Hilary Downes

Textural, isotopic and REE variations in spinel peridotite xenoliths, Massif Central, France

Abstract Sr and Nd isotope analyses and REE patterns are presented for a suite of well-documented mantle-derived xenoliths from the French Massif Central. The xenoliths include spinel harzburgites, spinel lherzolites and some pyroxenites. They show a wide range of textures from undeformed protogranular material through porphyroclastic to equigranular and recrystallised secondary types. Textural differences are strongly linked to trace element geochemistry and variations in radiogenic isotope ratios. Many undeformed protogranular xenoliths are Type IA LREE-depleted with MORB-type eSr values between − 30.7 and − 23.6, and eNd values + 13.9 to + 9.4. A second group of undeformed xenoliths are Type IB LREE-enriched with higher eSr values (− 22.7 to − 10.6) and lower eNd values (+ 11.9 to + 5.6). Deformed xenoliths with porphyroclastic, equigranular and secondary recrystallised textures are all Type IB (LREE-enriched, eNd 11.8). It is proposed that two separate events have given rise to the observed mixing arrays: (1) MORB-source depleted mantle was enriched by a component derived from an enriched mantle. Deformation and recrystallisation accompanied this event. (2) Subsequently, unenriched MORB-source mantle interacted with magmas chemically akin to the host basalts, and enrichment occurred with little deformation. Hypotheses of Tertiary mantle diapirism resulting in isochemical deformation and refinement of protogranular mantle to equigranular mantle are untenable because of differences in REE patterns and isotopic ratios between different textural groups.

Hf-Nd isotopic evolution of the lower crust

We report Hf isotopic data for over 50 well studied lower crustal samples from three Proterozoic and Phanerozoic regions in southwest Europe, eastern Australia and southern Mexico. We use these data to characterize the Lu–Hf isotopic composition of the lower crust and, in combination with existing Sm–Nd data, to constrain coupled Hf–Nd isotopic behavior and evolution within this reservoir. Although most of these samples have present-day parent/daughter (p/d) ratios consistent with Hf–Nd evolution within the terrestrial Hf–Nd array, some samples have divergent p/d ratios that would evolve out of the terrestrial array in 1 Ga or less. The present-day 176Hf/177Hf and 143Nd/144Nd isotopic compositions of all samples, with one lone exception, plot within the terrestrial array. This indicates that (1) some present-day p/d ratios may be a relatively recently acquired characteristic through magmatic or metamorphic processes not related to the time-integrated Sm/Nd and Lu/Hf ratios of their sources, and/or (2) the Lu/Hf and Sm/Nd p/d variations exist on a small hand-size scale but not necessarily on a larger scale. The lower crust, from this initial data set, is broadly similar to the upper crust in terms of both its present-day p/d values and time-integrated Lu–Hf and Sm–Nd evolution. As a result, the lower crust appears to have a Hf and Nd isotopic composition similar to that of all other crust and mantle reservoirs so far characterized.

Petrology and geochemistry of spinel peridotite xenoliths from the western Pannonian Basin (Hungary) : evidence for an association between enrichment and texture in the upper mantle

Twenty spinel peridotite xenoliths from Pliocene alkali basaltic tuffs and lavas of the western Pannonian Basin (Hungary) have been analysed for bulk rock major and trace elements, electron probe mineral compositions, and REE and Sr, Nd isotopes on separated and leached clinopyroxenes. The xenoliths are texturally diverse, including protogranular, porphyroclastic, equigranular and poikilitic textures which can generally be correlated with geochemical features. Protogranular xenoliths are relatively undepleted in Ca, Al, Ti and Na, whereas poikilitic xenoliths are more refractory. LREE-depleted patterns. and MORB-like εNd and εSr values are associated with protogranular peridotites. In contrast, xenoliths with complex textures are generally LREE-enriched. Much of the isotopic variation in the suite (εSr=−20.4 to +10.4, +Nd=+1.8 to +13.7) can be related to interaction between protogranular mantle and melts resembling the host alkali basalts, but a third (high εSr) component may be due to Miocene subduction beneath the region.

Crustal evolution of the Hercynian belt of Western Europe: Evidence from lower-crustal granulitic xenoliths (French Massif Central)

Abstract Granulite-facies lower-crustal xenoliths scavenged by the Tertiary basaltic pipes of Bournac and Roche Pointue (Massif Central, France) have been analysed for major and trace elements and radiogenic (Sr, Nd) isotope ratios. They display wide variations in major elements, rare-earth elements and isotopes ( 87 Sr 86 Sr = 0.7041–0.7216 ; ϵNd = −11.9 to +1.5), which tend to correlate with the three different rock types encountered: basic and acidic meta-igneous and metasediments. The basic liquids are of calc-alkaline affinity, and cumulates are abundant. There is a gap in silica content between the basic and acid meta-igneous xenoliths. The isotopic compositions suggest mixing of old pre-existing continental crust (represented by the metasediments) and mantle-derived basic melts which represent juvenile addition to the crust during the Hercynian orogeny. The basic meta-igneous rocks have been contaminated during their emplacement into the crust whereas the surrounding metasediments have suffered partial fusion and contamination by the same liquid, producing the Hercynian granitoids which have ϵNd and ϵSr initial ratios identical to those of the lower-crustal xenoliths. The residue from melting may be represented by the acid meta-igneous rocks, some of which have positive Eu anomalies. These results indicate that an important basaltic underplating event occurred during the evolution of Hercynian orogenic belt, although the exact timing of the underplating requires further constraint.

