HOERNES2 AND M. SATIR5 1DEPARTMENT GEOWISSENSCHAFTEN, MINERALOGISCH-PETROGRAPHISCHES INSTITUT, UNIVERSITA« THAMBURG, 20146 HAMBURG, GERMANY 2MINERALOGISCH-PETROLOGISCHES INSTITUT DER UNIVERSITA« T BONN, POPPELSDORFER SCHLOb, 53115 BONN, GERMANY 3GEOFORSCHUNGSZENTRUM POTSDAM, TELEGRAFENBERG, B 126, D-14473 POTSDAM, GERMANY 4INSTITUT FU« R GEOLOGIE, UNIVERSITA« T BERN, BALTZERSTRASSE 1þ3, CH-3012 BERN, SWITZERLAND 5INSTITUT FU« R GEOWISSENSCHAFTEN, UNIVERSITA« TTU« BINGEN, SIGWARTSTR. 10, 72076 TU« BINGEN, GERMANY RECEIVED JULY 8, 2011; ACCEPTED JUNE 13, 2012 ADVANCE ACCESS PUBLICATION SEPTEMBER 12, 2012 Basanites from theTertiary Siebengebirge area of Germany (part of amphibole-bearing spinel peridotite sources may suggest an origin the Central European Volcanic Province; CEVP) have high Mg# from the metasomatized part of the thermal boundary layer. (40·60), moderate to high Cr (4300 ppm) and Ni (4200 ppm) Application of new thermobarometric equations for the basaltic contents and strong light rare earth element enrichment, but system- magmas indicates relatively normal mantle potential temperatures atic depletion in Rb and K relative to trace elements of similar com- (1300^14008C); thus the inferred mantle ‘baby plume’ or ‘hot patibility in anhydrous mantle. Rare earth element melting models finger’ is not thermally anomalous. can explain the petrogenesis of these basanites in terms of partial melting of a spinel peridotite source containing residual amphibole. It is inferred that amphibole, indicated by the relative K and Rb KEY WORDS: alkali basalts; continental volcanism; crustal contamin- depletion and the melting model, was precipitated in the spinel peri- ation; partial melting; Siebengebirge | downloaded: 28.9.2021 dotite lithospheric mantle beneath the Siebengebirge, by metasomatic fluids or melts from a rising mantle diapir or plume. Alkali basalts and more differentiated rocks have lower Mg# and lower abun- INTRODUCTION dances of Ni and Cr, and have undergone fractionation of mainly Despite decades of intense investigation, significant ques- olivine, clinopyroxene, Fe^Ti oxides, amphibole and plagioclase. tions remain on the origin and evolution of rift-related Most of the basanites and alkali basalts approach the Sr^Nd^Pb alkaline basalts. In such areas, the volcanic products repre- isotope compositions inferred for the European Asthenospheric sent relatively small volumes of juvenile crust emplaced Reservoir component.Trace element constraints (i.e. low Nb/U and over short time periods, but the nature of the source and Ce/Pb ratios) and the Sr^Nd^Pb isotope composition of the differ- the processes that form these magmas are not well under- entiated rocks indicate that assimilation of lower crustal material stood. Although many of these usually sodium-dominated has modified the composition of the primary mantle-derived basalts have a chemistry similar to many ocean island bas- magmas. High 207Pb/204Pb ratios in the differentiated lavas point alts (OIB) (Alle' gre et al., 1981; Fitton & Dunlop, 1985; to assimilation of ancient lower crustal components having high Thompson & Morrison, 1988, among many others), it is U/Pb and Th/Pb ratios. Relatively shallow melting of inferred still highly debated whether the primary source of the https://doi.org/10.7892/boris.116076 ß The Author 2012. Published by Oxford University Press. All *Corresponding author: Phone: þ49(0)40-42838-2061. Fax: þ49(0)40- rights reserved. For Permissions, please e-mail: journals.permissions@ source: 42838-2422. E-mail: [email protected] oup.com JOURNAL OF PETROLOGY VOLUME 53 NUMBER 11 NOVEMBER 2012 melt is the shallow enriched subcontinental lithospheric 1995; Goes et al., 1999; Ritter et al., 2001; Wilson & mantle (SCLM), the shallow asthenospheric mantle or Patterson, 2001; Keyser et al., 2002). For the Rhenish deep plume-related mantle. The similarity to many OIB Massif, a columnar low P-wave velocity anomaly has been magmas has led to the proposal that the convecting as- detected beneath the Eifel (Ritter et al., 2001).This 100 km thenosphere is the source of most sodic alkaline magmas, wide structure extends down to 400 km depth and could in which the isotope compositions and the trace element be interpreted to be equivalent to an excess mantle tem- enrichment reflect mixing of enriched lithospheric mantle perature of 150^2008C in the absence of volatiles or partial and crustal material into the asthenosphere by delamin- melt. It should be noted that such geophysical consider- ation and subduction (White & Hofmann, 1982; ations require that the upper mantle is isochemical and McKenzie & O’Nions, 1983; Hart, 1988). Also