The circum-Mediterranean anorogenic Cenozoic igneous province

Partial melt compositions of natural carbonated peridotite (KLB-1 + 2.5 wt% CO2) have been determined at 3 GPa using the diamond aggregate method. Melt obtained at 1350 °C is carbonatitic, and with increasing temperature melt composition becomes silicate-rich. These melts are lower in SiO2 and Al2O3 and higher in MgO and CaO compared to dry partial melts of KLB-1 generated at the same pressure. The 1400 and 1450 °C liquids have melilititic compositions, which are in good agreement with those of natural alkali-basalts from continental regions. At higher temperatures, partial melts are richer in SiO2, and the compositions of liquids at 1475 and 1525 °C approximate nephelinite and basanite respectively. However, they deviate from natural alkaline rocks in that the CaO/(CaO+MgO) ratios are smaller, suggesting that the natural nephelinite and basanite magmas were formed at lower pressures. The compositional trend of natural alkaline rocks, melilitites-nephelinites-basanites, may reflect the effect of decreasing pressure.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97GL02956

Partial melt compositions of carbonated peridotite at 3 GPa and role of CO2 in alkali‐basalt magma generation

Primitive nephelinites and basanites from the Tertiary Hocheifel area of Germany (part of the Central European Volcanic Province; CEVP) have high Mg-number (>0� 64), high Cr and Ni contents and strong light rare earth element enrichment but systematic depletion in Rb, K and Ba relative to trace elements of similar compatibility in anhydrous mantle. Alkali basalts and more differentiated magmatic rocks have lower Mg-number and lower abundances of Ni and Cr, and have undergone fractionation of mainly olivine, clinopyroxene, Fe–Ti oxide, amphibole and plagioclase. Some nephelinites and basanites approach the Sr–Nd–Pb isotope compositions inferred for the EAR (European Asthenospheric Reservoir) component. The Nd–Sr–Pb isotope composition of the differentiated rocks indicates that assimilation of lower crustal material has modified the composition of the primary mantle-derived magmas. Rare earth element melting models can explain the petrogenesis of the most primitive mafic magmatic rocks in terms of mixing of melt fractions from an amphibole-bearing garnet peridotite source with melt fractions from an amphibole-bearing spinel peridotite source, both sources containing residual amphibole. It is inferred that amphibole was precipitated in the asthenospheric mantle beneath the Hocheifel, close to the garnet peridotite–spinel peridotite boundary, by metasomatic fluids or melts from a rising mantle diapir or plume. Melt generation with amphibole present suggests relatively low mantle potential temperatures (<1200 � C); thus the mantle plume is not thermally anomalous. A comparison of recently published Ar/Ar ages for Hocheifel basanites with the geochemical and isotopic composition of samples from this study collected at the same sample sites indicates that eruption of earlier lavas with an EM signature was followed by the eruption of later lavas derived from a source with EAR or HIMU characteristics, suggesting a contribution from the advancing plume. Thus, the Hocheifel area represents an analogue for magmatism during continental rift initiation, during which interaction of a mantle plume with the overlying lithosphere may have led to the generation of partial melts from both the lower lithosphere and the asthenosphere.

Petrogenesis of Tertiary Mafic Alkaline Magmas in the Hocheifel, Germany

Plio-Pleistocene volcanism in the Golan and Galilee (northeastern Israel) shows systematic variability with time and location: alkali basalts were erupted in the south during the Early Pliocene, whereas enriched basanitic lavas erupted in the north during the Late Pliocene (Galilee) and Pleistocene (Golan). The basalts show positive correlations in plots of ratios of highly to moderately incompatible elements versus the concentration of the highly incompatible element (e.g. Nb/Zr vs Nb, La/Sm vs La) and in diagrams of REE/HFSE (rare earth elements/high field strength elements) vs REE concentration (e.g. La/Nb vs La). Some of these correlations are not linear but upward convex. 87 Sr/ 86 Sr ratios vary between 0� 7031 and 0� 7034 and correlate negatively with incompatible element concentrations and positively with Rb/Sr ratios. We interpret these observations as an indication that the main control on magma composition is binary mixing of melts derived from two end-member mantle source components. Based on the high Sr/Ba ratios and negative Rb anomalies in primitive mantle normalized trace element diagrams and the moderate slopes of MREE–HREE (middle REE–heavy REE) in chondrite-normalized diagrams, we suggest that the source for the alkali basaltic end-member was a garnet-bearing amphibole peridotite that had experienced partial dehydration. The very high incompatible element concentrations, low K content, very low Rb contents and steep MREE–HREE patterns in the basanites are attributed to derivation from amphibole- and garnet-bearing pyroxenite veins. It is suggested that the veins were produced via partial melting of amphibole peridotites, followed by complete solidification and dehydration that effectively removed Rb and K. The requirement for the presence of amphibole limits both sources to lithospheric depths. The spatial geochemical variability of the basalts indicates that the lithosphere beneath the region is heterogeneous, composed of vein-rich and vein-poor domains. The relatively uniform 143 Nd/ 144 Nd («Nd ¼ 4� 0–5� 2) suggests that the two mantle sources were formed by dehydration and partial melting of an originally isotopically uniform reservoir, probably as a result of a Paleozoic thermal event.