Magmatic constraints on geodynamic models of subduction in the East Carpathians, Romania

Abstract The East Carpathian volcanic arc is the youngest region of calc-alkaline magmatic activity in Eastern Europe. A general age progression of the onset and cessation of magmatic activity occurs along the East Carpathian arc from older volcanic structures (ca. 12 Ma) in the NW to the youngest (

Geochemical variation in peridotite xenoliths and their constituent clinopyroxenes from Ray Pic (French Massif Central): implications for the composition of the shallow lithospheric mantle

Abstract Anhydrous mantle peridotite xenoliths from a single volcanic vent in the French Massif Central are compositionally varied, ranging from relatively fertile lherzolites to refractory harzburgites. Fertile lherzolites closely resemble previous estimates of undepleted mantle compositions but the average of the Ray Pic xenoliths is much less enriched in LILE and LREE than McDonoughs (1990) average mantle [McDonough, W.F., 1990. Constraints on the composition of the continental lithospheric mantle. Earth Planet. Sci. Lett., 101, 1–18]. The wide geochemical variation in the bulk rocks reflects significant heterogeneities that can be attributed to two major processes within the shallow lithospheric mantle. The first process is depletion, related to variable degrees of partial melting and melt extraction from an originally near-chondritic mantle. This process has largely controlled the major elements and much of the trace element variation between fertile lherzolites and refractory peridotites. LREE-depleted compositions are also produced by this process. During partial melting, HREE behaved coherently with the major oxides and the moderately incompatible trace elements (Y, V and Sc). A subsequent process of enrichment is indicated by high concentrations of incompatible trace elements in many of the xenoliths. Sr, Ba, K, Th, U, Nb and LREE abundance are independent of major oxide variations and reflect enrichment related to infiltration by alkaline silicate melts/fluids. Both fertile and refractory mantle were enriched but harzburgites were particularly affected. Modal metasomatism occurred only rarely and is indicated by Cr-diopside-rich veins and patches in a few samples. Their chemistry suggests that they were also formed by migration of similar magmas/fluids from the asthenospheric mantle, although the presence of wehrlitic patches may indicate interaction with carbonate melts. In both depleted and enriched xenoliths, trace element patterns for separated clinopyroxenes closely reflect those of the bulk rock, except for Rb, Ba and Nb, which are probably hosted by other phases.

Petrology and geochemistry of late Tertiary/Quaternary mafic alkaline volcanism in Romania

Abstract Alkaline volcanic activity occurred in the Persani Mountains (eastern Transylvanian Basin) and Banat (eastern Pannonian Basin) regions of Romania between 2.5 Ma and 0.7 Ma. This volcanism followed an extended period of subduction-related mostly andesitic and dacitic magmatism in the Eastern Carpathian arc. The Persani Mts. alkaline activity coincided with the last phase of subduction-related activity. Several lava flows and pyroclastic deposits in the Persani Mts. carry peridotitic mantle xenoliths and amphibole megacrysts. Major- and trace-element geochemistry indicates that the alkaline magmas are primitive, silica-undersaturated alkali basalts and trachybasalts (7.8–12.3 wt.% MgO; 119–207 ppm Ni; 210–488 ppm Cr) which are LREE-enriched. Mantle-normalised trace-element diagrams reveal an overall similarity to continental intraplate alkali basalts, but when compared with a global average of ocean island basalts (OIB), the Banat lavas are similar to average OIB, whereas the Persani Mts. basalts have higher Rb, Ba, K and Pb and lower Nb, Zr and Ti. These features slightly resemble those of subduction-related magmas, particularly those of a basaltic andesite related to the nearby older arc magmas. With 87 Sr 86 Sr varying from 0.7035-0.7045 and 143 Nd 144 Nd from 0.51273-0.51289, the Romania basalts are indistinguishable from those of the western Pannonian basin (Hungary and Austria) and Neogene alkali basalts throughout Europe. Amphibole megacrysts have similar isotopic signatures, and their REE patterns indicate derivation by crystallisation from a mafic alkaline magma. The age-corrected Sr and Nd isotopic compositions of a calc-alkaline basaltic andesite related to the preceeding period of subduction also lies within the field of the younger alkaline magmas. Pb isotopic ratios of the Romanian alkali basalts do not lie on the NHRL, but overlap the field of Tertiary alkali basalts from the western Pannonian basin, and have unusually high 207 Pb 204 Pb at a given 206 Pb 204 Pb . Thus it is probable that, although the Romanian alkali basalts have a strong asthenospheric (i.e. OIB-type mantle source) component, their Pb isotopic characteristics were derived from mantle which was affected by the earlier subduction.