important is isomineralogical, which is certainly an oversimplification. whether the variations in the chemical and isotopic In addition, the seismic wave anomalies are discontinous compositions of the magmas are the result of mixing of dif- beneath the Eifel and are more influenced by chemical ferent mantle endmembers or the result of crustal contam- and mineralogical anomalies rather than representing ination. To understand and model the formation and temperature gradients. Hence, the reported excess tem- evolution of rift-related volcanic provinces, we need to be perature should be viewed as an upper limit. On the able to discriminate between the various potential mantle other hand, based on xenolith studies, the metasomatized sources of the basalts and the sources of contamination. lithospheric mantle can be a potential source of the mafic Much of the controversy on the origin of rift-related lavas. Partial melting of such zones of metasomatized litho- provinces revolves around the interpretation of the trace spheric mantle, combined with interaction between as- element characteristics and Sr, Nd and Pb isotopic com- thenospheric melts and lithospheric melts, has been position of the basalts. This information has yielded proposed to explain the geochemical characteristics of the partly ambiguous results because both crust and enriched most primitive mafic alkaline rocks of the CEVP (Wilson mantle can impart similar characteristics and because the & Downes, 1991; Wilson & Patterson, 2001). In addition, chemical and isotopic compositions of the various end- crustal contamination of the mantle-derived magmas has member mantle reservoirs are created, at least in part, been widely documented within the CEVP (Massif et al through crustal recycling or are susceptible to modification Central: Wilson ., 1995a; Vogelsberg: Jung & Masberg, 1998; Bogaard & Worner, 2003; Jung et al., 2011; Rhon: through metasomatism. Hence, one interpretation is that « « Jung & Hoernes, 2000; Jung et al., 2005; Urach^Hegau: the lithospheric signatures of continental basalts are pri- Blusztajn & Hegner, 2002; Westerwald: Haase et al., 2004). marily signs of contamination of sublithospheric melts by Neither comprehensive whole-rock geochemical data nor either lithospheric mantle or continental crust. Another sufficient Sr^Nd^Pb isotopic data have been published for interpretation is that melting of continental lithosphere is the Tertiary Siebengebirge volcanic field. Such data are es- aided by the presence of water and/or low-melting tem- sential to constrain the role of fractional crystallization, perature mafic veins that will allow the lithospheric crustal contamination, and mantle source heterogeneities mantle to be the main contributor to continental basaltic in the petrogenesis of the alkaline lavas. In this contribu- volcanism (Gallagher & Hawkesworth, 1992; Harry & tion, major and trace element data and Sr^Nd^Pb^O iso- Leeman, 1995; Pilet et al., 2008). tope data are reported for primitive alkaline magmas For the Central European Volcanic Province (CEVP), from the Siebengebirge area; these data are used to con- the volcanic products converge on a common source com- strain the mantle source region of these basalts. Major position when plotted in conventional Sr^Nd^Pb isotope and trace element and Sr^Nd^Pb^O isotope data for diagrams. This common source has been termed PREMA more differentiated lavas from the same area are used to (PREvalent MAntle; Wo« rner et al., 1986), the European highlight the effects of fractional crystallization and crustal Asthenospheric Reservoir (EAR, Cebria¤ & Wilson, 1995), contamination in the genesis of these alkaline lavas. Low Velocity Component (LVC; Hoernle et al., 1995) or CMR (Common Mantle Reservoir; Lustrino & Wilson, 2007) and is believed to represent a plume-related reser- GEOLOGICAL SETTING voir. Other, probably minor, reservoirs are believed to ori- The Siebengebirge volcanic field forms part of an east^ ginate in the SCLM (Cebria¤ & Wilson, 1995; Granet west-trending belt of Tertiary^Quaternary volcanic fields et al., 1995; Hoernle et al., 1995). Support for a deep mantle in central Germany; these include the Eifel, Siebengebirge, origin for the most primitive magmas comes from Westerwald, Vogelsberg, Hessian Depression, Rho« n, Held- high-resolution mantle seismic tomography showing burg, and Oberpfalz (Upper Palatinate) (Fig. 1). The low-velocity domains in the mantle at various depths. trend of these volcanic fields is perpendicular to the main Consequently, an asthenospheric origin for the European NNE^SSW-trending Rhine graben rift system of Central volcanism linked