The Role of Lithospheric Mantle Heterogeneity in the Generation of Plio-Pleistocene Alkali Basaltic Suites from NW Harrat Ash Shaam (Israel)

Silica-poor, calcium-rich melilitites form a chemical and isotopic end-member of the spectrum of mafic magmas of the Tertiary-Quaternary volcanic province of western and central Europe. We propose that these unusual magmas are derived by partial melting of the thermal boundary layer (TBL) at the base of the European lithosphere. The processes involved in the evolution of the TBL have been constrained using major and trace element and Nd-Sr-Pb isotope data for Tertiary melilitites from the Urach, Hegau, and Rhine graben regions of Germany. The initiation of boundary layer evolution is limited in time by a major phase of Permo-Carboniferous rifting and associated magmatism postdating the Hercynian orogeny which would have destroyed the existing TBL by delamination. Model calculations indicate that the isotopic composition of the melilitite source cannot develop within the TBL over geologically reasonable periods of time (250–300 Million years) if the TBL evolves solely by incorporation of small degree (<0.1%) partial melts from an underlying, convecting, depleted (MORB-source) mantle reservoir. On the basis of this observation, the regional geodynamic setting and the melilitite data, we propose that an isotopically distinct mantle plume, impinging on the base of the European lithosphere during the Early Cenozoic, is involved in the petrogenesis of the melilitite magmas.

Melilitites: partial melts of the thermal boundary layer?

Analytical data on major elements and 31 trace elements in olivine nephelinites, nepheline basanites, basanitic alkali olivine basalts and their differentiates (tephrites, hawaiites, mugearites, benmoreites, latites, phonolites and trachytes) from Hegau, Kaiserstuhl, Rhon, Hessian Depression, Vogelsberg, Westerwald, Siebengebirge, E Eifel and Hocheifel are evaluated. They were based on 400 samples with new or unpublished data on about one third of the rocks. The Sr−Nd isotopic compositions for 78 rocks are included. The alkali basaltic volcanism is caused by adiabatic decompression of asthenospheric mantle updomed to a minimum depth of 50 km in connection with the Alpine continent collision. The chemical compositions of the primary basaltic melts from the different areas are similar containing about one hundred-fold enrichment of highly incompatible elements relative to the primitive mantle from partial melting of depleted and secondarily enriched peridotite. The elements Cs, K, Pb and Ti are specifically depleted in the basalts partly because of phlogopite being residual at partial melting. The Tertiary alkali basalts range in Nd-isotopic composition from 0.51288 to 0.51273 and in Sr-isotopic ratios from 0.7032 to 0.7042. These ranges indicate mixtures of HIMU, depleted and enriched mantle components in the metasomatically altered peridotite source which resembles that of certain ocean islands. The Nd-Sr-isotopic compositions of the Quaternary E Eifel are close to bulk Earth ratios. East and W Eifel plots differ distinctly from the Tertiary Hocheifel which is geographically intermediate. This isotopic difference, beside specific K/Na ratios, is probably caused by separate metasomatic pulses that immediately preceded the respective periods of volcanism. The metasomatically altered mantle had partly primitive mantle signatures (Nb/Ta, Zr/Sm and Th/U ratios) and partly ocean island (or MORB) source properties (Rb/Cs). A MORB source can be excluded because of the low K/Rb and high Th/U ratios. A correlation of δD with 87Sr/86Sr in amphibole and phlogopite and a slightly larger δ18O than in MORB is conformable with a seawater and crustal impact on the source of alkali basalts. Slightly higher than average water concentrations in the source of certain primary basaltic melts (indicated by amphibole phenocrysts in their basalts) are required for differentiation of these basalts in magma chambers of the upper crust. Model calculations are presented to explain compositions of differentiates which range from about 60% to about 20% residual melt. The latter are represented by phonolites and trachytes. The Nd- and Sr-isotopic signatures of the majority of differentiates indicate contamination by a granitic partial melt from the wall rocks of magma chambers. Olivine nephelinite magma was the common source of contaminated differentiates.

Emil Gohn

Papers

Cenozoic alkali basaltic magmas of western Germany and their products of differentiation