Mafic alkaline magmatism associated with the European Cenozoic rift system

Abstract Primitive mafic alkaline volcanic rocks from the Cenozoic rift system of western and central Europe have major and trace element and Sr-Nd-Pb isotopic characteristics which suggest the involvement of both lithospheric and asthenospheric mantle source components in their petrogenesis. The geochemical characteristics of the lithospheric component are in part constrained by those of spinel lherzolite and mafic granulite xenoliths entrained within the magmas. This component appears to be the product of partial melting of phlogopite/amphibole bearing mantle which was metasomatised as a consequence of magmatic activity during and preceeding the Hercynian orogeny and during subsequent phases of Permo-Carboniferous and Cenozoic extension. Partial melting of phlogopite is required to account for the generation of potassic magmas (leucitites and leucite nephelinites) with K 2 O Na 2 O ratios > 1. This component appears to differ between the individual Hercynian terrane blocks of Europe, reflecting, in part, their previous magmatic histories. The asthenospheric component has affinities with the source of HIMU OIB. This could represent a zone of enriched mantle in the thermal boundary layer at the base of the subcontinental lithosphere which is preferentially partially melted during extension. Alternatively it could, in part, be 500-400 Ma oceanic lithosphere subducted during the Hercynian orogeny. Although the data do not preclude the existence of deep mantle plumes, there is no need to invoke them in order to explain the HIMU characteristics.

Isotopic and trace-element arguments for the lower-crustal origin of Hercynian granitoids and pre-Hercynian orthogneisses, Massif Central (France)

Abstract The Cadomian-Hercynian basement of the Massif Central, France, contains both pre-metamorphic orthogneisses (600-410 Ma) and post-metamorphic undeformed granitoids (360-275 Ma). Major-, trace-element and Sr, Nd isotopic analyses of these intrusions indicate slight differences in source material for orthogneisses and two types of underformed granitoid: (a) granodiorite/monzogranites and leucogranites. Orthogneisses have moderately enriched REE patterns ( La Yb =5.3−13.3 ), variable HREE contents (YbN=4.2−25) and strong negative Eu anomalies ( Eu Eu ∗=0.23−0.36 ). Their ∈tNd-values are low (−5.8 to −1.7), indicating a strong crustal component in the source but ∈tSr-values are also generally low (−25 to +100) and overlap the field of age-corrected isotopic values for lower-crustal granulite xenoliths from the Massif Central. The source of the orthogneisses is therefore considered to be dominated by a lower-crustal meta-igneous component with additional input from a lower-crustal metasedimentary source. Granodiorites/monzogranites tend to show strong LREE enrichment ( La Yb =11−26 ) with small Eu anomalies ( Eu Eu ∗=0.54−0.75 ) and YbN values between 6 and 8.5. These granitoids form the least evolved end-members of a series trending to highly evolved leucogranites with similar La/Yb ratios (11–36) but with generally larger Eu anomalies ( Eu Eu ∗=0.24−0.62 ) and lower YbN values (3–8). Initial isotopic ratios of the leucogranites (∈tNd = −7.9 to −4.5 and ∈tSr usually > +100) mostly fall within the field of age-corrected lower-crustal metasedimentary granulite xenoliths from the region and it is proposed that these leucogranites were formed by partial melting of lower-crustal metasediments. Granodiorites and monzogranites have ∈tNd between −6.5 and −3.4 and ∈tSr-values

Tertiary-Quaternary intra-plate magmatism in Europe and its relationship to mantle dynamics

Abstract Anorogenic intra-plate magmatism was widespread in Europe from early Tertiary to Recent times, extending west to east from Spain to Bulgaria, and south to north from Sicily to central Germany. Magmatism is spatially and temporally associated with Alpine-Pyrenean collisional tectonics, the development of an extensive lithospheric rift system in the northern foreland of the Alps, and, locally, with uplift of Variscan basement massifs (Massif Central, Rhenish Massif, Bohemian Massif). The volcanic regions vary in volume from large central volcanoes (e.g. Cantal, Massif Central;Vogelsberg, central Germany), to small isolated plugs (e.g. Urach and Hegau provinces in southern Germany). Within the Mediterranean region, the Dinarides, the Pannonian Basin and Bulgaria, anorogenic volcanism locally post-dates an earlier phase of subduction-related magmatism. The major and trace element and Sr-Nd-Pb isotope characteristics of the most primitive mafic magmatic rocks (MgO > 6 wt%) provide important constraints on the nature of the mantle source and the conditions of partial melting. These are predominantly sodic (melilitites, nephelinites, basanites and alkali olivine basalts); however, locally, potassic magma types (olivine leucitites, leucite nephelinites) also occur. In several localities (e.g. Sicily; Vogelsberg and the Rhine Graben, Germany; Calatrava, central Spain) olivine and quartz tholeiites form a significant component of the magmatism. The sodic magmas were derived by variable degrees of partial melting (c. 0.5-5%) within a transitional zone between garnet-peridotite and spinel-peridotite mantle facies, close to the base of the lithosphere; the potassic magma types are interpreted as partial melts of enriched domains within the lithospheric mantle. Mantle partial melting was induced by adiabatic decompression of the asthenosphere, locally in small-scale, plume-like, diapirs, which appear to upwell from c. 400 km depth